Analysing the coupled effects of compressive and diffusion induced stresses on the nucleation and propagation of circular coating blisters in the presence of micro-cracks

Abstract

This paper presents the delamination of coating with micro-cracks under compressive residual stress coupled with diffusion induced stress. Micro-cracks in coating provide a passage for corrosive species towards the coating-substrate interface which in turn produces diffusion induced stress in the coating. Micro-cracks contract gradually with increasing compressive residual stress in coating due to thermal expansion mismatch which blocks the species diffusion towards the interface. This behaviour reduces the diffusion induced stress in the coating while the compressive residual stress increases. With further increase in compressive residual stress, micro-cracks reach to the point, where they cannot be constricted any further and a high compressive residual stress causes the coating to buckle away from the substrate resulting in delamination and therefore initiating blistering. Blistering causes the contracted micro-cracks to wide open again which increases diffusion induced stress along with high compressive residual stress. The high resultant stress in coating causes the blister to propagate in an axis-symmetric circular pattern. A two-part theoretical approach has been utilised coupling the thermodynamic concepts with the mechanics concepts. The thermodynamic concepts involve the corrosive species transportation through micro-cracks under increasing compression, eventually causing blistering, while the fracture mechanics concepts are used to treat the blister growth as circular defect propagation. The influences of moduli ratio, thickness ratio, thermal mismatch ratio, poisson's ratio and interface roughness on blister growth are discussed. Experiment is reported for blistering to allow visualisation of interface and to permit coupled (diffusion and residual) stresses in the coating over a full range of interest. The predictions from model show excellent, quantitative agreement with the experimental results.

A review of theoretical analysis techniques for cracking and corrosive degradation of film-substrate systems

“…Previously, Hutchinson models [22,23,36,39,40,43,44], defining film blistering mechanisms, were purely based on fracture mechanic concepts which were recently modified by Khan-Nazir [15,20] by developing modified interfacial toughness and mode mix equations for blister propagation under corrosive conditions. The developed equations provided an efficient method for the prediction of blister propagation within the frame work of fracture mechanics and thermodynamics (diffusion).…”

“…The corrosion resistance, tribological performance and film characteristics have been reported [10][11][12][13]. The crystalline structure of film materials and thin film porosity are not subjects of this review as the phenomenon of corrosion diffusing species through micro cracks in thin films have been reported [14][15][16][17][18][19][20][21] in detail.…”

Life assessment prognostic modelling for multi-layered coating systems using a multidisciplinary approach

“…Properties of alkyd coatings have been investigated by Armelin et al in detail 50 . Red oxide primer coating has Young's modulus of 6.14 GPa and the coefficient of thermal expansion is 21.6x10 QR /°S 24 . Sample 1 has mono-layer coat, sample 2 has bi-layer coating and sample 3 has tri-layer coating respectively.…”

A comprehensive predictive corrosion model incorporating varying environmental gas pollutants applied to wider steel applications

“…blistering, delamination, micro-cracks and corrosion damage measurement in real time has been reported [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Recently a simplistic model of variation in environmental parameters to understand the effects of changing relative humidity and temperatures in terms of corrosion initiation and propagation on bare steel samples has been published [8].…”