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Nation

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and Technology

tandards)

NIST Special Publication 758

Ceramic Tribology: Methodology

and Mechanisms of Alumina

Wear

Richard S. Gates, Stephen M. Hsu, and E, Erwin Klaus

will f ? T T T

NIST Special Publication 758

Ceramic Tribology: Methodology

and Mechanisms of Alumina

Wear

Richard S. Gates and Stephen M. Hsu

Ceramics Division

Institute for Materials Science and Engineering

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Gaithersburg, MD 20899

E. Erwin Klaus

Department of Chemical Engineering

The Pennsylvania State University

University Park, PA 16802

NOTE: As of 23 August 1988, the National Bureau of

Standards (NBS) became the National Institute of

Standards and Technology (NIST) when President

Reagan signed into law the Omnibus Trade and

Competitiveness Act

September 1988

U.S. Department of Commerce

C. William Verity, Secretary

National Institute of Standards and Technology

(formerly National Bureau of Standards) Ernest Ambler, Director
Library of Congress

Catalog Card Number: 88-600582

National Institute of Standards

and Technology

Special Publication 758

229 pages (Sept. 1988)

CODEN: XNBSAV

U.S. Government Printing Office

Washington: 1988

For sale by the Superintendent

of Documents,

U.S. Government Printing Office,

Washington, DC 20402

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402
FOREWORD

This report is born out of a cooperative effort between the Chemical

Engineering Department, at the Pennsylvania State University (PSU) , the

Tribology Group of the National Bureau of Standards, and the partial

support of the DOE ECUT Tribology Program. For years, the NBS has

encouraged and sponsored graduate cooperative programs in which promising

young scientists (some of them NBS staff members) come to NBS to conduct

research, part of which often becomes the graduate's thesis. While it is

not the objective of NBS to teach and train graduate students, such a program often furthers NBS programmatic goals by attracting high caliber

scientists working on areas where NBS mission lies. Many excellent

research papers have resulted and many students, upon graduation, have

chosen to stay at NBS to continue their research careers. Such programs

also draw many first rate university professors to NBS through the

participation of these students in NBS research programs and projects, thus

fostering NBS -university interactions and enhancing the scientific caliber

of the work and reputation at NBS and the participating university. At the same time, through the frequent contacts that NBS has with industries, a natural university/government lab/industry relationship evolves, bringing a team focus on many research projects of significant economic and

technological impact.

In 1984, primarily through the interactions of Dr. Stephen M. Hsu of NBS

and Professors Elmer Klaus and Larry Duda of PSU, a cooperative program in

Tribology was started. Three graduate students: Mr. Richard Gates ( a NBS

staff member), Mr. Jeffrey Yellets, and Mr. Douglas Deckman were enrolled

at the Chemical Engineering Department at PSU. Three reports have been

prepared to describe the fruit of their relentless efforts in the last

three years. These studies were conducted at NBS under the guidance of Dr.

Stephen Hsu with the close participation of Prof. Elmer Klaus who visited

NBS frequently.

Dr. James Eberhardt, Mrs. Terry Levinson, and Mr. David Mello of the DOE

ECUT Tribology Program have at the same time sponsored a major Tribology

program at NBS. That program also benefited from the studies conducted by

Mr. Gates, Mr. Yellets, and Mr. Deckman. These students, while not working

on sponsored projects directly, enabled NBS to explore some high risk, high

pay off projects parallel to ECUT projects. When appropriate, the students

were supported by ECUT for some time. To this, we gratefully acknowledge

the generous support of DOE ECUT, without whose support many ideas would

not be explored.

Stephen M. Hsu

Chief, Ceramics Division

iii

ABSTRACT

This report describes a systematic study that has been conducted to

develop methods for measuring the tribological properties of ceramic

materials under concentrated contacts. Step- loading four-ball and ball-onthree-flat wear tests were developed to provide friction and wear

characteristics of ceramic/lubricant combinations under various lubrication

conditions. These measurement techniques now enable one to study the

effect of different materials processing parameters, material

microstructures , and different lubricants on the friction and wear

performance of ceramics.

Water was found to react with alumina in a wearing contact to produce

lubricous products. A combination of x-ray diffraction and

thermogravimetric analysis techniques were used to investigate the kinetics

of alumina-water reactions. These experiments determined that transition

(gamma) alumina reacts with water at =200°C to form aluminum oxide

hydroxide (boehmite - AlO(OH)), while reactions at -100°C produce aluminum

trihydroxide (bayerite - Al(0H)3). A mechanism for lubrication of alumina

with water is proposed whereby stresses in the contact junction cause a phase transformation from alpha to gamma alumina. The gamma alumina

subsequently reacts with water to form a lubricous, layer lattice,

hydroxide

.

V

CONTENTS

Page

FOREWORD iii

ABSTRACT v

LIST OF TABLES xi

LIST OF FIGURES xiii

Chapter

I. INTRODUCTION AND SCOPE 1

II. CERAMIC WEAR TEST METHODOLOGIES: BACKGROUND AND LITERATURE

SURVEY

A. Basic Types of Tribological Studies .... 5 B. Current Efforts in Fundamental Ceramic Tribological

Studies 8 C. Key Differences Between Ceramics and Metals 12

D. Approach to Evaluation of Ceramic Wear Used in

This Report 18

III. EXPERIMENTAL APPARATUS AND OPERATING PROCEDURES

A. Wear Tester Selection 19

B. Specimen Material Selection 25

C. Lubricant Selection 25

D. Specimen Characterization Methods 27

E. Friction and Wear Measurement 38

F. Specimen Preparation 40

G. Operating Procedure Descriptions 42

IV. RESULTS AND DISCUSSION

A. Constant Condition Four-Ball Wear Testing Study

1. Base Conditions 45

2. Explanation of Severity Differences Between

Steel and Ceramic Tests Conducted at the

Same Operating Conditions 46

3. Four-Ball Constant Condition Test: Effect

of Test Duration 50

4. Four-Ball Constant Condition Test: Effect

of Load 55

B. Wear Test Severity Overlay 61

C. Analysis of Wear Scars 64

D. Four-Ball Constant Condition Test: Effect of Batch

to Batch Variations in Specimen Material 78

vii
E. Step-Loading Four-Ball Wear Testing Study

1 . Base Conditions 81

2. Step-Loading Test: Effect of Speed 86

3. Step-Loading Test: Effect of Specimen

Material 86

F. Ball-on-Three-Flat Testing 89

1. Comparison of Results to Four-Ball Tests .... 89

2. Ball-on-Three-Flat Test: Interpretation

of Friction Data 93

3. Ball-on-Three-Flat Test: Effect of

Specimen Material 96

G. Summary 99

V. ALUMINA/WATER TRIBOSYSTEM: INTRODUCTION AND LITERATURE SURVEY

A. Preliminary Explanation of Lubrication Mechanisms for

Alumina With Water 100

B. Structures and Nomenclature of Aluminum Oxides and

Hydroxides 103

C. Approach 112

VI. ALUMINA/WATER TRIBOSYSTEM: EXPERIMENTAL APPARATUS AND

OPERATING PROCEDURES

A. Test Approaches 113

1. Static Kinetic Tests 114

2. Powder Friction Tests 119

3. Dynamic Wear Tests 119

B. Reference Powders 120

C. Analytical Techniques 129

1. The rmogravime trie Analysis (TGA) 131

2. Differential Scanning Calorimetry (DSC) 141

3. X-Ray Powder Diffraction (XRPD) 141

4. Fourier Transform Infrared (FTIR) Analysis . . . 144

5. Scanning Electron Microscopy (SEM) 145

VII. ALUMINA/WATER TRIBOSYSTEM: RESULTS AND DISCUSSION

A. Kinetic Experiments on Powders 146

B. Powder Friction Tests 158

C. Wear Debris Analysis 162

VIII. SUMMARY AND CONCLUSIONS 175

IX. RECOMMENDATIONS FOR FUTURE WORK 179

REFERENCES 180

APPENDIX A - Selected Bulk Properties of Cer 006 and Cer 021

Alumina 184

viii
APPENDIX B - Coefficient of Friction Calculation for the

Four-Ball Apparatus 185

APPENDIX C - Equivalent Wear Scar Diameter Concept 187

APPENDIX D - Specimen Cleaning Procedure 190

APPENDIX E - Pressure-Temperature Relationship for the

Alumina/Water Bomb Reactor 192

APPENDIX F - X-Ray Powder Diffraction Patterns for Reference

Powders ALl through AL6 195

APPENDIX G - Interplaner Spacings, 2 Thetas, and Relative

Line Intensities for X-Ray Powder Diffraction

Patterns of Alumina's, Alixminum Oxide Hydroxides

and Aluminum Trihydroxides 201

APPENDIX H - An Estimate of the Hertzian Contact Diameter,

Mean Pressure, and Maximum Pressure for 52100

Steel, and Alumina Specimens, in the Four-Ball

Apparatus 205

APPENDIX I - Derivation of a Pseudo First Order Dependence

for the Gamma Alumina/Water Reaction at 194°C . . . 210

APPENDIX J - Mean Pressure Test Severity Plot for Wear Scar

Diameter vs Load (psi) 213

APPENDIX K - Mean Pressure Test Severity Plot for Wear Scar

Diameter vs Load (MPa) 214

NOTE: Certain commercial equipment, instruments, or materials are identified in this report to adequately specify the experimental

procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply

that the materials or equipment identified are necessarily the best

available for the purpose.

ix

Table

LIST OF TABLES

Title Page

1 Wear Test Design Philosophies 6

2 ASTM Wear Test Methods Applicable to

Ceramic Materials 11

3 Comparison of Physical and Mechanical

Properties of Selected Metals and

Ceramics 14

4 Typical Properties of Dense High Alumina

Ceramics 26

5 Summary of Selected Characterization Data

on the Alumina Ceramics Used in This Study 34

6 Nomenclature for Hydroxides of Aluminum 104

7 Analytical Techniques Utilized in the

Investigation of Alumina/Water Reactions 129

8 Thermogravimetric Analysis of Reference

Powders 138

9 Kinetic Data for Reaction of Gamma Alumina

(AL2) and Water at 194° C 154

10 Summary Friction Data for Alumina Based

Powders in Water 171

XI
iII
LIST OF FIGURES

Figure Title

1 Stress-Strain Relationship for Typical Metals

and Ceramics

2 Four-Ball Contact Geometry

3 NBS Modified Four-Ball Apparatus Design

4 Ball-on-Three-Flat Wear Test Contact Geometry

5 SEM Photomicrograph of Fracture Surface of Cer 004

Alumina - 2000X

6 SEM Photomicrograph of Fracture Surface of Cer 005

Alumina - lOOOX

7 SEM Photomicrograph of Fracture Surface of Cer 006

Alumina - 1500X

8 SEM Photomicrograph of Fracture Surface of Cer 021

Alumina - lOOOX

9 SEM Photomicrograph of Polished Surface of Cer 004

Alumina - 500X

10 SEM Photomicrograph of Polished Surface of Cer 005

Alumina - 500X

11 SEM Photomicrograph of Polished Surface of Cer 006

Alumina - 500X

12 SEM Photomicrograph of Polished Surface of Cer 021

Alumina - 500X

13 SEM Photomicrograph of Polished/Etched Surface of

Cer 004 Alumina - 5000X

14 SEM Photomicrograph of Polished/Etched Surface of

Cer 005 Alumina - 5000X

15 SEM Photomicrograph of Polished/Etched Surface of

Cer 006 Alumina - 5000X

16 SEM Photomicrograph of Polished/Etched Surface of

Cer 021 Alumina - 5000X

xiii
17 Comparison of Applicable Ranges for Optical

and Weight Loss Wear Measurement Techniques 41

18 Comparison of Hertzian Contact Pressure for 52100

Steel versus Alumina 49

19 Typical Wear Regimes for a Wear Test 51

20 Wear- in Behavior of Alumina under Unlubricated and

Paraffin Oil Lubricated Conditions 53

21 Friction Behavior of Alumina under Unlubricated and

Paraffin Oil Lubricated Conditions 54

22 Change in Friction Behavior with Time for Unlubricated

Alumina Tests at Different Loads 56

23 Comparison of Friction Behavior with Time for

Unlubricated Alumina Tests with and without Heating

Pretreatment 58

24 Comparison of Wear as a Function of Load for

Unlubricated, Paraffin Oil Lubricated, and Water

Lubricated Alumina Constant Condition Tests 60

25 Comparison of Coefficient of Friction as a Function

of Load for Unlubricated, Paraffin Oil Lubricated,

and Water Lubricated Alumina Constant Condition Tests 63

26 Low Magnification SEM Photograph of a Wear Scar

from an Unlubricated Alumina Test 65

27 High Magnification SEM Photomicrograph of Lower

Specimen Wear Scar from an Unlubricated Alumina

Wear Test 65

28 SEM Photomicrograph of Wear Debris from Unlubricated

Wear Test - lOOOX 66

29 SEM Photomicrograph of Wear Debris from Unlubricated

Wear Test - 10,000X 66

30 Low Magnification SEM Photograph of a Wear Scar

from a Water Lubricated Alumina Test 68

31 High Magnification SEM Photomicrograph of Lower

Specimen Wear Scar from a Water Lubricated Alumina

Wear Test 68

32 SEM Photomicrograph of Upper Specimen Wear Track

Water Lubricated Run 2748R - 25X 69

xiv
33 SEM Photomicrograph of Upper Specimen Wear Track

Water Lubricated Run 2748R - lOOX 69

34 SEM Photomicrograph of Upper Specimen Wear Track

Water Lubricated Run 2748R - 500X 71

35 SEM Photomicrograph of Upper Specimen Wear Track

Water Lubricated Run 2748R - lOOOX 71

36 SEM Photomicrograph of Upper Specimen Wear Track

Water Lubricated Run 2748R - 5000X 72

37 SEM Photomicrograph of Lower Specimen Wear Scar

Water Lubricated Run 2748R - 50X 72

38 SEM Photomicrograph of Wear Debris from Water

Lubricated Wear Test - lOOOX . 73

39 SEM Photomicrograph of Wear Debris from Water

Lubricated Wear Test - 5000X 73

40 Low Magnification SEM Photograph of a Wear Scar

from a Paraffin Oil Lubricated Alumina Test 75

41 High Magnification SEM Photomicrograph of Lower

Specimen Wear Scar from a Paraffin Oil Lubricated

Alumina Wear Test 75

42 SEM Analysis of Wear Debris from Paraffin Oil

Lubricated Wear Test - lOOOX 76

43 SEM Analysis of Wear Debris from Paraffin Oil

Lubricated Wear Test - lO.OOOX 76

44 Comparison of Wear as a Function of Load for Different

Batches of Alumina under Paraffin Oil Lubricated

Conditions (Constant Condition Tests) 78

45 Comparison of Wear as a Function of Load for Different

Batches of Alumina under Water Lubricated Conditions

(Constant Condition Tests) 79

46 Wear as a Function of Load for Unlubricated Alumina

Step-Loading Four-Ball Tests 81

47 Wear as a Function of Load for Water Lubricated Alumina

Step-Loading Four-Ball Tests 82

48 Wear as a Function of Load for Paraffin Oil Lubricated

Alumina Step-Loading Four-Ball Tests 84

XV
49 Effect of Speed on Step-Loading Four-Ball Wear Test

Results 86

50 Comparison of Wear Behavior as a Function of Load for

Paraffin Oil Lubricated Step-Loading Four-Ball Tests

on Silicon Nitride, Silicon Carbide, and Alumina 87

51 Comparison of Ball-on-Three-Flat and Four-Ball Wear

Results for Paraffin Oil Lubricated Alumina 90

52 Friction Traces for Paraffin Oil Lubricated Ball-onThree-Flat (BTF) Test 2727 R 93

53 Coefficient of Friction Values versus Load for

Paraffin Oil Lubricated BTF Test 2727 R 94

54 Comparison of Wear Results for NBS Produced Alumina

versus Commercial Alumina (Paraffin Oil Lubricated

BTF Tests) 96

55 Comparison of Friction Traces for NBS Alumina versus

Commercial Alumina 97

56 Structure of Al(0H)3 106

57 Layer Stacking Sequence for Gibbsite and Bayerite 107

58 Phase Diagram for AI2O3 - H2O System 109

59 Decomposition Sequences for Aluminum Hydroxides 110

60 High Pressure Bomb Reactor Design 114

61 Temperature-Pressure Relationship for Bomb Reactor 115

62 Temperature Inside Bomb versus Time during Heating 117

63 lOOOX Secondary Emission SEM Photograph of

Powder ALl 122

64 lO.OOOX Secondary Emission SEM Photograph of

Powder ALl 122

65 lOOOX Secondary Emission SEM Photograph of

Powder AL2 123

66 lO.OOOX Secondary Emission SEM Photograph of

Powder AL2 123

67 lOOOX Secondary Emission SEM Photograph of

Powder AL3 124

xvi
68 lO.OOOX Secondary Emission SEM Photograph of

Powder AL3 124

69 lOOOX Secondary Emission SEM Photograph of

Powder AL4 125

70 10,000X Secondary Emission SEM Photograph of

Powder AL4 125

71 lOOOX Secondary Emission SEM Photograph of

Powder AL5 126

72 lO.OOOX Secondary Emission SEM Photograph of

Powder AL5 126

73 lOOOX Secondary Emission SEM Photograph of

Powder AL6 127

74 lO.OOOX Secondary Emission SEM Photograph of

Powder AL6 127

75 Isothermal TGA of Powder ALB (Boehmite) 132

76 Temperature Scan TGA of Powder AL3 (Boehmite) 133

77 Temperature Scan TGA of Powder ALl (Boehmite) 135

78 Temperature Scan TGA of Powder AL2 (Gamma Alxomina) 136

79 Temperature Scan TGA of Powder AL6 (Gibbsite) 137

80 X-Ray Powder Diffraction Apparatus and Sample

Spectrum 141

81 Temperature Scan TGA of Reaction Product of Gamma

Alumina/Water at 100°C for 24 Hours 146

82 Temperature - Scan TGA of Reaction Product of Gamma

Alumina/Water at 194°C for = 1 Hour 147

83 Temperature Scan TGA of "Gel" Phase of Reaction

Product of Gamma Alumina/Water at 194°C for ~ 1 Hour 149

84 Temperature Scan TGA of "White" Phase of Reaction

Product of Gamma Alumina/Water at 194°C for ^ 1 Hour 150

85 Temperature Scan TGA of Reaction Product of Gamma

Alumina/Water at 194°C for 31 Minutes 151

xvii
86 Temperature Scan TGA of Reaction Product of Gamma

Alumina/Water at 194°G for 59 Minutes 152

87 Temperature Scan TGA of Reaction Product of Gamma

Alumina/Water at 194°C for 119 Minutes 153

88 Pseudo First Order Data Plot for Gamma Alumina/

Water Reaction at 194°C 156

89 Friction Traces for Powder Tests at 5 kg Load 158

90 Friction Traces for Powder Tests at 2 kg Load 159

91 Comparison of Final Coefficient of Friction Values

for Different Alumina Powders at 2 and 5 kg Load 160

92 X-Ray Powder Diffraction Spectrum for Wear Debris

from Water Lubricated Alumina Test 162

93 Temperature Scan TGA of Wear Debris from Water

Lubricated Alumina Test 164

94 FTIR Spectrum of Wear Debris from Water

Lubricated Alumina Test 166

95 Model for the Alumina/Water Tribosystem 167

96 Effect of Various Alumina Based Powders on Friction for Water Lubricated Alumina 172

97 Lubricating Effect of Boehmite Powder for

Water Lubricated Alumina 174

xviii
Chapter I

INTRODUCTION AND SCOPE

Ceramic or ceramic- like materials have been used in tribological

applications for thousands of years. A typical example is the

thresholds used in Assyrian villages 6000-7000 years ago during the

stone age . ^ Stone sockets were used as pivot points for wooden poles

providing a means of opening and closing the "door."

These early crude uses of ceramics eventually led to more

sophisticated applications better designed to take advantage of the

unique properties ceramics have to offer. Watches, for example, have

small crystals of diamond or sapphire ("Jewels") which are used as

pivot points for the inner workings. The low friction and wear

associated with these materials allows for many years of trouble free

operation. Still today, manufacturers of fine watches advertise the

number of jewels in the movement as a badge of quality.

More recently, advances in ceramic materials have produced new

structural or advanced ceramics. Current technology, being forced to

higher and higher performance from ever dwindling resources, has

placed greater demands on new materials . The result is a tremendous

increase in research in strong high temperature materials capable of

functioning in environments not suitable for conventional metals.

This research has been mainly centered on studies involving high

temperature " superalloys , " powder metallurgy, and advanced
2

ceramics. The superalloy and powder metallurgy suffer from the

drawback that they require certain rare "strategic" materials (cobalt,

chromium, tungsten, tantalum, nickel) which must be imported and thus

place the United States in a dependent role. Ceramic raw materials on

the other hand are commonly available from domestic suppliers.

Silicon and aluminum, two of the most abundant elements in the earths

crust, are common in many important structural ceramics.

Advanced ceramic materials therefore have several attractive

features which make them ideal candidates for high technology

applications. Their low thermal conductivity gives them insulating

properties which allow better thermodynamic efficiency for designs

such as low heat rejection engines. Their high temperature strength

and corrosion resistance allows them to operate under conditions

unsuitable for metals. Their high hardness would seem to make them

good candidates for low wear under the commonly held belief that wear

rate is inversely proportional to hardness.'' In addition, they can be

made from non- strategic raw materials.

Much of the screening work on advanced ceramics has focused on

the evaluation of candidate materials based on chemical and mechanical

property data.° Testing is then conducted by fabricating actual

ceramic parts and evaluating their performance . The result is

often an expensive trial and error approach which contributes little

or no fundamental understanding of the tribological behavior of

ceramic materials. What is needed to complete the picture is a
3

simple, inexpensive, repeatable system for evaluating the tribological

behavior of advanced ceramic materials and gaining a better

understanding of fundamental processes or mechanisms which govern

their behavior.

The objective of this study is to gain an understanding of the

wear test methodology that might be employed to evaluate the

tribological properties of ceramic materials. The scope of this study

is limited to structural or advanced ceramics under high contact

stress conditions. Lubricants are limited to paraffin oil, water, and

no lubricant to represent the three most common basic forms of

lubrication of hydrocarbon-based fluids, water-based fluids, and

unlubricated conditions

.

This report is organized in the following manner. First, a

background and literature survey is presented to familiarize the

reader with the present status of ceramic tribology. The wear testing

approach used in this report is also presented in the context of

previous research efforts. Second, the experimental apparatus and

operating procedures are described. Third, experimental results are

presented with interpretation and discussion. Initially, alumina is

used as a representative ceramic for most of the exploratory phase of

this work. Later, silicon carbide and silicon nitride ceramics are

evaluated using the fully developed methodology.
4

Interesting results for water- lubricated alumina prompted an indepth study of the possible lubrication mechanism for this system.

This is presented in the latter part of this report under the

alumina/water tribosystem headings and includes additional information

on a literature survey, experimental apparatus and operating

procedures, and results and discussion.
5

Chapter II

CERAMIC WEAR TEST METHODOLOGIES: BACKGROUND AND LITERATURE SURVEY

A. Basic Types of Tribological Studies

There are four basic wear test design philosophies that are

currently used in tribological testing. These can be classified as:

actual component studies; application oriented simulation studies;

material/lubricant characterization studies; and basic research.

These wear test design philosophies are listed in table 1 with

additional information on relative complexity, cost, and control.

Actual component studies can either involve a full scale working

piece of equipment with the appropriate components made of the

selected materials, or a test rig constructed to utilize the

components in the exact same way they would be used in service. An

example of the different approaches can be found in the use of ceramic

piston liners and rings in an adiabatic diesel engine. The actual

system approach would consist of fabricating piston liners and piston

rings and running them in the actual engine. The actual component

test rig approach would use the same components; however, they would

be placed on a simulation test rig that duplicates the primary

conditions (motions, loads, temperatures, etc.) but with better

control. There is usually some sacrifice of simulation in going to a

test rig (such as no combustion products or no thermal cycling) but
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this is a tradeoff for better control. Examples of this approach,

used for adiabatic diesel engines and gas turbines, are given in

19 1 references ^^'^'^^

Application-oriented simulation studies consist of using the same

materials as needed but not the actual components. Using the piston

ring and liner example, an apparatus that produces a reciprocating

line contact at appropriate speeds and loads would be used. This wear

test approach usually results in an increase in test control but,

again, by sacrificing some simulation aspects. Examples of this

approach, used for engine, tool, bearing, and grinding applications

are found in references

The material/lubricant characterization approach consists of

running friction and wear tests at a variety of conditions using the

actual material. This results in an understanding of the tribological

behavior of the material under selected severities (speed, load,

temperature, etc.). It may also be possible to gain an understanding

of the wear mechanisms for the material at different severities (i.e.,

a wear map) . The material thus characterized can be evaluated for a

particular end application. If, for example, it is found that a

particular ceramic material has a transition to a very high wear rate

above 6.9 MPa (1000 psi) and 800°C for slow speed sliding, the

material would be unsuitable for an application where pressures and

temperatures exceed these limits. Examples of studies using the
8

material/lubricant characterization approach are given in

references. °

^

The basic research approach focuses on understanding a

fundamental processes and mechanisms. These experiments are usually

highly controlled and often simplify a system to a select set of test

conditions. In many cases, very specialized equipment is required

therefore this is not always the least expensive approach. In some

cases, the distinction between some of these design philosophies is

not clear and they overlap. It is possible, for example, to conduct a

study incorporating philosophies from material/lubricant

characterization studies and basic research.

The material/lubricant characterization approach was the one

selected for this study. Besides the factor of cost (and budget),

this approach offers the attractive features of reduced complexity and

enhanced control to gain a better fundamental understanding of the

tribological characteristics of ceramic materials.

B . Current Efforts in Fundamental Ceramic Tribological Studies

There have been niimerous attempts to define the fundamental

tribological properties of certain ceramic materials using bench wear

tests. In general these studies can be divided into two groups:

single crystal ceramic studies and polycrystalline ceramic studies.
9

Single crystal studies are usually undertaken to study the

anisotropy of the tribological properties'^ although some studies

use single crystals merely as a source of pure material. Usually

single speeds and loads are used but many authors will select a few

speeds or loads , and note an apparent change in mechanism or wear rate

for different conditions.

Polycrystalline ceramic studies'^ are simpler in some ways

because the orientation becomes randomized and does not have to be

controlled during a test. In other ways they are more complex because

the additional variable of a second phase (grain boundaries) must be

considered. Frequently it is the grain boundaries that are the

weakest link in the structure of the polycrystalline ceramic. An

understanding of the tribological behavior of a single crystal ceramic

therefore does not necessarily endow an understanding of the

tribological behavior of polycrystalline forms of the same material.

Some authors use both single crystal and polycrystalline studies'^^ to

gain a complete understanding of the fundamental tribological behavior

of the single crystals and its relationship to the behavior of

polycrystalline materials.

The studies cited above represent attempts to understand the

tribological properties of ceramics from a variety of viewpoints, and

a wide range of conditions. Different wear test equipment is used

because there is no standard method or piece of equipment designed to

study the sliding friction and wear properties of ceramics. A
10

9

1

compilation of ASTM wear test methods conducted by Blau in late 1985"^-^

revealed only six test methods applicable to ceramics (table 2). Five

of these methods do not pertain to sliding wear and the sixth (G77) is

originally designed for metals. The scope section of this method

(G77) indicates that any materials can be used (plastics, ceramics);

however, representative precision data are given only for metals. In

addition, one of the specimens in method G77 is a 2 -inch diameter ring

with an inner taper. Dimensional tolerances are very tight (±0.025 mm

(0.001 in) for all dimensions, ±5 minutes for the taper angle) with a

surface roughness of approximately 0.2 /xm RMS. These factors would

make these specimens quite expensive to fabricate using ceramics. In

addition, post-test analyses (SEM for example) would be hindered by

the large size of the specimen.

More recently, new efforts are being made to develop

international standard test methods for ceramics under the guidance of

the Versailles Advanced Materials and Standards (VAMAS) project.

The ceramic wear test method development phase of the project centers

on comparison of wear and friction measurements using a pin on disc

(or ball on disc) tribometer. Many participants (including NBS) from

seven countries are participating in this study. An initial dry

(unlubricated) testing round robin offered promising results; however,

several procedural problems with sample cleaning, handling, and test

environment were encountered. Only a single speed and load is

currently being used in this study. In addition, the nature of the
11

Table 2

ASTM Wear Test Methods Applicable to Ceramic Materials

ASTM

Test Applications Materials Comments or Apparatus

C448 Porcelain Enamel

C501 Tile

C704 Refractory Brick

G32 Cavitation

G76 Erosion

G77 Sliding Wear

Ceramics

Ceramics

Ceramics

Metals

Ceramics

Plastics

Metals

Ceramics

Plastics

Metals

Ceramics

Plastics

NBS Lapping Machine

Taber Abraser

Room Temperature Jet

Erosion

Vibration in Liquid

Bath

Particles Against Flat

Block on Ring

Source: Blau (21)
12

effort (International, round robin) makes for slow progress in method

development

.

C . Key Differences between Ceramics and Metals

There are fundamental differences in the nature of interatomic

bonding between atoms in ceramics, and metals, that lead to

differences in chemical and physical properties. These differences in

turn lead to mechanistic differences in wear and friction.

Interatomic bonding in ceramic solids consists of a combination

of ionic, covalent, van der Waals , and hydrogen bonds, with ionic and

covalent bonding being the most important . "^-^ Covalent bonds are very

strong bonds formed when electrons are shared between atoms. The

resulting short bond lengths (1.54 A for C-C bond in diamond) produce

some of the strongest bonds known. Ionic bonds result when complete

transfer of electrons takes place between atoms producing charged

ions. The coulombic attraction between the charged ions (for example

Na+ and CI') produce relatively strong bonds with moderate bond

lengths (2.8 A for Na - Cl bond in halite). In both of these bond

types, electrons are localized to specific atoms or shared between

pairs of atoms. Van der Waals and hydrogen bonds are relatively weak

bonds. Van der Waals bonds arise from dipole forces between atoms.

Hydrogen bonds arise from the interaction of the slightly

electropositive H atom and an electronegative atom (0 or F for

example). Even though these atoms are bound to other atoms, there may
13

be enough residual electropositive and electronegative nature to set

up this weak interaction. Both of these weak bonds can be found

between layers of layered ceramic structures. For example, Van der

Waals bonding takes place between layers in graphite. Bond lengths

are characteristically large (3.4 A for the C-C bond between layers in

graphite)

.

Metals are held together through metallic bonding. These bonds

are the result of attractive forces (Coulombic) between positive

metallic atoms and a sea of free negative electrons. The result is

relatively strong bond strength and bond lengths similar to ionic

bonds ; however, these bonds are not as directional as ionic or

covalent bonds. This may result in an improved ability to yield to

relieve a stress.

The difference in the nature of the bonding between metals and

ceramics results in differences in their properties. The electron

mobility in metals for example allows them to have higher thermal and

electrical conductivity. Thermal conductivity is usually lower for

ceramics because conduction occurs mainly through phonon (thermal

vibration) transfer. In metals, both phonon transfer and electron

transport usually allow much higher thermal conductivity. (An

exception is diamond which is the best conductor known, perhaps due to

the very short bond length which may make phonon transfer very

efficient.) A comparison of typical physical and thermal properties

for selected ceramics and metals is given in table 3.
14

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15

In general, ceramics have lower densities, thermal conductivity,

thermal expansion, and electrical resistivity than metals. They also

have higher hardness and elastic modulus than metals, and tend to

retain their strengths to higher temperatures. Collecting data for a

comparison of this nature is very difficult for a variety of factors.

Ceramics and metals behave differently; therefore, different

properties are measured and highlighted. Tensile strength, for

example, is often not reported for ceramics because ceramics are

inherently weak in tension. Often, there is an order of magnitude

difference between compressive strength and tensile strength. Metals

on the other hand have tensile strengths much closer to their

compressive strengths ( = factor of 2 or 3) . The different behavior

for metals versus ceramics is also apparent when comparing their

stress strain relationships (fig. 1). This relationship is used to

determine the response of a material to applied stress. The initial

straight line portion of the figure is the elastic response. From the

slope, the elastic modulus can be calculated (large slope - high

modulus). At some point, the material behavior changes. For many

ceramics, failure occurs as brittle fracture. This failure point

defines the tensile strength of the material. For metals, the

material usually yields plastically before failing completely (usually

by necking - ductile failure). Thus, for metals, yield strength and

elongation are measured in addition to tensile strength.

In some cases, ceramic properties are so different from metals,

separate techniques are used to measure them. This makes comparison
16

CERAMIC

(BRITTLE)

Brittle Fracture

\

METAL

(DUCTILE)

Failure

Strain (C) —

>

Figure 1. Stress-Strain Relationship for Typical Metals

and Ceramics
17

of properties very difficult. Metals are relatively soft, therefore

hardness is reported as Brinell number or Rockwell number. These

values are usually based on indentation with a hard ball. Hardness

measurements for ceramics, on the other hand, require diamond

indentation techniques (Knoop or Vickers), which do not have a direct

conversion to the metal hardness values.

Another difference between metals and ceramics is the nature of

their processing. Metals are usually formed from a melt and cooled to

form the desired structure. Ceramics are usually sintered from a

powder. There may be a fundamental weakness introduced by this

technique which causes an inherent brittleness. The mere process of

sintering may cause impurities to collect in specific sites within

grain boundaries, for example, thus weakening the entire structure.

This behavior of impurities concentrating in phase boundaries has been

observed for metal systems and is actually used to purify them in the

technique known as zone refining.

The difference in the nature of the bonding also affects the

mechanisms by which these materials function tribologically . For

metal systems, one of the primary mechanisms of wear is through

adhesion. This is the result of micro welding of asperities and

subsequent removal of these welds from the parent material resulting

in many small "adhesive wear" particles. In ceramics, it appears that

the nature of the bonding does not allow adhesion (micro welding) to

take place easily between asperities. The dominant form of surface
18

degradation may instead be due to locally high stresses resulting in

microfracture.^^ In developing a methodology for wear testing of

ceramics therefore it is important to be aware of differences in the

nature of the bonding of the atoms , unique properties , and potential

mechanistic differences for ceramic materials. \

D . Approach to Evaluation of Ceramic Wear Used in This Report

The following approach to evaluating the wear of ceramic

materials was used base on an understanding of the literature and the

nature of ceramic materials. A tester was selected which allowed good

control of important test variables for a wide range of test

severities. A representative ceramic material was selected for a

parametric study of the effect of different test parameters on

friction and wear. A wide range of test severities were encompassed

in an effort to ensure that a bias of expected performance (based on

metals) was not limiting the study. Finally, a cycle of testing/data

analysis/interpretation was used to evolve into a more efficient and

informative test method as the study progressed.
19

Chapter III

EXPERIMENTAL APPARATUS AND OPERATING PROCEDURES

A. Wear Tester Selection

There are a wide variety of wear test devices available for

measuring the tribological properties of material lubricant

combinations. The Catalog of Friction and Wear Test Devices^^

published by the American Society of Lubrication Engineers contains a

detailed description of 234 different tribometers. Each apparatus has

particular strengths and weaknesses, however, given the requirements

of a ceramic material tribometer in the particular lubrication regime

we wished to study, a logical wear test device selection can be made.

The general requirements for the tribometer are: high contact

stresses to ensure that we are operating in the boundary lubrication

regime; good control of operating parameters of speed, load,

temperature, duration, atmosphere; high sensitivity of wear and

friction measurements; and fairly small specimens that can be easily

fabricated. The four-ball wear test geometry was selected as the best

apparatus to combine these features.

The four-ball wear test apparatus used in this study is an NBS

modification of a standard four-ball wear tester. Modifications were

made in order to enhance the load and temperature capabilities, and

repeatability of the standard device. The basic specimen

configuration (fig. 2) consists of a tetrahedral arrangement of four
Figure 2. Four-Ball Contact Geometry
21

half- inch diameter spheres. The three lower balls forming the base of

the pyramid are locked in place in a test cup. A fourth ball held in

a chuck, is nested on top of the three balls and provides three point

contacts. The upper ball rotates to provide a sliding motion between

the surfaces. An axial force is provided to adjust the loading

between the upper and lower specimens. A diagram of the overall NBS

four-ball assembly is shown in figure 3. The entire set of specimens

can be immersed in a lubricant and atmosphere control is obtained by

feeding the desired gases into the test chamber using flowmeters

equipped with needle valves. The specimen cup rests on a heating

block that is capable of providing temperatures up to 540°C. The

entire test cup/heater assembly rests on a frictionless air bearing

which serves to thermally isolate the assembly from the loading

piston, and allows for a high degree of sensitivity in friction

measurement. Friction produced in the contact points produces a

torque in the test cup assembly. This torque is measured, as a force

from a lever arm extending out of the assembly, using an electronic

force transducer. The force transducer sends an electric voltage

which is proportional to the applied force to a strip chart recorder.

Thus friction can be continuously monitored during a wear test.

The four-ball triboraeter satisfies all of the original ceramic

tribometer requirements. The three point contacts coupled with the

load capability provide test severities normally associated with

boundary lubrication. Good control of speed, load, temperature,

duration, and atmosphere are easily accommodated. Wear and friction
Drawbar ' 9

Baaring* •

Top Ball ChuckFriction Pickup

Teat Cup

Driva Pullay

Splndia

Environmental Qaa Inlet

Locking Nut

Thermocouple — Heating Block

Pneumatic Load Piston

Air Bearing

Figure 3. NBS Modified Four-Ball Apparatus Design
23

measurements are easy to obtain and sensitive enough for our purposes.

Half-inch diameter spheres are available in many ceramic materials.

In addition, the nature of the four-ball contact geometry with

frictionless air bearing allows the specimens to be largely selfaligning. This reduces the alignment problem common with wear test

devices, and allows for a high degree of sensitivity and

repeatability

.

A second configuration is possible with this apparatus by

replacing the three lower balls with 6.35 mm diameter (0.25 in), 1.59

mm thick (0.0625 in) disc specimens ("flats") as shown in figure 4.

This produces three point contacts between the upper ball and the

lower "flats" at exactly the same points as for the four-ball

contacts. The main difference between the two configurations is that

the four-ball test produces convex-convex surface contacts whereas the

ball-on-three-flat test produces convex-flat surface contacts. In

addition, the size and geometry of the flats provides a more thermally

conductive pathway between the wear contact and the metal test cup.

The ramifications of these differences will be discussed in a later

section.

The ball-on- three-flat geometry allows for testing of materials

not easily obtainable in ball form. Flats can be fabricated most

easily by cutting thin sections from 1/4- inch diameter rod stock which

is easily available, or by diamond core drilling of a 1/16- inch
24

Figure 4. Ball-on-Three-Flat Wear Test Contact Geometry
25

thick sheet of material. The combination of diamond core drilling and

sectioning allows flats to be fabricated from materials of almost any

shape

B . Specimen Material Selection

A single representative ceramic material was selected for initial

test method development work. This material was selected based on the

following criteria: Polycrystalline ceramic; mechanical, thermal, and

chemical properties typical of ceramic materials; and available with

good uniformity at relatively low cost. Based on these requirements,

polycrystalline aluminum oxide (alumina - AI2O3) specimens were

selected for preliminary testing. Alumina possesses mechanical,

thermal, and chemical properties fairly typical of ceramic materials

and is easily obtained in ball and flat form with good uniformity at

low cost. A listing of the approximate properties of this alumina

material is given in table 4.

C . Lubricant Selection

Three different lubricant environments were selected for initial

studies. Dry tests (no lubricant) were conducted in an effort to

establish a baseline for unlubricated friction and wear. A purified

paraffin oil was selected as a typical unformulated hydrocarbon

(nonpolar) lubricant of low viscosity (125/135 SUS) . Tests were also

conducted using deionized water (polar) as a lubricant. The water not
Table 4

Typical Properties of Dense High Alumina* Ceramics

Property Units Conditions Approximate

Values

Density

Young's Modulus

Hardness

g/cm^

GPa

#

Compressive Strength MPa

Tensile Strength MPa

Flexural Strength MPa

Thermal Expansion

Coefficient 10"^/°C

Thermal Conductivity W/cm °C

Specific Heat J/g "C

ASTM E18-67

Rockwell 45N

25°C

1000°C

25°C

1000°C

25°C

1000°C

25-200°C

25-1000°C

25°C

25-100°C

3.8-4.0

330-390

78-90

2000-3800

480-1930

200-310

100-220

275-550

170-410

5.3- 6.8

7.4- 8.6

0.2-0.33

0.75-0.88

Source: Condensed from Ceramic Source 1986, Vol. I, p. 334-335.

*Approximately 99% Alumina.
27

only served as a polar medium but also serves as a baseline for

possible usefulness of water based lubricants for ceramics.

D . Specimen Characterization Methods

A comprehensive review of the literature regarding tribology of

ceramics conducted with Jeff Yellets, " revealed a lack of adequate

specimen characterization in most cases. It is thought that detailed

pre and post-test characterizations can enhance the possibilities for

correct mechanistic interpretation of the data. I have therefore

tried to accurately characterize the materials used in this program.

In the course of this study, three separate batches of alumina

ball specimens were obtained (Cer 004, Cer 005, Cer 021). Later tests

were also conducted on flat alumina specimens obtained from rod stock

(Cer 006) . Selected bulk properties of some of these specimens are

given in Appendix A.

Specimens were characterized using profilometry , fractography

,

and ceramography . Profilometry is a technique which determines the

surface roughness of a sample. A diamond stylus is carefully drawn

over the surface of a sample. The stylus moves in response to the

microscopic contours of the sample as it is drawn over the surface.

This movement is amplified electronically and the electronic signals

are processed in such a way that a number representing the measured

roughness of the surface is obtained. Those wishing a more detailed
28

description of profilometry and its interpretation are referred to

reference 29. Ball specimens were obtained with a high degree of

uniformity of 12.7 mm ± 0.000635 mm (0.5 in ± 0.000025 in) and a

highly polished surface finish of 0.038 fim AA (1.5 fxin) . Flat

specimens were approximately 1 . 59 mm (0.0625 in) thick with a surface

finish of approximately 0.04 ij.m (1.6 ;uin) .

Fractography entails producing a fracture surface on a specimen

and observing the shapes and sizes of grains that become exposed.

Fracture surfaces were produced on ball specimens by cutting a notch

75% of the way through the ball with a diamond saw, then cleaving the

ball in half with a wedge. The rod stock was notched and cleaved in a

similar fashion. Once in the fractured state, specimens were mounted

on SEM studs, covered with thin conductive coatings ( = 20 nm) of

gold/palladium and analyzed in the SEM.

The scale marker on each of the SEM photomicrographs is

interpreted as follows. The left mark is the calibration mark. Its

length represents a unit length on the photograph which depends on the

number of dashes which follow the calibration mark. One dash

signifies the calibration mark is 1 /im long. Two dashes signifies the

calibration mark is 10 ^m long.

Fracture surfaces from the four different alumina batches were

analyzed in the SEM resulting in the photomicrographs shown in figures

5-8. A wide crystallite size distribution is seen for all four
Figure 5. SEM Photomicrograph of Fracture Surface of Cer 004

Alumina - 2000X

Figure 6. SEM Photomicrograph of Fracture Surface of Cer 005

Alumina - lOOOX
30

Figure 8. SEM Photomicrograph of Fracture Surface of Cer 021

Alumina - lOOOX
31

samples with a range of approximately 1 to 20 /xm. Grains appear to be

approximately equiaxial, and small ( ~ 1 fim) pores can be seen within

some of the crystallites.

Ceramography consists of cutting a section through a specimen and

polishing the cut surface with successively finer diamond polishing

compounds to reveal microstructural detail. Optical microscopy and

SEM are the most frequent analysis tools. In many cases chemical or

thermal etching is used to highlight the microstructure

.

SEM photomicrographs of the polished surfaces (figs. 9-12) reveal

the porosity of the surface. Samples Cer 005 and Cer 021 both appear

to have a similar porosity. Both large (

== 20 /im) and small (1-5 /xm)

pores are observed. Sample Cer 004 also shows approximately the same

bimodal distribution of pore size. Sample Cer 004 however appears to

have more of the smaller pores than Cer 005 and Cer 021. Sample Cer

006 appears to have a single small pore size distribution with pores

ranging from 0.5 to 8 /^m. In addition, the number of pores is much

higher than that observed for any of the other samples. The SEM

observations of the pores seems to agree quite well with the

calculated porosity for the different samples presented in table 5.

Polished samples were thermally etched at 1450°C for 5-10 minutes

to reveal the grain boundaries. This technique, suggested in a phone

conversation with a researcher at Adolph Coors Company, was claimed

to introduce no cracking or grain growth within their high alumina
Figure 9. SEM Photomicrograph of Polished Surface of Cer 004

Aliamina - 500X

Figure 10. SEM Photomicrograph of Polished Surface of Cer 005

Alumina - 500X
Figure 11 SEM Photomicrograph of Polished Surface of Cer 006

Alumina - 500X
34

Table 5

Summary of Selected Characterization Data on the

Alumina Ceramics Used in This Study

Approximate Crystallite Size Observed

Fracture Polished/ % Theoretical Porosity^

Sample Surfaces Etched Density^ ^

Surfaces

Cer 004 1-10 ^m 0.5-10 fxm 96.4 3.6

Cer 005 1-20 ^m 0.5-15 /zm 97.8 2.2

Cer 006 1-10 /xm 0.5-10 ^im 96.2 3.8

Cer 021 3-15 ^lm 1-10 nm 98.5 1.5

Measured via mass and volume calculations except for Cer 006 which

was supplied by the supplier literature,

100% - theoretical density.
35

samples . SEM photomicrographs of these surfaces show the individual

crystallites defined by their grain boundaries (figs. 13-16). For

these particular photomicrographs, regions were selected away from

larger pores in order to examine the cross sectional grain boundary

pattern. In all the photographs, a wide distribution of grain sizes

is observed. Both intracrystalline and intercrystalline pores are

observed. (Some of these intercrystalline pores may be due to pullout

of parts of grains during the polishing process.) Cer 004 has

individual crystallites ranging from approximately 0.5 to 10 yum. Cer

005 has crystallites ranging from approximately 0.5 to 15 //m; Cer 006

from 0.5 to 10 fim; and Cer 021 from 1 to 10 /xm. The tracks observed

going through the crystallites are residual polishing marks. SEM

photomicrograph observations from the polished-etched, and fractured

samples are summarized in table 5. The crystallite size information

from the fracture and polished/etched samples seem to give similar

information, and indicate subtle differences in micros true ture for the

different batches of alumina. Samples Cer 005 and Cer 021 appear to

be slightly coarser grained than the other two samples. They also

appear to be slight more dense. This agrees well with our

understanding of the sintering process where during the high

temperatures encountered, grain growth occurs, resulting in larger

•3 1 grains-*-^ (and lower final porosity) .
36

Figure 14. SEM Photomicrograph of Polished/Etched Surface of

Cer 005 Alumina - 5000X
Figure 16. SEM Photomicrograph of Polished/Etched Surface of

Car 021 Alumina - 5000X
38

E . Friction and Wear Measurement

As mentioned previously, friction is continuously recorded during

a test via an electronic force transducer. The paper output of the

strip chart gives a history of friction throughout the test.

Coefficient of friction can be calculated for any test where both the

friction and applied load are known, given the equation derived in

Appendix B.

0.02339 f

A* =

L

where f = measured friction force in grams,

L = applied 4 ball machine load in kgf , and

H = coefficient of friction.

Wear can be measured in a variety of ways; however, for the

purposes of our tests two principal methods were used: weight loss

and wear scar diameter. The weight loss method requires careful

weighing of specimens before and after a test using a semimicrobalance

with a repeatability of ±0.03 mg for a single measurement. This

method works quite well for tests in which wear is fairly severe

resulting in substantial weight loss; however, it is not adequate for

tests with low wear. In addition, this method has the advantage that

both upper and lower wear is measured and therefore the degree of

asymmetric wear can be observed.
39

The second technique for wear measurement, the wear scar diameter

measurement, consists of locating and measuring the scar under an

optical microscope fitted with a graduated reticle eyepiece. The wear

scar is usually oriented under the objective lens such that the

direction of sliding is horizontal (i.e., from left to right). Both

horizontal (left to right) and vertical (top to bottom) wear scar

measurements are recorded for each scar. The geometric average of the

two measurements [ab]-'-^^ on each scar are calculated and the

arithmetic average of the three scar values is taken as the diameter

(or equivalent diameter) of the wear scar. The equivalent diameter,

based on geometric value, was used because it was felt that this was a

more accurate representation of wear for our testing where

elliptically shaped wear scars were the rule rather than the

exception. Derivation of the equivalent diameter concept for a wear

scar is given in Appendix C. This technique also has the advantage

that if the wear scar is circular, the arithmetic average and

geometric average of the two measurements are the same. Therefore,

taking the geometric average merely extends the accuracy of wear scar

measurement representation to the elliptically shaped wear scar

regime.

Optical wear scar measurement has an advantage over weight loss

measurement in its ability to accurately reflect very low levels of

wear. The optical technique is capable of measuring as little as 0.1

mm diameter wear scars (although locating these wear scars is often a

tedious search) . This would represent a weight loss of approximately
40

0.015 ij,g of material (in the case of alumina), which is far less than

we can accurately weigh. Figure 17 compares the two wear measurement

methods used in terms of their useful ranges. In general optical wear

scar measurement is accurate in the range of 0.1 mm to 5.0 mm. The

lower limit is imposed from the resolution of the microscope

measurement system and increasing difficulty in locating these size

scars. The upper limit is due to the fact that very large wear scars

tend to be highly irregular in shape (approaching teardrop shapes) and

the wear scar diameter fails to accurately reflect wear in a

meaningful way. Weight loss measurements are accurate from

approximately 0.8 mm wear scar diameters and above. The lower limit

is imposed by the accuracy and repeatability of weight loss

measurement. This is caused by a combination of factors including

balance sensitivity but is mostly due to the repeatability of sample

cleaning and handling procedures.

F. Specimen Preparation

Ball specimens were received in a finished state and required

only solvent cleaning prior to testing. Flat specimens 6.35 mm (0.25

in) in diameter and 1 . 59 mm (0.0625 in) thick were cut (oversized in

thickness) from rod stock using a slow speed, low deformation, diamond

saw. They were polished using 240 grit then 11-14 ^m bonded diamond

abrasives followed by a series of successively smaller grain size

diamond polishing pastes of 6, 3, and 1 micron.
41 c

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42

This resulted in specimens of desired thickness with a surface

roughness of approximately 0.04 /im (1.6 nin) AA.

Just prior to testing, the specimens were cleaned in a series of

solvents to remove residual cutting and polishing oils, fingerprint

oils, and a variety of other possible contaminants. The complete

cleaning procedure, described in more detail in Appendix D consisted

of a series of ultrasonically agitated rinses in hexane/toluene (9:1),

hexane , and acetone, followed by drying in a stream of nitrogen gas.

This produced a clean surface which gave adequate repeatability for

most of the tests. In the special case of dry tests, it was found

that trace impurities in the acetone (probably water) remained on the

surface and reduced friction and wear for very low load tests. This

effect was corrected by heating the test cup under argon atmosphere to

110°C for 10 minutes to drive off the water then cooling it back down

before commencing the test.

G . Operating Procedure Descriptions

Two operating procedures were used for this study; one for

constant condition tests (CCT's) and one for step- loading tests

(SLT's). In addition, the procedures are slightly different depending

on whether four-ball (FB) or ball-on-three-flat (BTF) wear tests are

being conducted. For constant condition four-ball tests the specimens

and test parts that come in contact with the specimens or the

lubricants are cleaned with solvents. After drying with nitrogen gas.
43

the three lower specimens are positioned in the test cup and secured

in place with a locking ring. The upper rotating ball is pressed into

the ball chuck, and the ball-chuck assembly is secured onto the end of

the spindle. Ten milliliters of lubricant are placed in the lower

specimen holder and the assembly placed on the heater/air bearing.

The air bearing valve is adjusted to give lift to the air bearing and

the load is adjusted to zero to just support the weight of the entire

heater/air bearing, test cup assembly. This assembly is carefully

slid upwards until the upper and lower specimens just contact each

other

.

The friction force measuring chain is connected to the specimen

cup and the test cup is then balanced such that no force is exerted on

the electronic force transducer. This balancing step is necessary to

ensure the accuracy of the friction measurement at low loads. If the

test is being conducted at temperatures above ambient, the heater and

temperature controller are engaged. The timer is set to the desired

time, the load is applied, and the test commences when the desired

test temperature is reached. At the end of the test, the upper

specimen stops rotating, the load is removed, and the test cup and

upper specimen assemblies are removed and disassembled for post test

specimen analysis.

Step- loading tests are conducted in a similar fashion except

after a test segment is conducted at a particular load, the lower

specimen assembly is removed from the heater/air bearing. The

that
lubricant is removed and the surfaces of the wear scar gently wiped

with a cotton swab moistened with hexane if a hydrocarbon lubricant

was used. The scars are then measured with the lower specimens still

locked in place, using an optical microscope. New lubricant is added

to the test cup, the assembly is put back on the heater/air bearing

and balanced. The new load is applied and the next segment of the

test is conducted. This cycle continues until the desired maximum

load is completed.

The procedures for ball-on- three-flat tests are similar to the

four-ball tests except only one and a half milliliters of lubricant

are required. In order to ensure re-alignment of the specimens for

the step-loading ball-on- three-flat tests, double sided adhesive tape

is used to prevent the lower specimens from moving during handling for

wear scar measurements. The tape is applied as a thin (0.075 mm)

backing to the specimens and secures them to the bottom of the holder

recesses

.
45

Chapter IV

RESULTS AND DISCUSSION

A . Constant Condition Four-Ball Wear Testing Study

1 . Base Conditions

Ceramic wear test parametric studies were conducted by selecting

a set of base conditions and adjusting each parameter individually and

noting the effect. The initial base set of conditions selected were:

Speed: 600 rpm (23 cm/s)

Load: 20 kg

Temperature: Ambient (21 "C)

Duration: 10 minutes

Lubricant: 10 ml Purified Paraffin Oil, Water, or

unlubricated (dry)

Atmosphere: 0.25 i/min dry Air

Material: Alumina Ceramic Cer 004

These values of speed and load represent conditions intermediate

to the capabilities of the equipment (speed range: 0.6 - 10,000 rpm;

load range: 0.5 - 300 kg) that are well into the boundary lubrication

regime. These conditions produced wear scars of 3.16 mm for the dry

case, 1.70 mm for the paraffin oil lubricated case, and 1.01 mm for

the water lubricated case. Thus a ranking of wear for these

conditions is: Dry > Paraffin Oil lubricated > Water lubricated.
46

Coefficient of friction values for these tests produce a different

ranking however: Dry (0.64) > Water lubricated (0.29) > Paraffin Oil

lubricated (0.12). As a point of reference, these values can be

compared to results obtained for paraffin oil lubricated tests using

52100 steel specimens: Wear Scar = 0.34 mm, Coefficient of Friction =

0.085. At first it seems that the steel is far superior to the

alumina under the same test severity however it will be shown that the

same applied wear test conditions lead to different actual test

severities when ceramic specimens are used.

2 . Explanation of Severity Differences Between Steel and Ceramic

Tests Conducted at the Same Operating Conditions

Tests run on different materials (ceramics vs metals) on the

four-ball wear test under identical machine test conditions (speed,

applied load, temperature) actually run under different test severity.

This is because severity is also a function of material parameters and

ceramics have very different properties than metals. More

specifically, the severity is different for conditions of actual load

(pressure) experienced by the surface in contact, and also for

conditions of temperature.

In the case of pressure in the contact, we must look at the

geometry of the contact and how it is influenced by material

parameters. As shown before, the general geometry of the four-ball

contacts provide a distribution of force based on the applied load of
N = 0.408 L, where L is the applied load in kilograms, and N is the

normal load on the surface of a single ball in kilograms. This is

entirely based on geometry, and material parameters play no part.

f Material parameters do play a part in how this normal load is applied

to the surface. The general contact geometry for a single contact is

convex surface on convex surface. As a load is applied, these

surfaces press together deforming and providing a contact surface

(disc) which carries the load. The diameter of this disc is a

function of the applied load, the radii of the contacting surfaces,

and two material properties: Poisson's ratio and Young's modulus.

This relationship is described in the Hertz equatioii given in Appendix

H; however, in general, the diameter increases with decreasing Young's

modulus and Poisson's ratio. Poisson's ratio is the ratio of the

thickness decrease to the length increase (per unit width and length)

when a material is extended in tension.

^d/d

- .................. : . .......

where u = Poisson's ratio. Young's modulus is the proportionality

constant between stress (normal tensile) and strain in Hooke ' s law

where o is the normal stress, E is Young's modulus, and e is the

normal strain. The basic result of these material parameters on the

Hertz contact diameter is that harder materials deform less under
48

stress and produce higher contact pressures. A graph of calculated

maximum contact pressures (from Appendix H) for aliomina and 52100

steel are given in figure 18. They show that assuming elastic

contact, the maximum contact pressures for alumina are 50% higher than

steel given the same applied load. The pressure provided by steel at

twenty kilogram load is experienced by alumina at a bit less than six

kilograms. These calculations should not be taken too literally in

the sense that the assumption that alumina will behave elastically at

40 kg (750 kpsia) is not realistic. (The compressive strength of

alumina at room temperature is approximately 300 kpsia.) However,

this example serves as a caution that the contact severities for

alumina are higher because of its material properties.

The second severity parameter that may be different for different

material is temperature. During a wear test, friction in the contact

junction generates heat and raises the local temperature. This heat

dissipates into the lubricant and specimens and eventually an

equilibrium is established between rate of heat generation and

dissipation. The result is a steady state local temperature higher

than the ambient temperature of the test. Materials with lower

thermal conductivity do not allow heat to dissipate as rapidly and

give higher local temperatures. The difference in thermal

conductivity and thermal diffusivity between ceramics and metals

provide higher local contact temperatures for ceramics under the same

test conditions. A recent study by Munro-^'^ which modeled the

temperature distribution within test specimens of the four-ball wear
49

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test predicted very different temperatures for alumina specimens

versus 52100 steel specimens. Under conditions of 500 rpm, 40 kg

load, and coefficient of friction of 0.075 (lubricated) temperatures

of = 160°C were predicted for the contact junction using steel

specimens. Temperatures of = 400°C were predicted for alumina - a

difference of 240°C. In addition, thermal gradients in the alumina

were much sharper. The absolute values for the temperatures obtained

in his study are of course a function of the conditions he selected

(friction coefficient, convective heat loss values, etc.). The

conclusion he reached, that thermal conductivity of test specimens ha

a profound effect on contact temperatures, is still valid. Thus

higher temperature results in a higher thermal severity for alumina

specimens run under the exact same applied test conditions as 52100

steel.

These two examples of contact pressure and temperature

differences between alumina and steel specimens illustrate the need t

understand the actual contact severity conditions, not just the

applied test conditions. Therefore, data interpretation should take

these factors into account.

3 . Four-Ball Constant Condition Test: Effect of Test Duration

In general, a wear test can be composed of three different

regimes as shown in figure 19. Initially, very high wear rates

I) are observed as the surfaces wear-in to adjust to the test

wear

(Part
51

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conditions. This wear- in phase of a test is usually fairly severe and

can be very unpredictable especially with regard to friction.

Reproducibility of test results in this regime are usually poor.

Eventually, a condition can be reached in which wear rates are

relatively constant with time. This is the steady state wear regime

(Part II). Eventually, a third regime may be reached in which

essentially no wear occurs as the surfaces are so well worn in that

hydrodynamic lubrication can separate the surfaces. It is also

possible to switch to a new steady state wear rate (either lower or

higher) at any point during the test.

Tests were conducted using alumina under unlubricated (dry) and

paraffin oil lubricated conditions to see the effect of test duration

on wear. This was done to ensure that later tests would be run long

enough to allow them to progress beyond the severe wear- in regime of

the test. Tests were conducted at 600 rpm, 20 kg, ambient

temperature, using Cer 004 alumina specimens. Wear results from these

tests (fig. 20) indicate different wear behavior for the two cases.

Paraffin oil starts with a moderate wear scar of 1.17 mm at 1 minute

and increases in a nonlinear fashion up to 10 minutes duration. After

ten minutes, the wear rate (slope of line) drops to near zero. The

dry tests also start at a moderate wear scar of 1.40 mm at 1 minute.

However, the wear rate is more rapid than the paraffin oil lubricated

case. After ten minutes, the wear rate drops considerably to a much

lower value. The accompanying final coefficient of friction data for

these tests (fig. 21) show a tremendous difference between the two
Figure 20. Wear-in Behavior of Alumina under Unlubricated and

Paraffin Oil Lubricated Conditions
Figure 21. Friction Behavior of Alumina under Unlubricated and Paraffin Oil Lubricated Conditions
55

cases. The paraffin oil lubricated tests all have very low

coefficient of friction values (0.123) even for the high wear rate

wear- in regime. Repeatability of the friction measurement is good

(±0.009). The unlubricated tests have a much poorer repeatability

(±0.035). They also show a distinctly higher level of friction (// =

0.64) than for the paraffin oil lubricated case. In addition there

appears to be a slight rise in coefficient of friction with time.

Based on the results of these tests, a duration of ten minutes was

selected for subsequent tests. This duration allows the test to

complete the more severe wear- in phase of the test thus increasing the

repeatability. Tests longer than ten minutes only serve to lengthen

the time required for each test and do not appear to give any

additional information or better repeatability. This is especially

true for the paraffin oil lubricated case which has a very low wear

r . I \ \ I rate after ten minutes. \ I "4, U\/::;i

4 . Four-Ball Constant Condition Test: Effect of Load

\

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OCH"

Once the proper duration was selected, the next step was to see

the effect of load on wear and friction. Constant condition tests

were conducted at various loads and friction and wear values at the

end of the test were recorded. .tJssU!

Dry tests at different loads showed unusual friction behavior. A

comparison of friction traces from test at three different loads is

shown in figure 22. Loads of 20 kg or greater display the functional
56

600 -

500 -

400 -

o

c

o

4-o

300 •

200 -

100 -

— COF : 0.643

~- COF : 0.622

— COF : 0.168

Conditions : 600 rpm, 0.25 l/mln dry air, 10 min duration, alumina specimens

Figure 22. Change in Friction Behavior with Time for Unlubricated Alumina Tests at Different Loads
57

behavior shown in the upper friction trace. The coefficient of

friction is very high (0.65) from the very beginning of the test to

the very end. The friction trace is very rough and spiky, indicating

the severity of the wear process. These tests were also accompanied

by a combination of grinding and squeaking sounds from the test cup.

A test conducted at 15 kg starts out with a low coefficient of

friction (0.234) but during the course of the test, jumps to a higher

level of friction (/j, = 0.522). There appeared to be something

protecting the surface during the initial stages of the test that

produced low friction. A test at low load (10 kg) produced a low

level of friction which continued until the end of the test (/i =

0.168). The difference between the three tests was the severity of

the load. This has a direct bearing on the temperature in the contact

junction during a test since the temperature is proportional to

load.-^'^ This data therefore suggests that a surface contaminant was

present on the surface of the specimens after the cleaning procedure.

It may have been some kind of hydrocarbon impurity left over from the

cleaning solvents, hydrocarbons from the air in the lab, or water from

the solvents or lab atmosphere. These contaminants were able to

protect the surface during the low temperature low load tests. At

high loads, temperatures generated by friction are sufficient to

remove the contaminant from the surface, increasing the friction

level. This hypothesis was checked by repeating the ten kilogram test

with a procedural modification (fig. 23). In this test, just prior to

testing, the test cup was heated to 150°C for ten minutes under a flow

of argon. The specimens were then cooled (still under a flow of
Upper Trace : Specimens heated to remove contaminants

Lower Trace : No pre-heat treatment

O)

oTO

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o

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400

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POP

200 m

COF : 0.35 1

100 — COF

10

Time, mins

Conditions : 600 rpm, 10 kg, 0.25 i/min dry air,

10 min duration, aiumina specimens

Figure 23. Comparison of Friction Behavior with Time for

Unlubricated Alumina Tests with and without Heating

Pretreatment
argon) until they reached room temperature. The test was then started

and run under dry air and the friction recorded. As can be seen in

figure 23, the coefficient of friction immediately rose to a level of

0.58 and stayed there throughout the test, indicating complete removal

of the contaminant by the heating procedure

The friction and wear behavior of alumina was determined for

several loads under unlubricated , water lubricated, and paraffin oil

lubricated conditions. No unlubricated runs were conducted at loads

less than ten kilograms due to an instability of the wear test

apparatus under these conditions. Attempts to conduct tests at two

and five kilograms resulted in severe excessive vibration of the

entire test cup assembly. Apparently the vibration induced by wear at

these loads coupled with the inherent stiffness and mass of the test

assembly set up a resonant frequency response. Attempts to mitigate

this problem by adjusting the stiffness and mass of the test assembly

met with very limited success. This instability was not observed for

the lubricated runs at any load, nor for any of the unlubricated runs

at higher loads. JWear results for the unlubricated and lubricated tests are given

in figure 24. This data is plotted as log wear scar diameter versus

log load as is the usual procedure for four ball testing. This

convention is followed for several reasons. 1) Scales are compressed

and allow concise plotting of a wide range of data. 2) The

relationship between Hertzian wear scar diameter and load is a
60

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61

straight line of slope 1/3 when plotted log/log (Appendix H) . This

serves as a good reference point for the data since the Hertzian wear

scar diameter represents a minimum wear value at the specified load.

3) Data plotted in this fashion tend to fall on straight lines.

B. Wear Test Severity Overlay

During the course of this study it was found that a new method of

describing the relative severity of a test was needed. This was

because comparisons of data (wear scar diameter) were being made at

different loads. One solution was to define the severity at the end

of a test by the mean pressure produced in the contact junction. The

mean pressure is merely the normal force at the contact divided by the

area of the wear scar. This offers an easy method for comparing test

results since a lubricant/material combination which can sustain a

load of 50,000 psi would be better than one that causes more wear and

produces a larger wear scar such that the mean pressure drops to

10,000 psi. The relationship between wear scar diameter, load, and

pressure is such that a plot of log wear scar diameter versus load

will yield straight lines of constant mean pressure. An overlay

(transparency) can therefore be easily fabricated to determine the

mean pressure for a given load and wear scar size. Two test severity

figures are included in Appendix J and K of this report (J is in psi,

K is in MPa) . Transparencies can be made from these figures and used

as overlays to estimate mean pressures for any of the log wear scar

diameter vs log load plots. The sizes of all log/log plots of wear
62

scar diameter versus load have been standardized to allow easy

determination of mean pressures of the different phases of each test

using the overlays.

Figure 24 shows a large difference in wear between the three

different cases. The corresponding coefficient of friction levels at

the ends of these tests is given in figure 25. The unlubricated

alumina tests show the highest level of friction and wear throughout

the load range investigated. Mean pressure was approximately 10.3 MPa

(1,500 psi) throughout the load range showing that unlubricated, the

specimens can only support very low contact pressures. The water

lubricated tests show the second highest level of friction through

most of the load range however wear is lower than the paraffin oil

lubricated case for all but the lowest (5-7 kg) loads. Mean pressures

for the water case are approximately 207 MPa (30,000 psi) for the low

load region and 83 MPa (12,000 psi) for the high load region after the

wear transition. The paraffin oil lubricated tests show the lowest

level of friction for all except the 40 kg test. Paraffin oil has the

^ lowest wear at low loads but intermediate wear at higher loads. Mean

pressures for paraffin oil lubricated tests are approximately 345 MPa

(50,000 psi) for the low load region and 48 MPa (7,000 psi) for the

high load region after the wear transition. Both lubricated tests

appear to have transitions in their wear behavior at low loads of 8 kg

for paraffin oil and 11 kg for water lubricated tests. There appears

to be a fair amount of scatter in the wear behavior but straight lines

seem to represent the wear trends quite well. Interestingly enough
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the coefficient of friction values fail to give any indication of the

wear transition. All three cases tend toward higher coefficient of

friction values at higher loads. This may be due to a higher surface

plasticity at higher temperatures (at higher loads) . Both lubricated

cases show large friction transitions at very high loads (60 kg for

water; 40 kg for paraffin oil). This may be due to complete failure

of the lubricant at these loads. This is substantiated by the

observation that the friction levels appear to be approaching that of

the unlubricated tests.

C . Analysis of Wear Scars

Scanning electron micrographs of the wear scars and wear debris

produced from unlubricated, water lubricated, and paraffin oil

lubricated tests are presented in figures 26-43. The unlubricated

tests (figs. 26 and 27) show interesting morphology. The lower

specimen wear scar is teardrop shaped because of a large amount of

wear on the upper (rotating) ball. The lower magnification photograph

shows a relatively uniform distribution of small areas of apparent

plastic flow of material. Higher magnification (fig. 27) confirms the

apparent smearing of material along with evidence of large amounts of

very fine (submicron) debris littering the surface. Analysis of the

wear debris (figs. 28 and 29) confirms the very small size (0.2 yum) of

the debris. A separate particle size and shape analysis of wear

debris from the unlubricated test indicated that most of the wear
Figure 26. Low Magnification SEM Photograph of a Wear Scar

from an Unlubricated Alumina Test

Figure 27. High Magnification

Specimen Wear Scar

Wear Test

SEM Photomicrograph of Lower

from an Unlubricated Alumina
66

Figure 29. SEM Photomicrograph of Wear Debris from Unlubricated

Wear Test - lO.OOOX
67

particles were equiaxial and less than 0.1 /im.-^^ Given the size of

the debris, and the fact that the original material was composed of

1-20 fj,m sized grains, these photomicrographs indicate large amounts of

intragranular breakdown of material. This is probably due to a

combination of plastic deformation and abrasion. The apparent

plasticity seen is unusual for a brittle materials such as alumina at

room temperature and suggests high temperatures are generated from

frictional heating during the test.

Wear scars from water- lubricated alumina wear tests show a very

different behavior. Low magnification (fig. 30) indicates two

distinct regions within the wear scar. It appears that half of the

wear scar is covered with a film. The other half is relatively smooth

and contains much of the surface character (porosity, etc.) of the

original polished surface. Higher magnification photomicrographs

(fig. 31) of the surface shows some compacted debris filling in some

of the smaller pits (pores?) of the worn surface. A second, longer

duration test confirmed the presence of a "film" and provided a more

detailed look at its structure. Low magnification photomicrographs of

the wear track of the worn upper ball (fig. 32) clearly show the

presence of a "film". The upper wear track in figure 32 seems to be

made up of a smear of compacted material. The curved line apparent in

the middle of the picture is the impression of the lower specimen wear

scar left in the track at the end of the test as the specimens stopped

sliding. Direction of sliding of the ball on the track is right to

left. A higher magnification shot of this region (fig. 33) shows
Figure 30. Low Magnification SEM Photograph of a Wear Scar

from a Water-Lubricated Alumina Test

Figure 31. High Magnification SEM Photomicrograph of Lower

Specimen Wear Scar from a Water-Lubricated Alumina

Wear Test
Figure 33. SEM Photomicrograph of Upper Specimen Wear Track

Water-Lubricated Run 2748R - lOOX
thin smears of compacted material forming platelets which eventually

merge to form a fairly uniform film. The buildup outlining the

contact between the wear track and the lower ball indicates a small

pileup of material being pushed in front of the wear scar, evidence of

the fairly ductile nature of the film. At higher magnifications (fig.

34), the plate-like, smeared, nature of the film becomes apparent. It

appears that it is composed of small particles which smear into a very

smooth surface when adequate stress is applied (fig. 35). The

particles which make up this film are very small (fig. 36) and are on

the order of 0.1 fj,m or less. They appear to exhibit some cohesive

properties (they hold together when smearing); however, it appears

that these forces are fairly weak as evidenced by the cracks and

fissures seen in the film platelet of figure 36. Photomicrographs of

the lower wear scar from the same test confirm that the film is

present on both surfaces (fig. 37). The film was observed to be of

the same nature as the film for the upper wear track. Regions were

found where the film was very granular, and other regions were found

where the grains were compacted into a very smooth film. After the

wear tests were conducted, samples of the used lubricant (water) were

filtered (0.2 /im pore Nylon) and analyzed under the SEM. Two types of

wear debris were found as seen in figures 38 and 39. Very small

particles (most <0 . 1 /xm) were abundant. Several large "flakes" or

platelets were relatively common. The high magnification slot at

5000X (fig. 39) indicates that the platelet is composed of very small

particles (edge of flake) with a very smooth surface and a large

number of small particles adhering to the surface.
71

Figure 35. SEM Photomicrograph of Upper Specimen Wear Track

Water-Lubricated Run 2748R - lOOOX
Figure 36. SEM Photomicrograph of Upper Specimen Wear Track

Water-Lubricated Run 2748R - 5000X

Figure 37. SEM Photomicrograph of Lower Specimen Wear Scar

Water-Lubricated Run 2748R - SOX
73

Figure 39. SEM Phogomicrograph of Wear Debris from WaterLubricated Wear Test - 5000X
74

The small particles appear to have been placed on the surface after it

was formed. Whether they "grew" there or were attracted by some

cohesive force between the particles and the platelet is not known.

Wear scars produced from paraffin oil lubricated alumina tests

display a third wear scar character. At low magnification, figure 40

shows an enhanced isotropic roughness within the wear scar. No film

is apparent. Higher magnification (fig. 41) reveals distinct pits in

the surface. The sharp planes of the pits are facets of individual

grains and indicate that grain pullout has occurred. There is no

evidence of plasticity perhaps because of reduced temperatures in the

contact junction. These lower temperatures could be due to surface

cooling by the lubricant or reduced heat generation because of the low

level of friction for these tests (coefficient of friction =0.1, see

fig. 25). Analysis of the wear debris (figs. 42 and 43) indicate the

presence of large (10 fxm) particles produced by pullout of individual

grains ( intragranular fracture) as well as smaller particles (0.1-2

lim) which must have been formed from intragranular fracture of the

parent material.

The intragranular fracture may have occurred by gradual wearing

of the exposed surfaces of the grains as well as crushing of grains or

parts of grains that were pulled out of the original polycrystalline

matrix. Free wear debris in the contact junction may also contribute

to considerable abrasive (three body) polishing of the surface. The
Figure 40. Low Magnification SEM Photograph of a Wear Scar

from a Paraffin Oil Lubricated Alumina Test

Figure 41 . High Magnification SEM Photomicrograph of Lower

Specimen Wear Scar from a Paraffin Oil Lubricated

Alumina Wear Test
Figure 42. SEM Analysis of Wear Debris from Paraffin Oil

Lubricated Wear Test - lOOOX
77

surface and wear debris morphology support all three of these wear

mechanisms

.

Some researchers suggest that adhesion may play an important role

in the friction and wear of ceramics. Buckley and Miyoshi"^^ for

example suggest adhesion plays a critical role in the alumina/alumina

contact. Their experiments, conducted using sapphire under ultrahigh

vacuum, suggested that the adhesion between the surfaces was 1)

orientation dependent (anisotropic) and 2) extremely surface chemistry

conditions which they attributed to water vapor. Their experiments

with olive oil suggested hydrocarbons may have a similar effect on

reducing adhesion and would help to explain the reduce friction

observed in the four-ball results presented in this report.

Summarizing the results of the wear analysis: Dry wear appears

to produce very small particles of debris and some evidence of flow of

material is observed. Water lubrication results in the formation of a

surface film which protects the surface and reduces friction and wear.

Paraffin oil lubrication produces wear through a combination of

intragranular fracture (perhaps through those body abrasion) and grain

pullout

.
78

D . Four-Ball Constant Condition Tests: Effect of Batch to Batch

Variations in Specimen Material

The purpose of wear testing is to determine the tribological

properties of the selected material/lubricant combination. The

preliminary testing was conducted using early batches of alumina balls

with the bulk of the testing on Cer 004 alumina. A new batch of

alumina (Cer 005) was also tested using constant condition tests and

plotted as wear scar diameter versus load for both water and paraffin

oil lubricated conditions. Both aluminas were high purity and low

porosity. Wear data from these tests (figs. 44 and 45) show distinct

behavior for the two batches of alumina under both lubricating

conditions. Under water- lubrication (fig. 44) Cer 005 shows slightly

lower wear at low loads (mean pressure = 310 MPa (45,000 psia) vs 200

MPa (24,000 psia) for Cer 004) and slightly higher wear at higher

loads (mean pressure =48.3 MPa (7000 psia) vs 82.7 MPa (12,000 psia)

for Cer 004) with a lower transition load from low to high wear (7.5

kg vs 11 kg) . Under paraffin oil lubrication (fig. 45) a similar

behavior is seen: lower wear is observed for low loads (mean pressure

= 621 MPa (90,000 psia) vs 349 MPa (50,000 psia) for Cer 004), wear

rates at higher loads are similar (mean pressure =48.3 MPa (7000

psia), and the transition load is lower (3.5 kg vs 8.5 kg). Strict

interpretation of the reason for the wear difference in the two

materials cannot be made without an extensive examination of the

materials both before and after the tests. This data indicates,

however, that this wear test method is quite sensitive
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to small variations in materials as seen batch to batch. This high

sensitivity also suggests that it may be possible to study the effect

of processing variables on wear results. This would prove extremely

valuable to optimizing ceramic materials for tribological

applications

.

E . Step-Loading Four-Ball Wear Testing Study

1 . Base Conditions

Constant condition test results plotted as wear scar diameter

versus load give valuable information of the tribological properties

of ceramic materials. Unfortunately, it may take ten or more tests to

adequately describe the material behavior. This is both time

consxaming and potentially expensive. As a result, a step- loading

procedure was developed to derive the same information but using a

single test. This procedure was described in the experimental section

of this report.

Two unlubricated step- loading tests are shown in figure 46 for

Cer 005. The repeatability of the test is good and the data are

identical to the data obtained using constant condition tests. Mean

pressures of approximately 8.3 MPa (1,200 psi) are obtained.

Water- lubricated step-loading tests (fig. 47) also show good

repeatability for low and high load wear conditions, and also for the

transition load. When compared to the constant condition test
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results, the step- loading tests produce slightly higher low load mean

pressures of 345 MPa (50,000 psi) . The transition load is slightly

higher and the high load wear has much flatter response to load

variation. The mean pressure after the wear transition is 41 MPa

(6,000 psi). The difference between step-loading and constant

condition test results is due, I believe, to the higher severity at

the beginning of the constant condition tests at each load (virgin

surfaces)

.

Paraffin oil lubricated step- loading tests also show good

repeatability, especially for low load conditions, and transition load

values (fig. 48). The upper, high wear region shows slightly poorer

repeatability. Compared to the constant condition tests, the low wear

regime has a slightly higher mean pressure of 483 MPa (70,000 psi),

the transition load is slightly higher, and the high wear region has a

mean pressure of 41 MPa (6,000 psi) except for the high load level

portion of run 2659 R. These minor differences can be attributed to

the severity differences at the beginning of each load. For the

constant condition tests, the surfaces are unworn at the beginning and

stresses are very high. All but the initial load in the step- loading

tests start on a pre-worn scar. At large wear scars, the increase in

load may not be sufficient to cause a significant severity increase in

the test and a flat wear versus load plot may be observed. Besides

these minor differences, the step- loading gives essentially the same

information as the constant condition test but in one test.
85
86

2 . Step-Loading Test: Effect of Speed

A study was made of the effect of speed on step- loading wear

results. Tests at 1000, 200, and 10 rpm, shown in figure 49, indicate

an effect on both the low and high wear levels and also the wear

transition load. The differences in low and high wear levels (both

location and slope) are attributable to wear-in effects. At high

speed (1000 rpm) the wear- in has progressed to the steady state or low

wear regime and the plot of wear scar versus load results in lines

roughly parallel to the lines of constant pressure. At lower speeds

(200 rpm) , the specimens slide only 20% of the distance of the 1000

rpm test, steady state wear rates may not be achieved and the plot

produces lines with slopes greater than the lines of constant pressure

indicating continued wear- in. At still lower speeds (10 rpm) even

less wear- in is achieved and the plots produce lines with still

steeper slopes. The differences in transition loads may be due to a

dynamic rate phenomenon of temperature or possibly hydrodynamics.

3 . Step-Loading Test: Effect of Specimen Material

Three different ceramic materials were evaluated using the steploading four-ball test. Data from tests run on alumina (Cer 005),

silicon carbide (Cer 020), and silicon nitride (Cer 016), under

paraffin oil lubrication, are shown in figure 50. These materials

show markedly different behavior in this test. Alumina shows moderate
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levels of wear at low load (mean pressure = 483 Mpa or 70,000 psi) , a

transition at 5.5 kg, and a relatively high level of wear above 5.5 kg

(mean pressure = 48 MPa or 7,000 psi). Silicon carbide also exhibits

moderate levels of wear at low load (690 MPa or 100,000 psi), however

this moderate level of wear continues up to at least 60 kilograms

load. A slight transition is seen at 19 kg but wear is still only

moderate above this load. Silicon nitride shows very low wear at low

loads (mean pressure is 1,380 MPa or 200,000 psi) a small wear

transition is seen at 11 kg however wear increases only to a moderate

level up through at least 20 kg (mean pressure is 620 MPa or 90,000

psi).

F. Ball-on-Three-Flat Testing

Step- loading four-ball tests suffer from the drawback that four

balls are required for a test. Some ceramic materials are not readily

available in ball form however it is still desireable to understand

their tribological properties. Therefore, a modified technique was

developed to allow testing of ceramic materials not available in ball

form. The small discs used in this ball-on-three-flat (BTF) test are

easily fabricated from readily available rod stock.

1 . Comparison of Results to Four-Ball Tests

A ball-on- three-flat test was conducted on a sample of alumina to

see if results were similar to four-ball tests. The data, presented
in figure 51, indicate some similarity of results at low loads, but

differences are seen in both the transition load (13 kg in BTF vs 5 kg

in FB) and high load wear values. The four-ball tests give low load

mean pressures of 552 MPa (80,000 psi) , and mean pressures of only 48

MPa (7000 psi) are observed after the wear transition. The ball-onthree-flat test has slightly higher mean pressures before the wear

transition of 690 MPa (100,000 psi). After the wear transition the

mean pressure drops to approximately 20.7 MPa (3,000 psi) which is the

same low wear pressure as was observed for the dry sliding four-ball

case. These differences may be due to several factors. 1) The

difference in severity between the convex on convex contact of the

four-ball test and the convex on flat contact of the ball-on- threeflat test. 2) The difference in material (the four-ball test ran Cer

005 on Cer 005 while the BTF test ran Cer 021 on Cer 006). 3) The

much smaller thermal resistance of material in the case of the BTF

test, resulting in lower operating temperatures in the contact

junction.

I believe that the second two reasons may best explain the

results. While it is true that the calculated hertzian contact

pressure for a four-ball contact is 50% higher than the same load

using ball-on- three-flat, this is based on an initial condition (no

wear). Each load increment (except the first) for the step-loading

test however starts on a pre -worn wear scar, therefore, the

significance of the geometric differences between the two tests would

be reduced.
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The difference in materials probably plays the strongest role in

the differences in wear behavior between the four-ball and BTF tests.

Cer 006 is a 99.8% pure alumina while Cer 021 is a 99.5% pure alumina.

The purity of Cer 005 is not known exactly but is in the 99+% range.

We have already seen that these tests are sensitive enough to detect

wear performance differences among different batches of the same

material. The small compositional and structural differences between

Cer 006, Cer 021, and Cer 005 may therefore be enough to give

different wear results.

Temperature may also play an important role in this test. During

a test, an equilibrium is established between rate of heat generation

(friction) and dissipation (conduction through specimens and fluid).

This resulting equilibriiom establishes the contact junction

temperature for a load. In the case of the four-ball test heat must

be conducted through a 1/2" diameter low thermal conductivity ceramic

ball. In addition, there is only a small point contact between the

ball and the metal test cup. Resistance to heat flow is therefore

very high and high contact junction temperatures would be expected.

The ball-on- three-flat test, on the other hand, has a very small

amount (only 1/16 in) of material between the contact and the metal

test cup. In addition, the contact between the ceramic specimen and

test cup is over a large area (the entire back and sides of the

specimen). Heat transfer is enhanced, and therefore a different,

lower equilibrium temperature is obtained under the same conditions of

load and friction.
93

2 . Ball -on-Three-Flat Test: Interpretation of Friction Data

A small study was performed in order to determine how more useful

friction information could be gained from these ball-on-three-flat

(and also step- loading and constant condition) tests. Friction traces

from run 2727 R (fig. 52) were used to calculate coefficient of

friction values during each load increment. In addition to final

"steady state" coefficient of friction, values were calculated for

initial steady state, lower "steady" level, upper "steady" level, and

highest value for the seizure recovery spike of one was observed.

These values were plotted as coefficient of friction versus load in

figure 53. At low loads, friction is very steady with very little

difference between initial, final, high, and low values of friction.

As the load increases , variation in friction tends to increase , with

initial friction values starting low and final friction values being

high. Finally at 14 kg a spike occurs in the friction trace with a

coefficient of friction approximating the unlubricated case. The high

friction spikes coincide with the wear transition. The 16 kg load run

also contains a spike. Wear takes a corresponding increase at this

load and large variations in "steady" state friction are seen for

these two loads. The 18 and 20 kg loads do not have any spikes and

the wear is correspondingly low as seen by the small slope of the wear

scar vs load line in figure 51. Variations in "steady" state friction

are also very small.

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These results indicate the need for more than just the final

friction value for some tests. In this test, if only the final

coefficient of friction values were plotted, a relatively straight

line would be seen. There would be no hint of anything other than

normal wear behavior. The increasing variation of friction with

increasing load seem to give some indication that the test is

approaching a transition. The friction spikes correlate well with the

high wear regions of the test during and after the wear transition and

give evidence of the severe nature of the transition during this test.

3 . Ball-on-Three-Flat Test: Effect of Specimen Material

A paraffin oil lubricated ball-on- three-flat test was conducted

on a set of alumina specimens produced at NBS . This was done to see

if we could use the test to evaluate materials produced under

controlled processing conditions, and thus determine the effect of

processing variables on wear performance. The NBS alumina (fig. 54)

showed a very different behavior than the commercial grade alumina.

At low loads, the commercial alumina showed relatively low wear while

the NBS alumina had moderate wear. At higher loads, the commercial

alumina showed a transition to high wear while the NBS alumina still

exhibited only moderate wear. The mean pressure for the NBS alumina

was constant at 207 MPa (30,000 psi) throughout the load range. In

addition, the friction traces show evidence of sharp friction

transitions for many of the loads (fig. 55) indicating possible micro

wear transitions within each load. The NBS alumina may be
97

LOAD, kg

Figure 54. Comparison of Wear Results for NBS Produced Alumina

versus Commercial Alumina (Paraffin Oil Lubricated

BTF Tests)
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sacrificlally wearing at a moderate rate in order to avoid the

transition to high wear seen in the commercial product.

G . Summary

Three wear tests method for ceramic materials have been developed

and described: four-ball constant condition tests, four-ball steploading tests, and ball-on-three-flat ( step- loading) tests. Plots of

wear scar diameter versus load results in valuable information on

relative wear performance. This is enhanced by the use of a test

severity overlay which quickly provides the severity (mean pressure)

present during the test. In many cases friction data can also be

obtained that leads to a better understanding of the underlying

tribological processes. These techniques are very sensitive to

compositional and structural differences in ceramic materials and will

provide a means of studying the underlying relationship between

structure and tribological performance.
Chapter V

ALUMINA/WATER TRIBOSYSTEM: INTRODUCTION AND LITERATURE SURVEY

A. Preliminary Explanation of Lubrication Mechanisms for

Alumina With Water

During the development of a wear test method for ceramic

materials, it was found that water dramatically reduces the friction

and wear of alumina sliding on alumina over that of the unlubricated

case. Friction is reduced by 50-60%. Wear is reduced by 99% (as

measured by mass loss data) at high loads of 25-40 kg and even more at

lower loads before the wear transition. In an attempt to try to

understand the possible mechanisms by which water would lubricate

alumina, several possibilities were examined.

i) Fluid film provides some hydrodynamic lift reducing contact

between the surfaces and reducing friction and wear.

ii) Water acts as a coolant and serves to reduce temperatures

in the contact zone.

iii) Double layer effects or Rehbinder effects between the

surface and water which alter the near surface material behavior.

iv) Chemical reactions between water and alumina which produce

lubricous products.

Reduction of friction and wear through hydrodynamic lubrication

is a common and useful lubrication technique. The models used to
101

describe hydrodjmamic lubrication are derived from the Reynolds

equation and in general simplify to the relationship that pressure

(which supports the load) is proportional to surface velocity and

viscosity. ' This lubrication regime is usually found however under

conditions of relatively high sliding speed, low load, and viscous

fluids. The step loading test is run under conditions of relatively

low sliding speed, moderate to high loads, and water is not very

viscous. One would expect therefore that hydrodynamic lubrication

would not play a critical role. Despite this, some researchers have

claimed that hydrodjmamic lubrication is possible with water at slow

sliding speed with ceramics. Their experiment, conducted using

silicon nitride lubricated with water at low to moderate loads and low

speed (6.5 cm/s) , indicated very low coefficient of friction levels of

less than 0.002. It was claimed that chemical reaction between water

and the ceramic serves to chemically polish the surface to ultrasmooth

dimensions and thus allow for hydrodynamic lubrication. Our testing

however does not give as polished a surface. In addition, coefficient

of friction levels are not near the very low levels (<0.01) usually

associated with this type of lubrication. Therefore, this explanation

does not adequately explain the friction and wear behavior we have

observed with water and alumina.

Water does indeed provide a large degree of cooling, however, if

cooling were the sole explanation for the beneficial effects of water,

then other lubricants which are not as good a coolant would provide
102

less benefit. This is not the case, as paraffin oil gives lower

friction and wear than water at low loads.

Double layer effects or Rehbinder effects between surfaces and

/

fluids have gained some popularity as a way of explaining changes in

wear of materials under different environments. There appears to be

some correlation between wear, zeta potential, and material properties

such as hardness. Much of the work however shows changes in wear

rate by factors of two or four not the two order of magnitude that are

observed in this study. This explanation therefore would not appear

to be the predominant mechanism for reduction of friction and wear of

alumina by water.

At first, chemical reactions between water and alumina seems a

far-fetched possibility for lubrication. After all, aluminum oxide

already exists in its highest oxide state and is considered very

inert. Under the unusual conditions of stress and temperature that

exist in a tribological contact however chemical reaction may be

possible. SEM photographs of the wear scar indicated a film was

present in the wear scar and appeared to cover half of the wear scar.

These films were not present in either dry or paraffin oil lubricated

wear scars. The other key observation that indicated that chemical

reactions may play a role in lubrication is the fact that the hydrates

or hydroxides of alumina are layer lattice structures. That is, they

are arranged in layers with strong bonding among the atoms in the
103

layer and weaker bonding between layers. These weaker bonds are

easily sheared which would serve to reduce friction and wear.

Layer lattice (or lamellar) compounds have been used as solid

lubricants for many years. The most popular compounds, graphite,

and molybdenum disulfide, have been used for many different lubricant

applications including automotive lubricants. When stressed, the weak

bonds between the layers of atoms break and the layers slide easily

over one another. Because the bonds are weak, it does not take much

energy to break them; hence friction is very low.

It is the hypothesis of this report therefore that the unique

environment in a water lubricated contact junction produce reaction

products which are lubricous and reduce friction and wear. This is

not to say that the other factors described of hydrodynamics, cooling,

and double layer forces do not play a role in helping to reduce

friction and wear but that the chemical reaction to form lubricous

products is the dominant mechanism for lubrication in these tests.

B . Structures and Nomenclature of Aluminum Oxides and Hydroxides

There are many different forms of aluminum oxides and hydroxides

therefore a brief explanation of structures and nomenclature is in

order. Anyone desiring a more comprehensive explanation of the

structure or properties of these compounds are referred to references
104

41-44. The Alcoa paper on Oxides and Hydroxides of Aluminum by Wafers

and BeLl^-'- gives the best introduction to this area.

Aluminum oxide (AI2O3) exists in many forms. The most common

1 form is alpha (a) alumina (corundum) which is hexagonal. Other forms

of AI2O3 occur less frequently: gamma (tetragonal); delta

(orthobombic , tetragonal); eta (cubic spinal); theta (monoclinic) ; chi

(cubic, hexagonal); kappa (hexagonal); and iota (orthorhombic) . In

some cases (chi alumina for example) two crystal systems are listed.

This indicates a disagreement in indexing the x-ray powder diffraction

data by different authors and shows the difficulty and controversy

surrounding some of these transition aluminas (all aluminas except

alpha)

.

There is less controversy surrounding the existence and structure

of the aluminum hydroxides, table 6 lists the most common forms of

these compounds and gives some of the more common nomenclature

presently used by different researchers. I have chosen to refer to

the compounds as hydroxides instead of hydrates and use the symposium

names to further identify the form. This selection was made because

the crystal structures are not true hydrates. The water molecule

actually breaks an H-O bond to form separate H and OH groups which

become an integral part of the structure. Therefore the term hydrate

for compounds such as gibbsite is actually a misnomer.
105

Table 6

Nomenclature for Hydroxides of Aluminum

Nomenclature System

Chemical

Chemical Name Formula Symposium Alcoa

Aluminum Oxide

Hydroxides

or (Alumina

Monohydrate)

AIO(OH)

or

(AI2O3 SHoO)

Boehmite

Diaspore

Alpha Alumina

Monohydrate

Beta Alumina

Monohydrate

Aluminum

Trihydroxides

or (Alumina

Trihydrate)

A1(0H)3

or

(AI2O3 • 3H2O)

Gibbsite

or Hydrargillite

Alpha Alumina

Trihydrate

Bayerite Beta Alumina

Trihydrate

Nordstrandite

Source: Wefers and Bell (41), p. 8.
106

Gibbsite, bayerite, nordstrandite , and boehmite are layered

structures with relatively weak bonding between the layers. All three

trihydroxides have identical structures within each layer and differ

only in the stacking of the layers. Each layer consists of layers of

atoms of the sequence hydrogen, oxygen, aluminum, oxygen, hydrogen

(fig. 56) . Aluminum atoms are octahedrally coordinated to six

different oxygen atoms while each oxygen atom is bonded to two

aluminum and one hydrogen atoms. The stacking sequence for OH ions

for gibbsite (monoclinic) is AB*BA«AB«BA (fig. 57) . This

superposition of layers combined with the hexagonal arrangement of Al

ions within the layers leads to channels through the lattice parallel

to the C axis. Bayerite (monoclinic) is arranged in layers of OH

sequence AB«AB«AB'AB. This is because the OH ions do not lie directly

above each other as they do for gibbsite. Instead, the OH ions of the

second layer lie in the depressions between the OH groups formed by

the first layer. Nordstrandite (triclinic) is described in reference

41 as having the AB«AB sequence of bayerite with the OH ions of

adjacent double layers located opposite each other leading to the

stacking sequence AB»AB»BA»BA.^^ This shows the lattice of

nordstrandite to be a combination of the lattices seen for both

bayerite and gibbsite. Boehmite (orthorhombic) is also described as

having a layered structure although the layers are very different from

the layers seen for the trihydroxides. Diaspore (orthorhombic) has

more of a three dimensional structure. Wefers and Bell^^ give

approximate Moh's hardness values for the aluminum oxides and

hydroxides which show that the layer structures have much lower
107

Top View

Side View

=^ bond coming out of page

bond in plane of page — bond going into page

• aluminum

O oxygen

o hydrogen

Solid, filled atoms are in the plane of the page.

Solid, unfilled atoms are above the plane of the page.

Dashed atoms are below the plane of the page.

Figure 56. Structure of Al(0H)3
108

gibbsite bayerite

mMm

A

B

B

A

Source: Wafers and Bell (40) pl9

Figure 57. Layer Stacking Sequence for Gibbsite and Bayerite
109

hardness. For example, gibbsite is 2.5 to 3.5 on the Moh's scale;

boehmite is 3.5 to 4; diaspore is 6 1/2 to 7; and a AI2O3 is 9.

Two additional pieces of information are useful in interpreting

the relationship between the different forms of alumina and alumina

hydroxides. A phase diagram of the alumina water system (fig. 58)

shows the relationship between several phases. Low temperature (up to

100°C) seems to favor the trihydroxide . Between 100°C and 330°C

boehmite is formed. At highest temperatures alpha alumina is favored.

In addition, there are indications that higher pressures may favor

diaspore over boehmite. It is important to note on looking at this

phase diagram that it has been defined under conditions where pressure

is isotropic. In the case of a wearing surface, anisotropic shear

forces are applied. Therefore, this phase diagram should not be taken

as a literal representation of phases present at various temperatures

and pressures in a contact junction. It does, however, indicate that

low temperatures favor trihydroxides , intermediate temperatures favor

monohydroxides , and high temperatures favor alpha alumina. The second

useful item of information is the decomposition sequence for the

aluminum hydroxides from reference 41. The decomposition sequence is

useful because it gives an indication of possible reaction pathways

for formation of aluminum hydroxides assuming some reversibility for

the decomposition reaction. Figure 59 shows pathways for

decomposition of different aluminum hydroxides. Trihydroxides are

present at low temperatures. Depending on conditions of environment,

pressure, and temperature, they decompose to form either transition
0 100 200 300 400 500

Temperature, C

Source: Misra (43) p26

Figure 58. Phase Diagram for AI2O3 - System

112

aluminas (path b) or boehmite (path a) . Boehmite apparently

decomposes in a complex pathway through the transition aluminas of

gamma, delta and theta until it finally becomes alpha alumina. This

suggests, therefore, that it may be possible to form lubricous layer

structure alumina hydroxides from alpha altomina /and water, if the

decomposition reactions are reversible.

C . Approach

The objective of this part of the study is to determine whether

tribochemical reactions take place between water and alumina to form

lubricous compounds which reduce friction and wear. This is being

done using three different techniques. Firstly, kinetic experiments

were conducted using water and alumina powders to determine the

kinetics for these reactions. Secondly, wear tests were conducted

using aluminum hydroxide powders to determine whether they possessed

any lubricating capabilities. Finally, extensive analysis of the wear

scar surface and debris were performed in order to directly observe

evidence for the formation of these compounds in tribological

contacts

.
113

Chapter VI

ALUMINA/WATER TRIBOSYSTEM: EXPERIMENTAL

APPARATUS AND OPERATING PROCEDURES

A. Test Approaches

Several different approaches were used to investigate the

hypothesis that alumina-water reaction products are formed in the

tribocontact

o Static Kinetic Tests

o Powder Friction Tests

o Dynamic Wear Tests

The first approach consisted of trying to react alumina powder

with water and analyze the reaction products. This was termed the

static kinetic approach and attempted to show feasibility of aluminawater chemical reactions. The second approach involved running wear

tests with different powders in the contact to determine their effect

on friction. The third and most direct approach involved trying to

find evidence of the product in the wear scar and wear debris . The

first and second approaches were essentially a backup to the more

direct (but more difficult) analysis of wear products. If it was not

possible to find direct evidence of reaction products perhaps it could

be inferred through kinetics and performance data.
114

1 . Static Kinetic Tests

Static kinetic tests were conducted using the modified high

pressure micro-oxidation bombs shown in figure 60 and described in

detail by King.^^ The samples of alumina (0.5 g) anc^ water (25 ml)

are placed in a pyrex liner with a loose fitting pyrex cap. This is

placed in a cylindrical steel bomb fitted with removable top with a

pressure gage and valve. The top is secured to the cylinder with six

bolts and pressure is maintained through use of a high temperature 0-

ring seal. At the beginning of a test, the bomb is insulated at the

top and placed into an aluminum block heater containing a molten metal

heat transfer fluid. The test is run for a certain length of time

then terminated using water bath cooling. The contents of the pyrex

reaction tubes are then removed and the water and powder phases

separated using a centrifuge. The resulting powder-water slurry is

dried using a watch glass. The powder is carefully ground using an

agate motor and pestle, and saved for analysis. Several key features

allowed the test to be conducted in a repeatable and controlled

manner. One is the use of water as a reactant. The vapor pressuretemperature relationship of liquid water in equilibrium with saturated

vapor is very well documented (steam tables) . The pressuretemperature relationship of air in the closed bomb can also be

estimated using the ideal gas law. The combination of these two

pressure- temperature relationships allow us to estimate the

temperature in the reactor given the pressure. This relationship is

derived in Appendix E and plotted in figure 61. A second key
115

Insulation

Pressure Gauge

Valve

Steel Bomb

(w/O-Ring Seal)

Glass Liner

(w/Cap

Water

Alumina

Aluminum Heating Block

Molten Metal Heat

Transfer Fluid

Water Heat

Transfer/Pressurizing

Fluid

Figure 60. High Pressure Bomb Reactor Design
116

/
117

feature is the use of water heat transfer/pressurizing fluid between

the pyrex reaction vessel and the metal bomb casing. If this fluid is

not present, heat transfer between the steel bomb and the glass liner

tends to be much slower. In addition, boiling of the water reactant

during the rapid heatup resulted in splattering of aliomina on the

walls of the pyrex liner and sometimes completely out of the liner.

The water heat transfer/pressurizing fluid heats up faster than the

water inside the pyrex liner and boils. As it boils, it pressurizes

the system and reduces boiling of fluid inside the liner. Thus the

boiling and splattering of alumina on the inside walls of the pyrex

liner is greatly reduced.

It was estimated that less than one milliliter of liquid water

would be required to produce enough saturated water vapor at 200 °C to

fill the bomb. The fifteen milliliters of water used as the heat

transfer/pressurizing fluid is therefore more than sufficient to

ensure both liquid and vapor are present at the reaction temperature.

Half a gram of alumina is used as a reactant. Using reaction

stoichiometry , it was estimated that approximately a quarter of a gram

of water would be required to form the trihydroxide (tri -hydrate)

reaction product from all of the initial alumina. By using 25 ml of

water reactant, the water is present in large (lOOX) excess.

An experiment was conducted to determine the length of time

necessary to heat the bomb to the desired temperature (fig. 62) . The

temperature rises quite rapidly and within five minutes has
118
119

reached within two degrees of the final equilibrium temperature

(194°C). The intersection of the initial and final slopes are used to

determine the initial starting point for the reaction. This falls at

approximately one minute. At one minute, the temperature is 173°C or

21 °C below the final temperature. Reaction rates at this temperature

are approximately 1/4 the final rates and should not have a

significant contribution to the total reaction process. Therefore,

one minute is subtracted from all measured reaction times to give

actual reaction times used for kinetic data analysis.

2 . Powder Friction Tests

Powder friction tests were conducted using ball-on- three-flat

tests. First a dry test was conducted to provide a wear scar. Enough

dry powder was put into the test cup to completely immerse the lower

wear test specimens ( = 1 ml) and the friction test was conducted for

one minute. The powder was then completely removed and a second,

different powder was used. This was continued until each powder had

been run at least twice. In addition, two different loads (2 kg and 5

kg) were used.

3 . Dynamic Wear Tests

Ball-on- three-flat dynamic wear tests were used to generate wear

debris. Once generated, this debris was recovered using a ten

milliliter syringe to remove the water/suspended debris. Several
120

rinses of water were used to ensure recovery of as much debris as

possible. The mixture was centrifuged for ten minutes using a high

speed (13,000 g) centrifuge and the water was carefully withdrawn.

The resultant powder water mixture was dried on a watchglass

overnight, ground carefully to a fine powder using an agate motor and

pestle, and analyzed. Only very small amounts (1-2 mg) of material

could be recovered which tended to compound the analytical problems.

Preliminary tests produced powders that were reddish-orange in color

instead of white. Atomic absorption analysis indicated traces of iron

contaminant therefore an electroplated gold coating was applied to all

steel parts in contact with the specimens or lubricant (chuck, lower

specimen cup, spindle end, and threaded end of chuck retaining rod) to

eliminate this problem.

B . Reference Powders

Six reference powders were obtained and used in the course of

this study. They were used in the powder friction tests and also to

develop the analytical tests necessary to identify and quantify the

materials produced in static kinetic tests and dynamic wear tests.

The powders and the arbitrary identification numbers assigned to them

are

:

ALl: Boehmite (Al 0 .R 0 or AlOOH) [UC-1]

AL2: Gamma Alumina (7-AI 0 ) [UC-2]

AL3: Boehmite (Al 0 'H 0 or AlOOH) [UC-3]
121

ALA: Alpha Alumina (a-Al 0 ) (spray dried)

AL5: Alpha Alumina (a-Al 0 ) [UC-9]

AL6: Gibbsite (Al 0 • 3H 0 or Al(OH) )

The numbers and letters in the square brackets are an NBS designation

to identify the particular batch of powder that was produced. These

six samples represent four of the most common forms of alumina and

alumina hydrates (or hydroxides) , and were available in sufficient

purity and quantity to make them useful for analytical procedure

development

.

Samples of the powders were prepared for SEM analysis by

sprinkling powder on a stub painted with a thin coating of silver

conducting paint. After drying, the excess powder was removed and the

samples coated with a thin conductive coating of gold/palladium.

1,000X and lO.OOOX magnification photographs best illustrated the size

and morphology of the powders.

Sample ALl (boehmite) (figs. 63 and 64) consists of irregularly

shaped particles in a wide size range of 0.1 to 10 /im.

Sample AL2 (gamma alumina) (figs. 65 and 66) contains particles

which appear to be agglomerates. Particles appear to be in the 1-10

fjim range but are themselves composed of very small particles less than

0.1 /im in size.
122

Sample AL3 (boehmite) (figs. 67 and 68) has an entirely different

particle morphology although it is supposed to be the same basic

material as sample ALl . It appears to have been produced using a

spray drying process which tends to produce spherical particles.

These particles are mainly in the range of 5-20 /zm in diameter. Under

higher magnification (lO.OOOX) the material does seem to have a "waxy"

appearance similar to sample ALl.

Sample AL4 (alpha alumina) (figs. 69 and 70) is a spray dried

sample. The spherical particles are 1-15 /xm in diameter. Under

higher magnification (10,000X) the particles appear porous, and are

composed of fused microparticles approximately 0.1 /xm in size.

Sample AL5 (alpha alumina) (figs. 71 and 72) consists of

irregularly shaped particles of varying shapes and sizes.

Characterization of the shapes and size range is difficult due to the

wide variety available however most particles appear to be "chips"

approximately 20 /zm by 20 /im by 3 /xm thick. Upon closer inspection at

high magnification, the chips are composed of fused microparticles

approximately 0.1 /xm in size. These microparticles appear to be very

similar in size and morphology to the microparticles observed for

powder AL4.

Sample AL6 (gibbsite) (figs. 73 and 74) appears to be composed of

large polycrystalline agglomerates approximately 30 /xm in diameter.

Higher magnification reveals cleavage planes and crystalline character
Figure 53. lOOOX Secondary Emission SEM Photograph of

Powder ALl

Figure 64 lO.OOOX Secondary Emission SEM Photograph of

Powder ALl

Figure 67. lOOOX Secondary Emission SEM Photograph of

Powder AL3

Figure 68. 10,000X Secondary Emission SEM Photograph of

Powder AL3
Figure 69. lOOOX Secondary Emission SEM Photograph of

Powder AL4

Figure 70. 10,000X Secondary Emission SEM Photograph of

Powder AL4
Figure 71. lOOOX Secondary Emission SEM Photograph of

Powder AL5
Figure 73 lOOOX Secondary Emission SEM Photograph of

Powder AL6

Figure 74. 10,000X Secondary Emission SEM Photograph of

Powder AL6
129

consistent with the crystal system of gibbsite (monoclinic -001

cleavage)

.

X-ray powder diffraction (XRPD) patterns were run of all six

samples in order to confirm their structure. These spectra are given

in Appendix F. Samples ALA, AL5 , and AL6 all gave very sharp XRPD

peaks and indicated that they were indeed relatively pure samples of

alpha alumina and gibbsite. The other samples gave very diffuse peaks

that made identification more difficult. Samples ALl and AL3 were

confirmed to be boehmite even though the peaks obtained were quite

diffuse. It is believed that the diffuse peaks are due to very small

crystallite size for the samples. Sample AL2 also gave very diffuse

peaks. Comparison of the peaks to reference data suggest that the

sample may actually be delta alumina, or possibly a mixture of gamma

and delta alumina. Because of the similarity of these two deformed

polymorphs of alumina (both tetragonal) and the difficulty in

distinguishing between them, I have maintained the title of "gamma"

alumina for this sample but note that delta alumina may also be

present

.

C . Analytical Techniques

Several analytical techniques were used to study aluminas,

alumina hydroxides, alumina oxide hydroxides, wear scars, and wear

debris. The techniques and the samples they analyzed are given in

table 7. All of the techniques had some qualitative aspects, that
Table 7

Analytical Techniques Utilized in the Investigation of

Alumina/Water Reactions

Samples

Technique Acronym Analyzed Qualitative Quantitative

Thermogravimetric

Analysis TGA Powders

,

Wear Debris

Semi Yes

Differential

Scanning

Calorimetry

DSC Powders

,

Wear Debris

Semi Yes

X-Ray Powder

Diffraction XRPD Powders, Yes No

Wear Debris

Fourier Transform

Infrared FTIR Powders, Semi No

Wear Debris,

Wear Scars

Scanning Electron

Microscopy SEM Powders , Semi No

Wear Debris,

Wear Scars
131

they could sometimes distinguish between different samples because of

some aspect of their performance. X-ray powder diffraction is

especially useful as a qualitative analysis technique because, given

an adequate sample, it can distinguish between a wide variety of

materials. The key is that in many cases if a sample is not present

in sufficient quantity, purity, or large enough crystallite size, it

cannot be characterized using XRPD. Two of the techniques are also

quantitative if it is known what the sample is. Therefore by using

the combination of qualitative and quantitative techniques described

here, it may be possible to adequately characterize the necessary

materials.

1 . Thermogravimetric Analysis (TGA)

This technique essentially consists of a microbalance in tandem

with a furnace. A sample is heated on a gold pan in the presence of a

stream of oxygen and the weight loss from reaction/decomposition of

the material is monitored. The particular unit used for this study

was a Perkin Elmer TGS-2 equipped with a data analysis computer.

The analysis technique used was adapted from the procedure of

Carel and Cabbiness^^ with minor modifications. Basically the

procedure consists of heating a sample under a stream of oxygen (40

ml/minute) and observing the accompanying weight loss. The analysis

is divided into two parts. The first part consists of heating a

sample (usually = 10 mg) in a gold pan to 110°C and holding the
132

temperatures constant for 45 minutes and recording weight loss (as %

of the original weight) . This is termed the isothermal phase of the

analysis and determines the amount of free water associated with the

sample. A typical isothermal analysis represented by a sample of

boehmite is shown in figure 75. The % weight due to free water can be

read directly from the graph by measuring the point at which the curve

levels off.

The second part of the analysis consists of increasing the

temperature from 110°C to 910°C at a rate of 10°C/min and observing

the percentage weight change. This is termed the temperature scan

analysis. A representative temperature scan analysis is given in

figure 76 for boehmite and shows what information can be obtained with

this technique. The weight loss curve shows the relationship between

weight of the remaining sample and temperature. The decrease in

weight at approximately 300°C is due to water loss from the

decomposition of boehmite.

Al 0 • H 0 (s) 0 (s) + H 0 (g)

Knowing the stoichiometry of the reaction, we can then back-calculate

the amount of original material present.


135

A derivative curve can be superimposed on the weight loss curve

to allow for a more accurate determination of the maximum rate of

weight loss. These maxima (actually minima in the derivative curve)

were then used as representative of characteristic decomposition

temperatures of the different aluminum hydroxide phases. For mixtures

of different hydroxide phases, it is possible to separate the weight

loss contribution from individual phases using areas from the

derivative curves (as demonstrated by Carel and Cabbiness) ; however,

this was not necessary for samples encountered in this study.

Temperature scan analyses for some of the other reference powders

are displayed in figures 77-79 and summary data are tabulated in table

8. Boehmite (sample AL3) (fig. 76) consisted of one main peak at

313°C with another minor peak at lower temperature. This lower

temperature weight loss may be due to more strongly adsorbed

(chemisorbed? ) water being driven from the surface of the powder. The

other boehmite sample ALl (fig. 77) showed a derivative peak at 399 °C

and a slightly larger chemisorbed water peak than AL3 . At this point

it is not clear why the two samples have such different maximas

(perhaps it is due to the different particle size and morphology) but

this observation shows that some variations in decomposition

temperature can be expected. The total % weight loss results for the

two boehmite samples (28.3% and 24.6%) are higher than the theoretical

% weight loss of 15.0% for pure boehmite. This is due to the presence

of chemisorbed water and possibly small amounts of other hydroxide

phases. The gamma alumina sample (AL2) (fig. 78) shows a gradual
136

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138
Table 8

TGA Analysis of Reference Powders

Total Theor.

Decomp. Decomp.

Reference Free Weight Weight Temperature of

Powder Sample Name Water Loss Loss Derivative Peaks

# (%) (%) (%) #1 #2 #3

ALl Boehmite 5 9 28 3 15 02 399

AL2 7 Alumina 2 3 4 0

AL3 Boehmite 7 6 24 6 15 02 313

AL4 a Alumina 0 2 1 6 0

AL5 a Alumina 0 0 0

AL6 Gibbsite 0 34 8 34 64 234 300 521

0 34 9 34 64 237 306 530

0 34 9 34 64 235 299 525
140

weight loss over the duration of the temperature scan analysis. This

is due to water in the structure which helps to stabilize the deformed

structure of gamma alumina. The bound water content in gamma alumina

reference powder AL2 was determined to be only 4.0%. Gibbsite (AL6)

(fig. 79) gave a very distinct temperature scan TGA curve, with the

derivative curve indicating three distinct decomposition peaks. The

smaller peaks at 234 and 521 °C could be due to minor impurities of

other phases or they could be indicating that multiple decomposition

reactions are taking place in this sample. For example, some of the

gibbsite may transform to the monohydrate at 234°C via the reaction

Al 0 • 3H 0 (s) Al 0 • H 0 (s) + 2H 0

(g)

The main complete decomposition of gibbsite may occur at 300°C as

Al 0 • 3H 0 (s) Al 0 (s) + 3H 0

(g)

and finally, at 521°C the monohydrate that was formed at 234°C can

decompose

Al 0 . H 0 (s) Al 0 (s) + H 0

(g)

This hypothesis is also in agreement with the multiple reaction

pathways presented by Wefer and Bell in figure 59. It is thought that

a pressure buildup inside the particles of gibbsite due to release of

water and diffusion limitations favors the formation of monohydrate.^"
141

Total percentage weight loss for the gibbsite sample (AL6) of 34.8 -

34.9% agrees very well with the theoretical weight loss of pure

trihydrate of 34.64% (table 8). Comparison of the three separate

temperature scan runs indicates a good repeatability for both weight

loss measurement and characteristic decomposition temperature peaks.

Samples of alpha alumina (AL4 and AL5) showed little or no weight loss

for both isothermal and temperature scan TGA analyses.

2 . Differential Scanning Calorimetry (DSC)

This technique gives information similar to the TGA technique in

that decomposition temperatures can be defined. Quantification is

possible if the heat of reaction is known. DSC analysis of gibbsite

produced the same three peaks as the TGA analysis at approximately the

same temperatures. Since no new information was gained by this

technique, it was not used for further work.

3. X-Rav Powder Diffraction (XRPD)

This analytical technique consists of distributing a powder

sample on a surface in a random orientation and determining the angles

at which x-rays are diffracted (fig. 80). The angles and relative

intensity of the diffracted x-rays are characteristic of particular

crystalline materials. Only small amounts of material (0.5 g) are

required, and the technique is completely nondestructive as long as xray energies are not sufficient to disturb your sample.
x-ray source detector

gure 80. X-Ray Powder Diffraction Apparatus and Sample

Spectrum
Identification of powders from their power diffraction data are

made by comparing the peak locations (2^'s) and relative intensities

(I/I )of the unknown to similar data on known materials. This is best

accomplished by consulting the Joint Committee on Powder Diffraction

Standards (JCPDS) data base. This very valuable reference data is a

compilation of thousands of XRPD analyses on known compounds performed

by researchers around the world over approximately the past thirtyfive years. Data are reported as d-spacing (in Angstroms) and

relative intensity (I/I ) for each of the peaks on the x-ray powder

diffraction spectra. D-spacings are related to 26 (the actual angle

usually measured for each analysis) through the Bragg equation

A = 2d Sin 6

where A = wavelength of incident x-ray

d = d spacing in angstroms

6 = incident angle in degrees

In the case of my analyses, CuK^^i , radiation was used with a

wavelength of 1.54051 A which reduced the equation to

d Sin 6 = 0.770255
which describes the relationship between 6 and d spacing for the

apparatus used. In actual practice values of 29 are used to describe

the angles of peaks.

A search of the JCPDS data files produced a list of over 25

different reference data files pertaining to alumina and aluminum

hydroxides. Twelve of these data files were selected for referencing

The rest of the files were rejected for a variety of reasons ranging

from inability of the original author to index the d spacings to a

particular crystal system, to insufficient number or quality of peaks

Thus only the best quality data was used. A compilation of all of th

JCPDS file data are given in Appendix G along with d spacings,

calculated Id's, and relative peak intensities. Comparison of the

unknown peak Id's and relative intensities with the JCPDS data

permitted identification of the unknown material.

4 . Fourier Transform Infrared (FTIR) Analysis

FTIR is a specialized infrared analytical technique that can

detect the presence of characteristic functional groups in a sample.

In infrared spectrometry, infrared light is passed through or

reflected from a sample and the intensity of the light transmitted is

measured as a function of its frequency. The nature of the chemical

bonding in a sample allows it to absorb certain frequencies of

infrared radiation characteristic of the bond. By noting which

frequencies in the infrared spectrum are being absorbed by the sample
145

certain conclusions can be made regarding the chemical bonds in that

sample. Fourier transform is merely a specialized technique for

taking and storing the infrared spectral data. Instead of storing the

data as intensity versus frequency, the spectra is broken down into

constituent fourier series and the more concise fourler transformed

data Is stored.

FTIR data was collected in three different samples using two

different techniques

Focused reflectance within wear scar - microscope

size

Focused reflectance outside wear scar in debris -

0.25 mm spot size

Diffuse reflectance on powders - 1.0 mm spot size

5 . Scanning Electron Microscopy (SEM)

Scanning electron microscopy was performed on an ISI Super IIIA

SEM. Most samples were coated with approximately a 20 nm conductive

coating of gold/palladium, and analyzed in the microscope at 12 keV

accelerating voltage.

~ 0.25 mm spot

microscope =
146

Chapter VII

ALUMINA/WATER TRIBOSYSTEM: RESULTS AND DISCUSSION

A. Kinetic Experiments on Powders

Initial kinetic experiments were conducted on alpha alumina and

gamma alumina to determine their reactivity with water at different

temperatures. Low temperature experiments were conducted at

approximately 100°C in a covered beaker on a hot plate. Higher

temperature experiments were conducted in the bomb at approximately

200°C.

Alpha alumina was unreactive at low temperatures even for times

as long as 24 hours. Gamma alumina on the other hand appeared to

react. The TGA curve for the reaction product (fig. 81) shows a

distinct single decomposition peak at 272°C. X-ray powder diffraction

of the reaction product indicated bayerite was present. This is

consistent with the observation by Wefers and Bell^^ that bayerite

decomposes to a disordered eta alumina at 230-350°C.

Higher temperature bomb reactions determined that alpha alumina

was unreactive with water at 200°C for 3 hours. Gamma alumina however

was reactive and forced a whitish looking precipitate quite different

from the gelatenous looking gamma alumina water mixture. TGA analysis

of the homogeneous reaction product gave a small single peak at

approximately 570°C (fig. 82). The solid remaining after reaction
147

149

was also separated into two phases. The "gel" phase (fig. 83) gave a

TGA curve that looked like unreacted gamma alumina while the "white"

phase (fig. 84) gave a single distinct peak at 520°C. X-ray powder

diffraction patterns from the two samples confirmed that the "gel"

phase was gamma alumina and determined that the "white" phase was

boehmite. This data is in good agreement with the phase diagram

information in that low temperatures favor production of the

trihydroxide (bayerite) while higher temperatures favor production of

the monohydroxide (boehmite),

TGA was successfully used to qualitatively show that reactions

between water and gamma alumina were possible. The next set of

experiments were conducted to see if TGA could be quantitative and

allow an understanding of the kinetics of these reactions . Three

bombs were charged with gamma alumina and water and placed in the

heating block at the same time and heated to 194°C. The bombs were

removed at different times and quickly cooled, resulting in reaction

times of 31, 59, and 119 minutes. TGA analysis of the reaction

products (figs. 85-87) produced weight loss curves which could be

quantitatively analyzed. By knowing the weight loss due to water from

the decomposition of boehmite, the percentage boehmite (and then the

fractional conversion) in the reaction product could be calculated

(table 9) . Quantitative information is derived from the TGA

temperature scan runs in the following manner. First, straight lines

are drawn parallel to the weight loss curves before and after the

decomposition point. These lines tend to be roughly parallel to one
150
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Table 9

Kinetic Data for Reaction of Gamma Alumina (AL2) and Water at 194°C

Water Weight

Reaction Loss during Fractional

Time Decomposition Conversion in(l-f)

(min) (y%) (f)

0

31

0

2.05

GO

0.136 -0.146

59 4.12 0.274 -0.320

119 6.82 0.454 -0.605

y% , . ,= 15.02 for boehmite theoretical

theoretical
156

another. This serves to eliminate the bias from the residual gamma

alumina in the sample which exhibits a continuous weight loss with

temperature (as seen in figure 83). Next, a vertical line is drawn at

the minimum of the derivative curve. Finally, the length of the

vertical line between the parallel lines is measured the % weight loss

is obtained by comparing the length of this line to the vertical

scale

.

Once the rate data was obtained, a predictive model was sought to

describe the data. Using the simplest approach of assuming a

homogeneous system and mass action kinetics the equation for a first

order dependence was derived (Appendix I). The data (table 9) was

plotted (fig. 88) and seems to fit the model quite well. Other simple

orders were tried (0 order, 2nd order) but the data did not fit as

well. The rate constant for this reaction, determined from the slope

of the line, was 5.1 x 10" min~^.

The combination of tools of x-ray powder diffraction and TGA

offer a powerful technique for investigating the kinetics of these

systems. X-ray powder diffraction determines the structure of the

product. From this, the stoichiometry of the reaction can be

obtained. By knowing stoichiometry of the reaction, TGA can be used

to quantify the products and provide rate data. Ultimately, the

kinetics of the system can be understood.
30 60 90 120

Time, mins

gure 88. Pseudo First Order Data Plot for Gamma Alumina/

Water Reaction at 194°C
158

B . Powder Friction Tests

The kinetic experiments with powders demonstrated the feasibility

of forming reaction products in the contact junction however it does

not demonstrate that these products have any lubricating qualities.

Therefore, a series of dry wear tests were conducted using the

reference powders to see if these powders modified friction. Five

kilogram load tests (fig. 89) indicate both different friction levels

and different friction variability among the three main classification

of powders. Alpha alumina powder gives the highest level of friction

and greatest degree of variability. Large spikes of both high and low

friction show up regularly. Boehmite gives a slightly lower

coefficient of friction and less variability. Gibbsite gives a much

lower friction coefficient and a noticeably smoother friction trace.

Test at 2 kg load (fig. 90) produce the same coefficient of friction

ranking with a few differences in the traces. The alpha alumina

friction trace for example shows a bit less variability than was seen

in the 5 kg load test. A comparison of the final coefficient of

friction values for the six tests (fig. 91) shows good agreement among

the rankings with alpha alumina producing the highest friction,

boehmite offering a modest decrease in friction, and gibbsite

producing a large ( ~ 40%) decrease in friction. This data

demonstrates the solid lubricating ability of the hydroxides of

alumina, especially the trihydroxides

.


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C . Wear Debris Analysis

Analysis of the wear debris and wear scar was very difficult due

to the very small amount of material produced. This approach,

however, offered the best opportunity for direct evidence for the

hypothesis that water and alumina react to form lubricous products.

X-ray powder diffraction patterns were gathered using a computer

operated diffractometer , and a thin smear of powder on an oriented

quartz slide. The thin smear was used because there was insufficient

powder (by a factor of 20) to perform a normal powder diffraction

pattern. The computer control and data analysis was necessary to

allow for very long analysis times. This enhanced the signal to noise

ratio and gave the best opportunity for peak identification. A fortyeight hour run of the region from 10 to 70 degrees (45 minutes/degree)

indicated three components (fig. 92). The peak identification shows

the three components as alpha alumina (a) , a contaminant on the quartz

slide (c), and bayerite (b) . The bayerite peaks were very small and

represent the two strongest x-ray powder diffraction peaks for this

material indicating that bayerite is only present in small amounts.

Additional runs were performed at very slow scan speeds of 450

minutes/degree to enhance the detail in this region. These runs,

shown as enlargements in the figure, indicate definite small peaks are

present

.
163

10 20 30 40

2 theta

40 50 60 70

2 theta

Figure 92. X-Ray Powder Diffraction Spectrum for Wear Debris

from Water-Lubricated Alumina Test
TGA analysis of wear debris from a water- lubricated wear test is

shown in figure 93. Three general peaks can be seen. A broad peak is

seen with a minimum at 263°C. A second small peak as seen at 325°C.

A very sharp peak is observed at 249°C. The location of the sharp

peak is close to the location of the peak seen for the bayerite

reaction product from the kinetic tests. The difference between the

TGA peaks is that the wear debris bayerite peak is much sharper and

the minimum is at a lower temperature (249°C vs 272°C) . I believe

that the wear debris and kinetic test reaction products are the same

material and that the differences in TGA results are attributable to

the smaller particle size for the wear debris produced bayerite.

Bayerite particles produced in the wearing contact would tend to be

very small because of the high shear stresses in the contact junction

The particles produced in the kinetic tests were fairly coarse

grained. During the course of a TGA run, the temperature is

increased. At the decomposition temperature of bayerite, water is

lost and weight loss occurs. Weight loss can only occur if water is

allowed to diffuse through the crystal lattice and escape into the

atmosphere. In the case of the wear debris, particles are small,

diffusion distances are small, and weight loss is rapid resulting in

sharp peak. In the case of the kinetic experiments, the large

particles inhibit the diffusion of water and broaden and delay the

minimum of the peak. The difference in minimum is only 23°C which

corresponds to a delay of only 138 seconds. Diffusion limitations

could easily account for this difference.
165
166

Fourier transform infrared spectroscopy was used to find evidence

of hydroxides in the wear debris and wear scar. Analysis of the wear

scar itself did not give a clear indication of a hydroxide peak

probably because the layer was too thin. Analysis of some of the

debris piled outside the wear scar however (fig. 94) showed definite

presence of OH.

The three analysis techniques applied all indicate the presence

of a reaction produce between water and alpha alumina in the contact

junction. The x-ray powder diffraction and TGA experiments indicate

that the product is bayerite aluminum trlhydroxide (Al(0H)3). Since

the kinetic work indicated that alpha alumina is nonreactive with

water but gamma alumina is, something must take place in the contact

junction to produce gamma alumina (or perhaps another reactive

transition alumina) . This missing part is supplied by the research

performed by Hines, Bradt, and Biggers.^^ Their finding was that when

alpha alvimina is subjected to abrasion in an unlubricated wear test,

particles of delta alumina (a deformed alumina, very difficult to

distinguish from gamma alumina) were produced. This observation

complements the findings in this study and supplies the missing link

in the chain of events. The following sequence of events (see figure

95) therefore seems to be necessary for production of lubricous wear

products

:
167

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168

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169

1) High pressure, temperature, and shear stresses in the

contact junction produce a deformed alumina such as the

gamma phase transition alumina.

2) High temperature high pressure reaction between water and

the transition alumina produce an aluminum hydroxide. The

exact form depends on the conditions.

3) The aluminum hydroxide (in our case trihydroxide-bayerite

was observed), being a layer lattice structure, acts as a

solid lubricant and reduces friction and wear.

The model depicts a large chunk of material being removed from the

surface of the asperity, consisting of all three phases of material.

In actuality, I believe that two types of wear debris would dominate.

Where temperatures and stresses are optimum, very small particles of

easily sheared hydroxide particles will be produced. At locations

where stresses are too high and temperatures are too high (or too

low) , larger particles of debris (mostly alpha alumina) will be

produced.

A series of tests was conducted to observe the relative "solid

lubricating" ability of several alumina-based powders. First, a set

of alumina ball-on-three-flat specimens were run at 600 rpm and 10 kg

load for 2 minutes to provide a wear scar on the specimens. This was

done to reduce the contact pressure and allow the surfaces to conform.
170

Subsequent tests were conducted under the same conditions of speed,

load, and duration using 1.5 m2 of deionized water lubricant with

approximately 2% (w) powder in suspension. Coefficient of friction

values were taken as the steady state values obtained during the test.

These values were then ratioed to the coefficient of friction obtained

using deionized water alone to give a measure of the effect of the

powders on friction relative to the base case (water lubricated) . The

results of these tests are given in table 10.

Samples of alpha and gamma alumina powders in water increase

friction approximately 9 percent. This slight increase may be due to

the three-body "abrasive" contribution expected for non- lubricating

solids. The boehmite sample gave a 24 percent reduction in friction

indicating its potential as a solid lubricant. The gibbsite and

bayerite samples produced slight increases in friction of 13 and 7

percent, respectively. The lack of friction reduction from the

trihydroxides gibbsite and bayerite was disappointing. These powders

however had large (> 10 /im) crystallites that may have resulted in

more of an abrasive mechanism. This is supported by the relatively

rough friction traces obtained for these powders (fig. 96). Tests

conducted using water and the other powders (all with smaller

particles sizes) produced smooth friction traces.

An interesting effect was observed with the boehmite tests.

After the initial boehmite powder test, the cup was rinsed with water

and a new test was conducted on the same wear scars with pure
171

Table 10

Svunmary Friction Data for Alumina -Based Powders in Water

Coefficient of Friction

Lubricant*

Deionized Water 0.230 ± 0.007 1.00

a (AL5) 0.250 1.09

7 (AL2) 0.250 1.09

Boehmite (ALl) 0.175 ± 0.003 0.76

Gibbite (AL6) 0.259 1.13

Bayerite (Alcoa C-37) 0.245 1.07

Ratio of Friction

Coefficient

*All powders present at approximately 2% (w)

Test Conditions: Ball-on-Three-Flat Test Configuration

Aliamina Specimens: Ball - CER 021

Flats - CER 006

Speed: 600 rpm

Load: 10 kg

Duration: 2 minutes
172
173

deionized water. When this was done, some residual low friction was

still observed providing evidence that the lubricating "film" was

still present. A repeat of the rinse increased the friction slightly

back to the level of friction of the original deionized water case

(fig. 97). This "memory" effect indicates something about the

behavior of boehmite as a lubricant. First, it does not have to be

present in large quantities to work. Second, it appears to be fairly

tenacious in its ability to remain in the contact region even after

the circulating supply of powder has been removed. As the burnished

powder in the contact is gradually worn away and the circulating water

replaced, the friction gradually increases.
174

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175

Chapter VIII

SUMMARY AND CONCLUSIONS

This study has led to several valuable developments in the area

of ceramic tribology. Wear test methods have been developed, new data

analysis techniques have been employed, and a fundamental

understanding of the nature of tribochemical reactions between water

and alumina has been gained. The highlights of this study are:

i) A step-loading four-ball wear test procedure has been

developed to measure the friction and wear properties of

ceramic materials under a range of test severities using a

single set of specimens.

11) A ball-on-three-flat modification of this procedure has

been developed which gives information similar to the

four-ball test but allows testing of materials not

available in ball form, thus expanding the utility of the

test

iii) A test severity overlay technique was developed which

allows for quick interpretation of the severity of the

test in terras of mean pressure in the contact junction.

This provides a significant, easy method for comparison of

wear results over the entire range of load conditions.
176

iv) These procedures are sensitive to minor processing and

material parameters (batch to batch variations, etc.) and

therefore provide an opportunity to systematically study

the effects of processing and material variables on

friction and wear of ceramics.

v) Water- lubricated alumina gives much lower friction and

wear than the unlubricated case over a wide range of test

loads. SEM analysis revealed the presence of some sort of

film which may have played a lubricating role.

vi) Kinetic experiments using alumina powders confirmed the

possibility of reactions occurring between water and gamma

alumina. Aluminum oxide hydroxide - boehmite [AlO(OH) ] is

formed at 194°C and aluminum trihydroxide - bayerite

[Al(0H)3] is formed at ~ 100°C. Alpha alumina was not

reactive at these temperatures. Formation of aluminum

oxide hydroxide (at 194°C) followed a pseudo first order

dependence

.

vii) Friction testing conducted on powder samples in a dry

contact confirmed that aluminum oxide hydroxide (boehmite)

and aluminum trihydroxide (gibbsite) have lubricating

capability when compared to the parent alpha alumina.

Approximate reductions of 10-20% were found for boehmite

and 35-44% for gibbsite.
177

viii) Friction testing conducted on powder samples in water at

2% concentration confirmed that aluminiim oxide hydroxide

has lubricating capability when compared to the waterlubricated case. Reduction in friction of 24% was

observed.

ix) Analysis of wear debris using techniques of Fourier

transform infrared spectroscopy, thermogravimetric

analysis, and x-ray powder diffraction indicate the

I presence of aluminum trihydroxide phase (bayerite) in the

wear debris.

The water- lubricated alumina wear test results obtained in this

report point to very interesting interactions between water and

aluminum oxide in a tribocontact . A protective film is formed in the

contact which is a reaction product between water and alumina. Water

may also interact with these products (adsorption, etc.) to produce

other desirable lubricating behavior in addition to the contribution

of the easily sheared layer lattice structure (Gelatinous boehmite?)

These results have potential application in at least two important

areas. First, water- lubricated operations involving alumina may

benefit from additions of small amounts of boehmite powder. The

second application may be in the aluminum industry. Aluminum metal is

very susceptible to oxidation. Fortunately, the product of oxidation

forms a protective barrier to further oxidation; therefore, this
reaction is limited to the outermost surface of the metal. The outer

layer is aluminum oxide and may therefore benefit from the lubrication

mechanisms investigated in this report. Small amounts of boehmite

added to water-based lubricants (or perhaps even oil-based lubricants)

may result in reduced friction and wear.
179

Chapter IX

RECOMMENDATIONS FOR FUTURE WORK

This study has laid the groundwork for a wide variety of possible

research areas in ceramic tribology and ceramic tribochemistry . The

wear test methods developed during this study can be very useful in

studies relating wear performance with processing parameters and

material structure. They could also be used for ceramic/lubricant

interaction studies to see the effect of lubricants and lubricant

additives on the friction and wear behavior of ceramics. Finally,

studies of tribochemical reactions between ceramics and lubricants

would be of great interest. Using the case of alumina/water reactions

as an example, it would be interesting to find the influence of

different chemical parameters (pH, aqueous impurities, etc.) on the

formation rates of aluminum trihydroxide and aluminum oxide hydroxide.

These types of studies may have a tremendous impact on areas like

machinability of alumina, the use of alumina inserts in machining of

metals, and the compatibility of these alumina parts with water-based

working fluids
180

REFERENCES

1. Dowson, D., History of Tribology . p. 16, Longman, London, 1979.

2. ASTM Standardization News - "Advanced Materials" issue, 14, No.

9, October 1986.

3. Steinberg, M.A. , "Materials for Aerospace," Scientific American,

October 1986, pp. 67-72.

4. Liedl, G.L., "The Science of Materials," Scientific American,

October 1986, pp. 127-134.

5. Kear, B. H., "Advanced Metals," Scientific American, October

1986, pp. 159-167.

6. Bowen, H.K., "Advanced Ceramics," Scientific American, October

1986, pp. 169-176.

7. Rabinowicz, E., Friction and Wear of Materials . John Wiley &

Sons, New York, 1965.

8. Larsen, D.C., Adams, J.W., Johnson, L.R., Teotia, A.P.S., and

Hill, L.G., Ceramic Materials for Advanced Heat Engines.

Technical and Economic Evaluation . Noyes , Park Ridge, 1985.

9. The Adiabatic Diesel Engine . SAE SP 543, Society of Automotive

Engineers, Warrendale, 1983.

10. Adiabatic Engines: Worldwide Review . SAE SP 571, Society of

Automotive Engineers, Warrendale, 1984.

11. Advances in Adiabatic Engines . SAE SP 610, Society of Automotive

Engineers, Warrendale, 1985.

12. Shimauchi , T., Murakami, T., Nakagaki , T., Tsuga, Y. , and Umeda,

K. , "Tribology at High Temperature for Uncooled Heat Insulated

Engine," SAE Paper No. 840429, SP 571, Society of Automotive

Engineers, pp. 35-45, 1984.

13. Timoney, S.G., "Engine Rig for Screening Ceramic Materials," SAE

Paper No. 840433, SP 571, Society of Automotive Engineers, pp.

71-78, 1984.

14. Moorhouse, P., Mortimer, B., and Kamo , R. , "Solid Lubrication

Studies for Adiabatic Diesel Engines," SAE Paper No. 850508, SP

610, Society of Automotive Engineers, pp. 103-112, 1985.

15. King, A. G., "Ceramics for Cutting Metals," American Ceramic

Society Bulletin, 43, No. 5, pp. 395-401, 1964.
181

16. Bhushan, B. and Sibley, L. , "Silicon Nitride Rolling Bearings for

Extreme Operating Conditions," ASLE Transactions, 25, No. 4, pp.

417-428, 1982.

17. Mines, J. E., Bradt , R. C, and Biggers , J.V., "Grain Size and

Porosity Effects on the Abrasive Wear of Alumina," Wear of

Materials 1977, pp. 462-467.

18. Steijn, R.P., "Friction and Wear of Single Crystals," Wear, 7,

pp. 48-66, 1964.

19. Ishigaki, H. , Kawaguchi, I., Iwasa, M. , and Toibana, Y.

,

"Friction and Wear of Hot Pressed Silicon Nitride and Other

Ceramics," Wear of Materials 1985, pp. 13-21.

20. Miyoshi, K. , Buckley, D., and Srinivasan, M. , "Tribological

Properties of Sintered Polycrystalline and Single-Crystal Silicon

Carbide," American Ceramic Society Bulletin, 62, No. 4, pp. 494-

500, 1983.

21. Blau, P.J., "Wear Testing and Standardization," ASTM

Standardization News, pp. 34-36, October 1985.

22. Schwartz, L. and Steiner, B. , "Versailles Project on Advanced

Materials and Standards," ASTM Standardization News, pp. 40-44,

October 1986.

23. Kingery, W.D. , Bowen, H.K., and Uhlmann, D.R. , Introduction to

Ceramics . 2nd Edition, John Wiley & Sons, New York, 1976.

24. ASM Metals Reference Book . 2nd Edition, American Society for

Metals, Metals Park, pp. 85-89, 1983.

25. Perry, R.H. and Chilton, C.H. , Chemical Engineers Handbook . 5th

Edition, McGraw Hill, New York, NY, 1973, pp. 23-38 to 23-53.

26. Fischer, T.E. and Tomizawa, H. , "Interaction of Tribochemistry

and Microfracture in the Friction and Wear of Silicon Nitride,"

Wear, 105, pp. 29-45, 1985.

27. Benzing, R. , Goldblatt, I., Hopkins, V., Jamison, W.

,

Mecklenburg, K. , and Peterson, M. , Friction and Wear Devices . 2nd

Edition, American Society of Lubrication Engineers, Park Ridge,

1976.

28. Yellets, J. P., "The Design and Construction of a State-of - the-Art

High Temperature Tribometer," M.S. Thesis, Pennsylvania State

University, 1987.

29. Phone conversation with Dan Briggs, Adolph Coors Testing

Laboratories (by Dennis Minor - NBS , 11/12/86).
182

30. Booser, E.R., (Editor), CRC Handbook of Lubrication (Theory and

Practice of Tribology) Volume II. Theory and Design . Chapter 1,

"The Shape of Surfaces" by J.B.P. Williamson, CRC Press, Boca

Raton, pp. 3-16, 1986.

31. Kingery, W.D., Bowen, H.K., and Uhlmann, D.R., Introduction to

Ceramics . 2nd Edition, John Wiley and Sons, New York, p. 469,

1976.

32. Munro , R.G., "Temperature Considerations in the Study of Surfaces

Using a Four-Ball Apparatus," J. Appl. Phys .

, 57., 11, pp. 4950-

4953, June 1, 1985.

33. Blau, P.J., "Interpretations of the Friction and Wear Break-in

Behavior of Metals in Sliding Contact," Wear, 71 pp. 29-43, 1981.

34. Hsu, S.M., "The Effects of Chemical Reactions in Boundary

Lubrication," Ph.D. Thesis, The Pennsylvania State University,

pp. 275-280.

35. Dragoo, A., Robbins, R. , and Minor, D., "Powder

Characterization," Ceramics Technical Activities 1985, Institute

for Materials Science and Engineering, National Bureau of

Standards, U.S. Department of Commerce, NBSIR 85-3188, p. 45,

1985.

36. Buckley, D.H. and Miyoshi, K. , "Fundamental Tribological

Properties of Ceramics" from Proceedings of the 9th Annual

Conference on Composites and Advanced Ceramic Materials , pp. 919-

939, January 20-23, 1985. American Ceramic Society, Columbus,

1985.

37. Booser, E.R. (Editor), CRC Handbook of Lubrication (Theory and

Practice of Tribology) Volume II: Theory and Design . CRC Press,

Boca Raton, p. 71, 1984.

38. Tomizawa, H. and Fischer, T. E. , "Friction and Wear of Silicon

Nitride and Silicon Carbide in Water: Hydrodynamic Lubrication

at Low Sliding Speed Obtained by Tribochemical Wear" ASLE

Transactions 30, 1, pp. 41-46, 1987.

39. Ciftan, M. and Saibel, E. , "Rebinder Effect and Wear," Wear, 56,

pp. 69-80, 1979.

40. Booser, E.R. (Editor), CRC Handbook of Lubrication (Theory and

Practice of Tribology) Volume II: Theory and Design . CRC Press,

Boca Raton, p. 269, 1984.

41. Wefers, K. and Bell, G.M. , "Oxides and Hydroxides of Aluminum,"

Technical Paper No. 19, Alcoa Research Laboratories, 1972.
183

42. Dorre, E. , Hubner, H. , Alumina. Processing. Properties, and

Applications . Springer-Verlag , New York, 1984.

43. Gitzen, W.H. , Alumina as a Ceramic Material . American Ceramic

Society, Columbus, 1970.

44. Misra, C, Industrial Alumina Chemicals . ACS Monograph 184,

American Chemical Society, Washington, 1986.

45. King, K.J., "Development of a Pressurized System for Oxidation

Studies of Volatile Fluids," MS Thesis, The Pennsylvania State

University, 1983.

46. Carel, A.B. and Cabbiness, D.K., "Analysis of Aliomina by Combined

TG/X-Ray Diffraction," American Ceramic Society Bulletin, 64, No.

5, pp. 716-719, 1985.

47. Misra, C, Industrial Alumina Chemicals . ACS Monograph 184,

American Chemical Society, Washington, 1986, p. 81.

48 Hines, J.E., Jr., Bradt, R.C., and Biggers, J.V., "Delta Alumina

Formation during the Abrasive Wear of Polycrystalline Alumina,"

Wear of Materials 1979, pp. 540-550.
184

Appendix A

Selected Bulk Properties of Cer 006 and Cer 021 Alumina^ ^

Property Units

Values

Conditions Cer 006 Cer 021

Density

Compressive

Strength

g/cra^

MPa (kpsi)

Flexural Strength MPa (kpsi)

25°C

25°C

1000°C

3.82 3.90

>2071 (>300) 2620 (380)

331 (48)

193 (28)

379 (55)

Tensile Strength

Elastic Modulus

Poisson's Ratio

Thermal

Conductivity

Coefficient of

Linear Thermal

Expansion

MPa (kpsi) 25°C

GPa (10^ psi) 25°C

W/mK

10" V°c

25°C

100°C

400 °C

800°C

25-200°C

25-500°C

25-800°C

25-1000°C

345 (50)

29.4

23.0

6.7

7.3

7.8

8.0

262 (38)

372 (54)

0.22

35.6

25.9

12.1

6.3

7.1

7.6

8.0

8.3

<^ ^Product Literature for Coors AD995 (Cer 021) and AD998 (Cer 006).

Alumina supplied by Coors Porcelain Company, Golden, Colorado.
185

Appendix B

Coefficient of Friction Calculation for the

Four -Ball Apparatus

The coefficient of friction can be calculated for the NBS

modified four ball wear tester by considering the load application and

friction force measurement geometries. As described before, the fourball (or ball-on- three-flat) geometry produces a relationship between

applied load (L) and normal load (N) of N = 0.408L, where L is the

total applied load and N is the normal load on a single ball.

The friction force measurement system consists of an electronic

force transducer which measures the force applied at the end of a

lever arm (10.48 cm) due to the torque of the ball pot. The torque of

the ball pot is due to the friction force in the contact junction

which is on a much smaller lever arm. The actual distance between the

center of rotation and the center of the area of contact (wear scar)

is 0.366 cm. The ratio of the lever arms gives the force attenuation

factor (10.48/0.366 or 28.63). This indicates that an actual friction

force (Fg) of 28.63 g^ in the contact junctions will produce a torque

which will register only 1.0 gf force (measured force or F„) on the

electronic force transducer.

F^ = F^(28.63) [1]

The friction force due to one ball will of course be one third of the

total friction force.
186

Coefficient of friction ()u) is defined as the ratio of the actual

friction force to the normal force.

= !i [2]

Substituting, we have for a single ball

u = F,(28.63) ^ ^3 3^ F,

^3^

(3) (0.408)L L

where F^, and L are in the same units (kgf ) . In most cases , friction

force is measured in grams and applied load is measured in kilograms.

Thus

0.02339 F„

[4]

where Fj,, = measured friction force in g^

L = applied load in kgf
187

Appendix C

Equivalent Wear Scar Diameter Concept

For regular uniform wear scars, the arithmetic average of the optical

wear scar measurements (1 in the direction of sliding and 1

perpendicular to the direction of sliding for each of 3 lower

specimens) is reported as the wear scar diameter.

For irregular wear scars the concept of equivalent wear scar diameter

may be useful since it is not the length or width of the wear that is

most important but instead, the load supporting area of the scar.

Using the method recommended by Hsu,^^^ an imaginary rectangle can be

placed over the wear scar (I) such that the area of the rectangle is

equal to the area (A) of the elliptical wear scar (II). If this scar

were transformed into a circle of the same area (III) it would have a

diameter d - the "equivalent diameter". That is, the diameter of the

circle with an equivalent area.

II. Area = A , = a'b'

rectangle
188

III. Area = A . . = jrr^ = Trd^/A circle

A ^ , = A .

, rectangle circle

7rd2/4 = a'b'

d = 2(a'b'A)^^^

which would be more accurate than the simple arithmetic average of a

and b. Unfortunately we do not usually measure a' and b' instead we

measure a and b. In addition, the placing of a rectangle of equal

area over the wear scar is very subjective. There is however a method

that may be employed to estimate that the area of the wear scar if it

is elliptical.

The area of an ellipse with major axis a and minor axis b is

(a/2) (b/2)7r . This area can be set equal to the area of a circle:

A ,

. = A . elipse circle,

a b d2 _ _ TT = TT 2 2 4
189

= ab

= (ab)^/2

Thus the equivalent diameter can be obtained for elliptical wear scars

by taking the geometric average of the wear scar measurements.

dg = (ab)i/2

An example of values calculated for d for elliptical scars:

Major Minor Ratio d d Ad % Ad

Axis Axis Major/Minor Avg. Equivalent (d -d^) (Ad/d )xlOO

(Aspect Ratio) (d^) (d^) ^ ^^S

1. 0 1 0 1 0 1 0 1 0 0 0

1 1 1 0 1 1 1 05 1 049 .001 0 1

1 2 1 0 1 2 1 10 1 095 .005 0 5

1 3 1 0 1 3 1 15 1 140 .010 0 9

1 4 1 0 1 4 1 30 1 183 .017 1 4

1 5 1 0 1 5 1 25 1 .225 .025 2 0

1 6 1 .0 1 6 1 30 1 265 .035 2 7

1 7 1 .0 1 7 1 35 1 .304 0.46 3 4

1 8 1 .0 1 8 1 40 1 .342 .058 4 1

1 9 1 .0 1 9 1 45 1 .378 .072 5 0

2 0 1 .0 2 0 1 50 1 414 .086 5 7

d^ = (a+b)/2

dg = (ab)^/2

(l)S.M. Hsu, Ph.D. Thesis, Department of Chemical Engineering,

Pennsylvania State University, University Park, PA 1976, p. 81.
190

Appendix D

Specimen Cleaning Procedure

Thorough cleaning of wear test specimens and equipment is

important in ensuring good test precision and repeatability.

Therefore the following procedure was used to remove contaminants and

impurities from the specimens and equipment.

Specimens: 1) Immerse in a mixture of hexane^/toluene* (9:1) in an

ultrasonic bath for sixty seconds.

2) Pour off the liquid and repeat step 1) but only for

ten seconds ultrasonic treatment. Pour off the

liquid.

3) Rinse specimens in pure hexane

.

4) Immerse specimens in acetone* in an ultrasonic bath

for sixty seconds.

5) Pour off the liquid and repeat step 4) but only for

ten seconds ultrasonic treatment. Pour off the

liquid.

6) Rinse with fresh acetone.

7) Dry with a stream of nitrogen.**

*ACS grade solvents are used.

**In some very special cases (e.g., low load unlubricated runs) even

the ACS grade solvents contain enough impurities to affect the test

results. In these cases, follow the acetone rinse with ultrasonic

cleaning using a laboratory detergent (e.g., Micro) followed by

complete rinsing with deionized water.
191

Equipment: Same as for specimens in the case of loose parts. In the

case of fixed parts which cannot be moved or placed in an

ultrasonic bath, only rinsing with the appropriate

sequence of solvents is required.
Appendix E

Pressure Temperature Relationship for the Alumina/Water Bomb Reactor

Since there is a pressure gage attached to the bomb reactor, we

may get an indirect measure of the temperature in the reactor from the

total pressure. At any temperature T, the pressure in the reactor is

the sum of the partial pressure of air in the reactor plus the vapor

pressure of water in the reactor (-14.7 psia to correct from absolute

pressure to gage pressure)

.

P = P' . + P' ^ - 14.7

gage air HgO

where P = estimated gage pressure reading for the reactor at

gage & & f &

temperature T (psig)

P'^j^^ = partial pressure of air in reactor at temperature T

(psia)

P' = vapor pressure of water in reactor at temperature T H2O

(psia)

For a constant volume reactor where the air does not react, the

partial pressure of air can be estimated (at low pressures) using the

ideal gas law and
193

where i and f indicate initial and final states

In our case, = 21 °C = 294°K

Pi = 14.7 psia.

therefore

or

Pf = = 0.0500

Ti

. = 0.0500 T, air ^

where is in "K.

Values of P' are available in the steam table under vapor pressure

"2

of water above 100°C (for example CRC Handbook of Chemistry and

Physics 56th Ed, page D180)

A table can therefore be constructed by selecting several

temperatures, looking up the values of P' at each temperature,

calculating P' . , and ultimately finding the value for P at each ° air gage

temperature. A plot of temperature (°C) versus Pressure (P ) can

gage

then be constructed as a calibration (figure 61). The temperature of
the reaction can therefore be obtained by reading the gage pressure

and using the calibration chart (figure 61).
195

Appendix F

X-Ray Powder Diffraction Patterns for Reference Powders ALl through AL6
196
197

< AilSNBlNI
198

< A1ISN31NI
199
200

< AllSN3iNI
201

APPENDIX G

Interplaner Spacings, 2 Thetas, and Relative

Line Intensities for X-Ray Powder Diffraction

Patterns of Alumina's, Aluminum Oxide Hydroxides

and Aluminum Trihydroxides

a . 1 6 6 JCPDS File # JCPDS File # JCPDS File # JCPDS File #

10-173 10-425 16-394 23-1009

dA 29 dA 29 dA 29 I/I,, dA 29 1/1^

3 479 25 58 75

2 552 35 13 90

2 379 37 78 40

2 165 41 68 < 1 2 085 43 36 100

1 964 46 18 1

1 740 52 55 45

1 601 57 52 80

1 546 59 77 3

1 514 61 16 5

1 510 61 34 7

1 404 66 54 30

1 374 68 19 50

1 337 70 35 1

1 276 74 26 3

Trigonal

4 56 19 45 40

2 80 31 93 20

2 39 37 60 80

2 28 39 49 50

1 977 45 86 100

1 520 60 89 30

1 395 67 03 100

Cubic

7 6 11 63 4

6 4 13 82 4

5 53 16 01 4

5 10 17 37 8 4 57 19 41 12

4 07 21 82 12

3 61 24 64 4

3 23 27 59 4

3 05 29 26 4

2 881 31 01 8

2 728 32 80 30

2 601 34 45 25

2 460 36 49 60

2 402 37 41 16

2 315 38 87 8

2 279 39 51 40

2 160 41 78 4

1 986 45 64 75

1 953 46 46 40

1 914 47 4 6 12

1 827 49 87 4

1 810 50 37 8

1 628 56 48 8

I 604 57 40 4

1 538 60 11 8

1 517 61 03 16

1 456 63 88 8

1 407 66 38 50

1 396 66 98 100

Tetragonal

5 70 15 53 2

5 4 5 16 25 10

4 54 19 54 18

2 837 31 51 80

2 730 32 78 65

2 566 34 94 14

2 444 36 74 60

2 315 38 87 45

2 257 39 91 35

2 019 44 85 45

1 9544 46 42 8

1 9094 47 58 30

1 7998 50 68 14

1 7765 51 39 6

1 7376 52 63 4

1 6807 54 55 2

1 6216 56 72 6

1 5715 58 70 2

1 5426 59 91 25

1 5120 61 25 6

1 4883 62 33 25

1 4526 64 05 25

1 4264 65 37 10

1 3883 67 40 100

Monoclinlc

;9-Gallia Structure

Based on Cu Vi^^ radiation.
Boehmite

JCPDS File #

21-1307

Dlaspore

JCPDS File #

5-0355

Akdalalte

JCPDS File #

25-17

uA T /T dA zo T /T dA 9fi Zu T /T

o . i i

/i ft 1 An AH . 7 1

/ 1

11 Ro . fl9 o z 1 4 6

1

1 Q 9AZH 3.164 28. 18 65 3 99 z z . 9fi z o 1 nn 4 38 20 26 60

2. 346 38.,33 55 TJ . 9Z 11 LH 97z / . 7'^ 1 n 4 23 20 98 20

1 .980 45.,79 6

0^ . 05 30 3 24 27 5]^ 70 1.860 48.,93 30 oz . ATA J o . > 3 X1 sJ 28 1

3

20 1.850 49,,21 25 oz . JOO 7 fi7 9z . 7R 1 32 1

6

30

1 .770 51

.

,59 6

o

z . J JO J o . 1 7 RO 9Z . J J 'J J _)

.

A s 1X nyj

1 .662 55.,22 14 9z , J 1 / RO 1J J D 9z . -> J , Q R 1.527 60,,59 6 2. 131 A9H Z . "^R J^9Z 2. /t06 17 lA 9nz u

1 .453 64,.03 16 z

.

u / / A 1 SA AO oz . o o ,, 9z 7/ on

1 .434 64,.98 10 11 . 901 47.,81 3 9z . J z 38,.78 80

1 .412 66 . 12 2

1I . 815 QO z . 1 Q /i 1 *4 X , 1 R , X o

9z nu

1 . 396 66,.98 2

1 733 52,,78 3 z , 1 o / 1 1 ft

, / o J u 1.383 67 .69 6

1i . 712 53,.48 15 0z . 1 1 1 i 4/ 9Z

ft 9 , o Z

1 .369 68 .48 2 1

.

678 54,,65 3 2. 07 A 1

J. U

1 . 312 71 .90 16 1. 633 56,.29 43 1. 925 A "7. i / J u

1 . 302 72 .48 4 1, 608 57,.24 12 1

.

905 47 . 70 10

1

.

570 58,.76 4 1

.

859 48 .96 70 Orthorhombic 1. 522 60,.81 6 1

.

684 54 .44 30

1. 480 62,.72 20 1. 667 55 .04 10

1.,431 65 .13 7 1. 647 55 . 77 10

1

.

,423 65 .54 12 1. 622 56 . 70 50

1.,400 66 .76 6 1

.

547 59 .72 60

1.,376 68 .08 16 1. 511 61 .30 30

1

.

,340 70,. 17 5 1. 471 63 .15 10

1

.

329 70 .84 6 1

.

461 63 .63 20

1.,289 73,.39 6 1

.

418 65 .80 100

1.,279 74,,06 1 1

.

1 393 334

67

70

. 14

. 54 10010 Orthorhombic 1

.

328 70 .90 20

1

.

288 73 .46 10

1

.

280 73 ,99 25

1

.

264 75,,09 20

Hexagonal

Based on Cu ^ radiation.
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205

Appendix H

An Estimate of the Hertizan Contact Diameter,

Mean Pressure and Maximum Pressure for 52100 Steel

and Alumina Specimens in the Four-Ball Apparatus

Hertzian Contact Diameter

The Hertz equation for the radius of the contact area formed

between two elastic spherical surfaces under static load is as

follows^

,,diusa= /^ 3.N (k, ^k,) R,R, >^l/3

4(Ri + R2)

where R^ = R2 = 0.6350 cm (0.25 inches)

N = 0.408L

where L = the applied machine load in kg^

= k2 = ^

' [2:

ttE

where u - Poissons Ratio =0.30 for 52100 steel

E = Young's Modulus = 2.10 x 10^ kgf for 52100 steel

cm^

Using these values k = 1.379 x 10"^

1/3

a =

37r (0.408) (L) (2.758 x 10"^) (0.403)'\

(4) 1.270

[3;
206

a = 4.382 X lO"^ [L]^/^ (cm)

d = 8.764 X 10"3 [L]^/^' (cm)

d = 8.76 X 10- 2 [L]W3 (mm) [52100 steel] [6]

For alumina, values of Poisson's Ratio and Young's Modulus for alumina

were obtained from reference 2 for fully dense polycrystalline alumina

403.45 GN

m"

0.236

E = 403.45 GN

m'

lm2

lO^cm^

V 9.8N = 4.12 X 10^y

cm'

Substituting into equation [1]

djj = 7.09 X 10-2 [L]i/3 [Alximina] 7]

^Klaus, E.E., "Wear and Lubrication Characteristics of Some Mineral

Oil and Synthetic Lubricants," Ph.D. Thesis, The Pennsylvania State

Unviersity, 1952, p. 111. .

^ Alumina. Processing. Properties, and Applications . E. Dorre and

H. Hubner, Springer-Verlag Berlin, Heidelberg, 1984, p. 76.
207

Mean Pressure in a Contact Junction

mean

Force

Area

where Force = load applied in the 4-ball contact geometry = N

Area = area of the wear scar

for a uniform wear scar Area = nr^2 =_

For an area in cm^

if d is given in mm, Area =

For the 4-ball contact geometry;

Force = N = 0.408L

where L = the applied load in kgf

rd2

4(100)

fef\_ N _ (400) (0.408)L

mean cm2| 7rd2/4(100) 7rd2
208

mean

kg

cm'

(51.9)L

P (kpsi) = P I^M (0.014223) = (0-73817)L

mean mean | 2 I ^2 cm"

P (MPa) = P I ^ (0.09807) = ^^-^^^^ mean mean | 2 cm' d2

1 kpsi = 6.895 MPa

Wear Scar Diameters and Pressures in a Hertzian Contact

52100 Steel

Applied

Load,

mm

mean

MPa kpsi

max

MPa kpsi

5

10

20

40

100

200

300

0.149

0.188

0.237

0.299

0.405

0.511

0.584

1,147

1,440

1,812

2,277

3,103

3,898

4,477

166.3

208.9

262.8

330.3

450.0

565.4

649.3

1,720

2,160

2,719

3,416

4,655

5,847

6,716

249.4

313.3

394.3

495.4

675.1

848.1

974.0
Altimlna

209

P P

mean max Applied

Load,

kgf dg , mm MPa ' kpsi MPa kpsi

5 0 121 1,738 252 1 2,607 378 1 10 0 153 2,174 315 3 3,261 473 0

20 0 192 2,761 400 5 4,142 600 7 40 0 242 3,476 504 2 5,215 756 3 ICQ 0 329 4,702 682 0 7,053 1,023 0

200 0 415 5,910 857 2 8,865 1,285 8 300 0 475 6,767 981 5 10,151 1,472 3

P = 1.5 P for hemispherical load distribution,

max mean
210

Appendix I Derivation of a Pseudo First Order Dependence for the

Gamma Alumina/Water Reaction at 194°C

The reaction we are considering is the reaction between water and

7-alumina to form boehmite [the alumina monohydrate or aluminum oxide

hydroxide AlO(OH)]

AI2O3 + H2O ^ 2A10(0H)

this can be generalized as K

A + B « 2C

^2]

If we assume mass action kinetics and a homogeneous reaction model in

which mass and heat transfer is not limiting

dC. r = - _A = C3 - k.i fy

dt

if 1) Cg = Constant (i.e., excess B)

2) Either Cc is very small (low conversion) or k.^ is very small

(reaction not very reversible)

such that

k.i « ki C]
211

then

dt

[4]

where ' = Cj

dC

dt

A _ _ V ' r

J

Ca dC^ t

J

k dt

subject to conditions: @ t =0

Ca = Cj,

Jin C, = - kt

in ^ = - kt [5]

or

Ca = Ca e-k^ [6]
in terms of fractional conversion

212

Ca = Ca(1 - f)

or

= 1 - f

Substituting into eq [5]

in(l - f) = - kt

therefore a plot of in(l - f) versus t will produce a straight line of

slope -k if this reaction is pseudo first order.
APPENDIX J

Mean Pressure Test Severity Plot for Wear Scar

Diameter vs Load (psi)

- MEAN PRESSURE, pel

-

^^.^

^ 1 ^ 1 1 1 1 1 1 1 J

1 2 4 6 8 10 20 40 60 80

Load, kg
APPENDIX K

Mean Pressure Test Severity Plot for Wear Scar

Diameter vs Load (MPa)

Load, kg
NBS-n4A (REV. 2-ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET fSee instructions)

1. PUBLICATION OR

REPORT NO.

NIST/SP-758

2. Performing Organ. Report No 3. Publication Date

September 1988

4. TITLE AND SUBTITLE

CERAMIC TRIBOLOGY: METHODOLOGY AND MECHANISMS OF ALUMINA WEAR

5. AUTHOR(S)

Richard S. Gates, Stephen M. Hsu, and E. Erwln Klaus

6. PERFORMING ORGANIZATION (If joint or other than N6S. see instructions)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

GArTHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

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

SAME AS ITEM #6 ABOVE,

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 88-600582

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

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

bi bl iography or literature survey, mention it here) This report describes a systematic study that has been conducted to

develop methods for measuring the tribological properties of ceramic

materials under concentrated contacts. Step- loading four-ball and ball-onthree-flat wear tests were developed to provide friction and wear

characteristics of ceramic/lubricant combinations under various lubrication

conditions. These measurement techniques now enable one to study the

effect of different materials processing parameters, material

microstructures , and different lubricants on the friction and wear

performance of ceramics.

Water was found to react with alumina in a wearing contact to produce

lubricous products. A combination of x-ray diffraction and

thermogravimetric analysis techniques were used to investigate the kinetics

of alumina-water reactions. These experiments determined that transition

(gamma) alumina reacts with water at -200°C to form aluminum oxide

hydroxide (boehmite - AlO(OH)), while reactions at -100°C produce aluminum

trihydroxide (bayerite - Al(0H)3). A mechanism for lubrication of alumina

with water is proposed whereby stresses in the contact junction cause a phase transformation from alpha to gamma alumina. The gamma aliamina

subsequently reacts with water to form a lubricous, layer lattice, hydroxide.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

alumina, aluminum hydrooxides , ceramic, four-ball, friction, lubrication, paraffin oil,

transition alumina, tribochemistry , tribology, water, wear

13. AVAILABILITY

I Unl imited

I I For Official Distribution. Do Not Release to NTIS

fx"! Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[X^ Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

229

15. Price

USCOMM-DC 6043-P80

ij

I

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U.S. DEPARTMENT OF COMMERCE

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