Crystallization behavior of nanometer-sized Al2O3 composite coatings prepared by electroless deposition

Nanometer-sized Al2O3 particle-reinforced nickel-phosphorus matrix composite (Ni-P-Al2O3) coatings were prepared by electroless deposition. Particles were found to distribute homogeneously in Ni-P-Al2O3 coatings by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). It was found that the crystallization temperature of the composite coating decreased from the results of differential scanning calorimetry (DSC-7) and X-ray diffractometry. The activation energy and Avrami exponent of crystallization were lowered due to co-deposited particles in composite coatings. Nickel phosphide (Ni3P) precipitated after heat treatment at 230 °C for 24 h in Ni-P-Al2O3 coatings, while Ni-P coatings kept amorphous under such condition. Because of the existence of nanometer-sized particles, microhardness of the composite coatings greatly increased. With heat treatment, microhardness values of composite coatings as high as HV501150 were produced.