Amorphous Diamond-like Carbon Coatings Research Articles

Multilayer diamond-like amorphous carbon coatings produced by ion irradiation of polymer films

Diamond-like carbon (DLC) is known to be a versatile coating material, being found in a wide variety of surface engineering applications. In this study, we produced DLC coatings by ion irradiation and thus reconstruction of a thin polymer film. This procedure leads to a thin diamond-like carbon layer which is characterized by its high hardness. Due to the finite ion range during implantation, however, the maximum thickness of such coatings is limited. Nevertheless, several applications for DLC coatings demand a larger thickness. We thus increased the thickness of such DLC coatings by repeatedly applying the deposition and transformation process, thus creating thicker multilayers. We show that their physical and mechanical properties like roughness, nano-hardness, sp3 fraction as well as the very important adhesion of thick DLC films produced this way are very well suited for the desired applications. Although we find a small decrease in hardness and a little bit less sp3 fraction with increasing coating thickness, our results exhibit a typical hardness as well as sp3 fraction for a-C:H even for the largest coating thicknesses produced so far.

Hydrogen permeability of diamondlike amorphous carbons

The hydrogen barrier properties of the coatings of nitride and diamondlike amorphous carbon (DLC) were evaluated. Using plasma chemical vapor deposition and sputtering, ZrN, TiAlN, AlCrN, CrN, and DLC coatings were deposited with a thickness of about 3 μm. It was confirmed that all coatings have an impact on decreasing the hydrogen permeation rate. Specifically, by coating stainless steel with a DLC coating, the hydrogen permeation rate was reduced to 1/1000 or lower compared to that without a coating. By providing a buffer layer of Cr-N, the hydrogen barrier function was improved. DLC coatings with high hydrogen content had an especially high hydrogen barrier function. For hydrogen diffusion in coatings, the movement of atoms through hydrogen trap sites is important. There are both sp3 and sp2 bonds in DLC coatings, and excess hydrogen can be found in the interstitial space and the above-mentioned hydrogen trap sites. It is suggested that the hydrogen trap sites in a DLC coating with high hydrogen content are likely already filled with hydrogen atoms, and the movement of new hydrogen atoms could be limited.

On the study of the mechanical properties of Mo-B-C coatings

Mo 2 BC thin films show a favourable combination of high stiffness, hardness and elastic modulus together with moderate ductility. In this study we focused on the comparison of mechanical properties of Mo-B-C thin films with different structures (nanocrystalline or amorphous). The thin films were deposited on steel, hard metal and silicon substrates using DC magnetron sputtering. The mechanical properties of Mo-B-C films were studied using indentation techniques under both quasistatic and dynamic conditions using a wide range of loads from 50 μ N up to 1 N. The results showed that even amorphous Mo-B-C thin films had high hardness of 19.5 ± 0.5 GPa and elastic modulus of 276 ± 5 GPa. Their hardness is comparable with the common amorphous diamond-like carbon coatings. Moreover, their fracture toughness is significantly higher. The results of mechanical tests were correlated with microstructure observations carried out using scanning and transmission electron microscopy. The images of the deformed area under the residual indentation imprints showed no cracking even after high loads or after indentation with sharp cube corner indenter.

Diamond-Like Carbon Coatings on Ureteral Stents—A New Strategy for Decreasing the Formation of Crystalline Bacterial Biofilms?

Any catheter material placed in the urinary tract provides a surface for bacterial colonization and, therefore, it is susceptible to encrustation with crystalline bacterial biofilm. Encrustation and blockage by biofilms remain a major complication in patient care. Most patients with indwelling ureteral stents experience irritative symptoms related to these implants and many experience discomfort. Plasma deposited diamond-like amorphous carbon coatings are well-known for their excellent biocompatibility. A low temperature, low pressure plasma enhanced chemical vapor deposition technology was developed especially for coating polymeric medical implants with diamond-like carbon. We investigated the ability of diamond-like carbon to decrease the formation of crystalline bacterial biofilm as well as stent related side effects and discomfort. Diamond-like carbon coated ureteral Double-J stents were tested in vivo. In 10 patients with heavy encrustation, different underlying diseases and a stent removal frequency of less than 6 weeks due to encrustation a total of 26 diamond-like carbon coated stents were successfully tested for their ability to decrease the extent of crystalline biofilm formation. There was a 2,467-day period of experience with diamond-like carbon coated stents. No primarily stent related complications occurred. No crystalline biofilm formation was observed in vivo. Excellent and facile handling, a less painful replacement procedure and high tolerance of application were reported by physicians and patients. Due to low friction the coated stents could be placed and removed much more easily than standard stents. The frequency and severity of symptomatic urinary tract infections were distinctly decreased. Diamond-like carbon coating is a new strategy to improve the surface properties of ureteral stents. This novel surface effectively decreases friction, encrustation tendencies and biofilm formation.

The effect of temperature on the tribological mechanisms and reactivity of hydrogenated, amorphous diamond-like carbon coatings under oil-lubricated conditions

In this work we present the wear and friction behaviour of boundary-lubricated, hydrogenated, amorphous, diamond-like carbon coatings (a-C:H), in self-mated a-C:H/a-C:H contacts, at three different testing temperatures: 20, 80, 150 °C. We present results from Auger electron spectroscopy, X-ray photoelectron spectroscopy and Raman analyses relating to the chemical and structural changes in the diamond-like carbon coatings during sliding in the presence of mineral oil, with and without additives. We show, that chemical reactions between the a-C:H coatings and the oil additives take place, which are dependent on the temperature, on the presence of additives and the type of additives used. At high temperatures the extreme pressure additive interacts with the diamond-like carbon surface and forms a tribochemical layer with a four-times lower sulphur/phosphorous ratio than the additive formulation. In the absence of additives, however, graphitisation of the coating occurs under these conditions, which results in high-wear and low-friction behaviour. Another result from this study is that a-C:H coatings can oxidise during room-temperature experiments, suggesting that some interactions and adsorptions are also possible at lower temperatures.

Reciprocating sliding behaviour of self-mated amorphous diamond-like carbon coatings on Si3N4 ceramics under tribological stress

Amorphous diamond-like carbon films grown by magnetron sputtering have been deposited on silicon nitride based substrates for tribological purposes. A conductive Si3N4/30% vol.TiN composite was produced for bias substrate application. Friction and wear properties of carbon coated self-mated pairs were assessed using a reciprocal motion ball-on-flat setup in unlubricated conditions with applied normal loads of 3 N and 5 N. The worn surfaces were studied by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) in order to identify the prevalent wear mechanism. Unbiased and biased substrates behaved differently, the former undergoing premature delamination while the latter endured the tribological test conditions (3 N, ∼ 43 m). Very low friction coefficient values of ∼ 0.015 were sustained assuring remarkable wear behaviour. Surface grooving and wear debris accumulation in the sliding track lead to a roughness increase from the nominal rms value of ∼ 12 nm to ∼ 97 nm, although no weight loss and surface profile modification was quantifiable.

Influence of Silicon, Oxygen and Nitrogen Admixtures Upon the Properties of Plasma Deposited Amorphous Diamond-Like Carbon Coatings

AbstractAmorphous diamond-like carbon films (DLC) with various silicon, oxygen and nitrogen content were deposited by plasma enhanced chemical vapor deposition (PECVD) technique. The films were prepared from the mixture of methane and hexamethyldisiloxane (HMDSO) in r.f. capacitively coupled discharges (13.56 MHz). The reactive plasma was investigated by optical emission spectroscopy and capacitive coupled planar probe. A combination of RBS, ERDA, FTIR and XPS methods was used to study the films’ chemical composition and structure. The mechanical properties were studied using a depth sensing indentation technique. The films were mainly composed of C-C, C-H and C-Si bonds. The optimum deposition conditions for the preparation of DLC films, with enhanced thermo-mechanical stability, were determined.

Diamond-like amorphous carbon coatings for large areas of glass

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with a significant fraction of sp 3 bonding. DLC is a semiconductor having high mechanical hardness, chemical inertness and optical transparency. The films have widespread applications as protective coatings in areas such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMS). To date, such films have been deposited using mass selected ion beams, filtered cathodic vacuum arcs and pulsed laser ablation. These techniques are, however, limited in their areal coverage and throughput. Also, film adhesion to certain substrates, such as soda-lime glass, and the presence of micro-particles are still serious limitations. The primary motivation behind the present work is to develop a large area deposition technique, which produces highly transparent hydrogenated tetrahedral a-C thin films (ta-C:H) that acts as a scratch resistant layer and corrosion barrier on soda lime glass. Large area deposition of DLC, with sp 3 content as high as 80%, directly onto 1.5 m wide glass substrates is reported.

Defect characterisation in amorphous diamond-like carbon coatings

Amorphous diamond-like carbon coatings, produced by ion beam assisted physical vapour deposition (IBAD) and plasma enhanced chemical vapour deposition (PECVD) on Si(100) substrates have been studied by positron beam and other techniques. Both layers have similar positron annihilation characteristics and a density of 2.4±0.3 g cm −3, and are probably significantly hydrogenated. In the IBAD sample there is significant damage to the substrate, including a deep armorphised region and evidence of defect states at the interface to the carbon.

Improving the adhesion of amorphous carbon coatings on cemented carbide through plasma cleaning

Abstract Diamond-like amorphous carbon coatings 1 μm thick were deposited onto cemented carbide substrates by magnetron sputtering of a graphite target in argon under different substrate bias powers and chamber pressures. Scratch testing was used to assess the coating adhesion. X-ray photoelectron spectroscopy depth profiling was employed to quantify cobalt loss at the substrate surface as a function of bias power during plasma cleaning. It was found that under the same deposition conditions, the scratch adhesion strength increased with the bias power during plasma cleaning and reached a maximum at about 200 W or −210 V in terms of induced voltage. After that, further increases in bias power led to a decrease in adhesion. The increase was attributed to better cleaning of the sample surface and removal of surface cobalt while the decrease in adhesion was linked to an increase in residual stress which resulted in a different failure mechanism. Thus, an increase in the deposition power density, and therefore more severe ion bombardment, led to higher residual stress and lower adhesion. Under constant bias and deposition power, however, it was established that below a certain minimum chamber pressure spontaneous coating detachment occurs.

Amorphous diamond-like carbon coatings with graded metal interfaces

In laser cladding, high cooling rates create outcomes with superior mechanical and metallurgical properties. However, this characteristic along with the additive nature of the process significantly contributes to the formation of thermal stresses which are the main cause of any potential delamination and crack formation across the deposited layers. This drawback is more prominent for additive materials such as Stellite 1 which are by nature crack-sensitive during the hardfacing process. In this work, parallel to the experimental investigation, a numerical model is used to study the temperature distributions and thermal stresses throughout the deposition of Stellite 1 for hardfacing application. To manage the thermal stresses, the effect of preheating the substrate in a localized dynamic fashion is investigated. The numerical and experimental analyses are conducted by the deposition of Stellite 1 powder on the substrate of AISI-SAE 4340 alloy steel using a 1.1 kW fiber laser. Experimental results confirm that by preheating the substrate a crack-free coating layer of Stellite 1 well-bonded to the substrate with a uniform dendritic structure, well-distributed throughout the deposited layer, can be obtained contrary to non-uniform structures formed in the coating of the non-preheated substrate with several cracks.

Investigation of mechanical and tribological properties of amorphous diamond-like carbon coatings

We investigated the mechanical and tribological properties of amorphous diamond-like carbon (DLC) coatings deposited on Si(100) by a pulsed bias deposition technique. Tribological studies were performed using a pin-on-disc (POD) apparatus under a normal load of 6.25 N and at 10% relative humidity, with a ruby pin as a slider. Hardness measurements were performed using a nanoindenter and apparent fracture toughness using indentation techniques. We studied the influence of residual stresses on apparent fracture toughness. The data revealed that the thickness, hardness and compressive stress of the coating play different roles in the apparent fracture toughness. Crack initiation is influenced by the thickness and hardness of the coating, whereas crack propagation is influenced by the compressive stress in the film. The apparent fracture toughness of DLC coatings increased with coating hardness.

The electrochemical and wear behaviour of amorphous diamond-like carbon coatings and multilayered coatings in aqueous environments

R.f. plasma CVD deposited diamond-like carbon coatings (DLC) possess interesting wear properties in dry testing conditions or in vacuum applications. Extremely low friction coefficients are displayed in dry test conditions, the friction coefficient being dependent on relative humidity and presence of oxygen. The question arises whether these coatings are equally effective in an aqueous medium. Parameters controlling the through-coating porosity are discussed and their relative importance indicated. A coulometric quantitative porosity measurement has been used. Characterisation by light optical microscopy, profilometric measurements and EPMA suggest the importance of the microstructure of the substrate material. Further, pin-on-disk wear tests in water show a detrimental damage when compared to dry wear tests. De-adhesion of the coating occurs in an early stage with subsequently a rapid destruction of the coating. Important parameters in this failure mechanism are the initial contact stress and the development of this stress by initial wear.

Graphite particles in the diamond-like a-C films prepared with the pulsed arc-discharge method

Abstract Microparticle collection on the coating surface has been studied during the preparation of diamond-like amorphous carbon coatings with the pulsed arc-discharge method. The curved solenoid method was used for the particle filtering. The quality of the coatings was studied using profilometer, ESCA and SEM techniques and wear, friction and acid resistance tests. The coatings without and with filtering contained graphite particles (40% and less than 1% respectively) and the sp 2 sp 3 -bonding ratios were 51% and 78% respectively. The acid resistance of the nonfiltered films was poor. On the contrary, in pin-on-disc experiments with both types of coating against polyethylene, steel and Al2O3 pins, no clear differences in the friction and wear measurements were observed.

Tribological properties of amorphous diamond-like carbon coatings

A wide range of different amorphous diamond-like carbon (ADLC) coatings are now commercially available. Centre Suisse d'Electronique et de Microtechnique (CSEM) made a tribological characterization of amorphous carbon coatings obtained from seven different suppliers. Pin-on-disk tests using a CSEM tribometer showed generally lower friction and longer life in humid air than in dry air and/or vacuum. Tests are carried out with AISI 52100 ball bearing steel balls, under the conditions of 5 N load at 10 cm s -1 sliding speed, in dry air (relative humidity (RH), less than 3%) or humid air (RH, greater than 97%). Only one supplier proposed a coating showing self-lubricating properties in both dry and humid air. In vacuum, the friction of steel against hard coatings such as TiC or ADLC is high, but the sliding against each other shows good tribological properties. For sliding operation in air as well as in vacuum, the combination of a TiC coating against an ADLC coating can lead to stable and low friction and very low wear, under environmental conditions ranging from vacuum to humid air. In fact, the friction and wear behavior of this couple is even better than that of MoS 2-coated pairs.

Hydrogen-free amorphous carbon coatings deposited by the arc ion-plating process

Industry is very interested in the production of diamond and diamond-like coatings because of their unusual properties. In the meanwhile some deposition processes for diamond coatings are used industrially. These processes are carried out at high temperatures of 800–1000°C; therefore only some materials such as cemented carbides can be coated. Diamond-like amorphous carbon coatings are an alternative. These coatings show diamond-like properties and are very suitable for tribological applications. Some industrially used coating processes allow deposition temperatures of around 200°C. A relatively new method for the deposition on large substrates at low temperatures is the arc ion-plating process. In this process, carbon is evaporated and partially ionized by a high energy arc beam. The regulation of the coating properties, e.g. hardness and electrical resistance, can be achieved in a wide range by controlling such process parameters as the bias voltage or gas pressure. With the total absence of hydrogen and hydrogenous reactive gases, the growth of polymer-like parts in the coatings can be suppressed. This paper describes the deposition of amorphous diamond-like carbon coatings onto different substrate materials using the arc ion-plating process. Some characteristic properties of these coatings are shown by an example for a tribological application.

R.f. plasma-assisted chemical vapour deposition of diamond-like carbon: physical and mechanical properties

Hydrogenated amorphous diamond-like carbon coatings have been deposited on glass and silicon substrates using an r.f. plasma-assisted chemical vapour deposition process. The mechanical properties of these coatings such as stress (bending beam measurements), hardness and Young's modulus (nanoindentation measurements) were studied as a function of the r.f. bias voltage in the range 50–500 V. IR absorption spectroscopy and elastic recoil detection analysis were used to correlate these mechanical properties with the total hydrogen content and the hydrogen bonding configuration. Compressive stress, hardness and Young's modulus are shown to reach their maximum values simultaneously at a bias voltage of about 200 V. At the same time, the hydrogen-bonded sp 3:sp 2 ratio reaches a local minimum.

Characterization of chemical bonding and physical characteristics of diamond-like amorphous carbon and diamond films

Diamond-like (amorphous) carbon (DLC) films were prepared by dc magnetron sputtering and plasma enhanced chemical vapor deposition (PECVD) and diamond films were prepared by microwave plasma enhanced chemical vapor deposition (MPECVD). For the first time, chemical and mechanical characterization of the films from each category are carried out systematically and a comparison of the chemical and physical properties is provided. We find that DLC coatings produced by PECVD are superior in microhardness and modulus of elasticity to those produced by sputtering. PECVD films contain a larger fraction of sp3-bonding than the sputtered hydrogenated carbon films. Chemical and physical properties of the diamond films appear to be close to those of bulk diamond.