Title of project: A study on the evaluations of fatigue strength and materials integrity of hard surface coatings

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In the present study, fatigue strength and materials integrity of hard surface coatings have been evaluated. The coating material selected is a cermet (WC-12%Co) coating sprayed by high velocity oxygen fuel (HVOF) treatment. First rotating bending fatigue tests have been conducted at room temperature in laboratory air using specimens of medium carbon steel (S45C), low alloy steel (SCM435) and titanium alloy (Ti-6Al-4V) with the cermat sprayed coating and S45C with WFS sprayed coating of a 13Cr steel (SUS420J2). Plane bending fatigue tests were also conducted at stress ratios of 0, -0.5 and -1, The fatigue strength and fracture mechanisms were studied. Second, rolling contact fatigue of the WC-12%Co coated material was investigated on ring type plate specimen.Detailed observations of crack initiation and growth were performed and the results were compared with those obtained through stress distribution analysis performed by finite element method (FEM) and the crack initiation site was discussed, Third, inverse analysis of the velocity dispersion of laser excited surface acoustic wave (SAW) was used to estimate the elastic properties of three types of sprayed coatings, NiCrAlY, CrNiMoSiB and the WC-cermet. The obtained results were compared with those obtained by the mechanical methods of 3- or 4-point bending tests for the free standing specimen and/or sprayed specimen.Forth, with the aim of studying the effect of residual stress of the WC-cermet sprayed coating on fracture resistance, we evaluated the residual stresses of three coatings deposited by different spray conditions. AE monitoring and source characterization are utilized to study crack initiation and dynamics under quasi-static four-point bending condition.