L. Vandenbulcke

Characterization of diamond films deposited on titanium and its alloys

Abstract Titanium and its alloys have important applications for example in aerospace or as bioimplants. Some of these applications would be improved by diamond coatings. However the large thermal expansion mismatch between diamond and titanium or its alloys creates high residual stresses, up to about 7 GPa at 800 °C, which represent an important drawback. In this study, polycrystalline diamond films were deposited on pure titanium and Ti-6Al-4V in a classical tubular microwave plasma reactor from C-H(-O)-containing gas mixtures, at a temperature in the range 600–900 °C. Raman spectroscopy provided information about the diamond grain stress, which is obviously related to the deposition temperature. X-ray diffraction indicates the presence of titanium carbide or oxycarbide. Some other characterizations by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) are reported. It is shown that XPS coupled to argon ionic etching allows us to study the first steps of the deposition process. The structure and the chemical composition at the interface of a thicker deposit are obtained by TEM and EELS.

The oxidation behaviour of two- and three-dimensional C/SiC thermostructural materials protected by chemical-vapour-deposition polylayers coatings

The oxidation behaviour of two- and three-dimensional C/SiC protected by a chemicalvapour-deposition (CVD) ceramic coating was studied. The elements used to achieve the surface protection were silicon, boron and carbon, preferably forming SiC, B or B4C. The best results were obtained with the trilayer coatings, that is with, SiC as the internal layer, boron or boron carbide, as the intermediate layer and an external SiC layer. To get a good protection in a large temperature range, from 450 to 1500 °C, the total thickness of the trilayers must be higher than 160 μm and the intermediate layer thickness must be higher than 5 μm. Morphological characterization of oxidized samples has shown that, for intermediate oxidation temperatures, a glass was produced in the cracks. When the oxidation temperature was equal to or higher than 1300 °C, sealing of the cracks was rarely observed, but the oxidation resistance remained satisfactory.

Crystalline quality and residual stresses in diamond layers by Raman and x‐ray diffraction analyses

A major problem for diamond coating applications is that diamond films tend to exhibit poor adherence on many substrates and typically disbond at thicknesses of the order of a few micrometers principally because of residual stresses. Residual stresses are composed of thermal mismatch stresses and intrinsic tensile film stresses induced during film growth. Diamond films were deposited in a classical tubular microwave plasma reactor from hydrocarbon–hydrogen–oxygen gas mixtures. The stress level was investigated as a function of the gaseous composition (especially oxygen concentration) and deposition temperature (700–900 °C). Thermal stress was directly calculated from Hooke’s law using a biaxial Young’s modulus value of 1230 GPa for polycrystalline diamond. Total diamond film stress was directly determined by the radius of curvature method and by x‐ray diffraction using the sin2 ψ method. The microdistorsions and the size of the coherently diffracting domains have been determined from the broadening of the...

Characterization of boron nitride films deposited from BCl3-NH3-H2 mixtures in chemical vapour infiltration conditions

Boron nitride (BN) thin films deposited by isopressure and isothermal chemical vapour infiltration (ICVI) from BCl3-NH3-N2 mixtures have been characterized from a physicochemical point of view as functions of both the deposition conditions and the destabilizing action of moisture. As-deposited (deposited at 773 K and post-treated at 1273 K), the BN films are turbostractic (d0 0 2=0.36 nm, Lc=1.5 nm), have a low density (1.4 g cm−3) and contain oxygen (about 20 at%). A first oxygen content (191.5 eV by XPS) is inserted in the films during the CVI step in relation to the hygroscopy of intermediate solid products and the quasi-equilibrium between the formation of BN and B2O3. A second oxygen content (192.5 eV) is due to the hydrolysis of BN by moisture which induces a very drastic transformation of BN. This destabilization affects both boron and nitrogen atoms and leads to the formation of ammonium borate hydrates. Different post-treatments have been investigated to stabilize the BN films and it appears that nitriding under ammonia is the most efficient.

Mechanical properties of diamond films: A comparative study of polycrystalline and smooth fine-grained diamonds by Brillouin light scattering

Brillouin light scattering, Raman light scattering and x-ray diffraction were used to investigate the elastic and microstructural properties of polycrystalline and smooth fine-grained diamond films of varying diamond quality. They were deposited on a titanium alloy by a two-step microwave plasma-assisted chemical vapor deposition process at 600 °C. Their morphology and roughness were studied by scanning electron microscopy and atomic force microscopy. Their refractive indices were determined by the M-line spectroscopy technique. The diamond purity of all these coatings in terms of the sp3 bonding fraction was deduced from visible and UV Raman spectroscopy as a function of the deposition conditions. All the samples were found to be textured with a 〈011〉 crystallographic direction normal to the film plane, leading to essentially hexagonal symmetry of the elastic tensor. By taking advantage of the detection of a number of different acoustic modes, complete elastic characterization of the films was achieved. ...

Plasma-assisted chemical vapor deposition process for depositing smooth diamond coatings on titanium alloys at moderate temperature

Abstract A simple process has been perfected to deposit smooth fine-grained diamond coatings at 600°C on titanium alloys or titanium-coated surfaces. It consists of a two-step microwave plasma-assisted chemical vapor deposition (PACVD) procedure including first the deposition of a sacrificial sp 2 -carbon containing layer from a methane-rich CH 4 –H 2 mixture and then the diamond growth from a CO 2 –CH 4 inlet mixture. Scanning electron microscopy, X-ray diffraction, visible and UV Raman spectroscopy show that the coatings are smooth and mainly composed of crystalline diamond with a fine-grained morphology. The results are compared with the results obtained with classical rough polycrystalline coatings deposited from 8% CO–H 2 . Optical emission spectroscopy reveals important differences between the plasma species produced for the deposition of these smooth coatings and the plasma species produced for the deposition of both polycrystalline coatings from 1% CH 4 –H 2 or 8% CO–H 2 mixture and nanocrystalline films from Ar–CH 4 (–H 2 ).

TWO-STEP PROCESS FOR IMPROVED DIAMOND DEPOSITION ON TITANIUM ALLOYS AT MODERATE TEMPERATURE

A simple two-step process is reported here to deposit diamond coatings on titanium alloys at temperatures equal to or lower than 600 °C. The first step allows us to increase the carbon nucleation rate and to deposit a sacrificial layer which contains more than about 25% sp2 carbon. Its thickness is selected both to limit the interaction of titanium element with the plasmas used for diamond growth during all the second step, even when an oxygen-containing mixture is used, and to diffuse completely at the end of the process. After the first step, the formation of titanium carbide is observed by x-ray diffraction and x-ray photoelectron spectroscopy, which does not reveal any oxygen incorporation in the coating-substrate interfacial region. These results are related to the final strong diamond adherence.

Chemical vapour deposition of boron carbide from BBr3CH4H2 mixtures in a microwave plasma

Abstract Boron carbide coatings are deposited at moderate temperatures, equal to or lower than 600°C, from BBr 3 CH 4 H 2 mixtures in a microwave plasma. The deposition rate, the composition and the microhardness of the deposits are studied as function of the deposition parameters. Amorphous B-C films are obtained in a large composition domain, ranging from pure boron to 40 at.% C, with deposition rates between 5 and 25 μm/h −1 , principally as a function of temperature, total flow rate and inlet concentration of CH 4 . The amount of bromine incorporated in the solid is measured to be between 0.2 and 5 at.% according to the deposition conditions, and the deposit microhardness varies in the range 1000–5000 kg mm −2 . The deposition process is discussed and it appears that the activated intermediate species containing carbon inhibit the deposition rate as the methane inhibits the deposition rate of boron carbide in the low temperature chemical vapour deposition of BCL 3 CH 4 H 2 .

Tribological performance of diamond-coated Ti–6Al–4V alloy with respect to diamond characteristics

Abstract Polycrystalline and smooth fine-grained diamond coatings with different characteristics were deposited on Ti–6Al–4V at 600°C in a microwave plasma. Surface roughness of as-deposited polycrystalline and fine-grained diamond coatings was in the 15–140 nm (r.m.s.) range. Some polycrystalline coatings were polished after deposition to a final roughness of 5 nm. Sliding experiments were conducted in ambient air with a rotating pin-on-disk tribometer. The friction coefficient of uncoated Ti–6Al–4V pins during sliding against the two types of diamond coatings was in the 0.05–0.1 range after a run-in period. The fine-grained diamond coatings provided low friction coefficients and low wear rates to the titanium counterbodies. All the results showed the great influence of the surface roughness and the influence of the diamond quality on the tribological results. Sliding tests between two diamond-coated counterfaces evidenced a self-polishing mechanism leading to ultra smooth wear tracks and a low final wear rate. The advantages of the fine-grained diamond coatings with respect to the polycrystalline coatings were also pointed out in that case. The great adhesion of the coatings was deduced from the absence of coating failure, crack and stress relaxation in the disk track even at high load.

Nondestructive evaluation of micrometric diamond films with an interferometric picosecond ultrasonics technique

Synthetic diamond thin or thick films are suitable materials for many applications because of their weak friction coefficient and their large hardness, Young modulus, and thermal conductivity. A series of diamond thin films of variable quality obtained by plasma-assisted chemical vapor deposition lying on titanium alloy substrates has been studied by picosecond ultrasonics. Femtosecond laser pulses allow the generation and detection of picosecond acoustic pulses that can be used to study the elastic properties of thin micrometric and submicrometric films. Acoustic fields generated in the substrate and propagating in the transparent diamond film are detected by an interferometric technique; they give rise to oscillations and abrupt step-like changes in the transient reflectivity variations. An analysis of these different features allows the determination of various characteristics of these films, such as longitudinal elastic constant, thickness, and roughness in terms of the diamond quality.