High‐toughness mullite ceramics fabricated by hot‐press sintering of pyrophyllite and AlOOH at low temperatures

Abstract

High-toughness mullite ceramics were fabricated through hot-press sintering (HPS) of pyrophyllite and AlOOH, which were wet-milled and well mixed using a planetary ball mill. The impacts of sintering temperatures and contents of AlOOH on mullite phase formation, densification, microstructure and mechanical properties in ceramic materials were investigated through XRD, SEM and mechanical properties determination. The results indicated that high-toughness mullite ceramics could be successfully prepared by HPS at temperatures higher than 1200 • C for 120 min. Increasing the sintering temperature from 1000 to 1300 • C significantly enhanced the flexural strength and fracture toughness of samples. The highest flexural strength of 297.97±25.32 MPa and fracture toughness of 4.64±0.11 MPa⋅m 1/2 were obtained for samples sintered at 1300 • C. Further increase of temperature to 1400 • C resulted in slight decrease of flexural strength and fracture toughness. Compared with the mullite ceramics prepared only using pyrophyllite as raw material, incorporation of AlOOH into raw material significantly increased the mechanical properties of final mullite ceramics. And stoichiometric AlOOH and pyrophyllite as starting material gave the best performance in fracture toughness. The high-toughness of mullite ceramics were ascribed to the high mullite phase content, fine mullite whiskers and in situ formed, intertwined three-dimensional network structure obtained through HPS at a low temperature of 1300 • C.

Significantly improved mechanical properties of mullite ceramics by adding AlOOH with different sizes and morphologies

“…The absence of initial mullitization in the cases of Pyr‐AlOOH‐F and Pyr‐AlOOH‐U is due to the fact that the γ‐Al 2 O 3 transformed from AlOOH at high temperature is dissolved in the silica‐rich liquid phase of clay and participates in the mullitization reaction, and the heat required for this reaction is higher than that released by the generation of primary mullite. Therefore, no exothermic peak of primary mullite formation is observed around 1100°C 13 . As for all samples, an exothermic peak appears around 1250°C, meaning the formation of the secondary mullite phase 44 …”

“…Preliminary studies by our group showed that the addition of AlOOH to pyrophyllite improves the mechanical properties of the obtained mullite ceramics compared to the ceramics using only pyrophyllite as a raw material. 13 AlOOH has higher reactivity due to its dehydration and decomposition as transition state γ-Al 2 O 3 during high-temperature calcination, which delays or prevents the formation of cristobalite, and achieves complete mullitization at low temperatures. 14 In fact, the morphology and size of the second-phase particles, including alumina source, significantly influence the properties and structure of ceramic.…”

“…The amount of SiO 2 in natural clays can be depleted by adding alumina sources, allowing excess SiO 2 to further form mullite as well. Preliminary studies by our group showed that the addition of AlOOH to pyrophyllite improves the mechanical properties of the obtained mullite ceramics compared to the ceramics using only pyrophyllite as a raw material 13 . AlOOH has higher reactivity due to its dehydration and decomposition as transition state γ‐Al 2 O 3 during high‐temperature calcination, which delays or prevents the formation of cristobalite, and achieves complete mullitization at low temperatures 14 …”