An Evaluation of Chemical Aging/Oxidation in High Performance Composites Using the Vickers Micro-Indentation Technique

Abstract

Vickers micro-indentation was utilized to follow the changes in micro-hardness and thereby evaluate the chemical aging/oaidation phenomena in two different high performance polymeric composite systems. The two systems under study were1M8/954-2, a carbon fiber-reinforced thermoplastic-toughened thermoset composite, and IM8/ITX, a semicrystalline thermoplastic composite. Oxidation profiles with aging, were obtained on [0°]8 s (unidirectional) composite specimens by indenn across the thickness of the samples in the fiber direction. Samples were aged up to 6 months in environmental chambers at 150°C in three different environments: an inert nitrogen environment, a reduced air pressure of 13.8 kPa (2 psi air), and ambient air pressure. The experimental results show a considerable drop in overall hardness of both IM8/954-2 and 1M8/ITX composites after a week of aging in all of the three environments, possibly attributable to a thermal stress-reieving effect. Very little additional change was observed after 6 weeks aging. Thereafter, the hardness profiles show drops in hardness starting at the specimen edges initially, and then gradually spread to the specimen interior with time. The above observations were indicative of diffusion controlled oxidation in the samples. Also, the hardness drops in the composites appeared to be sensitive to oxygen partial pressure in the aging environment. Samples aged in ambient air were most affected, those in nitrogen least affected, and specimens aged at the reduced pressure were intermediate. A similar diffusion controlled oxidative process was also seen from Vickers indentation results on aged 1M7/5260 thermoset (bismaleimide) composites. This study is a preliminary effort and one of the first of its kind to study the effect of chemical aging in composites using the micro-indentation technique. This work has left some questions unanswered, nevertheless it has shown potential for utilizing micro-indentation as a tool for studying composite behavior in the future. Some suitable modifications to this test technique could enhance its usefulness, thereby gaining greater insights in the area of composite durability.