Research at the Army Materials Technology Laboratory (AMTL) and elsewhere has shown that many glass properties including elastic modulus, hardness, and corrosion resistance are improved markedly by the substitution of nitrogen for oxygen in the glass structure. Oxynitride glasses, therefore, offer exciting opportunities for making high modulus, high strength fibers. Processes for making oxynitride glasses and fibers of glass compositions similar to commercial oxide glasses, but with considerable enhanced properties, are discussed. We have made glasses with elastic moduli as high as 140 GPa and fibers with moduli of 120 GPa and tensile strengths up to 2900 MPa. AMTL holds a U.S. patent on oxynitride glass fibers, and this presentation discusses a unique process for drawing small diameter oxynitride glass fibers at high drawing rates. Fibers are drawn through a nozzle from molten glass in a molybdenum crucible at 1550 C. The crucible is situated in a furnace chamber in flowing nitrogen, and the fiber is wound in air outside of the chamber, making the process straightforward and commercially feasible. Strengths were considerably improved by improving glass quality to minimize internal defects. Though the fiber strengths were comparable with oxide fibers, work is currently in progress to further improve the elastic modulus and strength of fibers. The high elastic modulus of oxynitride glasses indicate their potential for making fibers with tensile strengths surpassing any oxide glass fibers, and we hope to realize that potential in the near future

Messier, Donald R.

Patel, Parimal J.

Rich, R. E.

English

NASA Technical Reports Server

N9 2- 22 7 06
OXYNITRIDE GLASS FIBERS
Parimai J. Patel, Donald R. Messier and R.E. Rich
Research Ceramic Engineer
ATI"N: SLCMT-EMC
Army Laboratory Command
Materials Technology Laboratory
Watertown, Massachusetts 02171-0001
ABSTRACT
Research at the Army Materials Technology Laboratory (AMTL) and elsewhere has shown that many
glass properties including elastic modulus, hardness, and corrosion resistance are improved markedly by the
substitution of nitrogen for oxygen in the glass structure. Oxynitride glasses therefore offer exciting opportunities
for making high modulus, high strength glass fibers. Discussed in this paper are processes for making oxynitride
glasses and fibers of glass compositions similar to commercial oxide glasses, but with considerably enhanced
properties. We have made glasses with elastic moduli as high as 140 GPa (20 Msi) and fibers with moduli of
120 GPa (17 Msi) and tensile strengths up to 2900 MPa (420 ksi). AMTL holds a U.S. patent on oxynitride glass
fibers, and this presentation discusses a unique process for drawing for drawing small diameter (10 urn)
oxynitride glass fibers at high drawing rates (1500 m/min). Fibers are drawn through a nozzle from molten glass
in a molybdenum crucible at 1550°C. The crucible is situated in a furnace chamber in flowing nitrogen, and the
fiber is wound in air outside of the chamber, making the process straightforward and commercially feasible.
Strengths have been considerably improved by improving glass quality to minimize internal defects. Though the
fiber strengths have been comparable with oxide fibers, work is currently in progress to further improve the
elastic modulus and strength of the fibers. The high elastic modulus of oxynitride glasses indicate their potential
for making fibers with tensile strengths surpassing any oxide glass fibers, and we hope to realize that potential
in the near future.
INTRODUCTION
The incorporation of nitrogen into an oxide glass network has been shown to improve several
properties of the glass. Increasing nitrogen content leads to increases in glass transition temperature,
viscosity, density, hardness, corrosion resistance, and elastic modulus. 1 For applications of high-performance
resin matrix composites, the elastic modulus and strength need to be as high as possible and the density as
low as possible. Presently, the continuous fibers used for advanced composites have been s-glass and
e-glass fibers. S-glass fibers are from the Mg-Si-A1-O system while e-glass fibers have a Ca-Si-A1-O system.
Work done at the U.S. Army Materials Technology Laboratory has focused on the processing of bulk
oxynitride glass that are analogous to the s-glass composition for the purpose of drawing the glass into small
diameter, high strength, high elastic modulus fibers for use in high-performance resin matrix composites.
Bulk glasses have been produced with elastic modulus values of up 140 GPa. Earlier research at MTL as
shown that the fibers retain all the properties of the bulk glass.
The oxynitride glass fiber program first focused on oxynitride glasses that had improved properties
over oxide glasses. Once the bulk glasses were produced, they were attempted to be drawn into fibers. The
criteria for the glass was to have high modulus, low density, and be fiberized easily. The third factor is the
crucial one. Bulk oxynitride glasses with superior properties have been produced at M.T.L. However,
drawing thin glass fibers from these compositions has not always been easy. It is this qualifying factor that
determines which compositions can be studied. After further investigation,two glass compositions were
chosen for examination. Both had compositions similar to commercial s-glass. However, one had 2.72
atomic percent nitrogen and the other had 4.13 atomic percent nitrogen. These fibers had moduli that
were greater than the oxide glass fibers. High modulus is the most important criteria for composites and it
is in this aspect that the oxynitride glass fibers are superior. However, the tensile strengths of the oxynitride
glass fibers were too low, especially when considering their potential. The effort was then put through to
258
https://ntrs.nasa.gov/search.jsp?R=19920013463 2020-03-17T11:41:12+00:00Z
improve the tensile strength of the fibers. The following paper describes that process and the
accomplishments made in the fiber properties.
PROCEDURE
Glass."
Batches were prepared using the compositions listed in Table I. To optimize the processing of the
glasses, two base compositions were used so that comparison would be possible. As shown, one
composition, NSS, was a MgO-AI203-SiO2-Si3N 4 composition. This glass contains 2.72 mole % nitrogen
incorporated into the glass network. The other glass composition, NS 7.5, had 4.13 mole % nitrogen of the
same batch materials. These two compositions were used for their good drawing characteristics thus
allowing the optimization of the processing of high strength fibers.
Table I: Oxynitride Glass Base Composition
Species
MgO
A1203
SiO2
Si3N 4
NS8 (2.71 at. % N)
Wt. Percent
10.13
25.34
59.79
4.74
NS7.5 (4.13 at. % N)
Wt. Percent
10.20
25.51
57.13
7.16
Figure I outlines the process that has been developed for the oxynitride glasses. The oxide
constituent of the glasses (MgO-AI203-SiO2) were dry mixed for six hours. Wet mixing with acetone was
found undesirable due to iron contamination of the acetone. The oxides were melted in air at 1600°C for
two hours and quenched in water. The oxide glass was then mixed with silicon nitride (Union Carbide #
1929) and placed in a molybdenum foil crucible. The crucible was placed into a alumina crucible and
loaded into a water-cooled, tungsten-mesh resistance furnace. The sample was melted at 1650°C in nitrogen
at a dwell time of 2 hours and a subsequent furnace quench of 50°C per minute. The sample was removed,
crushed, and remelted at the identical thermal cycle used for the first melt. At this point, the glass was
ready to be characterized and drawn into fibers.
259
Figure h Oxynitride Glass Fabrication Process
atch Oxide Constituents I
IMelt in nitrogen, 1650°C I
ICrush and Remelt[
Fibers
Once the glass was produced, it was ready to be drawn into fibers. The fiber drawing process used
is very similar to commerdal processing of continuous glass fibers. Figure II is a schematic representation
of the fiber drawing process. The glass is placed in a molybdenum single nozzle crucible. The furnace
assembly is located in a furnace chamber with flowing nitrogen. After ample time for the nitrogen flow to
fill the tube the furnace is inductively heated to 1500°C- 1600°C, depending on composition. Once the
temperature has stabilized, a nut is removed from the bottom fixture of the silica glass tube. A thin alumina
rod is then fed through to the bushing where a glob of glass has accumulated at the nozzle. The glass
adheres to the cooler alumina rod allowing the glass to thin out into a fiber as the alumina rod is pulled
through the bottom of the hole. The fiber is broken off the rod and pulled manually to the winder where it
is taped on. As one can see from Figure II, only the glass is exposed to the nitrogen atmosphere. The fiber
is pulled in air. This design allows the process to be straightforward and commercially feasible. The winder
motor speed is increased until it reaches a speed of 2400 r.p.m, or a draw rate of 1500 meters per minute.
The fiber diameter, at this speed, is roughly between 8 and 12 microns. The only limitation for the fiber
diameter is the drum winder speed. Presently, 2400 r.p.m, is the maximum speed of the winder. At this
point, the fiber can be drawn continuously until the glass supply has diminished. However, for the purpose
of characterization of the fibers, the draw is interrupted several times for sampling.
260
Figure II: Schematic Representation of Oxynitride
Glass Fiber-Drawing System
_TO CONTROLLER
MOLYBDENUM
CRUCIBLE ©
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ZIRCONIA ,.
INSULATION .t,
N_OUT_
TWO-COLOR PYROMETER
QUARTZ WINDOW
I
FURNACE ASSEMBLY
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0 4--- INDUCTION COIL
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FIBER
TAKE UP REEL
261
Characterization
Prior to the fiber draw, the bulk glass was characterized using optical techniques. The glass was observed
through a microscope with transmitted fight to determine the presence of any crystalline material or
unmelted batch. If the glass was determined to be of good quality, the glass would drawn into fibers. The
density of the glass was also calculated using Archimede's method. Microhardness was also measured on
polished sections using a Knoop indenter and a .98 N load.
During the draw of the fibers, the fiber is broken several times for testing. Sampling is a crucial
step in the characterization of the glass fibers. The fibers that are drawn do not have any sizing or any
protective coating on them due to the experimental nature of the work. They are thus easily damaged and
great care must be taken in their handling. The small diameters of the fibers further increased the difficulty
in handling. The fibers were pulled off the line of the draw once the proper draw speed had been attained.
The fibers were taped onto an I-shaped holder. A tape of seven fiber mounts with a gage length of 30 mm
were placed behind the fiber. The fiber was taped to the mounts leaving the fiber within the slot untouched.
Once all the fibers have been mounted, they were cemented at ends of the cut out slots. After the cement
had dried, the mounts were cut off the tape and broken in tension. The fibers were broken at a crosshead
speed of 1 mm per minute. The diameter of the fibers and the force required to break the fibers were
recorded for future calculation of the tensile strength. Once the fibers were broken, if the fiber did not
shatter, the fracture surface was studied using a S.E.M. Surface fracture analysis was performed on all fiber
draws to determine the fracture origin of the fiber. In addition to the fiber tensile strength, the elastic
modulus of the fibers was also determined via a sonic modulus instrument that measured the speed of
sound through the fibers.
RESULTS
As cited before, oxynitride glasses show considerable potential for advance composites. There is a
marked improvement in the properties over oxide glass fiber counterparts. However, the tensile strength was
relatively weak. One focus of work at the Materials Technology Laboratory was to produce the highest
modulus, highest strength, lowest density oxynitride glass fibers possible. Using the procedure described
earlier, great strides have been made in attaining that goal.
One of the important properties of the oxynitride glasses is the high elastic modulus, an important
property necessary for advanced structural resin matrix composites. For aerospace application, the fibers
also need to be of the lowest possible density. The modulus of the commercial e-glass with a density of
2570 kg/m 3 is 73 GPa. 2 The density and modulus of s-glass is 2480 kg/m 3 and 85 GPa3, respectively. An
NS8 glass has been found to have a modulus of 105 GPa with a density of 2500 kg/m 3. The NS7.5 glass
,with more nitrogen, has a modulus of 112 GPa. Further examination into additives has led to glasses with
modulus values of 120 GPa at a density of 2700 kg/m 3. Due to the the importance of density, one
convention of looking at the modulus is the Elastic Modulus to Density Ratio (E/p). As seen on Figure III,
the glasses made at M.T.L. have shown an improvement over conventional oxide glasses. The e-glass and
s-glass had ratios of 28 m-6 and 34 m-6 respectively. The oxynitride glass fibers displayed higher ratios. The
NS 7.5 has a modulus to density ratio of 46 m-6. NS8-G has an elastic modulus of modulus/density ratio of
43 m-6 compared to 28 m-6 of e glass and 34 m-6 of s glass. This glass also had tensile strengths comparable
to oxide glass fibers.
262
Figure III: Modulus/Density Comparison
5O
M
O
D 40
U
L
U
30S -6
/ (m )
D
E 2O
N
S
I
T 10
Y
0
28
46 43
34
r -- T -- I -- T
E-Glass S-Glass NS 7.5 NS 8-G
Glass
The tensile strength for the fiber is dependent on the composition as well as the processing. The
base compositions, NS 8 and NS 7.5, were utilized for their good drawing properties and elastic moduli.
Processing of the glass was ma,_nized using these glass compositions. Figure IV illustrates the fiber tensile
strength increases incurred due to the process changes implemented. As the process improvements have
been made, the tensile strengths have become greater and greater. The MTL "E"glass refers to commercial
e-glass marbles that were acquired from PPG Industries. These marbles were drawn in our fiber drawing
apparatus as a standard of the fiber drawing and fiber testing techniques. The fibers tested a mean of 2649
MPa which were 15 to 20 percent weaker than industry standard. This is due to handling of the glass fibers.
The tensile strength goal for the oxynitride glass fibers was to reach the strengths of commercial e-glass.
The best attempt has been the NS 8-G glass fiber whose strength surpassed the MTL "E" with a mean
strength of 2694 MPa. The highest tested value for the NS 8-G batch was 3320 MPa. This composition, as
previously mentioned, had an elastic modulus of 121 GPa with a density of 2700 kg/m 3. This indicates the
potential of higher strengths from tighter processing control. It is the first oxynitride glass fibers drawn at
AMTL that couples the previous high modulus values with high tensile strength indicating the real potential
for use in high performance resin matrix composites.
263
T
E
N
S
I
L
E
S
T
R
E
N
G
T
H
4000
3000
2000
lOOO
(MPa)
0
Figure IV:Mg-Si-AI-O-N Glass Fiber Strength
Increases via Process Improvements
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FIBER DRAWING EXPERIMENT
Fut0r_ Work;
The oxynitride glass fibers produced thus far have already surpassed the oxide fibers with respect to
elastic modulus and specific modulus. They have shown the potential for higher tensile strengths than their
oxide counterparts. However, the high elastic modulus of the oxynitride glasses can still be further increased
with incorporation of higher levels of nitrogen. Concurrently, the potential for higher strengths can also be
improved via further process improvements. The goal is to determine the highest elastic modulus glass fibers
producible with low density and high tensile strength and subsequently produce composite materials using the
fibers.
REFERENCES
1 D.R. Messier, R.P. Gleisner, R.E. Rich, "Yttrium-Silicon-Aluminum Oxynitride Glass Fibers," J. Am.
Ceram. Soc., 72 [11] pp. 2183-86, (1986).
2 P.F. Auborg and W.W. Wolf, Advances in Ceramics, 18, pp. 51-63, (1986)
3 R.F. Lowrie, Modern Composite Materials, pp. 270-323, Addison-Wesley (1967)
264


Oxynitride glass fibers

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