ICMCTF2004 Session E1-2: Friction and Wear of Coatings I: Lubrication and Surface Effects

Tuesday, April 20, 2004 1:30 PM in Room Royal Palm 1-3

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		E1-2-1 Third Body Dynamics and Mechanics of MoS₂ and MoST Coatings G.Y. Lee, K.J. Wahl, I.L. Singer (U.S. Naval Research Laboratory) The tribological behavior of MoST (Ti-Mo-S) and MoS₂ coatings have been investigated by in situ tribometry, which allowed real time observation of third body dynamics. Coatings were deposited by either closed field unbalanced magnetron sputtering (MoST and MoS₂) or ion beam assisted deposition (MoS₂). Reciprocating sliding tests were performed in dry and ambient (40-50% RH) air environments at sliding speed of 1 mm/s. Transparent sapphire hemispheres were used as counterbodies to provide reasonably high contact stress (1.1 GPa mean Hertzian stress at 24N load) and allow optical observation of third body dynamic processes. The rate of third body transfer film buildup on the sapphire counterfaces was monitored in situ using Newton’s rings. Titanium containing coatings showed rapid thickening of third body transfer film on the sapphire hemisphere as compared to MoS₂ coatings in humid environments. Optical observations of third body dynamics demonstrated that for all coatings and environments the major velocity accommodation mode was interfacial sliding, where most of the sliding took place between the transfer film and the wear track. Profilometry and scanning instrumented indentation were used to evaluate hardness and modulus of the wear tracks and third body transfer films. Transfer film hardness was lower than that of the parent coatings. The role of third body chemistry and mechanics in the tribological performance of MoS₂ based coatings will be discussed. [1] N.M. Renevier, V.C. Fox, D.G. Teer, and J. Hampshire, Surf. Coat. Tech. 127 (2000) 24-27.
1:50 PM		E1-2-2 New Method for Deposition MoS₂ and WS₂ Coating J.H. Richter (Richter Precision inc.); A.R. Torosyan (National Academy of Sciences of the Republic of Armenia) We describe improved tribological properties of the systems obtained using our new mechanochemical method for the syntheses of sulfide coatings MoS₂ and WS₂. The new method leads to decreased friction coefficients and increased durability of coatings. Optimal conditions of mechanochemical synthesis were found experimentally. At hypo-optimal concentrations of sulfur the coating contains numerous flaws and cracks, while at hyper-optimal quantities of sulfur the surface quality deteriorates and roughness is increased. When key synthesis parameters are maintained in the optimal window the process of synthesis is well-controlled and reproducible, and thickness of the coating can be regulated. Plane samples of low carbon steel and bearing steel balls were mechano-chemically coated with Mo-MoS₂ and W-WS₂ layers. The properties were tested using a pin-on-disc wear tester. Testing was carried out under contact pressure P=300-350 Mpa and sliding speed of 0.8m/s, corresponding to extreme wear conditions. Control parameters for testing were friction coefficient f and the durability C, defined as the number of cycles before coating failure. The results are given below to compare the performance of the mechano-chemical coatings with conventional vapor phase deposited coatings, in the form (Method of Synthesis: Fric. coeff. f - No. cyc. to failure) (Treatment of Mo in H2S: 0.05 - 30000) (Treatment of Mo in S vapor: 0.05 - 50000) (Mechano-chemical Mo-MoS2: 0.03 - 70000). Mechano-chemical coating allows appreciable improvement of tribological properties of coatings for severe operating conditions. The novel deposition method is carried out at room temperature from initial elemental substances, and the coatings are synthesized under the regime of dry friction and impact, so that they develop micro- and macro-structures already well adapted for conditions of severe contact loading and display stable wear from the start.
2:10 PM		E1-2-3 Lubrication Mechanisms of MoS₂/Graphite/Sb₂O₃ Coatings J.S. Zabinski (Air Force Research Laboratory); B.S. Phillips (Universal Technology Center); J.J. Hu, A.A. Voevodin (Air Force Research Laboratory) Lubrication by MoS₂ is very effective in dry and vacuum environments, but is degraded in moist atmospheres. By adding graphite to an MoS₂-based solid lubricant mixture or by encapsulating MoS₂ in a DLC matrix, lubrication lifetimes in high humidity are significantly enhanced. The solid lubricant surface adapts to the environment during rubbing such that MoS₂ coats the contacting surfaces in dry environments and graphite, or graphite-like carbon coats surfaces in moist air. The addition of Sb₂O₃ significantly enhances the performance of MoS₂ and MoS2/graphite compositions. Our studies suggest that Sb₂O₃ forms a sub layer that supports the solid lubricants and helps to prevent cracking and delamination. To understand the role of Sb₂O₃ in improving lubrication, wear tracks from burnished coatings and thin films deposited by PLD/PVD were studied using Raman, XPS, and cross-section TEM. To elucidate the importance of wear induced sub layers, artificial bilayers and multilayers were fabricated using PLD/PVD techniques. These structures were evaluated to determine the influence of the layer structure on tribological properties. The MoS₂/graphite/Sb₂O₃ compositions exhibited adaptive behavior in vacuum and moist air, which was related to the presence of Sb₂O₃ sub layers. Tribological behavior will be discussed in terms of surface chemistry and structure.
2:30 PM		E1-2-4 Tribological Characteristics of Bonded Film Lubricants of Organically Modified Hybrid Ceramic Binder Materials for High Temperature Applications H. Kong (Korea Institute of Science and Technology, South Korea); H.G. Han, E.S. Yoon (Korea Institute of Science and Technology) In order to enhance the thermal stability of binder materials of bonded type solid lubricants, several metal-alkoxide based sol-gel materials such as methyltrimethoxysilane (MTMOS), titaniumisopropoxide (Ti(Opri)4), zirconiumisopropoxide (Zr(Opri)4) and aluminumbutoxide (Al(Obut)4) were modified chemically by both epoxy and acrylic silane compounds. Friction and wear characteristics of the bonded typed solid lubricants, whose binders were of several hybrid ceramic materials, were tested with a reciprocating tribo-tester under a high temperature. Wear life was evaluated with respect to the heat-curing temperature, friction temperature, type of supplement lubricants, and ratio of binder materials. Test results showed that the Si-Zr hybrid ceramic materials modified by epoxy-silane compounds had a higher wear life compared to others. Sb₂O₃ was also found to be the most effective supplement lubricants in the high temperature, and XPS analyses revealed that it was caused mainly by a strong anti-oxidation effect to MoS₂ particles. Results also showed that the higher heat-curing temperature resulted in the higher wear life, and the higher friction temperature resulted in the lower wear life.
2:50 PM		E1-2-5 Tribological Characteristics of Coatings for High Temperature Applications B.S. Phillips, J.S. Zabinski (Air Force Research Laboratory) Lubricant compositions for operating over a broad temperature and humidity range were studied. The base coatings consist of a mixture of graphite, MoS₂, Sb₂O₃, and Cs powders. During terrestrial, or humid, conditions the graphite is the active lubricant and in dry conditions the MoS₂ acts as the active lubricant. Cs powders were used to increase the temperature operating range and Sb₂O₃ powder was used for its beneficial effects on MoS₂. The temperatures studied ranged from room to 600°C. The goals of this study are to determine a suitable high temperature additive as well as study the mechanisms controlling the tribological properties. The experiments were run using a pin-on-disk tribometer with the capability of achieving a maximum sample temperature of 700°C. The test specimens were 1 diameter Si₃N₄ disks sliding against Si₃N₄ balls. Chemical and surface analysis of the sliding surfaces was accomplished with x-ray photoelectron spectroscopy (XPS), raman spectroscopy, and scanning electron microscopy (SEM).
3:10 PM		E1-2-6 Solid Lubrication of Ti₆Al₄V Interfaces Subjected to Low and High Temperature Gross Slip Fretting C.H. Hager Jr. (Universal Technology Corporation); J.H. Sanders, J.S. Zabinski (Air Force Research Laboratory) Fretting is a destructive form of wear that occurs at component contacts and can accelerate crack initiation and interface degradation. Prevalent in Ti-alloy contacts, fretting wear often occurs in blade/disk interfaces of fan and compressor stages in turbine engines causing premature component failure. Currently surface coatings and solid lubricants are implemented to reduce interfacial damage. However, the extreme temperatures of turbine engines limit current solid lubricants. Typical lubricants used are MoS₂ and Graphite based. Current research is focused on the use of composite lubricants, or lubricants with additives that expand effective operational range of the particular solid lube. This presentation shows the conclusions from the testing/evaluation of solid lubricant mixtures of low and high temperature solid lubricants burnished onto the surface of Ti₆Al₄V and subjected to gross slip fretting wear from room temperature to 450°C (842°F).
3:30 PM		E1-2-7 Evaluating the Abrasion Resistance of Coatings with Abrasive Finishing Tape K.G. Budinski (Bud Labs) In 2002 the ASTM G2 Committee on Wear and Erosion approved an abrasion test for metals that used a specified amount of rubbing of a continuous fixed abrasive loop on a stationary test specimen. The test method (ASTM G 174) was restricted to metals because only metals were used in interlaboratory tests. This paper discusses the extension of this testing method to coatings. Test were conducted to reduce the load, sliding distance, and abrasive size to make the test less agressive. The 30 micrometer aluminum oxide used in the G 174 test method penetrates thin fim TiN and similar coatings in only several meters of abrasive contact. The goal of the project was to have the abrsion rate slow enough to allow the penetration through the coating to be tracked and stopped before penetration. This would allow calculation of the coating wear rate without complications from conjoint substrate wear. The project goal was achieved by a combination of reducing test load and abrasive size. A test method is proposed for addition to the G174 standard as an abrasion test for coatings.
3:50 PM		E1-2-8 Frictional Behavior of Rolled Surfaces Coated with Polymer Films D.N. Chvedov, R. Jones (Alcan Int., Canada) Rolled sheet, coated with a lacquer or polymer film, is often subjected to forming operations. In some cases, the polymer coating incorporates an internal lubricant, which migrates to the surface during curing. Alternatively, an external lubricant may be applied to the surface after the coating operation. These lubricants help to improve the runability of the coated sheet through forming operations such as stamping or deep drawing. In the present study, samples of rolled aluminum sheet were coated with polymer films of various thicknesses. The coating contained various concentrations of a wax lubricant additive. For certain sample panels the wax was also applied externally, on top of the coating. Two cooling regimes of coating application were studied. The cooling profile affects the mechanical properties of coating, along with the amount of wax migrating to the surface. Friction testing was performed by means of the "ball on flat" friction test apparatus. It was found that the coating thickness has the most prominent effect on the coefficient of friction. In the case of thin coatings, an increase in wax content on the surface gave rise to an increase in the coefficient of friction. At intermediate polymer coating thicknesses, the coefficient of friction was found to be substantially independent of the wax level. However, for thicker coatings, increasing the wax level on the surface caused a drop in the coefficient of friction. These observations can be explained in terms of the indentation of the coating film by the ball, which affects the area of contact during the friction test. A model is proposed which takes account of both the coating thickness, and wax lubricant level in a single analytical expression. This model is based on the concept of hydrodynamic lubrication. The modeling part of the present work was supported by the study of indentation of the coated sheet by steel ball.
4:10 PM		E1-2-9 Active Friction Modulation of Thermosensitive Polymer Films: Towards Surfaces with Switchable Friction States. S. Sundararajan, P. Hattan (Iowa State University) The ability to actively control friction properties of a surface is of great interest to both micro/nanotechnology and macroscale applications. We present our investigations on the effect of varying thermal conditions on the micro/nanotribological properties of selected polymer films using scanning probe microscopy (SPM). The grafted films studied are based on the poly-N-isopropylacrylamide (PNIPAAM) molecule, which is known to be heat sensitive and switch from hydrophilic to hydrophobic states at and above the transition temperature (30 to 35°C)¹. PNIPAAM based surfaces have been investigated for a variety of "smart surface" applications^2,³. However, very few studies have addressed their tribological behavior. In this study, micro/nanoscale friction and adhesive forces between the films and a hydrophobically functionalized Si₃N₄ tip is measured as a function of ambient temperature to identify switchable friction states of the films. The study will provide a better understanding of lubrication mechanisms at the molecular level and advance us towards realizing surfaces for active friction control. ¹ H.G. Schild, Poly(N-isopropylacrylamide) experiment, theory and application, Prog. Polym. Sci., vol 17 (1992) pp. 165. ₂ J. Hoffmann et al., Photopatterning of thermally sensitive hydrogels useful for microactuators, Sensors and Actuators A-Physical, vol 77, (1999) pp. 139-144. ₃ L. Liang et al., Thermosensitive poly(N-isopropylacrylamide)-clay nanocomposites with enhanced temperature response, Langmuir, vol. 16, (2000) pp. 9895-9899.
4:30 PM		E1-2-10 Decomposition of Perfluoropolyether (PFPE) Lubricant in Plasma Shamim Mirza, K. Nakayama (National Institute of Advanced Industrial Science and Technology, Japan) Perfluoropolyethers (PFPEs) are widely used as lubricants on magnetic media in hard disk driver (HDD), and play a critical role in protecting both the carbon overcoat and the underlying magnetic layer from damage caused by slider-disk contacts during normal operation. Because the lubricant layer on the disk surface is extremely thin (1-20 nm), lubricant stability is an important factor that directly influences the long-term reliability of HDD [1]. However, one of the major tribological problems at the head-disk interface (HDI) is the decomposition of PFPE lubricants, which leads to failure of HDI. The decomposition of PFPE has been explained with some mechanisms, e.g., thermal and catalytic reactions. However, these mechanisms do not explain fully the decomposition of PFPE. On the other hand, Nakayama et al. [2] discovered microplasma in the gap of sliding contact. This suggests that the high energetic microplasma causes the decomposition of the lubricants. So, the purpose of this work is to study the decomposition of perfluoropolyether (PFPE) lubricant in the plasma produced by the electrical-discharging. Reflection-absorption Fourier transform infrared (FTIR) technique and gas-chromatograph/mass-spectroscopy (GC/MS) have been used to analyze the decomposition products of PFPE lubricant produced by the plasma. The observed results showed that PFPE lubricant is decomposed in the plasma. This suggests that PFPE lubricants are decomposed by the tribo-microplasma, which generated at head-disk sliding contact. [1] C. M. Mate, Tribol. Lett., 4, 119 (1998). [2] K. Nakayama, and R. A. Nevshupa, J. Phys. D: Appl. Phys., 35, L53, (2002). .
4:50 PM		E1-2-11 Tribological Behavior of PTFE Film with Nanodiamonds D.S. Lim, J.Y. Lee (Korea University, South Korea) The effect of nano diamond addition on tribological behavior of polytetrafluroethylene (PTFE) film was investigated. Nano diamonds produced by detonation method were added up to 2 w/o to PTFE film to improve the wear resistance and thermal stability. Tribological tests of nano diamond reinforced PTFE coatings have been performed by using ball on plate reciprocating wear tester at room and 150°C . The wear resistance of PTFE composite coating increased with increasing content of nano diamond up to 1 w/o but decreased with further addition of nano diamond of all tested temperature ranges. Coefficient of friction and width of wear track changed from 0.073 to 0.064 and 0.84 mm to 0.42 mm respectively when 1 w/o of nano diamond was added to PTFE film compared to PTFE coating without nano diamonds. Possible mechanisms for enhancement of tribological properties by nano diamond addition were discussed based on SEM and TEM observation and DSC analysis.