Relative performance of detonation gun sprayed Cu-Ni-In and alumina coatings on AA 6063 under plain fatigue and fretting fatigue loading

The present study compares the plain fatigue (PF) and fretting fatigue (FF) behaviour of two coating materials Cu-Ni-In and alumina that were deposited by detonation gun spray process on Al-Mg-Si alloy (AA 6063) samples. Multipass coating was formulated on all four sides of the gauge portions of the fatigue specimens to a thickness of about 150 to 160 mu m and subsequently grinding operation was carried out to reduce the thickness to about 100 mu m. Coatings were characterised in terms of microstructure, microhardness, residual stresses and surface roughness. Uniaxial PF and FF experiments were conducted on uncoated and coated specimens. Cu-Ni-In coating exhibited relatively lower hardness compared with alumina coating but higher than that of the substrate. Ground Cu-Ni-In coating exhibited very high surface compressive residual stress compared with ground alumina coating and substrate. The coated samples experienced higher friction force compared with uncoated specimens. Cu-Ni-In coated specimens exhibited slightly higher friction force compared with alumina coated samples. Cu-Ni-In coated samples exhibited superior PF and FF lives compared with uncoated and alumina coated specimens. This has been explained in terms of differences in the values of surface hardness, surface roughness, residual stress, porosity, adhesion and friction force. As there were many factors, which influenced PF and FF lives, it was not possible to find out the relative importance of each factor. However, it is speculated that the presence of very high compressive residual stress at the surface was mainly responsible for the superior performance of Cu-Ni-In coated specimens.