Experimental investigation of galling occurrence in sliding contact under high pressure of a manganese phosphate versus steel interface: Comparison of two greases

In OCTG (Oil Country Tubular Goods) field, surface treatments and greases are used to make connections reliable by avoiding galling in metal-metal contacts. This study compares the anti-galling performances and wear behavior of two tribo-systems: steel versus manganese phosphate coated steel contact tested with two different greases. The damage scenario and the kinetics of the involved tribological phenomena will be investigated thanks to a test based on the ASTM G196 standard and appropriate surface analyses. A rotational oscillatory movement is applied on two cylindrical samples under static normal loads. Friction coefficient is computed using torque-sensor records. Counterparts are made of carbon steel of yield strength 550 MPa. One has been subjected to a manganese phosphate treatment to generate a protective layer of few microns thick. The other counterpart is used as machined. Two different industrial greases with anti-wear and extreme pressure additives are tested. Results showed a significant difference of tribological behavior between both greases. XPS analyses are done to understand the composition of the interfacial materials. The protective mechanisms of both greases are compared. Pb-containing grease allows the instantaneous formation of a protective layer whereas Pb-free grease forms a thin and durable protective layer after quite long running-in phase. Manganese phosphate layer participate in the protective layer formation. •Pb-containing grease allows the instantaneous formation of a protective layer.•Pb-free grease forms a thin and durable protective layer after quite long running-in phase.•Manganese phosphate layer participate in the protective layer formation.