CVD deposition and characterization of coloured Al 2O 3/ZrO 2 multilayers

Abstract

Coloured Al 2O 3/ZrO 2 multilayers have been deposited onto WC–Co based inserts by a CVD process. Through physical as well as optical analysis of such multilayers, colour is believed to originate from interference. The coatings are obtained with good process reproducibility. It was found that the ZrO 2 process used in the multilayer, with ZrCl 4 as the only metal chloride precursor, results in a mixture of tetragonal and monoclinic ZrO 2 phases. However by adding a relatively small amount of AlCl 3 during such a process results in ZrO 2 layers being composed of predominantly tetragonal ZrO 2 phase. Corresponding multilayers seem to have a more fine grained and smoother morphology whereas multilayers containing monoclinic ZrO 2 phase seem to be less perfect with existence of larger grains of ZrO 2 which are believed to scatter light and alter the reflectance of such a multilayer. In addition to this, such multilayers were found to be free of or with greatly reduced amount of thermal cracks, normally present in pure CVD grown Al 2O 3 layers. It is believed that, in the studied Al 2O 3/ZrO 2 multilayers, the observed tetragonal ZrO 2 phase is the result of a size effect, where small enough ZrO 2 crystallites energetically favor the tetragonal phase. However as the ZrO 2 crystallite size distribution is shifted to larger sizes it is believed that a mixture of crystallites with both stable and metastable tetragonal phases as well as a stable monoclinic phase is obtained. The proposed metastable tetragonal ZrO 2 phase may in fact explain the absence of thermal cracks in such multilayers through a transformation toughening mechanism, well known in ZrO 2 based ceramics.