Tetrahedrally coordinated hydrogen-free amorphous diamond-like carbon coating (denoted as ta-C) presents ultralow friction under boundary lubrication conditions at 80°C in presence of OH-containing molecules. To understand the mechanism of ultralow friction, we performed gas-phase lubrication experiments followed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses and this using two simple molecules: deuterated glycerol and hydrogen peroxide. The experiments were complemented by computer simulations using the ReaxFF reactive force field. These simulations suggest a ta-C surface rich in sp 2 carbon with some reactive sp 1 carbon atoms, in agreement with previous energy filtered transmission electron microscopy (EFTEM) results. Sliding simulations show that the carbon surface atoms react with glycerol and hydrogen peroxide to form OH-termination. Moreover, the hydroxylation is then followed by the chemical dissociation of some of the glycerol molecules leading to the formation of water. This is in agreement with the secondary ion mass spectrometry (SIMS) analyses and mass spectrometer results obtained with gas-phase lubrication experiments with the same molecules. Both experimental and computer simulations strongly suggest that the hydroxylation of the carbon surface is at the origin of ultralow friction together with the formation of waterrich film in the sliding interface. IntroductionIn previous works, we have shown that some polyols (glycerol for example) and corresponding esters (glycerol monooleate "GMO" for example) are able to reduce friction of DLC surfaces under lubrication in elasto-hydrodynamic, mixed, and boundary conditions. 1 Tests have been performed in a poly alpha olefin base oil containing GMO and a friction coefficient as low as 0.02 has been recorded, in the temperature range 20-100°C. This friction level is well below the one obtained with polar molecules in the same conditions. Because pure glycerol is able to give the same friction level, we can deduce that the monolayer model used for long-chain molecules does not apply in this case. In previous work, hydroxylation of carbon surfaces has been proposed as an alternative mechanism; however, no clear evidence by analytical tools have been shown because of the need to clean the surfaces before analysis. In this work, we would like to confirm this assumption by using both a gas-phase lubrication apparatus and computer simulation. Two simple molecules have been chosen: deuterated glycerol and hydrogen peroxide. Surfaces were made of ta-C carbon in all cases. Because water is certainly released from the hydroxylation process, a detailed study of the role of water in friction reduction is also presented. I. Elaboration and Characterization of ta-C CoatingsA ta-C diamond-like carbon (DLC) coating with a thickness of about 0.9 µm was deposited on a polished carburized steel disk and a hardened steel pin from a graphite target by arc-ion plating, a physical vapor deposition (PVD) process. In addition, secondary ion mass spectrometry ...

Energy-filtering transmission electron microscopy (EFTEM) analysis coupled with the technique of samples preparation, focused ion beam, was used to study physical, chemical, and mechanical properties of diamond-like carbon coatings (DLCs). Two different coatings (ta-C and a-C:H) were investigated, presenting different tribological behaviors in a boundary lubrication regime with glycerol. Electron energy loss spectroscopy appears to be a very powerful technique to characterize such DLC coatings. Special attention was paid to the maximum energy of the plasmon peak, which was used to evaluate some physical and mechanical properties of DLC coatings (density, sp3∕sp2 ratio, hardness). For ta-C superlubric coating, EFTEM results show a rearrangement of the DLC bulk structure under the friction process. Typically, the transformation of sp3 carbon into sp2 carbon was clearly observed and permits a self-adaptation of the coating, allowing it to support shearing without any delamination in spite of important compressive residual stresses in the coating. Moreover, the formation of graphitic carbon is evidenced at the top surface. This graphitic layer certainly helps the lubrication by glycerol in forming OH termination.

Diamond‐like carbon (DLC) films are gaining attention for its use in a wide range of tribological applications because of their low friction coefficient and high wear resistance. Recently, ultra‐low friction or superlubricity as low as 0.006 has been observed between hydrogen‐free DLC films lubricated with oil containing a glycerol mono‐oleate (GMO) additive. Consequently, there are many efforts to understand their detailed tribological behaviour. In this study, first, a characterisation of a pristine hydrogen‐free tetrahedral amorphous DLC coating (denoted as ta‐C) is presented. The technique used for ta‐C characterisation is the energy‐filtered transmission electron microscopy studied on a focus ion beam cross section of the coating. Then, to simulate the action of GMO, which is used as an additive in engine oils, and to understand its mechanism of action in boundary lubrication conditions, simple and shorter molecules such as pure glycerol and hydrogen peroxide are used in friction tests. These two molecules and GMO have the same alcohol chemical function. Friction tests in the presence of pure glycerol and hydrogen peroxide are presented. Moreover, to understand the properties of these two molecules, the overtone of GMO and their reaction mechanism in boundary lubrication, liquid phase lubrication was simulated by gas phase lubrication. Results show that very low friction coefficients are obtained with no apparent wear. Finally, the mechanisms of ultra‐low friction are investigated by the wetting method and the X‐ray photoelectron spectroscopy (XPS) technique. These two techniques are used to identify the physical (wetting method) and chemical (XPS) changes occurring at the ta‐C surface after friction. Copyright © 2007 John Wiley & Sons, Ltd.

The fuel economy and reduction of harmful elements in lubricants are becoming important issues in the automotive industry. An approach to respond to these requirements is the potential use of low friction coatings in engine components exposed to boundary lubrication conditions. Diamondlike carbon (DLC) coatings extensively studied as ultralow friction films to protect the surfaces of ductile metals for space applications are expected to fulfil this part. The main purpose of this work is to investigate the friction and wear properties of glycerol lubricated DLC coatings under boundary lubrication conditions. The DLC material consists of tetrahedral hydrogen free amorphous diamond-like carbon (denoted as ta-C) as shown by the time of flight secondary ion mass spectroscopy (ToF-SIMS) analyses and the nanoindentation measurements. The friction coefficient below 0?01, called superlubricity, and no measurable wear were obtained by sliding the ta-C/ta-C friction pair in the presence of pure glycerol as a lubricant at 353 K. The mechanism by which glycerol is able to reduce the friction in the millirange was revealed by ToF SIMS analyses inside and outside wear scars formed by friction experiments using deuterated glycerol and 13 C glycerol.