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Environmental legislations prescribe globally more and more environmentally acceptable lubricants (EALs), based on esters and polyglycols. The compatibility of EALs with tribo-materials used in hydraulic System such sliding parts and oil seals needs to be considered. Aluminium alloys are widely used as lightweight material for sliding parts in many applications. It is necessary to provide optimized design on its surface because of the inferior wear resistance under mixed/ boundary lubrication. The AA6061 alloy (uncoated and coated by different conversion coatings) and an alternative AI-Fe-XY alloy (uncoated) were tested by a continuous sliding test (cylinder-on-disk) under mixed/boundary lubrication with commercially available EALs at 80°C against 100Cr6 Steel. Appropriate combinations of aluminium alloys (coated or uncoated) and commercially available EALs have the potential to compete with conventional tribo-system like steel/steel lubricated with mineral hydraulic fluids under mixed/boundary lubrication.
Self-mated magnesia stabilized zirconia (Mg-PSZ) ceramic sliding couples have been investigated at 100 N load (P0max= 1324 MPa) in oscillating sliding conditions in different humidity conditions in air and in hot steam. Temperatures have been varied up to 400 °C and pressures up to 6 bars. The results show that the wear behavior of MgO-ZrO2 under high Hertzian contact pressures is strongly dependent on temperature and is similar for both dry oscillating and oscillating in hot steam. However, although the evolution in wear rates on temperature is similar and the wear rates of MgO-ZrO2 plunged above 300 °C in hot steam and air by nearly three orders of magnitude, SEM micrographs revealed in hot steam at 400 °C smooth wear tracks. In contrast, hot steam enhanced the tribochemistry of self-mated alumina couples and reduced wear rates. Hot steam decreased the coefficients of friction of MgO-ZrO2 with increasing temperature, but not the wear rates.
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above-mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2 alumina and antimony impregnated carbon.
3 different base-stocks (poly-a-olefin, polypropylene glycol and unsaturated TMP ester) and these blended with 2 different additives (1wt% ZnDTP or 1wt% dibenzyl disulfide(DBDS)) were used as lubricants to evaluate the tribological properties of AA6061-T6 treated by means of plasma electrolytic oxidation in a SRV tribometer (Toi|=80 °C, Fn = 50N, v= 50 Hz, 2mm stroke, 2hrs). The base oils of PPG and the TMP-ester had better lubricities than the PAO with/without the additives. ZnDTP tend to reduce wear, but when added to PAO and PPG increased the friction. The unsaturated TMP-ester +1wt%DBDS showed 45% lower friction than PAO + ZnDTP (reference), whereas PPG + DBDS had 30% lower friction.