High Temperature Oxidation Behavior Of High Velocity Oxygen-Fuel Sprayed TiB₂-NiCr Coating

Titanium diboride is an advanced ceramic material with excellent mechanical behavior. TiB₂-M（M=Fe, Ni, Cr） coating has better abrasive resistance than traditional Cr₃C₂-Ni Cr and TiC-NiCr coating and low oxide rate below 730℃ in existing studies. It has potentiality to use in boiler environment with ash erosion and molten salt deposits corrosion. But the researches of TiB₂-M system coatings have not been understood and revealed sufficiently.The well-wettability metal Ni Cr with TiB₂ was used as binder phase. 60 wt.%TiB₂-32 wt.%Ni-8 wt.%Cr composite powder was produced by spray-drying technology, and HVOF-sprayed TiB₂-Ni Cr coating was deposited on carbon steel. XRD,SEM and TEM were used to study the composition, microstructure and high temperature oxidation behavior of the HVOF-sprayed TiB₂-Ni Cr coating. The following conclusions were obtained:The calcined agglomerated powder with the apparent density 1.54 g/cm3, fluidity 9.7 s/50 g and a median diameter 36.4 μm, which 76.4 vol.% of powders with the range size in 15⁴5 μm by changing the solid content of slurry, the temperature of spray-drying, the centrifugal rate of agglomerated disc and the temperature of heat treatment. The boride coating was deposited by HVOF spray with micro-hardness values 867.1±32.4 HV0.3, fracture toughness values 2.2 ± 0.5 MPa·m1/2 and porosity 0.51%. The metal phases in the raw powders had changed into Ni2 B and Cr B during the calcine process, and these phases in coating were unchanged. The SEM images showed, TiB₂ phase homogenously distributed in the coating. There is no obviously diffusion in the interface between the TiB₂ and binder phase, that confirmed by TEM. The coating has good oxidation resistance, the oxidation kinetic content Kp are determined to be 1.95×10-13 and 1.48×10-11 g2/cm4·s. The coating have outstanding hot corrosion resistance in Na2SO4-K₂SO₄（7:3） mixture salt environment at 500℃. The mass gain curves are parabolic at 500℃,700℃ and 900℃. The oxidation kinetic content Kp are determined to be 1.8×10-12, 1.2×10-10 and 1.3×10-9 g2/cm4·s. A dense Ti O2-Ni5 Ti O7-Ni5 Ti O4（BO3）2 oxide scale was finally formed in molten salt corrosion environment to separation the salt and coating to prevent the further corrosion.