Hot Pressed Boron Nitride K. M. TAYLOR
. The C a r b o r u n d u m Co., Niugara Fulls, N . Y .
This paper describes a self-bonded boron nitride body made by hot pressifig.
The body has an ivorylike appearance with a n average boron nitride content of about 97% and a density of about 2.1 grams per cubic centimeter. In crystalline structure and machining and lubricating properties it resembles graphite. Also, like graphite, i t shows directionalism i n its properties, indicating partial orientation of the platelike crystals with respect to direction of forming pressure. It is much stronger at room temperature but somewhat weaker at 1000" C. than graphite. It has high electrical resistivity unlike graphite. It is stable in air to 700° C. and oxidizes only slowly from 100' to 1000° C. It has low weight loss i n chlorine at 700' C., is resistant to some corrosive liquids, and is not wet by molten glass.
I
T HAS been known for many years that boron nitride has
attractive refractory, eleatrical, and lubricating properties. However, its use has been limited to a few laboratory and other highly specialized applications, because it has been expensive to manufacture and quite difficult to fabricate the powder into a dense mechanically strong body. I n 1950 the Carborundum Go., under a Navy Bureau of Ordnance contract ( 4 ) ,made a preliminary study of the possibilities of making refractories based on boron nitride. Although the specific body described in the present paper was not developed under that contract, information was obtained which was later helpful in its fabrication.
materials. It is mechanically strong and can be readily machined, drilled, threaded, or cut with a band saw (Figure l). The maximum temperature a t which it can be used is about 1600" C. At this temperature expansion and rupture usually occur. The exact cause of failure has not been determined. However, this property is being studied and there are indications that the useful temperature range can be extended considerably. The present body shows directionalism in its physical properties, resulting from partial orientation of the thin platelike crystals during hot pressing. This characteristic is also being further studied. The body appears to have excellent resistance to thermal shock. PROPERTIES OF BORON NITRIDE
Composition. The boron nitride content of the hot pressed material ranges from 95 to 99% with an average of about 97%. The following composition is representative of the average product: % Boron nitride Boric oxide Silica Alumina Carbon
Figure 1. Hot pressed boron nitride may be machined, sawed. or threaded
Pure boron nitride is a bulky white powder consisting of 43.670 boron and 56.4% nitrogen. It has a crystalline structure similar to that of graphite and like graphite has a slippery feel. I t s true density is approximately 2.25 grams per cc. According to other investigations (2),boron nitride does not melt under atmospheric pressure but sublimes at about 3000" C. The body described in this paper is made by hot pressing the powder in graphite dies a t relatively high temperatures under moderate pressure. It has an ivorylike appearance and a slippery feel. The apparent density is about 2.1 grams per cc. It has an average hardness of 2 on Moh's scale. I n spite of its soft nature, its resistance to sandblast erosion is equivalent to that of plate glass, indicating less brittleness than in most ceramic 2506
97.0 2.4
0.26 0.15
0.og
The percentage of boron nitride is based on the boron oontent after deducting the oxidic boron from the total boron. Direct determinations of nitrogen were made but the results were erratic. Direct determinations of oxygen by a fusion method gave results in good agreement with the total combined oxygen of the above analysis. Strength. Although a relatively soft material, hot pressed boron nitride has higher strength at low temperatures than many commercial refractories, being several times as strong as plain graphite. Strength is maintained with only small loss up to about 350" C. From 350" to about 700" C. the strength drops rapidly and then a t still higher temperatures again becomes less sensitive to temperature changes. Directionalism is evident in both compressive and transverse strength but is more pronounced in the latter. These points are demonstrated in Tables I and I1 and in Figure 2. Modulus of Elasticity. The modulus of elasticity of hot pressed boron nitride is low for a ceramic material. The value is higher than for graphite a t low temperatures but decreases
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
ABRASIVES AND REFRACTORIES rapidly from 350" to 700" C. From 700" to 1000" C. there appears t o be no significant change. Directionalism is quite marked as can be seen from Table I11 and Figure 3. The relatively low modulus of elasticity is consistent with the good thermal shock properties which this body exhibits. 0
.
0
3;
14
"i
---
\
t TESTING FORCE
z 4 a
--\
\
\
'\
PERPENDICULAR TO' PRESSING DIRECT ION
4
!-
2
I
I
1
I
200
400
600
800
T E M P E R A T U R E,
Figure 2.
offers a refractory body with interesting possibilities for
...rocket nozzles and combustion chamber liners ...high temperature bushings, gaskets, and internal pump parts ,..atomic reactor structural materials ...insulators and dielectrics
TESTING FORCE PARALLEL TO PRESSING DIRECTION
n.
HOT PRESSING OF BORON NITRIDE POWDER
O
I
1 0
C.
Effect of temperature on transverse strength of hot pressed boron nitride
Thermal Conductivity. Hot pressed boron nitride is a good conductor of heat compared with other ceramic materials. This is true especially when thermal conductivity is measured perpendicular to the pressing direction a t high temperatures. Thermal conductivity changes only slowly with temperature as can be seen in Table IV. This is in marked contrast to some ceramics such as beryllium oxide where the thermal conductivity is quite high a t low temperatures but decreases sharply with increasing temperatures.
is 40.5 X 10-6 deg.-l in the C direction. I n the A direction the mean coefficient of expansion is -2.9 x 10-6 deg.-lat 20" C., becoming zero a t 770" C. Our study indicates that hot pressed boron nitride having an apparent density of about 2.1 grams per cc. expands approximately ten times as much parallel to the pressing direction as perpendicular to this direction when heated from room temperature to 1000" C. Thermal expansion in the two directions is shown graphically in Figure 4. Table V lists the coefficients of thermal expansion in the two directions over several temperature ranges. From these data it is seen that hot pressed boron nitride, compared with other ceramic materials, has moderately high thermal expansion parallel to the direction of pressing but extremely low expansion perpendicular to this direction. The thermal expansion of hot pressed boron nitride is also influenced by substantial changes in density. This can be seen from Figure 5 which shows expansion parallel to the pressing
Table I. Compressive Strength of Hot Pressed Boron Nitride at Room Temperature versus Direotion of Testing
-
Form
v)
a TESTING FORCE PAR A L L E L D l RECT ION
Direction of Testing Force Parallel to pressing direction Perpendicular t o pressing direction
Table 11.
LL
0
.-
3
0 2 4 ~ PERPENDICULAR ! ~ ~ ~ F O R TO C E ~ . I
PRESSING DIRECTION
o,:
I
I
200
400
. I
N-
I
I
600
800
I I IOQO
Compressive Strength, Lb./Sq. Inch 45,000 34,000
Transverse Strength of Hot Pressed Boron Nitride versus Temperature
Temperature, 25 350 700 1000
C.
Transverse Strength When Testing Force Is Applied, Lb./Sq. Inch Perpendicular t o Parallel t o pressing direction pressing direction 7280 15,880 6700 14,800 3,840 1900 1080 2,180
TEMPERATURE. 'C.
Figure 3.
Effect of temperature on modulus of elasticity of hot pressed boron nitride
Thermal Expansion. Directionalism is more pronounced in thermal expansion than in any of the other physical properties measured. This is because of the marked difference in the thermal expansion of the boron nitride crystal in the C and A directions. According to Pease (3) who studied this property by x-ray techniques, the mean coefficient in the range of 0" to 800" C. December 1955
Table 111.
Modulus of Elasticity of Hot Pressed Boron Nitride versus Temperature
Temperature, 25 350
700 1000
C.
Modulus of Elasticity When Testing Force Is Applied, Lb./Sq. Inch X 106 Parallel to Perpendicular to pressing direction pressing direction 12.4 4.9 8.0 3.2 1.5 .5 1.6
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PRODUCT AND PROCESS DEVELOPMENT Table IV.
Thermal Conductivity of Hot Pressed Boron Nitride Thermal Conductivity Measured, Cal./Cm.P/Cm./Sec./O C. Parallel to Perpendicular t o pressing direction pressing direction 0.036 0.069 0.034 0.067 0.032 0.065 0.030 0.063 0.029 0.064
Temperature, O
c.
300 500 700 900
1000
Table V. Coefficient of Thermal Expansion of Hot Pressed Boron Nitride with Density of 2.12 GramsjCc. Expansion Measured Cm./Crn./o C. X 10-6 Parallel to Perpendicular to pressing direction pressing direction 10.15 0.59
Temperature Interval, O C. 25- 350 25- 700 25-1000
8.06
0.89
7.51
0.77
PARALLEL TO PRESSING DIRECTION
per second. At frequencies of lo3cycles per second or higher the dielectric constant changes only slowly with temperature to 500" C. The dielectric constant was also determined with the electric field perpendicular to the direction of pressing at a frequency of 1O1O cycles per second. A value of 4.80 was obtained a t room temperature with only slight variations to 500' C. Data relative to dielectric constant determinations are shown graphically in Figure 6. All determinations were made on baked and desiccated samples. The dissipation factor ( I ) , tan 6, was measured with the electric field parallel to the direction of pressing at frequencies of lo2 to 108 cycles per second and with the electric field perpendicular to the direction of pressing at a frequency of 1010 cycles per second. All data are for baked and desiccated samples. Figure 7 shows that the dissipation factor a t room temperature is quite low a t all frequencies tried. At low frequencies, as 102 cycles per second, the dissipation factor increases rapidly with temperature. However, at high frequencies, as lo7 and 1Olo cycles per second, the change with temperature to 500' C. is relatively slow. Chemical Stability. Hot pressed boron nitride has excellent stability in air at 700' C. Even a t 1000" C. the rate of oxidation is relatively slow as can be seen from Table VIII.
a.
I
w
/ PERPENDICULAR TO 25% ZOO
0
Figure
400 600 T E M P E R A T U R E , 'C.
4.
800
Thermal expansion pressed boron nitride
of
1000
hot
direction as a function of density. Within limits, therefore, expansion can be controlled by pressing to a predetermined density. Other methods of controlling directionalism are being investigated. Electrical Properties. Although hot pressed boron nitride resembles grayhite in some respects, it differs markedly from graphite in electrical characteristics. Resistivity measurements were made with the electric field parallel t o the direction of pressing. The effect of temperature on the resistivity of dry specimens is shown in Table VI. As can be seen from these data, the resistivity is high at low and moderate temperatures, being roughly of the same order as electrical porcelain. Although resistivity drops rapidly a t high temperatures it is still 3.1 X 104 ohm-cm. a t 1000' C. The effect of variation of humidity on resistivity a t room temperature is shown in Table VII. The dielectric constant ( 1 ) of hot pressed boron nitride, measured-with the electric field parallel to the direction of pressing, is 4.15 a t room temperature a t frequencies from lo2 to IO8 cycles
Table VI.
Volume Resistivity of Dry Hot Pressed Boron Nitride versus Temperature
(Measurements made with electric field parallel t o pressing direction) Resistivity, Ohm.-Cm. Temperature, C. 1.7 X 1018 25 2.3 X 10'0 500 3.1 x 104 1000 6 X 10' 1500
2508
-7WC
Ol
I
1.70
1.80
I
1.90
I
2.00
I
I
2.10
2.20
PRESSED D E N S I T Y , GRAMS/CC.
Figure 5. Effect of density on thermal expansion of hot pressed boron nitride measured parallel to pressing direction
Table VII. Effect of Humidity on Volume Resistivity of Hot Pressed Boron Nitride a t Room Temperature (Measurements made with electric field parallel to pressing direction) Resistivity, Ohm.-Cm. Relative Humidity, % 1013 20 50 7 x 1010 90 5 x 109
Table VIII. Oxidation of Hot Pressed Boron Nitride in Air and Stability to Chlorine a t Elevated Temperatures Loss in Weight, Mg./Sq. Cm.
Exposure Time, Hr.
700' C.
2 10
Oxidation in Air 0.014 0.062 0.138 0.235
30 60
c.
1000~
0.35 0.85 4.8 10.0
Chlorine a t Elevated Temperature 2.7 3 17.0 0.25 20 0.55 40
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ABRASIVES AND REFRACTORIES
Table IX. Effect of Some Inorganic and Commercial Organic Solvents on Hot Pressed Boron Nitride Bars (2'/, X '/2 X Inch, When Immersed 7 Days at Room Temperature) Immersion Liquid
Weight Loss, Mg./Sq. Cm. Inorganic Xone 10 7 1.3 17.5 8.9 8.9
Loss in Transverse Strengtha. %
None 60 23 55 70 82
Commercial organic solvent Carbon tetrachloride 1 3 Gasoline 1.6 Benzene .4 Alcohol, 95% 14 6 Acetone 13 0 a
1. Rocket nozzles and combustion chamber liners 2. High temperature bushings, gaskets, and shroud rings 3. Internal pump parts for handling certain molten metals 4. Structural material in atomic reactors 5. Electric insulators, dielectric material, separators in vacuum tubes
20 Kone None 48 32
Average transverse strength at start of test, 7280 lb./sq. inch.
I
'"t
I
i"'
8.0
T E M P E R A T U RE, OC.
Figure 7 . Effect of temperature on dissipation factor of hot pressed boron nitride at several frequencies
"0
z 100
300 400 500 TEMPERATURE, O C . 200
Figure 6. Effect of temperature on dielectric constant of hot pressed boron nitride at several frequencies Data at 10'0 cycles per second t a k e n w i t h electric field perpendicular to direction of pressing; at other frequencies, field is parallel
Chlorine has but little effect on hot pressed boron nitride a t 700" C. but attacks it appreciably a t 1000° C. Results of tests a t these two temperatures are shown in Table VIII. Hot pressed boron nitride is rapidly decomposed by fused sodium hydroxide and is weakened by prolonged immersion in boiling water. Thus, a bar 2l/4 X l / 2 X l/, inch lost about 7001, of its original transverse strength when immersed in boiling water for 7 days. However, the bar was dimensionally stable. Bars of the same size immersed in several other inorganic liquids at room temperature showed varying degrees of stability (Table IX). No changes were noted in the dimensions of any of the bars. Results of immersion tests in some commercial grade organic solvents are also shown in Table IX. It will be noted that those which contain water, as acetone and 9570 alcohol, have the greatest weakening effect on the hot pressed bars. I n all casea the bars maintained their original dimensions. APPLICATIONS
The properties of the hot pressed boron nitride body described in this paper suggests a number of applications. December 1955
1)ata a t 1 0 ' 0 cycles per second t a k e n w i t h electric field perpendicular to direction of pressing; at other frequencies, field is parallel
600
6. Rupture disks for high temperatures in corrosive atmospheres 7. Equipment for handling molten glass, as forming tools, mold liners, and lens fusion blocks.
Hot pressed boron nitride has already been tried in a preliminary way for some of these uses with good results. Work is being continued on this body and further improvements in its properties are expected. ACKNOWLEDGMENT
The data on dielectric constant and dissipation factor were obtained by the Laboratory for Insulation Research, Massachusetts Institute of Technology. The author is also indebted to various members of Research Staff of The Carborundum Co. for suggestions and data on physical properties. LITERATURE CITED
Massachusetts Institute of Technology, Laboratory for Insulation Research, Cambridge, Mass., private correspondence, 1954. (2) Alott, W.. Trans. Am. Electrochem. Soc., 34, 255 (1918). (3) Pease, R. S., Acta. Cryst., 5 , 356 (1952). (4) U.S.N. Bur. Ordnance Research Contract, NOrd. 10982, 195051. (1)
RECEIVED f o r review April 6, 1955.
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ACCEPTED
May 1 1 , 1t)55
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