Spark plasma sintering β-SiAlON-BN ceramic composites
β-SiAlON-BN ceramic сomposites - promising materials that can be used in conditions of high mechanical stress, thermal shock, abrasion and exposure to aggressive chemical agents. β-SiAlON-BN composites are highly promising for metallurgical applications such as tubes for metal pouring, pipe heaters, nozzles, dozer units, annular breakers, buckets, crucibles, lining plates, thermocouple casing, sensor level gages, etc.
Among methods for producing powder sialon based materials combustion synthesis (CS) has attracted increasing interest due to the possibility of obtaining products of high purity in a short reaction time. The most rapid and effective sintering method of ceramic powders, as well as other solid materials, is spark-plasma sintering (SPS). Thus, using a combination of these methods is possible to obtain high-density ceramics composition with improved physical mechanical properties.
Synthesis of initial materials for sintering was based on infiltration-mediated combustion powder mixture of Si, Al and B in nitrogen gas under pressure of 10 MPa. CSed β-Si5AlON7 and h-BN powders, were prepared by a high-energy planetary steel ball mill. The milled powders, which weighed about 0.5 g, were placed into a graphite die of 10,4 mm inner diameter and were sintered using a spark plasma sintering system Labox 625 under vacuum condition of lower than 10 Pa. The heating rate was 50 °C/min. The sintered compacts were heated from room temperature to 600 °C without load and were then heated to the desired temperatures (1550-1800 °C) at compressive stress of 50 MPa. The compacts were maintained at the desired temperature for 5 min before the power was turned off. Temperature monitoring during sintering between 600 C and the final sintering temperature was carried out using an optical pyrometer, focused on a hole on the surface of the carbon die.
The particle size distributions of the milled powders were determined via laser scattering method by using Fritsch Analysette 22. The used powders and sintered compacts were characterized by XRD (DRON-3.0) and SEM (JEOL 6610L). Sample densities were determined by hydrostatic weighing. Flexural strength (f) were measured by using universal testing machine (Instron-5966).
As a result, β-SiAlON-BN samples having a relative density of 95-98% were obtained by SPS while sintering pure β-SiAlON powders without additives allows to obtain a density of 85-90% of the samples. Using Y2O3 intensified sintering of pure β-SiAlON powder and practically no effect on the shrinkage kinetic compositions of samples β-SiAlON-BN. Introduction Y2O3 considerably increases the sintering intensity of β-Si5AlON7 at temperatures above 1400 °C, i.e. after formation of the eutectic with liquid oxide compounds. Introduction of h-BN also contributes to obtaining a more dense materials at the SPS, but the mechanism of action of this compound is different. h-BN improves compactibility of sintered powder mixtures. When 30 wt. % content of particles of non-wettable oxide melts, h-BN particles are almost completely isolated of β-Si5AlON7 particles from each other. Thus, the relative density of β-SiAlON-BN close to the theoretical value can be achieved without the introduction of Y2O3 activator.
The level of the relative density is a determining parameter for the strength characteristics of sintered ceramic composites. Flexural strength ceramic composition samples β-SiAlON-BN increased as compared with the samples obtained under the same conditions but without BN. Deterioration of the mechanical characteristics was observed with BN content increases above 10 wt. %. It is also found that the change in the strength properties of the sintered samples relative to their similar densities dependencies identified earlier in other published works, in the ceramics of similar composition and at least not inferior to them by the value of the magnitude of attainable strength.
Thus, in this work, high-density β-SiAlON–BN (0-30 wt. %) ceramic composites with improved strength properties are obtained at the SPS from CSed raw powders (β-Si5AlON7 and BN). Such obtained machinable ceramics seem promising for fabrication of items operating in severe conditions of strong thermal shock and corrosion-active medium.