Externally applied stress sign and film elastic properties effects on brittle film fracture

Abstract

Rectangular stainless steel samples with TiN film deposited on the front lateral surface were loaded in three-point bending to the maximum normal strain of 6%. Scanning electron microscopy showed that vertical cracks appeared in the tension zone when the tensile strain exceeded 1.5%, while horizontal cracks appeared in the compression zone when the compressive strain exceeded –2.9%. Film cracks in the compressive zone originate from the tensile stress imposed by the plastically deformed substrate due to the Poisson’s expansion. Taking plastic deformation and Poisson’s expansion of the substrate in compression into account, theoretical analysis of normal stress distribution along the cracked film segment in compression is presented. Substrate strain and film elastic properties affect film cracking in the compressive zone. At larger compressive strain, some transverse cracks along with buckling cause the film spallation. The presented method is useful for studying brittle film fracture with variable strain levels in a single sample.

Keywords:

• Bending
• compression
• cracking
• deposition
• thin films
• fracture
• thin-film mechanics
• TiN film
• stainless steel substrate
• Poisson’s expansion

Funding

This work was supported by the National Natural Science Foundation of China [grant number 51271022], [grant number 51431004]; Beijing Higher Education Young Elite Teacher Project [YETP0353]; National Basic Research Program of China [2012CB937502]; the National Science Foundation [IRES 1358088].