Showing papers on "Ceramic matrix composite published in 2009"

Ceramic matrix composites containing carbon nanotubes

Abstract: Due to the remarkable physical and mechanical properties of individual, perfect carbon nanotubes (CNTs), they are considered to be one of the most promising new reinforcements for structural composites. Their impressive electrical and thermal properties also suggest opportunities for multifunctional applications. In the context of inorganic matrix composites, researchers have particularly focussed on CNTs as toughening elements to overcome the intrinsic brittleness of the ceramic or glass material. Although there are now a number of studies published in the literature, these inorganic systems have received much less attention than CNT/polymer matrix composites. This paper reviews the current status of the research and development of CNT-loaded ceramic matrix composite (CMC) materials. It includes a summary of the key issues related to the optimisation of CNT-based composites, with particular reference to brittle matrices and provides an overview of the processing techniques developed to optimise dispersion quality, interfaces, and density. The properties of the various composite systems are discussed, with an emphasis on toughness; a comprehensive comparative summary is provided, together with a discussion of the possible toughening mechanism that may operate. Last, a range of potential applications are discussed, concluding with a discussion of the scope for future developments in the field.

Mechanical hysteresis in ceramic matrix composites

Abstract: Ceramic matrix composites (CMCs) are interesting materials for aeronautic applications because of their good mechanical properties at high temperatures even under air. Contrary to bulk ceramics, CMCs have a non-brittle mechanical behaviour due to the high strength of fibres, and due to optimized fibre/matrix interactions after matrix multi-cracking. Interactions between fibres and matrix act mainly at the interface level. When fibres and matrix are debonded in a zone close to a matrix crack, bridging fibres slide with friction in the surrounding matrix. This friction is characterised by the interfacial frictional shear stress (τ) and is the main phenomenon leading to energy dissipation during cyclic fatigue of CMCs. Internal friction evolutions measured from stress/strain loops during cyclic fatigue or static fatigue can be described by shear-lag models with a variation of the interfacial shear stress. For example to-and-fro slides of bridging fibres can lead to an interfacial wear and a decrease of the interfacial shear stress. For static fatigue, recession of interfaces is also a way to modify the interfacial shear stress during time. Hence internal friction is an interesting parameter to be measured on CMCs because it allows to follow the evolutions of fibre/matrix interactions during fatigue.

The synthesis and consolidation of hard materials by spark plasma sintering

Abstract: Dense ceramic matrix composites consisting of boron carbide, titanium diboride, hafnium diboride were synthesized by spark plasma sintering (SPS). Specifically, a functionally graded boron carbide–aluminum composite with a precipitous microstructural gradient was created by a unique SPS die/punch off-set technique. Additionally, pseudo eutectic titanium diboride–boron carbide composites were synthesized by SPS using mechanically alloyed elemental powders. Lastly, hafnium diboride–20 vol.% silicon carbide composites were created from high energy ball milled precursor powders. This work demonstrates the SPS equipment’s promising potential to synthesize bulk, hard, refractory composites.

Carbon Nanotube Reinforced Composites: Metal and Ceramic Matrices

Abstract: 1. Introduction - Types of carbon nanotubes (CNTs) - Synthesis of carbon nanotubes - Importance of CNT-metal nanocomposites - Importance of CNT-ceramic nanocomposites - Current challenges in developing CNT reinforced metal and ceramic matrix nanocomposites 2. Carbon nanotube -metal nanocomposites - Processing of CNT-metal nanocomposites - Dispersion of CNTs in metals - Interfaces in CNT-metal nanocomposites - Aluminum, magnesium and titanium-based composites - Microstructural characterization 3. Physical Properties of CNT-metal nanocomposites - Electrical behavior - Percolation concentration 4. Mechanical characteristics of CNT-metal nanocomposites - Tensile deformation behavior - Strengthening and toughening mechanism - Structure-property relationship - Comparison with nanoparticle-reinforced metals 5. Carbon nanotube-ceramic nanocomposites - Preparation of CNT-ceramic nanocomposites - Oxide based nanocomposites : Alumina, magnesia, silica, titania and zirconia matrices - Carbide-based nanocomposites: Silicon carbide, boron carbide - Nitride-based nanocomposites: Silicon nitride - Microstructural characterization 6. Physical Properties of CNT-ceramic nanocomposites - Electrical behavior - Percolation concentration - Thermal behavior 7. Mechanical characteristics of CNT-ceramic nanocomposites - Strengthening and toughening - Microdeformation mechanism - Fracture toughness 8. Conclusions - Future prospects - Potential applications of CNT-metal nanocomposites - Potential applications of CNT-ceramic nanocomposites

Effects of the fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiC fiber reinforced SiC matrix composites

Abstract: SiC fiber reinforced SiC matrix (SiCf/SiC) composites, employing two types of KD SiC fibers (from National University of Defense Technology, China) with different fiber surface characteristics as reinforcements, were fabricated by precursor infiltration and pyrolysis (PIP) process. The fiber surface characteristics were evaluated by SEM, XPS and Raman analysis. The effects of fiber surface characteristics on the interfacial microstructure and mechanical properties of the KD SiCf/SiC composites were investigated. The results show that the tensile strength of the KD-2 SiC fibers (with silicon-based oxide surface layers) is about 85% that of the KD-1 SiC fibers (with pyrocarbon (PyC) surface layers), but the flexural strength of the KD-2 SiCf/SiC composite is only around 15% that of the KD-1 SiCf/SiC composite. SEM, TEM and elemental mapping analysis show that the large strength difference between the two composites is ascribed to the interfacial microstructure and the degree of fiber damage, which are arising from the different fiber surface characteristics.

Ceramic matrix composite blade having integral platform structures and methods of fabrication

Abstract: A method of making a ceramic matrix composite blade includes laying up at least one ply of the plurality of fibrous ceramic plies in a preselected arrangement to form an airfoil and dovetail preform, laying up at least one additional ply of the plurality of fibrous ceramic plies on the airfoil and dovetail preform in a second preselected arrangement to form an integral platform as part of the non-rigidized blade preform, rigidizing the blade preform to form a rigidized blade preform, and densifying the rigidized blade preform to form a ceramic matrix composite blade having an integral platform structure. A tool for making the ceramic matrix composite blade and a ceramic matrix composite blade are also disclosed.

Mechanical behaviour and lifetime modelling of self-healing ceramic-matrix composites subjected to thermomechanical loading in air

Abstract: The simulation of the behaviour and lifetime of composites with self-healing ceramic matrix requires the coupling of models issued from both mechanics and chemistry. The mechanical macromodel is based on a partitioning of damage according to the various degradation mechanisms. Analysis windows on the microscale enable the reconstruction of the crack network indicators and of the opening states of the cracks as functions of the loading. The self-healing mechanism is modelled by creating an oxide plug in an open crack and simulating the diffusion of oxygen through its evolving geometry. An evolution law for fibre strength as a function of oxygen concentration provides an illustration of the influence of complex thermomechanical loading on the composite's lifetime.

Improvement of flexure strength and fracture toughness in alumina matrix composites reinforced with carbon nanotubes

Abstract: In this study, alumina matrix composites reinforced with carbon nanotubes (CNTs) were fabricated by CNT purification, mixing, compaction, and sintering processes, and their relative density, electrical resistance, hardness, flexure strength, and fracture toughness were evaluated. 0–3 vol.% of CNTs was relatively homogeneously distributed in the composites, although some pores existed. The three-point bending test results indicated that the flexure strength increased with increasing volume fraction of CNTs, and reached the maximum when the CNT fraction was 1.5 vol.%. The fracture toughness increased as the CNT fraction increased, and the fracture toughness of the composite containing 3 vol.% of CNTs was higher by 40% than that of the monolithic alumina. According to the observation of the crack propagation path after the indentation fracture test, a new toughening mechanism of grain interface bridging-induced CNT bridging was suggested to explain the improvement of fracture toughness.

Dispersion assessment and studies on AC percolative conductivity in polymer-derived Si-C-N/CNT ceramic nanocomposites

Abstract: Nanocomposites comprise polysilazane-derived SiCN ceramic charged with carbon nanotubes (CNTs) have been prepared by dispersion of multi-walled CNTs with a diameter of 80 nm in a cross-linked polysilazane (HTT 1800, Clariant) using a simple roll-mixer method. Subsequently, the composites were warm pressed and pyrolyzed in argon atmosphere. Scanning electron microscopy (SEM) and 3D Raman imaging techniques were used as major tools to assess the dispersion of CNTs throughout the ceramic matrix. Furthermore, studies on the effect of the volume fraction of CNTs in the nanocomposites on their electrical properties have been performed. The specific bulk conductivities of the materials were analyzed by AC impedance spectroscopy, revealing percolation thresholds (ρc) at CNT loadings lower than 1 vol%. Maximum conductivity amounted to 7.6 × 10−2 S/cm was observed at 5 vol% CNT. The conductivity exponent in the SiCN/CNT composites was found equal to 1.71, indicating transport in three dimensions.

Slip casting of nanozirconia/MWCNT composites using a heterocoagulation process

Abstract: The addition of carbon nanotubes (CNTs) is expected to increase the fracture toughness of ceramic matrix composites, but an uniform dispersion of the nanotubes in the matrix is essential This is a complex issue in aqueous medium because of the nanotubes hydrophobicity In this work, the stability and rheological behaviour of nanozirconia concentrated suspensions, from 20 to 33 vol% solids, with and without multiwall carbon nanotubes (MWCNTs) was optimised in order to obtain homogeneous green samples by slip casting The manufacture of nanozirconia/MWCNTs composites was performed using a heterocoagulation process in which the CNTs were homogeneously coated by the ceramic particles through strong electrostatic attractive forces between the two phases and further consolidation by a slip casting route After sintering, the effect of the MWCNT on the hardness and fracture toughness of the nanostructured zirconia samples was evaluated

Development and characterisation of high-density oxide fibre-reinforced oxide ceramic matrix composites with improved mechanical properties

Abstract: A low cost and reliable ceramic matrix composite fabrication route has been developed. It involves the coating of 2D woven ceramic fibres (Nextel™ 720) with oxide nano-size ceramic particles by electrophoretic deposition (EPD) followed by impregnation of the coated fibres with ceramic matrix and warm pressing at 180 °C to produce the “green” component ready for pressureless sintering. The effects of two different weak interface materials, NdPO4 and ZrO2, on the thermomechanical properties of the composites are also examined. Damage mechanisms, such as debonding, fibre fracture, delamination and matrix cracking within the composite plates subjected to tensile loading are analysed using acoustic emission technique and correlated with microstructure. It is shown that the composites with NdPO4 interface, 10% porosity and 40 vol.% fibre loading have superior themomechanical properties in terms of strength and damage-tolerant behaviour in multilayer plate form. The improved sinterability and microstructure stability at moderate temperatures ensure both the fibre integrity and load transfer efficiency resulting in high strength damage-tolerant composites. The final components produced are considered to be suitable for use as shroud seals and insulating plates for combustor chambers in aircraft engines.

The effects of multiwalled carbon nanotubes on the hot-pressed 3 mol% yttria stabilized zirconia ceramics

Abstract: The bulk composites of 3 mol% yttria stabilized zirconia ceramics reinforced by multiwalled carbon nanotubes were prepared by ball milling, spray-drying and hot-pressing processes. The effects of MWCNTs’ contents and heterocoagulation pretreatment on the mechanical properties of 3Y–ZrO 2 /MWCNTs’ composites were investigated at room temperature. Experimental results showed that the heterocoagulation pretreatment played a vital role in homogeneous dispersion of MWCNTs in the ceramic matrix. The flexural strength of 989.8 ± 20.0 MPa and fracture toughness of 5.77 ± 0.06 MPa M 1/2 were obtained for the composite with 1.0 wt.% of MWCNTs’ content, which were 135.3 MPa (or 8.4%) higher in flexural strength and 0.92 MPa M 1/2 (or 21.1%) higher in fracture toughness than those of blank 3Y–ZrO 2 , respectively. The mechanisms of strengthening and toughening of the composites could be attributed to the synergic effects of bridging, pulling out of MWCNTs and their promotive effects on the phase transformation of the ceramics.

Low pressure hot pressing of B4C matrix ceramic composites improved by Al2O3 and TiC additives

Abstract: B 4 C matrix ceramic composites toughened by Al 2 O 3 and TiC were prepared by low pressure hot pressing. The relative density, Vickers hardness, fracture toughness and flexural strength of the new fabricated composites were measured. Microstructure observations of the fracture surfaces and the indentation cracks of the B 4 C matrix ceramic composites were analyzed, and an X-ray diffraction phase analysis was performed. The experiment results showed that chemical reactions took place during the low pressure hot pressing process and resulted in the B 4 C/Al 2 O 3 /TiB 2 composite. The densification rate of the B 4 C matrix ceramic composites was enhanced and the mechanical properties were improved via the introduction of Al 2 O 3 and TiC additives. The Vickers hardness, fracture toughness and flexural strength of the composite with the addition of 4.7 wt.% Al 2 O 3 and 10 wt.% TiC were 24.8 GPa, 4.8 MPa m 1/2 and 445 MPa, respectively.

Alumina-based hetero-modulus ceramic composites with extreme dynamic strength – phase transformation of Si3N4 during high speed collisions with metallic bodies

Abstract: Till today several kinds of ceramics and ceramic matrix composites are developed for extreme environmental conditions. Most of these ceramics have microstructures with relatively „big” crystals, having high rigidness and strong inclination to nick, pitting and rigid fractures, so they are not usable for collision with metallic or other bodies under high speeds like 800 m/sec or more. On the basis of several years experiments in development and testing of ceramic materials and corundum matrix composites the authors successfully developed new alumina-matrix composite materials reinforced with Si 2 ON 2 , SiAlON, AlN and Si 3 N 4 . These new alumina based ceramic matrix composites were tested under collisions with different metallic bodies having high densities and speeds higher than 800 m/sec. During the collisions the kinetic energy of flying metallic objects distributing to fracture energies, heatings and recrystallizations both of ceramic and metallic bodies. In the centres of collisions, where oxygen was absent, the authors have found new, high density “diamond-like Si 3 N 4 “ materials with cubic crystals, where nitrogen atoms distributed in the centres of the cubes. These new crystal structures of Si 3 N 4 in the alumina matrix have extreme dynamic strength and hardness, like diamond. Having surplus of oxygene in the centres of collisions this new “diamond-like Si 3 N 4 “ was not observed, when a very strong oxydation of metallic bodies was taken place. Using the energy conception of collision, the authors mathematically described the energy engorgements of destruction of ceramic materials and heating of participating bodies as well as energy engorgement used for the phase transformations of ceramic and metallic particles during their collision.

Reinforced oxide coatings

Abstract: A reinforced coating may include an oxide matrix and a reinforcement. In some embodiments, the reinforcement may include at least one of SiC and Si 3 N 4 . The reinforced coating may be deposited over a substrate, which may include a superalloy, ceramic or ceramic matrix composite (CMC). The reinforced coating may be deposited over the substrate alone or in combination with one or more additional oxide layers and/or a bond coat. The reinforced coating may be deposited using plasma spraying, physical vapor deposition, cathodic arc deposition, chemical vapor deposition, slurry dip coating, sol-gel coating, electrophoretic deposition, or another suitable deposition process.

Comprehensive method for local application and local repair of thermal barrier coatings

Abstract: A method for the local initial application of a thermal barrier coating layer ( 3 ), or for the local repair of coating defects and/or deteriorations of components ( 1 ) in the hot gas path of a gas turbine engine, which components are coated with a thermal barrier coating layer, includes at least the following steps: (I) in the case of repair, normally overall inspection of the whole component ( 1 ) for the determination of the location of defect/deterioration, as well as of corresponding type of defect/deterioration of each place for a multitude of locations of the component ( 1 ); (II) if needed, preparation of the surface in at least one location; (III) local application of a ceramic tissue together with a wet chemical thermal barrier coating layer deposition material for the formation of a patch ( 5 ) of ceramic matrix composite; (IV)a intermediate inspection of the patch and/or the surface; (IV)b in the case of a repetitive and/or multi-step repair method, subsequent layer application of a ceramic tissue together with a wet chemical thermal barrier coating layer deposition material for the formation of a patch ( 5 ) of ceramic matrix composite at this location; (V) if needed, surface finishing at the at least one location; and (VI) final inspection of the at least one location. Steps (IV)a, (V) and (VI) can be omitted with the provision that at least one of steps (IV)a or (VI) is carried out.

Sol–gel route to carbon nanotube borosilicate glass composites

Abstract: Dense borosilicate glass matrix composites containing up to 3 wt% of multiwalled carbon nanotubes were produced by a sol–gel process. The three different silicate precursors employed (tetramethylsilane (TMOS), methyltriethoxysilane (MTES) and methyltrimethoxysilane (MTMS)) yielded transparent xerogels which were subsequently crushed and densified by hot pressing at 800 °C. The dispersion of the carbon nanotubes was aided by using an organic–inorganic binder (3-aminopropyl triethoxysilane) which limited flocculation of the CNTs in the silica sol. After densification, the borosilicate glass composites containing up to 2 wt% CNTs showed significant improvements in hardness and compression strength, as well as thermal conductivity, whilst percolation effects lead to a dramatic increase in electrical conductivity above 1 wt%. This simple approach to disperse CNTs into a technical silicate glass matrix via the sol–gel process focusses specifically on the borosilicate system, but the procedure can be applied to produce other inorganic matrix composites containing CNTs.