Abstract
Eletroless Ni-P exhibiting sufficient properties of hardness, corrosion protection and wear resistance has been applied in various fields of industries. The as-deposited Ni-P coating with supersaturated solid solution which could be strengthened by proper heat treatment is widely used in the optical industries due to its good cutting ability at low temperature and thermal stability at high temperature. However, the strength of Ni-P coating would degrade rapidly with increasing operation temperature higher than crystallization temperature of Ni3P. Suppressing the crystallization of N3P to elevated temperature is thus crucial to extend the application field of Ni-P based coating. To enhance the thermal stability, a third element, tungsten, is incorporated into the Ni-P based coating. Thermal stability of electroless Ni-P-W deposit can be enhanced by the dissolution of tungsten into the nickel matrix as compared to binary electroless Ni-P films. Thermal analysis shows that the introduction of W in the Ni-P coating by co-deposition retards the Ni3P precipitation up to 405.5oC and retains the strengthening effect to a higher temperature of 450□C. For the Ni-P-W coating with high P content, the strengthened effect is extended to an even higher temperature of 500oC. From the calculation of Debye-Scherer equation, the lattice constant of Ni calculated with five major peaks in X-ray diffraction is obtained. The value of 3.564A is bigger than pure nickel of 3.52A, indicating the dissolution of tungsten into Ni matrix. The kinetic parameter of activation energy in the Ni-P-W coating analyzed by the Kissinger as well as Augis and Bennett methods with different heating rates from 1 to 50oC/min is 307kJ/mol which is much higher than that in the Ni-P coating. Microstructure evolution indicates that all coatings in the as-deposited state show amorphous structure. The precipitation of Ni3P accompanied with W dissolving into the Ni matrix is revealed to be the final product of the phase transformation in Ni-P-W coatings after thermal treatment.The microhardness of the as-deposited coatings is enhanced by co-deposition of tungsten. With increasing heat-treated temperature, the hardness of the Ni-P-3.5W coating increases rapidly, reaching the maximum value of 1540HK at 400oC. For Ni-P-W coating with high P content, the peak hardness is 1460HK at 450oC and retains the high value to 500oC. For thermal cycle test at 375oC, the crystallization behavior is crystallization of nickel, dissolving of tungsten into nickel matrix and finally the precipitation of Ni3P phase. The hardness increases slightly due to the nickel refining and the dissolution of tungsten into nickel matrix through six times of thermal cycle. After eight cycles, more increase in hardness is revealed with the precipitation of Ni3P. When the temperature of the thermal cycle test is raised to 400 or 450oC, hardness retains a high reliability even after 8 times of thermal cycle.