Evaluation of Anti-Friction Properties of Solid Lubricant Coatings for a Piston Skirt of a High-Force Diesel

Anti-friction properties of solid lubricating coatings dedicated for the decrease in friction and increase in seizure resistance in the piston skirt-cylinder liner couple of high-forced combustion engines have been compared. Various modifications of coatings based on a polymer binder with addition of high-purity molybdenum sulfide and graphite have been deposited onto the segment surface. The graphite-based coating (foreign manufacturer), which is now successfully employed for steel pistons in diesels of heavy vehicles, has been used as the reference specimen. The studies were carried out at the friction temperature of 110 degrees C in the range of normal loads of 50-450 N at a constant reciprocating frequency of 20 Hz, which is comparable to operating conditions of tribocoupling in diesel engines. Analysis of the results has shown that deposition of all coatings significantly decreases the friction coefficient, which amounts to 0.12-0.14 in some coatings (containing simultaneously molybdenum disulfide and graphite), which corresponds to the condition of contact of friction surfaces in the presence of lubricating material and the absence of direct metal contact in the friction couple.