Ceramic/metal interface fracture toughness

Fracture toughness of Si3N4/S45C joint with an interface crack

Ceramic/metal joints have been increasingly applied in a wide range of engineering fields because the ceramic has stable mechanical properties at high temperature and good resistance to wear, erosion and oxidation. However, the difference of material properties between metal and ceramic induces stress singularities at the interface edge. The stress singularity together with the thermal residual stress degrades the strength of ceramic/metal joint and makes the evaluation of the strength difficult.

In an article published in AZojomo*, researchers have carried out fracture toughness tests for Si3N4/S45C specimens with interface cracks of different lengths. It was found that the specimen with a crack of 4 mm has higher apparent fracture toughness than those with cracks of 1mm and 2mm due to the reduction of the residual stress. Fracture propagated into Si3N4 from the crack tip in the direction of 40° for cracks of 1mm and 2mm while it propagated along the interface for crack of 4mm.

Elasto-plastic analysis was carried out considering S45C as the linear hardening material and Si3N4 as the elastic material. It was found that the stress around the crack tip is dominated by an elasto-plastic singular stress field, which is substantially the same as the elastic singular stress field of an interface crack. Evaluation of the fracture path and toughness was carried out based on the stress intensity factors of the elasto-plastic singular stress field.