Strength measurements of thin brittle ZrO2 coatings produced by magnetron sputtering on steel substrates

Conventional techniques for determining mechanincal properties usually cannot be applied to ceramic, coatings because of the difficulties in handling thin brittle films with a thickness of about 5 mu m. However, analysis of crack spacing resulting from a four-point bending test of coating and substrate may reveal some mechanical coating properties of the coating such as tensile strength, Weibull modulus, adhesion and eventually the intrinsic thin film stress. To examine this ZrO2 stabilised by Y2O3 (YSZ) coatings were deposited by reactive magnetron sputtering on Inconel 617 substrates. These samples were subjected to tensile loading during four-point bending tests within a Scanning Electron Microscope. The crack spacing was initially examined and the load applied and subsequently increased in several steps up to 1.0 GPa. A simulation of the crack spacing, taking into account a finite adhesion of the coating on the substrate and stress relaxation in the vicinity of the cracks due to elastic and plastic deformation of coating and substrate, reveals the mechanical properties of the coating. Depending on deposition conditions and surface preparation, the Weibull modulus was m approximate to 2...6 and a fracture stress at the first failure approximate to 20-1500 MPa for the YSZ stabilized coatings depending on the residual compressive stress. The fracture toughness was generally K-IC<3.6 MPa m(1/2). (C) 1997 Elsevier Science Ltd.