AL2O3-COATED CEMENTED CARBIDES - OPTIMIZATION OF STRUCTURE, NUMBER OF LAYERS AND TYPE OF INTERLAYER

The effect of the Al2O3 Coating structure, the number of layers and the type of interlayer on steel and cast iron machining was investigated. The coatings were applied to cemented carbides. First, coatings of single, double and triple layers of Al2O3, all backed by a TiN interlayer, were compared. Secondly, coatings of a single layer of Al2O3 backed by TiN, TiC and TiCN were compared. Thirdly, coatings of a single layer of Al2O3 backed by an oxidized TiC layer were compared by varying the TiC oxidation intensity. The results show that the application of a coating causes a major strength decrease, the larger the number of layers, the greater is the strength decrease. Double and triple Al2O3 layers did not show consistently improved machining results compared with those for a single layer. It was found that an uncontrolled Al2O3 microstructure nucleates on TiN, TiC and TiCN, leading to inconsistent machining results. The oxidation intensity of the TiC layer prior to Al2O3 deposition controls the adhesion, structure and grain size of the Al2O3 layer. Optimum machining results for steel and cast iron were obtained with a single layer of fine-grained alpha-Al2O3 backed by an oxidized TiC layer, which also had a desirable black coating colour.