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IMBSSR 83-2664

NATIONAL BUREAU OF STANDARDS LIBRARY

A111D1 M SEflDE

Structural Reliability of

Yttria- Doped, Hot- Pressed Silicon

Nitride at Elevated Temperatures

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

inorganic Materials Division

Washington. DC 20234

March 1983

Annual Report

October 1, I SSI-September 30, 1982

h'

Prepaied for Department of Defense

Air Force Office of Scientific Research

Washington, DC

I

APR ft 7 q 0 3 NBSIR 83-2664

STRUCTURAL RELIABILITY OF

YTTRIA-DOPED, HOT PRESSED SILICON

NITRIDE AT ELEVATED TEMPERATURES

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Measurement Laboratory

Center for Materials Science

Inorganic Materials Division

Washington, DC 20234

March 1983

Annual Report

October 1, 1 981 -September 30, 1982

Prepared for Department of Defense

Air Force Office of Scientific Research

Washington, DC

2

P

C o

oX

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

STRUCTURAL RELIABILITY OF YTTRIA-DOPED , HOT-PRESSED

SILICON NITRIDE AT ELEVATED TEMPERATURES

S. M. Wiederhorn and N. J. Tighe

ABSTRACT

The strength of yttri a-doped , hot-pressed silicon nitride was

investigated as a function of temperature and applied load. Data collected at

1200 °C are presented in the form of a strength degradation diagram for an

applied stress of 350 MPa. At this temperature, the behavior of the yttriadoped material is found to be superior to that of magnesia-doped silicon

nitride, in which creep results in the formation of microcracks that lead to

strength degradation. By contrast, the yttria-doped material does not suffer

from microcrack formation, or strength degradation at 1200 °C. At higher

temperatures strength degradation does occur, and as a consequence, an upper

limit of 1200 °C is recommended for yttria-doped, hot-pressed silicon nitride

in structural applications.

1
1 . INTRODUCTION

Modern structural ceramics are intended for use in applications such as

heat engines and heat exchangers, in which mechanical and thermal loading can

degrade the mechanical strength of these materials. The strength can also be

degraded by exposure to hostile environments at elevated temperatures, which

cause both localized and generalized chemical corrosion of ceramics. Although

materials scientists and engineers are aware of the long term susceptibility

of ceramic materials to strength degradation, the engineering techniques for

addressing time-dependent strength degradation in ceramic materials are

limited. The usual approaches that are used to improve lifetime employ

empirically derived safety factors, or fracture mechanics based crack growth

theories. Whereas both of these approaches are of value to predict structural

reliability, experience in high temperature heat engine programs suggests that

more effective lifetime prediction techniques are needed to assure the

structural reliability of ceramics.

In an earlier paper, a new approach was suggested to improve component

reliability. 1 The approach is more probabilistic in nature than either the

fracture mechanics, or the safety factor approach, and is based on the

assumption that both the applied stress and the strength of a set of

components can be described by their own probability density function. The

failure rate of a set of components is determined by the time evolution of the

two' probabi 1 i ty density functions. Concepts of fracture mechanics can be used

as part of this probabilistic approach to structural reliability to provide a

mechanistic understanding of the time evolution of the probability density

function for strength.

2
To characterize the mechanical behavior of a structural ceramic, both the

strength and the time evolution of strength must be determined. As noted in

reference 1, it is possible to represent this time evolution of strength

graphically on a diagram that relates strength, exposure time and cumulative

failure probability. Information on crack propagation and flaw generation can

be incorporated into the method to give a picture of the time-evolution of

component strength. Strength degradation diagrams (or maps) developed by

application of the method give a general picture of the strength behavior of

ceramics, and as such, are similar to the deformation maps and fracture maps

2 first suggested by Ashby.

An example of a strength degradation map is given in figure 1 for

* magnesia-doped, hot-pressed silicon nitride (NC 132) . Details of the methods

used to develop this map are given in references 1 and 5. The map represents

the strength evolution of four-point, bend specimens subjected to an initial

maximum fiber stress of 250 MPa at 1200 °C. As can be seen from the figure,

specimen strength is plotted as a function of exposure time. The curves in

rt The commercial designation of these materials (NC 132 and NCX 34) is presented

for purposes of identification only, and does not imply endorsement by the

National Bureau of Standards.

rt* The fiber stress is calculated from the simple elastic beam formula, which

does not take into account the possibility of stress relaxation due to

creep, or the possibility that creep in tension may differ from creep in

3 4

compression. A creep exponent N ~1 has been reported in the literature ,

for NCX 34 which supports the use of the simple elastic beam formula in the

present study.

3
the figure represent the cumulative failure probability of specimens that were

broken at temperature after specified times of exposure. In figure 1, 12 to

18 specimens were broken at each time increment to obtain sufficient

statistical data to locate the failure probability curves. Also included in

figure 1 are lines representing the applied stress, and the static fatigue

limit. Failure of a component at a given probability level occurs when the

probability curve representing the strength of a component, intersects the

line for applied stress. Subcritical crack growth occurs when the strength of

the component is less than the stress that corresponds to the static fatigue

limit. Thus, in figure 1 subcritical crack growth is expected for specimens

that have strengths between 250 and 350 MPa.

Figure 1 also suggests that there are several regions to the strength

behavior. For periods less than 0.5 hours exposure, stress relaxation and

crack healing in the vicinity of machining damage result in a strength

increase. Between 0.5 hours and 2 hours of exposure the change in strength is

slight, suggesting that relaxation in the vicinity of the machining flaws has

been completed. Between 2 and 4 hours of exposure, pit generation in the

ceramic surface caused by oxidation results in a radical change in the

distribution of strength for the test specimens. Pit formation occurs quite

rapidly and in a random way between 2 and 4 hours of exposure, so that there

is no relation between the initial set of flaws caused by machining and the

pits resulting from localized oxidation. Between 4 and 16 hours of exposure,

the pits become less severe as sources of failure and the probability curves

plateau suggesting that new pits are not generated after this time. After

approximately 1 week of exposure, creep of the specimens results in the

formation of cavitation cracks which gradually degrade the strength. Thus,

for magnesia-doped, hot-pressed silicon nitride, there are at least 4 regions

4
of strength degradation: (1) subcritical crack growth; (2) machining flaws;

(3) pits; and (4) cavitation cracks. An advantage of a diagram such as that

shown in figure 1 is that it gives perspective to the strength behavior

vis-a-vis potential strength degradation mechanisms. The diagram delineates

those regimes of stress where peak overloads should be avoided and the time

frame over which strength degradation is to be expected.

The objective of the present study is to present a similar set of

strength data obtained on yttri a-doped , hot-pressed silicon nitride (NCX 34).

This material was selected for study because of its excellent resistance to

5

creep at elevated temperatures. Furthermore, the material does not suffer

strength degradation as a result of pit formation at elevated temperatures.

Some billets of yttri a-doped , hot-pressed silicon nitride, however, do suffer

from strength loss at an intermediate temperature "700-900 °C as a result of

6-8

grain boundary cracking. As will be shown, the billets used in the present

set of studies did not suffer from this type of strength degradation.

Strength data for the yttri a doped material will be presented in the form of a

strength degradation diagram, which highlights the difference in behavior of

the two materials at 1200 °C.

2. EXPERIMENTAL PROCEDURE

Investigations were conducted on two billets of yttria-doped , hot-pressed

silicon nitride, which were cut into both strength test-bars (3 X 4 X 50 mm)

and fracture mechanics, double-torsion, test plates (2 X 25 X 75 mm). The

double torsion specimens were grooved (2 mm wide by 1 mm deep) to control the

direction of crack growth during fracture. The bars were tested in four point

bending using a silicon carbide bend fixture with an upper and lower span of

10 mm and 40 mm respectively. All test bars were randomly selected from their

5
respective billets to avoid systematic errors resulting from point-to-point

variation in the billets. Thirty double torsion test plates and 80 strength

test bars were cut from one billet, herein designated as billet A, whereas,

150 strength test bars were cut from the second billet, billet B.

The data for the strength degradation maps were obtained primarily from

billet B. Strength studies were conducted using a test facility developed for

5 this purpose. The facility uses pneumatic, "frictionless 11 bellows to apply

the load to each specimen, and an electronic load cell to measure the load.

Because the loading system is compact, three such systems were used in each

furnace to obtain strength data. Using four furnaces, 12 specimens could be

tested simultaneously. By attaching linear differential transformers to each

silicon carbide loading ram, the displacement of each specimen could be

monitored during exposure.

The data for the strength degradation map were obtained by subjecting

each specimen to a sustained load, at a given temperature for a fixed period

of time. Most of the strength data were obtained at 1200 °C using an initial

applied stress of 350 MPa. However, data were also obtained as a function of

temperature and applied load. Strength data at elevated temperatures were

obtained by exposing and breaking the specimens at temperature without first

cooling them to room temperature.

Billet A was used for several different studies. The double torsion

specimens were used to obtain measurements of the critical stress intensity

factor, Kj£, at room temperature; specimens were broken at a constant rate of

9 1/2 loading . Based on 7 measurements, values obtained for were 5.4 MPa-m

1/2 with a standard deviation of 0.3 MPa-m

Strength test bars from billet A were used to evaluate the effect of

mechanical damage on the strength. Specimens were indented with a Knoop

6
indenter using a load of 20 N before exposing them to elevated temperatures

The crack that forms beneath the Knoop indentation simulates the type of

damage that normally occurs when ceramics are machined. By studying the

effect of load, temperature, and exposure time on the strength of specimens

that contain indentation cracks, the effect of high temperature exposure on

machining damage could be elucidated. The technique was also used to

establish the static fatigue limit for this material at 1200 °C.

3. RESULTS

Catastrophic failure of yttri a-doped , hot-pressed silicon nitride due to

6-8

chemical instability has been reported in the temperature range,

700-900 °C. To investigate this possibility in the present study, exposure

tests were conducted in the temperature range 600 °C through 1300 °C. After

168 hours of exposure, specimen strength was measured at elevated temperature.

At temperatures of 600 °C through 1000 °C, and at 1300 °C, three specimens

were used for each strength measurement. At room temperature, ten specimens

were used, while at 1200 °C six were used (the six at 1200 °C were subject to

an applied load of 350 MPa for 100 hr). As can be seen from figure 2, there

is no evidence of strength loss in the intermediate temperature range of

interest. In fact, the strength appears to be higher in this temperature

range than at room temperature. The drop in strength at ~1300 °C is typical

of hot-pressed silicon nitride tested at elevated temperatures . ^

Examples of the probability distribution curves used to plot the strength

degradation map are given in figures 3 and 4. At room temperature a bi modal

distribution curve is obtained; the slope of the curve at low levels of

probability is less than the slope at high levels of probability. A similar

bimodal probability distribution for NCX 34 was reported recently by Easier

et al.^ After the specimens are exposed at 1200 °C a unimodal strength
distribution is obtained (figure 4). The weak specimens from the low

probability end of the distribution in figure 3 are eliminated from the

distribution in two ways: (1) crack growth, in which case the weak specimens

break while subjected to load at elevated temperatures; (2) crack "healing",

in which case the strength of specimens increase as cracks get shorter or

blunter. Of these two possibilities, the second appears to have dominated the

present study.

Crack healing can be demonstrated by testing specimens that have been

intentionally weakened with a Knoop indentation (using a load of 20 N) before

testing. Data from a set of specimens that has been tested in this way are

illustrated in Table 1. As can be seen from the table, specimens that were

exposed to 1200 °C in air for at least 100 hours, were stronger by ~50 percent

(~490 MPa -> ~740 MPa) after exposure to high temperatures. The increase of

strength occurs regardless of the level of the load applied to the specimens

during the annealing treatment. In some cases, healing was so complete that

the fracture did not initiate from the indentation, but from some other flaw

at the surface. The fact that an initial applied stress of 450 MPa could be

sustained by the specimens without failure during the 100 hour annealing

period, suggests that at 1200 °C subcritical crack growth in the yttria doped

material is minimal, i.e. the v-Kj curve probably has a steep slope at this

temperature.

•Despite the above observation of crack healing, crack growth must have

occurred in some of the specimens tested in the "as-ground" condition because

of the breakage that occurred during the annealing period. Of the 78

specimens that were subjected to 350 MPa at 1200 °C, two broke within the

first 5 minutes of the test, probably as a result of subcritical crack growth

from surface flaws; a third broke after ~43 hours of exposure. In total,

8
approximately 4 percent of the specimens broke in the course of the test.

This small number of failures supports the suggestion that crack growth in

yttria-doped silicon nitride is minimal at 1200 °C.

The strength degradation map for the yttria-doped, hot-pressed silicon

nitride is given in figure 5. The static fatigue limit in figure 5 was

determined from the data in Table 1. Based on the fact that specimens with a

mean strength of ~486 MPa were able to sustain a load of 450 MPa for 100

hours, we estimate that the static fatigue limit had to be less than ~1.03

(486/450) times the applied stress, i.e. ~380 MPa in figure 5. Thus, the

fatigue limit and the applied stress are nearly the same for NCX 34, suggesting

that at 1200 °C subcritical crack growth plays a minor role in failure

process. This conclusion is supported by stress rupture data obtained on 12

NCX 34 by Quinn , who showed that rupture of this material occurred over a

very narrow range of applied stresses at 1200 °C.

By comparing figure 5 with figure 1, we see that the fracture behavior of

the yttria doped, hot-pressed silicon nitride differs considerably from that

of the magnesia-doped material. After a slight decrease in strength for short

exposure times (<2 hr), the strength of the yttria doped material remains

relatively constant with exposure time. Between 100 and 1000 hours of

exposure, the strengtn increases significantly. This long-term behavior is in

sharp contrast to the results obtained for the magnesia doped material, which

exhibits a serious drop in strength after ~100 hours of exposure. By

comparing the diagrams for the two materials, the mechanical superiority of

the yttria doped material at 1200 °C is apparent.

The effect of temperature on the strength degradation of NCX 34 was

studied by collecting strength data as a function of temperature for a fixed

sustained load of 350 MPa and an exposure time of 100 hours. Figure 6

9
illustrates the dramatic effect of temperature on strength. Below 1200 °C,

the shape of the curve is very similar to that obtained for specimens that

were exposed to elevated temperatures, but which were not forced to support an

applied stress (figure 2). The strength at 1250 °C is about the same as at

1200 °C for all probability levels. However, between 1250 °C and 1300 °C, the

strength drops precipitously. Of the six specimens tested at 1300 °C, five

broke in the course of the test; only one survived the full 100 hours at 350

MPa. The one survivor was well on its way to failure since its strength, 395

MPa, was only ~45 MPa above the applied stress. For the specimens that failed

at 1300 °C, the failure times were: 0.25, 1, 1.5, 3 and 6 hours; there was no

prior warning from time-displacement curves of any of these specimens that

failure was imminent.

Failure of the silicon nitride at 1300 °C appears to be related to the

relative ease of creep deformation of this material at this temperature.

Measurements of specimen displacement as a function of time and temperature

suggest that the displacement is a power function of the exposure time, figure

7, and that the creep rate increases as the temperature is increased. This

parabolic creep behavior occurred over the entire period of measurement (~100

13-15

hours). In agreement with other studies on hot pressed silicon nitride,

steady state creep was not observed within this time frame. A plot of the

-1

logarithm of the displacement rate, In 5, as a function of (RT) indicates an

Arrhenius behavior with an apparent activation energy for creep of ~110

kcal/mol figure 8. Both the absence of steady state creep, and the apparent

activation energy are consistent with earlier measurements on other grades of

4 13-15

hot-pressed silicon nitride. ’ With regard to the failure that occurred

at 1300 °C (fig. 6), the higher creep rate shown in figures 7 and 8 correlates

with the shorter lifetime of the specimens at higher temperatures . However,

10
as noted below, a quantitative comparison of component lifetime with creep

rate was not achieved, so it appears that lifetime and creep rate are not

simply related for NCX 34.

4. DISCUSSION OF RESULTS

The mechanical behavior of ceramic materials is considerably more

complicated at elevated temperatures than at low temperatures

. ^ Failure

occurs by diffusive creep or by viscous flow processes that control crack

nucleation and crack growth at elevated temperatures. As noted by Evans,

the processes that affect failure include the nucleation and growth of

cavities, the linking up of cavities to form small cracks, and the growth of

small cracks (or pre-existing flaws) to form critical size cracks for

spontaneous failure. Provided these processes occur sequentially, the slowest

one constitutes the rate limiting step in the failure process.

Extensive theoretical investigations on the subject of high temperature

failure suggest that the failure of structural ceramics is in many ways

16 similar to that in metals. Diffusion at elevated temperatures appears to

control cavitation creep, which eventually results in creep rupture in both

metallic and ceramic materials. Therefore, rupture by cavitation creep and

crack growth is expected to be common to both types of materials. Extensive

experimental studies on metals have shown that creep rupture in tension can be

described by parametric relationships between failure-time, t^, temperature,

T, and steady-state creep rate, s. Two such relations are the Monkman-Grant

, . . 17

rel ati on

£ = constant ( 1 )
and the Orr-Sherby-Dorn relation, 18

t

f exp (-Q/RT) = f (cr) (2)

The term, Q, in equation (2) is the activation energy for the rate limiting

step in the failure process.

Recent theoretical investigations suggest that equations similar to

1 and 2 also apply to creep rupture of ceramic materials at elevated

16 19

temperatures. ’ Since all of the proposed failure mechanisms are activated

processes, the form of the equation for failure is identical to equation 2

with regard to temperature. However, the equations may be quite different

with regard to applied stress, cr, because f(cr) depends on the failure

-1

mechanism. Thus, for steady state viscous creep, e « a and t^ « cr . By

-14 . ... 20

contrast, t^ <* a if crack growth is involved in the rate limiting step

In a qualitative way, equation 2 describes the failure results of the

present study. As the temperature is increased, the exponential term in

equation 2 also increases, requiring a decrease in the failure time for the

equality to hold. The relative failure times at 1200 °C and 1300 °C can be

estimated from the present set of data. At 1300 °C, the median failure time

of the yttri a-doped , hot-pressed silicon nitride was ~ 2 hours. Assuming that

equations 1 and 2 apply, and that Q is given by the activation energy for

creep (110 kcal/mol), a failure time of ~20 hours is predicted at 1200 °C.

Although this prediction is in the right direction, it does not agree with our

finding that the failure time at 1200 °C exceeded ~1000 hours. Thus, the

theoretical analyses embodied in equations 1 and 2 are not adequate to explain

the data obtained in the present study. We suspect that the failure mechanism

changed as the temperature was increased from 1200 to 1300 °C. This change in

12
mechanism accounts for the lack of agreement between our stress rupture data

and equation 2. Further studies will be needed to identify the mechanism of

failure in NCX 34 at 1300 °C. This finding is in marked contrast to the

results on rnagnesia-doped, hot-pressed silicon nitride (NC 132) published

21 recently by Quinn and Quinn. Using an activation energy of ~168 kcal/mol,

these authors showed that stress rupture data from this material fit equation

2 over the temperature range 1100 to 1300 °C.

The difference in stress-rupture behavior of the magnesia and yttria

doped material results from differences in the microstructure and chemical

8 22-24

composition of the two materials. ’ High- resol ution , transmission

electron microscopy studies have shown that magnesia-doped, hot-pressed

silicon nitride has a thin layer of vitreous silicate at grain boundaries and

packets of vitreous silicate at triple junctions of the material. At elevated

temperatures , the vitreous phase softens resulting in a reduction of the

mechanical load capacity of the material due to creep. By contrast, the

structure of the yttria-doped materials is more complex. Second phase,

crystalline yttrium silicon-oxynitride is usually observed at the triple grain

junctions. The crystalline structure is, however, very sensitive to the exact

composition of the material because of the complex phase diagram that controls

the Y^O^-SiC^Si structures. In some yttria-doped, materials an amorphous

phase has been detected at the grain boundaries; in others (NCX 34), no

amorphous phase is apparent. The consequence of adding yttria to silicon

nitride is to produce a material that is resistant to creep and creep fracture

at elevated temperatures. Of the two materials discussed, the yttria doped

material is far more resistant to creep and creep-cracki ng at 1200 °C. The

mi crostructures of the two materials after similar amounts of deformation at

1200 °C are illustrated in figure 9. In the magnesia doped material (fig. 9a)

microcracks are common at grain boundaries that are perpendicul ar to the

13
applied tensile stress. In some cases, large voids open at triple point

junctions, and microcracks link up to form cracks that extend over many grains

of the material. By contrast, the yttria doped material (figure 9b) maintains

its integrity despite the deformation. There is no evidence for void, or

microcrack formation in this material, which suggests that the yttria doped,

hot-pressed silicon nitride exhibits sufficient ductility (via dislocation

motion, diffusive creep, or grain boundary sliding) at 1200 °C to avoid the

microcrack formation that is the main cause of long-term strength degradation

in the magnesia doped material.

From a practical point of view, the results in figure 5 suggest a sharp

limit to the allowable temperature for the use of the yttria doped material

in structural applications. At temperatures below 1250 °C, the strength of

this material is virtually identical to that of specimens that were not

subjected to external loads during exposure (figure 2). Furthermore, the

strength does not appear to degrade with time. Above 1250 °C, however, the

drop in strength was so precipitous at an applied stress of 350 MPa, that for

practical purposes the strength should be considered to be zero for long term

structural applications. This finding is consistent with the results reported

by Quinn,

^who used a stepped temperature, stress rupture test to study the

mechanical response of NCX 34 to temperature. In order to use NCX 34 at

temperatures in excess of 1300 °C, the applied stress had to be reduced

substantially (<200 MPa for 1300 °C). Considering these two sets of data, it

may be concluded that 1200 °C is a safe upper limit for the temperature at

which NCX 34 can be used in structural applications.

Acknowledgement: The authors are pleased to have had the support of the Air

Force Office of Scientific Research for this work. The authors are also

grateful for the helpful discussions with E. R. Fuller, Jr. and R.J. Fields.

14
REFERENCES

1. S. M. Wiederhorn, "A Probabilistic Framework for Structural Design", in

Fracture Mechanics of Ceramics, Vol. 5 and 6, R. C. Bradt, A. G. Evans,

D.P.H. Hasselman, and F. F. Lange eds. , Plenum Publishing Coro., New

York (1983).

2. M. F. Ashby, "First Report on Deformation-Mechanism Maps," Acta Met. 20,

887 (1972).

3. G. W. Hollenberg, G. R. Terwilliger and R. S. Gordon, "Calculation of

Stresses and Strains in Four-Point Bending Creep Tests," J. Am. Ceram.

Soc. 54, 196-9 (1971).

4. D. C. Larsen and J. W. Adams, "Property Screening and Evaluation of

Ceramic Turbine Materials," Semi Annual Technical Report No. 9, Contract

F33615-79-C-5100 , prepared for U.S. Air Force, Wright Aeronautical

Laboratories , Wright Patterson AFB, Dayton, Ohio 45433, by: IIT Research

Institute, Materials Technology Division, Chicago Illinois 60616,

December 1980.

5. N. J. Tighe and S. M. Wiederhorn, "Effects of Exposure on the Reliability

of Silicon Nitride", in Fracture Mechanics of Ceramics, Vol. 5 and 6,

R.C. Bradt, A.G. Evans, D.P.H. Hasselman and F.F. Lange, eds., Plenum

Publishing Corp. , New York (1983).

6. G. E. Gazza, ""Hot Pressed Si,N^,“ J. Amer. Ceram. Soc. 56, 662 (1973).

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

Stability Studies in the Si ^N^-SiO^-Y^O^ Pseudoternary System," J. Am.

Ceram. Soc. 60, 249-52 (1977).

15
8. N. J. Tighe, K. Kuroda, T. E. Mitchell and A. H. Heuer, "In-Situ

Oxidation of Y^O^Doped Si^N^," pp. 310-13, Electron Microscopy 1980,

Vol. 4, High Voltage, P. Brederoo and J. Van Landuyt, eds. , North Holland

Publishing Co., New York (1980).

9. A. G. Evans and S. M. Wiederhorn, "Crack Propagation and Failure

Prediction in Silicon Nitride at Elevated Temperatures," J. Mat. Sci., 9,

270 (1974).

10. A. F. McLean, E. A. Fisher, R. J. Bratton and D. G. Miller, in Brittle

Materials Design, High Temperature Gas Turbine, Technical Report AMMRC

CTR 75-28 to Army Materials and Mechanics Research Center, Watertown, MA,

Oct. 1975.

11. T. E. Easier, R. C. Bradt, and R. E. Tressler, "Effects of Oxidation and

Oxidation under Load on Strength Distributions of Si^N^," J. Amer. Ceram.

Soc. 65, 317-319 (1982).

12. G. Quinn, "Characterization of Turbine Ceramics after Long Term

Environment Exposure", Army Materials and Mechanics Research Center,

AMMRC TR 80-15, April 1980.

13. R. Kossowsky, D. G. Miller, E. S. Diaz, "Tensile and Creep Strengths of

Hot-Pressed Si^N^," J. Mat. Sci. 10, 983-997 (1975).

14. M. S. Seltzer, "High Temperature Creep of Silicon-Base Compounds," Bull.

Am. Ceram. Soc., 56, 418-423 (1977).

15. B. S. B. Karanaratne and M. H. Lewis, "High Temperature Fracture and

Diffusional Mechanisms in Si-Al-0-N Ceramics," J. Mat. Sci. 15, 449-462

(1980).

16. A. G. Evans, "Elevated Temperature Fracture," in Fracture Mechanics of

Ceramics, Vol. 5 and 6, R.C. Bradt, A.G. Evans, D.P.H. Hasselman and F.F.

Lange, eds., Plenum Publishing Corp. , New York (1983).

16
17. F. C. Monkman and N. J. Grant, "An Empirical Relationship between Rupture

Life and Minimum Creep Rate in Creep-Rupture Tests," Proc. ASTM, 56,

593-620 (1956).

18. R. L. Orr, 0. D. Sherby, and J. E. Dorn, "Correlations of Rupture Data

for Metals at Elevated Temperatures," Trans., ASM, 46, 113-128 (1954).

19. A. G. Evans and C. H. Hsueh, "Creep Fracture in Ceramic Polycrystals," in

Creep and Fracture of Engineering Materials and Structures, Pineridge

Press, Ltd., Swansea, U.K. , 1981, pp. 409-431.

20. T.-J. Chuang, K. I. Hagawa, J. R. Rice and L. B. Sills, "Non-Equl ibri urn

Models for Diffusive Cavitation of Grain Interfaces," Acta Met. 27,

265-284 (1979).

21. G. D. Quinn and J. B. Quinn, "Slow Crack Growth in Hot-Pressed Si,N^," in

Fracture Mechanics of Ceramics, Vol. 5 and 6, R.C. Bradt, A.G. Evans,

D.P.H. Hasselman and F.F. Lange, eds. , Plenum Publishing Corp. , New York,

(1983).

22. D. R. Clarke and G. Thomas, "Grain Boundary Phases in a Hot-Pressed MgO

Fluxed Silicon Nitride," J. Amer. Ceram. Soc. 60, 491-5 (1977).

23. D. R. Clarke and G. Thomas, "Mi crostructure of Y^O., Fluxed Hot-Pressed

Silicon Nitride," J. Amer. Ceram. Soc. 61, 114-118 (1978).

24. N. J. Tighe, "Analysis of Oxide and Oxide/Matrix Interfaces in Silicon

Nitride," NBSIR 82-2574, July 1982, to be published in The Proceedings of

the American Ceramic Society, 1983.

17
Table 1: Indentation Study

Test Strength Fracture

Condition (MPa)

*

Ori gi n

Room Temperature 491 (16) Indentation

1200°C (1/2 hr.

)

486 (70) Indentati on

1200°C (100 hr. ) 743 (25) 2 at Indentation

1 not at Indentation

1200°C (163 hr.

450 MPa

Load)

699 (52) 6 at Indentation

3 not at Indentation

* Number in parentheses gives the standard deviation of the measurement.
Specimen

Strength

(MPa)

Fig. 1. Strength degradation diagram for magnesia-doped, hot-pressed

silicon nitride (NC 132). Temperature of exposure 1200 °C;

applied load 250 MPa.

19
200 400 600 800 1000 1200 1400

Temperature, °C

Fig. 2. Strength of NCX 34 after exposure to test temperature for

168 hours. Specimens were exposed and tested at temperature

without cooling to room temperature.

20
Fig. 3. Initial room temperature strength distribution of yttri a-doped

hot-pressed ni tride.

21
NCX 34 1 200°C 350 MPa

Fig. 4

J3

(0 n

ok— Ql

<D

Z3

Strength (MPa)

Strength distribution after 2, 4, and 16 hours of exposure at

1200 °C at a sustained load of 350 MPa.

22
700

600

S 500 -

in

400 Fatigue Limit —

— 0.5

1

2 4

1 1 161 1

Applied Stress

1 1 10 100 Exposure Time, Hours

1000

Fiq 5. Strength degradation diagram for yttria-doped , hot-pressed silicon

' ’ nitride (NCX 34). Temperature of exposure 1200 °C; applied stress

350 MPa.

23
1000 -

Fig. 6. Effect of temperature on the strength of NCX 34 which is subject

to a sustained load of 350 MPa for 100 hours.

24
1

Fig. 7

NCX 34 Creep, 4-Point Bending

350 MPa

1300

1 I—

<-> 0.1

0.01 !-

1.0 10.0 Time (Hours)

100

Creep behavior of NCX 34 at 350 MPa maximum fiber stress,

point displacement as a function of time.

Center

25
Fig. 8. Estimate of the activation energy for creep for NCX 34.

26
Fig. 9. Transmission Electron Micrographs of hot-pressed silicon nitride:

(a) NC 132, (b) NCX 34. The arrows in (a) indicate boundries within

the foil that have cracked as a result of creep. The yttria doped

material (b) gives no evidence for crack formation. Y indicates the

yttria rich phase; W indicates the tungsten rich phase.

27

NBS-114A IREV.,2-80)

U.S. DEPT. OF,COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) 83-2664 March 1983

4. TITLE AND SUBTITLE

STRUCTURAL RELIABILITY OF YTTRIA-DOPED , HOT-PRESSED SILICON NITRIDE AT ELEVATED

TEMPERATURES

5. AUTHOR(S)

S. M. Wiederhorn and N. J. Tighe

6. PERFORMING ORGAN I ZATION (If joint or other than N BS, see instructions) 7. Contract/ Grant No. AFOSR- I SSA- 83-0001

3

national bureau of standards

DEPARTMENT OF COMMERCE 8. WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Type of Report & Period Covered

Annual

Oct. 1 , 1 981 -Sept. 30, 1

Department of Defense

Air Force Office of Scientific Research

Washington, DC

10. SUPPLEMENTARY NOTES

QE] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here)

The strength of yttria-doped, hot-pressed silicon nitride was investigated as a function of temperature and applied load. Data collected at 1200 °C are presented in

the form of a strength degradation diagram for an applied load of 350 MPa. At this

temperature, the behavior of the yttria-doped material is found to be superior to

that of magnesia-doped silicon nitride, in which creep results in the formation of

microcracks that lead to strength degradation. By contrast, the yttria-doped

material does not suffer from microcrack formation, or strength degradation at

1200 °C. At higher temperatures strength degradation does occur, and as a consequence, an upper limit of 1200 °C is recommended for yttria-doped, hot-pressed

silicon nitride in structural applications.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) ceramics; creep; creep rupture; fracture; silicon nitride; strength

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