Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

Strength degradation of SiC fibers with a porous ZrB2-SiC coating: Role of the coating porous structure

ZrB₂-SiC coatings with varied porous structures were deposited on SiC fiber tows using the sol-gel method and cured at 1400 ℃ in vacuum. Tensile strength of the coated SiC fibers were much lower than that of the uncoated fibers. The bimodal distribution in the Weibull plot of the coated SiC fibers demonstrated that the fracture of the coated fiber can be attributed to two types of defects: the porous structure of the coating and the fiber defects. Detailed morphology and microstructure characterization of the coating and fiber combined with strength calculation were carried out to investigate the individual contribution of the fiber defects and the porous coating layer respectively. The results revealed that apart from the fiber damage during the coating process the porous structure of the fiber coating has a non-negligible effect on the fiber strength, presumably due to a relatively strong bonding between the fiber and coating.     

Chemical Interactions Between Cement and E-Glass Fibers with a CaO–BaO–SiO2–TiO2 Coating

E-glass fibers were coated with a 15CaO–15BaO–20SiO₂–50TiO₂ thin film by the sol–gel method. Mechanical and chemical tests were performed on coated and uncoated fibers in cement and cement extract solutions to investigate the interactions between cement and gel-glass film. The results show that the resistance of E-glass fibers to the alkali cement medium is enhanced by the 15CaO–15BaO–20SiO₂–50TiO₂ coating. The significant roles of TiO₂, CaO, and BaO in the protection fibers from the alkaline attack of cement are described. Some evidence is presented that the alkali corrosion of the coated fibers results in the formation of a thick and compact Ti film that suppresses further corrosion reaction.     

Atomic force and scanning electron microscopy study of zirconia-coated silicon carbide fibers

The peculiarities of morphology of the initial Hi-Nicalon™ and Tyranno-SA™ fibers and also the zirconia-coated fibers before and after exposition to air at 1000 °C were studied using atomic force (AFM) and scanning electron microscopy (SEM). The quantitative AFM analysis allowed us to evaluate a number of roughness parameters of the initial and coated fibers. After application of coating, the roughness of Tyranno-SA™ fiber was increased approximately by a factor of 2 in comparison with that of the initial fiber for the same scanned squares, whereas the roughness parameters of Hi-Nicalon™ fiber retained its value after application of coating. Moreover, the difference in the roughness parameters for coated Tyranno-SA™ and Hi-Nicalon™ fibers is enhanced after exposition to air at 1000 °C. The grain sizes of coatings are also greatly distinct for both types of the coated and exposed to air fibers. It suggests that the microstructural properties of coatings studied are greatly dependent on the properties of fiber itself. The martensitic relief for the zirconia coating on Tyranno-SA™ fiber was directly observed by AFM.The obtained results on roughness of coated ceramic fibers could be useful for the evaluation and optimization of the mechanical behavior of CMC's, e.g. SiC/ZrO₂/SiC_f.     

The yttria-stabilized zirconia and interfacial coating on Nicalon™ fiber

Sols of yttria-stabilized zirconia may be used as simple, readily processable and accurate controllable precursors for the ZrO₂ interfacial coatings on SiC-based Nicalon™ fibers. The ZrO₂ interfacial coatings of predictable crystal phase compositions were obtained in dependence of yttria dopant level. The morphology, composition and oxidation resistance of coated fibers were evaluated by SEM, EDS, XPS, XRD, and Raman analysis. All coatings obtained are uniform, continuous and adherent to substrates. The delamination within the ZrO₂ interfacial coating was found. Possible reasons of this phenomenon are discussed. The peculiarities of the behavior of Y-stabilized ZrO₂-coated fibers in air at elevated temperature are considered.     

Evaluation of Porous ZrO2-SiO2 and Monazite Coatings Using NextelTM 720-Fiber-Reinforced Blackglas™ Minicomposites

A procedure was developed to fabricate oxide-fiber-reinforced minicomposites with a dense matrix and evaluate two oxidation-resistant interface coatings, porous oxide (zirconia-silica mixture) and monazite. The coatings were evaluated using Nextel^TM 720-fiber-reinforced Blackglas^TM-matrix minicomposites. Boron nitride (BN) coated and uncoated fibers were used as controls for comparison. The evaluation was based on ultimate failure strengths, fractography, and fiber pushin tests. All the composites that used fiber coatings had ultimate strengths significantly better than the control that used uncoated fibers. In addition, porous-oxide-coated fibers were found to be similar to BN-coated fibers in strength, fractography, and fiber pushin behavior. Monazite-coated fibers resulted in similar ultimate strengths but showed no appreciable fiber pullout. Fiber pushin tests showed that monazite debonds readily but frictional resistance is higher than for BN or porous oxide fiber coatings.     

Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites

A porous oxide fiber coating was investigated for Nextel^™ 610 fibers in an alumina matrix. Polymeric-solution-derived yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) with a fugitive carbon phase was used to develop the porous fiber coating. Ultimate tensile strengths of tows and minicomposites following heat treatments in argon and/or air were used to evaluate the effect of the porous fiber coating. The porous YAG fiber coatings did not reduce the strength of the tows when heated in argon, and they degraded tow strength by only ∼20% after heating in air at 1200°C for 100 h. Minicomposites containing porous YAG-coated fibers were nearly twice as strong as those containing uncoated fibers. However, after heating at 1200°C for 100 h, the porous YAG coatings densified to >90%, at which point they were ineffective at protecting the fibers, resulting in identical strengths for minicomposites with and without a fiber coating.     

LaPO4 coating on alumina-based fiber: Strength retention of fiber and improvement of interfacial performances

The characteristics of the interface are the key factors that determine the mechanical properties and fracture behavior of fiber-reinforced ceramic matrix composites. Design and preparation of coatings which can preserve fiber strength and maintain appropriate interfacial bonding strength are of great challenges. LaPO₄ coating is a promising weak interface coating for oxide fiber reinforced oxide ceramic matrix composites. Through this coating, the toughening mechanism of the composite such as fiber pulling out and fiber debonding is triggered. The LaPO₄ coating was deposited on the surface of alumina-based fibers by a solution precursor heterogeneous precipitation method. The effects of different precursors and different deposition temperatures on fiber strength were studied, and the mechanism of the strength degradation of the coated fiber was analyzed. It was found that the fibers coated with phytic acid precursor and deposited at 90 °C had the highest tensile strength compared to other coated fibers. The retention of strength is attributed to its loosely stacked coating. Besides, a single fiber pullout test was carried out to evaluate the effect of the coating on the interface of the composites. The results show that the composites coated by depositing citric acid precursor and phytic acid precursor at 90 °C can reduce the interfacial bonding strength by 32.5% and 46.7%, respectively compared to uncoated composites. This study has potential application value in the preparation of ceramic matrix composites used in oxidation and high temperature environments.     

Characterization and property of microarc oxidation coatings on open-cell aluminum foams

The ceramic coatings were prepared on open-cell aluminum foams by microarc oxidation (MAO) treatment in an alkaline-silicon electrolyte. The morphology, microstructure, elemental distribution, and phase composition of the MAO coatings were investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction, respectively. The corrosion behaviors of the coated and uncoated foams were evaluated by electrochemical polarization measurement. The results show that the MAO coatings cover the surface of open-cell aluminum foams. The coatings were composed of an external porous layer and an internal dense layer. The main phase of the MAO coating phase is γ-Al₂O₃. The coated aluminum foams exhibit more positive corrosion potential and lower corrosion current density compared with the uncoated aluminum foams.     

Processing,structural, and biological evaluations of zirconia scaffolds coated by fluorapatite

Highly porous zirconia (ZrO₂) scaffolds fabricated by the replication method were coated with fluorapatite (FA). The FA coating was obtained by dipping the ZrO₂ scaffolds into stabilized aqueous FA slips having different viscosity values (≤5.0 mPa.s). The influence of the FA slip viscosity and the immersion time on the reduction in the scaffold porosity and microstructure of the coated scaffolds were investigated. Cell spreading and survival of bone marrow‐derived stromal cells (BMSC) and pre‐osteoblastic MC3T3‐E1 cells on the uncoated and coated scaffolds were examined using fluorescence and SEM microscopy, and MTT assay.The FA slip with the lowest viscosity value did not lead to a continuous film along the strut network and the macropores remained uncoated. The slips with the highest viscosity value produced a partial blocking of macropores. The porous structure obtained after coating with slips of 2.2 mPa.s viscosity for 2 seconds exhibited a low reduction in porosity and pore size (400‐420 μm), due to the formation of the FA layer, and a continuous film distributed along the strut surfaces. Morphology, spreading, and survival of BMSC and MC3T3‐E1 cells over a 7‐day culture period evidenced good biocompatibility of FA‐coated ZrO₂ scaffolds processed by dip coating.     

Raman study of yttria-stabilized zirconia interfacial coatings on Nicalon™ fiber

The undoped, 3Y- and 9Y-stabilized ZrO₂ interfacial coatings on SiC-based fiber type Nicalon™ were fabricated by sol–gel approach and studied using Raman spectroscopy. Raman spectroscopy proved to be a very successful method for revealing beyond question the monoclinic, tetragonal and cubic modification in the as-prepared and exposed to air ZrO₂-coated Nicalon™ fibers. The quantitative phase analysis in the tetragonal or tetragonal/monoclinic two-phase interfacial zirconia coatings was done using an accurate calibration curve directly determined from the Raman spectra of standard mixtures with known monoclinic and tetragonal phase ratios. It was found that the undoped ZrO₂ coating on Nicalon™ fiber was composed of monoclinic together with tetragonal modification in approximately equal fractions whereas after exposition to air the t → m phase transformation occurred in full extent. The 3YSZ coating also underwent the t → m transformation, with the extent of this transformation being different for various areas of the same filament and for various filaments.A monitoring of the t → m phase transformation within ZrO₂ coating on Nicalon™ fiber using micro-Raman spectroscopy makes it possible quantitatively to evaluate an ability of ZrO₂ as oxidation resistance and readily deformable weak interfacial coating for CMC's.     

Chemical vapor deposition of ZrO2 and C/ZrO2 on mullite fibers for interfaces in mullite/aluminosilicate fiber-reinforced composites

For the realization of crack deflection and fiber pull-out in aluminosilicate fiber-reinforced dense mullite-matrix composites, suitable fiber/matrix-interfaces are an important requirement in order to obtain sufficiently weak bondings between fibers and matrices. Two types of chemical vapor deposited (CVD) fiber/matrix-interfaces have been studied in the present work porous ZrO₂ and C/ZrO₂-double layers. In the latter case, carbon was burned out to form a gap during the processing of composites (fugitive coating). Porous ZrO₂ coatings were produced by an optimized CVD-process with Zr-acetylacetonate as a precursor. The constancy of the layer thickness depended on the deposition temperature. It was found that at a temperature of approximately 300°C and a pressure of 5 hPa, suitably uniform layers with thickness ranging between 100 and 300 nm were achieved. The coatings contained approximately 15 wt% carbon which produced, after annealing in air, a porous structure. The deposition kinetics can be described by a first order reaction. The carbon layer in C/ZrO₂-double layers was produced by using propane. The thickness of carbon layer was 10 and 100 nm, respectively. Aluminosilicate fiber/mullite matrix composite prepegs were fabricated by infiltration of coated and unidirectionally oriented fiber (0°) with a slurry, containing a pre-mullite powder, calcined at 1100°C. Uniaxial hot-pressing of dried prepegs was carried out at <1250°C for 15 min, at 20 MPa. Prepegs with ZrO₂ fiber/matrix-interfaces were hot-pressed in air, while the samples with C/ZrO₂-interfaces were processed in flowing argon. After hot-pressing, samples with C/ZrO₂-interfaces were heat-treated in air (1000°C) in order to burn out the C-layer (fugitive coating). These composites yielded a controlled fracture with a high deflection rate and a favorable fracture strength of about 200 MPa, due to crack-deflection and fiber pull-out. Composites with ZrO₂-interfaces, on the contrary yielded no crack deflection or pull-out. Therefore, they are less damage tolerant than those having C/ZrO₂ double layer systems.     

Plasma etching and plasma polymerization coating of carbon fibers. Part 2. Characterization of plasma polymer coated carbon fibers

Unsized AS-4 carbon fibers were subjected to RF plasma etching and/or plasma polymerization coating in order to enhance their adhesion to vinyl ester resin. Ar, N₂ and O₂ were utilized for plasma etching, and acetylene, butadiene and acrylonitrile were used for plasma polymerization coating. Etching and coating conditions were optimized in terms of plasma power, treatment time, and gas (or monomer) pressure by measuring the interfacial adhesion strength. Interfacial adhesion was evaluated using micro-droplet specimens prepared with vinyl ester resin and plasma etched and/or plasma polymer coated carbon fibers. Surface modified fibers were characterized by SEM, XPS, FT-IR, α-Step, dynamic contact angle analyzer (DCA) and tensile strength measurements. Interfacial adhesion between plasma etched and/or plasma polymer coated carbon fibers and vinyl ester resin was reported previously (Part 1), and characterization results are discussed is this paper (Part 2). Gas plasma etching resulted in preferential etching of the fiber surface along the draw direction and decreased the tensile strength, while plasma polymer coatings altered neither the surface topography of fibers nor the tensile strength. Water contact angle decreased with plasma etching, as well as with acrylonitrile and acetylene plasma polymer coatings, but did not change with butadiene plasma polymer coating. FT-IR and XPS analyses revealed the presence of functional groups in plasma polymer coatings.     

Hydrothermally grown uniform TiO2 coatings on ZrO2 fibers and their infrared reflective and thermal conductive properties

Improving the infrared reflectivity of ZrO₂ polycrystalline fibers is of great benefit to its thermal applications. In the present research, we cast a highly uniform TiO₂ coating with a thickness ranging from dozens to hundreds of nanometers on ZrO₂ fibers by utilizing hydrothermal growth. The coating bonds tightly to the ZrO₂ fibers via Zr–O–Ti chemical linkages, and the thickness of the coating can be tailored by varying the hydrothermal growth time. The TiO₂ coating, acting as a sheath towards electromagnetic radiation, not only reflected light with wavelengths ranging from the visible region to the infrared region and up to 8 μm but also shielded the Raman signals of the ZrO₂ fibers. The present research provides an efficient way to cast controllable and uniform coatings on flexible fiber materials. The obtained ZrO₂ fibers coated with TiO₂ may have applications such as reinforcement for bulk ceramics, thermal barrier coatings, aerogels, etc., thus performing the dual functions of mechanical strengthening and thermal insulation.     

Mechanical properties of ceramic matrix composites with siloxane matrix and liquid phase coated carbon fiber reinforcement

In order to evaluate the benefits of continuous liquid phase coating (CLPC) for carbon fibers, coated fibers as well as uncoated fibers were applied in the preparation of unidirectionally reinforced ceramic matrix composites (CMCs) with polysiloxane based matrix. Fibers coated with precursor based ceramic or carbon coatings were transferred into prepregs by continuous fiber impregnation with liquid polysiloxane and filament winding. The wet prepregs were cut to shape, laminated and then pressed and cured in the mold at 150 °C for 1 h. The cured polymeric matrix composites were calcined and densified by subsequent precursor infiltration/calcination cycles. The flexural strength of the CMCs was measured by 4-point bending tests, the microstructure was determined by optical and scanning electron microscopy. The application of CLPC coated fibers led to a significant improvement in composite strength and young's modulus compared to identical reference samples with uncoated carbon fibers.     

Effect of ceria and zirconia nanoparticles on corrosion protection and viscoelastic behavior of hybrid coatings

Organic–inorganic hybrid nanocomposite coatings contain inorganic particles that are dispersed in organic phase in nanometric dimensions. Ceria and zirconia colloidal dispersions are uniformly distributed in the epoxy silica-based hybrid nanocomposite by sol–gel method and coated on 1050 aluminum alloy substrate with spin-coating technique. The hybrid sol is prepared by organic–inorganic precursors formed by hydrolysis and condensation of 3-glycidoxypropyltrimethoxysilane and tetraethylorthosilicate (TEOS) in acidic solution using bisphenol A as networking agent and 1-methylimidazole as initiator in the presence of various ratios of ZrO₂ and CeO₂ colloidal nanoparticles. Particle size distribution, surface morphology and inorganic components distribution were determined by scanning electron microscopy (SEM) and EDXA techniques. SEM and Si, Zr, Ce mapping micrographs proved the uniform distribution of nanoparticles in the coatings. Transmission electron microscopy indicated that the nanoparticles dimension stay at the nanoscale level. The glass transition temperature (T _g) and loss properties (damping) of coatings were evaluated by dynamic mechanical thermal analysis. The corrosion protection of the coatings on the 1050 AA substrate was studied by potentiodynamic measurements. The results indicated that by introducing ceria nanoparticles in 1:1 molar ratio to TEOS in coating composition, corrosion protection was improved. However, the simultaneous presence of two nanoparticles (i.e., ceria and zirconia in 1:1 molar ratio) in the coating compositions increased the corrosion protection efficiency up to 99.8 %. The multiple glass transitions and shifting to higher and wide range of temperatures by adding ceria and zirconia nanoparticles indicated a better network interaction between inorganic nanoparticles and organic molecular chains which also led to better corrosion protection of the coating in this composition.     

UV photo‐stabilization of tetrabutyl titanate for aramid fibers via sol–gel surface modification

Tetrabutyl titanate was used as sol–gel precursor of a nanosized TiO₂ coating to improve the photo‐stability of aramid fibers. The nanosized TiO₂ coating was characterized by XRD and XPS. The influence of the TiO₂ coating on photo‐stability of aramid fibers was investigated by an accelerated photo‐ageing method. The photo‐stability of aramid fiber showed obvious improvement after coating. After 156 h of UV exposure, the coated fibers showed less deterioration in mechanical properties with the retained tensile strength and elongation at break greater than 36 and 50% of the original values, respectively, whereas the uncoated fibers degraded completely and became powdery. SEM analysis showed no significant surface morphological change on the coated fiber after the exposure, while some latitudinal crack fractures appeared on the uncoated aramid fiber. The effect of the nanosized TiO₂ coating was also well demonstrated by examining the difference of distributions of C1s in XPS deconvolution analysis on the surface of uncoated/coated fibers with increasing UV exposure time. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3113–3119, 2007     

Supersonic atmospheric plasma sprayed ZrO2 and ZrB2-SiC/ZrO2 coatings on Cf/Mg composites for anticorrosion

To improve the corrosion resistance of the carbon fiber reinforced magnesium matrix composites (C_f/Mg composites), ZrO₂ and ZrB₂-SiC/ZrO₂ composite coatings were prepared by supersonic atmospheric plasma spraying (SAPS) on C_f/Mg composites. The microstructure and phase composition of the coatings before and after the corrosion test were investigated. Open circuit potential and potentiodynamic polarization tests were measured at room temperature. Results revealed that the corrosion current density ($i_{\mathit{corr}}$) of the ZrO₂ coated C_f/Mg composites decreased by one order while the ZrB_2-SiC/ZrO₂ coated C_f/Mg composites reduced by two orders. Compared with C_f/Mg composites, the corrosion potential ($E_{\mathit{corr}}$) of the ZrO₂ and ZrB₂-SiC/ZrO₂ coated C_f/Mg composites increased by 220.5?mV and 1021.8?mV respectively, indicating that the ZrB₂-SiC/ZrO₂ composite coatings greatly improve the corrosion resistance of C_f/Mg composites. The uniform distribution of the SiC particles with small grain size in ZrB₂ is responsible for the densification of the coating. The ZrB₂-SiC/ZrO₂ composite coatings provide a barrier for the substrate to impede the entry of Cl^- in the corrosion solution, thus exhibiting a better corrosion resistance than the ZrO₂ coating.     

Surface defect healing effect of silica coatings on silicon nitride fibers

In this study, silica coatings with different thickness were prepared on silicon nitride fibers by a continuous dip-coating method. The effects of the coatings on the mechanical properties of the silicon nitride fibers were investigated. The SiO₂ coatings with uniform thickness were prepared from a sol solution with a concentration of 0.75 wt% and then heat-treated at 400 °C, and the strength of the fibers was improved by the treated coating. The tensile strength of a coated fiber was approximately 26% higher than that of an uncoated fiber because the thin coating healed the surface defects. Our study also confirmed that the size of sol particles must match that of the flaws on the fiber surface before these flaws could be effectively repaired. Finally, a probable mechanism will be proposed here to explain this effect. The present results demonstrate that the strength of silicon nitride fibers can be enhanced by coating them through the sol–gel process, and the findings are expected to provide guidelines for repairing strength-limiting flaws in other fibers.     

Precipitation Coating of Monazite on Woven Ceramic Fibers: I. Feasibility

Monazite coatings were deposited on woven cloths and tows of Nextel™ 610 fibers by heterogeneous nucleation and growth using solution precursors. Initial experiments revealed two coating regimes in which monazite was either precipitated both in solution and onto the fiber surfaces or only onto the fiber surfaces depending on the precursor solution concentration and fiber surface area. In both cases, regions of tightly packed fibers within cloth were uncoated. Image analysis of coated fiber cross sections revealed a strong correlation between fiber separation and coating thickness, suggesting that the coating of tightly packed fibers was limited by transport of the reactants in solution to these areas. By adopting a coating procedure in which the tightly packed regions are saturated with reactants before precipitation, more uniform coatings of monazite were obtained throughout the cloth; however, the strength of as-coated and heat-treated fibers was degraded and remains problematic.