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 radiated engineering ceramics. The findings showed that both of the CO2 and the fibre laser radiated samples induced tensile residual stress into the engineering ceramics through the high temperature gradient that was inhibited by the laser treatments in comparison to the as-received samples. The results were more significant for the fibre laser surface treated ZrO2 engineering ceramic in particular as opposed to the CO2 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 radiated by the fibre laser as opposed to the CO2 laser which produced lower processing temperature