Stefan Rosiwal

Dependence of the growth rate, quality, and morphology of diamond coatings on the pressure during the CVD-process in an industrial hot-filament plant

The pressure dependence of the growth rate, quality and morphology of CVD-diamond films in an industrial hot-filament plant (CC800D) were investigated by SEM and Raman. Additionally, the concentration of atomic hydrogen near the filament was determined via a calorimetric measurement method. At a substrate temperature of 850 °C the smallest growth rate (0.1 μm/h) and the best quality of diamond coatings were obtained at the pressure with the highest hydrogen concentration (20 mbar). The growth rate increases strongly with decreasing pressure and achieves the maximum value of 0.7 μm/h at 3 mbar. At the same time the diamond coating quality decreases.

CVD diamond coated titanium alloys for biomedical and aerospace applications

Abstract The titanium alloys Ti-6Al-4V and Ti-6Al-7Nb, which are commonly used in aerospace and biomedical applications, were coated with diamond films using a Microwave-Plasma-CVD process. The internal compressive stresses of these films may reach −8 GPa, strongly depending on the substrate temperature during deposition. The stresses can be effectively lowered by grit-blasting of the substrates prior to deposition to values of −2 GPa. This measure is expected to improve the adhesion of the diamond coatings even further. The solid particle erosion of diamond thin films on Ti-6Al-4V substrates grown with various methane concentrations in the process gas was compared with a commercial TiAlN PVD hard coating. Under an incidence angle of 90 °, a superior erosion resistance of diamond coatings with 4% and 8% methane was demonstrated. The biocompatibility of diamond films was tested in vitro using the human osteoblastic cell line hFOB 1.19. The performance of the coatings was comparable to that of pure titanium, i.e. diamond coatings are very suitable for medical applications. Retarded cell proliferation as observed in some cases is attributed to the high surface roughness due to grit-blasting and the hydrophobic character of the diamond films.

Structural changes of tungsten heating filaments during CVD of diamond

Hot tungsten filaments (2000 °C) are used for the activation of the gas phase during hot filament chemical vapor deposition (CVD) of diamond. In the present investigation, technically pure and doped tungsten filaments were subjected to typical process conditions. The changes in the tungsten filaments microstructure were investigated. During the first 60 s, a rapid transformation of W into W2C occurs due to the high reactivity of the metallic surface. Afterwards, the W2C shell grows inward at a linear rate. The activation energy of this process is 30 kJ mol − 1 , which corresponds to the dissolution of methane on the filament surface which is the rate determining step. Once W has transformed completely into W2C, a WC shell starts to grow from the surface. In doped tungsten a parabolic behavior can be found for the growth of WC which occurs when the diffusion of carbon through the growing shell is the rate-determining step. Additionally, the temperatures of the filament and substrates drop considerably. Both filament and substrate temperature drops are discussed in detail with the help of a model describing the power balance.

Erosion resistance of CVD diamond-coated titanium alloy for aerospace applications

Abstract The solid-particle erosion resistance of Ti–6Al–4V can be improved significantly by applying a chemical-vapor-deposited diamond coating under conditions where physical-vapor-deposited coated substrates fail. In particular, diamond coatings grown with relatively high methane concentrations (4–10%) in the process gas atmosphere exhibit superior performance. The reason for this behavior is a higher in-plane strength of these coatings due to growth defects like twin formation and secondary nucleation.

Microwave-plasma-CVD of diamond coatings onto titanium and titanium alloys

Abstract CVD-diamond films have been deposited onto pure titanium and Ti-6Al-4V substrates. It was found that with optimized processing conditions, these films show very good adhesion, even though they are highly stressed (−4 to − 6GPa) and relatively thick (4–6 μm). The reason for this might be the formation of a strong TiC-diamond interface. Calculations for the critical energy release rate G before indentation testing lead to a value of G=95 J m−2, which is comparatively high. The internal compressive stresses which developed in the diamond films are about 4 GPa lower than expected based on calculations. Several stress relief mechanisms were identified. These include the appearance of tensile growth stresses, the formation of a titanium carbide interlayer, plastic flow in pure titanium substrates and the effect of phase transformations (β-titanium→α-titanium + hydride) in both substrates while cooling. In addition, the application of a cooled sample stage induces a thermal gradient in the titanium base substrate during the coating process with the result of lower average specimen temperature. This leads to smaller contraction and lower film stresses during cooling. The coating process affects the microstructure and mechanical properties of the titanium base substrate. Pure titanium seems to be more sensitive against hydrogen at elevated temperatures than Ti-6Al-4V. The alloy Ti-6Al-4V possesses a CVD-diamond processing window up to deposition temperatures of 770 °C where no severe changes in the grain structure occur. In principle, dissolved hydrogen and hydride precipitations in Ti-6A1-4V and in pure titanium can be removed after the coating process by a vacuum furnace annealing. Using this procedure, mechanical properties of titanium base substrates can be almost completely restored after CVD-diamond deposition.

Tribological optimization of CVD diamond coated Ti-6Al-4V

Abstract CVD diamond coatings on Ti-6Al-4V substrates were found to be highly effective in reducing fretting fatigue which, for example, can occur in the root section of aerospace compressor blades. Optimization of a mechanical polishing process leads to significant improvements in the tribological properties of these diamond coatings. This is shown by extremely low wear rates and low coefficients of friction against bearing steel, alumina and diamond. The wear and friction behavior was investigated in a ball-on-disk tribometer and characterized by profilometry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and micro-Raman spectroscopy.

Growth stages of chemical vapor deposited diamond on the titanium alloy Ti6Al-4V

Abstract CVD-diamond films were deposited ontothe titanium alloy Ti6Al-4V. Samples were investigated in a ground and ultrasonically pretreated stage and after different times of deposition. The coating time was varied between 1 and 210 min. SEM, XPS and Micro Raman spectroscopy were used to characterize different stages of deposition. A model describing the process of chemical vapor deposition of diamond onto Ti6Al-4V is developed.

The influence of diamond chemical vapour deposition coating parameters on the microstructure and properties of titanium substrates

Abstract In contrast to their excellent mechanical properties, titanium alloys possess poor wear characteristics. Diamond coatings appearto be a promising solution for the wear problem. Using standard deposition parameters for silicon (diamond scratched surface; microwave chemical vapour deposition, 500 W, 800 °C for 4 h, 50 mbar with an atmosphere of 1% CH 4 + 99%H 2 ) good coatings on pure titanium were obtained. The mechanical properties of the titanium are strongly influenced by the processing temperature and the gas atmosphere. As a result of the hydrogen adsorption, grain coarsening and formation of titanium hydride occur and subsequently the low cycle fatigue strength decreases by an order of magnitude. However, by annealing in vacuum (800°C for 2 h, furnace cooled) the hydrogen can be removed again and the initial mechanical properties can almost completely be restored. Based on scanning electron microscopy micrographs, Auger analysis, X-ray diffraction and microhardness measurements a basic understanding of the microstructural changes was developed. The surface structure may be visualized as consisting of five zones, created during the deposition process (diamond layer/nucleation zone/reaction layer/gradient layer/affected substrate).

High temperature diffusion chromizing as a successful method for CVD-diamond coating of steel—Part II

Well adherent CVD-diamond coatings were deposited onto 41Cr4 steel substrates using chromium carbide diffusion interlayers. The influence of coating parameters on microstructure and composition of chromium carbide layer and substrate was investigated. In situ heat treatment allows for a variation of physical and mechanical properties of the substrate. We assume that the adhesion of CVD-diamond coatings depends inter alia on the carbon concentration on top of chromium carbide layer. This possible effect is discussed as well.

Graphite interlayer formation during CVD diamond coating of iron base alloys : The analogy to metal dusting

In the present paper the various difficulties in diamond depositionon iron was investigated in more detail. A microwave-CVD process with the same deposition parameters was used for all substrates. Results for commercially pure iron, and the binary iron alloys FeSi10 and FeSi20 are presented in comparison with titanium and copper