Coaxial helical gas assisted laser water jet machining of SiC/SiC ceramic matrix composites

SiC/SiC ceramic matrix composites (CMCs) offer an excellent combination of properties at high temperature such as high specific strength, chemical inertness and irradiation tolerance. Those superior properties make CMCs beneficial for use in high-temperature structural applications that are exposed to extreme environments such as aerospace and nuclear energy. However, well machining qualities can hardly be achieved by conventional machining techniques owing to these properties. Laser water jet (LWJ) machining is a promising solution, which is capable of ablating materials with less/no heat defects, well machining precision and consistency. Nevertheless, the machining capacity of LWJ is still limited by the stability of water jet to a great extent. A water layer may form on substrate surface during the impingement of LWJ, which also sets up obstacle for sufficient ablation. Therefore, a novel coaxial helical gas atmosphere is introduced to promote the machining capacity of LWJ in this paper. A theoretical model is established to describe the gas-water two-phase flow field during the ejection and impingement of coaxial gas assisted LWJ (CGALWJ). The influences of gas component and pressure on the stable length of water jet and surface water layer status are analyzed based on numerical simulations and experiments. Scribing experiments are further carried out on CMCs substrates with thickness of 3 mm. Groove with maximum depth-to-width ratio of 13.6 as well as through cutting are realized without the drawing of SiC fibers, formation of recast layer and delamination. The theoretical and experimental results provide solid foundation for the high-quality machining of ceramic matrix composites and other hard-to-process materials.