This paper discusses the work accomplished, thus far, in the development of the Fiber Twist Test (FTT) as an alternative and improved technique for testing the interface properties of continuous fiber composite materials. The unique features of the fiber twist test include the absence of Poisson effects during testing. This eliminates the nonlinear variation, with changing embedded fiber lengths, of fracture modes and accompanying debond energy, and of friction along the interface. Since the fiber is not removed from its surrounding matrix during testing, the interface area remains constant. This facilitates a more accurate measurement of friction properties at the interface.

Interface and torsional shear stresses in the fiber twist test (FTT) were computed using the ABAQUS finite element program. Interface stress singularities were compared with an elasticity solution for the torsion of a fiber embedded in an elastic half space [1]. Single fiber composite systems having perfectly bonded interfaces and fiber/matrix shear modulus ratios of Gf /Gm = 1 – 3 were considered. The decay rates and depths of the interface shear stress σrθ and the torsional shear stress σZθ in the fiber and matrix were evaluated for each Gf/Gm ratio.

A two-step forced-flow, thermal-gradient, chemical vapor infiltration process (FCVI) was proposed to reduced processing time while maintaining uniformly high densities. GTCVI, a finite-volume computer code developed specifically for the FCVI process was used to model thermal gradient effects on processing time and density. An optimum thermal gradient was determined and used to process material with uniformly infiltrated bundles.

The development of thick-walled, tubular ceramic composites has involved investigations of different fiber architectures and fixturing to obtain optimal densification and mechanical properties. The current efforts entail modeling of the densification process in order to increase densification uniformity and decrease processing time. In addition, the process is being scaled to produce components with a 10 cm outer diameter.

Carbon fiber-carbon matrix composites were fabricated using the forced flow-thermal gradient chemical vapor infiltration (FCVI) process. Preforms were prepared by stacking 40 layers of plain weave carbon cloth in a graphite holder. The preforms were infiltrated using propylene, propane, and methane. The present work showed that the FCVI process is well suited for fabricating carboncarbon composites; without optimization of the process, we have achieved uniform and thorough densification. Composites with porosities as low as 7% were fabricated in 8–12 h. The highest deposition rate obtained in the present study was ∼3 μm/h which is more than an order of magnitude faster than the typical value of 0.1–0.25 μm/h for the isothermal process. It was also found that the use of propylene and propane as reagents resulted in faster infiltration compared to methane.

Fabrication process of advanced C/C composites was developed by combining the CVI and liquid impregnation-pyrolysis processes. Time-controlled temperature-gradient (TC-TG-) CVI process was developed to accelerate the carbon matrix deposition in the C-fiber preform within relatively short process-time. The temperature gradient in the preform was changed according to a controlled program. The present TC-TG-CVI process gave twice as much weight gain as that of the conventional steady TG-CVI process. The CVI process, however, showed limitation to fill up the large pores between the bundles. Liquid impregnation-pyrolysis was, therefore, carried out after the TC- TG-CVI process to improve the infiltration of these pores. Polycarbosilane (PCS) with the yield of about 50 wt% was used as the liquid precursor to form a SiC-rich matrix to improve the oxidation-resistance. SiC matrix was observed in the pores between bundles after pyrolysis of PCS. The weight gain after pyrolysis was found to be nearly saturated after 10 cycles of the impregnation-pyrolysis process.

SiC-SiC composites exhibit excellent mechanical and corrosion properties, which are desired for a variety of advanced applications including fusion reactors, heat exchangers and advanced turbine engines. However, SiC-SiC composites fabricated by the CVI methods have unacceptably low through-the-thickness thermal conductivity, which limits their use in these applications. CVD SiC co-deposition with either beryllium or boron compounds significantly enhanced the thermal conductivity of CVD SiC at temperatures up to 1000°C. Results pertinent to the doping processes and the measured thermal conductivity of doped CVD SiC are reported. Mechanisms for the enhanced thermal conductivity of the CVD SiC are discussed based on the microstructural characteristics of the doped specimens.

This paper describes a number of processing techniques that are currently in use for the development and production of continuous fiber reinforced ceramic composite materials. The limited number of available processing routes are compared with respect to the resulting material properties.

As it appears, the Chemical Vapor Infiltration technique is one of the most extensively developed methods. During the last decade, at least five different modifications of the isobaric isothermal CVI principle have been developed; each route having its own benefits. CVI techniques have now been developed to the extent that industrial commercialization is being realized. Projected cost aspects of the various CVI manufacturing techniques have been examined and compared.

Interfacial properties of 1-D SiC/RBSN composites were measured by the matrix crack spacing method and by the fiber push-out method, and the results were compared. The composites consisted of 8 to 33 vol% of aligned SCS-6 SiC fibers (142 μm diameter) in a relatively porous (20 to 40 vol%) Si3N4 matrix. The effects of fiber volume fraction and test temperature on the interfacial properties have been investigated. The advantages and limitations of both methods in evaluating the interface properties of fiber-reinforced ceramic matrix composites and the factors influencing the interfacial measurements are discussed.

The possibility of improving the interfacial shear strength of the SiC fiber reinforced SiC matrix composite system was examined. A ceramic fiber coating was chosen based on availability and chemical stability with the fiber and matrix. Fiber push-out tests conducted on as-received and coated fiber reinforced composite samples allowed characterization of the interfacial shear strength. Average debond shear and frictional sliding stresses were calculated. The effects of sample thickness and second phase addition in the matrix were also evaluated. Tested samples were examined by SEM to determine the location of the interfacial failure and to determine if any interface reactions had occurred. The coating was then evaluated based on the resulting interfacial shear strength, failure location, and integrity of the interface as compared to those properties of samples reinforced with as-received fibers.

A fiber treatment was used to change the bonding strength of the Nicalon NLM 202 SiC fiber from weak to strong, in a series of 2D-SiC/SiC composites with multilayered interphases. The materials with the pre-treated fibers were compared to the same materials but reinforced with as received fibers. The stress-strain behavior and the fracture toughness were examined as a function of crack patterns identified by TEM. All the materials could be grouped into two distinct families: (i) materials reinforced with untreated fibers have a weak fiber bonding and are characterized by a low strength and a low toughness and (2) materials with the pre-treated fibers have a strong fiber bonding and are characterized by a high strength and a high toughness. This latter behavior is identified by TEM. It corresponds to a new interfacial behavior with a cohesive mode of interfacial cracking, involving branching and deflection by the successive interfaces. In the former family, the adhesive interfacial failure mode corresponds to the classical debond/sliding mechanism.

Oxide - carbon multilayer coatings were continuously applied to various fibers of nominal SiC composition. A liquid-phase coating system that allows application of the coatings in a controlled atmosphere at relatively rapid rates was employed. Sugar - ammonium hydroxide solutions were used for carbon coatings, and aqueous sols were used for the oxides. Carbon was also deposited simultaneously with alumina by chemical vapor deposition of a hydrocarbon in the coating furnace. The coatings were extensively characterized by optical microscopy and TEM. Problems with embrittlement by oxide coatings and poor adherence of oxide coatings on carbon, and some possible solutions to these problems, are discussed.

The microstructures of Nicalon silicon carbide (SiC) fibers and layered coatings of boron nitride (BN) followed by chemical vapor infiltrated silicon carbide (CVI-SiC) were characterized using optical and electron microscopy. Two different precursors and reactions were used to produce the BN layers while the deposition of CVI silicon carbide were nearly identical. Coated tows were examined in cross-section to characterize the chemistry and structures of the constituents and the interfaces. One BN precursor yielded three sublayers while the other gave a relatively homogeneous nanocrystalline layer.

A BN rich layer grown on Nextel™312 fibers by appropriate ammonia treatments was evaluated using various complimentary techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM)/ Parallel Electron Energy Loss Spectroscopy (PEELS in TEM). Three different ammonia treatments were studied. Ammonia treatment resulted in crystallization of the Nextel™312 fiber. The BN rich surface layer formed due to ammonia treatment was clearly detected in XPS and PEELS both before and after oxidation. The layer thickness was estimated to be between 5–10 nm. The layer was stable after oxidation treatment at 600°C for 100 hours. High resolution TEM observations of the fiber surface revealed a variable BN rich layer thickness. Patches of turbostratic BN were observed under certain conditions, however mostly the layer appeared to be amorphous.

Pseudo-porous SiC/C coatings were deposited on Nextel™440 and Nicalon™ fibers by CVD. The morphology and chemistry of the coatings was evaluated, both before and after oxidation, using Scanning Electron Microscopy (SEM), X-Ray Diffraction Analysis (XRD), X-Ray Photoelectron Spectroscopy (XPS) and Auger spectroscopy. Coated fibers were subjected to two different oxidation treatments to assess coating stability: a) oxidation at 600°C for 20 hours, and b) oxidation at 1000°C for 20 hours. Pseudo-porous SiC/C on Nicalon™ fibers appear to be more oxidation resistant than the same coatings on Nextel™440 fibers.

Fiber coatings have been used to modify fiber-matrix interfacial forces, and thus control mechanical properties of continuous fiber ceramic composites. It has been shown that the properties and thickness of the interlayer influence composite properties such as matrix cracking and ultimate strength, toughness and interlaminar shear. The effects of fiber coating properties and thickness on fiber-reinforced SiC matrix composites fabricated employing CVI techniques have been examined. Correlations between interface condition, mechanical properties and failure mechanisms have been made.

The effect of zirconia (ZrO2) interfacial coatings on the interfacial shear behavior in sapphire reinforced alumina was examined in this study. Zirconia coatings of thicknesses ranging from 0.15 to 1.45 μm were applied to single crystal sapphire (Saphikon) fibers using a particulate loaded sol dipping technique. After calcining at 1100°C in air, the coated fibers were incorporated into a polycrystalline alumina matrix via hot pressing. Interfacial shear strength and sliding behavior of the coated fibers was examined using thin-slice indentation fiber pushout and pushback techniques. In all cases, debonding and sliding occurred at the interface between the fibers and the coating. The coatings exhibited a dense microstructure and led to a higher interfacial shear strength (> 240 MPa) and interfacial sliding stress (>75 MPa) relative to previous studies on the effect of a porous interphase on interfacial properties [1]. The interfacial shear strength decreased with increasing fiber coating thickness (from 389 ± 59 to 241 ± 43 MPa for 0.15 to 1.45 μm thick coatings, respectively). Sliding behavior exhibited load modulation with increasing displacement during fiber sliding which is characteristic of fiber roughnessinduced “stick-slip”. No effect of fiber coating thickness on the interfacial sliding stress was observed for single pushout or pushback events. Heat treatment at 1550°C in air coarsened the fiber surface roughness, resulting in significantly higher interfacial shear strengths (>30%) and interfacial sliding stresses (>60%) relative to the coated fibers in an as-hot-pressed condition. Interfacial sliding resistance decreased significantly after the first sliding cycle. Evidence of substantial “stick-slip” behavior was eliminated from the load displacement plots after one pushout/pushback cycle; however, the pushback plots exhibited evidence of fiber reseating, followed by a decreasing trend in load with increasing displacement of the fiber back to the original position. These results directly support the interphase fragmentation scenario proposed for interface fatigue in ceramic composites.

The high interfacial shear strengths and sliding stresses measured in this study, as well as the potentially strength degrading surface reconstruction observed on the coated fibers after hot pressing and heat treatment, indicate that dense zirconia coatings are not suitable candidates for optimizing composite toughness and strength in the sapphire fiber reinforced alumina system.

A CaO-Al203-SiO2 (CAS)/Nicalon glass-ceramic matrix composite has been subjected to elevated temperature oxidation heat-treatments between 375 and 1200°C, for up to 100 hours. Micro- and macro-mechanical properties have been determined by fiber push-down, using a mechanical properties microprobe, and flexure testing, respectively. Aging between 450 and 800°C results in significant property degradation, with reduced bending modulus and flexure strength, increased fiber sliding stress, and a transition to a purely brittle failure mode. Aging degradation is due to oxidative removal of the carbon interlayer, with the subsequent formation of a silica bond between fiber and matrix. At higher temperatures, carbon is retained due to the formation of a protective silica plug at exposed fiber ends, with the subsequent retention of composite properties. Short duration pre-treatment schedules, at 1000 or 1100°C, were developed to prevent intermediate temperature property degradation.

Preceramic polymers are attractive precursor materials for the production of low cost continuous fiber reinforced ceramic composites (CFCC) into near net shape components. Currently, CFCC components based upon a Blackglas™ matrix reinforced with Nextel™ 312 fibers with a BN rich surface layer are being investigated for gas turbine engine applications. The effects of oxidation on tensile and bend properties were investigated after exposure to flowing air at 600°C for 20–1000h. A significant reduction in the tensile properties accompanied by increases in fiber pull-out was observed after oxidation for 96 hours. After 500 hours oxidation, strength decreased by 50% relative to as-prepared composites. These results indicate that oxidation beyond 200 hours may be embrittling the composite.

Static fatigue tests of a Nicalon fiber-reinforced SiC matrix composite were conducted in four-point bending over a temperature range of 425° to 1150°C in air at selected stress levels. The composite consisted of a Nicalon cloth with a 0.3 μm graphite interfacial coating and a Forced Chemical Vapor Infiltration (FCVI) SiC matrix composite; samples were tested with or without a final protective SiC seal coat. The results indicated that the fatigue life of the Nicalon-SiC composite decreased with an increase in either applied stresses or test temperatures. However, the composite exhibited a fatigue limit of ∼ 100 MPa at temperatures ≤ 950°C which decreased to ∼ 70 MPa at 1150°C. Both electron microscopy and thermogravimetric studies suggested that the lifetime of the composites was dictated by the oxidation of graphite interfacial layer at temperatures ≤700°C and by oxidation of graphite coating accompanied by formation of silicate interfacial layer via oxidation of the Nicalon fiber (and the SiC matrix) at temperatures ≥ 950°C. Use of a SiC seal coat effectively retarded the oxidation reactions and increased the lifetime by at least one order of magnitude at 425°C. On the other hand, the SiC seal coat made little (if any) difference in fatigue life at 950 °C.