Influence of pre-formed boundary layers on wear transition in sliding lubricated contacts

Abstract

Influence of boundary layers on wear transition from mild to severe conditions in sliding lubricated contacts is experimentally studied. A servohydraulic controlled ball-on-disc tribometer is used. During running, additives in lubricants form boundary layers, which influence the lubrication positively in mixed and boundary lubricated contacts. However, it is not clear if and to what extent boundary layers are formed during the initial critical running-in of a contact. The possibilities of forming wear-reducing boundary layers in advance by a simple preforming procedure have therefore been studied in this work. In order to reduce the number of tests and at the same time improve the possibility of statistically verifying the results, the tests are designed as a factorial design experimental series. The factors varied are sliding speed (0.1 and 1 m s −1), additives (triphenyl-phosphorothionate (TPPT) and a phosphorusnitrogen based additive) and formation of boundary layers (pre-formed or not). The pre-forming is done by heating the test discs to 200 °C, covering them with the lubricant with additives, holding them at this temperature for 3 min and thereafter cooling them in the oven with the door open. The results clearly show that pre-formed boundary layers have a strong positive influence on the mild to severe wear transition. The results also show that ‘in situ’ formed layers with both TPPT and PN additives have a positive influence on the transition level.