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 R-a = 0.8 mu m. Experimental studies involved finish machining with varied feed and cutting speed, while depth of cut was kept at a(p) = 0.25 mm. Apart from the resulting surface finish R-a 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 TiA1N/TiSiN coated cemented carbide, have shown acceptable surface integrity results. (C) 2018 The Authors. Published by Elsevier Ltd.