Dynamic strength of fine-grained alumina ceramics obtained by Spark Plasma Sintering

Abstract

The results of dynamic compressive testing of alumina samples with different grain sizes sintered from nano and fine α-Al2O3 powders are presented. The ceramics were obtained by spark plasma sintering (SPS). The effect of heating rate (Vh), sintering temperature (Ts), holding time (ts), cooling rate (Vc) on hardness, fracture toughness, dynamic strength stress (σY) of Al2O3 has been studied. An amorphous layer of nanometer thickness was present on the surface of nanopowders in the initial state. After sintering, the grain boundaries of the ceramics had a crystalline structure; no inclusions of the amorphous phase were found. It has been suggested that during the SPS process, an amorphous structure containing an excess free volume is transformed into a crystalline phase with the formation of dislocation-type defects at the grain boundaries, which create long-range internal stress fields. It is shown that nanopores less than 50 – 100 nm in size are present at the grain boundaries of sintered ceramics. It is shown that the nonmonotonic nature of the dependence of σY on the temperature and time of the SPS is due to the simultaneous change in the density, the nonequilibrium state of the grain boundaries, and the grain size of the ceramic. It is shown that a decrease in the degree of nonequilibrium of the grain boundaries of alumina due to an increase in the SPS temperature or an increase in the holding time makes it possible to increase the dynamic strength of alumina. It has been established that an increase in the cooling rate leads to the formation of compressive residual stresses and a slight increase in σY of ceramics. The maximum dynamic strength (σY = 1755 MPa) for alumina ceramics with average grain size 1.6 – 2 µm obtained by SPS (Vh = 50 °С/min, Ts = 1520 °С, ts = 50 min).