2 coating systems: carbon graded titanium carbide and carbon graded titanium diboride, were recommended, from a thermodynamic study, for the protection of the Sigma fibre against attack by a titanium matrix. The intrinsic problems of thermal degradation (surface initiated failures), handling sensitivity and surface contamination (lack of coating adhesion) were overcome.
The viability of 2 fibre coating formulations has been demonstrated, resulting in a commercial source of fibre appropriate for use in titanium based composites. The factors influencing the degradation of silicon carbide monofilament in the presence of titanium have also been evaluated.
The objective was to produce a silicon carbide fibre capable of reinforcing titanium based composites without the risk of mechanical degradation. 2 coating systems, carbon/graded titanium carbide and carbon/ graded titanium diboride, were recommended from a thermodynamic study for the protection of the fibre against attack by a titanium matrix.
Problems caused by thermal degradation, surface initiated failures, handling sensitivity, surface contamination, lack of adhesion coating associated with the initial fibre have been overcome. This was achieved by the provision of a carbon coating immediately onto the fibre surface.
The concept was proved using the multilayer coating recommended by the thermodynamic study to afford some protection to the fibre when incorporated in a titanium matrix. Additional features which may have to be addressed to optimise the coating performance were identified. Continuous lengths of carbon and carbon/titanium diboride fibre can now be produced.
Research has been conducted into the production of a silicon carbide fibre capable of reinforcing titanium based composites. The starting point for the development was an already available silicon carbide monofilament which had the following limitations: thermal degradation with surface initiated failures; handling sensitivity; surface contamination and lack of coating adhesion. A key finding was the need to provide a carbon coating immediately on to the fibre surface in order to overcome these problems. The carbon layer also maintains the surface integrity of the fibre and, after additional diffusion barrier coating, provides a weak interface to divert crack propagation.
The concept of using multilayer coatings, recommended by a thermodynamic study, to afford some protection to the fibre when incorporated in a titanium matrix has been proved. The coating performance can be further optimized by altering its microstructure to reduce grain boundary diffusion.
Continuous lengths of carbon and carbon titanium boride coated fibre have been produced and basic process parameters for 2 titanium metal matrix composite consolidation routes have been established. Composites prepared using these processes have been assessed and yield tensile strengths comparable to those using United States sourced fibres.
A prototype was made available on 14/10/92
THE POTENTIAL VALUE OF SILICON CARBIDE REINFORCED TITANIUM COMPOSITES IN APPLICATIONS DEMANDING HIGH STIFFNESS AND THERMAL STABILITY HAS BEEN DEMONSTRATED IN A MAJOR R&D ACTIVITY CURRENTLY UNDERWAY IN THE UNITED STATES OF AMERICA.
THESE NOVEL HIGH PERFORMANCE MATERIALS HAVE NOT YET BEEN EXPLOITED IN INDUSTRY DUE TO THE FACT THAT DURING THE FABRICATION OF THE COMPOSITE INTO USEFUL COMPONENTS VARIOUS FIBRE/MATRIX INTERACTIONS HAVE A DEGRADING EFFECT ON THE MECHANICAL PROPERTIES.
THE OBJECTIVES OF THE PROJECT ARE TO IDENTIFY THE NATURE OF THE DEGRADATION BY DETERMINING THE MAJOR WEAKENING MECHANISMS, TO COUNTER THIS DEGRADATION BY SURFACE TREATMENT OF THE FIBRE OR BY MATRIX ALLOYING AND TO PRODUCE SAMPLES OF COMPOSITE EXHIBITING PROPERTIES WHICH WILL SATISFY SOME OR ALL OF THE CURRENTLY IDENTIFIED INDUSTRIAL APPLICATIONS.
Funding SchemeCSC - Cost-sharing contracts
SW1W 0JU London
OX11 0RA Didcot - Oxfordshire