MULTILAYER CRACKING RESISTANCE IN BENDING

Degradation and failure of thin, hard coatings in tribological applica tions is often governed by crack initiation and growth. For this reaso n, 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 c oatings. Several studies support this assumption, but until today no r eliable method that is capable of quantitatively comparing the crackin g resistances of single and multilayer coatings has been presented. Th is work utilises a new experimental method for determination of cracki ng resistances of tribological vapour deposited coatings. Coated beams of high speed steel are subjected to four point bending while simulta neously detecting the applied load and the acoustic emission resulting from crack formation and growth. As the bending device can be inserte d in a scanning electron microscope (SEM), it is also possible to obse rve the beams during bending. The SEM allows the high magnifications n ecessary to reveal cracks and observe subtle differences in cracking b ehaviour between, e.g. single layer and multilayer coatings. Several s ingle layer and multilayer coatings, consisting of various combination s of evaporated Ti and TIN and sputtered NbN and TaN, are evaluated. T he 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 coat ings have indeed higher cracking resistance than the single layer coat ings. (C) 1997 Elsevier Science S.A.