ICMCTF2002 Session E1-2: Reduction of Friction Through Coating/Surface Modification

Tuesday, April 23, 2002 1:30 PM in Room California

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		E1-2-1 Different Tribological Behavior of MoS₂ Coatings Under Fretting and Pin-on-Disk Conditions X. Zhu (Xian Jiaotong University, China); W. Lauwerens, P. Cosemans, M. Van Stappen (WTCM Surface Treatment, Belgium); J.P. Celis (Catholic University of Louvain, Belgium); L.M. Stals (Limburgs Universitair Centrum, Belgium); J. He (Xian Jiaotong University, China) The performance of tribological coatings is often evaluated by fretting or pin-on-disk measurements. However, only a few papers concerned the application of the two methods on the same coating. In this paper, a comparison of the tribological behavior of sputtered MoS₂ coatings in fretting and pin-on-disk tests under different conditions is studied. The relationship between the mechanical properties of the coatings and their wear performance in both test methods is discussed. MoS₂ coatings were deposited by magnetron sputtering from a MoS₂ target. The coating thickness and adhesion were evaluated by respectively the ball crater and scratch test. The hardness as well as the toughness were obtained by a microhardness test with a Vickers indentor. After the tribological tests the surface morphology of the wear scar in the coating was observed by SEM. Also transfer films on the counterbody were investigated. It appeared that the fretting and pin-on-disk tests can give considerably different results. Coatings that show excellent performance in fretting tests have shown a low endurance in pin-on-disk tests. Sliding speed appears to be a crucial factor. Also the material of the counterbody, steel or corundum, influences the result in both tests in a different way. The different behavior of a coating in both tests is related to the toughness of the coating and to the way that the debris is involved in the friction and wear mechanisms.
1:50 PM		E1-2-2 Fretting Wear Behavior of a Cu-Ni-In Plasma Coating V. Fridrici, S. Fouvry, Ph. Kapsa (LTDS - CNRS / ECL, France) In some aeronautical applications, two parts made of a titanium alloy are to be in contact and a relative motion between them may occur. But, the bad tribological behavior of Ti-6Al-4V rubbing against itself is well known (high friction coefficient, high wear rate). In order to improve this behavior, surface coatings are developed and used. In this paper, we report on the fretting-wear behavior of a Cu-Ni-In plasma spray coating, used as a retainer for a solid lubricant film (molybdenum disulphide for instance), deposited on the root of the fan blades. This study is representative of the case when the solid lubricant is worn off out of the contact. Cu-Ni-In coating was plasma sprayed (about 150 microns - 200 microns in thickness) on a shot-peened Ti-6Al-4V cylinder (simulating a fan blade) and this coated cylinder was tested against a flat, shot-peened Ti-6Al-4V surface (simulating the fan disk of the turbine engine), under fretting wear solicitation (small amplitude reciprocating motion). The Cu-Ni-In coating is rough and softer than the shot-peened Ti-6Al-4V surface. This property leads to an increase of the contact area and a decrease of the normal contact stress. However, the Cu-Ni-In coating does not modify the transition displacement amplitude between partial slip and gross slip conditions. The effects of normal force and displacement amplitude (from +/- 10 microns to +/- 50 microns) on the fretting wear behavior of the Cu-Ni-In coating were investigated. A dissipated energy method was applied to describe the wear properties of the coating. Moreover, the influence of the Cu-Ni-In coating on the cracking of the flat Ti-6Al-4V counterbody has been studied : under the same experimental conditions, the coating reduces the length of the cracks in the Ti-6Al-4V specimen.
2:10 PM	Invited	E1-2-3 The Role of Transfer Films in the Wear of Poly(methylmethacrylate) A. Chateauminois (Ecole Superieure de Physique et Chimie Industrielles, France) Investigations of interface tribology have widely demonstrated that wear processes are strongly dependent on the formation and the behavior of transfer films which are formed by aggregation and compaction of wear debris within sliding contacts. In this study, the complex interrelationships between the frictional processes, the formation of transfer films and the ultimate wear degradation have been investigated within dry contacts between poly(methylmethacrylate) and a steel counterface. Low amplitude sliding motions, referred to as fretting, have been selected for their aptitude to trap the wear debris within coherent transfer films. The wear processes within these fretting contacts were analyzed using complex contact zone kinematics conditions, which combined various extents of, and combinations of , linear and sliding motions. It was demonstrated that the wear degradation of the PMMA substrate was critically dependent upon the formation, under specific contact loading conditions, of a highly coherent polymer transfer film. The efficiency of the wear particles compaction processes was emphasized by nano-indentation experiments, which showed that the mechanical properties (modulus, hardness) of the transfer film were close to those of the virgin PMMA substrate. These results were rationalized by considering the interrelationships between the sites of energy dissipation within the contact and the changes in the contact load carrying capacity which are associated to the mechanical response of the transfer film during the shearing process. The problem of the localization of a significant part of the frictional energy dissipation within the restricted volume of the transfer film is especially addressed.
2:50 PM		E1-2-5 Friction-induced Structural Transformations of DLC Coatings under Different Atmospheres J.C. Sánchez-López (Instituto de Ciencia de Materiales de Sevilla, Spain); A. Erdemir (Argonne National Laboratory); C. Donnet (University J. Monnet., France); TC Rojas (Instituto de Ciencia de Materiales de Sevilla, Spain) The structural transformations occurred during friction under different chemical environments of diamond-like carbon (DLC) coatings with growing hydrogen contents have been investigated by Raman and electron energy-loss spectroscopies. Tests were performed using uncoated steel balls at contact stresses of 1 GPa in ambient air (RH= 30-40%), dry air (RH<1%) and dry nitrogen (<1%). The lowest friction coefficient (f<0.02) is obtained for the most hydrogenated sample in dry nitrogen where the formation of a third-body layer is observed onto the steel surface. The Raman spectra obtained from the counterfaces after sliding revealed a remarkable increase and narrowing of the "D" and "G" peaks when going from humid to dry atmospheres. The analysis of peak positions and I_D/I_G ratio pointed out to an increasing order and an enlargement of the sp² clusters under friction. The shape and position of the C K-edge spectra for the transfer layer are also affected in the same way although evidences of extended graphite layer formation were not found. The developments of these different trends are correlated with the hydrogen-to-carbon ratio of the gas precursor used during the synthesis and the type of surrounding atmosphere.
3:10 PM		E1-2-6 Friction Mechanisms of Amorphous Carbon Nitride Films under Variable Environments: a Triboscopic Study J.C. Sánchez-López (Instituto de Ciencia de Materiales de Sevilla, Spain); M. Belin (Ecole Centrale de Lyon, France); C. Donnet (University J. Monnet., France); C. Quirós, E. Elizalde (Universidad Autónoma de Madrid, Spain) In this work, a carbon nitride film (CN_x) prepared by ion-beam assisted deposition onto silicon was investigated using reciprocating sliding tests under different surrounding atmospheres (ambient air, dry air, dry nitrogen and UHV) and testing conditions (contact pressure, nature of the counterface). The CN_x coating provided a "superlow" friction coefficient of 0.007-0.008 in dry nitrogen atmosphere independently of the mating material (steel or sapphire balls). The build-up of a smooth transfer film onto the ball surface was detected by optical microscopy in this situation. The evolution of the friction coefficient was spatially recorded along the track as sliding progressed. This technique, called "triboscopy", combined with variable amplitude tests (VAT) performed on the same wear track, monitored the changes of time-dependent local phenomena related to the contact area. It revealed that the nature of both counterfaces (ball and film) has to be modified during a short running-in period before the achievement of the superlow friction regime. Nitrogen molecules play a specific role in the lubrication mechanism of the interface maintaining a high passive nature of the sliding counterfaces. When other surrounding atmospheres are used, the superlow friction coefficient is not attained even if the counterfaces were previously rubbed in dry nitrogen. The assembly of these tools allowed to obtain a better understanding of the tribological behavior in terms of friction coefficient, material transfer and chemistry of the sliding interface. The conclusions obtained about the processes appearing in the interface are discussed and compared to the current interpretation for the lubricity behavior of diamond-like and MoS₂ coatings.
3:30 PM		E1-2-7 Quantifying Transfer Film Thickness by In Situ Raman Tribometry T.W. Scharf, I.L. Singer (US Naval Research Laboratory) Many authors have demonstrated that the friction and wear behavior of DLC coatings are controlled by a transfer film that forms and accumulates in the sliding contact. We have recently shown that this transfer film also controls the friction and wear of a type of DLC coating known as DLN, diamond-like nanocomposite.¹ DLN, like DLC, is an amorphous carbon-based coating, but it also contains silicon and oxygen. In situ optical microscopy and Raman spectroscopy have been used together to monitor the formation, thickening, thinning, and loss of transfer films at the interface during reciprocating sliding of transparent, sapphire hemispheres against DLN coatings. ² As the transfer film thickened, Raman spectra in the C-C band region (1300 to 1600 cm^-1) evolved from that of the coating to the transfer film. A shift in the G-band peak, a feature of the spectrum, was shown to correlate with the relative amount of transfer film. Here we use in situ Raman spectroscopy to quantify the thickness of transfer films based on mean free path calculations of the weighted sum of the coating plus transfer film spectra. The thicknesses of the transfer films at various stages of sliding were correlated to the friction behavior. ¹T.W. Scharf and I.L. Singer, Role of Third Bodies in Friction and Wear of Diamond-like Nanocomposite Coatings Studied by In Situ Tribometry, submitted Trib. Trans. ²T.W. Scharf and I.L. Singer, Monitoring Transfer Films and Friction Instabilities with In Situ Raman Tribometry, submitted Trib. Lett.
3:50 PM		E1-2-8 A Comparison Study of Alkyl- and Aryl-phosphate Vapor Phase Lubricants D. Sung, A.J. Gellman (Carnegie Mellon University) Conventional fluid lubrication is not feasible for use with high efficiency engines operating at extremely high temperatures. Vapor phase lubrication has been proposed and tested as an alternative method for the lubrication of mechanical engine components operating at temperatures in excess of 550 °C. In vapor phase lubrication, lubricants are continuously delivered in the vapor phase and react on the hot surfaces of working parts to form a thin solid lubricating film as the existing film is being worn away. The phosphorous containing organics have been most widely tested as vapor phase lubricants and shown the excellent lubricating ability. Especially the arylphosphates are known to have better performance than alkylphosphates. This work attempted to gain insight into the origin of the differences in the performance of alkyl and arylphosphates as vapor phase lubricants using temperature programmed desorption spectrometry and Auger electron spectroscopy. The alkylphosphates decompose via alkyl intermediates that readily undergo β-hydride elimination and desorb into the gas phase as olefins, thus removing carbon from the surface. In contrast the arylphosphates generate aryloxy intermediates by the P-O bond scission and aryl intermediates by further C-O bond scission. Both of these intermediates cannot undergo β-hydride elimination and thus decompose to deposit carbon onto the Fe surface. The higher efficiency for carbon deposition may be the primary reason for the superior performance of the arylphosphates as vapor phase lubricants.
4:10 PM		E1-2-9 Influence of EP and AW Additives on the Tribological Behaviour of Hard Low Friction Coatings B. Podgornik, S. Jacobson, S. Hogmark (Uppsala University, Sweden) Hard coatings, especially hard low-friction coatings are becoming more and more important also in the field of machine components. By both improving the wear resistance and giving excellent frictional properties, hard low-friction coatings certainly provide a great opportunity to improve durability and to reduce frictional losses of machine components. However, the majority of machine components is operating under lubricated conditions, where interactions between the surface and the lubricant play a significant role. Especially under boundary lubrication conditions these interactions will determine the tribological behaviour of the contact. A main limiting factor to extend the use of hard coatings in machine component applications is the lack of knowledge about how these inert coatings perform under lubricated conditions, using today's oils, originally designed for the steel/steel contact situation. The aim of the present work was to investigate the influence of conventional extreme-pressure (EP) and anti-wear (AW) additives on the tribological performance of hard low-friction coatings in the boundary lubrication regime. Tests were performed in a load-scanning test rig, where WC-C coated steel bars were loaded against coated and uncoated bars, while the steel/steel combination was used as a reference. This test configuration involves two crossed bars that are forced to slide reciprocally against each other under a constant speed, allowing the normal load to gradually increase during the forward stroke and to correspondingly decrease during reverse stroke. Load-scanning tests were performed under boundary lubrication conditions using pure poly-alpha-olefin oil (PAO), PAO mixed with commercial sulphur-based EP additive or ZDDP-based AW additive, and fully formulated gearbox oil. The coefficient of friction was monitored as a function of time and normal load. After the test, the worn surfaces were analysed by means of OM, SEM and EDS. This investigation shows that also under these boundary lubrication conditions introduction of hard low-friction coatings significantly improves the tribological performance. Furthermore, the coatings were found to have a great influence on the running-in process.
4:30 PM		E1-2-10 Wear Resistance of a High Nitrogen Austenitic Stainless Steel Coated with Nitrogenated Amorphous Carbon Films A. Di Schino, L. Valentini, J.M. Kenny (University of Perugia, Italy); Y. Gerbig, H. Haefke (CSEM, Switzerland) Nitrogen alloyed austenitic stainless steels exhibit attractive properties as high strength and ductility, good corrosion resistance and reduced tendency to grain boundary sensitation. The high austenitic potential of nitrogen allows reducing the nickel content in steel offering additional advantages such as cost reduction, increase of N solubility in the melt and less tendency to Cr₂N formation. Due to the above mentioned properties, in the last years many efforts have been developed in producing high nitrogen austenitic stainless steels, but to our knowledge, no results have been reported on the wear resistance of this class of steels, and on the effect on the wear resistance of nitrogenated carbon films coatings. In this paper, the wear resistance of such materials is, for the first time, investigated and compared to that measured for the same samples coated with nitrogenated amorphous carbon (a-C:H(N)) films deposited by means of plasma enhanced chemical vapour deposition. To this aim, pin on disk (PoD) tests were performed in order to investigate wear stability of coated and uncoated substrates. Results are compared to those of analogue measurements on a standard AISI 304 stainless steel.
4:50 PM		E1-2-11 Tribomechanical Properties of Self Lubricated Oxide Ceramic Hard Coatings M.V. Kireitseu, M.A. Belorserkovski (Institute of Machine Reliability, Belarus) A lot of researches indicate a great importance of thermal sprayed coatings for wear resistance. However some valuable properties such as self-lubrication ability of thermal sprayed coatings become hesitated some times. In the paper self-sealing effect of alumina layer formed by micro arc oxidizing of thermally sprayed aluminum layer has been investigated under preliminary sealing by PTFE suspension, oil, MoS₂, water and dry contact. The tests revealed an effect of sealant type used, applied load and structure on wear rate and coefficient of friction of the coatings. The data on wear rate revealed that wear rate of PTFE sealed alumina coatings reaches the lowest wear rate 0.04-0.10 m/km (applied load 10 MPa, sliding speed 2 m/s). The PTFE sealant appears on the coating surface from pores, however, under vacuum in the microscope camera the oil appears on the coating surface from pores. Optical microscopy of the alumina surface showed that PTFE particles penetrate into the pores and voids of surface layer. The rest of the PTFE particles are distributed in the lubricant film between the friction contacted surfaces. Coefficient of friction investigated at pin-on-disk scheme. Alumina-based coatings show 0.02-0.04 coefficient of friction under the 0.5-17.5 MPa load. Porosity of alumina layer increases coefficient of friction at start up. It is expected that the coatings is to be used in hard-loaded sliding bearings and pairs as well as in repairing of shafts and worn out surfaces in automobile and aircraft industry.