Structure, Properties, and Oxidation Resistance of Prospective HfB2-SiC Based Ceramics

This work is devoted to the fabrication of heterophase powdered and sintered ceramics based on the hafnium diboride and silicon carbide by means of self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The structure of SHS-powders consists of hafnium diboride grains and agglomerated polyhedral 2-6 mu m silicon carbide grains. The produced composite powders are characterized by an average size of similar to 10 mu m and maximum size of 30 mu m. The phase composition of the synthesized powders and sintered ceramics are identical. The sintered ceramic is characterized by a high degree of structural and chemical uniformity, a residual porosity of 3.8%, a hardness of 19.8 +/- 0.4 GPa, strength of 597 +/- 59 MPa, and fracture toughness of 8.8 +/- 0.4 MPa m(1/2). Plasma torch tests (PTTs) are carried out to determine oxidation resistance under the influence of high-enthalpy gas flow. The phase composition and microstructure of the surface of the sample after testing are investigated. HP ceramics demonstrated outstanding resistance towards oxidation in a high-enthalpy plasma torch at 2150 degrees C and heat flow of 5.6 MW/m(2) for 300 s. During plasma torch testing, a dense protective oxide layer 30-40 mu m thick is formed on the surface of HfB2-SiC ceramic. The layer consists of an HfO2 hull filled with amorphous borosilicate glass SiO2-B2O3. From the composite HfB2-SiC SHS powder, by hot pressing, experimental samples of model bushings of the combustion chamber of a low thrust liquid rocket engine are designed to conduct PTTs in realistic operating conditions.