Quantification of metallic coating failure on carbon fiber reinforced plastics using acoustic emission

Carbon fiber reinforced plastic substrates were subsequently coated with an electrochemically applied nickel and an electroplated copper layer. The coating-substrate system was loaded in four-point bending and the acoustic emission from coating failure was recorded during loading. The acoustic emission signals were analyzed using pattern recognition and frequency analysis techniques. This approach yields three distinguishable types of acoustic emission signals, which are correlated to three different failure mechanisms: i) nickel cracking ii) copper cracking and iii) delamination between the two coating layers. To confirm the correlation between the types of acoustic emission signals and the respective failure mechanisms and to assess the validity of the acoustic emission method to describe mechanical failure, the micro-mechanical fracture energies released during mechanical loading were calculated based on microscopic measurements of the crack progress utilizing scanning electron microscopy and scanning acoustic microscopy. These energies were compared to the associated acoustic emission signals energy for each failure mechanism. We found the calculated micro-mechanical energy values to be proportional to the measured accumulated acoustic emission energy of the associated acoustic emission signal type. We conclude that the reported failure classification method offers the possibility to compare fracture toughness values in multilayered coatings with multiple failure types, derived solely from acoustic emission energies.