Surface Mechanical Properties of Ti Alloys Produced by Excimer Laser Mixing of Ti on AISI 304 Stainless Steel
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SURFACE MECHANICAL PROPERTIES OF Ti ALLOYS PRODUCED BY EXCIMER LASER MIXING OF Ti ON AISI 304 STAINLESS STEEL T. R. JERVIS*, J-P. HIRVONEN*, M. NASTASI*, T. G. ZOCCO*, J. A. MARTIN**, G. M. PHARR***. AND W. C. OLIVER**** *Materials Science and Technology Division "**Chemistry and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545
***Department of Materials Science, Rice University, Houston, TX 77251 "****Metals and Ceramics Division,Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT We have investigated the properties of surface alloys of Ti formed by excimer laser melting and mixing of Ti overlayers on AISI 304 stainless steel substrates.The thickness of the initial Ti surface layers and the fluence and number of pulses used to mix the surface were varied to investigate a broad range of processing conditions. Samples were uniformly translated at different speeds under repetitive pulses of homogenized laser radiation at 308 nm to vary the number of pulses incident on the sample. We investigated the surface structure and composition, the wear and friction behavior, and the surface hardness of the resulting alloys. The extent of mixing is observed by Rutherford backscattering spectroscopy to be proportional to the number of pulses incident on the sample. Amorphous surface layers are formed at some mixing levels with a composition which indicates that Ti substitutes for Fe in the stainless steel on a one to one basis. Incorporation of C from surface and interface contamination is observed but at a level much below that found in comparable ion implanted materials. Friction coefficient measurements indicate that there exists an optimum level of mixing for each layer thickness. Excessive mixing degrades both the coefficient of friction and the wear performance in dry sliding tests. As with the tribological measurements, the surface hardness showed a dependence on amount of mixing. The surface hardness of the as deposited samples was higher than that of the laser mixed samples, initially decreased with mixing, and finally increased with further mixing. INTRODUCTION Surface processing techniques have been shown to alter the surface properties of materials as they relate to a number of applications including tribology. In particular, Ti ion implantation [I] and ion mixing of Ti/Fe multilayers[2] have been shown to dramatically improve the tribological properties of bulk materials which are normally not suitable for tribological applications. These techniques generally form surface layers with excellent adhesion to the substrate material. Melting and mixing of added surface layers using ultraviolet light from an excimer laser can quickly and efficiently process large areas of metal surfaces. The concentration of an added constituent can be chosen arbitrarily to meet the application requirements. Also, because the process itself is very fast, the mixing can be performed in air or in a shield gas environment, eliminating the time required for vacuum system pumpdown. The use of infrared (IR)
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