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Growth and characterization of diamond and diamond like carbon films with interlayer
h [electronic resource] /
by Roja Gottimukkala.
[Tampa, Fla] :
b University of South Florida,
ABSTRACT: Diamond and diamond-like carbon films, with their exceptionally good mechanical, chemical, and optical properties, are the best materials as protective hard coatings for electronic devices and cutting tools. The biocompatibility of these materials makes it suitable for bone implants. The wide range applications of these films are hindered because of the high compressive stresses developed during the deposition. Use of carbide and nitride interfacial layers has emerged as one of the methods to reduce the compressive stresses.The present research focuses on the study of different materials as the interfacial layers for diamond and tetrahedral amorphous carbon films. For tetrahedral amorphous carbon AlN, Ta, TiN, TiC, TaN and W were investigated as the interlayer materials. The interlayer was deposited at different substrate temperatures to study the temperature induced changes in the residual stress. The tetrahedral amorphous carbon with TiN interlayer deposited at 300Â¨C and 600Â¨C exhibited a maximum reduction in the stress.TiN and TiC were deposited as interlayer for the diamond films on Ti-6Al-4V alloy. TiC has improved the adhesion of diamond with the substrate and exhibited less compressive stresses compared to TiN.
Thesis (M.S.)--University of South Florida, 2005.
Includes bibliographical references.
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Adviser: Ashok Kumar, Ph.D.
Pulsed laser deposition.
Chemical vapor deposition.
x Mechanical Engineering
t USF Electronic Theses and Dissertations.
Growth and Characterization of Diamond and Diamond like Carbon Films with Interlaye r by Roja Gottimukkala A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Department of Mechanical Engineering College of Engineering University of South Florida Major Professor: Ashok Kumar, Ph.D. Frank Pyrtle, III, Ph.D. Muhammad Rahman, Ph.D. Date of Approval: November 4, 2005 Keywords: Pulsed Laser Deposition, Chemical Vapor Deposition, Stress, Frict ion Coefficient, Nanoindentation Copyright 2005, Roja Gottimukkala
DEDICATION To God and My Family
ACKNOWLEDGEMENTS I am indebted to everyone who helped me throughout my research work to make this work successful. I thank God and my family for their love and support, for encouraging me to seek for myself a meaningful education. Deepest thanks to my a dvisor, Dr. Ashok Kumar for his consistent trust and support. I would also like to express my sincere gratitude to thesis committee members, Dr. Frank Pyrtle, III a nd Dr.Muhammad Rahman for being on the committee. Just a word Â‘thanksÂ’ is not adequate to a cknowledge the help and encouragement given by my friend Harish during the research w ork. Thanks are due to Raghu, for his encouragement and valuable suggestions. Thanks are in order for all my colleagues and my friends (especially Sriram and Swetha) for their encouragement and moral support during the research period.
i TABLE OF CONTENTS LIST OF TABLES iii LIST OF FIGURES iv ABSTRACT vi CHAPTER 1 INTRODUCTION TO DIAMOND AND DIAMOND RELATED MATERIALS 1 1.1 Different Forms of Carbon 1 1.2 Diamond 2 1.2.1 History of Diamond 4 1.2.2 Structure of Diamond 5 1.2.3 Properties of Diamond 6 1.2.4 Growth of Diamond 8 1.2.5 Deposition Techniques 8 18.104.22.168 Hot Filament Assisted CVD 8 22.214.171.124 DC Plasma Assisted CVD 9 126.96.36.199 RF Plasma Assisted CVD 9 188.8.131.52 Microwave Plasma Assisted CVD 10 184.108.40.206 Electron Cyclotron Resonance Â–MPACVD 10 1.2.6 Applications of Diamond Films and Coatings 10 1.3 Diamond-like Carbon (DLC) Films 11 1.3.1 History of Diamond-like Carbon Films 12 1.3.2 Structure of Diamond-like Carbon Films 13 1.3.3 Mechanical Properties 14 1.3.4 Deposition Techniques 15 220.127.116.11 Filtered Cathodic Arc Deposition 15 18.104.22.168 Pulsed Laser Deposition 16 22.214.171.124 Magnetron Sputtering 16 1.3.5 Applications 17 1.4 Literature Review 17 1.5 Overview of the Thesis 19 CHAPTER 2 EXPERIMENTAL TECHNIQUES 21 2.1 Diamond-like Carbon Films 22 2.1.1 Pulsed Laser Ablation 22
ii 2.1.2 Experimental Procedure 26 2.2 Diamond Films 27 2.2.1 CVD Deposition Technique 27 126.96.36.199 Microwave Plasma Assisted CVD 30 2.2.2 Experimental Procedure 31 CHAPTER 3 STRUCTURAL AND MECHANICAL CHARACTERIZATION 32 3.1 Raman Spectroscopy 32 3.2 Nanoindentation 34 3.3 Nanoindentation Data Analysis 36 3.3.1 Doerner and Nix Method 36 3.3.2 Oliver and Pharr Method 36 3.3.3 Basic Equations Involved 36 3.4 X-Ray Diffraction 39 3.5 Friction Test 41 3.5.1 Description of UMT 42 CHAPTER 4 RESULTS AND DISCUSSION 44 4.1 Amorphous Carbon Films without Interlayer 44 4.1.1 Raman Spectroscopy 45 4.1.2 Nanoindentation 48 4.2 Tetrahedral Amorphous Carbon Films with Interlayer 50 4.2.1 Raman Spectroscopy 51 4.2.2 Nanoindentation 56 4.2.3 Friction Test 61 4.3 Diamond 62 4.3.1 Raman Spectroscopy 63 4.3.2 X-ray Diffraction 64 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 66 REFERENCES 68
iii LIST OF TABLES Table 1.1 Properties of Various Forms of Carbon 2 Table 1.2 Properties of Diamond 3 Table 1.3 Properties of Diamond Compared to Other Semiconductors 7 Table 2.1 Properties of Diamond and DLC Materials 21 Table 2.2 History of CVD Diamond 30 Table 4.1 Residual Stress in the Tetrahedral Amorphous Carbon Films Obtained from Raman Spectra 47 Table 4.2 Hardness and YoungÂ’s Modulus for Tetrahedral Amorphous Carbon Films Deposited at Various Temperatures 49 Table 4.3 Reduction in Stress (GPa) Obtained from Raman Analysis 55 Table 4.4 Summarized Results Obtained from Nanoindentation for Layered Tetrahedral Amorphous Carbon Films 57 Table 4.5 Friction Coefficient of Layered ta-C Films 61 Table 4.6 Increase in Stress (GPa) Obtained from Raman Analysis 64
iv LIST OF FIGURES Figure 1.1 Structure of Diamond 6 Figure 1.2 Ternary Phase Diagram of DLC Films Showing sp 3 sp 2 and H Contents 14 Figure 2.1 Schematic Representation of Laser-Target Interactions 23 Figure 2.2 PLD System at USF 25 Figure 3.1 Renishaw Raman Spectrometer at USF 34 Figure 3.2 Plot of Load vs Displacement Data 35 Figure 3.3 a) Schematic of Indenter and Specimen Geometry b) Load Displacement Curve 38 Figure 3.4 Schematic of Nanoindenter XP 39 Figure 3.5 X-Ray Diffraction 41 Figure 3.6 Universal Micro Tribometer with PC Based Feed Back Control 42 Figure 4.1 Raman Spectra of the Tetrahedral Amorphous Carbon Films Deposited at Different Temperatures 45 Figure 4.2 Modulus vs Displacement Plot of Tetrahedral Amorphous Carbon Films Deposited at Different Temperatures 49 Figure 4.3 Hardness vs Displacement Plot of Tetrahedral Amorphous Carbon Films Deposited at Different Temperatures 50 Figure 4.4 Raman Spectra of ta-C Film with Ta as Interfacial Layer 52 Figure 4.5 Raman Spectra of ta-C Film with Different Interlayers Depos ited at 100C 53 Figure 4.6 Raman Spectra of ta-C Film with Different Interlayers Depos ited at 300C 53 Figure 4.7 Raman Spectra of ta-C Film with Different Interlayers Depos ited at 600C 54
v Figure 4.8 Reduction in Stress vs Interlayer Material at Different Tem peratures 56 Figure 4.9 Modulus vs Displacement Plot of ta-C Film with the Interlayers Deposited at 100C 58 Figure 4.10 Hardness vs Displacement Plot of ta-C Film with the Interlayer s Deposited at 100C 58 Figure 4.11 Modulus vs Displacement Plot of ta-C Film with the Interlayers Deposited at 300C 59 Figure 4.12 Hardness vs Displacement Plot of ta-C Film with the Interlayer s Deposited at 300C 59 Figure 4.13 Modulus vs Displacement Plot of ta-C Film with the Interlayers Deposited at 600C 60 Figure 4.14 Hardness vs Displacement Plot of ta-C Film with the Interlayer s Deposited at 600C 60 Figure 4.15 Raman Spectra of Diamond Films with Different Interfaces 64 Figure 4.16 XRD of original substrate and TiCdiamond film 65
vi GROWTH AND CHARACTERIZATION OF DIAMOND AND DIAMOND LIKE CARBON FILMS WITH INTERLAYER Roja Gottimukkala ABSTRACT Diamond and diamond-like carbon films, with their exceptionally good mec hanical, chemical, and optical properties, are the best materials as prote ctive hard coatings for electronic devices and cutting tools. The biocompatibility of these ma terials makes it suitable for bone implants. The wide range applications of these fi lms are hindered because of the high compressive stresses developed during the deposit ion. Use of carbide and nitride interfacial layers has emerged as one of the methods t o reduce the compressive stresses. The present research focuses on the study of different material s as the interfacial layers for diamond and tetrahedral amorphous carbon films. For tetrahedral amorphous carbon AlN, Ta, TiN, TiC, TaN and W were investigated as the interla yer materials. The interlayer was deposited at different substrate temperatures to study the temperature induced changes in the residual stress. The tetrahedral amorphous carbon with TiN interlayer deposited at 300C and 600C exhibited a maximum reduction in the stress.
vii TiN and TiC were deposited as interlayer for the diamond films on Ti-6Al-4V all oy. TiC has improved the adhesion of diamond with the substrate and exhibited less compressive stresses compared to TiN.
1 CHAPTER 1 INTRODUCTION TO DIAMOND AND DIAMOND RELATED MATERIALS 1.1 Different Forms of Carbon Carbon is the sixth most abundant material that exists in about mil lion different compounds in different forms. Diamond, graphite, fullerene, and amorphous carbon are the four known allotropes of carbon. The atomic arrangement makes the carbon to exist in different forms. Graphite is soft and slippery in contrast to di amond which is the hardest material known to man. Diamond is an abrasive whereas gr aphite is a good lubricant because the graphite layers stick to each other. These extremely diverse properties can be attributed to the type of bonding between carbon atom s. Diamond has sp 3 hybridized orbital in which four carbon atoms form covalent bonding w ith the adjacent atoms resulting in a tetrahedral structure. Graphite has sp 2 bonding where three carbon atoms form a trigonal structure in a plane and p bonding exists between the layers. Graphite has low coefficient of friction because of thi s p bonding between the layers. The other type of bonding is sp 1 where in the two valence electrons of carbon atom form bonds in a plane and the other two electrons form the p bond. Diamond-like carbon has a mixture of sp 3 and sp 2 bonding. It is hard to make diamond because carbon tends to be in graphite form under normal conditions as it is more sta ble. Even though graphite is the stable form, diamond once formed would never
2 alter to graphite. Amorphous carbon films exhibit mechanical, electrical and opt ical properties similar to that of diamond but they lack the crystalline structure. Table 1.1 Properties of Various Forms of Carbon Density (g/cm 3 ) Hardness(GPa) % sp 3 at% H Band Gap(eV) Diamond 3.515 100 100 5.5 Graphite 2.267 0 ~0.04 Glassy C 1.3-1.55 2-3 ~0 0.01 a-C,evap. 1.9-2.0 2-5 1 0.4-0.7 a-C,sputt 1.9-2.4 11-15 2-5 0.4-0.7 a-C:H, hard 1.6-2.2 10-25 30-60 10-40 0.8-1.7 a-C:H,soft 0.9-1.6 <5 50-80 40-65 1.6-4 Polyethylene 0.92 0.01 100 67 6 ta-C 3.0 55-65 mainly <1 1.2 Diamond Diamond, the marvel material, represents the extreme value for any material property. The word "diamond" derives from the Greek word adamas which means "impossible to tame". Diamond is rare in nature because carbon reacts with ma ny elements forming different compounds and graphite formation is faster than diamond under ordinary conditions. Owing to its high hardness, diamond is used in grinding, polishi ng and making weapons. Diamond studded rotary drill bits and saws were use d to dig oil wells
3 and bore holes through hard rocks. These are only a few applications of diamond. Naturally available diamond is difficult to be engineered into dif ferent physical configurations. Hence, the synthesis of diamond as a thin film or a coating has gained interest in both research and industry. Though diamond can be found from diff erent sources, about 90 tons of single crystal diamond are synthetically ma de from graphite at high temperatures and pressures every year, and are cheaper c ompared to natural diamond. Polycrystalline diamond can be sintered and the diamond thin film s are grown using the most promising chemical vapor deposition technique. Though the c ost of these films is more than naturally occurring diamond, their application c an be justified economically. Basic properties of diamond are given in Table 1.2 Table 1.2 Properties of Diamond Property Value Crystal structure FCC (sp 3 bonded, tetrahedral) Atomic density 1.76 X 10 23 cm -3 Lattice constant 3.567 A o Hardness 1.0 X 10 4 kg/mm 2 Tensile strength >1.2 GPa Compressive strength >110 GPa Coefficient of friction 0.03 Sound velocity 1.8 X 10 4 ms -1 Density 3.52 gcm -3 YoungÂ’s Modulus 1.22 GPa PoissonÂ’s Ratio 0.2 Thermal expansion coefficient 1.1 X 10 -6 k -1 Thermal conductivity 2500 W/m-k
4 Table 1.2 (continued) Thermal shock parameter 3.0 X 10 8 Wm -1 Debye temperature 2200 K Optical index of refraction(at 591 nm) 2.41 Optical transmittivity 225 nm (UV) to long IR (> 25 m),IR absorption band from 2-7 m Loss tangent at 40 Hz 6.0 X 10 -4 Dielectric constant 5.7 Dielectric strength 1.0 X 10 -7 Vcm -1 Electron mobility 2200 cm 2 (Vs) -1 Hole mobility 1600 cm 2 (Vs) -1 Electron saturated velocity 2.7 X 10 7 cms -1 Hole saturated velocity 1.0 X 10 7 cms -1 Work function Negative for  surface -1 eV Bandgap 5.45 eV Resistivity 10 13 -10 16 -cm 1.2.1 History of Diamond The existence of diamond can be dated back to biblical times. It is believed that they are first found in the river basins of Godavari and Krishna in India. Though i t is a widely known fact that diamond is famous as a precious stone in jewelry, it was shown that the diamond had many other applications. It was first used in grinding and polishing. But unfortunately, the naturally available diamond cannot be engineered i nto different physical configurations. The saga of the efforts to produce syntheti c diamond started in early 1950Â’s. The first fruitful effort to grow diamond at low pres sures was by William Eversole of Union Carbide Corporation in 1952. He produced diamond on a diamond
5 seed crystal using carbon monoxide as source gas. No later, Genera l Electric came up with the idea of growing diamond at high temperature and high pressur e in 1955. In 1960Â’s Angus proposed the Chemical Vapor Deposition (CVD) technique as the most feasible method for growing diamond thin films. The diamond obtained was i n metastable phase. They found that the diamond could be efficiently grow n in excessive hydrogen. Though the molecular hydrogen ceased the growth of the g raphite to a good extent, the need for removing the deposited graphite was accomplished by atomic hydrogen. Gardener first generated the atomic hydrogen using a hot tungsten filament and this was used to etch the graphite deposits. In 1970Â’s Deryagin reported the growth of diamond films on non diamond substrates . In 1980, researchers at Nati onal Institute for Research in inorganic Materials (NIRIM) used a hot filam ent to activate methane and hydrogen. The success of NIRIM in growing metastable diamond at a rapid rate augmented the interest of many groups in the low pressure diamond growth. 1.2.2 Structure of Diamond The structure of diamond, to which the various extreme properties ca n be attributed, is shown in Figure 1.1 All the important applications of diamond have the most common face centered cubic crystal with four atoms spaced at a quarter of a cube diagonal. Diamond also has a hexagonal structure known as lonsdaleite, mostl y found in meteorites.
6 Figure 1.1 Structure of Diamond 1.2.3 Properties of Diamond Hardness is the most important characteristic of diamond that make s it an ideal material for cutting and grinding tools. The hardness of the diamond is due to its atomic arrangement. Different tools like drill bits, wire drawing dies and extrusion dies are coated with diamond films but care should be taken regarding the adhesion of these film s. Because the diamond reacts and dissolves in ferrous materials, they are not applied to ferrous materials. Its other properties include high thermal conduct ivity, chemical inertness, and low coefficient of thermal expansion, high yield str ess, optical transparency, exhibits excellent tribological properties and elec trical properties. Though natural diamond exhibits exceptional properties, it cannot be engineered to all desired combinations of these properties. Diamond as a thin film or coating has potential to exhibit more combinations for specific applications. The high hardness and optical transmissivity of diamond makes it an ideal material for radome s and windows. Diamond with large dielectric constant, low mass density and low coeffic ient of thermal expansion is good to use as a capacitor material. The high thermal conduct ivity of diamond makes it the best choice for heat spreaders and heat exchangers. Most of its applications lie in the
7 electronic devices. Diamond films are used to coat magnetic tap es and diamond fibers are used in composites for reinforcement. Due to high band gap of 5.5ev, dipoles ar e not present in the diamond and hence the spectral transmission is wide. Few fundamental properties of other competing materials are lis ted in Table 1.3. Though materials like silicon, germanium and silver have the same struct ure, it can be observed that most of the properties of diamond are much better than its competitors. Table 1.3 Properties of Diamond Compared to Other Semiconductors Property Silicon SiC Diamond Lattice constant (A) 5.43 4.358 ( for -SiC) 3.567 Thermal expansion (in 10 -6 C) 2.6 4.7 1.1 Density(gcm -3 ) 2.328 3.216 3.515 Melting point( in C) 1420 2540 4000 Max Operating Temp ( C) 225 1900 Bandgap(eV) 1.1 3.0 5.45 Saturated electron velocity( X 10 7 cms -1 ) 1.0 2.5 2.7 Maximum electron velocity( cms -1 ) 1 X 10 7 1 X 10 7 Electron mobility (cm 2 (Vs) -1 ) 1500 400 2200 Hole mobility(cm 2 (Vs) -1 ) 600 50 1600 Break down (X 10 5 Vcm -1 ) 3 40 100 Dielectric constant 11.8 9.7 5.5 Resistivity ( cm) 10 3 150 10 13 Thermal conductivity (Wcm -1 K -1 ) 1.5 5 25 Absorption edge (m) 1.4 0.4 0.2 Refractive index 3.5 2.65 2.42 Cohesive Energy (eV) 4.64 6.34 7.36
8 Table 1.3 (continued) YoungÂ’s Modulus (GPa) 130 450 1200 Shear Modulus (GPa) 80 149 577 Hardness(kg/mm 2 ) 1000 3500 10000 Fracture Toughness 1 5.2 5.3 1.2.4 Growth of Diamond Two different growth mechanisms are in existence for growing di amond. One is the equilibrium growth at high pressure and the other being the metastable growth at subatomic pressures. The equilibrium growth of diamond can be carried out by a static or dynamic process. In a static process, the metastable graphite is dissolved into a liquid solvent and in a high-pressure press diamond precipitates as a stable phase whereas in a dynamic process graphite directly converts to diamond at high pre ssure and high temperature. Hydrocarbon gases are used for the metastable growth of diamond at sub atmospheric pressure. All the methods available for metastable grow th of diamond differ in the means of generating hydrogen atoms. 1.2.5 Deposition Techniques This section gives a brief overview of the popular deposition techniqu es for growing diamond films. 188.8.131.52 Hot Filament Assisted CVD A tungsten filament placed above the substrate is heated at high t emperature and a mixed gas of CH 4 diluted in H 2 is passed through the reactor for the growth of diamond films.
9 Unless the filament is hot, neither diamond nor the graphite can be de posited. The disadvantage is that the filament is carburized with the feed gas and thereby effects the growth of diamond. The atomic hydrogen helps the formation of single bonded carbon atoms and results in the formation of tetrahedral arrangement. 184.108.40.206 DC Plasma Assisted CVD In this method, a DC bias is applied between the plate and the s ubstrate. The plasma produced generates atomic hydrogen and carbon precursors for the growt h of diamond. High nucleation densities of 10 8 cm -2 at a growth rate of 20 /hr are possible. The advantage of this method is that no substrate pretreatment is requi red when diamond is grown on non-diamond substrates. However, the diamond films produced in DC plasmas were reported to have high stresses and contain high concentration of hydrogen and other impurities due to the plasma erosion of the electrodes. It was obser ved that when a positive bias is applied to the substrate and negative to the plate, dia mond is deposited and in converse, graphitic carbon is produced when the applied bias is reversed. 220.127.116.11 RF Plasma Assisted CVD The deposition is carried by inductive and plasma heating at a subst rate temperature above 800C. It was found that the high power in the discharge is neces sary for efficient diamond growth. The grown diamond is found to have small grain size and good adhesion to the substrate. However, the high power results in the contami nation forming SiC from the silicon crucible.
10 18.104.22.168 Microwave Plasma Assisted CVD MPACVD is the popular technique for the development of diamond films. Surface wave sustained plasma excites the gas phase. The substrate is placed in the middle of the plasma and heated in the range of 800C-1100C by a heater. With this m ethod, deposition is performed continuously from tens to hundreds of hours. The disadva ntage of this system is that the quality of the film decreases with the increase i n deposition rate. 22.214.171.124 Electron Cyclotron Resonance Â–MPACVD In this method, the plasma is produced when the axial magnetic fi eld due to the electromagnets has a value of 875 G (ECR condition). Electrons rec eive resonance impulses by preferentially absorbing the generated microwave s. The magnetic mirror confinement occurs at the ends of the plasma chamber and the ECR c ondition in the downstream. When the two electromagnets are placed close to the microwave window, ECR condition occurs with in the plasma chamber. The drawbacks from most of these methods were the low growth rates, high temperatures and the diffi culty in depositing on non diamond substrates. So the initial goals were to produce high quality diamond films on non diamond substrates at a higher growth rate and suitable temperat ure at which the growth of graphite would be inhibited. 1.2.6 Applications of Diamond Films and Coatings Optical coatings for the protection of softer materials agains t micro particle abrasion.
11 Antireflection coatings for diamond optics. Oil-less diamond bearings are expected to have better performanc e than existing lubricated ones. Marine life does not attach to the diamond surfaces Diamond as a protective coating against salt-water corrosion. Diamond coatings can significantly reduce water drag and elimin ate the need for liquid surfactants Semi conductor devices such as high frequency amplifiers, high power microwave devices due to its electron, hole mobilityÂ’s, break down str engths, and reasonable band gap at high temperatures and relatively low leakag e currents. Surface Acoustic Wave (SAW) applications due to high acoustic velocities. Diamond photodiodes Diamond field emitters due to its negative electron affinities (NEAÂ’s ) Heat exchangers, condensers to furnaces due to high thermal conductivity Fabrication of high aspect ratio MEMS and NEMS components 1.3 Diamond-like Carbon (DLC) Films Diamond-like carbon (DLC) films are amorphous in structure and has the properties almost comparable to that of diamond. The properties of these films pr edominantly depend on the deposition methods and process conditions employed. Even today, the nature of these materials is not understood to the full extent. Thes e films are broadly classified as hydrogenated and non hydrogenated. The hydrogen conte nt predominantly affects the structure of DLC films. Hydrogen content is less than 1% in non-hyd rogenated
12 DLC films where as, it can be about 60% in hydrogenated DLC films. Usually, these films may have a combination of sp 3 sp 2 and sometimes even sp 1 bonds. Diamond-like hydrocarbons (a-C:H) are mostly deposited either by hydrocarbon ion beams or r.f. self bias plasma assisted CVD. The presence of hydrogen not only stabilizes the dangling bonds but also contributes in achieving wide optical gap and hi gh electrical resistivity. The other major group is hydrogen free diamond-like ca rbon films popularly known as tetrahedral amorphous carbon as the films have a higher frac tion of sp 3 bonds. The mass densities for these films vary from 1.4 to 2.0 g/cm 3 DLC films usually have high compressive stress and the stress level increases with the increase in hydrogen content. The adhesive strength decreases with the increase in the hydrogen c ontent. a-C:H films doped with Nitrogen have shown a reduction in compressive stress. DLC films hav e a smoother surface compared to diamond and hence have many applications that cannot be fulfilled by diamond films. Amorphous carbon (a-C) is deposited eithe r by sputtering or by the pulsed laser ablation. PLD is the technique used for this research. The carbon plumes formed by laser vaporization are condensed on the substrate re sulting in hydrogen free diamond-like carbon films. These films have carbon with gre ater mass densities and high hardness. 1.3.1 History of Diamond-like Carbon Films Aisenberg and Chabot in 1971, first deposited diamond-like carbon films using carbon ions and these films had many properties of diamond. DLC are meta stable and amorphous in nature. They can be deposited at temperatures less than 325 C . Kaplan et al in 1985 found that hydrogenated DLC films, depending on the deposition m ethod,
13 contains hydrogen varying from 10% to 60%. In 1986, Robertson reported that e ven though DLC lack long-range order they possess short or medium range or der . In 1971, Aisenberg and Chabot succeeded in depositing hydrogen free DLC f ilms using C + ions in Argon environment. 1.3.2 Structure of Diamond-like Carbon Films The structure of DLC consists of both three fold coordinated sp 2 and four fold coordinated sp 3 While the sp 2 controls the electrical properties like band gap, sp 3 controls the mechanical properties like hardness and rigidity of the film. The four fold coordinated sp 3 is the characteristic of hard diamond. The four valance electrons of carbon atom are tetrahedrally bonded to each other forming the str ong s bond. The three folded sp 2 carbon atom is the characteristic of graphite. In graphite, the thre e valence electrons of carbon atom form a strong s bond and the fourth atom forms a weak and unsaturated p bond normal to the s bond.
14 Figure 1.2 Ternary Phase Diagram of DLC Films Showing sp 3 sp 2 and H Contents Figure 1.2 represents the compositions of DLC films with respect to sp 3 sp 2 and H contents. From the phase diagram, it can be concluded that the hydrogenat ed amorphous carbon films has less percent of sp 3 bonds. The ta-C has the highest sp 3 fraction. Even though the films have same sp 3 and hydrogen content, the properties of the film vary depending on the sp 2 clusters in the film. Sputtered amorphous carbon films have more of sp 2 content. The a-C:H films are located at the center of the pha se diagram and has varying amounts of sp 3 sp 2 and H contents. With the increase in the hydrogen content, sp 3 content decreases and the optical transmission and band gap increases. 1.3.3 Mechanical Properties The mechanical properties like hardness, friction, wear resist ance and elasticity depend mainly on the strength of the sp 3 bond. The presence of hydrogen does not affect the elastic property of the film. It mostly depends on the sp 3 and sp 2 content. This property is the measure of resistance to deformation. The intrinsic stresses in the film also determine
15 the quality of the film. The intrinsic stresses limit the ma ximum thickness of the DLC films that in turn limits its use in applications demanding the thi ck coatings. It has been shown that the stresses in the film can be reduced by reducing t he sp 3 content in the film . The sp 3 bonds usually strain the links resulting in high compressive stress es. Dekempeneer et.al. observed the decrease in the stress with the change in bias voltage from -200V to -400V . Most of the important applications of DLC can be attributed to its friction and wear properties combined with its hardness. Pin on disk is the most popular method used for measuring the coefficient of friction. 1.3.4 Deposition Techniques Diamond-like carbon films are divided into two main sections. One being the hydrogenated and the other is non-hydrogenated DLC. Numerous techniq ues are available to deposit diamond-like carbon films. But not all the techniques can produce hydrogen free a-C coatings. In this section, different deposition te chniques available for depositing hydrogen free a-C thin films are discussed. 126.96.36.199 Filtered Cathodic Arc Deposition This is the best technique to produce hard tetrahedral amorphous carbon thin films. The apparatus is equipped with a magnetic filtering technique that e fficiently removes the macro particles and hence improves the smoothness of DLC film even at the room temperature. Graphite is used as the cathode source and carbon ions a re produced in vacuum, between the graphite cathode and anode. The arc currents used are in the range
16 of 40 Â– 90 A. A bias voltage of 100 to 300V is applied to control the ion ene rgy at the substrate. The sp 3 content in these films is very high and thus the films exhibit hi gh hardness compared to the films deposited by other techniques. 188.8.131.52 Pulsed Laser Deposition Pulsed laser ablation is considered as the best technique for the deposition of hydr ogen free diamond-like carbons. The sp 3 content is high in PLD deposited films because the laser energy transforms the sp 2 bonds to sp 3 bonds. The shortwave length of the excimer laser results in the deposition of atomically smooth surface of the films. 184.108.40.206 Magnetron Sputtering N.H Cho et al, reported the change in structure and physical properti es of sputter deposited amorphous carbon with the change in the power density. They showe d that graphitic features increase with the increase in the power densi ty. Hydrogen free DLC can be deposited by sputtering a carbon target in the presence of Argon gas. Even hydrogenated diamond can also be deposited using a mixture of Argon and Hydrogen gas while sputtering. Paik  showed that the amount of sp 3 bonds in the film depends on the ion energy. It was observed that the sp 3 content increased with the increase in the ion energy. The pyrolitic graphite target is sputtered in the presenc e of Argon gas to get the amorphous diamond-like carbon films. Hydrogenated DLC can also be prepare d by passing a suitable hydrogen precursor gas.
17 1.3.5 Applications Because of the low friction, high wear resistance and reasonable ha rdness DLC films have many applications. Few of them are listed below: DLC is used as protective coatings from razor blades to magnetic tapes. With its self-lubricating property, DLC facilitates the motion of hard disk beneath a floating head. DLC is used as anti reflecting coating for Silicon solar cells. High hardness combined with a low coefficient of friction makes D LC the best material for MEMS applications DLC coated cutting tools exhibited an increased lifetime. Biocompatible material for orthopedic implants. Cold cathode field emitters Anti-scratch coatings for optical applications, infrared optics, sunglasses, optical Lenses. IR sensors and lenses. Optical and electronic components. 1.4 Literature Review Diamond and diamond-like carbon coatings has been the topic of intere st to the researchers because of its excellent mechanical, tribological and optical properties. But unfortunately, the high compressive stress of these films demotes its numerous applications. Due to the formation of sp 3 bonds, the compressive stresses are more in these films and hence have the poor adhesion. Stresses in the films can be relieved by
18 carrying out the deposition at high substrate temperature. Unlike diamond, which is stable even at high temperatures, DLC graphitizes at a temperature near to 400C. To overcome these problems many researchers have succeeded in developing meta l-doped DLC films. These films have showed better adhesion and improved wear properties TiC and WC were successfully incorporated in DLC films and improvement in the film properties has been obtained . A.A.Voevidin et.al.  designed a stack of multilayered interlaye rs with different ceramics and soft metals to improve the adhesion of DLC films on s tainless steel substrate. The layers of the selected materials were deposite d in such a way that the modulus increases from bottom to the top layer. A combination of magnet ron sputtering and PLD were used to deposit functionally gradient metal/ carbide/ DLC coatings. The use of ductile Ti interlayer increased the coating scratch re sistance, Ti and TiC interlayers enhanced the friction coefficient and the wear rates, compared to t he single layered DLC films. C.Donnet et.al. , conducted experiments to study the relationship be tween the deposition conditions and the film composition, the properties like stress, and friction of the interlayered DLC films. Ti/a-C:H films were deposited us ing a hybrid technique of magnetron sputtering and d.c plasma enhanced chemical vapor deposition wi th the bias voltage between -35V to -260V to optimize the deposition conditions exhibit ing ultra low friction in ultra high vacuum. The investigation revealed that dependi ng on deposition conditions, DLC films exhibit a wide range of friction behavior, th e lowest coefficient of friction being exhibited by the films deposited at lowest bias.
19 Mosaner et.al.  succeeded in reducing the internal stresses in DLC films by thermal annealing. The films deposited using PLD were annealed in air to study the effect of annealing temperature and time on the reduction of stresses in the films. By thermal annealing, they deposited films that are more than one micron thick with comparatively low stresses as the internal stresses are relieved due to relaxati on of chemical bonds in the film. Sood et.al. , first investigated the growth of diamond film on Ti-6A l-4V, by varying the dose of carbons ions implanted into the samples at room temperature The deposition of diamond was carried out by microwave chemical vapor deposition at a substrate temperature of 1000C. With the increase in the ion dose, a reduction in t he nucleation density was reported. The Raman spectra revealed the presence of diamond particles, and the films were peeled off due to the presence of high internal stress. Fu et.al. , studied the effect of deposition conditions on the stress es developed in the diamond films on a Ti substrate. The compressive residual stresses increased with the increase in the deposition time, high concentrations of CH 4 also increased the stresses, and this resulted in the delamination of the films. They proposed the me thod of using interlayers to reduce the stresses and observed that the presenc e of nitride layer decreased the compressive stresses to a significant extent and improved the adhesion of the diamond films on Ti alloys. 1.5 Overview of the Thesis Diamond and diamond related coatings have superlative mechanical, t hermal, optical and electronic properties. These coatings demonstrate high chemic al inertness to almost all
20 the environments. The major concern associated with these films is that they suffer from high compressive stresses that cause the delamination of the film s and this is the main drawback of diamond and diamond related materials that reduce their applications. Development of stresses in thin films is due to temperature cha nges, lattice mismatch, high energy of deposited ions and implantation of foreign atoms in the deposited films. Intrinsic stresses are developed during the growth of thin film s. As the thickness of the films increase, the stresses increase causing the failure of the films by cracking, buckling and delamination even before the wear. Films with compressive stres ses are more desirable compared to tensile stresses as the former increase s the strength whereas the latter causes cracks in the films. Stresses due to lattice mismatch can be avoided by using nitride and carbide interlayers. The other factor influencing the qua lity of films is the adhesive strength at the interface. For thin films, the stresse s and the adhesion between the substrate and the film are interrelated. Improvement in the adhe sion energy paves path for the prevention of buckling and delamination caused due to high st resses. Even the coefficient of friction of films decreases with the decrease in the stre sses. The present research focuses on the reduction in the stresses for both diamond and diamond-like carbon (DLC) films using different materials as i nterlayers. Micro Raman spectroscopy is used to find the structure and also the stresses in the films. The mechanical properties and the coefficient of friction are evalua ted using the Nanoindenter XP and the ball on disk method respectively.
21 CHAPTER 2 EXPERIMENTAL TECHNIQUES Diamond and diamond-like carbon coatings are primarily deposited usi ng vapor deposition techniques like chemical vapor deposition (CVD) and physical vapor deposition (PVD). Though both are atomistic deposition techniques, deposition in CVD is because of chemical reaction, and in PVD deposition is due to condensation. Table 2.1 Properties of Diamond and DLC Materials Thin Film Bulk Property CVD Diamond a-C a-C:H Diamond Graphite Crystal Structure Cubic a o =3.561 A Amorphous mixed sp 2 and sp 3 bonds Amorphous sp 3 /sp 2 Cubic a o =3.567 A Hexagonal a=2.47 A Form Faceted crystals Smooth or rough Smooth Faceted crystals Hardness (H v ) 300012000 1200-3000 900-3000 700010000 Density (g/cm 3 ) 2.8-3.5 1.6-2.2 1.2-2.6 3.51 2.26 Refractive Index 1.5-3.1 1.6-3.1 2.42 2.15 Electrical Resistivity ( /cm) >10 13 >10 10 10 6 -10 14 >10 16 0.4 Thermal Conductivity (W/m.K) 1100 2000 3500 Chemical Stability Inert Inert Inert Inert Inert Hydrogen Content (H/C) 0.25-1 Growth Rate (m/hr) ~1 2 5 1000 (synthetic)
22 This chapter gives a brief overview of the different techniques us ed for the deposition of diamond related materials. PLD and MPCVD techniques are used for diamond-like carbon and diamond respectively. Table 2.1 gives the properties of diamond a nd DLC materials. 2.1 Diamond-like Carbon Films 2.1.1 Pulsed Laser Ablation Pulsed laser ablation is one of the most renowned techniques for thin f ilm deposition. Thin film is formed by the condensation of the ablated target material, with or wi thout the background gas, on the surface of the substrate. J.Cheung is the first one to come up with the numerous advantages of Pulsed Laser Deposition. Since then PLD is used for the deposition of different materials like ferroelectrics, metals high temperature super conducting thin films and ceramics. The main advantage of PLD is its ability to deposit materials containing more than 5 to 6 compounds without changing the stoic hiometry of the material that is otherwise difficult to achieve with other available methods. Other advantages include the deposition of multilayered epitaxial films w ith the aid of target carousel, deposition in vacuum, inert or reactive gases and high energi es of the ablated species.
23 Figure 2.1 Schematic Representation of Laser-Target Interactions  Though PLD is simple to operate, it is difficult to understand the abl ation process because of the complexity involved in the laser beam solid interac tion. Figure 2.1 shows the schematic of laser target interactions. There are many proposed models, the details are given elsewhere . No single model discusses the effect of all the aspects on the
24 laser solid interactions as models are developed taking a particul ar feature into account and neglecting the others. It can be better explained as a two st ep process 1) heating of the material surface with the absorption of photons and 2) formation of the molten layer that vaporizes. The target material is irradiated by the las er energy. The electromagnetic radiation absorbed by the solid surface is converted to electronic excitation and then to thermal, chemical and mechanical energy, which in turn, causes evapo ration, ablation, excitation and plasma formation. The main drawback associated with the PLD is the formation of micron sized particulates on the films. They are deposited because of the target inhomogenities, fluctuations in the laser energy and so on. Mainly three approaches have been developed to re duce these particulates. Particulates can be controlled by reducing the las er energy fluctuations and inhomegenities in the target . The other approach is the use of vel ocity filter that ejects the massive particles with relatively smaller ve locities than those of atomic and molecular species . Third one being the off axis deposition in whic h the substrate is placed parallel to the laser plume . The other concerns are poor f ilm reproducibility, non uniformity of thickness when deposited over large areas. PLD has emerged as one of the most competent technique for deposi ting hydrogen free amorphous carbon thin films. Extensive research has been done to stud y the effect of laser density, substrate temperature  on the properties of diamond -like carbon thin films. Depending on the laser density and the substrate temperat ure, the optical and electrical properties can be tailored between diamond and graphite . The main
25 advantage of pulsed laser ablation over CVD is that the deposition te mperature can be low as the kinetic energy of the depositing species is very high. C VD is preferred if the substrate can withstand the high temperature. The PLD system at USF uses the KrF excimer laser, the mos t popular for pulsed laser ablation. It has a wavelength of 248 nm with photon energy of 5eV. The system has a laser generating unit, vacuum chamber, optical elements to focus the laser beam on to the target, target carousel that can hold four target holders and substra te holder, mass flow controllers, vacuum pump and a substrate heater that can go to a maxi mum temperature of 650C. The chamber is equipped with two rotary feed through, one of which a llows selecting the required target for the deposition. Figure 2.2 PLD System at USF
26 The other being the target rotator that helps to avoid the crater formation due to prolonged exposure to laser at a particular position of the target. F ilms can be deposited in Ultra high vacuum, the order of 10 -7 torr. Figure 2.2 shows the schematic of the PLD system at USF. 2.1.2 Experimental Procedure Lambda Physik KrF excimer laser was used for the deposition of diamond-like carbon films and the interfacial films. The PLD system contains tar get holder that can accommodate four different targets at a time. This feature c an be used to deposit multilayers without breaking the vacuum. The deposition chamber was eva cuated to millitorr range using a mechanical pump and then high vacuum is cre ated using the turbo molecular pump. All the depositions were carried out in a pressure ra nge of 10 -7 Torr. High power UV optics were used to focus the laser on the target at a pulse rate of 10 Hz and the laser energy is fixed at 300 mJ. The substrate, Si(100) is cleaned ultrasonically in methanol and acet one. Pyrolitic graphite (99% purity) is used for the deposition of ta-C films that are deposited at different temperatures, room temperature (RT), 100C, 200C, 300C, 400C on Si( 100) substrates, respectively. The substrate to target distance was m aintained at 4 cm for all the depositions. Interlayered DLC were deposited with the aid of mutliple target holder. The desired targets were mounted on to the holder prior to the experiment and w ere rotated to bring the required target under the laser irradiation. In this research T iC, TiN, AlN, TaN, W and Ta were used as the interlayers and their effect on the per formance of DLC films was
27 investigated. The interlayers are deposited at 100C, 300C and 600C wherea s the ta-C is deposited at 100C for all the experiments. 2.2 Diamond Films It is difficult to grow a diamond that is free of defects. The ph ysical properties of diamond are greatly affected by the presence of defects like point defects, dislocations and stacking faults. Diamond lattice has boron, nitrogen, hydrogen and oxy gen as substitutional defects. Presence of Nitrogen leads to less ther mal conductivity . With the increase of nitrogen concentration diamond becomes colorless and a lso it affects the different material properties. For HPHT the growth rates shoul d be as low as possible to reduce the defects and it is difficult to maintain the required condi tions for a long time. The flexibility of varying the conditions makes the CVD technique a better option compared to HPHT. Hence, diamonds with fewer defects can be effici ently produced using the CVD method. 2.2.1 CVD Deposition Technique The deposition of diamond was initially carried by methods such as i on beam method, ion implantation, and chemical vapor deposition techniques (CVD). Of these methods, CVD process has drawn much attention as the films can be grown on any dimensional scale, shape and thickness. CVD of diamond from hydrogen rich hydrocarbon containing gases has been the most successful method. In this process, the plas ma ball is produced inside the reactor (deposition chamber) very near to the heated substrate by applying high power and disassociating the feed gases (Hydrogen and Hydrocarbon specie s (CH 4 )) into free radicals such as CH 3 CH 2 and CH. These excited hydrocarbons generate carbon
28 atoms and some of these excited carbon atoms deposit as a diamond fil m while others decay their energy during their migration and deposit as graphiti c carbons or non diamond carbons. In presence of hydrogen, a chain reaction converting t he non diamond carbons to hydrocarbons take place and again produces the diamond species The growth of diamond is ensued as long as this chain reaction continues. At the sa me time, a spontaneous graphitization takes place. It is necessary to inhibit the nucleation of graphite while adding the carbon atoms to the surface. Hence, most of the gr owth processes involve termination of diamond surface with hydrogen or a halogen since both hydrogen and halogen can form a high energy bond with carbon than the carbon atom itself. The advantages of hydrogen or halogen termination are listed below: 1) To prevent the formation of any unwanted C-C bonding on the diamond surface and thereby preventing the growth of graphitic species. 2) The partial pressure of the carbon atoms in the vapor phase is so low that the free atoms are not probably the major source of carbon for the growth of dia mond. More likely, a reaction of CH 4 or other hydrocarbon with the diamond surface might result in the deposition of a carbon atom. 3) Prevents the rapid growth at nominal temperatures. As the growth t emperature is increased to a certain extent, smaller amount of the surface is terminated and provides a larger number of denude sites for arriving carbon atoms t o form a tetrahedral bond.
29 There are other methods such as addition of oxygen surfactants, whi ch have less bond energy than C-C so that the arriving carbon atoms can replace these surfactant atoms. However, the molecular hydrogen alone is not sufficient to complete ly suppress the growth of graphite. Hence, atomic hydrogen is used to remove the sa me. Also, incase of conventional hydrogen termination, only one in every 10 4 hydrogen atoms is replaced by a carbon atom. The growth of diamond in a metastable region would be possible if t he following conditions are satisfied. 1) Formation of mobile species containing a single carbon atom by coll ision of CH 4 with diamond surface. 2) Rapid diffusion of carbon containing surface species 3) Formation of diamond by addition of carbon surface species to a vacant site. The difficulty in the diamond synthesis was due to extremely shor t life time of the excited carbon atoms. The life time of the carbon atoms can be incr eased either by incorporating atomic hydrogen species, by disassociating a carbonace ous compound, electric discharge, electron bombardment, UV irradiation, shocks and she ars, x-rays. The growth of CVD diamond has been introduced; different methods exist ing and their time line is listed in Table 2.2.
30 Table 2.2 History of CVD Diamond Method employed Year HFCVD 1981 MPCVD and RFCVD 1982 Electron assisted CVD 1982,1985,1987 DC plasma CVD 1982 RF thermal plasma CVD 1987 DC plasma jet CVD 1988 Arc Discharge plasma CVD 1988,1989 Magneto microwave plasma CVD 1987 220.127.116.11 Microwave Plasma Assisted CVD MPACVD is an electrode less process and thereby prevents the cont amination of the films due to the electrode erosion. Due to the short wavelength or hi gh frequency generator (12.4 GHz), higher plasma density is produced and the growth of diamond is enhanced when compared to a low frequency HF-CVD method (13.6 KHz). A ver y intensive localized discharge is obtained with a little tendency to spread. It is suitable for producing atomic hydrogen, nitrogen and oxygen. The plasma generated by mi crowave is stable for a long time. E max = (QE) 2 /8 2 f 2 M E max = maximum ion energy Q = Ion charge M = mass of ion f = frequency
31 The maximum energy has to be small so that the ions produced will not etch the diamond surface. The disadvantage is the strong tendency to deposit the is olated particles on the non diamond substrates. 2.2.2 Experimental Procedure The interlayer coatings such as TiN and TiC, were deposited using PLD as it combines the advantage of stoichiometric deposition at lower temperature s with little contamination. Prior to the deposition, the substrates were cleaned usi ng acetone and methanol and were loaded in to the vacuum chamber. Using a turbomolecular pump, the chamber is evacuated to a base pressure of 10 -7 Torr. Both the interlayers are deposited at a substrate temperature of 300C. The samples with interlayers are treated in diamond particle s uspension for 25 minutes using an ultrasonic bath and then cleaned in methanol. The substrate was then loaded in to the MPCVD chamber and the deposition was carried out in the presenc e of methane and hydrogen at a temperature of 800C.
32 CHAPTER 3 STRUCTURAL AND MECHANICAL CHARACTERIZATION Characterization describes those features of composition and struct ure (including defects) of a material that are significant for particular preparat ion, study of properties, or use, and suffice for reproduction of the materials. This chapter reviews the techniques used to characterize the diamond and diamond-like carbon coatings. 3.1 Raman Spectroscopy Raman spectroscopy has emerged as a powerful technique to identify t he molecules and study their structural properties. It is based on the fact tha t the molecules interact with the electromagnetic field of the incident radiation and vibrate at c haracteristic frequencies of those materials. In 1928, Chandrasekhara Venkata Raman discovered the p henomenon of Raman Effect. It is a simple phenomenon, where the monochromatic light is focused on the sample and the scattered light is analyzed for the required information. Raman spectra are observed in the UV-visible region and is concerned with the vibrational transitions that appear in the 10 4 to 10 2 cm -1 region. The spectrum consists of Rayleigh scattering that has same frequency as the incident radiation and a weak line corresponding to the Raman scattering that has a shift in frequency Depending on the
33 increase or decrease in the frequencies, the lines are calle d stokes and anti stokes, respectively. Usually Raman spectra are observed for the vibrati onal and rotational transition. A transition is said to be infrared active if there i s a change in the dipole moment of the molecule and is Raman active if there is a change in polarizability of the molecule. The commercially available Raman spectrometer has five main c omponents. Continuous wave laser like Ar+ at a wavelength of 514.5 nm are used as exci tation source. Sample illumination and scattered light collection system, sample holder, monochromator or spectrograph and detection system are the other important components i n the Raman spectrometer. Laser is focused on the sample placed under the micros cope that excites the sample and the scattered light is collected in the same path a s the incoming laser. The scattered light is dispersed on to a charge coupled device (CCD) de tector. The sample size needed for Raman spectroscopy is small as the focused lase r beam is 1-2 mm in diameter. The disadvantage of Raman is that it needs a powerful laser source to observe weak Raman scattering. Applications: Raman spectroscopy is widely used in chemistry as this techni que depends on the vibrational transitions that are specific to the chemical bonds It has many applications like real time monitoring of anesthetic and respirat ory gas mixtures at the time of surgery.
34 Figure 3.1 Renishaw Raman Spectrometer at USF 3.2 Nanoindentation The mechanical behavior of thin films at micro level or less c annot be deduced from the properties of the bulk materials. Even the behavior is not same for a free standing film and the one attached to a substrate. This can be mainly attribute d to the change in the microstructure, dislocation density and vacancy concentration differe nces. Nanoindentation is one of the popular techniques for measuring the mecha nical properties of thin films. In this method, an indenter is driven into the material and the applied load and displacement are continuously monitored. Recently, differe nt depth sensing indentation instruments have been developed for performing the nanoindentation. With these instruments, it is possible to make shallow indentations at the required locations. There are many proposed methods to analyze the data obtain ed from these
35 instruments  and issues to be considered in the analysis of data and are discussed briefly by Cripps . In nanoindentation a hard indenter, typically made from diamond, with known tip geometry is driven into the material to be tested by increasing normal load. W hen a preset maximum value is reached, the load is reduced until partial or com plete relaxation. The residual impression is measured and the hardness is defined as the ratio of maximum load, P, to the residual area A r rA P H = Now, the problem is finding the indentation area. Direct measurement of the residual area of a small indent is difficult. To solve this problem depth sensing inde ntation method was developed. In this method, at each stage of the experiment the position of the indenter relative to the sample surface is precisely monitored with a sensor. The obtained data is analyzed to find the contact area without having to measure the res idual impression manually. Figure 3.2 shows the typical load vs displacement plot for obtained data. Figure 3.2 Plot of Load vs Displacement Data
36 3.3 Nanoindentation Data Analysis Once the load-displacement data is collected, there are different proposed methods to analyze the data and obtain mechanical properties. 3.3.1 Doerner and Nix Method This method states that if the change in the contact area is s mall during unloading, the indenter can be treated as a flat punch. They assumed that all the material in contact with the indenter is plastically deformed and obtained equations to find the contact area from load displacement data . 3.3.2 Oliver and Pharr Method This method predicts that the unloading data for an elastic contact f or different indenter tip follows a power law that is given as, m h Pa= where ,P, is the indenter load h is the elastic displacement, and a and m are constants. 3.3.3 Basic Equations Involved There are set of governing equations available to calculate the hardness and modulus values from the given load vs displacement data. The Nanoindentation equipm ent works on the basis of these classic equations. To find the hardness, the contac t area at maximum load is used. If the shape of the indenter tip is accurately known, a tip area function can be generated.
37 ( ) c c h f A = h c is the contact depth ,where the indenter is in actual contact with the sample. The hardness is now given as, c A P H max = The slope of the initial unloading gives the contact stiffness, r E A dh dP Sp2 = = where E r is the combined modulus or the reduced modulus that combines the modulus of indenter and the specimen and is given by ( ) ( ) s s i i r E E E 2 2 1 1 1n n+ = where i, corresponds to indenter and s, to the sample The tip used during this research is a berkovich tip. The analysis of test data for a berkovich indenter is discussed here. The berkovich indenter has an incl uded half angle of 65.3. The relationship between the projected area, A, of the indentati on and the actual depth of contact, h p, is given as 2 2 2 5. 24 3. 65 tan 3 3 p p h h A = = Once Â‘h p Â’ is calculated, the hardness is obtained using projected contact area.
38 Figure 3.3 a) Schematic of Indenter and Specimen Geometry b) Load Displacement Curve h e = is the elastic deformation of the specimen h p = the actual contact depth h r = is the depth of contact beneath the specimen free surface h t = is the total penetration depth In order to reduce the complexity involved in the numerical calculat ions, berkovich indenter is considered as a cone with a semi angle of 70.3 that gives the same area to depth ratio. For completely elastic contact, the relationship between the load and the depth of penetration for a cone is given by 2 tan 2 h E pp a= The derivative of the above equation gives the slope, i.e., stiffness as h E dh dP Sp atan 2 2 = =
39 With the appropriate substitutions for h, the elastic modulus for the spe cimen tested using a berkovich diamond indenter is given as 5. 24/ 1 2 1 *p bp h dh dP E = Nanoindentation is performed for all the samples an d the results are discussed in the fore coming chapter. Figure 3.4 shows the schematic of t he nanoindenter XP used for this research. Figure 3.4 Schematic of Nanoindenter XP 3.4 X-Ray Diffraction Each of the elements existing has different propert ies like structural, physical, mechanical, thermal and electrical, etc. These prop erties have to be studied to better understand how different elements can have their ap plications. Different analytical tools
40 have been invented to study the structural, physica l, morphological, optical and electrical properties etc. Of these, the study of structural p roperties becomes important as they largely determine the remaining properties of the m aterials. One of the important techniques used to study the structural properties is XRD (X-Ray Diffractometer). The atomic planes of a crystal cause an incide nt beam of X-rays (if wavelength is approximately the magnitude of the interatomic dist ance) to interfere with one another as they leave the crystal. The phenomenon is called Xray diffraction . As an x-ray beam travels through any substance, its intensity d ecreases with the distance traveled through the substance. Only a small range of charac teristic x-rays are widely used for diffraction. When the voltage on an x-ray tube is increased abov e a certain value then the wavelengths of characteristic lines of target metal are obtained. These characteristic lines are denoted by K K and K These characteristic lines were discovered by W.H .Bragg. He and his son W.L.Bragg have done a foremost exper imentation of finding out the crystal structures of NaCl, KCl and KBr etc. The fa mous Braggs law was formulated based on which the diffraction studies are carried out.
41 Figure 3.5 X-Ray Diffraction  ) (sin 2 q l d n = n= order of diffraction =wavelength d= separation between the planes =angle of incidence beam 3.5 Friction Test Universal Micro Tribometer (UMT), a tribological test ing apparatus, is used for the friction test of the specimen samples .The apparatu s, which has load sensor and feed back control from a PC, is operated using the software c ontrols on the PC. Machine can be operated according to several test procedures that can be programmed as sequences, which in turn can be edited and are very flexible. It eliminates the need to program the whole procedure again for low cycle tests. The X an d Z direction forces and
42 displacements are measured using the load sensor an d the software automatically plots all the values of the coefficient of friction, force du e to friction in the X direction and force (applied) in Z direction. Static and sliding coeffi cient of friction can be obtained from the graph. Figure 3.6 Universal Micro Tribometer with PC Based Feed Back Control 3.5.1 Description of UMT Main features of the universal micro tribometer are as follows: 2D force sensor to measure the friction and normal load with a force range of 1100 N or 0.22 Â– 22 lb with a resolution of 0.1 N or 0.022 lb. PC based 12-channel data acquisition and 3 motor co ntrollers.
43 Testing block, which is made of high-density cast i ron vibration damped frame. Upper vertical positional system has 150 mm of trav el at 0.001-10 mm s -1 with 1 micron resolution. Upper lateral positioning system has 75 mm of trave l at 0.01-10 mm s -1 with 2 micron resolution. Tribometer is facilitated with load feed back contr ol system and suspension for the force sensor. Automatic sequencing of tests and data acquisition Additional sensors like the contact acoustic emissi on detector and electrical contact resistance.
44 CHAPTER 4 RESULTS AND DISCUSSION 4.1 Amorphous Carbon Films without Interlayer The mechanical properties of silicon, which is an i mportant material in electronic packaging, are improved with a amorphous carbon (a: C) coating. Owing to the compressive stresses in the film, the a:C exhibits a poor adhesion with the silicon. This work focuses on the use of carbide and nitride inte rlayer to improve the adhesion between silicon and a:C by reducing the stresses. Amorphous carbon was deposited on Silicon substrate using the PLD method as discussed in sect ion 2.1.2. The structural and the mechanical properties of the a-C films were studied as a function of deposition temperature. The structural properties of the films were characterized by the Raman spectroscopy and the mechanical properties by nanoi ndentation and friction test. This section presents the results and discussion of the tetrahedral amorphous carbon without any interlayer. All the films were deposited at fiv e different temperatures Â– Room Temperature (25C), 100C, 200C, 300C, and 400C. Using an optical microscope, it was observed that t he films buckled at edges, which indicated the poor adhesion of the films to the sub strate. The wrinkles spread into the films with the increase in the time and is attribut ed to the relaxation of stresses in the
45 films over time. It can be inferred that buckling i s due to high compressive stresses developed in the films during the deposition. 4.1.1 Raman Spectroscopy It is a known fact that for a tetrahedral amorphous carbon film, a broad peak centered at a Raman shift of 1570cm -1 is observed. A peak at 1350cm -1 is due to the D band corresponding to the disordered graphite. Figure 4.1 Raman Spectra of the Tetrahedral Amorphous Carbon Films Deposited at Different Temperatures Figure 4.1 shows the Raman spectra for the films de posited at various temperatures. If the shoulder at 1350 cm -1 is less, it implies that the film has more sp 3 content. The film deposited at room temperature, 100C does not show a prominent peak at 1350 cm -1
46 indicating the presence of higher sp 3 fraction than sp 2 Even the film deposited at 200C did not show a significant peak at 1350 cm -1 As the deposition temperature is increased from 200C to 400C, the films exhibited a hump at 1350 cm -1 and the width of the shoulder increased. This indicates the decrease in the sp 3 fraction and hence can be concluded that the films exhibit more graphitizatio n, as seen from the Raman spectra. From the Raman Spectroscopy, it can be inferred tha t there is a decrease in the sp 3 content with the increase in the deposition tempera ture. If the amorphous carbon films exhibit a high content of sp 3 bonding they are addressed as tetrahedral amorphou s carbon films. From the Raman spectra it is confirmed that these films exhibit a high content of sp 3 and hence can be termed as tetrahedral amorphous c arbon films (ta-C). Here after this notation is used for all the films. Raman Spectroscopy was used to determine the stress es in the films. The equilibrium separation between the atoms change with the increa se in the stresses and this in turn changes the vibrational frequencies. This change ca n be observed in the Raman shift. If the atoms are under tension, the vibrational freque ncies decrease and hence the Raman shift decreases. The atoms in the compression exhib it an increase in the vibrational frequencies and the Raman shift increases. The stre sses developed in the films can be calculated using
47 D =w w sE = D w Shift in the Raman wave number w = wave number of the reference E= elastic modulus of diamond-like carbon Table 4.1 gives the reduction in the stress in ta-C films without interlayers, calculated using the G-band at 1570 cm -1 as the reference. The peaks were obtained using a m ultiple peak Gaussian fit. With the increase in deposition temperature, the peak shifted to the left, indicating a decrease in the stress, which is evident from the decrease in the stress values given in the Table 4. 1. The modulus value i s assumed as 412GPa for the calculations. Table 4. 1 Residual Stress in the Tetrahedral Amorphous Carbon Films Obtained fr om Raman Spectra Sample Deposition Temperature (C) Residual Stress (GPa) DLC RT 1.892 DLC 100 1.664 DLC 200 2.294 DLC 300 2.602 DLC 400 6.2522
48 4.1.2 Nanoindentation It is important to retain the hardness of DLC films while making efforts to improve its other properties. Nanoindentation is the best techn ique to measure the mechanical properties of thin films like hardness and modulus. Berkovich diamond tip was used as the indenter. Nine indents were performed on each s ample at different locations using Nanoindenter XP. To reduce any errors in the calcul ations the average values of hardness and modulus were accepted. The modulus and hardness values were obtained using Oliver and PharrÂ’s approach. For every set of exper iment, indentations were also performed on fused Silica to confirm that the tip i s in perfect shape. Figure 4.2 and Figure 4.3 shows the variation of mo dulus and hardness with the indenter displacement. From the nanoindentation test, it was observed that with the increase in the temperature, the hardness and the modulus of the ta -C films increase up to a temperature of 100 C and then starts to decrease in the entire range. This is because of the increase in the sp 2 bonds at higher temperatures, which was earlier co nfirmed from the Raman spectra. The films deposited at 100 C exhibited hi gh fraction of sp 3 content and hence has the highest hardness and modulus value. The val ues of youngÂ’s modulus and hardness were obtained using Oliver and PharrÂ’s app roach and the results are summarized in Table 4.2.
49 Table 4.2 Hardness and YoungÂ’s Modulus for Tetrahedral Amorphous Carbon Films Deposited at Various Temperatures Sample Deposition Temperature(C) YoungÂ’s modulus (GPa) Hardness(GPa) Sample 1 Room Temperature 370.096 51 Sample 2 100 412.784 59.320 Sample 3 200 363.947 43.451 Sample 4 300 250.721 22.493 Sample 5 400 198.264 16.473 0 100 200 300 400 500 600 700 800 2030405060708090 Displacement Into Surface (nm)Modulus (GPa) DLC RT DLC 100 C DLC 200 C DLC 300 C DLC 400 C Figure 4.2 Modulus vs Displacement Plot of Tetrahedral Amorphous Carbon Films Deposited at Different Temperatures
50 0 10 20 30 40 50 60 70 2030405060708090 Displacement Into Surface (nm)Hardness (GPa) DLC RT DLC 100 C DLC 200 C DLC 300 C DLC 400 C Figure 4.3 Hardness vs Displacement Plot of Tetrahedral Amorphous Carbon Film s Deposited at Different Temperatures 4.2 Tetrahedral Amorphous Carbon Films with Interlayer The delamination of the ta-C films due to high comp ressive stresses is an obstacle for its application. These stresses can be reduced either b y increasing the deposition temperature or by decreasing the energy of carbon species. Howe ver, from the discussion in earlier section, it is evident that the increase in the dep osition temperature results in the graphitization of the films. The films with higher sp 2 content exhibit a lower hardness, which is not desirable. The stresses in thin films are reduced efficiently by using interfacial layers, materials that can form carbide s and which exhibit lower stresses at the interface are the best materials as interlayers. To serve as an efficient interlayer, the modulus of such materials should lie between the mo dulus of the substrate and the deposited films.
51 This section discusses the enhancement in the adhes ion of ta-C films with the substrate by the presence of interlayer. Carbides, nitrides a nd metals that have affinity for carbon were used as interfacial layers. Using PLD, compoun ds such as TiC, TiN, AlN, TaN, and metals like Ta and W were deposited as interfacial layers. The stresses in the interface materials vary with the deposition temperature and hence the investigation was done at three different deposition temperatures of 100C, 3 00C, and 600C. The deposition was carried out for 20 minutes. For all the samples, th e tetrahedral amorphous carbon was deposited on these interfaces at 100C because the as deposited samples have shown higher sp 3 content at this temperature. This particular tempe rature was opted because the undoped ta-C at this temperature exhibited high fra ction of sp 3 bonds. With the aid of the multiple target holders, the required multilayer th in films were successfully deposited. The results of Raman spectroscopy, Nanoindentation and friction test are discussed in the following sections. 4.2.1 Raman Spectroscopy Raman spectra of these samples revealed that with t he increase in deposition temperature of interlayer, the G-peak moved towards left, indic ating the relaxation of stresses in the films. All the selected interlayer materials follow ed the same trend. Figure 4.4 shows the Raman spectra of ta-C films with tantalum as interl ayer. From the Figure 4.4 it can be observed that there is no significant D-peak at 135 0 cm -1 indicating that the films are predominantly sp 3 bonded.
52 Figure 4.4 Raman Spectra of ta-C films with Ta as Interfacial Layer High quality ta-C films exhibit a relatively symmet ric G-band and a very less significant D-band in the Raman spectra. The following figures show the Raman spectra of ta-C films with the interlayers deposited at different t emperatures. The G-band for all the spectra was located using multiple Gaussian fit. Th e reduction in stress in ta-C films were calculated by measuring the shift in the peaks and the resulting values for all the films are summarized in the Table 4.3. For the calculations, modulus of diamond-like carbon is taken as 412 GPa.
53 Figure 4.5 Raman Spectra of ta-C Film with Different Interlayers Deposit ed at 100C Figure 4.6 Raman Spectra of ta-C Film with Different Interlayers Deposit ed at 300C
54 Figure 4.7 Raman Spectra of ta-C Film with Different Interlayers Deposit ed at 600C ta-C films with AlN as an interlayer did not show c hange in the compressive stress with the change in the deposition temperature. ta-C with Ta as interface exhibited a considerable amount of reduction in stresses at hig h deposition temperature of Tantalum. The stress reduction was more in the films with TiN as the interface. Films with TiC interlayer, deposited at 600C exhibited the maximu m reduction in the stress.
55 Table 4.3 Reduction in Stress (GPa) Obtained from Raman Analysis Sample Interlayer material Deposition temperature (C) G-band (cm -1 ) Residual stress (GPa) Sample 6 AlN 100 1549.06 2.71 Sample 7 AlN 300 1548.97 2.72 Sample 8 AlN 600 1553.66 1.49 Sample 9 Ta 100 1557.08 0.59 Sample 10 Ta 300 1556.81 0.58 Sample 11 Ta 600 1542.64 4.41 Sample 12 TiN 100 1542.72 4.38 Sample 13 TiN 300 1540.18 5.06 Sample 14 TiN 600 1539.37 5.27 Sample 15 TaN 100 1544.26 3.97 Sample 16 TaN 300 1543.80 4.1 Sample 17 TaN 600 1542.79 4.36 Sample 18 TiC 100 1549.54 2.57 Sample 19 TiC 300 1548.23 2.92 Sample 20 TiC 600 1538.92 5.39 Sample 21 W 100 1543.8 4.1 Sample 22 W 300 1546.82 3.30 Sample 23 W 600 1542.67 4.39
56 0 1 2 3 4 5 6 Residual Stress (GPa) AlNTaTiNTaNTiCW Interlayer Material 100 300 600 Figure 4.8 Reduction in Stress vs Interlayer Material at Different Tem peratures 4.2.2 Nanoindentation The hardness and the modulus of all the samples wer e tested using Nanoindenter XP and the results were summarized in Table 4.4. The plots of modulus vs displacement and hardness vs displacement were also included. Of all the samples, films with TiN as interlayer exhibited highest hardness. All the film s exhibited almost same hardness owing to the fact that all the selected interlayers are h ard materials.
57 Table 4.4 Summarized Results Obtained from Nanoindentation for Layered Tetrahedral Amorphous Carbon Films Sample Interlayer Material Deposition temperature(C) YoungÂ’s Modulus(GPa) Hardness(GPa) Sample 6 AlN 100 248.353 25.143 Sample 7 AlN 300 240.265 24.542 Sample 8 AlN 600 269.077 26.267 Sample 9 Ta 100 224.831 28.023 Sample 10 Ta 300 258.793 27.731 Sample 11 Ta 600 248.675 25.75 Sample 12 TiN 100 329.536 29.454 Sample 13 TiN 300 306.367 31.909 Sample 14 TiN 600 289.879 28.122 Sample 15 TaN 100 208.798 27.104 Sample 16 TaN 300 293.346 26.653 Sample 17 TaN 600 200.872 25.738 Sample 18 TiC 100 217.172 26.974 Sample 19 TiC 300 221.542 25.115 Sample 20 TiC 600 245.854 26.165 Sample 21 W 100 237.266 28.02 Sample 22 W 300 224.166 26.021 Sample 23 W 600 194.416 24.056
58 0 50 100 150 200 250 300 350 400 450 500 102030405060 Displacement Into Surface (nm)Modulus (GPa) AlN Ta TaN TiC TiN W Figure 4.9 Modulus vs Displacement Plot of ta-C Film with the Interlayer s Deposited at 100C 0 5 10 15 20 25 30 35 40 45 50 102030405060 Displacement Into Surface (nm)Hardness (GPa) AlN Ta TaN TiC TiN W Figure 4.10 Hardness vs Displacement Plot of ta-C Film with the Interlaye rs Deposited at 100C
59 0 50 100 150 200 250 300 350 400 450 500 102030405060 Displacement Into Surface (nm)Modulus (GPa) AlN Ta TiN TaN TiC W Figure 4.11 Modulus vs Displacement Plot of ta-C Film with the Interlayer s Deposited at 300C 0 10 20 30 40 50 60 102030405060 Displacement Into Surface (nm)Hardness (GPa) AlN Ta TiN TaN TiC W Figure 4.12 Hardness vs Displacement Plot of ta-C Film with the Interlaye rs Deposited at 300C
60 0 100 200 300 400 500 102030405060 Displacement Into Surface (nm)Modulus (GPa) AlN Ta TaN TiC TiN W Figure 4.13 Modulus vs Displacement Plot of ta-C Film with the Interlayer s Deposited at 600C 0 10 20 30 40 50 60 102030405060 Displacement Into Surface (nm)Hardness (GPa) AlN Ta TaN TiC TiN W Figure 4.14 Hardness vs Displacement Plot of ta-C Film with the Interlaye rs Deposited at 600C
61 4.2.3 Friction Test Using the ball on disk method, friction test was pe rformed on all the samples against stainless steel balls. A load of 4N was applied for one minute at a translational velocity of 2mm/sec. As deposited ta-C films that were tested w ith UMT against stainless steel ball, demonstrated a coefficient of friction of 0.02. Fro m the test results, it was observed that the coefficient of friction of the ta-C films depos ited on interlayers decreased with the lowest being exhibited by the films with TaN interl ayer. The table below summarizes the friction test results. Table 4.5 Friction Coefficient of Layered ta-C Films Sample Interlayer Material Deposition temperature(C) Friction coefficient Sample 6 AlN 100 0.013 Sample 7 AlN 300 0.014 Sample 8 AlN 600 0.012 Sample 9 Ta 100 0.010 Sample 10 Ta 300 0.018 Sample 11 Ta 600 0.014 Sample 12 TiN 100 0.016 Sample 13 TiN 300 0.014 Sample 14 TiN 600 0.012 Sample 15 TaN 100 0.014 Sample 16 TaN 300 0.011
62 Table 4.5 (continued) Sample 17 TaN 600 0.018 Sample 18 TiC 100 0.019 Sample 19 TiC 200 0.017 Sample 20 TiC 300 0.012 Sample 21 W 100 0.015 Sample 22 W 200 0.017 Sample 23 W 300 0.019 4.3 Diamond In order for the stresses in diamond film to be min imum, the difference in the coefficient of thermal expansion of the substrate and the film should be as low as possible. Intrinsic stresses are developed during the growth of the fil ms due to the impurities in the layer whereas the thermal stresses are developed when the sample is cooled to room temperature rapidly. The diamond coatings on Ti-6Al -4V alloy, mostly used in aerospace airframe, engine parts, and bone implants, were inv estigated in this research. This alloy exhibits poor adhesion with the diamond films and h ence the effect of interlayers on the improvement of adhesion is studied. Initially, diamond is deposited on Ti-4Al-6V substr ate. The film has peeled off due to the adhesion problems arising because of the mismatch o f coefficient of thermal expansion. Adhesion of these films can be enhanced by using an interlayer that induces chemical or mechanical bonding between the diamond and the subs trate. The interlayer should
63 provide a transition of modulus and thermal expansi on coefficient from substrate to the diamond film, which reduces the stresses. TiN and TiC were investigated as the interlayers to enhance the adhesion of diamond films on Ti-4Al-6V alloy. The experimental procedur e was discussed in section 2.2.2. Raman spectroscopy and XRD were used to characteriz e the structural properties of the films. 4.3.1 Raman Spectroscopy Raman spectroscopy was performed on the diamond fil ms deposited on Ti-6Al-4V with TiC and TiN interlayers. Raman peak of diamond on b are silicon substrate is taken as reference. Figure 4.15 shows the Raman spectra of d iamond on bare silicon and on other interlayers. The diamond peak on Si was obtained at 1332.57 cm -1 The samples with TiN as an interlayer demonstrated a peak at 1345.71 cm -1 and 1570 cm -1 The peak at 1332.57 cm -1 attributed to diamond (sp 3 bonding), shifted to 1345.71 cm -1 indicating the presence of high compressive stresses. The peak at 1570 cm -1 is attributed to sp 2 bonding and indicates the presence of graphite. Diamond films w ith TiC interlayer have shown a minor shift in the peak from 1332.57 cm -1 to 1333.4 cm -1 Hence, these samples have less compressive stresses compared to the TiN films. Str esses developed in these films was calculated using the Raman shift. Considering the m odulus of diamond deposited on silicon, which in this case is 900GPa, the increase in stresses was calculated. The values are summarized in Table 4.5.
64 Table 4.6 Increase in Stress (GPa) Obtained from Raman Analysis Outer Layer Interlayer material Raman shift(cm -1 ) Increase in Stress(GPa) Diamond TiC 1333.4 0.56 Diamond TiN 1345.71 8.87 Figure 4.15 Raman Spectra of Diamond Films with Different Interfaces 4.3.2 X-ray Diffraction In order to improve the adhesion of the films, a th ick coating of TiN and TiC were deposited using PLD. Diamond films on TiN did not a dhere well at the edges, whereas the TiC interlayer resulted in good adhesion of dia mond films on Ti alloy substrate.
65 Figure 4.16 shows the XRD pattern of the substrate and the TiC interlayered diamond film. Peaks corresponding to TiC and diamond are re presented in Figure 4.16. Figure 4.16 XRD of Original Substrate and TiC Â–Diamond Film Diamond TiC Intensity (a.u)
66 CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS Compressive stresses, poor adhesion, and poor wear resistance are found to be the crucial factors with the tetrahedral amorphous carbon and d iamond films that are extensively used in the microelectronic device packaging, cutti ng tools and wear resistant magnetic disks. The interfacial bonding and internal stresse s determine the adhesion of thin films. This research investigated the growth and character ization of diamond and diamond-like materials on Ti alloy and Silicon substrates, parti cularly to improve the adhesion by reducing the internal stresses. Tetrahedral amorpho us carbon (ta-C) primarily contains the sp 3 bonds and has properties close to diamond. The har dness of as deposited ta-C films decreased with the increase in the deposition temperature because of the increase in the sp 2 character at higher temperatures. Raman analysis r evealed a decrease in the internal stress with the increase in the deposition temperature but at the same time, the Dband corresponding to the sp 2 content in the films has become significant. An interlayer that has affinity to form carbide wit h a modulus value between tetrahedral amorphous carbon and silicon has the potential to r educe the internal stresses. Films with TiN interlayer, deposited at 300C and 600C has th e maximum stress reduction of 5.06 and 5.27 GPa respectively. The nanoindentation test s on these samples revealed that they did not exhibit any significant change in the hardn ess values with the change in the
67 deposition temperature of interlayers. Film with Ti C interlayer deposited at 600C, also exhibited a stress reduction of 5.39 GPa. Hence, th e interlayers of TiN at 300C and 600C, TiC at 600C can be considered to have the p otential to reduce stresses in the ta-C films. Friction test results demonstrate a decrease in the coefficient of friction of the interlayered ta-C films. Diamond films grown on the Ti-6Al-4V were peeled of f due to high interfacial stresses. The adhesion of these films was improved with an in terlayer of TiN and TiC. The Raman spectra of these films have revealed that the compr essive stresses are more with a TiN interface when compared to TiN interface. XRD was u sed to study the microstructure of these films. The peak corresponding to diamond and TiC were observed in the XRD results. The future for diamond and diamond-like carbon film s is bright in orthopedic applications. With low friction coefficient and imp roved adhesion diamond-like carbon films and diamond can be efficiently used in joint replacement for improving the mechanical properties of bulk material and hence en hancing the integration with the bone.
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