Fatigue strength of diamond coatings' interface assessed by inclined impact test

The inclined impact test is an efficient method for characterizing the fatigue strength of interfaces of nanocrystalline diamond coatings (NCD). During this test, an oscillating oblique load induces repetitive shear stresses into the region between film and its substrate. Inclined impact tests were conducted on NCD coated specimens with a thickness of ca. 5 μm at forces up to 850 N and 1.5 million loading cycles. The related imprints were evaluated by confocal microscopy measurements and EDX micro-analyses. Dependent on the applied load, after a certain number of impacts, damages in the film interface region develop resulting in coating detachment. In this way, residual stresses of the film are released leading to its lifting (bulge formation). The bulges are destroyed by further repetitive impacts and the coating is totally removed. The geometry of the developed NCD coating's bulges can be effectively described by appropriate Finite Element Method (FEM) supported calculations. Based on the attained impact test results, Woehler-like diagrams were developed for monitoring the fatigue failure or endurance of NCD coatings at various impact conditions. Employing such diagrams, the fatigue strength of NCD coatings can be assessed. According to the attained Woehler-like diagrams, a threshold load of 730 N exists for the film delamination by fatigue in the investigated case.