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I. INTRODUCTION

WHENEVER dry sliding occurs, mechanical energy is transformed into heat through the surface. Volumetric processes also occur in and around the real area of contact. This frictional heating, and the thermal and thermomechanical phenomena associated with it, can significantly influence the tribological behavior of the sliding components.[1,2,3] Shape memory alloys exhibit properties that differ from those of conventional engineering materials. The shape memory effect,[4,5] superelasticity,[6,7] and damping effect[8,9] are the properties most closely linked to the martensitic transformation. This transformation can be caused by either temperature or stress. When friction occurs, temperature and stress are the two important parameters; they are more determinant parameters for shape memory alloys than in conventional materials. Superelasticity involves the production of a martensitic transformation when an external stress is applied. The only important difference between thermal and stress-induced martensite is that the plates being formed through stress are of just one or a small number of variants, instead of being members of a self-accommodating group.[10] The variant formed is the one whose corresponding strain most relaxes the applied tensile stress in a crystal whose length is kept constant, the same variant that for a given tensile stress gives the largest tensile strain. For copper-based shape memory alloys, the tension associated with stress-induced martensite is directly related to the transformation temperature Ms.[11,12,13] The bigger the transformation temperature Ms, the smaller the stress required to induce martensite. There is equivalence between temJ. PEÑA, Assistant Professor, is with the Dept. Ciencia de Materiales e Ingeniería Metalúrgica, ETSEIB, Universidad Politécnica de Cataluña, Diagonal 647, 08028-Barcelona, Spain, and the Dept. Materiales y Diseño, Elisava Escuela Superior de Diseño, Ample 11–13, 08002 Barcelona, Spain. F.J. GIL, Professor of Materials Science, is with the Dept. Ciencia de Materiales e Ingeniería Metalúrgica, ETSEIB, Universidad Politécnica de Cataluña, Diagonal 647, 08028-Barcelona, Spain. Contact e-mail: francesc.xavier. [email protected] J.M. GUILEMANY, Professor of Physical Metallurgy, Dept. Ingeniería Química y Metalurgia, Universidad de Barcelona, Martí i Franqués 1, 08028-Barcelona, Spain. Manuscript submitted September 21, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

perature and stress in the thermoelastic process of martensite formation: a decrease in temperature is equivalent to an increase in stress, both of which stabilize the martensite phase. Several authors[14–20] have shown that the NiTi alloy, which exhibits shape memory properties, is often more wear-resistant than conventional materials, such as some steels and Ni- and Co-based alloys. We have shown previously that CuZnAl shape memory alloys have a high dry wear resistance,  alloys proving to be more wear-resistant than the martensitic ones.[21] II. EXPERIMENTAL PROCEDURE Wear tests were per