The use of plasma immersion ion processing in the synthesis of protective coatings for Al die casting

Plasma immersion ion processing (PIIP) has been used to synthesize adherent coatings on steel substrates that are wear-resistant and provide wetting resistance to molten aluminum. Synthesis procedures were developed for two coatings, boron carbide and a chrome-carbi-oxide. The coatings were produced using a plasma generated by the pulsed glow discharge method with various gas mixtures. Boron carbide coatings were produced using mixtures of diborane (B2H6) diluted with He and mixed with acetylene (C2H2) gas, and the chrome-carbi-oxide coating was produced using Cr(CO)6. The boron carbide coatings were found to be amorphous with hardness values in range of 12–13 GPa. The Cr-based coatings were somewhat harder with values in the range of 14–20 GPa. Both coatings show some resistance to reaction with molten Al; the boron carbide coating showed limited reaction with Al while the Cr–C–O coating showed no reaction. The non-wetting properties of the chrome-carbi-oxide coating have been ascribed to the formation of a thin Al2O3 membrane around the molten Al that kinetically and thermodynamically inhibits a reaction between the Al and the Cr–C–O film.