High strength joints of ceramic to ceramic and metal to metal have been achieved with glass ceramic joining media. The development of graded expansion glass ceramic joining media has enabled SiC-Nilo K joints to be achieved. Braze joints have been made of ceramic to metal with various ceramic and metal combinations and joint shear strengths up to 200 MPa, together with high tensile and bending strengths, have been achieved. Metallic interlayers such as copper and nickel between ceramic and metal, to relieve expansion mismatch stress, have been shown to be advantageous. The joint strength decreased with rise in temperature in excess of 500 C. Using diffusion bonding techniques, high shear strengths of greater than 200 MPa, together with high tensile and bending strengths have been achieved for Si3N4-to-Nilo K combinations with nickel interlayers. Finite element analysis, using the GEC ALSTHOM MELISSA programme, has demonstrated the influence of the type and thickness of stress relieving interlayers and showed that peaked stress is in the ceramic near the interface. Finite element analysis results have been correlated with strength data.
Silicon carbide and silicon nitride, together with derivatives such as Sialon (silicon aluminum oxygen nitrogen) are being considered for use in a number of advanced engineering applications because of their refractoriness and their ability to withstand harsh environmental conditions. However, they are often bonded to other components made of dissimilar materials such as metals and alloys. The research addressed 2 major problems in the bonding of these materials to metals:
providing the necessary transition between the widely disparate structures of the ceramics and the metals;
bridging the major differences in the linear thermal expansion characteristics which would have led to the development of high stresses in the joins.
The research led to the following conclusions:
the strength of the ceramic to metal joints, at ambient temperature appears to be governed by the ability of the ceramic to withstand applied stress additional to that induced by the expansion mismatch between ceramic and metal;
at elevated temperatures the strength of brazed joints is influenced by the ability of the braze to resist applied forces;
high strengths have been achieved in both brazed and diffusion bonded assemblies showing efficiency of the techniques developed for producing high quality ceramic to metal joints;
a new bonding technology has been developed employing glass ceramic and braze joining media together with diffusion bonding technology.
THE PROJECT IS AIMED AT DEVELOPING NOVEL BONDING TECHNIQUES FOR JOINING ENGINEERING CERAMICS SIC AND SI3N4 TO A NUMBER OF INDUSTRIAL METALS AND ALLOYS. THE PRINCIPAL TECHNICAL PROBLEMS WHICH ARE BEING ADDRESSED ARE
A) THE TRANSITION BETWEEN THE METALLIC AND CERAMIC ELECTRONIC STRUCTURES,B) THE THERMAL EXPANSION MISMATCH BETWEEN CERAMIC AND METAL COMPONENTS.
THE RESEARCH IS INTENDED TO ADVANCE THE PRESENT STATE-OF-THE-ART BY MEANS OF THREE PARALLEL APPROACHES INVOLVING THE DEVELOPMENT OF BONDS VIA LIQUID PHASE (METALLIC AND GLASS-CERAMIC) AND VIA SOLID STATE (METALLIC) BONDS, THE METALLIC JOINING MEDIA INCORPORATING NOVEL ELEMENTS SUCH AS GE, SI AND TI. A FURTHER ASPECT OF THE RESEARCH IS THE DEVELOPMENT OF GRADED THERMAL EXPANSION JOINTS TO BRIDGE THE WIDE DISSIMILARITIES BETWEEN EXPANSIONS OF THE CERAMICS AND THOSE OF THE METALS.
WORK CARRIED OUT IN THE FIRST PART OF THE PROJECT HAS:
A) SELECTED AND CHARACTERISED CERAMICS AND METALS FOR FURTHER RESEARCH,B) ENABLED GOOD BONDS TO BOTH CERAMIC AND METALLIC END MEMBERS TO BE ACHIEVED USING ALL APPROACHES,
C) COMMENCED DEVELOPMENT OF GRADED EXPANSION MEDIA FOR THE INTERMEDIATE PHASES.
INITIAL JOINTS HAVE BEEN EXAMINED AND FURTHER UNDERSTANDING OF CHEMICAL AND PHYSICAL PHENOMENA INVOLVED IN THE JOINTS WILL BE DEVELOPED. AS THE PROJECT DEVELOPS BOND QUALITY STRENGTH WILL BE DETERMINED OVER A RANGE OF TEMPERATURES. THE DATA OBTAINED WILL ENABLE DESIGN CRITERIA FOR THE BONDS TO BE ESTABLISHED AND THUS OPEN UP NEW APPLICATIONS OF THESE CERAMICS AND ENABLE COST EFFECTIVE DESIGNS OF STRUCTURES USING THEM TO BE MADE.
Funding SchemeCSC - Cost-sharing contracts