ICMCTF2008 Session E3-1: Tribology of Amorphous and Nanostructured Films

Tuesday, April 29, 2008 1:30 PM in Room California

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		E3-1-1 Wear and Friction Maps of a-C:H 20% Films P.A. Radi, L.V. Santos, L.F. Bonetti, V.J. Trava-Airoldi (INPE/MCT-Instituto Nacional de Pesquisas Espaciais, Brazil) DLC films exhibit attractive mechanical, optical, electrical, chemical and tribological properties. However the high costs with trial material for each application results in a very low percentage of tested products for industrial applications. Wear and friction maps are the way to estimate wear and friction behavior in a wide load and speed range. This paper presents the wear and friction maps for high quality DLC films with 20% hydrogen content (DLC-20%) on titanium alloy (Ti₆Al₄V) substrate produced under strictly controlled growth conditions. The techniques required to ensure high adhesion and film reproducibility are in the methodology. Hardness was measured and was considered as an additional reference in terms of DLC films quality. All tests were carried out at different loads and speeds on Ti₆Al₄V substrate with DLC-20% samples, using Ti₆Al₄V ball as counter body. The results show DLC films can resist wear well with varying friction coefficients, depending on the load. These wear and friction maps help identify the load and speed constraints of DLC films for industrial applications.
1:50 PM		E3-1-2 Structure and Mechanical Properties of Si-C:H/a-C:H Multilayered Coatings Grown by LF-PECVD C. Chouquet, C. Ducros (CEA Grenoble, France); S. Barrat (LSGS Ecole des Mines, France); A. Billard (LERMPS UTBM Montbéliard, France); F. Sanchette (CEA Grenoble, France) Diamond-like Carbon films (DLC) offer a wide range of exceptional mechanical and tribological properties that make them one of the most valuable engineering materials for automotive field. Depending on elaboration conditions, DLC performances can often be limited by high internal compressive stress, low thermal stability and increased friction with humidity. Recent studies have confirmed that novel concepts like doped or multilayered DLC films can improve properties. In this study, multilayered coatings consisting in a stack of amorphous hydrogenated carbon films (a-C:H) and amorphous hydrogenated silicon carbide films (Si-C:H) have been deposited by low frequency plasma enhanced chemical vapour deposition (LF PECVD). Cyclohexane-hydrogen and tetramethylsilane-argon mixtures have been respectively used for the a-C:H and Si-C:H layers. Period thickness was varied between 25 and 300nm and TEM micrographs revealed very sharp interfaces even for the thinnest periods. Nanoindentation measurements were applied to determine hardness and Young modulus, and residual stress was evaluated using a bending method. No significant influence of period thickness was highlighted but nanolayering allows reducing residual stress whereas hardness remains equivalent to that of an a-C:H monolayer. Other characterisations such as bending and tensile tests were also performed to compare multilayered films mechanical behaviours with those of monolayer coatings. Ball on disk tribometer experiments were realised, friction coefficient and wear rate evolutions were evaluated and discussed as a function of test conditions, period thickness, Si-C:H layer properties and top layer nature.
2:10 PM	Invited	E3-1-3 Tribological Thin Films in Lubricated Rolling Contacts R.D. Evans (The Timken Company) Tribological thin films are used in rolling element bearings to increase fatigue life and reduce wear in boundary lubrication, improve debris resistance, protect against false brinelling damage, and delay failure in the case of lubricant loss. Of particular interest are nanocomposite metal carbide reinforced amorphous hydrocarbon coatings such as WC/a-C:H, which are deposited at temperatures < 180°C and have simultaneous high hardness and low elastic modulus relative to steel. Two fundamental considerations in selecting thin film coatings for use in high stress, lubricated contacts are coating wear mode and traction. Fatigue life tests were conducted on tapered roller bearings with various types of WC/a-C:H coated rollers to explore the effects of film structure and architecture on performance. Film wear modes are explored in cross section with transmission electron microscopy of samples prepared with focused ion beam milling. In addition, Stribeck curves were generated for various thin film coating types using a ball-on-flat test configuration to examine surface composition and texture effects on lubricated rolling traction.
2:50 PM		E3-1-5 Characterisation and Tribochemistry of Tetrahedral Hydrogen-Free Amorphous Carbon in Presence of Glycerol C. Matta, M.-I. De Barros Bouchet (Ecole Centrale de Lyon, France); L. Joly-Pottuz (INSA de Lyon, France); B. Vacher, T. Le Mogne, J.-M. Martin (Ecole Centrale de Lyon, France); T. Sagawa (Nissan Motor Co., Ltd., Japan) In this paper, a unique tribological system that produces extremely low friction under boundary lubrication conditions with no apparent wear is reported. This system is composed of a pair of thin tetrahedral hydrogen-free amorphous carbon coating (ta-C couple) lubricated with pure glycerol at 80°C. First, the chemical properties of ta-C DLC were studied by Energy-filtering transmission electron microscopy (EFTEM) coupled to focus ion beam (a technique of sample preparation) and the combination of XANES, ToF-SIMS and Auger spectroscopies. The sp³ structure of the ta-C coatings was confirmed and the presence of a very thin graphitic layer at the extreme surface was detected. Moreover, the mechanical properties were studied by nanoindentation measurements. Then to understand the mechanism of ultra-low friction ToF-SIMS analysis were performed inside and outside the tribofilm and by using deutered and ¹³C glycerol. The result revealed the hydroxylation of the extreme surface of the coatings. XPS and EFTEM studies also confirmed the hydroxylation. The origin of the ultra-low friction seems to be due to the possible formation of low-energy van der Waals interaction between the OH-terminated carbon film surfaces and the OH function of glycerol. This mechanism is in very good agreement with molecular dynamics (MD) simulations and with results obtained by using another model of OH-terminated surface. Based on these findings, pure glycerol coupled with ta-C may be promising candidates in the field of biotechnology applications.
3:10 PM		E3-1-6 On the Hydrogen Lubrication Mechanism(s) of DLC Films: An Imaging TOF-SIMS Study O. Eryilmaz, A. Erdemir (Argonne National Laboratory) In this study, we investigated the effects of hydrogen on sliding friction and wear behavior of diamondlike carbon (DLC) films in dry nitrogen and high vacuum environments. The highly-hydrogenated DLC films provided superlow friction (i.e., less than 0.01) regardless of the test environments, while the films that are hydrogen-free or -poor had to be tested in hydrogen-containing test environments or be subjected to a hydrogen plasma treatment in order to attain low friction. When tested in dry nitrogen or vacuum, these films exhibited very high friction and wore out very quickly. All test were performed using pin-on-disk machines under 2-10 N loads and at 0.2 to 0.5 m/s sliding velocities. 2- and 3-D TOF-SIMS images of sliding surfaces revealed strong evidence for the existence of a hydrogen-rich surface layer on sliding surfaces of hydrogen-free or -poor DLC films after tests in hydrogen-containing test environments. Based on the results from both the tribological and surface analytical studies, we propose a hydrogen-lubrication mechanism that can further explain the superlubricity of hydrogen-rich DLC films and the much improved tribological behavior of hydrogen-free or poor DLC films in hydrogen containing test environments.
3:30 PM		E3-1-7 Nanoscale Roughness Effects on Superlow Friction of Diamond-Like Carbon Films V. Turq, J. Fontaine, B. Vacher, J.L. Loubet, D. Mazuyer (Ecole Centrale de Lyon, France) Diamond-like carbon coatings exhibit the unique combination of high hardness and wear resistance with low friction abilities. For hydrogenated amorphous carbon films (a-C:H), achievement of "superlubricity", with coefficient of friction lower than 0.01, has indeed been reported in vacuum or inert environment. However, in ultra-high vacuum, sudden drastic friction increase is observed with some coatings after few tens to hundreds of superlow friction. Occurrence of local adhesive phenomena can account for such dramatic change, but their origin remains unknown. This paper reports observations of a-C:H surface morphology by high resolution Scanning Electron Microscopy (SEM) and quantification of a-C:H surface topography by Atomic Force Microscopy (AFM). These techniques reveal "nano-scale" roughness on the surface of DLC surfaces that are usually considered as very smooth. Indeed, with pads of about 10 nm in height and 100 nm in width, the average roughness Ra remains small, around only few nanometers. Nevertheless, the wear of these pads seems to control the occurrence of strong adhesion between sliding counterfaces and thus the further evolutions of the coefficient of friction. The relations between superlubricity and such nano-scale roughness will be thus discussed.
3:50 PM		E3-1-8 Hard Nanostructured Sulfur-Doped CH_x-TiB₂ Coatings for Improved Friction and Mechanical Performance B. Zhao, Y.W. Chung (Northwestern University) Hydrogenated amorphous carbon films are known to attain ultra-low friction performance only in dry environments. Our work demonstrated that sulfur doping of hydrogenated carbon films results in ultra-low friction performance in both dry and humid environments. However, these films have a hardness of 7 - 10 GPa and an elastic modulus around 80 GPa, which are too low for some high stress applications. Formation of nanolayer or nanocomposite coatings is known to improve hardness. With the aim to produce hard, low-friction coatings, we synthesized nanostructured films of sulfur-doped hydrogenated carbon and titanium diboride using dual-target magnetron sputtering. This paper will discuss the film structure and how such structure correlates with its tribological and mechanical properties. Effect of annealing and high temperature tribological testing will also be discussed.
4:10 PM		E3-1-9 Comparative Performance of Nanocomposite Coatings of TiC or TiN Dispersed in a-C Matrixes D. Martínez-Martínez, C. López-Cartes, A. Fernández-Camacho, J.C. Sánchez-López (Instituto de Ciencia de Materiales de Sevilla CSIC-US, Spain) Titanium carbide (TiC) and nitride (TiN) are two of the most used materials in the field of protective coatings, due to their optimal mechanical and tribological properties. The addition of the second phase can provide extra benefits to the coating, like improved hardness, reduced friction and/or oxidation resistance. In this work, we present two series of coatings in which TiC and TiN (hard) are mixed at the nanometric level with a lubricant (soft) phase like amorphous carbon (a-C). Both series of TiC/a-C and TiN/a-C nanocomposite coatings were prepared by double magnetron sputtering of C and Ti(N) targets in a Ar atmosphere (P=5x10^-3 Torr). The properties of coatings are found to be controlled mainly by the power ratio applied to each magnetron. The chemical composition has been measured by Electron Energy Loss Spectroscopy (EELS), and evolves from pure C to pure TiC or TiN, going through the nanocomposite structures presented above. These structures are confirmed by Transmission Electron Microscopy (TEM) and diffraction techniques, like X-Ray Diffraction (XRD) and Electron Diffraction (ED). The mechanical and tribological properties are found to be mainly controlled by the hard/soft phase ratio present in the coating. The hardness values are maximal for pure-phase coatings, like TiC (27 GPa) and TiN and a-C (20 GPa). However, tribological properties found the optimal performance for both TiN/a-C and TiC/a-C nanocomposite coatings with high amount of carbon (80-85%), showing low friction values (f around 0,1) and high wear resistance (k about 10^-7 mm³N^-1m^-1). The toughness of the coatings is also briefly discussed.
4:30 PM		E3-1-10 Magnetron Sputtered Ti-DLC Coatings on Rubber Substrates X.L. Bui, Y.T. Pei, J.Th.M De Hosson (University of Groningen, Netherlands) Dynamic rubber seals operate in sliding contact mode at a relatively high speed and under no or only marginal lubrication condition, being the major sources of friction in lubrication systems or bearings and also the cause of loss of the function and failure of lubrication systems due to severe wear. Ti-containing diamond-like carbon (Ti-DLC) coatings have been deposited on FKM (fluorocarbon), ACM (acrylate) and HNBR (hydrogenated nitrile butadiene) rubbers and also on Si wafer as reference via unbalanced magnetron reactive sputtering from a Ti target in C2H2/Ar plasma. The columnar structures, which resulted in a rough morphology, were often observed on coatings deposited on rubbers even at very high bias voltage of -300V whereas a bias voltage of only -100V could restrain the columnar structure of coatings on Si wafer. A high bias voltage guaranteed high adhesion strength thus excellent tribological performance of coatings deposited on rubbers but it brought negative effects for coatings on Si wafer. When sliding against ø6mm 100Cr6 steel ball counterpart, very low coefficients of friction were achieved (< 0.2 for coated rubbers versus >1 for uncoated rubbers) and the coated rubbers maintain functional up to the normal load of 5 N without coating failure. The flexibility of coated rubbers has been evaluated by three point bending tests.
4:50 PM		E3-1-12 Thin Coatings for Lubricated Slip-Rolling Contacts Under High Initial Hertzian Contact Pressure C.-A. Manier, D. Spaltmann, G. Theller, M. Woydt (Federal Institute for Materials Research and Testing (BAM), Germany) The light-weight approach in today's automotive engineering requires tribosystems, which can withstand higher contact pressures. The application of high-performance coatings represents one approach among others. This paper presents a novel coating-substrate system in comparison to newly developed DLC- coatings in a bench mark test procedure exerting slip-rolling conditions in the presence of liquid lubricants. The novel coating-substrate system can withstand at least 10 million cycles under initial Hertzian contact pressures of up to P_max = 3.500 MPa and oil temperatures of at least 120°C. In comparison, the newly developed DLC-/ta-C-coatings are slip-rolling resistant for at least up to 10 million cycles at room temperature (some of them also at 120°C oil temperature) under Hertzian contact pressures of P_max = 2.600/2.940 MPa. A bench mark compares the slip-rolling resistance of these novel coatings with other state-of-the-art DLC coatings.