aThe presence of water in lubricating oil affects the performance of lubricated rolling contacts. This work particularly focuses on the role of water in altering the evolution of zinc dialkyldithiophosphate (ZDDP)-derived reaction layers. A series of tribological tests were carried out using a rolling four-ball machine. Different initial water amounts were added to 2 wt% ZDDP in poly-alpha-olefin (PAO). The reaction layer evolution and its chemical properties were studied using XPS and SEM. SEM analyses show that water seems to inhibit the growth of the ZDDP-derived reaction layer. XPS analyses reveal that the reaction layer undergoes a depolymerisation of the polyphosphate chain when water is present in lubricated contacts. Surface distress was observed in 2 wt% ZDDP in poly-alpha-olefin, whereas when water was mixed with pure base oil, the surface distress increased. Different mechanisms of the influence of water on the ZDDP-derived reaction layer evolution are discussed.

No abstract

The friction and wear of alumina and zirconia ceramics doped with various weight percentages (0.5% wt, 1% wt and 5% wt) of CuO was studied. Dry sliding tests by using a pin-on-disc tribotester were conducted on these materials against commercially available Al2O3, ZrO2, SiC, and Si3N4 ceramic balls.The results show that CuO give a significant reduction of friction only when the alumina and zirconia doped with CuO were sliding against Al2O3 balls. The coefficient of friction of CuO doped in alumina sliding against Al2O3 balls reduces from 0.7 to 0.4 and hardly depends on the normal load and the velocity. On the other hand, CuO doped in zirconia can reduce the coefficient of friction (when sliding against Al2O3 balls) from 0.8 to a value of about 0.2 and 0.3 depending on the normal load.SEM pictures taken from the wear track showed that smooth patchy layers were formed. These smooth patchy layers, which carry the normal load, are responsible in reducing the coefficient of friction.

International audienceIn the present study, an existing theoretical micropitting model, based on the competitive interaction between surface fatigue and mild polishing wear, is employed to predict the effects of different base oil/additive solutions and the relative humidity of the environment on micropitting damage. These effects, though expected to be tribochemical in nature, manifest themselves mechanically; for example, by possibly affecting the fatigue strength of the surface and increasing or reducing the boundary friction and/or the mild wear rate, which altogether may cause an appreciable influence on the degree of micropitting. The present model takes into account only the last two issues (i.e., the change in boundary friction and wear) due to the presence of additives; nevertheless, it is able to predict their performance under micropitting conditions in most cases, showing good agreement with the corresponding experimental data. The results of the present work establish the basis of a blueprint for selecting oil additives for bearing applications

No abstract