Mullite Matrix Research Articles

Damage Detection in Fibre Reinforced Ceramic and Metal Matrix Composites by Acoustic Emission

In this work damage micro-mechanisms of two different types of fibre reinforced composites are investigated by acoustic emission, AE. Ceramic based oxide fibre reinforced mullite matrix composite and metallic based SiC fibre reinforced titanium matrix composites exhibit different fracture mechanisms during loading and AE technique could pinpoint these damage mechanisms based on the AE responses detected simultaneously. The results show that in a ceramic matrix composite, the identification of fibre fracture and matrix cracking requires careful analysis of the AE data as both fibres and matrix break in brittle manner. Whereas the separation of fibre fracture from the ductile tearing of matrix ligaments could be easier in metallic based composites, such as titanium matrix composites.

Preparation of mullite-based iron magnetic nanocomposite powders by reduction of solid solution

In this article, the preparation of mullite-based iron magnetic nanocomposite powders by hydrogen reduction of Fe-doped mullite solid solution with a nominal composition of Al5.4Fe0.6Si2O13 is reported. The formation process of Al5.4Fe0.6Si2O13 solid solution was analyzed using X-ray diffraction analysis (XRD), Fourier Transform Infrared Spectrum (FT-IR), thermogravimetric, and differential thermal analysis (TG-DTA). It is found that doping with Fe3+ cation affects the crystallization temperature of mullite. During the hydrogen reduction process, more than 89% Fe3+ cation in solid solution were transformed into α-Fe phase when reduction temperature reached 1200 °C. Microstructure characterization of nanocomposite powders reduced at 1300 °C reveals that there are two types of α-Fe particles in mullite matrix. Fe nanoparticles with a size of approximately 10 nm were precipitated within the mullite grains, while Fe particles larger than hundreds of nanometers were located at the surfaces of the mullite grains. The measurement of the magnetic properties of nanocomposite powders indicates that large particles and nanoparticles of α-iron have the ferromagnetic and superparamagnetic behavior at room temperature, respectively.

Capillary rise properties of porous mullite ceramics prepared by an extrusion method using organic fibers as the pore former

Porous mullite ceramics with unidirectionally oriented pores were prepared by an extrusion method to investigate their capillary rise properties. Rayon fibers 16.5 μm in diameter and 800 μm long were used as the pore formers by kneading with alumina powder, kaolin clay, China earthen clay and binder with varying Fe 2O 3 contents of 0, 5 and 7 mass%. The resulting pastes were extruded into cylindrical tubes (outer diameter (OD) 30–50 mm and inner diameter (ID) 20–30 mm), dried at room temperature and fired at 1500 °C for 4 h. The bulk densities of the resulting porous ceramics ranged from 1.31 to 1.67 g/cm 3, with apparent porosities of 43.2–59.3%. The pore size distributions measured by Hg porosimetry showed a sharp peak at 10.0 μm in the sample without Fe 2O 3 and at 15.6 μm in the samples containing Fe 2O 3; these pores, which arose from the burnt-out rayon fibers, corresponded to total pore volumes ranging from 0.24 to 0.34 ml/g. SEM showed a microstructure consisting of unidirectionally oriented pores in a porous mullite matrix. Prismatic mullite crystals were well developed on the surfaces of the pore walls owing to the liquid phase formed by the Fe 2O 3 component added to color the samples. The bending strengths of the tubular samples ranged from 15.6 to 26.3 MPa. The height of capillary rise, measured under controlled relative humidities (RH) of 50, 65 and 85%, was greater in the ceramics containing Fe 2O 3 than in those without Fe 2O 3, especially in the thinner samples. The maximum capillary rise reached about 1300 mm, much higher than previously reported. This excellent capillary rise ability is thought to be due to the controlled pore size, pore distribution and pore orientation in these porous mullite ceramics.

Microstructure and Properties of Mulite-FeCr Multifunctional Composite

Mullite-based FeCr multifunctional composites were prepared by reducing (Fe, Cr)-doped mullite solid solution powders with different Cr/Fe ratio and successively hot pressing. Structural characterization revealed that the dispersion of intragranular FeCr alloy nanoparticles and intergranular iron grains in mullite matrix was obtained in nanocomposite powders. Dense mullite-FeCr composites were fabricated from nanocomposite powders by sintering at 1650 °C. The microstructures, mechanical and ferromagnetic properties of composites were strongly affected by the Cr/Fe ratio in initial solid solutions.

Creep of Nextel™720/alumina–mullite ceramic composite at 1200 °C in air, argon, and steam

The tensile creep behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200 °C in laboratory air, in steam and in argon. The composite consists of a porous alumina–mullite matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress–strain behavior was investigated and the tensile properties measured at 1200 °C. The elastic modulus was 74.5 GPa and the ultimate tensile strength was 153 MPa. Tensile creep behavior was examined for creep stresses in the 70–140 MPa range. Primary and secondary creep regimes were observed in all tests. Creep run-out (set to 100 h) was achieved in laboratory air for creep stress levels ⩽91 MPa. The presence of either steam or argon accelerated creep rates and reduced creep lifetimes. Composite microstructure, as well as damage and failure mechanisms were investigated.

Performance of Mullite Matrix Microwave Absorber Coatings Fabricated by Plasma Spraying Technology

The clusters of mullite matrix(steatite,carbon)composite absorber powders were produced by using spraying spheroid granulator,and the composite powders were used to fabricate composite microwave absorber coatings by plasma spraying technique.The results show that during the plasma spraying,the carbon is oxidized,and the steatite is decomposed to enstatite and quartz for the high temperature plasma gas.The experiments show that the mean adhesive strength of coatings decreases with the coating thickness increasing,which reaches 2MPa with thickness of 0.8mm.The crack extracted the coatings rupture surfacewith wavelet maximum module exists in the glass phase interface.For these coatings,with the coatings thickness increasing,the reflectivity coefficient transforms to low frequency; while with the complex permittivity of enstatite increasing,it decreases and transforms to high frequency. The reflectivity coefficients of the coatings with thickness of 0.8mm are all below -5dB in the frequency range from 15GHz to 18GHz.

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.

Development of mullite/zirconia composites from a mixture of aluminum dross and zircon

In order to obtain mullite/zirconia composites, mixtures of aluminum dross and zircon were sintered. Aluminum dross was collected and purified by a milling, sieving and washing process. Stoichiometric mixtures of aluminum dross and zircon were sintered at several temperatures (1400, 1450 and 1500 °C) for several periods of time (2, 4 and 6 h). After the purifying treatment the dross contained mainly Al 2O 3, AlN, MgAl 2O 4, SiO 2 and metallic Al. A homogeneous mullite matrix with small zirconia particles was obtained by sintering the aluminum dross–zircon samples at 1500 °C for 6 h.

Strengthening Effect of In-Situ Dispersed Hexagonal Boron Nitride in Ceramic Composites

High strength particulate ceramic composites are in general reinforced by strong dispersoids, such as strong ceramic particles (SiC, TiB2, ZrO2, et al) and strong metallic particles (Mo, W, et al). In this work high strength ceramic composites with in-situ synthesized hexagonal boron nitride (h-BN) have been prepared and characterized. As an example, we manufactured mullite-BN composites by reactive hot pressing (RHP) using aluminum borates (9Al2O3·2B2O3 and 2Al2O3·B2O3) and silicon nitride as starting materials. The obtained material RHPed at 1800°C showed a strength of 540 MPa, which was 1.64 times higher than that of the monolithic mullite ceramics. TEM observation revealed that the composite had an isotropic microstructure with a fine mullite matrix grain size of less than 1 μm and a nano-sized h-BN platelets of about 200 nm in length and 60∼80 nm in thickness. The high strength was suggested to be from the reduced matrix grain size and the small toughening effect by the h-BN platelets. In addition, this kind of ceramic composite demonstrates low Young’s modulus that is beneficial to the thermal/mechanical shock resistance, and excellent machinability.

Reinforcement of mullite matrix with multi-walled carbon nanotubes

Dense multi-walled carbon nanotubes (MWNTs)-mullite ceramic matrix composites were firstly prepared by hot pressure sintering with MWNTs, Al 2O 3 and SiO 2 powders as starting materials. It was found that when the sample was sintered at 1600 °C, the addition of 5 volume percent MWNTs led to 10% increase in bending strength and 78% increase in fracture toughness, respectively, compared with the monolithic mullite. The reinforcement mechanism was discussed based on the microstructure investigation. The broken nanotubes and pullout of MWNTs at interfaces are efficient in transferring the load from the mullite matrix to the nanotubes, leading to the improvement of the mechanical properties.

Fracture and Fatigue Behaviour of Mullite/Molybdenum Composites

The fracture and fatigue behaviour of a mullite/molybdenum composite is investigated. The attention is focused on the measurement of fracture toughness, KIc, on long through the thickness cracks by using SENB specimens, and on the growth of indentation cracks under static, monotonic and cyclic loads. Molybdenum was chosen to reinforce the mullite matrix because of the similar thermal expansion coefficients for both phases. It is essential to know and take into account the shape of the initial indentation cracks as well as the eccentricity change after extension under monotonic and cyclic stress. This study shows that, in mullite/molybdenum composites, static fatigue effects are negligible, but these composites are susceptible to mechanical degradation under cyclic loads. It is shown that the fatigue crack growth rate exhibits a high dependence on Kmax and that the fatigue sensitivity, defined as the ratio between fatigue crack growth rate threshold and KIc, is much lower than for other materials processed by powder metallurgy.

Thermal conductivity of highly porous mullite material

The thermal diffusivity of highly porous mullite materials (35–60 vol.% porosity) has been measured up to 1000 °C by the laser flash method. These materials were fabricated by a direct consolidation method based on the swelling properties of starch granules in concentrated aqueous suspensions and showed mainly spherical shaped pores of about 30 μm in diameter. From the point of view of heat conduction, they behave as a bi-phase material of voids dispersed in the continuous mullite matrix. The temperature dependence of thermal conductivity for the different porosities was modeled by a simple equation that considers the contribution to heat conduction of the mullite matrix and the gas inside the pores, as well as the radiation. The thermal conductivity of the matrix was taken from the measurements done in a dense mullite while the conductivity in the voids was assumed to be that of the testing atmosphere.

Progress in Processing and Performance of Porous‐Matrix Oxide/Oxide Composites

All‐oxide continuous fiber‐reinforced ceramic matrix composites are enabling materials for high‐temperature structural applications in oxidizing environments. However, their industrial use requires further improvements in material performance as well as a significant reduction of the processing costs. This article gives some insight into a novel colloidal processing route. A porous mullite matrix was designed to obtain damage‐tolerant behavior as well as high‐temperature long‐term stability. Laminated composites were formed with conventional techniques similar to the manufacture of polymer matrix composites. This simple and low‐cost process leads to homogeneous microstructures with improved material properties compared with the state of the art in continuous fiber‐reinforced oxide/oxide composites. The developed composites in the present study exhibit favorable mechanical properties both at room temperature and after thermal aging for 1000 h at temperatures up to 1300°C.

Fabrication of Highly Porous Mullite Materials

Porous mullite, formed by large spherical‐shaped pores evenly distributed within a mullite matrix, was processed by a direct consolidation method based on the starch swelling properties. Materials with porosities up to 60 vol% and with an average pore size of ∼30 μm were prepared. Porosity matched the starch volume fraction for contents ≥45 vol%, while for lower contents the porosity was above the volume fraction of starch. Formation of crack like defects either surrounding or connecting pores occurred during the drying–sintering processes in the high starch content mullite.

積層した織物(炭化ケイ素とアルミノケイ酸塩)/ムライトマトリックス複合材料の相，微構造及び力学物性に及ぼす熱処理の影響 (第２報)

積層した織物(炭化ケイ素とアルミノケイ酸塩)/ムライトマトリックス複合材料の相，微構造及び力学物性に及ぼす熱処理の影響 (第２報)

Fracture behaviour of mullite fibre reinforced–mullite matrix composites under quasi-static and ballistic impact loading

The fracture behaviour and damage development in mullite fibre reinforced–mullite matrix composites have been investigated using chevron notch technique and ballistic impact tests. Fracture toughness ( K Ic) values in the range of 1.8–3.3 MPa m 1/2 were determined using the chevron notched specimen technique. A large variability of K Ic data due to the complex (heterogeneous) composite microstructure was found. Extensive fibre pull-out occurred during failure, which was due to a favourable matrix/fibre interfacial bond given by NdPO 4 coating of the fibres. The materials response under ballistic impact loads was studied using a gas gun. The projectiles were glass balls of 10.15 mm in diameter and weighing 1.4 g. The projectile velocity was in the range 77.6–207.5 m/s. The remanent load carrying capability of composite samples after ballistic tests was measured to quantify ballistic impact induced microstructural damage. The composites retained some of their load bearing capacity even after penetration of the projectile, since structural damage caused by projectiles remained localised, preventing catastrophic failure. Penetration by the projectile occurred at impact energy of about 4 J for the conditions investigated. Understanding crack propagation and damage development under ballistic impact loads may open new opportunities for the use of these composites in lightweight armour applications.

Mullite-Boron Nitride Composite with High Strength and Low Elasticity

Mullite–boron nitride (BN) composite with high strength, low Young's modulus, and highly improved strain tolerance was prepared by reactive hot pressing (RHP) using aluminum borates (9Al2O3·2B2O3 and 2Al2O3·B2O3) and silicon nitride as starting materials. Compared with the monolithic mullite, the composite RHPed at 1800°C showed 1.64 times (540 MPa) the strength, 70% (153 GPa) the Young's modulus, and 2.34 times (3.53 × 10−3) the strain tolerance. Transmission electron microscopy observation revealed that the composite had an isotropic microstructure with a fine mullite matrix grain size of less than 1 μm and nanosized hexagonal BN (h-BN) platelets of about 200 nm in length and 60–80 nm in thickness. The high strength was suggested to be from the reduced matrix grain size and the small toughening effect by the h-BN platelets.

Microstructural characterisation of an alumina/mullite composite tested in creep

Abstract This paper presents the results of creep testing of a ceramic matrix composite (CMC) in which a matrix of mullite (Al2O3/SiO2) and SiOC (Umox) is reinforced with a cross-ply architecture [(0/90)3]s of alumina (Nextel™610) fibres. Seven specimens have been tested under different loading (50–80 MPa) and temperature (1000–1200 °C) conditions. The interfacial behaviour has been analysed through fibre push-in tests performed with a nanoindentation system. Fractographic and microstructural analyses have been performed in each specimen looking for correlations between measured parameters such as fibre and bundle pullout, crack density, crack path, oxidation, and interfacial mechanical properties.

FEM study and optimisation of a CMC outer liner

The possible employment of ceramic matrix composites (CMCs) in place of metals in the hot parts of the turbo machines is studied. The use of this class of materials should allow a relevant increase in the temperature of the gas with an increase in the overall efficiency. The CMC under study is a 12 layers laminate composed of alumina fibres embedded in a mullite matrix mounted with a crossply architecture. Thermal and stress analyses of a FE model of an outer liner is performed, and results are presented. These results are not completely satisfactory, and an optimisation process is applied to the component to identify that best material configuration, which results in six layers with an angleply architecture. An evident improvement is obtained, as the resulting stress level is significantly reduced.

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al2O3/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.