TiB whisker coating on titanium surfaces by solid-state diffusion: Synthesis, microstructure, and mechanical properties
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SURFACE coating is an effective way to engineer high hardness and wear resistance as well as resistance to oxidation and corrosion in metallic materials.[1] The most common way of increasing the surface hardness and wear resistance of steel, for example, is carburization, where a high volume fraction of Fe3C and alloy carbides such as Cr23C6, TiC, VC, and WC can be formed, depending on the alloy composition.[2] There is considerable interest in surface hardening of titanium (Ti) for many engineering applications. Ti and its alloys are characterized by their low density, high specific stiffness and strength levels, and excellent corrosion resistance compared to steel.[3] Although the major area of Ti application has been in the aerospace industry, which makes up about 80 pct of its usage, its level of use in chemical, petrochemical, and sports applications is increasing. However, the areas in which Ti alloys have been unable to replace steels and other hard metals are contact and wear-resistant applications where high surface hardness and wear resistance is required. The use of Ti and its alloys in these applications is largely limited by the poor wear resistance[4] and low oxidation resistance[5] of Ti, even at moderate temperatures of about 400 ⬚C. Therefore, it is of interest to explore ways of surface modification to improve the wear, oxidation, and corrosion resistance of Ti. S. AICH, formerly Graduate Student, Department of Metallurgical Engineering, the University of Utah, is Graduate Student with the Department of Mechanical Engineering, University of Nebraska–Lincoln, Lincoln, NE 68588-0656. K.S. RAVI CHANDRAN, Professor, is with the Department of Metallurgical Engineering, the University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] Manuscript submitted February 26, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
Titanium can be coated with TiN and TiC by ion implantation,[6] laser gas nitriding,[7] and electron-beam surface alloying.[8] A recent study[9] used an electron beam to remelt the TiB2 particles placed on Ti surfaces, forming a composite coating of Ti and TiB in the surface layers. The increases in Vickers hardness levels at the surface ranged from 300 to 800 kgf/mm2 (under a 300 g indentation load). Although the increase in hardness is substantial, these coating processes are relatively expensive, time consuming, and often involve costly and complicated equipment. In contrast, a solid-state process such as pack diffusion is a low-cost and simple process that has been applied to industrial carburizing, nitriding, and boriding of steels for several years.[2,10–13] If such low-cost, solid-state processes for Ti could be developed, then the application of surface-hardened Ti alloys in wear-resistant applications can be accelerated. The present study is concerned with the formation of surface coatings in which TiB whiskers are formed by solidstate diffusion of B in Ti in by pack diffusion. There is no prior study of this nature. Although boriding of Ti can be accom
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