Surface state and wear of cutting tools

Due to the heavy deformation of the material machined several constituents like C, Si, Mg, Mn, S and others are squeeze out of the crystal lattice and deposit on the clearance and rake face of the cutting tool. In contact with air these elements may oxidize. With lamellar and vermicular graphite iron Si-O-Fe-Mn-S and Si-O-Fe layers, respectively, were formed. With globular grey cast iron Mg-Fe-O-C depositions dominated. At the beginning the layers may cover the whole tool-work piece contact area. But, with increasing wear, cutting forces and temperature the layers tend to move towards regions with lower cutting pressure. According to the brutto composition of the layers compounds with a relatively low melting point or with good deformability at cutting temperature may form which might reduce (up to a critical load) friction between tool and work piece or chip. The formation of a continuous "lubrication film" is more likely on smooth surfaces than on rough ones. But, the film does not necessarily protect the tool. Si-O-Fe films occurring in machining CGI were shown to consist of an amorphous SiO2 multi-layer containing sub-micron crystalline particles or iron or an iron compound/alloy. Molten or doughy "glass" containing small, abrasive particles and flowing across the tool surface under high pressure, at high temperature and with high speed may initiate chemo-mechanical polishing as observed with coated tools. A low melting compound in contact with uncoated cemented carbide may penetrate between the WC grains, result in disintegration of the WC skeleton, loss of whole grains and rapid wear. The complex conditions of film formation and the investigation of different wear states may contribute to the different, sometimes contradicting observations found in the literature.