Thermal barrier coatings, typically comprising of MCrAlY bond coat and YSZ top coat, have been used on super alloys to improve the thermal efficiency of turbine engines. These coatings have low thermal conductivity and appropriate surface properties. This study investigates improvement of thermal insulation, wear, and corrosion resistances of AZ91D Mg alloy by applying TBC coatings .Plasma flame has an obvious thermal effect on the substrate during deposition of TBCs on Mg alloy substrate by air plasma spray process and leads to serious oxidation of the substrate surface. Ni-P interlayer with low (3 wt.%) and high (9 wt.%) phosphorus was deposited to protect the Mg substrate from thermal oxidation. Ni–P layer is pre-deposited on top of Mg alloy by electroless plating, followed by deposition of MCrAlY bond coat by HVOF and YSZ top coat by APS process respectively. The microstructure and phaseconstituents of coatings were investigated by SEM, XRD and EDS. Coefficient of thermal conductivity was obtained by HT11 armfield linear heat transfer apparatus. It is well known that adhesion mechanism of YSZ coatings to the metallic substrate is basically a mechanical anchorage as is common for spray coatings. Interfacial indentation is used to create and propagate a crack in the interface between a coating and its substrate. This methodology allows defining an apparent interface toughness, which can represent the adhesion of the coating on its substrate. The interfacial indentation test (IIT) was performed at the interface of YSZ top coat/AZ91D substrate. For this purpose, polished cross-sections of the YSZ-coated samples were indented using a Vickers diamond pyramid with 9.81 N, 19.62 N, 29.43 N, 49.05 N, 98.1 N contact loads. For each indentation, half-diagonal of the indent and the length of the cracks were measured using an optical microscope. pore area were calculated with ImageJ software. The surface roughness of the specimens were measured with theMitutoyo average surfaceRoughneTester. Pin-on-disk wear test based on ASTM-G99 was used to investigate tribological behavior of the applied coatings. Wear behavior of coatings was investigated in design of experiments using full factorial approach. Three factors namely coating material, linear speed in three levels and normal load in two level were selected for DOE. SEM images illustrate porous surface and some micro-cracks of YSZ coating. Thermal conductivity of coatings is comparable with MCrAlY/YSZ on super alloys. The wear loss values of the ceramic lamella increase with the increase in the applied loads as expected. It is observed that coating without Ni-P interlayer exhibits highest value of wear rate and coating with Ni-P high phosphorus interlayer has lowest values of wear rate. Keywords: MCrAlY/YSZ, Ni-P interlayer, AZ91D Mg alloy, Adhesion, Thermal conductivity, Wear, DOE
