Andreas Lachenwitzer

Chemical and mechanical analysis of tribofilms from fully formulated oils Part 1 – Films on 52100 steel

Abstract The authors report, for the first time, a comprehensive chemical and mechanical characterisation of antiwear films prepared from a fully formulated oil that is commercially available. Wear increases substantially when using the fully formulated oil compared to using ZDDP alone. X-ray absorption near edge structure (XANES) spectroscopy at the P K- and L-edges, S K-edge, Mo L-edge, B K-edge, Ca L-edge, O K-edge and Fe and Zn L-edges permits chemical characterisation of the major elements in the thin films. Ca phosphates, ZnS and MoS2 are the main P and S species formed, contrary to previous studies involving only ZDDPs, whereby Zn phosphates are the dominant species. These findings can be accounted for by using the hard and soft acid and bases (HSAB) principle. Small amounts of CaCO3 are present, but no B was detected, implying it does not become incorporated into the film. Atomic force microscopy (AFM) reveals continuous pads with a relatively uniform indentation moduli (125 ± 10 GPa), separated by trenches that are essentially comprised of uncovered steel substrate.

Nanoscale chemistry and mechanical properties of tribofilms on Al–Si alloy (A383): interaction of ZDDP, calcium detergent and molybdenum friction modifier

Abstract The interaction of a friction modifier and a calcium phenate detergent additive, with zinc dialkyl dithiophosphates (ZDDPs) in the formation of antiwear films on A383, has been studied using synchrotron radiation and nanoindentation techniques. X-ray absorption near edge structure (XANES) spectroscopy has shown that films prepared from oils containing both ZDDP and detergent, and ZDDP and molybdenum dithiocarbamate (MoDTC), are chemically similar to, but thicker than those made from oils containing only ZDDP. In addition, wear was greatly reduced in the presence of the detergent which was correlated with the basicity and the presence of the friction modifier. The phosphorus K and L edge XANES spectra show that the tribofilms are polyphosphate glasses of similar nature to those found on steel, but characterised by a shorter chain length. The sulphur K edge shows a MoS2 like film and under certain conditions, the presence of a sulphate species is detected. High resolution topographic images and mechanical properties were determined by atomic force microscopy and imaging nanoindentation. The films formed in the presence of the detergent exhibited similar mechanical responses independent of the conditions tested. The indentation modulus of the films on the Al matrix always appear much softer than the films formed on the Si grains whether or not the lubricant contains only ZDDP, or both ZDDP and MoDTC.

Chemical and mechanical analysis of tribofilms formed from fully formulated oils Part 2 – Films on Al–Si alloy (A383)

Abstract The interactions of a commercially available fully formulated oil on an Al–Si alloy have been studied, and a chemical and mechanical characterisation of the resulting surface films has been undertaken to compare with the results found on 52100 steel. ZnS and Ca phosphate are the primary species found. No Mo species were found in the film, but B was detected; the latter observation contrasts with the results found for 52100 steel. A surprisingly consistent indentation modulus was found for the films formed on A383, considering the complex microstructure of the alloy, which has been shown previously to influence the mechanical response of the film produced from zinc dialkyl dithiophosphates (ZDDPs) in oil. The stiffness of the films on A383 is statistically indistinguishable from that found on 52100 steel, and the topography of the film also looks unchanged compared to the steel study.

Nanoscale Studies of the Role of ZDDPs in the Wear Protection in the Automobile Engine

The improvement of fuel consumption is an important driving force for research and development in the automobile industry in order to minimize greenhouse gas emissions as well as improving fuel economy. Aluminum alloys are a class of alternative materials that are being used to replace cast iron in motor components due to the concomitant weight savings which result in improved fuel efficiency, and cost savings. Our research focuses on these alternative Al-based alloys as well as traditional steel interfaces, and the protective films that form on the surfaces. Currently the zinc dialkyl-dithiophosphates (ZDDPs) have been used as engine oil additives for over 60 years. They are important chemically-active additives, known for their antioxidant and antiwear characteristics. ZDDPs are known to form a protective film (tribofilms) at rubbed surfaces, typically on iron containing metals surfaces commonly used in the automotive industry; however ZDDPs and the products formed are not well suited for the environment as they can readily poison the catalytic converters and their efficacy on Al-Si alloys is not well established.Copyright