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HIGH TEMPERATURE APPLICATIONS OF STRUCTURAL CERAMICS

QUARTERLY PROGRESS REPORT

January - March 1980

Samuel J. Schneider

Project Manager

U.S DEPARTMENT OF COMMERCE

Center for Materials Science

National Bureau of Standards

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Under Contract No. DE-AI05-800R20679

"This report was prepared as an account of work sponsored by the United

States Government. Neither the United States nor the United States

Department of Energy, nor any of their employees, nor any of their

contractors, subcontractors, or their employees, makes any warranty,

express or implied, or assumes any legal liability or responsibility

for the accuracy, completeness, or usefulness of any information,

apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights."

TABLE OF CONTENTS

I. SUMMARY OF PROGRESS TO DATE

Articles Published and Talks Presented

II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS

1. High Temperature Fracture of Structural Ceramics

2. Crack Growth Mechanism Maps

3. Thermochemical Data

4. Kinetics of Oxidation

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I. SUMMARY OF PROGRESS TO DATE

Brief Summary

1. High Temperature Fracture of Structural Ceramics

Four-point bend tests on notched bars of SisAlONy were completed and the

results compared with previous work on a yttria-doped silicon nitride (NCX 34)

and with literature values for other silicon nitrides. The NCX 34 appears

most promising and further tests will be carried out on it and other research

grade materials recently received. Reduced scale testing rigs are being made

to test small billets of experimental materials.

2. Crack Growth Mechanism Maps

Preliminary maps for Si 3 N 4 have been drawn. A computer program for automatically

drawing maps from K-v data has been written. Data for SiC has been collected.

A new mechanical analysis of 4-point bending has been completed to get K-v

data from load-deflection curves.

3. Thermochemical Data

Experiments with heating Si 3 N 4 in 95N 2

: 5 H 2 at ^1750 °C in various container

materials indicated that Mo was a suitable container for short term heat

treatments. High temperature x-ray diffraction experiments were initiated with mixed a-3 Si 3 N 4 specimens.

4. Kinetics of Oxidation

Comparison by x-ray powder diffraction of two billets of Si 3 N 4 containing

approximately 8 wt % Y 2 O 3 failed to show any difference in the phase

analysis that could account for the observed difference in low- temperature

oxidation resistance. The two billets contained nearly equal amounts of

WSi 2 before oxidation, but this compound could not be detected in a specimen

that had been severely damaged by oxidation for 20 hours at 735 °C. The

study was undertaken at the request of Dr. Nancy Tighe as part of her work

on NBS-DoE contract EA-77-A-01-6010.

Articles Published and Talks Presented

"A Review of Deformation and Fracture Mechanism Maps with an Introduction to

Crack Growth Mechanism Maps," R. J. Fields, Penn State Materials Science and^

Engineering Seminar, Pennsylvania State University, College Park, Pennsylvania,

February 14, 1980.

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II DETAILED DESCRIPTION OF TECHNICAL PROGRESS

1. High Temperature Fracture of Structural Ceramics (E. R. Fuller, Jr.,

R. J. Fields, L. Chuck, and D. Harne)

Progress : Contact was established with two sources of research grade silicon

nitrides and sialons. We have received from Dr. K. Kobayashi of the National

Industrial Research Institute of Kyushu the following materials: 15 billets of

Si-Al-O-N compounds which include billets of single phase

z equals 1, 1.3, 2.3, and 3 as well as billets of two phase sTalons^conii sting

of 15R-A1N and 12H-A1N polytype phases. He has sent along the complete fabrication procedures for these materials.

We have also received a number of bars of silicon nitride + garnet materials from

Prof. Tien of the University of Michigan. These hot-pressed B'-Si^N. bars contain

about 10% refractory garnet and varying minute amounts of platinum'^cRloride

nucleating agent. They are in a variety of heat-treated conditions.

Two billets of Kobayashi 's sialon (z=l) were cut into notched bars (10 x 5 x 50 mm. with a 3 mm. deep notch in the 10 mm. direction) for toughness measurements, and

double torsion specimens (25 x 75 x 1 mm.) for crack growth and toughness measurements. The notched bars have been tested in four point bending. These results

may be compared with the data which was obtained here for yttria doped silicon

nitride (NCX 34) prior to the inception of the present project. Since both

materials were tested by the same technique, the results are directly comparable.

To measure a valid K^p, the fracture must initiate from a sharp crack or notch.

In both of the above^rfiaterials, a sharp stable crack was grown from the blunt

machined notch by bending the specimen at an elevated temperature (1400 °C for

NCX 34 and 1500 °C for the sialon). Since this deformation introduced residual

stresses around the crack tip, the specimen was annealed at the pre-cracking

temperature for one hour in order to relax these stresses. These pre-cracked

bars were then loaded to fracture at various displacement rates and temperatures

in air.

A fracture toughness was determined as the critical stress intensity factor at

which the specimen fractured catastrophically. This measure of Kxp is not in

accord with method E399 of ASTM because that procedure does not allow for

time dependent effects. Nevertheless, the same critical point that we record

here has frequently been used in fracture studies of ceramics, and identified

as K,p. The toughness of the sialon and the NCX 34 are plotted as a function

of temperature in Fig. 1 together with published data for NC 132, HS 130

(commercial grades of hot-pressed silicon nitride) and HS 350 (a commercial

grade of reaction-bonded silicon nitride).

The hot pressed sialon is distinctly less tough than any of the hot pressed

silicon nitrides, though it is comparable to or better than, the reaction

bonded silicon nitrides. Additionally, we observed evidence of melting on the

fracture surface (Fig. 2) and severe oxidation of the specimen's surface at

1500 °C (Fig. 3). The melting may be due to the sialon not being single phase.

On fracture surfaces obtained at room temperature we observed light needles and

dark spots (Fig. 4). EDAX did not indicate the presence of any heavy metal

impurity. We concluded that complete mixing had not occurred in the fabrication

process.

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This information is being communicated to the source of the material. It is

hoped that greater homogeneity will improve this material as further work on this particular lot of Si^AlON^ is not justified.

In contrast, the NCX 34 tested in the same manner showed a much higher toughness

than all previously tested silicon nitrides. This material also showed a strong

effect of strain rate on toughness (Fig. 5), again, in contrasct to the sialon

which showed no such dependence. The results of the work on NCX 34 have been

written up for publication. Further experiments on this material will be carried

out as it is the most promising material tested so far.

A three-station, instrumented creep and creep-fracture test facility has been

designed and will be constructed when approval is obtained for the equipment

expense. A prototype double-torsion fixture has been constructed for use in

a universal testing machine equipped with a high- temperature vacuum furnace.

The performance of this fixture is being evaluated on a well -character! zed

glass at moderate temperatures and results indicate that the design is reliable.

A similar fixture will be fabricated from tungsten and/or SiC for use at high

temperatures in vacuum and in hydrogen-argon or hydrogen-nitrogen gaseous

mixtures. A special "cold-tip" has been constructed to reduce the partial

pressure of water vapor in these hydrogen gases to the vapor pressure at 77 K.

Plans : More fracture toughness and crack growth measurements will be made on the NCX 34, the silicon nitride - garnet material and on single phase sialon of

other compositions. Fracture mechanics specimens will be prepared for two

additional commercial grades of silicon nitride: a hot-pressed material (NC 132)

and a reaction bonded material (NC 350). A high- temperature, double-torsion

fixture will be fabricated for use in the vacuum/reducing environment furnace.

Reduced scale, four-point bend rigs are being made to handle specimens made

from some of the small billets, of research grade materials.

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G HS 130* ^ NC 132'*

_I_NCX 34**

I O SigAION^**

NC 350*

-» poblishod data

present work

Fig. 1 Critical stress intensity factors for various materials as a function of temperature.

Fig« 2 Crack surface of Si^AlONy showing evidence of melting at 1500°C.

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Fig. 3 Originally smooth side surface is severely oxidized after 1 hour

at 150QOC.

Fig. 4 Room temperature fracture surface exhibits white needles and

dark spots.

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Fig. 5 Critical stress intensity factor for NCX 34 at 1400°C as a function

of strain rate.

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2. Crack Growth Mechanism Maps (R. J. Fields and E. R. Fuller, Jr.)

Progress : A preliminary crack growth mechanism map (Fig. 6 ) has been con- structed for a commercial, hot-pressed silicon nitride (HS-130). A computer

program has been written in BASIC which maps K-v (stress intensity factor

versus crack velocity) data with contours of constant temperature onto a map

of K-T (stress intensity factor versus temperature) with contours of constant

crack velocity. The program uses a theoretically based equation for therermally activated, mechanically assisted crack growth to analyze the data.

Only a small amount of crack growth data for SiC was found in the literature.

It was decided to use an analysis of fracture life time as a function of stress

to determine the coefficient (A) and exponent (n) for a crack growth law of

the form

v=AK'^

Even so, the data on this material are sparse and either more data will have to

be found or actual experiments carried out before a map can be drawn.

An original analysis of four point bending was made in which stress intensity

factor-velocity (K-v) data could be obtained without any assumption about the

general functional form as required above. This analysis will be used in Task

One to provide data for use there and in this task. The analysis is based on knowing the compliance of a notched bend bar as a function of crack length. We

have determined this function and can invert it numerically. Hence, we know

crack length as a function of compliance. The compliance of a creep resistant

material in which a crack is growing is closely approximated by the secant modulus.

So at any point on the load deflection curve, we can get crack length and load,

and from these two quantities, K. Between successive points on the curve we can get Aa/At (change in crack length per interval of time), i.e. crack velocity,

and associate this with the mean K value.

Plans : Establish the validity of the above K-v analysis against known crack

lengths in sialon and NCX 34 (yttria doped 3

' Si 3 N4 ). If it works, generate

K-v curves for these materials at various temperatures.

Complete data collection for SiC and use life-time analysis to get K-v curves for

this material. Construct crack growth mechanism map for SiC if possible without

further experimental measurements.

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1000 1100 1200 1300 1400

T (^C)

Figure 6. Crack growth mechanism map constructed for silicon nitride.

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3. Thermochemical Data (R. S. Roth, J. L. Waring)

Progress : A program was initiated to investigate the thermodynamics

of the so-called a B transition in Si 3 N 4

. Candidate materials

from five commercial sources of SisTi^ were analyzed by x-ray powder

diffraction. Various mixtures of the following phases were found

to occur: aSi 3 N 4 ; BSi 3 N 4

", Si2N20; Si and unknown phase(s), depending

on the source. Only one small specimen prepared by th€ CVD process was found to be single-phase aSi 3 N 4 and none contained single-phase

BSi 3 N 4

. From the above it is evident be seen that specimens of

single phase material must be obtained in sufficient quantities for use as starting materials for future preparations.

Because single phase Si 3 N 4 was not immediately available, some preliminary

work was initiated to determine an appropriate container material

heat Si 3 N 4

. An available sample containing B-Si 3 N 4 + a-Si 3 N 4 + Si was heated in three container materials in forming gas (95% N 2 + 5% H 2 ) at about 10 atm partial pressure of oxygen to the melting point of Pt

(1769 °C) by induction heating. The susceptor for these experiments

was a covered Mo crucible 1 5/8" long, by 1 3/16" diameter with a .056"

sight orifice. In the interior of the outer susceptor crucible was placed

a second small test crucible 7/16" long x 1/2" diameter of either Mo,

BN, or W containing the 3 phase mixture of Si 3 N 4

. The test crucible and

the susceptor were isolated by Zr02. The duration of each heating was approximately two hours and temperature was measured with a disappearing filament optical pyrometer.

In the first experiment no reaction was detected between the Mo test

crucible and "Si 3 N4

." In the second experiment a test crucible was fabricated from hot pressed BN obtained from a commercial supplier.

This test crucible-plus-"Si 3N4" specimen was heated similarly. After cooling,

the Si3N4 specimen contained glassy-like intrusions and the walls of the BN

crucible were severely eroded. The BN from this supplier appears to

be unsuited for this work. The material causing the problem may be

derived from an impurity within the hot pressed BN.

In the last experiment of the series, a W crucible with "Si 3 N4

" was heated similarly. The surface of the W crucible appeared to be

severely etched with grain growth. The x-ray pattern of material

taken near the wall of the W test crucible appeared to contain only

a and B Si 3 N4 and did not appear to be significantly different from

the material heated previously in Mo.

To investigate the effect of W in contact with Si 3 N 4 in forming gas,

a mixture of 95% Si 3 N 4 and 5% W was heated at 1750 °C. The resultant

reaction products were examined by x-ray powder diffraction and found

to contain a mixture of a-Si 3 N 4 + B-Si 3 N4 and a small amount of a phase

which was identified as WSi 2

- The phase was probably formed by the

reaction of the free Si in the starting material with the container W.

Although the existence of a similar phase MoSi 2 has been reported in the

literature it was not detected in the experiment utilizing a Mo container.

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A. modified M.R.C. high temperature powder diffractometer has been

set up for Si 3 N 4 work. This apparatus employs a ribbon element

which serves as a heater and specimen holder. From the foregoing

discussion. Mo was selected as the most suitable heater material for

this work. For the first high temperature x-ray experiment, a heater

with a thickness of about .005" was fabricated. A 0.030 g specimen

of Si 3 N 4 was placed in the cavity of the heater. The specimen was aligned with respect to the diffractometer and the x-ray source. Diffraction intensity was too low, initially, due to both absorption

by the beryllium window between the detector and the specimen, and to an insufficient amount of a specimen material composed of low atomic

number elements (Si & N). To compensate, a larger than normal amount

of specimen was required to obtain adequate x-ray intensity. The

x-ray chamber was evacuated and purged several times with forming gas

(95% N 2 , 5% H 2 ). The forming gas was allowed to flow and escape the

system through a bubbler. At maximum voltage the power dissipated

(lOOA at 1.6 V) was only 160 watts and the temperature was only 900 °C.

To complicate the experiment, the test material temperature did not

adhere to the surface of the heater and large temperature gradients were observed on the surface.

In the second experiment, a .001" thick Mo strip was employed as a heater and a maximum temperature of 1320 °C was obtained in forming

gas before burnout. The power dissipated was approximately 336

watts.

In the third experiment a .003" heater was employed and full power

efficiency was obtained. In addition, the Si 3 N 4 was applied by a different procedure in an attempt to produce a thin adherent coat.

The Si 3 N 4 was very finely ground and suspended in acetone and

drops of the suspension were placed on the Mo heater. This procedure

yielded a thin, uniform coat with sufficient thickness to produce

proper x-ray intensity. This material was heated to temperatures

in excess of 1450 °C. At 1486 °C for 5 minutes, the diffraction

pattern for the Si 3 N 4 disappeared. Room temperature examination

showed that all of the material volatilized from the surface of the

Mo heater. The platinum rhodium thermocouple welded to the bottom

of the heater was attacked by the volatilized material although it was not in contact with the Si 3 N 4 .

In the last experiment the specimen mounting procedure was duplicated;

the temperature was increased at approximately 100 °C intervals. The same three phase mixture (B-Si 3 N 4 , a-Si 3 N 4 , +Si ) of Si 3 N 4 was examined by x-ray diffraction from room temperature to about 1400 °C.

The relative amounts of each phase remained approximately the same over the range - nothing disappeared. The reported unit cell for

B-Si 3 N 4 is hexagonal with a = 7.603A, c^ = 2.909A. In this experiment

the following diffraction p^aks were monitored over the temperature

range: (210) (101) for B-Si 3 N 4 and (210) (102) for a-Si 3 N 4. At about

700 °C some apparent splitting of the (101) in B-Si 3 N 4 was observed,

but this may be merely instrumental in nature. More work will be

needed to delineate the problem.

Plans : Continue the high temperature investigation of the so-called

a B transition in Si 3 N 4 . Assemble, test and calibrate a hightemperature, inert atmosphere furnace for long term equilibrium heating.

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4. Kinetics of Oxidation (F. A. Mauer and C. R. Robbins)

Progress : In collaboration with Dr. Nancy Tighe of NBS two billets of

hot pressed Si 3 N 4 , containing approximately 8 wt% Y 2 O 3 as a densif ication

aid, have been examined by x-ray powder diffraction. Dr. Tighe has

studied the oxidation resistance and microstructure of these materials

by optical methods and TEM as part of her work on NBS-DoE contract EA77-A-Ol -6010. One of the billets (labeled NASA billet No. 3) showed

conspicuously poor oxidation resistance at low temperature ('\^735 ®C)

compared to the second, although it was supposed to be identical in

composition and processing. The x-ray examination was undertaken at the

request of Dr. Tighe to detect differences in the phase composition of

the two billets that could account for the difference in low temperature

oxidation resistance. It provided an opportunity for us to gain familiarity with the phases encountered in the system Si 3 N 4 -Si 02 -Y203 and to

determine suitable heating schedules for oxidation studies.

Both of the billets examined had a nominal Y 2 O3 content of 8 wt%,

but an analysis of billet No. 3 showed that the actual content was 5.5

wt%. In the earier test of oxidation resistance a bar approximately

100mm long and 6.4mm x 3.2mm in cross section had been cut from billet

No. 3 and heated in air for 20 hours with a gradient of ''^650 ”C over the

length of the bar. The high temperature end, heated to 1380 °C, showed

no change, whereas the end heated at 735 °C had changed from black to

grey, had swollen measurably, and had cracked into several pieces.

The x-ray study (carried out in collaboration with Clyde L. McDaniel

of Division 562) consisted of a comparison of the phase composition for

four samples: 1 ) material from the grey (low temperature) end of the

bar, 2) material from the black (high temperature end, 3) material from

billet No. 3 as received (no oxidation), and 4) material from the second

billet, which had shown no evidence of low temperature oxidation. Phase

identification is based on comparison with reference patterns for phases

in the system Si 3 N 4 -Si 02 -Y 203 and related systems^

In all cases, the primary phase was 3 -Si 3 N4 which constituted 80 to

90% of the sample. There was no detectable a-Si 3 N 4 phase. In all cases there were 6 or 8 weak lines that have been tentatively identified as the H-phase of Lange et.al. (1976). Although there does not seem to be

general agreement, the composition of this phase as given by Lange is

YioSi 7023 N 4

. A more prominent phase, which occurs in nearly equal

concentration in samples 2, 3, and 4, but not in sample 1, has been

identified as Si 2 W. The amount is estimated to be from 3% to 10%. The source of contamination is undoubtedly WC balls used in milling the

powder. WC has often been mentioned as an impurity in Si 3 N 4

. For

example, Lange estimated the WC contamination of his samples to be in

the range 1.5 - 3 wt% and reported that "the trace amounts of WC also

disappeared during oxidation". No oxides of silica v;ere detected in any

of the x-ray patterns (including patterns of the surface of the oxidized

bar as well as of pulverized material).

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These results fail to show any difference in the phase composition

of the "good" and "bad" Si3N4(Y203) as received. Loss of the Si2W

contaminant was the only conspicuous change accompanying disintegration

of the material from billet No. 3 during heating in air at 735 °C. Any

oxidation that occurred must have resulted in glassy phases or crystallites too small to produce good x-ray diffraction lines. It is quite

possible that many hours of heating are required to crystallize the

equilibrium silicate phases.

One question remaining is, what became of the Si2W? Did it form a glassy phase on oxidation, or did it form a volatile compound?

Plans :

Samples are being prepared for examination by energy dispersive x- ray spectroscopy in the SEM to compare the tungsten content of material

from the grey (low temperature) end of the bar with that from the black

(high temperature) end. The results should give important clues as to

the role of tungsten in the oxidation of hot-pressed Si3N4. Samples

will be oxidized at higher temperatures and for longer times in order to

obtain detectable amounts of oxidation products and to promote devitrification so that the phase composition can be determined by x-ray powder

diffraction.

REFERENCDS

1. F. F. Lange, S. C. Singhal, and R. C. Kuznicki, "Phase Relations

and Stability Studies of the Si3N4-Si02-Y203 Pseudoternary", Westinghouse Research Report, No. 76 -9D4-P0WDR-R1 , 1976.

2. L. J. Gauckler, H. Hohnke, and T. Y. Tien, "The System Si3N4-

Si02-Y203", J. Am. Ceram. Soc. 63 , 35 (1980).

3. J. Ito and H. Johnson, "Synthesis and Study of Yttrialite", Amer.

Mineral. 1940 (1968).

4. Powder Diffraction File, JCPDS--International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA 19081.

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