Tribology of tool–chip interface and tool wear mechanisms

The tribological phenomenon at the tool–chip interface controls chip formation and tool wear. As metal cutting speed increases from low to moderate speeds, the tribological conditions at the tool–chip interface tend to change from sliding to seizure, giving rise progressively to partially segmented chips. Once seizure sets in, thermoplastic shear occurs in the secondary shear zone, raising the local temperature at the tool–chip contact. Thermally activated processes set in. Dissolution of the tool into the chip takes place by a diffusion mechanism, causing tool crater wear. The maximum depth of the crater is located at some distance away from the cutting edge of the tool. At higher cutting speeds chip segmentation is caused by thermoplastic shear localisation at the primary shear zone causing high temperature rise and rapid tool wear close to the cutting edge of the tool. The change in tribological phenomenon at the tool–chip interface as a function of cutting speed is discussed. Quantitative data are reported to show the onset of seizure in metal cutting and the consequences of seizure in promoting dissolution wear by diffusion mechanism. The calculated temperature distribution under seizure conditions is compared with measured crater depth profiles to show that crater wear is coupled with phase transformation. Under conditions of seizure, the role of coatings in suppressing dissolution wear is demonstrated in both ductile iron and low carbon steel using TiN and HfN, which are compounds having the least solubility in the workpiece (steel matrix).