A study on the tribomechanical properties of oxide-boride-nitride ceramic matrix composite coatings fabricated over substrates with metallic electroplated layer

Abstract

Ceramics are widely used in protective applications owing to their high strength and stiffness at high temperature, low density, excellent wear, corrosion and high temperature oxidation resistance. Composites consisting of a ceramic matrix reinforced with ceramic particulates, are potential coating materials for wear resistance, high hardness, corrosion resistance etc. As the properties of the ceramics are different from metallic substrate. So, at the interface of the ceramic coating and substrate abrupt changes in properties take place that causes mismatch in thermomechanical properties between coating and the substrate. For a coating to perform better during its service life, it should develop stronger adhesion with substrate surface. There are various ways to improve the adhesion of the coating. One way is to use of bond coat applied over the substrate before developing the top coating and second is to use of functionally graded interlayer to achieve better adhesion. In present work a metallic layer with the help of electroplating is inserted between ceramic composite coating (base on oxide-boride-nitride prepared by LSA) and steel substrate to increase the adhesion between them. Ni, NiCr and NiAl coating were deposited using electroplating method on different steel substrates. Precursor powder of top coating were preplaced over electroplated substrate and then a laser was scan over the preplaced powder substrate to develop the top coat. The coating were characterized by scanning electron microscope, Xray diffractometer, reciprocating tribometer, Vickers hardness tester, scratch tester, to find the microstructure, different phases, wear and friction, micro hardness, scratch hardness and fracture toughness respectively. Also the tribological properties were compared between top coating with insertion of metallic electroplating coating and top coating without insertion of metallic electroplating coating. iv The present work shows that, There is an improvement in the tribological property of coatings having intermediate electroplated metallic coating between top coat and substrate as compare to coating without electroplated layer between them. Also the fracture toughness value and micro hardness value of the CMC coatings developed over NiAl electroplated substrate is higher than other coatings. This shows the better adhesion of coating with the substrate due to the ductile interface between top coat and substrate. The ductile interface increases the resistance to crack propagation by distributing the stress at the cracks tip or by the plastic deformation of cracks (by making the sharp edges of cracks to blunt). In this way the tribomechanical property and adhesion strength of coating increase by reduces the crack propagation at the interface of coating and the substrate