Properties of Mullite-Zirconia Composites Prepared through Reaction Sintering Kaolin, α-Al₂O₃, and ZrO₂

Abstract

Mullite–zirconia composites were synthesized through reaction sintering Algerian kaolin, α-Al2O3, and ZrO2. Phases present and their transformations were characterized using x-ray diffraction. Quantitative phase analysis was performed following the Rietveld method. Hardness and fracture toughness were measured by Vickers indentation. The flexural strength was measured using a Universal Testing Machine. It was found that the microstructure of samples sintered for 2 hours at 1600°C was composed of mullite grains which have whiskers’ shape and ZrO2 particles. In the composite containing 16 wt.% ZrO2, the ratio of tetragonal zirconia transformed to monoclinic zirconia was relatively small and did not exceed 18%. However, in the composite containing 32 wt.% ZrO2 around 75% of the tetragonal structure changed to monoclinic structure. Also, it was found that the increase of ZrO2 content from 0 to 32 wt.% decreased the microhardness of the composites from 14 to 10.8 GPa. However, the increase of ZrO2 content from 0 to 24wt.% increased the flexural strength of the composites from 142 to 390 MPa then decreased it with further increase of ZrO2 content. The fracture toughness increased from 1.8 to 2.9 MPa.m1/2 with the increase of ZrO2 content from 0 to 32 wt.%; and the rate of the increase decreased at higher fractions of ZrO2 content. The average linear coefficient of thermal expansion (within the range 50 to 1450°C) for samples containing 0 and 16 wt.% ZrO2 sintered at 1600°C for 2 hours was 4.7 x10-6 K-1 and 5.2 x 10-6 K-1 respectively.

Additive manufacturing and mechanical characterization of high density fully stabilized zirconia

“…Groups 3 and 6 had the highest hardness values (15.3±0.4 GPa and 15.4±0.2 GPa, respectively). To compare the fracture toughness, the equation proposed by Liang et al [21] was chosen due to its popularity for zirconia [22][23][24][25][26][27][28][29][30][31][32]. According to their method, the value of fracture toughness is estimated from the following equation: (1) where K IC is the fracture toughness in MPa•m 0.5 , H is the hardness in MPa, E is the Young's modulus in MPa, φ is a constant equal to 3, and a and c are the half diagonal length of the indent and half length of the crack in m, respectively.…”

“…where ν is Poisson's ratio which was assumed to be 0.29 [22][23][24][25][26][27][28][29][30][31][32].…”

Sintering Characteristics of a Powder Injection Molded Ceria-Stabilized Zirconia–Mullite Composite

“…The amount of tetragonal phase transformed to monoclinic phase affects the mechanical properties of the composites. A low transformation will result in enhanced strength and For the mullite-32 wt% ZrO 2 system, a 75% change in phase transformation resulted formation of microcracks resulting in inferior properties [13]. The larger conversion of t-ZrO 2 to m-ZrO 2 could explain the microstructure and properties for the present data obtained at 1600 C. The maximum fracture toughness of 4.1 AE 0.23 MPa Á m 1=2 observed in the present study is significantly higher than the values obtained in most of the prior studies where other technologies like uniaxial and cold isostatic pressing was used to fabricate the zirconia-mullite composites ( Table 1).…”

“…A brief summary of properties achieved and processing methods used in prior studies are summarized in Table 1 [4][5][6][7][8][9][10][11][12][13][14][15][16]. The information presented in Table 1 clearly shows that shape forming of the zirconia-mullite composites is typically reported using either uniaxial or isostatic pressing.…”