D Raab et al. Materials Science and Engineering A 417(2006)341-347 Table 3 Mechanical properties of the manufactured composites(mean values) Three-point bending test Biaxial strength test Bending strength(MPa) Continuous fiber Short fiber Continuous fiber hort fiber Continuous fiber Short fiber 440 uncoated extel M 440/40nm BN coating NextelTM 440/150 nm BN coating 98 4M44 The relative error of bend ngth and biaxial strength data is 10%. The relative error of Kie data is 10%(NA, not assessed The occasional defects are thought to be caused by an uncon- 3.3. Nextel M 440 fiber reinforcement trolled coating process. Nevertheless, a turbostratic hexagonal structure was demonstrated for the boron nitride phase coat- Because of the thermal expansion coefficient mismatch ings by TEM investigations, as shown in previous investigations (a>M)in these composites, radial tensile stresses surround- ing the Nextel fibers develop. There is, however, a residual The desized Zen Tron glass fiber in Fig 4 shows a relatively hoop compressive stress field in the matrix, which is beneficial smooth surface as expected for an amorphous material. On the for the strength of the composite material contrary, the tin dioxide CVD coating exhibits a relatively rough As expected, unidirectional and short NextelM440 fiber crystalline structure in AFM imaging. Raman spectrometry and reinforced composites showed increased strength in three-point X-ray diffraction(XRD) have shown that the tin oxide coating bending tests for both 40 and 150 nm BN fiber coatings, as consists mainly of cassiterite phase [211(results not shown here). compared to uncoated fibers In the case of continuous fiber rein- Besides the changes in fiber chemistry due to interactions with forcement, the fracture toughness increased up to 3. 3 MPam /2 tin ions and the possible development of surface microstresses, for composites with 40 nm thick boron nitride interfaces. For the a mechanical aspect of fiber damaging should not be ruled out, short fiber reinforced composites, no enhancement of the frac caused by the formation of relatively large cassiterite crystals. ture toughness was observed. This is a consequence of the low This crystal growth can lead to surface stresses, which in turn fiber content in these composites(see Table 2)and due to the fact might prompt the formation of microflaws on glass fiber sur- that the basic toughening mechanisms(mainly crack deflection and crack bridging)are different from those active in contin- uous fiber composites, e.g. fiber debonding and pull-out [5,6 3. 2. Composite characterization Moreover it can be assumed that the edges of the short fibers are not coated, and therefore diffusion reactions may occur between The thermal expansion coefficients(a)determined for differ- fibers and matrix which should cause strong interface bonding ent composites containing short or continuous fiber reinforce- leading to brittle fracture In biaxial flexural strength tests, no ment are listed in Table 2. The a-values for the unidirectional significant differences between the strength values of the com- continuous fiber reinforced composites were measured in the posites with uncoated and BN-coated fibers(150 nm thick) were fiber direction. The experimental results are in broad agreement found for both continuous and short fiber reinforcement. Only with values calculated by the rule of mixtures using the a-values composites with short fiber reinforcement with a 40 nm thick BN for matrix and fibers given in Table 1 The results of the mechanical strength tests are summa- rized in Table 3. The fracture behaviour of the composites was investigated by both three-point bending and biaxial flexural tial applications of the composites as structural components, e.g. 280 rength tests. The latter test is relevant considering the poten- in architecture, where flat composite panels will be enclosed in a metal or plastic frame, thus possibly exposed to biaxial stresses due to thermal expansion mismatch of the materials involved [22]. Moreover, the biaxial test method is useful to obtain rel- ive data to compare the behaviour of conti and short fiber reinforced composites. The test allows for com- Displacement [mm] parison of the influence of the fiber orientation in continuous fiber composites and the quasi-isotropic behaviour of short fiber Fig. 5. Load-displacement curves composites regarding both mechanical strength and work of reinforced, interface: 150nm BN,(2) ngth test: (1) short fibre bre reinforced, interface 50 nm BN, (3)short fibre reinforced, no coating.344 D. Raab et al. / Materials Science and Engineering A 417 (2006) 341–347 Table 3 Mechanical properties of the manufactured composites (mean values) Fibers Three-point bending test Biaxial strength test Bending strength (MPa) Fracture toughness (MPa m1/2) Bending strength (MPa) Continuous fiber Short fiber Continuous fiber Short fiber Continuous fiber Short fiber NextelTM 440 uncoated 61 78 1.0 1.43 61 59 NextelTM 440/40 nm BN coating 91 NA 3.3 NA NA 76 NextelTM 440/150 nm BN coating 98 88 2.3 1.45 59 63 ZenTronTM uncoated 51 61 1.1 1.45 53 38 ZenTronTM/70 nm SnO2 coating 84 63 0.9 1.01 54 64 The relative error of bending strength and biaxial strength data is 10%. The relative error of Kic data is 10% (NA, not assessed). The occasional defects are thought to be caused by an uncon￾trolled coating process. Nevertheless, a turbostratic hexagonal structure was demonstrated for the boron nitride phase coat￾ings by TEM investigations, as shown in previous investigations [21]. The desized ZenTronTM glass fiber in Fig. 4 shows a relatively smooth surface as expected for an amorphous material. On the contrary, the tin dioxide CVD coating exhibits a relatively rough crystalline structure in AFM imaging. Raman spectrometry and X-ray diffraction (XRD) have shown that the tin oxide coating consists mainly of cassiterite phase [21](results not shown here). Besides the changes in fiber chemistry due to interactions with tin ions and the possible development of surface microstresses, a mechanical aspect of fiber damaging should not be ruled out, caused by the formation of relatively large cassiterite crystals. This crystal growth can lead to surface stresses, which in turn might prompt the formation of microflaws on glass fiber sur￾faces. 3.2. Composite characterization The thermal expansion coefficients (α) determined for differ￾ent composites containing short or continuous fiber reinforce￾ment are listed in Table 2. The α-values for the unidirectional continuous fiber reinforced composites were measured in the fiber direction. The experimental results are in broad agreement with values calculated by the rule of mixtures using the α-values for matrix and fibers given in Table 1. The results of the mechanical strength tests are summa￾rized in Table 3. The fracture behaviour of the composites was investigated by both three-point bending and biaxial flexural strength tests. The latter test is relevant considering the poten￾tial applications of the composites as structural components, e.g. in architecture, where flat composite panels will be enclosed in a metal or plastic frame, thus possibly exposed to biaxial stresses due to thermal expansion mismatch of the materials involved [22]. Moreover, the biaxial test method is useful to obtain rel￾evant quantitative data to compare the behaviour of continuous and short fiber reinforced composites. The test allows for com￾parison of the influence of the fiber orientation in continuous fiber composites and the quasi-isotropic behaviour of short fiber composites regarding both mechanical strength and work of fracture. 3.3. NextelTM 440 fiber reinforcement Because of the thermal expansion coefficient mismatch (αF > αM) in these composites, radial tensile stresses surround￾ing the NextelTM fibers develop. There is, however, a residual hoop compressive stress field in the matrix, which is beneficial for the strength of the composite material. As expected, unidirectional and short NextelTM 440 fiber reinforced composites showed increased strength in three-point bending tests for both 40 and 150 nm BN fiber coatings, as compared to uncoated fibers. In the case of continuous fiber rein￾forcement, the fracture toughness increased up to 3.3 MPa m1/2 for composites with 40 nm thick boron nitride interfaces. For the short fiber reinforced composites, no enhancement of the frac￾ture toughness was observed. This is a consequence of the low fiber content in these composites (see Table 2) and due to the fact that the basic toughening mechanisms (mainly crack deflection and crack bridging) are different from those active in contin￾uous fiber composites, e.g. fiber debonding and pull-out [5,6]. Moreover, it can be assumed that the edges of the short fibers are not coated, and therefore diffusion reactions may occur between fibers and matrix which should cause strong interface bonding leading to brittle fracture. In biaxial flexural strength tests, no significant differences between the strength values of the com￾posites with uncoated and BN-coated fibers (150 nm thick) were found for both continuous and short fiber reinforcement. Only composites with short fiber reinforcement with a 40 nm thick BN Fig. 5. Load–displacement curves of selected samples (borosilicate glass/NextelTM 440 fiber) for biaxial flexural strength test: (1) short fibre reinforced, interface: 150 nm BN, (2) continuous fibre reinforced, interface: 150 nm BN, (3) short fibre reinforced, no coating