NBSIR 82-2518(R)

Div. 561

V

HIGH TEMPERATURE APPLICATIONS OF STRUCTURAL CERAMICS

QUARTERLY PROGRESS REPORT

January - March 1982

Samuel J . Schneider

Project Manager

Center for Materials Science

National Bureau of Standards

U. S. Department of Commerce

Washington, D. C. 20234

PREPARED FOR THE UNITED STATES

DEPARTMENT OF ENERGY

Under Contract No. DE-A105-800R20679

"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,

expressed 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 fights."

TABLE OF CONTENTS

PAGE

INTRODUCTION I

Subtask A. High Temperature Fracture of Structural Ceramics. . . 2

Subtask B. Crack Growth Mechanism Maps 4

Subtask C. Microstructure and Phase Alteration 5

Subtask D. Microstructure and Fracture in Reactive

Environments 7

X

NBS-5. 12 - 1 IICH TEMPERATURE APPLICATIONS

OF STRUCTURAL CERAMICS

E. R. Fuller, R. J. Fields, T. J. Chuang,

R. Krause, A. Dragoo and S. J. Schneider

National Bureau of Standards

Center for Materials Science

Washington, D. C. 20234

INTRODUCTION

The achievement of higher efficiency thermochemical engines and

heat recovery systems requires the availability of high temperature,

high performance structural materials. Structural ceramics such as

SiC, Si^N^ and certain Al^O^-Si^N^ combinations have received particular

attention for these applications due to their basic characteristics of

good strengths coupled with good corrosion and thermal shock resistances.

Even with these positive attributes, improved reliabilities and extended

lifetimes under service conditions are necessary for structural ceramics

to gain industrial acceptance and use. The problems are mechanical

and/or chemical in nature and are enhanced by the fact that these

materials are subjected to high temperatures, reactive environments and

extreme thermal gradients.

With an objective of improved performance for heat engine/heat

recovery applications the NBS program on structural ceramics addresses

these problems through the determination of the critical factors which

influence mechanical and mlcrostructural behavior. The activities of

the program are grouped under four major subtasks with each designed to

develop key data, associated test methods and companion predictive

models. The status of the subtasks are detailed in the following

sections

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NI5S-').1?(A) - IIJCIl Il'MIM'RATimK I'MMC'J'UHE OF STKIJCJ’URAL CFF<j^^

DISCUSSION OF CURRENT ACTIVITIES

Silicon cMiblcU' •-.pc'c liiM-nu (u-SiC <ind KT-SIC) were friicLurecl In the

( c'liipri a I n re i aiij^e 1200 ®C Lo 1600 "C. The Lliroiigli-noLched .epee linens of

(t S I C and KT-SI(^ showed no r.Igns of stable crack {growth prior Lo fracture.

Tlu' K for t lu'se t hronj^^h-notched specimens is higher than for a sharply

iiacki'd spc'c linen because of the bluntness of the notch (p 250 pm).

To avoid aiiib I gnlt les In the K _ determined by fracture from blunt

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notches, chevron-notched specimens (see Figure 1) were also tested,

rills type of spt'cimen Lends to form a sharp crack while loading.

Fracture then occurs from this sharp crack. However, analysis of

from this type of specimen is computationally more difficult and we are

at present evaluating the data. Chevron-notched specimens of KT-SiC

usually showed evidence of creep crack growth prior to rapid fracture,

hut a-SiC samples did not.

The toughness of NCX-3A at room temperature (R.T.) was also

determined this quarter using the Indentation technique. This may be

compared in Table 1 with the R.T. toughness determined using singleedge notched bend bars. Precracking of these bars was accomplished at

1400 "C by slow bending. The values of K obtained by bending are significantly higher than those obtained by the indentation technique.

The discrepancy is probably due to the fact that cracks introduced at

elevated temperatures (i.e. creep cracks) are not nearly as sharp as those produced by indentation at R.T.
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chevron.

Table 1. Fracture Data of Various Structural Ceramics

Material Specimen Type Temperature Kj^(MPat^

Evidence of

Slow Crack

Growth

a-SiC thru-notched 1500 "C 4.06 No

a-SiC chevron 1500 '“C 9.3* No

a-SiC chevron 1600 °C 10.9* No

KT-SiC chevron 1050 °C 12.1* —

KT-SiC chevron 1200 °C 24.4* Yes

KT-SiC chevron 1350 "C 23.6* —

KT-SiC chevron 1400 15.8* Yes

NCX-34 indentation R.T. 6.0 —

NCX-34 thru-notched R.T. 9.0 —

*This data is preliminary
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NBS-5. 12(B) - CRACK GROWTH MECHANISM MAPS

DISCUSSION OF CURRENT ACTIVITIES

As noted in part A above, no stable crack growth was observed in

a-SiC. Notched specimens which had been held under load at 1500 °C for

different lengths of time are now being examined microscopically. As

shown in Figure 2 and 3, the areas near the notch were heavily cavitated.

This suggests that the mechanism of failure is generalized degradation

by creep cavitation as opposed to creep crack growth. We have obtained

self-dif fusivity data for SiC from the Diffusion Data Center at NBS in

order to investigate the above contention.

Most of the data for the crack growth mechanism map of NCX-34 has

been digitized and will be used to construct such a map in the immediate

future.

Figure 2. a-SiC notched specimen held under 40 kg load for 26 hours at

1500 ®C and then fractured. Notch root is visible at bottom

of micrograph. Cavities in the material are visible in

adjacent areas. (300X)
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Figure 3. 5000X magnification of cavities seen in Figure 2.

NBS-5. 12(C) - MICROSTRUCTURE AND PHASE ALTERATION

DISCUSSION OF CURRENT ACTIVITIES

Oxidation studies were carried out on NC203 and NC430 SiC for times

up to eight hours in air. Initial oxidation proceeded rapidly in the

case of siliconized SiC (NC430) . This SiC continued to exude silicon

with each heat treatment. The primary oxidation products formed on this

material appeared to be cristobalite plus some tridymite. After a total

of eight hours of oxidation at 1400 ®C in air, the hot-pressed SiC

(NC203) showed the first evidence of oxide layer formation. The x-ray

diffraction peak obtained was tentatively assigned to cristobalite.

Work continued on the construction of a furnace for high temperature

x-ray diffraction studies. Bonded ceramic fiber components and materials

were received, and pieces for the furnace chamber walls are being cut

and fashioned to the required shapes. Tests of the furnace are expected

to begin in April.
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Two approaches are under investigation for the fabrication of the

lu'atlng eieiTient. The first approach is the utilization of the sample as the heating element. Since the SiC samples generally have very high

resistances, it is necessary to apply a stable conductive coating to the

underside of tlie samples. Since we were Informed that a stable plasmasprayed coating was difficult to achieve on SIC, we undertook the Investigation of several flred-on compositions consisting of silicon carbide,

silicon, carbon, dopants, and binders. Although It has been possible to

form "hard" coatings, a low-resistance coating has not been achieved to

date

As a second approach, heating elements have been machined from the

central portion of SIC heating elements. Although this approach appears

to be feasible, present constraints on the design may Introduce too high

a limit on (20)-range at low (20) angles. Also, the larger thermal mass

Introduced Into the furnace Is undesirable. Nevertheless, this approach

Is being pursued to put the furnace Into operation and to gain some

experience with It.

Activities are also continuing to develop a model for Induced microstresses and microstrains. Since bulk materials generally contain

cracks and inclusions without well characterized regularities, mathematical idealizations generally involve special circumstances which permit

their solution. Presently, an attempt is being made to strike a balance

between what is desirable physically and what is tractable theoretically.

Idealizations are being sought which will render the problem of a simultaneous array of cracks and inclusions tractable. It is hoped that

such a model will retain the essence of the physical situation for a broad class of materials while possessing symmetries useful for a mathematical solution.

Some primary difficulties we have encountered in the development of

the model are as follows. Complex stress functions and conformal mapping

techniques have been applied successfully to isolated extended inclusions

in a contiguous infinite medium. An inclusion has a well defined

bounding surface within which the material properties are continuous and
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uniform. Cracks, on the other hand, have regions of plasticity at the

crack ends, and the mathematical difficulties associated with this

feature have caused an alternative to the conformal mapping techniques

to be sought. This has led to an idealization in which a crack is

treated as a slit which mathematically becomes a straight line cut in a plane. Such a cut is formally a displacement discontinuity. But, edge

and screw dislocations are also displacement discontinuities. Consequently, it is possible to find a distribution of these dislocations to

represent a slit-like crack. The advantage of this is that the strain

state for an edge or screw dislocation is known; and, hence, the strain

state for the crack can be computed from the equivalent distribution of

dislocations. The problem with this approach is that it works only for

simple displacement discontinuities; which, therefore, excludes extended

inclusions. Consequently, a mixed problem of cracks and inclusions

probably will have to be developed in the complex potential formalism.

Two considerations which must yet be given attention are the applicability of numerical solution techniques for the mixed problem and the

possibility of using rather general Green's function formulations.

NBS-5. 12(D) - MICROSTRUCTURE AND FRACTURE IN REACTIVE ENVIRONMENTS

DISCUSSION OF CURRENT ACTIVITIES

The construction of an apparatus for fracture mechanical testing of

selected structural ceramics in controlled gaseous environments at high

temperature is almost completed. An existing gear-driven loading

machine was readied for use, its worn out gear drive having been replaced.

This machine can produce a displacement rate as low as 0.5 ym/min for

crack propagation studies. A commercially manufactured gas-tight box

furnace (15 cm cube hot zone) was delivered the first week of February.

Molybdenum disilicide heating elements were installed in the furnace,

and the furnace was wired to an automatically controlled power supply.

The furnace was established level in the frame of the loading machine,

and silicon carbide loading rods were appropriately secured in special

bellows assemblies attached to the top and bottom ports of the furnace.

A thermostated bath was set up to generate a given partial pressure of
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w;i(«'r v.ipor wliicli will Ix’ conil) i iird with n j^p'ir: [low Ihronpli tlx; fnrnnce.

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tx'ollnp, of 1 Ix' fmii.'K'o ;;hrll .’iiid Ix'llow.x i (;,s to jircvciiL w-itcr

ix'iuh'U!;.! ( ion In (ho fniii.icc'. !>np|)]io.s of oxyjxxi, nifroj^oii, (.irlion

dioxidt', .'ind snlfnr dioxido {piso.s .ilonp, with ihoir flow roii t rol 1 or .<> are

avill.iMi'. Tho I r oonnoci ionn to the fnin.icx* .ind Lo an e.s I .ahl I aht-d

I'xli.nisl synl oin will c()nii>loto tho .nsaoinbl y of Lbe app.iratns.

Iioid'lo I or;: Ion spooiinons .ind clx-vron not died, four point bonding

spi'c iiiu’iis art* lu'ing in'oji.irod for rr.ick propagation and fracture tougbncBS

sfiulios, roc.pix't Ivt'ly . Both sintori'd .ilpb.a .silicon carbide and sllilonlxc'd (K'f) .silicon esubide blllc'L.s wore obtained coiiiniorcially and are

being diamond .s.iwed to in.ike the specimens. The fracture iiK?cbanlcal

t »';'.ts will coiimu'uce in the imiiu'diate future.

A second furnace and attendant gas lines were assembled for reaction

studli’s between SIC and gases other than oxygon which are components of

flue gases from coal-fired power plants. These gases Include SO2, CO2

CO, N2, and water vapor. Since N2 is expected to be relatively nonreactive at oxygen activities contemplated in this study, it will be

used primarily as a carrier or dilutant gas for this study. Sulfur

dioxide (1000 ppm in N2) is the first gas selected for study. It will

be used dried and containing controlled amounts of water vapor. We are

awaiting delivery of a SO2-N2 mixture.
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