Effect of processing on carbon fibers Misoriented crystallite linking two Tensile stress causes basal Catastrophic failure occurs if crystallites parallel to fiber axis. plane rupture in La direction crystallite size> critical flaw size (b) Fig. 6. Reynolds and Sharp mechanism for tensile failure of carbon fibers [20] Each of these show that the PAN-based failure mode explains the difference between the flaw extensively folded and sensitivity(and, therefore, the tensile strength) of the interlinked turbostratic layers of carbon with nongraphite, fibrillar PAN-based carbon fiber and interlayer spacings considerably larger than those of graphitic, mesophase pitch-based fiber. Recent work raphite. As a result, PAN-based carbon fibers have by Dobb er al. [24] indicates that inter-crystalline a low degree of graphitization. The turbostratic layers and intra-crystalline disorder, most likely caused by within PAN-based carbon fibers appear to follow the the fibrillar structure of the PAN-based carbon fiber. original fibril structure of the PAN precursor fiber. is responsible for the superior compressive strength Although the turbostratic layers within these fibrils of this class of carbon fiber tend to be oriented parallel to the fiber axis, they are Obviously, the fundainental fibrillar structure of not highly aligned. As first proposed by Johnson PAN-based carbon fibers is created during initial [20]. it is this fibrillar structure that makes PAN fiber formation. However, little if any research into based fibers less prone to faw-induced failure. He this fiber formation process has been done since based his argument on the brittle-failure mechanism PAN's adoption as a carbon fiber precursor. Instead, proposed by Reynolds and Sharp. As discussed as the above revicw bove, the crystallites within PAN-based carbon based carbon fiber research has concentrated on fibers are not perfectly aligned, and misoriented abilization and carbonization, by contrast, research itively common (see Fig. 6(a) in pitch-based carbon fibers has concentrated on When a stress is applied parallel to the fiber axis, the perfecting precursor chemistry and the development ystallites align until their movement is restricted by of structure during fiber formation. As will be seen, a disclination in the structure( Fig. 6(b)). If the stress pitch researchers appear to have chosen the more is sufficient, the misoriented crystallite will rupture critical area for control and optimization of fiber and relieve the stress within the fiber(Fig. 6(c)). properties When the size of the ruptured crystallite(perpendicu lar to the fiber axis)is larger than the critical faw size,a catastrophic failure occurs, and the fiber 3. PITCH-BASED CARBON FIBERS breaks. Even if the rupture crystallite is smaller than Its highly condensed aromatic structure the critical flaw size, catastrophic failure can occur if phase pitch (the precursor for pitch-ba he crystallites surrounding the disclination are con- fibers) relatively good thermal stability ous enough to allow a crack to propagate inte this, mesophase pitch precursor fibers are me/t sn,or boring crystallites. According to Johnson, this As previously mentioned, melt spinning is the pre-Effect of processing on carbon fibers 349 Misoriented crystallite linking two crystallites parallel to fiber axis. (4 ! I Tensile stress causes basal plane rupture in La direction. (b) Catastrophic failure occurs if crystallite size 5 critical flaw size. cc> Fig. 6. Reynolds and Sharp mechanism for tensile failure of carbon fibers [20]. Each of these studies show that the PAN-based carbon fibers contain extensively folded and interlinked turbostratic layers of carbon with interlayer spacings considerably larger than those of graphite. As a result, PAN-based carbon fibers have a low degree of graphitization. The turbostratic layers within PAN-based carbon fibers appear to follow the original fibril structure of the PAN precursor fiber. Although the turbostratic layers within these fibrils tend to be oriented parallel to the fiber axis, they are not highly aligned. As first proposed by Johnson [20], it is this fibrillar structure that makes PAN￾based fibers less prone to flaw-induced failure. He based his argument on the brittle-failure mechanism proposed by Reynolds and Sharp. As discussed above, the crystallites within PAN-based carbon fibers are not perfectly aligned, and misoriented crystallites are relatively common (see Fig. 6(a)). When a stress is applied parallel to the fiber axis, the crystallites align until their movement is restricted by a disclination in the structure (Fig. 6(b)). If the stress is sufficient, the misoriented crystallite will rupture and relieve the stress within the fiber (Fig. 6(c)). When the size of the ruptured crystallite (perpendicu￾lar to the fiber axis) is larger than the critical flaw size, a catastrophic failure occurs, and the fiber breaks. Even if the rupture crystallite is smaller than the critical flaw size, catastrophic failure can occur if the crystallites surrounding the disclination are con￾tinuous enough to allow a crack to propagate into neighboring crystallites. According to Johnson, this failure mode explains the difference between the flaw sensitivity (and, therefore, the tensile strength) of the nongraphite, fibrillar PAN-based carbon fiber and graphitic, mesophase pitch-based fiber. Recent work by Dobb ef al. [24] indicates that inter-crystalline and intra-crystalline disorder, most likely caused by the fibrillar structure of the PAN-based carbon fiber, is responsible for the superior compressive strength of this class of carbon fiber. Obviously, the fundamental fibrillar structure of PAN-based carbon fibers is created during initial fiber formation. However, little if any research into this fiber formation process has been done since PAN’s adoption as a carbon fiber precursor. Instead, as the above review indicates, nearly all recent PAN￾based carbon fiber research has concentrated on stabilization and carbonization. By contrast, research in pitch-based carbon fibers has concentrated on perfecting precursor chemistry and the development of structure during fiber formation. As will be seen, pitch researchers appear to have chosen the more critical area for control and optimization of fiber properties. 3. PITCH-BASED CARBON FIBERS Its highly condensed aromatic structure gives meso￾phase pitch (the precursor for pitch-based carbon fibers) relatively good thermal stability. Because of this, mesophase pitch precursor fibers are melt spun. As previously mentioned, melt spinning is the pre-