Ultrasonic Nondestructive Characterization For Properties Of Modification Layers Of Surface Materials Based On Spectral Analysis

Modification layers of materials surface were applied extensively in aerospace, petrifaction, micro-electronics and automobile manufacture engineering fields etc, ultrasonic nondestructive characterization for its properties, such as geometry scale, elastic modulus, bonding strength and residual stress etc, was very significant in quality evaluation of modification layers. Because modification layers thickness was small and included heterogeneous thin layers, ultrasonic signals from heterogeneity interfaces overlapped and interfered. Thus, ultrasonic characteristic parameters of modification layers were hard to be acquired. In the paper, one ultrasonic testing and signal processing system of pulse-echo with immersed focusing has been set-up for characterization of modification layers of materials surface, by optimizing ultrasonic testing method and designing experiment parameters based on ultrasonic propagation speciality in multi-layer. The crystal diameter of ultrasonic transducer was 7mm, center frequency was 10 MHz, and the curvature radius of acoustic lens was 5.5 mm, focus in water was 15.6 mm, displacement precision of transducer was 0.01 mm. In addition, computer was used to process ultrasonic signals by Origin software.The ultrasonic testing system was used to measure the thickness of ZrO2 ceramic coatings (270-400 μm) plasma sprayed on superalloy. According to spectral analysis for thickness measurement of multi-layer materials, the frequency interval △f of periodic minima in normalized power spectra was obtained. Based on the relationship (h=v/2△f) between acoustic velocity v and frequency interval △f , the thickness h of ZrO2 ceramic coatings can be calculated, and the coatings thickness of 400 μm, 339 μm, 277 μm accorded with those of metallographic method. This ultrasonic testing system was also used to test Cr2O3 ceramic coatings on stainless steel irradiated by high-intensity pulsed ion beam (HIPIB) at 300 A/cm2 with 1-10 shots, and the spectral analysis technique was adopted to process ultrasonic signals. With irradiation number increasing from one to ten,amplitude of normalized power spectra at 5.371 MHz, 6.836 MHz, 8.301 MHz has changed from bigness to smallness, and the amplitude undulation at low irradiation number was more serious than high irradiation number. The amplitude undulation of normalized power spectra accorded with surface roughness of modification layers of Cr2O3 coatings irradiated by HIPIB. The ultrasonic nondestructive characterization for thickness of surface coatings and properties of irradiated modification layers has contributed to the quality evaluation of modification layers of surface materials.