Coating optimisation for high speed machining with advanced nanomechanical test methods

Abstract

Advanced nanomechanical testing has been used to evaluate key factors influencing tool life (1) a plasticity index (PI, the plastic work done/total work done during indentation), at room and elevated temperature (2) hot hardness and (3) fatigue fracture resistance, and determine their relative importance in different cutting applications. The optimum combination of hardness and toughness/plasticity to minimise wear and extend the life of coated WC–Co cutting tools was found to vary with the severity and nature of the cutting conditions. For interrupted cutting the plasticity index is critical, with high values (i.e. not extremely high H/ E) resulting in extended tool life. Elevated temperature nanoindentation showed decreasing hardness and increasing PI with temperature. In high-speed turning hot hardness is the dominant factor whilst for interrupted cutting high hot hardness should be combined with improved plasticity for longer tool life. A novel test technique nano-impact, was used to simulate the interrupted contact (and cyclic loading) conditions occurring in milling applications and evaluate the fatigue fracture resistance of coated tools. It was able to successfully rank coatings in terms of tool life in end milling and reproduce the evolution of tool wear in the cutting test. In elevated temperature nano-impact testing the probability and extent of fracture during the test decreased at elevated temperature, consistent with the higher PI. Results from the advanced nanomechanical tests can be used in combination to predict which coatings have longer life in severe cutting conditions.