The role of hard coatings in carbide milling tools

The role of hard coatings in prolonging the life of cemented carbide tools is now well‐documented for continuous turning applications. In contrast, relatively few studies have elucidated the effect of such coatings in milling, where both mechanical and thermal cyclic loading impose special requirements on the tool cutting edge. This investigation describes the wear mechanisms observed in coated carbide milling tools which have been the object of recent commercial tool development. Coated carbide tools (by CVD and PVD) of varying cutting edge geometry, substrate, and coating composition were subjected to fly‐cutter milling tests under a nominal range of cutting conditions. Examination of the used cutting edges under optical and electron microscopy showed both thermal‐fatigue‐induced vertical cracks, perpendicular to the cutting edge, as well as lateral cracks parallel to the edge which result from mechanical cyclic stresses. The wear patterns and the sequential progression of these cracks were characterized on the used tools. It is noted that the coatings tend to inhibit crack propagation rather than reduce crack initiation and serve to delay the onset of typical failure mechanisms found in uncoated tools. The fracture toughness of the cutting edge is more critical in milling than in turning applications. A discussion is given on matching the requirements of edge geometry, material properties of the substrate, and the hard coating to optimize the performance of the coated carbide milling insert.