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–BCl3 post-discharge. The ZrB2 growth mechanism consists of a diffusion step of zirconium from the substrate to the gas–film interface. Zirconium atoms react with boron chlorides to synthesize a zirconium diboride thin film. Zirconium chlorides, which are by-products of this reaction, are evacuated by the gas flow. Consequently, a substrate weight loss is noted during the ZrB2 film growth. ZrCl4 addition in the reactive gas mixture significantly lowers this weight loss by decreasing the rate of zirconium chloride formation. The zirconium diboride growth mechanism occurs even if no zirconium tetrachloride is introduced in the reactor. However, the use of a ZrCl4 flow rate modifies the film composition. Indeed, oxygen concentration in the coating 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 homogeneous gas phase reaction between zirconium tetrachloride and the oxygen-containing molecules. The use of a ZrCl4 flow rate improves the adhesion of the films. Tin accumulation at the film–substrate interface, which is induced by diffusion of zirconium atoms, slows down when ZrCl4 is added to the gas mixture. Thus, the use of a zirconium tetrachloride flow enables the synthesis of thicker ZrB2 films without the coating peeling off.