Fiber-reinforced ceramic composite material comprising a matrix with a nanolayered microstructure

Abstract

A fiber-reinforced ceramic matrix composite material exhibiting increased matrix cracking strength and fracture toughness is produced by sequentially depositing a plurality of 5-500 nanometer-thick layers of a primary ceramic matrix material phase periodically separated by 1-100 nanometer-thick intermediate layers of a secondary matrix material phase onto the reinforcing fibers upon their consolidation. The resultant nanolayered matrix enhances the resistance to the onset of matrix cracking, thus increasing the useful design strength of the ceramic matrix composite material. The nanolayered microstructure of the matrix constituent also provides a unique resistance to matrix crack propagation. Through extensive inter-layer matrix fracture, debonding and slip, internal matrix microcracks are effectively diverted and / or blunted prior to their approach towards the reinforcing fiber, thus increasing the apparent toughness of the matrix constituent. This unique toughening mechanism serves to dampen energetic co-planar macrocrack propagation typically observed in conventionally manufactured ceramic matrix composites wherein matrix cracks are usually deflected at the fiber / matrix interphase region.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of application Ser. No. 09 / 764,809 filed Jan. 16, 2001.FIELD OF THE INVENTION [0002] The present invention relates to a ceramic matrix composite material composed of a refractory fiber reinforcement, a fiber coating or fiber coating system, and a nanolayered ceramic matrix having increased matrix cracking strength and methods of producing same. BACKGROUND OF THE INVENTION [0003] Fiber-reinforced ceramic matrix composite materials are actively being developed for a variety of high-temperature military, aerospace and industrial applications. While possessing high specific strength and toughness, the utility of current ceramic matrix composites are severely limited by their susceptibility to oxidation embrittlement and strength degradation when stressed at or beyond their matrix cracking strength and exposed to high-temperature oxidation. Thus, for the current state of technology, the linear-elast...

AI Technical Summary

Benefits of technology

[0019] The present invention comprises a novel material and manufacturing process for fabricating the same. The materials and process described herein results in producing a fiber-reinforced ceramic matrix composite material with increased resistance to matrix cracking. According to the invention, a matrix constituent with a nanolayered microstructure exhibiting increased strength and toughness is produced by sequentially depositing a plurality of thin layers of a primary ceramic matrix phase, interposed by a plurality of very thin intermediate layers of a secondary matrix phase onto the reinforcing fibers upon their consolidation.

Problems solved by technology

While possessing high specific strength and toughness, the utility of current ceramic matrix composites are severely limited by their susceptibility to oxidation embrittlement and strength degradation when stressed at or beyond their matrix cracking strength and exposed to high-temperature oxidation.

The principal disadvantages of ceramics as structural materials are their low failure strain, low fracture toughness and catastrophic brittle failure characteristics.

Because of these inherent limitations, monolithic ceramics lack the properties of reliability and durability that are necessary for structural design acceptance.

The interface must provide sufficient fiber / matrix bonding for effective load transfer, but must be weak enough to debond and slip in the wake of matrix cracking, leaving the fibers to bridge the cracks and support the far-field applied load.

Despite the many possible high-temperature ceramic matrix composite systems, however, the number of practical systems is limited by the currently available reinforcing fibers.

Carbon fiber-reinforced SiC ceramics (C / SiC), however, are susceptible to severe strength degradation when exposed to high-temperature oxidizing environments for prolonged periods.

This limitation is due to the extensive process-induced matrix microcracking resulting from the relatively large thermal expansion mismatch between the carbon fiber reinforcement and the surrounding SiC matrix.

The resultant matrix cracks provide access to environmental intrusion, particularly oxidation, which accelerates the degradation of the compliant fiber coating (e.g., pyrolytic carbon and boron nitride) and the reinforcing fiber.

Although these ceramic fibers are more oxidation resistant than carbon fibers, the resultant composites also experience irreversible oxidation embrittlement and strength degradation when stressed at or beyond their matrix cracking strength and subsequently exposed to high-temperature oxidation.

Continued loading beyond the onset of matrix cracking results in the formation of many regularly spaced matrix cracks (i.e., multiple matrix cracking) typically accompanied by a nonlinear decrease in composite stiffness.

Matrix microcracking is therefore a fundamental life-limiting issue for ceramic matrix composites being considered for use in extended-life thermostructural applications.

From an engineering mechanics standpoint, the high elastic modulus of the CVI-derived SiC matrix relative to the reinforcing fiber is a disadvantage for load transfer.

The relatively high matrix stiffness is thus a disadvantage from the standpoint of matrix microcracking.

Thus, the early onset of matrix microcracking and subsequent oxidation embrittlement and strength degradation is a primary performance limitation of current state-of-the-art materials.

Once the composite elastic limit is exceeded, the structural designer is faced with using a nonlinear and potentially time-dependent microcracked material system.

One, however, nontrivial approach towards increasing the matrix cracking strength in ceramic matrix composites is by increasing the mechanical properties (e.g., strength and fracture toughness) of the matrix constituent.

While attempting to deposit “massive” bodies of SiC, unanticipated thermochemical process instabilities (i.e., chugging) within the “cold-wall” chemical vapor deposition (CVD) reactor resulted in producing a material with an unusual layered microstructure.

Despite the extraordinary mechanical and physical properties of this termed “ultra-structured” material, however, commercialization was hindered by problems of reproducibility.

In short, processing difficulties associated with the inability to control the naturally occurring chemical instability within the cold-wall reactor during deposition prevented this product from becoming commercially successful.

Specifically, the uncontrollable, and not well-understood “cyclic” instability phenomenon was not easily scaled to larger or hot-wall reactors, resulting in low yield, poor reproducibility and poor process economics.

These patent disclosures, however, did not address the problem of increasing the strength and / or toughness of the matrix constituent itself and were directed instead to the product and method of depositing the oxidation-resistant multilayered ceramic fiber coating material.

This secondary layer is deposited so thinly that its crystallites do not have a chance to grow, and thus do not achieve any preferred orientation.

Virtually no structural components have ever been produced by conventional methods with grains less than 0.4 μm (400 nm) in size.

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