Alumina Mullite Zirconia Research Articles

Wear behavior and microstructure of alumina-mullite-zirconia composites prepared by a novel method: Coating of zircon powder by aluminum alkoxide

The formation of the mullite phase is the main challenge in the preparation of alumina-mullite-zirconia (AMZ) composites. To overcome this limitation, a novel method based on the coating of zircon powder with aluminum alkoxide was proposed in this study. Reaction sintering of alumina and coated zircon was carried out at 1630 °C for 3 h. The microstructural, physical, mechanical, and tribological properties of samples were compared with the conventional AMZ composites prepared by common mixing of alumina and zircon. The microstructural analysis indicated the higher alumina phase of the sample prepared with the conventional method. On contrary, the samples prepared with the proposed method included higher percentages of mullite phase. In terms of mechanical properties, the conventional AMZ samples performed better. However, due to the beneficial effect of the mullite phase in tribological applications, the samples prepared with this new method show superior wear resistance. Especially, the samples prepared from 30 wt% aluminum alkoxide exhibited the best wear resistance. The delamination and adhesive wear mechanisms govern the wear process.

Introducing MnO2 and ZnO Additives for the Development of Alumina–Mullite–Zirconia Composites

Introducing MnO2 and ZnO Additives for the Development of Alumina–Mullite–Zirconia Composites

Development of AMZ composites via mutual attraction of particles in wet processing

ABSTRACTIn this paper, three approaches were investigated for preparing alumina‐mullite‐zirconia (AMZ) composites. The weight ratio of alumina to zircon was selected to be 85/15. In the first approach, common well‐known reaction sintering or solid‐state mixing of alumina and zircon were used. In the second approach, suspensions of the raw materials were prepared to examine the effect of wet processing method. The third approach was based on the use of aluminum alkoxide coated zircon and alumina. The sintering of samples was carried out at the temperature of 1630°C for 3 hours. Phase composition and Rietveld refinement method, microstructural observation, EDS analysis, as well as physical and mechanical properties were used and determined to characterize the sintered samples. The results showed that aluminum alkoxide coating can develop the mentioned reaction sintering and mullite formation. However, the desired mechanical properties were not obtained. Wet processing approach resulted in more interesting data about the formation of mullite and could improve the microstructure homogeneity of final composites. Higher amount of tetragonal zirconia, good densification, high hardness, and fine microstructure were obtained by the wet approach. These interesting results were attributed to the fine discrete particles provided by mutual attraction in the wet preparation method.

A new method for preparing AMZ composites: Coating zircon powder by aluminum alkoxide

This paper presents a new method for preparing alumina-mullite-zirconia (AMZ) composites. For this purpose, zircon particles were coated by aluminum alkoxide using a wet processing method. The effects of zircon particle size and aluminum alkoxide content)10, 30 and 50 wt % (on the reaction sintering of AMZ composites were investigated. The sintering of samples was carried out for 3 h at the maximum temperature of 1630 °C. Scanning electron microscopy (SEM) was used to investigate the coating process. Energy dispersive spectroscopy (EDS) analysis was also performed to detect the alkoxide coating on zircon particles. The phase constituents of the sintered samples were characterized using X-ray diffraction (XRD) method and estimated using the Rietveld refinement method utilizing MAUD software. The results indicated that aluminum alkoxide promoted the formation of mullite phase and improved the microstructural homogeneity of the final composites. This new approach proposed that using aluminum alkoxide coated zircon can support obtaining desirable phases in the fabrication of AMZ composites. In this way, milling of zircon particles can help alkoxide to provide a homogenous coating.

Microstructure and mechanical properties of hot-pressed Al2O3–mullite–ZrO2–SiC composites

In this study, the effects of SiC content, sintering temperature and Al2O3/SiO2 ratio on the microstructure and mechanical properties of hot-pressed alumina–mullite–zirconia–silicon carbide (Al2O3–mullite–ZrO2–SiC) composite ceramics were investigated systematically. The research results indicate that the composite ceramics have excellent combined properties when SiC content is 20 vol% and sintering temperature is 1530 °C, with Vickers hardness, fracture toughness and flexural strength of 17.5 GPa, 12.3 MPa m1/2 and 970 MPa, respectively. The fracture toughness and flexural strength are higher than those of monolithic Al2O3 ceramics and Al2O3-based composites, which are fairly attributed to the refined microstructures of composites and the synergistic effects of crack deflection, pull-out, branching, and phase transformation toughening mechanisms induced by short columnar Al2O3 grains and SiC platelets formed in the ceramics, as well as the ZrO2 particles existed in the ceramics.

Phase Evolution, Microstructure, and Mechanical Properties of Alumina-Mullite-Zirconia Composites Prepared by Iranian Andalusite

This paper investigates the effects of Iranian andalusite and short milling times on alumina–mullite–zirconia composites. Andalusite powder was added at 0, 2.5, 5, and 10 wt% to an alumina–zircon mixture and the raw materials were milled for 1 or 3 h. The sintering of samples was carried out at the temperatures of 1550°C, 1600°C, and 1650°C for 3 h. Microstructural changes, phase composition, physical properties, and mechanical strength of the sintered composites were characterized by scanning electron microscopy, X-ray diffraction, density, and strength measurement tests. Results show that andalusite promoted the decomposition of zircon and accelerated the reaction sintering of alumina–zircon, which leads to the formation of much more mullite phase and improvements to the composites’ thermal shock resistance up to about 50%.

Effect of short milling time and microwave heating on phase evolution, microstructure and mechanical properties of alumina–mullite–zirconia composites

Abstract Alumina–mullite–zirconia composites were prepared using alumina and zircon powders pressed uniaxially at 250 MPa and sintered in a microwave furnace held at 1 550 °C for 90 min. The effects of short milling and sintering time on the density, phase evaluation and mechanical strength of the sintered composites were analyzed and compared with composites sintered in a conventional furnace. The goal was to decrease sintering time and temperature over that for conventional heating. The results showed that, although the densities were similar for both methods, the hardness, mechanical strength and fraction of the tetragonal zirconia phase of the microwave-sintered composites were much higher. The milling time yielded better densification and higher mechanical properties. It was found that the shorter sintering time in a microwave furnace requires longer milling time of the powders to obtain the same composite properties.

Thermo-mechanical properties of mullite/zirconia reinforced alumina ceramic composites

Alumina–mullite–zirconia ceramic composites were prepared by reaction bonding of alumina and zircon mixtures after firing at different temperatures 1300°, 1400° and 1500°C. Constant amount of magnesia was added as a sintering aid. The technological parameters of the sintered ceramic composites, i.e. the mechanical properties and densification parameter as well as thermal shock resistance, have been investigated. The phase compositions and microstructure of the sintered ceramic composites were detected by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results indicated that alumina–mullite–zirconia ceramic composites fired at 1500°C for 2h were achieved a good densification parameters and mechanical properties as well as excellent thermal shock resistance. In addition, these ceramic composites were showed enhancement in Vickers’ microhardness and fracture toughness values.

Effect of SiC additions on microstructure, mechanical properties and thermal shock behaviour of alumina–mullite–zirconia composites

Abstract This paper describes the effect of additions of silicon carbide (SiC) particles on density, microstructure, mechanical properties and thermal shock behaviour of slip-cast alumina–mullite–zirconia composites. Alumina, zircon and SiC powders were used to prepare highly concentrated (51 vol.% loading) and stable aqueous suspensions by using Dolapix dispersant (0.5 wt.%). The sintering process was carried out in two stages, first at 1600 °C and then 1500 °C, with 2 and 5 h holding, respectively. Densities and thermal shock properties as well as strength, toughness and hardnessof the prepared composites were evaluated. Two-stage sintering process of alumina–mullite–zirconia–SiC (AMZS) composites led to a remarkable grain size reduction and an increase of density. The maximum value of fracture toughness and minimum value of grain size were obtained by using 20 vol.% silicon carbide. Results showed that the addition of SiC particles (20 vol.%) improved the thermal shock resistance (Δ T = 1000 °C) of alumina–mullite–zirconia composites even after 10 heating cycles.

Phase composition of alumina–mullite–zirconia refractory materials

Refractories in the Al 2O 3–SiO 2–ZrO 2 system are widely used in many applications, for ceramic rollers in particular, and are characterized by high mechanical strength, excellent thermal shock resistance, resistance to corrosion by alkaline compounds and low creep at high temperature. Their performances greatly depend on the amount and chemical composition of crystalline and glassy phases, which were investigated by quantitative XRPD (RIR–Rietveld) and XRF in order to assess the effect of various Al 2O 3/SiO 2 ratios of starting batches and different alumina particle size distributions. Refractories consist of mullite, corundum, zirconia polymorphs and a vitreous phase in largely variable amounts. The mullite percentage, unit cell parameters and composition vary with sintering temperature, being mostly influenced by the Al 2O 3/SiO 2 ratio of the batch. Its orthorhombic unit cell increased its volume from 1400 to 1500 °C, while its stoichiometry became more aluminous. The corundum stability during firing is strongly affected by the Al 2O 3/SiO 2 ratio, but not by the particle size distribution of alumina raw materials. Zirconia raw materials are involved in the high temperature reactions and about one-third of the available ZrO 2 is dissolved in the glassy phase, ensuring excellent resistance to alkali corrosion, mainly depending on the fraction of coarse alumina. The phase composition of the vitreous phase increased with sintering temperature, being over 20% when the fractions of coarse alumina in the starting batch are between 0.2 and 0.5.

Observation of dislocation assisted high temperature deformation in mullite and mullite composites

Fine grain alumina–mullite–zirconia composites demonstrate high strain rate superplastic flow (10 −2 s −1) under compression at 1400–1500 °C. Transmission electron microscopy (TEM) studies reveal dislocation activity in mullite grains of the deformed material, indicating that dislocations are generated and propagated during deformation as an accommodation mechanism for superplastic deformation. To further study dislocation accommodated slip in mullite, polycrystalline mullite in ratios of 3Al 2O 3·2SiO 2 and 2Al 2O 3·1SiO 2 were fabricated by reactive sintering of nanocrystalline alumina and colloidal silica. The strain rate of the resultant mullite was four orders of magnitude lower than the alumina–mullite–zirconia composite material. Dislocation generation accommodated the deformation of nominally single-phase polycrystalline mullite compositions at 1450 °C under 40 MPa. Three types of dislocations were observed, with a few dislocations having the character b = [0 0 1]. Dislocation accommodated deformation at high temperatures is significant in mullite and the complex structure of mullite may activate multiple slip systems at high temperatures.

Creep behaviour of alumina–mullite–zirconia nanocomposites obtained by a colloidal processing route

The high-temperature creep behaviour of high-purity alumina (A) and an alumina–mullite–zirconia nanocomposite (AZS) has been studied. The alumina–mullite–zirconia nanocomposite was prepared by using a colloidal processing route (powder–alkoxide mixtures). Creep tests were carried out in air in a 4-point-bending-fixture from 1200 to 1400 °C under constant stresses ranging from 30 to 220 MPa. Creep parameters (stress exponent n and activation energy Q) were correlated with microstructural features in order to determine the dominant creep mechanisms for both materials. The slow crack growth region (SCG), given by pairs of critical stress and the corresponding critical strain rate at the temperatures 1200, 1300 and 1400 °C of both materials was studied. It was found that the creep rate of AZS was two orders of magnitude lower than the creep rate of undoped alumina A. The dominant creep mechanism of A is assumed to be a combination of grain boundary and lattice diffusion controlled creep. The creep mechanism for AZS is different and depends on the temperature. It is supposed that lattice diffusion controlled creep (Nabarro–Herring) is the dominant creep mechanism at 1200 °C, whereas at 1300 °C it is supposed to be grain boundary sliding accommodated by grain boundary diffusion. Comparing the slow crack growth region of both materials, a dramatic improvement was observed. The slow crack growth region of alumina is shifted nearly twice concerning the applied stresses for AZS at the temperatures 1200, 1300 and 1400 °C.

Microstructural features of an Al 2O 3–ZrO 2–SiO 2 refractory material treated with CO 2 laser melting

In this study, an Al 2O 3–ZrO 2–SiO 2 system refractory with the alumina–mullite–zirconia (A–M–Z) structure was treated by CO 2 laser surface melting aiming at improving its surface density and modifying the corresponding microstructure, which were considered to be important factors influencing the erosion resistance of refractory materials. The microstructure and phase structure features of the refractory induced by the laser beam have been investigated by scanning transmission microscopy incorporating energy dispersive X-ray technique and X-ray diffraction analysis. After laser surface melting, the microstructure was changed into a fine and dense dendrite structure in the top and center region of the laser-melted zone and the non-dendrite structure between a dendrite region and the substrate region. The phase m-ZrO 2 or c-ZrO 2 was uniformly dispersed into the dendrite instead of the non-uniform particle-like shape in the as-received refractory, and the dominant phases were changed from α-Al 2O 3+Al 6Si 2O 13 to m-ZrO 2+α-Al 2O 3+Al 2.427O 3.64 due to the selected evaporation of constituents SiO 2 and Al 2O 3.

CO 2 laser-induced structure changes on an alumina–mullite–zirconia refractory

An Al 2O 3–ZrO 2–SiO 2 system refractory with alumina–mullite–zirconia structure (AMZ) has been laser-melted to improve its surface density and to modify its microstructure in this study. Investigations into the laser-melted samples showed that laser scanning velocity had a significant influence on the surface morphology, the microstructure and the phase constituents of the laser-treated zones when other laser parameters were kept constant. Laser overlapping treatment at 8 mm s −1 produced a crack-free, smooth and dense surface on the refractory but melting treatments at a lower scanning velocity of 4 mm s −1 and a higher scanning velocity of 12 mm s −1 generated obvious cracking surface and rough surface respectively. After laser melting treatment, the microstructure was changed into a fine and dense dendrite structure in the central upper region of the laser-molten zone and a planar structure between the dendrite region and the substrate region; the dominant phases were changed from α-Al 2O 3+Al 6Si 2O 13+m-ZrO 2 to m-ZrO 2+α-Al 2O 3+Al 2.427O 3.64 due to the selected evaporation of constituents SiO 2 and Al 2O 3, and the phase m-ZrO 2 or c-ZrO 2 was uniformly dispersed into the dendrite instead of the nonuniform particle-like shape in the as-received refractory. An increase in scanning velocity resulted in more planar structure region and higher contents of the nonequilibrium phases c-ZrO 2 and Al 2.427O 3.64.

Mechanical properties and thermal shock behaviour of alumina–mullite–zirconia and alumina–mullite refractory materials by slip casting

Strength and modulus values of slip-cast alumina–mullite–zirconia and alumina-mullite refractories have been determined as a function of quench temperature by air and water quenching. The resistance to fracture initiation and propagation caused by the thermal stresses was increased with the addition of zircon, as supported by the R and Rst parameters. There was no catastrophic decline in strength on thermal shock at high temperature or with repeated cycling, where a large proportion of intergranular fracture in the fracture surfaces was observed. There was ∼50% loss of strength after the 1st cycle, and further cycling did not markedly change the retained strength of the material to the 40th cycle. The inevitable cracking occurred after thermal shock testing was kept below the critical crack length due to the transformation toughening effect of zirconia, thus maintaining a high degree of integrity, associated with the high values of K1c and γi.

The microstructural features of an alumina–mullite–zirconia refractory material corroded by molten glass

An alumina-mullite-zirconia refractory penetrated by a standard soda-lime-silica glass was statically tested at 1370 °C for 72 h. Microstructural features and EDAX analysis of the compositions in the corroded region were examined by SEM. During penetration, alumina particles started dissolving in the glassy-phase; and thus randomly oriented needle-like alumina crystals were formed and began growing up in the penetrated region. On the other hand, zirconia made an effective barrier in the corroded zone, and therefore the formation of needle-like alumina crystals disappeared steadily towards to the end of the penetration region, with a low solubility rate of SiO2, Na2O and CaO, leading to a high corrosion resistance of the refractory.

Alumina–mullite–zirconia composites: Part II Microstructural development and toughening

Densification characteristics and fracture toughness of the alumina–mullite–zirconia (AMZ) composites fabricated by the colloidal mixing route were examined. The densification of boehmite–silica–zirconia precursor compacts was characterized by three distinct stages. Stage I was represented by a rapid shrinkage between 1100 and 1300°C and was caused by the viscous flow sintering of the SiO2 component. Stage II was characterized by a temporary cessation of the shrinkage caused by the mullitization. Stage III was represented by a restoration of the shrinkage for temperatures above 1450 °C. The α-Al2O3 seeding facilitated the formation of elongated grains in the AMZ composites, and these elongated grains correspond to α-Al2O3. The toughening caused by the microcrack nucleation was comparable to that by the t → m martensitic transformation and increased with increasing total content of zirconia.