Biocompatibility and Surface Integrity of Zirconia Ceramics Treated by Laser-plasma Driven Shock-waves

This is a multi-disciplinary paper focused on the preliminary results of deploying laser shock treatment (LST) to ZrO₂ ceramics. This work has significance in several industrial sectors for components that will benefit from strengthening. These components are, namely: dental implants; cutting and drawing tools; valves, bearings, pressure vessels, heat exchangers and high-performance scissors and knives, where modification of hard, brittle materials properties such as that of a ZrO₂, can yield a performance boost. To elucidate the influence of LST on ZrO₂ ceramics, an Nd: YAG laser was used, exhibiting (operating at discrete) laser energies of 17 mJ, 85 mJ and 170 mJ, a 2 mm spot size, a pulse repetition rate of 5 Hz and pulse duration of 10 ns was deployed at 532 nm wavelength. Investigation of ZrO₂ ceramic surface integrity revealed a transition from a crack-free topology to a surface dominated by fractures, as the laser energy increased from 85 mJ to 170 mJ. Residual stresses obtained by incremental hole drilling (IHD) were measured to be tensile in the upper layer and the sub-surface layer. However, compression of -595 MPa was found in the sub-surface of the ZrO₂ ceramic to a depth of 350 µm after LST, particularly in the transverse direction. Biological analysis metabolic activity measurements, indicated a rise in activity for all samples as the contact time increased (24 h, 3 d and 7 d). The as-received surface revealed a more limited change in metabolism, retaining similar biochemical levels during the first 3 days followed by a slight increase after a week of cultivation, bringing about significant increase in activity in all LST surfaces, relative to the as-received condition. This suggested that a ZrO₂ LST exhibited enhanced cell response, particularly, after 7 days of contact-time. The overall outcome of this introductory paper, not only showed that ZrO₂ ceramics, can be laser shock treated/peened, to induce some possible beneficial mechanical and physical effects, microstructural and surface topography changes, but also showed that LST can facilitate improvement in cell response to the ZrO₂ ceramic. This opens-up, new prospects for treatment of ZrO₂ based ceramics for biological applications such as tooth implant screws, where avoiding fractures from mechanical loading, strength enhancement, as well as biocompatibility are all important.