Ideally, engine journal bearings are expected to operate under hydrodynamic lubrication regime to limit wear and promote minimal friction by sliding. Nevertheless, engine journal bearings fail since wear caused by severe conditions in actual engine operation, such as start/stop, misalignment, lubricant degradation, overheating, and debris contamination, producing a transition from hydrodynamic to mixed and boundary lubrication regimes and wear increased in both engine journal bearings and crankshaft. Thus, this work aims to study the influence of boundary lubrication, engine mineral oil aging, and debris contamination on wear of engine journal bearings. An adapted microscale abrasion tester using a ball-on-concave flat configuration was used to reproduce boundary lubrication under controlled conditions. Steel balls having a similar surface than crankshafts and concave flat samples cut from actual engine journal bearings were tested. The tests were run under boundary lubrication at a constant load, speed, and sliding distance at 26 ℃ and 100 ℃ using separately clean fresh and aged engine mineral oil, and then, tests were conducted using the oils contaminated with hard abrasive particles. The engine mineral oil was degraded by a laboratory aging process approaching oxidation of an engine mineral oil used in actual use conditions. Oxidation, additives depletion and changes in viscosity were evaluated. The wear volumes and scar morphologies of engine journal bearing samples were analyzed. The results suggested that high temperature was the main contributor for wear increase in engine journal bearings, while oil aging and debris did not influence considerably on the wear. However, the oils contaminated with hard particles produced a wear decrease in engine journal bearing samples but increased wear in rotary balls.

Some times operating conditions, namely, misalignment, overheating and the start/stop of engine generate boundary lubrication conditions increasing wear of journal bearings (JBs). Thus, debris are a consequence of wear and fatigue either from JBs or other lubricated mechanical components. Debris are commonly immersed in the oil and recirculated through the entire lubrication system interacting with all the lubricated mechanical elements and accelerating wear rate due to three-body abrasion. The aim of this work is to evaluate the effects of abrasive particles on the wear behaviour of sections of an actual JB coated with a soft alloy (Pb-Cu-Al) by replicating boundary lubrication in JBs using a micro-scale abrasion test setup. Steel balls were used to replicate the shaft counter face for the tests. Initially, the tests were carried out with a slurry prepared with distilled water and SiC micro-particles at a concentration of 20% vol. On the other hand, a SAE 10W-30 engine oil was blended with SiC micro-particles at different concentrations to replicate an engine oil contaminated with abrasive particles. The slurry was tested at 26°C while the contaminated oil was tried at two different oil temperatures (26 and 100°C). The wear scars produced were measured and analysed by optical microscopy, SEM and contact profilometry. It was found that clean oil generated higher wear than oil contaminated with SiC particles at different concentrations since a layer of SiC particles was generated on the scars by embedment of many particles in the soft coating. It acted as protective layer for the JB's coating reducing wear. However, it generated significant wear in the ball surface.