Microstructure And Oxidation Resistance Of The Mechanically Alloyed Si-B-C-N-Al Powders And Ceramics

Si-B-C-N-Al powders have been prepared by mechanical alloying (MA)technique using aluminum (Al) and aluminum nitride (AlN) as aluminum source,respectively. The effects of aluminum source and content on microstructure, valencebond, thermal stability and oxidation resistance were systematically investigated.And Si-B-C-N-Al ceramics were fabricated by hot pressing (HP) technique. Theinfluence of sintering parameters, aluminum source and content on phasecompositions, microstructure and properties of the bulk ceramics has been studied.The effects of aluminum source and oxidation temperature on the oxidationbehaviour of materials were investigated and the oxidation mechanism was alsodiscussed.The experimental results show that in the A3powders, which were preparedusing crystalline silicon (Si), hexagonal boron nitride (h-BN), graphite (C) andaluminum (Al) as starting materials, the particles are completely amorphous; theas-prepared AN3powders using AlN as aluminum source are predominantlyamorphous with a very small volume fraction of AlN nano-crystalline phase. Theanalysis on bonds shows that the solid reactions among raw materials occurred byMA, which resulted in the generation of new chemical bonds such as Si-C, B-C-N,C-C, B-N and Al-N. Compared with the A3powders, the AN3powders have similarbonding types but no oxygen-containing bonds and have relatively simpleenvironment of aluminum atoms. The oxidation resistance of AN3is better than thatof A3. The activation energies and oxidation resistance of the powders reduced withthe increase of aluminum content. The thermal stability of Si-B-C-N-Al powderswas excellent at high temperature in argon.There are four ceramics prepared using different sintering technique. Theincrease of sintering temperature and pressure has little effect on the phasecompositions and3C-SiC、BCN、AlN and AlON are the main phases in the fourceramics. While the increase of sintering temperature and pressure has a great effecton densification, the ceramics prepared at1900°C under a pressure of50MPa havethe highest density and are most dense.The aluminum source and content have a greater impact on the phasecompositions. The ceramics with lower aluminum content contain4H-SiC phase andthe ceramics using AlN powders as aluminum source does not have AlON phase.The microstructures are fine and homogeneous in the two ceramics using differentaluminum source, lamellar BCN and3C-SiC with straight stacking faults areobserved, disordered atom areas were also found in3C-SiC、 AlN and BCN grains.The thermal expansion coefficient values of the three ceramics were4.8~5.1×10-6/°C at1300°C.The weight change of the two ceramics using different aluminum source showsthe similar trends with the oxidation time increasing when the oxidationtemperatures are1200、1300and1400°C, that is first weight loss, then rapid weightgain and finally into a relatively stable stage of weight change. The phasecompositions on the oxidation surface are the same for the two ceramics afteroxidation for80h at1200、1300and1400°C. The oxidation products are SiO2、mullite and C, the diffraction peaks of mullite and C increase in intensity withoxidation temperature raising, while the crystalline diffraction peaks of SiO2at1200°C changed into amorphous diffuse scattering peaks at1400°C, this change isbeneficial on improving the oxidation resisitance of the materials. The thickness ofoxide scales increased with increasing oxidation temperature for the two ceramics;while at the same oxidation temperature, the oxide scales of the ceramics using Alpowders as aluminum source are slightly thicker than those of the ceramics usingAlN powders as aluminum source. The oxidation resistance of the ceramics usingAlN powders as aluminum source is superior to the ceramics using Al powders asaluminum source with comprehensive comparison.The analysis on the oxidation mechanism of the ceramics shows that thediffusion mechanism controls the oxidation at1200°C, the oxidation behaviour iscontrolled by the oxidation reaction and diffusion mechanism when the temperatureis higher than1300°C.