Processing and properties of a silicon nitride multilayer composite toughened by metallic laminae

The objective of this research was to develop a silicon nitride composite with toughness and damage tolerance sufficient for use in applications require controlled failure behavior. The technical approach formed a laminate composite material using alternating silicon nitride-metallic layers. The number of metallic layers used in the composite was one fewer than the number of silicon nitride layers, resulting in both outer layers being silicon nitride. Such a configuration enables the outer surfaces of the composite material to take advantage of the greater wear resistance, chemical stability, and thermal stability exhibited by silicon nitride relative to the metallic material. The metallic layers arrested any cracks in the outer layers, thus producing a toughened silicon nitride and avoiding the normal fast-fracture behavior of monolithic silicon nitride.; Four different silicon nitride-metallic material combinations were examined. The metallic materials explored were (1) chromium, (2) titanium, (3) molybdenum, and (4) tantalum. The various material combinations resulted in composites with different mechanical properties. Three different mechanisms for increasing the toughness and damage tolerance of silicon nitride were examined by forming laminate composites using the different material combinations. The types of toughening mechanisms explored were (1) brittle reinforcing phase with weak interfaces, (2) brittle reinforcing phase with strong interfaces, and (3) ductile reinforcing phase with weak interfaces. The silicon nitride-tantalum material combination resulted in composites with a ductile reinforcing phase and weak interfaces. The silicon nitride-tantalum composites exhibited the highest flexure strength and fracture toughness of the composite material combinations examined.