Metallographic observations on the wear process of TiN-coated cutting tools

The wear process of are-evaporated TiN coatings on cutting tools of AISI M2 high speed steel was studied during uninterrupted cutting of an annealed low alloyed tool steel, AISI O2. Wear on rake and flank faces of coated and uncoated tools was documented by metallography at various cutting times. With coated tools, wear on the rake face is initiated by the formation of superficial cracks in the coating, parallel to the cutting edge. Adhesion of work material at the cracks then leads to attrition of the TiN layer, and damage progresses by propagation of the cracks and further attrition. Superimposed on this process is a continuous contribution of abrasive wear (microabrasion), which is particularly strong at the flank face. The reduction in the wear rate caused by coatings is ascribed primarily to a more favourable contact geometry. This effect persists even after coating breakthrough, in that the remaining coating at the tool edge keeps the built-up edge small, producing thin chips which flow rapidly over the tool surface. The resultant strong heating of the contact area in the breakthrough zone leads to the formation of a flow layer in which a large part of the shear deformation of the chip is concentrated. The flow layer acts as a high temperature lubricating him, prolonging the protection of the substrate even after partial removal of the coating. Uncoated tools do not form a flow layer in their crater area.