In situ observation of the cracking behavior of TiN coating on 304 stainless steel subjected to tensile strain

The validity of a periodic cracking model capable of evaluating the interfacial shear strength of a ceramic film on metal substrate system was tested in a highly residual-stressed TiN coating-304 stainless-steel substrate system. In situ observation of the cracking behavior of the coating was conducted while the TiN-coated tensile specimen was being pulled inside the chamber of a scanning electron microscope. Four sequentially developed morphologies of the crack and/or inter-crack-spacing of the coating, i.e. multiplication, stabilization, cross-linking and spallation, were observed. Inter-crack-spacings of the fragmented coating were analyzed and identified with their respective initiation mechanisms, thereby testing the claims made by the model. Stringent criteria for selecting the characteristic inter-crack-spacing were articulated to improve the accuracy of the calculated maximum interfacial shear stress. Lastly, a modified periodic cracking model, which incorporates the contribution from residual stress and the new rule for selecting the characteristic inter-crack-spacing was presented. The interfacial shear strength of the subject system evaluated through the modified model is 0.65 +/- 0.06 GPa, which is of the order of magnitude of the substrate's yield shear strength. (C) 1999 Elsevier Science S.A. All rights reserved.