Thermal aging effects on mechanical and tribological performance of PEEK and short fiber reinforced PEEK composites T. Sınmazc ¸elik * , T. Yılmaz Kocaeli University, Mechanical Engineering Department, Engineering Faculty, Veziroglu Campus, _ I zmit, 41040, Turkey Received 30 March 2005; accepted 15 July 2005 Available online 6 September 2005 Abstract The effects of thermal aging on the properties of unfilled and random oriented short fiber reinforced PEEK and its composites have been studied. After the isothermal aging process, there is a remarkable decrease in degree of crystallinity but more organized crystallize structure achieved. As a result of transcrystalline layer formation, there was a considerable increase in the flexural mod- ulus of materials. Thermal aging affects the impact properties of filled and unfilled PEEK dramatically. F max , E max and E Æ F max results of both filled and unfilled aged PEEK and its composites are dramatically decreased. Thermal aging makes materials more brittle and there was a significant decrease in toughness. % Crystallinity is not the unique parameters to determine polymerÕs per- formance. The orientation of crystals is another important parameter in microstructure and plays important role in mechanical and tribological properties of PEEK and its composites. There is a close relationship between thermal aging and microstructure. But there is not a linear relationship between microstructure and tribological properties. Microstructural changes after thermal aging serves developed mechanical properties. Increased mechanical properties results in improved tribological properties. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Polymer–matrix composites; Short-fiber composites; Friction/wear; Impact behavior; Heat treatment 1. Introduction Polyetheretherketone (PEEK) is a semi crystalline polymer with attractive properties with superior specific strength and stiffness. These are excellent materials for a variety of structural applications as in aerospace, auto- motive and chemical industries. PEEK and its compos- ites are one of a new generation of engineering polymers having high temperature properties. It is possible to control the microstructure of PEEK and its composites, especially the crystallinity by suit- able thermal history. Many studies of the thermal his- tory effects on the morphology of PEEK are reported in the literature [1,2], motivated by strong dependence of the mechanical performance on the microstructure of the polymer. For example, both the tensile and shear strength of PEEK increase with an increase in % crystal- linity. The level of crystallinity also affects the resistance to hostile environments [3]. The specific microstructure parameters whose influences are evaluated by those studies are the degree and type of crystallinity and the size of the spherulites. The influence of the thermal his- tory on the mechanical properties of PEEK matrix com- posites concluded that the effect of the degree of crystallinity is more important than that of the order of crystallinity [4]. The more organized crystallinity formed by isothermal crystallization results in better sta- tic and fatigue performances compared to those result- ing from annealing crystallization [5]. There are at least two important factors that can influence the fiber/matrix interface in thermoplastic ma- trix composites; neither factor has a significant effect on 0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.07.007 * Corresponding author. Tel.: +90 262 3351148; fax: +90 262 3352812. E-mail address: tamersc@yahoo.com (T. Sınmazc ¸elik). www.elsevier.com/locate/matdes Materials and Design 28 (2007) 641–648 Materials & Design