On the establishment of an appropriate method for evaluating the residual stresses after laser surface treatment of ZrO₂ and Si₃N₄ engineering ceramics

A new approach of predicting the residual stresses following a CO₂ and fibre laser surface treatment of Si₃N₄ and ZrO₂ engineering ceramics is proposed. The approach used a three-point bending test to predict the localized residual stresses present within the engineering ceramics by using condition of static equilibrium and bending moment. This allowed one to determine the local microscopic residual stresses present and an understanding of the stress state within the surface and through the cross section of the as-received and the laser irradiated engineering ceramics. The findings showed that both of the CO₂ and the fibre laser irradiated samples induced tensile residual stress into the engineering ceramics through the high temperature gradient that was inhibited by the laser surface treatments in comparison to the as-received samples of the two engineering ceramics. The results were more significant for the fibre laser surface treated ZrO₂ engineering ceramic in particular as opposed to theCO₂ laser radiated ZrO₂ engineering ceramic. It was postulated that the difference in the depth of energy absorbed by the fibre laser in comparison to the CO₂ laser as well as the temperature difference between the two lasers was inducing different level of thermal energy into the sub-surfaces of the ZrO₂ and the Si₃N₄ engineering ceramics. This inherently had generated variation in the residual stress induced within the bulk of the engineering ceramics as there was more expansion and contraction produced within the samples irradiated by the fibre laser as opposed to the CO₂ laser which produced lower processing temperature.