The effect of electroless Ni–P coatings on the fatigue life of Al 7075-T6 fastener holes with symmetrical slits

In this paper, the fatigue behaviour of Al 7075-T6 fastener holes with symmetrical through slits was studied. The holes were coated with electroless nickel (EN) plating with a high phosphorous content of 10–13 wt% and a thickness of 40 μm. Uncoated open-hole, EN coated open-hole, uncoated bolted hole and EN coated bolted hole specimens were fatigue tested. Bolted samples were clamped with a high tightening torque of 7 Nm. The established S–N curves showed 282–1348% improvements in the fatigue life due to the combined effect of EN coating and bolt clamping, depending on the level of maximum alternating stress. Excellent adhesion was observed between the coating and the aluminium substrate along the crack path. Tensile tests results showed a considerable reduction of 54% in the ductility of the coated material while both the yield and ultimate strengths were found to slightly increase by approximately 6% in comparison with the uncoated aluminium alloy.

► Ni–P coatings improved fatigue life of Al 7075-T6 fastener holes with slits by 20%.
► Ni–P coatings further improved fatigue life of bolted holes by additional 113–412%.
► Larger crack propagation zones for Ni–P coated and bolted holes at low cyclic loads.
► Excellent adhesion of the coating to the aluminium substrate under fatigue loading.