Papers

ICMCTF2000 Session E1-2: Reduction of Friction Through Coating/Surface Modification

Tuesday, April 11, 2000 1:30 PM in Room California

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		E1-2-1 Tribological Properties of Thermal Sprayed Magnéli-Type Coatings with Different Stoichiometries (Ti_nO_2n-1) O. Storz, H. Gasthuber (Daimler Chrysler AG, Germany); M. Woydt (Federal Institute for Materials Research and Testing (BAM), Germany) The results presented in this paper show the possibility of producing titania based Magnéli-type coatings by thermal spraying as well as the tribological properties of these coatings deposited by this technique. For that purpose a spray powder with a defined phase composition were applied on aluminum substrates by atmospheric plasma spraying. Crystalline Magnéli phases of the Ti_nO_2n-1-type with a crystallographic shear structure could be identified after spraying in the coating by means of X-ray diffraction and TEM analysis. Dry sliding experiments were conducted on a high-temperature pin-on-disc test rig against polycrystalline alumina (99,7% Al₂O₃) and sintered silicon carbide (SSiC) at 23°C and 1 m/s with 2000 m of sliding distance. The COF for all tests remain high between 0.7 and 0.8. The best wear results under dry sliding conditions are achieved with the Ti6O11/ Al₂O₃ couple, showing wear rates of 1,210-6 mm3/Nm for the Ti₆O₁₁-coating and 2.310^-8mm³/Nm for the Al₂O₃-ball. After running-in, the wear rates further decreased. Another significant fact, are the PV-values greater of 20 Watt/mm² associated with the low wear rates. Results from tests at 400°C will also be presented. The tribological behavior of the sprayed coatings indicate strong anti wear properties under dry sliding conditions and lower wear rates than for polycrystalline Magnéli-type phases reported previously. The tribological behaviour under dry oscillating sliding at different relative humidities will also be presented.
2:10 PM		E1-2-3 Influence of Molybdenum Composition in Chromium Oxide-based Plasma-sprayed Coatings on Their Tribological Behavior H.-S. Ahn, I.W. Lyo (Korea Institute of Science and Technology, Korea) Plasma-sprayed Cr₂O₃-based coatings containing molibdenum were studied to gain a better understanding of the influence of molibdenum composition in the coatings on their tribological behaviour. Cr₂O₃/Mo composite powders (Cr₂O₃-5wt%Mo and -20wt%Mo) and a Cr₂O₃ powder were fabricated using a spray-drying method and plasma-sprayed coatings of these powders were produced to evaluate their tribological performance. Wear tests were conducted with a reciprocating motion at room temperature and 450°C under dry sliding condition. Measurement of friction coefficient was made in association with the sliding cycle. The physical characteristics of worn surfaces was investigated by scanning electron microscopy and chemical composition of the coating surfaces was analyzed using a X-ray photoelectron spectrometer and a X-ray diffractometer. The study showed that the friction coefficient of coatings containing molibdenum were lower than that composed of Cr₂O₃ alone at both test temperatures. The addition of molibdenum in the Cr₂O₃ coatings did not significantly improve the anti-wear performance of the coatings at both test temperatures. Cr₂O₃/Mo composite coatings tested at 450°C exhibited considerably lower friction coefficient than that tested at room temperature. Dispersed smooth films were formed in the worn surface for all coatings. These wear protecting layers, formed by plastic deformation of adhered and compacted debris particles to the surface, strongly influence the friction of the coatings as already observed by the authors with different plasma-sprayed coatings. The chemical composition of these films varied depending on the test temperature. XPS analysis of the smooth surface films indicated that MoO₃ composition was dominantly formed as Mo species. MoO₃ composition in the smooth film appears to be more favorable than MoO₂ in reducing friction.
2:30 PM		E1-2-4 Tribological Properties, Phase Generation and High Temperature Phase Stability of Tungsten- and Vanadium- Oxides Deposited by Reactive MSIP-PVD-Process for Innovative Lubrication Applications St. Bärwulf, E. Lugscheider, O. Knotek, K. Bobzin (University of Technology Aachen, Germany) The tungsten and vanadium oxides are promising to be usable as solid lubricants at elevated temperatures because of their ability to form Magnéli-phases. So far they were mainly investigated as powdery material for tribological applications or as a component of ceramics. As a matter of fact metal-oxides are interesting for tribological insets at atmospheric conditions because of their expected oxidation stability and low adhesion. The study intends to report about investigations made to find stable process windows for the deposition of tungsten and vanadium oxides in a reactive d.c. mode by the MSIP-PVD-process and especially the influence of the oxygen content in the sputtering atmosphere as well as the deposition temperature on the phase generation. A simplified "phase diagram" of the binary systems V-O and W-O as a function of the deposition temperature (378 - 650 K) and the oxygen content (0 - 50%) will be presented. The high temperature phase stability of films is of essential interest for a tribological inset as a solid lubricant at elevated temperatures. It will be shown, that the vanadium-oxides are phase stable up to 878 K and the tungsten- oxides up to 1100 K (measured in a high-temperature-xrd facillity). Further the deposited metal oxide-phases and selected mechanical and tribological properties, like the friction coefficient versus various materials, will be discussed. A reduction of friction against steel down to 0,2 could be reached for polished and (V-O, W-O) coated samples at room temperature. The coatings were analyzed by various testing methods to characterize the tribological, mechanical and structural properties, like SEM, scratch testing, nanoindentation, XRD and pin on disk.} Note: Requested an Oral Session.
2:50 PM		E1-2-5 Time Dependent Friction Behavior of Near-Frictionless Carbon (NFC) Coatings J.A. Heimberg, K.J. Wahl, I.L. Singer (Naval Research Laboratory); A. Erdemir (Argonne National Laboratory) Reciprocating sliding tests were performed on near-frictionless carbon (NFC) coatings in dry nitrogen and dry air environments. NFC is a diamondlike carbon that exhibits low friction, low wear behavior ¹. Coated sapphire balls (0.635 mm diameter) were loaded to 9.8 N against coated steel flats; track lengths were between 1 and 5 mm. Time dependent friction behavior was investigated by varying the sliding velocity between 10 and 3000 micron/sec, which changed the wear tracks’ exposure times to the environment. At high velocities (greater than 500 micron/sec), the average friction coefficient in steady-state was µ=0.003. At low velocities, the friction increased with sliding cycle. These data can be fit to an equation that models oxygen adsorption onto carbon. Analysis of the spatially resolved friction data revealed a second time-dependent effect: a transition from low, constant friction during part of the cycle to increasing friction during the remaining portion. Time constants calculated from this transition were found to be independent of sliding velocity and dependent on the environment. ¹ ¹ “Super-Slick: A Near-Frictionless, Diamond-Hard Carbon Coating Is Getting Its First Tests in Commercial Applications,” Mechanical Engineering, John DeGaspari, 4/29/99
3:30 PM		E1-2-7 Properties and Tribological Performance of MOCVD Zirconium Carbonitride Films in Comparison with Commercial CVD and PVD Coatings C.P. Allenbach, M. Morstein, N.D. Spencer (Laboratory for Surface Science & Technology, ETH Zurich, Switzerland) Zirconium carbonitride (ZrCN) coatings were deposited on tool steel by low-temperature MOCVD using three different precursors, Zr(NEtMe)₄, Zr(pip)₄ and the binuclear complex Zr₂(µ-NtBu)₂(NHtBu)₄. These organometallic compounds allow deposition of ZrCN in the temperature range of 400-600°C using a system pressure of 500 Pa or lower and an ammonia/nitrogen mixture as reactant gas. The films were characterized with respect to their chemical composition, microstructure, hardness and roughness. A larger amount of oxygen or graphitic carbon impurities was found to correlate with a decrease in film hardness. The unlubricated friction behavior of ZrCN films deposited between 450 and 500°C on polished HSS disks was measured in a dry atmosphere versus various non-ferrous alloy pins as counterbodies, as well as steel. A range of industry-standard hard PVD and CVD coatings was deposited on the same kind of steel substrates for comparison. All these films, including TiN, TiCN, TiAlCN, κ-Al₂O₃ and diamond-like carbon (DLC), were then systematically tested using the same pin-on-disk set-up. The friction evaluation shows that the MOCVD zirconium carbonitride films markedly reduce the coefficient of dry-sliding friction compared to uncoated steel, for all investigated counterbody materials. Depending on the pin material, a considerable friction reduction of up to 50% was also obtained relative to all commercial coatings except DLC. Transfer of pin material to the coating was observed in some cases both by profilometry and imaging XPS (X-ray photoelectron spectroscopy). Under our test conditions, the wear rates of industrial coatings were practically zero, whereas the ZrCN coatings, due to their relatively lower hardness, exhibit noticeable wear.
3:50 PM		E1-2-8 The New Development of Sputtered Carbon Based Coatings Incorporated with Cr, Ti, B and Nitrogen S. Yang, D.G. Teer (Company Teer Coatings Ltd, United Kingdom) Sputtered carbon and carbon/chromium coatings were recently reported to have excellent tribological performance with high load bearing capacity [1,2]. The effect of introducing Cr, Ti, B, and nitrogen to deposit composite or multilayer coatings has been studied. The deposition was carried out to produce around a 2.0 ?m CrTiBCN base layer and then followed by a 2.0 ?m CrTiBC tribological top layer using unbalanced magnetron sputtering with one chromium, one titanium di-boride, and two carbon targets with a three fold substrate rotation mechanism. The base layer was found to have a hardness of around 2500 HV, critical load of scratch Lc ~ 80 N, and friction coefficient >0.4 in air similar to those of conventional hard coatings (TiN for example). The tribological top layer was shown to have both high hardness (~2200 HV) and low coefficient of friction (~ 0.06 under a load of 80 N in pin-on-disc test). The nitrogen gas flow rate for deposition of the base layer and the sputtering currents of chromium and titanium di-boron targets for the deposition of the tribological top layer were optimised to produce coatings with the best combination of properties of high hardness, low friction, and low wear rate. Pin-on-disc tests in air and reciprocating wear tests in air, water, and oil were used to determine the tribological performance of the coating, and the results indicated that this new composite coating could be used as a hard, solid lubricant films for a range of mechanical components. ¹ ¹ S. Yang, D. Camino, A.H.S. Jones and D.G. Teer, Presented at ICMCTF 99 CONFERENCE, San Diego, USA, in press in Surf. Coat. Technol.. ² S. Yang and D.G. Teer, Presented at AEPSE’99 conference, Beijing, P.R. China, to be published
4:10 PM		E1-2-9 Sulfides and Boron Nitride Based Coatings for Applications in Dry Machining T. Eckardt, K. Bewilogua, T. Müller, J. Penz, N. Kläke (Fraunhofer Institute for Surface Engineering and Thin Films, Germany) Dry machining has found increasing demand in industry for cutting and forming tools. One attempt to reduce or even eliminate the use of liquid cooling agents is by coating tools with solid lubricants. Different machining operations will most likely require specific coating systems because each case differs for example in load, shear stress or temperature generation. Chalcogenide based compounds, like molybdenum and tungsten sulfide, have been considered quite promising for dry cutting and forming operations. Due to their low wear resistance, however, they are most commonly stabilised by simultaneous sputtering with titanium. Additionally, in combination with titanium nitride hard coatings their tribological behavior was improved. After systematic variation of deposition parameters in the magnetron sputter process several tools were coated and tested under industrial conditions. Their properties were determined with regard to final operation, e. g. drilling, sheet metal forming or milling. It was also investigated wether there is a correlation between results of laboratory and under application conditions. Furthermore other coating systems based on boron and nitrogen were investigated. Either a boron carbide or a titanium diboride/boron nitride sputter target was used. In both cases the coating composition is modified by introducing reactive gases like e. g. nitrogen. In both systems boron nitride was identified which is supposed to work as a solid lubricant while TiB₂ and B₄C respectively, act as wear protection. Results from tests with various coated tools will be presented and compared.
4:30 PM		E1-2-10 Influence of Soft Coatings on Chip Formation and Cutting Performance H.K. Toenshoff, T. Friemuth, C. Podolsky, H. Seegers (University of Hannover, Germany) Today, special research activities are concentrated on the optimization of coating properties to improve the performance of cutting tools. (Ti,Al)N-based hard coatings are well known for their excellent microhardness even at elevated temperatures as well as for their good oxidation and wear resistance. Further improvements concerning the wear behaviour may be achieved by reducing the friction between tool and workpiece material in cutting. In this paper, the cutting performance of hard and soft coated carbide tools is investigated in turning and drilling of tempered steel. Chip structure is analyzed in order to investigate the effect of friction. The cutting forces and the wear behaviour of the drilling process are highlighted for (Ti,Al)N and (Ti,Al)N+WC/C-coatings. A superior wear behaviour of soft coated tools was found in the initial wear phase. An influence of soft coatings on the wear behaviour after the running-in period could not be observed.
4:50 PM		E1-2-11 Study on Ion Sulphuration Layer As a New Solid Lubrication Coating on Steels and its Wear Mechanism under Dry Condition J. Liu, N. Zhang, D. Zhuang, X. Fang, M. Guan (Tsinghua University, China) The most commonly used solid lubricants as a coating at present time are some laminated materials with hexagonal structure and low shear strength, like graphite and MoS2. FeS possesses also a hexagonal structure, low shear strength and some advantages, like high melting point(1,100C), much larger thickness of coating, low price and no obvious interface between coating and substrate. So the sulphuration process has been recognized as a friction-reducing technique for a long time. The traditional sulphuration method was the simple and cheap electrolytic process. However, it was not widely propagated due to the pollution problem. A new ion sulphuration technique and equipment was developed based on the principle of ion nitriding in 90’s. Until now it has been widely applied to many machine components successfully in China, like rolling bearings, gears, dies, cutting tools, cylinders, piston rings and so on. The service life of sulphurized components can all be prolonged obviously to different extent. In this paper the sulphide layer was made on 1045 and 52100 steels by the low temperature ion sulphuration technique. Its tribological properties under dry condition were investigated on a reciprocating SRV testing machine. Its friction coefficient, width of wear scar as the wear volume and endurance were measured. The morphology and composition of wear scars and wear debris were analysed by SEM+EDX. Based on the experimental results the wear mechanism of sulphide layer was discussed. The conclusion of this research can be drawn as follows, the sulphide layer can adhere to the rubbing surfaces and impede their direct contact, the friction coefficient and wear resistance of sulphrized surfaces can be significantly improved by its solid lubrication property, the higher hardness of substrate can increase its endurance, and the sulphide can promote the formation of oxide in the same layer, which is also beneficial to reducing friction coefficient and wear resistance. Keywords: Ion sulphuration, solid lubrication, tribological property, wear mechanism