Amorphous Carbon based coatings are well known and typically used for high wear resistant and low friction in mechanical contacts. Since carbon based films can be fabricated at rather low temperatures < 150°C the application covers a broad variety of different metal, ceramic and to some extent also organic substrates. Especially for micro mechanical and micro electro mechanical systems (MEMS) dry lubricants are very essential for applications due to the absence of additional sticking and adhesion phenomena usually induced by liquid lubricants. Recent tribological developments in MEMS technology, application and characterization, using thin carbon based films are reviewed in this paper.

However, besides the development of thin tribological coatings further improvement of the frictional behaviour can be realised by optimizing the contact situation. Therefore an optimized topography and surface profile is generated by means of micropatterning. This novel results on micro structured surfaces coated with a-C:H and a-C films will be discussed. Furthermore an oscillating friction tester for the investigation of flat to flat contact, closing the gap between the common pin-on-disc tester and the single asperity investigations by means of AFM method will be presented. The perspectives and limitations of the micro areal tester as well as the behaviour of areal microcontacts and micro structured surfaces will be discussed.

The goal of the investigations was a further reduction of friction by use of microstructures and load-optimized contact areas. Therefore the resulting contact area, correlated with the resulting local pressure, was varied over three orders of magnitude by variation of both width and spacing of the micropattern in the range of 5-100µm. For optimised combination of sliding carbon film and micro patterned surfaces the friction coefficient was reduced by more than 30% compared to full area contact.

Tribological properties of the diamond-like carbon (DLC) film are sensitive to the test environment such as relative humidity. Recently, we investigated the tribochemical reactions between the film, counterface materials and the environment in order to understand the humidity dependence of the tribological behaviors@footnote 1@ . In the present work, uncoated and DLC coated steel ball was used to investigate the effect of the counterface materials on the humidity dependence. Tribological properties were investigated by using a ball-on-disc type tribometer in environment chamber. The DLC film was deposited on Si (100) wafer using benzene (C₆H₆) by r.f-plasma assisted chemical vapor deposition. The tribological test was performed in ambient air of relative humidity ranging from 0 to 90%. When using uncoated steel ball, the friction coefficient of DLC film increased from 0.025 to 0.2 as the humidity increased from 0% to 90%. In the case of DLC coated ball where the Fe-rich debris was not generated during sliding, the humidity dependence of the friction coefficient was much smaller: friction coefficient was 0.08 at the relative humidity of 90%. These results confirmed that the humidity dependence of the friction coefficient is related to the formation of Fe-rich debris by the wear of steel ball. The atomic bond structure of the transfer layer on the uncoated steel ball was much more graphitic than that on the DLC coated ball. These results suggest that the graphitic properties of the transfer layer are the major reason for the humidity dependence. Fe in the Fe-rich debris would enhance the formation of the graphitic transfer layer that is sensitive to the humidity.

footnote 1@ S.J. Park, J.-K. Kim, K.-R. Lee, D.-H. Ko, Diamond. Relat. Mater. 12 (2003) 1517.

The tribology program at NASA-Glenn is investigating diamond like carbon (DLC) films and carbon based nano-particles. The service conditions range from high temperature atmosphere to room temperature vacuum. Some of the lubricants and surface coatings of tribological significance that we have evaluated include neat nano-particles, both grown in situ and as bulk material deposited on the substrate, nano-particles dispersed in oils (nano-particulate based greases) and hydrogenated amorphous DLC films. These applications prove to be highly substrate dependent. The substrates we have considered for tribological couples include 440°C stainless steel (SS), quartz, sapphire and others coupled to similar material or one of the other substrates listed. In this presentation we will discuss results of our friction and wear testing for these systemin both a spiral orbit tribometer (SOT) and a unidirectional pin-on-disc (PoD) tribometer.

The SOT is a rolling contact tribometer which mimics the motion of a ball bearing. Our SOTs operate at ambient temperatures and in air, inert gas or vacuum. Typically SS on SS are the tribocouples lubricated with materials for space bearings such as low volatility oils or films such as DLC. The oil-nano-particle mixtures have the consistency of a grease. Nano-particles tested in Krytox 143AC include: nano-onions, single walled nanotubes, multi-walled nanotubes (MWNT) and fluorinated versions of these same base nano-carbons.

The PoD unit employed uses identical atmospheres to the SOT but is capable of high temperature operation. The majority of this testing uses a quartz disc with sapphire or SS pins. For bulk nano-particles, a thin layer (same materials as for the SOT) is deposited on the disc. Alternatively, we have grown MWNTs in situ by first depositing a thin metal catalyst layer and subsequently growing the MWNTs by chemical vapor deposition.

Modern mechanical machinery and engines have to run under high loads, speeds and temperatures and tighter tolerances and have to deliver smaller energy consumption, longer life and increased environmental friendliness. In many cases, the tribological systems and the materials choice are the performance- and lifetime limiting factors. Carbon based coatings play a very important role as an element for the design of those modern systems. In an industrial environment, a variety of coatings from this family of materials is being applied.

Hard diamond-like coatings will influence the counterbody, especially during early stages of operation. This can affect the performance of the tribosystem by reducing potential lifetime and friction benefits. A modular coating design for a metal-free hydrogenated diamond like carbon layer is proposed to avoid this circumstance. By suitable coating architecture and coating process, the counterbody surface is stabilized and system performance improved. The coating properties are demonstrated. The tribological behaviour of the coatings in several applications is being shown and benefits are being discussed.