Investigation of Si3N4–TiN/Si3N4–Si3N4 trilayer composites with residual surface compression

The present study involved the fabrication of three-layered composites consisting of outer layers that contained Si3N4 and an inner layer that contained TiN in a Si3N4 matrix. Surface compressive stresses were developed upon cooling due to the relatively higher thermal expansion coefficient (CTE) in the inner layer. The flexural strength of layered Si3N4 composites was substantially greater than that of monolithic Si3N4. This was attributed to the surface compressive stress. The effects of TiN composition and inner layer thickness on the mechanical properties were investigated. Layered samples containing 20 vol. % TiN had lower flexural strength than Si3N4–10% TiN/Si3N4–Si3N4 due to the formation of microcracks in the inner layer. Crack behaviors in layered samples were affected by the residual stress, interface, and free sample surface. Both theoretical and experimental results indicated that the strength and toughness of layered composites were substantially greater than those of monolithic materials. The determination of fracture toughness in three-layered materials by the surface indentation technique should be done carefully due to the influence of residual stress.