Surface Hardening of Titanium Articles With Titanium Boride Layers and its Effects on Substrate Shape and Surface Texture

There is widespread interest in engineering improved properties into the surface layer of manufactured articles. One method for doing so involves a novel boriding process that creates hardened surface layers by the growth of a dual layer $TiB2+TiB$ coating on titanium articles. The objective of the present work was to demonstrate the fundamental feasibility of this process by producing uniform thick boride coating layers on titanium articles and to polish them to a very fine surface texture suitable for biomedical implant bearing surfaces. A powder pack diffusion boriding process was used to grow dual layer $TiB2+TiB$ coatings on simple shapes. Lapping processes were used to polish the borided articles. Evaluation was carried out using measurements of surface texture, geometric form, and hardness, and by metallurgical analysis. Boriding on as-received titanium articles resulted in shape distortion that hampered the subsequent polishing efforts. Hence, further articles were treated with stress-relief annealing prior to boriding, at temperatures below and above the $β$-transus of the substrate article. Annealing itself caused some form distortion, which was eliminated by lapping. Then, after boriding the annealed articles, varying surface textures and shape distortions were observed. Articles annealed above the $β$-transus had surface textures with significant peak-to-valley roughness $(14–55μm)$⁠, and the texture appeared to be patterned upon the substrate microstructure. However, form distortion seemed to be alleviated. For articles annealed below the $β$-transus, form distortion was not alleviated, and the articles exhibited wavy surface textures with a high peak-to-valley roughness (up to $50μm$⁠). Whether combined or independent, the surface texture changes and shape distortion that occurred during boriding thwarted the polishing processes; the articles could not be uniformly polished to a roughness less than $0.05μm$ within the coating thickness. To achieve uniformly polished dual layer $TiB2+TiB$ surfaces on titanium articles using the pack boriding technique, it appears that the substrate raw materials should be free of residual stresses and consist of a fine microstructure.