Recent developments in CVD deposition technology have made texture-control of α-Al₂O₃ realistic and feasible on an industrial scale and textured α-Al₂O₃ coatings are attracting increased attention due to the improved wear properties. Consequently, the better understanding of mechanical as well as thermal barrier properties of these kinds of coatings is important.

The growth textures of CVD α-Al₂O₃ coatings can be predetermined by optimized nucleation and growth processes. The following textures have been reported for α-Al₂O₃ in the patent literature: (012), (104), (110), (116), (300) and (001). In commercial α-Al₂O₃ coatings the most commonly occurring textures are: (012), and (104). The control of texture from (012) to (014) has resulted in substantial improvement of commercial products. The existence of the (001) texture with promising cutting properties has only recently been reported.

This presentation reviews the recent advances in the field of CVD of Al₂O₃. The development of the first generation CVD Al₂O₃ layers with poor coating uniformity and undefined crystal structures to modern α-Al₂O₃ coatings will be described. The latest developments in the area of texture control of α-Al₂O₃ will be treated in more detail. Examples of controlled deposition of (012), (110), (104) and (001) textured alumina layers will be shown. The performances of the textured alumina coatings in metal cutting will be compared and discussed. Results obtained by using XRD, TEM, SEM and practical cutting tests will be presented.

Today CVD-alumina is one of the most important components in modern coating systems for cutting tools. Commercial alumina coatings can be deposited with different structures. They vary in modification, texture and grain size. A main target in the last years was to get a better knowledge about correlations between texture and wear behaviour of Al₂O₃ layers.

In this work different commercial hard metal inserts coated with α- and κ-Al₂O₃ top layers are characterised extensively regarding to structure as well as physical and mechanical properties. At first the alumina layers are analysed concerning their modification, texture and grain size. Following microhardness, Young’s modulus, stress, thermal conductivity and the friction coefficient were determined for each individual structure. It was found that α- and κ-Al₂O₃-layers show distinct differences in microhardness and Young’s modulus. α-Al₂O₃ layers offer Vickers hardness up to 2800 HV[0.01] and Young’s modulus up to 480 GPa while κ-Al₂O₃-layers do not exceed hardness values of 2400 HV[0.01] and Young’s modulus of 370 GPa. Furthermore correlations between grain size and hardness as well as Young’s modulus are received. The lowest thermal conductivity is measured for <104> textured α- and κ-Al₂O₃-layers. All α-Al₂O₃ layers with other textures show a higher thermal conductivity. Based on the structure-property-relations received in this work application advices are given for an optimal application of cutting tools coated with alumina layers of different structures.

Today Ti_1-xAl_xN coatings represent a standard PVD coating for cutting tool applications. Fcc-Ti_1-xAl_xN is a metastable homogeneous phase with excellent properties as high hardness, high oxidation resistance and high cutting performance. An important aim is the preparation of coatings with a maximum aluminium content. The higher the aluminium content the higher the oxidation resistance. But PVD processes are limited to the deposition of monophase Ti_1-xAl_xN coatings with a maximum x = 0.65. Higher x leads to the co-deposition of the softer hexagonal wurtzite phase.

In this work the preparation of monophase fcc-Ti_1-xAl_xN coatings with x up to 0.9 is described. A LPCVD process using TiCl₄, AlCl₃, NH₃, H₂, N₂ and Ar is applied. At deposition temperatures between 800°C and 900°C very hard Ti_1-xAl_xN layers with 0.7 < x < 0.9 were prepared. The layers possess a high hardness around 3000 HV and Young’s moduli up to 540 GPa. A strong adherence on hardmetal inserts is achieved by deposition of a thin TiN bonding layer. Scratch test measurements showed critical loads of about 100 N. The stress in the coatings depends on the deposition conditions and varies between 650 MPa and -200 MPa. In different wear tests a high performance of the CVD-Ti_1-xAl_xN was observed. Coated inserts showed a reduced wear by 30% in comparison with a state-of-the-art PVD coating in reaming of steel and cast iron. In milling 42 CrMo4V the new CVD-Ti_1-xAl_xN also exceeds the best PVD coatings available on the market today.