Modification of fracture toughness parameter K_1c following CO₂ laser surface treatment of Si₃N₄ engineering ceramic

Surface treatment of a Si₃N₄ engineering ceramic assisted by a CO₂ laser was conducted to identify changes in the fracture toughness parameter K_1c. The K_1c was determined by using a Vickers macrohardness indentation method before and after the CO₂ laser surface treatment. Optical microscopy and infinite focus variation techniques were then adopted to measure the crack length and to investigate the surface morphology as well as the crack geometry within the engineering ceramic. Thereafter, computational and analytical methods were employed to calculate the K_1c. Chemical analysis was further conducted to elucidate the change in composition as a result of the CO₂ laser surface irradiation. The results showed that a decrease in the near surface hardness of 7 and 44% in the resulting crack lengths was found with the Si₃N₄ engineering ceramic following the CO₂ laser surface treatment. This inherently led to an increase in the K_1c. A rise in the K_1c of 64% for the Si₃N₄ engineering ceramics was found (under the applied conditions) in comparison to the as received surface. This occurred due to the Si₃N₄ engineering ceramic being oxidised and further forming a new surface layer, which was somewhat softer than that of the as received or laser unaffected surface. Compositional analysis showed that the formation of the new surface layer as a result of the CO₂ laser surface treatment was found to be SiO₂.