Abstract Tool life decreases considerably due to the softening of the surface layer of a tool caused by a high thermal load. In order to optimize the surface structure of tools, the influence of the thickness of a hard coating film on the plastic deformation of the tool was investigated systematically. The distribution of temperature and softening in the tool for backward extrusion processes were calculated by finite-element analysis. On the basis of the results, the plastic deformation resistance of the tool corner was analyzed under various tribological conditions including sliding velocity and friction, incorporating the presumed depth of the softened layers. To prevent the surface layer from deforming plastically, the following two points should be reflected in the design of the thermal-loading cycles and the surface structure of the tool. The first is that the thickness of the thermally softened layer under the hard layer should become less than that of the hard layer. The second is that if a thermally softened layer of about 1 mm thickness develops, the decrease in the deformation resistance of the softened layer should be within 20% of the deformation resistance of the bulk material.