Papers

ICMCTF2000 Session E1: Reduction of Friction Through Coating/Surface Modification

Tuesday, April 11, 2000 8:30 AM in Room California

Tuesday Morning

Start	Invited?	Item
8:30 AM		E1-1 Mechanisms of Vapor Phase Lubrication with Organo-Phosphate Lubricants N.H. Forster (Air Force Research Laboratory); C.S. Saba (University of Dayton Research Institute) This presentation summarizes what is known about the mechanisms of vapor lubrication with organo-phosphate lubricants. The lubricant concentration to provide good results in air environment over extremely broad temperature ranges will be cited. The presentation also covers what is known about the structure of the deposition film, chemical reactions, reaction rates, and lubrication mechanisms. As a summary of findings, it is known that lubricant decomposes at the vapor phase boundary to form phenoxy groups, phenol-alkyl groups, phosphate groups, and fused aromatic rings such as anthracene. These groups react with ferrous substrates to form relatively thick lubricating films (i.e., 200 nm) under highly loaded dynamic conditions. The deposition film is extremely tenacious and produces very low coefficients of friction, µ = 0.07, at sliding speeds of 10 m/s, temperatures of 500°C, and contact stresses of 1.5 GPa. Post-test analyses, of the deposition film grown at 450°C, indicates the film is composed of iron poly-phosphate, D and G peeks indicative of a short-range graphite, and chemical gradients of iron, carbon, phosphorus, and oxygen. Controlled experiments on static iron foils support a previous hypothesis that mass diffusion of iron from the substrate to the vapor boundary is the rate-limiting step. The low friction with this form of lubrication is largely attributed to presence of graphite and organic species at the vapor boundary. However, the most important aspect of the lubrication mechanisms is thought to be the relatively thick lubricating film and active replenishment under dynamic conditions. This is largely attributed to polymerization of the phosphate group that serves as binder between the lubricating species and the ferrous substrate. Additionally, poly-phosphate probably assists in transport of iron through vacancies in the phosphate structure and hence, promotes chemical activity.
9:10 AM		E1-3 Low Friction and Wear Thin Films from Inortanic Fullerene-like (IF) MoS₂ Nanoparticles M. Chhowalla, G.A.J. Amaratunga (University of Cambridge, United Kingdom) Layered materials such as graphite, molybdenum and tungsten disulphide (MoS₂and WS₂) are commonly used as solid state lubricants in space technology, ultra high vacuum conditions and recently in tool cutting applications. Their lamellar lattice allows for easy shearing in the hexagonal plane, giving them a low coefficient of friction. However, MoS₂ and WS₂ do not perform well in humid environments due to the formation of metal oxide and hydrogen sulphide at the contact area between mating metal surfaces. The reaction primarily occurs at the unterminated bonds of the hexagonal planes, deteriorating the layered structure and inducing high frictional wear. Recently, it was demonstrated that inorganic fullerene-like (IF) WS₂ nanoparticles, similar to carbon bucky ‘onions‘ and nanotubes, are effective solid lubricants. The superior properties of cage structured IF-WS₂ nanoparticles is attributed to their chemical inertness and elasticity. Here we show that low friction and wear thin films of MoS₂ can be created by depositing closed, hollow nanoparticles in the form of a thin film onto a substrate. The films show a very low friction coefficient in dry nitrogen atmosphere. However, the truly novel result is that the nanoparticle MoS₂ films also show low friction and wear even in humidity. The low friction is attributed to the presence of curved S-Mo-S planes that prevent oxidation and preserve the layered structure.
9:30 AM		E1-4 MoS₂ -Metal Mixtures: Wear Mechanisms and Film Structure A. Savan, E. Pflüger, E. Jeanpetit (CSEM Surface Engineering, Switzerland); P. Crausaz (University of Applied Sciences, Switzerland); M. Simmonds (Sheffield Hallam University, Materials Research Institute, United Kingdom) Many investigations ¹, ², ³ have noted the increased sliding lifetimes and decreased friction coefficients obtainable for sliding contacts in humid air environments when using mixtures of MoS₂ and various metals or compounds. It has been observed that this pin-on-disk tribometer result can be achieved by at least 2 general routes. The first is an MoS₂ -metal film that very rapidly wears a flat on the static steel ball (pin) and itself shows a correspondingly wide and deep wear track. The second is an MoS₂ -metal film that has a very narrow, almost invisible wear track and little or no apparent wear of the ball, nearly until end of life of the film. Metal additives such as Ti, Ni or Al give the first type of tribological behavior, while additives such as Au or WSe₂ give the second type. The development of the disk wear track and the corresponding ball and transfer film are followed, with SEM, EDX and RBS analyses of these zones to detect and correlate chemical and structural evolution of the films, and thereby attempt to account for the different wear mechanisms. ¹ ¹ D.N. Dunn, K.J. Wahl and I.L. singer, MRS Spring Meeting, Symposium T, 13-17 April 1998, San Francisco, CA. ² J.S. Zabinski, M.S. Donley, S.D. Walck, T.R. Schneider and N.T. Mcdevitt, Tribology Transactions vol. 38 (1995), 4, 894-904. ³ M.R. Hilton, r. Bauer, S. Didziulis, M.T. Dugger, J.M. Keem and J. Scholhamer, Surf. Coat. Technol., 53 (1992) 13-23.
9:50 AM		E1-5 Correlation of Plasma Conditions to Structural Properties for a MoS₂/TiN Solid Lubricated Coating System Deposited by Unbalanced Magnetron sputtering A. Woolverton (Advanced Coatings and Surface Engineering); J.J. Moore (Colorado School of Mines); W. Weaver (British Nuclear Fuels, United Kingdom) Recent interest in novel coatings for wear applications has focused on PVD deposited solid lubricated coatings. A common coating system used for this purpose consists of MoS₂ doped with a metal such as Ti. Coatings have been deposited for this study by unbalanced magnetron co-sputtering of MoS₂ and Ti targets in Ar and Ar/N₂ plasmas. This study correlates plasma conditions to compositional and structural properties of coatings deposited under the same conditions. A Hiden EQP quadrupole mass spectrometer has been used to analyze plasma species and energies. The following process parameters have been varied: target power, bias, and pressures of Ar and N₂ gases. Coatings have been characterized in terms of composition, structure and friction behavior. Finally, the coatings are compared to commercially produced solid lubricants of similar composition.
10:30 AM		E1-7 Tribological Properties under Vacuum of Metal Doped MoS₂ Magnetron Sputtered Films A. Garcia, M. Brizuela, I. Braceras, J.L. Viviente, J.I. Onate (INASMET, Spain) MoS₂ is the most widely used lamellar compound solid lubricant film for space applications. This coating is highly suitable for application on precision mechanisms due to their uniform coverage, excellent adhesion and low friction properties. However, there is an interest in further improving MoS₂ films by enhancing their endurance life and resistance under moist conditions. This would alleviate storage conditions and extend their ground test capability and the spin off in a range of ground industrial applications, such as in wear components or tools for special machining operations. This work reports on the first series of laboratory experiments conducted with a new generation of solid lubricant films that after ground qualification will eventually be tested in the TriboLAB instrument onto the EuTEF at the International Space Station (ISS). MoS₂ films have been deposited on AISI 440C steel in a Magnetron Sputtering PVD equipment. These films were all metal doped with Ti as a getter material and inner layers of TiN and Ti were deposited on some films. Dynamic micro-indentation measurements showed that the hardness under load of these films (at 10 mN max.) ranged from 3800 to about 10000 N/mm² with a elastic recovery near 30%. Ground tribological tests were performed initially at atmospheric pressure at controlled varying relative humidity raging from 50 to 70 % RH and subsequently under vacuum in a ball on disc tribometer. It was found that the steady state friction coefficient was about 0.1 when tested under air with relative humidity up to about 60 % RH. When tested under vacuum, most of the films produced a steady state friction coefficient of about 0.01 with average lives of 25000 sliding metres, reaching in some cases an endurance as high as 44000 metres before failure. As it occurs with other MoS₂ films, there is transference of material to the counterface ball during wear testing, therefore providing a low friction behaviour during the tribological tests.
10:50 AM		E1-8 Properties of MoS₂/Metalllic, Non Metallic Composite Coatings Deposited by Closed Field Unbalanced Magnetron Sputter Ion Plating N.M. Renevier (The University of Birmingham, United Kingdom); D.G. Teer (Teer Coatings Ltd, United Kingdom) As previously reported [¹, ²] the properties of MoS₂ coatings can be improved by the co-deposition of a small amount of titanium. These MoS₂ /Ti coatings known as MoST^TM, were harder, much more wear resistant and also less sensitive to atmosphere water vapour than pure MoS₂ during tribological testing. These coatings have given excellent industrial results for a wide range of cutting and forming applications. Excellent results in terms of tribological properties have been reported for other metal such as Molybdenum, Chromium, Zirconium and tungsten [³, ⁴]. The influence of non metallic components such as Carbon and nitrogen has been studied using reciprocating wear tests and pin on disc tests (friction and wear) and hardness will be reported. Structural analysis using different techniques such as reflection X-ray, optical microscopy, scanning electron microscopy and transmission electron microscopy will be reported. ¹ - V.C. Fox, N.M. Renevier, D.G. Teer and J. Hampshire, V. Rigato, Proceedings of the PSE Conference in Garmisch Partenkirchen, 14-18 September 1998, Germany, in press in Surf. Coat. Technol.. ² - N.M. Renevier, V.C. Fox, D.G. Teer and J. Hampshire, ICMCTF Conference in San Diego, April 1999, USA. in press in Surf. Coat. Technol.. ³ - N.M. Renevier, V.C. Fox, D.G. Teer and J. Hampshire, ICMCTF Conference in San Diego, April 1999, USA. To be published, ⁴ - V. Rigato, G. Maggioni, A. Patelli, D. Boscarino, N.M. Renevier, D.G. Teer, PSE ASIA Conference in Begin, September 1999, China, to be published
11:10 AM		E1-9 Properties of Rubbed and Unworn Bulk MoS₂ Material, MoS₂ and MoS₂ /Titanium Composite Coatings Deposited by Closed Field Unbalanced Magnetron Sputter Ion Plating N.M. Renevier, D.G. Teer (Company Teer Coatings Ltd, United Kingdom) As previously reported [¹, ²] the properties of MoS₂ coatings can be improved by the co-deposition of a small amount of titanium. These MoS₂ /Ti coatings known as MoST^TM, were harder, much more wear resistant and also less sensitive to atmosphere water vapour during tribological testing. These coatings have given excellent industrial results for a wide range of cutting and forming applications. These as deposited coatings are almost amorphous or very little crystalline. Samples have been rubbed in reciprocating wear tests and pin on disc tests their friction and wear rate measurements have been reported. Scratch test and hardness tests have been performed in the rubbed and unworn area. The counterparts as well as the coated samples were studied. Structural analysis has also been carried out using different technique such as reflection X-ray RHEED, optical microscopy, scanning electron microscopy and transmission electron microscopy on unworn and rubbed samples. A preferred orientation was observed dependant of the number of cycles and on the load. ¹ ¹ - V.C. Fox, N.M. Renevier, D.G. Teer and J. Hampshire, V. Rigato, Proceedings of the PSE Conference in Garmisch Partenkirchen, 14-18 September 1998, Germany, in press in Surf. Coat. Technol.. ² - N.M. Renevier, V.C. Fox, D.G. Teer and J. Hampshire, ICMCTF Conference in San Diego, April 1999, USA. in press in Surf. Coat. Technol..
11:30 AM		E1-10 Enhancement of Abrasive Ability of Diamond Film Coated with Solid Lubricant Film J-J. Rha, K-H. Lee, W-S. Baek (Korea Institute of Machinery & Materials, Korea); Y-J. Baik (Korea Institute of Science & Technology, Korea); S.-R. Lee (Korea Institute of Machinery & Materials, Korea) After diamond film was synthesized on (100) Si wafer by hot filament method, MoS_{sub> 2thin layer as solid lubricant was coated on diamond film through sputtering. XRD analysis was conducted for phase identification and microstructures were observed by SEM. Diamond film had facet grains, therefore very rough surface. The thickness of solid lubricant film was so thin, the roughness of diamond film was not changed. Tribo-test was conducted in diamond film and diamond film coated with MoSsub 2. Typical tribo-test condition was that load was 50g, rotation speed was 1.6 m/s and counter material was used Ruby ball 3mm in diameter. Friction coefficients were monitored in tribo-testing and wear scars of Ruby balls were measured in order to analyze abrasive ability of diamond film. The values of friction coefficient of all samples were reached initially as high as the range of 0.6 - 0.8, decreased gradually as increasing sliding distance and finally saturated at 0.1. Decreasing of the friction coefficient was due to graphitization of diamond film that was in contact with Ruby ball. The difference of two kind of specimen was that the saturation time of lubricated diamond film was longer than non-lubricated diamond film. Wear scar of Ruby ball slid on lubricated diamond film was larger than non-lubricated diamond film. These were the reason that solid lubricant film delayed the graphitization of diamond film and forced edge of diamond to penetrate deeply into Ruby ball.}
11:50 AM		E1-11 No Title Listed A.V. Pokhmurska, O.V. Kaminski (Institute for Applied Problems of Mechanics and Mathematics of National Academy of Sciences of Ukraine) Coatings have been obtained by electric arc spraying and plasma spraying of powder wires technologies. Different schemes of laser treatment were used to create the hardened zones on the sample surface. Laser treatment has been carried out in such a regime in which the coating and a thin layer of a base are being alloyed. The strength of coatings increases after laser treatment. porosity after laser treatment decreases from 10 to 0 %, making it possible to apply such coatings as corrosion resistant ones. Corrosion resistance evaluation of samples with coatings carried out by analysis of polarization curves demonstrates the corrosion current decrease Load-bearing capacity, friction coefficient and wear resistance have been investigated. The wear characteristics of both electric arc sprayed and plasma sprayed coatings are improved too. The largest improvement was observed in the case of the material with high hardness has been used as the counter-specimen.