# Adhesive Threshold Predictive Modelling for Coatings in the Presence of Interfacial Impurity

This source preferred by Zulfiqar Khan

Authors: Nazir, H. and Khan, Z.

Start date: 16 June 2015

The current research has developed a unified model that provides a foundation for an efficient analytical design incorporating the concepts of both thermodynamics and fracture mechanics. The mathematical equations within the unified model for both the components (thermodynamics and fracture mechanics) have been designed independently and then fused to form a governing law for debondment initiation and propagation.

Simulations based on experimental observations provide the first evidence that the interface debondment of a coating from its substrate depends on the size of impurities at the interface. Simulations were designed to find the critical value of impurity size ric at the interface which act as a boundary point between coating fail-safe and coating fail conditions. If initial impurity size r_i at the interface exceeds the critical value of impurity size ric, it results in detachment of the coating from the substrate. As the coating detaches, the propagation of the interfacial debondment is subjected to mode-mix conditions, which depends upon six mode-mix parameters. These mode-mix parameters decide the rate of debonding propagation and amplitude of deflection from the substrate. The six mode-mix parameters are: mode I toughness г_IC, interface roughness/material parameter Γ_IC/E_1 L_a/(R_a^2 ) , debonding driving force F, debondment amplitude w, mode-mix function j and interface toughness Z. The critical values of all six mode-mix parameters are found particular to condition when initial impurity size r_i at the interface equals to the critical value of impurity size ric. The respective mode-mix parameters exhibit critical values at critical impurity size ric. The critical values of all the mode-mix parameters are very small close to zero (→0), except mode-mix function j and interface toughness Z, which possess maximum values at critical data point. The simulations employed an iterative method to find the critical size of impurity and mode-mix parameters over a range of temperature (0 deg C to 60 deg C) and moles of salt in vapour (0.2mol to 0.3mol)). A series of experiments employed ASTM-B117 test in conjunction with analyses. These experiments showed excellent, quantitative agreement with the simulation trends predicted by the theoretical model. The critical values provide guidelines which will be used by coating manufacturers to design for durability. Furthermore, the model provides a foundation for future research within the area of coating failure analysis and prediction.

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