CVD TiC/alumina and TiN/alumina multilayer coatings grown on sapphire single crystals

Multilayers of TiC/α-Al2O3 and TiN/κ-Al2O3, consisting of three (1 mm thick) alumina layers separated by thin TiC or TiN (∼20 nm thick) layers, have been deposited onto c- and r-surfaces of single crystals of α-Al2O3 by chemical vapour deposition (CVD). The aim of this paper is to describe and compare the detailed microstructure of the different multilayer coatings by using transmission electron microscopy (TEM).The general microstructure of the alumina layers is very different when deposited onto different surfaces of α-Al2O3 single crystal substrates. On the c-surface the alumina layers grew evenly resulting in growth of single crystal layers of TiC or TiN and alumina throughout the coating. However, when deposited on the r-surface the alumina layers generally grow unevenly. The κ → α transformation occurs in TiN/κ-Al2O3 multilayer coatings when deposited on both c- and r-surface substrates. In each layer the transformation starts at the interfacial pores in the upper part and proceeds inwards. The inner layer starts to transform first before the transformation proceeds to the outer layers. The microstructure of the transformed α-Al2O3 layers is different compared to as-deposited α-Al2O3, e.g. several voids and dislocations are present within the transformed α-Al2O3 layers. No pores were observed within the as-deposited alumina layers while a small number of pores was observed at the interfaces below the TiC and TiN layers. However, linkage of pores was observed within the transformed α-Al2O3 layers. There are also pores present in the substrate below the innermost TiC and TiN layers. It is believed that they are formed due to chemical etching during the deposition of the TiC and TiN layers. Single crystal TiC/alumina and TiN/alumina layers grow epitaxially on the c-surface substrates with close-packed planes growing on close-packed planes. On the r-surface, epitaxy is present only at some rare locations in αr-sub-TiC/Al2O3. However, in αr-sub-TiN/Al2O3 no epitaxy was observed.