FINAL REPORT

"DEVELOPMENT OF A CONTINUOUS SPINNING PROCESS

FOR PRODUCING SILICON CARBIDE - SILICON

NITRIDE PRECURSOR FIBERS"

CONTRACT NO. NAS8-34648

Prepared For

Marshall Space Flight Center

fNASA-CB-178458) DEVELOPMENT OF A

CONTIGUOUS SPINNING PBOCESS FOE PRODUCING

SILICON CARBIDE - SILICON NITEIDE PBECOBSOfi

FIBEBS Final Report i8^*;1**8636"^ 11D

Labs., inc.). .-« 3 p HC .A03/BF. A01 CSCL 11D

N86-16269

Onclas

05255

JAN198

RECEIVE D

NASASTlFACiUH;

By

Bjorksten Research Laboratories, Inc.

P.O. Box 9444

Madison, Wisconsin 53715
TABLE OF CONTENTS

Page

CONCLUSIONS 1

OBJECTIVE 2

SUMMARY 3

RECOMMENDATIONS 4

INTRODUCTION 5

DISCUSSION 6

Fiber Extruder 6

Figures 1-1 7 ' 8-25

Figure 19 28

Spinning of Silicon Carbide - Silicon Nitride

Precursor Fibers 29

Table 1 30

Crosslinking Studies 33

Pyrolysis Studies 35

Figures 20-2 3 37-40
CONCLUSIONS

The silicon carbide - silicon nitride precursor polymer produced by

NASA's Huntsville laboratories can readily be fiberized, crosslinked and

pyrolyzed. The product is a metallic black fiber, presumably of a composition

of the type C Si N .

x y z

However, the properties of the resultant fiber have yet to be evaluated

and knowledge of how the three steps, i.e. fiberization, crosslinking and

pyrolysis, interact is almost nil.

- 1 -
f

I

OBJECTIVE

The objective of this research program is to design and construct an

apparatus for the continuous production of silicon carbide - silicon nitride

precursor fibers.

- 2 -
SUMMARY

The objective of the program was met by developing a small scale extruder based on a cylinder/piston assembly. The piston was caused to advance

into the cylinder cavity at a precisely controllable rate by a variable speed

DC motor via a differential screw mechanism. The resin in the cylinder cavity

was extruded through variously sized spinneret plates, the fiber thus obtained

being taken up onto a variable speed contact wheel.

Preliminary work with a sample of silicon carbide - silicon nitride precursor resin supplied by NASA demonstrated that the resin could readily be

fiberized at 200-210 F. It was found possible to fiberize the resin directly

into ambient air, a controlled atmosphere cabinet not being necessary.

However, work with two subsequent batches of resin was rather less successful. These appeared to be gelled and also to contain substantial amounts of

foreign material. Filtration of the resin was resorted to, and even then only

small quantities of fiber could be obtained, with considerable difficulty.

The fibers so obtained could be readily crosslinked by maintaining them

at 50°C and 100% relative humidity. After 65 hours of this regimen, the fiber

softened at about 400 C, after 90 hours at 600 C, and after 9 days full crosslinkage and a..softening temperature of 800 C was attained.

Fully crosslinked fibers were successfully pyrolyzed at 1000 C, both with

and without tension. Pyrolyzed fibers were of a metallic black color, with

cyrstals (probably from an extraneous source) embedded in the surface.

- 3 -
RECOMMENDATIONS

1. Evaluate the C Si N fibers obtained in this program with respect to such

x y z ° r

properties as thermal resistance, tensile strength, elongation, modulus,

elastic recovery, oxidation resistance, electrical conductivity, and

other characteristics of interest.

2. Explore the pertinent steps in the production of the precursor resin to

determine how gelled product may be avoided and how resin properties such

as composition and molecular weight may be controlled.

3. Determine how resin properties, fiberization, crosslinking, and pyrolysis

parameters interact and define the optimum parameters for each of these

with the goal of producing the optimum fiber.

- 4 -
INTRODUCTION

Carbon fibers have excellent physical properties, leading to their widespread use in high performance composite materials. However, their high

electrical conductivity, while an advantage in some applications, is a distinct

disadvantage in others.

The production of C Si N fibers by the pyrolysis of suitable presursor

x y z

resins of the polycarbosilazane type has been reported, both in the scientific

and the patent literature. While the resultant shiny, black, amorphous fibers

appear to have good physical properties, i.e. 1.9 X 10 psi tensile strength

and 2.6 X 10 psi modulus, little else is known of their properties.

In order to determine if C Si N fibers are a viable alternative to carbon

x y z

fibers, considerable further information about their production and properties

must be obtained. The present program represents'preliminary efforts in that

direction.

- 5 -
DISCUSSION

1. Fiber Extruder

Based on previous experience in these laboratories, a fiber extruder was

designed. The operation of the design may be visualized by reference to Figure 1,

a side view of the extruder.

The drive pulley (1) is powered by a main drive motor (not shown). A 1-16

screw (3) is locked to the drive shaft (2) by a key (5). Rotation of the pulley

and drive shaft drives the 1-16 screw downward through the fixed nut (6). A

1/2-20 screw (7) with a removable piston (10) is threaded into the core of the

1-16 screw and restricted from rotation by a key (8). As the 1-16 screw is

driven downward by the drive motor, the 1/2-20 screw is simultaneously withdrawn

upward. The 1-16 screw advances downward 0.0625" per driveshaft revolution,

while the 1/2-20 screw withdraws 0.0500". The net result is a 0.0125" per revolution piston advance.

By removing the heat shields (18), the main drive assembly (secured by four

screws not shown), and withdrawing the 1/2-20 shaft key (8) the entire drive

unit and piston can be removed from the extruder block. This permits manual

loading of the resin in the melting cup (11).

Resin melting is accomplished by cartridge heaters (16) installed in the

aluminum block (.17). Temperature is maintained by a temperature controller unit

(not shown) and thermocouples positioned in the melting pot and at the spinneret

orifice. Hollow, heat shields (18) thermally insulate the apparatus. A removable

- 6 -
socket head screw (15) secures the spinneret plate (.14) and optional filters

(13) at the base of the melting pot/extrusion cylinder cavity.

Figure 2 is a cross-sectional view of the unit (at Section AA in Figure 1).

It illustrates positioning of the aluminum support beams (6) which span from

just above the screw guides and key holder (4) to the 1-16 fixed nut (Figure 1).

The gaps between beams allow access to the melting pot when the piston and drive

unit are removed.

- 7 -
Hollow_J)rive Shaft

Drive^ulley © ' ' 1-lfi^crew

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ORIGINAL PAGE 98

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Key

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Filters

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15) Socket Head & Screw

Figure 1. Extruder

- 8 -
1) Heat Shields

ORIGINAL PAGE S3

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Melting Cup

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FULL SCALE

Figure 2. Cross-Section AA

- 9 -
Figures 3-15 provide detailed engineering drawings of the various

subassemblies, as follows:

Figure 3 - Differential screw and piston

Figure 4 - Melting cup

Figure 5 - Cylinder

Figure 6 - Nut and holder

Figure 7 - Lower guide and key holder

Figure 8 - Body

Figure 9 - Body

Figure 10 - Bearing mount

Figure 11 - Coupler assembly

Figure 12 - Body - bottom view

Figure 13 - Body - bottom view

Figure 14 - Body - top view

Figure 15 - Coupler, coupler sleeve and drive tube

Finally, Figures 16 and 17 depict the extruder in, respectively, assembled and

disassembled states.

- 10 -
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- 16 -
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- 20 -
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- 21 _
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- 22 -
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- 24 -
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- 25 -
The extruder was driven by a 1/4 horsepower, 1750 rpm Graham DC motor

with a variable speed (variable voltage) controller. An SM-cyclo double speed

reducer was used to gear the motor output down in a 121:1 ratio. The system

so fitted provided extrusion rates from 0.051 cc/min to 0.740 cc/min.

Heat was supplied to the extruder by eight 100 watt cartridge heaters in

the main extruder body (see Figure 12). These were controlled by an Omega model

149 proportioning temperature controller. Temperature control of the extruder

was found to be excellent, the maximum excursion from the control point being

no more than 1 F (either with or without the annular and top heat shields in

place).

It was felt that a safety shut-off switch was necessary in order to prevent

mechanical damage to the extruder. However, the compact design and the inacessibility of the moving parts relating to position penetration made any kind of

limit switch impossible. The problem was solved by installing a cam on the input shaft to trigger a count mechanism which would shut the unit off after a

predetermined number of revolutions. By carefully calibrating the differential

screw the counter prevents the piston from "bottoming out". In addition, the

displayed count provides an exact indication of the piston position within the

extrusion cylinder.

A 1/8 horsepower, 1750 rpm Graham DC motor with a variable speed control

was utilized for the take-up function. This was coupled directly to a 11" diameter, 4" wide expanding contact wheel, upon which the fiber was wound. The

entire take-up system was mounted on a rolling cartridge mount to permit traversing

the take-up. This system provided take-up rates from 500 ft/min to 5,000 ft/min.

- 26 -
Spinneret plates were punched from 0.042" thick molybdenum and 0.046"

stainless steel. Several plates were provided with 100 and 200 micron orifices

via an electron beam process, while 400 and 800 micron orifices were produced

by microdrilling.

Finally, the system was calibrated in terms of readily observable parameters, namely the armature voltages of the extruder and take-up motors. These

voltages are good indicators of motor speed. Charts were prepared relating

these two parameters in terms of their respective armature voltages for each of

three fiber diameters, 15, 25 and 30 microns. The speed dependency of each

motor as a function of armature voltage was determined. The volumetric rate of

extrusion was then directly relatable to the extrusion motor speed and armature

voltage through the piston corss-section area and the differential screw thread

ratios and pitches. Similarly, the linear rate of take-up was relatable to

the corresponding motor parameters through take-up wheel diameter. By implementing the armature voltages in the table, reasonable speeds for the extruder

and the take-up can be achieved to produce fiber of a predetermined diameter.

A system evaluation was undertaken with polystyrene resin. A spinneret

plate with a 0.032" orifice was installed. However, molten polystyrene resin

did not flow freely from the melting cup into the extruder piston cavity. The

problem apparently was caused by an air lock, and was alleviated by the installation of two vent pipes in the melting cup. Thereupon, once the optimum temperature had been defined, a full charge of polystyrene resin was extruded and wound

onto the take-up wheel with no fiber breakage.

The entire extruder assembly, without heat shields but with the control

panel, is depicted in Figure 19.

- 27 -
ORIGINAL PAGE IS

OF POOR QUALITY

1 I

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D.C. Drive Motor (Variable Speed)

(Graham, Model 6016, 1/4 HP.)

Reducer (121:1)

(Ho. 18095-MC SH-CYCLO)

8 Cartridge Heaters (Chromalox, PCN 275231

3/8" Diameter X 3" Long, 100 Watts)

Heater

Variac

Take-Up Speed Control

Collapsible Drive Coupler

Extruder

:Rev Counter

(Eagle Signal Ind. Controls Model CX302A6)

Temperature Controller

[Omega Model 143)

iT.C. Sockets

Ilain Power

Take-Up Motor (Variable Speed)

(Graham, Model 6005, 1/8 HP) Speed Controls - Models 601 and 602

(Graham Company)

Figure 19.

Extruder Assembly With Control

Panel (Heat Shields Removed From

Extruder Body)

- 28 -
2. Spinning of Silicon Carbide - Silicon Nitride Precursor Fibers.

For preliminary trials with silicon carbide - silicon nitride precursor

polymer the extruder was fitted with a 0.016" orificed spinneret plate.

Two small chips of the MW-3468 resin were placed in the extruder melting

cup and the temperature was raised to 200 F. The resin softened but did not

melt. The temperature was raised by 5 increments and allowed to stabilize for

fifteen minutes at each setting. At 215 F the resin was fully melted but was

too viscous to flow. The temperature was increased by 10 increments to 260 F

at which point the resin slowly ran down the melting cup ports into the extruder

piston cavity. At 300 F the resin flowed freely down the piston cavity walls

onto the spinneret plate.

A new charge of resin was placed in the melting cup and melted at 300 F,

filling the piston cavity. As extrusion began the resin dripped from the

spinneret orifice, indicating the resin was too hot. The temperature was reduced to 225 F and allowed to stabilize. Extrusion at this temperature yielded

a fiber that started to draw nicely, but broke freqently.

A new charge of resin was placed in the melting cup, melted at 274 F and

extruded. The extruder motor voltage regulates extrusion speed and was set at

25 volts for this as well as the prior run. ^ake-up wheel speed was maintained

at 240 feet per minute. Several hundred feet of 60 micron diamter fiber were

drawn with a 210 F extruder temperature with occasional fiber breakage. The

extruder voltage was cut to 20 volts and fiber diameter decreased to 30 microns.

Occasional fiber breakage continued.

- 29 -
The extruder temperature was cut back to 200 F and the fiber take-up

speed was increased to 290 feet per minute. The resulting fiber measured 26

microns in diameter and several hundred feet were drawn with very few fiber

breaks.

The drawn fiber was very brittle and hard to handle. Slight pressure

caused the fiber to disintegrate.

Further fiberization runs demonstrated that the MW-3468 resin requires a

melt temperature in the range of 200-210 F for optimum fiberization. Two sets

of conditions to produce ca. 30 micron fiber are given in Table 1.

Table 1.

Extrusion

Del. rate

cc/min

0.0612

0.117

Armature potential

volts

10

18

Take-up

Speed Armature potential Extruder

ft/min volts Temperature, F

275 4.5 200

478 8.0 210

Some fiber breakage was experienced. A few of the breaks appeared to be

caused by "gelled" resin, rather than foreign material. It was decided to

counter this problem with the use of filters on subsequent runs. Temperature

control was excellent, usually within + 1 F of the control point. Speed control

was satisfactory.

- 30 -
Initial attempts to spin fiber from a second resin shipment were unsuccessful due to a gelled component in the resin. At 225 F, a temperature at which

fiber was successfully drawn from the first resin sample, the resin exited the

0.032" spinneret orifice in spurts of grainy, bubble impregnated rods, 0.032"

in diameter and approximately 0.50" long (see photograph). The gelled resin's

high viscosity caused high resistance to extrusion which deformed the threads

of the extruder main drive screw. A new screw with more thread surface contact

was constructed to improve the durability of the extruder.

The extruder temperature was slowly increased to find the melting point

of the resin. At 350 F resin was observed dripping from the spinneret orifice.

Within a few minutes, all the nongelled resin had dripped out of the piston

cylinder and only the unmelted gelled resin remained. The temperature was increased to 400 F and extrusion was attempted. Once again, the resin exited the

orifice as grainy, bubble-filled rods. It became apparent that the resin would

have to be filtered prior to extrusion to separate the gelled component.

A portion of the remaining resin sample was filtered directly into the

extrusion cylinder through an 0.008" mesh screen. Filtration was done under an

inert nitrogen atmosphere at 375°F. The gel remaining on the screen, about 25%

of the initial amount, was pale yellow, grainy and bubble filled. A section of

hollow glass tube remained on the screen as well, apparently a stirring rod tip

which broke off during resin manufacture.

The filtrate resin was dark amber, clean and homogeneous in appearance.

Extrusion at 225 F yielded resin that drew into fiber smoothly with only

occasional fiber breakage. Twenty-five to thirty micron diameter fiber was

- 31 -
drawn continuously in two to three minute periods several times. Two pirns of

2000 to 3000 feet each were drawn.

A third batch of resin from Huntsville was filtered through an 0.008"

mesh screen at 375 F under an inert atmosphere. Fiberization was unsuccessful,

however, as the resin seemed too weak to draw. It was easily started by hand

drawing from the spinneret orifice, but drew down in diameter quickly and broke

before reaching the take-up wheel. The limited amount of resin after filtration

made it difficult to determine the nature of the fiberization problem.

Half of the remaining resin was filtered and drawn into fiber. It drew

fairly easily but the limited amount of resin after filtration yielded only a

few hundred feet of fiber, approximately 40 microns in diameter.

- 32 -
3. Crosslinking Studies

Preliminary crosslinking studies were conducted with the 26 micron fiber

which was obtained at an extruder temperature of 200 F and a take-up rate of

290 ft/min from the first batch of polymer.

One sample was placed in a desiccator at 100% relative humidity (RH) while

another sample was placed in a 0% RH desiccator. After five days the samples

were compared. The 100% RH exposed fiber seemed slightly less brittle than

the 0% RH exposed fiber. The 100% RH sample desiccator was placed in an oven

at 60 C to foster fiber crosslinking. Examination after 24 hours showed increased fiber flexibility and improved tolerance to pressure.

These results prompted a comparison of resin melted in a 40% RH room atmosphere with resin melted in a 0% RH nitrogen atmosphere. Both samples had the

same degree of brittleness when cooled. These results led to the conclusion that

there appeared to be no advantage to drawing the fiber in an inert atmosphere.

At this point the planned construction of an inert gas fiberization cabinet was

abandoned, and all fiberization studies were conducted at ambient humidity conditions.

As already indicated, fiberization studies with the second batch of resin

yielded two pirns of 2,000 - 3,000 feet each. The first pirn was placed in an

oven at 75°C (165°F), 100% relative humidity to crosslink the fiber. Examination after 40 hours exposure indicated that the oven temperature was too high,

as the fiber had become tacky and had agglomerated into a multi-fiber mass. The

second pirn was crosslinked at 50°C (122°F), 100% relative humidity. Daily examination indicated increasing tensile strength and decreasing brittleness.

- 33 -
To determine if a fiber was fully crosslinked, an individual fiber was suspended

across a channel in a metal block. The fiber was observed as the block was

heated. When the fiber softened, it sagged, indicating the melting point.

After 65 hours at 50°C, 100% relative himidity, the fiber softened at 400°C.

After 90 hours exposure, the fiber withstood 600 C, at which point the aluminum

block supporting the fiber began to soften and blister.

To counter this problem a high temperature melting point device was set

up using an induction heater to evaluate hydrolyzed fiber and to determine

whether it had been crosslinked sufficiently to sustain pyrolysis temperatures

without intolerable deformation. Eight short fibers were suspended over a 5/8"

channel in a one inch graphite block. A quartz tube housed the block during

the heating process which was performed under an inert atmosphere. The fiber,

which had been exposed to 50 C, 100% relative humidity for nine days, was

heated slowly. At approximately 800 C (block temperature) the fiber began to

curl. Examination of the fiber in contact with the block revealed a darkening

in color with a tapering color density gradient toward the cooler suspended

portion. The fiber had been pyrolyzed without melting, indicating a high

degree of crosslinking. The nine day hydrolysis period was probably more

exposure than was necessary, but budgetary constraints limited further evaluation

of crosslinking parameters.

- 34 -
4. Pyrolysis Studies

Pyrolysis studies were conducted with a Fisher No. 472 furnace. This was

modified with two vacuum sphere assemblies, one at each end, which made it

possible to place the fibers under suitable tension in a controlled vacuum/

atmosphere. The furnace so modified is depicted in Figure 20.

A trial run found the temperature controller to be malfunctioning, but

manual control proved adequate for our experiments. The temperature profile

of the 8" heat zone was examined at 750 C with the aluminum combustion tube

in place. The half profile is outlined at one inch intervals below:

Thermocouple Distance From

Center of Heat Zone 0 1" 2" 3" 4" 5"

Temperature (Degrees C) 750 745 733 707 653 568

Considering the 100 C difference from center to either end, the center temperature

of the furnace was run to 1000 C to assure fiber pyrolysis over the entire 8"

heat zone.

A bundle of approximately 40 of the hydrolyzed fibers, 12" in length, was

placed on a molybdenum fiber guide tray and pyrolyzed in the high temperature furnace

under an inert nitrogen atmosphere. No tension was applied to the fibers. The

temperature was slowly increased from 600 C to 1000 C over a three hour period.

Little shrinkage or curling was apparent in the pyrolyzed fiber which was shiny

black in appearance. Clear crystals were apparent on the surface of the fibers

in random positioning (Figure 21).

A second bundle of hydrolyzed fiber was prepared for pyrolysis. Tungsten

wire, 0.002" in diameter, was tied around each end of the 12" fiber bundle to

- 35 -
facilitate tensioning of the short fibers during pyrolysis. The entire bundle

was tensioned as a whole in this first experiment because the brittleness of

the fibers made individual handling very difficult. A steel machine nut

weighing approximately 1.3 grams was attached to the tungsten wire on one end

of the fiber bundle and the wire on the other end was secured to a fiber guide.

The weighed end was suspended over a pulley in the guide system, thus positioning

the fibers within the heat zone while maintaining them under tension (Figure 22 ).

Fiber shrinkage due to pyrolysis was monitored by following the height of the

suspended nut with a cathetometer.

During initial heating the fiber expanded approximately 0.5 centimeters.

The fiber began to shrink at a center temperature of approximately 650 C and

continued to do so at a fairly constant rate as the temperature approached

900 C. At 900 C the rate of shrinkage decreased, but shrinkage continued as

pyrolysis extended to the material in the heating zone boundaries. Total fiber

shrinkage was approximately 3.5 centimeters at the 1000 C shutdown point. Microscopic examination revealed a 20% decrease in fiber diameter accompanying

pyrolysis; 25 to 30 micron diameter fibers decreased to 20 to 24 microns. This

examination also showed crystals present on the straight metallic black fiber

(Figure 23 ) •

A second attempt to pyrolyze a fiber bundle under tension was interrupted

by a break in the fiber at 830°C. In addition to removing tension, the break

caused the fiber bundle to move partially out of- the heat zone and removed the

tension. As a result, only about 4" of the fiber bundle was fully pyrolyzed.

- 36 -
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Figure 21.

Crystals Present on Pyrolyzed

Fiber 125X Magnification

- 38 -
Sphere Combustion

Tube End

PAGE IS

POOR QUALITY

Fiber Exit

Disc

Pulley

Support

Pulleys

Figure 22.

Weight Attached to Tungsten

Wire to Tension Fiber Bundle

During Pyrolysis

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Tungsten Wire

Interface

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and Weight

Fiber Exit

Holes

Oven

Reflection By

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Fiber

Tensioning

Weight

- 39 -
ORIGINAL PAGE IS

OF POOR QUALITY

Figure 23.

Fiber Bundle Pyrolyzed Under

Tension 65X Magnification

- 40 -
An examination of both untreated and hydrolyzed fiber revealed particulate

impurities attached to the fiber surface. It was not determined whether these

were crystalline or noncrystalline, nor whether they were related in any way to

the definitely crystalline outcroppings noted on the pyrolyzed fibers. While

they were indeed small enough to pass through the 0.008" resin screen, they were

attached only superficially at the fiber surface and appeared not to have been

wet by the matrix when it was in the fluid state. It seems likely that they

represent detritis picked up through electrostatic attraction during or after

fiberization.

- 41 -