Doctoral Dissertations

Date of Award

5-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Peter K. Liaw

Committee Members

T. M. Besmann, C. J. McHargue, J. D. Landes, A. Bleier, D. P. Stinton

Abstract

It is well established that the fiber-matrix interface plays a key role in determining the mechanical properties of ceramic matrix composites. Currently, Nicalon-fiber/SiC-matrix composites and Nextel-fiber/SiC-matrix composites owe their good mechanical properties at room temperature to either carbon (C) or boron nitride (BN) interfaces, which provide a weak interfacial bond. However, C and BN interfaces encounter oxidation problems at elevated temperatures resulting in degradation of mechanical properties of these composites in oxidizing environments. Hence, alternative oxidation-resistant interface materials need to be identified and developed. Recent analytical and finite element modeling studies have indicated that low modulus interfacial coatings may be desirable to reduce residual radial compressive stresses at fiber-matrix interfaces which develop upon cooling from processing to room temperature. Two oxides, mullite and aluminum titanate, were chosen as interface materials for Nicalon/SiC and Nextel/SiC composites based on their relatively low modulus and their good oxidation resistance at elevated temperatures. Mullite and aluminum titanate precursor sols were developed for coating applications. High temperature X-ray diffraction studies in air identified that mullite crystallizes between 950°C and 1050°C, while aluminum titanate forms between 1200°C and 1400°C. The crystallization temperature of mullite was lowered by 100°C compared to those reported in the literature. Nicalon tows are embrittled during the formation of mullite at 1000°C, and hence, there is a need to protect the fibers by a relatively inert material during the sol-gel oxide processing, The aluminum titanate formation temperature is high and would damage Nicalon and Nextel fibers. However, Nicalon fibers dip-coated in an aluminum titanate sol and heat-treated at 1000°C for up to 10 h in air, forming an alumina-titania (Al23-TiO2) mixture, were not embrittled. Based on the non-embrittlement of the fibers, the Al23-TiO2 mixture was chosen as an alternative to aluminum titanate for an interface material. Additionally, there is a need to protect the oxide coatings from the HCl attack, a by-product of the SiC matrix deposition reaction. To protect the fibers and oxides during the processing, a C coating was selected because of its inertness and ease of processing. To study the effect of protecting the fibers by an inert material during sol-gel oxide coating, oxide/C, and C/oxide/C interfaces were considered for Nicalon/SiC and Nextel/SiC composites. Since composites with C/oxide/C interfaces exhibited higher flexural strengths than composites with oxide/C interfaces in the as-processed condition. Also, Nicalon/SiC composites with CVoxide/C interfaces retained damage-tolerant behavior and substantial amount of their as-processed strengths even after 24 h oxidation at 1000°C in air than Nextel/SiC composites with C/oxide/C interfaces. On these basis, Nicalon/SiC composites with C/oxide/C interfaces Transmission electron microscopy studies C/oxide/C interfaces were studied. were chosen for further investigation, identified that the obtained oxide coatings were very thin (< 0.1 |J,m), and hence, that the effect of modulus on the mechanical properties could not be investigated in the present study. A control sample of a Nicalon/SiC composite with a thin C (< 0.1 |im) interface was also fabricated. Nicalon/SiC composites with C and C/oxide/C interfaces were fabricated and flexure tested at room temperature in both as-processed and oxidized conditions. Flexural strengths of the composites with C, and C/oxide/C interfaces decreased with increasing exposure time at 1000°C, but they retained graceful failure even after 500 h oxidation. Composites with C/oxide/C interfaces retained substantial flexural strengths than a composite with a C interface after 200 h oxidation at 500°C. After 200 h oxidation at 750°C, composites with C and C/oxide/C interfaces underwent catastrophic failure and drastic reduction in strengths (> 73%). Nicalon/SiC composites with C/oxide/C interfaces performed poorly at 750°C, so Nicalon/SiC composites with BN/oxide/BN interfaces were fabricated and tested in both as-processed and oxidized conditions. Additionally, a control sample, Nicalon/SiC composite with a BN interface, was fabricated and flexure tested. Composites with BN and BN/Al23-TiO2/BN interfaces retained substantial strengths and exhibited graceful failure even after 200 h exposure at 500°C, 750°C, and 1000°C, whereas the composite with a BN/mullite/BN interface failed predominantly in a catastrophic manner after exposure at 750°C and 1000°C.

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