電析Ni-P合金めっきの熱処理による皮膜構造変化と硬さとの関係

抄録

Electroplated Ni-P films with a range of P contents were prepared an electroplating method based on a Watt type bath. Vicker's hardness of each specimen was measured after annealing at 200-800°C. The hardening mechanisms for the Ni-5wt%P and Ni-11wt%P films were studied in detail by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Hardness of the Ni-5wt%P film increased gradually at annealing temperatures up to 400°C and then decreased at higher annealing temperatures. The hardness of the Ni-11wt%P films, on the other hand, showed no increase at annealing temperatures up to 300°C, and a sudden increase at 400°C, with no further change at higher annealing temperatures.
The mechanisms of hardening were different for the two films. As-plated Ni-11wt%P film had an amorphous structure, while hardened specimens exhibited the two phase structure of Ni₃P-Ni with an Ni₃P matrix, and its hardening is attributed to the formation of Ni₃P. As-plated 5wt%P film had an f. c. c structure, and the specimens annealed at 400°C showed little structural change except for a gradual increase in the lattice parameter as annealing time increased, indicating that oversaturated P is excluded from the Ni lattice. Given that no Ni₃P precipitates were observed, it was concluded that segregation of P to the grain boundaries was the mechanism of hardening. The softening of this film at 800°C was considered to be due to the formation of an Ni-rich structure containing the Ni₃P phase.