Mechanism of thermal shock crack extension in metal/ceramic sintered functionally graded materials

The thermal shock fracture mechanism of metal/ceramic functionally graded materials was studied by burner heating test. Dependence of thermal shock crack initiation and propagation on controlled compositional gradients was virtually shown and discussions were made on the basis of fracture mechanics with special reference to the effect of compositional profile on crack extension behavior. Three types of FGMs, having the same thickness of graded layer with different compositional profiles, were fabricated by powder metallurgical process. The fracture toughness of each composition was determined by newly devised repeated vickers indentation method directly on FGM specimens. The fracture toughness increased with increase in the metal phase content, owing to toughening mechanisms of thermal-strain-misfit and crack deflection. The FGMs were joined on cooling substrates and used for burner heating test. The crack formation was always observed on the ceramic surface during cooling due to large residual tensile stresses. By comparison between the fracture toughness and mode I stress intensity factor, vertical cracks in convex-profile FGMs were deflected toward the direction parallel to the surface. The depth of the parallel cracks beneath the surface may correspond well to a location of mode II stress intensity being equal to zero. On the other hand, initiated vertical cracks in concave-profile FGMs were considered to arrest without deflection.