Single-source-precursor derived bulk Si3N4HfBxN(1-x) ceramic nanocomposites with excellent oxidation resistance

In the present work, bulk Si3N4/HfBxN1-x ceramic nanocomposites were successfully fabricated via a polymer-derived ceramic approach. The chemical reaction to form the single-source precursor was confirmed by FT-IR and XPS, in which both Si−H and N−H groups of perhydropolysilazane react with borane dimethyl sulfide complex and tetrakis(dimethylamido) hafnium(IV). The investigation of the polymer-to-ceramic transformation of the synthesized precursors indicates that Hf- and B-modified PHPS exhibits high ceramic yields of up to 100 wt % after pyrolysis at 1000 °C under ammonia. Moreover, XRD and TEM results show that the SiHfBN ceramics with a molar ratio of B : Hf=5 and 10 resist crystallization at temperatures up to 1500 °C and separate after annealing at 1700 °C into nanocomposites comprising of an α-Si3N4 matrix with embedded ternary HfBxN1-x phases, solid solutions of rock salt-type HfN and HfB. Based on the investigation, warm-pressing was applied to fabricate bulk SiHfBN specimens, and the oxidation behavior of samples annealed at 1700 °C was recorded at 1500 °C over a range of oxidation times between 1 and 50 h. The weight changes of Si3N4/HfBxN1-x ceramics with B : Hf molar ratios of 2 : 1, 5 : 1 and 10 : 1 are 4.31 %, 4.37 % and 2.57 %, respectively. The formation of HfSiO4, B2O3 and SiO2 during oxidation plays a crucial role for the improvement of the oxidation resistance of the Si3N4/HfBxN1-x ceramics.