Optimization of the Scratch Test for specific coating designs

The proper design of wear resistant coatings applied to cutting tools comprises the optimization of the mechanical properties (Young's modulus, yield strength, adhesion, intrinsic stresses, fracture, fretting etc.) of the coating-tool system. The goal is to find material and structural solutions which keep the resulting stress–strain field under typical application conditions below the stability limits of the system. Based on nanoindentation measurements obtained from the coating-tool system which should be optimized, a scratch test is dimensioned with respect to load range and indenter geometry. The measured data from this “Physical Scratch Test” are used to simulate spatial stress profiles and to calculate the von Mises stress characteristics and the maximum normal stresses in the scratch direction. In a further step, the simulations are used to suggest scratch parameters for a “Fine Tuned Scratch Test” which increase the sensitivity of the test for specific depth regions in the coating-tool architecture and allow improved and more sensitive investigations of critical interfaces, transition layers and surface-near substrate regions. The tests were performed at PVD coated inserts (nitrides and oxides) and compared with the results obtained from cutting tests. ► Dimensioning a scratch test increases its depth sensitivity significantly. ► Analytical model enables investigation of stress evolution during scratch tests. ► Linking critical stresses to failure moments takes scratch tests to physical level. ► Physical scratch tests help to understand failure mechanisms in applications.