Surface treatment of silicon nitride using contact-less energy beams

Abstract

This research investigated the surface treatment of Hot Pressed Silicon Nitride (HP Si₃N₄) predominantly using lasers, in order to enhance the surface properties, in particular the fracture toughness (K_1c). K_1c is a fundamental mechanical property that quantifies the resistance of the material to crack propagation. Empirical equations derived by previous workers were used to calculate the K_1c from Vickers hardness indentation test.

Conventional Shot Peening of metallic substrates generate beneficial surface compressive stresses by plastic deformation that enhances fatigue properties. Polycrystalline ceramics do not tolerate plastic deformation so the mechanism of mechanical shot peening cannot apply to ceramics. Literature in the field of laser surface treatment of ceramics has identified mechanisms of minimising surface cracks and porosity by softening and redistributing minor amorphous/ glassy phases to in-fill surface defects.

A number of empirical equations, derived by previous workers were assessed and one equation [Ponton 1989] in particular was identified as the most suitable. A low powered CO₂ (Carbon dioxide) laser, 2 KW, Nd: YAG (Niodinium, Yittrium, Aluminium Garnet) industrial laser assisted by an industrial Kuka robot, plasma augmented processing and ultrasonics was employed.

As machined (virgin surface) of HP Si₃N₄ and ground & polished surfaces were first examined for their hardness and the K_1c of the material was calculated prior to the surface treatment using the energy beams. It was found that an average of an 18 % increase was obtained in the materials hardness for the HP Si₃N₄ by grinding & polishing in comparison with the “as machined” surface.

Surface treatment using the CO₂ laser showed a decrease in the hardness by an average of 35 %. Nd: YAG treatment of HP Si₃N₄ also decreased the hardness by an average of 27 %. It was believed that a change in the materials composition occurred due to surface oxidation. It was believed that the treated area of the material was changed from Si₃N₄ to silicon dioxide. Crack lengths found after conducting the Vickers indentation test for Nd: YAG processing were much larger than the cracks obtained from CO₂ laser processing due to rapid thermal shocking which did not occur with CO₂ processing. This decreased the K_1c value of the samples treated by the Nd: YAG laser. Although, the value of K_1c obtained from the Nd: YAG laser treatment averaged up to 7.12 MPa √m and an average of 14 % of the samples showed an increase in the materials K_1c. CO₂ laser processing was found to be more successful in increasing the K_1c in comparison with other energy beams applied.

Date of Award	2008
Original language	English
Awarding Institution	Coventry University
Supervisor	Colin Page (Supervisor)