Low temperature growth mechanism of zirconium diboride films synthesised in flowing microwave Ar-BCl3 post-discharges

Zirconium diboride films are synthesised at 460 degrees C by a flowing micr owave Ar-BCl3 post-discharge CVD process to coat Zircaloy-4 substrates. As a zirconium tetrachloride flow is of no use to synthesise these films, they are grown by a mechanism which consists of a diffusion step. To identify h ow this step occurs, characteristics of the films are compared with those t hat could be expected when three different diffusion processes are consider ed: boron diffusion, zirconium diffusion and BCl3 diffusion. As the last pr ocess requires a high porosity of the film (about 50%) to occur, it cannot be referred to describe the ZrB2 synthesis mechanism. The physico-chemical and structural characteristics of ZrB2 films do not help to distinguish the two processes involving a diffusion step in solid state. Only a profilomet ric study of the film growth direction has shown that zirconium diffuses in to the Zircaloy-4 substrate from the core to the solid-gas interface. This mechanism implies that zirconium tetrachloride is also synthesised at the s urface of the film. The influence of this mechanism on the behaviour of the Zircaloy-4 alloying elements has also been analysed. A precise description of the tin behaviour is provided and its consequences, on adhesion of ZrB2 films are examined. (C) 1999 Elsevier Science S.A. All rights reserved.