Al2O3–NiCrAl composites and functional gradient materials fabricated by reactive hot pressing

Al2O3–NiCrAl composites were fabricated by mixing NiO, Al and Cr powders and then reactive hot pressing. The high temperature alloy NiCrAl was formed by the reaction of extra Al, Cr and the Ni reduced from NiO. The Al2O3–NiCrAl composites with various Al2O3 fractions were successfully fabricated by the proper addition of extra Ni, Cr, Al or Al2O3 powders. A five-layer functional gradient material of YSZ–NiCrAl was fabricated using the Al2O3–NiCrAl composites with 25, 52.2 and 75 vol.% Al2O3 as interlayers. The results from XRD analysis, optical microscope observation and thermal cycling test show that the composites fabricated by this method consist of α-Al2O3 phase and (Ni, Cr, Al) solid solution mainly, together with a small amount of NiAl. The α-Al2O3 grains formed by this in situ reaction between NiO and Ni are ultrafine and uniformly distributed. The three-point bending strength for the composite with 52.2 vol.% Al2O3 is 363.5 MPa. No cracking was found in the functional gradient materials after 10 thermal cycles up to 1000 °C. It shows that the composites and the functional gradient material fabricated in this work have good strength, good resistance to oxidation and thermal shock.