QUARTERLY PROGRESS REPORT

April - June 1981

Samuel J . Schneider

Project Manager

RLE COPY

HIGH TEMPERATURE APPLICATIONS OF STRUCTURAL CERAMICS S*'

I

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

6c

lOO

"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 rights."

TABLE OF CONTENTS

PAGE

I. INTRODUCTION 1

II. DETAILED DESCRIPTION OF TECHNICAL PROGRESS . 1

Subtask A. High Temperature Fracture of Structural Ceramics. . 1

Subtask B. Crack Growth Mechanism Maps 3

Sub task C. Microstructure and Phase Alteration 4

Subtask D. Microstructure and Fracture in Reactive

Environments 7

i

NBS-5.2.10 - HIGH TEMPERATURE APPLICATIONS

OF STRUCTURAL CERAMICS

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

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_0„-Si_N, combinations have received particular

3 4 2 3 3 4

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 microstructural 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.

*

NBS 5.2.10(A) - HIGH TEMPERATURE FRACTURE OF STRUCTURAL CERAMICS

RESULTS FROM PRIOR QUARTERS

Four-point bend tests on notched bars of Si^AlON^ were completed

and the results compared with previous work on a yttria-doped SiC (NCX34)

and with literature values for Si^N^. Reduced scale testing rigs using

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SiC were manufactured to test small billets of experimental materials.

Fifteen different compositions of Si^N^ + Y.A.G. materials obtained from

Professor Tien of University of Michigan using these rigs were tested at

1300 ®C in order to avoid oxidation. Further studies of Tien's Si^N,

were conducted to ascertain critical stress intensity factors and to

clarify some inconsistencies in earlier results. A literature survey of

available data on fracture toughness, crack growth behavior and creep

properties of SiC, Si^N^ and sialon was completed. Four-point bending

tests under constant displacement rate were performed on several SiC and

sialon specimens. For a-SiC tested in air, the results showed little

oxidation and high resistance to slow crack growth up to 1000 °C. For

sialon specimens (Si, A1 0 Nq ) , it was found that as z increases, the

susceptibility to crack growth and oxidation decreases. The asymptotic

behavior of the stress intensity factor as a function of crack length

for 4-point bending at small and large crack lengths was investigated in

more detail. This work is useful for future numerical solution schemes.

DISCUSSION OF CURRENT ACTIVITIES

Experimental Work

High temperature four-point bending tests under constant displacement

rate condition have been performed on notched SiC specimens. Several

ways of initiating cracks failed due to the high degree of hardness

before a special chevron notch was successfully introduced into the beam

specimens. This produces a short, stable crack growth period. However,

the crack branches shortly into two subcracks which we speculate may be

due to shearing effects. Techniques to avoid this problem are currently

under investigation.

A paper entitled "Effects of Composition on the Fracture Toughness

and Slow Crack Growth of Sialon" is being prepared and will be presented

at the International Symposium on Fracture Mechanics of Ceramics,

University Park, Pennsylvania July 17, 1981.

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Theoretical Work

The P-A charts generated from the Instron testing machine have been

digitized and thus can be input directly to the Cromenco computer. A

computer program has been developed which converts the P-A curves into

V-K crack growth law. Conversely, given the V-K relationship, say

V = Ak'^, it is possible to predict the P-A curve. Regarding a and c as the dependent variables and A as independent variable, we obtain a system of first order nonlinear ordinary differential equations for the

unknowns c and a as follows:

2 dc 2AB . .n+2 A

T7 = —r~Y(a) —

^ = (^>4 V(a)"

dA : n

A c With the initial conditions: C = C^, a = a^, when A = 0, a computer

program for solving this system of coupled equations has been developed

which successfully predicts P-A curve for any given A and n. A paper

addressing this problem will be prepared shortly.

NBS 5.2.10(B) - CRACK GROWTH MECHANISM MAPS

RESULTS FROM PRIOR QUARTERS

Preliminary maps for Si^N^ have been drawn. A computer program for

automatically drawing maps from K-V data has been written. Data for SiC

has been collected with the aim of constructing a crack growth mechanism

map. An evaluation of K was made because its definition by ASTM E-399

conflicts with common usage in the ceramic community. A preliminary

crack growth mechanism map for a commercial hot pressed SiC was constructed

« from published information. The literature survey of fracture and crack

growth mechanisms was completed for SiC, Si^N^ and sialon.

A crack growth model based on diffusion mechanisms was developed.

The model entails a grain-boundary crack growing in steady state due to

applied stress by stress assisted surface and grain-boundary diffusion.

By further assuming the grain on either side of the crack behaves elastically.

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the model predicts a unique K-V relationship and a threshold K below which

no crack growth will take place. A comparison of this prediction to a set

of creep crack growth data on sialon at 1400 °C showed good agreement.

DISCUSSION OF CURRENT ACTIVITIES

The paper entitled "A diffusive crack growth model for creep fracture"

(NBSIR 81-2255) was submitted to J. Am. Ceram. Soc. on 4/10/81 for

publication.

An investigation into the energy release rate on this crack growth

model was also included in the report. The results show that J is the

total amount of energy consumption rate which consists of a portion

consumed in the creation of new crack surfaces and the remaining portion

dissipated in the matter transport processes.

The theoretical prediction of V-K functional relationship based on diffusion-controlled crack growth is being converted into K-T space in

order for the future data to fit in the diffusive crack growth area of

the mechanism maps.

NBS-5.2.10(C) - MICROSTRUCTURE AND PHASE ALTERATION

RESULTS FROM PRIOR QUARTERS

X-ray analysis of sialon samples having the formula

Si^ A1 0 No (z = 1 to 3) 6-2 z 2 8-2

showed that the lattice parameters varied linearly with z. Surface analysis

of 6-Si^N^ and garnet samples indicated the presence of B-Si^N^,

B-cristobalite, 6-Y

2

Si20y and mullite.

X-ray analysis were performed on various "as-received" commercial silicon

carbide ceramics. Carborundum's a-SiC was shown to be composed of. both 6H

and 4H polytypes; Norton's recrystallized SiC, to be composed exclusively

of the 6H polytype; Norton's NC203, to contain the 6H polytype, WC, a trace

of AI

2

O

2

and a trace of an unidentified phase; and Norton's NC403, to have

the 6H polytype, free Si and a well-developed, but unidentified phase.

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DISCUSSION OF CURRENT ACTIVITIES

Preliminary x-ray diffraction data for NC-203 silicon carbide

fracture specimens were analyzed. These specimens had been oxidized

at temperatures from 1200 to 1500 °C for 1 week. An unoxidized specimen

showed predominantly alpha silicon carbide with a lesser amount of

tungsten carbide. The development of a tridymite surface layer was evident on specimens oxidized at 1200, 1300 and 1400 “C. At 1500 °C, a glassy phase was observed to be present along with the tridymite in the

surface layer.

A literature survey covering the period from 1977 to the present was begun to assemble high-temperature thermal and mechanical data for SiC and

Si02 (tridymite) . This data will be used to estimate internal stresses

generated in the SiC substrate and in the oxide layer during high-temperature

oxidation of SiC. In addition, the previous studies related to the oxidation

kinetics of SiC are being surveyed.

A concept for a high-temperature x-ray furnace is illustrated by the

schematic shown in the attached figure. By using the specimen as its

own heating element, it is anticipated that the problems with heat loss

and temperature uncertainty in the specimen will be reduced, if not eliminated.

Electrical power will be coupled to the specimen through a pair of lowresistance, siliconized SiC rods. Temperature will be measured by locating

a thermocouple junction (not shown) adjacent to, but not touching, the bottom

of the specimen. In addition to the cylinder of bonded alumina fiber

insulation separating the specimen and the power input ends of the SiC rods,

a. second hollow cylinder C^ot shown) of similar insulation will surround

the specimen with the exception of a window for the x-ray beam. Finally,

the furnace will be enclosed in a gas-tight, water-cooled metal chamber

similar to a previous NBS design.

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Q.

Q.

CO

O

CO

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NBS-5.2.10(D) - MICROSTRUCTURE AND FRACTURE IN REACTIVE ENVIRONMENTS

RESULTS FROM PRIOR QUARTERS

Various alternate systems were considered for an apparatus in which

to conduct high temperature mechanical tests under gaseous environments

of fossil fuel combustion products. The following decisions have been made

concerning the system and its operation:

1. Heating System - MoSi

2

element electric furnace has been chosen. A gas

furnace was initially considered. Due to limitations on possible gaseous

environments, this system was rejected. The electric furnace decided upon

will allow a much wider range of gas compositions to be employed.

2. Gaseous Environment System - Provided by gas mixing from compressed

tank supply. Steam will be obtained either from a steam generator or by

bubbling gas composition through water at a specific temperature to give the

desired vapor pressure.

3. Loading System - A choice will be made between either an existing Instron

displacement rate controlled machine or a hydraulic ram coupled to a load

cell and specially assembled for the present purpose.

4. Specimen Configuration - Single-edge notched bars will be tested in

3 or 4 point flexural loading. These specimens will be 25 to 50 ram long

and will be used to determine K . Double torsion specimen 15 x 20 x 2 up

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to 25 X 72 X 2 mm will be used for crack velocity measurements.

DISCUSSION OF CURRENT ACTIVITIES

Design of the apparatus for high-temperature mechanical testing in

simulated fossil fuel environments was presented at the DoE program review

on May 8, 1981. The apparatus consisted of a gas-tight, high-temperature

C1600 “C) electrically heated furnace with attachment fixtures to allow

application of a load with a universal testing machine. Simulated

environments will be gaseous mixtures of steam and of up to four component

gases Ctypically, N

2

, ©

2

, CO

2

, and SO

2

) . Procurement of the two major

components of apparatus Cthe gas mixing system and the furnace with

temperature controller) is in process.

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Specimen configurations proposed last quarter for obtaining hightemperature crack growth and fracture data were the single edge notched

bar (SENB) in bending and the double torsion (DT) configuration. Both

specimens have their advantages and disadvantages, but their main disadvantages from a testing viewpoint are that the SENB specimen is difficult

to precrack in a stable manner and the DT specimen has a highly curved

crack front making the analysis somewhat questionable. Although we will

utilize both of these specimens, an alternate testing configuration is

being evaluated for possible usage to obtain high-temperature crack growth

and fracture data. It is the double cleavage drilled-compression (DCDC)

specimen used by Janssen to obtain crack growth data of glass at room

temperature in various liquid environments. The specimen, a compression

bar with a center drilled hole, precracks stably initiating two symmetric

cleavage cracks. A stress- intensity-factor solution for this specimen

has been given by Janssen using a finite element code for his specimen

dimensions. We are currently examining stress-intensity-factor solutions

for other specimen dimensions in an effort to determine if there is a regime of constant K behavior for this specimen.

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