Microstructure Evolution of Thermally Sprayed TiB2-Ni Cermet Coating: Comparison Between APS and HVOF Process

TiB2-Ni cermet coatings were deposited by atmospheric plasma spraying and high-velocity oxy-fuel (HVOF) spraying with an agglomerated powder. The microstructure of the coatings was investigated by scanning and transmission electron microscopy, electron microprobe analysis and x-ray diffraction analysis. A model describing the microstructure evolution during the spray process was proposed. As the sprayed particles experienced high-temperature processing in the plasma jet, the original Ni3B binder was quickly oxidized and then a part of TiB2 particles was dissolved in the melt resulting in the formation of a ternary Ni-Ti-B phase and fine TiB2 precipitates. Oxide was also observed at the periphery of the sprayed splats. The combustion flame having lower temperature significantly alleviated the dissolution of TiB2 as well as oxidation of Ni3B binder, so that most of the original phases were retained in the HVOF-sprayed coating. The mechanical properties of the coating sprayed by the two different techniques showed a significant difference, in terms of coating hardness and fracture toughness.