Sulphur doped DLC films deposited by DC magnetron sputtering

Diamond‐like carbon (DLC) and sulphur doped diamond‐like carbon (S‐DLC) films were synthesised at different sulphur molar percentage of 0%, 2%, 5%, 8% and 10% by direct current (DC) magnetron sputtering process using novel compressed sulphur‐graphite targets at relatively low power density. Films were characterised for their morphologies, structural, electrical and optical properties. Scanning electron microscope images reveal changes in the quality of the obtained films shown by the denser packing of DLC grains at different sulphur percentage. The conductivities of S‐DLC films were found to be in the range of 6.0 × 10^?3–0.6/Ω cm. The optical band gap energies were found to be in the range of ～1.4–2.0 eV. Both electrical and optical measurements exhibit nonlinear responses with optimum at around 5% sulphur molar percentage (minimum for conductivity and maximum for optical band gap energy). These trends of change in both conductivity and optical band gap energy are consistent with the variation in bond characters of the films indicated by Raman spectroscopy. © 2011 Canadian Society for Chemical Engineering     

Growth and properties of hydrogen-free DLC films deposited by surface-wave-sustained plasma

Hydrogen-free diamond-like carbon (DLC) films were deposited by a new surface-wave-sustained plasma physical vapor deposition (SWP-PVD) system in various conditions. Electron density was measured by a Langmuir probe; the film thickness and hardness were characterized using a surface profilometer and a nanoindenter, respectively. Surface morphology was investigated using an atomic force microscope (AFM). It is found that the electron density and deposition rate increase following the increase in microwave power, target voltage, or gas pressure. The typical electron density and deposition rate are about 1.87 × 10¹¹–2.04 × 10¹² cm^− 3 and 1.61–14.32 nm/min respectively. AFM images indicate that the grain sizes of the films change as the experimental parameters vary. The optical constants, refractive index n and extinction coefficient k, were obtained using an optical ellipsometry. With the increase in microwave power from 150 to 270 W, the extinction coefficient of DLC films increases from 0.05 to 0.27 while the refractive index decreases from 2.31 to 2.11.     

Ultra-high temperature ablation behavior of MoAlB ceramics under an oxyacetylene flame

The ultra-high temperature ablation of a polycrystalline, fully dense, predominantly single phase MoAlB ceramic discs under an oxyacetylene flame is examined. The linear ablation rate decreases from 1.3 μm/s during the first 30 s to - 0.7 μm/s after 60 s when the surface temperature reached about 2050 °C (with a flame temperature around 3000 °C). Up to 60 s, the MoAlB is ablation resistant due to the formation of a protective and viscous surface Al₂O₃ layer. As the ablation time is prolonged, the protective Al₂O₃ scale becomes porous and is almost fully destroyed at the central ablation region after 120 s. This accelerates the formation of large amounts of volatile species (mainly B and Mo oxides), resulting in a reduction in the ablation resistance.     

Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD

The effects of CH₄ / C₂H₄ flow ratio and annealing temperature on the defect states and optical properties of diamond-like carbon (DLC) films deposited by novel microwave surface-wave plasma chemical vapour deposition (MW SWP CVD) are studied through UV/VIS/NIR measurements, atomic force microscopy, Raman spectroscopy and electron spin resonance analysis. The optical band gap of DLC has been tailored between a relatively narrow range, 2.65–2.5 eV by manipulating CH₄ / C₂H₄ flow ratio and a wide range, 2.5–0.95 by thermal annealing. The ESR spin density varied between 10¹⁹ to 10¹⁷ spins/cm³ depending on the CH₄ / C₂H₄ flow ratio (1 : 3 to 3 : 1). The defect density increased with increasing annealing temperature. Also, there is a strong dependence of spin density on the optical band gap of the annealed-DLC films, and this dependency has been qualitatively understood from Raman spectra of the films as a result of structural changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm).     

Chemical vapour deposition of diamond on nitrided chromium using an oxyacetylene flame

In this paper we present our first preliminary results on chemical vapour deposition (CVD) of diamond onto nitrided chromium using an oxyacetylene flame. Polycrystalline diamond films were obtained after deposition at very low substrate temperatures (<400°C). At these low temperatures there was extremely weak bonding, or no bonding at all, between the deposited layer and the substrate. To obtain stronger bonding, four growth experiments were carried out at initially higher substrate temperatures (700–1000°C). Whilst growth continued, the substrate temperatures were lowered step by step to 250°C. It was observed that on lowering the substrate temperature by more than about 500°C from the initial temperature, delamination occurred, suggesting that the thermal stresses exceeded the bonding strength. Subsequently, adherent diamond coatings were grown on the freshly exposed substrate surface whilst further lowering the substrate temperature. These diamond coatings were characterized using scanning electron microscopy and the adhesion of the diamond coatings to the substrates was assessed by means of the scotch tape test.     

Tribological properties of DLC films deposited on steel substrate with various surface roughness

Tribological properties of diamond-like carbon (DLC) films in water were investigated concerning with the influence of surface roughness and various mating materials. The DLC films were deposited by pulsed-bias CVD method on AISI630 stainless steel. The substrate roughness (R_a) is in the range of 1.4–740 nm. AISI 440C, AISI 304 stainless steel and brass balls were used as a mating ball. The friction coefficients of DLC films against with AISI 440C stainless steel ball indicated under 0.1 irrespective of the roughness. The film having smooth surface (R_a=1.4 nm) had severe damage at a load of 9.4 N. However, the film having rough surface (R_a=263 nm) had no damage at the same load. The specific wear rate of the steel ball increased with increase of roughness of the surface. In the case of AISI 304 stainless steel ball, the specific wear rate of the ball showed similar tendency. The friction with brass ball showed relatively high friction coefficient in the range of 0.12–0.25. However, the damage on the films could not be observed after friction test. It is considered that the roughness of the surface is important factor for the rupture of the film in water environment.     

Properties of barium hexaferrite thick films deposited by electron beam evaporation

This paper presents results obtained on barium ferrite thick films prepared by electron beam evaporation. First of all, we have looked at the influence of substrate temperature on the films properties. Then, we have fixed the substrate temperature to 700 °C and the effects of films thickness on the different properties has been investigated. Indeed for a substrate temperature of 700 °C we have observed that the layers are crystallized in the BaM phase with in-plane preferential orientation. However, a secondary non magnetic phase (BaFe₂O₄), which can modify the magnetic properties, appears on some layers.     

Photoconductivity of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited by a new pulsed DC discharge plasma chemical vapour deposition (CVD) using hydrogen and methane gas mixture. When methane concentration (Cm) i.e. CH₄/(H₂ + CH₄) was increased from 3 to 40%, the graphitization of the carbon film increases as evident from Raman study. When Cm was increased to 30%, DLC film shows photoconducting property. The white light photoconductivity (S = I_l/I_d, where I_l is light current and I_d is dark current) measured with solar simulator under AM 1.5 condition was approximately 20 at room temperature. The photoconductivity was not clear when Cm was lower than 20%. ESR measurements also show that the electron spin density was slightly decreased with decreasing concentration of methane. Thus we can conclude here that at higher concentrations of methane at 30%, Sp² content of the film increases and the DLC film becomes photoconducting.     

Ultra-high temperature ablation behavior of Ti2AlC ceramics under an oxyacetylene flame

The linear and mass ablation rates of Ti₂AlC ceramics under an oxyacetylene flame at a temperature up to 3000 °C were examined by measuring the dimensions and weight change of the ablated samples. The linear ablation rate was decreased from 0.14 μm s⁻¹ for the first 30 s of the ablation to 0.08 μm s⁻¹ after 180 s. Ti₂AlC ceramics gained small amounts of weight upon ablation, which is attributed to the formation of oxidation products on the ablated surface. The ablation surface exhibits a two-layer structure: an oxide outer layer, consisting mainly of α-Al₂O₃ and TiO₂ and some Al₂TiO₅, and a porous sub-surface layer containing Ti₂Al_1−xC and TiC_xO_y. With increasing ablation time, the content of TiO₂ and Al₂TiO₅ in the outer layer increased, and more pores developed in the sub-surface layer. The thermal oxidation of Ti₂AlC under the flame and scouring of the viscous oxidation products by high-speed flow of gas torch are the main ablation mechanisms.     

Antibacterial activity of DLC and Ag–DLC films produced by PECVD technique

Diamond-like carbon (DLC) films have been the focus of extensive research in recent years due to its potential application as surface coatings on biomedical devices. Doped carbon films are also useful as biomaterials. As silver (Ag) is known to be a potent antibacterial agent, Ag–DLC films have been suggested to be potentially useful in biomedical applications. In this paper, DLC films were growth on 316L stainless steel substrates by using Plasma Enhanced Chemical Vapour Deposition (PECVD) technique with a thin amorphous silicon interlayer. Silver colloidal solution was produced by eletrodeposition of silver electrodes in distilled water and during the deposition process it was sprayed among each 25 nm thickness layer DLC film. The antibacterial activity of DLC, Ag–DLC and silver colloidal solution were evaluated by bacterial eradication tests with Escherichia coli (E. coli) at different incubation times. With the increase of silver nanoparticle layers in Ag–DLC, the total compressive stress decreased significantly. Raman spectra showed the film structure did not suffer any substantial change due to the incorporation of silver. The only alteration suffered was a slightly reduction in hardness. DLC and Ag–DLC films demonstrated good results against E. coli, meaning that DLC and Ag–DLC can be useful to produce coatings with antibacterial properties for biomedical industry.     

Influence of plasma nitriding treatment on the adhesion of DLC films deposited on AISI 4140 steel by PVD magnetron sputtering

Duplex surface treatments composed of diamond like carbon (DLC) coating followed by plasma nitriding have drawn attention for a while. In this study, AISI 4140 steel substrates were plasma nitrided at different treatment temperatures and times. Then, DLC films were deposited on both untreated and plasma nitrided samples using PVD magnetron sputtering. The effect of different plasma nitriding temperatures and times on the structural, mechanical and adhesion properties of DLC coatings was investigated by XRD, SEM, microhardness tester and scratch tester, respectively. It was found that surface hardness, intrinsic stresses, layer thickness values and phase distribution in modified layers and DLC coating were the main factors on adhesion properties of duplex coating system. The surface hardness and residual stress values of AISI 4140 steel substrates significantly increased with both DLC coating and duplex surface treatment (plasma nitriding + DLC coating). Increasing plasma nitriding temperature and time also increased the diffusion depth and the thickness of modified layers. Hard surface layers led to a significant improvement on load bearing capacity of the substrate material. However, it was also determined that the process parameters, which provided lower intrinsic stresses, improved the adhesion properties of the duplex coating system.     

Properties of diamond films deposited by multi-cathode direct current plasma assisted CVD method

Four diamond films were prepared by the multi-cathode direct current plasma-assisted chemical vapor deposition (DC-PACVD) method and optical and thermal properties were characterized. Optical transmission and thermal conductivity were strongly dependent on the power density and the methane concentration. Impurities such as, H, Na, Al, Si, K, Ca and Ta were detected by SIMS analysis. The Ta concentration in diamond films was found to be around 300 ppm by RBS measurement and Ta inclusion originated from the Ta cathode kept above 2100°C. Optical and thermal properties of the diamond film deposited with a growth rate of 4 μm/h at 0.37 kW/cm² (17 kW on φ76 mm substrate) and 5% CH₄ were similar to that of the type IIa natural diamond.     

Ceramization and oxidation behaviors of silicone rubber ablative composite under oxyacetylene flame

Silicone rubber ablative composite filled with silica and carbon fibers was prepared and tested using an oxyacetylene torch. After the material was fired, the structure, composition and thermal-oxidative properties of the composite were analyzed. The results showed that a pyrolysis layer, a ceramic layer and a silica layer were formed in turn by decomposition, ceramization and oxidation reactions of the virgin ablative composite. Aromatic carbon was formed in the porous pyrolysis and ceramic layers by the degradation of the silicone rubber matrix, which transformed into inorganic carbon in the zone close to the silica layer. Crystallite growth of silicon carbide, the content of which is 10.2 wt% of the ablative products, is revealed in the ceramic layer. Oxidation of the compounds in the ceramic layer yields a silica layer, which is composed primarily of by silica. The thermo-oxidative stabilities of the ablative layers were better than that of the virgin material as a result of the formation of an inorganic ceramic structure.     

Capabilities of combined studies of DLC films by X-ray methods

We demonstrate the possibility of determining a large group of physical properties of DLC films using only one group of methods based on X-ray interference studies. These include methods the determination of the film thickness, material density and roughness of the surface. We present the analysis of possibilities to use the method of the two-crystal X-ray spectrometer to evaluate internal stress and to deduce the modules of elasticity and thermal expansion coefficients of the film. It is shown that this method can be used for the in-situ control of the film parameters during the film deposition in the technological chamber.     

Biological activity of silver-doped DLC films

The technique of combined DC metal arc and carbon pulsed arc was used to deposit thin solid films containing up to 6.5 at.% of silver. The microstructure and antibacterial properties of silver-doped diamond-like carbon (DLC) films have been investigated. Silver nanoparticle of flat disk shape located inside of an amorphous carbon matrix revealed excellent antibacterial properties concerning Staphylococcus аureus bacteria. Titanium substrates with DLC films doped with silver have an inhibiting effect on growth of some tumors, in particular, on rat neoplastic С6 glioma. This result is new and opens further possibility for application of DLC:Ag composite in medicine.     

Ablation behavior and mechanism of boron nitride - magnesium aluminum silicate ceramic composites in an oxyacetylene combustion flame

In the present study, ablation behavior and properties of BN-MAS (magnesium aluminum silicate) composites impinged with an oxyacetylene flame at temperatures up to 3100 °C were investigated. As ablation time ranged from 5 to 30 s, the mass and linear ablation rates increased from 0.0027 g/s and 0.001 mm/s to 0.0254 g/s and 0.087 mm/s, respectively. A SiO₂-rich protective oxide layer formed during the ablation process, which contributed to the oxidation resistance of the composites. Ablation products mainly consisted of magnesium-aluminum borosilicate glass, mullite, spinel and indialite. The thermal oxidation of h-BN during flame ablation and scouring of MAS by high-speed gas flow were the main ablation mechanisms.     

Charge carrier lifetime in DLC films

Diamond-like carbon (DLC) layers deposited at room temperature in 13.56 MHz radio-frequency methane (CH₄) plasma have been studied. The results of transient currents for DLC thin films are reported. The carrier's lifetime was determined based on the transient current analysis for the surface and bulk recombinations: t_rs=0.3 ms, t_rv=0.11 ms. These values seem to be relatively high for structures of this type. The diffusion length for DLC films L*=0.67×10⁻⁴ cm. Other parameters such as the diffusion coefficient D*=4×10⁻⁵ cm²/s and surface recombination rate S=0.37 cm/s are exceptionally small here.     

Ablation behavior of (TiZrHfNbTa)C high-entropy ceramics with the addition of SiC secondary under an oxyacetylene flame

The ablation behavior of high-entropy ceramics (HECs) was investigated in this study using an oxyacetylene flame at 2000 °C. Spark plasma sintering was used to construct a dense HEC (TiZrHfNbTa)C with a 20 vol% of SiC addition (HEC-20SiC). The densification of HEC-20SiC can be improved to a certain extent by adding SiC particles, increasing the hardness of HEC-20SiC to up to 24.6 GPa, and the crack deflection observed through the addition of SiC particles were considered to be the strengthening and toughening mechanisms. After ablation, Hf₆Ta₂O₁₇, Ti_5.1Ta_4.9O₂₀, Nb₂Zr₆O₁₇, TaZr_2.75O₈, and SiO₂ can be detected on an ablated surface and HEC-20SiC possesses the minimum mass ablation rate (?1.9 mg s^?1) and line ablation rate (2.1 μm s^?1) among the comparative ceramics. On the one hand, the SiC phase forms gaseous CO, CO₂, and SiO as well as viscous SiO₂ during ablation and some part of the heat can be dissipated by the evaporation of gaseous CO, CO₂, and SiO; further, pore defects can be healed by viscous SiO₂, thus inhibiting the diffusion of reactive oxygen species. On the other hand, the HEC phase with a lattice-distortion caused by single-phase solid-solution can effectively inhibit the invasion of reactive oxygen species and the outward migration of metal atoms. The invasion rate of reactive oxygen is considered to be the main step during HEC-20SiC ablation, and it is believed that higher principal component HECs can improve ablation performance even further.     

Characterization of DLC:Si films by the gas effusion technique

In this work an investigation of hard DLC:Si films by the gas effusion technique is presented. Effusion of hydrogen, methane and higher hydrocarbons was studied for films with silicon contents of up to 40 at%. Three major contributions to the effusion spectra could be identified: (i) a desorption-limited mechanism from the internal surfaces of a network of voids which could be observed even for large hydrocarbon molecules, indicating that low silicon content material possesses a porous structure; (ii) a sharp peak related to the abrupt graphitization of the films which dominates the spectra for hydrogen and methane effusion in the low concentration range and is gradually shifted to high temperatures as the silicon content is increased; and (iii) a diffusion-limited mechanism that appears for high silicon content films, suggesting that the films undergo a transition from porous to a relatively compact structure.     

Cyclic ablation behavior of mullite-modified C/C-HfC-SiC composites under an oxyacetylene flame at about 2400 °C

Mullite-modified C/C-HfC-SiC composites were prepared via precursor infiltration and pyrolysis (PIP). The phase composition, microstructure, and cyclic ablation behavior under oxyacetylene flame with a heat flux of 4.18 MW/m² were investigated, and a comparison with the C/C-HfC-SiC composites showed that the mullite-modified composites have better ablation resistance. Results displayed a HfO₂ skeleton structure wrapped by SiO₂ and a dense layer of HfO₂-SiO₂ in the center and transition regions after the first ablation for 30 s, respectively. The structures transformed into HfSiO₄-wrapped carbon fibers and "island" shape HfO₂-HfSiO₄-SiO₂ layer after the second ablation for 30 s. Then both structures underwent severer peeling of HfSiO₄ and consumption of SiO₂ after the third ablation for 30 s. The modified composites exhibited better mass ablation rates after forming HfSiO₄, which were 0.36 mg/s cm² and 0.38 mg/s cm², respectively.