ICMCTF2001 Session E1-2: Reduction of Friction Through Coating/Surface Modification

Wednesday, May 2, 2001 1:30 PM in Room California

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		E1-2-1 The Influence of Temperature and Humidity on Wear of MoS₂ and MoS_T Films R Aharonov, N LoBiondo (IonBond Inc.) MoS₂ and MoS_T (MoS₂ with addition of Ti) were deposited by Unbalanced Magnetron Sputtering. The films were deposited with and without a cold trap in the vacuum system in an effort to assess the influence of residual water vapor on film's tribological properties. The coatings were evaluated in high temperature tribometer at temperatures up to 500°C and relative humidity up to 90 % and also under water. Friction coefficient, wear volume, wear rate, and durability were measured. In every case the MoS_T coating was dramatically less sensitive to moisture then the conventional MoS₂ coating. Structural features of the MoS_T coating coupled with the deposition technique is responsible for the observed behavior.
1:50 PM		E1-2-2 Microstructure, Chemistry and Tribology of MoS_x/Au and MoS_x/WSe_y Co-Sputtered Composites M.C. Simmonds (Sheffield Hallam University, United Kingdom); A. Savan (CSEM, Swiss Center for Electronic and Microtechnology, Switzerland); C.P. Constable (Sheffield Hallam University, United Kingdom); H. Haefke (CSEM, Swiss Center for Electronic and Microtechnology, Switzerland) Co-sputtered film of MoS_x with WSe_y and MoS_x with Au were deposited in order to investigate the relationships between tribological behaviour and film microstructure and chemistry. The mechanical properties of the films were investigated by pin-on-disk wear testing in 50% RH air as a function of dopant concentration. X-ray diffraction indicates that the microstructure is strongly affected by the inclusion of WSe_y or Au and that there is a strong correlation between between microstructure and tribological performance with an increase in the basal orientation corresponding to improved film lifetime. X-ray diffraction and Raman measurements indicate a large increase in the c-axis of the MoS_x/WSe_y films; although XPS measurements show little evidence to suggest that W and/or Se may be substituting into the MoS₂ structure. Film structure and wear tracks have been investigated using a combination of HRSEM, SEM-EDX mapping and imaging Auger. Results reveal differences in structure on the nanometer scale and differing wear mecahnisms, both of which may contribute to the observed tribological behaviour.
2:10 PM		E1-2-3 Chemical Effects in the Tribology of Sputter-Deposited Metal/MoS₂ Films J.R. Lince (The Aerospace Corporation) There has been increasing interest in sputter-deposited and IBAD MoS₂ films, with the observation that oxidation-enhanced failure of the films in air can be reduced by cosputtering with metallic elements. This has expanded the utility of such films beyond solely vacuum use. Studies indicate that many different metallic elements show some benefit, presumably due to the formation of a nanocomposite structure with increased density, fracture toughness, and a more amorphous crystal structure. The influence of the metals on the tribochemistry of the films has not been well-studied. We have explored the chemistry of cosputtered metal/MoS₂ films, both the composition of the as-grown films, and the changes in their composition on wear in vacuum and various atmospheres. The results for MoS₂ cosputtered with several metals, including Ti, Ni, and Au, are correlated with previous photoelectron spectroscopy (PES) studies of the interfacial chemistry of metals deposited on single crystal MoS₂ surfaces. For example, PES shows that Ti reacts strongly with MoS₂ to produce TiS₂,¹ which we also see in cosputtered Ti/MoS₂ films. The effect of pin-on-disk testing of the films in vacuum is studied using XPS within the wear track on the disk without exposure to atmosphere between testing and analysis. Wear and transfer film formation will also be presented. The results are compared to similar studies in inert gas and air. 1. J. R. Lince, D. J. Carré, and P. D. Fleischauer, "Schottky Barrier Formation on a Covalent Semiconductor without Fermi Level Pinning: The Metal/MoS₂(0001) Interface" Phys. Rev. B 36 (1987) 1647.
2:30 PM		E1-2-4 Elimination of Lubricants in Industries in Using Self-Lubricating Wear Resistant Coatings Based on MoS₂ N.M. Renevier, D.G. Teer (Teer Coatings Ltd., United Kingdom) The problem that has been haunting for long manufacturing industries (automotive, aerospace, domestic appliance and electronics) is the lubrication. Most processes are impossible without lubrication, otherwise wear and galling will occur. On the other hand, many lubricants are toxic and some lubricants are even flammable, and therefore the use of lubricants is a big environment, safety and health concern. This paper demonstrates how innovative self-lubricated coating such as MoS₂/Titanium composite coatings can be used to solve these problems and are suitable for the specific requirements with characteristics such as low frictional coefficient, high wear resistance and low cost. For example, demand coatings such as MoS₂/Titanium composite coatings is increasing, therefore forming and cutting process can be performed “dry” without use additional lubricants, that is, the oil based lubricants can be eliminated from processes. The benefits of MoS₂/Titanium composite coatings are both ecological and economical. The ecological side is obvious. The economical benefits can be obtained from the elimination of the direct costs of lubricants and the application apparatus, from longer tool life and less tool maintenance, from improved quality of machined parts as well as from the indirect costs of parts cleaning and waste disposal. Industrial results from forming and cutting processes will be presented and understanding analysis will be performed. Parameters such as life-time, cutting force, surface finishing… will be correlated to laboratory friction and wear tests as well as microstructure observations.
2:50 PM		E1-2-5 Role of Transfer Film on Wear Life of Ti-doped MoS₂ Coatings in Moist Air R.N. Bolster (Naval Research Laboratory); I.L. Singer (Naval Research Laboratory, United States) Ball-on-disk tests were performed on two Ti-doped and an undoped MoS₂ coating in sliding contact against hard steel balls in dry and moist (40% and 90% RH) air. Tests were run either to 20 kcycles or until the friction coefficient rose above 0.2 (life tests). As reported by others, the wear life of Ti-doped coatings was 10 to 100 times longer than undoped coatings in moist air. Wear tracks and ball scars were investigated after tests run 20 kcycles. Optical interferometry was used to measure the depth of wear tracks and estimate the thickness of transfer films on the balls. Transfer films of MoS₂ were seen on the contact area of all the balls that survived the 20 kcycle tests. Depth-sensing Berkovich indentation tests were performed on the coatings and on transfer films; for the latter, indents were placed about 40 micrometers apart along the contact area, overlapping both transfer film and film-free areas. Ti-doped coatings were harder (9-11 GPa) than undoped (5 GPa) coatings. Hardness values for (sufficiently thick) transfer films were about the same for all three coatings, ranging from 1 to 4 GPa. We will discuss the role of transfer films in the sliding behavior of MoS₂ and suggest that the enhanced wear life of the doped coatings can be attributed to the lower relative hardness of the transfer film relative to that of the coating.
3:10 PM		E1-2-6 Fretting Wear Behavior of Moisture-Resistant MoS_x Coatings X. Zhang (Katholieke University of Leuven, Belgium); W. Lauwerens (Limburg University, Belgium); L. Stals (Limburg University, Beligum); J. He (Xi'an Jiaotong University, PR China); J.P. Celis (Katholieke Universiteit Leuven, Belgium) The fretting wear behavior of sulphur deficient MoS_x coatings with different crystallographic orientations has been investigated in ambient atmosphere of controlled levels of relative humidity. The coefficient of friction and the wear rate of MoS_x coatings appear to be dependent not only on fretting parameters like contact stress, fretting frequency, and relative humidity, but also strongly on the crystallographic orientation of the coatings. For random oriented MoS_x coatings, the coefficient of friction and the wear volume increase significantly with increasing relative humidity. On the contrary, basal oriented MoS_x coatings exhibit little sensitivity to different levels of relative humidity. The coefficient of friction of MoS_x coatings decreases with increasing contact stress and decreasing fretting frequency. An abrupt decrease of the coefficient of friction and an increase of the wear rate occur above a threshold contact stress value and fretting frequency for random oriented MoS_x coatings. A method that allows to analyse the differences in wear resistance of random and basal oriented MoS_x coatings, is proposed based on a correlation of wear volume with dissipated energy.
3:30 PM		E1-2-7 Preparation of Ni-P-MoS₂ Solid lubricant Films by Using Different Surface Detergents J.H. He, W.L. Chang (Feng Chia University, Taiwan) Electroless plated Ni-P-MoS₂ solid lubricant films has attracted intentions due to its cost-saving, age-hardenability as well as satisfactory performance. Surface detergent, rarely explored can however dramatically influence the film growth behavior and the consequent film properties. An electroless nickel plating was used in this study to prepare the Ni-P-MoS₂ solid lubricant films on steel using different types of commercial surface detergent Hydrophile-Lipophile Balance (HLB) value, in particular. The pH value, bath temperature and MoS₂ powder concentration of the bath were examined to reveal the influence of surface detergent on growth behavior of the deposits. X-ray diffractometer and scanning electron microscope were used to characterize crystal structure and observe surface morphology, respectively. Film thickness was measured by optical microscope. Energy dispersive spectrometer was used to determine the MoS₂ content in the deposited films. Experimental results show that detergents with HLB value over 11, enables the uniform dispersion of MoS₂ powders into the bath either at the room temperature or the elevated bath temperature with the exception of Triton-100-reduced which suffers deterioration itself at a higher temperature. MoS₂ content in the film depends strongly on the deposition parameters while the growth rate of deposit does not. Maximum codeposited MoS2 content, ten fold higher than Tween-80, happens to the deposited film by using Tween-20. Whatever kind of surface detergents is used, the codeposited MoS₂ is highly (002) preferred orientated. A model is proposed to elucidate the detergent effect on the growth behavior of the growing films.
3:50 PM		E1-2-8 Mechanical Solicitations and Dissipated Energy Influence on a Copper/2D Woven Carbon-Carbon Composite Couple Crossed by an Electrical Current A. Samah (National Polytechnics Institute of Lorraine, France); D Paulmier (LPMM-ERMES, CNRS UMR 7554, France) The tribological behaviour of a sliding contact of a copper/2D woven carbon-carbon composite couple crossed by a direct electrical current is studied with a pin on disc tribometer. The influence of the total heat energy dissipated through the contact on the friction coefficient µ, the contact resistance and the specific wear rate is analysed. It appears that, when the electrical intensity I ≥ 30A and the copper is cathode, µ is low and decreases when the total dissipated energy increases. When the copper is anode, the trends of the specific wear rate of the c-c composite changes and goes through a minimum. The contact temperature increases with the electrical intensity increase but depends on polarity. The growth rate of the oxide layer increases with the intensity according to the Cabrera and Mott theory. Scanning Electron Microscopy and Electron diffraction Spectroscopy permit to explain the results.
4:10 PM		E1-2-9 Enhancement to the Tribological Properties of Titanium Using Plasma Electrolytic Oxidation and Plasma-Immersion Ion Assisted Deposition Techniques A.L. Yerokhin, A. Leyland, C. Tsotsos, A.D. Wilson, A. Matthews (University of Hull, United Kingdom) Recent studies¹ have demonstrated that oxide ceramic surface layers produced by Plasma Electrolytic Oxidation (PEO) have a strong potential to improve the tribological performance of Ti-alloys. In the present work, detailed investigations are undertaken to optimise PEO parameters for the creation of the ceramic layers on a Ti-6Al-4V alloy. The effects of modified aluminophosphate electrolytes, current density and oxidation time on the layer thickness, structure, microhardness and adhesion are discussed. Tribological evaluation is performed using microabrasion and reciprocating-sliding testing against cermet (WC-Co) and metal (SAE 52100 steel) counterfaces. The layer thickness and microhardness are found to increase from below 30 µm to 70-90 µm and from less than 6 GPa to 10-12 GPa respectively, with increasing content of Al in a rutile matrix, in which harder inclusions of Al₂O₃ and Al₂TiO₅ are formed. This promotes a significant improvement of the surface wear resistance; however, in the case of a steel counterface, increased values of friction coefficient (µ=0.6-0.7) are observed due to the transfer of Fe-Ni compounds from the counterface onto the oxide ceramic surface. To further improve the performance of such PEO treated surfaces, the application of a thin (0.5-2.0 µm) DLC topcoat has been carried out using a Plasma-immersion Ion Assisted Deposition technique. It is shown that a thin DLC film can effectively reduce chemical interaction between the counterfaces in the contact area, whilst the thick PEO layer provides adequate load support for the surface to resist contact deformation. These 'duplex' surface treatments create an advantageous combination of low friction µ=0.1-0.15/0.15-0.25 against the ceramic/metal counterfaces) and low wear rates (down to 10^-9 mm³/Nm) for both of the counterface pairings. ¹ A.L.Yerokhin et al, Surface & Coat.Technol.,130(2000)195.