Research On Ultrasonic Reflection And Scattering Mechanism And Characterization Of High Performance Heterogeneous Coatings

High performance coatings,such as thermal barrier coatings and wear-resistant coatings,are widely used in aeronautic,astronautic,and nuclear engineerings.Due to the heterogeneity of coatings caused by tiny pores and cracks,it is significant to develop the nondestructive testing(NDT)technology to accurately characterize and evaluate various properties(heterogeneity,elastic modulus,interfacial roughness,etc.)of the heterogeneous coatings,for development and application of the coatings.To overcome these questions,we will clarify the ultrasonic reflection and scattering mechanism in heterogenous coatings and establish the relationships between ultrasonic parameters and coating characteristics.Thermal spray technology or vapor deposition technology makes the structure and properties of coatings be distinct from those of the bulk materials,such as the effect caused by the thin thickness of the coatings,random distribution of tiny pores and cracks,specific microstructure with the growth in the certain direction and the coating attached to a substrate,which leads to many characteristic parameters(density,elastic modulus,velocity,etc.)of the coating are heterogeneous and fluctuation in a certain range.These factors make the characterization and evaluation of heterogeneous coating very difficult.In this paper,based on the wave-type conversion analysis,phase-screen approximation theory,and self-consistent theory,we establish the ultrasonic reflection coefficient amplitude spectrum(URCAS)of heterogeneous coatings by considering the effect of heterogeneity of coatings on the density,velocity,and attenuation coefficient.The theoretical relationships between the geometric features,interfacial roughness,microstructure features(porosity,pore size)and the spectrum characteristics are built.A two-parameter inversion utilizing the cross-correlation algorithm has been proposed for characterizing the properties of heterogeneous coatings based on the URCAS.On this basis,the effects on measurement results by testing conditions and material heterogeneity is analyzed through numerical simulation and experimental measurements.Finally,the validity of the proposed ultrasonic techniques is verified by experimental measurements.The main contents are as following:(1)A novel ultrasonic technique was proposed to characterize the heterogeneity of heterogenous coatings,which utilized the offset coefficient of resonant frequency(OCRF)of the URCAS.Ultrasonic simulations were carried out on seven sets of 7YSZ coating models with different porosities(0-25.12%).The simulation results showed that the OCRF ?decreased with porosity and pore size increasing.The values of ? were 1.000,0.998,0.996,0.986,0.978,0.964,and 0.963 corresponding to the models with porosities of 0,0.25%,1.00%,4.02%,9.04%,16.08%,and 25.12%,respectively.Ultrasonic experiments were carried out on the 7YSZ specimen utilized 50 MHz immersion focusing transducer.Experimental results showed that all the y of 7YSZ specimen fluctuated during 0.88?0.98,which had the same fluctuating law as the porosites.It was verified that the variation of y could be used to characterize the heterogeneity of the heterogenous coatings.(2)A nondestructive method for characterizing the elastic modulus and Poisson's ratio of heterogenous coatings was developed using an ultrasonic small angle incidence scheme in coatings with four layers and three interfaces.Through wave-type conversion analysis,the longitudinal and the transverse waves at the interfaces were simultaneously derived at an incident angle of 4.1°.The URCAS was used to determine the thickness and longitudinal wave velocity of specimens through a two-parameter inversion utilizing the cross-correlation algorithm.The elastic modulus and Poisson's ratio of coatings were calculated utilizing the inversion results and modified density.Ultrasonic experiments were carried out on four WC-Ni specimens sprayed using the high velocity oxygen fuel(HVOF)method at spray angles of 30°,45°,60°,and 90°.The thicknesses measured by the ultrasonic method were in good agreement with those observed by SEM with less than 9.3%error.The porosities determined from cross-section SEM photographs were 4.67%,1.76%,0.92%,and 0.19%,respectively with increasing spray angle.The elastic moduli of the specimens measured by the ultrasonic method were in the range of 315 GPa-351 GPa,and the Poisson's ratios were during 0.221?0.245.Metallurgical analysis indicated that increasing the spray angle increases both the density and the bonding strength of particles which leads to an enhancement of the elastic modulus of the coatings.The proposed ultrasonic method is valid for nondestructive characterization of the elastic modulus and Poisson's ratio of the coatings.(3)In order to nondestructively characterize the interface roughness of heterogenous coatings,the ultrasonic reflection coefficient amplitude spectrum(R-URCAS)involving interface roughness was derived utilizing the phase-screen approximation theory.The interface roughness was determined by a two-parameter inversion utilizing cross-correlation algorithm.The effects of ultrasonic wavelength,shape variations,and the heterogeneity of coatings on the roughness measurements were analyzed through numerical calculation.Ultrasonic experiments were carried out on standard roughness specimens and WC-Ni specimen utilizing water immersion transducer.The roughness Rq of the standard roughness specimens were 4.2?m,8.5 ?m,14.2 ?m,and 28.6 ?m measured by confocal laser scanning microscope(CLSM),respectively.Experimental results showed that the roughness of standard roughness specimens obtained by the proposed ultrasonic measurement were in good agreement with the CLSM observations,and the relative errors were less than 8.2%.Compared with metallographic method,the the absolute error of WC-Ni coating specimen determined by ultrasonic method was less than 1.4?m and the relative error was less than 11.0%.(4)The thermally grown oxide(TGO)growth at the interface of ceramic coating/bond coating in 7YSZ coatings was evaluated based on URCAS.A theoretical analysis was performed about the influence of acoustic impedance match relationship between the ceramic coating and its adjacent media on URCAS.The immersion ultrasonic pulse echo method was carried out on the 7YSZ specimen before and after oxidation under 1050?×1 h for 15 cycles.The resonant peaks of URCAS obtained before and after oxidation showed that TGO which generated between the ceramic coating and bond coating due to the oxidation,changed the acoustic impedance match between the ceramic coating and its adjacent media.The numerical calculation results indicated that the resonant frequency f2 shifted from 11.6 MHz for TGO thickness of 5.0 ?m to 11.9 MHz for TGO thickness of 8.0 ?m.This method can be use to nondestructively characterize the generation of TGO in TBCs,which is important to practical engineering application.