The Naval Surface Warfare Center has developed processes for the preparation of mullite (3(Al2O3)(dot)2(SiO2)) whiskers and mullite-whisker felt. Three patents on the technology were issued in 1990. The processes are based on chemical reactions between AlF3, Al2O3, and SiO2. The felt is formed in-situ during the processing of shaped powdered precursors. It consists of randomly oriented whiskers which are mutually intergrown forming a rigid structure. The microstructure and properties of the felt and size of the whiskers can be modified by varying the amount of Al2O3 in the starting mixture. Loose mullite whiskers can be used as a reinforcement for polymer-, metal-, and ceramic-matrix composites. The felt can be used as preforms for fabricating composite materials as well as for thermal insulation and high temperature, chemically stable filters for liquids (melts) and gases

Haught, Deborah A.

Talmy, Inna G.

English

NASA Technical Reports Server

MULLITE WHISKERS AND MULLITE-WHISKER FELT 
Inna 6. Talmy and Deborah A. Waught 
Naval Surface Warfare Center, Dahlgren Division 
Silver Spring, MD 20903-5000 
ABSTRACT 
Naval Surface Warfare Center has developed processes for the preparation of mullite (3A120g 2Si0,) 
whiskers and mullite-whisker felt. Three patents on the technology were issued in 1990. The processes are 
based on chemical reactions between AlF3, A1203, and Si02. The felt is formed in-situ during processing sf 
shaped powdered precursors. It consists of randomly oriented whiskers which are mutually intergrow= forming 
a rigid structure. The microstructure and properties of the felt and size of the whiskers can be modsed by 
varying the amount of A1203 in the starting mixture. Loose mullite whiskers can be used as a reinforcement for 
polymer-, metal-, and ceramic-matrix composites. The felt can be used as preforms for fabricating composite 
materials as well as for thermal insulation and high temperature chemically stable filters for liquids (melts) and 
gases. 
INTRODUCTION 
The current interest in the development of refractory-oxide fibers and whiskers is spurred by the 
increasing demand for high-temperature structural materials for use in an oxidizing atmosphere. As a reinforcing 
material, whiskers are preferable to fibers because they are single crystals and their properties are not affected 
by grain growth and grain boundary-induced creep at high temperatures. The low free energy and high rnodu%aas 
and strength of whiskers compared to polycrystalline materials make it possible to use whiskers to reinforce 
matrices of the same composition. 
Currently, only non-oxide whiskers such as Sic  and Si3N4 are commercially available. However, they 
cannot be used in an oxidizing atmosphere at temperatures above 100CPC. Additionally, SiC whiskers are 
electrically conductive and not suitable for use in dielectric ceramics. 
Mullite (3A120f 2Si02) is a promising candidate for whiskers because of its excellent chemical stability, 
low thermal expansion, and good high-temperature strength and creep resistance. Since mullite has a relatively 
low dielectric constant and loss tangent, the whiskers may be suitable for toughening materials for dielectric 
applications (such as radomes). 
Both loose whiskers (or fibers) and fibrous preforms can be used as reinforcement in the fabrication 
of composites. The use of fibrous preforms instead of loose whiskers (or fibers) excluded problems of their 
deagglomeration and uniform distribution in matrix materials. Matrices can be infiltrated into the preforms by 
vacuum impregnation, sol-gel processing, chemical vapor infdtration and melt infiltration techniqules. 
PREPARATION OF MULLITE WHISKERS AND FELT 
The NAVSWC method for preparation of both mullite whiskers and felt is based on the fo"ollo~ng two 
reactions1": 
2 4 F 3  + B i o 2  -------------- > A12(Si04)F2 + SiF4 (g) 
Topaz 
(1) 
6A12(Si04)F2 + Si02 --------- > 2(3Al2o3 2Si02) + 3SiF4 (g) 
Mullite 
(2) 
https://ntrs.nasa.gov/search.jsp?R=19930016397 2020-03-24T07:25:43+00:00Z
In reaction (I), AlF3 and Si02 are heated at 700 - 905' C to form topaz as an intermediate product and 
in reaction (2), topaz is thermally decomposed at 1250 - 1400' C to yield mullite. The whiskers are grown as a 
result of a vapor-phase chemical reaction. Heating in a SiF4 atmosphere is necessary for the preparation of 
high-qualily whisker product. Up to 75% A1203 can be substituted for AIF3 in the starting materials. In this 
case, the mullite whiskers form as a result of chemical interactions involving all of the starting components, as 
well as the gaseous SiF4 generated in reactions (1) and (2) - assuming that corresponding portions of AlF3 
required for the process are produced by a reaction between SiF4 and A1203. 
The whisker preparation process involves mixing the raw powders and firing the loose powders in closed 
Si3-containing system. Mullite whiskers prepared at 1250" C in optimum SiF4 atmosphere are shown in Figure 
I. As produced, they form loose aggregates which are easily separated. The whiskers of rectangular cross 
section have a narrow size distribution with average aspect ratio of 30. 
The felt preparation process involves mixing and shaping (by any conventional method) the powdered 
precursors (A&, Si02 and optional A1203), followed by firing at 1250 - 1400" C. A significant advantage of this 
process is that there are no whiskers in the precursor mixture. The whiskers are formed inside the product 
during firing. Because loose respirable whiskers are not handled at any step of the process, health hazards 
associated with handling whiskers are eliminated. The final felt product is about 80% porous with low 
dimensional changes (about 1% expansion) compared to the green shapes which is a valuable feature for the 
preparation of near-net shape composite preforms. Figure 2 shows the microstructure of the felt consisting of 
randomly oriented (in 3 dimensions) and uniformly distributed individual mullite whiskers and spherulites which 
are mutually intergrown or mechanically interlocked. With a bending strength (3 point) of about 3 MPa (427 
psi), the felt is rigid and can be used in composite processing utilizing various impregnation techniques. 
TO decrease the amount of relatively expensive AlF3 and accordingly decrease the amount of hazardous 
Si% generated by the reactions, and possibly modify the size of the whiskers, A1203 (avg. particle size 0.05 p m) 
was substituted for AlF3 in amounts up to 75 mole %. The whisker size gradually decreases with increasing 
Al,O, content up to 75% A1203 for both loose whiskers and felt (Figure 3). In the felt, the number of 
spherulites also gradually decrease with increasing A1203 content. The linear expansion and porosity of the felt 
slightly decrease but the bending strength significantly increases with more than 25% A1203 substitution (Table 
1.). The almost two-fold increase in strength with 75% A1203 substitution can be attributed to much smaller 
whislcers in the felt structure. The felt has pores of 1 to 30 pm depending on the A1F3\Al2O3 ratio with very 
narrow size distribution (Figure 4). 
Table 1. - Properties of Mullite-Whisker Felt With Substitutions of A1203 for AlF3 
APPLICATIONS OF MULLITE WHISKERS 
Muglite whiskers have the potential to be used as reinforcement in a variety of polymer, ceramic and 
metal matrix composites. Unfortunately, the full potential of these whiskers as a reinforcement has not been 
completely exploited. Mullite whiskers were successfully used as reinforcement for celsian (Ba0.A120,~EPS10,) 
and fused silica (SO2) composites6. The loose whiskers were uniformly mixed with the ceramic powders and 
then hot pressed to full density. The fracture toughness of fused silica was doubled by the addition of 20 vol% 
mullite whiskers with a cross section of 4-12 pm. For the celsian matrix, 20 vol% 12-26 p m whisker additions 
increased the toughness by 40%. Properties of the composites are shown in Table 2. 
Table 2. Properties of Mullite Whisker Composites 
Mullite Whisker Flexural Strength 
* Short Chevron Notch Beam Method 
APPLICATIONS OF MULLITE-WISKER FELT 
The mullite-whisker felt can be used as a preform for the preparation of polymer-, metal-, and ceramic- 
matrix composites or by itself as thermal insulation or high temperature filters for liquids or gases. It can be 
infiltrated with a variety of matrices using various processing techniques such as vacuum impregnation, chemical 
vapor infiltration (CVI), and melt (glass or metal), sol-gel or pre-ceramic polymer infiltration to produce 
composites. Polymers with viscosities of up to 10,000 cp have been used to infiltrate the felt. Researchers at 
Ceramatec have demonstrated that mullite whisker composites can be fabricated using sol-gel techniques7. AAea 
8 infiltration cycles, the density of the sample was 2.06 g/cm3 with 17% porosity. The process is being optimized 
to yield composites with densities greater than 95% dense. 
A forced flow thermal gradient chemical vapor infiltration technique developed at Oak Ridge National 
Laboratory was used to infiltrate the felt with a S i c  matrix. Since the deposition parameters used for the 
infiltration experiment were optimized for a Nicalon preform,the felt was only infiltrated to about 85% density 
with variable densities through the thickness of the sample. The properties of the produced composites are given 
in Table 3. 
Fully dense mullite-whisker felt composites were prepared by a vacuum and isostatic pressure-assisted 
infiltration technique using cast aluminum alloy ~ 3 5 6 ~ .  The alloy was preheated to 681P C and the felt was 
preheated to 551P C. A 800 psi pressure of nitrogen was required to fully infiltrate the preform. Due to the Back 
of bonding between A356 A1 and mullite, the mechanical properties of the composite did not reflect the 
reinforcing potential of the whiskers. There was little transfer of the load from the matrix to the mullite 
whiskers. It will be necessary to increase the affinity of the matrix towards the reinforcement. Many opeions 
exist in matrix selection; addition of a small percentage of Li or Mg to the matrix may be sufficient, or 
contemporary Al-Li alloys such as 2090, 8090, 2091 etc. may be useful in enhancing bonding at the 
whisker/matrix interface without degrading the strength of mullite. 
Table 3. Properties of SIC Infiltrated Mullite-Whisker Felt 
* Single Edge Notch Beam Method 
The felt was also evaluated for possible use as a filter in coal gasification processing. Conversion of coal 
Lo a gaseous fuel requires filtering of hazardous impurities from final gases to meet environmental and turbine 
eq ipment  requirements in advanced coal-fueled power generation systems. The criteria for successful use and 
operation of porous filters requires not only thermal, chemical, and mechanical stability of the material, but also 
long-term structural durability and high reliability. Preliminary results indicate that the mullite-whisker felt is 
a very promising candidate. It showed good chemical stability in the tested gas atmosphere and had very high 
collection efficiency. 
SUMMARY 
Processes for the preparation of mullite whiskers and rigid mullite-whisker felt have been developed and 
patented. The 80% porous felt consists of randomly oriented mullite whiskers mutually intergrown, forming a 
rigid structure. The felt is formed in-situ during processing shaped powder precursors; thereby eliminating the 
health hazards associated with handling loose whiskers. Both the mullite whiskers and mullite-whisker felt are 
promising new reinforcements for polymer-, metal- and ceramic-matrix composites. Additionally, the felt can 
be used by itself as thermal insulation or high temperature chemically stable filters for liquids (melts) and gases 
such as for coal gasification. 
The authors would like to acknowledge the assistance of Dr. M. Norr in the SEM studies. We would 
also like to thank Rick Lowden of Oak Ridge National Laboratory for infiltrating the felt using his CVI process 
and characterizing the composites. This work was funded by the Office of Naval Technology through the 
Weapons and Spacecraft Materials Block Program headed by Dr. W.T. Messick. 
REFERENCES 
1. Talrny, I. and Haught, D., "Preparation of Mullite Whiskers", U.S. Patent No. 4911902, 27 
March 1990. 
2. Talmy, I. and Haught, D., "Preparation of Mullite Whiskers From AIF,, SO2,  and A1203 
Powders," U.S. Patent No. 4910172, 20 March 1990. 
3. Talmy, I. and Haught, D., "Rigid Mullite Whisker Felt and Method of Preparation", U.S. Patent 
No. 4948766, 14 August 1990. 
4. I. Talmy and D. Haught, "Preparation of Mullite Whiskers", Proceedirtgs of Fiber-Ta 87 
Corzferertce, NASA Conference Publication 3001, p. 69-78 (1987). 
5. I. Talmy and D. Haught, "Preparation and Properties of Rigid Mullite-Whisker Felt", 
Proceedirtgs of the 12th Conference on Composite Materials and Structures, NASA Conference 
Publication 3018, p. 1-11 (1988). 
6. C.L. Conner, "Mullite Whisker Composite Fabrication", NAVSWC TR 90-344 (12 August 1990). 
7. I.G. Talmy, DA. Haught, and S. Limaye, "Porous Mullite Whisker Felt for Preshaped 
Composite Preform", Materials and Processing Report, Vol. 4, No. 7, 3 (1989). 
8. D. Haught, I. Talmy, D. Divecha and S. Karmarkar, "Mullite Whisker Felt and Its Application 
in Composites", Materials Science and Engineering, A144, p. 207-214 (1991). 
FIGURE 1. SCANNING ELECTRON MICROGRAPHS OF MULLITE WHISKERS 

FIGURE 3. SCANNING ELECTRON MICROGRAPHS OF MULLITE WHISKER FELT WITH 
SUBSTITUTION OF (A) 0%, (B) 25%, (C)  50%, AND (D) 75% A1203 FOR AIF3 



Mullite Whiskers and Mullite-whisker Felt

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