Development and Kinetics of TiB 2 Layers on the Surface of Titanium Alloy by Superplastic Boronizing
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de (TiB2) is well known for its excellent properties such as high hardness, high melting point, high elastic modulus, and good wear resistance. Owing to these properties, it is mainly used for cutting tools, wear resistance, and corrosion resistance part.[1] Conventionally TiB2 can be produced by a variety of processing methods, including the sintering, hot pressing, hot isostatic pressing, microwave sintering, and dynamic compaction, in which the production cost for all of these methods is high. It is crucial to densify the TiB2 since the boride has relatively low self diffusion coefficient due to the relatively strong covalent bonding of the constituents and also because of the oxygen-rich layer (mainly TiO2 and B2O3) that easily form on the surface of Titanium.[1–3] A monolithic TiB2 with 98 pct density has been successfully obtained through the sintering process at high temperature, 2750 K (2477 °C) and about 3 GPa of pressure is needed in order to compact the boride.[4] Meanwhile, the TiB2 coating formed by a pack boronizing process is recognized as inexpensive and not a very complex process. In this coating process, generally dual boride layer consists of TiB and TiB2 are formed. Though, it is desirable only to have TiB2 layer NOR TAIBAH TAAZIM, Student, ISWADI JAUHARI, Senior Lecturer/Supervisor, and MOHD FAIZUL MOHD SABRI, Associate Professor, are with the Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. Contact e-mail: [email protected] YUKIO MIYASHITA, Associate Professor, is with the Department of Mechanical Engineering, Nagaoka University of Technology, 1503-1 Kamitomioka, 940-2188, Nagaoka, Niigata, Japan. Manuscript submitted June 25, 2015. Article published online February 18, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
since TiB is considered having much lower hardness.[5,6] Thus some of the boronizing techniques may produce only TiB2, however the boride layer thickness is relatively thin.[5,7,8] The reason is because TiB2 formation requires a high diffusion rate process. TiB2 will only be formed after the boride grows to saturate. According to the TiB phase diagram, the compound TiB2 may exists at 30.1-31.1 wt pct of boron compositions.[6,9] The laser boriding is an example of a process that produced only the TiB2 layer with the thickness layer in excess of 150 μm, without the formation of the TiB layer.[10] Superplasticity has been applied in the surface hardening processes of metals, especially steels—under the tensile mode, initial pressure, compression mode, and dual compression method conditions.[11–15] The initial pressure condition is considered the simplest method because it doesn’t require extensive equipment to perform—only a simple special designed clamp. Since the process involves the interaction between the solid powders and solid substrate, the principle of the initial pressure condition is almost similar to the diffusion bonding concept where during the initial stage of diffusion bonding, the asperities on each of the fayin
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