Spraying of the brittle ceramic zirconium diboride by a wire explosion technique

Abstract

A wire explosion technique was employed for the spraying of zirconium diboride, a high-melting-point, brittle ceramic. To study its spraying mechanism and the feasibility of high-density coating, the explosion of a ZrB2 ceramic wire was examined with time-resolved measurements of the apparent resistance of an exploding wire, expansion of a discharge channel, wire fragmentation, and gas flow around the wire. The explosion under argon gas of 0.1 MPa in base pressure was started with heating up to the melting point of ZrB2, and with sequential gas breakdown around the wire. The breakdown was followed by the formation of a cylindrical shock wave and an electrical discharge channel, and by their expansion. Wire breaks were found at a later period of the electrical discharge, and resulted in the fragmentation of the wire of less than 1 mm in size, and in the transformation of almost the whole wire into fine-molten particles. On the other hand, at the explosion under air of 13.3 Pa in base pressure no shock wave was found, but larger fragments of the wire and the transformation of about 50% of the wire volume into fine particles were observed. ZrB2 coated substrates were analyzed with x-ray diffraction and scanning electron microscopy. The spraying under the high-base-pressure condition provided a much denser coating than that under the low base pressure. However, a small number of cracks was found on the sprayed film surface, and it was assumed that their formation was induced due to the brittleness of ZrB2. These measurements have revealed the close relation of the wire explosion to the base pressure of surrounding gas, and, in particular, the result that the high-base-pressure explosion associated with a cylindrical shock wave could provide a high-density coating. In consideration of the existence of high-base-pressure gas and the structure of a shock wave and an associated discharge channel, it is suggested that adequate heating and high-temperature holding of the wire plays an important role in the spraying. The spraying of ZrB2 wires presented here has revealed the feasibility of high-density coating of refractory, brittle ceramics with the wire explosion technique.