This work studies braking (simulated-stop) behavior of a polyacrylonitrile (PAN)-pitch carbon-carbon composite under different surface conditions. The results indicate that broken-in (BI) specimens exhibited much higher friction coefficients and wear than those of as-polished (AP) specimens under the same braking conditions. The specimens braked from higher speed always suffered higher wear either due to their higher friction coefficients or longer braking times. The friction behavior of the present composite was found most sensitive to the debris morphology. The friction behavior of BI specimens, which had undergone a transition during the BI treatement, was more complex and harder to predict than that of AP specimens, which did not undergo transiti before or during braking. The severe stuctural damage on the BI/2000 surface, which could not be mended, resulted in an unstably high friction surface and large wear.
Because of their low density, excellent thermal and mechanical properties, chemical inertness and self-lubricating capability, carbon-carbon (C-C) composites have become the top choice as aircraft brake disk material. Current fighters, such as the USF-14, F-15 and F-18, the French Mirage 2000, as well as such commercial aircraft as the Boeing 747, Airbus, Concorde, Canadair Challenger and Gulfstream Ⅲ, have employed C-C brakes.
The C-C composites currently used as aircraft brake disks are eigher chemical vapor- infiltrated (CVI) matrix reinforced with PAN-based fabric laminates, or mesophase pitch-based chopped fiber yarns reinforced with phnolic resin char-CVI hybrid matrix. Although these C-C composites have been used as aircraft brake material for years, not much information regarding their friction and wear behavior is available in the open literature.
Awasthi et al reported that different kinds of composite, wear parameters, environments and the prior history of composite surface had influences on tribological behavior of carbon-carbon composites. They found that friction coefficients under static or low speed conditions were generally low, but could be much higher immediately following a landing stop when the brake disks were hot.
Some recent reports have been focused on the tribological behavior of C-C composite under low and medium conditions. Ju et al. and Chen and Ju compared the tribological behavior among six different carbon-carbon composites including three 2D PAN-pitch, one 2D PAN-CVI, one 2D pitch-resin-CVI and one 3D PAN-pitch. They observed that the pre-transitional friction coefficients were as low as 0.1-0.2. During transition, the initially-formed thin, smooth lubricative film was suddenly disrupted and turned into a thick powdery debris layer that caused friction coefficient to abruptly rise up to 0.5-0.9. The powdery debris on certain composites was easily “ironed” into smooth and tight lubricative film to cause both friction and wear to decline.
The present work studied the simulated-stop tribological behavior under different initial surface conditions and speeds of a PAN-pitch based carbon-carbon composite.
Materials and methods:
The 2D PAN-pitch composite for this study was fabricated at the laboratories of Materials Rand D center. This composite was fabricated using woven high-modulus PAN-based fiber preforms, which were pressure-infiltrated and densified with molten pitch as matrix. Detailed processing of this composite has been described elsewhere. The density of the composite was 1.81 g/cm3 with a tensile strength of 148 Mpa.