Ultrasonic Nondestructive Characterization Of Density And Elastic Modulus For Surface Coating Based On Acoustic Pressure Reflection Coefficient Method

Ultrasonic characterization and evaluation for surface coating (SC) properties, such as thickness, density, longitudinal velocity, transverse velocity and elastic modulus, has become the hot issue of coating testing. The coating is so thin that the echoes from the front and back of the coating overlap, this factor limits the application of ultrasonic method in characterizing SC. Based on analysing the ultrasonic propagating in the interface between the water/coating and identifying the overlapped waveform, a novel method-the ultrasonic reflection coefficient method is proposed to determine the density and elastic modulus from the normal and oblique incidence pressure reflection coefficients.The primary principle of the ultrasonic reflection coefficient method is:according to the analyses of ultrasonic propagation in the water/coating/substrate structure, the pressure reflection coefficient of the coating can be calculated by simplifying the overlapped waveform and comparing the echoes reflected from coating and bulk specimens if the pulse duration is short enough under the condition of oblique incidence. Then, the density and elastic modulus of coating is determined according to the relations between the parameters of ultrasonic and coating properties. To check up the feasibility of this method, it was firstly applied to the homogeneous aluminum cladding on the steel substrate. Based on ultrasonic echo method and waterlogging coupling mode, when the ultrasonic incidents vertically, a probe with 8 mm diameter and 12 MHz central frequency was used to measure the pressure reflection coefficient. Moreover, a pair of probes with 10 mm diameter and pulse duration of 0.35μs were used to measure the pressure reflection coefficient at oblique incidence. Then the density and elastic modulus of the aluminum cladding were calculated to be 2.718 g/cm3 and 66.12 GPa respectively. The relative error in the measurement of density between the Archimedes method and the reflection coefficient method is 0.37% and the relative error of elastic modulus between the measurement result and values in reference is 2.76%. These results show the present method is available for measuring the density and elastic modulus of the coatings. On this basis, the measurement was carried out on the microwave absorbing coating containing carbonyl iron powder which has certain degree inhomogeneous. The density and elastic modulus are 4.281 g/cm3 and 17.32 GPa. The relative error in the measurement of density between the Archimedes method and the reflection coefficient method is 1.32%. Furthermore, the measurements were carried out on ZrO2-7%Y2O3 coatings with obvious inhomogeneous which were prepared by EB-PVD method, and partial specimens are irradiated by High-Intensity Pulsed Ion Beams (HIPIB) with the 1 and 5 shots. The densities of three kinds of coating are 4.900 g/cm3,4.903g/cm3 and 4.909 g/cm3, respectively, and elastic modulus are 31.28 GPa,31.90 GPa and32.73 GPa, respectively.The corresponding ranges of density and elastic moduli are similar to the results given in the reference and accord with the results from SEM and microhardness tests. It shows that the ultrasonic reflection coefficient method can be used to ultrasonic characterization for homogeneous and inhomogeneous coating/cladding density and elastic modulus, it offers a new way to nondestructive evaluate surface coating properties and quaility.