황화물계 3가 크롬도금욕에서 크롬-탄소 및 크롬-탄소-인 합금도금의 전착과 결정화 거동

Abstract

Chromium-carbon (Cr-C) and chromium-carbon-phosphorus (Cr-C-P) alloy deposits using trivalent chromium sulfate baths containing potassium formate were prepared to study their current efficiency, hardness change and phase transformations behavior with heat treatment, respectively. The current efficiencies of Cr-C and Cr-C-P alloy deposits increase with increasing current density in the range of 15-35 A/dm2. Carbon content of Cr-C and phosphorous of Cr-C-P layers decreases with increasing current density, whereas, the carbon content of Cr-C-P layer is almost constant with the current density. Cr-C deposit shows crystallization at 400oC and has (Cr+Cr23C6) phases at 800oC. Cr-C-P deposit shows crystallization at 600oC and has (Cr+Cr23C6+Cr3P) phases at 800oC. The hardness of Cr-C and Cr-C-P deposits after heating treatment for one hour increase up to Hv 1640 and Hv 1540 and decrease about Hv 820 and Hv 1270 with increasing annealing temperature in the range of 400~800oC, respectively. The hardness change with annealing is due to the order of occurring of chromium crystallization, precipitation hardening effect, softening and grain growth with temperature. Less decrease of hardness of Cr-C-P deposit after annealing above 700oC is related to continuous precipitation of Cr23C6 and Cr3P phases which retard grain growth at the temperature.