Residual stresses in plasma electrolytic oxidation coatings on Al alloy produced by pulsed unipolar current

Plasma electrolytic oxidation (PEO) coatings have been produced on aluminium substrates using a pulsed unipolar current mode at different duty cycle and frequency, along with reference samples produced using direct current. Residual stresses attributed to the α-Al2O3 constituent of the coating have been evaluated by X-ray diffraction using the Sin2ψ method. It is found that compressive normal and shear stresses were present in the coatings, which indicated the existence of stress gradient across the coating thickness. Depending on treatment parameters, internal normal stress in the coatings ranges from − 111 (± 19) MPa to − 818 (± 47) MPa and shear stress ranges from − 45 (± 27) MPa to − 422 (± 24) MPa have been found. Correlations between internal stress and coating thickness, surface morphology and phase composition have been discussed. PEO regimes favourable for the production of coatings with minimal stress, dense morphology and relatively high content of α-Al2O3 phase are recommended.