Effect of Composition and Thickness on the Tribological Performance of Epoxy-MoS2 Composite Coatings in Reciprocating Block on Ring Tests

Abstract

Over the past two decades, there has been an increasing adoption of protective and lubricant coatings for their usage in threaded connections in the oil and gas industry to reduce both failed connections due to galling and environmental contamination due to lubricant spillage. In this work, the influence of composition and thickness on the tribological performance of MoS2-epoxy matrix composite coatings in reciprocating block-on-ring tests was studied. Epoxy resins with 2.5, 5, and 10 wt.% MoS2 were deposited on SAE 4140 steel blocks using a manual procedure, and after curing, hardness and thickness were measured. The tribological evaluation was performed using a block-on-ring test at low speed (30 mm/s), with reversing sliding direction and continuously varying loads between 0 and 5000 N. These conditions are similar to those encountered on the thread flanks during make-and-break operations in OCTG (Oil Country Tubular Goods) threaded connections. Wear surfaces were examined by optical and scanning electron microscopy (SEM). The wear resistance of coatings containing 2.5 and 5 wt.% of MoS2 strongly depends on their thickness, with a threshold around 70 μm. Coatings with lower thicknesses displayed constant wear rates, which decreased as the MoS2 concentration increased. On the other hand, coatings with thicknesses exceeding 70 μm showed an increase in wear rate proportionate to their thickness. In the case of coatings with 10% MoS2, the friction and wear were not influenced by coating thickness. These coatings exhibited the lowest average coefficient of friction (COF) values (0.06 ± 0.015) and wear rates (3.1 ± 1.3 × 10−8 mm3/N mm) compared to the other compositions tested. This performance is attributed to the formation of tenacious tribofilms on the surfaces.