Study on the Evaluation of Coating Adhesion and Strength of Metal Coated with Ceramic Thin Film

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The ceramic coating for various kinds of machine components improves the wear resistance, heat resistance, cutting ability or corrosion resistance. Coating technology on metals will be utilized more widely for various kinds of machine components and structures which require high functional properties. The aim of this investigation is to clarify the coating adhesion, strength of thin film and the effect of hard thin film coated on the fatigue behavior of metals. Scratch test, static tensile test, residual stresses measurement in film by the X-ray method, and load-controlled high-cycle and strain controlled low-cycle fatigue tests were conducted, using specimen of 0.37% carbon steel coated with TiN by physical vapor diposition (PVD) and chemical vapor diposition (CVD) methods. Main conclusions obtained in this study are as follows:1. The scratch adhesion of TiN film coated by PVD is higher than that coated by CVD. Coating failure modes in the scratch test are different in each other; PVD … More film is failed by chipping as a result of the bending of film and CVD film is spalled by buckling.2. From the static tensile tests, it was found that flaws on PVD and CVD coating films occurred at the total tensile strains of 0.40% and 0.87%, respectively, and that they increased with the strain. It has been shown that scratch adhesion and fracture of coating film under tensile strain does not share any features.3. The fatigue life of specimens coated is affected by the applied stress ratio and the strain amplitude level. This is due to the crack initiation behavior in the substrate induced by the rupture of the coating film. The fatigue life of coated specimen decreased as compared with that of an uncoated one, if coating film fails during fatigue process as the results of fatigue damage of film itself or accumulation of plastic deformation of substrate metal. Many cracks were induced at the substrate by flaws in the coating film. The increase in fatigue strength of the coated specimen was observed under the region of low total strain amplitude and under the stress ratio of -1, because crack initiation was delayed by the hard coating film on the specimen surface which acted as a barrier to the egress of dislocations.4. The rotating bending fatigue tests were conducted in air and in 3.0% NaCl solution using the pre-flawed coating specimens. The fatigue life of pre-flawed coating specimen remarkably decreased as compared with that of an uncoated specimen. It was explained by the observation of fracture surface and the estimation of fatigue life based on the small fatigue crack propagation law that many cracks occured along the flaw of film are coalesced in each other and grow to large one crack at the early stage of fatigue process. Less