C. Matta

On the possible role of triboplasma in friction and wear of diamond-like carbon films in hydrogen-containing environments

Hydrogen-free diamond-like carbon (DLC) films (both amorphous (a-C) and tetrahedral amorphous carbon (ta-C)) suffer high friction and severe wear losses when tested in inert and/or high vacuum environments. However, they provide anomalous superlow friction and wear coefficients in the presence of hydrogen gas, water vapour and alcohol molecules in the test environment. In this paper, we used such films in a systematic study to further confirm that hydrogen indeed plays an important role in their friction and wear behaviours. To study the effect of hydrogen, we conducted sliding tests in a hydrogen-containing test chamber and analysed the chemistry of their sliding contact surfaces using a time-of-flight secondary ion mass spectrometer. Clearly, the sliding contact regions of the carbon films became very rich in hydrogen after tribological tests in the hydrogen-containing chamber. In an attempt to understand the fundamental tribochemical mechanisms involved, we performed additional tests on these DLC films using a highly instrumented tribometer that permitted us the visualization of triboplasmas generating at or in the vicinity of the sliding surfaces. In this test system, we confirmed the formation of a triboplasma inside the contact area of the DLC films as evidenced by the characteristic UV radiation. Based on these observations, we believe that the formation of such triboplasmas within the contact zones of these DLC films may have triggered unique tribochemical reactions between hydrogen and carbon atoms on their sliding surfaces and thus resulted in very low friction and wear during tests in hydrogen-containing environments.

Superlubricity mechanism of diamond-like carbon with glycerol. Coupling of experimental and simulation studies

We report a unique tribological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coating of hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) at 353 K in a ta-C/ta-C friction pair lubricated with pure glycerol. To understand the mechanism of friction vanishing we performed ToF-SIMS experiments using deuterated glycerol and 13C glycerol. This was complemented by first-principles-based computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. These simulations show that DLC with the experimental density of 3.24 g/cc leads to an atomistic structure consisting of a 3D percolating network of tetrahedral (sp3) carbons accounting for 71.5% of the total, in excellent agreement with the 70% deduced from our Auger spectroscopy and XANES experiments. The simulations show that the remaining carbons (with sp2 and sp1 character) attach in short chains of length 1 to 7. In sliding simulations including glycerol molecules, the surface atoms react readily to form a very smooth carbon surface containing OH-terminated groups. This agrees with our SIMS experiments. The simulations find that the OH atoms are mostly bound to surface sp1 atoms leading to very flexible elastic response to sliding. Both simulations and experiments suggest that the origin of the superlubricity arises from the formation of this OH-terminated surface.

Superlow friction of ta-C lubricated by glycerol: An electron energy loss spectroscopy study

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 compre...

Improved mixed and boundary lubrication with glycerol-diamond technology

Abstract 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. Diamond-like 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 13C glycerol.

The Future of Boundary Lubrication by Carbon Coatings and Environmentally Friendly Additives

This paper presents a tiibological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coaling of Diamond-Like-Carbon in a DLC friction pair lubricated with OH-containing additives. The DLC material is hydrogen-free tetrahedrally amorphous DLC (denoted as ta-C).

Superlubricity of Steel Surfaces in Presence of Polyhydric Alcohols

Anomalous low friction of hydrogen-free tetrahedral hybridized carbon (ta-C) coated surfaces lubricated by pure glycerol was observed at 80°C. In the presence of glycerol, the friction coefficient is below 0.01 at steady state, corresponding to so-called superlubricity regime. This new mechanism of superlow friction is attributed to easy glide on tribo-formed OH-terminated surfaces. In addition to the formation of OH-terminated surfaces but at a lower temperature, we show here some evidence that superlow friction of polyhydric alcohols could also be associated with tribo-induced degradation of glycerol, producing a nanometer-thick film containing organic acids and water. Second, we show novel outstanding superlubricity of steel surfaces directly lubricated by a solution of myo-inositol in glycerol at ambient temperature (25°C). For the first time, under boundary lubrication at high contact pressure, friction of steel is below 0.01 in the absence of any long chain polar molecules. Mechanism is still unknown but could be associated with friction-induced dissociation of glycerol and interaction with steel surface.Copyright