Formation and investigation of nanocomposite gradient coating with upper layer of hafnia

Nanostructured hafnia films and gradient coatings NiCoCrAlY – (7YSZ + NiCoCrAlY) – 7YSZ – (7YSZ + 9YSH) – 9YSH were deposited by low pressure plasma spraying using a supersonic nozzle creating Prandtl-Meyer expansion fan. The coatings have been analyzed by scanning electron microscopy, nuclear backscattering spectrometry (NBS), XRD analysis, scratch testing, thermal analysis and thermal insulation testing. The coating structure are characterized by the mixture of lamellar and nanostructured regions composed of nanoparticle with sizes of 0.02–0.04 μm. Profiling of the coatings elemental composition by NBS method has indicated the presence of transition zones of the mixed composition at the layers boundaries. Synchronous thermal analysis has showed stability of the 9YSH coating structure at temperatures up to 1200 °C and a tiny exothermic process around 1400 °C. In thermal insulation test the thermal conductivity of gradient coatings ceramic layers was estimated as the range of 0.2–0.7 W/m·K at temperatures of 1200–2700 °C. The gradient coating with thickness of 150 μm reduced the heat flow to the cooling system to 0.34 from the initial one. •A nozzle creating Prandtl-Meyer expansion fan was used for plasma spraying.•The coatings structure consists the mixture of lamellar and nanostructured regions.•The mass thicknesses of the mixed layers exceed the thicknesses of the main layers.•The coating reduced the heat flow to the cooling system to 0.34 from the initial one.