Random multi-phase medium model and its application in analysis of ultrasonic propagation characteristics for AlSi-polyester abradable seal coating

Abstract

Ultrasonic nondestructive characterization of multi-phase heterogeneous coatings is challenging due to the complex elastic spatial distribution. In this paper, a two-dimensional Random Multi-phase Medium Model (RMMM) for heterogeneous coatings containing solid-solid-gas three phases is developed, which is able to simulate the elastic spatial distribution of coatings by taking into account material microstructure. This model is then applied to correlate ultrasonic wave velocity and attenuation coefficient with component contents and microstructural characteristics. Microscopic observations were performed for AlSi-polyester abradable seal coatings to examine the morphology and content of AlSi, polyester, and pore. Further statistical analysis of the autocorrelation lengths, orientation angle, mean value, standard deviation, and roughness factor of the coating density distributions was carried out. These statistical characteristics provided quantitative constraints on the construction of a random medium, which were then taken as the basis to establish the RMMM by virtue of a bidirectional peak-valley searching algorithm. The RMMMs for AlSi-polyester abradable seal coating with a thickness of 1.0 mm, polyester content of 47.6%–44.1%, porosity of 0.3%–4.0% were presented. Based on these RMMMs, the effects of component contents, sizes, and spatial distributions on velocity and attenuation were analyzed using two-dimensional Finite-Difference Time-Domain (2D-FDTD) method. Results indicate that when porosity increases from 0.3% to 4.0%, the longitudinal wave velocity decreases from 2251 m/s to 2150 m/s whose relative variation ratio is about 4.5%, while the attenuation coefficient increases from 4.45 dB/mm to 6.56 dB/mm whose relative variation ratio is up to 47.4%. The simulated results show a good consistency with those of the experiments. The modeling method is expected to be effective for the precise interpretation of ultrasonic propagation in other multi-phase heterogeneous coatings.