ICMCTF2018 Session B3: Deposition Technologies and Applications for Diamond-like Coatings

Hydrogen free and hydrogen containing amorphous carbon materials are commonly referred to as diamondlike carbons. Due to the hardness and the ability to deposit coatings at temperatures compatible with most engineering materials, diamondlike carbons have become widely used as tribological coatings for addressing friction and wear in mechanical components. There are few if any intrinsic properties of these synthetic materials since their mechanical, structural, and compositional properties are strongly dependent upon the deposition process conditions. Furthermore, a high level of fundamental knowledge on how material properties relate to the tribological performances of diamondlike carbon coatings in various application environments has not been achieved. Application environments can be categorized by the type of tribological contact (rolling, sliding, mixed mode), lubrication condition, temperature, atmosphere, loading, and other items. This presentation shall discuss several examples where subtle differences in the deposition processes of diamondlike carbon coatings were responsible for transitioning unacceptable tribological performances in specific application environments to acceptable ones.

Traditionally, watch manufacturers use liquid lubricants, sometimes applied to up to 100 individual lubrication points, to achieve the required low and stable friction behaviour between components and sufficient lifetime performance. Despite the careful treatment of parts and use of specific (often customized) lubricants, after a few years, the effectiveness of the lubricants degrades through evaporation, (wear) particle contamination, etc., and as a consequences watches break down or do not perform as required. In addition, some lubrication points are very difficult to access.

The goal of this study was to evaluate whether, for certain watch components, solid-lubricating coatings can be used to replace liquid lubricants.

Eight different types of Diamond Like Carbon (DLC) coatings and a titanium-stabilized MoS₂ coating (all commercially available, different suppliers), which had been preselected from a wider range of possibilities, were evaluated in laboratory tribological tests. The tests were carried out in air under reciprocating sliding conditions using an uncoated steel ball (4.5 mm diameter) sliding on a coated steel plate at 2N load, 10 mm/s sliding speed and at two levels of humidity, 30% and 70% RH. These conditions were chosen in order to simulate the actual use conditions as closely as possible. All tests were carried out three times to gain insight into the repeatability of the results.

The friction force was recorded continuously as a function of sliding distance up to 720m, corresponding to a test duration of 20 hrs. The average wear rate of the coating was determined at the end of the test, based on surface profilometry. Scanning Electron Microscopy (SEM) was used to examine the coatings and associated steel balls after testing.

Clear differences were observed in the friction and wear behaviour of the different coatings but this did not correlate well with coating hardness. Transfer layer formation from the coating onto the steel ball was observed in all cases and it is hypothesized that the formation of this layer is related to the running-in behaviour, i.e. the sliding distance needed in order to achieve a stable friction force. The characteristic appearance of the transfer layer, as observed using SEM, varied considerably between the coatings tested. Increasing the humidity from 30% to 70% had relatively small effects on the friction and wear behaviour.

HIPIMS (High Power Impulse Magnetron Sputtering) is a magnetron sputtering technology devoted to produce thin film coatings with enhanced properties. The technology offers advantages such as denser coatings, higher hardness values and smoother surfaces. This paper presents studies of the role the HIPIMS discharge, as well as an added positive voltage reversal pulse right after the negative HIPIMS pulse, have on coating properties and productivity in industrial applications.

The instant advantage is that the magnetron surface will be immediately discharged, which will reduce the tendency to arcing. However, there are several other effects observed during the performance of reactive sputtering, such as enhanced high energetic positive ion bombardment towards the substrate. Due to raise of the plasma potential, higher incorporation of reactive species into the depositing film, enhanced deposition rates and elastic hardness values, as well as crystallinity will be affected.

Minimization of energy loss due to friction of sliding parts in automotive components becomes increasingly important to increase overall energy efficiency of vehicle. Surface engineering through application of DLC coating is effective to lead to enhancement of wear resistance and sliding property, and hence widely used in recent automotive industry. A product range of DLC coatings in automotive components includes, for instance, fuel injection system, valve lifter, rocker arm, piston ring, gears. Items for application have been expanding more and more in recent years. Depending on its application, suitable deposition techniques shall be selected and the process parameters including design of adhesion layer are optimized accordingly. In addition to conventional hydrogen containing DLC coatings, hydrogen free DLC having a high sp3/sp2 ratio, also referred to as ta-C (tetrahedral amorphous carbon), draws a great practical attention due to its unique features of high hardness >40 GPa and a possible superlubrication effect in a particular lubricating condition. Deposition of ta-C with the controlled property and ensured adhesion is of practically significance. A productivity of coating process, i.e., throughput, is equally important especially for an industrial mass production scale.

In this work we compare systematically various DLC coatings deposited by various industrial vacuum coating technologies of, for instance, unbalanced magnetron sputtering, cathodic arc, also referred to as arc ion plating, and plasma enhanced CVD process. These are presented with respect to the coating properties, applications, and productivity. Our particular interest is also placed on ta-C coatings deposited by cathodic arc. We have developed a round-bar type target specially designed for ta-C coatings for industrial scale. The target was a pure graphite with a typical diameter of 20 mm. A stable arc discharge was sustained at a target surface either in vacuum or in inert gas atmosphere. The coatings were characterized as a low hydrogen content of <1%, high sp3 fraction of >80%, and high hardness of up to 70 GPa. Also the coatings exhibited a relatively low surface roughness in as deposited condition, implying a less emission of macroparticles without any mechanical and/or magnetic filter. For better understanding of intrinsic mechanical properties of coatings, Micro Slurry Erosion (MSE) test was performed where erosion rate of material against blasting was quantitatively evaluated and served as a representative material parameter. The results of MSE of some selected coatings are discussed in relation to the material properties and respective coating parameters.

Designing promising mechanical systems with ultra-low friction performance and establishing superlubricity regimes are desirable not only to greatly save energy but also to reduce hazardous waste emissions. However, very few macroscale superlubricity regimes for engineering applications have been reported. Here, we demonstrate that sustained superlubricity can be achieved at engineering scale when the contact pressure is higher than 2 GPa. Such engineering superlubricity originates from the in situ formation of curved graphene ribbons or onion carbon tribo-films at the sliding interface. Experimental data also demonstrate the wear of the amorphous carbon film containing some nano-diamond particles against Al₂O₃ ball is consistent with atomic attrition. A feasible two-stage mechanism is proposed to explain the friction and wear behaviors. This finding in amorphous carbon films will not only enrich the understanding of superlubricity behavior, but also be helpful to establish more superlubricity regime for more engineering applications.