Development of high toughness silicon nitride ceramics

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Due to the combination of properties such as high strength, fracture toughness and good high temperature strength, silicon nitride ceramics have become the leading structural ceramics in many industries including automotive, cutting tools, aerospace and armour industries. Recently, silicon nitride has been identified for yet another application such as biomaterials for the manufacture of artificial bones and implants. In all of the above noted applications, the key requirements are the combination of high fracture strength (well over 1000MPa), fracture toughness (well above 10 MPa.m1/2), hardness (well above 10 GPa) and Young's modulus (over 300 GPa). The long term objective in this program is the development of a microstructure capable of providing higher resistance to crack propagation and resulting in a new generation of silicon nitride based ceramics with mechanical properties superior to those of the existing material and possibly close to that of some lower toughness metals and alloys such as aluminum and aluminum alloys. The short term objective is to study the role of non-oxide additives such as nitrides (boron nitride, zirconium nitride, etc.) in the densification, microstructure development and mechanical properties. The strategy here is to encourage the growth of elongated ß-Si3N3 grains and to modify the interface in order to intensify the pull-out and crack bridging. Crack bridging has been identified as the major and most powerful mechanism of toughening in silicon nitride based ceramics. The present research program is based on the applicant's preliminary studies where it has been shown that major increases in fracture toughness are possible when selected additives are used. In one instance, fracture toughness of >14 MPa.m1/.2 was measured. To the applicant's knowledge this value for fracture toughness is the highest ever recorded in the open literature. Successful outcome of this program will lead to the creation of a new class of ceramics with a combination of properties unmatched in the past. It will also provide the opportunity for training of graduate students, PDFs and Research Associates in this rapidly growing area of advanced ceramics in automotive, aerospace and energy sectors.