Surface Hardening of Ceramic Oxide and Metal Alloy Single Crystals by Ultrasonic Cavitation
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James R. Brewster, Y. Powell-Friend and L. A. Boatner Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056
Abstract Cavitation effects have long been considered to be undesirable phenomena that resulted in the damage and failure of metallic components. In the present work, we establish that, in fact, controlled applications of ultrasonic cavitation phenomena can be used to enhance the surface properties of both ceramics and metals. Polished (100) surfaces of single-crystal MgO and single crystals of 70%Fe-15%Ni-15%Cr (stainless-steel) were subjected to ultrasonically induced cavitation by exposure to 20 kHz excitations (at - 100 W/cm 2 ) in isopropanol. Knoop micro-indentation hardness measurements on untreated and ultrasonically treated areas of the surfaces revealed a hardening that increased with the duration of the ultrasonic treatment up to a saturation level. Relative increases in the surface hardness of up to 30% in the case of MgO and -250% in the case of Fe-Ni-Cr were obtained. It was found that the rate of hardening was not uniform over the surface but was more rapid on those portions of the surface that were directly under the edge of the ultrasonically vibrating hom tip.
Introduction The damage of solid surfaces by the collapse of cavities in liquids is considered to be a major deleterious effect that has accordingly been comprehensively studied. The most important cases of damage involve applications in which fluids are subjected to high rates of shear such as the erosion of propellor blades and high-speed lubricated bearings. Here we present an opposite approach to this phenomenon, that is, the application of cavitation phenomena, as induced by ultrasound, to enhance rather than damage the surface properties of both metals and ceramics. Ultrasonically induced cavitation has been used in the past as a research tool for gaining fundamental insight into the nature of cavitation erosion (1) and for the selection of materials for applications in high-cavitation environments (2). The nature of the microstructural changes that take place when the surface of a metal single crystal is exposed to hydrodynamically generated cavitation has also been studied (3). In studies of this nature, changes in the hardness of the surface were only a side issue to the major effect of interest , i.e. the rate of damage through the removal of material from the surface. The present approach to these phenomena, is to arrest the ultrasonic treatment short of that which would cause any material removal in order to produce a controlled, localized hardening of the surface In the present experiments, the material surfaces were exposed to cavitation produced by a 375 w, 20 kHz ultrasonic generator driving a 13 mm diameter titanium tip. The vibrating face was placed parallel to the specimen surface at a distance of 0.5 mm. The liquid medium in all of the experiments was isopropanol. The specimens were single crystals of 70%Fe-15%Ni-15%Cr, an analog of 300 series stainless-steels, and crystals of MgO. Since bo
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