Quantitative characterization of the interfacial roughness and thickness of inhomogeneous coatings based on ultrasonic reflection coefficient phase spectrum

Abstract

Aim at nondestructively characterizing the interfacial roughness of coatings, ultrasonic reflection coefficient phase spectrum (URCPS) as a function of interfacial roughness is derived based on the phase screen approximation theory [1]. For inhomogeneous coatings, the attenuation coefficient α(f) shows a non-negligible effect on the URCPS. The relationship of α(f) on frequency f is used to decouple the URCPS. The constructed URCPS is used to determine the interfacial roughness and thickness of specimens through a two-parameter inversion utilizing the cross-correlation algorithm. The effects of the coating inhomogeneity (such as porosity) on the roughness measurement are analyzed through numerical calculation. A series of simulations with interfacial roughness from 6.2 to 12.7 μm indicate that measurement errors of the thickness are all less than 8.0%. The relative errors of the measured roughness of models without porosity and models with the porosity of 3% are less than 11.8%. For the models with porosity 5%, when the roughness is larger than 10.3 μm, the relative error is still larger than 11.7%. Ultrasonic experiments were carried out on a tungsten carbide (WCNi) coating utilizing water immersion, flat transducer. The WC-Ni coating, with unknown interfacial roughness, was sprayed on a stainless steel using the high-velocity oxygen fuel (HVOF) method. Experimental results show that the interfacial roughness of specimen obtained by the proposed ultrasonic measurement are in good agreement with that of SEM observations, the absolute error of the measured roughness is less than 1.4 μm and the relative error was less than 11.0%.