Study On High Strain Rate Deformation Of Alumina, Silicon Carbide Ceramics And Al₂O₃/SiC Nanocomposites

In order to fully understand the bullet-proof mechanism of ceramic armour and furtheroptimize the testing technique of high strain rate deformation properties of ceramic materials,and aim at the continual problem: ceramic targets for bullet-proof properties testing, which areusually carried out at ballistic velocities, are seriously damaged after impact and cannot be usedfor the post-characterization of microstructure and dynamic response. The projectiles with thecontrolled speed and tip quality were fired by gas gun to impact the ceramic specimens at100m/sto mimic the bullet-proof properties testing, the microstructure, residual stress and plasticdeformation on both the target surface and cross-section were analysed in the project. The SplitHopkinson Pressure Bar (SHPB) method was also used to test the compression strength ofceramics under high strain rate.In this thesis, the effects of grain sied and nano particles as the second phase on themechanical properties of ceramics were studied. The alumina ceramic with fine grain size1.7μmhad higher Vickers hardness of18.3GPa. Compared with the alumina sintered at1550°C, thestrength and toughness of Al2O3/SiC nanocomposites increased by60%and45%respectively,when5wt%nano silicon carbide particle was added.The influence of different material mechanical properties on the bullet-proof efficiency wasanalysed in the project, the vital property for ceramic armour appeared to be hardness. Thecomparisons of the imparct region microstructure and crack categories beneath the impact ofdifferent materials were made, and the residual stress and material plastic deformation wereexamined by Cr3+fluorescence or SiC Raman spectroscopic technique. The results show that:with the increase of material hardness, the dimension of impact crater reduced and the conicalcracks appeared. The cracks in SiC sample with23.8GPa hardness were almost conical and hadless than45°angle to the surface. The surface materials of SiC and B4C/Al8B4C7Ceramics werepartly fractured due to the low toughness. The alumina sample had the highest compressionresidual stress level at3.25GPa, the microcracks in the impact region were induced by sufficientbiaxial stress. The highest microcrack density was observed in the centre area of Al2O3/SiCcrater and more energy of the penetrating projectile can be consumed. It is feasible to use theYoffe’s model to predict the crack patterns and positions under sharp impact. Because of the high temperature above1000°C was reached when the high speed impactoccurred, the Young’s Modulus of both projectiles and ceramic materials in the small impactregion reduced. Hence the projectiles and targets deformed significantly, and the bigger impactarea with lower level compression residual stress was found when compared to the quasi-staticspecimens. The highest compression stress for quasi-static samples was also existed in aluminaat5.19GPa. However compared to the dynamic tests, the residual stress and material plasticdeformation level beneath the quasi-static indentation were nearly the same but with moreshallow material deformation region.According to the effect studies of the tungsten carb anvils’ shape, ceramic specimen shapeand dimension designing, incident pulse shaping and interfacial frictions on the Split HopkinsonPressure Bar technique, the testing equipment and condition were modified and to assure thespecimens were in stress equilibrium during dynamic compression, so the preconception ofone-dimensional and uniform planar elastic wave propagation can be achieved. In this project theSHPB compression strength of different ceramics at1200-1800/s high strain rate obtained, thestrength increased with the reduction of grain size in alumina and the maximum result was3.63GPa. Compared with alumina ceramics, the5wt%addition of nano SiC particles had littleimprovement of the SHPB strength for Al2O3/SiC nanocomposites. Due to the inertia associatedwith the crack tip, ceramic materials exhibit rate-sensitive failure strength under the high strainrate load.The high strain rate deformation level of alumina was smaller than other materials; howeverAl2O3/SiC nanocomposites showed uniform and low residual stress in the crater region, the conecracks in high hardness materials was less harmful to ceramic armour. This study is a supplementto the bullet-proof mechanism of ceramics, and it can provide theoretical basis for designing andpreparing ceramic armours.