Laser-Assisted Surface Engineering of Silicon Nitride to Minimize the Surface Defects Generated During Grinding Operation: Part I

Abstract

Silicon Nitride (Si3N4), a structural ceramic, is being increasingly used for various applications such as bearings, rotors, valves, etc., due to its highly useful mechanical and physical properties. To achieve the desired close tolerance and better surface integrity for such applications, abrasive machining using diamond wheel grinding, is primarily used. Due to the extreme mechanical properties of the silicon nitride, sub-surface and surface defects like micro-cavities, micro-cracks, smeared areas and debris are formed on the ground surface of the component due to grinding. The final flexural strength of the components are jeopardized to a greater extent due these surface defects. In this research work, a novel process is developed and established to eliminate/minimize these surface defects. The process involves annealing of defects using continuous wave carbon-dioxide (CW CO2, λ = 10.6μm) laser radiation. The laser is applied as a radiation source to selectively soften and reflow the secondary non-Si3N4 phase material to fill/anneal the defects present in the surface and sub-surface. Experiments were performed at three different power densities and at a constant scanning speed in air ambient. Power densities and scanning speed were selected based on the output of the model. It is found during the investigation that the laser treatment reduces the surface roughness and increases the micro-hardness without any change in the surface stoichiometry. Also, the sub-surface and surface defects are minimized and the flexural strength, which is an important property for the related applications, is found to be increased after treatment. The detrimental influence of machining defects on the mechanical failure of the sample during the four-point bend testing was found to be significantly reduced.