Spark plasma sintering as a post-spray treatment for thermally-sprayed coatings

Spark plasma sintering (SPS) is a new process for the densification of porous materials. The sparks are generated between the particles and cause the material to be heated rapidly. In this study, SPS was applied under compressive loading to thermal spray coatings consisting of a porous solid matrix. Owing to heating by the SPS method, the matrix became ductile and by simultaneous compression, the coating was deformed resulting in the densification of the porous sprayed coating. A magnesia stabilized zirconia (ZrO2–25 wt.% MgO) powder was used as the flame-sprayed material on a steel substrate. In the SPS process, the thermal spray coated steel was exposed to rapid heating under vacuum at temperatures of 900, 950 and 1000 °C under compressive load for 10 min. Investigations into the metallurgical and physical properties of the coatings confirmed that the as-sprayed coating had high porosities of up to 22%, and after SPS treatment, the porosity fell to below 5%. The bond strength and microhardness of the coating was also increased. Changing the SPS temperature and compressive load parameters could control these properties. X-Ray diffraction analysis showed that a phase transformation of the zirconia influenced the physical properties.