Investigation of the effect of internal stresses on the ballistic resistance of fine-grained aluminum oxide obtained by spark plasma sintering

The results of tests for ballistic resistance of ceramic samples based on aluminum oxide (Al2O3 + 0.25MgO) obtained by high-speed electric pulse plasma sintering (EIPS) are presented. Samples 30 mm in diameter, 6.5 and 7 mm thick had a high relative density (more than 99%), a uniform fine-grained microstructure (average grain size ~1–1.5 mm), and increased hardness (more than 17 GPa). Using the finite element method, using the ANSYS Workbench package, it is shown that during rapid cooling at a rate of 50 °C/min, an uneven temperature field is formed in the samples and a difference in temperatures arises on two opposite sides of the ceramic samples. It has been established that non-uniform cooling during SPS leads to the formation of compressive internal stresses (up to –450 MPa), and there is a significant difference in the magnitude of internal stresses on opposite sides of ceramic samples. The differences in the parameters of the microstructure and microhardness on opposite sides of the samples are insignificant. Ballistic resistance tests were carried out at a speed of 700 m/s, using cylindrical-conical impactors made of hardened steel 52100. The ballistic resistance of the ceramics was evaluated by the anti-barrier action, i.e., the penetration depth of the striker into the aluminum witness sample located behind the tested ceramic plate. It is shown that the barrier action during testing of a ceramic plate with a large value of compressive internal stresses (from –390 to –450 MPa) is 1.5–2 times less than during ballistic testing of a ceramic plate, in the surface layer of which internal compressive stresses are small (from –60 to –90 MPa). The results obtained for the first time demonstrated the importance of the formation of compressive fields of internal stresses in ceramics, which makes it possible to further increase the ballistic resistance of protective ceramic plates.