D Raab et al. Materials Science and Engineering A 417(2006)341-347 demands, the interfaces should not influence optical wave guid- Temperatur→ Pressure ing in the visible wavelength, which is essential to achieve a transparent, or at least translucent, material [9-13] In this study, the role of two different interfaces in oxide fiber reinforced borosilicate glass matrix composites was exam- g ined and discussed. Zen Tron M glass fibers were coated with tin dioxide and Nextel TM 440 fibers with boron nitride via chem- 8400 ical vapour deposition(CVD), as described elsewhere [17, 18] E300 The fibers were characterized by tensile strength testing and A200 987654321 atomic force microscopy(AFM). a borosilicate glass matrix (type 756f) was reinforced with desized or coated fibers, both in chopped and continuous form. The fiber volume content was kept low(<12 vol%)to allow for comparison with ear- Time [min] lier related investigations on similar glass matrix composites Fig. 1. Hot-pressing conditionsemployed for optimal composite manufacturing tin-tetramethyl in the presence of oxygen at 450C. Details of 2. Experimental procedures the Cvd coating procedure are given elsewhere [ 17,1 Fiber tensile strength tests were carried out using an 2. Materials INSTRON 4467 facility with a 10N load cell according to DIN Norm ( din V ENV 1007-6, 2002). At least 50 desized and The properties of the glass matrix and of the fibers used as coated fibers were measured for each condition reinforcements are shown in Table 1. because of the refrac. The surface characteristics of desized and coated fibers were ive index mismatch between fibers and matrix, translucent but determined by atomic force microscopy (TMX 2000 Explorer not transparent composites were expected. Moreover, due to the SPM, TopoMetrix). The measurements were carried out in the thermal expansion mismatch between fiber and matrix in com- contact mode under atmospheric conditions posites reinforced with Zen Tronglass fiber, a residual micro- compressive stress field in radial direction should develop in the 2.3. Composite fabrication and characterization atrix. while in the case of Nextel TM fibers. radial microtensile stresses are expected The fiber rovings were infiltrated using a standard slurry infil- tration process, which has been described in the literature [5] 2.2. Fiber coating and characterization Glass particles of size doo <70 um were used. For short fiber composites, the infiltrated rovings were cut to a mean length Coating of the Next! TM fiber with boron nitride films of of 0.3 mm. Composite manufacturing was carried out by hot thickness 40 or 150 nm and of the Zen Tron M glass fiber with pressing at 750 C applying a pressure of 5 MPa for 15 min a tin dioxide film of 70 nm thickness was carried out by con- Fig. I shows the temperature-pressure-time schedule of the hot tinuous CVD under atmospheric pressure in a vertical hot-wall pressing cycle. A short holding time at maximum temperature reactor Before coating, the tiber roving was thermally desized. and the application of the uniaxial pressure during cooling main- The deposition of boron nitride was carried out by reaction tained until the beginning of the glass transformation range were of boric acid-trimethy late and ammonia at 1100C. analo- found to be advantageous for achieving composites with very gously, tin dioxide was obtained by thermal decomposition of low porosity (1%) Table 2 presents an overview of the different composites ricated. The volume fraction of fibers of all composites Table I Properties of the glass matrix and fibers used lass matrix. Fi 三 thermal expansion coefficients of the manufactured composites of fiber a20,,0°c(ppm/K) Fiber concentration silicate) Continuous Short Continuous Short fiber Manufactured by TELUX AGY Refractive index, nD 1.523 Nextel m 440 desized 4.8 9 Thermal expansion coefficient Nextel M 440. 40 nm 5.15 NA (20,,300°)(10-6K-) BN coatin mation temperature(C) 500 Nexte//M,s 440,150mm4.97 s modulus( GPa) BN coating ng strength(MPa) Zen Tron desized 4.36 strength(GPa) 52- 207 Fiber diameter(um) 10-12 SnO coating342 D. Raab et al. / Materials Science and Engineering A 417 (2006) 341–347 demands, the interfaces should not influence optical wave guid￾ing in the visible wavelength, which is essential to achieve a transparent, or at least translucent, material [9–13]. In this study, the role of two different interfaces in oxide fiber reinforced borosilicate glass matrix composites was exam￾ined and discussed. ZenTronTM glass fibers were coated with tin dioxide and NextelTM 440 fibers with boron nitride via chem￾ical vapour deposition (CVD), as described elsewhere [17,18]. The fibers were characterized by tensile strength testing and atomic force microscopy (AFM). A borosilicate glass matrix (type 756f) was reinforced with desized or coated fibers, both in chopped and continuous form. The fiber volume content was kept low (<12 vol%) to allow for comparison with ear￾lier related investigations on similar glass matrix composites [7]. 2. Experimental procedures 2.1. Materials The properties of the glass matrix and of the fibers used as reinforcements are shown in Table 1. Because of the refrac￾tive index mismatch between fibers and matrix, translucent but not transparent composites were expected. Moreover, due to the thermal expansion mismatch between fiber and matrix, in com￾posites reinforced with ZenTronTM glass fiber, a residual micro￾compressive stress field in radial direction should develop in the matrix, while in the case of NextelTM fibers, radial microtensile stresses are expected. 2.2. Fiber coating and characterization Coating of the NextelTM fiber with boron nitride films of thickness 40 or 150 nm and of the ZenTronTM glass fiber with a tin dioxide film of 70 nm thickness was carried out by con￾tinuous CVD under atmospheric pressure in a vertical hot-wall reactor. Before coating, the fiber roving was thermally desized. The deposition of boron nitride was carried out by reaction of boric acid–trimethylate and ammonia at 1100 ◦C. Analo￾gously, tin dioxide was obtained by thermal decomposition of Table 1 Properties of the glass matrix and fibers used Glass matrix, 756f Fiber, ZenTronTM (S-glass) Fiber, NextelTM 440 (alumino silicate) Manufactured by TELUX AGY 3M Refractive index, nD 1.49 1.523 1.616 Thermal expansion coefficient (20, ..., 300 ◦C) (10−6 K−1) 4.8 2.8 5.3 Transformation temperature (◦C) 500 816 – Young’s modulus (GPa) 45 87 190 Bending strength (MPa) 52 – – Tensile strength (GPa) – 4.58 2.07 Fiber diameter (
m) – 10 10–12 Fig. 1. Hot-pressing conditions employed for optimal composite manufacturing. tin-tetramethyl in the presence of oxygen at 450 ◦C. Details of the CVD coating procedure are given elsewhere [17,18]. Fiber tensile strength tests were carried out using an INSTRON 4467 facility with a 10 N load cell according to DIN Norm (DIN V ENV 1007-6, 2002). At least 50 desized and coated fibers were measured for each condition. The surface characteristics of desized and coated fibers were determined by atomic force microscopy (TMX 2000 Explorer SPM, TopoMetrix). The measurements were carried out in the contact mode under atmospheric conditions. 2.3. Composite fabrication and characterization The fiber rovings were infiltrated using a standard slurry infil￾tration process, which has been described in the literature [5]. Glass particles of size d90 < 70
m were used. For short fiber composites, the infiltrated rovings were cut to a mean length of 0.3 mm. Composite manufacturing was carried out by hot pressing at 750 ◦C applying a pressure of 5 MPa for 15 min. Fig. 1 shows the temperature–pressure–time schedule of the hot￾pressing cycle. A short holding time at maximum temperature and the application of the uniaxial pressure during cooling main￾tained until the beginning of the glass transformation range were found to be advantageous for achieving composites with very low porosity (1%). Table 2 presents an overview of the different composites fab￾ricated. The volume fraction of fibers of all composites was Table 2 Measured thermal expansion coefficients of the manufactured composites Condition of fiber surfaces α20, ..., 300 ◦C (ppm/K) Fiber concentration (vol%) Continuous fiber Short fiber Continuous fiber Short fiber NextelTM 440 desized 4.81 4.74 9 9 NextelTM 440, 40 nm BN coating 5.15 N/A 12 N/A NextelTM 440, 150 nm BN coating 4.97 4.50 12 6.5 ZenTronTM, desized 4.36 4.53 9 9 ZenTronTM, 70 nm SnO2 coating 4.30 4.18 5 5