ALBERT

Notes: An electrical column explosion technique was proposed for the spraying of zirconium-diboride powder. For the study on its spraying mechanism and the feasibility of high-density coating, the explosion of a column specimen composed of an alumina tube containing the powder was examined under argon gas of 0.1 MPa with time-resolved measurements of the heating process of an exploding column, behavior of a discharge channel, and spraying of ZrB2 powder through column explosion. From these measurements, there is convincing evidence that the column explosion was started with electrical discharge of argon gas among ZrB2 particles inside the tube, and with sequential temperature increase of the particles to the melting point. Electrical discharge and rapid heating inside the column specimen during a few tens of μs were expected to initiate microcracks due to shocklike intense stress in the tube, and further to cause fracture of the column. In fact, another gas breakdown was induced outside the column, and it was followed by the formation of a cylindrical shock wave and an electrical discharge channel, and by their radial expansion. The fracture of the column was found at a later period of electrical discharge, and it caused the dispersion of lots of small fragments. Consequently, ZrB2 powder was ejected symmetrically. ZrB2-coated substrates were analyzed with a scanning electron microscope and an energy-dispersive x-ray spectroscope. It was confirmed that the coating resulted from the deposition through impacts of fine molten particles on the substrate. The substrate was covered completely with ZrB2 deposition through spraying only a few times. No pore was found under observation of several μm in size in the deposition layer and the layer-substrate interface. Hence, the ZrB2 spraying experiments presented have revealed the high feasibility of high-density coating of conductive refractory ceramics powder with this column explosion technique.

Notes: 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.