Multilayer cracking resistance in bending

Abstract

Abstract Degradation and failure of thin, hard coatings in tribological applications is often governed by crack initiation and growth. For this reason, cracking resistance is one of the most important coating materials properties. Multilayer coating structures are commonly assumed to have cracking resistances superior to those of conventional single layer coatings. Several studies support this assumption, but until today no reliable method that is capable of quantitatively comparing the cracking resistances of single and multilayer coatings has been presented. This work utilises a new experimental method for determination of cracking resistances of tribological vapour deposited coatings. Coated beams of high speed steel are subjected to four point bending while simultaneously detecting the applied load and the acoustic emission resulting from crack formation and growth. As the bending device can be inserted in a scanning electron microscope (SEM), it is also possible to observe the beams during bending. The SEM allows the high magnifications necessary to reveal cracks and observe subtle differences in cracking behaviour between, e.g. single layer and multilayer coatings. Several single layer and multilayer coatings, consisting of various combinations of evaporated Ti and TiN and sputtered NbN and TaN, are evaluated. The multilayer coatings all contain a large number of lamellae and are produced by repeated, alternating deposition of two of the materials. The different types of coatings are found to behave quite differently during the bending procedure, and it is shown that the multilayer coatings have indeed higher cracking resistance than the single layer coatings.