Ceramics have been widely used for personnel and vehicle armour because of their desirable properties such as high hardness and low density. However the brittle nature associated with the ceramic materials, i.e. low toughness, reduces their ability to withstand multiple ballistic hits. The present work is focused on ceramic armour materials made from alumina and zirconia toughened alumina (ZTA). The effects of grain size and zirconia phase transformation toughening on the mechanical and high strain rate properties in both materials were investigated in detail. Alumina, 10%, 15% and 20% nano ZTA with 1.5 mol% yttria stabiliser were produced with various grain sizes. The processing of the materials started from suspension preparation, spray freeze drying of the suspension and die pressing to produce homogeneous green bodies with densities above 54%. Then, the green bodies were sintered using conventional single stage and/or two stage sintering to produce the samples with full density and a range of grain sizes (0.5 to 1.5 µm alumina grains and 60 to 300 nm zirconia grains). The effects of the processing conditions on the microstructures were studied and the optimum processing route for each sample was determined. The mechanical properties of the alumina and ZTA samples were investigated, including Vickers hardness, indentation toughness, 4-point bend strength and wear resistance. The results showed that, with an increasing amount of zirconia addition, evident increases of the toughness, strength and wear resistance properties were observed, whilst the hardness reduced slightly correspondingly. The effect of density and grain sizes on the hardness and toughness were studied as well: larger alumina grain size led to a higher hardness and negligible change in toughness, whilst the zirconia grain coarsening enhanced the phase transformation toughening effect and the samples displayed a higher toughness. In addition to the investigation of the mechanical properties, the alumina and nano ZTA samples were subjected to high strain rate testing, including split Hopkinson pressure bar (SHPB) (8-16 m/s) and gas gun impact testing (100-150 m/s). The high strain rate performances were compared in terms of their fracture behaviours, fragmentation process and fragment size distribution. Raman spectroscopy was used to measure the amount of zirconia phase transformation in ZTA samples after the high strain rate testing. The residual stress and dislocation density in alumina grains after testing were quantitatively measured using Cr3+ fluorescence spectroscopy. The results indicated that zirconia phase transformation can reduce the residual stress and dislocation densities in the ZTA samples, resulting in less damage, lower plastic deformation and less crack propagation. In addition, a nano zirconia material with 1.5 mol% yttria stabiliser (1.5YSZ) was subjected to a gas gun impact test with a very high impact speed (142 m/s); a deep projectile penetration was observed, due to the low hardness of the pure zirconia, whilst the sample stayed intact. The result further confirmed that the zirconia phase transformation toughening effect can improve the sample's high strain rate performance.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:574213 |
| Date | January 2013 |
| Creators | Huang, Shuo |
| Publisher | Loughborough University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://dspace.lboro.ac.uk/2134/12513 |
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