Development of a thermal barrier material using combustion synthesis

The present investigation employs combustion synthesis as a method to produce a functionally graded Ni3Al/Al2O3+TiB2 composite material for use as a thermal barrier system for nickel-based alloys at elevated temperatures. Starting materials were Ni, Al, TiO2 and B2O3 in powder form. Adiabatic thermodynamic calculations used to determine the maximum theoretical temperature reached during combustion suggest that up to 1600 K may be reached in the Ni+Al metallic layer, easily sufficient to initiate the ceramic-based reaction. The latter reaction is predicted to reach 3000 K. Experiments were first conducted in an induction furnace to establish conditions necessary for combustion to occur. Subsequent experimentation, with applied pressure during combustion, was conducted in a Gleeble 1500 thermomechanical test unit modified to accept the samples of interest. Characterisation of the combustion products by means of hardness measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis confirmed that the products were Ni3Al and Al2O3+TiB2. Also, the mechanical integrity was unchanged after 10 thermal cycles in the modified Gleeble unit. Finally, the coating thickness required to keep a Ni-based substrate below 850°C in a 1100°C environment is estimated to be 1.8 mm, based on thermal conductivity calculations using a finite element method.