DOE/NASA/0339-2

NASA CR-175054

iranm

GKOIOGI)©

S. Musikant, S.C. Samanta,

P.R. Architetto, and E. Feingold,

General Electric Company

Space Systems Division.

December 1985

Prepared for

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Lewis Research Center

Under Contract DEN 3-339

for

{NASA-CR- 17.5054) TRANSFORMATION TOUGHENED

CERAMICS 'FOR THE EEAV Y DUTY DIESEL ENGINE

TECHNOLOGY PBGGBAE, PHASE 2 final Report,

Aug. ,1984 - Jul. 1985 (General Electric Co.)

186 P_HC_A_05/MF :AQJ '_. CSCL 13P G3/85

N86-22451

Unclas

05J19
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DOE/NASA/0339-2

NASA CR-175054

Transformation Toughened Ceramics

for the Heavy Duty Diesel Engine

Technology Program — PHASE II

S. Musikant, S.C. Samanta,

P.R. Architetto, and E. Feingold,

General Electric Company

Space Systems Division.

December 1985

Prepared for

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Lewis Research Center

Under Contract DEN 3 339

for

U.S. DEPARTMENT OF ENERGY

Conservation and Renewable Energy

Office of Vehicle and Engine R&D
TABLE OF CONTENTS

SECTION

1

2

3

5

6

TITLE PAGE

INTRODUCTION AND BACKGROUND 1

TRANSFORMATION TOUGHENING (TT) 4

EXPERIMENTAL PROCEDURE 5

3.1 Materials and Processing 5

3.2 Characterization 6

RESULTS AND DISCUSSION 7

4.1 Process Optimization-Maximization of Percent Retained

Tetragonal Phase 7

4.2 Scale Up-Large Billet Preparation and Microstructrual

Characterization 7

4.3 Mechanical Properties of TT Mullite 8

4.4 Microstructural and EDX Analysis 8

4.5 Improved TT Mullite By use of Al203-Rich Mullite

. Matrix , 9

4.6 Microstructure of Mullite (AlgO Rich) 10

4.7 Discussion of Mechanical Test Results 10

4.8 Sintering Schedule Versus Percent Retained Tetragonal

& Grain Size 11

4.9 Thermophysical 12

SUMMARY AND CONCLUSIONS 13

REFERENCES 15

Appendix I
LIST OF TABLES

NUMBER TITLE PAGE

I HIGH PURITY MULLITE (193 CR) - DATA SHEET 16

II DESCRIPTION OF TT MULLITE COMPOSITIONS 17

III TT BILLETS FOR CHARACTERIZATION 19

IV TEST MATRIX 21

V MECHANICAL PROPERTIES OF TT MULLITE 22

VI TT MULLITE COMPOSITION BILLETS (ALUMINA RICH) 23

VII MECHANICAL PROPERTIES OF TT MULLITE (ALUMINA RICH) ... . 25

VIII 4-PT FLEXURE TEST RESULTS 26

VIIIA MULLITE-COMPOSITIONS OF BILLETS 27

VIIIB SPECIMEN DESIGNATIONS USED IN APPENDIX I 28

IX COMPRESSION TEST RESULTS FOR MULL 1:1 ZH (IITRI SM11) AT

VARIOUS TEMPERATURES 29

X THERMOPHYSICAL CHARACTERIZATION DATA FOR MULLITE AND ALUMINA

SPECIMENS 30
LIST OF FIGURES

NUMBER TITLE PAGE

1 . THE ZIRCONIA-HAFNIA SYSTEM 31

2 SCHEMATIC - TRANSFORMATION TOUGHENING 32

3 PREPARATION OF 1:1 Zr02-Hf02 POWDER MIXTURE BY SOL-GEL. . . 33

4 SCHEMATIC FABRICATING PROCESSES 34

5 XRD PATTERNS FOR SPECIMENS SHOWING FRACTION OF RETAINED

TETRAGONAL " . 35

6 FRACTURE SURFACE MULLITE, ROOM TEMP FLEXURE 36

7 FRACTURE SURFACE MULLITE - 10 V % Zr02, ROOM TEMP FLEXURE . 37

8 FRACTURE SURFACE MULLITE - 10 V % 2:1 Zr02-Hf02, ROOM TEMP

FLEXURE 38

9 FRACTURE SURFACE MULLITE - 10 V % 1:1 Zr02-Hf02, ROOM TEMP

FLEXURE 39

10 FRACTURE SURFACE MULLITE, 1000°C FLEXURE 40

11 FRACTURE SURFACE MULLITE - 10 V % Zr02, 1000°C FLEXURE. . . 41

12 FRACTURE SURFACE MULLITE - 10 V % Zr02-Hf02, 1000°C FLEXURE 42

13 MICROSTRUCTURE MULLITE - 10 V % Zr02-HF02 POLISHED AND

THERMALLY ETCHED SURFACE 43

14 COMPOSITION OF A MULLITE - Zr02/Hf02 GRAIN BOUNDARY (A) AND

A MULLITE GRAIN (B) IN MULLITE - 10 V % Zr02-Hf02 ... 44

15 MICROSTRUCTURE OF MULLITE NO. !A/Zr02-Hf02, 75/15 V % (TT 05

2185-1), SINTERED & HIPed 45

16 MICROSTRUCTURE OF MULLITE NO. !A/Zr02-Hf02, 90/10 V % (TT 05

2185-2), SINTERED AND HIPed 45
LIST OF FIGURES (CONTINUED)

NUMBER TITLE PAGE

17 MICROSTRUCTURE OF MULLITE NO. 2A/Zr02-Hf02, 85/15 V %

(TT 052185-3), SINTERED AND HIRED 46

18 MICROSTRUCTURE OF MULLITE NO. 2A/Zr02-Hf02, 90/10 V %

(TT 052185-4), SINTERED AND HIRED 46

19 MICROSTRUCTURE OF MULLITE NO. 2A/Zr02-Hf02, 90/10 V %

(TT 061485), SINTERED 47

20 SINTERING STUDY 90 V % MULLITE - 10 V % ZrOo.5HfQ.202 • • • 48

21 COMPARISON OF SECOND PHASE PARTICLE SIZE FOR TWO SINTERING

SCHEDULES 49

IV
SECTION 1

INTRODUCTION AND BACKGROUND

The objective of this program is to develop a transformation toughened structural

ceramic with improved high temperature properties and scale up the process

to sizes representative of components for a heavy duty engine diesel engine.

During the first year (Phase I) of this study, the feasibility of using

solid solution as a toughening agent in mullite (3 Alg^. 2Si02) and ajumina

(A1203) has been demonstrated. Specimens of mullite and of alumina containing

10-20v/o of Zr02-Hf02 (1:1 molar) solid solution were fabricated by sintering

or hot pressing at 1590-1620°C, characterized and tested for physical, mechanical

and thermal properties. Significant toughening of both matrix materials were

exhibited.^)*

Based on the need for a low thermal conductivity and high thermal shock resistant

ceramic in advanced diesel engines, transformation toughened (TT) mullite was

chosen as the prime candidate material for this study. The second year (Phase

II) efforts concentrated on the optimization of processes and scale up for

transformation toughened mullite. The material was characterized in sufficient

detail to provide the component designer/user a data base necessary for

preliminary design.

During the second year (Phase II) effort three tasks plus a reporting task

were completed. The three technical tasks are described below:

*Numbers in ( ) refer to references.
Task 1 - Process Optimization

Process variables were investigated to achieve high strength transformation

toughened mullite. The variables investigated were:

1. Powder preparation method for mullite matrix material.

2. Powder preparation method for the toughening agent precursor, Zrg.s

Hf0.502•

3. Composition, namely volume % of the second plate additive or toughening

agent Zr0.5Hfo.52.

4. Powder milling, blending and cold pressing (including iso-static

pressing) procedures.

5. Near net shape consolidation by sintering and/or HIP ing.

The resulting specimens were characterized by X-ray diffration (XRD) and scanning

electron microscopy (SEM). Flexural strength or modulus of rupture, compressive

strength and fracture toughness of the materials were determined at room

temperature as well as at elevated temperatures. The characterizations were

repeated for selected specimens after being subjected to a thermal soak in.

air at 1000°C for approximately 100 hours.

Task 2 - Process Scale Up

The best process developed in Table 1 was scaled up to a size consistent with

a diesel engine component. A typical size to be considered for scale up is

a piston cap approximately 5 1/2" O.D. X 1/2" thick. The actual scale up

achieved was on the order of 1" X 1" X 6" long.

Billets were fabricated to the scaled up dimensions and test specimens were

cut from these billets for detailed characterization.
Jask 3 - Characterization

Selected specimens from the scaled up billets were characterized as follows:

1. Microstructure by SEM.

2. Modulus of rupture at RT, 500°C, 800°C, 1000°C.

3. Compressive strength at RT and 800°C.

4. Fracture toughness at RT by indentation technique and fracture toughness

at RT, and 800°C by 4 point bend bar with multiple indents.

5. Coefficient of thermal expansion,RT to 1200°C.

6. Thermal conductivity at 110°C, 700°C and 1250°C.
SECTION 2

TRANSFORMATION TOUGHENING (TT)

, Hf02 and solid solutions of Zr02-Hf02 exist in three crystalline phases..

These are a low temperature monoclinic form, a high temperature tetragonal

form, and a still higher temperature cubic form. There have been a large number

of investigations (2-5) of transformation toughened Zr02 and transformation

toughening of other ceramic matrices such as A1203, where Zr02 has been used

as the toughening compound. Transformation toughening by ZrC>2 depends on the

metastable retention of the tetragonal phase upon cooling from elevated

temperature and the athermal tetragonal-mono!inic (t-*m) transformation of Zr02

grains (with an increase'in volume) under the influence of an advancing crack.

Near the t--m equilibrium temperature, transformation toughening does not occur

because the transition takes place thermally. TT is most effective at

temperatures far below the equilibrium t m temperature.

For Zr02» the equilibrium temperature for the monoclinic-tetragonal phase

transition is about 1000°C (1832°F) during heat up. For Hf02, m-^-t

transformation occurs at approximately 1749°C (3164°F). As shown in Figure

1, Phase Diagram of the Zr02-Hf02^) system, a 1:1 molar Zr02~Hf02 solid solution

has a m-^-t transformation temperature of about 1420°C (2588°F) and thus such

a solid solution increases the temperature range (compared to that of Zr02

alone) where transformation toughening is effective. In this program, the

1:1 molar Zr02~Hf02 was employed for the most part as the toughening agent.

The temperature of interest for engine applications extends to 1371°C (2500°C),

although diesel engine operation is generally limited to a temperature <814°C

(1500°F).

In summary, transformation toughening (TT) of a consolidated, dense mullite

body by inclusions of Zr02-Hf02 solid solution depends on the retention of

mestastable tetragonal phase of the Zr02-HfC>2 solid solution and transformation

of mestastable tetragonal to the thermodynamically stable monoclinic phase

under the influence of an advancing crack. The process is shown schematically

in Figure 2.
SECTION 3

EXPERIMENTAL PROCEDURE

3.1 MATERIALS AND PROCESSING

Major raw-materials used for preparation of TT mullite compositions are mullite

(the matrix) and Zr02~Hf02 (1:1 molar) as the toughening phase. High purity

mullite (3Al203-2Si02) powder from Baikowski International Corporation, Charlotte,

N.C., was used as the matrix material. Initially Baikowski mullite, Grade 193,

was planetary ball milled (PBM), wet, 'for 1^ hour, dried and then PBM for 15

minutes to get the starting matrix powder. Later on, another new, finer variety.

of Baikowski mullite, grade 193 CR, became available and thus it was substituted

for the mullite used earlier. The data sheet from Baikowski International on

193 CR mullite is shown in Table I. It was found that this mullite powder in

the as received condition is equivalent to the old Baikowski mullite (193 grade)

after planetary ball milling as described above.

solid solution toughening phase powder was prepared from an aqueous

solution of zirconium oxychloride (ZrOCl2-8H20). and hafnium oxychloride (HfOCl2-

8H20) in_ desired proportions via sol-gel process. For the most part, 1:1 molar

Zr02~Hf02 was used as the toughening agent. The method of Zr02-Hf02 powder

preparation by sol-gel technique followed by drying, calcination and planetary

ball millin g is described in Figure 3. Zr02~Hf02 powder in other proportions,

like, 2:1 molar and 3:1 molar were also prepared in the same manner and each

material was used as a toughening phase in a limited number of TT mullite

compositions.

TT mullite batch compositions were prepared by planetary ball milling both the

mullite matrix and ZrO'2-Hf02 toughening phase powders in requisite proportions

(90-85/10-15 vol %) for 15-30 minutes. Some batches containing A1203 additions

to the mullite matrix were prepared using Norton Co., grade 38-900 A1203- The

alumina and mullite were pre-blended by PBM for 15 minutes before millin g with

the Zr02-Hf02 powder. A few batches of mullite-1:! molar Zr02~Hf02 (90/10 v/o)

were also attrition milled in n-propanol for 6-9 hours in order to prepare much

finer particle size batch materials.
Powder batches thus prepared were pressed into billets by cold isostatic pressing

(CIP) at 40-55 KSI. The billets were then densified by sintering in air at

1600-1650°C for 0.5-1.0 hour, followed by, in some cases, hot isostatic pressing

(HIP) at 1500-1650°C, 29-30 KSI argon.

Figure 4 shows various possible fabricating processes for a TT material exercised

during this study and the precursor GE IR&D study.

3.2 CHARACTERIZATION

The resulting TT materials were characterized for fraction of retained tetragonal

phase by X-ray diffraction (XRD) as described in Reference 1 and microstructure

by scanning electron microscopy (SEM). Mechanical properties of selected samples

including modulus of rupture, fracture toughness and compressive strength of

the materials at room and elevated temperatures were determined. These mechanical

tests were performed by Illinois Institute of Technology Research Institute

(IITRI), Chicago, Illinois. Modulus of rupture and elastic modulus were

determined on TT specimens (3 mm X 4 mm X 45 mm on 20/40 mm supports) by 4-point

flexure test as per MIL-STD-1942(MR) . .

Most of the fracture toughness KIQ, measurements were performed in 4 point flexure

by multiple indent method as per Cook and
SECTION 4

RESULTS AND DISCUSSIONS

4.1 PROCESS OPTIMIZATION-MAXIMIZATION OF PERCENT RETAINED TETRAGONAL PHASE

During the initial stage of the investigation, procesing of mu11ite-l:l

(90/10 v/o) was optimized in order to achieve high percent retained tetragonal

of Zr02~Hf02 solid solution phase. A high percent retained tetragonal which

is transformable is desired to enhance the potential for TT during crack

propogation. The mullite matrix powder and Zr02~Hf02 toughening agent were

wet milled for various periods of time in a planetary ball mil l to produce

starting powder of varying particle sizes and distributions. Ten (10.0) gm

batches were prepared by PBM for 15-30 minutes. Three pellets (0.5" diameter)

of each powder batch were then cold pressed and sintered at 1610°C/30 minutes

and percent tetragonal phase retained was determined by XRD. Details of the

materials, processing conditions and XRD results are given in Table II. Based

on the data, it seemed that further wet millin g of the mullite beyond 1% hours

did not show much benefit. However, size reduction of Zr02-Hf02 toughening

phase by extended millin g provides some improvement in percent tetragonal phase

retainment^ The highest result of 34% retained tetragonal was obtained on

a mix composition (TT101984B) with 1% hours milled mullite and 1:1 Zr02-Hf02

milled for 8 hours. Thus, such starting powders were used for further batch

preparation. It was also found (TT020785) that the new, deagglomerated Baikowski

mullite, 193 CR, was a good substitute for \h hrs PBM mullite, type 193.

Based on the data in Table II, it was also found that percent tetragonal phase

retainment was improved to about 40% (TT102984B vs TT101984B) by a two-stage

millin g operation. Thus, later on this technique was also employed for batch

preparation. Results in Table II also show about 49% retained tetragonal for

a mullite-2:l molar Zr02/Hf02 composition and about 46% retained tetragonal

for a mullite-3:l molar Zr02Hf02 composition.

4.2 SCALE UP-LARGE BILLET PREPARATION AND MICROSTRUCTURAL CHARACTERIZATION

Large amounts (about 400.0 gms each prepared in 10.0 gm batches) of two mullite

Zr02/Hf02 compositions were prepared for large-scale billets for

characterization. Billets of mullite and a mullite-Zr02 composition were also

made for comparison.
Batch compositions, processing methods, sintered density and percent retained

tetragonal by XRD of sintered pieces are described in Table III. Each powder

batch composition was pressed into a large billet (approx. 8 3/4" X IV X 1")

by CIP'ing at 40 KSI. The billets were then cut into about 2.5" long pieces

and sintered at 1610°C for 30 minutes. . XRD studies indicated quite high fraction

of tetragonal phase retainment. Figure 5 illustrates the XRD patterns for

the Zr02-Hf02 and ZK>2 containing specimens.

4.3 MECHANICAL PROPERTIES OF TT MULLITE

Test specimens from sintered TT ceramic billets, as mentioned above, were

machined and fabricated by BOMAS Machine Specialities, Inc., Somerville, MA.

The test specimens were finished and tested in accordance with MIL-STD-1942(MR)

by IITRI, Chicago, Illinois, for mechanical properties (4-point .flexural

strength, elastic modulus and fracture toughness) at room temperature and

elevated temperatures (500, 800, 1000°C). They were also characterized for

their microstructures by SEM as well as optical microscopy. Table IV shows

the test matrix for these TT materials.

Mechanical properties of various TT mullite materials, as described in Table

III, are given in Table V. The mullite-10 v%l:l Zr02/Hf02 composition showed

the best mechanical properties of the various compositions studied. It showed

a flexural strength or MOR of about 38.2 KSI at room temperature and 33.3 and

35.4 KSI at 800°C and 1000°C respectively. MOR showed a dip (27.2 KSI) at

500°C probably due to some intrinsic mechanism. This trend was also seen in

other compositions. Fracture toughness, KJQ, for mullite-10 v% 1:1 Zr02/Hf02

was 2.73 and 2.44 MPa at room temperature and 800°C respectively.

Complete mechanical test results are given in the Appendix I authored by J.

W. Adams of IITRI.

4.4 MICROSTRUCTURE AND EDX ANALYSIS

Figures 6 to 9 show SEM fratographs of the fracture surfaces of flexure test

specimens (described in Table V) after test at room temperature. Figures 10

to 12 show fracture surfaces after 1000°C flexure tests of mullite, mullite-10

v% Zr02 and mullite-10 v% 1:1 Zr02/Hf02 respectively. Figure 13 shows the

microstructure of mullite-10 v% 1:1 Zr02Hf02 after polishing and thermally
etching at 1500°C, 15 mins. It shows round Zr02/Hf02 solid solution grains

<1 urn and elongated 1-5 urn mullite grains. It is believed finer grain sizes

are necessary for further improvement in mechanical properties.

Also, stoichiometry of a mullite-10 v% 1:1 Zr02/Hf02 specimen was investigated

by energy dispersive X-rays (EDX) analysis. Figure 14 illustrates the difference

in composition in the mullite-Zr02-Hf02 grain-boundary (A) and the mullite grain

(B) showing a definite increased silicon peak in the grain boundary.

A quantitative analysis of the mullite-Zr02-Hf02 grain boundary by EDX shows

a Al/Si wt % ratio of 67.2/32.7, which is much lower than a theoretical Al/Si

wt % ratio of 74.3/25.7 in a stoichiometric nominal mullite (71.8/28.2 wt %

Al203/Si02). EDX analysis of other TT materials (Table V) also shows Si02 rich

mullite compositions which would lead to a Si02-rich glassy phase in grain

boundaries at processing temperatures. The presence of such a high silica glassy

phase in grain-boundaries is believed to have contributed to degrading the

flexural strength of the TT materials tested up to this point in the study.

•

4.5 IMPROVED TT MULLITE BY USE OF AL?0^-RICH MULLITE MATRIX

In order to minimize the formation of a Si02-rich liquid phase in a TT ceramic

at the sintering temperature, it was decided to prepare two Al203-rich mullite

matrix materials by adding 10 and 18 vol % ^2Q3

 to 193 CR Baikowski mullite.

The mullite matrix, #1A, had a calculated composition of about 72.0/28.0 wt

% Al203/Si02 and another mullite matrix, #2A, had 75.0/25.0 wt % Al203/Si02-

Phase diagram work by Klug and Prochazka at GE-CRD(8) has shown that mullite

can contain up to 75 wt % A1203 at 1650°C.

As shown in Table VI density of the sintered TT materials increased by about

1.5 - 2.0% after containerless HIP'ing. Density of the sintered plus HIP'ed

materials ranged between 98.0 - 99.0%. Although compositions containing 10

vol % 1:1 ZK>2-Hf02 exhibited higher percent retained tetragonal compared to

those containing 15 vol % Zr02-Hf02, (10 - 11% vs. 5 - 6% of the Zr02-Hf02

content) after sintering, further processing by HIP'ing resulted in approximately

the same percent retained tetragonal (5-6%) for both levels of Zr02-Hf02 addition
probably due to grain growth. This is considered to be a low percent retained

tetragonal, on the order of 0.5% of the total volume of the specimen in the

case of the 10 v/o 1:1 Zr02~Hf02 addition.

4.6 MICRQSTRUCTURE OF MULLITE (AlpO^ RICH)

We also examined the microstructure of all the TT materials by a scanning electron

microscope (SEM). Photomicrographs showing Zr02-Hf02 solid solution (ss)

toughening phase distribution and grain size in these sintered and/or HIP-ed

materials are shown in Figures 15 to 19. They show fairly uniform distribution

of the toughening phase (Zr02~Hf02 ss) and sizes range predominantly between

0.2 to 1.0 urn. The first four materials (TT 052025-1- to 4, Table V), which

have been sintered and then HIP-ed also show some larger Zr02-Hf02 ss grains

(about 1-3 urn) due to the extended heating cycle compared to TT 061485 specimen,

which was made by sintering only.

4.7 DISCUSSION OF MECHANICAL TEST RESULTS

The mechanical test data is presented in full detail in Appendix I. Specimen

designations used in Appendix I are described in Table VIIIB.

The results tabulated in Table VIII (billets described in Table VIIIA) show

that the addition of 1:1 molar Zr02-Hf02 solid solution to mullite significantly

enhances the MOR at RT and elevated temperatures. The effect on KJQ is stronger

at both RT and elevated temperature. Thermal soak of these materials at 1000°C/

140 hrs shows a RT strength (MOR) degradation of about 25%. Scanning electron

microscopy has shown grain growth during such thermal soaks and the strength

degradation is attributed to such grain growth.

Of the TT mullite specimens, Specimen #SM6 in Table VIII appears to be about

the best of the compositions although additional work is expected to show much

improved properties over #SM6. In this specimen fracture toughness was increased

by 50% at RT and by 75% at 800°C and MOR was improved by 45% at RT, 30% at

800°C and 10% at 1000°C over straight mullite.

There appears to be a drop-off in strength at temperatures in the 500-800°C

range for mullite and the TT-mullites (for example specimens SMI and SM6).

10
The compression data for one billet SM11 appears encouraging. As can be seen

in Table IX. If one takes the highest data point for each condition, even at

the current state of development of the TT-mullite, very high values were measured

on single cylindrical test specimens 6.35 mm dia X 12.7 mm long. These test

points- were: RT 375 Ksi; 800°C 267 Ksi; 1000°C 174 Ksi; and after 1000°C/100

hr thermal soak, RT 178 Ksi.

4.8 SINTERING SCHEDULE VERSUS % RETAINED TETRAGONAL & GRAIN SIZE

Although the highest 4 point flexural strength of about 48 Ksi was obtained

with an alumina-rich TT mullite, it is clear that further improvement in

mechanical properties of such TT materials is needed. It is believed that still

finer grain size is needed to achieve higher strength. Thus, a number of batches

of mul.lite #1A-1:1 molar Zr02-Hf02 (90/10 v%) were prepared by attrition millin g

in n-propanol for 9 hours. Powder batches were then CIP'ed into billet form

at 55 Ksi and sintered in air at various temperature between 1500-1650°C for

30 minutes. The idea is to keep the grain sizes of the toughening Zr02~Hf02

phase (as well as matrix phase) small as one in the range which would maximize

the percent retainment of the transformable tetragonal phase and hence increase

the strength and fracture toughness of the TT material. Density and % retained

tetragonal in the sintered TT material(s) are shown in Figure 20. It appears

that a sintering temperature of 1600°C will be preferred to 1650°C because percent

retained tetragonal reached a maximum of about 23% (at > 96% theoretical density)

at 1600°C. In addition, as shown in SEM microstructures in Figure 21, as

expected, Zr02~Hf02 grain sizes are also much smaller at 1600°/30 minutes than

those obtained after sintering at 1650°C/30 minutes, approximately 1.0 urn compared

to approximately 0.5 urn (Figure 20).

Thus, it is believed that additional optimization in mullite-Zr02-Hf02

compositions and processing techniques will be effective for further improvement

in the mechanical properties of transformation toughened mullite.

11
4.9 THERMOPHYSICAL

Thermophysical properties of TT materials were not measured during this Phase

II study'1). However, thermal expansion coefficient (CTE), thermal diffusivity

and thermal conductivity data on various TT mullite and TT alumina determined

in Phase (1) are listed in Table IX. The CTE's of the mullite materials were

about 60% of those measured for the alumina matrix materials. Also, the thermal

conductivity of the mullite matrix specimen was seen to be between 23% and 40%

of the comparable alumina matrix specimen over the temperature range (110-1250°C)

investigated.

12
SECTION 5

SUMMARY AND CONCLUSIONS

Transformation toughened (TT) mullite is a good candidate as a structural ceramic

component in advanced diesel engine applications due to its low thermal

conductivity, low thermal expansion coefficient and inherently high thermal

shock resistance. During Phase II of this DOE-NASA sponsored program, attempts

were made to develop optimized mullite-ZK)2/Hf02 compositions and processes

for scale up and characterization in order to provide the component designer/user

the data base necessary for design.

Investigations of various mullite-Zr02/Hf02 compositions included processing

via preparation of Zr02-Hf02 (1:1 molar) toughening phase by a sol-gel method,

planetary ball milling or attrition millin g of starting materials and powder

batches, cold isostatic pressing into billet form and then sintering or sintering

followed by hot isostatic pressing. Characterization of TT materials included

percent Zr02-Hf02 solid solution tetragonal phase by X-ray diffraction, density,

mechanical properties, MOR or flexural strength, fracture toughness, Kj^, elastic

modulus, compressive strength at room and elevated temperatures upto 1000°C,

thermal expansion and conductivity, and microstructures by scanning electron

microscopy.

The best TT material developed so far had a composition of 85 vol % mullite

- 15 vol % 1:1 molar Zr02~Hf02 (Specimen SM-6 in Table VIII). The mullite was

a modified Baikowski mullite prepared by adding extra A1203 (Baikowski mullite/

A1203 = 90/10 vol %) in order to eliminate high silica content glassy phase

at the grain boundaries and thus improve the mechanical properties.

This mullite - Zr02/Hf02 transformation toughened material showed a 4 point

bend flexure strength of 48 KSI and 34 KSI at room temperature and 800°C

respectively.

13
Fracture toughness, Kjr,, was found to be 3.3 MPaxI m at room temperature and

2.57 at 800°C. In comparison, untoughened mullite (made by the same process)

had a MOR of 33 KSI and KIC of 2.3 MPa4~~m~at room temperature and a MOR 26 KSI

and KIC, 1.5 MPaJlTat 800°C (Table X).

However, it has been suggested that a TT material should have at least 60 KSI

room temperature flexural strength (4 point bend) to be seriously considered

for a piston cap design study for diesel engine as originally planned under

Phase III of the current program.

Based on the experimental data obtained so far, it is apparent that further

work towards optimization of mullite - Zr02/Hf02 compositions and processing

methods is needed.

RECOMMENDATIONS FOR FUTURE WORK

Such advanced work would be aimed at further reductions in particle size by

_attrition milling and lowest possible temperature processing. Probably, the

best approach would be to cold isostatically press the powder, relatively low

temperature sinter (bisque fire), encapsulate, and HIP at the lowest possible

temperature, ^1500°C.

In addition, all sources of defects such as large pores, large, uncomminuted

particles in the batch, and defects in the CIP'ed green body wil l have to be

eliminated since many flexural fracture surfaces during the investigation showed

such flaws, as described in Appendix I.

14
SECTION 6

REFERENCES

1. "Transformation Toughened Ceramics For The Heavy Duty Diesel Engine Technology

Program, Annual Report - Phase I", S. Musikant, E. Feingold, H. Rauch and

S. Samanta, NASA CR-174689, October 1984.

2. "Transformation Toughening, Part 4, Fabrication, Fracture Toughness .and

Strength of Al203-Zr02 Compositions", F. F. Lange, J. Of Materials Science,

17, p. 247-254 (1982).

3. "Design of Transformation Toughened Ceramics", N. Claussen, M. Ruhle, Advances

in Ceramics, Vol 3, Science & Technology of Zirconia, American Ceramic

Society, 1981.

4. "Stress-Induced Transformation of Tetragonal ZrO? Particles in Ceramic

Matrices", Nils Claussen, J. Am. Ceramic Society, 61 (1-2) 85-86 (1978).

5. "Transient Thermal Stress Behavior in Zr02-Toughened A1203", J. Am. Ceramic

-Society, 64 (1) 37-39 (1981).

6. "The'System Zirconia - Hafnia", R. Ruh, H. Garrett, N. Tallam and R. Domagela,

J. Am. Ceramic Society, 51, p. 23 (1968).

7. "Modified Indentation Toughness Technique", R. F. Cook and B. R. Lawn, J.

Am. Ceramic Society, C-200, November 1983.

8. F. J. Klug, PhD Thesis, "Alumina-Silica Phase Equilibria in the Mullite

Region", December 1984, Rensselaer Polytechnical Institute.

15
TABLE I

HIGH PURITY MULLITE

REF. 193 CR

(BAIKOWSKI, INTERNATIONAL

CHARLOTTE, NORTH CAROLINA)

Chemical Formula: 3Al205-2Si02

Minimum Purity: > 99.2%

% Converted to

Mullite Phase: > 98%

Surface Area: 2 m2/gm

Bulk Density: 0.6 gm/cc

Tap Density: 0,9 gm/cc

Pressed Density: 1,30 gm/cc (2000 psi)

Agglomerate Size Distribution

Cumulative Weight Percent

< 1 .On m 24

< 1.5 M m 34

< 3.On m 85

< 6.On m 100

Mullite can also be supplied in an

as-calcined agglomerated form with

a larger particle size distribution.

16
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20
TABLE IV TEST MATRIX

Temperature, "Ca

Property

Flexure strength and

elastic modulus

Unexposed

Exposed 100 hr/

1000'C

Fracture toughness

Unexposed

Exposed 100 hr/

1000'C

Fracture surface

analysis

25

3

3

3

3

All

bUO 800 1000

3 3 3

3 3 3

3

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samples optically,

selected samples In

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aNumbers 1n table represent sample replica tes.

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28
TABLE IX

COMPRESSION TEST RESULTS FOR MULL 1:1 ZH (IITRI SM11)

AT VARIOUS TEMPERATURES

Sampl e

Number

1

2

3

Mean

4

5

6

Mean

7 -

8

9

Mean

10

11

12

Mean

Compressi ve

Strength

psi

375,250

144,600

122,200

214,120

260,710

266,870

132,480

220 ..020

173,980

52,150

113,065

25°C

177,780

36,360**

137,370

117,170

MPa

2587.3

997.0

842.5

1475.6

1797.5

1840.0

913.4

1517.0

1199.5

359.6

779.6

El

106

25°C

32

26

29

800°C

-

-

1000°C

-

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After 1000°C/100

1225.7

250.7

947.1

807.8

26

13

20

astic Modulus

psi GPa

.6 224.7

.8 184.6

.7 204.7

_ _-

-

_

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hr Exposure

.7 184.1

.9 95.7

.3 139.9

Poisson's

Ratio

0.29

—

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0.26

0.28

0.27

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(ni)T (nT)T (

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ADDITIVE

1:1

Spec. No. Til 1684

2:1 Zr02 -

Spec. No. TT120784A

All Zr02

Spec. No. TT20784B

RETAINED

TETRAGONAL PHASE AS

FRACTION OF ADDITIVE

0.40

0.66

0.78

FIGURE 5 X-RAY DIFFRACTION PATTERNS FOR SPECIMENS

SHOWING FRACTION OF RETAINED TETRAGONAL

35
UJo:IDXLUUJo:01UJQ.EUJOOo:UJUJu<ru.C:=3U)UJo:u<rQ:u.UJoru

36
ORIGINAL PAGE 13

OF POOR QUALITY

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ORIGINAL PAGE IS

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Of POOR QUALITY

FIGURE 15 MICROSTRUCTURE OF MULLITE NO. !A/Zr02-Hf02, 85/15 V%

(TT052185-1), SINTERED AND HIPed

FIGURE 16 MICROSTRUCTURE OF MULLITE NO. !A/Zr02-Hf02, 90/10 V%

(TT05218502), SINTERED AND HIPed

45
10KV X5080 19U 951

FIGURE 17 MICROSTRUCTURE OF MULLITE NO. !A/Zr02-Hf02, 85/15

(TT052185-3), SINTERED AND HIPed

TE 2A/ZR02-HF02

FIGURE 18 MICROSTRUCTURE OF MULLITE NO. 2A/Zr02-Hf02, 90/10

(TT052185-4), SINTERED AND HIPed

46
ORIGINAL PAGE IS

OF POOR QUALITY

FIGURE 19 MICROSTRUCTURE OF MULLITE NO. 2A/Zr02~Hf02, 90/10 V%

(TT061485), SINTERED

47
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48
ORIGINAL PAGE IS

0E POOR QUALITY

N

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49
APPENDIX I

Report No. IITRI-M06138-1

MECHANICAL PROPERTIES OF TOUGHENED MULLITE

General Electric Company

Space Systems Division

Valley Forge Space Center

P.O. Box 8555

Philadelphia, Pennsylvania 19101

Attention: Dr. Solomon Musikant

Prepared by

Jane W. Adams

IIT Research Institute

10 West 35 Street

Chicago, Illinoi s 60616

11 November 1985
u.

OF p0,T ORIGINAL PAGE IS

OF POOR QUAJLfTY

FOREWORD

This report contains the results of mechanical property evaluation of

various toughened mullite compositions that General Electric is developing for

DOE/NASA. The materials employ mullite as a matrix for a dispersed toughening

phase of ZrO /HfO solid solutions. 2 2

Flexure strength and fracture origin analysis, elastic modulus, and

fracture toughness were evaluated for eleven developmental materials. Data

were compared to transformation-toughened zirconia values where appropriate.

Respectfully submitted,

I IT RESEARCH INSTITUTE

W. Adams

Associate Engineer

Approve

S. A. Bortz, Manager

Nonmetallic Materials and Composites

Materials and Processing Technology

N T RESEARC H INSTITUT E

ii IITRI-M06138-1
1. FLEXURAL STRENGTH

The flexural strength was determined in the quarter 4-point configuration

on test samples that were of nominal dimensions 3 x 4 x 45 mm. The upper and

lower spans were 20 and 40 mm, respectively, and the strain rate was 9.5 x

10"5 sec"1 (a crosshead speed of 0.5 mm/min). The sample and fixture dimensions conform to fixture configuration B in MIL-STO-1942 (MR), "Flexure

Strength of High Performance Ceramics at Ambient Temperature."

The static elastic modulus was determined by measuring the outer fiber

tensile strain during flexural strength testing. Resistance strain gages were

used to measure deformation at 25°C. At elevated temperature a precision

electromechanical instrument was employed, where deformation was recorded at

three positions on the sample tensile face within the region of pure bending

(i.e., within the upper span length). This was accomplished by extending SiC

rods from an LVDT coil and core up through the furnace to the sample. In this

manner, the outer fiber tensile strain was directly recorded, without the need

to employ potentially inaccurate methods of subtracting out the fixture/load

rod deformations.

The fast fracture bend strength results are presented in Tables 1 to 10

and Figure 1. The results of samples that were exposed for nominally 100 hr

at 1000°C in static laboratory air prior to residual room temperature strength

measurement are also included in the tables. Both toughened and non-toughened

materials were tested. Generally the toughened materials retained ~75% of

their room temperature strength in fast flexure testing at 800°C, although

MULL 1:1 ZH, 10 v/o, and MULL 2A ZH, 10 v/o (a) retained only about 50%, while

MULL 2A ZH, 10 v/o (b) retained 88%. Samples which were exposed at 1000°C/140

hr then tested at room temperature also had strengths ~75% of the unexposed

room temperature values, with the exception of MULL 2A ZH, 10 v/o (a) which

had a 25% increase in strength. In most cases the elastic moduli decreased as

a function of temperature and exposure by a few percent, with MULL 2A ZH, 10

v/o (b) showing the least variation (although this material was not exposed at

1000°C).

N T RESEARC H INSTITUT E

1 IITRI-M06138-1
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All fracture surfaces were examined in a stereoscopic optical microscope,

and selected samples in an SEM. Fracture origins are indicated in Tables 1 to

10. Most fracture origins were related to processing. Open or closed pores

were small, singular, and generally round, either below the tensile surface,

or exposed during test sample preparation by diamond grinding. "Processing

defect" is a more general category, encompassing poorly sintered regions, pore

clusters, or large irregularities in the overall structure, which may have

appeared during cold pressing. Subsurface large grains in these materials

were glassy in appearance. Examples of the various types of critical flaws

are shown in Figure 2.

N T RESEARC H INSTITUT E

3 IITRI-M06138-1
TABLE 1. FLEXURE TEST RESULTS FOR NULL (IITRI SMI)

AT VARIOUS TEMPERATURES

4-Point Failure

Sample

Number

1

2

3

Mean

4

5

6

Mean

7

8

9

Mean

10

11

12

Mean

Sample

Number

1

2

3

4

5

6

7

8

9

10

11

12

Flexure Strength Strain,

psi

a.

32,840

32,580

34,880

33,430

24,350

23,030

21,550

22,980

27,670

23,180

26,970

25,940

31,740

31,760

35,410

32,970

Open Closed

Pore Pore

*

•

•

«

•

•

a

MPa %

Elastic

ID6 psi

Modulus

GPa

Strength at Various Temperatures

25°C

226.4 0.12

224.6 0.12

240.5 0.13

230.6 0.12

500°C

167.9

158.8 0.07

148.6 0.06

158.4 0.07

800°C

190.8 O.U8

159.9 0.07

186.0 0.09

178.9 0.08

1000°C

218.9 0.13

219.1 0.11

244.2 0.14

227.4 0.13

b. Fracture Origins

Critical Flaw

Processing Subsurface

Defect Large Grai

•

•

•

27.0

27.8

26.8

27.2

_ _

32.5

36.7

34.6

34.3

33.4

29.3

22.6

23.9

28.2

25.4

25.8

Type

Machi ning

n Fl aw

186.4

191.6

184.6

187.5

_ _

223.8

253.3

238.6

236.4

230.0

201.7

222.7

165.0

194.8

175.5

178.4

Undetermi ned

*

•

IITRI-M06138-1
TABLE 2. FLEXURE TEST RESULTS FOR MULL-Z (IITRI SM2)

AT VARIOUS TEMPERATURES

4-Point Failure

Sample

Number

1

2

3

Mean

4

5

6

Mean

7

8

9

.Mean

10

11

12

Mean

Sample

Number

1

2

3

4

5

6

7

3

9

10

11

12

Flexure Strength Strain,

psi

a.

29,980

46,390

36,550

37,640

24,040

22,280

29,210

25,180

32,000

25,240

33,990

30,410

32,220

24,880

36,530

31,210

Open Closed

Pore Pore

*

•

•

•

•

•

•

•

HP a %

Elastic

106 pSi

Modulus

GPa

Strength at Various Temperatures

25°C

206.8 0.11

319.9 0.17

252.0 0.13

259.6 0.14

500°C

165.8 0.06

153.6 0.06

201.4 0.09

173.6 0.07

800°C

220.7 0.13

174.1 0.09

234.4 0.10

209.7 0.11

1000°C

222.2 0.13

171.6 0.10

251.9 0.17

215.2 0.13

b. Fracture Origins

Critical Flaw

Processing Subsurface

Defect Large Grai

•

•

*

>

27.1

27.7

27.9

27.6

37.9

35.3

34.1

35.8

28.2

27.3

33.1

29.5

25.3

23.7

21.2

23.4

Type

Machi ning

n Fl aw

187.2

191.3

192.1

190.2

261.3

243.5

234.9

246.6

194.2

188.5

228.5

203.7

174.3

163.7

145.9

161.3

Undetermi ned

IITRI-M06138-1
TABLE 3. FLEXURE TEST RESULTS FOR MULL 1:1 ZH (IITRI SM3)

AT VARIOUS TEMPERATURES

4-Point Failure

Sample

Number

1

2

3

Mean

4

5

6

Mean

7

8

9

Mean

10

11

12

Mean

Sample

Number

1

2

3

4

5

6

7

8

9

10

11

12

Flexure Strength Strain,

psi

a.

37,160

41,070

36,210

38,150

27,540

28,030

25,410

26,990

34,130

32,300

33,440

33,290

32,410

35,830

38,130

35,460

Open Closed

Pore Pore

•

«

•

•

•

•

»

MPa %

Elastic

106 psi

Modulus

GPa

Strength at Various Temperatures

25°C

256.3 0.13

283.2 0.15

249.7 0.13

263.1 0.14

500°C

189.9 0.08

193.3 0.08

175.3 0.10

186.2 0.09

800°C

235.4 0.13

222.7 0.12

230.6 0.12

229.6 0.12

1000°C

223.5 0.15

247.1 0.20

262.9 0.17

244.5 0.17

b. Fracture Origins

Critical Flaw

Processing Subsurface

Defect Large Grai

*

•

•

•

27.7

27.2

28.0

27.6

34.9

37.4

26.4

32.9

26.4

27.0

28.0

27.1

21.4

18.3

22.8

20.8

Type

Machining

n Fl aw

191.0

187.4

193.2

190.5

240.8

257.7

181.8

226.8

182.1

186.0

193.1

187.1

147.7

126.0

157.1

143.6

Undetermined

•

IITRI-M06138-1
TABLE 4. FLEXURE TEST RESULTS FOR MULL 2:1 ZH (IITRI SM4)

Sample

Number

4-Point

Flexure Strength

psi. MPa

Fai lure

Strain, Elastic

% 106 pSi

Modul us

GPa

a. Strength at Various Temperatures

1

2

3

Mean

4

5

6

Mean

7

8

9

Mean

10

11

12

Mean

33,700 232.

22,410 154.

35,330 243.

30,480 210.

No materi

25°C

4 0.12 28.5

5 0.08 27.7

6 0.12 28.4

2 0.11 28.2

500°C

al tested at 500°C

800°C

196.7

191.3

195.6

194.5

No material tested at 8UO°C

29,080 200.

33,050 227.

25,710 177.

29,280 201.

1000°C

5 0.17 17.0

9 0.16 19.9

3 0.13 19.8

9 0.15 18.9

117.5

137.3

136.5

130.4

b. Fracture Origins

Sample

Number

1

2

3

10

11

12

Open Closed Processi

Pore Pore Defect

•

•

Critical Flaw Type

ng Subsurface Machining

Large Grain Flaw

.

Undetermi ned

•

•

•

IITRI-M06138-1
TABLE 5. FLEXURE TEST RESULTS FOR MULL-ZH-HIP* (IITRI SMS)

Sample

Number

1

2

3

Mean

4

5

6

Mean

4-Point

Flexure Strength

psi

a.

39,050

36,550

42,140

39,250

29,310

19,610

27,500

25,470

MPa

Strength at

269.3

252.0

290.6

270.7

202.1

135.3

189.7

175.6

Failure

Strain,

%

Elastic

106 psi

Modul us

GPa

Various Temperatures

25°C

0.12

0.11

0.13

0.12

80JTC

0.09

0.06

0.08

0.08

32.6

32.8

32.3

32.6

33.3

33.1

33.8

33.4

224.6

226.3

223.0

224.6

229.6

228.5

233.4

230.6

b. Fracture Origins

Critical Flaw

Sample

Number

1

2

3

4

5

6

Open Closed

Pore Pore

•

•

Processing Subsurface

Defect Large Grai

•

•

Type

Machining

n Flaw

Undetermi ned

•

•

*B111et No. T041285-2.

IITRI-M06138-1
TABLE 6. FLEXURE TEST RESULTS FOR MULL 1:1 ZH (IITRI SM6)

Sample

Number

4-Point

Flexure Strength

psi MPa

a. Strength at

1

2

3

Mean

4

5

6

Mean

XI

X2

- X3

Mean

43,990

47,690

52,540

48,070

32,320

36,950

32,100

33,790

25°C

33,630

32,360

41,900

35,960

303.3

328.8

362.3

331.5

222.9

254.8

221.4

223.0

Fai lure

Strain,

%

Elastic

10^ psi

Modul us

GPa

Various Temperatures

25°C

0.14

0.15

0.17

0.15

800°C

0.09

0.10

0.10

0.10

31.8

31.9

32.3

32.0

36.6

35.9

32.0

34.8

219.3

219.9

222.7

220.6

252.3

247.8

220.5

240.2

After 1000°C/140 hr Exposure

231.9

223.2

288.9

248.0

0.12

0.11

0.15

0.13

29.0

29.5

28.7

29.1

199.8

203.6

197.9

200.4

b. Fracture Origins

Critical Flaw

Sample

Number

1

2

3

4

5

6

XI

X2

X3

Open Closed

Pore Pore

•

•

Processing Subsurface

Defect Large Grai

•

•

•

•

Type

Machining

n Flaw

Undetermined

•

•

•

IITRI-M06138-1
TABLE 7. FLEXURE TEST RESULTS FOR MULL 1:1 ZH (IITRI SM7)

Sample

Number

1

2

3

Mean

4

5

6

Mean

4-Point

Flexure Strength

psi

a.

45,510

38,000

42,120

41,880

21,580

23,560

22,710

22,620

MPa

Strength at

313.8

262.0

290.4

288.7

148.8

162.5

156.6

156.0

Failure

Strain,

%

Elastic

106 pSi

Modulus

GPa

Various Temperatures

25°C

0.14

0.12

0.13

0.13

800 °C

0.07

0.07

0.07

0.07

32.7

32.7

32.7

32.7

31.3

32.8

32.4

32.1

225.5

225.5

225.5

225.5

214.4

226.0

223.3

221.2

b. Fracture Origins

Critical Flaw

Sample

Namber

1

2

3

4

5

6

Open Closed

Pore Pore

e

•

•

«

Processing Subsurface

Defect Large Grai

0

•

Type

Machining

n Fl aw

Undetermined

10 MTRI-M06138-1
TABLE 8. FLEXURE TEST RESULTS FOR MULL 2A ZH (IITRI SMS)

Sample

Number

1

2

3

Mean

4

5

6

Mean

4-Point

Flexure Strength

psi

a.

38,700

47,410

37,360

41,160

24,490

30,440

24,550

26,490

MPa

Strength at

266.8

326.9

257.6

283.8

168.9

209.9

169.3

182.7

Failure

Strain,

%

Elastic

106 pSi

Modulus

GPa

Various Temperatures

25°C

0.12

0.15

0.12

0.13

800°C

0.09

0.09

0.07

0.08

32.8

32.0

31.6

32.1

27.5

31.7

33.8

31.0

226.1

220.6

217.9

221.5

189.7

218.5

233.2

213.8

25°C After 1000°C/140 hr Exposure

XI

X2

- X3

Mean

29,880

38,000

27,860

31,910

206.1

262.1

192.2

220.1

0.10

0.13

0.09

0.11

29.8

29.6

31.0

30.1

205.8

203.8

213.5

207.7

b. Fracture Origins

Critical Flaw

Sample

Number

1

2

3

4

5

6

XI

X2

X3

Open Closed

Pore Pore

Processing Subsurface

Defect Large Grai

•

•

•

•

•

•

Type

Machini ng

n Flaw

Undetermi ned

•

•

«

11 IITRI-M06138-1
TABLE 9. FLEXURE TEST RESULTS FOR MULL 2A ZH (IITRI SM9)

Sample

Number

1

2

3

Mean

4

5

6

Mean

XI

X2

X3 -

Mean

4-Point

Flexure Strength

psi

a.

29,960

27,190

36,720

31,290

15,040

18,740

14,380

16,050

25°

36,960

40,780

39,140

38,960

MPa

Fai lure

Strain,

%

Elastic Modulus

10* psi GPa

Strength at Various Temperatures

206.6

187.5

253.2

215.7

103.7

129.2

99.2

110.7

25°C

0.09

0.08

0.11

0.09

800°C

0.05

0.06

0.05

0.05

33.9

33.7

34.0

33.9

31.2

32.4

29.8

31.1

233.7

232.4

234.4

233.5

214.9

223.5

205.8

214.7

C After 1000°C/140 hr Exposure

254.9

281.2

269.9

268.7

0.12

0.13

0.13

0.13

31.2

31.8

30.5

31.2

215.5

219.0

211.7

215.4

b. Fracture Origins

Sample

Number

1

2

3

4

5

6

IX

X2

X3

Open Closed Processi

Pore Pore Defect

•

•

•

«

Critical Flaw

ng Subsurface

Large Grai

0

*

•

•

Type

Machi ni ng

n Flaw

•

Undetermi ned

•

12 IITRI-M06138-1
TABLE 10. FLEXURE TEST RESULTS FOR MULL 2A ZH (IITRI SM10)

4-Point

Sample

Number

1

2

3

Mean

4

5

6

Mean

Flexure Strength

psi

a.

43,720

32,750

40,020

38,830

28,650

41,700

32,320

34,220

MPa

Strength at

301.4

225.8

277.5

268.2

197.6

287.5

222.9

236.0

Failure

Strain,

%

Elastic

106 psi

Modulus

GPa

Various Temperatures

25°C

0.14

0.10

0.13

0.12

800°C

0.09

0.14

0.10

0.11

31.8

32.0

31.0

31.6

32.3

30.5

31.4

31.4

219.3

220.6

213.7

217.9

223.0

210.4

216.7

216.7

b. Fracture Origins

Critical Flaw

Sample

Number

1

2

3

4

5

6

Open Closed

Pore Pore

•

•

•

Processing Subsurface

Defect

Type

Machining

Large Grain Flaw

•

Undetermi ned

•

•

13 IITRI-M06138-1
ORIGINAL PAGE, IS

OF POOR QUAUIY

25KU X50 1006, 100.0U IITRI

(a) Subsurface elongated pore was the fracture origi n

for this MULL 2A ZH, 10 v/o, (b) sample tested at

800°C (sample number SM10F6).

(b) Higher magnification view of (a) above.

Figure 2. SEM micrographs of representative types

of fracture origins (tensile surface is uppermost

in each photograph) of samples tested in 4-point

flexure at various temperatures.

14
ORIGINAL PAGE; fS

r

 POOR QUALITY

(c) This MULL 1:1 ZH sample failed at a subsurface pore

cluster in flexure at room temprature (sample

number SM3F2).

Figure 2 (cont.)

15
(d) Fracture surface of MULL 1:1 ZH, 15 v/o ZH tested

at room temperature after ]000°C/]kO hr static air

exposure. An open pore was the critica l flaw

(sample number SM6X1).

(e) Detail of (d) above.

Figure 2 (cont.)

16
ORIGINAL PAGE fS

POOR QUALITY

(f) A large subsurface inclusion was the fracture origi n

of this MULL 1:1 ZH, 10 v/o sample tested at room

temperature (sample number SM7F2).

(g) A higher magnification view of (f) above. In

optical microscope such inclusions appeared

glassy.

an

Figure 2 (cont.)

17
(h) This exposed (1000°C/1'»0 hr) MULL 2A ZH, 15 v/o

sample tested at room temprature had a processing defect fracture origin in the form of an

incompletely sintered region at the tensile

surface (sample number SM 8X1).

(i) Detail of (h) above showing the lower density

region which is ^20 ym diameter.

Figure 2 (concluded)

18
2. COMPRESSIVE STRENGTH

Compression testing was performed on one batch of material at room and

elevated temperature, and at ambient temperature after static air exposure at

1000°C/100 hr. Elevated temperature testing was performed in a Kanthal wire

wound furnace placed in an Instron testing machine. The load platens used for

all tests were made of Udimet 700. The upper platen fitted into a hemispherical seat in the upper load rod to give some self-alignment to the load train.

At elevated temperatures, flat and parallel plates of Norton NC-132 HP-Si N

were placed between the sample and platens to lessen the chance that the small

diameter sample would cause an indentation on the loading platens.

Samples were 6.35 mm diameter by 12.7 mm long. The crosshead speed was

8.5 x 10~3 mm/sec. Elastic modulus and Poisson's ratio were determined at

room temperature using 2-element, 90°, "Tee" rosette resistance strain gages.

Modulus was not measured at elevated temperature. The results are presented

in Table 11. Compressive strength varied from 250 to 2587 MPa (36 to 375

ksi). "Whether this is due to material variation or nonuniform loading during

testing is not known. Samples were completely destroyed during testing

(explosive brittle fracture), so fracture analysis was not possible. In the

case of sample 11, the large pore (which presumably contributed to the

sample's low strength) was discovered prior to testing. The low strength of

sample 8 may also be the result of a similar subsurface processing defect.

*Special Metals Corp., New Hartford, NY.

N T RESEARC H INSTITUT E

• 19 IITRI-M06138-1
TABLE 11. COMPRESSION TEST RESULTS FOR MULL 1:1 ZH* (IITRI SM11)

AT VARIOUS TEMPERATURES

Sample

Number

1

2

3

Mean

4

5

6

Mean

7

8

9

Mean

10

11

12

Mean

Compressi ve

Strength El

psi

375,250

144,600

122,200

214,120

260,710

266,870

132,480

220,020

173,980

52,150

113,065

25°C

177,780

36,360**

137,370

117,170

MPa 106

25°C

2587.3 32

997.0

842.5 26

1475.6 29

800°C

1797.5

1840.0

913.4

1517.0

1000°C

1199.5

359.6

779.6

After 1000°C/100

1225.7 26

250.7

947.1 13

807.8 20

astic Modulus

psi GPa

.6 224.7

.8 184.6

.7 204.7

_

_

-

-

hr Exposure

.7 184.1

.9 95.7

.3 139.9

Poisson's

Ratio

0.29

—

--

--

--

--

0.26

0.28

0.27

*Notes on mailing package for this material were: #1 AM-5, sinter

1650/1 hr, same composition as TT052185-2, except attrition mil l

batch 9 hr.

**Sample contained large open pore at surface.

20 IITRI-M06138-1
3. FRACTURE TOUGHNESS

Fracture toughness values, K , were determined using a modified indentation-strength method. The objective was to observe the relative toughness of

each material as a function of temperature.

The indentation-strength method of Chantikul et al .,

L was used for this

work, where strength is related to material toughness by the equation:

Kc - nR

v (E/H)"8 (oP1/

3)3'4

R

where: n is a dimensionless constant

E/H is the ratio of elastic modulus to hardness, GPa/GPa

a is the indentation flexure strength, MPa

P is the indentation contact load, MN

Initial measurements were necessary to determine the proper indentation load,

P. To determine the proper indentation load, samples having indentation loads

of 5 to 10 kgf (4.9 x 10"5 to 9.8 x 10~5 MN) were tested in 4-point flexure at

ambient temperature. An indentation load of 10 kgf (9.8 x 10~5 MN) was chosen

as the one which would assure a controlled flaw large enough (relative to

grain size and natural flaws) to be the fracture origin.

Indentations were made using a Vicker's Hardness test machine. They were

made at room temperature, in ~40% relative humidity, just prior to flexure

testing. The indentation load duration was 15 sec. The indentation axes were

oriented parallel to the sample length and width. The samples were not postidentation annealed, nor was any oil placed on the indentations, since it was

felt that at elevated temperature those treatments would be ineffectual.

Table 12 summarizes the test and calculation parameters. The material

toughness was calculated according to the equation above. Two assumptions

D

were made in order to perform the calculations. First, the constant n was

assumed to be 0.59, based upon the reference work 1'

2

 and work done by Schioler

at AMMRC on transformation-toughened zi rconias. 3>1 * Second, the ratio of

I T RESEARC H INSTITUT E

21

IITRI-M06138-1
TABLE 12. PARAMETERS FOR KC CALCULATION

Samples

• 3.00 x 4.00 x 45 mm flexure bars

• As-received, as-machined surfaces

Controlled Flaw

• One Vicker's indentation at center of

tensile surface

• Indentation load = 10 kg = 98N

• No post-indentation anneal

» Indentations not covered with immersion oil

Flexure Testing

9 4-point configuration

• Lower span = 40 mm, upper span = 20 mm

• All testing in air

• Relative humidity = 41%

• One break per sample

K^ Calculation

KC . „; (E/H, (o

^

nv = 0.59 assumed

E/H considered equal to 17 for all test

conditions

Fracture surfaces of all samples examined

for vali d failure

Reference 1.

22 IITRI-M06138-1
elastic modulus to hardness was calculated from the average modulus and hardness values and was assumed constant over the range of temperature. Lawn5

notes that variations of 10 < E/H < 50 adds no more than ±10% error in KC

measurement. This is true primarily because E/H is raised to a small power

(1/8) in the equation to determine KC.

Tabular results of fracture toughness as a function of temperature are

presented in Tables 13 and 14, and is plotted in Figure 3. The highest toughness, 3.8 MPa'm1'

2, was exhibited by the hipped material at room temperature.

At elevated temperature the MULL 2A ZH (15 v/o, SMS) material had a toughness

twice that of mullite alone (i.e., 3.0 vs. 1.5 MPa'm1'

2). Non-toughened

mullite experienced a 35% decrease in toughness at 800°C, while the toughened

materials maintained about 90% of their room temperature values. For comparison, commercially available transformation-toughened zirconias evaluated by

the same method showed loss of toughness at 750°C (as compared to room temperature values) of 60% for Mg-PSZ materials and ~70% for Y-PSZ.6

N T RESEARC H INSTITUT E

23 IITRI-M06138-1
TABLE 13. FRACTURE TOUGHNESS OF UNEXPOSED TOUGHENED

MULLITES DETERMINED BY THE INDENTATION-STRENGTH

METHOD AT ROOM TEMPERATURE (P = 98 N)

Sample

No.

Indented 4-Point

Flexure Strength Kr , ,

psi MPa MPa*mi' 2

MULL (IITRI SMI)

1

2

3

Mean

1

2

3

Mean

1

2

3

Mean

1*

2*

3

4

Mean

1

2

Mean

11,460

12,200

12,830

12,160

MULL-Z (I

16,930

18,200

17,730

17,620

MULL 1:1 ZH

14,570

16,360

14,290

15,070

MULL 2:1 ZH

19,740

20,450

16,1)30

17,260

18,370

MULL-ZH-HIP

23,940

23,510

23,730

79.0

84.2

88.5

83.9

ITRI SM2)

116.8

125.5

122.3

121.5

(IITRI SMS)

100.5

112.8

98.6

104.0

(IITRI SM4)

136.2

141.0

110.6

119.0

126.7

(IITRI SM5)

165.1

162.3

163.7

2.2

2.3

2.5

2.3

3.0

3.1

3.1

3.1

2.7

2.9

2.6

2.7

2.8

2.9

2.9

3.0

2.9

3.9

3.8

3.8

*These samples were indented at P = 49N.

24 IITRI-M06138-1
TABLE 14. FRACTURE TOUGHNESS OF UNEXPOSED TOUGHENED

MULLITES DETERMINED BY THE INDENTATION-STRENGTH

METHOD AT 800°C (P = 98 N)

Sample

No.

Indented 4-Point

Flexure Strength

psi MPa

Kc> / MPa'nl' 2

MULL (IITRI SMI)

4

5

6

Mean

4

5

6

Mean

4

5

6

-Mean

1

2

3

Mean

1

2

3

Mean

1

2

3

Mean

1

2

3

Mean

7,110

21,690

6,460

6,790

MULL-Z (I

32,910

13,420

18,210

15,815

MULL 1:1 ZH

13,020

12,150

13,860

13,010

MULL 1:1 ZH

13,810

13,520

14,310

13,880

MULL 2A ZH

16,580

17,610

17,570

17,250

MULL 2A ZH

11,970

10,420

12,770

11,720

MULL 2A ZH (

9,870

10,330

11,340

10,510

49.0

149.6

44.5

46.8

ITRI SM2)

227.0

92.5

125.6

109.1

(IITRI SM3)

89.8

83.8

95.6

89.7

(IITRI SM6)

95.2

93.3

98.7

95.7

(IITRI SMS)

114.3

121.5

121.1

119.0

(IITRI SM9)

82.6

71.9

88.1

80.9

IITRI SM10)

68.1

71.2

78.2

72.5

1.6

Fracture not

at indentation

1.5

1.5

Fracture not

at indentation

2.5

3.2

2.8

2.5

2.3

2.6

2.5

2.6

2.5

2.6

2.6

2.9

3.1

3.1

3.0

2.3

2.1

2.4

2.3

2.0

2.1

2.2

2.1

25 IITRI-M06138-1
3.5

3.0

* 2.5

Q_

2.0

.5

1 .0

MULL-Z

i

MULL 1:1 ZH

i

-MULL

OMULL-ZH-HIP

AMULL 2:1 ZH

A MULL 1:1 ZH (15 v/o)

• MULL 2A ZH (15 v/o)

*MULL 2A ZH (10 v/o, a)

• MULL 2A ZH (10 v/o, b)

200 ^00 600

Temperature, °C

800 000

Figure 3. Fracture toughness determined by the indentation-strength method

at ambient temperature and 800°C (Vickers Indentation, P = 98 N).

26
4. SUMMARY

The toughened mullites of this study had higher strengths and Kj^ values

than non-toughened mullite. There were trends toward higher room temperature

and elevated temperature strengths. Retained room-temperature strength after

static soaks in laboratory air at 1000°C seemed to indicate that overaging is

not a problem for this material, as with some transformation-toughened

zirconias (TTZ). The elastic moduli decreased slightly as a function of temperature and exposure. Processing defects such as pores, large grains, and

poorly bonded regions were the primary fracture origins. These materials are

less tolerant of defects than are TTZ's; it might be expected that strengths

would increase as they are removed. Compression strength was evaluated for

one material, giving strengths of -1500 MPa (220 ksi) at 25° and 800°C.

Fracture toughness was evaluated by the indentation-strength method using

the same sample and fixture size as used for flexural strength evaluation.

Although these toughened mullites are not as tough as transformation-toughened

zirconias, they retain toughness better at elevated temperature than do TTZ

materials. Strength and toughness did not always correlate well for these

materials: MULL 2A ZH, 15 v/o, had the highest toughness at 800°C, but only

nomimal strength at that temperature. MULL 2A ZH, 10 v/o (b) had one of the

highest strengths overall and retained it well at 800°C, yet had low toughness

at 800°C.

N T RESEARC H INSTITUT E

IITRI-M06138-1

27
REFERENCES

1. P. Chantikul, G. R. Anstis, B. R. Lawn, and D. B. Marshall, "A Critical

Evaluation of Indentation Techniques for Measuring Fracture Toughness:

II," J. Am. Ceram. Soc. 64[9], 539-43 (1981).

2. R. F. Cook and B. R. Lawn, "A Modified Indentation Toughness Technique,"

Communications of the Amer. Ceram. Soc., C-200 (1983).

3. L. J. Schioler, AMMRC, personal communication (1985).

4. L. J. Schioler, R. N. Katz, A. C. Gonzalez, and B. R. Lawn, "Effect of

Overaging on the Room Temperature Strength of Partially Stabilized

Zirconia," Amer. Ceram. Soc. 8ul1., _64[2], pp. 326-327 (1985).

5. B. R. Lawn, NBS, personal communication (1985).

6. D. C. Larsen and J. W. Adams, "Long-Term Stability and Properties of

Zirconia Ceramics for Heavy Duty Diesel Engine Components," DOE/NASA/

0305-1, NASA CR-174943 (August 1985).

I I T RESEARC H INSTITUT E

IITRI-M06138-1

28
1. Report No.

NASA CR-175054

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle

TRANSFORMATION TOUGHENED CERAMICS FOR THE HEAVY DUTY

DIESEL ENGINE TECHNOLOGY PROGRAM PHASE II

5. Report Date

December 1, 1985

6. Performing Organization Code

7. Authors)

S. Musikant, S. C. Samanta, P. Architetto, E. Feingold

8. Performing Organization Report No

10. Work Unit No.

9. Performing Organization Name and Address

General Electric Company - Space Division

P. 0. Box 8555

Philadelphia, PA 19101

11. Contract or Grant No.

DEN-3-339

12. Sponsoring Agency Name and Address

U. S. Department of Energy

Office of Vehicle & Engine R&D

Washington, DC 20585

13. Type of Report and Period Covered

Contractor Report

August 1984 - July 1985

14. Sponsoring Agency Code

DOE/NASA/0339-2

15. Supplementary Notes

Phase II Final Report Prepared Under Interagency Agreement DE-AI01-80CS50194

Project Manager, M. Bailey, Propulsion Systems Division, NASA Lewis Research

Center, Clevland, OH 44135.

16. Abstract

The objective of this program is to develop an insulating structural ceramic for

application.in a heavy duty "adiabatic" diesel engine. The approach is to

employ transformation toughening (TT) by additions of zirconia-hafnia solid

solution (ZHSS). The feasibility of using ZHSS as a toughening agent in mullite

and alumina has been demonstrated in the first year (Phase I) of this work.

The results of the Phase I effort are documented in NASA CR-174689.

This report covers the second year (Phase II) effort. Based on Phase I results,

a decision was made to concentrate the Phase II effort on process optimization

of the TT mullite. A strong factor in that decision was the low thermal conductivity and high thermal shock resistance of the mullite.

Results of the Phase II effort indicate that optimum toughening of mullite by

additions of ZHSS is difficult to achieve due to apparent sensitivity to

morphology. The 48 ksi room temperature modulus-of-rupture (I'OR) achieved in

selected specimens is approximately 50% of the original strength target. The

MOR deteriorated to 34 ksi at 800°C. The KIC was measured at 3.8 MPa/m

at room temperature.

17. Key Word* (Suggested by Authors))

Diesel Engine, Ceramics, Mullite, Transformation Toughening, Fracture Toughness,

Zirconia, Zirconia-Hafnia, Hafnia, Processes, Properties

19. Security CiassH. (of this report)

Unclassified

18. Distribution Statement

Unclassified-l'nl imited

STAR Category 85

DOE Category UC-96

20. Security Classif . (of this page)

Unclassified

21. No. of pages 22. Price*

85

'For sale by the National Technical Information Service, Springfield, Virginia 22161