Control of Interfacial Properties through Fiber Coatings: Monazite Coatings in Oxide–Oxide Composites

Fiber pushout tests were used to quantify the effects of fiber coating thickness on the mechanical properties of two model composite systems: a monazite-coated (LaPO₄-coated) alumina (Al₂O₃) fiber in an Al₂O₃ matrix and a LaPO₄-coated yttrium aluminum garnet (YAG) fiber in an Al₂O₃ matrix. Interface properties were quantified using the Liang and Hutchinson (LH) pushout model and mechanistically rationalized by considering the change in residual thermal stresses with changes in the coating thickness. Measures of the pure Mode II interfacial fracture energy, the coefficient of friction, and a radial clamping pressure are extracted by fitting the LH equations to the experimental results. Using the approach that has been developed herein, a methodology is available for measuring the interfacial properties, predicting the effect of coating thickness, and selecting the coating thickness to     

Carbon Coating of Nextel 550 and 720 by Pitch–Toluene Pyrolysis

The carbon coatings on Nextel 550™ and 720™ were studied in batch (furnace) and continuous (solution fiber coating apparatus) modes, and in an inert atmosphere. Ashland Aerocarb 240™ pitch in toluene was used as the precursor material for carbon coating. The effect of processing variables (i.e., pitch concentration, Nextel fiber sizing, and oxidative curing) was studied. Characterization was done by tactile, scanning electron microscopy (SEM), and thermal gravimetric–differential thermal analysis (TG-DTA). Coating adherence, thickness, and uniformity were investigated. Applied carbon coatings adhered well onto the filaments; however, the thickness and uniformity of the carbon coatings were not good, mainly because of melting and wicking of the pitch inside the tow prior to pyrolysis.     

Monazite Coatings on Fibers: II, Coating without Strength Degradation

Washed and unwashed rhabdophane (LaPO₄· x H₂O) sols were used to apply monazite coatings to 3M Nextel 720 and 610 fibers. This precursor was designed to minimize stress corrosion from gaseous decomposition products at high temperature. The coatings were heat-treated in-line at 900°–1300°C, in air, using a continuous vertical coater with immiscible liquid displacement. Coatings were characterized by optical microscopy, scanning electron microscopy, and transmission electron microscopy. The sol was characterized with light-scattering and zeta-potential measurements. Precursor phase evolution was studied with differential thermal analysis/thermogravimetric analysis and X-ray diffractometry. The washed sol had a higher pH and lower weight loss than the unwashed sol. The as-coated fibers were tensile tested, along with coated fibers heat-treated in air at 1200°C for 100 h. The precursor was slightly phosphate-rich, and this excess phosphate reacted with alumina in the fiber to occasionally make very small (<10 nm) pockets of AlPO₄ at the coating–fiber interface after 100 h at 1200°C. Both washed and unwashed sols made coated fibers with higher tensile strengths than those of coated fibers made from other precursors, and the washed sol may actually have slightly increased fiber strength when in-line heat treatments at <1200°C were used. A small amount of AlPO₄ may also have helped seal preexisting flaws. Degradation mechanisms during fiber coating are discussed in this paper.     

Monazite Coatings on SiC Fibers I: Fiber Strength and Thermal Stability

Commercially available SiC fibers were coated with monazite (LaPO₄) using a continuous vertical coater at 1100°C. Coated fibers were heat treated in dry air, argon, and laboratory air at 1200°C for 1–20 h. The tensile strengths of uncoated and coated fibers were measured and evaluated before and after heat treatment. Fiber coating did not degrade SiC fiber strength, but heat treatment afterwards caused significant degradation that correlated with silica scale thickness. Possible strength degradation mechanisms for the coated fibers are discussed. Coating morphology, microstructure, and SiC oxidation were observed with scanning electron microscopy and transmission electron microscopy. Monazite reacted with SiC to form lanthanum silicate (La₂Si₂O₇) in argon, but was stable with SiC in air. Despite the large coefficient of thermal expansion difference between monazite and SiC, micron thick monazite coatings did not debond from most types of SiC fibers. Possible explanations for the thermomechanical stability of the monazite fiber coatings are discussed.     

Two Phase Monazite/Xenotime 30LaPO4–70YPO4 Coating of Ceramic Fiber Tows

Equiaxed yttrium–lanthanum phosphate nanoparticles (Y_0.7,La_0.3)PO₄·0.7H₂O were made and used to continuously coat Nextel^™ 720 fiber tows. The particles were precipitated from a mixture of yttrium and lanthanum citrate chelate and phosphoric acid (H₃PO₄), and characterized with differential thermal analysis and thermogravimetric analysis, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The coated fibers were heat treated at 1000°–1300°C for 1, 10, and 100 h. Coating grain growth kinetics and coated fiber strengths were determined and compared with equiaxed La-monazite coatings. The relationships between coating porosity, coating hermeticity, and coated fiber strength are discussed.     

环氧涂料与碳纤维复合材料涂覆性能研究

以环氧树脂和聚酰胺为基料,通过添加KH 560改性的纳米SiO₂、SiC微粉、滑石粉和颜料等物质制备了一种适合用于碳纤维复合材料板材涂装的环氧涂料,探讨了聚酰胺与环氧树脂的质量比及改性纳米SiO₂、SiC、滑石粉的添加量对漆膜性能的影响。结果表明:涂料的最佳配方以甲、乙组分总质量计为:聚酰胺与环氧树脂质量比为1∶3,改性纳米SiO₂、SiC微粉、滑石粉填入量分别为1%、6%、8%,按此配方得到的环氧涂料涂覆在活化液处理后的碳纤维板材上,漆膜具有很好的耐酸碱性、耐水性和耐盐雾性,且柔韧性较好,高温处理后漆膜不发生起皱、脱皮等现象,达到了中车涂装的技术标准;环氧涂料和碳纤维板材存在物理和化学等相互作用,且环氧分子聚合过程中产生的内应力较小,因此附着性能优良,适合工业化作业。     

Deposition of Oxide Coatings on Fiber Cloths by Electrostatic Attraction

Electrostatic attraction has been used to deposit alumina and mullite coatings onto woven cloths of Nicalon fiber. This approach produced uniformly coated cloths while minimizing the dependence on the wetting relationship between the fiber and the coating precursor. The use of low sol concentrations (#1 g/L) facilitated the removal of excess sol from the cloth. Water was used to further displace excess aqueous sol from the cloth to give minimal fiberfiber bridging while maintaining uniform fiber coatings. The use of a polyelectrolyte enabled multiple deposition of coatings onto the fibers in the cloth. Multiple coating steps resulted in completely coated fiber surfaces; coverage was limited to 50% with only a single coating step. Vacuum infiltration was necessary to remove entrapped air from the cloths during coating. A discussion of wetting and infiltration of fiber cloths has been presented, which highlights issues that are critical for obtaining uniformly coated fibers and minimal fiber bridging in a woven-fiber cloth or preform.     

Precipitation Coating of Monazite on Woven Ceramic Fibers: II. Effect of Processing Conditions on Coating Morphology and Strength Retention of Nextel™ 610 and 720 Fibers

Woven cloths of Nextel^™ 610 and 720 fibers were coated with monazite by precipitation. The cloths were first saturated with concentrated precursor solutions, and then submerged in warm water to initiate precipitation onto the fiber surfaces. Coatings were characterized by scanning electron microscopy, and transmission electron microscopy; thermogravimetric analysis was performed on LaPO₄ owders precipitated in solution under the same conditions as the coatings were deposited. Coating thickness distributions were measured and analyzed. Coated fiber strength was measured following heat treatment for 2 h at 1200°C. Processing conditions which retain a substantial fraction of the uncoated fiber strength are identified, and are discussed in the context of current understanding of strength degradation in coated fibers. Strength retention of coated Nextel^™ 610 fibers following heat treatment was broadly insensitive to precursor solution chemistry and was more strongly affected by intercoat firings which govern the final coating microstructure. For fixed processing conditions, more strength degradation was observed in Nextel^™ 720 due to higher residual stresses in the fiber.     

Formation of Porous Polymer Coatings on Complex Substrates Using Vapor Phase Precursors

In this work, porous poly(methacrylic acid) (PMAA) coatings are formed on complex substrates using vapor phase precursors. The porous coatings are created by partially polymerizing solid monomer deposited onto the substrate. The conformality of the porous PMAA coatings is studied on the external and internal surfaces of cylindrical substrates. Flow effects lead to thickness variations in the θ‐direction while thermal gradients and monomer depletion effects lead to thickness variations in the z‐direction. These variations can be reduced by modifying the flow rate of the monomer vapor, by reducing the radiative heat on the substrate, or by increasing the dimension size of the substrate. This work shows that vapor phase processing methods can be a viable alternative to solution phase methods and the observed trends can be utilized in a range of vapor phase technologies to optimize porous coatings for use in tissue engineering, sensors, and separations.

     

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.     

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.     

Continuous Sol–Gel Coating of Ceramic Multifilaments: Evaluation of Fiber Bridging by Three-Point Bending Test

Nextel^™ 610 multifilaments were continuously coated with aqueous sols of yttria-stabilized zirconia (8YSZ). For the transformation of the sol–gel precursor to a crystalline oxide coating detailed microstructural characterization is given. The main subject of the study was the bridging of fibers by interphase material, a general phenomenon of sol–gel coatings. A three-point bending test is proposed as a semi-quantitative method to determine the characteristic stiffening of bridged fiber bundles. This method is successfully used to evaluate the influence of sol concentration on fiber bridging and to prove the benefit of additional surfactants to aqueous sols. Differences in film formation on the individual fiber surfaces leading to homogeneous or inhomogeneous film thickness and variation in degree of fiber surface coverage are explained with a three-fiber model. Based on model and experiment it is shown that even surface coverage and homogeneous film thickness can better be accomplished by multiple coatings from low concentrated sols than by single coatings from high concentrated sols. Finally, the three-point bending test is used to qualitatively investigate the mechanical properties of the material bridges which can be correlated with results from structural investigations.     

Monazite Coatings on Fibers: I, Effect of Temperature and Alumina Doping on Coated-Fiber Tensile Strength

Monazite was continuously coated onto Nextel 720 fibers, using an aqueous precursor and in-line heat treatment at 900°–1300°C. Some experiments were repeated with alumina-doped precursors. Coated fibers were heat-treated for 100 h at 1200°C. Coatings were characterized by optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. Coated-fiber tensile strengths were measured by single-filament tensile tests. The precursors were characterized by X-ray diffractometry, differential thermal analysis/thermogravimetric analysis, and mass spectrometry. Coated-fiber tensile strength was lower for fibers coated at higher deposition temperatures. Heat treatment for 100 h at 1200°C decreased tensile strength further. The coatings were slightly phosphate-rich and enhanced alumina grain growth at the fiber surface, but phosphorus was not detected along the alumina grain boundaries. Fibers with alumina-doped coatings had higher tensile strengths than those with undoped coatings after heat treatment for 100 h at 1200°C. Alumina added as α-alumina particles gave higher strengths than alumina added as colloidal boehmite. Alumina doping slowed monazite grain growth and formed rough fiber–coating interfaces after 100 h of heat treatment at 1200°C. Possible relationships among precursor characteristics, coating and fiber microstructure development, and strength-degradation mechanisms are discussed in this paper.     

Boron Nitride Coatings on Oxide Substrates: Role of Surface Modifications

Physical mixtures of a BN polymeric precursor with oxides such as MgO, Al₂O₃, and TiO₂ have been found to yield crystalline, tightly adherent coatings of hexagonal BN on the oxide surface, after appropriate heat treatments. In every instance, the BN (0001) basal planes are parallel to the oxide surface. Generally, the coating conforms very well to the contours of the oxide surface. However, an exception was the silica surface where no coatings were observed. It appears that the surface silanols condense to form siloxanes at high temperatures, yielding a surface that is unreactive and unable to bind to the BN. Modification of the silica surface by incorporation of a monolayer of titania dramatically alters the ability of the silica to stabilize BN coatings. It was found that stable BN coatings could be obtained when the silica surfaces were thus modified.     

用纤维光学技术测定液体混合程度对筛板塔效率的影响

本研究采用纤维光学技术及以任意方式注入示踪剂、两点检测之激励-响应手段,实测了普通筛板和大孔径筛板的液相涡流扩散系数.通过对试验数据的回归分析,在较宽的操作条件范围内,得到了涡流扩散系数与堰高、液流强度及空塔气体动能因子的关联式,并发现普通筛板和大孔径筛板可用同一关联式很好地关联.本文关联式所预测的板效率与实测的板效率相比相当符合.     

浆粕性质对离子液体法再生竹浆纤维性能的影响

以6种不同性质的竹浆粕为原料,选用1-乙基-3-甲基咪唑醋酸盐{[EMIM] Ac]为溶剂,采用干喷湿纺的方式制备了再生竹浆纤维,探讨了竹浆粕性质对再生竹浆纤维制备过程中溶解、流变行为以及纤维性能的影响.结果表明:离子液体对竹浆粕具有非常强的溶解能力;6种竹浆粕/[EMIM] Ac纺丝液都属于切力变稀流体;高聚合度和高α纤维素的竹浆粕制备的纤维的断裂强度和模量较高,但是纺丝过程不连续,发生断头较多.     

Precipitation Coating of Rare-Earth Orthophosphates on Woven Ceramic Fibers—Effect of Rare-Earth Cation on Coating Morphology and Coated Fiber Strength

Monazite (La, Ce, Nd, and GdPO₄) and xenotime (Tb, Dy, and YPO₄) coatings were deposited on woven Nextel^™ 610 and 720 fibers by heterogeneous precipitation from a rare-earth citrate/phosphoric acid precursor. Coating phases and microstructure were characterized by SEM and TEM, and coated fiber strength was measured after heat treatment at 1200°C for 2 h. Coated fiber strength increased with decreasing ionic radius of the rare-earth cation in the monazite and xenotime coatings, and correlates with the high-temperature weight loss and the densification rate of the coatings. Dense coatings with trapped porosity and high weight loss at a high temperature degrade fiber strength the most. The degradation is consistent with stress corrosion driven by thermal residual stress from coating precursor decomposition products trapped in the coating at a high temperature.     

凝固浴浓度对离子液体法纤维素纤维结构以及性能的影响

探讨了以离子液体1-丁基-3-甲基咪唑氯盐([BMIM]Cl)为溶剂制备的纤维素纤维纺丝工艺条件中凝固浴浓度对纤维结晶结构以及力学性能的影响。实验表明:在相同拉伸比和气隙条件下,凝固浴浓度对再生纤维素纤维的结构以及性能影响较大。随着凝固浴浓度的增加,纤维的结晶度和无定形取向都呈现先增大后减小的趋势,纤维的横向晶粒变小,拉伸强度、初始模量也呈先增大后减小的趋势。     

Trapping of Radioactive Inert Gas Radon on an Oxide Glass: Scintillating Glass Fiber Bundle Method

Two macroporous fiber bundle radon sensors were developed by close-packing the 50 m fibers drawn from scintillating oxide glass in the quartz tubes. Radon (²²²Rn) diffuses from the source, through these sensors in parallel or in series, to the end of the linear closed system filled with air under ambient temperature and pressure of 290.15 K 307.95 K and 760 Torr, respectively. The strategy is to provide radon atoms and alphas emitted from radon and its progeny with large accessible scintillating glass surface areas for efficient trapping and detection, respectively. The variations of count rates over 115 days indicate both the significant trapping of radon on the glass surfaces as well as the dependence of this trapping process on temperature.     

Synthesis of Si-N and B-N Preceramic Coatings on Silica Gel by Chemical Surface Coating: A Porosity Study

Silica gel has been modified by successive gas reactions with (SiCl₄-NH₃) and (BCl₃-NH₃) in order to obtain Si-N and B-N preceramic polymers, which are chemically bound to the substrate surface. The effect of the polymer synsthesis on the porosity characteristics of the substrate is determined as a function of the number of applied modification cycles, using N₂ adsorption-desorption isotherms recorded at 77 K. The elemental analysis data in combination with the adsorption isotherms yield a more detailed picture of the density and the homogeneity of the coatings. Using the pore size distributions, the contribution of pore blocking and pore narrowing can be calculated as a function of the amount of reaction cycles. A new calculation method for the pore blocking and pore narrowing, which is not based on any pore shape model, is also presented and compared with the former calculation method.