Deposition Of Diamond Coatings Research Articles

Effect of Plasma Power on the Deposition of Diamond Coatings on Pure Titanium

Diamond coatings appear to be a promising solution for the improvement of tribological behavior of titanium alloys. By means of microwave plasma assisted chemical vapor deposition (MW-PACVD), diamond coating was deposited on pure titanium using CH4/H2 gas mixtures under different plasma powers. Surface and interface characterization of the deposited coating under different plasma powers was carried out using SEM, grazing incidence x-ray diffraction (GIXD) and Raman spectroscopy. Adhesion of diamond coating with substrate was evaluated using an indentation tester. Results showed that adhesion of diamond coatings was not good under high plasma power, whereas the crystallinity of diamond coating was not good under low plasma power. The higher the plasma power, the larger the diamond crystal size, the less content of non-diamond carbon and the poorer the adhesion strength. During the diamond deposition, growth of TiC competed with diamond formation for the available carbon content. Relatively low plasma power inhibited TiC formation more than diamond formation. Under a high plasma power, the formation of a thick and porous TiC layer appeared to promote interfacial debonding and spallation of the diamond coating.

Hydrogen embrittlement of titanium during microwave plasma assisted CVD diamond deposition

During diamond deposition on a titanium substrate using a gas mixture of H2–CH4 (196 : 4), hydrogen easily diffused into the substrate and led to significant microstructural coarsening and a severe loss in Charpy impact energy. In order to prevent the rapid diffusion of hydrogen into the substrate during diamond deposition, three techniques were studied. The first method was to use a post-vacuum annealing treatment to achieve dehydrogenation of a diamond coated titanium specimen. However, the Charpy impact energy could not be restored significantly even after a few hours of annealing. The second method was to apply a barrier interlayer between the diamond coating and titanium substrate. Results showed that even though the sputtered TiN coating and plasma nitrided layer could prevent the rapid diffusion of hydrogen and carbon into the titanium substrate, the deposited diamond coatings had poor adhesion to the substrate. A graded interlayer produced by plasma nitriding followed by plasma carbonitriding was effective in preventing the rapid diffusion of hydrogen and also improving the nucleation rate and adhesion of the diamond coating. The third method was to use a gas mixture of Ar–H2–CH4 instead of the conventional H2–CH4 . With the use of an Ar–H2–CH4 (180 : 16 : 4) mixture, a smooth and nanocrystalline diamond coating was deposited, and there was little change in the substrate microstructure or Charpy impact energy after diamond deposition.

Effects of pre-treatments and interlayers on the nucleation and growth of diamond coatings on titanium substrates

During diamond deposition on titanium substrates, two processes exist: (1) diffusion of hydrogen into a titanium substrate and the formation of hydride thereby degrading the mechanical properties of the substrate; and (2) competition among the rapid diffusion of carbon atoms into substrates, the formation of carbide and the nucleation of diamond crystals (thereby affecting the nucleation and growth rate of the diamond coating). To increase the diamond nucleation rate and prevent the rapid diffusion of hydrogen and carbon into the substrate, different surface treatments and interlayers were studied in this paper. Results showed that polishing with diamond pastes and ultrasonic pre-treatment in diamond suspensions will significantly increase the nuclei density of diamond crystals. However, the diffusion of hydrogen into the substrate could not be prevented . Pre-etching of the titanium substrate using hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Results showed that on a TiN interlayer, there was no significant improvement in diamond nucleation and growth, and the deposited diamond coatings showed poor adhesion. New diamond crystals were formed on the DLC interlayer in which DLC acted as the precursor for diamond nucleation. However, the so-formed diamond coating showed spallation. The plasma nitrided layer could prevent the rapid diffusion of hydrogen and carbon into the titanium substrate, but results showed a relatively low nucleation density of diamond crystals and poor adhesion. A graded interlayer combining plasma nitriding followed by plasma carbonitriding was effective in preventing the rapid diffusion of hydrogen and carbon into the substrate and improving the nucleation rate and adhesion of diamond coating.

Tribological performance of mechanically lapped chemical vapor deposited diamond coatings

Polycrystalline diamond layers were deposited onto the titanium alloy Ti–6Al–4V via microwave plasma chemical vapor deposition (CVD). It is shown that the diamond film reduces wear and friction immensely compared with uncoated titanium/titanium tribo systems. An additional polishing process of the CVD-diamond layer improves the wear behavior even further. An influence of air humidity is found.In addition, different ceramic materials (Al2O3, SiC, Si3N4, ZrO2) were tested against as-deposited and polished CVD-diamond layers. Al2O3 and Si3N4 exhibit very good wear behavior accompanied by very low friction against as-deposited CVD-diamond. SiC and ZrO2, however, show somewhat inferior wear and friction behavior. Suggestions are made to explain the different performance. Again, a wear increase is found with increasing humidity. Polishing of CVD-diamond layers improves the tribological performance further.

Surface and interface characterization of diamond coatings deposited on pure titanium

Diamond coatings appear to be a promising solution for the improvement of tribological behavior of titanium alloys. By means of microwave plasma assisted chemical vapor deposition (MW-PACVD), diamond coating has been deposited on pure titanium substrates using CH4/H2 mixtures at moderate temperature (550–600°C). The surface and interface characterization of deposition coating with increasing deposition duration up to 21 h has been studied using SEM and grazing incidence X-ray diffraction (GIXD). TiC formation on the substrate surface was detected after 15 min deposition. Growth of TiC competed with diamond formation for the available carbon. The formation of a thick porous TiC layer appeared to promote interfacial debonding and spallation of the diamond coating. During the deposition of diamond coatings, hydrogen diffused into Ti substrate, and the formation of titanium hydride was detected by GIXD. The formation of TiC and diamond layers did not inhibit the formation of titanium hydride. This led to profound microstructural changes and a severe loss of impact strength. A two-step process with higher ratio of CH4 during the first step deposition appeared to be beneficial as a result of the higher nuclei density of diamond crystals. The diamond coating so formed was observed to be in more intimate contact with the substrate.

Erosive wear behaviour of thick chemical vapour deposited diamond coatings

This paper reports work carried out on 60–200 μm thick CVD diamond coatings deposited on tungsten and cemented tungsten carbide substrates. The erosive wear behaviour of these coatings relative to cemented tungsten carbide is described. Erosion tests used quartz silica sand, on average 194 μm in diameter, in air at a velocity of 268 ms −1. The erosion rates and micro-mechanisms, and their effect on coating life are presented as a function of coating thickness and the surface conditions of as-grown or lapped coatings. The eroded surfaces were studied by scanning electron microscopy (SEM) and surface profilometry. Ultrasonic imaging and taper polishing of tested samples were also performed to reveal sub-surface damage and to elucidate its contribution to coating degradation. The results suggest that the samples erode by a gradual chipping of grains in the early stages followed by the accumulation of damage at the coating–substrate interface. It is this latter feature which eventually leads to catastrophic failure of the coating along the interface. These features are discussed in the context of the classical erosion damage features normally exhibited by brittle materials as well as the coating microstructure. The propensity for coating debonding would suggest that improved coating adhesion would further enhance erosive wear behaviour of thick CVD diamond coatings.

Deposition of diamond coating on pure titanium using micro-wave plasma assisted chemical vapor deposition

The nucleation and growth of diamond coatings on pure Ti substrate were investigated using microwave plasma assisted chemical vapor deposition (MW-PACVD) method. The effects of hydrogen plasma, plasma power, gas pressure and gas ratio of CH4 and H2 on the microstructure and mechanical properties of the deposited diamond coatings were evaluated. Results indicated that the nucleation and growth of diamond crystals on Ti substrate could be separated into different stages: (1) surface etching by hydrogen plasma and the formation of hydride; (2) competition between the formation of carbide, diffusion of carbon atoms and diamond nucleation; (3) growth of diamond crystals and coatings on TiC layer. During the deposition of diamond coatings, hydrogen diffused into Ti substrate forming titanium hydride and led to a profound microstructure change and a severe loss in impact strength. Results also showed that pre-etching of titanium substrate with hydrogen plasma for a short time significantly increased the nuclei density of diamond crystals. Plasma power had a significant effect on the surface morphology and the mechanical properties of the deposited diamond coatings. The effects of gas pressure and gas ratio of CH4 and H2 on the nucleation, growth and properties of diamond coatings were also studied. A higher ratio of CH4 during deposition increased the nuclei density of diamond crystals but resulted in a poor and cauliflower coating morphology. A lower ratio of CH4 in the gas mixture produced a high quality diamond crystals, however, the nuclei density and the growth rate decreased dramatically.

Diamond coatings applied to mechanical face seals

The desired properties of a mechanical face seal are low friction and low wear rate. To fulfil these requirements the seal material needs a low friction coefficient, high hardness, high corrosion resistance, high thermal conductivity and a high thermal shock resistance. Diamond displays a low friction against almost all materials. Furthermore, diamond shows the highest hardness of all materials, is extremely corrosion resistant, has the highest thermal conductivity (four times that of copper) and has a high thermal shock resistance. Therefore, diamond has the potential of becoming a very competitive seal material. The use of diamond in this kind of application has not yet been realised since it is an expensive material and very difficult to shape into complex geometries. However, it has recently become possible to deposit diamond coatings onto various substrate materials with the desired shape and thereby give the component diamond properties. The objective of this study is to investigate the performance of diamond coated cemented carbide as a face seal material. Laboratory dry sliding tests have been performed as well as component tests with water as the pressure medium. Commercial face seals were used as reference. The results show that diamond coatings considerably lower the friction in mechanical face seals and are therefore an interesting alternative to seal materials commercially available today. The benefits from using diamond coatings include lower running cost, increased reliability and the possibility to use a more simple seal design.

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.

Diamond film synthesis in low pressure premixed flames of different fuel types

Various aspects of diamond film growth in low pressure flames are presented. Experiments are performed in a low pressure flat flame facility and diamond films are deposited on molybdenum substrates over areas of at least 2 cm2 or 5.5 cm2. Pure acetylene-oxygen (C2H2-O2), ethylene-oxygen (C2H4-O2), and methane-oxygen (CH4-O2) premixed flames are studied and compared for their use in depositing high quality diamond films. It is found that uniformity and quality are very sensitive to substrate temperatures Ts and flame equivalence ratios φ and vary between flames of different fuel types. The growth behavior in response to changes in fuel types is interpreted in terms of burning velocities, which determines the residence times of intermediate species in the post-flame region and strongly influences the flux of reactants to the substrate surface. These results can be used to project alternative, perhaps more cost-effective flame synthesis strategies to deposit diamond coatings.

Residual stress, Young's modulus and fracture stress of hot flame deposited diamond

Chemical vapour deposition (CVD) diamond coatings deposited on various substrates usually contain residual stresses. Since the residual stress affects the adhesion of the coating to the substrate, as well as the performance of the coating/substrate composite in many technical applications it is of importance to study the magnitude of these stresses. In the present study the hot flame method was used to deposit diamond coatings on cemented carbide inserts by scanning the surface with a nine flame nozzle. By varying the oxygen to acetylene flow ratio and the deposition time coatings of different qualities and thicknesses were obtained. The residual strain/stress of the coatings was measured by three different methods: X-ray diffraction using the sin 2 (Ψ) method, Raman spectroscopy and disc deflection measurements. To extract the residual stress from the strain data the Young's modulus was obtained from bending tests of diamond cantilever beams manufactured from free standing diamond films. The latter technique was also used to determine the fracture stress of the diamond films. All deposited coatings displayed a residual compressive strain/stress state. The residual strain in the diamond coatings did not vary with coating thickness (1.5 μm to 20 μm) but was found to increase from −1.8 × 10 −3 to −2.2 × 10 −3 with decreasing diamond quality. The compressive residual stress was found to decrease from −2 GPa to −1.3 GPa with decreasing diamond quality. This is mainly due to a decrease in Young's modulus (from 1.1 TPa to 0.6 TPa) with decreasing diamond quality. Also the fracture stress was found to decrease (from 1.8 GPa to 0.8 GPa) with decreasing diamond quality. The three methods used for measuring the stress state in the coatings, X-ray diffraction, Raman spectroscopy and deflection measurement, all give the same result. The deflection technique has the advantage that no information about the elastic properties of the coating is needed, whereas Raman spectroscopy has the best lateral resolution (≈5 μm) and is the fastest method (≈5 min).

Diamond film synthesis in low pressure premixed flames of different fuel types

Various aspects of diamond film growth in low pressure flames are presented. Experiments are performed in a low pressure flat flame facility and diamond films are deposited on molybdenum substrates over areas of at least 2 cm 2 or 5.5 cm 2. Pure acetylene-oxygen (C 2H 2-O 2), ethylene-oxygen (C 2H 4-O 2), and methane-oxygen (CH 4-O 2) premixed flames are studied and compared for their use in depositing high quality diamond films. It is found that uniformity and quality are very sensitive to substrate temperatures T s and flame equivalence ratios φ and vary between flames of different fuel types. The growth behavior in response to changes in fuel types is interpreted in terms of burning velocities, which determines the residence times of intermediate species in the post-flame region and strongly influences the flux of reactants to the substrate surface. These results can be used to project alternative, perhaps more cost-effective flame synthesis strategies to deposit diamond coatings.

Influence of oxygen on the deposition of diamond coatings by microwave plasma CVD

The effect of oxygen addition in the plasma composition and diamond structure has been investigated during diamond film growth by microwave-assisted chemical vapour deposition (MWCVD) from CH 4+ H 2 gas mixtures. Optical emission spectroscopy (OES) and mass spectrometry have been used for detection of chemical species present in the plasma. The results for the gas mixtures with and without O 2 show noticeable changes in the plasma composition, as detected by the presence of oxygenated species (mainly CO and OH), produced by the oxygen addition to the discharge. We have related the diamond content in the films with the CO mole fraction present in the discharge. The etching of the non-diamond compounds seems to be due to OH radicals as well as to atomic oxygen present in the discharge which are able to etch carbon at higher rates.

Deposition of diamond coatings on particles in a microwave plasma-enhanced fluidized bed reactor

Diamond growth on non-diamond particles in a fluidized bed reactor by microwave plasma-enhanced chemical vapor deposition (MPECVD) is reported. Si and SiO 2 powders are used as seed particles without pretreatment, and a 0.5–2.0% CH 4 in H 2 mixture with addition of 0–3% O 2 is used as the fluidizing gas as well as the carbon source. SEM images show that good quality diamond is deposited on Si and SiO 2 particles after 8 h at 9 Torr with flow rates of 160 seem of 2.0% CH 4 in H 2 and 3 sccm of O 2.

Low amplitude oscillating sliding wear on chemically vapour deposited diamond coatings

The tribological behaviour under low amplitude oscillatory sliding of diamond coatings obtained by chemical vapour deposition (CVD), also called fretting wear, was investigated. In a ball-on-flat configuration, CVD diamond coatings deposited on flat WC-Co substrates are worn against CVD-diamond-coated silicon nitride balls as well as against uncoated corundum balls. The influence of fretting parameters such as the contact load, displacement amplitude and test duration on the wear of the CVD diamond coatings deposited on flat WC-Co substrates has been studied. A criterion was derived to distinguish between different fretting regimes. Damage mechanisms such as elastic asperity interaction, abrasive sliding, particle ploughing and adhesive interaction are related to the fretting regimes. A correlation between the wear volume generated on CVD-diamond-coated WC-Co and the dissipated friction energy has been established. Based on this correlation, a prediction of the wear damage induced on CVD diamond coatings by low amplitude oscillatory sliding is shown to be possible when the contact conditions are known.

Friction and wear properties of diamonds and diamond coatings

Abstract The recent development of chemical vapor deposition techniques for diamond growth enables bearings to be designed which exploit diamond's low friction and extreme resistance to wear. However, currently produced diamond coatings differ from natural diamond surfaces in that they are polycrystalline and faceted, and often contain appreciable amounts of non-diamond material ( i.e. graphitic or amorphous carbon). Roughness, in particular, influences the friction and wear properties; rough coatings severely abrade softer materials, and can even wear natural diamond sliders. Nevertheless, the best available coatings exhibit friction coefficients as low as those of natural diamond and are highly resistant to wear. This paper reviews the tribological properties of natural diamond, and compares them with those of chemical vapor deposited diamond coatings. Emphasis is placed on the roles played by roughness and material transfer in controlling frictional behavior.

DIAMOND CHEMICAL VAPOUR DEPOSITION

Preparation, characterisation and selected applications of vapour deposited diamond coatings and free-standing membranes are reviewed. General trends and implications of a C/H/O-phase diagram, which provides a common basis for all diamond CVD methods are discussed.