Wear Resistance of Ceramic, Metallic and Composite Coatings
Wednesday, April 12, 2000 8:30 AM in Room California
E2-1 An Experimental Approach to Determine the Critical Parameters Related to the Mechanical Stability of Coated Systems
M. Ignat (Domaine Universitaire, France); Y. Berthier (INSA Lyon, France)
Coatings are sporadically used today in various technological applications because they offer improvements in wear, corrosion and/or external thermomechanical performance. However, there are a large number of applications where coatings are not employed due to potential unreliability of the coating. A critical deficiency in this area is our poor knowledge regarding the mechanical stability of the films on their substrates. Coated systems, comprised of the coating and the substrate, are subjected to stresses. These can be internal stresses produced by growth conditions or thermoelastic mismatch, or external stresses applied monotonically, or cyclically. The stresses may be capable of producing bulk and or interface failures. We shall develop this concept from results obtained using scratch, wear and microtensile tests. The emphasis will be on the critical conditions which activate degradation of the coated system.
E2-3 An Energy Description of Hard Coatings Wear Mechanisms
S. Fouvry, P. Kapsa (Ecole Centrale de Lyon, CNRS, France)
Numerous hard coatings have been developed to improve the wear resistance of sliding components under dry of lubricated situations. High micro hardness is usually required to resist against abrasion wear mechanisms. Although microhardness measurements can not be considered as a sufficient parameter to design a tribological application. Wear analysis and friction coefficient identification are systematically required. To quantify the wear kinetics, the Archard approach is classically applied. It consists to relate the wear volume with the product of the sliding distance and the normal load. A wear coefficient is then extrapolated which is supposed to establish the wear resistance of the studied material. The present study demonstrates that this approach can only be applied if the friction coefficient is maintained constant. It appears much more relevant to consider the interfacial shear work dissipated through the contact as a significant wear parameter. This energy approach is extended to the study of different coatings TiN, TiC, TiCN and a high speed steel substrate for fretting and reciprocating sliding conditions. Wear energy coefficients are identified which permit to classify the wear resistance of the studied tribosystems. It is shown that most of the energy required to damage the material is dissipated in the third body interface. The notion of activating energy coefficients is then introduced which consists to identify the partition of the total dissipated energy consumed through the different damage processes such as the breakage of ionic bounds and the oxidation processes.
E2-4 An Induction Period for the Abrasion of Steel by Diamondlike Carbon
S.J. Harris (Ford Scientific Research Lab); G.G. Krauss, T.J. Potter (Ford Scientific Research Labs); M. Grischke (Balzers Limited, Liechtenstein)
We have shown previously that over the course of thousands of cycles, the abrasiveness (or polishing ability) of diamondlike carbon falls drastically in pin-on-disk experiments using steel pins, and that the rate of this fall can be characterized by a single parameter. In this work we show that there is a very short induction period of only a few cycles during which the abrasiveness does not fall. In spite of its short duration, nearly all of the abrasion that occurs takes place during the induction period. Since a coating’s ability to protect against rolling contact fatigue depends on its ability to polish, this short induction period can be critical for determining the value of a coating. We will discuss factors which maintain the abrasiveness during the induction period and factors which subsequently turn the abrasiveness off.
E2-5 Relationship of Hardness and Microstructure to the Wear Resistance of DLC Coatings
J. Zhang, L.E. Seitzman (Caterpillar Inc.); L. Allard, L. Riester (Oak Ridge National Laboratory)
Coating hardness is often described as a controlling property of wear behavior. The accuracy of this assumption is evaluated in this investigation. DLC and metal-containing DLC coatings were deposited on 52100 steel. The coatings were analyzed for chemical and phase composition, microstructure, thickness, hardness, indentation modulus, and roughness. They were also tested for wear resistance under two- and three-body conditions. Coating wear did not correlate with hardness or indentation modulus. The strongest correlation appeared to be with coating microstructure. These results are discussed in terms of susceptibility of the microstructure to sub-surface shear.
E2-7 Deposition and Tribological Evaluation of Alumina/Aluminum Nano-Composite Coatings
M.Y. Chen (Air Force Research Laboratory); R.Y. Lin, G. Beaucage (University of Cincinnati)
Alumina/aluminum based composites with excellent physical and mechanical properties offer great potentials for lightweight, wear resistant, and high temperature applications. Wear resistant nanostructure materials represent a unique class of materials that can offer superior properties compared to monolithic materials. In this study, rapid infrared processing technology is used to deposit nanostructured nanocrystalline alumina/aluminum based composite coatings with and without secondary reinforcement of SiC and/or MoS@sub 2@ particulate. Frictional behavior of these composite coatings were characterized. The effects of secondary reinforcements on the friction and wear behavior were examined. Pin-on-disc experiments were performed between 440C stainless steel balls and disc samples of alumina/aluminum based composites. Wear tracks on the composite discs and material transfer to the steel balls were examined using a scanning electron microscope equipped with an energy dispersive X-ray spectroscope. The relationship between residual stress (resulting from mismatch of the coefficients of thermal expansion on the ceramic and metal phase) and the pertinent material properties: adhesion, hardness, toughness, friction, and wear resistance will be presented.
E2-8 Tribological Performance of Sprayed WC/Co Coatings
Y.R. Liu, Y. Qiao, T.E. Fischer (Stevens Institute of Technology)
The work presented is part of a research program aimed at exploring the possible advantages of nanometer scale microstructures in WC/Co composite coatings. Several laboratories have prepared coatings by High Velocity Oxygen Fuel (HVOF) or High-Energy Plasma Spray (HEPS) methods and used either commercial (Metco 2004) or nanostructured (Nanocarb) powders as feed. Microstructure, mechanical properties and abrasion resistance and unlubricated sliding resistance of these coatings were investigated. Their performances were also compared to that of sintered bulk WC/Co with WC grain sizes ranging from 4 micrometers to 70 nanometer. The results are as follows: The hardness of the coatings varies from 3 to 13GPA, which is much lower than that of sintered samples (10 to 23 GPA) because of the porosity of the former. Phase analysis by x-ray diffraction (XRD) revealed various amounts of decarburization in the coatings, which contain WC, W@sub2@C and W. Decarburization was coupled with loss of cobalt. The abrasive and sliding wear resistance is limited by the hardness of the samples. For a given hardness, the wear resistance is lowered by decarburization, which produces a hard but brittle phase. Nanocarb powders are more prone to decarburization than commercial material. This is due to the morphology of the powders and not to the size of the WC particles: Nanocarb powders have the shape of thin-walled hollow spheres that heat up rapidly in the gun. The work has shown that, in order to obtain the desired structure and properties of the coatings, the spray technique must be adapted to the powder used.
E2-9 Tribology of Sol-gel aAumina Coated Al Alloys
S. Wilson, H.M. Hawthorne (National Research Council, Canada); T. Troczynski, Q. Yang (University of British Columbia, Canada)
Alumina coatings, 50-60 micron thick (HV25g = 4.1-5.2 GPa), were deposited using a sol gel technique on 6061 Al and A356 Al alloys. Each was subjected to dry sliding wear tests against hard bearing steel balls (SAE 52100 steel, HV 8.9 GPa, 6.35 mm diameter.) and softer mild steel pins (AISI 1018 steel, HV 2.9 GPa) at different sliding speeds and contact loads. Coating performance against the two steels was compared using wear rate and damage maps constructed on load vs. sliding speed axes. Wear at low contact loads and sliding speeds was characterized by steel oxide transfer to coating surfaces which was more pronounced for wear against the mild steel. Coating damage by micro cracking was also observed and was more severe at higher contact loads and for wear against the harder bearing steel. An attempt has been made to correlate damage modes in sliding contact with that from controlled scratch experiments on the coatings.
E2-10 Analytical Modelling of Mechanical Contact Problems on Multilayer Systems
N. Schwarzer (Technical University of Chemnitz, Germany)
In the paper the author presents theoretical modelling of contact problems for the optimisation of the elastic mechanical parameters of the coating to the point where the stress distribution will never become critical within a given load range and various contact geometries. Utilising the method of image charges (well known from electrodynamics) in contact mechanics [1, 2] one obtains rather simple, strait forward and efficient procedures for the evaluation of the elastic field within a layered material. All stress and displacement components may be expressed in terms of elementary functions, when the corresponding solution for the homogeneous half space is known in such an elementary form, too. Because there exist a huge number of those solutions for various indenter shapes and load distributions the author can use a wide collection of known results to model relatively complex contact problems including shearing load components for the case of layered materials. The discussion will be illustrated by means of a fictive compound consisting of a pliant metal substrate and a suitable film which shall protect the substrate from plastic flow and avoid film cracking simultaneously. Utilising the theoretical procedure mentioned above for up to three layers an “optimal” coating will be proposed which shows a distinct depth dependent variation of the elastic parameters (Young’s modulus, Poisson’s ratio). Experimental results concerning this topic will be presented elsewhere .  N. Schwarzer, F. Richter, G. Hecht: Elastic Field in a Coated Half Space under Hertzian pressure distribution, J. of Surface & Coatings Technology (1999) 114, 292 - 304  N. Schwarzer: Arbitrary load distribution on a layered half space, ASME, Journal of Tribology, submitted  T. Chudoba, N. Schwarzer, F. Richter: “Determination of Mechanical Film Properties in a Bilayer System due to Elastic Intendation Measurement with a Spherical Indenter”, this conference
E2-11 Effect of Annealing Treatment and Fe@sub 2@O@sub 3@ Addition on the High Temperature Tribological Behavior of the Plasma Sprayed Zirconia Coating
D.S. Lim, J.H. Shin (Korea University, South Korea); H.-S. Ahn (Korea Institute of Science and Technology, South Korea)
Fe@sub 2@O@sub 3@ addition to the zirconia and annealing treatment after plasma spraying were performed to improve high temperature tribological behavior. The wear experiments were carried out using a reciprocating ring-on-disk tribometer at selected temperature in the range of 20~600@super o@C. The addition of Fe@sub 2@O@sub 3@ to the zirconia coating lowered the wear resistance and the coefficient of friction. This decrease is due to stabilization of tetragonal phase and the increase of toughness and microhardness. Minimum wear and friction was obtained with 5 mol% Fe@sub 2@O@sub 3@ added zirconia coatings. This result was explained by residual stress and microcracks created due to % Fe@sub 2@O@sub 3@ addition. Fe@sub 2@O@sub 3@ added plasma sprayed zirconia coatings were annealed at 500@super o@C up to 10 cycles to improve tribological performance. The annealing treatments decreased the wear rate due to the release of tensile stress. The wear rate of annealed and 5 mol% Fe@sub 2@O@sub 3@ added coating was decreased up to 40% and 50% compared untreated zirconia coating at room temperature and 400@super o@C, respectively.
E2-12 Multifunctional Co-sputtered Cermet Coatings
F.M. Kustas (Engineered Coatings, Inc.); B. Mishra, J. Zhou (Colorado School of Mines)
Co-sputtering of ceramic carbides & metals is a unique method to produce cermet coatings with an attractive set of mechanical properties, such as extremely high hardness and exceptional toughness. The flexibility of the co-deposition method enables the selection of materials that can provide more than one engineering function, such as hardness, toughness, oxidation protection, etc. Selection of materials for the cermet coatings was based on previous work for co-sputtered metals, where a large difference in atomic size and a large difference in melting temperature was suggested as a means of producing coatings with higher densities (and superior mechanical properties) and discrete reinforcements, respectively. Several different cermet coatings, consisting of materials with nearly comparable thermophysical properties (e.g., melting temperature and coefficient of thermal expansion) and others consisting of materials with significantly different properties, were co-sputter deposited for testing and characterization. These coatings include titanium carbide (TiC) & tungsten (W), silicon carbide (SiC) & molybdenum (Mo), and SiC & aluminum (Al). Coating composition, structure (from x-ray diffraction), scratch adhesion, hardness, and wear resistance are reported for these cermet coatings.