Properties of some oxynitride glass fibers

] O U R N A L OF

Journal of Non-CrystallineSolids 184 (1995) 133-136

ELSEVIER

Properties of some oxynitride glass fibers H. Kaplan-Diedrich, G.H. Frischat * Institut fi~r Nichtmetallische Werkstoffe, Technische Universitfit Clausthal, Zehntnerstr. 2 a, 38678 Clausthal-Zellerfeld, Germany

Abstract

Using a hole-bushing drawing process, oxynitride glass fibers could be produced successfully in three different compositions. Although this method has resulted thus far in fibers of non-constant diameters only, several physical and chemical properties could be determined. Fibers of the glass system MgO-A1203-Y203-SiO2 with an N content of 15.8 at.% partly displayed tensile strengths up to 13 GPa (20 txm diameter fibers), a Young's modulus of 200 GPa and an extremely high chemical resistance against 1N NaOH solution. Despite the scatter in the reported data, these oxynitride fibers seem to be the most stable glass fibers investigated thus far.

1. I n t r o d u c t i o n

The incorporation of nitrogen into silicate glasses results in improved mechanical and chemical stability [1-5]. These improvements are mainly related to the higher packing density of the oxynitride glasses and the less polarized S i - N bond in comparison with the S i - O bond. It has also been shown that oxynitride glass fibers display advantages over pure oxide glass fibers [6-8]. These fibers were drawn from the melt, but a gel ammonolysis method was also used [9]. Some of the mechanical properties of these fibers reported are very promising. For example, values of Y o u n g ' s modulus, E, of 115-170 GPa were claimed for the different compositions. Although this highest E value of 170 GPa for a C a - M g - S i - A I - O - N composition [8] was questioned [10], an E value of 140 GPa seems to be real for Y - S i - A 1 - O - N fibers [6]. Compared with commercial ' E ' glass fibers,

* Corresponding author. Tel: +49-5323 72 2463. Telefax: +49-5323 72 3119.

where Young's moduli of around 73 GPa are obtained, some oxynitride glass compositions may be developed into high-modulus fibers. Recently, bulk oxynitride glasses of the system M g O - A 1 2 0 3 - S i O 2 were prepared [11]. They proved to be very stable alkali-resistant materials and showed Y o u n g ' s moduli of up to = 130 GPa. Starting from this base glass system, three different fiber compositions were developed, and it could be shown that high-modulus glass fibers with E values of around 200 GPa may be obtained. However, fiberization of these compositions needs special attention.

2. E x p e r i m e n t a l

2.1. Glass and fiber preparation

Table 1 gives the compositions of the glasses prepared. Glass N 21 has a composition identical to the one used in Ref. [11], whereas glass NY 27 was derived from N 21 by substitution of Y203 for AI203. Glass NY 160 is a composition already

0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0022-3093(94)00605-9

134

H. Kaplan-Diedrich, G.H. Frischat/Journal of Non-Crystalline Solids 184 (1995) 133-136

chosen by Messier et al. [6]. However, because of its high N content it could not be fiberized successfully by those authors. The raw materials, M g O (reagent grade) and A120 3 (water-free) from Merck, Darmstadt, A l N (powder, 99%) and Si3N 4 (powder) from Ventron, Karlsruhe, Y203 ( 0 . 8 - 3 . 0 Ixm) from Starck, Goslar, and SiO 2 (powdered rock crystal, < 100 ~ m ) from Heraeus, Hanau, were mechanically mixed carefully in acetone, dried and pressed into cylinders using a 160 kN load. These were then immersed into the graphite crucible of the fiber drawing apparatus. The heating rate (inductive heating) was 40 K / m i n , the melting temperature 1600°C for a holding time of 15 min. After that, the melts were cooled to the respective fiberization temperatures. For the drawing, a single-hole bushing process was used (see Ref. [12] for details). Since oxynitride glass melts have a surface tension almost twice as high as that of oxide glass melts ( 5 0 0 - 6 0 0 N mm 1 compared with 3 0 0 330 N m m -~ for melts of ' E ' glass composition), the drawing process turned out to be difficult. Although different alternatives were tried and a broad choice of drawing parameters was used, the melt stream oscillated to some extent after leaving the orifice, and this resulted in non-constant fiber diameters [12]. 2.2. Property measurements

The bulk oxynitride glasses were analyzed for Mg, Al, Y, and Si by the X-ray fluorescence method

(PW 1410, Philips, Kassel), for N by the Kjeldahl method, accuracy 5%, and were checked for a C content (Coulomat 702, StrOhlein, D i i s s e l d o r f / Kaarst), accuracy + 0.003 wt%. The density, p, was determined by the pycnometer method, accuracy + 0 . 0 0 1 g cm -3. Measurements of the glass transition temperature, Tg, were conducted with a dilatometer (402 ES, Netzsch, Selb), accuracy + 5 K. The elastic moduli E, G and /~, respectively, were examined using an ultrasonic technique (USiP11, Krautkr~imer, KOln), accuracy + 5 GPa, + 3 GPa, and + 0.01, respectively. The diameters of the fibers were determined by means of an optical microscope (Axiophot, Zeiss, Oberkochen), accuracy 8.3% for a diameter of 30 p~m. The tensile strength was measured by glueing the fibers (2 cm length) into A1 frames (strength proof tester R S A 10, Schenck, Trebel). The loading rate was 0.1 m m s -1. The relative error, including the maximum error of the fiber diameter, was 19%. The E modulus of the fibers was determined using a bending method described in Refs. [13,14]. Fibers of > 30 b m diameter and 1 0 - 1 3 cm length were fixed at one end and their bending according to the own weight was measured with an accuracy of 0.25 mm. The reproducibility of this method is very good, thus three independent measurements using the same fibers differed by only 1%. The G modulus of the fibers was obtained by a torsion pendulum method, using 10 cm long fibers. This method proved to be inaccurate and an error of about 35% has to be tolerated. The Poisson ratio, /~, was calculated from

Table 1 Glass compositions (synthesis, mol %) and some properties of bulk and fiber glasses Glass

MgO

Al203

AI2N 2

Y203

SiO2

SiN4/3

N conp (g cm -3) Tg E G /x tent(wt%) (°C) (GPa) (GPa)

N 21 40 NY 27 40 NY 160 -

10 7.6

5 7.5 17

7.5 25.1

41.25 41.25 50.30

3.75 3.75 -

3.69 4.27 4.37

Glass fibers

G (GPa) ~

Am/m

SiL/Si o (mgg-l)

AIL/AIo (mgg-1)

E (GPa)

(mgg 1)

N21 160±23 57±16 0.43_+0.09 47 ± 3 38±6 27 ± 2 NY27 207± 4 72± 4 0.46±0.05 4.5± 0.5 21±7.5 6.5±0.3 NY 160 213 ± 24 73 ± 14 0.46 ± 0.02 56 ± 10 a 29 ± 2.5 a 12 ± 3 a a Measured on glass powder.

2.882 3.311 3.941

820 116 826 189 940 136

43 71 51

0.33 0.33 0.33

H. Kaplan-Diedrich, G.H. Frischat /Journal of Non-Crystalline Solids 184 (1995) 133-136

the E and G values obtained by /x = ( E / 2 G ) - 1. The error in /x includes the errors in the quantities E and G. The chemical stability of the oxynitride glass fibers was determined (in the case of composition NY 160 grained glass was used, diameter 315-500 p~m), using experiments similar to those described in Ref. [11]. I N NaOH was used with a ratio fiber (powder)/solution = 1/100. The testing temperature was 60°C with a time of 28 days. The total weight loss, Am, was determined by weighing the dried samples, and the contents of leached Si and Al were measured from the solution by atomic absorption spectrometry (AAS) (SP 2000, Pye Unicam, Kassel). The accuracy of the measurements was _ 0.5 mg of S i / L , and +0.1 mg of A l / L . Am/m is given in mg g - l , and SiL/Si 6 and A l L / A l e are given as ratios of the amounts of the elements in the solution, related to the contents of the respective elements in the glass. Corroded fibers were also inspected by scanning electron microscopy (SEM) (JSM-U3, Jeol, Japan).

3. Results

Chemical analysis indicated that the oxide and nitrogen contents did not differ significantly from the calculated values. The carbon content of glass N 21 was 0.03 wt%, that of NY 27 was 0.02% and that of NY 160 was 0.05%. These contents do not influence the glass properties. However, since the carbon was found to be present as agglomerates, the particles may influence the fiber strength. Table 1 gives the densities, glass transition temperatures and the elastic moduli of the bulk glasses. The fibers produced had diameters varying between about 20 and 100 Ixm, with values from 40 to 70 p,m as the most probable ones. The tensile strengths of these fibers showed two regimes: that of lower strength displayed values between about 0.5 and 2 GPa, whereas fibers in the high-strength regime showed maximum tensile strengths up to 9 GPa (N 21), 13 GPa (NY 27), and 15 GPa (NY 160). It could be clearly shown that the thinner the fibers are, the more of them lie within the high-strength regime. Thus, nearly all fibers with a diameter ~ 20 Ixm showed high strength whereas, with increasing diameter, high-strength fibers become much less probable. Of course, fibers of con-

135

stant diameters were used for the strength measurements only, and the same was true for the other property determinations. Table 1 also contains the experimentally obtained values E, G and /x of the fibers as averages of different measurements. Finally Table 1 also gives the chemical stabilities of the fibers (powder in the case of glass NY 160) against 1N NaOH.

4. Discussion

Commercial ' E ' glass fibers of 20 Ixm diameter show a tensile strength of 3.5 GPa [15]. The lowstrength fibers produced in this work clearly have lower strengths; however, this is in line with the findings of Messier and Patel [10], who also reported low strength values for their oxynitride glass fibers. Carbon agglomerates, colloidal silicon droplets, bubbles, crystals and other defects may cause an early breaking of the fibers. Several of these flaws could be visualized by scanning electron microscopy (SEM) on the fiber surfaces. Messier and Patel [10] and Murakami and Sakka [16] reported theoretical strength values of 20-40 GPa for oxynitride glasses. Although our high-strength fibers still do not reach these values, the data reported here are by far the highest ever published for glass fibers. Obviously the surfaces of these (20 p~m diameter) fibers are practically free of any strength-reducing flaws. Likewise, the Young's moduli of our glass fibers are very high compared with those of existing data [6,8,10]. They clearly exceed the E values of the compact glasses and are in line with commercial SiC fibers (180 GPa) or PAN-based carbon fibers (240 GPa) [10]. The G moduli also display high values, and (apparent) Poisson's ratios o f / z = 0.45 may possibly be interpreted as evidence for a preferred orientation of the fibers [17]. The chemical stability tests show that glass fiber NY 27 is an extremely alkali-resistant material, which is very promising. SEM investigations of the corroded fiber surfaces display a crystalline layer (as in the case of Ref. [11] of hydrotalcite, Mg6AIzCO3(OH)16- 16H20, composition), which may act as a protecting diffusion barrier against further attack. The thickness of the layer is in the Ixm range.

136

H. Kaplan-Diedrich, G.H. Frischat /Journal of Non-Crystalline Solids 184 (1995) 133-136

5. Conclusions In this w o r k three different types o f oxynitride glass fiber w e r e drawn. Partly they s h o w e d extremely high strengths, high elastic moduli, and w e r e v e r y alkali-resistant. H o w e v e r , scatter in the data is still high, o b v i o u s l y due to difficulties during the preparation process. Therefore, the results reported in this publication should be taken as preliminary, but indicate the potential for fabricating high-perform a n c e fibers. The authors gratefully a c k n o w l e d g e the financial support g i v e n by the D e u t s c h e F o r s c h u n g s g e m e i n schaft ( D F G ) , B o n n - B a d Godesberg.

References [1] H.O. Mulfinger, J. Am. Ceram. Soc. 49 (1966) 462. [2] G.H. Frischat and C. Schrimpf, J. Am. Ceram. Soc. 63 (1980) 714. [3] R.E. Loehman, J. Non-Cryst. Solids 56 (1983) 123.

[4] C. Schrimpf and G.H. Frischat, Glastechn. Ber. 57 (1984) 97. [5] G.H. Frischat and K. Sebastian, J. Am. Ceram. Soc. 68 (1985) C-305. [6] D.R. Messier, R.P. Gleisner and R.E. Rich, J. Am. Ceram. Soc. 72 (1989) 2183. [7] P.J. Patel, D.R. Messier and R.E. Rich, in: Technology 2001 (NASA Conf. Publ. 3136, Vol. 2, 1991) p. 258. [8] H. Minakuchi, H. Osafune, K. Kada, K. Kanamura and H. Fujii, in: Science and Technology of New Glasses, Proc. Int. Conf. on New Glasses, ed. S. Sakka and N. Soga (Ceramic Society of Japan, Tokyo, 1991) p. 329. [9] M. Sekine, S. Katayama and M. Mitomo, J. Non-Cryst. Solids 134 (1991) 199. [10] D.R. Messier and P.J. Patel, J. Non-Cryst. Solids 182 (1995) 271. [11] B. Steffestun and G.H. Frischat, J. Am. Ceram. Soc. 76 (1993) 699. [12] H. Kaplan-Diedrich, PhD dissertation, Technische Universit~it Clausthal (1993). [13] V.A. Ryabov and D.V. Fedoseev, Glass Technol. 11 (1970) 36. [14] B.J. Norman and D.R. Oakley, Glass Technol. 11 (1970) 45. [15] G. P~ihler and R. Briickner, Glastechn. Ber. 54 (1981) 52. [16] M. Murakami and S. Sakka, J. Non-Cryst. Solids 101 (1988) 271. [17] G. P~ihler and R. Briickner, Glastechn. Ber. 58 (1985) 33, 45.