ICMCTF2011 Session B2-2: CVD Coatings and Technologies

WC-Co composite has been widely employed to cutting tools due to high hardness and ductility; however, W is a rare resource and localized on earth. Therefore, alternate material should be sought to substitute W. Since Ti is more abundant and Ti-based compounds have high hardness and strength, TiN-Ni cermet can be a promising cutting tool material having competitive performance of WC-Co composite. The surface of these cutting tools should be generally coated by thermally insulative and anti-abrasive α-Al₂O₃film, preventing temperature increase and degradation of mechanical properties of substrate material. α-Al₂O₃ film has been commonly prepared by chemical vapor deposition (CVD) on cutting tools using halide precursors, typically AlCl₃, at more than 1300K. Metalorganic CVD (MOCVD) is also applicable to prepare Al₂O₃; however amorphous or γ- Al₂O₃ has likely been formed even around 1300K. In order to prepare α-Al₂O₃ film on TiN-Ni cermets, a low temperature deposition of α-Al₂O₃ and a high deposition rate are key issues for practical application of TiN-based cermet cutting tools.

Alumina coatings present great technological interest in numerous application domains, such as microelectronics, catalysis or surface protection. This study focuses on the implementation of different aluminum oxide coatings processed by metal-organic chemical vapor deposition from aluminum tri-isopropoxide on commercial TA6V titanium alloy to improve its high temperature corrosion resistance. Previous work allowed establishing processing conditions – microstructure relationships for such coatings. Films grown at 350°C and at 480°C are amorphous coatings and correspond to the formulas AlOOH, and Al₂O₃, respectively. Those deposited at 700°C contain g- Al₂O₃ nanocrystals. Their mechanical properties and adhesion to the substrates were investigated by indentation, scratch and micro tensile tests. Hardness and stiffness of the films decrease with increasing the deposition temperature and reach a plateau corresponding to the onset temperature crystallization of their crystallization (700°C). The hardness of the coatings prepared at 350°C, 480°C and 700°C is 5.8 ± 0.7 GPa, 10.8 ± 0.8 GPa and 1.1 ± 0.4 GPa, respectively. Their stiffness is 92 ± 8 GPa (350°C), 155 ± 6 GPa (480°C), and 11.5 ± 2.8 GPa (700°C). Scratch tests cause adhesive failures on the films grown at 350 °C and 480°C whereas cohesive failure is observed for the nanocrystalline one, grown at 700°C. Micro tensile tests show a more progressive cracking on the latter films than on the amorphous ones. Similar micro tensile tests were performed for Al oxide coated and uncoated Ti alloy substrates after corrosion with NaCl deposit during 100 h at 450°C. The coated samples have a mechanical strength similar to that of the as processed uncoated samples not corroded. After corrosion test, the mechanical strength of the coated Ti alloys is higher than that of the uncoated alloys. It is concluded that the films allow maintaining the good mechanical properties in the targeted operating conditions. However, after corrosion test only the film deposited at 700°C yields an elongation at break comparable to that of the as processed samples without corrosion. The other coatings do not allow the fall of ductility recorded after corrosion test on TA6V. As a conclusion; the as established processing – structure – properties relation paves the way to engineer MOCVD aluminium oxide complex coatings which meet the specifications of the high temperature corrosion protection of titanium alloys with regard to the targeted applications.

Al₂O₃ coatings have been one of the most important coatings for the cutting tools. Al₂O₃, which maintains high hardness and excellent oxidation resistance under such a severe cutting condition, is that dominate the tool-life. Al₂O₃ has a several different crystal system, metastable κ phase and stable α phase Al₂O₃ have been used for the cutting tool. A lot of researches have been done about the heat transformation of κ→α phase. Recently, reported that B- and Ti-B-doped κ-Al₂O₃ slowly phase transformed to α-Al₂O₃ than non-doped one, however, investigation into the heat transformation mechanism of κ→α phase on B- and Ti-B-doped κ-Al₂O₃ has yet been successful.

In this paper, Ti or Zr-doped κ-Al₂O₃ coatings were deposited using a hot wall CVD equipment, with AlCl₃-ZrCl₄ or TiCl₄-CO₂-HCl-H₂ gas mixture. After being heated these Al₂O₃ coatings at several heat-treatment processes, crystal system and morphology of these Al₂O₃ coatings were analyzed. Heat transformation mechanism of Ti or Zr-doped κ-Al₂O₃ will be discussed.

The microstructure and wear properties of CVD α -Al₂O₃ layers with (10-12) and (0001) growth textures were compared with MTCVD Ti(C,N) layers in single point turning of AISI 4140 steel. The experimental coatings were investigated by FEG-SEM, EBSD and a combination of FIB and analytical TEM techniques prior to and after machining. Substantial texture effects on wear performance of the α -Al₂O₃ layers were observed. When compared to (10-12) textured layer, an enhanced ability of the (0001) textured layer to undergo plastic deformation was confirmed. The deformation was localised in the near surface region of the layer. In contrast to the α -Al₂O₃ layers, the Ti(C,N) layer exhibited a more uniform plastic deformation across the entire layer thickness. The wear characteristics of the layers are further interpreted in the light of thermal, mechanical and frictional conditions occurring at the tool–chip contact interface.

Aluminum Nitride (AlN) is a III-V wide band gap semiconductor material with a high thermal conductivity and good chemical inertia. These properties make it suitable for several applications in the opto-electronic field, the most important is the fabrication of UV diodes. Thick AlN layers have been processed by HTCVD (High Temperature Chemical Vapor Deposition) using AlCl₃ and NH₃ diluted in H₂.

The thermodynamic and kinetic modeling of AlN growth on AlN templates at different temperatures and N/Al ratios have been made in previous studies [1-2]. The conclusion of (Boichot and al. [1]) was that experiments should be conducted at constant supersaturation and thickness to understand the actual influence of N/Al ratio on AlN layer quality. The relationship between temperature and supersaturation was established by (Claudel and al. [2]).

In the first part of the study, experiments were performed on graphite substrates to study the preferential orientation of AlN crystals by varying the temperature and N/Al ratio. It is demonstrated that low N/Al ratio allows the control of growth orientation of the AlN crystal facets along the c-axis.

In the second part, experiments were carried out on c-plane sapphire substrates to investigate the effect of supersaturation and N/Al ratio on the morphology of grown layers. The layers are characterized by SEM, XRD, XPS and SIMS to evaluate both crystal quality and O and Cl contamination.

The main conclusion is that the control of surface quality in term of epitaxial relationship between sapphire and h-AlN is possible through a precise setting of supersaturation and N/Al ratio in the gas phase. It is also concluded that epitaxial growth along c-axis is not thermodynamically favored in classical CVD conditions (low pressures, high temperature and N/Al up to 1).

References

[1] R. Boichot, A. Claudel, N. Baccar, A Milet, E. Blanquet, M. Pons, 2010, Epitaxial and pollycristalline growth of AlN by high temperature CVD: Experimental results and simulation, Surface and Coating Technology, article in press.

[2] A. Claudel, E. Blanquet, D. Chaussende, M.Audier, D.Pique, M.Pons, 2009, Thermodynamic and experimental investigations on the growth of thick aluminum nitride layers by high temperature CVD, Journal of Crystal Growth 311, 3371–3379.

The requirements of chemically vapour deposited ( CVD ) coatings for protective application has increased enormously over the past 30 years. The progress from simple monolayer coatings to of today’s complex coating systems were made possible through innovations in coating equipments and the continual optimization of the processes; this in parallel with improved substrate materials, and the progress in preparation periphery, such as pre- and post-treatment operations. A potential for further coating improvement in industrial applications lies with defined structures and new material combinations. One route to obtain better controlled structures is by adding small amounts of metallic or non-metallic elements combined with controlled nucleation for well known processes. This will be illustrated on the example of boron additions to high and moderate temperature Ti-based coatings. The influence of the concentrations of dopants in connection with coating architecture (e.g. multi- or nano-layers) on coating morphology, hardness, friction and wear behavior will be reported.

Application examples of doped coatings and combined coatings will also be presented. Particularities related to diffusion phenomena of the highly mobile boron atom will be discussed.

TiCN/Al₂O₃ hard coatings grown by chemical vapor deposion (CVD) are state-of-the-art in metal cutting applications using indexable cemented carbide inserts. A TiCN base layer grown by a medium-temperature (MT) CVD process is a crucial feature for wear resistance and toughness of the tools. In order to influence the structure and properties of this base layer, five different MT-TiCN coatings with increasing amounts of CO in the feed gas were deposited using an industrial-scale low-pressure CVD system. The coatings were deposited using a TiCl₄-CH₃CN-H₂-N₂-CO feed gas system with a total flux of 60 l/min. The deposition temperature was 900 °C, the deposition pressure 100 mbar. Phase composition, preferred orientation of crystallites and residual stresses were characterized by X-ray diffraction (XRD). Surface topography and fracture cross-sections were investigated by scanning electron microscopy (SEM). Surface roughness was measured using a white light profilometer. Indentation hardness and indentation modulus of the coatings were determined using nanoindentation with a Berkovich indentor. The oxygen content of the different coatings was analyzed by using electron probe microanalysis (EPMA) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The thickness of the coatings was measured by light optical microscopy (LOM) on polished cross-sections. EPMA and ToF-SIMS measurements confirmed a good correlation between the oxygen amount in the coatings and the CO flow. With increasing CO amount up to 2.2 vol. % in the gas supply, the originally equiaxed grains change to plate-like grains, while the grain size decreases to a few hundred nanometers. Furthermore, the addition of CO makes the preferred orientation of crystallites weaker until it nearly vanishes at 2.2 vol.-% CO in the feed gas. Also the surface roughness decreases with rising CO addition. TEM analyses showed that the incorporation of oxygen leads to a homogenous distribution of voids within the TiCN grains and to increasing density of twin boundaries. The residual stress shows a minimum while hardness and indentation modulus reach a maximum for CO amounts of 1.5 vol.-% in the feed gas.