Solid solution magnesium aluminate spinel with the alumina‐rich composition MgO·1.2 Al2O3 has been prepared as a transparent polycrystalline ceramic with average in‐line transmission at 550 nm of 84.8 ± 2.7% and >82% throughout the visible spectrum. Starting powders are prepared from mixtures of high purity Mg(OH)2 and γ‐Al2O3 thoroughly mixed in an aqueous slurry. Water is removed by rotary evaporation. The solids are collected, dried, calcined, mixed with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa Ar. Final grain sizes ranged between 300 and 1000 μm. Samples exhibited flexural strength of 176.8 ± 46.2 MPa; hardness of 12.3 ± 0.2 GPa; and elastic modulus of 292.9 ± 7.5 GPa. Control samples of stoichiometric magnesium aluminate spinel (MgO·Al2O3) were prepared with the same procedure and exhibited comparable values for transmission and physical properties.

It has long been known that a relation exists between a material's hardness and its gross impact performance; however, the nature of this relationship has not been understood to a degree useful in materials development. Many studies have shown that harder ceramics tend to display better ballistic performance. In addition, some research has suggested that a material's potential for inelastic deformation (or its "quasi-plasticity"a bulk property) may also play an important role in its resistance to penetration. Methods of quantifying the bulk plasticity of a ceramic material are, however, extremely limited. The current study continues an investigation into a recently proposed technique to (1) quantify bulk quasi-plasticity in SiC materials, and (2) use the "plasticity" value along with a hardness value to predict the transition velocity of potential armor ceramics. The transition velocity values predicted by this approach generally show excellent agreement (within 5% in most cases) with experimentally determined velocities. In addition, the robustness of this predictive technique is demonstrated through the use of multiple operators and multiple hardness testing units. †

Magnesium hydroxide, Mg(OH)2 (both 99% and 99.99% purity grades), γ‐Al2O3, and AlOOH have been investigated for the direct production of transparent MgAl2O4. The highest average in‐line transmittance through 3.5–4 mm thick samples is 84.2 ± 1.0% at 550 nm, attained by using mixtures of 99.99% pure Mg(OH)2 and γ‐Al2O3. Other formulations exhibited 77–80% visible transmission, suggesting that further optimization can improve sample consistency. All samples exhibited a Knoop hardness of ~12 GPa, and elastic modulus of ~280 GPa. Biaxial flexural strength measurements ranged from 85 to 136 MPa depending on starting materials.

The indentation-size effect (ISE) in a commercial SiAlON was investigated by conducting Knoop indentation between 0.1 and 20 kg. The resulting ISE was analyzed utilizing Meyer's Law, Proportional Specimen Resistance (PSR) model, and a Multifractal Scaling Law (MFSL). Meyer's law and the PSR model fits to the hardness-load data were not excellent. Further analysis based on the PSR model and MFSL revealed three piecewise linear fits corresponding to load regimes 0.1-0.3, 0.5-2, and 5-20 kg. Physical inference of MFSL fit parameters suggested that these three load regimes correspond to where indentation behavior is governed by deformation mechanisms limited to single grains, grain boundaries, and multiple grains, respectively. Independent of the ISE analysis results, comprehensive examination of indents by scanning electron microscopy revealed that changes in deformation mechanisms could also be grouped into these three load regimes. Corresponding changes in deformation mechanisms were microcleavage cracking, grain-boundary cracking, and macrocracking, respectively. These observations are consistent with the findings of both the PSR model and MFSL with respect to the physical aspects of the governing mechanisms. It is concluded that these mechanisms are responsible for the observed ISE in this commercial SiAlON.

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