Zirconia-toughened Mullite Research Articles

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

ABSTRACTHigh-performance mullite-based composite ceramics were prepared successfully using natural kaolin and alumina as raw materials and ZrO2 as an additive. The influence of sintering temperature and ZrO2 content on the sintering behaviour and mechanical properties of zirconia-toughened mullite ceramics was studied systematically. With increasing sintering temperature from 1450°C to 1560°C, the primary phases of as-sintered composite ceramics were mullite and corundum with a small amount of ZrO2, and the bulk density of the composite ceramics increased from 2.29 to 2.72 g cm–3. Furthermore, the ZrO2 phase transition promoted transgranular fracture, and ZrO2 grains were pinned at the grain boundaries, thereby enhancing the mechanical strength of the composite ceramics. Moreover, the AZS12 sample, with 12 wt.% ZrO2 and sintered at 1560°C, had the greatest flexural strength and fracture toughness of 91.6 MPa and 2.47 MPa m–1/2, respectively. Adding ZrO2 to the composite ceramics increased their flexural strength by ~37.6%.

Development of mullite fibers and novel zirconia-toughened mullite fibers for high temperature applications

Polycrystalline mullite fibers and novel zirconia-toughened mullite (ZTM) fibers with average diameters between 9.7 and 10.3 μm containing 3, 7 and 15 wt.-% tetragonal ZrO2 (ZTM3, ZTM7, ZTM15) in the final ceramic were prepared via dry spinning followed by continuous calcination and sintering in air. A shift in the formation of transient alumina phases and tetragonal ZrO2 to higher temperatures with increasing amounts of ZrO2 was observed. Concomitantly, the mullite formation temperature was lowered to 1229 °C for ZTM15 fibers. X-ray diffraction revealed formation of the desired tetragonal crystal structure of ZrO2 directly from the amorphous precursor. Room temperature Weibull strengths of 1320, 1390 and 1740 MPa and Weibull moduli of 9.5, 7.1 and 9.0 were determined for mullite, ZTM3 and ZTM15 fibers, respectively. Average Young’s moduli ranged from 190 to 220 GPa. SEM images revealed crack-free fiber surfaces and compact microstructures independent of the amount of ZrO2.

Powder Injection Molding of Ceria-Stabilized, Zirconia-Toughened Mullite Parts for UAV Engine Components

Powder injection molding (PIM) of ceria-stabilized, zirconia-toughened mullite composites were investigated in the present article with the goal of obtaining performance enhancement in complex geometries for energy and transportation applications. A powder-polymer mixture (feedstock) was developed and characterized to determine its suitability for fabricating complex components using the PIM process. Test specimens were injection molded and subsequently debound and sintered. The sintered properties indicated suitable properties for engine component applications used in unmanned aerial vehicles (UAVs). The measured feedstock properties were used in computer simulations to assess the mold-filling behavior for a miniature turbine stator. The results from the measurements of rheological and thermal properties of the feedstock combined with the sintered properties of the ceria-stabilized, zirconia-toughened mullite strongly indicate the potential for enhancing the performance of complex geometries used in demanding operating conditions in UAV engines.

SiC/ZTM Nanocomposite with Needlelike Mullite Grains and Enhanced Mechanical Properties

Zirconia-toughened mullite (SiC/ZTM) nanocomposites were prepared by a chemical precipitation method. The samples showed good sinterability and could be densified to >98.7% of the theoretical density at 1350°–1550°C. Because of the addition of mullite seeds in the starting powder and the pinning effects of ZrO2 and SiC particles on mullite grain growth, a fine-grained microstructure formed. Mullite grains were generally equiaxed for the sample sintered at 1400°C; whereas, for the sample sintered at 1550°C, most mullite grains took a needlelike morphology, and SiC particles were primarily located within mullite grains. The strength and toughness increased with the increasing sintering temperature, and reached their respective maximum of 780 MPa and 3.7 MPa·m1/2 for the sample sintered at 1550°C.

Influence of Microstructure Evolution on the Electroconducting Behavior of Intragranular TiN/ZTM Nanocomposites

Electroconductive zirconia-toughened mullite (TiN/ZTM) intragranular nanocomposite was fabricated by hot-pressing a powder mixture of nano-sized TiN, ZrO2(2Y), and mullite gel. The material showed a good sinterability and could be highly densified at a low temperature of 1300°3C. Sintering temperature strongly influenced the microstructure and electrical resistivity of the material. The electrical resistivity increased monotonously from 20 Ω-cm to 1.5 times 106°3Cm, as the sintering temperature was increased from 1300° to 1500°3C. TEM results indicated that such a phenomenon could be ascribed to the changes in the microstructure of the material, which led to a decrease in the connectivity of the TiN network in the sample as the sintering temperature was increased.

Flame stabilization, operating range, and emissions for a methane/air porous burner

A porous burner in which a premixed flame is stabilized within the solid matrix is a promising technology for various applications due to its low emissions. In this article, experimental measurements of flame stabilization, operating range, and emissions are reported for several porous burners. The burner consisted of two sections of reticulated porous ceramic: a section of 23.6 pores per centimeter (ppcm) followed by a section of 3.9 ppcm. Two different porous media materials, yttria-stabilized zirconia/alumina composite (YZA) and zirconia-toughened mullite (ZTM), were used in two separate, but identical, burners. Results reveal that the interface between the two sections of porous ceramic stabilized the flame effectively over a range of conditions in the YZA burner. In the ZTM burner, however, theflamepropagated through the interface and into the upstream section. A third burner was constructed of YZA in which the length of the upstream section was reduced. The operating range for this burner was determined for a range of equivalence ratios. Firing rates ranged from 675 to 3951 kW/m 2 . Unburned hydrocarbons were elevated at low firing rates but were low at higher firing rates. Carbon monoxide (CO) emissions were less than 15 ppm. Nitric oxide (NO x ) emissions were below 10 ppm for all cases.

Microstructure and Mechanical Properties of SiC/zirconia-toughened Mullite Nanocomposites Prepared from Mixtures of Mullite Gel, 2Y-TZP, and SiC Nanopowders

SiC/ZTM (zirconia-toughened mullite) nanocomposites were prepared by hot pressing mixtures of mullite gel, 2Y-TZP, and SiC nanopowders. The intimate mixing of Al2O3 and SiO2 components in the starting powder prevented intermediate ZrSiO4 phase formation during sintering. Addition of nano-sized SiC significantly retarded the matrix grain growth, making the microstructure much finer and more uniform. Transmission electron microscopy and high-resolution transmission electron microscopy revealed that many SiC nanoparticles were found in mullite and ZrO2 grains, and low-energy grain boundaries and mullite–liquid interfaces parallel to the {110} planes of rodlike mullite grains were formed. It is deduced that the formation of rodlike mullite grains is the result of the preferential development of these low-energy grain boundaries and mullite–liquid interfaces. The mechanical properties of the SiC/ZTM nanocomposite showed significant improvement over those of ZTM, and further enhancement in the mechanical properties was achieved by combinative strengthening with nano- and micro-sized SiC.

Sliding friction and wear of alumina-reinforced zirconia-toughened mullite composites

The friction and wear characteristic of self-mated alumina-reinforced zirconia-toughened mullite (ZTM/A) composites has been investigated using a block-on-ring tribometer in different lubricants at varying loads. Load-dependent wear transitions were observed for these ceramics. The wear transition was usually accompanied by an abrupt change of friction coefficient and wear rate. The addition of Al 2O 3 effectively reduces the wear rate of ZTM/A composites before the wear transition in water and at middle loads in machine oil. Scanning electron microscope (SEM) micrographs show that the main pre-transition wear mechanism of ZTM/A composites is plastic deformation, ploughing and occasionally grains pulling out, while fracture is the dominant mechanism of post-transition. Al 2O 3 can restrain the t→m phase transformation of zirconia to some extent and improve the resistance of these materials to wear by fracture, ploughing and plastic deformation.

Mechanical properties of zirconia-based ceramics asfunctions of temperature

Abstract Commercially available ceramics, MgO–ZrO 2 , CeO 2 –ZrO 2 , and an in-house fabricated zirconia-toughened mullite were examined in this study for use as a structural component in diesel engines. The fast fracture strengths of these materials were measured by loading ASTM C-1161-B specimens in four-point flexure at 30 MPa/s and at 20, 200, 400, 600, and 850 °C. The dynamic fatigue or slow crack growth susceptibility was assessed at 20 and 850 °C by combining the fast fracture strengths with strength data obtained by testing the same specimens in four-point flexure at 0.30 and 0.003 MPa/s stressing rates, as specified in the ASTM C 1368 standard. Fracture toughness was measured following the ASTM C-1421 standard and using chevron notch specimens in three-point flexure at room and elevated temperatures. The strength of the zirconia-toughened mullite was invariant to increases in the temperature and decreases in the loading rate, while the MgO–ZrO 2 and CeO 2 –ZrO 2 materials exhibited strength degradation as temperatures increased and the loading rates decreased. Temperature was observed to have the greatest influence on facture toughness. As temperatures increased, the fracture toughness values dramatically decreased for all the materials examined in this study. Improvements in the fracture toughness are needed most for these ceramic materials in order to meet the structural requirements and to develop a more durable and reliable diesel engine component.

Preparation of zirconia-toughened mullite with dissociated zircon

A method for the preparation of zirconia-toughened mullite using dc plasma is presented. An indigenously developed 50-kW thermal plasma reactor is used for the preparation of zirconia-toughened mullite using a dissociated zircon and alumina mixture. Both XRD and SEM studies of the product confirmed the formation of mullite and zirconia. The zirconia grew as equiaxed grains of uniform size (~5 μm) in a mullite matrix.

Preparation of zirconia toughened mullite by thermal plasma

A method for preparation of zirconia toughened mullite using DC plasma is presented. An indigenously developed 50 kW thermal plasma reactor is used for preparation of zirconia toughened mullite (ZTM) from premixed zircon and alumina mixture. ZTM of different compositions were prepared by varying alumina content in the mixture. Finer grain size (<1 μm) of zirconia is observed in aluminium-rich mullite compared to that of the silicon-rich mullite.

Properties of zirconia-toughened mullite ceramics improved by B2O3 additive

Zirconia-toughened mullite (ZTM) ceramics has been prepared by using an electrically fused mullite as a raw material and its mechanical properties and microstructure as a function of impurities in the raw material were studied. These impurities led to a decrease in the mechanical properties of ZTM ceramics by changing the properties of the glassy phase in the ceramics, especially at high temperature. The mechanical properties of the ceramics were improved by adding B2O3, and the toughness at room temperature increased from 4.4 MPa m1/2 to 5.9 MPa m1/2 while that at 800°C increased from 2.9 MPa m1/2 to 4.4 MPa m1/2. The toughness of the ceramics on the addition of B2O3 at room temperature was increased by 34% and that at 800°C by 52%. The influence of the impurities on the ZTM ceramics and the improvement of the ceramic properties by the addition of B2O3 were studied and their mechanisms were discussed.

Microwave Sintering of Zirconia Toughened Mullite (ZTM)

ABSTRACTMicrowave sintering of zirconia toughened mullite (ZTM) has been performed in a single mode applicator. In comparison with conventional sintering, microwave processing of ZTM leads to a higher density and finer grain size. Microstructure of microwave sintered ZTM was characterized by TEM and HRTEM techniques. The pinning of intergranular ZrO2 dispersoids retarded the grain growth of mullite matrix. The observation of a considerable number of trans-granular microcracks indicates that microcracking toughening is the main toughening mechanism for ZTM.

Synthesis of cellular inorganic materials by foaming sol-gels

A method for synthesizing low density, cellular inorganic materials from sol-gels has been developed. The method utilizes viscosity control during gelation to stabilize bubbles generated from freon droplets dispersed in the sol. Cellular silica, mullite and zirconia-toughened mullite have been fabricated with relative densities ranging from 10 to 45% and average cell sizes ranging from 30 to 1000 μm in diameter in dense matrices. This new class of low density, sol-gel-derived materials offers many opportunities in high temperature insulation, low permittivity substrates and catalyst supports, to name a few.