Abstract

Ultrasonic propagation characteristics in thermal spray materials are affected by the complex microstructures of these coatings. Factors such as oxide stringers, porosity, splat size and morphology can all affect wave propagation. This dependence causes many concerns when one performs nondestructive evaluation of such coatings. However, we find that the unique properties of thermally sprayed coatings can be used for the fabrication of the cladding region of clad solid ultrasonic wave guides (or buffer rods) consisting of a core and a cladding. These wave guides are used as precious quality control devices for on-line monitoring of industrial materials processes. This paper is devoted to the tailoring of the ultrasonic behavior of metallic coatings produced by arcspraying. Experimental results show that by a proper control of the core-cladding system it is possible to minimize ultrasonic spurious signals occurring in a clad wave guide while the tailoring of the splat morphology, intersplat oxidation and porosity enable the appropriate ultrasonic absorption within the coating. The unwanted spurious signals are normally generated at the periphery of the core. Examples of the use of thick (up to 5 mm) stainless steel or bronze coatings as claddings on steel, aluminum or zirconium rods in order to provide good ultrasonic wave guides are given. Compared to a conventional non-clad rod the signal to noise ratio of such clad rods has been improved by more than 40 dB due to the reduction of the spurious signals. The potential applications of these clad buffer rods for thickness measurements and in-situ monitoring of materials at elevated temperatures are demonstrated.

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