Tribological Applications of Shape Memory and Superelastic Effects
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Tribological Applications of Shape Memory and Superelastic Effects Wangyang Ni1,2, Yang-Tse Cheng1, and David S. Grummon2 1Materials and Processes Laboratory, General Motors Research and Development Center, Warren, Michigan 2Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan. ABSTRACT We provide an overview of our recent studies on novel tribological applications of shape memory and superelastic effects: (i) the use of shape memory NiTi alloys as self- healing surfaces, and (ii) the use of the superelastic NiTi as an interlayer between a hard coating and a soft substrate to improve interfacial adhesion, decrease friction coefficient, and improve wear resistance. 1. INTRODUCTION NiTi alloys are well-known for the shape memory and superelastic effect [1]. Since the first discovery of shape memory effect in NiTi in 1960’s, there have been extensive research efforts on both the fundamental understanding and industrial applications of NiTi alloys. Recent years have seen an increasing interest in its wear applications. For example, shape memory alloys exhibit desirable wear properties under cavitation erosion [2-4] and dry sliding wear conditions [5-12]. Our recent studies, summarized in this paper, show that the shape memory alloys may be used in the following two new ways for tribological applications: (i) self- healing surfaces based on the shape memory effect, and (ii) superelastic interlayers between hard coating and soft substrate to reduce friction coefficient and wear loss. 2. SELF-HEALING SURFACES In many applications, contact induced surface damages are unavoidable. It is thus highly desirable that a tribological system can detect and heal such damages automatically. The shape memory property offers a possible means to achieve self- healing in tribological systems. However, it was unknown whether shape memory effect exists at microscopic length scales and under complex loading conditions that are frequently encountered in tribological applications, although the shape memory effect was well established at macroscopic length scales and under simple loading conditions such as tensile, compression, or shear. To establish the basis for using shape memory materials as self- healing tribological surfaces, we have conducted a systematic investigation of the magnitude of shape recovery on the surfaces of a martensitic NiTi alloy using indentation and scratch tests. The indentation and scratch experiment can simulate an asperity contact under normal and tange ntial loading, respectively. Fig. 1 (a) shows the 3-D profiles of a spherical indent on the martensitic NiTi before and after recovery [13]. The recovery of the indent was induced by heating the specimen to a temperature above the austenite finish temperature. The specimen almost completely recovers to its original shape after being heated. The shape memory effect under uniaxial loading has been intensively studied. Here, we demonstrated that the shape memory effect also exists under indentation loading
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