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New ceramic fibre maintains physical properties at high temperatures.

It is intended as a reinforcement in high-performance composites with plastic, metal or ceramic matrices

A new ceramic fibre under development at Dow Corning Corp. shows unusually stable properties at high temperatures. Dow Corning HPZ ceramic fibre |1~ is a continuous, multifilament material prepared from hydridopolysilazane polymer by a pyrolytic process. The fibre is amorphous with a typical elemental composition of 58% silicon, 28% nitrogen, 10% carbon, and 4% oxygen. Figure 1 shows the amorphous structure of the fibre.

HPZ fibre is intended as a reinforcement in high-performance composites with plastic, ceramic, or metal matrices. It shows excellent tensile strength and modules, as well as high volume resistivity.

High-temperature performance

The uniqueness of HPZ fibre lies in its stability at high temperatures, which may be important in fibre reinforcement of ceramic matrix composites (CMC). The high strength and excellent toughness of CMCs are a result of the mechanical properties of the reinforcing fibres and weak interfacial bonding between the ceramic fibres and the matrix. In many composite systems, the reinforcing fibre can actually be a limitation because of thermal decomposition and loss of mechanical properties at high fabrication or use temperatures. Ceramics within the SiCo and SiNCO systems have been shown to degrade in inert environments at elevated temperatures. |2-7~ Above about 1000|degrees~C, equilibrium partial pressures of degradation byproducts become significant, the most important byproducts being carbon monoxide, nitrogen (when present), and silicon monoxide. |8,9~

When sufficient carbon is available, decomposition can then proceed until oxygen is depleted. The residue is primarily composed of SiC with excess carbon or silicon, depending on the original composition, and nucleation and growth of SiC crystallites are favored.

This combination of decomposition and crystal growth increases the porosity and graininess of most ceramic fibres, often resulting in reduced fibre properties. However, X-ray diffraction shows that the amorphous structure of HPZ fibre (SiNCO) is relatively unchanged after 65 hours at 1100|degrees~C and 24 hours at 1300|degrees~C, Fig. 2.

Amorphous SiCO and SiOCTi ceramic fibres show some growth of microcrystalline SiC after 65 hours at 1100|degrees~C and well-developed-SiC peaks (crystallite sizes 10|+ or -~2.5nm) with shoulders indicating smaller amounts of -SiC phases after 24 hours at 1300|degrees~C. The initial X-ray diffraction pattern for a crystalline aluminum borosilicate fibre, shows a mullite pattern with crystallite size 30|+ or -~5nm. Little change is apparent in this fibre after 65 hours at 1100|degrees~C, but some peak sharpening indicating an approximate doubling of crystallite size is apparent after 24 hours at 1300|degrees~C.

This increased decomposition and crystal and grain growth results in a general trend of decreasing strength as again conditions become more severe. Although SiCO and SiOCTi have high initial strength, they suffer a significant loss in strength after a 2-h, 1400|degrees~C argon (with trace oxygen levels) heat soak, Fig. 6. Aluminum borosilicate fibre has lower strength before aging, but retains about 80% of the initial value after heat soak. HPZ fibre retains about 50% of its initial tensile value, ending with tensile strength after heat soak similar to that of the aluminum borosilicate. Recent testing suggests that future HPZ products may approach 100% tensile retention after aging.

Under oxidizing conditions (100 h at 1000|degrees~C with flowing, wet air), SiCO and SiOCTi fibres lose strength in a similar manner to the findings in inert atmosphere, Fig. 7. HPZ fibre and aluminum borosilicate fibre show good retention of strength at these conditions.

The room-temperature electrical properties of the fibres after heat aging essentially reflect the changes in bulk chemistry occurring. Figures 8 and 9 show that the dielectric behavior of SiCO and SiOCTi fibres change significantly as the chemistry changes due to loss of CO. In sharp contrast, HPZ and aluminum borosilicate fibres are electrically very stable as a result of their more stable chemistry.

Although room temperature tests after heat treatment can be useful in judging the performance of a fibre, measurement of properties at high temperature is necessary to effectively design composites for high-temperature use. Workers at Pennsylvania State University have developed equipment and procedures for elevated-temperature and generally maintain strength up to about 1200|degrees~C in fast-fracture tests of this type. Aluminum borosilicate fibres show lower strength at room temperature and lose strength rapidly when tested at elevated temperatures, especially above 1000|degrees~C. Above 1200|degrees~C, SiOCTi fibre loses strength rapidly while only SiCO and HPZ fibre hold usable strength levels at 1400|degrees~C. Elevated-temperature modulus data, Fig. 11, generally parallel the strength behavior, although here the chemical stability of HPZ is more apparent.

A qualitative test of oxidative thermal stability has also been conducted. The fibre tow was suspended vertically with a weight placed on it to give a mild stress and subjected to a 1250|degrees~C flame in air. The time that the tow was able to support the weight in the flame was the measure of the fibre's oxidative stability. Initially, fibres were tested with a 12-g weight. The aluminum borosilicate fibre failed in under one second; HPZ, SiCO, and SiOCTi fibres survived flame exposure for over 15 minutes. These latter three fibres were then subjected to constant stress levels for more rigorous comparison. Figure 12 shows the performance results. HPZ ceramic fibre outperformed SiCO and SiOCTi fibres at both 0.25 and 0.40 ksi/filament stress.

These data are limited to direct fibre tests of small-diameter, textile-grade ceramic fibres generally considered to be the more desirable reinforcements for ceramic matrix composites. Although direct fibre tests are useful for initial comparison of fibre properties and material selection, such comparisons do not necessarily predict composite performance. Detailed studies of fibre-matrix compatibility and the critical interface region are necessary to determine performance of composites.

General physical properties

Current physical properties of the fibre are listed in Table 1. A range of combinations of fibre properties can be produced. Most commonly available are a low modulus grade (about 22Msi tensile modulus, 2.3 g/cc density).

Figure 13 shows the failure probability of HPZ versus fibre tensile strength. The relatively high value of the slope of the line indicates the good uniformity of the fibre.

HPZ ceramic fibre is supplied with various surface treatments designed for optimum TABULAR DATA OMITTED compatibility with various organic and inorganic matrix materials. The most commonly used sizing is polyvinyl alcohol (PVA), which has been found to facilitate further handling such as weaving or braiding. This sizing can be removed by washing with warm water.

HPZ fibre is available in developmental quantities as continuous yarn wound on 3-in.-diameter bobbins and in several styles of cloth. Two sizes of 500 and 1000 filaments are currently available in 22 Msi and 28 Msi tensile modulus versions. It is available in plain weave and 8-harness satin weave.

Safety and handling

Precautions normally associated with inorganic fibres of this type should be taken. The toxicological properties of fibre fragments or dusts produced during fibre (and composite) processing have not been thoroughly studied; hence, exposure that might result in inhalation of such fibres or dust should be eliminated by the use of appropriate ventilation or respirators. The fibres may be irritating to the skin; gloves are recommended. Safety glasses are also recommended.

Future research

Future research efforts at Dow Corning are directed toward modulus variations, high-temperature stability, surface treatments, increasing filaments per tow, and evaluation in composites.

References

1. HPZ fibre has been under study for several years as part of a DARPA/Air Force contract. (LeGrow, G.E., T.F. Lim, J. Lipowitz, and R.S. Reaoch, Am. Ceram. Soc. Bull., 66(2), 363-367 (1987).

2. Bhatt, R.T. and M.D. Kraitchman, NASA Tech Memo. 86891, USAAVSCOMM Tech Rpt. 85-c-4, April 1985.

3. Anderson, C.H. and R. Warren, Composites 15(1), 16-24 (1984).

4. Mah, T., et al., J. Materials Sci., 19, 1191-1201 (1984).

5. Simon, G., and A.R. Bunsell, J. Materials Sci., 19, 3649-3657 (1984).

6. Clark, T., et al., Ceramic Engineering and Science Proceedings, 6(7-8), 576-588 (1985).

7. Lipowitz, J., Ceramic Bulletin, 70(12), 1888-1894 (1991).

8. Luthra, K.L. J. Am. Ceramic Soc., 69(10), C-231-C-233 (1986).

9. Rao, Y.K., and H.G. Lee, Trans J. Br. Ceram Soc., 82, 123-128 (1983).

10. Pysher, D.J., K.C. Goretta, R.S. Hodder, and R.E. Tressler, J. American Ceramic Soc., 72, 284 (1989).
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Title Annotation:Dow Corning Canada Inc.
Author:Jones, R.E.; Rabe, J.A.; Peterson, A.L.
Publication:Canadian Chemical News
Date:Nov 1, 1992
Words:1402
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