Crack removal behavior in ultra-precision machining of sapphire

Processing ceramics like sapphire is challenging due to the brittle response exhibited during machining which poses a limitation on maximum material removal rate and throughput of the machining process. Due to this limitation, material has to be removed in small increments in order to create the required form and shape. This study investigates the material response during overlapping machining operation and whether existing surface cracks can be removed by taking advantage of the anisotropy of sapphire. Scribing cuts were performed on the A- and R-plane of sapphire at various depths and the results show that cracks can form during overlapping machining even when the depth of cut is below the critical depth of cut. Experiments on the R-plane of sapphire show that existing surface cracks can be removed by machining over the cracks in a different direction with higher critical depth of cut. Based on these results, development of effective machining strategy to produce crack free surface on sapphire is discussed.