Abstract
An automatic scratch tester with an acoustic emission (AE) detector has been developed by the CSEM-Centre Suisse d'electronique et de microtechnique S.A.-(former LSRH), it is an automatic device intended to test the mechanical strength-adhesion and intrinsic cohesion-of hard, brittle coatings formed by chemical vapor deposition (CVD) or physical vapor deposition (PVD) on softer, tougher substrates. The test consists in scratching the surface of a coated substrate specimen with a smoothly rounded diamond point. The load applied at the point is increased continuously as the point moves on the surface, and a piezoelectric accelerometer is used to detect the AE produced as the coating is damaged. Preliminary scratch test results are shown using PVD TiN-coated cemented carbide testpieces. The influence on critical load (Lc) of coating thickness, deposition rate and coating pressure is studied. Coating pressure is varied from a lower end of 8×10-4 Torr to 2×10-3 Torr. Coating thickness is changed by the distance of the testpieces from the evaporator crucible. In case of higher coating pressure, critical load increases with coating thickness in the range of 1-4μm, however, coatings thicker than 6μm showed a sudden fall in the critical load value. This suggests that a critical thickness or deposition rate exists for adhesion in PVD TiN coating. In case of lower pressure, reproducibility of the Lc value was rather poor and at thicknesses of 1-8μm there was no apparent difference in the critical load observed.
