Effect of ZrCl4 addition on ZrB2 film synthesis in flowing Ar-BCl3 post-discharges

Zirconium diboride films are synthesized on Zircaloy-4 substrates at 733 K by reactive chemical vapor deposition enhanced by a flowing microwave Ar-BC l3 post-discharge. The ZrB2 growth mechanism consists of a diffusion step o f zirconium from the substrate to the gas-film interface. Zirconium atoms r eact with boron chlorides to synthesize a zirconium diboride thin film. Zir conium chlorides, which are by-products of this reaction, are evacuated by the gas flow. Consequently, a substrate weight loss is noted during the ZrB 2 film growth. ZrCl4 addition in the reactive gas mixture significantly low ers this weight loss by decreasing the rate of zirconium chloride formation . The zirconium diboride growth mechanism occurs even if no zirconium tetra chloride is introduced in the reactor. However, the use of a ZrCl4 flow rat e modifies the film composition. Indeed, oxygen concentration in the coatin g is constant when no ZrCl4 is used. It decreases along the substrate when ZrCl4 is added to the reactive gas mixture. This decrease is due to a homog eneous gas phase reaction between zirconium tetrachloride and the oxygen-co ntaining molecules. The use of a ZrCl4 flow rate improves the adhesion of t he films. Tin accumulation at the film-substrate interface, which is induce d by diffusion of zirconium atoms, slows down when ZrCl4 is added to the ga s mixture. Thus, the use of a zirconium tetrachloride flow enables the synt hesis of thicker ZrB2 films without the coating peeling off. (C) 2000 Elsev ier Science B.V. All rights reserved.