ICMCTF1999 Session E2: Wear Resistance of Ceramic, Metallic and Composite Coatings

Thursday, April 15, 1999 8:30 AM in Room California

Thursday Morning

Start	Invited?	Item
8:30 AM		E2-1 Improving Tribological Properties of Sputtered Boron Carbide Coatings by Process Modifications T. Eckardt, K. Bewilogua (Fraunhofer Institute for Surface Engineering and Thin Films, Germany); G. van der Kolk, T. Hurkmans, T. Trinh, W. Fleischer (Hauzer Techno Coating Europe, The Netherlands) In this paper the modification of magnetron sputtered boron carbide coatings will be discussed. Using argon as a sputter gas and and boron carbide (B4C) targets, hard but rather brittle films with good adhesion were deposited. These coatings have high friction coefficients against steel. Adding small amounts of other elements during the sputter process improves significantly the tribological behaviour of boron carbide. An incorporation of hydrogen leads to a structure modification that is similar to amorphous carbon coatings. The reduction of dry friction is not correlated to decreasing roughness of the film which was examined by AFM but is a real effect of chemical bondings that have changed in the material. The amount of necessary reactive gas is very small compared to other reactive Me-DLC deposition processes. This is an important advantage of these modified boron carbide coatings. Several parameters of the sputter process like gas flow rate and substrate bias have been varied and investigated. Furthermore chemical composition, film morphology, wear resistance, wettability and tribological behaviour were determined to characterize these coatings for potential applications. Fields of application are all kinds of sliding parts under high load and increased temperature conditions in automobile engines and mechanical engineering generally.
8:50 AM		E2-2 Metal/Carbide Co-sputtered Wear Coatings B. Mishra (Colorado School of Mines); F.M. Kustas (Engineered Coatings, Inc.); J. Zhou (Colorado School of Mines) Carbide materials offer extremely high hardness, but are limited due to high brittleness. Thin films of carbides have potential for wear coatings if their adhesion to engineering material substrates and durability can be increased. Additions of metals to ceramics is a well known method to increase toughness, as demonstrated by the class of ceramic/metal (cermet) bulk materials. Using the same approach employed in the cermet industry, carbide and refractory metals were deposited by co-sputtering to investigate the effects of metal additions on the mechanical and tribological properties of the thin-film coatings. Carbide (B₄C, TiC) and refractory metals (Mo, W) were co-deposited onto stainless and bearing-steel substrates using unbalanced magnetron sputtering. Coating elemental composition, blade deflection (residual stress) scratch adhesion, microhardness, and wear tests were performed on selected coatings to determine trends in properties with metal content ranging from zero to ~50 atomic percent. Results on stainless steel substrates suggest higher coating residual stress for coatings with lower metal content, and greater coating adhesion for films with lower microhardness. Addition of metal bond layers significantly improved the cermet coating scratch adhesion (up to 55 N) to stainless steel. Extremely high microhardness, over 5000 Knoop (10 gm load), was measured for a B₄C-15 atomic percent Mo co-sputtered film.
9:10 AM		E2-3 Comparison of Fretting Wear of CrN and TiN Coatings at Different Levels of Relative Humidity H. Chen, P.Q. Wu (Katholieke Universiteit Leuven, Belgium); C. Quaeyhaegens (Limburgs Universitair Centrum, Belgium); K.W. Xu (Xi'an Jiaotong University, China); L.M. Stals (Limburgs Universitair Centrum, Belgium); J.W. He (Xi'an Jiaotong University, China); J.-P. Celis (Katholieke Universiteit Leuven, Belgium) Chromium nitride (CrN) coating holds high potential to replace the titanium-based coating, particularly for some special applications. In this paper, chromium nitride and titanium nitride (TiN) coatings were deposited on tool steel substrates by ion beam assisted deposition and triode ion-plating techniques respectively. Fretting wear of these coatings has been investigated in air of different levels of relative humidity (RH). The wear loss volume was evaluated by profilometry and morphologies of wear scar were studied by scanning electron microscopy. The phases of CrN layer were analysed by glancing angle X-ray diffraction method. Results show that the friction coefficient of the CrN coatings is about 0.3, a little higher than that of TiN coatings at 50% and 85% RH. However, TiN coatings exhibit a high friction coefficient of about 0.8 at RH<10%, but CrN coatings remain at much lower friction coefficient in about the same level as that at high RH. In addition, the wear loss volume of CrN coatings is smaller than that of TiN coatings, especially at low relative humidity. Results of wear rate expressed by wear volume per joule of dissipated energy indicate that CrN exhibits higher wear resistance than TiN. These properties make the CrN coatings appreciable to be used in ambient air without lubrication, especially in very dry working environment.
9:30 AM		E2-4 Influence of Geometry and Target-to-Substrate Distance on the Properties of Cathodic Arc Deposited Chromium Nitride Coatings R.E. Hawbaker (Caterpillar Inc.); C.Q. Dam (Consultant); P.H. McCluskey (Caterpillar Inc.) Implementation of thin coatings to industrial applications by physical vapor deposition introduces issues related to part geometry and coating uniformity throughout the chamber. In this study, chromium nitride coatings were grown using cathodic arc deposition on 52100 steel flats (coupons). The coupons were placed inside and on the end face of annular cylinders distributed throughout the deposition chamber. Each cylinder contained four coupons (one on the face and three inside). Coated system properties were investigated as a function of coupon location within the cylinder and location of the cylinder within the deposition chamber. Properties such as hardness, adhesion, coating composition and morphology, and wear behavior were evaluated. The results are discussed in terms of the differences between batch processing large numbers of parts with complex shapes versus processing small numbers of test coupons with simple geometries.
9:50 AM		E2-5 Development and Testing of Various Wear-Resistant PVD Hard Coatings for Cobalt-Base Alloys B.V. Cockeram (Bettis Atomic Power Laboratory) One approach to reducing the costs and hazards associated with nuclear plant radiation exposure from activated cobalt wear debris is covering the cobalt-base materials with a wear resistant coating. The hardnesses of some cobalt-base wear alloys are significantly lower than conventional PVD hard coatings, and support of the hard coating is a concern. Four approaches have been taken to minimize the hardness differences between the substrate and PVD hard coating: (1) use a thin Cr-nitride hard coating with layers that are graded with respect to hardness, (2) use a thicker, multilayered coating (Cr-nitride or Zr-nitride) with graded layers, (3) use nitriding to harden the alloy subsurface followed by application of a multilayered coating of Cr-nitride, and (4) use of nitriding alone. Since little work has been done on application of PVD hard coatings to cobalt-base alloys, some details on process development and characterization of the coatings is presented. Scratch testing was used to evaluate the adhesion of the different coatings. A bench-top pin-on-disc test and rolling contact test were used to evaluate the wear resistance of the coatings. The test results are discussed, and the more desirable coating approaches are identified.
10:30 AM	Invited	E2-7 Mechanical and Tribological Properties of Spray-Coated Conventional and Nanostructured WC/Co Composites T.E. Fischer, L. Liu, Y. Qiao (Stevens Institute of Technology) We review the hardness, toughness, and wear resistance of WC/Co composite coatings sprayed and of sintered materials. For sintered bulk cermets we review the effect of the WC grain size and the benefit of nano structured materials. The hardness of sintered cermets increases and their toughness decreases with decreasing cobalt mean free path. Nanostructured cermets (WC size 70 nm) further increase the hardness without detriment to toughness. Abrasive and sliding wear resistance increase with the hardness of the material, but nanocomposites show an additional increase in abrasion resistance. For coatings, we review the effect of spray technique and WC grain size in the powder. The coatings have lower hardness and toughness than the sintered materials. They present anisotropic mechanical properties because of the lower adhesion between splats. The wear mechanisms consist of polishing of WC grains and removal of splats in the coatings. The wear rate and mechanisms are influenced as much by the coating technique as by the WC grain size because of the removal of the deposition splats. The sliding wear resistance of the coatings, however, is only slightly lower than that of the sintered materials.
11:10 AM		E2-9 Wear Mechanism Maps for TiN Coated High Speed Steel S. Wilson, A.T. Alpas (University of Windsor, Canada) Dry sliding wear experiments were conducted on PVD TiN coated high speed steel discs over a range of contact loads (5N-300N) and sliding speeds (0.1-5m/s) against high speed steel pins using a pin-on-disc test configuration. Friction heating effects were monitored by recording bulk pin surface temperatures and correlated with wear rates and mechanisms for TiN coatings 3 µm and 5 µm thick. Steady state wear rate, wear mechanism and bulk temperature data are summarized on load versus sliding s peed axes in the form of wear maps and compared to equivalent data for uncoated high speed steel. Coating surface roughness and thickness were found to influence wear rate and mechanism transitions at specific combinations of contact load and sliding speed which corresponded to characteristic critical bulk temperature of approximately 200 deg.C. Wear mechanisms were divided into three categories: (i) mild polishing of TiN and transfer of oxidized pin debris material at low loads and speeds; (ii) increased polishing damage and spallation at intermediate loads and speeds; (iii) plastic deformation and rapid wear of the TiN and pin arising as friction heating becomes increasingly adiabatic at high loads and speeds. Scanning electron microscopy of the subsurface regions of worn coatings reveals increasing TiN and substrate plasticity as conditions become more severe. These damage mechanisms are correlated with those observed in elevated temperature scratch tests on each TiN coating and uncoated substrate material.
11:30 AM		E2-10 Evaluation of the Wear Resistance of Multilayered PVD TiN/TaN on HSS Substrates M. Nordin, M. Larsson, S. Hogmark (Uppsala University, Sweden) The tribological performance of thin hard coatings is, for a given substrate material, mainly determined by the coating hardness, coating fracture resistance, the contact temperature and chemistry (in the prevailing tribosystem). For a given application, an improved tribological performance can therefore e.g. be accomplished by increasing the coating fracture resistance while retaining the hardness, or vice versa. A possible way to accomplish this is to introduce a multilayered structure in the coating. The multilayer structure will act as a crack inhibitor and thereby increase the coating fracture resistance with retained, or even increased, hardness. Three PVD TiN/TaN multilayer coatings, with different lamella thicknesses, were deposited on high speed steel substrates. The multilayered coatings were investigated with respect to their mechanical properties (composite hardness, residual stress, fracture resistance and adhesion) and tribological properties (abrasive and erosive wear rates). In addition, coating and lamella thickness, morphology and microstructure of the coatings were studied. The multilayered coatings were compared to single layered TiN and TaN. It was observed that the fracture resistance and adhesion increased with decreasing lamella thickness. The decrease in lamella thicknesses also contributed to a decrease in the abrasive and erosive wear rate. A detailed study of the wear mechanisms will be presented.
11:50 AM		E2-11 Comparison of the Fretting Wear Behaviour of PVD TiN Coated and Uncoated High Speed Steel at Different Testing Conditions P.Q. Wu, H. Chen, J.-P. Celis (Katholieke Universiteit Leuven, Belgium) The fretting wear behaviour of materials in vibrating contacts is dictated by mechanical contact conditions, environmental conditions, and materials properties. These factors may interact with one another, thus making wear a complex system property. In this paper, the influence of some testing parameters on the fretting wear behaviour of PVD TiN coated and uncoated ASP23 steel is systematically reported. These parameters include relative humidity, normal force, displacement stroke, fretting frequency and number of cycles. Criteria proposed earlier in literature for determining the changes in wear mode and fretting regime, and wear rate have been confirmed. Based on Hertzian contact theory, the wear mechanism of TiN coatings will be discussed for different fretting parameters. This study indicates how important the proper selection of the testing parameters on the friction and wear of materials is. Keywords: TiN, coatings, steel, fretting, wear