Surface roughness and sub-surface deformation measurements in machining of niobium

Single-phase niobium is used in superconducting accelerator components in particle accelerating facilities. Functionality of such accelerator components is placing high demands on the surface quality. Single-phase niobium, as opposed to niobium alloys, is a highly ductile metal, which in a cutting process normally leads to such surface defects as adhesion, built-up edges, side flow, and burr formation. In this study, six different tool materials: uncoated and two types of coated cemented carbide, PCBN, PCD and single-crystal diamond have been tested in longitudinal turning of niobium at different cutting parameters, in order to achieve the target value of Ra = 0.8 μm. Experimental studies involved finish machining with varied feed and cutting speed, while depth of cut was kept at ap = 0.25 mm. Apart from the resulting surface finish Ra and cutting forces, the following surface quality parameters were analysed: side-flow, adhesion, burr formation and sub-surface deformation. Only two of the tool materials, PCD and nanolaminate TiAlN/TiSiN coated cemented carbide, have shown acceptable surface integrity results.