Shanghai Institute of Ceramics, Chinese Academy of Sciences
Additive-free hot-pressed silicon carbide ceramics – a material with exceptional properties
Speaker：Prof. RNDr. PAVOL ŠAJGALÍK
Slovak Academy of Sciences, Bratislava, Slovakia
Freeze-granulated and afterwards under infrared lamp annealed silicon carbide powder was densified to the full density without any sintering aids by hot-pressing/ultra-rapid hot-pressing at 1850 °C. This densification temperature is at least 150-200 °C lower compared to the up to now known solid state sintered silicon carbide powders. Presented silicon carbide hot-pressed ceramics have excellent mechanical properties. Vickers hardness is 29 GPa and indentation fracture toughness is 5.25 MPa.m1/2. Samples densified by ultra-rapid hot-pressing have also full density and hardness of 27.4 GPa and fracture toughness of 5.3 MPa.m1/2. Creep rate of ultra-rapid hot-pressed samples at 1450 °C and 100 MPa load is 3.8 x 10-9 s-1 and at 1400 °C and the same load conditions is 9.9 x 10-10 s-1. Partial phase transformation /- SiC was observed in the granulated and hot-pressed/ultra-rapid hot-pressed samples.
Densification behavior of granulated and annealed silicon carbide powders is discussed. The possible explanation of high density silicon carbide ceramics is based on the formation of the transient liquid phase. The transient liquid is possibly formed by aluminum, carbon and silica. Aluminum is present as an impurity in the starting powder. The increased concentration of aluminum can be explained by its segregation to the triple-points at the hot-pressing temperature. Carbon is present as a residuum of the organic agents used for the granulation silicon carbide powder. Silica is present as a main impurity of the starting silicon carbide powder.
The oxidation behaviour of this way prepared SiC ceramics at 1350-1450°C/0-204h was investigated This way prepared SiC ceramics is characterised by an excellent oxidation resistance (4.91x10-5 mg2/cm4h at 1450°C).