| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 93 === Properties of Ni-P and Co-P electrodeposits can be best related to the phosphorus content and microstructure of the deposit. In this study, Ni-P and Co-P deposits were electroplated using the direct current in nickel and cobalt sulfamate solutions, respectively, with the addition of 0~40 g l-1 phosphorous acid (H3PO3). Experimental results indicate that deposit phosphorus content increased, while the current efficiency decreased with increasing bath H3PO3 concentration. Concomitant with the increase in deposit phosphorus content, the structure of the deposit evolved from coarse columnar grains for pure Ni and Co deposits to the mixture of column and laminate, then fine laminates, and finally amorphous structure. Both the deposit microstructure and proton discharge during electroplating affected the hardness and internal stress of the deposits. The hardness of Ni-P electrodeposits increased after 1 h of annealing at temperatures less than 400℃. This increase is due to the precipitation of P-bearing compounds. More P-bearing compounds precipitated and effectively inhibited the growth of recrystallized Ni grains for the deposit with higher phosphorus contents. Consequently, the deposit with more phosphorus generally exhibited higher peak hardness after 1 h of annealing at distinct temperatures. When annealing at temperatures exceeding 500℃, growth of Ni grains and coarsening of Ni3P precipitates prevailed; thereby resulting in a decrease in deposit hardness. Co-P deposits generally had higher softening temperature than Ni-P deposits because after annealing numerous twins formed in Co grains, while Ni grains were rather free of crystalline defects.
Unlike dc electroplating, pulse electroplating enhanced the codeposition of phosphorus. And most importantly, pulse plating improved the current efficiency and reduced the internal stress associated with the deposit. Deposits containing high phosphorus contents could be plated under high current efficiency using the low duty cycle pulse currents. Furthermore, for the pulse currents with low duty cycle, namely 10%, both the grain size and internal stress of the deposit decreased with increasing pulse frequencies. The presence of saccharin in the solution modified the electrocrystallization of Ni-P deposit and reduced the internal stress of the deposit. Adsorption of this organic molecule, however, impeded adsorption and subsequent reduction of H3PO3. Therefore, less phosphorus was codeposited when saccharin was added to the solution.
|
|---|
