Phase transformation and sintering behaviour of mullite and mullite–zirconia composite materials

Abstract

Mullite is one of the most promising engineering materials for applications at elevated temperatures, but has poor mechanical properties at ambient temperature; therefore, it is usually reinforced with particles, fibres or whiskers to improve its properties. Among particles added to mullite are ZrO 2 particles which improve its fracture toughness through the well known process of phase transformation from tetragonal to monoclinic in zirconia particles. The aim of the present work is to explore the utilisation of Algerian kaolin, a-Al 2 O 3 and ZrO 2 to synthesise mullite-ZrO 2 composites through reaction sintering and investigate phase transformation and sintering behaviour of the composites. The raw materials were mixed through planetary ball milling followed by attrition milling. Compacted samples were sintered at temperatures between 1100 and 1600uC for 2 h. The bulk density was measured by the water immersion method. X-ray diffraction (Rietveld method) was used to characterise phases present in the sintered samples. It was found that the zirconia phase retained its tetragonal structure with the addition of up to 16% zirconia. The formation of primary mullite in all samples was complete at 1250uC. The cristobalite started to form at 1150uC, and disappeared at 1300uC in the samples of mullite, and at 1250uC when ZrO 2 was added. The zircon compound ZrSiO 4 started to form at 1250uC and completely disappeared at 1400uC. The increase in ZrO 2 ratio promoted the formation of grains with spherical shape.

In situsynthesis and thermal shock resistance of mullite used for thermal storage ceramics in solar thermal power generation

“…The presence of mullite crystals together with interconnected pores gives the samples a higher bending strength and a better thermal shock resistance. This phenomenon was also reported by Lin 19 and Sahnoune et al 20 According to the high magnification of SEM results, many crystals connect to each other closely in Fig. 5f, resulting in a perfect bending strength of A3 sample, which is sintered at 1600uC.…”

Influence of zirconia on the sintering behaviour and mechanical properties of reaction-sintered mullite-based composite ceramics

“…The decomposition of kaolin to form mullite also yields cristobalite (Sainz et al , 2000; Sahnoune et al , 2013; Aswad et al , 2021), which transforms into an amorphous phase at high temperatures and impacts the mechanical properties of mullite composite ceramics negatively. To improve the mechanical strength and fracture resistance of mullite composite ceramics, some recent papers have focused on the modification of mullite through the introduction of secondary phases (Ji et al , 2013; Mahmood et al , 2017; Qian et al , 2020; Qin et al , 2020; Sarker et al , 2022).…”

Microstructure evolution and mechanical behavior of a mullite fiber heat‐treated from 1100 to 1300°C

“…Considering that the CMCs reinforced by mullite fibers are produced and applied at 1100-1300 • C, 20,21 the high temperature will have remarkable effects on the microstructure and properties of mullite fibers. Thus, it is significant to investigate the mullite fibers about microstructure evolution and mechanical behaviors in the temperature range of 1100 to 1300 • C.…”