Michael A. Kmetz

CVD Mo, W and Cr oxycarbide, carbide and silicide coatings on SiC yarn

SiC yarn has been coated with molybdenum oxycarbide and tungsten oxycarbide without degrading the fibre. Conversion of the coatings to the metals has been accomplished by heat treatment in nitrogen. These metal coatings have then been converted to suicide coatings by heating in SiCl4. An SiC/WOC minicomposite has been formed by longer deposition of tungsten oxycarbide on a silicon carbide yarn.

An investigation into a multilayered BN/Si3N4/BN interfacial coating

In order to prevent environmental degradation of the interface, a triplex coating was employed as the interface in ceramic matrix composites (CMC). This interface consists of an initial BN layer followed by a Si3N4 layer and lastly another BN layer. Single strand unidirectional mini-composites using BN/Si3N4/BN coated ceramic grade Nicalon™ fibers as the reinforcement and chemical vapor infiltrated (CVI) SiC as the matrix were fabricated to understand the initial properties of the interfacial coating. Field emission scanning electron microscopy (FE-SEM) confirmed the thickness of the triplex coating before and after mini-composite fabrication. FE-SEM micrographs after mechanical and environmental testing of the single strand unidirectional mini-composites showed the consequences of using the triplex interfacial coating. Finally, eight ply continuous fiber reinforced (CFR) CMCs with the BN/Si3N4/BN triplex interface and the traditional BN/Si3N4 duplex interface were fabricated using the polymer impregnation and pyrolysis (PIP) process. The PIP process has gained popularity in recent years and this allows for the fabrication of larger CMC panels as compared with the CVI process. Mechanical testing for the PIP-fabricated CFR-CMC panels showed that the composites using the triplex interface had better mechanical properties than those fabricated with a BN/Si3N4 duplex interface after environmental testing.

Chemical vapour deposition of tungsten and TiN on SiC fibres

Handling of uncoated SiC/W (Sigma) fibres resulted in a drop in their strengths. Coating with TiB2 or boron at about 1000 °C by chemical vapour deposition resulted in a further reduction in strength. Coating with tungsten or TiN by lower temperature chemical vapour deposition processes did not degrade the fibre and the coated fibres also retained their strengths after heating in hydrogen at 1040 °C.

Double Interface Coatings on Silicon Carbide Fibers

Interface coatings on fibers are important in ceramic matrix composites. In addition to providing toughness, the interface coating must also protect the reinforcing ceramic fibers from corrosive degradation. A double interface coating has been applied onto silicon carbide fibers. The double interface coating is comprised of a combination of nitride and oxide coatings. Among the nitrides, boron nitride and titanium nitride were utilized. These nitrides were deposited by CVD. The metal oxides of choice were aluminum oxide and zirconium oxide which were applied onto the nitride coatings by MOCVD. The phases on the coated fibers were determined by XRD. The surface coating microstructures were observed by SEM. The effect of the coatings on the tensile strengths was determined by Instron tensile strength measurements.