D Raab et al. /Materials Science and Engineering A 417 (2006)341-347 23 0 nm 2,3μm0pm 150 nm B desized s-glass fiber 70 nm tin dioxide Zen Tron 1g. 4. AFM images of fiber surfaces: desized and tin oxide-coated Zen tron Fig. 2. Comparison of single fiber tensile strength data for desized and coated glass fibers NextelM 440 fibers and for desized and coated Zen TronM fibers(gauge length measured Fig. 2 shows the results of these tests for Nextel M and in the range of 5-12%. Discs of 50 mm in diameter and -3 mm zen tron TM fibers for desized NextelTm 440 fibers as well as thickness were fabricated. Microstructural examination was car- for the 40 and 150 nm BN-coated NextelTM fibers. no significant ried out by scanning electron microscopy (SEM)(CamScan 44, differences in fiber tensile strength values were found. The data Cambridge Instruments). From hot-pressed discs, rectangular scattering is also comparable. However, a pronounced decrease test bars of dimensions 40 mm x 6 mm x 3 mm were cut and pol- in fiber tensile strength was found for the tin dioxide-coated ished. Three-point bending strength tests were performed using Zen Tron TM glass fiber in comparison to the desized fiber.Fiber an INSTRON 4467 facility with a 2N load cell according to degradation due to the thermal treatment during the CVDcoating DIN Norm DIN ENV 658-3. At least five samples were tested process is excluded as a cause of strength loss because of the for each condition. For biaxial flexural strength testing discs relatively high strength of the desized fiber treated at nearly the were thinned to I mm thickness and polished with SiC-paper same temperature as that used for the CVD coating process. A ( um). The apparatus used for measuring the biaxial strength diffusion of tin ions into the glass fiber during the coating process test has been described elsewhere [19]. At least 10 samples were cannot be ruled out however and it should be considered further used for each composition and the results were averaged (see Section 3.2 below). Chemical reactions at the Sno/glass The thermal expansion coefficient of composites was mea- interface and locally stressed regions on the fiber surface as sured using a dilatometer(NETZSCH TMA 402). Test bars of a result of the thermal treatment during CVD might be other dimensions 3 mm x 3 mm x 25 mm were used for the measure measure- contributing reasons leading to the decrease of tensile strength of coated Zen TronM fibers Preliminary assessment of fracture toughness(KI Figs. 3 and 4 show AFM images of desized and coated obtained by the indentation technique, using loads of IoN. The fibers surfaces for both NextelTM and ZenTronTM fibers, respec Heckel-equation[20] was used to calculate Kl from the length tively. Fig. 3 shows that the fine-grained structure of the desized of the cracks emanating from the corners of Vickers impres- Nextel TM 440 fiber becomes smudged by the CVD coating pro- cess. Grain boundaries are still visible in the Bn coating and 3. Results and discussion there is no significant difference between the structure of coat- ings of 40 and 150 nm thickness. earlier investigations have also 3.. Fiber characterization shown no significant changes in surface roughness of BN films prepared by a similar CVD coating process [18]. In some cases, e To evaluate the influence of the CVD coating process on the larger particles were found on the coated fiber surface, as shown chanical properties of fibers, the fiber tensile strength was for example on the sample image for the 40 nm coating(Fig 3) 2,3pm 0 desized Nextel 440 fiber 50 nm boron nitride 150 nm boron nitride Fig 3. AFM images of fiber surfaces: desized and boron nitride-coated Nextel M fibersD. Raab et al. / Materials Science and Engineering A 417 (2006) 341–347 343 Fig. 2. Comparison of single fiber tensile strength data for desized and coated NextelTM 440 fibers and for desized and coated ZenTronTM fibers (gauge length 26 mm). in the range of 5–12%. Discs of 50 mm in diameter and ∼3 mm thickness were fabricated. Microstructural examination was car￾ried out by scanning electron microscopy (SEM) (CamScan 44, Cambridge Instruments). From hot-pressed discs, rectangular test bars of dimensions 40 mm × 6 mm × 3 mm were cut and pol￾ished. Three-point bending strength tests were performed using an INSTRON 4467 facility with a 2 N load cell according to DIN Norm DIN ENV 658-3. At least five samples were tested for each condition. For biaxial flexural strength testing discs were thinned to 1 mm thickness and polished with SiC-paper (3
m). The apparatus used for measuring the biaxial strength test has been described elsewhere [19]. At least 10 samples were used for each composition and the results were averaged. The thermal expansion coefficient of composites was mea￾sured using a dilatometer (NETZSCH TMA 402). Test bars of dimensions 3 mm × 3 mm × 25 mm were used for the measure￾ments. Preliminary assessment of fracture toughness (KIc) was obtained by the indentation technique, using loads of 10 N. The Heckel-equation [20] was used to calculate KIc from the length of the cracks emanating from the corners of Vickers’ impres￾sions. 3. Results and discussion 3.1. Fiber characterization To evaluate the influence of the CVD coating process on the mechanical properties of fibers, the fiber tensile strength was Fig. 4. AFM images of fiber surfaces: desized and tin oxide-coated ZenTronTM glass fibers. measured. Fig. 2 shows the results of these tests for NextelTM and ZenTronTM fibers. For desized NextelTM 440 fibers as well as for the 40 and 150 nm BN-coated NextelTM fibers, no significant differences in fiber tensile strength values were found. The data scattering is also comparable. However, a pronounced decrease in fiber tensile strength was found for the tin dioxide-coated ZenTronTM glass fiber in comparison to the desized fiber. Fiber degradation due to the thermal treatment during the CVD coating process is excluded as a cause of strength loss because of the relatively high strength of the desized fiber treated at nearly the same temperature as that used for the CVD coating process. A diffusion of tin ions into the glass fiber during the coating process cannot be ruled out, however, and it should be considered further (see Section 3.2 below). Chemical reactions at the SnO2/glass interface and locally stressed regions on the fiber surface as a result of the thermal treatment during CVD might be other contributing reasons leading to the decrease of tensile strength of coated ZenTronTM fibers. Figs. 3 and 4 show AFM images of desized and coated fibers surfaces for both NextelTM and ZenTronTM fibers, respec￾tively. Fig. 3 shows that the fine-grained structure of the desized NextelTM 440 fiber becomes smudged by the CVD coating pro￾cess. Grain boundaries are still visible in the BN coating and there is no significant difference between the structure of coat￾ings of 40 and 150 nm thickness. Earlier investigations have also shown no significant changes in surface roughness of BN films prepared by a similar CVD coating process [18]. In some cases, larger particles were found on the coated fiber surface, as shown for example on the sample image for the 40 nm coating (Fig. 3). Fig. 3. AFM images of fiber surfaces: desized and boron nitride-coated NextelTM fibers