Surface characterization and compositional evaluation of a fibre laser processed silicon nitride (Si3N4) engineering ceramic

Fibre laser surface treatment of a cold isostatic pressed (CIP) Si3N4 engineering ceramic was performed using various processing gas compositions to observe the changes on and within the surface of the engineering ceramic; in particular, surface topography, material removal, chemical composition, surface hardness and distribution of the heat affected zone (HAZ). Material removal, oxidation, morphological changes were found with all fibre laser irradiated samples as the surface decomposed. Surface finish and the material removal varied with changes in the gas composition. Fibre laser processing of the Si3N4 with N2 assist gas proved to be the most effective combination for effecting micro-structural changes as small sized elongated grains were found as a result of the faster cooling rate. Consequently, O2, ambient air and compressed air produced considerable amount of morphological changes. This was found to be due to material removal and de-composition of the upper most surface layer as well as the Si3N4 ceramic’s interaction with the assist gas compositions and the atmosphere at elevated temperatures. Maximum material removal was found when O2 assist gas was employed, taking in account of the O2 generating an exothermic reaction and consequently excessive heating. The compositional analysis revealed a chemical change occurring within all fibre laser irradiated surfaces of the Si3N4 engineering ceramic as fibre laser irradiated Si3N4 was transformed to SiO2.