Altering the wetting properties of orthopaedic titanium alloy (Ti–6Al–7Nb) using laser shock peening

This work focuses on exploiting the effects of laser shock peening (LSP) to control the wetting characteristics of bio-material surfaces integrated with surface characteristics such as surface energy, macro and nano-topography. In particular, the effects of laser energy and beam footprint overlap of LSP were explored on Ti–6Al–7Nb alloy, quantified by using the measurement of dynamic contact angle, followed by determination of the surface-free energy and the work of adhesion. Surface modification by LSP was conducted at laser energy of 3 J, 5 J, 7 J, & overlap of 33%, 50%, 67% at 3 mm laser spot diameter. An incremental hole drilling method was employed for near to surface residual stress measurement. The results showed that compressive residual stress of between −42MPa and −516 MPa were formed on the sub-surface of LSPned Ti–6Al–7Nb. The results showed that surface roughness, surface-free energy and work of adhesion were proportional to laser energy, contact angle, however, was found to be inversely proportional to laser energy at consistent overlap. Additionally, surface-free energy and work of adhesion are proportional to overlap, but surface roughness and contact angle have a negative correlation with overlap. The correlation between laser energy and contact angle can be explained by Wenzel's theory while the relationship between overlap and contact angle is described by Cassie-Baxter model. This investigation on effects of LSP on the wetting characteristics not only addresses the required parameters for cell response on LSP modified titanium alloys, but also identifies that a metallic material strengthening process such as laser shock peening can also modify the wettability of a solid metallic surface as well as benefit the mechanical properties of metallic implants.