Simo Varjus

Friction and wear properties in dry, water- and oil-lubricated DLC against alumina and DLC against steel contacts

Abstract Diamond-like carbon (DLC) films can be divided into two major categories according to their hydrogen content. These categories have similarities in tribological performance, but the films also behave in a different manner in different tribological conditions. The results of amorphous hydrogenated carbon films (a-C:H) and hydrogen-free hard carbon films (a-C) are reported in this study. The a-C:H films were deposited using the radio frequency (rf) plasma technique, and the hydrogen-free hard carbon films using pulsed vacuum arc. The coatings were characterized and investigated with respect to their tribological performance in dry (50% RH), water-lubricated and oil-lubricated slow sliding conditions (0.004 m s −1 ). The a-C and a-C:H films had a low friction coefficient in dry sliding conditions (0.15 to 0.22), which was further decreased by 10–40% under boundary lubrication. The a-C:H(Ti) films exhibited good self-lubricating properties (0.10) in dry sliding conditions and the a-C films had the lowest friction coefficient in water- (0.03) and oil-lubricated (0.08) conditions. The hydrogen-free hard carbon films showed excellent wear resistance in dry, water- and oil-lubricated conditions, but hydrogenated a-C:H films suffered from severe wear in aqueous conditions. The performance of a-C:H films could be improved by titanium alloying. In dry sliding conditions, the tribolayer formation of DLC films influenced the friction and wear performance, but in oil-lubricated conditions boundary lubrication layers were formed, which governed the tribological mechanisms in the contact.

Differentiating the tribological performance of hydrogenated and hydrogen-free DLC coatings

Diamond-like carbon (DLC) coatings cover a wide range of different types of carbon-based coatings, which generally have properties such as low friction and high wear resistance. DLC films can be divided into two major groups based on their hydrogen content, namely hydrogenated and hydrogen-free carbon coatings. This presentation describes the research work on amorphous hydrogenated carbon films (a-C:H) and hydrogen-free tetrahedral amorphous carbon films (ta-C). These coating types offer low friction performance and good wear resistance, but they also have some dissimilarities in the friction and wear properties due to their different hydrogen content and microstructure. The a-C:H films deposited for this study were produced by the rf plasma deposition technique and the ta-C films by the pulsed vacuum arc technique. The tribological performance of the a-C:H and ta-C coatings was evaluated by pin-on-disc tests carried out in normal atmosphere. Selected coatings were also evaluated in dry atmosphere. The wear resistance of the ta-C films was higher compared to the a-C:H films, but the ta-C films caused higher wear of the counterpart. The increase in normal load and sliding velocity decreased the friction coefficient of the a-C:H coating against the steel and alumina counter-face, whereas the ta-C coating showed more stable friction performance. The micro-Raman studies showed clear graphite formation for the a-C:H film, whereas the graphite formation on the ta-C film was not so evident. In dry conditions the ta-C had a high friction coefficient, which could be reduced by doping the film with hydrogen. The results show that the hydrogen content together with graphitisation plays an important role in the friction performance of DLC films.

Effect of tribofilm formation on the tribological performance of hydrogenated carbon coatings

Amorphous hydrogenated carbon (a-C:H) films were deposited by the r.f. plasma technique on stainless steel substrates. Pin-on-disc experiments were carried out over a wide range of normal loads (5–40 N) and sliding velocities (0.1–3.0 m s−1) in order to study the friction and wear performance of the coating against steel and alumina. The friction coefficient of a-C:H films against both steel and alumina pins decreased with increasing load and sliding velocity. On the pin wear surface, tribolayer formation was detected. The wear of the alumina pins increased with increasing load and sliding velocity when they contacted the coating. However, the thick tribolayer formed on the wear surface of the steel pin protected it from excessive wear when high loads and sliding velocities were applied. The wear surfaces were analysed by secondary ion mass spectroscopy and Auger electron spectroscopy. The analyses revealed that the thick tribolayer formed on the pin wear surface mainly consisted of the oxides of the pin material. However, evidence of carbon was found in the sliding deposit formed in front of the contact area of the pin and also in some cases on the pin wear surface. Carbon played an important role in the low friction behaviour although the amount of carbon was low. It is assumed that a thin tribolayer with low shear strength, consisting of carbon species, is formed on the disc wear surface. The coating wear increased when the normal load was increased. Some transfer of pin material was observed on the coating wear surface.

Characterization of wear surfaces in dry sliding of steel and alumina on hydrogenated and hydrogen-free carbon films

Abstract Hydrogenated amorphous carbon coatings were deposited by r.f. plasma and hydrogen-free carbon films in pulsed arc discharge on stainless steel substrates. The coatings were characterized and evaluated in tribological tests. Pin-on-disc tests were used over a wide range of test parameters: normal load, 5–40 N; sliding velocity, 0.1–3.0 m s −1 . The wear of both coatings was of the same order of magnitude (0.7 × 10 −3 −5.1 × 10 −3 mm 3 ). However, the wear of the counterface was one order of magnitude higher for the hydrogenfree carbon coatings. Increasing the normal load generally caused an increase in coating wear and in most cases also an increase in counterface wear. When the steel pin was sliding against the hydrogenated carbon coating with a high sliding velocity and load, a rather thick tribofilm was formed on the pin wear surface, lowering the coefficient of friction and reducing the pin wear. The tribofilm formed on the alumina pin sliding against the hydrogenated carbon film also seemed to reduce the friction coefficient but could not prevent the pin wear. A tribofilm was also formed on the pin wear surface when the hydrogen-free carbon coating was sliding against the steel and alumina pins, but the layer was not able to protect the pins. The tribofilm did, however, lower the coefficient of friction, which was rather insensitive to the different test parameters used. According to secondary ion mass spectroscopy analyses, material transfer of the pin was detected on the disc (coated) wear surfaces. The tribofilms formed on the pin wear surfaces consisted of pin material, hydrogen, oxygen, and carbon.

Load-carrying capacity evaluation of coating/substrate systems for hydrogen-free and hydrogenated diamond-like carbon films

Diamond-like carbon (DLC) coatings have shown excellent tribological properties in laboratory tests. The coatings have also been introduced to several practical applications. However, the functional reliability of the coatings is often weakened by adhesion and load-carrying capacity related problems. In this study the load-carrying capacity of the coating/substrate system has been evaluated. The DLC coatings were deposited on stainless steel, alumina and cemented carbide with two different deposition techniques: the tetrahedral amorphous carbon (ta-C) coatings were deposited by a pulsed vacuum arc discharge deposition method and the hydrogenated carbon (a-C:H) films by radio frequency (r.f.) plasma deposition method. The load-carrying capacity of the coated systems was evaluated using a scratch test, Rockwell C-indentation test and ball-on-disc test. The effect of substrate material, substrate hardness, coating type and coating thickness was studied. An increase in substrate hardness increased the load-carrying capacity for the coated systems, as expected. The two coating types exhibited different performance under load due to their different physical and mechanical properties. For the load-carrying capacity evaluations the ball-on-disc configuration was found to be most suitable.

Microstructural changes in DLC films due to tribological contact

Abstract The hydrogenated (a-C:H) and hydrogen free (ta-C) films were tested in sliding wear tests. The tests were carried out in air at room temperature (RH 15±5%). The coefficient of friction in the tests was in the range 0.03–0.12 for the a-C:H films and 0.17–0.28 for the ta-C films. The wear volume of the DLC films was sufficiently low to enable the direct comparison of the film properties of the worn film to the original film. The microstructure of the films after wear tests has been characterized by micro RAMAN, the mechanical properties were measured by nano indentation and the elastic modulus was measured with the laser-acoustic method. No change of the micro structure of the DLC films was observed. The observed reduction of the elastic modulus of the ta-C film is explained by the evolution of mechanical defects such as micro-cracks and cracking at the nodular defects typical in non filtered vacuum arc depositions

Improvement of a-C:H film adhesion by intermediate layers and sputter cleaning procedures on stainless steel, alumina and cemented carbide

Abstract The adhesion of amorphous hydrogenated carbon (a-C:H) films deposited in a radio frequency (r.f.) plasma discharge on stainless steel, alumina and cemented carbide with different intermediate layers (Ni, Ti and TiC) and sputter cleaning procedures was studied. The composition of the carbon films and the intermediate layers as well as the interface between the coating and the substrate was determined by secondary ion mass spectroscopy (SIMS). The adhesion experiments were carried out using a scratch tester. Tested specimens were also studied by scanning electron microscopy (SEM) to reveal the morphology of the coatings and the scratches. Without any intermediate layer, the a-C:H coatings generally had insufficient adhesion to the substrate materials studied. For stainless steel and cemented carbide substrates, the TiC intermediate layer and, for alumina substrates, the titanium intermediate layer gave the best adhesion values evaluated by the scratch test. Also, the sputter cleaning of the substrates prior to deposition was necessary for sufficient adhesion of the coating. The intermediate layers also change the failure mode of the coating in the scratch test in some cases.

Low friction ta-C films with hydrogen reservoirs

Ž. 3 Diamond-like tetrahedral amorphous carbon ta-C films, which contain no hydrogen and high content of sp bonds have a Ž. very high hardness and elastic modulus up to 600 GPa . They have also excellent tribological properties in atmospheres that Ž. contain humidity 50% RH or also when submerged in water. However, in dry atmospheres the co-efficient of sliding friction against bearing steel, for instance, increases to values of over 0.7. In this paper ta-C films were modified by adding hydrogen to the films. The ta-C films were deposited by using non-filtered pulsed vacuum arc deposition in the ambient pressure of hydrogen and hydrocarbon gases. A novel layered structure containing titanium layers with hydrogen and carbon have also been grown by using filtered metal arc in combination with the pulsed carbon arc. The hydrogen content and the composition of the films were Ž. measured by using time-of-flight elastic recoil detection analysis TOF-ERDA . The hardness of the films was measured by nanoindentation. The tribological performance of the coatings were determined by using pin-on-disk tests. The counter part Ž. material was steel AISI 52100 , the normal load applied 5 N and the sliding velocity 0.02 ms. In the modified films the hydrogen content was up to 16 at.% in ta-C and up to approximately 50 at.% in the titanium layers. A co-efficient of friction of approximately 0.2 at dry atmosphere was measured for hydrogen-containing ta-C and also for layered structures where a hydrogen-containing Ti layer was buried under a hydrogen free ta-C layer. The results indicate that the lowered co-efficient of friction is a result of hydrogen transport to the contact surfaces with the aid of surface diffusion through the pinholes of the ta-C layer. 2001 Elsevier Science B.V. All rights reserved.

Transfer layers in tribological contacts with diamond-like coatings

The tribological properties of amorphous hydrogenated carbon (a-C:H) films deposited on steel AISI440 B in a radio frequency (RF) assisted plasma were studied. The films were studied using a pin-on-disc machine with steel AISI 52100 and alumina as the counterface materials. The sliding velocities varied from 0.1 to 3.0 m/s and the normal forces from 5 to 40 N. The tests were carried out unlubricated in room air at 22±2 °C temperature and with 50±5 % relative humidity. Tribofilms formed on the pin and coating wear surfaces were studied by secondary ion mass spectrometry (SIMS). The coefficient of friction strongly depended on both the sliding velocity and the load. For the steel pin sliding against coating the coefficient of friction varied from μ=0.42 to μ=0.1 and for the alumina pin sliding against coating the coefficient of friction varied from μ=0.13 to μ=0.02. The formation of a tribofilm on the pin had a significant effect on both the friction and the wear properties of the coating.