Microstructural Developments and Mechanical Properties of Electroless Ni-B Coating

Abstract

Phase transformation behavior, micro structural development, mechanical and tribological properties of electroless Ni-B coating was characterized using different characterization techniques. As deposited electroless Ni-B coating containing 94 wt. % of NI and 6 wt. % of B is amorphous. It crystallizes via two exothermic reactions one at 3000C and another at 430˚C. It has been observed that there is also slow evolution of the heat in between this two exothermic reactions. XRD studies display that as deposited coating undergoes multi-stage crystallization events. At the first exothermic peak NI3B phases crystallizes, in between two a phase mixture of Ni and Ni3B and at the second exothermic peak NI2B + Ni3B crystallizes. Evolution of the free Ni in the complete crystalline coating is not predicted by the equilibrium phase diagram of the Ni-B system. Microscopic observation of the as deposited coating displays a novel compositionally modulated microstructure comprises of different length scales ranging from micrometer to nanometer level. In situ TEM study along with composition analysis were carried out in order to track the crystallization pathway and microstructural development. This kind of composition fluctuation of the coating is intrinsic to the deposition process. In best of our knowledge this kind of microstructure is the first time reported example of phase separation in a binary metal-metalloid system without spinoidal decomposition. Effect of this kind of microstructure and phase evolution on the mechanical and tribological properties of the coating is very profound. Increase in the nanocrystalline borides content of the coating increases the hardness value of the coating as well as improved tribological properties of the coating. In the low load regime (5 N and less) wear resistance of the coating is provided by the oxide layer formed on the wear track by preventing the direct contact between the coating and counterface. Local temperature rise due to friction and nancrystalline nature of the coating enhances the tendency of oxide layer formation. Characterization of the oxide layer was carried out using SEM, EPMA, Nanoindenation and Raman Spectroscopy. Whereas in case high load regime (above 5 N) this oxide layer breaks off and direct contact between the coating and counterface is established. This increases the wear rate of the coating. Material is removed from the coating through subsurface crack formation and propagation by low cycle fatigue mechanism. Effect of amorphous phase and free Ni on the tribological properties of the coating is detrimental by promoting a strong adhesion between the coating and steel counter face, whereas nanocrystalline borides shows opposite effect. A nano tribological studies using lateral force microscopy shows that nanocrystalline borides decreases the coefficient of friction of the coating. Phase evolution and microstructural characterization also shows that above 450˚C there is a significant diffusion of the boron from the coating to the steel substrate. This restrict the high temperature tribological studies of the coating up to a temperature range of 450˚C. Wear data along with worn track characterization demonstrate the fact that above 100˚C even in low load regime wear rate is very high. Wear of the coating is mainly governed by the plastic deformation of the coating and breakage of the protective oxide layer. Analytical calculation as well experimental observation shows that during the time of wear the temperature at the local contact region reaches a very high value even up to 1100˚C. This may soften the coating and causes the wear though plastic deformation of the coating.