Effect of silicon nitride coating thickness on fatigue of silicon microbeams

•We conduct low cycle fatigue experiment on Si3N4/Si cantilever microbeams.•Increasing surface coating Si3N4 thickness will decrease fatigue life.•Higher frequency of 100 Hz leads to longer fatigue life of 0.4 μm coating specimen.•Si3N4/Si beams can sustain higher strain amplitudes than those for Si beams.

In this paper, two silicon nitride layers with thickness, 0.2 and 0.4 μm, are coated onto single crystal silicon (SCS) in order to achieve Si3N4/Si cantilever microbeams. The effect of LPCVD silicon nitride surface coatings on fatigue properties of SCS cantilever microbeams is investigated. Fatigue testing is conducted at both 40 Hz and 100 Hz. Typical S–N (strain amplitude–fatigue cycle) curves of the beams are achieved and correlated fatigue failure modes are investigated. It is found that thinner Si3N4 coating of 0.2 μm results in better fatigue lives of Si3N4/Si beams than thicker Si3N4 coating of 0.4 μm. Both thinner and thicker coated beams have major fatigue crack planes along {1 1 1} planes; however, thicker coated beams possess specific failure mode of delamination, which is not found in thinner coated beams. Delamination reduces the reinforcing effect of thicker Si3N4 coating and leads to its shorter fatigue life. For thicker coated beams, fatigue life at 100 Hz is longer than that at 40 Hz. The mechanism for delamination and the effect of cyclic frequency is investigated, and factors for better fatigue life are proposed.