Surface formation of plasma-electrolytic cobalt-containing coatings on piston alloys of aluminum

Abstract

Based on the review of functional coatings on piston alloys of aluminum, a conclusion was made on the feasibility of using oxide coatings to increase the strength and durability of parts of the piston group of internal combustion engines. It is shown that the synthesis of the oxide layer on alloys of the Al-Si system (silumins) can be carried out by plasma electrolyte treatment in alkaline electrolytes with the addition of salts of dopant metals, in particular cobalt. This will allow to obtain coatings that have activity in the processes of catalytic combustion of fuel. It is proposed to form cobalt-containing oxide coatings on piston alloys by plasma-electrolytic oxidation in the electrolyte of 0.4 mol/dm3 K4P2O7, 0,1 mol/dm3 CoSO4 at current densities of 3–5 A/dm2 in the “decreasing power” mode. It is established that under these conditions during 15 min of oxidation on high-silicon alloys AL30 and AK21 an oxide layer with a content of up to 4.7 at.% Co is formed. The presence of up to 6.4 at.% of phosphorus in the synthesized coating will increase the heat resistance of the oxidized surface. At the same time, the silicon content in the surface layers decreases 4–5 times compared to the source material. It is shown that the morphology and phase composition of the coating changes with the incorporation of the additive component into its composition. The inclusion of cobalt occurs in the form of thermodynamically stable oxide Co3O4 (CoO·Co2O3), which crystallizes in the spinel crystal cell. This, along with a high degree of surface development is a prerequisite for improving the functional properties of the obtained oxide layers. The developed method was used for applying a cobalt-containing coating on the surface of a piston made of alloy AL30. It is established that in order to obtain a uniform oxide layer, certain technological parameters should be observed. The predicted use of a piston coated with cobalt-containing oxide coating will reduce the amount of toxic substances with exhaust gases and hourly fuel consumption, which is promising for intracylinder catalysis.