Preparation of plasma-sprayed coatings of ZrN under a controlled nitrogen atmosphere

Spraying of 22 — 45 μm ZrN particles was performed in a controlled-atmosphere chamber using an Ar-N₂ plasma. The power levels were varied from 20 to 45 kW. The nitrogen pressure was regulated at either 60, 150 or 700 Torr. Use of the flattening test ensured that the degree of melting of the particles could be controlled. The crystalline structure, microhardness, thermal expansion, microstructure and porosity distribution of the coatings deposited onto steel substrates were examined and compared for the different spraying conditions. The flattening test showed that spraying at 700 Torr improves the heat transfer and leads to dense deposits with good interlamellar cohesion. The microhardness of such deposits varies between 945 and 1045 HV. The sprayed layers are very sensitive to oxidation at temperatures higher than 500 °C, the oxidation modigying their structure and causing a sharp variation in their expansion coefficient (from α = 8 × 10^-6 °C^-1 below 500 °C to α = 125 × 10^-6 °C^-1 between 600 °C and 1000 °C). This phenomenon limits the use of ZrN to temperatures lower than 500 °C.