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Embrittlement study of electroplated nickel plates using indentation technique
Keywords: electroplating, nickel, embrittlement
Electroplated nickel has been known to have brittle fracture problem and the phenomena originated from the fact that the Nickel films contain small amounts of impurities such as hydrogen, sulfur and phosphorus which enable plating process and improve mechanical properties (hardness) by precipitation hardening process. However, the effects of the impurities on embrittlement phenomena of the Ni films have not been studied in a quantitative manner. In this study, correlation between the impurities and brittle fracture of the Ni films are presented systematically. Experimental results show that fracture toughnesses of the Ni films are closely related with microstructure of the Ni films and impurities content. The specimens for microstructure evaluation and indentation test were prepared on steel plates. Nickel films were deposited on the steel plates using electroplating method. Steel sheet was used for cathode material and the distance between anode and cathode was controlled as 55mm. Current density used for the electroplating was 70mA/cm2. The thicknesses of the Ni film and steel substrates were controlled as 50 and 400um, respectively. Some impurities such as hydrogen, sulfur and phosphorus are inevitably contained in the Ni film during the deposition process. Though content of H in the Ni films cannot be measured, contents of S and P were determined with ICP analysis as 0.07 " 0.10 for P and 0.02 " 0.05 for S. Heat treatments were conducted with prepared Ni films on steel substrate in tube-type furnace. Aging temperature was controlled from 250 to 700oC and aging time was varied from 2 to 50 hrs. Microstructures and grain sizes of the Ni films which underwent aging processes were evaluated with SEM measurements, with support of focused ion beam (FIB) etching technique. For the specimens aged for 50hrs, measured grain sizes changed from about 100nm (250 oC) into several microns order (600 oC). Even though same aging condition developed similar microstructures for the specimens with different impurities content, fracture toughnesses measured using crack lengths which occurred through indentation tests were varied with impurities content in the specimens. The specimens with low impurity content developed fewer and shorter cracks and consequently showed higher fracture toughness values.
Yoonchul Sohn,
Samsung Advanced Institute of Technology
Yongin, Gyeonggi-do

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