The Principle And Experimental Research Of Ultrasonic Characterization For Layered Medium

Ultrasonic nondestructive characterization for the properties of modification layers of materials surface (such as geometry scale, elastic modulus, bonding strength and residual stress etc) has been widely studied recently. Since the thickness of modification layers is small and it includes heterogeneous thin layers, ultrasonic signals from heterogeneity interfaces overlapped and interfered which makes the ultrasonic parameters hard to be acquired. This paper solves wave fluctuant equation and gets the expression form of reflection coefficient when wave transmits in three layered structure as water/coating/substrate, based on the principle of ultrasonic transmits in multi-layered structure. The relationship between the minimum of the mode of ultrasonic reflection coefficient, ultrasonic velocity and coating depth is acquired. The changing of ultrasonic attenuation in the coatings can be analyzed when the velocity is calculated.Using this method, Cr₂O₃ ceramic coatings which were 50μm and sprayed on stainless steel were evaluated nondestructively. High-intensity pulsed ion beam (HIPIB) method was used to prepare three specimens which have different characterization. The reflection wave signal and the reflection coefficient spectrum of the coatings were obtained using ultrasonic immersed focusing method. The ultrasonic velocity was calculated and the relationships between ultrasonic attenuation coefficient and frequency of the coatings were compared. The results indicate that the velocity increase with the increasing of irradiation number, for the specimen with 0,1, 5 irradiation number the velocities are 2002m/s, 2099m/s and 2197m/s. The relationship between ultrasonic attenuation and frequency is α=Afⁿ, the values of A and n decrease with the increase of irradiation number. For the specimens with 0, 1, 5 irradiation number, the values of A and n are 0. 046, 0. 026, 0. 020 and 1. 702,1. 658, 1. 649. The total values of the attenuation decrease with the increasing of irradiation number. For the specimens with 0,1, 5 irradiation number, when the ultrasonic frequency is 20MHz, the attenuation values are 7.51Np/mm, 3.76Np/mm, and 2.86Np/mm. After analyzing the cross section SEM results of the ceramic coatings, it is founded that the decrease of the attenuation relates to the decrease of the porosity and micro cracks in the coatings. The ultrasonic nondestructive characterization for irradiated coatings has contributed to the quality evaluation of modification layers of surface materials.