ICMCTF2010 Session B3: CVD Coatings and Technologies

Chemical vapor deposition of coatings containing metallic alloys is expected to open for these materials the gate to numerous application fields. The design, validation and optimization of this white spot in processing science request an insight on different aspects, the stone corner of which is precursors’ selection. The use of organometallic and molecular compounds widens the possibilities of CVD, provided they are appropriately designed and evaluated. Other important points are optimized reactor design, dedicated methodologies and on line, in situ and ex situ diagnostics for the gas phase and the growing surface.

In the presentation, the correlation among processing conditions, microstructure and properties of AlCu coatings is used as a paradigm to illustrate the above. AlCu coatings were processed in a vertical, cold wall, stagnant flow reactor. Dimethylethylamine alane and either copper cyclopentadienyl triethyl phosphine or copper N,N’-Di-isopropylacetamidinate were used as aluminium and copper precursors, respectively. Silicon and stainless steel were used as substrates. Deposition was performed either simultaneously or sequentially, followed by post deposition annealing. Gaseous by-products were identified by on line and in situ mass spectrometry and helped in modeling the process by considering fluid flow patterns, temperature profile and chemical kinetics. The microstructure, elemental composition and the nature of the phases (including complex metallic alloys such as the approximant phase Al4Cu9) present in the films were determined. They were correlated with the processing conditions and with the properties of the films, namely their wetting and tribological behavior. Based on these results, strategies for the processing of higher order alloys are discussed.

a-C:H:Si coatings have attracted significant attention due to their properties of high hardness, chemical inertness, low friction coefficient and high wear resistance. However, when deposited on “soft” steels the substrate deforms plastically when the applied load is high. By pre-nitriding steels the support of the thin hard coating can be remarkable improved. The corrosion resistance can be significantly increased by an additionally post-oxidizing process before coating deposition. The deposition of a-C:H:Si coatings on pre-nitrided and post-oxidized heat treatable steels is a promising technique in many areas of mechanical engineering, where low friction coefficients and low wear rates are needed or the corrosion behaviour has to be enhanced.

In this work a triplex treatment process (plasma nitriding and postoxidation followed by plasma deposition of a-C:H:Si) was developed in a commercially available DC-PACVD system in one run and further scaled up. The process gases H₂, C₂H₂, Ar and HMDSO were fed into the vacuum chamber by mass flow controllers. An important point during upscaling is the assurance for a homogeneous distribution of plasma density and energy. The results show a decreasing deposition rate during upscaling while leaving the plasma density constant. It was also found that the shape and even the position of the charging have a great influence on the plasma process and the properties of the deposited coatings. Chemical analysis of the a-C:H:Si films exhibits a decreasing Si-content with increasing charging.

The determination of the composition of the a-C:H:Si films was carried out using GDOES (glow discharge optical emission spectroscopy) whereas the adhesion was evaluated by a scratch tester. The hardness measurements were conducted by a nano-indenter and the topography of the layers was characterized by means of SEM (scanning electron microscopy) whereas the tribological properties were assessed with a ball-on-disc test.

The availability of a relevant computation of aluminum nitride CVD (Chemical Vapor Deposition) growth process is one of the steps toward the understanding of crystal quality dependence to experimental conditions. The objective of this study is a comprehensive modeling of this CVD process from aluminum trichloride (AlCl₃) and ammonia (NH₃) used as reactants, at high temperature (1200-1600°C). Recent studies of Dollet et al. (2002) and Swihart et al. (2003) provide sufficient kinetic data to build a reliable set of 94 homogeneous chemical reactions. At the same time, the thermodynamic data of 5 from the 28 involved chemical species were lacking. We propose here the ab-initio thermodynamic estimated properties for these species. The critical point of the simulation is surface reaction data. Nearly none is published on this subject. We take inspiration from the sticking coefficients of the main surface reactants evaluated by Dollet et al. (2002) to start our simulation, refining them with experimental data. We chose to adopt a ‘simplified’ 20 surface reactions set, not taking into account the adatoms diffusion and recombination on the surface, but only considering direct sticking through activation energies.

We have obtained two main results after the simulation. Firstly, the chemical reaction set could be greatly reduced by a wise analysis of the reaction activation barriers. First dehydrogenation of NH₃ and dechlorination of AlCl₃ are highly improbable even at the process temperature, ensuring that the main species flow nearly unchanged from the reactant inlet to the growing surface. The only noticeable reaction is the formation of the unstable, but non negligible, AlCl₂NH₂ intermediate. The second result is that the growth rate is easily fitted for low values (<20 µm/h), but a discrepancy between simulations and experiments provided by Claudel et al. (2009) appears for high growth rates. This difference cannot be reduced by optimizing the activation energies of the surface reactions. We think that the phenomenon involved is the change from a 2D to a 3D growth. Indeed the surface rugosity, so the total number of atomic site available for crystal growth, varies with growth rate. Despite the lack of numerous data on epitaxial growth, we foresee to have a better agreement with numerical results in this case.

A.Claudel, E.Blanquet, D.Chaussende, M.Audier, D.Pique, M.Pons. Journal of Crystal Growth, Vol. 311 (2009), Issue 13, 3371-3379.

A. Dollet, Y. Casaux, G. Chaix, C. Dupuy. Thin Solid Films 406 (2002), 1–16.

M. T. Swihart, L. Catoire, B. Legrand, I. Gökalp, C. Paillard. Combustion and Flame 132 (2003) 91-101.

Excellent high temperature properties, chemical stability and low radioactivity under neutron irradiation make silicon carbide (SiC) and SiC-base ceramics composite attractive materials for use in advanced nuclear energy systems. The objective of this work is the fundamental study of (i) the high temperature stability of tungsten (W) coatings on bulk SiC and (ii) the role and potential of an aluminium nitride (AlN) diffusion layer. The multilayered material, bulk SiC/AlN/W is produced by high temperature chemical vapour deposition (HTCVD).

AlN films were first grown on SiC-3C at 1000°C by HTCVD chlorinated process. The precursors used are ammonia NH3 and aluminium chlorides AlClx species formed in situ by reaction of Cl2 on high purity Al wire. W coating on AlN at 700°C was grown by HTCVD using WF₆ on H₂. The interfacial zones SiC/AlN/W were characterized using scanning electron microscope, X-Ray diffraction and electron microprobe microanalysis. No cracking zone has been observed and no interface reaction or parasitic phases have been revealed between the different layers at high temperature (1000°C). This is in agreement with thermodynamic computations. Simple thermomechanical calculations were performed to optimize the thickness of each layer to reduce stress and cracking. This new multilayered structure based on bulk SiC allows paving the way of the use of SiC composites in nuclear energy systems.

Ti_1-xAl_xN is a well established material for cutting tool applications exhibiting a high hardness and an excellent oxidation resistance. A main route for improving the performance of Ti_1-xAl_xN is the incorporation of further elements. Now a new LPCVD process allows the deposition of a very aluminium-rich TiAlCN. The process works with a gas mixture of TiCl₄, AlCl₃, NH₃, H₂, N₂, Ar and ethylene or acetonitrile as carbon source. At deposition temperatures between 800°C and 900°C hard TiAlCN layers can be prepared. In this work structure, composition, properties and cutting performance of CVD-TiAlCN coatings were investigated.

Aluminium-rich TiAlCN was obtained at 800°C and 850°C consisting of a major fcc-Ti_1-xAl_xC_yN_z phase. Lattice constants and WDX analysis indicate only a low carbon content < 1 at.-%. At 900°C the metastable fcc-TiAlCN disappears and co-deposition of Ti(C,N) and h-AlN occurs. From TEM analysis it is evident that in the TiAlCN layer deposited at 850°C the carbon is mainly located at the grain boundaries and not in the crystals of the fcc-Ti_1-xAl_xC_yN_z phase. The layers deposited at 800°C and 850°C possess a high hardness around 3000 HV. A strong adherence on hardmetal inserts is achieved by a bonding layer system consisting of an initial TiN layer followed by a TiAlN gradient layer. Scratch test measurements showed high critical loads of about 100 N. TiAlCN prepared at 850°C showed also an amazing thermal stability under vacuum conditions up to 1350°C. The wear behavior was investigated by different milling tests. For milling steels 42CrMo4V and C45 as well as cast iron GGG70 coatings containing aluminium-rich fcc-Ti_1-xAl_xC_yN_z with x > 0.8 exceed the performance of different state-of-the-art CVD and PVD coatings.

Titanium oxynitride coatings were deposited by atmospheric pressure MOCVD using titanium tetra-isopropoxide and N₂H₄ as reactive gas. The films composition was controlled by variation of the N₂H₄ mole fraction. The variation of the N content in the films results in changes in structural, electrical and optical properties. When a large excess of the nitrogen source is used the resulting film contains around 20 at. % of nitrogen and forms dense and amorphous ternary solid solution. Relatively high growth rate of these amorphous TiO_xN_y coatings was obtained. Correlations between the structure, the morphology, the composition and the growth rate of the films are presented and discussed in relation with deposition conditions and preliminary properties.

CVD-alumina is one of the most important components in modern coating systems for cutting tools. There were many activities in the last years to improve the properties of these coatings like hardness and wear resistance using different dopants.

The aim of this work is to examine the influence of chromium addition on the structure and properties of alumina CVD coatings. There is a lack on suitable chromium-containing precursors for thermal CVD processes. In this work a new chromium precursor based on a ternary aluminium chromium chloride is applied.

For determining the temperature range where the ternary aluminium chromium chloride is formed investigations using the chemical transport of chromium(II)chloride with aluminium(III)chloride are performed. Gaseous AlCl₃ reacts at a minimum temperature of 270°C with CrCl₂ and the ternary Al₂CrCl₈ is formed.

Deposition of chromium-containing layers is carried out in a LPCVD process using the new precursor, CO₂ and H₂. Composite coatings consisting of alumina, aluminium chromium oxide and chromium carbides are prepared at deposition temperatures between 920°C and 1030°C. Depending on deposition temperature and CO₂ concentration different types of layers were obtained. A first coating system consists of Al_2-xCr_xO₃ and chromium carbides Cr₇C₃ and Cr₂₃C₆ showing a hardness of 2600 HV[0.01]. A total chromium content of 13 at.-% in the layer is analyzed by WDX. In Al_2-xCr_xO₃ an x-value of 0.015 is calculated from the lattice constants. Additionally the existence of Al_2-xCr_xO₃ is verified by ruby luminescence. A second coating type exhibit a composite structure consisting of Al_2-xCr_xO₃ and Cr₇C₃ with a low total chromium content about 1 at.-%. This layer possesses a high hardness up to 2800 HV[0.01].

Thermodynamic calculations show a good agreement between theoretical and experimental phase compositions in the coatings. The wear behavior of the prepared coating types was investigated by milling tests.