Cracking and Adhesion of Ceramic Films
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CRACKING AND ADHESION OF CERAMIC FILMS ALAN ATKINSON AND RICHARD M GUPPY AEA Technology, Building 429, Harwell Laboratory, Didcot, OXIl ORA, UK. ABSTRACT Brittle films of ceramics on metal substrates become mechanically unstable when a critical thickness is exceeded. Experiments have been carried out to investigate this instability for films under tension and compression. Films of cerium oxide gel under tension were prepared by spin-coating a ceria sol onto stainless steel substrates. Films exceeding a critical thickness failed by throughfilm cracking. The relationship between crack spacing and film thickness was consistent with partial interfacial delamination accompanying film cracking. Films of nickel oxide under compression were prepared by the oxidation of nickel and NiA1 and NiCr alloys. The adhesion of the films to the substrates was measured in a double bending beam configuration and by scratch testing. A critical thickness was observed at which adhesion became very low and which depended on the composition of the metal substrate. The observed behaviour is consistent with a buckling instability in the film. In both types of film the conditions for crack propagation appear to determine instability, but the nature of the failure-initiating defects remains obscure. INTRODUCTION Ceramic films have a wide range of applications in optics, electronics, catalysis, tribology and corrosion protection. In all these applications it is essential that the ceramic remains adherent to the substrate in service and, in most applications, it is necessary for the film to be free of gross defects such as cracks. Therefore the mechanical stability of ceramic films is a matter of generic importance across the whole spectrum of their application. The film may fail mechanically either from intrinsic stresses originating in the film growth process or from externally applied stresses in service or testing. Intrinsic (or residual) stresses tend to approximate to uniform biaxial tension or compression acting in the plane of the film. Extrinsic stresses are often more complicated and their distribution is very sensitive to the way in which the external loads are applied. Extrinsic stresses rarely act alone, but are superimposed upon the intrinsic stresses. The concept of "adhesion" between the film and substrate may be expressed in several different ways. At its most basic level it may be regarded as the fracture energy, Gi, of the interface. Alternatively adhesion could be regarded as the strength of the interface in normal loading; which would be determined by both Gi and a population of flaw sizes. However, in practice the normal load must be applied in addition to the intrinsic stress. Thus the most easily measured practical "adhesion" is the externally applied normal load required to cause film detachment. In all cases the driving force for the failure of the film is the release of elastic energy stored within the system. Since the stored energy associated with the intrinsic stress increases with film thickness, thicker films are m
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