Evolution of residual stress in Ti–Al–Ta–N coatings on hard metal milling inserts

Abstract

Coated WC–Co hard metal milling inserts applied in milling application show thermal fatigue induced by interrupted tool-workpiece contact and wear as the two main damage mechanisms. Depending on the magnitudes of thermal and mechanical loads, either wear or thermal fatigue in form of combcracks may be dominant and determine the insert's lifetime. The present work illustrates the evolution of residual stress in an arc-evaporated Ti–Al–Ta–N coating for two different milling test setups, in one of which wear acted as the dominant damage mechanism. In the other test setup thermal fatigue fostered the formation of combcracks. Earlier work revealed a location on the tool's rake face, referred to as region of interest, with a significant buildup of tensile residual stresses in the WC phase of the substrate using synchrotron facilities. The residual stress state in the coating was determined in this region of interest by a cover method on the tools' rake faces after a defined number of cuts by X-ray diffraction using in-house facilities. In the wear dominated test setup, compressive residual stresses remained present until the end of tool life in coatings. Tensile residual stresses were found in coatings on inserts in which thermal fatigue was dominant.