Abstract:
The main objectives of this Ph.D. research work are the development of enhanced metal
matrix nanocomposite coatings by employing a new technique through gas carburizing of
electrodeposited nanocrystalline and amorphous alloy precursors. The carburizing
environment was created by introducing a flowing mixture of vaporized 95.5% alcohol (0.25
ml/min, liquid) and argon (0.5 L/min, gas) into the carburizing furnace. Six different alloy
coatings namely Ni-W, Co-W, Co-Ni-W, Fe-W and Fe-Ni-W with tailored microstructure
obtained by controlling the process parameters like current density, pH and temperature were
synthesized. The in-situ carbide formation of binary and/or ternary carbides has been
obtained in the bcc/fcc matrices. In Ni-WC composite coatings, WC is formed in the FCC Ni
matrix. The size of the carbide particles ranges between 100 and 500 nm. TEM study has
revealed the presence of carbide phase in the form of very small precipitates inside the Ni
grains, the size of such precipitates is between 10 and 40 nm. The Co-W and Fe-W
electrodeposited alloy coatings are not much compact at high deposition current densities,
incorporation of Ni increased the adhesion and compactness of the coatings.
The carbides of Co-Ni-W composite were needle like that at less carburizing time were at
the top surface , while these were present at the depth also with increase in carburizing time
as revealed by the section SEM images. Dendritic structure of carbides of Fe3W3C and WC
were observed in Fe-W-C carbide composite coatings with the through thickness carbide
formation.
The hardness of the composite coatings was found to increase with the tungsten content
and the carburizing temperature. The maximum hardness of about 1200 KHN is achieved for
Fe-Ni-W composite coatings.
Supersaturated nature of electrodeposited amorphous and nanocrystalline alloys, in
addition to high diffusivity, have been attributed for the formation of carbide phases
uniformly distributed in the matrix at a temperature range of 700–850°C.
The new technique adopted for the fabrication of metal-matrix composite coatings is
based on the idea of being cost effective and the attainment of finer microstructure of the
alloy coatings and carbides through electrodeposition and low temperature gas carburizing
processes.
