Papers

ICMCTF1998 Session E2: Wear Resistance of Ceramic, Metallic and Composite Coatings

Tuesday, April 28, 1998 8:30 AM in Room California

Tuesday Morning

Start	Invited?	Item
8:30 AM		E2-1 Surface Engineering by Design - Modeling Surface Treatments for Improved Tirbological Perforamnce T. Bell (University of Birmingham, United Kingdom) Designers are requiring mechanical systems to perform in increasingly severe conditions, such as intensive loads, high speed and harsh environments; these conditions are challenging the limits of conventional materials. Increasingly, manufacturers are turning to surface treatments such as PVD coatings and plasma nitriding to answer these challenges. One limitation to rapidly deploying such advanced surface engineering technologies is a lack of design guidance. Coatings and substrate combinations are often selected haphazardly, because property-performance relationships are lacking. To address this problem, researchers are developing models for prediciting these relationships. The philosophy required to build such models is illustrated with an advanced contact mechanics model for prediciting performance of duplex surface treatments.
9:10 AM		E2-3 Tribological Design Concept of Coatings K. Kato (Tokoku University, Japan) The hardness, thickness and boundng strength of a coating on a substrate should be designed in relation to the elasticity and hardness of the substrate, and the hardness and elasticity should be always modified by considering the temperature distribution at the contact region. On the other hand, the change of coating thickness by successive wear must be always considered in relation to the resultant changes of stress and strain distributions. The possible transition of wear mode by the change of coating thickness must be considered in practice. In all those considerations, the mating surface against the coating should also be designed or carefully chosen. Those points of consideration are discussed and a recommendable way of coating design for tribological usage is proposed.
10:10 AM		E2-6 Modelling of Tribological Stresses in Coated Surfaces R.D. Arnell (University of Salford, United Kingdom) This paper briefly describes the normal contact of rough, homogeneous materials, and shows the importance of the plasticity index in determining the relative amounts of elastic and plastic contact of the contacting asperities. The discussion is then extended to include the effects on the near-surface stress fields of the tangential tractions which are present during sliding. A recent finite element model of the tribological stresses at coated surfaces is then described. The model ascribes realistic elastic/plastic/strain hardening properties to the coating, substrate, and counterface, and can be used to predict stresses between sliding asperities. Finally, potential developments of the model to allow its application to rough surfaces are described.
10:50 AM		E2-8 Interlayer Design - Hard Coats for Tribo-Protection S. Ramalingam, L. Zheng (University of Minnesota) Recent progress in PVD, CVD and diamond film deposition processing has led to wider use of thin films for tribo-protection. Coatings used for wear protection are the mono nitrides and carbides of transition metals, alumina, multi-layer films of one or more of these and/or diamond films including DLC films. In virtually every instance, the elastic constant Ec of the hard coating is higher than that of the substrate Es on which the film is deposited. Poisson's ratio's and coefficient of thermal expansions also differ. When hard coats are used to protect high strength alloys of Al and Ti elastic mismatch, expressed in terms of the Es/Ec ratio can range below 0.1. In high stress tribo-contacts where the hard coats can confer excellent wear protection in a number of hostile environment applications, the mismatch in elastic constants at the film-substrate interface can result in high film flexure, shear and lift-off stresses at the interface. Propensity for coating failure can become unacceptably high, especially in closed tribo-systems (unlike tool coatings which function in open tribo-systems). One way to avoid such problems is to raise the hard coat film thickness. A less expensive approach is possible by interposing a pre-selected metal or alloy layer of preferred thickness so that the mismatch stresses rendered less hazardous. An interlayer material can be chosen to provide a less hazardous mismatch between the hard coat and the interlayer, and the interlayer and the substrate. This two degree of freedom interlayr design problem for a given hard coat-substrate pair is addressed in this presentation for complete and incomplete tribological contacts. Analytical methods useful for interlayer design will be presented and discussed. They will be supplemented with FEM calculations (in the plastic range) so that the coating designed can be characterized with indentation testing.
11:30 AM		E2-10 TiN Coating Wear Mechanisms in Dry Sliding Contact Against High Speed Steel S. Wilson, A.T. Alpas (University of Windsor, Canada) Titanium nitride (TiN) coatings deposited on cutting tool and material surfaces susceptible to wear, help to extend the operating life and range of conditions for which they are used. However, correlation of their tribological performance with coating mechanical and microstructural properties remains a challenge, given that sliding contact and attendant friction heating effects at different loads and speeds, impose a large range of thermomechanical and tribochemical phenomena which are difficult to quantify. In this paper, a wear map approach is adopted to evaluate the tribological behaviour of a TiN coating (3 micron thick) physically vapour deposited (PVD) onto M2 tool steel. Dry sliding wear tests were conducted on TiN disc specimens against M2 steel pins, using a pin-on-disc sliding configuration at different sliding speeds (0.1-5.0 m/s) and contact loads (0.1-500N). Scanning electron microscopy, energy dispersion specroscopy and X-Ray analysis techniques were employed to characterise modes and mechanisms of material removal. Coating wear rates and transitions in wear behaviour are summarised on contact load versus sliding speed axes in a similar manner to other wear map investigations ¹ ^, ². The effects of sliding speed and load on wear mechanisms and transitions are discussed in terms of coating thermomechanical and tribochemical behaviour. ¹ S.Wilson and A.T. Alpas, Wear Mechanism Maps for Metal Matrix Composites,Wear, (1997) in press. ² S.Wilson and A.T. Alpas, Effect of Temperature and Sliding Velocity on TiN coating Wear" Surface and Coatings Technology, accepted for publication (July 1997).
11:50 AM		E2-11 Tribological Improvement of Bulk and Electroplated Nickel by Implantation of Titanium and Carbon M.T. Dugger, S.M. Myers, J.A. Knapp, D.M. Follstaedt, T.R. Christenson (Sandia National Laboratories) Ion implantation of titanium and carbon into nickel results in substantial improvements in the strength and wear resistance of the nickel surface. These benefits are present in pure polycrystalline nickel as well as fine-grained electrodeposited nickel used to fabricate small machine parts. Optical profilometry was used to examine surface topography after sliding wear. The wear rate during unlubricated sliding against hardened steel is reduced for bulk nickel and for electrodeposited material relative to the unimplanted materials. The friction coefficient is also reduced by this treatment, due to a change in the interaction between the sliding bodies from predominantly adhesive to abrasive. These changes in tribological behavior are associated with changes in the microstructure and mechanical properties of the implanted material. The implantation treatment creates a fully amorphous layer at the surface, where titanium and carbon concentrations are 16 and 22 atomic percent, respectively. The material below the amorphous layer is slightly enriched in Ti and C, and is crystalline but highly dislocated. Finite element modeling of nanoindentation load versus depth measurements for this layered structure indicates a maximum flow stress of about 5 GPa in the amorphous material, compared to 0.15 GPa in the bulk. Changes in tribological behavior are discussed in terms of wear theories and the observed changes in mechanical properties of the implanted materials. We hypothesize that in addition to the wear enhancement due to strengthening of the surface, that a difference in the composition of the surface impacts the onset of adhesive interactions between the implanted material and the counterface.
12:10 PM		E2-12 Tribology of Nitrogen Implanted Silicon S.J. Bull, S.V. Hainsworth (University of Newcastle, United Kingdom) Ion implantation has been used to modify the surface and near-surface properties of materials for some time now and its use to improve the wear resistance of metallic components is well documented. The effects of ion implantation on the wear performance of ceramics and coatings is much less well understood. Although it has been shown that ceramics can be hardened by ion implantation and crack suppression and changes in friction are also possible there is little or no long-term data available to determine if these changes in mechanical properties have a beneficial effect on wear performance. In this study we have investigated the effect of 50kV nitrogen ion implantation on the wear performance of {100} and {111} oriented single crystal silicon in the dose range 5x10¹³ to 6x10¹⁷ ions cm^-2. The samples were characterised by scanning electron microscopy, nanoindentation and scratch testing prior to reciprocating wear testing against either a steel sphere or a diamond cone. There was no change in hardness after ion implantation except for the very highest dose sample when nitrogen bubble formation occurred turning the surface layer into a honeycomb which is softer than the original material. However, there is a considerable reduction in the wear of the silicon in reciprocating wear tests against a Rockwell C diamond slider at 10N normal load due to the suppression of fracture in and around the wear track. This arises due to the presence of compressive stresses induced by the implantation treatment, but also is also due to reductions in surface tractions produced by changes in friction after implantation.