Laser surface nitriding of Ti6Al4V alloy coupled with an external stress field
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We report on an approach for laser surface nitriding of Ti6Al4V alloy coupled with an applied stress field. A surprising finding was that, with increasing the applied stress levels, the decreased residual stress, the nitrogen concentration near the surface, and the surface microhardness of the nitrided layer were associated with the increased friction coefficient. Across the depth of the nitrided layer, the hardness, the elastic modulus, and the wear resistance (H/E) measured by nanoindentation decreased gradually and were ascribed to the gradient of nitrogen concentration in the melt zone. I. INTRODUCTION 1
Since the pioneering work of Katayama et al. in 1983, laser gas nitriding of titanium alloys has been extensively investigated with the aim of producing a hardened surface layer and thus improving the tribological properties.2–6 Compared with the conventional solidstate nitriding methods such as chemical vapor deposition, plasma nitriding, and physical vapor deposition, laser gas nitriding offers several advantages such as a high nitrogen concentration, fast treatment, accurate position control, and without any undesired heating effect on substrate through the reaction between a laser molten top layer and nitrogen. Nevertheless, avoiding surface cracking is still a critical problem in laser nitriding processing due to the predominantly tensile residual stress being generated within the processed layer over the last decades.7 The roughness and hardness of the surface layer can cause a concentration of tensile stress, which leads to perpendicular cracking when the critical value is exceeded during the cooling process.8 Previous studies have shown that adding some chemical elements such as nickel and chromium within the laser nitrided layer are of benefit to preventing the formation of crack according to the report of Fu and Batchelor.9 In addition, preheating the substrate material to reduce the cooling rate or diluting the nitrogen environment can reduce the tendency of cracking.10–13 However, preheating large work pieces is not convenient in industry and both surface hardness and wear resistance decrease significantly when the mixture of argon and nitrogen have been used.3 In fact, diffusion and convection during the nitrogen take-up and the formation of titanium nitrides will affect a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0050
344
http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 2, Feb 2010 Downloaded: 19 Mar 2015
the microstructure and properties of the laser nitrided layer.14,15 Previous research16 indicates that mechanical stresses will alter the activation energy for diffusion and thus affect the process of atom migration in solid matter. In this regard, it is worthwhile to investigate the effect of external applied stress field on the quality of the laser gas nitrided layer and thus provide an alternative for laser surface nitriding of Ti6Al4V alloy. II. EXPERIMENTAL
Conventional Ti6Al4V alloy (Ti64) with dimensions 50 mm 10 mm 5 m
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