WC/C coating protection effects on 7075-T6 fatigue strength in an aggressive environment

Thin hard coatings are typically employed to improve surface material characteristics such as hardness, wear and contact fatigue resistance, and friction behavior. Their effect on the component fatigue strength may vary, depending on the substrate surface finishing and to the microstructural modification of the base material due to the Physical Vapor Deposition (PVD) process thermal loads, particularly critical for a 7075-T6 low ageing temperature (121 °C) alloy. The present work investigates the effects of a low temperature (180 °C) Tungsten Carbide/Carbon (WC/C) PVD coating, in terms of corrosion protection under fatigue loads. The effects of an aggressive environment over uncoated and coated 7075-T6 hourglass specimens were analyzed, by rotating bending (R = -1) step loading fatigue tests at 2·105 cycles. A chemically aggressive external condition has been investigated, considering a methanol environment. Results show a certain protective capability of the WC/C coating, which is however offset by the fatigue strength drop found for coated samples at 2·105 cycles.