Pierre A. Willermet

Mechanism of formation of antiwear films from zinc dialkyldithiophosphates

Recently developed experimental techniques have allowed better characterization of lubricant-derived antiwear and antifriction surface films. These techniques and the information obtained in earlier studies are being applied to fully formulated engine oils with the objective of assisting implementation of advanced fuel efficient and emissions friendly oil technologies. Published results from those studies were combined with some new experimental results and with background information from related experiments and from experiments delineating the antioxidant chemistry of ZDTP to yield a more global view of antiwear film formation. This more global view has allowed us to propose an improved mechanism for the formation of lubricant-derived antiwear films, which accounts for a substantial body of experimental data, together with some unique observations resulting from our own investigations.

Chemical characterization of tribochemical and thermal films generated from neutral and basic ZDDPs using X-ray absorption spectroscopy

X-ray absorption near edge structure (XANES) spectroscopy at the phosphorus L-edge and sulphur L-edge has been used to characterize the chemical nature of tribochemical and thermally generated films from several ZDDP antiwear agents in the neutral and basic forms. Using the P and S L-edge XANES spectra of model compounds with known structure as fingerprints, the chemical structures of P and S species in the films have been identified. P appears in all the films as polyphosphates in different proportions of short and long chain polyphosphates. In some films, polyphosphates are accompanied by unchanged ZDDP. Generally films generated from neutral and basic ZDDPs show similar P and S chemistry (polyphosphates and sulphides) but contain different proportions of unchanged ZDDP. However, the aryl ZDDP films have different polyphosphate structure compared to the alkyl ZDDP films. The sulphur proportion in the tribochemical films is decreased a great deal, but remains in the reduced form. However, S in the thermo-oxidatively generated films, appears both in the reduced and oxidized form, depending on the ZDDP and the temperature.

Amorphous hydrogenated carbon films for tribological applications. I. Development of moisture insensitive films having reduced compressive stress

Abstract Although earlier investigations on the tribological behaviour of amcrphous hydrogenated carbon (AHC) films in sliding contact with steel showed encouraging results, four open issues were identified. They were: (a) dependence of friction and wear on humidity (i.e., the friction coefficient and the wear increased with humidity), (b) limitations on film thickness (i.e., films greater than 2 μm thick delaminated due to large compressive stress), (c) deposition of films on substrates other than silicon and (d) lubricant compatibility (i.e., formation of lubricant-derived antiwear films on AHC film surfaces). Steps were taken to address some of these open issues by incorporating silicon in AHC films. Friction and wear tests were conducted on AHC films containing various amounts of silicon. Incorporation of silicon in AHC films rendered the friction coefficients and the wear of a steel counterface insensitive to moisture. Silicon incorporation in AHC films also significantly reduced compressive stress. This allowed deposition of 10 μm thick films. These effects were achieved without any compromise with the friction coefficient and the film wear if the amount of silicon in the film was kept within a certain concentration range. In addition, silicon-containing AHC films were thermally more stable than silicon-free films. Experiments conducted with two lubricants resulted in significantly lower wear of the silicon-free AHC films than that obtained for unlubricated sliding. Similar friction coefficients were obtained for AHC film/steel and steel/steel combinations in lubricated sliding.

The composition of lubricant-derived surface layers formed in a lubricated cam/tappet contact II. Effects of adding overbased detergent and dispersant to a simple ZDTP solution

The surface films formed by a set of oils of related additive chemistry, but with differing detergent and dispersant contents, have been chemically characterized using a combination of surface analysis techniques. The films were formed in a cam/tappet friction apparatus with a direct acting bucket tappet geometry. In the absence of overbased detergent, the amorphous films were composed essentially of inorganic zinc phosphates formed by the ZDTP anti-wear additive, and evidence of higher molecular weight phosphates (e.g. metaphosphates) was found. Adding overbased detergent and dispersant resulted in partial replacement of zinc by the detergent metal and loss of the higher molecular weight phosphates in favour of ortho- and pyro-phosphates.

Lubricant-derived tribochemical films—An infra-red spectroscopic study

Lubricant-derived tribochemical films were prepared in a cam/tappet tribometer using fully formulated engine oils containing zinc dialkydithiophosphate. A direct acting bucker tappet geometry with a removable friction disc in the tappet was employed. The friction discs could easily be removed for analysis after the completion of the friction experiments. Reflection-absorption infra-red was used to characterize the films, together with XPS and Auger spectroscopies. The results were consistent with an inorganic, amorphous phosphate film with cations derived from the lubricant additives. Building on these results, additional experiments were undertaken to determine the nature of the films better. Model materials for the films were prepared by synthesizing phosphate glasses and by thermally and thermo-oxidatively decomposing zinc diisopropyl dithiophosphate. The results indicated that the tribochemical films were not phosphate glasses as expected, but rather predominantly amorphous orthophosphates and pyrophosphates formed by thermooxidative decomposition of zinc dialkyldithiophosphate.

AN ANALYSIS OF VALVE TRAIN FRICTION IN TERMS OF LUBRICATION PRINCIPLES

Friction losses in a motored 1.6L valve train can be reduced by roller tappets, by needle bearing inserts placed in the rocker arm/fulcrum contact and in the cam journals and by reducing spring tension. Friction reducing engine oil additives reduce valve train friction substantially, but oil viscosity has only a limited effect. These results can be quantitatively accounted for by a simple friction model based on lubrication theory. Both the model and the experimental results are consistent with the idea that the friction losses in the valve train are mainly due to boundary and mixed lubrication.

Amorphous hydrogenated carbon films for tribological applications II. Films deposited on aluminium alloys and steel

Abstract This paper describes the methods for the deposition of AHC films on aluminium alloys (2024, 7075 and an additional Al-Si alloy) and AISI 4340 steel. Both unmodified and silicon modified AHC films were deposited. AHC films could be deposited on aluminium alloys without any interlayer. The deposition of AHC films on steel required an interlayer which could be aluminium, silicon or chromium. Thin films (1–2 μm) deposited on aluminium alloys and steel influenced durability of films and friction coefficients in contact with steel. These were believed to be due to plastic deformation of substrates. Deposition of a thicker coating system (interlayer + AHC) reduced friction coefficients and also improved film durability. The durability of films deposited on steel substrates was evaluated under both unlubricated and lubricated conditions for 5.5 million cycles under 4.4 N load and up to 2.5 m/s sliding speed. Although there was wear, the films survived 5.5 million test cycles under unlubricated sliding, but in the presence of two lubricants, the film wear was very small and could not be measured. It was observed that the wear of the steel counterface in contact with silicon-containing AHC films could be higher than that against an uncoated steel in the presence of certain lubricants.

Experimental evaluation of tappet/bore and cam/tappet friction for a direct acting bucket tappet valvetrain

This paper reports on tappet/bore friction and torque at the camshaft that were measured for a direct acting bucket tappet using a cam/tappet friction torque and friction coefficient as a function of cam angle were derived from those measurements. The results showed that, for the particular geometry tested, tappet/bore friction torque accounted for about 13% of the total cam/tappet/bore friction torque at 250 cam rpm. This fraction decreased with increasing speed. Tappet bore friction was greatest at about {plus minus}40 degrees of cam angle, where side loads on the tappet bore were highest. In contrast, earlier results for a center pivot rocker arm design showed tappet bore friction to be negligible.

Effects of Composition and Surface Finish of Silicon Nitride Tappet Inserts on Valvetrain Friction

Abstract In order to build more fuel efficient engines, new materials and lubricant formulations are being sought to reduce frictional losses. The valvetrain contributes about 6-10% of the total frictional losses in an engine. To reduce valvetrain frictional losses, polished silicon nitride tappet inserts were evaluated for their friction reduction potential. Silicon nitrides obtained from three sources were polished using three different processes: the suppliers conventional diamond polishing technique and two non-diamond polishing techniques-“Ford Finish” and chemo-mechanical polishing. The valvetrain friction torque was measured in a laboratory apparatus using a single cam lobe rotating against a direct acting mechanical bucket tappet with production engine hardware. The friction torque values obtained with surfaces prepared by all three processes differed significantly although their initial centerline average surface roughnesses were similar. All three silicon nitride surfaces prepared by “Ford Finish” showed lower friction torque than the production steel surface when an engine oil containing no friction modifier was used. With a low friction oil containing a friction reducing additive, friction torque values were significantly lower and polished silicon nitride surfaces did not offer additional friction reduction benefit relative to the production steel surface. A simple calculation showed a maximum of about 0.5 % fuel economy benefit can be gained due to polishing with the engine oil without friction modifier but very little, if any, with low friction oil. However, silicon nitride inserts may be useful for weight reduction and increased durability.

The Aluminum Beaker Oxidation Test for MERCON™ World-Wide Service ATF

The oxidation stability of automatic transmission fluids can be evaluated using several approaches. These include motored transmission oxidation tests. The Aluminum Beaker Oxidation Test is one of the tests specified for MERCON tm service ATF. This paper provides a summary of the test procedure, reviews the data supporting the validity of the test, and presents information on the repeatability and reproductibility of the test method