Fabrication of high-strength transparent MgAl 2 O 4 spinel polycrystals by optimizing spark-plasma-sintering conditions
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By optimizing the heating rate during spark-plasma-sintering (SPS) processing, a highstrength transparent spinel (MgAl2O4) can be successfully fabricated for only a 20-min soak at 1300 C. For the heating rates of 10 C/min, the spinel exhibits an excellent combination of in-line transmission (50–70%), four-point-bending strength (>400 MPa), and hardness (>15 GPa). The excellent optical and mechanical properties can be ascribed to the superimposed effects of the sub-micrograin size, fine-pore size, and low porosity, which are related closely to the heating rate during the SPS processing. The present study demonstrates that to attain a high-strength transparent spinel at low temperatures and short sintering times, the low-heating-rate SPS processing is more efficient compared with the high-heating-rate SPS processing. I. INTRODUCTION
The spark-plasma-sintering (SPS) processing has been widely used for the densification of several materials including metals, ceramics, and composites.1 This method is because, compared with the hot pressing (HP) or hot isostatic pressing (HIP) techniques, SPS can attain high-heating rates of typically 50 C/min1–4 and, hence, can complete the powder densification within a short processing time. The short duration of the SPS processing has been regarded as the primary advantage over the HIP and HP techniques. On the contrary, Kim et al.5–7 recently demonstrated that for the fabrication of high-density and sub-micrograined polycrystalline alumina (a-Al2O3), a low-heating-rate SPS processing is more effective than the widely used high-heating-rate processing. For a heating rate of a = 100 C/min, the sintered alumina had a low density and was opaque, whereas for a = 8 C/min, it showed a good transparency for only a 20-min soak at 1150 C. By applying the low-heating-rate SPS processing (a = 10 C/min), a transparent polycrystalline spinel (MgAl2O4) can also be fabricated at 1300 C.8,9 A noticeable advantage of the low-heating-rate SPS processing is that a sub-micro microstructure with grain sizes smaller than 1 mm is attainable, probably due to the low temperature and the short processing time for the densification; for example, the transparent spinel shows a sub-micrograin size of 0.4 mm. In most of the earlier studies on transparent spinels,10–17 the grain size has generally exceeded 10 mm. This result is due to fact that a spinel a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0335 J. Mater. Res., Vol. 24, No. 9, Sep 2009
powder is a hard-to-sinter material, thus spinel powders doped with sintering aids, such as LiF,10,12,17 CaO,13,14 and B2O3,16 have usually been consolidated at high temperatures of >1400 C for long times using HP or HIP techniques. Thus, densification using the sintering aids or at high temperatures causes grain coarsening. The grain size of a 1 wt% LiF-doped spinel reached 20 to 30 mm by SPS processing at 1600 C for only a 20-min soak.17 The grain coarsening is usually accompanied by the reduction
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