CRACKING RESISTANCE OF THIN HARD COATINGS ESTIMATED BY 4-POINT BENDING

In this work, a fast and easily performed four-point bending test for evaluation of cracking resistance of thin hard coatings is presented. A bending device, small enough to be put in an SEM and observed in sit u, has been designed. By crack formation studies, a measure of coating cracking resistance is obtained. Two methods of crack detection are u tilised: detection of acoustic emission and direct observation in the SEM. The two: methods yield virtually identical values of the cracking resistance. However, the acoustic evaluation is much faster and easie r to perform and is therefore to be preferred in long test series. SEM observations, on the other hand, allow a more straightforward interpr etation. In this paper the test is used to determine the coating strai n corresponding to crack initiation in TiN and CrN coatings on high sp eed steel. The test yields values of cracking resistance for the coati ngs in the range 0.1% (TiN) to 0.7% (CrN). In the SEM studies, cracks were found to nucleate predominantly at defect sites in the coating an d propagate, highly aligned, perpendicular to the length of the beam. The cracks usually terminated when reaching the substrate. However, if hitting carbide at the substrate-coating interface, the crack also co ntinues through the carbide, and terminates when reaching the substrat e matrix.