ICMCTF2016 Session B2-1: CVD Coatings and Technologies

Ti(C,N) has many interesting properties, such as a high hardness and high temperature stability. It is a widely used material both as bulk material and in coatings, for instance in abrasive applications.

When used as a coating these properties may be connected to the texture and the microstructure of the coating. Therefore, it is important to control the coating process in such a way that the coating can be tailored towards the specific application. In order to achieve this, a greater understanding of how process parameters affect the growth of the coating is needed.

In this study MT-CVD Ti(C,N) coatings were deposited on polished WC-Co substrates from a gas mixture of TiCl₄, CH₃CN, H₂, N₂, and HCl. The variation of process parameters, such as gas phase composition and pressure had an effect on texture and microstructure. It was found that the deposition temperature, when varied between 760 and 910°C, had a small effect on microstructure and texture, but influenced the deposition rate. The introduction of a growth rate inhibiting gas on the other hand influenced the microstructure and texture.

The modifications made to alter the texture and microstructure had very little effect on the coating composition. With this in mind it can be concluded that the changed coating properties were not caused by a changed chemical composition.

In this work Ti(C,N) has been deposited on WC-Co (cemented carbide) cutting tools by the means of a moderate temperature CVD process (MT-CVD).

It is well known how to deposit such Ti(C,N) coating with preferential <211> texture with certain microstructure and composition. It is through exploring such references understood that the process itself i.e. the combination of a unique set of process parameters to a large extent controls all of these critical properties such as texture , microstructure and composition of such Ti(C,N) coatings.

However although this is true in many cases the role of the nucleation surface can be important as well. The present work combines this process knowledge by growing such Ti(C,N) coatings on tailored nucleation surfaces in order to control especially the texture of such Ti(C,N) coatings without changing the microstructure nor the composition of such coatings.

The coatings were studied by the means of electron microscopy, and x-ray diffraction.

The results of this work shows that it is possible to deposit different Ti(C,N) coatings using the same set of critical Ti(C,N) process parameters but by varying the nucleation surface.

Laser-CVD of alumina has been reported, where the deposition rate of 200-300 μm h^-1 and formation of alpha-phase at relatively low substrate temperature were achieved. We examined laser-CVD of alumina/TiN multilayer coating on Ni-bonded Ti(C,N) cermet. In the laser-CVD of TiN, a columnar TiN was formed on alumina substrates, but on Ni-bonded Ti(C,N) cermet substrates, the deposit contained a large amount of Ni-Ti, and the substrate seriously degraded, because of an outward diffusion of Ni from the substrate. We found that the control of NH₃ (reactive gas) pressure was effective in suppressing the Ni diffusion, and the columnar TiN was successfully formed on the cermet substrate. Finally, we have prepared the alumina/TiN multilayer coating in a single laser-CVD process with converting source gases during deposition.

We are now developing a new atmospheric laser-CVD process using an electrospray technique as a liquid source supply. Electrospray can provide tiny droplets and the droplets repeatedly split into smaller droplets by Rayleigh fission. Alpha-alumina with columnar structures have been prepared from organic solution of Al(acac)₃. The deposition rate was about 200 μm h^-1, which was comparable to that by the laser-CVD under low pressure.

Transition metal nitrides attract interest in coating and surface engineering, due to excellent mechanical properties, high hardness and oxidation resistance. The idea that NbTiN alloy thin films could combine the excellent superconducting properties of NbN and the mechanical and metallic (low residual resistivity) properties of TiN is already demonstrated in practice by PVD techniques. Despite the sufficient number of reports on the electrical characteristics of NbN and NbTiN films, only a few studies are oriented in the structural and thermodynamic description. In this contribution, we will report for the first time (to the best of our knowledge) the deposition of Nb_1-xTi_xN thin films by high temperature chemical vapor deposition (HTCVD) process. We will study the role of Ti in the stabilization of the (Fm-3m) cubic Nb_1-xTi_xN structure, while a comparison with NbN films will be made.

NbN and Nb_1-xTi_xN thin films are grown using a HTCVD reactor. The chlorination of the Nb (99,99%) and Ti (99,99%) metallic source occurs in-situ, in a Cl₂ (99,999%) chlorination chamber. The reaction by-products with NH₃ as well as the deposition procedure is described elsewhere [1]. The deposition temperature is varying in the range of 1000^oC to 1300^oC and the substrates used are (0001) and (11-20) oriented single crystal sapphire. Films are characterized using XRD, SEM and AFM in order to provide the necessary structural and surface morphology information. The thermodynamic analysis of both chlorination and deposition steps is carried out with Factstage thermochemical software, using SGTE thermodynamic database.

We will show that for T_deposition<1100^oC the deposited films are a mixture of (111) oriented (Fm-3m) cubic NbN and (P6₃/mmc) hexagonal NbN, while only cubic NbN grows at higher temperature. With the addition of Ti, (111) cubic Nb_1-xTi_xN oriented films are deposited, completely free of hexagonal phases, with enhanced crystalline quality and surface roughness. By the XRD analysis (in-plane, pole figure and reciprocal space mapping) we determine the orientation relationship of the NbN and NbTiN films with the sapphire substrate and calculate the residual stress. Assisted by the thermodynamic analysis of NbTiN growth in the HTCVD process, we will discuss the possibility to control the chlorination process of Nb and Ti source and consecutively the Ti content in the grown films. The last, is believed to affect the structural but also the superconducting properties of the thin films.

[1] F. Mercier, Surface & Coating Technology, 260,124, 2014

Many important applications of the group-13 nitrides are suffering from the lack of suitable substrates for the epitaxy of the high crystalline quality material that would have performance close to a theoretically predicted. The reason for that is lattice and thermal expansion coefficients mismatch between a substrate and an epitaxial material. The best solution for this issue is use of a native substrate to perform the homoepitaxy. Currently employed substrates for fabrication of high power devices and LEDs based on group-13 nitrides are alumina (sapphire, α-Al₂O₃) since it is relatively cheap, silicon carbide (SiC) due to the very low lattice mismatch and gallium nitride (GaN) because of its moderate lattice mismatch. Another alternative that is emerging is homoepitaxial growth on physical vapor transport (PVT) deposited AlN substrates that are expected to be promising once price will decrease and size increase. The most appealing technique in terms of price would be the direct epitaxy of AlN on Si (111), but for now, no thick layer have been produced due to large lattice mismatch that induce defect formation as well as high thermal expansion coefficient mismatch, which is an origin for the crack formation in the AlN layer.

In this work we investigate possibility to deposit c-axis oriented wurtzite AlN on Si (111) substrates at high growth rate by employing high temperature chemical vapor deposition (CVD). As precursors for growth of AlN NH₃ and AlCl₃ are employed. Later is produced in situ by feeding Cl₂ gas through a chlorination tube filled with Al pallets and heated up to about 600 °C. As a carrier gas hydrogen is used. Such chlorinated chemistry allows for high growth rate and high purity of the resultin g thin films. Here we present results on deposition of AlN on Si (111), patterned Si (111) and carburized Si (111). Deposited layers have been characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. First two were employed to investigate crystalline structure at both macro- and microscopic scales. This is necessary to find imperfections of the crystalline structure and determine their origin. Last characterization technique is used to observe morphology and thickness of deposited films that are also important parameters for characterizing growth process.