CONTROLLED MICROSTRUCTURAL DEVELOPMENT OF SI(3)N(4) TO ENHANCE FRACTURE TOUGHNESS

SILICON NITRIDE IS AN ADVANCED CERAMIC MATERIAL WITH EXCEPTIONAL PROPERTIES. HOWEVER, A SEVERE PROPERTY LIMITATION IS OBSERVED IN THE LOW FRACTURE TOUGHNESS OF SI(3)N(4). THE POTENTIAL FOR UTILIZATION OF SI(3)N(4) CERAMICS IN MANY APPLICATIONS WOULD BE SIGNIFICANTLY IMPROVED IF ITS FRACTURE TOUGHNESS COULD BE INCREASED WITHOUT SACRIFICING HIGH TEMPERATURE STRENGTH. IT IS PROPOSED THAT THE ABOVE CAN BE ACCOMPLISHED THROUGH MANIPULATION OF THE MICROSTRUCTURE OF SI(3)N(4) TO PRODUCE A MIXTURE OF ACICULAR AND EQUIAXED GRAINS. THIS CAN BE ACHIEVED BY NANOCOATING INDIVIDUAL SI(3)N(4) PARTICLES WITH RARE EARTH OXIDES (Y(2)O(3), ND(2)O(3), AL(2)O(3) OR SOME COMBINATION). CONTROL OF THE AMOUNT AND TYPE OF COATING, AND RATIO OF COATED TO UNCOATED PARTICLES WILL ALLOW CONTROL OF THE RESULTING SI(3)N(4) MICROSTRUCTURE TO EFFECT THE DESIRABLE PROPERTIES. ALSO, THE UNIFORM DISTRIBUTION OF THE COATING IN A CONTROLLED AMOUNT TO LIMIT GLASSY PHASE FORMATION WILL RESULT IN A HOMOGENEOUS COMPOSITE WITH DECREASED PORTABILITY FOR HIGH TEMPERATURE DEGRADATION.