Enhanced mechanical and corrosion properties of electroless Ni-P coatings with multi-walled carbon nano tubes incorporation and annealing treatment

Abstract

Electroless Ni-P-CNT coating finds its application in aerospace, automotive, electronics, oil and gas, medical, defence and marine industries for its superior mechanical, tribological, thermal and electro-chemical properties. The different poly-alloy and composite coatings are also getting developed for purely academic researches. The physical, electrochemical, and mechanical properties of the electroless Ni-P coating were improved by adding multi-walled carbon nano tubes (MWCNT) to the electroless Nickel-Phosphorous (Ni-P) coating. Further, to examine the influence of the annealing temperature on the characteristics of both electroless Ni-P alloy and Ni-P-CNT composite coatings deposited onto copper substrate, the as-coated samples were subjected to heat treatment at various temperatures. To evaluate their microstructural, mechanical and electrochemical properties, the coatings were studied using Scanning Electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction, Vickers microhardness testing, and potentiodynamic polarization. The microhardness and corrosion rate of the electroless Ni-P coating were improved from 587 ± 76 VHN_10gf to 784 ± 78 VHN_10gf and from 19.46 ± 0.27 to 9.97 ± 0.22 μm/Y. respectively by the inclusion of CNTs into the Ni-P coating matrix. The inclusion of CNTs in the Ni-P matrix causes the formation of a barrier layer to enhance the passivation of the electroless Ni-P coating, resulting in an increase in the corrosion resistance. The addition of CNTs to the Ni-P matrix has reduced the phosphorus content resulting improved crystalline behaviour and high hardness. These properties were further improved after annealing at 400 °C, possibly due to solid solution strengthening, precipitation hardening, and generation of an enhanced phosphorous surface, resulting in the formation of an electrochemically inactive nickel film. Moreover, a hardness value of 1119 ± 85 VHN_10gf has been achieved by electroless Ni-P-CNT coating after annealing at 400 °C temperature.