Improvement of the hardness and wear resistance of (TiC, TiN)/Ti-6Al-4V surface-alloyed materials fabricated by high-ene
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RODUCTION
TITANIUM alloys, having excellent specific strength and stiffness, have been mainly used for structural and engine parts of ultrasonic airplanes, for materials for a petrochemical plant, and for surgical implants, but their applications to parts requiring wear resistance have been limited.[1,2,3] Thus, studies on the development of new advanced materials whose surface properties are enhanced over titanium alloys have been conducted to achieve surface hardening or surface alloying.[4,5,6] One of the recent research efforts in surface hardening is the fabrication of surface-alloyed materials by direct irradiation using high-energy heat sources such as a laser or electron beam.[7–11] Physical deposition techniques such as ion implantation[12] and plasma spray coating[13] and thermochemical surface treatments such as nitriding,[14] carburization, and boriding[15] have been used in order to improve the surface properties of titanium alloy substrates. The former techniques improve the resistance to indentation and deformation of the substrate material, but are prone to surface damage and interfacial separation under repeated loading conditions, as well as due to the shallow thickness of the fabricated layer.[16] In the latter cases, fabricated at high temperatures, a relatively thick nitride or carbide layer can be made, but the substrate may be twisted or the surface properties may deteriorate because of the high fabrication temperatures (750 ⬚C
to 900 ⬚C). To overcome these shortcomings, surface alloying has been performed by depositing ceramic powders such as TiC,[17] TiB2,[17,18] and SiC[7] on a titanium alloy substrate and by irradiating it with a high-energy laser or electron beam. Upon irradiation, the ceramic powders and the substrate surface are melted, and then titanium-based surfacealloyed materials can be fabricated, as ceramic elements penetrate into the substrate and form hard precipitates. Particularly, irradiation with a high-energy (in the range of several megavolts) electron beam has several advantages: (1) it results in a strong interface between a surface-alloyed layer and a substrate, (2) it has little influence on the substrate properties because of the short irradiation time, and (3) it results in homogeneous heating and cooling. Compared with the laser-beam method, this method has twice the thermal efficiency and produces a thicker surface-alloyed layer, although it requires the use of a high-voltage electron accelerator.[19] In this study, a simple process is suggested to fabricate Ti-6Al-4V surface-alloyed materials by evenly depositing TiC or TiN powders on a Ti-6Al-4V alloy substrate and then irradiating it with a high-energy electron beam. Flux was used to protect these ceramic powders from air and to promote homogeneous melting. Three kinds of surface-alloyed materials were fabricated by varying the ceramic powders, in order to comparatively analyze the microstructural modification and the variations in hardness and wear resistance. II. EXPERIMENTAL
JUN CHEOL OH, Research Assistan
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