Abstract:

The attractive set of properties offered by alumina makes it an excellent candidate to protect mechanical components operating in harsh environments. However, when processed on metallic substrates, the high temperature stability, chemical inertia, and high hardness contrast with the low toughness, low solubility and very different thermal expansion coefficient. Processing such a sound coating on a metallic substrate is a challenge, aggravated when attempting to do so with a hardfacing technique. This study addresses this challenge and discusses procedures to process ceramic coatings on a Ni based alloy using Plasma Transferred Arc (PTA) technique. It aimed to process a functionally graded coating that can take advantage of the superior properties of alumina and simultaneously accommodate the differences in properties between the alumina and a metallic substrate. Powder mixtures of a metallic oxide and Al were processed on a metallic substrate to produce a multilayer coating composed of a layer of alumina on top of an aluminide layer that accounts for the bond between the metallic substrate and the alumina layer and smooth the difference in properties. Results showed that the use of processing parameters typical of hardfacing superalloys led to discontinuous set droplets on top of the metallic substrate. Discussion focus on the selection of processing parameters, particularly, deposition current and velocity that allow to proccess a sound multilayer coating. These parameters control penetration into the substrate and act on the balance between adhesion and cohesion forces, that to group the droplets and maintain the melt pool, respectively.