To im prove the performance of joints between metals and ceramics by reducing interfacial stresses, composites of metals and ceramics will be used as intermediate layers. These interlayers will be graduated in composition ranging from pure metal to pure ceramic, so minimizing mismatching of properties which leads to stress.
The objective of this project is to improve the effectiveness of joining ceramics to metals and reduce the tendency to adhesive failure resulting from interfacial tresses arising from mismatching of thermal expansion coefficient and elastic modulus.
Forming techniques are developed for graduated joints composed of layers of cermets made either by cosintering or pressure infiltration of porous ceramic preforms which can be used to effect joints between ceramics and metals.
Cermets have been produced by both cosintering of nickel and zirconia and the infiltration of molten metal (aluminium) into porous preforms of alumina. Crack propagation studies show the preponderance of ceramic type (brittle) behaviour in up to 70% metal (nickel/zirconia).
Porous preforms of graduated porosity have been successfully prepared on a small sample size (up to 50 mm diameter) by die pressing, assembly of tape cast layers, pressure filtration and the use of burnable porosifiers. Delamination between layers of different composition has been prevented by the use of a corrugated boundary. This was achieved by pressure infiltration by gas as the pressure failed to fully infiltrate fine porosity. Mechanical pressurisation of the molten metal (squeeze casting) results in a high failure rate because of the thermal shock to the ceramic preforms. This technique was only successfully applied to samples with diameter of up to 500 mm. Larger ones were always cracked.
MANY JOINING METHODS HAVE RESULTED IN JOINTS WHICH FAILED EARLY IN SERVICE NOT BECAUSE THE JOINT ITSELF LACKED HIGH TEMPERATURE STRENGHT BUT BECAUSE THE STRESSES DEVELOPED ARE CONCENTRADED ON A THIN INTERLAYER. THE INTENTED AIM OF THIS PROJECT IS TO DEVELOP CERAMIC COMPOSITES OF DENSE CERAMIC ON ONE FACE AND A METAL-RICH CERMETALLIC TEXTURE ON THE OTHER, THE DENSE SURFACE BEING EXPOSED TO THE HIGH TEMPERATURE REGION OR THE AREA WHERE GOOD WEAR RESISTANCE IS REQUIRED AND THE CERMETALLIC SURFACE IN CONTACT WITH THE METAL SUBSTRATE BEING PROTECTED. THIS PROCESSIVE GRADATION OF TEXTURE WILL SPREAD THE CONCENTRATION OF STRESSES OVER A GREATER AND MORE DUCTILE REGION.
BY REDUCING CRACKING DUE TO TENSILE FAILURE AT THE BASE OF THE COMPONENT, THE CERMETALLIC CONCEPT WILL ALSO ENHANCE THE THOUGHNESS OF THE CERAMIC.
THE NOVELTY LIES IN DEVELOPING A CERAMIC PREFORM OF GRADED POROSITY BY PARTICLE SIZE SELECTION AND THE USE OF A CAREFULLY CONTROLLED POWDER CONSOLIDATION PROCESS TO ENSURE THAT THE CONCENTRATION RATES OF LAYERS DIFFERING WIDELY IN TEXTURE DO NOT CAUSE DELAMINATION DURING SINTERING OR INTRODUCE HIGH STRESSES INTO THE COMPACT. THIS APPLIES ALSO TO THE DEVELOPMENT OF THE METAL INFILTRATION TECHNIQUES.
IT IS CONSIDERED THAT THE APPROACH OF INFILTRATING PREFORMED COMPACTS WITH METAL IS FAR SUPERIOR TO THE USE OF METAL-CERAMIC POWDER MIXES WHERE THE PRESENCE OF METAL WILL ADD TO THE PROBLEM OF DIFFERENTIAL SHRINKAGE DURING SINTERING.
THE ECONOMIC POTENTIAL OF THIS WORK LIES IN THE PROVISION OF RELIABLE COMPONENTS WHICH WILL ENABLE MORE SUFFICIENT AND MORE MARKETABLE COMBUSTION ENGINES. HOTTER RUNNING ENGINES ENSURE MORE STOICHIOMETRIC COMBUSTION AND HENCE ELIMINATION OF TOXIC EMISSION AND THE COST OF SUPPLEMENTARY EQUIPMENT TO COMBAT THIS.
Funding SchemeCSC - Cost-sharing contracts