Study chemical reaction of –Si–OH surface layer by solid and ionic form to surface quality when polishing with chemical–mechanical slurry

Abstract

The yttrium aluminium garnet (YAG) and sapphire materials are commonly used in laser and optical devices. Producing an ultra-precise surface quality is necessary for the application in optical devices. However, YAG and sapphire materials belong to difficult-to-machine materials with high brittleness and hardness. Therefore, ensuring the main criterion of producing a quality surface in the nanometre form with the ability to remove the material when finishing this material is challenging. Eliminating machining residues using chemical–mechanical slurry (CMS) is essential in creating ultra-precise components in optical devices. Based on this feature, this work investigates the efficiency of the CMS polishing process by comparing the surface reaction modes with the ionic and solid reaction modes when polishing YAG and sapphire materials. The study procedures aim to clarify the polishing performance corresponding to these two reaction types. Experimental analyses show the balance between CMS polishing technology’s mechanical and chemical effects with the ionic reaction model that can be generated. Results also show that the ionic surface reaction modes give more uniform material removal than the solid reaction on YAG and sapphire crystal surfaces. Therefore, when polished by CMS technology with ionic surface reaction modes, the surface quality is better than solid reaction. In the CMS polishing with ionic responses, adding more Na₂SiO₃–5H₂O content to the CMS is necessary if the chemical reaction is weak. This process will facilitate the –Si–OH components to be more distributed on the workpiece’s surface, thereby improving reaction speed and correcting the balance between mechanical and chemical in polishing by CMS. This balance action results in an ultra-smooth YAG crystal surface without scratches, with a roughness obtained in the nanometre form (Ra = 1.031 nm) after polishing by CMS-2.