This paper aims to analyze the effect of the nature of the core material, the geometry of the buffer rods, as well as the nature and thickness of the cladding material on the ultrasonic propagation properties of these guidance components. The development of ultrasonic waveguides was studied in order to address the need for operation under strict conditions. The effects of the composition and thickness of the cladding as well as the type and geometry of the core were investigated with straight and tapered aluminum oxide and 1018 steel rods for the core and with plasma-sprayed aluminum oxide or aluminum oxide-chromium oxide for the coating. The results show that tapered plated buffer rods perform better than their unclad straight counterparts. They also indicate that the addition of chromium oxide to aluminum oxide is promising in a broader temperature range in which these conductors can be used. Paper includes a German-language abstract.

Ceramic-clad ceramic components were developed to address the need for buffer rods capable of operating under severe conditions. The buffer rods were produced using plasma spraying to build up a 500-nm thick layer of alumina on a solid alumina rod. Both conventional plasma spraying and high power plasma spraying techniques were employed to deposit the alumina coating directly onto a pre-roughened surface. Characterization of the resulting coatings indicated a level of porosity of several percent, substantially higher than that of the dense core. The room temperature wave propagation characteristics exhibited by these clad buffer rods were significantly improved over that of the unclad components.

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.