Performance evaluation of polybenzimidazole coating for aerospace application

Abstract

In Present study, characteristic behavior of PBI as a protective coating for aircraft application is presented. Performance of PBI coating is evaluated after exposure to hot wet environment and liquid immersion. Critical properties including hardness, scratch resistance and adhesion of PBI coating are assessed and results are presented. M21 epoxy based carbon fiber composite is used as substrate material. Atmospheric pressure plasma treatment (APPT) is performed to prepare the surface of substrate prior to application of PBI coating. Lap shear tests demonstrate that APPT has improved the adhesion strength of PBI bonded joints to about 250%. SEM analysis shows that strong composite/coating interface resulted in cohesive failure of the bonded joints. Lap shear test results on conditioned samples reveal that composite bonded joints has depicted about 15% decrease in strength after exposure to hot/wet environment under the conditions of 80 °C and 95% relative humidity (RH). However, mode of failure of bonded joints has not changed even after 1000 h of conditioning. PBI coated samples were also immersed in water and Skydrol to evaluate the effect on hardness and scratch resistance of PBI coating. Scratch tests on conditioned samples reveal that the visco-elastic recovery has increased from 58% (unexposed sample) to 71% with PBI coated panel immersed in skydrol. However, no major effect on visco-elastic recovery is observed with the samples immersed in water and the sample conditioned in hot/wet environment. Nano-indentation test results indicate that PBI coated panel immersed in skydrol has not depicted any decrease in elastic modulus ad hardness; whereas coated panel immersed in water for 1000 h has shown a minor decrease in both modulus and hardness. All these results indicate that PBI coating has great potential to retain its properties under harsh environment and it can be used as protective coating for aerospace application.