Effect of Chromium Content in the Steel Substrate on the Coating Formation and Tribological Properties of Manganese Phosphate Coatings

In the literature and industrial experience, the effect of insufficient and poor manganese phosphate coating formation on steel with higher chromium content (> 1 wt%) is well known, but the reason for this limitation or the appearance and tribological performance of such coatings is not described sufficiently. This work describes the differences in structure and tribological properties of manganese phosphate coatings on different steels with up to 3 wt% of chromium and reveals the formation of amorphous and crystalline phosphate phases by transmission electron microscopy (TEM). The tribological experiments were performed on a high load test setup with API modified as lubricant. Besides TEM also scanning electron microscopy, and focused ion beam cross sections were employed to investigate the microstructure and appearance of the coatings. Furthermore, this work describes the formation of tribologically superior coatings on martensitic steel with up to 3 wt% chromium.