ICMCTF2007 Session B3: CVD Coatings and Technologies

The recent developments in the CVD process technology have made texture-control of α-Al₂O₃ realistic and feasible on an industrial scale. Consequently, the understanding of thermal barrier and cutting properties of these kinds of α-Al₂O₃ coatings is important.

The growth textures of CVD α-Al₂O₃ coatings can be predetermined by optimized nucleation and growth processes. The following preferred growth directions (textures) are 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 typically: <012>, <104> and <110>. The existence of the <001> texture has only recently been reported and it is not commercially utilized.

The aim of the present study is to evaluate the thermal barrier and wear properties of textured α-Al₂O₃ coatings. The following α-Al₂O₃ coatings were deposited for this purpose: <012>, <104> and <001>. A Ti(C,N) coating deposited to a same thickness as the alumina layers was used as a reference. X-ray diffraction was used to characterize and confirm the preferred growth orientations of the α-Al₂O₃ coatings. Orthogonal turning tests were carried out on a modern CNC lathe equipped with a dynamometer for cutting force measurements and IR-CCD camera for temperature measurements. The orthogonal turning was achieved through axial turning of tubular samples made of the work piece material (AISI 2140). The textured α-Al₂O₃ coatings exhibited clear diffences in wear resistance. Sustantial differences, for example, in crater wear resistance could be detected. Even though all the alumina layers were in general much better thermal barriers than the Ti(C,N) layer the differences in thermal barrier properties between the textured alumina layers appeared to be small.

The macroscopic modeling of CVD reactors has the ambition to pave the way towards the linking of the film properties to process parameters. The models used are based on thermodynamic, kinetic and transport databases and they involved thermodynamic, kinetic and heat and mass transfer calculations which can be linked between them.

This presentation describes specific examples of the application of databases and software packages to solve CVD modeling problems coming across the growth of materials apparently well known, like SiC single crystals or SrTiO₃ thin layers with high dielectric permittivity. The benefits of this kind of approach will be emphasized. In the same time, the dangers related to the use of: wrong data (even in good data bases), inaccurate phase diagrams, MOCVD precursors whose thermal decomposition is unknown are illustrated, showing that without high quality data, even the most advanced model can’t be of any value.

Ti_1-xAl_xN coatings are well-established coatings for cutting operations. Most important for superior cutting performance is a supersaturated solid solution of Al in the TiN lattice providing excellent wear- and oxidation-resistance. The growth of these metastable coatings is favored by non-equilibrium deposition techniques like physical vapor deposition (PVD) or plasma enhanced chemical vapor deposition (PECVD). Nevertheless, the formation of metastable structures has also been found in coatings produced by thermal CVD but only little work has been published and investigations on structure and mechanical properties are still missing.

In this work (Ti,Al)N coatings were deposited on cemented carbide between 550 and 700°C by thermal CVD using AlCl₃, TiCl₄, Ar, NH₃, and H₂ gas mixtures at atmospheric pressure with varying AlCl₃ partial pressures p(AlCl₃). Increasing p(AlCl₃) as well as deposition temperature result in a significant increase of the Al-content up to 35 at.%. Depending on the Al-content, X-ray diffraction analysis indicates the formation of single-phase Ti_1-xAl_xN coatings, or additional formation of fcc-AlN and hcp-AlN, respectively. The formation of both Ti_1-xAl_xN and fcc-AlN results in enhanced mechanical properties compared to single-phase and, particularly hcp-AlN containing coatings. Coating morphology as characterized on scanning electron microscopy cross-sections yields extremely fine structures. The results obtained demonstrate the feasibility of thermal CVD to deposit (Ti,Al)N coatings and provide a suitable basis for further developments.

CVD Ti(C,N)/alumina multilayers have been shown to be successful as wear resistant coatings on cemented carbide cutting tools. The alumina layers are generally deposited by conventional CVD, while the intermediate Ti(C,N) layers are deposited by moderate temperature chemical vapour deposition (MTCVD). Previous work has mainly been concentrated on the alumina layers, while this work focuses on the MTCVD Ti(C,N) layers.

For conventional CVD of Ti(C,N) the deposition temperature is approximately 1000°C. In MTCVD of Ti(C,N) more reactive organic precursors are used, which facilitates a lower deposition temperature at 700-900°C. This results in decarburisation of the cemented carbide substrate during deposition, which prevents formation of the brittle n-phase in the surface region.

Deposition of the investigated MTCVD coatings was carried out in a computer-controlled hot-wall CVD reactor. Commercial cemented carbide inserts with a composition of 94 % WC and 6 % Co were used as substrates. The Ti(C,N) coatings have a thickness of 8 µmm and three different types of doping are used: CO, CO₂ and AlCl₃.

The MTCVD Ti(C,N) coatings investigated in this work were pre-examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD was used to determine the texture coefficients and the SEM was used to study the surface morphology. The main part of this work was carried out by transmission electron microscopy (TEM) using a Phillips CM 200 FEG working at 200 kV. Cross-section TEM thin foils were prepared using a combined FIB/SEM (focused ion beam/scanning electron microscope) instrument, whereby electron transparency was achieved throughout the coating thickness. The detailed microstructure of the different coatings will be described in this work.