Preparation of Ni-B-SiC composite coatings by electroless plating.

Ni-B-SiC composite coatings were deposited from two different types of electroless nickel bath. Bath 1 which had NaBH4 as a reducing agent, gave produced coatings with a higher boron content than coatings deposited from Bath 2 which contained DMAB. In both cases, the SiC content of the coatings increased with increases in the concentration of SiC particles and the rotation rate of the agitator. The hardness of the coatings was increased from 600Hv to 950Hv by the incorporation of 5-6wt. % SiC. The hardness of coatings deposited from Bath 1 was further increased to 1080-1420Hv by heating in a vacuum for 1h, but the hardness of coatings deposited from Bath 2, on the other hand, was greatly reduced by the heating. X-ray analysis of the coatings deposited from Bath 1 indicated that the large increment in hardness induced by heating is attributable to precipitation hardening owing to the formation of Ni3B. The softening of the Bath 2 coatings is explained by the recrystallization of nickel assisted by the codeposited thallium derived from a stabilizer.