Structural changes induced by heating in electroless nickel-phosphorus alloys

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Structural changes induced by heating were studied in electroless nickel-phosphorus alloy coatings at three phosphorus compositions: 1%–3%, 6.5%–8% and 10.5%–11% (by weight). X-ray diffraction patterns and pole figures for the (111) and (200) diffraction lines were obtained for as-plated coatings and coatings heated to temperatures from 235°C to 800°C. Microhardness tests were run on each coating. The low phosphorus composition (1%–3% P) as deposited was crystalline. It was single-phase nickel with an indication of a weak (100) texture. The strength of this texture increased with temperatures up to about 535°C. The Ni 3P phase was detected at 425°C. The intermediate composition (6.5%–8% P) as deposited exhibited the diffraction pattern of an amorphous material. The f.c.c. nickel structure with preferred orientation appeared after heating above 350°C. The texture was (110) and increased in strength with temperature to 800°C. A weak secondary texture, (100), was detected at higher temperatures. Ni 3P was detected above 400°C. The high phosphorus alloy (10.5%–11% P) as deposited appeared amorphous. After heating to 385°C both nickel and Ni 3P diffraction patterns were observed, but there was no confirmed preferred orientation in nickel. A duplex texture, consisting of (110) and (111), was found after heating to 425°C. Hardness of the coatings increased with temperature to a maximum of 1000 HK 200 at about 350°C. Hardness decreased for higher heating temperatures, but by less for higher phosphorus compositions. In the intermediate and high phosphorus alloys, Ni 3P was the matrix phase after heating to 800°C.