Showing papers on "Ceramic matrix composite published in 2019"

High-entropy environmental barrier coating for the ceramic matrix composites

Abstract: A novel high-entropy material, (Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 ((5RE0.2)2Si2O7) was prepared by the sol-gel method and investigated as a promising environmental barrier coating (EBC) for SiC-based composites. The results of X-ray diffraction and transmission electron microscopy indicated that rare-earth elements were distributed homogeneously in the single monoclinic phase. Moreover, it was found that the new material (5RE0.2)2Si2O7 had good phase stability, well-matched coefficient of thermal expansion with SiC-based composite, and excellent resistance to water-vapor corrosion. The water-vapor corrosion test of (5RE0.2)2Si2O7 coated Cf/SiC composites further confirmed that (5RE0.2)2Si2O7 was suitable for application as EBC material and could provide effective protection to Cf/SiC composites from water-vapor damage.

Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology

Abstract: Spray drying, binder jetting and chemical vapor infiltration (CVI) were used in combination for the first time to fabricate SiC whisker-reinforced SiC ceramic matrix composites (SiCW/SiC). Granulated needle-shaped SiCW was spray dried into SiCW spherical particles to increase flowability and thereby increase printability. Then, binder jetting was employed to print a novel SiCW preform with two-stage pores using the SiCW spherical particles. The subsequent CVI technology produced pure, dense, and continuous SiC matrix with high modulus and strength. Consequently, SiCW/SiC with appropriate mechanical properties was obtained. Finally, the challenges of the novel method and the ways to improve the mechanical properties of SiCW/SiC are discussed.

On understanding the microstructure of SiC/SiC Ceramic Matrix Composites (CMCs) after a material removal process

Abstract: The unique material nature (e.g. hard, brittle, heterogeneous and orthotropic) of SiC-based Ceramic Matrix Composites (CMCs) highly affects the outcomes of machining process by inducing high thermo-mechanical loads during material removal. This can result in severe material damage which in turn causes a reduction of the in-service life of critical structural ceramic components (such as in aero-engines or nuclear reactors). In this study, the phenomenon by which the material removal mechanism during drilling influences the CMC surface integrity are discussed by characterising the fracture and deformation phenomena on the CMC's constituents - i.e. SiC and Si materials. Moreover, the strain induced to the surface, together with the changes in chemical composition are characterised via micro Raman spectroscopy and related to the principles of residual stresses upon cutting. This results in a novel understanding of the material removal process that governs cutting of SiC-based CMCs while emphasising how the different microstructure, morphology and nature of ceramics behave under the same cutting conditions. This study has therefore led to a comprehension of how the microstructure of complex hierarchical ceramic materials such as SiC/SiC CMCs is affected by a mechanical cutting process and opens avenues to understand the structure damage under other machining operations (e.g. milling, grinding).

Mechanical behaviour of carbon fibre reinforced TaC/SiC and ZrC/SiC composites up to 2100°C

Abstract: The microstructure and elevated temperature mechanical properties of continuous carbon fibre reinforced ZrC and TaC composites were investigated. Silicon carbide was added to both compositions to aid sintering during hot pressing. Fibres were homogeneously distributed and no fibre degradation was observed at the interface with the ceramic matrix even after testing at 2100 °C. The flexural strength increased from 260 to 300 MPa at room temperature to ∼450 MPa at 1500 °C, which was attributed to stress relaxation. At 1800 °C, the strength decreased to ∼410 MPa for both samples. At 2100 °C plastic deformation resulted in lower strength at the proportional limit (210–320 MPa), but relatively high ultimate strength (370–440 MPa). The sample containing ZrC had a lower ultimate strength, but higher failure strain at 2100 °C due to the weak fibre/matrix interface that resulted in fibre-dominated composite behaviour.

Research progress of ceramic matrix composite parts based on additive manufacturing technology

Abstract: Ceramic matrix composites (CMCs) are materials that can be engineered for high-temperature applications in various fields including aerospace, marine, etc. It is very difficult to fabricate CMCs us...

Electromagnetic wave absorption and mechanical properties of silicon carbide fibers reinforced silicon nitride matrix composites

Abstract: It is difficult for ceramic matrix composites to combine good electromagnetic wave (EMW) absorption properties (reflection coefficient, RC less than -7 dB in X band) and good mechanical properties (flexural strength more than 300 MPa and fracture toughness more than 10 M P·m1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si3N4 matrix composites (SiCf/Si3N4) were designed and fabricated via chemical vapor infiltration technique. Effects of conductivity on EM wave absorbing properties and fiber/matrix bonding strength on mechanical properties were studied. The SiCf/Si3N4 composite, having a relatively low conductivity (its conduction loss is about 33% of the total dielectric loss) has good EMW absorption properties, i.e. a relative complex permittivity of about 9.2-j6.4 at 10 GHz and an RC lower than −7.2 dB in the whole X band. Its low relative complex permittivity matches impedances between composites and air better, and its strong polarization relaxation loss ability help it to absorb more EM wave energy. Moreover, with a suitably strong fiber/matrix bonding strength, the composite can transfer load more effectively from matrix to fibers, resulting in a higher flexural strength (380 MPa) and fracture toughness (12.9 MPa▪m1/2).

Degradation of ytterbium disilicate environmental barrier coatings in high temperature steam atmosphere

Abstract: The use of Ceramic Matrix Composites (CMCs) in the hottest part of an aero engine promises great improvements in fuel efficiency by decreasing component weight and allowing higher gas inlet temperatures. However, an environmental barrier coating (EBC) is required to protect the CMC from the corrosive water vapour contained in the combustion environment. Here, CMC specimens were coated with a silicon bond coat and ytterbium disilicate (Yb2Si2O7) layer using air plasma spraying. The specimens were subsequently exposed to a water steam environment at 1350 °C for hundreds of hours. Stress evolution and phase stability were measured throughout to observe possible degradation. Cross-sectioning of the samples revealed the occurrence of sintering, the formation of a thermally grown oxide along the silicon/EBC interface, and a reaction between the ytterbium disilicate and silica. However, no coating failure was observed, even after 750 h of isothermal exposure to the hot steam environment.

A new CaF2-YAG:Ce composite phosphor ceramic for high-power and high-color-rendering WLEDs

Abstract: Thermally robust and optically transparent phosphor ceramics are now attracting great attention as promising luminescent materials for high-power white light-emitting diodes (WLEDs). Here, we report a new CaF2-YAG:Ce phosphor ceramic in which the yellow-emitting YAG:Ce particles are embedded in a non-luminescent CaF2 ceramic matrix. The composite phosphor ceramic was fabricated at a very low sintering temperature of 700 °C by using a simple hot-pressing method. No detectable interfacial reaction occurs between the YAG:Ce particle and the CaF2 matrix. Owing to the excellent optical properties and high thermal conductivity of CaF2, the prepared CaF2-YAG:Ce phosphor ceramic shows a high quantum efficiency (IQE = 83.2%), and a low thermal quenching (only 9% loss in luminescence at 150 °C). WLEDs, fabricated by combining the CaF2-YAG:Ce phosphor ceramic with a 460 nm blue chip of 2 W, exhibited bright white light with a luminous flux of 359.7 lm, a color temperature of 4021–7941 K and a color rendering index of 70.1–82.7. The composite phosphor ceramic showed a better performance than the traditional single-phase YAG:Ce ceramic, enabling it to be used in high-power WLEDs.

Microstructure of spark plasma sintered TiB2 and TiB2–AlN ceramics

Abstract: In this research study, the effects of aluminum nitride (AlN) additive on the densification behavior and microstructure development of titanium diboride (TiB2) based ceramic matrix composite were investigated. In this way, a monolithic TiB2 ceramic and a TiB2–5 wt% AlN ultrahigh temperature ceramic composite were fabricated by spark plasma sintering (SPS) process at a temperature of 1900 °C for a dwell time of 7 min under an externally applied pressure of 40 MPa in vacuum conditions. The relative density measurements were carried out using the Archimedes principles for evaluation of bulk density and rule of mixtures for calculation of theoretical one. Compared to the additive-free monolithic TiB2 ceramic sample with a relative density of ~96%, the addition of AlN as a sintering aid greatly improved the sinterability of TiB2 matrix composite so that a near fully dense sample with a relative density of ~100% were obtained by the spark plasma sintering process. The removal of harmful oxide impurities of titania (TiO2) and boria (B2O3) from the surfaces of starting TiB2 powder particles and in-situ formation of new phases such as aluminum diboride (AlB2) and Al2Ti as an intermetallic compound of aluminum and titanium, not only improved the sinterability of the composite ceramic, but also significantly prevented the extreme growth of TiB2 grains.

Processing and mechanical properties of new hierarchical metal-graphene flakes reinforced ceramic matrix composites

Abstract: Hierarchical tantalum-graphene flakes reinforced zirconia (3Y-TZP) ceramic matrix composites were fabricated by wet processing route and freeze drying followed by spark plasma sintering (SPS). The microstructures and mechanical properties were investigated. The results show that graphene and Ta particles are homogeneously dispersed in the ceramic matrix and the optimum sintering temperature for complete densification of composites and thermal reduction of the graphene oxide is 1500 °C. The addition of dual reinforcements of tantalum microflakes and graphene nanoflakes results in significant improvement in the mechanical properties of the ZrO2 matrix. Approximately a 30% increase in flexural strength vs the zirconia-Ta composite and a 175% increase in fracture toughness vs the monolithic zirconia have been achieved by introducing 0.5 vol% GO and 20 vol% Ta particles.

Thermomechanical Fatigue of Ceramic-Matrix Composites

Abstract: In this paper, the thermomechanical fatigue (TMF) of fiber-reinforced ceramic-matrix composites (CMCs) is investigated using the hysteresis-based damage parameter. The micro stress field of the damaged CMCs of matrix cracking and fiber/matrix interface debonding is obtained considering the temperature-dependent fiber/matrix interface shear stress. The fiber/matrix interface debonded length and unloading/reloading slip length are determined using the fracture mechanics approach. Based on the damage mechanisms of fiber sliding relative to the matrix in the interface debonded region, the TMF hysteresis loops models and hysteresis-based damage parameters are developed for the partially and completely debonding to analyze the damage evolution inside of fiber-reinforced CMCs. The effects of temperature, phase angle and loading sequences on the damage development of SiC/SiC composite are discussed. When TMF temperature range increases, the fatigue hysteresis loops area, residual strain increase, and the hysteresis modulus decreases, due to the increase of the fiber/matrix interface slip length. Under TMF loading, the phase angle affects the interface debonding and sliding range, and the hysteresis loops shape, location and area of the fiber-reinforced CMCs. The experimental TMF damage evolution of 2D SiC/SiC and cross-ply SiC/MAS composites are predicted.

A novel vibration-assisted slurry impregnation to fabricate Cf/ZrB2-SiC composite with enhanced mechanical properties

Abstract: The three dimensional needle-punched carbon fiber reinforced ZrB2-SiC composite (Cf/ZrB2-SiC) with highly uniform distribution was fabricated successfully via a novel vibration-assisted slurry impregnation and low-temperature (1450 °C) hot pressing technique using nanosized ZrB2 powders. The carbon fiber/ceramic matrix interfaces were clear without obvious reaction products detected by the high resolution transmission electron microscopy (HR-TEM), indicating the degradation of carbon fiber was effectively inhibited. The Cf/ZrB2-SiC composite exhibited a typical non-brittle fracture feature with a high work of fracture of 1104 J/m2, which was approximately twice that of composite fabricated only by slurry impregnation and hot pressing. The enhancement in work of fracture was attributed to multiple toughening mechanisms of continuous carbon fibers such as extensive fiber bridging and pull-out accompanied by obvious crack deflection and branching. This work provides a valuable potential of preparing continuous carbon fiber reinforced ceramic composites with uniform component distribution and enhanced mechanical properties.

Polymer-derived Ta4HfC5 nanoscale ultrahigh-temperature ceramics: Synthesis, microstructure and properties

Abstract: Nanoscale Ta4HfC5 ceramics were synthesized from curing and pyrolysis of novel polymer precursors which were synthesized by cohydrolysis and polycondensation of acetyl acetone coordinated tantalum alkoxide and hafnium alkoxide followed by blending with phenolic resin as carbon source. Pyrolysis of the polymer precursor at 1600 °C in vacuum produced Ta4HfC5 nanocrystallites with an average grain size of 21 nm and well-distributed elements, encapsulated by an amorphous carbon shell. Near full dense Ta4HfC5 monoliths can be prepared by spark plasma sintering (SPS) at 1600 °C with 10 vol% MoSi2 as an additive, of which the Vicker micro-hardness and flexural strength achieved 17.58 GPa and 466 MPa, respectively. The polymer precursor method shed light on the fabrication of ceramic matrix composites. Besides, the high electrical conductivity of 1.5 × 104 S/m entitled the ceramics to a prospective of utilization in microwave absorbing/shielding fields under harsh conditions.

Anisotropy and Enhancement of Thermoelectric Performance of Sr0.8La0.067Ti0.8Nb0.2O3- δ Ceramics by Graphene Additions

Abstract: A-site deficient SrTiO3 ceramics are very promising n-type oxide thermoelectrics but currently limited by their low performance compared to more conventional materials. We show that incorporation of graphene or graphene oxide can significantly improve the transport properties and hence ZT of the ceramic matrix. Powders of Sr0.8La0.067Ti0.8Nb0.2O3−δ were prepared by the mixed oxide route; ceramics and composites with ≤3 wt% graphene or graphene oxide were densified by spark plasma sintering (SPS) at 1473 K for 5 minutes. The microstructures obtained were uniform with an average grain size of 5 μm; the carbon additions were uniformly distributed. Composites employing ‘as-prepared’ powders exhibited three orders of magnitude increase in electrical conductivity, a reduction in thermal conductivity from 4.00 to 2.64 W m−1 K−1, but very modest thermoelectric figure of merit (ZT) values, less than 0.1. Graphene additions yielded superior thermoelectric performance to graphene oxide. Composites prepared with ‘pre-reduced’ oxide powders and 1 wt% graphene were at least 99% dense, with further improvement in electrical conductivity. There was strong anisotropy in their transport properties due to the alignment of the graphene flakes perpendicular to the pressing direction; electrical conductivity was significantly higher perpendicular to the pressing direction; thermal conductivity was lowest parallel to the pressing direction. The highest thermoelectric figure of merit (∼0.25 at 1000 K) was achieved for samples containing graphene measured parallel to the pressing direction. The control of thermoelectric transport properties by additions of carbon species, and the resulting anisotropy in properties could guide the development of processing routes to produce future target materials.

Development of a slurry injection technique for continuous fibre ultra-high temperature ceramic matrix composites

Abstract: A simple and effective slurry injection method for producing dense and uniform ultra-high ceramic matrix composites from preforms of high fibre density was developed. As this method is based on slurry injection the homogeneity is not constrained to small preform sizes; dense components of high fibre volume can be produced in theoretically any size and shape. Samples produced by this method demonstrated high and consistent densities, with the injection method obtaining densities an average 27% higher and 87% lower in variability when compared to conventional vacuum impregnation. Tomography demonstrated no bias in the ceramic powder distribution for samples produced by injection, whereas samples produced by vacuum impregnation alone displayed poor powder penetration to the centre of large samples. The new approach yielded composites that were as strong and/or more consistent in strength compared to vacuum impregnation. Thermo-ablative testing demonstrated significant improvements in protective capability for materials produced by this route.

Rapid Heating of Silicon Carbide Fibers under Radio Frequency Fields and Application in Curing Preceramic Polymer Composites.

Abstract: Silicon carbide (SiC) fibers are widely used as a reinforcement in ceramic matrix composites due to their high mechanical strength and superior thermal resistance. Here, we investigate the rapid ra...