Tribochemistry involves chemical reactions occurring at sliding contact interfaces in the presence of gaseous and/or liquid media. It often leads to the formation of a solid reaction film (also termed boundary film) which controls friction and wear and hence the efficiency and reliability of moving mechanical systems (such as engines). Here we demonstrate tribochemical conversion of methane to graphene, nano-onion, and disordered carbons on the sliding surfaces of Ni-, Cu-, and CuNi-containing VN coatings at atmospheric pressure and room temperature, providing 2–3 orders of magnitude reduction in wear and ∼50% reduction in friction compared to those of the uncoated steels. Transmission electron microscopy confirms that graphene forms preferably on metal rich nanoclusters of the composite coatings, while the carbon nano-onions are scattered throughout the carbon tribofilm. Ab initio molecular dynamics simulations elucidate underlying mechanisms involved in the tribochemical conversion of methane to carbon-based nanostructures in support of microscopic observations. These scientific findings may lead to new materials technologies that can use methane as a source for continuous and in situ lubrication. For example, there is an urgent need to curtail the uses of lubricating oils in natural gas compressors and engines as they contaminate the natural gas being compressed or burnt.

No abstract

From corrosion protection to embrittlement and lubrication, the presence of phosphorus at iron surfaces is critical for a range of processes and applications. However, phosphorus adsorption on iron has never been studied experimentally or theoretically. Here P chemisorption on the most stable surface of iron is analyzed by means of density functional theory. The most stable adsorption geometry and coverage are identified, and the changes induced by P adsorption on the structural and electronic properties of the metal are discussed. A systematic comparison among P, S, N, and O uncovers a peculiar behavior of the P and S species: attractive adatom–adatom interactions at high coverage lead to the formation of two-dimensional overlayers that reduce the metal reactivity and work function. This effect is important in iron embrittlement and for the functionality of P- and S-containing additives included in lubricant oils.

Polytetrafluoroethylene (PTFE), commercially known as Teflon, is one the most effective insulating polymers for a wide range of applications because of its peculiar electronic, mechanical, and thermal properties. Several studies have attempted to elucidate the structural and electronic properties of PTFE; however, some important aspects of its structural and electronic characteristics are still under debate. To shed light on these fundamental features, we have employed a first-principles approach to optimize the two coexisting PTFE structures (monoclinic and orthorhombic) at high pressure by using the characteristic zigzag planar chain configuration. Our electronic analysis of the optimized structures shows charge transfer from carbons to fluorines, supporting the PTFE electronegative character. In addition, band structure calculations show that the band gap is estimated to be around 5 eV, which correlates with previous studies. Moreover, the analysis of the valence and conduction states reveals an intrachain and an interchain character of the charge distribution, suggesting additional insights into the PTFE electronic properties.

While (In x Ga 1−x ) 2 O 3 alloy is a crucial system for the Ga 2 O 3 -based ultrawide bandgap semiconductor application, its successful phase control has been struggling because of its heterostructural nature and rich polymorphs. Here, we identified the thermodynamic phase diagrams for both the bulk state and epitaxial state of (In x Ga 1−x ) 2 O 3 alloy by using comprehensive density afunctional theory (DFT) calculations and regular solution models, which is consistent with previous experimental reports. By comparing the phase diagrams under a strain-free condition and an epitaxial strain condition, we demonstrate that the epitaxial strain is a significant factor in the successful growth of alloys in heteroepitaxy processes. While the alloying of (In x Ga 1−x ) 2 O 3 is limited by a miscibility gap under the strain-free condition, the Al 2 O 3 heteroepitaxy substrate opens more metastable regions for various polymorphs. With the choice of a suitable substrate, we also suggest the phase control strategy for (In x Ga 1−x ) 2 O 3 alloys in orthorhombic polymorphs.