GOALI: Functional Superelastic Interlayers for Control of Friction and Wear In Hard-Coated Components

Michigan State University, East Lansing MI

Investigators

Abstract

Abstract 0510294 In a collaborative project between Michigan State University, the University of Michigan, and General Motors Corporation, the investigators are developing an adaptive functional multilayer tribological coating system having dramatically improved adhesion, extremely high wear resistance, and excellent resistance to damage from overload, thermal cycling, and high-cycle fatigue. The work is motivated by the fact that although the use of metal-oxy/nitride/carbon hard coatings is well-established for application to tool materials, much less progress has been made on efficient wear-coating systems for use in machine components. The functionality of machine parts often dictates the use of soft and compliant materials such as Mg, Ti, Zn, Al, Cu, Ag, or Au. These all have poor elastic/plastic support properties, and entail large compliance and CTE mismatches to potential hard coating materials. Transformational superelasticity is an isothermal form of the shape-memory effect. It is what gives NiTi eyeglass frames ('Flexon') their extremely high resilience. Applied as vacuum-deposited thermo-functional interlayers, NiTi thin films increase resilience, accommodate large mismatch strains, moderate contact stresses, reduce friction, and limit damage accumulation during large, high-cycle strain reversals in both processing and service. Some of the same mechanisms also limit the interfacial stresses that degrade adhesion, potentially allowing use of much thicker hard coatings (>10 microns) than has previously been thought possible. Measurements of tribological response by dry sliding wear, bend adhesion, scratch loading, and nanoindentation are being made under conditions of constant surface-chemistry and contact morphology. CrN, which is a low-cost coating having medium-high hardness, high stiffness, and very low thermal expansion, is being used as a model material for sliding contact against a hardened carbide ball. Aluminum alloy 6061-T6 is being used as a model substrate material. Experimental variables are the relative NiTiX/CrN layer thicknesses and topography, interlayer processing, load, and temperature. Tribological probes are augmented by many other techniques. Their various mechanical and tribological properties will be investigated. The solid mechanics thrust is focused on numerical modeling of interfacial shear, bearing, and compatibility stresses resulting from both processing and sliding contact loading. The emphasis is on predicting plastic strains in the substrate and strain distributions in the superelastic interlayer. The educational component involves undergraduates in the proposed work via summer internships and independent study projects. This continues our historically successful efforts to (1) challenge students from diverse groups with hands-on research, and (2) provide opportunity for advanced undergraduates to contribute to the literature, and to become competitive for NSF graduate research fellowships.