Experimental study on “ductile-brittle” transition in micro-grinding of single crystal sapphire

Micro grinding is catching an increasing concern in manufacturing industries from scholars all over the world. The researches on “Ductile-brittle” transition in micro-grinding of brittle materials are important to the machining results, and this transition process could be affected by various factors, such as micro grit-material interaction, grinding force, material crystal structure, and so on. In this study, a series of scratching tests are carried out by two different apex angle indenters to analyze the effect caused by grain shape or indenter shape, corresponding machined surfaces, and scratching forces are observed and measured. It is found that scratched surface of obtuse indenter is almost intact and there are smooth fish bone shape apophyses on it; scratched surface of acute indenter is severely cracked. In respect of scratching force, force generated by obtuse indenter is much lower and much more stable than acute indenter. Based on the above differences, a new grinding force model considering grain shape effect in micro-grinding is built; micro grinding experiments conducted by different grain sizes micro grinding tools were designed and carried out to verify the proposed grinding force model. Three machining modes during the grinding experiment are defined; grinding force and quality of the machined surface are observed and compared. Rough critical undeformed chip thickness for different machining modes are given, when grain size of grinding tool is 800 #, h m ≤ 0.265 nm or 500 # h m ≤ 0.8 nm, it is ductile mode; tool 800 #, 0.265 nm < h m ≤ 1.35 nm or 500 # 1 nm ≤ h m ≤ 4 nm, it is ductile-brittle mode; tool 800 #, h m ≥ 2 nm or 500 # h m ≥ 5 nm, it is brittle mode. According to the experimental results, it is found that small grain size grinding tool is easier to generate a worse surface quality and a big crack size. Crack size is rising with the increasing of feeding velocity, and is decreasing with the rising of rotation speed. The conclusions made in this study provide a theoretical reference and experimental basis for ductile zone micro-grinding of hard and brittle materials.