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Dry sliding tribological characteristics of hard, flat materials with low surface roughness
h [electronic resource] /
by Subrahmanya Mudhivarthi.
[Tampa, Fla.] :
University of South Florida,
Thesis (M.S.M.E.)--University of South Florida, 2003.
Includes bibliographical references.
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ABSTRACT: This thesis focuses on identifying hard material pairs with low roughness, high coefficient of static friction, high wear resistance and high modulus of elasticity, suitable for sliding in dry friction conditions under a normal load. A wide range of materials including various steels, various coatings on tool steels deposited by various deposition techniques and different ceramics were examined and considered for tribological testing. Procedures and sequences were developed for conducting tribology tests on the material pairs. High endurance low cycle tests were conducted and based on the performance of material pairs with respect to friction, wear and surface roughness a small set of material pairs and coatings was selected for further testing. High endurance high cycle tests were performed on an additional seventeen pairs of material pairs selected for long term sliding. Material pairs were selected for low endurance tests based on high corrosion resistance along with all the above specified design parameters. Low endurance tests were conducted to identify material pairs sliding for a short distance in humid environments. Results are tabulated and pictures of the material pairs after wear tests are presented. It was found that four material pairs for high endurance applications and two pairs for the low endurance applications performed very well in regard of design specifications. These material pairs find a major application in friction clamps of an Inchworm motor resulting in enhancement of force output of the motor.
Adviser: Hess, Daniel P.
x Mechanical Engineering
t USF Electronic Theses and Dissertations.
Dry Sliding Tribological Characteristics of Hard, Flat Materials with Low Surface Roughness by Subrahmanya Mudhivarthi 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: Daniel P Hess, Ph.D. Thomas G Eason, Ph.D. Glen H Besterfield, Ph.D. Date of Approval: September 26, 2003 Keywords: friction clamps, inchworm motors, friction, wear, coatings Copyright 2003 Subrahmanya Mudhivarthi
DEDICATION To Lord Shirdi Saibaba and My Family
ACKNOWLEGEMENTS I express my gratitude to everyone w ho helped me throughout my research work, without whose assistance, this work w ould not have been successful. First of all I thank God and my Family for their love and support, which was the driving force behind this endeavor. I express my deep gratitude and thankfulness to Dr. Daniel Hess, major professor, for providing me w ith this opportunity to conduct the thesis and also for his guidance and support thr oughout my research work. I am grateful to Dr Glen Besterfield and Dr. Thomas Ea son for accepting to be on the committee. I thank Mr. Frank Giglio, Mr. James Steven s and Mr. Andy Kent, my colleagues for their help during the research work and also for providing me immense support in many ways. I take this opportunity to thank Dr. Ashok Kumar, Dr. Arun Sikder and Mr. Parshuram Zantye for their cooperati on while using the equipment during the research work. I thank all my friends 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 1 1.1 Overview 1 1.2 Background 4 1.2.1 Literature review of inchworm motors 5 1.2.2 Relevant works in the past on tr ibological testing of materials 7 1.3 Outline 9 CHAPTER 2 MATERIALS 10 2.1 Requirements 10 2.2 Metals 11 2.2.1 AISI 52100 alloy steel 11 2.2.2 Croblox 11 2.2.3 440C stainless steel 11 2.2.4 Lapped A2 tool steel 12 2.3 Coatings on lapped tool steel 12 2.3.1 Tungsten carbide 12 2.3.2 Titanium nitride 13 2.3.3 Diamond like carbon 13 2.3.4 Tetrabond 14 2.3.5 MoST coating with TiCN under layer 14 2.4 Coatings on feeler gage tool steel 15 2.4.1 Molydenum disulphide 15 2.4.2 Graphite 15 2.5 Ceramics 16 2.5.1 Partially stabilized zirconia 16 2.5.2 Sapphire 17 CHAPTER 3 TESTING 18 3.1 Hardness measurement 18
ii 3.2 Surface roughness measurement 21 3.3 Friction and wear testing 23 3.3.1 Description of UMT 24 3.3.2 High endurance tribology tests Â– low cycle 24 3.3.3 High endurance tribology tests Â– high cycle 26 3.3.4 Low endurance tribology tests 27 CHAPTER 4 RESULTS AND DISCUSSION 30 4.1 High endurance low cycle test results 30 4.2 High endurance high cycle test re sults 38 4.3 SEM and EDX analysis 54 4.4 Low endurance test results 55 4.5 Surface roughness measurement of prototype 59 4.6 Discussion and applications 62 CHAPTER 5 CONCLUSIONS 65 REFERENCES 68
iii LIST OF TABLES Table 1 Physical properties of 52100 steel 11 Table 2 Physical properties of a typical ceramic 16 Table 3 Hardness of metals and ceramics 21 Table 4 Pretest surface roughness data of materials 22 Table 5 High endurance low cycle test results 31 Table 6 High endurance hi gh cycle test results 40 Table 7 Low endurance test results 55
iv LIST OF FIGURES Figure 1 Representation of inchworm motor 2 Figure 2 Nanoindenter apparatus with the PC based control 18 Figure 3 Close up view of the nanoindenter work area 18 Figure 4 Charts presenting hardness and modulus of elasticity plotted against displacement into surface 19 Figure 5 Taylor hobson surtronic 3P profilometer 21 Figure 6 Universal micro tribometer with PC based feed back control 22 Figure 7 The upper test fixture and specimen 28 Figure 8 The lower test fixture, specimen and clamps 29 Figure 9 TiN coated A2 tool steel afte r 500 cycle test with severe coating removal 35 Figure 10 MoST coated A2 tool steel afte r 100 cycle test with severe coating removal 36 Figure 11 Feeler gage after 500 cy cle test with severe abrasion 37 Figure 12 Tetrabond coated A2 steel from test no. 8 with severe abrasion after 10,000 cycles of reciprocating sliding 49 Figure 13 TiN coated A2 steel from test no. 9 with severe abrasion after 10,000 cycles of reciprocating sliding 50 Figure 14 52100 steel gage block specimen from test no. 15 after 200,000 cycles 51 Figure 15 Ceramic gage block specimen from test no. 15 after 200,000 cycles 52
v Figure 16 Wear particles from 52100 st eel gage block from test no. 15 on mating ceramic gage block post 100,000 cycle test at 500,000 cycles of cumulative testing 53 Figure 17 SEM picture of wear pa rticle at 15000 X magnification 54 Figure 18 Top croblox gage block speci men from test no. 17 after 125 cycles 57 Figure 19 Bottom croblox gage block specimen from test no. 17 after 125 cycles 58 Figure 20 Motor prototype part surf ace nearest actuators after 1.243 mile (2km) sliding endurance test (m ade of 52100 steel gage block) 60 Figure 21 Motor prototype part surface farthest from actuators after 1.243 mile (2km) sliding endurance te st (made of 52100 steel gage block) 61
vi DRY SLIDING TRIBOLOGICAL CHARACTERISTICS OF HARD, FLAT MATERIALS WITH LOW SURFACE ROUGHNESS Subrahmanya Mudhivarthi ABSTRACT This thesis focuses on identifying hard material pairs with low roughness, high coefficient of static friction, high wear resi stance and high modulus of elasticity, suitable for sliding in dry friction conditions under a normal load. A wide range of materials including various steels, vari ous coatings on tool steels deposited by various deposition techniques and different ceramics were examin ed and considered for tribological testing. Procedures and sequences were developed for conducting tribology tests on the material pairs. High endurance low cy cle tests were conducted and based on the performance of material pairs with respect to friction, wear and surface roug hness a small set of material pairs and coatings was select ed for further tes ting. High endurance Â– high cycle tests were performed on an additional seventeen pair s of material pairs se lected for long term sliding. Material pairs were selected for low endurance te sts based on high corrosion resistance along with all the above specifi ed design parameters. Low endurance tests were conducted to identify material pairs sliding for a short distance in humid environments. Results are tabulated and pictures of the material pairs after wear tests are presented. It was found that four materi al pairs for high endurance appl ications and two pairs for the low endurance applications performed very well in regard of design specifications. These material pairs find a major application in fric tion clamps of an Inchworm motor resulting in enhancement of force output of the motor.
1 CHAPTER 1 INTRODUCTION 1.1 Overview Contacting surfaces while slidi ng against each other experience a force of resistance at the interface. This is called frictional fo rce. A surface while sliding against another experiences repeated stress due to mechan ical contact, resulting in removal or displacement of mass or volume of the material This removal or disp lacement is termed as wear. Friction and wear aspects of any movi ng part need high attention for its safe and efficient operation. It is difficult to identify suitable materials for applications where high friction and low wear are desired at the interf ace. The purpose of this thesis is to suggest material pairs with high coefficient of sta tic friction and high wear resistance that can perform efficiently under a normal load in dry sliding conditions for long term and short term applications in different environments Parameter specifications for the material pairs are designed as high har dness, high static friction, high wear resistance, low sliding friction, roughness for average peak and valley in the range of 10-15 in. Friction clamps present on either side of a pi ezoelectric actuator stack in an Inchworm motor (precise sub micro positioning devices refer Figure.1) drive assembly is an application where high friction and low wear is a crucial require ment. These clamps consist of a stack of material shims or gage bl ocks. Function of these clamps is to transfer the force from the piezoelectric actuator to the housing. The piezo electric actuator extends in a step size of the order of microns and nanometers. Friction clamps must work efficiently accommodating the micron level st ep size of the actuator, which can be achieved only if extremely low surface roughness is maintained on the material shims of the friction clamps. The material shims in the stack not only need to maintain minimum wear and low surface roughness but also need to have sufficient static friction to hold
2 the high force without occurrence of slip. For such an applic ation, investigating friction and wear characteristics of the material sh ims proves significant. Figure below is a block diagram of an Inchworm motor. Figure 1 Representation of inchworm motor Inchworm motors are used in actuators us ed for high resolution positioning purposes. The Inchworm motor uses an optical position en coder to directly measure the shaft position with a micron level resolution. For inst ance commercial Henderson-Burleigh Inchworm motor  has a resolution less than 1 nm and position accur acy less than 1 m. Inchworm motors eliminate errors caused by ov ershoot and backlash in traditional motor systems. Step sizes can be programmed a nd commanded in any multiple of the encoder resolution. One of its applications is to help cell physiologist s in positioning microelectrodes for electro phys iology recordings. These reco rdings require micrometer scale control of target velocity and target acceleration steps. These steps must be achieved with least vibration in order to prevent damage to cells, processes or connections. The Inchworm PZT elements respond in microseconds with very high stiffness to achieve high acceleration and velocity. For instan ce the commercial LSS-8000 Inchworm motor  achieves its top speed of 1.5mm per s ec within 2 m of starting. The motor's Piezo-stacks Friction pairs
3 dynamic velocity range also is important; it can move at any velo city from 1nm/s to 1.5mm/s without gear changes. This helps in achieving clean penetrations of the shaft. The goal is to avoid dimpling or ripping th e membrane which results in severe cell damage. Microscale inchworm motors have sufficient force output whereas Mesoscale and Macroscale inchworm motors produce force output in the range of 0.04 Â– 5 Kgf. This moderate upper limit can be increased to 9 Kgf but friction and stick-slip problems are expected to arise . A piezoelectric actuator stack, clamping stacks and friction clamps constitute the drive assembly of the inchworm motor. The PZT act uator supplies sufficiently large forces, the clamping systems transfer this force to the housing. Force transmitted to the positioning shaft is the force received by the housing. Th e force output depends on the efficiency of clamping system. If the clamping system is not efficient, it cannot support much of the force supplied by the piezoelectric stack and sl ip occurs between the shims in the stack resulting in the low force output If the clamping system is capable of holding the force and transmitting it without slip in the stac k, force output will be much more than the moderate force output of av ailable inchworm motors. One of the main challenges in the developm ent of an inchworm motor is to find a solution to increase force output and hence th e performance of the motor. This can be achieved by increasing the ability of the cl amping system to hold greater force. High static friction between the material shims in the clamps is required in order to help the clamping system function effectively by holding the force supplied by the PZT stack to a greater extent and transfer the force to the housing without slip occu rring in the clamp. At the same time surface roughness and wear on the surface should be low in order to ensure smooth travel without vibration in the positioning shaft. Current research will assist in selecting materials that are optimum for this application. Also this research suggests material pairs for other applic ations in corrosive environm ent but short term sliding.
4 Optimum working materials for this research are chosen on the basis of their mechanical properties like hardness, roughne ss and tribological properties like coefficient of friction and wear resistance. Corrosion resistance is an other important property that is taken into consideration when materials are chosen for the low endurance testing. Alloy steel, ceramics, steel substrates coated with variou s thin hard coatings constitute the set of material pairs used. Coatings improve wear resistance, thermal resistance and corrosion resistance of a substrate. Sliding performa nce of the materials can be improved by using an appropriate coating. Prope rties of a coating depend on the method of deposition. All the materials are tested for their ha rdness and pretest roughness measurements. Friction testing and wear tes ting is done on the universal mi cro tribometer. Some of the pairs are rejected without testing for wear as th eir static friction is very low. Based on the roughness data and the intensity of wear, further testing is d one on the specimen samples. After eliminating a lot of pairs a final set of material pairs is selected for the high endurance applications and lo w endurance applications. 1.2 Background Friction and wear at the interface of two cont acting surfaces need to be controlled in order to ensure safe operation of machin ery. Applications of dry friction can be categorized into various types, based on the re quirement of friction force. In day to day life, living beings can walk only because of sufficient friction betw een the feet and floor, vehicles can be driven safely as long as there is enough fric tion between tires and ground, sufficient friction between the gripping surface and any object is absolutely necessary to lift the object without any slip. For many indust rial applications li ke couplings, joints, etc. friction and wear should be maintained low. In precision positioning systems where accurately controllable motion is desired, stat ic friction in friction clamps needs to be sufficiently high. Maintaining th is friction at an optimum le vel is important to achieve enhanced output and better performance of the positioning systems. Along with the friction maintenance, wear and surface roughness need to be controlled at the interface of the moving parts to provide a longer life period to the moving components and avoid
5 undesired vibrations in the output motion. In this research a search is conducted for material pairs operating in a situation of high friction and low wear. The following sections present relevant research works in the recent past on Inch worm motors and also in tribological testing of materials operating in dry sliding friction. 1.2.1 Literature review of inchworm motors The piezoelectric actuators and motors worki ng on inchworm motor principle are some of the most investigated devices in the recent period. Inchworm motors are being developed to get an improved force output with smooth travel. The following are some of the works done on the inchworm motors in recent past. Judy et al constructed a piezo electric stepper motor for submicrometer controlled movement. This motor has a piezoelectric driving material (PZT) measuring 25.4mm X 12.7mm X 1.6mm. Velocities of 5.7-476 m/s and displacement step size of 0.07-1.1 m were achieved with this motor. Displacement step size and velocities were controlled by application of PZT extension voltages rangi ng from 60-340 V. This motor was intended for applications in electron microscopy, s canning tunneling micros copy, alignment of optical fibers and magnetic recording. They suggested the use of an electrostatic saw tooth clamp for better clamping in the future works. Miesner et al created a linear motor wo rking on the inchworm motion principle. Piezoelectric and magnetostrictiv e materials were used to generate the motion. Motor was operated at an electrical re sonance, switching power inte rnally between inductive and capacitive components. Terfenol-d rods drove the center expanding material of the drive assembly. These rods were surrounded by a ma gnetic coil which forms an inductive coil. The normal electrical phase relationship between these components provided neutral timing for the inchworm. Motor direction was controlled by the magnetic bias on the rods. The motor exhibited a stall load of 26 lb and no load speed of 1 in/s.
6 Zhang et al designed an inchworm-type li near piezomotor that consists of three piezoelectric actuators. Finite element methods were used to optimize the stiffness of elastic hinges and to calculate stress-strai n field on the flexure frame. Piezomotor was capable of traveling speed of 1.6 mm/s over a travel range of 300 mm with a positioning resolution of 5 nm. The force output wa s 200 N and a stiffness of 90 N/ m. Yeh et al demonstrated linear inchworm moto rs that can operate with moderate to high voltages. Their inchworm mo tor design decouples the actuator force to achieve large force and large displacement while consum ing low power. This inchworm motor was fabricated on a silicon wafer and had mini ature dimensions. The motor dimension was 1.5mm x 1mm x 15m on a silicon handle wafer. Motors were operated for over 13.5 hours for a total of 23.6 million cycles without stiction. They demonstrated motors with 80 m of motion, stepping rates of 1000 full st eps/second corresponding to 4mm/s shuttle velocity, and hundreds of N of force. In their design to improve clamping mechanism, saw tooth shape was used on the shuttle and the clutch. Chen et al developed a mesoscale actuator device, which is similar to piezoelectric device inchworm motors. The only difference wa s they replaced the friction clamping by a mechanical interlocking microridges. Th e design operated in th e range of 0.2-500 Hz frequency and did not support hi gh external loads, one of th e reasons being the limited stress capabilities of the micr oridges. Their future work included designing a clamping mechanism to support larger external loads In all the above research works, effectiv e performance of clamping mechanism was an important aspect of concern. The current research helps in improving the friction clamping system in the inchworm motor by sel ecting the appropriate material pairs that can be used in the fr iction clamp. Extensive se arch is done for selec ting the material pair that provides high static friction, low slidi ng friction, high wear resistance and maintains low surface roughness. A Universal Micro Tri bometer is used to simulate the shim interaction in the motor. Friction and wear tests are done to know the tribological
7 performance of material pairs. Below are some of the relevant works done in tribological testing. Tribological performance of materials sliding under dry contact friction is being investigated from many years. The following are some of the relevant works done in the past years analyzing the tribological proper ties of materials in dry sliding friction conditions. The apparatus used in the current research is Universal micro tribometer. Some other apparatus that are used for tri bology testing are pin on disk high frequency wear test rig (HFWTR), ball on three flats (BTF) wear apparatu s, four ball wear test rig (FBWT), pin-on-disk tribometer etc. Works done on them ar e briefly described below. 1.2.2 Relevant works in the past on tribological testing of materials Mesyef et al investigated wear mechan isms in ceramic-ceramic and ceramic-metal sliding contact. High friction coefficient of about 0.5 for metal-ceramic pair and 0.8 for ceramic-ceramic pair were observed. They at tributed wear mechan ism of metal-ceramic was transfer of metal to cer amic surfaces. Wear mechanism of ceramic-ceramic pair was due to intensive plastic deformation of su rface layers. Ceramic on metal wear was attributed to surface polishing and fract ure on the surface. Their observations and conclusions supported the decisi on to select ceramics and me tals for current research. G W Stachowiak et al examined the fricti on and wear characteristics of metallic materials in sliding contact with the oxide ceramics using a pin-on -plate configuration and reciprocating motion. They concluded that the combinations showed high friction, ceramic materials showed good wear resistance in sliding contact than metals. Their conclusions supported the selecti on of ceramics for our research. Gahr conducted wear tests in air with pin-on-ring tribom eter for ceramic-ceramic and ceramic-steel pairs. Ceramic materials included alumina, zirconia, SiSiC and metals were hardened, normalized, spheroidized 0.2-0.9 % C steels. They concluded ceramic materials offer greater resistance to wear than steels. They also found that ceramicceramic and zirconia-steel, SiSiC-steel comb inations exhibited hi gh friction coefficient and offered great resistance to wear.
8 Ko et al conducted wear tests during the process of search for wear-corrosionresistant materials. They concluded that stai nless steels and to an extent carbon steels showed higher corrosion-scaling resistance than hard faced materials. They also stated that hardness ratio of the material pair has significant effect on the wear of the surfaces. Also concluded that hard smooth surfaces e.g. chrome plated steel provides good wear resistance when sliding under severe abrasi ve conditions. These conclusions helped in selecting pairs for low endurance testing. Kim et al investigated frictional be havior of extremely smooth and hard solids (silicon, sapphire, SiC, quartz, glass etc.,) in air. The study was conducted to obtain the minimum friction under dry slidin g conditions at a normal load of 5 grams. They stated that above materials exhibite d low friction and wear coeffi cients. This behavior of smooth and hard solids in air was attributed to low probability of as perity interaction and wear particle generation. The Normal load in these tests was very low. Wear occurs when two surfaces slide against each other; it depends on various factors like hardness, size of wear debris produced et c. In sliding contact the asperities on the harder surface penetrate into and plow the softer surfa ce. Ploughing increases friction force and also produces wear particles, which in turn get trapped at the interface resulting in more friction and wear. Plastic deformati on and fracture are believed to be occurring at the interface, resulting in wear. Also signi ficant evidence is av ailable that two body abrasion is generally inversel y proportional to hardness of th e surface and proportional to the normal load and sliding distance of many pure metals. The deformation and ploughing components can be reduced by reducing surface roughness, reducing the difference in hardness of the sliding surfaces, and preventing the wear debris from getting trapped at the interface. Size and shape of the wear debris can be very useful in characterizing the wear. Mild wear can be ch aracterized by finely divided wear debris (typical 0.011 micron in particle size). The worn surface will have relatively low surface roughness. Severe wear in contrast results in mu ch larger particles, typically of the order of 20-200 micron in size, which may be visible with the naked eye .
9 1.3 Outline Further in this report there are four chapters that describe the prope rties of the materials considered for testing, the experimental set up and testing procedures, results, discussion and the conclusion. Second chapter deals with th e details of all the materials that were used in the current research. It describes all the mechanical properties of the various substrates and the hard thin coatings on them. Third chapter of the report explains the expe rimental setup and the procedure by which the specified properties of the materials like hardness, roughness, fr iction coefficient are determined and it also focuses on the actual ex perimental set up and the procedure for the wear testing of the specimen samples. The design of the fixtures and clamps is also presented in the chapter. Fourth chapter of the thesis report presents the friction data and the roughness data. This data forms the basis to decide if the tests were to be continued for particular specimen sample pair. This chapter also presents pict ures of the specimen sa mples after the wear tests were done. The wear pattern can be seen in these pictures which gives an idea of the level of wear on the surface. This chapter also presents discussion of the results and the suggestions for applications of material pair s. There are two categories of applications where these materials can be used. High endurance applications Low endurance applications. Final chapter presents the summary of the re search, explaining in detail the criteria for rejecting many of the pairs and deductions from results. Research is concluded suggesting the material pairs that perfor m well in specified conditions for various applications.
10 CHAPTER 2 MATERIALS 2.1 Requirements Design parameters and constraints are develope d for materials being tested which include high dimensional stability, high parallelism, high hardness, high surface finish or low surface roughness, high static friction coeffici ent, high wear and corrosion resistance. A wide range of materials are chosen in this re gard including steels, thin film coated steel substrates and ceramics which are listed below: Metals o Feeler gage tool steel o 52100 steel gage block o Croblox gage block o 440 C stainless steel gage block o Lapped A2 tool steel Coatings on lapped A2 tool steel o WC coating by Balzers  o TiN coating by IonBond  o DLC coating by Balzers o Tetrabond coating by IonBond o MoST coating by IonBond Coatings on Feeler gage tool steel o MoS2 coating o Graphite coating Ceramics o Zirconium oxide ceramic gage block (PSZ) o Sapphire rectangle
11 2.2 Metals 2.2.1 AISI 52100 alloy steel 52100 alloy steel is a through hardening st eel which has high hardness and youngs modulus of elasticity. H eat treated materials (steels) are preferred in applications where high dimensional stability is of much importa nce. Increasing manganese content in the alloy can increase hardenability of 52100 allo y steels. This has high contact fatigue strength. This alloy steel also o ffers high corrosion resistance. Table 1 Physical properties of 52100 steel Properties 52100 Hardness 413 HV Tensile strength 1379 MPa YoungÂ’s modulus 207 GPa Density 7.85*10 E 3 Kg/m3 Thermal expansion 1.24*10 E-5 /oC Thermal conductivity 43.25 W/m-K 2.2.2 Croblox The Croblox is carbide of chromium. Chro mium is a corrosion resistive material. Chromium carbide is hard and it offers high resistance to wear. Chromium carbide when sliding against steel exhibits high coefficien t of friction and high w ear resistance when compared to TiN coating in similar conditions . 2.2.3 440 C stainless steel This is a high carbon content martensitic st ainless steel. This steel has a moderate corrosion resistance and has hardness up to RC 60. This steel has a melting point of 1482 oC and has a modulus of elasticity of 200 GPa. This has a high wear resistance and finds applications in measuring instruments, gage blocks, valve components etc. This steel can
12 have superior machinability when alloyed with sulphur. These are usually used in a hardened condition. Hardness, strength and m achinability can be improved by subcritical annealing. In short these steels can be used where high hardness and wear resistance are the premium requirements, as the toughness a nd corrosion resistance are moderate. The above desired characteristics can be obtained by quenching and stress relieving processes for the high carbon steel. 2.2.4 Lapped A2 tool steel Lapped A2 tool steel is an alloy steel whic h is metallurgically pure and has high wear resistance and high hardness. Th is steel is also known for its dimensi onal stability. A2 tool steel has enhanced mechanical and tr ibological properties with the addition of chromium and molybdenum (1.1 %). This ha s 5 % of chromium which increases the toughness, wear resistance and s lightly imparts corrosion resist ance. Corrosion resistance can be further improved by using thin hard coati ngs on A2 tool steel. This tool steel finds application in knife edges, saw edges as this has high wear resistan ce. A2 tool steel can be cold treated to improve its toughness wit hout increasing its hardness which results in less brittle steel with high toughness. 2.3 Coatings on lapped tool steel 2.3.1 Tungsten carbide The main reason tungsten carbide is being used as thin coating is it maintains its hardness even in elevated temperatures. Tungsten ca rbide system exists in two phases, WC and W2C. Both phases have a hexagonal structure. Th e micro hardness of these coatings is in the range of 800 to 2100 HV. Mechanical and tribological properties of the coating are strongly influenced by the type of deposition. Tungsten carbide can be deposited in three ways: Thermal spraying Sputtering Chemical Vapor Deposition (CVD) process.
13 2.3.2 Titanium nitride Titanium Nitride was first coated commercially on tool s by the CVD method. In the recent days because of this coating plasma assisted Physical vapor deposition (PVD) process got much importance. Because of the excellent tribological properties, titanium nitride has attracted considerab le research and it is certainly, in tribological terms, the most explored hard thin coating. The abrasive wear performan ce of titanium nitride coatings is dependent on the coating method. Lee and Bayer (1985) compared the abra sive wear resistance of titanium nitride coatings produced by R.F diode sputtering, D.C. Magnetron sputtering, vacuum arc deposition and ion plating. The ion plating me thod was shown to be superior, which may result from the process characteristics that provide high levels of ionization efficiency . There is a variation in the c ontact mechanism and the wear process in sliding contacts between titanium nitride and st eel depending on the contact pa rameters such as geometry, speed, load, roughness etc. 2.3.3 Diamond like carbon (DLC) The amorphous carbon coatings with (a-C: H) or without hydrogen (a-C) produced by the Ion beam assisted evaporation, sputtering, i on plating, and Plasma Enhanced Chemical Vapor Deposition (PECVD) processes are very hard and are normally called Diamond like carbon (DLC) coatings. Diamond like ca rbon is the name commonly accepted for hard carbon coatings which have similar mech anical, optional, electrical and chemical properties to natural diamond, but which do not have a dominant crystalline lattice structure. They are amorphous and consist of a mixture of SP3 and SP2 carbon structures with SP2 bonded graphite like clusters embedded in an amorphous SP3 bonded carbon matrix.
14 The elasticity of the DLC coatings depends on the structure of the carbon layer. The diamond like carbon structure is a metastable form and can only exist up to a certain temperature level. Thermal graphitisation of th e graphite film takes place, starting from temperatures in the range of 300 to 600 oC . With increasing plasma deposition energy, the mass of the released molecules d ecreases and the thermal stability increases considerably. The topography of diamond like co atings is typically smooth and does not have a jagged micro roughness like the pyramidal diamond surface. Deposition of diamond coatings requires very hi gh substrate temperature (~ 1000 oC). Diamond like carbon coatings have a widely varying adhesion to the subs trate. The adhesion of the coating to the substrate depends on the internal stresses. Wear characteristics of this coating depend on the adhesion to a certain extent. 2.3.4 Tetrabond This coating is from the family of non-hydrogenated DiamondLike Carbon (DLC) coatings. The coating is extremely hard, 80-100 GPa, made of tetrahedral amorphous carbon with SP3 fraction of 85% or more. This type of coating can also be called amorphous diamond coating The Tetrabond coati ng has been successfu lly used in wear and abrasion applications, mach ining aluminum, its alloys and abrasive materials, such as Graphite. In spite of a difference in the structure of hydrogenated and non hydrogenated diamond like carbon coatings, not much variati on in the tribological properties is noted. These are harder than the hydrogenated diam ond like carbon coati ngs. These coatings when deposited on metals exhibit more coe fficient of friction th an when deposited on ceramics. 2.3.5 MoST coating with TiCN under layer MoST coating is a solid lubricating coating coated generally by PVD process. It offers ultra low coefficient of friction than ma ny other surface coatings like Teflon and Graphite. But the differentiating factor of this coating from many other coatings like Teflon is its high hardness similar to that of TiN. This coating has high wear resistance.
15 This coating is mainly composed of sul phur and molybdenum. MoST coating improves production of stainless steels a nd spring steels. As this is a solid lubricating coating, it eliminates need of lubrication in many applications. It elimin ates galling and this coating also improves tool performance. 2.4 Coatings on feeler gage tool steel 2.4.1 Molybdenum disulphide Molybdenum disulphide coating is a lamellar solid coating. This is a dry lubricating coating and it is usually used where thicker coat ings are necessary or an initial wear in is required. This coating has a layered struct ure of molybdenum and sulphur atoms. This coating can perform well upto temperatures of 400 oC in atmospheric conditions and upto 800 o C in vacuum. The method of depositing this coating material is simple as it can be done in powder or spray. This can also be co ated by sputtering and plasma spraying. This can be coated on all kinds of metals and stee ls. Tribological and ther mal properties of this coating are enhanced when alloyed with va rious metals during the deposition of the coating. This coating finds application in roller bearings, ball bearings, automotive and engine parts, linear guides and for all mech anical components in moving contact. This coating gets reduced in its thickness for the run in period and then it stabilizes for the rest of its life period exhibiting a very low friction coefficient. At the end of its life time it wears away forming blisters which lead into powder. The characteristics of these films depend on the method of deposition. 2.4.2 Graphite Graphite coating is a solid film lubricant or a lamellar solid coating, like the molybdenum disulphide coating. Graphite coatings are mo re stable form of carbon, because of this reason graphite coating can operate at high te mperatures and offers low coefficient of friction but can take only moderate loads. Th ese prevent abrasive wear for certain extent by providing lubrication while sliding. These coatings are mo re clean and easy to work with when compared to any lubricant between the sliding metals. The graphite coating
16 deteriorates in vacuum environment. Galvan ic corrosion is possible with the graphite coating. The co efficient of fr iction decreases with an increa se in relative humidity in the atmosphere. The graphite coating exhibits hi gh friction coefficient in vacuum and very low friction coefficient in air. 2.5 Ceramics 2.5.1 Partially stabilized zirconia (PSZ) One of the ceramic gage blocks used in th e current research is Zirconium oxide (ZrO2). The gage block used here is also called Part ially Stabilized Zirconia (PSZ). Zirconium oxide is a heat resistant and wear resist ant and has high operating temperatures. Along with excellent tribol ogical properties ZrO2 also has the following characteristics: High fracture toughness. High corrosion resistance. High hardness. Low conductivity. This zirconium oxide in its pure form has a cr ystal lattice structure at high temperatures and at low temperatures it has tetragonal and monolithic structure, however if the oxide is stabilized by addition of calcium magnesium or ytrium oxides hardness, strength and particularly toughness can be incr eased a lot. Below are some of the physical properties of a typical ceramic. Table 2 Physical properties of a typical ceramic Properties Ceramic Hardness 157-3600 HV Tensile strength 517-2400 MPa YoungÂ’s modulus 150-550 GPa Density 2.2 *10 E3 1.7 *10 E4 Kg/m3 Thermal expansion 2.3*E-6 1.78*E-5 /oC Thermal conductivity 1.6-176 W/m-K
17 2.5.2 Sapphire Sapphire is another ceramic gage block used in the research. Sapphi re is hard and has high wear resistance. It has good surface finish. These qualities suit the design specifications for the current research. Its high brittle nature makes it difficult material to work with. It is a scratch resistant material due to its high hardness. It is harder than most of the materials with exceptions like diamond. It is chemically inert. It has a high thermal conductivity (42 W/moK at 20 oC) and has high melting temperature of 2040 oC. Sapphire is drawn from Alumina (Al2O3). Sapphire comes out as a single crystal cylindrical piece, which is cut by diamond tools into different shapes. The specimen samples are ordered from L. S. Starrett Company and th e coatings on tool steel and feeler gage steel were done by Balzers and I onbond companies [18, 19]. Above mentioned materials are tested in this rese arch for their hardness, youngs modulus and surface roughness before the friction and wear tests. Surface roughne ss data is collected after every run for every specimen sample which aided the wear analysis of the surface.
18 CHAPTER 3 TESTING Specimen samples are tested for their mechan ical and tribological characteristics using precision equipment including Nanoindenter, Ta ylor Hobson Profilometer and Universal Micro Tribometer(UMT). Hardness and modul us of elasticity are obtained from Nanoindenter, surface roughness from prof ilometer and tribological testing was performed on UMT. Detailed description of th e apparatus, values of hardness, surface roughness of the tested material pairs are presented below. 3.1 Hardness measurement Hardness testing is performed on the Nanoi ndenter, a high precision instrument which measures the mechanical prope rties of the different materi als and different thin film coatings on the material substrates. This measures the hardness, youngÂ’s modulus and also gives the loading and unloading characterist ics of the thin film coatings or of the material substrates. This testing is done with a sharp indenter indent ing the material at a nanoscale. The material properties are measur ed from simple measurements of load, displacement and time. This method of m easuring the hardness is similar to the conventional testing when the obtained data is compared. The indenter usually used in testing is the Berkovich diamond. This indenter has a pyramidal shape having three sides. The data obtained from the equipment is acqui red into an excel work sheet, loading and unloading characteristics ar e simultaneously plotted. The hardness and modulus of elasticity are plotted against th e displacement of the indenter into the surface of the tested specimen. The following pictures show the Na noindenter apparatus that has PC based control and a close up of the actual work area of the indenter and charts plotting various data obtained from Nanoindenter.
19 Figure 2 Nanoindenter apparatus with the PC based control Figure 3 Close up view of the nanoindenter work area
20 0 10 20 30 40 50 60 0200400600800100012001400 Displacement Into Surface (nm)Hardness (GPa) A2toolsteel.xls Ceramicgageblock.xls DLC.xls Feelergage.xls Graphitefeelergage.xls Molyfeelergage.xls MoST.xls Steelgageblock.xls Tetrabond.xls TiN.xls WCC.xls 0 100 200 300 400 500 600 700 800 900 1000 0200400600800100012001400 Displacement Into Surface (nm)Modulus (GPa) A2toolsteel.xls Ceramicgageblock.xls DLC.xls Feelergage.xls Graphitefeelergage.xls Molyfeelergage.xls MoST.xls Steelgageblock.xls Tetrabond.xls TiN.xls WCC.xls Figure 4 Charts presenting hardne ss and modulus of elasticity plotted against displacement into surface
21 During testing of Tetrabond, WC and DLC coat ings, indentation depth was more than 30 percent of the coating thic kness (upper limit for indenta tion depth), resulting in a marginal error in measurement. However mechanical properties are calculated from the data obtained around 10 % of indentation depth, which mini mizes the percentage of error. This testing was not repeated as the above mentioned samples were eliminated during wear tests. The table below lists the hardness values of some of the materials, which are considerably hard. Table 3 Hardness of metals and ceramics 3.2 Surface roughness measurement After hardness testing, specimen samples ar e tested for their surface roughness. The surface roughness is one of the most important parameters taken into consideration. The material samples have to maintain very low surface roughness as to provide effective holding capability to the clamp against the actua tor stack with a stroke length of the order of microns. The surface roughness of materials is presented in two terms Ra and Rtm. These are the most common terms in which the surface roughness is presented. Ra is the arithmetic mean of the deviation of the roughne ss profile from the mean position. This is the usual way to present the roughness measurem ent. The Rtm value is the average peak to valley height of the profile in the a ssessment length. Taylor Hobson Surtronic 3P Profilometer is used to measure the surface r oughness of the materials. The cut-off in the profilometer is set at 0.03Â” (0.8 mm) when th e measurements were taken. The picture of the Profilometer and surface roughness data are as follows. Material Hardness Feeler gage tool steel 48-62 Rc 52100 steel gage block 62-65 Rc Croblox gage block 70-72 Rc 440C stainless steel gage block 55 Rc Zirconium-oxide ceramic gage block68-70 Rc Sapphire rectangle 9 mohs
22 Figure 5 Taylor hobson surtronic 3P profilometer Table 4 Pretest surface roughness data of materials Material Ra ( in) Rtm ( in) Feeler gage tool steel 6-9 25-52 A2 tool steel 2-4 7-25 52100 steel gage block 2-3 11-14 Croblox gage block 1-2 5-9 440C stainless steel gage block 1-4 2-10 TiN coated A2 8-12 46-54 Tetrabond coated A2 8-10 62-71 MoST coated A2 17-22 131-160 WC coated A2 9-14 28-66 DLC coated A2 4-8 19-40 Graphite coated feeler gage 15-31 102-104 MoS2 coated feeler gage 6-15 30-62 Zirconium-oxide ceramic gage block 2 6-14 Sapphire rectangle 2-3 6-9
23 3.3 Friction and wear testing Friction and wear testing of the specime n samples is done on the universal micro tribometer (UMT). UMT is a Tribological test ing apparatus with a load sensor, which has feed back control from a PC. The apparatus is operated using the software controls on the PC. Several test procedures can be programmed as sequences and machine can be operated accordingly. These sequences are editable and thus are very flexible, which eliminates the need to program the whole procedure again for low cycle tests. The load sensor of the machine measures the X and Z direction forces and displacements. The software automatically plots all the values of friction coefficient, friction force, force due to friction in X direction and force (applied) in Z direction. Static and sliding coefficient of friction can be obtained from the graph. Figure 6 Universal micro tribometer with PC based feed back control
24 3.3.1 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 1-100 N or 0.22 Â– 22 lb with a resolution of 0.1 N or 0.022lb. PC based 12-channel data acquisi tion and 3 motor controllers. Testing block, which is made of high-dens ity cast iron vibration damped frame. Upper vertical positional system ha s 150 mm of travel at 0.001-10 mm s-1 with 1 micron resolution. Upper lateral positioning system ha s 75 mm of travel at 0.01-10 mm s-1 with 2 micron resolution. Tribometer is facilitated with load feed back control system and suspension for the force sensor. Automatic sequencing of tests and data acquisition Rotational drive for the base plate (whi ch is fixed in our experimentation). Additional sensors like the contact ac oustic emission detector and electrical contact resistance. The interaction of the material shims is si mulated using the Universal Micro-Tribometer shown in Figure 6. The fixtures used for this application are designed which can be seen in the Figures 7 and 8. The specimen fixed to the upper fixture appl ies vertical load on the specimen which is fixed to the lower fixture which in turn is fixed to the base plate. The upper specimen fixed to the upper vertical and lateral positioning system reciprocates over the lower one simulating the motion of material shims in Inchworm motors. 3.3.2 High endurance tribology tests low cycle Nine material pairs are select ed initially for the low cycle tribology testing. Feeler gage, graphite coated feeler gage, MoS2 coated feeler gage, A2 t ool steel coated with TiN, Tetrabond, WC, MoST, DLC are paired with them selves and the last combination is AISI 52100 alloy steel over th e Zirconium oxide ceramic gage block
25 These nine pairs are tested under dry sliding conditions with a test sequence described as follows: The two specimens are fixed so that th e upper specimen comes into contact with the lower specimen near about at its center. Set the normal load to be 44.5 N ( 10 lb ) for 1 sec. Move the upper specimen to the le ft by 5 mm(0.19 inch) at V= 0.5 mms-1 (0.0016404 ft/s) Move the upper specimen to the right by 5 mm at V= 0.5 mms-1 which completes the reciprocating cycle. Repeat the reciprocating cycle for N-1 cycles where N is 10, 100 or 500 cycles. The test time taken for 10 cycles is 3 minutes, 100 cycles take 33 minutes and 500 accordingly takes 2 hours. Clean the test sp ecimens before and after the testing with acetone. Test procedure for the low cycle tribology testing is as follows: The above described test sequence is pe rformed for N=10 cycles during which the friction force Fx, normal force Fz, sliding displacement z and coefficient of friction are measured. Clean the test specimens with acetone, photograph them and take the surface roughness measurements and assess wear on basis of roughness data. Perform the test sequence on the tribomet er for 100 cycles but measure Fx, Fz, and z for first and last 10 cycles, that is run for 10 cycles measuring the parameters, run 80 cycles and then run 10 cycles measuring the parameters. Clean the test specimens with acetone, photograph them and take the surface roughness measurements and assess w ear on basis of roughness data. Perform the test sequence on the tribomet er for 500 cycles but measure Fx, Fz, and z for first and last 10 cycles, that is run for 10 cycles measuring the parameters, run 480 cycles and then r un 10 cycles measuring the parameters. Clean the test specimens with acetone, photograph them and take the surface roughness measurements and assess w ear on basis of roughness data.
26 3.3.3 High endurance tribology tests Â– high cycle The results obtained from the low cycle tri bology tests formed a basis to do further testing on the material shims under severe trib ological conditions. Hi gh cycle tribological tests are conducted over a long period under th e same normal load but a different sliding velocity. Modifications to the earlier procedure are made with respect to number of cycles and sliding velocity. The test sequence is as follows: The lower specimen is fixed on to the lowe r fixture an then the upper specimen is fixed to the top fixture and ad justed to be at the near centre of the lower specimen. Set Fz= 44.5 N ( 10 lb ) Move the upper specimen in a reciprocator y motion with an amplitude of 5 mm at a velocity V= 7.5 mm s-1. Repeat the whole reciprocating cycle for N-1 cycles. Note: 10,000 cycles run takes about 3.7 hour period. The test procedure for high cy cle tribological testing is: Obtain the upper and lower specimen samples for the testing. Measure the surface roughness and take the pictures of the specimen before testing. The friction measurement for the spec imen combination is found with 44.5 N vertical load, sliding velocity of 0.5 mm s-1 and sliding distance of 2 mm and this procedure is repeated in the same direc tion for 3 times with one second pause in the motion. Perform the test sequence which is described above for 10000, 50000 or 100000 cycles. Clean the test specimens with acetone and take the surface roughness measurements. Wear is assessed on the basis of the roughness data.
27 3.3.4 Low endurance tribology tests After the high endurance testing on all the ma terial pair combinations, low endurance tests are conducted to identify material pa irs with the highest friction, capable of maintaining acceptable surface roughness and with standing 49.21 inch (1.25m) of sliding without resulting in high levels of wear. Materials, which can offer high corrosion resistance, are chosen for testing as th e low endurance applications involved harsh environments. The test procedure and test se quence are the same as described previously but are programmed for only 125 cycles with a test run time of 2.77 minutes to the nearest approximation. Fixtures are designed to hol d the specimen samples in th e UMT. The fixtures are mounted one to the base and the other to the upper lateral and vertical positioning system. The fixtures are made of A2 tool steel and clamps are made of Al uminum. Design of the fixtures is presented in follo wing pages in figures 7 and 8.
28 Upper test specimen: Upper test fixture: Figure 7 The upper test fixture and specimen 0.36Â” 0.5Â” 0.15Â” 0.35Â” 6-32 UNC 1.5Â” 0.5Â” 0.25Â” 0.13Â” 1.0Â” 0.5Â” 0.05Â” 0.2Â” 0.15Â”
29 Lower test specimen: Lower test fixture: Clamps: Figure 8 The lower test fixture, specimen and clamps 1.0Â” 0.2Â” 6-32 UNC 0.35Â” 0.252Â” dia (for M6-1 screw) 0.409Â” dia 1.0Â” 3.0Â” 0.15Â” 0.35Â” 0.3Â” 0.5Â” 2.0Â” 0.05Â” 0.25Â” 2.0Â” 0.75Â” 0.25Â” 0.3Â” 0.35Â” 0.1Â” 0.15Â” 0.07Â” 0.144Â” dia 0.24Â” dia
30 CHAPTER 4 RESULTS AND DISCUSSION The results obtained from the friction and wear tests are tabulated in this chapter. Specimen samples are tested for their su rface roughness after each run. Further tests on each sample are conducted only if the wear a nd surface roughness data after every test is in acceptable range. The pictures of specime n samples after considerable testing are presented in this chapter. Pictures showing the wear debris collected and the average size of the particle in wear debris during the testing of 52100steel over zirconium oxide are also presented. 4.1 High endurance low cycle test results The results from the low cycle tribology tests are presented in the Tables 3-6. Testing on several combinations is terminated as they exhibited unacceptable levels of wear and surface roughness. The coating removal that oc curred with the TiN coated A2 tool steel after the 500 cycle test can be seen in Figure 9. The damage to the MoST coated A2 steel after 100 cycles is illustrated in Figure 10. The severe abrasion to the feeler gage steel after 500 cycles is presented in Figure 11. Similarly the other two feeler gages with coatings are also severely abraded and c ould not meet the wear and surface roughness requirements. Surface roughness of the material s with coatings decreased following the initial repeated sliding due to surface r un in. ( see Table 5 and 6 or reference ). WC coated A2 tool steel on itself a nd the 52100 alloy steel upon ceramic gage combinations produced the most desirabl e tribological perfor mance and acceptable surface roughness when an additional 500 cycle test is conducted. The following tables (from 5-8) are the results from the low cycle tr ibological tests and the pictures of some of the specimens that could not survive the tests.
31 Table 5 High endurance low cycle test results TiN coated A2** Tetrabond coated A2 WC coated A2 Before coating Ra ( in) 2-4 2-4 2-4 Rtm ( in) 7-25 7-25 7-25 After coating Ra ( in) 8-12 8-10 9-14 Rtm ( in) 46-54 62-71 28-66 Surface indication Bright Matte matte Hardness* (GPa)20 27 14 Modulus* (GPa) 379 402 191 10 cycle test static 0.20-0.22 0.18-0.32 --sliding 0.18-0.20 0.16-0.29 0.13-0.20 Ra ( in) 6-9 5-6 3-8 Rtm ( in) 37-69 26-46 22-51 Wear indications Light abrasion Mild scuffing Mild abrasion 100 cycle test static ------sliding 0.18-0.23 0.07-0.08 0.14-0.16 Ra ( in) 5-10 3-6 4-8 Rtm ( in) 23-69 20-46 22-51 Wear indications Light abrasion Mild scuffing Mild abrasion 500 cycle test static ------sliding 0.50-0.56 0.045-0.050 0.18-0.20 Ra ( in) 9-44 3-6 3-7 Rtm ( in) 41-364 23-42 16-47 Wear indications Coatingremoval Severeabrasion Mild scuffing Mild abrasion Average over 40 in ** A2 tool steel (4 GPa average ha rdness & 236 GPa average modulus over 40 in)
32 Table 5 (continued) MoST coated A2 DLC coated A2 Steel&ceramic gage blocks Before coating Ra ( in) 2-4 2-4 2-3 / 2 Rtm ( in) 7-25 2-25 11-14 / 6-14 After coating Ra ( in) 17-22 4-8 NA Rtm ( in) 131-160 19-40 NA Surface indicationmatte Bright Bright Hardness* (GPa) 7 17 13 / 16 Modulus* (GPa) 147 232 267 / 248 10 cycle test static ------sliding 0.10-0.32 0.08-0.10 0.12-0.18 Ra ( in) 9-27 4-6 3-5 / 2-11 Rtm ( in) 45-177 16-38 14-26 / 3-32 Wear indications Coating removal Mild abrasion Mild abrasion 100 cycle test static ------sliding 0.20-0.25 0.04-0.06 0.12-0.14 Ra ( in) 7-23 3-6 2-3 / 2-4 Rtm ( in) 22-51 17-41 5-9 / 3-9 Wear indications Severecoating removal Mild abrasion Mild abrasion 500 cycle test static --0.065-0.070 --sliding --0.056-0.060 0.12-0.15 Ra ( in) --3-7 2-3 / 2-5 Rtm ( in) --16-43 5-7 / 4-11 Wear indications --Mild abrasion Mild abrasion Average over 40 in
33 Table 5 (continued) Feeler gage Feeler gage with graphite coating Feeler gage with MoS2 coating Before coating Ra ( in) 6-9 6-9 6-9 Rtm ( in) 25-52 25-52 25-52 After coating Ra ( in) NA 15-31 6-15 Rtm ( in) NA 102-104 30-62 SurfaceindicationBright Matte Matte Hardness* (GPa) 8 0.3 0.7 Modulus* (GPa) 258 6 41 10 cycle test static 0.3-0.6 0.25-0.26 0.36-0.46 sliding 0.3-0.6 0.20-0.23 0.35-0.45 Ra ( in) 7-17 4-42 9-30 Rtm ( in) 36-73 24-235 35-167 Wear indications Moderate abrasion Coating removal&transfer Coating removal&transfer 100 cycle test static 0.65-0.70 --0.52-0.58 sliding 0.60-0.65 0.13-0.22 0.50-0.54 Ra ( in) 11-24 9-23 9-31 Rtm ( in) 21-133 34-149 31-144 Wear indications Severe abrasion Severe coating removal&transfer Severe coating removal&transfer 500 cycle test static 0.68-0.72 ----sliding 0.60-0.65 ----Ra ( in) 13-32 ----Rtm ( in) 57-123 ----Wear indications Severe abrasion ----* Average over 40 in
34 Table 5 (continued) Tetrabond on A2 WC on A2 DLC on A2 Steel&ceramic gage blocks 2nd 500 cycle test static --------sliding 0.05-0.06 0.16-0.19 0.12-0.14 0.17-0.20 Ra ( in) 3-6 2-4 3-7 2-4 / 2-3 Rtm ( in) 15-42 16-34 24-37 3-10 / 3-10 Wear indications Mild scuffing Mild abrasion Mild abrasion Mild abrasion
35 Figure 9 TiN coated A2 tool steel aft er 500 cycle test with severe coating removal
36 Figure 10 MoST coated A2 tool steel after 100 cycle test with severe coating removal
37 Figure 11 Feeler gage after 500 cy cle test with severe abrasion
38 4.2 High endurance high cycle test results The specimens which survived in the low cycl e tribology tests are test ed for an additional 20,000 cycles, in two 10,000 cycl e sets. All the four spec imens performed reasonably well. The results are tabulated in Table 9. Si xteen more specimen pairs are selected to conduct more high cycle high endurance tests. The results are tabulated in the tables 10 17. Four samples (sample nos. 1, 2, 5 and 6) were not tested beyond the friction testing, as their static friction coefficient was very lo w. Testing of six (sample nos. 4, 7, 8, 9, 12 and 13) of the remaining specimen pairs is terminated because of their unacceptable surface roughness values and wear after 40,000 cycles. Figures 12 and 13 illustrate the failure of the Tetrabond coati ng on A2 tool steel gage block (test no. 8) and TiN coating on A2 steel gage block (tes t no. 9) after 10000 cycles of sliding. High abrasion was found on the surface and the coating was removed. The 52100 steel on the ceramic gage block comb ination performed very well with respect to the wear and surface roughness, three test samples (nos. 3, 14 and 15) were analyzed and the results were found to be repeatable as shown in Tables 14 and 15. Sample no 15 (52100 steel over ceramic) was analyzed till 7, 00,000 cycles which is equal to 4.35 mile (7 Km) distance of sliding and found to perf orm very well with respect to wear and surface roughness (see Table 16 ). Pictures of this sample after 2, 00,000 cycles, which is equal to 1.243 mile (2 Km) of sliding, were taken and illustrated in figures 14 and 15. The oxide formation on the steel gage block can also be seen within the fine scratches. Very fine scratches on the ceramic block canno t be seen in the picture. The particles generated while sliding are co llected and found to be oxidi zed particles (see figure no 16). Ceramic on ceramic gage block combination is analyzed as sample no 16. This material combination performed very well with respect to wear and surface roughness, but the friction coefficient for this pair was a comp aratively more (around 0.32 ). This may be because of change in orientation of the ceramic blocks while clamping between the 50,000 cycle intervals.
39 Sapphire gage block in combination with 52100 steel and zirconium oxide ceramic gage block (test no. 10, 11) also performed very we ll with respect to th e wear and surface roughness values. The sapphire block was brittle and clamping it to the fixture without fracturing it was considerably di fficult. To see that the sapphire block doesnÂ’t fracture rubber layer was kept as a barrier be tween the gage block and the clamp. Only pairs that performed well after 40000 cycles were: Sapphire gage block over the AISI 52100 steel gage block, Sapphire gage block over the PSZ ceramic gage block, 52100 steel gage block over th e PSZ ceramic gage block, PSZ ceramic gage block over itself. Following are the high cycle high endurance test results and pictures of the specimen after high cycle sliding.
40 Table 6 High endurance high cycle test results Tetrabond on A2 WC on A2 DLC on A2 Steel&ceramic gage blocks PRETEST* Ra ( in) 3-6 2-4 3-7 2-4 / 2-3 Rtm ( in) 15-42 16-34 24-37 3-10 / 3-10 Wear indications Mild scuffing Mild abrasion Mild abrasion Mild abrasion POST 1st 10,000cycles Ra ( in) 3-6 3-4 3-5 2-7 / 2-7 Rtm ( in) 16-41 23-34 25-33 5-11 / 2-16 Wear indications Mild scuffing Mild abrasion Mild abrasion Mild abrasion POST 2st 10,000cycles Ra ( in) 2-6 3-6 3-5 2-8 / 2-9 Rtm ( in) 14-43 18-37 18-33 8-46 / 2-13 Wear indications Mild scuffing Mild abrasion Mild abrasion Mild abrasion *Pretest condition of these specimens c onsists of 10+100+500+500 cycle tribotests
41 Table 6 (continued) Test Number 1 2 3 4 WC* & ceramic g.b.** DLC* & ceramic g.b 52100 g.b. & ceramic g.b WC* & 52100 steel g.b. PRETEST Ra ( in) 9-14 / 2 4-8 / 2 2-3 / 2 9-14 / 2-3 Rtm ( in) 22-66 / 6-14 19-40 / 6-14 11-14 / 6-14 22-66 / 1114 static 0.089-0.091 0.093-0.11 0.16-0.17 0.18-0.20 sliding 0.087-0.092 0.093-0.10 0.14-0.15 0.18-0.21 Status based on static cancel cancel Proceed proceed POST 1st 10,000cycles Ra ( in) 2-3 / 1-2 4-10 / 2-3 Rtm ( in) 6-10 / 3-14 24-36 / 615 Wear indications Mild abrasion Mild abrasion Status Proceed proceed POST 2nd 10,000cycles Ra ( in) 2-3 / 1-2 5-11 / 2-13 Rtm ( in) 4-9 / 3-4 33-65 / 456 Wear indications Mild abrasion Mild abrasion Status Proceed cancel POST 40,000cycles Ra ( in) 2-5 / 2 Rtm ( in) 5-18 / 4-9 Wear indications Mild abrasion coated on A2 tool steel ** g.b. = gage block
42 Table 6 (continued) Test Number 5 6 7 WC* & croblox g.b.** Croblox g.b. & ceramic g.b Tetrabond* & ceramic g.b PRETEST Ra ( in) 9-14 / 1-2 1-2 / 2 8-10 / 2 Rtm ( in) 22-66 / 2-9 2-9 / 6-14 62-71 / 6-14 static 0.11 0.10-0.11 0.19-0.21 sliding 0.11 0.10-0.11 0.17-0.21 Status based on static Cancel cancel Proceed POST 1st 10,000cycles Ra ( in) 5-15 / 2-5 Rtm ( in) 68-93 / 5-16 Wear indications Mild abrasion Status Proceed POST 2nd 10,000cycles Ra ( in) 4-13 / 1-3 Rtm ( in) 79-94 / 6-8 Wear indications Abrasion Status Cancel POST 40,000cycles Ra ( in) Rtm ( in) Wear indications coated on A2 tool steel ** g.b. = gage block
43 Table 6 (continued) Test Number 8 9 10 Tetrabond* & 52100 steel g.b. TiN & A2 steel Sapphire & 52100 steel g.b. PRETEST Ra ( in) 8-10 / 2-3 8-12 / 2-4 2-3 / 2-3 Rtm ( in) 62-71 / 11-14 46-54 / 7-25 6-9 / 11-14 static 0.32-0.36 0.20-0.24 0.14-0.15 sliding 0.32-0.36 0.16-0.23 0.13-0.14 Status based on static Proceed proceed Proceed POST 1st 10,000cycles Ra ( in) 11-25 / 6-10 65-226 / 64-255 2 / 2-3 Rtm ( in) 79-156 / 12-51 458-1088 / 280-1257 4-8 / 7-12 Wear indications Severe abrasion & corrosion Severe abrasion & corrosion Mild abrasion Status Cancel cancel Proceed POST 2nd 10,000cycles Ra ( in) 2-3 / 2-3 Rtm ( in) 4-6 / 5-7 Wear indications Mild abrasion Status Proceed POST 40,000cycles Ra ( in) 2-3 / 1-2 Rtm ( in) 5-10 / 6-9 Wear indications Mild abrasion coated on A2 tool steel ** g.b. = gage block
44 Table 6 (continued) Test Number 11 12 13 Sapphire & ceramic g.b.** DLC & 52100 steel Croblox gb & 52100steel gb PRETEST Ra ( in) 2-3 / 2 4-8 / 2-3 1-2 / 2-3 Rtm ( in) 6-9 / 6-14 19-40 / 11-14 2-9 / 11-14 static 0.16-0.17 0.14-0.15 0.41-0.66 sliding 0.15-0.17 0.13-0.14 0.56-0.66 Status based on static Proceed Proceed Proceed POST 1st 10,000cycles Ra ( in) 2 / 1-4 4-7 / 2-3 2-12 / 2-10 Rtm ( in) 4-6 / 5-9 35-40 / 5-12 3-46 / 4-44 Wear indications Mild abrasion Mild abrasion Abrasion Status Proceed Proceed cancel POST 2nd 10,000cycles Ra ( in) 2-3 / 1-2 5-7 / 2 Rtm ( in) 4-7 / 4-5 27-48 / 6-8 Wear indications Mild abrasion Mild abrasion Status Proceed Proceed POST 40,000cycles Ra ( in) 1-2 / 2 4-12 / 2-19 Rtm ( in) 3 / 3-7 21-96 / 14-86 Wear indications Mild abrasion Abrasion coated on A2 tool steel ** g.b. = gage block
45 Table 6 (continued) Test Number 3 52100 g.b. & ceramic g.b POST 60k cycles total Ra ( in) 1-6 / 1-2 Rtm ( in) 4-17 / 2-11 Wear indications Mild abrasion Status proceed POST 100k cycles* total Ra ( in) 2-4 / 2-3 Rtm ( in) 3-15 / 2-10 Wear indications Mild abrasion Status proceed POST 150k cycles total Ra ( in) 2-12 / 2-3 Rtm ( in) 7-46 / 4-13 Wear indications Mild abrasion Status proceed POST 200k cycles total Ra ( in) 2-17 / 2-3 Rtm ( in) 5-47 / 4-8 Wear indications Mild abrasion 100k cycles = 1km
46 Table 6 (continued) Test Number 14 15 Steel g.b. & ceramic g.b. Steel g.b. & Ceramic g.b. PRETEST Ra ( in) 2 / 1-2 1-2 / 1-2 Rtm ( in) 3 / 2-6 2-3 / 2-3 static 0.13-0.14 0.16-0.18 sliding 0.12-0.13 0.16-0.17 POST 50,000cycles Ra ( in) 2-8 / 2 7-8 / 2-4 Rtm ( in) 2-39 / 2-5 19-24 / 3-12 Wear indications Mild abrasion Mild abrasion POST 100,000cycles Ra ( in) 2-7 / 2 3-9 / 2-7 Rtm ( in) 2-40 / 2-5 5-34 / 4-20 Wear indications Mild abrasion Mild abrasion POST 150,000cycles Ra ( in) 3-9 / 2 4-10 / 3-9 Rtm ( in) 4-42 / 3-4 11-37 / 8-35 Wear indications Mild abrasion Mild abrasion POST 200,000cycles Ra ( in) 1-9 / 2-3 3-10 / 2-6 Rtm ( in) 4-55 / 3-8 6-46 / 10-37 Wear indications Mild abrasion Mild abrasion
47 Table 6 (continued) Test Number 15 Steel g.b. & Ceramic g.b. POST 300,000cycles Ra ( in) 4-13 / 2-5 Rtm ( in) 7-65 / 4-10 Wear indications Mild abrasion POST 400,000cycles Ra ( in) 3-8 / 2-5 Rtm ( in) 15-58 / 6-24 Wear indications Mild abrasion POST 500,000cycles Ra ( in) 2-12 / 2-4 Rtm ( in) 17-72 / 8-12 Wear indications Mild abrasion POST 600,000cycles Ra ( in) 3-14 / 3-5 Rtm ( in) 8-71 / 6-33 Wear indications Mild abrasion POST 700,000cycles Ra ( in) 7-13 / 2-12 Rtm ( in) 30-55 / 4-39 Wear indications Mild abrasion
48 Table 6 (continued) Test Number 16 Ceramic g.b. & Ceramic g.b. PRETEST Ra ( in) 1-2 / 1-2 Rtm ( in) 2-3 / 2-3 static 0.14-0.15 sliding 0.13-0.14 POST 50,000cycles Ra ( in) 2-4 / 2-4 Rtm ( in) 2-9 / 5-11 static 0.25-0.27 sliding 0.24-0.25 Wear indications Mild abrasion POST 100,000cycles Ra ( in) 2-3 / 1-4 Rtm ( in) 2-5 / 2-11 static 0.17-0.19 sliding 0.15-0.17 Wear indications Mild abrasion POST 150,000cycles Ra ( in) 1-7 / 2-5 Rtm ( in) 2-24 / 2-13 static 0.34-0.35 sliding 0.32-0.33 Wear indications Mild abrasion
49 Figure 12 Tetrabond coated A2 steel fr om test no. 8 with severe abrasion after 10,000 cycles of reciprocating sliding
50 Figure 13 TiN coated A2 steel from te st no. 9 with severe abrasion after 10,000 cycles of reciprocating sliding
51 Figure 14 52100 steel gage block specimen from test no. 15 after 200,000 cycles
52 Figure 15 Ceramic gage block specimen from test no. 15 after 200,000 cycles
53 Figure 16 Wear particles from 52100 steel gage block from test no. 15 on mating ceramic gage block post 100,000 cycle test at 500,000 cycles of cumulative testing 0.10Â”
54 4.3 SEM and EDX analysis Wear particles are analyzed using SEM and E DX analysis. From this analysis the size (diameter) of an average particle is noted based on which level of wear can be estimated. Average particle size is measur ed to be of the order of 50.39 in, which assists to estimate the wear as mild wear. Also presen ce of various elements and oxidation of the 52100 steel gage particles were detected. Th e picture showing the particle size is presented below. Figure 17 SEM picture of wear particle at 15000 X magnification 50.39 in
55 4.4 Low endurance results After the high endurance tests, five material pairs are chosen for low endurance tests. These low endurance tests are designed and programmed for 125 cycles of testing. All the material pairs performed reasonably we ll with respect to wear and roughness. Croblox sample with edge wear is shown in the figure nos. 17 and 18. Similar edge wear was found in all the tested samples. Of a ll the material pairs 52100 steel on itself displayed the highest friction coefficient. 440C stainless steel on itself displayed moderately lesser friction than 52100 pair bu t it offers better corrosion resistance which makes it the most suitable combination for the applications in corrosive environments. Table 7 Low endurance test results Priority Number 17 18 19 Croblox g.b. & croblox g.b. Croblox g.b. & 52100 steel g.b. 52100 steel & 52100 steel g.b. PRETEST Ra ( in) 1-2 / 1-2 1-2 / 1-2 1-2 / 1-2 Rtm ( in) 2-3 / 2-3 2-3 / 2-3 2-3 / 2-3 static 0.19-0.24 0.30-0.35 0.50-0.70 sliding 0.17-0.19 0.25-0.30 0.50-0.70 POST 125cycles Ra ( in) 2 / 2 2-4 / 2-9 2-3 / 2-3 Rtm ( in) 5-8 / 3-7 4-12 / 3-22 5-13 / 5-16 Wear indications Mild abrasion Mild abrasion Mild abrasion
56 Table 7 (continued) Priority Number 20 21 440C stainless steel gb on itself 440C stainless steel gb & ceramic gb PRETEST Ra ( in) 1-4 / 1-4 1-4 / 1-2 Rtm ( in) 2-10 / 2-10 2-10 / 2-3 static 0.60-0.65 0.25-0.30 sliding 0.50-0.65 0.25-0.30 POST 125cycles Ra ( in) 3-6 / 2-4 2 / 1-2 Rtm ( in) 9-25 / 4-14 4-6 / 2-5 Wear indications Mild abrasion Mild abrasion
57 Figure 18 Top croblox gage block specimen from test no. 17 after 125 cycles
58 Figure 19 Bottom croblox gage block specimen from test no. 17 after 125 cycles
59 4.5 Surface roughness measurement of prototype Surface roughness measurement of prototype of an inchworm motor after an endurance test for 1.243 mile (2 km) of sliding distance is taken with Taylor Hobson Surtronic 3P profilometer. 52100 alloy st eel in combination with zirconia gage block material is used in the prototype. The surface nearest to the actuators is presented in Figure 19. The measured Ra values are 2-6 in and the Rtm values are 6-36 in. Some oxidation on the surface can be seen in the picture. The ot her surface is shown in the Figure 20. The measured roughness values are Ra of 2-7 in and Rtm of 10-33 in. Performance of these prototype parts is comparable with the 52100 steel-ceramic pair (tests no. 3,14 and 15 presented earlier in this ch apter) after 200,000 cycles of test ing which is equal to 1.243 mile (2 km) of sliding distance.
60 Figure 20 Motor prototype part surfa ce nearest actuators after 1.243mile (2km) sliding endurance test (made of 52100 steel gage block)
61 Figure 21 Motor prototype part surface farthest from actuators after 1.243 mile (2km) sliding endurance test (made of 52100 steel gage block)
62 4.6 Discussion and applications Results from the friction and wear testing are analyzed to select the material pairs that performed well with respect to friction and wear. Surface roughness data of the samples after each run is acquired. Se verity of wear is estimate d from the photographs with significant magnification and surface roughness of specimen samples after each run. If the wear intensity is low and surface roughness is within the specified limits, then further testing on the sample is performed. Hardne ss had a vital effect on the wear of the surfaces. It can be noticed from the results th at the softer materials could not survive the tests for a longer period. Coated specimens di d not survive the high cy cle tribology tests. Also it was observed from the results that coatings resulted in increased surface roughness of the substrate material. Ceramics being the hardest materials of all suffered very less abrasion and maintained reas onably low surface roughness. 52100 alloy steel exhibited low surface roughness even though there was marginal wear on the surface. Wear debris collected during the testing of 52100 alloy steel gage block on PSZ (partially stabilized zirconia) gage bl ock is analyzed in the SE M and through EDX process from which it was evident that the particles were from the steel gage block and the size of wear particle being of the order of a micron, wear could be char acterized as mild abrasion. After analyzing the results, four pairs from the high endurance testing and one pair from the low endurance testing are selected. Two types of applications can be suggested according to the test procedures and the sliding distance for which the material samples were tested. High endurance applications Low endurance applications High endurance applications are those wherein the materials pairs slide for a long time or for a long distance under a normal load. In these app lications material pa irs need to slide against each other maintaining low surface roughness, low wear and high frictional force at the interface. One of such applications can be found in the friction clamping of an inchworm motor drive mechanism. Inchworm motors are used in precise positioning
63 systems with a nanometric step size (resolu tion). For an improved force output of the inchworm motors efficient friction clamping is necessary. In a novel design developed by DSM engineers  for efficient friction cl amping, there are material shims stacked in such a way that they slide against each othe r under a normal load. Ma terial pairs used in these clamps have to provide high coeffici ent of friction at the interface and have to maintain less wear and low surface roughness. Material shim interaction was very closely simulated and tested on the universal micro tribometer. Using the material pairs selected in this research work would enhance the holding ability of the clamp stack resulting in better performance of the motor. These materi al pairs are machined accurately in regard of dimension, parallelism, flatness, r oughness. Because of the Low roughness and flatness only a minor component of the appl ied force will be consumed in holding the expanding actuator stack thus adding to th e improved force output of the Inchworm motor. Inchworm motors are used in scanning electron microscopy as a positioning system operating at a nanometric resolution. In such app lications the step size will be of the order of several nanometers to a micron. To provide efficient clamping fo r an actuator with such a step size, the material shims in th e clamp have to be extremely smooth. This research suggests material pairs that can be used in the friction clamps for such applications. Encoder Inchworm stage is a nanopositioning product which finds applications in e-beam lithography, positioning of electrical probe s and positioning samples for micromachining in a focused ion beam milling machine. This high-resolution option combines an Inchworm motor based precision cross-roller bearing stage and the latest position encoder technology in a fully integrated positioning package. In achieving high resolution of the order of several nanometers and accuracy in su ch applications, friction clamping plays an important role. The current research helps in improving the clamping efficiency for such precision positioning systems resulting in high force output and smooth travel.
64 Inchworm motors are also used in Nanorobot systems where the motor has to deliver nanometric resolution with a long travel. Agai n for a better performance of the motor in such applications holding the actuator stack effectively for such nanometer level stroke size, clamp needs to have an extremely smoot h material which offers high static friction with low wear on the surface. Suggested mate rial pairs in such clamps provide most desirable results. Low endurance applications are those where ma terials slide for a short distance but under a normal load in harsh and moist environm ents. Material pairs selected for these applications were tested for 2.77 minute run which is equal to 49.21 inch (1.25 m) sliding distance. Material pairs in these conditi ons should possess high corrosion resistance. Specimen samples were also tested for the high coefficient of friction at the interface. There are many defense applications where th e suggested material combinations can be used for safe and effective operation.
65 CHAPTER 5 CONCLUSIONS This thesis focuses on the tri bological performance of the material pairs sliding under normal load in the absence of a lubricant. Ma terial pairs are tested for their friction and wear characteristics while sliding in dry friction conditions. Design specifications are developed for the material pairs being tested including high static co efficient of friction, high wear resistance, low surface roughness and high hardness, high flatness and parallelism of the surface. Material pairs, which perfor med well and met the design specifications, are selected for applications in various environments. Several materials and many coatings on tool steel substrates are selected for testing. Coatings enhance the tribological, mechanical and thermal propertie s of a substrate, which made them probable specimens for testing. Before tribological testin g all the material pairs are tested for their hardness and surface roughness using high prec ision equipment like Nanoindenter and Taylor Hobson Profilometer. Va rious test sequences are deve loped for tribological testing of material pairs on Universa l micro tribometer. These sequences are editable which made it easier to test the materials for diffe rent number of cycles. Low cycle tests are conducted initially on nine pairs and then hi gh cycle tests are conducted on sixteen more pairs. High cycle tests ar e initially conducted for 10000 cy cles and as the testing proceeded they are increased to 50000 cycles which equals 0.31 mile (0.5 Km) of sliding distance. After each run surface roughness meas urements are taken with Profilometer and samples are photographed. Based on the data obtained from the profilome ter and from the intensity of wear on the surface, further testing was c onducted. Results indicate that ma terials with low hardness did not survive the tests. Also from the da ta analysis it can be concluded that using coatings increases surface roughness and many of the coated specimens did not meet the
66 specifications and hence their testing was te rminated. High endurance and low endurance applications are suggested in this th esis work for the material pairs. From the obtained results it can be concl uded that ceramic on ceramic, steel on ceramic and ceramic on steel pairs performed extremel y well with respect to friction, wear and surface roughness specifications. Material pair s selected for high endurance applications based on their performance are listed below. Sapphire gage block over 52100 alloy steel gage block Sapphire gage block over PSZ ( partially st abilized zirconia) ceramic gage block 52100 alloy steel gage block over PSZ ceramic gage block. PSZ ceramic on PSZ ceramic gage block. Sapphire performed really well in both the comb inations with respect to the friction, wear and surface roughness but it was very brittle in nature. Ther e was considerable difficulty in clamping it without fracturi ng the surface. The problem was resolved by using a rubber layer between block and the clamp. 52100 alloy steel block over PSZ gage block pair performed extremely well of all the above listed combinations. This combinati on was tested for 7,00,000 cycles of sliding which is equal to 4.35 mile (7 Km) of slidi ng distance. Even after this many cycles of testing wear and surface roughness levels were very low and exhibited significant coefficient of static friction at the interface. PSZ gage block on itself performed extremel y well with respect to wear and surface roughness. The scratches on both the surfaces were not noticeable. This pair also provides good corrosion resistan ce. Friction coefficient at th e interface was high; this might be due to a slight disorientatio n of specimen samples during the testing. From the low endurance tests it was found that combination of 440C stainless steel on itself performed very well. It offers a fric tion coefficient in th e range of 0.5-0.65. 440C
67 stainless steel offers high corrosion resistance This pair performed very well in terms of wear and surface roughness for intended distan ce of sliding. Edge wear occurred on both the surfaces but still the roughne ss of the surface was well in the acceptable limits. This combination is selected for applications wh ere high friction and high corrosion resistance is desired for a shor t sliding distance. Using the material pairs selected in this research work in friction clamps of inchworm motor drive assembly, performance of fricti on clamps will be improved, resulting in a high force output of the motor, maintaining the precision during positioning applications.
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