Improving fretting fatigue behaviour of Al 7075-T6 bolted plates using electroless Ni-P coatings

In this paper, the effect of electroless nickel-phosphorous coatings on the fatigue and fretting fatigue behaviour of Al 7075-T6 bolted plates has been investigated. A double-lap bolted joint specimen was designed and manufactured from the aluminium plates and subsequently coated with Ni-P coatings of 40 μm in thickness with a high phosphorous content of 10-13 wt.%. Then, different tightening torques were applied to clamp the plates together with the aim of studying the effect of clamping force on the fretting fatigue life of the joints. Ni-P coatings were found to protect Al 7075-T6 clamped plates against fretting fatigue damages even at low fatigue loads where the uncoated joints showed a large reduction in their fatigue life due to the fretting effects. Fretting fatigue life of moderately and firmly clamped plates was successfully improved by approximately 30-40% and 50-60% respectively after the application of Ni-P coatings. Furthermore, fatigue cracks were found at the outer surface of the aluminium substrate at the edge of the hole associated with nodular defects whose deleterious effect was more pronounced at high cyclic loads. The Ni-P deposit presented a very good adhesion to the substrate at low and moderate loads; however, considerable delaminations and fracture of the coating film at high cyclic loads was observed.