NBSIR 73-297

Fracture and Deformation of Alumina

S. M. Wiederhorn

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 31, 1973

Final Report

Prepared for U.S. Army Research Office-Durham

Department of the Army

Durham, North Carolina 27706

Contract IMo. RDRD-MS-11549-MC

NBSIR 73-297

FRACTURE AND DEFORMATION OF A ALUMINA

S. M. Wiederhorn

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 31, 1973

Final Report

Prepared for U.S. Army Research Office-Durham

Department of the Army

Durham, North Carolina 27706

Contract No. RDRD-MS-11549-MC

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

FRACTURE AND DEFORMATION OF ALUMINA

ABSTRACT

This report summarizes work conducted during the past eight years on

the fracture and deformation of ceramic materials. Accomplishments discussed

in this report include: an elucidation of fracture processes in alxaminum

oxide and sodium chloride; the development of techniques to study the

deformation of alimin\jm oxide during abrasion; and the development of

techniques for measuring fracture mechanics parameters on ceramic materials.

FRACTURE AND DEFORMATION OF ALUMINA

This project was initiated to elucidate the mechanisms of fracture

of brittle materials, with the hope of assuring the reliability of these

materials in structural applications. Materials selected for study were

aluminxam oxide and sodium chloride. Alxaminum oxide was selected because

of its importance in ranging lasers, electronic substrates, ceramic

armor and air bearings. Sodium chloride was selected because its mechanical

properties were representative of a large group of ceramic materials having

the sodium chloride structure. These include the alkali halides which

today are important optical components of lasers.

Fracture mechanics techniques were used for these studies because

of their advantage in elucidating fmdamental processes occurring during

fracture. Fracture mechanics techniques are used to measure the fracture

surface energy or the stress intensity factor of a material. Since these

are \iniquely related to the crack tip stresses, they are the controlling

mechanical parameters for crack extension. Thus by using fracture mechanics

techniques one may relate various fracture phenomena, such as delayed

fail\ire, directly to the stresses at the crack tip.

The technique used for these studies was the double cantilever beam

(DCB) technique developed by Oilman to measure the surface energies of

single crystals. Because of the approximate nature of the equations

used by Gilman, a complete elastic solution of the DCB configuration
- 2 -

was obtained in the course of the project. This assured an accurate

equation for calculating stress intensity factors from load and specimen

dimensions. The complete elastic solution, obtained by the collocation

procedure, agreed with the one used by Oilman for cracks that were long

with respect to other specimen dimensions. However, for short cracks there

were significant differences between the two solutions because of a

contribution to the elastic energy from shear stresses in the crack arms

and stresses in regions of the specimen past the crack tip. The elastic

solution obtained in this program for the DCB configuration is now

widely used by other investigators.

Initial experimental work revealed that the fracture of sodium

chloride was severely influenced by plastic deformation at the crack tip,

and that the fracture of sapphire was severely affected by water in the

environment. Based on these observations, our experimental work developed

along two lines: one, investigating the effect of environment on fracture;

and the second, investigating the effect of plastic deformation.

Work on the plastic deformation of ionic crystals was conducted on

sodium chloride. Two types of plastic deformation were observed

at a crack tip, similar in many respects to plane stress and plane

strain deformation observed in metals. If plane stress deformation

occurred, the crack intersected slip bands when it propagated and

the energy for fracture was higher than the theoretical surface

energy. However, if plane strain deformation occurred at the crack
- 3 -

tip, dislocation motion in the vicinity of the crack resulted in

stress concentrations that aided crack motion. Consequently, the load

for fracture, and the fracture surface energy were considerably less than

predicted theoretically. These findings were consistent with a theoretical

model proposed by Clark [Phil. Mag. 1_, 393 (1962)] to explain observations of fracture in MgO, a material having the sodixim chloride

structure.

Environmental studies on single-crystal aliiminum oxide (sapphire)

indicated that the crack motion in this material is very similar to that

occurring in glass. Generally, fracture can be divided into three

regions of behavior. At low stress intensity factors (region I) the crack

velocity is influenced by the water in the environment and is strongly

dependent on the applied load. At higher stress intensity factors (region

II) crack motion depends only on the moisture in the environment. At

still higher stress intensity factors (region III) the crack motion depends

only on the stress and is independent of environment. This behavior

parallels that observed on glass, and suggests that at all but the highest

stress intensity factors, subcritical crack growth in sapphire is due to a

stress corrosion reaction with water in the environment. These results

agree with strength studies on polycrystalline alumina in which

delayed failure has been attributed to moisture in the air. This delayed

failure is undoubtedly due to the growth of cracks such as were observed

in the current study.
- 4 -

This project was next directed towards an elucidation of the fracture

process in sapphire. There is considerable controversy about the mechanism

of fracture in this material. Because sapphire is very hard, exhibiting

plastic flow only at temperatures greater than 900°C, it is expected to

fail by brittle fracture at lower temperatures. However, its decrease in

strength with temperature between 25 and 600°C is not consistent with the

classical theories of brittle fracture. Therefore, some other explanation

is necessary to account for the fracture behavior of sapphire. One group

of investigators believes that the strength decrease results from a plastic

flow process. They believe that fracture is intimately related to plastic

flow and that as the temperature increases, the plastic flow and, consequently,

fracture became easier. A second group disagrees with this conclusion

suggesting instead that the reason for the strength decrease is that the

fracture of sapphire is determined by thermally activated crack growth.

It was our feeling that these opposing theories could be critically evaluated

by studying the crack tip structure of sapphire using transmission electron

microscopy, and by accurately characterizing the fracture behavior of this

material via fracture mechanics techniques. Consequently, the fracture of

sapphire was studied using fracture techniques developed earlier in this

program. Also, new techniques were developed so that crack tips in sapphire

could be examined by transmission electron microscopy.

Our studies of the fracture energy of sapphire can be divided into

two parts: an evaluation of the fracture energy as a function of crystal
- 5 -

orientation; and an evaluation of the fracture energy and crack velocity

as a function of temperature under vacuum conditions. Fracture energies

(3) as a function of crystal plane were measured on three different

planes of sapphire, the basal (0001) plane, the rhombohedral twin (0112)

plane, and the secondary prismatic (1010) plane. Fracture energies for

2 the rhombohedral and prismatic planes were 6.0 and 7.3 J/m , respectively,

but for the basal plane, fracture energies were estimated to be greater

2

than 40 J/m . Crack propagation on this plane by the double cantilever

beam technique was not possible.

Fracture energy measurements were also made in vacuum by developing

a new test apparatus capable of a vacuum of 10 ^ torr and of a temperature

(12) as high as 750°C. Studies using this equipment showed that the fracture

energy of sapphire (1010 plane) depended on temperature, decreasing from

2 2 7.3 J/m at room temperature to approximately 4.5 J/m at 750°C. This

change in fracture energy was consistent with strength measurements,

suggesting that the strength decrease probably results from the effect

of temperature on the crack propagation rate. It was also observed

in this study that the temperature dependence of the fracture energy

depended on the crystallographic orientation of the fracture plane,

since cracks on the rhombohedral plane did not propagate on this

plane at temperatures above approximately 200° C even though this

was the favored plane at room temperature.

In smother aspect of our work, new techniques of specimen preparation

were developed for transmission electron microscopy studies of dislocation
- 6 -

interactions with crack tips. The techniques developed for this study

were based on earlier work at NBS by N. J. Tighe, who demonstrated the

value of ion bombardment thinning as a method of specimen preparation.

Specimens suitable for the electron microscope were prepared by first

placing indentations in the surface of one-eighth inch sapphire or alumina

disks. These indentations always initiated surface cracks which could be

studied by transmission electron microscopy provided the specimens were

thinned preferentially from the side of the disk opposite that containing

the crack. Examination of these specimens revealed dense dislocation

(8) activity associated with the indentation. However, little if any

(12) dislocation motion was associated with the crack tip. This observation

led to two additional studies in which the defect surface structure

(cracks, slip bands and twins) formed during the grinding and polishing

of sapphire was investigated. ^^'^^'-^^^ Dislocation and twin rearrangement

resulting from high temperature annealing were also studied.

Results from the transmission electron microscopy studies of crack

tips indicated that little if any dislocation motion occurred when cracks

were propagated at temperatures less than 400*'C. Above 400°C, some

dislocation motion was observed, but dislocation motion was the exception

rather than the rule. Cracks that were propagated by mechanically

stressing often healed spontaneously when the mechanical stresses were

released. This type of healing occurred when two crack surfaces came

back together in almost perfect registry. A different type of crack
- 7 -

healing occurred when specimens were annealed at high temperatures. In this

case, healing occurred by diffusion and sintering of the crack tip. Pore

growth and crack recession was always associated with crack healing at

high temperatures. This high temperature annealing probably accounts

for the increase observed in the strength of sapphire after annealing.

The most important result of the fracture and transmission electron

microscopy studies is that dislocation motion does not accoiint for the

temperature dependence of the strength of sapphire. Strength degradation

is more consistent with an activated crack growth process. However, the

causes of this activated process are as yet ill-defined. Since the

activation energy for the process is approximately 200 Kcal/mol, the fracture

mechanism may involve oxygen or aluminum ion diffusion near the crack

tip. Additional work will be necessary to further clarify the mechanism

of fracture in this very important material.

In summary, work on this project has resulted in: the development

of new techniques for determining fracture energy; a deeper understanding

of the fracture process in alkali halide materials and sapphire; an

exact elastic solution of the double cantilever beam configuration; the

development of a new technique for studying surface damage in ceramic

materials; and a characterization of surface damage resulting from the

abrasive wear of alumina during polishing, grinding and abrasion operations.

A list of all publications and technical reports published under Army

Research auspices is given below.
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1. Moisture Assisted Crack Growth in Ceramics, S. M. Wiederhom,

Intl. J. of Frac. Mech. 171 (1968).

2. Critical Analysis of the Theory of the Doiable Cantilever Method

of Measuring Fracture-Surface Energies, S. M. Wiederhom, A. M.

Shorb, and R. L. Moses, J. Appl. Phys. 39, 1569 (1968).

3. Fracture of Sapphire, S. M. Wiederhom, J. Am. Ceram. Soc. 52_,

. 485 (1969)

.

4. Fracture of Ceramics, S. M. Wiederhom, Proc. of Symposium on Mechanical and Thermal Properties of Ceramics, April 1-2, 1968,

National Bureau of Standards, Washington, D. C, NBS Spec. Publ.

303, May 1969, pp. 217-41.

5. Plastic Deformation and the Fracture Surface Energy of Sodium

Chloride, S. M. Wiederhom, R. L. Moses, and B. L. Bean, J. Am.

Ceram. Soc. 52, 18 (1970).

6. Crack Propagation in Polycrystalline Ceramics, S. M. Wiederhom,

Proc. 15th Sagamore Army Materials Res. Conf., Sagamore Conference

Center, Lake Raquette, New York, Aug. 20-23, 1968. From Ultrafine- Grain Ceramics , Syracuse Univ. Press, Syracuse, N. Y., 1970,

pp. 317-38.

7. Discussion of "Observation of Plastic Deformation in Sapphire Base

Ceramics," A. H. Heuer, B. J. Hockey, and S. M. Wiederhom, Metallurgical Transactions, 2_' 1^^^ (1971).

8. Plastic Deformation of Aluminvim Oxide by Indentation and Abrasion,

B. J. Hockey, J. Am. Ceram. Soc. 54_, 223 (1971).

9. Observations by Transmission Electron Microscopy on the Subsurface

Damage Produced in Aluminum Oxide by Mechanical Polishing and Grinding, B. J. Hockey, Proc. 20, Brit. Ceram. Soc. Mtg., "Textural

Studies of Ceramics," London, England, Dec. 16-17, 1970, June 1972,

. pp. 95-115.

10. Observations on Mechanically Abraded Aluminum Oxide Crystals by

Transmission Electron Microscopy, B. J. Hockey, Proc. of Symposiiam on "The Science of Ceramic Machining and Surface Finishing,"

NBS, Gaithersburg, Md., Nov. 2-4, 1970, NBS Spec. P\abl. 348, May

1972, pp. 333-39.
- 9 -

11. Use of the Hardness Test in the Study of the Plastic Deformation of

Single Crystals, B. J. Hockey, to be published in Proc. of Symposium

on "The Science of Hardness Testing and Its Research Applications,"

Detroit, Mich., Oct. 18-21, 1971.

12. Effect of Temperature on the Fracture of Sapphire, S. M. Wiederhorn,

B. J. Hockey, and D. E. Roberts, to be published in Phil. Mag.

THE FINDINGS IN THIS REPORT ARE NOT TO BE CONSTRUED

AS AN OFFICIAL DEPARTMENT OF THE ARMY POSITION, UNLESS

SO DESIGNATED BY OTHER AUTHORIZED DOCUMENTS.

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3. REPORT TITLE FRACTURE AND DEFORMATION OF ALUMINA

4. DESCRIPTIVE NOTES ("TVpe of report and Indusivo dates) Final

5. AUTHOR(S) (First name, middle initial, taat name)

Sheldon M. Wiederhorn

«. REPORT DATE

31 July 1973

7a TOTAL NO. OF PAGES

1°

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6a. CONTRACT OR GRANT NO. RDRD-MS-11549-MC

b. PROJECT NO.

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11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY U. S. Army Research Office-Durham

Box CM, Duke Station

Durham, North Carolina 27706

13. ABSTRACT

This report summarizes work conducted during the past eight years on the fracture

and deformation of ceramic materials. Accomplishments discussed in this report

include: an elucidation of fracture process of aluminum oxide and sodium chloride;

the development of techniques to study the deformation of aluminum oxide during

abrasion; and the development of techniques for measuring fracture mechanics

parameters on ceramic materials.

1^1^ ^ORM ^ "*^^*^** ^ORM M?*. I JAM •4. WHICH IS ©••OI.BTB FOB AHMV 0«B. UNCLASSIFIED

Security Classification
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1 4. KEY WORDS

LINK A LINK B LINK C RO L E W T ROLE W T ROLE W T

Electron microscopy; fracture; mechanical

properties; plastic deformation; sapphire;

sodium chloride.

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