Development of a thermal barrier material using combustion synthesis

The present investigation employs combustion synthesis as a method to produ ce a functionally graded Ni3Al/Al2O3 + TiB2 composite material for use as a thermal barrier system for nickel-based alloys at elevated temperatures. S tarting materials were Ni, Al, TiO2 and B2O3 in powder form. Adiabatic ther modynamic calculations used to determine the maximum theoretical temperatur e 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 re action. The latter reaction is predicted to reach 3000 K. Experiments were first conducted in an induction furnace to establish conditions necessary f or combustion to occur. Subsequent experimentation, with applied pressure d uring combustion, was conducted in a Gleeble 1500 thermomechanical test uni t modified to accept the samples of interest. Characterisation of the combu stion products by means of hardness measurements, X-ray diffraction, scanni ng electron microscopy and electron probe microanalysis confirmed that the products were Ni3Al and Al2O3 + TiB2. Also, the mechanical integrity was un changed after 10 thermal cycles in the modified Gleeble unit. Finally, the coating thickness required to keep a Ni-based substrate below 850 degrees C in a 1100 degrees C environment is estimated to be 1.8 mm, based on therma l conductivity calculations using a finite element method. (C) 1999 Elsevie r Science S.A. All rights reserved.