Spraying of zirconium-diboride powder by an electrical column explosion technique and its mechanism

Abstract

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.