Thickness Measurement Of YSZ Coatings On Nickel-based Superalloy Substrate With Ultrasonic Method

YSZ thermal barrier coating on the surface of nickel-based superalloy substrate with the method of electron beam physical vapor deposition method (EB-PVD) is widely used on hot end components of aero-engine. Coating thickness is a key quality index of thermal barrier coatings, it has affected service life, bond strength, and so on. Ultrasonic pulse echo technique has already been used as one of the most usual methods for non-destructive testing of coating thickness with the characteristics of simple, convenient and wide detection range. However, the nickel-based superalloy will formed dendritic segregation and solid solution strengthening in the preparation process, the non-uniformity of micro structure will affects the propagation of ultrasound, and thus affects the coating thickness measurement.In this paper, the anisotropy and non-uniformity of nickel-based superalloy substrate has been analyzed by means of ultrasonic pulse echo technique, ultrasonic C-scan equipment and EBSD technology, thereby analyzed the influence to acoustic pressure reflection coefficient spectrum, finally characterized the anisotropy and non-uniformity of substrate by deviation of dominant frequency, and optimal selected ultrasonic echo according to the deviation of dominant frequency of acoustic pressure reflection coefficient spectrum. The main contents are as follows:(1) The expression of the functional relation was deduced between acoustic pressure reflection coefficient amplitude and elastic modulus of substrate. The influence to velocity and echo amplitude of ultrasonic are analyzed from a theory angle.(2) Measured the YSZ coating thickness of specimen with nickel-based superalloy substrate by ultrasonic method, show that the maximum relative error of nickel-based superalloy substrates up to52.5%, the maximum standard deviation is28.2μm. However the results of stainless steel substrate specimen are stable and reliable, the maximum relative error is9.3%, and the maximum standard deviation is3.32, which means the anisotropy and non-uniformity of nickel-based superalloy substrate has a great influence on the coating thickness measurement;(3) Compared ultrasonic C-scan image with EBSD results, show that in the conditions of signal grain in the propagation direction of ultrasonic, echo amplitude of acoustic pressure reflection coefficient spectrum changes along with crystal orientation, and dominant frequency of amplitude spectrum keeps invariant; in the conditions of multiple grains in the propagation direction of ultrasonic, acoustic pressure reflection coefficient spectrum aliasing and dominant frequency deviate. Further study on ultrasonic testing of coating thickness by numerical simulation of CIVA, the relationship between crystal orientation of nickel-based superalloy and acoustic pressure reflection coefficient spectrum is consistent with experimental results.(4) Optimal selected the nickel-based superalloy substrate specimen with coating thickness of120,100,80,60μm using the deviation of dominant frequency of the acoustic pressure reflection coefficient amplitude spectrum, separately. The maximum absolute error is respectively11.5,9.7,7.5,7.69p,m and the maximum relative error is respectively9.6%,9.7%,9.5%,12.8%. The result of specimen of120,100,80μm meet the requirements of engineering application, and specimen of60μm has to be studied further.