Design of coatings to minimize tool crater wear

A quantitative study of the micromechanisms of crater during the high speed machining of an AISI 1045 grade steel with a tungsten carbide tool has confirmed that diffusional wear is the dominant mechanism of crater wear. Neutron activation analysis is used to determine the amount of tungsten dissolved in the chips as a function of cutting speed. The concentration of tungsten dissolved in the chips increases from a negligible amount (0.2 ppm) to as much as 10.5±0.5 ppm as the cutting speed is increased from 100 to 240 m min-1, whereas the contribution from mechanical wear as given by tungsten carbide content remains nearly constant at 0.8 ppm. The depth of penetration of tungsten into the chips machined at 150 m min-1 and 240 m min-1 respectively were determined using secondary-ion mass spectrometry analysis. The tungsten concentration distribution from the interface into the chip in each case is characteristic of a diffusion profile. Ultrafine grains (0.5 μm) observed in the secondary shear zone contribute to grain-boundary-enhanced diffusion of tungsten into the chips. Diffusional wear of tungsten carbide was decreased markedly by coating the tool with HfN. The thermodynamic potential for dissolution of HfN in steel as measured by the equilibrium solubility product of HfN at 1200δC in austenite is six orders of magnitude less than that of WC. Thus, the magnitude of the thermodynamic potential for dissolution of a coating into the workpiece at typical tool-workpiece interface temperatures is an important criterion in the design of a coating to minimize tool crater wear.