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NIST Special Publication X24

Ceramic Bearing Technology

Proceedings of the NIST/DARPA

Workshop on Ceramic Bearing Technology

April 17-18, 1991

Gaithersburg, Maryland

Said Jahanmir, Editor

mi

NIST Special Publication 824

Ceramic Bearing Technology

Proceedings of the NIST/DARPA

Workshop on Ceramic Bearing Technology

April 17-18, 1991

Gaithersburg, Maryland

Edited by

Said Jahanmir

Ceramics Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

Sponsored by

Defense Advanced Projects Agency

National Institute of Standards and Technology

Issued November 1991

U.S. Department of Commerce

Robert A. Mosbacher, Secretary

National Institute of Standards and Technology

John W. Lyons, Director
National Institute of Standards U.S. Government Printing Office

and Technology Washington: 1991

Special Publication 824

Natl. Inst. Stand. Technol.

Spec. Publ. 824

219 pages (Nov. 1991)

CODEN: NSPUE2

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
Executive Summary

The objectives of the workshop were to assess the status of ceramic bearing

technology, and identify the key research topics needed to expand the range of

applications for ceramic bearings. The principal advantages of advanced

structural ceramics, such as silicon nitride, over metallic counterparts are

lower density, higher abrasion resistance, higher chemical inertness, and

electrical and magnetic insulation. Advanced structural ceramics have been

evaluated during the past twenty years as balls, rollers, and races in bearing

applications. Currently, silicon nitride hybrid bearings (ceramic balls with

steel races) are being used quite successfully as instrument bearings and as

machine tool spindle bearings. In general, progress in research and in the

implementation of research results for utilization of ceramic bearings has been

slow. Therefore, this workshop was organized to identify the major impediments

and recommend possible solutions.

A total of eleven invited presentations were given at the workshop which

was attended by seventy- five representatives from industry, government, and

universities. The presentations and subsequent discussions covered present and

potential future applications of ceramic bearings, and topics related to

processing, machining, quality control, design, testing, and performance

evaluation. The following recommendations are based on the presentations and

the subsequent discussions by the workshop participants:

0 Research activities should focus on cost reduction, while maintaining

or improving the current level of performance reliability. All

aspects of bearing production from powder processing to bearing

assembly should be examined for means of cost reduction.

0 Research in powder processing, compaction, and densification should

address optimization of microstructure for best bearing performance,

as well as cost control.

0 New and improved cost-effective machining techniques should be

developed for ceramic bearing races. Special attention should be

given to dimensional tolerance and surface integrity of the finished

components

0 Non- destructive evaluation techniques and quality control methods

should be developed for use in various stages of bearing production.

It should be recognized that only the applications requiring a high

level of reliability should be subjected to expensive inspections.

0 Although the current silicon nitride bearing materials are adequate

for use in hybrid bearings, new and improved materials that are being

developed should be evaluated for use in high- temperature allceramic bearings.

iii
0 New and improved computerized design procedures and life prediction

methods should be developed for ceramic bearings. These computer

programs should include dynamic analysis of the total bearing system,

performance data, and models for wear, fatigue, and lubricant

degradation.

0 Detailed understanding on the mechanisms of wear and rolling contact

fatigue, and the role of microstructure on these mechanisms are

needed to allow developments of new and improved bearing materials.

0 Performance data on presently available materials should be collected

and made available to the bearing industry to allow proper selection

of bearing materials.

iv
Foreword

DARPA sponsorship of this workshop is related to a program initiative in

ceramic bearings technology. Over the last three months I have been talking to

many of the participants in this workshop concerning needs and opportunities in

this technology area. The viewpoints expressed by those in companies

characterized as ceramic fabricators, ball finishers, bearing companies, or end

users, as well as the tribologists in both companies and government laboratories,

were very useful to me in getting an understanding of the needs in this

technology area. Because of the differing perspectives of those involved in

developing and applying ceramic bearing technology, I thought a topical workshop

would be useful in exchanging viewpoints and promoting interactions.

The Defense Department has an interest in ceramic bearing technology

because it is an enabling or enhancing technology for weapon systems and

platforms. The low density of ceramics compared to metals makes them attractive

as high-speed rolling elements since centrifugal forces are reduced. Ceramic

bearings out-perform metal bearings in most corrosive and/or erosive

environments; also ceramic bearings are required for high- temperature

applications. Additional applications result from material properties including

high electrical resistivity, non-magnetic, and reduced catalytic activity for

decomposition of lubricants.

Factors limiting the greater utilization of ceramic bearings include their

cost relative to metal bearings and questions of reliability and predictability

of performance. It is hoped that this workshop will contribute to identification

of research opportunities, which will result in increased usage of ceramic

bearings in both military and non-military applications.

William S. Coblenz

Defense Sciences Office

Defense Advanced Research

Projects Agency

v
Acknowledgments

The financial support for this activity was provided jointly by the Defense

Advanced Research Project Agency (DARPA) and the National Institute of Standards

and Technology (NIST) . Funds from DARPA were received under ARPA Order No. 8080,

Program Code No. 1Y10

.

This workshop was conducted at the suggestion of Dr. William Coblenz in

an informal meeting at NIST. The editor is grateful to Dr. Coblenz for his

encouragements and advice. Dr. Larry Fehrenbacher of Technology Assessment and

Dr. Karl Mecklenburg of the U.S. Air Force assisted in the selection of the

technical presentations and the invitation list. The workshop speakers,

discussion group chairs, and participants made this workshop a complete success.

Special thanks are due to Kathy Kilmer and Lori Phillips of NIST for the

site selection and the detailed arrangements of the workshop. I am particularly

indebted to Karen Lusk and Margaret Robinson for the preparation of this report

and taking care of all other details that are necessary for conducting a

workshop

.

Disclaimer

The opinions expressed in this report are those of the editor and authors,

and are based on the presentations and discussions at the workshop. These

opinions and recommendations do not necessarily represent the views of the

Defense Advanced Projects Agency, or the Government of the United States.

Information on product names, manufacturers, or suppliers are included in

this report only for clarification. This does not imply endorsement of the

products or services by NIST.

Cover Photograph

The cover photograph is courtesy of Dr. John W. Lucek, CERBEC, Ceramic

Bearing Company.

vi
Table of Contents

Page No

Executive Summary iii

Foreword v

Acknowledgements vi

1. Workshop Summary 1

2. Discussion Group Summaries 15

Ceramic Processing and Blank Fabrication 17

Subhas Malghan, NIST, and Jim Hannoosh, CERBEC

Machining, Quality Control, and NDE 21

K. Subramanian, Norton, and Grady White, NIST

Design, Performance Testing, and Life Prediction 27

James Dill, Mechanical Technology, Inc.

and Marshall Peterson, NIST

3. Technical Presentations 37

Advantages of and Issues in the Application of 39

Ceramic and Ceramic Hybrid Bearings

Jim Dill, Mechanical Technology, Inc.

Payoffs and Challenges in Utilization of 55

Ceramic Components in Spacecraft Mechanisms

Paul D. Fleischauer, The Aerospace Corp.

Hybrid Ball Bearing for Naval Applications 69

Gerry J. Phillips, David Taylor Research Center

Instrument Bearing Requirements and Issues 99

Robert A. Westerholm, Litton Guidance

and Control Systems

High-Temperature Applications and Challenges 107

of Ceramic Bearings

Lewis B. Sibley, Tribology Systems, Inc.

Effects of Composition, Microstructure , and 121

Processing on Ceramic Rolling Element Bearing Performance

R. Nathan Katz , Worcester Polytechnic Institute

vii
Effects of Machining and Finishing on Performance 131

Mike N. Gardos, Hughes Aircraft Company,

and Julian R. Pratt, Spheric, Ltd.

Requirements and Issues in Quality Control 141

and NDE of Ceramic Bearings

B. T. Khuri-Yakub, Stanford University

Ductile Regime Grinding of Brittle Materials 169

Tom A. Dow, Steven C. Fawcett, and

Ronald 0. Scattergood, North Carolina State University

Design and Life Prediction Issues for Ceramic Bearings 181

Crawford Meeks, AVCON, Inc.

Tribology Issues Related to Machining 199

and Performance of Ceramic Bearings

Said Jahanmir, NIST

Appendix A: Workshop Agenda 219

Appendix B: Workshop Attendance List 221

viii
Section 1

Workshop Summary

1

Workshop Summary

The objectives of the workshop were to assess the status of ceramic bearing

technology, and identify the key research topics needed to expand the range of

applications for ceramic bearings. The principal advantages of advanced

structural ceramics, such as silicon nitride, over metallic counterparts are

lower density, higher abrasion resistance, higher chemical inertness, and

electrical and magnetic insulation. Advanced structural ceramics have been

evaluated during the past twenty years as balls, rollers, and races in bearing

applications. Currently, silicon nitride hybrid bearings (ceramic balls with

steel races) are being used quite successfully as instrument bearings and as

machine tool spindle bearings. In general, progress in research and in

implementation of research results in utilization of ceramic bearings has been

slow. Therefore, this workshop was organized to identify the major impediments

and recommend possible solutions.

A total of eleven invited presentations were given at the workshop which

was attended by seventy- five representatives from industry, government, and

universities. The presentations covered both present and potential future

applications of ceramic bearings, and discussed topics related to processing,

machining, quality control, design, testing, and performance evaluation.

Following the invited presentations , the workshop was divided into three

discussion groups: 1) Ceramic Processing and Blank Fabrication, 2) Machining,

Quality Control, and NDE, and 3) Design, Performance Testing, and Life

Prediction. After extensive discussions, the chairman of each group presented

the summary of the discussions to the workshop participants. These summaries

and the abstracts of invited presentations, as well as the viewgraphs used in

these presentations, are included in this report. The workshop agenda and the

list of attendees are included in the Appendix.

The following summary is based on the presentations and discussions at the

workshop. The applications where ceramic bearings can be used, and the

associated benefits, are listed. The major issues in design and life prediction;

processing and materials; machining, non- destructive evaluation, and quality

control; and bearing performance are described in detail. This discussion is

concluded by a list of major recommendations for research.

Applications and Benefits :

The present and future applications for ceramic bearings can be classified

into five categories depending on the design and the requirements of the

application.

1) Marginally Lubricated systems : In these applications hybrid bearings

lubricated with grease and oil have the advantages of lower heat generation

and better overall performance due to reduced contact area (determined by

the elastic modulus of the materials). Specific applications include:

machine tool spindle bearings, fuel -lubricated turbine engine bearings,

fuel pumps, fuel hydraulic systems, fuel controls, pump motors and

generators for naval auxiliary machinery, gyroscopes, space gimbles,

3
momentum and reaction wheels, solar array drives, turbopumps, antenna and

sensor pointing systems, instrument bearings, and bearings for computer

disc drives.

2) High-Speed Liquid Lubricated Applications : In high-speed bearings it

may be possible to improve bearing life by using silicon nitride rolling

elements with steel races. The principal advantage is the reduction of

the outer race stresses due to lower centrifugal force as a result of lower

density of silicon nitride. Applications include: high-speed spindle

bearings, turbine engine mainshaft bearings, and air cycle machines.

3) Low- Speed Solid Lubricated Applications : At low to moderate speeds and

at temperatures to 450° C, solid lubricated hybrid bearings show improved

wear life when compared with steel bearings. Smaller contact area and

reduced microslip in hybrid bearings give an overall reduced wear and

improved life. Applications include: turbine engine auxiliaries, air

motors, nozzle actuators, space gimbals, and x-ray tube rotating cathodes.

Some of the applications cited for marginally lubricated systems may

benefit from solid lubrication if the technology becomes available.

4) High-Speed Solid Lubricated Bearings : The primary benefits of hybrid

bearings capable of operation at high- speeds and temperatures to 450° C with

solid lubrication are reduced weight, and reduction of engine size due to

the elimination of the liquid lubrication system and gearbox. Other

advantages are engine storability, and lower torque and starting

requirements at low temperatures.

5) High-Speed Solid Lubricated All-Ceramic Bearings : Solid lubricated allceramic bearings capable of operating at temperatures up to 750° C and at

high-speeds would offer significant advantages in advanced turbine engines.

Other applications of all-ceramic bearings include flywheel bearings for

electric-powered automobiles, turbomachinery , advanced heat engines, and

bearings for nuclear reactors. Much past research has been directed toward

this type of bearing; but significant issues in materials, lubricants,

mounting, lubricant supply, and cooling need to be resolved.

Utilization of advanced structural ceramics in bearings can result in many

technical benefits, which will undoubtedly result in lower operating cost of the

overall mechanical systems. Major benefits are as follows:

0 The lower density of silicon nitride will reduce the cage forces at

high acceleration rates. Also, reduced weight is beneficial during

launch of space mechanisms.

0 Ceramic rolling elements may reduce noise, resulting in quieter

bearings

0 All -ceramic bearings offer chemical inertness compared with steel

bearings; they are superior in applications where lubricated by

process fluids.

4
0 In electrical applications, ceramic parts improve electrical

isolation and eliminate damage due to electrical discharge.

0 Improved precision of ceramic balls can improve performance, i.e.,

smooth operation with low vibration, in a wide range of applications.

0 Reduced sensitivity to loss of lubricant and higher fatigue life can

increase reliability and increase the overall life of the bearing.

0 Higher hardness of the ceramics may result in lower damage due to

particulate contamination in the bearing.

Design and Life Prediction:

A rolling element bearing is a highly complex mechanical component since

numerous variables must be considered in its design. The design process consists

of analysis and optimization of key design variables. Some of the important

design variables for a rolling element bearing include: geometric parameters

(ball diameter, contact angle, pitch diameter, raceway curvature, and ball-cage

pocket clearance), external loads (radial and axial forces, moments, unbalanced

forces, and vibrations), internal loads (centrifugal and gyroscopic forces,

dynamic collision of cage and rolling elements, and skidding forces), material

and lubricant parameters, and operational variables (such as temperature and

speed) . It should also be recognized that some of these variables are

interdependent. Conventional design techniques mostly consider the geometric

aspects and load-carrying capacity of bearings. Although this may be sufficient

for low-speed, low-load, well-lubricated bearings, it is not possible to use

these techniques to design reliable, high-speed bearings made with new advanced

materials such as ceramics. Development of ceramic bearings requires dynamic

analysis of the total bearing system including the motion of individual balls,

the cage, and the races.

The design process is not complete until the bearing geometry, materials,

lubricants, and integration of the bearing with shaft and the housing has been

achieved. At this point the potential failure modes must be identified and the

variables adjusted to accommodate normal wear of the system. Finally, the life

of the bearing and the reliability factor for the intended application must be

determined. Several advanced design and analysis programs are currently

available, which can be used on personal computers or work stations. These

programs not only can be used as a guide to design ceramic bearings, but also

can be used to analyze the effect of bearing parameters on the performance, and

to warn against potential failures. These programs, however, have been developed

for design of bearings for certain applications, and may not be suitable for the

wide range of applications where ceramic bearings can be implemented. Since the

design requirements and performance issues depend heavily on the application,

and are not the same for all anticipated applications, these programs must be

modified.

The specific design issues for each application category are different;

these issues are discussed in the discussion summary of the Design and

Performance Testing. For hybrid marginally lubricated bearings, for example,

5
which have the potential for the most wide -spread use, the important issue is

design predictability. In order to improve the design predictability of hybrid

bearings, material life factors are needed for both the steel and ceramic parts

run together. Development of these life factors will require performance data

on fatigue, wear and lubricant degradation, as well as heat generation and its

effect on performance. In addition, the lower thermal expansion coefficient of

ceramics compared to metals must be incorporated in the design. Furthermore,

the balance between increased stresses due to higher elastic modulus of the

ceramic balls and the lower centrifugally induced loads at high speeds must be

carefully accounted for in bearing design and life calculations. While these

are key factors in design, the general consensus is that given adequate data on

material properties and life factors, designers know how to account for these

in the design.

Processing and Materials Issues :

Based on limited available data, it can be concluded that performance of

silicon nitride is superior to other advanced structural ceramics for bearing

applications. The current route of processing silicon nitride bearing components

is by powder beneficiation, compaction and densification. Processing of powders,

starting with fine powders through densification, is intensive in capital and

energy; therefore, it is expensive. An accurate assessment of the cost of blank

formation is not possible with the available data, but it is obvious that by

starting with lower-cost raw materials, and by reducing rejects or recycle, the

overall cost can be decreased. In addition, the cost can be dramatically reduced

if volume of production can be increased. In the immediate future, current

methods of powder processing are expected to dominate. Therefore, further

research and developments in powder processing are required to establish lowcost methods for production of densified silicon nitride materials suitable for

bearing applications.

A large number of unit operations are involved in powder processing and

densification. Each operation is considered to be a cause for increased cost

and a source of introduction of impurities. At present, due to small batch

operations, the cost of process monitoring and operation are high. Also, the

end properties are overly sensitive to small changes in the process parameters.

Because of difficulties in controlling and reproducing all the processing

parameters, batch-to-batch variation and the uniformity of composition and

microstructure have become major problems. To enhance reproducibility and

uniformity, a large number of measurements are usually made on the powder,

slurry, green body, and densified ceramic components. But, the measurements

contribute to increased production cost of the bearing components. Hence, there

is a need for identification of specific measurements that have a major impact

on the processing parameters.

All conventional methods of shape forming (dry pressing, injection molding,

and colloidal filtration) involve a number of steps that require powder handling

in different stages. Though densification by hot iso-static pressing (HIP) is

the preferred mode of processing, it is expensive and leaves behind a glassy

surface layer which must be removed by machining. If fully utilized, the cost

of HIP can be decreased and cost due to rejections can be reduced. However,

6
pressureless sintering of suitable compositions should be explored as possible

cost-effective densification alternatives for bearing materials.

The starting powder , additives, processing method, and processing variables

strongly influence the final composition and microstructure of the material.

No data exist on the optimization of microstructure for best wear resistance or

bearing performance. In most cases, existing materials have been tested under

only a few selected test conditions for fatigue and for wear. Published data,

although very limited, suggest that the amount of grain boundary phase, the

inclusion content, and the porosity level must be kept as small as possible.

The influence of grain size, grain aspect ratio, additive chemistry, alpha/beta

ratio, and grain boundary composition require further investigation.

One reason for the lack of systematic studies on the effect of

compositional, microstructural , and processing variables is the difficulty of

holding any of these constant as the others are varied. An alternate approach

is to identify the detailed mechanisms controlling failure, i.e., spalling and

wear, and investigate the effect of important parameters affecting the failure

process using statistically designed experiments. The result of such studies

could be used to optimize the processing conditions that would provide the best

microstructure for bearing performance.

Machining and Finishing Issues :

The performance and reliability of ceramic rolling element bearings are

controlled not only by microstructure and composition, but also by surface

roughness and integrity, which are controlled by machining. Formation of defects

and residual stresses due to machining can be detrimental and promote early

failure by fatigue and spalling. In addition to the contribution of machining

damage to performance, machining also contributes to the high cost of ceramic

bearings. Therefore, it is imperative to optimize the present machining methods

and find innovative and cost-effective methods to machine ceramic bearing

components that are free of machining generated defects.

Although there are no standard procedures for machining ceramic balls and

races, it appears that diamond grinding and polishing are the most commonly used

techniques. The machining procedure for silicon nitride balls consists of

grinding and polishing of near net- shape spheres to obtain the desired tolerances

and surface finishes. Currently, this process is slow and requires several

hours. However, excellent tolerances and surface finishes can be obtained.

Recent studies have shown that the polishing rate and the tolerances, as well

as the surface finish, depend on several factors including the microstructural

parameters, processing procedure and mechanical properties of the material. For

example, hot-pressed materials seem to be inferior to HIP'ed materials, because

of the microstructural anisotropy obtained in hot pressing. Grinding and

polishing rates depend on the fracture toughness and hardness of the material

through the influence of microstructural parameters on these properties. It

should be pointed out that formation of machining defects and residual stresses

also depend on the microstructure and the processing conditions, since each

material may respond differently to the applied surface forces and thermal

conditions during the machining process. Nevertheless, ball grinding and

7
polishing can be considered as an established technique. As already stated, it

is time-consuming and costly; any improvements that result in the reduction of

cost and improvements in the surface quality would be beneficial. An example

of a new technique is polishing by magnetic levitation, which appears to produce

excellent quality at higher production rates.

Machining techniques for ceramic rollers and races have not been

established, although diamond grinding and polishing are being used. Machining

of races for all-ceramic bearings from a block of silicon nitride is costly, and

often results in a large rejection rate. New machining techniques and/or

optimization of the grinding process are badly needed. It should be recognized

that machining is a complex process and involves a large number of interdependent

variables. For example, in grinding one must consider the machine tool, the

grinding wheel, the grinding fluid, the machining parameters, and of course the

workpiece. Therefore, machining in general, and grinding in particular, must

be considered as a system. The effect of these parameters on the machining rate

and the quality of finished components must be determined, in order to optimize

the machining practice.

Before improvements in machining operations can be made, specific knowledge

of the surface requirements of the finished bearing must be known: e.g., shape,

size, expected stresses, and expected modes of failure. With this knowledge in

hand, it would be possible to know if existing machining technology would be

adequate to finish the bearings to an acceptable reliability level or whether

modifications to current machining practices (or even completely new practices)

would be required. Regardless of the particular machining process employed,

there is a need for generalized guidelines for machining which would outline

standard procedures as well as recognized risks and benefits associated with

those procedures.

Studies have shown that quality of finished surfaces and the degree of

precision obtained in grinding of high-strength ceramics, such as silicon

nitride, is very much influenced by machine stiffness. Since rolling element

bearings require a high level of precision, it is important to recognize the

effect of machine tool design on the quality of finished components. Stateof-the-art stiff machine tools can currently be used in a creep-feed grinding

mode to remove large amounts of material at high rates. These recent

developments must be evaluated for machining of silicon nitride races for allceramic bearings

Another recent development in machining of "brittle" materials is the

ductile regime grinding process . The concept of ductile regime grinding is based

on the idea that at very small depths of cut the material can be removed by

plastic deformation rather than fracture, which is usually observed for brittle

materials. The critical depth of cut for transition to ductile regime depends

on hardness and fracture toughness of the material being machined. For silicon

nitride and silicon carbide the critical depth of cut is in the range of 200 to

300 run. It may be possible to increase the critical depth of cut by analyzing

the effect of chemical environment on the process of chip formation, with the

goal of improving the grinding process through selection of proper chemical

compounds for addition to the grinding fluid. This approach may also be

instrumental in the reduction of the overall machining cost by decreasing the

8
wear rate of the diamond grains used in grinding.

A major factor contributing to the high cost of ceramic bearings is the

small number of bearings currently being produced. As the demand increases, the

cost per component is expected to decrease. Presently, machining is done in a

batch mode. In order to achieve automation in ceramic machining, and allow for

a flexible manufacturing environment, research in on-line sensors, process

models, control strategies, and mechanistic understanding is needed. The need

for increased research in ceramic machining has also been recognized by NIST and

DOE. It is important to coordinate the machining activities of DARPA with other

agencies to achieve a faster technology development and eliminate the potential

of duplication.

NDE and Quality Control :

Performance of ceramic bearings can be influenced by any factors that

negatively affect the failure process. For example, large grains, voids,

inclusions, and microcracks near surfaces can promote excessive fatigue and/or

wear by acting as sites of crack nucleation. In addition to these surface or

near-surface defects, one must also determine the dimensional tolerance and

surface topography. Furthermore, any internal defects such as large inclusions,

cracks, and density variations may cause premature fracture and failure of the

bearing components. The fear of premature failure may be the single most

important barrier to the greater use of ceramics in bearing applications.

In order to ensure reliable bearing performance, non- destructive evaluation

(NDE) and quality control are necessary. Reliability is, in practice, made up

of two components: true reliability and perceived reliability. For many

applications, current ceramic bearings are already sufficiently reliable, but

the consumer's perception is contrary to this fact. Because of the perceived

low reliability, utilization of ceramic bearings for certain applications

requiring high reliability is limited. However, it should be pointed out that

for many near- term applications, good process control coupled with sampling plan

and destructive analysis or some established NDE techniques may be adequate.

Advanced NDE techniques are costly and may raise the price of the bearing

components; they may even hinder use of ceramic bearings in lower-risk

applications that do not require such stringent inspections.

In order to select an existing NDE technique or develop a new one, one must

first determine the potential failure modes for the intended application. It

is critical that the failure modes be determined in real bearings rather than

in simulated tests such as rolling contact fatigue. Next, it is necessary to

identify what kinds of flaws must be detected, since flaw size, shape, and

location all influence the selection of NDE procedure. Clearly, the

identification of the flaws to be detected is determined by the reliability

criteria appropriate for the specific application. Once the types of critical

flaws have been identified, it should be possible to judge whether known

evaluation techniques are applicable or whether new techniques need to be

developed, either from scratch or by modifications of existing procedures.

9
In the last fifteen to twenty years, several techniques have been developed

for the non- destructive evaluation of dense ceramics. These include: x-ray

computed tomography, microfocus x-ray, thermal imaging, nuclear magnetic

resonance, and ultrasonics, in addition to visual optical inspection and

fluorescent penetrant technique. Most of these techniques are suitable for

detection of flaws larger than 50 microns in the bulk sample, with the exception

of high-frequency ultrasonics and thermal wave measurements with laser. The

latter techniques, although still in the experimental research stage, seem to

be capable of detecting smaller surface flaws generated on and near the surface

from machining operations

Detection of machining generated defects is crucial to bearing performance,

because surface fatigue cracks initiate at or very near the surface. Also, as

the quality of ceramic materials improve and the density of the bulk defects is

reduced through improved processing techniques, it is expected that surface

defects become more detrimental to the bearing performance. It is, therefore,

necessary to extend the capabilities of the thermal wave imaging and ultrasonic

technique for the detection of machining generated defects. Future research must

address the NDE of actual ceramic bearing components, i.e., balls, rollers, and

races, and must be capable of inspecting the entire surface.

It should also be emphasized that application of NDE and quality control

should not be limited to the finished component. All other steps in the

manufacture of the bearing require inspection and control. Each of these steps

is interrelated with the others, and maximum reliability requires that improved

understanding occur at each step. It is important to consider where NDE should

be applied in the manufacturing process, and whether certain rough-part

geometries and certain stages during manufacturing, for example green machining

or powder processing, are easier to inspect. Inspection of powder, slurry, and

green compacts for defects and rejection of defective parts at the early stages

of production can decrease the production cost. In-process monitoring of powder

processing unit operations for particle size distribution, agglomerate

concentration, dispersion chemistry, green density, and geometric dimensional

variations of green bodies can lead to significant improvements in the overall

process reliability.

Performance-Related Issues :

Wear and rolling contact fatigue (i.e., spall formation) are the two

general mechanisms of failure in ceramic bearings. Fatigue cracks are usually

initiated on the surface or near the surface from inhomogeneities in the

material; for example, microcracks , inclusions, cavities, and grain boundaries

Initiation and propagation of fatigue cracks are influenced by the physical and

thermal properties of the material and the environment. In the latter, chemical

reactions between the environment and the crack tip can accelerate the

propagation of the crack, resulting in premature failure. Wear, or gradual

removal of material, is generally accompanied by deformation and fracture, and

often cyclic fatigue at a scale much smaller than the spall formation. Plowing

and damage by hard foreign debris and wear debris also can cause considerable

damage to the bearing. Tribochemical reactions between the surface and the

10
environment and removal of the reaction film is another mechanism for wear, as

documented for silicon nitride.

Although past research has provided some insights into potential failure

mechanisms in ceramic bearings, the details of these processes and the role of

important variables influencing failure have not been explored systematically.

In the absence of such information, laboratory test programs have been used to

evaluate fatigue and sometimes wear of ceramic bearing materials. One such

example is the rolling contact fatigue (or RCF) testing, in which a circular rod

of silicon nitride is rotated against three steel balls. Although this test is

useful to rank different materials, it is generally believed that many

potentially acceptable materials are rejected because the high loads used in

the RCF test do not simulate the conditions in real bearings. Furthermore,

recent experience with ceramic bearings indicates that rolling contact fatigue

is not the only failure mechanism. In this respect it is important to determine

the primary failure mechanism for each class of applications and optimize the

material design for that application. It is possible that optimization of

material properties for resistance to fatigue may negatively affect the wear

resistance. For applications requiring resistance to surface fatigue there is

a need to develop standardized tests conducted on relevant bearing components,

e.g., ceramic balls instead of cylindrical rods. Such tests should also allow

the evaluation of the effects of machining damage on performance.

Although the wear mechanisms of advanced structural ceramics have been

the subject of recent investigations, much of the current understanding may not

be relevant for ceramic rolling contact bearings, due to the differences in the

test conditions used in wear studies and the operating conditions in bearings.

But it is clearly established that wear occurs by deformation and fracture, and

is often influenced by tribochemical reactions. Recent studies have shown that

the rate of wear can suddenly increase by two orders of magnitude if a threshold

value of contact load is exceeded. The wear process at higher loads is related

to the fracture of material near the surface. In order to avoid the transition

to severe wear, one must reduce the coefficient of friction by lubrication,

and/or increase the fracture toughness of the material.

Transition from mild to severe wear by fracture is highly sensitive to the

coefficient of friction, because of the effect of friction on the tensile

component of stress at the trailing edge of the contact. Therefore, even a small

reduction in the coefficient of friction can result in a large increase in the

transition load, and a major reduction in the rate of wear. The coefficient of

friction can be reduced by hydrocarbon lubricants and polar additives. Although

limited data have been published, the details of boundary lubrication mechanisms

of silicon nitride are not known. Particularly, the adsorption mechanism of

polar compounds on ceramic surfaces and the role of chemical structure on the

adsorption process need to be analyzed. This type of information can be used

to develop effective lubricants for ceramic bearings.

Chemical reaction between the ceramic surface and the lubricant is also

an important mechanism for lubrication. Chemically reacted films with low shear

strength can provide a low coefficient of friction. The current liquid lubricant

technology is based on the accumulated knowledge on reaction mechanisms between

metals and lubricants. A parallel set of information on reactivity of ceramics

11
with lubricants is needed to develop lubricant additives for ceramic bearings.

In many applications, lubricant degradation through oxidation is the lifelimiting process. In this respect, it is necessary to develop a better

understanding of thermal and oxidative breakdown of liquid lubricants, including

greases, for use with silicon nitride bearings.

For high-speed, high- temperature applications the key to success is finding

adequate race and cage materials and a lubrication system that can work to

provide stable bearing operation both mechanically and thermally at the speeds

and temperatures required. Hydrocarbon liquid lubricants have a limited

performance at elevated temperatures or under vacuum environments due to severe

degradation and evaporation. Solid lubricants are the only viable alternative.

The main problems, however, include adhesion of the solid lubricant coating to

the substrate, high wear rate of the coating and inadequate response of the

coatings to the temperature variations. It is necessary to pursue other hightemperature solid lubricants and lubricant delivery methods such as powder

lubrication and the self - lubricating cage materials. In the latter, lubrication

is provided by a composite cage containing the solid lubricant as a second phase.

Recent research results have confirmed that in these materials lubrication is

achieved by the formation of a transfer film, which contains materials from both

the composite and the counterface , as well as tribochemical reaction products.

Therefore, a better understanding of the formation of the transfer film and the

relationship between microstructure of the composite and mechanical properties

of the transfer film are needed to optimize the microstructure and composition

of self -lubricating materials.

Recommendations :

The following recommendations are based on the technical presentations and

discussions at the workshop:

0 Research activities should focus on cost reduction, while maintaining

or improving the current level of performance reliability. All

aspects of bearing production from powder processing to bearing

assembly should be examined for means of cost reduction.

0 Research in powder processing, compaction, and densification should

address optimization of microstructure for best bearing performance,

as well as cost control.

0 New and improved cost-effective machining techniques should be

developed for ceramic bearing races. Special attention should be

given to dimensional tolerance and surface integrity of the finished

components

0 Non- destructive evaluation techniques and quality control methods

should be developed for use in various stages of bearing production.

It should be recognized that only the applications requiring a high

level of reliability should be subjected to expensive inspections.

12
0 Although the current silicon nitride bearing materials are adequate

for use in hybrid bearings, new and improved materials that are being

developed should be evaluated for use in high- temperature allceramic bearings.

0 New and improved computerized design procedures and life prediction

methods should be developed for ceramic bearings . These computer

programs should include dynamic analysis of the total bearing system,

performance data, and models for wear, fatigue, and lubricant

degradation.

0 Detailed understanding of the mechanisms of wear and rolling contact

fatigue, and the role of microstructure on these mechanisms, is

needed to allow developments of new and improved bearing materials.

0 Performance data on presently available materials should be collected

and made available to the bearing industry to allow proper selection

of bearing materials.

13

Section 2

Discussion Group Summaries

15

Ceramic Processing and Blank Fabrication

Discussion Group Summary

S. G. Malghan and J. G. Hannoosh

A number of issues were discussed by the participants. The following four

issues were specifically identified as the major hurdles for the widespread

application of ceramic bearings:

• Blank cost reduction

• Powder -processing -micros tructure- performance interrelationships

• Reliability

• Uniformity of terminology

Blank Cost Reduction

The current cost of production of silicon nitride bearings from powder as

the starting material are approximately $400/kg. The main problem to address

is the reduction of the production cost by at least 80%. The major costcomponents of the production of bearings are:

• Powder, and other starting materials

• Processing, including densification

• Finishing, including machining

• Inspection, including non- destructive evaluation.

The starting powder costs range from $60-100/kg for fine-sized pure

powders. However it appears that by starting with a lower- cost powder, such as

silicon, the overall cost does not decrease substantially. Hence, an accurate

assessment of the cost is not possible with the available data. However, it is

obvious that by starting with lower- cost raw materials, and by reducing rejects

or recycle, the overall cost can be decreased. Since the conversion of Si,

Si02 , or SiClA to Si3 NA is an energy- and capital-intensive process, the overall

cost should decrease substantially with increased volume of production. In

addition, the current practice of Si 3 N4 powder production and processing may not

be the best approach in the long run. An example is Sullivan Mining process,

in which powder is not the starting material.

However, in the immediate future, current methods of processing powders

are expected to dominate. Therefore, powder processing studies are required to

establish not only the methods of overcoming deficiencies in the specific

powders, but also the impact of specific impurities on the final properties of

the ceramic.

Processing of powders, starting with fine powders through densification,

is a capital-, energy-, and manpower- intensive activity. A large number of unit

operations are involved, in which each subprocess is considered to be a cause

for increased cost and a source of introduction of impurities. All three

conventional methods of shape forming (dry pressing, injection molding, and

17
colloidal filtration) involve a number of steps that require powder handling in

different stages. Though densification by hot isostatic pressing is the

preferred mode of processing, it is expensive and leaves behind a glassy surface

layer. Pressureless sintering and suitable compositions should be developed as

cost-effective densification alternatives.

Additional issues discussed by the participants were: batch-to-batch

oxygen variation of powders; control of surface layer during glass -encapsulated

HIP; research versus manufacturing issues; development of powder specifications

that yield reproducible properties; failure mechanisms (fatigue- -static

mechanical, thermal, etc.) at low stresses; development of new materials and

processes with different types of powders; application of statistical methods

to solve technical and engineering problems; process modelling based on

fundamental understanding of flow behavior and heat transfer; and kinetics during

densification by HIP especially for large components (currently balls larger than

10 cm diameter cannot be processed by HIP)

Finishing of dense silicon nitride bearings can often constitute up to 50%

of the overall cost. Machining and surface finishing are time-consuming, costly

steps that often induce surface defects. Near -net -shape forming and costeffective machining are potential areas of research.

Inspection of powder, slurry, green body, and dense silicon nitride is one

of the primary requirements in the production of defect- free bearings. At

present, due to small, batch operations, the end-properties are overly sensitive

to small changes in the process parameters. To enhance reproducibility, a large

number of measurements are being made. Data on batch-to-batch variation of

powders and effect of such variation is not available, but such changes are

considered detrimental to reproducibility.

Powder -Processing -Micro structure -Performance Interrelationships

In general, no data exist on the optimization of powder-processingmicros true ture versus rolling and sliding contact. In most cases, existing

materials have been tested under a few selected wear conditions. Specifically,

the effect of grain size, chemical compositions, powder processing methods, and

densification method have not been studied. Systematic variation of

microstructure or composition of silicon nitride has also not been examined.

Though silicon nitride has emerged as the most promising material for

bearings applications, other candidate materials of suitable microstructure

should be examined. In the case of silicon nitride, most of the data exist on

MgC03 or MgO as the sintering aid. Other compositions have not been addressed

with respect to their suitability for wear applications.

An understanding of the interactions between silicon nitride composition

and the lubricant used during bearing operation should be developed for a given

application. Lubricant chemistry is an important issue in providing a lower wear

rate and in the development of self- lubricating systems.

18
Reliability

Due to extensive in-process evaluation during powder processing and nondestructive testing of green and sintered bodies, bearings reliability has

increased. However, these efforts add to the manufacturing cost. Accurate

quantitative assessment of bearings produced by different manufacturers would

help the users community.

Uniformity of Terminology

In order to assist the users of bearings, the terminology related to

ceramic bearings has to be made uniform and interrelated to conventional

terminology used in the bearing industry.

19

Machining. Quality Control and NDE

Discussion Group Summary

K. Subramanian and G. White

This session was attended by about 30 people. The consensus seemed to be

that the central theme of this session should be reliability of the bearing and

that "reliability" reflects not only the machining and evaluation processes but

all other steps in the life of the bearing: raw material, processing, and

applications. Each of these steps is interrelated with the others, and maximum

reliability requires that improved understanding occurs at each step. In

addition, "reliability" is made up of two components: true reliability and

perceived reliability. For many applications, current ceramic bearings are

sufficiently reliable, but the consumer needs to be educated to this fact.

For other applications, there remains a need for improvement, not only in

the processes of machining and bearing evaluation, but also in the determination

of what "reliability" means; i.e., what are the failure modes for specific

applications? This question is critical for the determination of specifications

both for machining and non- destructive evaluation procedures. In addition, the

group felt strongly that tests of different failure modes must be related to real

bearings rather than to simulations.

In terms of machining, the group felt that, before improvements in

machining operations could be made, specific knowledge of the surface

requirements of the finished bearing must be known: e.g., shape, size, expected

stresses, expected modes of failure. With this knowledge in hand, it would be

possible to know if existing machining technology would be adequate to finish

the bearings to an acceptable reliability or whether modifications to current

machining practices (or even completely new practices) would be required.

Regardless of the particular machining processes employed, the group recognized

the need for the existence of a generalized guideline for machining practices

which would outline standard procedures as well as recognized risks and benefits

associated with those procedures.

Needs for evaluating the bearings are very similar to those stated above

for machining practices. First, it is necessary to identify what kinds of flaws

must be detected; i.e., flaw size, shape and location all influence the type of

test procedure employed. Clearly, the identification of the flaws to be detected

is determined by the reliability criteria appropriate for the specific

application. Once the types of critical flaws have been identified, it should

be possible to judge whether known evaluation techniques are applicable or

whether new techniques need to be developed, either from scratch or by

modifications of procedures used in other applications. One question which was

not resolved concerned the methodology of developing new evaluation procedures

.

Should a standard (material, size, shape, etc.) test specimen be created which

can be used throughout the industry, or should each company generate specific

test specimens suitable for each application? Clearly, there are tradeoffs in

either case, and probably, some combination of the two should be the goal. There

was universal agreement, however, that evaluation tests must reflect real needs

21
applications

.

Finally, the idea of education reoccurred repeatedly during discussions.

In the first case, there needs to be a great deal more exchange of basic

information among the companies involved with ceramic bearings. The feeling was

emphasized that proprietary concerns were reducing the competitiveness of

American bearing companies as a whole, as well as individually. In the second

place, the bearing consumer needs to be educated as to the benefits and

reliability of ceramic bearings. In addition, the feeling was expressed that

consumer demand for ceramic bearing reliability was far more stringent than

similar demands on metal bearings and that this discrepancy reflected further

need for education of the consumer.

22
Reality Perception

Reliability

Quality

Material

Consolidation

Process

Reliability

Finishing * Process

1. Simultaneous and concurrent improvements

of all linkages are critical - Both fundamental and applied

2. Quality is achieved when all linkages are

- Specified

- Controlled

23
Reliability

1. Failure Mode

- How is it defined?

- Application dependnent

2. Failure Modes determine specifications for - Machining

- NDE

3. Tested Failure Modes must relate to real

bearings

4. Reliability includes failure avoidance as we!!

as acceptance/rejectance criteria

Machining:

1. What is needed

- Surface requirements

- Shape

- Geometry

- Surface considerations

2. How to machine

- Develop new technology

- Modify known technology

- Transfer technology

3. Methodology

- Generalized guideline for beginning

- Reduce cycle time by optimization and

system integration

24
Evaluation:

1. What to evaluate

- Flaw size \. - Flaw type

- Surface/bulk flaw

Determined by

reliability criteria

2. How to evaluate

- Develop new techniques

- Modify known techniques

- Transfer technology

3. Methodolgy

- Standardized specimen (material, size,

shape, application) - Commercial specimen

25

Design. Performance Testing and Life Prediction

Discussion Group Summary

J. Dill and M. B. Peterson

Issues in these areas depend heavily on the application and are not the

same for all anticipated uses of ceramic bearing components. Applications can

be broken down into classes which have similar requirements. Classes which were

discusses included:

o High-Speed Liquid Lubricated Hybrid Bearings

o Liquid Lubricated Hybrid Bearings for Instrument and Space Applications

o Hybrid Bearings for Low-Noise Operation

o Process Fluid Lubricated Bearings

o Low- Speed Solid Lubricated Bearings

o High-Speed Solid Lubricated Hybrid Bearings

o Solid Lubricated Instrument and Space Mechanism Bearings

o High-Speed Solid Lubricated All -Ceramic Bearings

Once an agreement was reached on these classes of bearings with common

issues, the key issues for each class were discussed. Since the issues of design

and life could not be discussed without discussing materials and NDE, those

issues were also discussed, especially relative to how they impact design and

life. This summary will address the issues by class and then end with some

general comments. High-speed liquid lubricated hybrid bearings and liquid

lubricated hybrid instrument bearings are two of the most widespread of the

initial applications and therefore were discussed first.

High-Speed Liquid Lubricated Hybrid Bearings

Table 1 itemizes the main concerns relative to this type of application

and identifies some of the applications. The starred items indicate the most

critical issues which must be resolved if ceramics are to see wide use in this

type of bearings.

For liquid lubricated hybrid bearings to achieve wide acceptance, the major

design issue which must be resolved is the current inability to predict the life

of a specific bearing design in a given application. To bring hybrid bearing

design to the current level of all-steel bearing design will require the

development of materials life factors for both the steel and ceramic parts when

run together. Development of these life factors will require both element tests

such as rolling contact fatigue rod tests and full bearing testing. Fatigue can

only be defined for each component in the materials couple when tested together

(including the lubricant) since the lubricant/material combination will affect

the results.

27
Table 1: HIGH-SPEED LIQUID LUBRICATED

HYBRID BEARINGS

o Definition of Skidding Margin and Damage

o Lubricant Formulation for Use With Ceramic Components

o Prediction of Fatigue Life in Specific Applications*

o Effects of Ceramic Materials on Metallic Parts

o Failure Detection for Non-Metallic Parts

o Non- Destructive Evaluation*

o Secondary Damage to System on Failure of Ceramic

o Cage Materials Optimized for Use with Ceramics*

o Race Coatings for Wear/Lubrication Enhancement*

o Hybrid Lubrication Concepts (Solid Film/Liquid)

o Performance with High-Temperature Liquid Lubes (PPE/PFAE)*

o Industry Standards and Material Specifications

For high-speed liquid lubricated bearings, in addition to fatigue life,

heat generation and lubrication requirements (i.e. , grease versus oil mist versus

oil jet) are also key issues. The balance between increased stresses due to the

higher elastic modulus of the ceramic component and the lower centrifugally

induced loads at high speeds mast be carefully accounted for in bearing design

and life calculations. While these are key factors in design, the general

consensus is that given adequate data on material properties and life

multipliers, designers know how to account for these factors.

NDE is an issue primarily for higher reliability applications such as

aircraft turbine engines where extreme levels of reliability are required. In

applications such as machine tool spindles, a benefit could be realized from

improved NDE, but because of the lower- risk, NDE requirements are not as

critical. There is a concern that if the stringent NDE required for turbine

engine bearings is applied to components for lower-risk applications such as

spindles, the price increase will overshadow the potential benefits and eliminate

the use of ceramic components. NDE must be appropriate to the application. For

example, a large part of the price differential between a class 7 spindle bearing

and a class 7 turbine engine bearing (which can be a factor as high as 10 for

similar sizes) is due to the added inspection and documentation required for the

turbine engine bearing.

With the present state of the art of the ceramic materials and NDE, it is

better to focus on the lower- risk applications where there is a clear benefit.

For many turbine engines, it is not possible to show a clear benefit with ceramic

rolling elements even if the stringent NDE requirements can be met. Also, if

a theoretical benefit can be shown in a turbine engine by using a hybrid bearing,

in most cases, the life of the current all-steel bearings is more than adequate

and is not considered an issue by the manufacturers. At the present time, the

payback on promoting turbine engine applications of hybrid bearings is probably

not worth the efforts which would be required to get them accepted.

For many high-speed applications such as machine tools, it may be desirable

to develop improved cage materials for use with ceramic balls. Testing has shown

28
that in low-lubricity liquids the standard silver-plated steel cage used in many

high-speed bearings is not adequate. Data on the wear of different cage

materials with lubricants of interest will increase the understanding of bearing

life in hybrid bearing applications.

To take full advantage of the wear resistance of the ceramic rolling

element materials, it may be necessary to improve the wear resistance of the

races by the use of hard coatings. While it has been shown that hard coatings

such as titanium nitride and titanium carbide can improve race fatigue and wear

performance, coating processes are variable in their reliability. A wellcontrolled coating process with demonstrated performance could further enhance

hybrid bearing performance over that of a hybrid bearing with bare steel races

For higher- temperature advanced turbine engines, such as those being

developed under the IHPTET initiative, there may be a benefit to using hybrid

bearings with advanced high- temperature lubricants such as polyphenyl ethers and

perfluoroalkyl ethers. The poorer lubricating properties of these lubricants

may require ceramic hybrid bearings to achieve adequate performance for the

systems. As these engines evolve over the next ten years, this technology should

follow a natural evolution, especially if ceramic bearing components can be

developed to wider usage in other applications first.

Liquid Lubricated Hybrid Bearings for Instrument and Space Applications

In these types of bearings , generally loads are low and speeds are

moderate. Table 2 details the key issues for this type of bearing. Again, the

issues felt to be of primary concern by the group are starred.

Table 2: Liquid Lubricated Hybrid Bearings

for Instrument and Space Applications

o Lubricant Formulation for Use With Ceramics

o Lubricant Life and Degradation Models*

o Lubricant Supply Mechanisms*

o Designs to Accommodate Thermal Expansion Mismatch*

o Evaluation of Cage vs. Cageless Designs*

o Ceramic Materials for Precision Finishing

o Race Coating for Wear Reduction and Lubrication*

o Database on Performance of Ceramic Balls

o Definition of Vibration Reduction From Improved Precision

o Definition of Sphericity Improvements vs. Steel

o Effects of Improved Precision of System (Pointing and Control)

o Definition of Reduction in False Brinelling and Fretting

Fatigue is not generally a problem in this type of bearing. The major

bearing failure mode is generally degradation of the lubricant. In this class

of bearing, a thin film of lubricant is generally applied to the race for the

life of the bearing. The retainer may be a porous phenolic type of material

which supplies some additional lubrication via release of an impregnated liquid

lubricant. Testing has shown that substitution of a ceramic ball for a steel

29
one in an instrument bearing will improve bearing life by reducing the

degradation rate of the lubricant. The amount of this improvement needs to be

quantified and adequate lubricant life models developed. Since the phenolic

cages were designed for use with steel balls, alternate cage materials and

lubricant supply mechanisms may need to be developed to achieve optimum

performance

.

Relative to design, component designs which accommodate the lower thermal

expansion of the ceramic balls are required. The information is available to

develop these designs, but system designers must be aware of the best techniques

for mounting to accommodate the thermal expansion mismatch.

For ceramic hybrid bearings, the use of full complement designs needs to

be explored. Tests have shown benefits for full complement designs with all

steel bearings, but to date, little effort has gone into investigating full

complement hybrid bearings. In all-steel full complement bearings, the ballto-ball contacts can be a problem. In a hybrid full complement bearing, the

ceramic to ceramic ball contacts may perform better. This issue needs to be

evaluated.

To achieve full benefit of the reduction of lubricant degradation by

eliminating active metal asperity sites in the contacts of the bearing, it may

be necessary to coat the races with a ceramic hard coating such as titanium

nitride. There was no discussion of any experience with testing which would

indicate if a further benefit can be realized in a hybrid bearing with coated

races over one with uncoated races.

Hybrid Bearings for Low-Noise Operation

Table 3 lists the key issues for low noise bearings. In general, lownoise bearings run at moderate loads and speeds, and their life is more often

defined by the length of time that they remain quiet than by the fatigue life

of the components. The starred items were those felt to be of primary concern.

Table 3: Hybrid Bearings for Low-Noise Operation

o Definition of Wear Mechanisms

o Performance Data on Quiet Life

o Definition of the Effect of Better Geometry on Noise

o Influence of Cage Interactions on Noise

o Lubricant Formulation for Use With Ceramics*

o Race Coatings and Coating Process Control*

o Definition of Skidding Margin Benefits

o Grease Formulation*

o Ceramics Capable of Providing Better Sphericity in Balls*

Quiet life is influenced by race wear and damage caused by debris and

marginal lubrication in grease lubricated bearings. Testing indicates that

ceramic hybrid bearings do have a longer quiet life, but this needs to be better

quantified.

30
Debris damage to the balls is reduced in a hybrid bearing, but remains the

same on the races. Further improvements in bearing quiet life may be realized

if reliable hard coatings can be developed for low-noise bearings.

For some of the highest performance applications, fatigue life may be a

problem which can be improved by using ceramic rolling elements. The life

improvement can be achieved because the lower density of the ceramic rolling

element results in a better skidding margin in the bearing, meaning that the

bearing can be run at reduced preload thus resulting in a longer life with proper

design.

Because greases for low-noise bearings have been formulated for all-steel

bearings, they need to be re-evaluated to determine if the same formulations are

optimum for hybrid bearings. Improved grease formulations may be possible with

hybrid bearings, and these need to be evaluated.

Because it can be very difficult and costly to replace a bearing in a lownoise shipboard application, high reliability is required for all bearing

components. While NDE requirements similar to those for manrated turbine engines

are not necessary, a qualified quality control process which has demonstrated

the ability to reliably produce quality ceramic balls, time after time, is

required for low-noise bearings if they are to be used.

Process Fluid Lubricated Bearings

For process fluid lubricated bearings, Table 4 lists the areas discussed

relative to where hybrid or all -ceramic bearings might be of benefit and what

some of the issues are. The general feeling was that in this area, the key

concerns are dependent on the specific application and will closely parallel the

issues of concern with the liquid lubricated equivalent application. The major

difference is that process fluids are generally low- lubricity materials. Use

of a hybrid bearing generally provides better performance than an all-steel

bearing, but is very application-specific.

All-ceramic bearings, even made from lower-performance ceramics, may have

applicability in the chemical industry if they can demonstrate performance

advantages over steel bearings which frequently corrode quickly. Bearing

suppliers felt, however, that cost is generally a key issue in the chemical

industry and that prices have to drop significantly before they will be used.

Table 4: Process Fluid Lubricated Bearings

APPLICATIONS TECHNICAL ISSUES

o Performance Data

o Chemical Resistance Data

o Cage Materials

o Wear

o Coatings for Races

o Mounting Ceramic Races

o Chemical Industry

o Polyethylene Reactors

o Water/Slurries

o Freon/Freon Replacement

o Rocket Propellants

o Jet Fuel

31
Low- Speed Solid Lubricated Bearings

There are a wide range of low- speed solid lubricated applications for

hybrid and all-ceramic bearings which are being considered. Some of these and

the technical issues associated with them are listed in Table 5.

Table 5: Low-Speed Solid Lubricated Bearings

APPLICATIONS

HIGH TEMPERATURES

o T/E Nozzle Actuators

o Control Surface Actuators

o Rolling Mills

o High -Temperature Maunfacturing

Processes

o X-ray Rotating Cathodes

TECHNICAL ISSUES

HIGH TEMPERATURES

o Separator Materials

o Design

o Corrosion of Steel Races

o Oxidation of Ceramic

o Torque/Friction Increase

o High Loads

o Brinelling of Steel Races

o Thermal Cycling Effects

o Ceramic Material

APPLICATIONS

LOW TEMPERATURES

o Control Surface Actuators

o Fire Control Radar

o Swash Plate Bearings

o Electro-optic Actuators

TECHNICAL ISSUES

LOW TEMPERATURES

o Fatigue Life

o High Loads

o Expansion Coefficients

o Contamination Effects

High- temperature applications include some of the first applications in

turbine engines such as exhaust nozzle actuators. Low- temperature applications

of particular interest include radar and electro-optical actuators.

For these applications, adequate lubricants at the expected temperature

of operation, separator materials, fatigue life or wear life, and proper mounting

to accommodate the low thermal expansion coefficient of the ceramic are key

issues. For x-ray cathode bearings, the insulating properties of the ceramic

may be a problem since the current steel bearings are used to make the electrical

connection to the cathode.

High-Speed Solid Lubricated Hybrid Bearings

Table 6 lists the key issues which must be addressed in solid lubricated

hybrid bearings for high speeds. The factors listed are in addition to those

listed in Table 1 for high-speed liquid lubricated hybrids. Starred items

indicate key concerns

.

32
Table 6: High-Speed Solid Lubricated Hybrid Bearings

o Adequate 300- 500° C Race and Cage Materials*

o Solid Lubricant Formulation and Supply*

o Cooling Techniques

o Ceramic Material Optimization

o Wear and Fatigue Life Models*

o Clearance Control Over Wide Temperature Range

There is a significant payoff which can be realized in limited life turbine

engines such as those used in cruise missiles and drones by using solid

lubricated hybrid bearings. In order for this to be accomplished, however,

significant materials issues must be addressed. Key to success is finding

adequate race and cage materials and a lubrication system which can work to

provide stable bearing operation both mechanically and thermally at the speeds

and temperatures required. To date, investigation of solid lubricant coatings

has been unsuccessful. It may be necessary to use powdered solid lubricants in

an air carrier to achieve adequate life at the speeds of interest.

To arrive at a successful demonstration of such a bearing, the bearing

design must be approached as a system. The cooling required will be determined

by the friction coefficient which can be achieved and the ability to minimize

the heat generation in the bearing. Optimum race materials with one lubricant

may not be optimum with another.

A significant advantage of a solid lubricated hybrid bearing over an allceramic one is the ease of mounting and elimination of concerns about the brittle

fracture of the inner race at high speeds. Such a bearing will probably be

limited to 500° C because of the limitations of steel race materials. The

composition of the ceramic material will also play a more significant part in

the overall performance of such a hybrid bearing. It may be necessary to develop

a ceramic composition optimized for such bearings, which is different from

current materials that have been optimized for use in liquid lubricated

applications

Solid Lubricated Instrument and Space Mechanism Bearings

The issues which must be faced in solid lubricated instrument and space

mechanism bearings, were regarded as essentially the same as those for liquid

lubricated bearings for these applications. The main difference is that instead

of lubricant degradation life being an issue, solid film wear life is the primary

bearing life determining factor. Since this type of bearing is generally a lowtemperature application, operating at low loads and moderate speeds, the specific

chemistry of the ceramic is not a critical issue. Lubricant film adherence to

ceramic materials is clearly a key issue since current solid films were developed

to adhere to metallic bearings.

33
High-Speed Solid Lubricated All-Ceramic Bearings

Table 7 details the key issues which must be addressed for this

application. While the majority of the research funded by the Department of

Defense to date in ceramic bearing technology has focused on this application,

significant materials and systems design issues still remain for this type of

bearing.

Table 7: High-Speed Solid Lubricated All-Ceramic Bearings

o Cooling Requirements and Techniques

o Improved Race Fracture Toughness*

o Mounting Techniques for Ceramics on Metallic Shafts

o Lubrication Techniques and Lubricants*

o Cage Materials*

o Definition of Load Capacity

o Performance Testing and Life Demonstration*

o Shock Load Capability

o Wear Models and Wear Life Prediction

o Allowable Stress Levels

o Rolling Contact Fatigue Life Models

o Optimized Ceramic Materials

o Surface Modification for Improved Performance

o Non- destructive Evaluation of Ceramic Components

o Reinforced Ceramics for Bearing Applications

If all-ceramic bearings are ever to achieve the operating speed required

for use in a turbine engine, a critical issue which must be addressed is the low

fracture toughness of the current ceramic materials. Fracture toughness of

11-16 MPa-m1/2 must be achieved for safe operation at high-speeds. High fracture

toughness is particularly important with the higher surface traction values

present in a solid lubricated bearing which will tend to increase the tensile

stresses in the surface of the bearing races.

The development of such bearings requires a systems approach with lubricant

composition, ceramic composition, and cage material all requiring optimization.

Despite reasonable levels of funding for this area over the past 10 years, the

materials problems remain significant. Speeds achieved remain on the order of

50-75% of what is required. As the materials technology develops, performance

testing is required to evaluate the selection of the best materials.

The major design issue is mounting of the ceramic race if the system

requires use of a metallic shaft. While a number of approaches have been tried,

to date none has been completely satisfactory.

Of all of the applications of ceramic bearing components, this application

remains the most developmental despite the significant efforts which have been

devoted to this area. At the present, it is not possible to judge whether

current ceramics are adequate since lubricants have not been found which work

with them.

34
General Comments About Ceramic Bearings

In discussing ceramic bearing components in general, the following issues

were identified as the key ones:

o Cost

o Machining Techniques

o Near -Net -Shape Parts (Reduced Machining Costs)

o Material Composition

o Tougher Ceramic Materials

The sense of the group was that for ceramic bearing components to become

more widely used in the applications where their current performance is adequate,

costs must be reduced. While customers are willing to pay a premium for improved

performance, especially in military systems, price is an issue. This is

especially true in the lower-performance applications where steel bearing costs

can be quite low even for a high-precision bearing.

For all-ceramic bearings, costs are particularly an issue because of the

high cost of ceramic races even relative to ceramic balls. Table 8 lists some

ideas generated on potential applications for all -ceramic bearings with the

current state of the art of the materials. Some of the applications may evolve

because of the systems benefits which can be realized.

Table 8: All-Ceramic Bearing Applications

o Replacement for Non-Magnetic Bearings , Like BeCu

o Applications Requiring Electrical Isolation

o Chemically Aggressive Environments

o Fire Control Radar (Reduced Radar Interactions)

o Chemical Laser Blowers

o High -Temperature Mechanisms

o High -Load Non-Magnetic Gimbals

o Highly Abrasive Environments

o High- Stiffness Mechanisms

35

Section 3

Technical Presentations

37

Advantages of and Issues in the Application of Ceramic

and Ceramic Hybrid Bearings

James F. Dill

Mechanical Technology Inc

.

Latham, NY 12110

This presentation focuses on the current and future potential applications

for ceramic and hybrid ceramic bearings. The application focus is on military

and government systems, but extensions are made where possible to commercial

applications

.

The potential applications are broken down into four possible classes

including high DN and marginally liquid lubricated ones and hybrid and all

ceramic solid lubricated ones. The requirements and performance advantages are

discussed for each class of application.

For marginally liquid lubricated bearings, there is documentated evidence

that heat generation and lubricant degradation are reduced in hybrid ceramic

bearings. This means that performance advantages can be realized in a wide range

of applications including grease and mist lubricated bearings, process fluid

lubricated systems and lubricated for life systems like gyroscopes and gimbal

bearings

.

In high DN applications, the low density of the ceramic rolling elements

in a hybrid bearing can improve life by reducing the centrifugally induced

stresses on the outer race. If the outer race stress controls the bearing life,

a life improvement will be realized. Whether such an improvement can be

achieved, is a function of the details of the externally applied loads and the

centrifugally induced stresses. For some of the more frequently mentioned

applications of this type, such as turbine engine mainshaft, current bearing life

of all-steel bearings may be more than adequate making any theoretical

improvement of little importance when the higher costs of ceramic components are

considered.

Solid lubricated hybrid bearings for low speed applications may be the

first turbine engine application of this technology. When hybrid bearings are

used in such applications as exhaust nozzle actuators significant improvements

in wear life may be possible.

Solid lubrication of all-ceramic bearings has been the focus of much of

the government funded research and development. If this technology can be

successfully developed it could have a significant impact on the design of

advanced turbine engines and other high speed machinery. However, substantial

technical barriers still remain to the application of this technology in any real

system. Emphasis on research in this area has possibly skewed the interest of

American producers relative to foreign suppliers, especially the Japanese, who

are much more focused on near term applications like liquid lubricated hybrid

bearings. This emphasis has given the Japanese at least a perceived advantage

in technology implementation at the current time.

39
Factors inhibiting the full utilization of ceramic bearing technology

include material properties such as fracture toughness and thermal expansion

coefficient, finishing techniques including machining and NDE, and cost issues.

The current cost of hybrid bearings relative to all-steel bearings of 2-5 times

cannot be justified in many of the lower risk applications where experience could

be gained with the material. If applications are to expand significantly prices

need to be reduced to 1.2-1.5 times that of all-steel bearings.

The presentation concludes with a summary of the key issues for liquid and

solid lubricated applications, NDE, and materials testing which must be addressed

if the application of ceramic bearing components is to be expanded.

40
Advantages of and Issues in the

Application of Ceramic and Ceramic Hybrid Bearings

bv

Dr. Jim Dill Mechanical Technology Inc.

April 17, 1991

MECHANICAL TECHNOLOGY INCORPORATED

7J.TI

HIGH COST LIMITS APPLICAT

LIMITED APPLICATIONS IN

HIGH COST LIMITS APPLICATIONS

LIMITED APPLICATIONS INCREASE

HIGH COST LIMITS APPLICATIONS

LIMITED APPLICATIONS INCREASE COST

HIGH COST LIMITS APPLICATIONS

41
Applications of Hybrid Bearings

As '4 '^/^<?/&/Jy&/

Ciulse Missile Malnshafls y y •y y y y

Air Startar6 V •y y y •y

Auxiliary Power Unlls V y y V -y

Gyro Platforms V •y

Gyro Gimbals V y

Machine Tool SplndleB V V V y y

Turbochargers V V y

X-ray Tubes y y V •y

Submarine Machinery y y y

Vacuum Pumps y -y V

Centrifuges •y V •y y

Disk Drives V V V V -y V

Ctiemlcal Applications -y *y •y V

Nuclear Applications V y y •y

Dental Drills V V y y

Helicopter Gearboxes V m V V V y

Furnace Mechanisms V V

Satellite Gimbals y V

Satellite Momentum Wheels V V y V V V

Marginally Liquid Lubricated Applications

In marginally liquid lubricated systems, there is documented

evidence of lower heat generation and better overall performance

in hybrid bearings.

Technical Issues: • Reduced contact area and contact microslip reduce

heat generation.

• Possibly some reduction in surface initiated failures (may be due to dissimilar materials).

Applications: • Grease and oil mist lubricated machine tool spindles • Fuel lubricated expendable turbine engine bearings

• Computer disc drives

42
Marginally Liquid Lubricated Applications

Possible Extensions: • Fuel pumps, fuel hydraulic systems, fuel controls • Space Gimbals, momentum wheels, etc. • Motors, APU's

• Turbopumps (?)

Comment:

• May be desirable to explore possible performance

advantages of hybrid bearings with high temperature

liquid lubricants like PPE and PFAE.

Status:• Current materials are more than adequate for this application. Reduced costs will expand use.

High DN Liquid Lubricated Applications

In high DN bearings (generally 2 + MDN), it may be possible to improve bearing life by using ceramic rolling elements to reduce

outer race stresses.

Technical Issues: • Low density of Silicon Nitride (40% of steel) reduces

the centrifugally induced forces on outer race.

• If centrifugal force due to rolling elements controls bearing life, an improvement will be seen.

• High modulus of ceramic can cancel benefits of lower

density in many applications.

Applications/Extensions: • High speed spindles • Turbine engine mainshaft (?) • Air cycle machines

• Any high DN bearing application

43
Effect of Centrifugal Force on Outer Rig Stress

4 1 1 1 1 1

Imin-'] 0 15 30 45 60 75 xlO 3

Inner ring speed n

0 1 2 3 * x 10* [mm-min'M PU x n

High DN Liquid Lubricated Applications

Comment:

• Depends heavily on balance of external load and total speed range expected. Demonstration of advantage depends

on life calculation technique.

• May solve a current nonproblem, i.e., fatigue life is not an issue in the current cruise missile engine for example

or in most man-rated engines.

• Essentially, a ball bearing problem since roller bearings

are generally not life limited because of low stresses.

Status:• Current ceramic materials are more than adequate.

• Clear advantage not seen in many applications.

44
High DN Solid Lubricated Hybrid Bearing Applications

A hybrid bearing capable of operation to 2.5 x 10 DN and

temperatures to 600 - 1000°F with solid lubrication would permit

several advantages in cruise missile engine design to be realized.

Advantages Cited: • Weight, cost and engine envelop reduction via elimination

of lubrication system and gearbox.

• Possible advantages in engine storability and low temperature

starting may be achieved also.

Comments:

• Partial benefit could be realized by eliminating gearbox

and using electrical lube pumps.

• Fuel lubricated bearings may offer comparable advantages.

• A supplemental solid lubricant supply like powder lubrication may be required to meet strategic missile engine life require- ments.

Status:• Current work has shown little improvement over

previous all steel bearing work.

• Significant work on tribomaterials system (races/rolling

elements/lubricants) and lubrication approach required.

Low Speed Solid Lubricated Lubricated Applications

At low to moderate speeds and temperatures to 600 - 1000°F, solid lubricated hybrid bearings show improved wear life when compared to

all steel bearings.

Technical Issues: • Smaller contact area, reduced microslip and dissimilar materials in contact combine to give overall reduced wear

and improved life.

• Differences in expansion of races and rolling elements may

make clearance control over entire temperature range difficult.

Applications/Extensions

• Turbine engine auxiliaries - air motors, nozzle actuators • Space gimbals and other mechanisms

• X-ray tube rotating cathodes

Comments:

• This may be the first real turbine engine application • High friction in vacuum may require improved solid lubricants

Status:• Current ceramic materials adequate

• Improved solid lubricants and steel races could expand applications

45
High DN Solid Lubricated All Ceramic Bearing Application

Solid lubricated all ceramic bearings capable of operating at temperatures up to 1500°F or above and 2.5 - 3.0 MDN would offer significant advantages in advanced turbine engines.

Technical Issues/Status:

• The bulk of ceramic bearing research funded by the Air Force and Navy has been directed ultimately toward this application.

• Fracture toughness of current materials is inadequate.

• Significant issues in materials, lubricants, mounting,

lubricant supply and cooling need to be resolved.

• Despite advances in material science, results to date have

been disappointing when compared to 25-year old Fairchild- Stratos work.

• Ceramic material requirements differ from those for liquid lubricated hybrid bearings.

• Research emphasis in this area has biased American efforts

relative to Japanese.

Mainshaft Bearing Lives and Expected

Failure Mode as a Function of Speed

for a Constant Bore Size

Silicon N itride M-50 ~18 ksi Vin.

1.0

' f K(c -4.1 ksi Vin. \^Silicon Nitride Kte ~7 ksi Vin.

ue

Life \ \ Rolling Contact \ \ Fatigue Modes —

\ \ Expected

Mixed Modes

Fracture Modes

Expected

'c3 0.5

<D>

CD rr

n I l il L_»- 0 1.0 1.5 2.0 2.5 3.0

DN ~ Millions

91616

46
Other Applications Where Ceramic Components

Offer an Advantage

• The lower density of silicon nitride will reduce cage forces

at high acceleration rates. Hybrid bearings may perform better than all steel as backup bearings for magnetic bearings.

• Ceramic rolling elements may reduce noise resulting in quieter

bearings.

• All ceramic bearings offer chemical inertness superior to steel. The Japanese are investigating lower performance SiC and Si3N4

materials in process lubricated applications.

• In electrical applications, ceramic parts improve isolation.

• Improved precision of ceramic balls can improve performance

in a wide range of applications.

Manufacturing Related

Applications Issues

Cost:

- Hybrid Bearings Currently 2-5 X Steel

- Ball Cost 10 X Steel

Machining:

- More Efficient Machining Techniques

- Minimization of Material Removal

- Variation of Parameters with Composition

- Narrower Spread of Ball Sizes (Race Matching)

NDE:

- Bulk Defects

- Surface/Machining Damage

- Flaw Size

47
Steel vs Ceramic Part Finishing

Steel Ceramic

• Bulk of Machining in Softened State

• Bulk of Machining in Hardened

State

• Final Grinding of Hardened

Parts Removes Small Amount

of Material

• Diamond Grinding Required to Remove Relatively Thick Surface

Layer from HIPPING.

• Even in Hardened State, Steel Parts have a much

Lower Hardness than Ceramic

• High Hardness Makes Machining

Very Difficult and Costly

• NDE Done on Simple Shapes,

Rods and Tubes

• Near Net Shape Forming Precludes

Inspection of Simple Shapes

• NDE for Bulk Defects Done

on Raw Stock

• NDE Later in Process Done on

Higher Value Added Part

• Defect Size within Current NDE Capabilities

• Lower Toughness Results in Critical Flaw Size at Limits

of NDE

Materials Related

Application issues

Composition:

- Bearing Quality Silicon Nitride Not a Single Composition Like Steels - Optimum Material Composition May Differ for Liquid Lubed and High Temperature Solid Lubed Applications

Physical Properties

- Fracture Toughness - critical for all ceramic bearings

- Thermal Expansion - control of internal clearance

- Elastic Modulus - increases stresses at high applied loads

- Density - decreases stresses at high speeds

48
Design Related

Application Issues

Analysis:

- Life Prediction Techniques (endurance limit) - Optimization of Internal Geometry

- Control of Clearance Over Wide Temperature Range

- Mounting of All Ceramic Bearings

Performance Testing: - Rolling Contact Fatigue • of ceramic

• effects on steel - Wear

- Full Scale Bearing Tests

Suspended Tests

Reasons

gHH Ball Spoiling Eg Max. Cycles EEB Rod Wear £22 Rod Spoiling

49
Volumetric Wear/Cycle

120 Hour Tests

Material

Material A

Track Life & Wear

1 2 3 4 5 6 7 6 9 10 Track Number

Life LH Wear

50
Material C

Track life & Wear

KEY TECHNICAL CONCLUSIONS

Liquid Lubricated Applications

• A number of current silicon nitride materials

are adequate for many hybrid bearing applications.

• Parts are expensive both because of blank costs and finishing costs.

• Applications for liquid lubricated all ceramic

bearings are limited (corrosive environments, nonmagnetic requirements,

electrical isolation).

• Advantages/disadvantages must be carefully weighed

- lower centrifugal forces on outer race do not always lead to increased bearing life.

• Hybrid roller bearings have a narrower range of usage

and are more difficult to produce - consequently, they

have received less development attention.

51
KEY TECHNICAL CONCLUSIONS

Nondestructive Evaluation

• NDE is limited and thus limits applications

to some extent.

• Costly NDE additions driven by perceived high

end market - T/E mainshaft - may raise prices

and kill real lower risk applications that don't require such stringent controls.

• For many/most near term applications, good process

control coupled with sampling plan and destructive

analysis or NDE may be adequate. Especially when

coupled with a process control specification.

• Need to consider where NDE is applied in the manufacturing

process and if certain rough part geometries are easier to inspect.

• Need to compare NDE requirements for ceramics in a given application with steel components in the same

application.

KEY TECHNICAL CONCLUSIONS

Solid Lubricated Applications

Low Speed/Low Temperatures:

• Advances in Solid Lubrication for Space

Mechanisms Have Been Significant

• Current Ceramics are Adequate

High Speed/High Temperatures: • Improved Fracture Toughness Required to Meet DN Needs for All Ceramic Bearings

• Hybrid Bearings with High Temperature Metallic Races May Be Nearer Term Than All Ceramic Bearings

• Mounting Remains a Major Issue

• Significant Advances in Solid Lubrication Are Still Required

• Optimum Ceramic Compositions Can Only be Determined

in Conjunction with Lubricant Development

52
'I

!

KEY TECHNICAL CUNCLUblUNb

Materials Testing

• High Stress Rolling Contact Fatigue Tests May Overload Some Good Materials

• Optimizing a Material for Rolling Contact

Fatigue May Negatively Effect Other Properties

(Fracture Toughness vs Hardness)

• Standardized Tests are Needed for Both Materials Optimization and Machining Optimization

• Need to Understand Both Fatigue and Wear Properties

of Ceramic Materials

• Composition/Finish of Ceramic Parts May Affect Steel

Part Life

53

Payoffs and Challenges in Utilization of Ceramic Components

in Spacecraft Mechanisms

Paul D. Fleischauer

Mechanics and Materials Technology Center

The Aerospace Corporation

El Segundo, CA 90009

Technology advancements in spacecraft mechanical systems and components

are critical if proposed lifetime and performance requirements for advanced

programs are to be met. Equivalent advancements have been made at a far more

rapid pace in other technology areas, vis. electronics, optics, power, and

thermal control; and, consequently, tribological mechanisms are becoming the

limiting systems for present programs. Ceramic parts and coatings offer a

relatively uncharted avenue for improvements in operating performance and life

of precision bearings and gears and for some actuators. Although the volume of

parts and production for space applications is small, at over 100 million dollars

per spacecraft, the potential payoffs are very large. As an example, a recent

extension in the predicted life of a satellite from three to five years, due to

the incorporation of a synthetic oil in its scanner bearings , may mean that two

of the projected vehicles in the current series will not need to be built

(savings of $200 to 300 million) . Ceramic bearings in that system could extend

the life even further.

The mechanisms that experience tribological problems and continue to plague

spacecraft designers and engineers include solar array drives, momentum and

reaction wheels, antenna and sensor pointing systems, sliding electrical contacts

such as those found in slip-ring assemblies, and cryopumps and coolers. Problems

are manifested as pointing errors, deployment failures, attitude instabilities,

electrical noise generation, and cooling/pumping abnormalities. Causes of the

problems invariably are associated with lubricant failure, friction noise, and

wear

.

The approach we have found to be successful in resolving these types of

operational problems combines fundamental materials properties research with

analytical models of the application and simulation testing in the laboratory.

The results are finally inserted into life tests and then into satellite

construction. The payoffs are realized in terms of improved surface finishes

and wear resistance of moving, contacting parts (which produce lower torques and

torque noise, longer operating lifetimes, and less power consumption), reduced

weight (which lowers launch and operating costs) , and improved strength (which

provides the possibility for reducing the complexity and cost of design)

.

Specific examples of applications of ceramics for space systems include

the demonstration that TiN coatings or Si 3 NA balls in boundary applications of

perfluoropolyalkylether oils can extend operating life up to a factor of ten,

and the testing of coatings and solid ceramic parts in the spin bearings and

gears of certain momentum wheels. The opportunities for additional insertion

of ceramics are significant and are just beginning to be appreciated.

55
The challenges for effective use of ceramics in space applications are

probably similar to those in other areas. They include establishing the utility

of lubricants and additives on relatively unreactive, often primarily covalent,

ceramics compared to reactive metals; determining their wear rates when

lubricated in sliding, rolling, and mixed contact regimes; and understanding the

behavior of lubricants in boundary and EHD regimes on ceramic surfaces. The

latter point is essential if models of bearing and gear performance are to be

applied to determine satellite design criteria.

Some research on bonding properties of ceramic surfaces has been done, but

much more is needed to fully exploit these materials in space applications. For

example, a common additive for mineral (and synthetic) oils under boundary

conditions, lead naphthenate , provides protection on steel during wear by forming

a protective lead metal layer. The formation of this layer appears to require

the presence of metallic iron on the steel surface. Nonmetallic nitrides

probably cannot provide the necessary reducing agent for lead formation, so this

probably is not a good additive for those materials. Other chemical

compatibilities and reactivities need to be well understood before the proper,

effective combinations of material and lubricant can be identified.

56
Payoffs and Challenges in the Utilization of Ceramic

Bearings in Spacecraft Mechanisms

Paul D. Fleischauer

Mechanics and Materials Technology Center

The Aerospace Corporation

El Segundo, CA 90245

Presented to; NOT/MRM

tamk learing Teehftiolefg Worksnep

17 April 111!

Ceramic Components in Spacecraft Mechanisms

OUTLINE

Introduction - Spacecraft Mechanisms

Types of Theological Problems

Lubricant Degradation vs. Parts Failure (Wear)

Payoffs for Ceramics - Performance Improvements

Challenges for Ceramics - Research/Testing Needs

Coatings vs. All Ceramic Parts

Summary

57
MOVING MECHANICAL ASSEMBLY FUNCTIONS

DEPLOYMENT k

ARTICULATION

SENSOR k

ANTENNA

POINTING

STABILIZATION/

ATTITUDE CONTROL

CONDUCTION THROUGH

ROTATING INTERFACES

58
TRIBOLOGY FOR SPACE SYSTEMS IN THE NINETIES

Tribokgy rapidly becoming weak link

TRIBOMECHANISM/COMPONENT PERFORMANCE

Current tribilagg problems wiih active spacecraft

• Number one recurrent problem is wiih bearings in CMGs

also with momentum/reaction wheels k gyroscopes

i Second leading problem is with actuators, gears, k gimbals

operating in boundary lubrication regime

. Third problem concerns low noise operation of slip rings

• Other problems with turbo machinery, cryopumps, coolers

59
Lubrication Problem Solving

M/91-6400

Ceramic Components in Spacecraft Mechanisms

Pageffs hi Ceramics - hthm&m® Imprsvimifite

Improved surface finishes and wear resistance

Lower torque & noise - Better pointing control

Longer operational life

Less power consumption

Reduced weight - Lower (launch fit orbit) costs

Improved strength - Potential reduction in complexity/cost

60
Lubrication of DMSP OLS

PROBLEM:

LUBRICANT LIFE FOR

5-YEAR MISSION

OLS OSCILLATING ASSEMBLY

61
Control Moment Gyro

FIXED END

FLOATING END

Tribology Components: Requirements/Technologies

Tribology <§s ^ ^ <^"" ^

Requirements <> x#

^ ^ ^

^ ^ & </ <T ^ # <f

Spin Bearings X II X X X

Gimbal Bearings X 13 X X 1x0 X X

Sensor Supp. Brgs. X X X X X X X

Motor, Synchronous XDIXlXX XXX

Solar Array Brgs. X 11 X 0! X X B X X

Mixed Regime

Gears X X X H X X H X x

Harmonic Drives X 1 X LI X '

Hi x

Technology % % % Q % % #

"Solution" \ ^ % %

X X \ \ *

%-.

62
Wear Life as a Function of Hertzian Stress

LU

LL-J

EC<

LU

10'

w% 10 7

10'

10

<B> 440C/440C

© Si 3 N 4/440C

A TiN/TiN

- .

I

I 1.10 1.15 1.20 1.25 1.30 1.35

HERTZIAN STRESS, Pa x 10 9

1.40

Ceramic Components in Spacecraft Mechanisms

Challinps for Cinmiei = RistgrehfTisting Needs

Effectiveness of lubricants with ceramic vs metal surfaces

Surface chemical ineractions

Electronic structure considerations

Lubricated wear rates in rolling, sliding, & mixed regimes

Boundary versus EHD lubrication

Surface roughness

Film thickness

63
440 C Stainless Surface Composition

6-7 minutes- removes Fe oxides

1 0-1 1 minutes - removes Cr oxides

THE AEROSPACE CORPORATION

EL SEGUNDO, CALIFORNIA

Lead Naphthenate on 52100 Bearing Steel

LEAD IRON

COoo

1 'a

l(a) u

aAa

L-AA , 154 150 146 142 138 134 130 715 713 711 709 707 705 703

BINDING ENERGY, eV BINDING ENERGY, eV

All samples were heated to 250 C

Unscratched, alkaline washed

Scratched, solvent cleaned

Unscratched, solvent cleaned

64
VALENCE LEVEL PHOTOELECTRON SPECTRUM

Molecular Orbital Energy Level Diagram

From DV-Xa Calculation

C 2p

c 3v si„cA^4

E (eV)

-

' 7

Si 3p

C 2s

Si 3s

-9

- -10

65
Core Level XPS Spectra

Sputter/Anneal Cycles

O Is Si 2p

107 101

Binding Cntrgy (eV)

66
Ceramic Componenis in Spacecraft Mechanisms

Surface additives often require chemical reaction with steel

Lead formation requires metallic iron

TCP forms "active" phosphate

Ceramics generally covalent, nonmetallic

Knowledge of bonding orbitals/energy levels allows

selection of proper additive(s)

Tests (component level, life tests) required for verification

Test Facilities For Ceramic Bearings

Boundary Wear Tester

Dry Nj Gox

Jijr* it-*, a* J"D

TJTTB

CMG Spin Bearing Spindles

Dcod Weight Load

Soaring .

I p=u

Molor

— Supports

'

-Tnchomalor

Thrust or Angular Contact Br g . Life

Gimbal Bearing

Performance

"Z3-

i-U67
Ceramic Components in Spacecraft Mechanisms

Ceramics provide potential for significant increases in

performance and life of spacecraft mechanisms

Payoffs include reduced torque and noise, reduced wear,

reduced weight; all of which lower cost ($$$]

Challenges involve lubrication schemes and verification testing

Must convince programs to use what is available!!

68
Hybrid Ball Bearing For Naval Applications

Gerry J . Phillips

David Taylor Research Center

Annapolis, MD 21402

Ball bearings comprised of steel rings and silicon nitride balls have

undergone extensive testing and evaluation for application in naval auxiliary

machinery. These hybrid bearings have outperformed standard all-steel bearings

in every test conducted to date and shipboard evaluations are being pursued.

Key advantages demonstrated over standard bearings include extremely good wear

resistance, electrical insulation, lower preload requirements and excellent shock

resistance

.

69

HYBRID BEARING DEVELOPMENT

NOISE

TESTS

SKID

TESTS

ELECTRIC

ARCING

TESTS

CONTAMINATION

TESTS

SHOCK

TESTS

FATIGUE

LIFE

LITERATURE

SEARCH

SINGLE

BEARINGS

DUPLEX

BEARINGS

STEEL

BEARINGS

r _L_ n

I TDC l

l STEEL l

1 I

HYE BEAURID

INGS

TDC

HYBRID

STEEL

BEARINGS

HYBRID

BEARINGS

NOISE TEST RESULTS

3/4" BALL BEARINGS

ANDERONS

BAND LOW MID HIGH

HYBRID 15 4 7

STEEL 18 8 15

NT-4 LIMITS 24 17 18

NT-3 LIMITS 24 20 20

FAFNIR SIZE 310 CARTRIDGE TYPE

71
NOISE TEST RESULTS

1-1/16" BALL BEARINGS

ANDERONS

BAND LOW MID HIGH

HYBRID 11 5 13

STEEL 12 6 19

NT -4 LIMITS 32 21 22

NT -3 LIMITS 32 24 32

BARDEN SIZE 2315 ANGULAR CONTACT

NOISE TEST RESULTS

1-3/16" BALL BEARINGS

ANDERONS

BAND LOW MID tilQti

HYBRID 26 44 169

BARDEN STEEL 22 10 19

NT-4 LIMITS 32 23 25

NT-3 LIMITS 40 24 32

CERBEC SIZE 318 DEEP GROOVE

72
FATIGUE LIFE

LITERATURE SEARCH

• FATIGUE DATA AVAILABLE FROM NAVAIR PROGRAMS 1970-1980

• Si3N4 (NC 132) SUPERIOR WEAR AND FATIGUE LIFE

• SPALLING FAILURES SIMILAR TO THAT OF STEEL BALLS

• BILLET-PRESSED MATERIAL SIGNIFICANTLY BETTER THAN

NEAR-NET-SHAPE CERAMIC MATERIAL

• FATIGUE LIFE STRONGLY INFLUENCED BY FINISHING PROCESS

• FATIGUE LIFE 10 x THAT OF M50 BEARING STEEL POSSIBLE

RECOMMENDATION

• ENDURANCE PROOF TEST FOR NEW BEARING DESIGNS

ELECTRIC ARCING TEST RESULTS

STANDARD BEARING

• CAN BE DAMAGED BY ELECTRIC ARCING

• EXTENT OF DAMAGE DEPENDS ON ENERGY

OF DISCHARGE

HYBRID BEARING

• CERAMIC BALLS DO NOT CONDUCT

ELECTRICITY

• BEARING CANNOT BE DAMAGED BY ELECTRIC

ARCING

73
5

ccQ

ui

l±J a.

Cfl CJ2

DC<

LUm

SKID TEST HYBRID BEARING SN 6

THRUST 500 LBS ROYCO 28 GREASE

4 MAR 1988

5.00

4.98

4.96

4.94

4.92

4.90

4.88

4.86

4.84

4.82

4.80

4.78

4.76

4.74

4.72

4.70

I*

******* *^*«

12 16 20

RPM/100

24 28 32 36

INDIVIDUAL BEARING SKID TEST RESULTS

03

_J

II—(A

OCI

H

o

o

DC>

C3

1000

800 -

1000 2000 3000 4000

SHAFT SPEED (RPM)

74
BEARING LOADING ILLUSTRATED

POSITIVE

FORCE

SKID TEST RESULTS

DUPLEX PAIRS (DB) 3600 RPM 1000-LB PRELOAD

75
CONTAMINATION TESTS

HARDWARE

MAIN FEED PUMP MOTOR

7315 DUPLEX BEARING (1,000 lb Preload)

DUPLEX BRG UPPER END CAP REMOVED

ACCELEROMETERS ON BRG HOUSING

FOBM ON BRG RING

CONTAMINATION TESTS

CONTAMINANT

CARBON DUST - From DC Generator

8 GRAMS - Over 5 Hours

76
CONTAMINATION TESTS

SEQUENCE

STEEL BEARINGS FIRST

CONTAMINANT ADDED UNTIL

NOISE LEVELS DRIVEN UPWARD

- REPEAT SAME CONTAMINATION

AMOUNT/TIMES WITH HYBRID

REPEAT SAME WITH TDC BRGS

77
RANGEi S3dBBV

A: MAG

120

dBA

STATUS: PAUSED

RMSi 10

40

HYBRtP

Xt 1800 Hz

Y: 88. 72 dB

A

START: 0 Hz BW: 37. 5 Hz STOP: 10 000 Hz

CONTAMINATION TEST RESULTS

(ANDERONS)

BEARING CONDITION

STANDARD

BEARING

HYBRID

BEARING MIL-B-T7931

LOW

BAND

BEFORE 14 AFTER (WITH CARBON DUST) 32 AFTER (CLEANED) 2813

19

13

40

MEDIUM

BAND

BEFORE 7 AFTER (WITH CARBON DUST) 80 AFTER (CLEANED) 807

1010

25

HIGH

BAND

BEFORE 24 AFTER (WITH CARBON DUST) 440 AFTER (CLEANED) 4201650

55

26

78
TDC*

CORROSION PROTECTION

- WEAR RESISTANCE

FATIGUE LIFE

« LUBRICATION

10x Better Than Stainless

5x Better Than Std Brgs

2x Longer Than Std Brgs

Less Sensitive To

Inadequate Lubrication

• THIN DENSE CHROME

CONTAMINATION TEST

TDC HYBRID

15

IMPACT ACTIVITY

1.8

1.6 jz

O

»<

<

1.2 Q_

NOISE LEVEL

0.8 100 200 300 TIME, MINUTES

400 500

79
CONTAMINATION RESULTS

MIL STD HYBRID TDC

SPEC BRG BRG HYBRID

LOW BEFORE 40 14 13 18

BAND AFTER 32 19 20

CLEANED 28 13 15

HID BEFORE 25 7 7 7

BAND AFTER 80 10 10

CLEANED 80 10 13

HIGH BEFORE 26 24 16 18

BAND AFTER 440 50 82

CLEANED 420 55 83

outer

Inner

80
52'/. CERAMIC

^7 1

STEEL INNER RACE

CONTACT

STEEL^

x 57* CERAMIC INNER RACE

CONTACT

81
HYBRID BEARING SHOCK TESTS

MIL-S-901C

BLOW

NO.

PLATFORM

ORIENTATION

MOTOR

CONDITION

DROP

HEIGHT

TABLE

TRAVEL

1 LEVEL RUNNING 1.75' 3"

2 LEVEL STILL 2.75' 3"

3 LEVEL RUNNING 2.75' 1.5"

4 30 DEGREES STILL 2.25 3"

5 30 DEGREES RUNNING 4.0' 3"

6 30 DEGREES STILL 4.0' 1.5"

82
HYBRID BEARING SHOCK TESTS

MIL-S-901C

PEAK G'S

1 2 3 4 5 6 BRG CAP V 74 88 86 66 82 78 STEEL

SHAFT 129 84 102 BEARINGS

BRG CAP H 12 33 10 41 37 -43

PEAK G'S

1 2 3 4 5 6 BRG CAP V 73 92 86 68 78 72 HYBRID

SHAFT 137 BEARINGS

BRG CAP H 22 18 18 20 16 41 MAX BEARING SHOCK = 137 g's = 51,642 lbs. AFBMA STATIC THRUST CAPACITY = 46,772 lbs

83
SHOCK TEST RESULTS

BALL DAMAGE

TYPICAL STEEL BALL TYPICAL CtRAMIC BALL

(2.5x) ' 2 ' 5*>

84
SHOCK TEST RESULTS

AXIAL POSITION OF ROUNDNESS TRACES

85

NEW BEARING ROUNDNESS

TRACES

87
SHOCK TEST RESULTS

STEEL BEARING NO. 8

88
SHOCK TEST RESULTS

STEEL BEARING NO. 8

89
90
SHOCK TEST RESULTS

STEEL BEARING NO. 7

91
SHOCK TEST RESULTS

HYBRID BRG. NO. 1

92
93
SHOCK TEST RESULTS

HYBRID BRG. NO. 2

94
SHOCK TEST RESULTS

HYBRID BRG. NO. 2

95
SHOCK TEST RESULTS

RING RACEWAY DAMAGE

ECCENTRICITY DENTS

NEW BEARING 20

UPPER STL #8 50 10

LOWER STL #7 45 5

UPPER HYB #1 55 10

LOWER HYB #2 60 5

ADVANTAGES OF HYBRID BEARINGS

• LIGHTER WEIGHT - 8TD BRG 7.0 LB, HYB BRG

5.7 LB

• HIGHER SPEED - GREATER DN CAPACITY

• REDUCED SKIDDING - LOWER PRELOAD

• BETTER WEAR RESISTANCE

• LONGER FATIGUE LIFE

• LESS SENSITIVE TO LUBRICATION PROBLEMS

• SUPERIOR CORROSION RESISTANCE

• ELECTRICALLY NON-CONDUCTIVE

• HIGH TEMPERATURE CAPABILITY

96
HYBRID SUMMARY

5jc ALL TESTING OF CERAMIC BALLS SO FAR

HAVE SHOWN POSITIVE RESULTS.

5fc SILICON NITRIDE IS AN EXCELLENT MATERIAL

FOR BEARING APPLICATIONS.

5fc TDC RACEWAY COATINGS ARE NOT RECOMMENDED

AT THIS TIME.

>fc SHIPEVAL OF HYBRID BEARINGS IS RECOMMENDED.

Instrument Bearing Requirements and Issues

Robert A. Westerholm

Litton Guidance and Control Systems

Woodland Hills, CA 91365

Ceramics for instrument ball bearings have advantages beyond recognized

high strength, corrosion resistance, and high temperature properties for which

they are usually selected.

The author's company manufactures inertial navigation systems which use

instrument bearings in both moderately high speed gyroscope rotors and low speed

or oscillatory gimbals. Rolling element instrument bearings may be defined as

bearings which:

1. Accurately maintain the position of a rotating element,

2. Are smooth running with limited vibration,

3. Have very low, very stable torques.

Applications include gyroscopes, tape and disc drives, encoders,

flowmeters, etc.

Because of moderate load requirements instrument bearings do not fail by

fatigue of the bearing races or balls but by combined lubricant breakdown and

surface wear.

Re -lubrication is usually impossible, bearings are often inside hermetic

containers and sometimes in spacecraft. Therefore, they are lubricated once for

life.

A program to develop improved ceramic coatings for ball bearings was begun

under subcontract to Hughes Aircraft Co. and the sponsorship of DARPA and AFML.

An extensive survey of potential materials and processes identified chemical

vapor deposited (CVD) hard coatings from a Swiss laboratory as a potential

technology for instrument ball bearings.

Sputtered hard coatings, not surprisingly, did not adhere strongly enough

for ball bearings. Tests of CVD coatings were first aimed at proving the

strength and adherence of the coatings. The test vehicle was a half- inch

diameter bearing from a production gyroscope for which test equipment and a large

body of experience existed. The bearings contained eight, three thirty- second

inch diameter balls with a conventional porous phenolic retainer impregnated with

a hydrocarbon oil.

Bearings of several configurations were tested while axially loaded

(thrust) at about ten times normal loads; maximum hertzian stress were about

300,000 to 400,000 psi. Material combinations included titanium carbide (TiC)

coating over tungsten carbide (WC) balls, titanium nitride (TiN) coating over

99
titanium carbide (TiC) balls, titanium carbide (TiC) coating over steel balls

and steel races, and all-steel bearings.

Results were that (1) coating integrity was excellent; there were no

coating failures. (2) Performance of both coated steel and coated ceramic balls,

as well as coated steel balls with coated steel races, was uniformly excellent

with virtually no wear, while all-steel bearings generally suffered significant

wear. The hybrid bearings with ceramic ball surfaces on steel performed as well,

for these conditions, as ceramic balls with ceramic coated races.

A life test program followed. With loads about three times normal bearings

were run in excess of 20,000 hours in many instruments. Bearing life was

extended from two to more than five times

.

As a result, bearings with TiC coated balls were eventually qualified

introduction gyros; and many thousands of gyros have since been delivered. In

addition silicon nitride balls have been incorporated in gimbal bearings for

production systems and additional applications are being considered.

To summarize: Ceramic and ceramic coated balls in steel bearings have been

found to substantially improve life, reliability, and production yields in

guidance system gyroscopes and gimbals while at the same time improving

performance. Improved materials and processes, reduced cost, and adapting

designs to ceramic properties are goals being pursued.

100
LITTON GUIDANCE & CONTROL SYSTEMS INTRODUCTION

o OBVIOUS ATTRIBUTES OF CERAMICS

o OTHER USEFUL ATTRIBUTES

BACKGROUND - LITTON BEARING APPLICATIONS

o INERTIAL GUIDANCE SYSTEMS

o GYROSCOPES WITH SMALL INSTRUMENT BEARINGS

o GIMBAL SYSTEMS WITH LARGER "INSTRUMENT1 BEARINGS

INSTRUMENT BEARINGS

o ACCURATELY MAINTAIN THE POSITION OF A ROTATING

ELEMENT

o SMOOTH RUNNING, LOW VIBRATION

o LOW, UNIFORM TORQUES

o LOW TO MODERATE LOADS

o LESS THAN 30 MM DIAMETER

o RELUBRICATION NOT APPLICABLE

o MINIMAL LUBRICANT AVAILABLE

vugrawl 17 April 1991

LITTON GUIDANCE & CONTROL SYSTEMS

INSTRUMENT BEARING FAILURE MODES

o METAL FATIGUE FAILURES DO NOT OCCUR

o LUBE BREAKDOWN ACCOMPANIED BY LIGHT WEAR

o PRECISION LOCATION AFFECTED

o TORQUES AFFECTED

vugrawl 17 April 1991

101
LITTON GUIDANCE & CONTROL SYSTEMS

PROGRAM TO IMPROVE GYRO BEARING PERFORMANCE IN GYRO

BEARINGS

o ENTERED AN EVALUATION PROGRAM TO IDENTIFY IMPROVED

MATERIALS AND OR PROCESSES

o DARPA/AFML/PROGRAM TO DEVELOP HIGH TEMP, SOLID

LUBRICATED BEARINGS

o EXTENSIVE SEARCH FOUND CVD CERAMIC COATED BEARINGS

IN ADVANCED STATE OF DEVELOPMENT

vugrawl 17 April 1991

LITTON GUIDANCE & CONTROL SYSTEMS

EVALUATION OF MATERIALS/METHODS

o FIRST EFFORT TO USE SPUTTERED COATINGS FAILED

o CVD CERAMIC COATINGS WERE PROMISING

o TEST PROGRAM USED A STANDARD LITTON GYRO BEARING

FOR A TEST VEHICLE

o BEARING TEST EQUIPMENT AND EXTENSIVE BASELINE DATA

ON STANDARD BEARINGS IN PLACE

vugrawl 17 April 1991

102
LITTON GUIDANCE & CONTROL SYSTEMS

MATERIAL COMBINATIONS TESTED

o TITANIUM CARBIDE COATING OVER SOLID TUNGSTEN CARBIDE

BALLS (TiC/WC)

o TITANIUM NITRIDE COATING OVER SOLID TITANIUM CARBIDE

BALLS (TiN/TiC)

o TITANIUM CARBIDE OVER 440 STEEL BALLS (TiC/440C)

o 440C AND 52100 STEEL RACES

o TITANIUM CARBIDE COATING OVER 440C RACES

vugrawl 17 April 1991

LITTON GUIDANCE & CONTROL SYSTEMS

TEST CONDITIONS AND OBJECTIVES

o OPERATE AT EXAGGERATED STRESS LEVELS TO TEST

INTEGRITY OF COATING

o RUN MANY MILLIONS OF CYCLES TO TEST ENDURANCE

o TEST FIVE TIMES LOAD, 22500 RPM, UP TO 300 HOURS,

WITHOUT FAILURE

o LIFE TEST UP TO 20,000 HOURS

vugrawl 17 April 1991

103
LITTON GUIDANCE & CONTROL SYSTEMS

TEST RESULTS

o EXCELLENT PERFORMANCE OF TiC COATED STEEL BALLS

o PERFORMANCE OF THE TiC COATED STEEL BALLS ON STEEL

RACES EQUALLED THAT OF SOLID CERAMICS AND TiC COATED

RACE COMBINATION

vugrawl 17 April 1991

LITTON GUIDANCE & CONTROL SYSTEMS

SUBSEQUENT ACTION

o EXPLORED OTHER POTENTIAL SOURCES FOR COATINGS AND

CERAMICS

o INVESTIGATED MEANS OF COST REDUCTION

o BEGAN FORMAL QUALIFICATION TESTING IN GYROSCOPES

o SUCCESSFULLY INCORPORATED TiC COATED STEEL BALLS IN PRODUCTION GYROS

vugrawl 17 April 1991

104
LITTON GUIDANCE 8, CONTROL SYSTEMS

RESULTS IN SYSTEMS

o LIFE IN GYROSCOPE BEARINGS IS SHOWN TO BE INCREASED

FROM TWO TO FIVE TIMES

o PRODUCTION YIELDS INCREASED DRAMATICALLY

o POOR PERFORMING BEARING LOTS, ONCE COMMON, HAVE

ALMOST DISAPPEARED

vugrawl 17 April 1991

LITTON GUIDANCE & CONTROL SYSTEMS

SOLID CERAMIC BALLS

o BENEFITS IN EXTENDED LIFE OF BEARINGS ARE SIMILAR TO

COATED BALLS

o LOW THERMAL COEFFICIENT CREATES A MISMATCH WITH

MATERIALS USED IN INSTRUMENT CONSTRUCTION

o COST TO USER ARE EXPECTED TO BE SIGNIFICANTLY LOWER

THAN COATED BALLS

o SiN BALLS HAVE BEEN INCORPORATED INTO GIMBAL

BEARINGS WHERE DESIGN MODS COULD BE MADE

o SiN BALLS CURRENTLY ON TEST FOR A NEWER DESIGN

GIMBAL-LIKE APPLICATION

vugrawl 17 April 1991

105
LITTON GUIDANCE & CONTROL SYSTEMS

WHY HYBRID BEARINGS WORK

o DISSIMILAR MATERIALS RESIST ADHESION IN HIGH PRESSURE

CONTACT, ELIMINATE ASPERITY WELDING AND ADHESIVE

WEAR

o REDUCED HEAT, ABSENCE OF FRESH, NASCENT STEEL WEAR

PARTICLES REDUCES LUBRICANT BREAKDOWN RATES

o HARD, SMOOTH FINISHES RESIST WEAR, REDUCE FRICTION

vugrawl 17 April 1991

LITTON GUIDANCE & CONTROL SYSTEMS

SUMMARY

o CERAMICS, IN HYBRID STEEL BEARINGS CAN PROVIDE

SPECTACULAR LIFE AND RELIABILITY IMPROVEMENTS IN ORDINARY ENVIRONMENTS

o THE DEVELOPMENT OF SOURCES FOR MATERIALS, COATING

METHODS AND ALTERNATIVES MATERIALS AND PROCESSES

ARE IMPORTANT GOALS.

vugrawl 17 April 1991

106
High-Temperature Applications and Challenges of Ceramic Bearings

Lewis B. Sibley

Tribology Systems, Inc.,

Paoli, PA 19301

Application of silicon nitride to ball and roller bearings is reviewed,

including recent developments of unique self-contained solid- lubricant systems

for use over wide temperature ranges . New ceramic materials developments for

bearings with improved ductility over current monolithic silicon nitrides are

described. Building on early experience with special bearings for nuclear

reactors, tubromachines and adiabatic diesel engines, a methodology for computer

modelling of solid- lubricated ceramic bearings is used in the design of reliable,

integral, long-life systems for new critical applications. An interdisciplinary

team has been organized to provide the synergy of materials and bearing

technologies needed to meet the most demanding design objective.

Tribology Systems, Inc. (TSI) is in the process of significantly extending

the state-of-the-art in the design of solid- lubricated all-steel and ceramic

bearings. TSI is working in four critical areas in this regard. First is the

recent availability (enhanced under subcontract from TSI) of the PKG , Inc . '

s

ADORE dynamic bearing analysis computer program. Second is the USAF- funded

development of advanced new- generation solid lubricants by TSI. Third is the

TSI-developed, self-contained, internal lubricant replenishment system. Last

is the TSI temperature - compensating bearing/shaft/housing mounting system for

all-ceramic bearings.

By using this modern computer dynamic analysis, the motions and forces of

all the elements in a bearing are computed in very small time steps to establish

the basic dynamic stability of the bearing and the equilibrium balance of wear

rates of the solid- lube films and other wearing parts in the bearing. The

dynamic analysis is uniquely suited for solid lubrication, compared to the

classical computer quasi-static analyses used in the past for bearing engineering

in the industry, which with conventional cut-and-try methods, occasionally (and

unpredictably) result in catastrophic failures of solid- lubricated bearings.

Dynamic analyses of conventional bearing designs at typical high-speed operating

conditions with solid lubrication do not reach equilibrium but predict excessive

wear and increase in friction sometimes sufficient to cause the seizure of

current -de sign bearings. Stable operation is predicted by using a unique (patent

pending) lube -ring design and bearing internal geometry together with solidlube materials in the pockets and lands and coatings on the balls or rollers and

races having the right balance of friction and wear properties.

Greatly improved compositions of solid- lubricant materials and engineering

ceramics have been identified for operation at high temperatures and both low

and high speeds in bearings of these special designs. Also, a unique proprietary

method of mounting all -ceramic inner and outer bearing rings on the metal shaft

and housing structures having several times the thermal expansivity of the

ceramic has been developed. Metal compensator rings pressed onto the shaft and

in the housing have tapered faces contacting the ceramic bearing rings designed

107
o that the bearing mounting clamp forces do not vary with the large temperature

hanges expected in high-performance machines.

108
HIGH-TEMPERATURE APPLICATIONS AND CHALLENGES OF CERAMIC BEARINGS

by

Lew Sibley

Presented at

NIST/DARPA CERAMIC BEARING TECHNOLOGY WORKSHOP

April 17, 1991

Rockville, MD

51

TRIBOLOGY

SYstems 'nc J^A225 Plank Avenue 5^ Paoli, PA 19301

Telephone (215) 889-9088

109
BREAKTHRU IN LONG-LIFE SELF-LUBE BALL AND ROLLER BEARINGS

TRIBOLOGY SYSTEMS , INC.

PAOLI , PA

CERAMIC BEARINGS - A TECHNOLOGY WHOSE TIME HAS COME

O NO RUST

0 THREE TIMES AS HARD AS BEARING STEEL

0 THREE TIMES THE TEMPERATURE LIMIT OF STEELS

0 SIXTY PERCENT LIGHTER THAN STEEL

O EASIER TO LUBRICATE (DOESN'T GALL)

SOLID LUBRICANTS - BETTER MATERIALS AND DESIGNS

0 FOUR TIMES THE TEMPERATURE LIMIT OF OILS

O FORM TRANSFER FILMS IN BEARINGS FOR REPLENISHMENT

O INTERACTIONS WITH BEARING DYNAMICS CLARIFIED

110
THir T»A«UfE-ttlLU0NS0fiT»6« VU+T*

HI
CERAMIC ELEMENT BEARING

ENDURANCE TESTING

RUNNING

TIMES (HR) BEARING S/N

TCCT Ktri

1 Cj 1 IN w. TEST

TOTAL AT

SUSPENSION #1 #2 n fclvi AniSo

1 813 813 07 11 CAGES FAILED

2 1019 oa 02 #2 CAGE FAILED , 2A 3 1022 06 02 VISCOSEALS

FAILED,

O.O.R. BALLS

3 .06 ' 11 VISCOSEALS

CAM cn r

SKF

PERF. 03

SOLAR

PERF. 12(33) - 10 - 03

4 10 03 ROTOR DYNAMICS,

#1 HIGH AT

4A 295 307 10 03 DRIVE FAILURE

GRAND TOTAL 42S4 [AGGREGATE, ON (6) BEARINGS]

from Taylor, K. M .

, Sibley, L. B. , and Lawrence, J. C,

"Development of a Ceramic Rolling Contact Bearing

for High-Temperature Use," Wear . (3), 226-40 (1963).

Figure I. Special Solid Lubricated 204 Size Ceraolc Ball Bearing

After 12 Hours Operation at 1500 F and 6000 RPM under 300,000 psl Contact Stress

112
DYNAMIC MEASUREMENTS OF SOLID-LUBRICATED CERAMIC CONTACTS

0 SPECIAL TEST RIG CLOSELY SIMULATES BEARINGS

IN HIGH-PERFORMANCE MACHINES

0 BOTH HIGH SPEEDS (20,000 FPM } AND TEMPERATURES (1500F)

O IMPACT FRICTION MEASUREMENTS OF SOLID-LUBE CAGE POCKETS

0 TRACTION MEASUREMENTS OF CERAMIC BEARING CONTACTS

COMPUTER SIMULATIONS OF BEARING DYNAMICS

0 USING TEST RIG DATA TO DESIGN SELF-LUBE BEARINGS

0 BALANCE INTERNAL GEOMETRY AGAINST SOLID LUBE FILM WEAR

0 RESULTS IN STABLE BEARING OPERATION OVER LONG LIFE

O BEARING CARTRIDGE DESIGNED FOR DIRECT MOUNTING IN

MACHINES

FAILURE MODES OF TPI BOLOG ICAL ELEMENTS

- SPALLING FATI6UE AT DESIGN LIFE

- LUBRICATION DISTRESS t CHEMICALLY REACTIVE BOUNDARY FILMS

- EXCESSIVE PARASITIC LOADS S DEBRIS PENT INITIATED SPALLS

- SMEARING OR SCUFFING UNDER SEVERE SLIDING

- SKID DAHAGE FROM INERTIAL LOADS

- HIGH SPEED LUBRICANT STARVATION

- CORROSION, OVERHEATING AND LUBRICANT INCOMPATIBILITY

- FAIL-SAFE VIBRATION VERSUS CATASTROPHIC SEIZURE

113
Computer Modeling of Rolling Bearings

Practical Significance of the

Quasi-Static and Dynamic Models

Quasi-Static Models Dynamic Models

Overall load distribution Contact stresses Nominal lubricant film thickness Fatigue life Bearing stiffness

Cage instability

Rolling element skid and skew

Lubrication effects Wear modeling

Bearing heat generation Applied bearing torques Dynamic applied loads

Irregular bearing geometry

Optimization of manufacturing tolerances Bearing noise

PKG Inc

ADORE

Advanced Dynamics Of Rolling Elements

PKG Inc

Presented by Lewis B. Sibley - 1990 STLE Annual Meeting, 7-10 May 1990, Denver, Colorado EXAMPLE

Cage Pocket Clearance Optimization In a High-Speed Solid- Lubricated Roller Bearing

Guide Land Clearance = 0.015 In

Poc Clearance = 0.005 in Poc Clearance = 0.010 in Poc Clearance = 0.015 in Poc Clearance = 0.020 in

114
ADVANCED DYNAMICS Of ROLLING ELEMENTS

o

z

IdO

<

z

o

zo O

zU

u

O

a

1.25

1-

0.75

0 50

0.25

O

2 _ O i <~> O

<2o

z

ADORE-1.5

CAGE

- Data Set 1

Ball/Cage Interactions

with cage friction coefficient of 0.15

Performance simulated

over 10 shaft revolutions.

M= 1.24E-01 S= 9.56E-01

0.5

PLOT NO. 9

SOLID LUES BALL BRG RUN gj

M= 190E-01 S= 1.30E+00

M= 1.98E-01 2*6 S= 1.55E+00

BEARING ROTATION (REV)

Figure 4. ADORE Computer Simulated Plots of Solid Lubricated

Ball/Cage Impact Loads

115
ADVANCED DYNAMICS OF ROLLING ELEMENTS

ADORE-1.5

PLOT NO. 13

' M= 4.41E-01 S= 2.74E-01

M= 4.22E-01 S= 2.80E-01

M= 3.96E-01 S= 2.59E-01

BEARING ROTATION (REV)

Figure 5. ADORE Computer Simulated Plots of Solid Lubricated

Ball/Cage Relative Motions Producing Impacts In Pig.

116
Lead Acid

Battery Pack

Drive Motor

Electronic Controller

Unique

Mobility Drive Motor

Alternator

(a) Conventional electric Drive

Lead Acid Driva Unique

Battery Motor Mobility Motive Pack Electronic Drive Motor Power

Controller Alternator ~+— n

Flywheel/Electric

Surge Power Unit

Flywheel

Electric Controller

(b) Hybrid Electric Drive

Fig. 1: Schematic ol Conventional and Hybrid Electric Drives

117
UNIQ 206

PLOT NO. 1ft

POWER LOSS- (LBF*IN/S) xlB'

6

3

5

4

3

2

1

@ J i I i L

BEARING ROTATION (REV) xl0i

M= 1.82E+03

S= 1.50E+03

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

1A

UNIQ 286

TINE AVERAGE WEAR RATES (IN**3/S)

CAGE xlB7

6

5

4

3

2

1

0

PLOT NO. 3C

J_Z L

BEARING ROTATION (REV) x!0

1

H- 1.98E-07

$= 1.84E-0?

0.6 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

3C

118
UN1Q 20G

<1 LOSS (LBF*IN/S) xi0~ 3

PLOT NO. lfl

1.2

1.1

1.6

0.9

0.8

0.7

0.6

-jj ujj

1 uililljJljL-^dlijjLlii.

J 1 -L I 1 J _. 1 Sr 4.84E+0] M 0.2 0.4 0.6 0,8 LB

BEARING ROTATION (RED) xlG"1 1.2

If)

UNIQ 206

TIKE AUERAGE WEAR RATES <IN**3/S)

CAGE X10 11

6

PLOT NO. 3C

5 [

4

3

2

1

6.0

r

r.

r

r

fa 3.27E-R8

S= L97E-88

0.2 0.4 0.6 0.8

BEARING ROTATION (REM xl0

-1

1.0 1.2

2C

119
FEA Model of Ceramic Bearing Inner Ring

120
Effects of Composition. Microstructure . and Processing on Ceramic

Rolling Element Bearing Performance

R. Nathan Katz

Worcester Polytechnic Institute

Worcester, MA 01609

Ceramic rolling element bearings provide properties which can enhance the

performance of many systems. During the past decade ceramic rolling element

bearings (CREB's) have been commercially applied in several niche markets ranging

from machine tool spindle bearings to North Sea oil well pump bearings. However,

further exploitation of these bearings, particularly in very high performance

military applications such as aircraft engine mainshaft bearings, will require

simultaneous reductions in cost and improvements in reliability. In

accomplishing these goals, understanding of the role of composition,

microstructure and processing is essential. It will be seen that the available

literature documenting the effect of these three variables on bearing performance

is extremely limited. In fact there is no documentation of any studies where

composition, microstructure or processing have been independently varied and

bearing performance measured. Trends discussed in this paper will therefore be

inferred or suggested based on what literature is available.

In defining the role of composition on the performance of CREB's there are

two levels to consider; differences between compositional "families" of ceramics

(i.e., zirconia versus silicon carbide versus silicon nitride), and differences

within a single "family" (i.e., silicon nitride with 1 % MgO versus silicon

nitride with 4 % yttria) . While it is "well known" that fully dense silicon

nitrides are superior to other ceramic rolling elements, there is remarkably

scant documentation in the literature. McLaughlin [1] performed comparative

rolling contact fatigue (RCF) tests on several grades of silicon nitride, silicon

carbide, and a transformation toughened zirconia, all at a Hertz stress of

860,000 psi. The zirconia and HIP silicon carbide were both found to have

average RCF lives 2 to 3 orders of magnitude less than any of the silicon

nitrides

.

There have been no documented, systematic studies of the effect of

compositional variations within a given material family (i.e., silicon nitride

with starting powder, processing parameters, grain size, etc. held constant but

type and percent of densification aid varied). However, several studies exist

where several different silicon nitrides were tested in an identical manner.

These studies (for example the data of Lucek [2]) tend to suggest that the less

additive the better the RCF behavior of silicon nitride.

Detailed descriptions of microstructural variables such as grain size,

morphology, phase content and distribution, porosity, etc., are universally

lacking in the literature documenting RCF or bearing test results. A paper

presented by Mandler and Musolff [3] suggests that porosity is a key

microstructural variable in the RCF of silicon nitride. Available data can be

interpreted to suggest that the influence of the differing percentages and

compositions of densification aid is a result of differing type and percent of

121
grain boundary glass phase.

The influence of processing is clearer. Lucek [2], has demonstrated

dramatic improvements in RCF performance of silicon nitride with 1% MgO, as one

changes process from hot pressing to HIP'ing to HIP'ing low WC content powder.

One reason for the lack of systematic studies of the effects of

compositional, microstructural , and processing variables is the difficulty of

holding any of these constant as the others are varied. Thus, an alternative

strategy to address the influence of basic material parameters on bearing

performance is required. Taking into account that the failure mode of silicon

nitride in RCF is spall formation, if one could identify the materials properties

related to the formation of the spall, one could design microstructures for

optimized bearing performance. Such as alternate strategy will be presented.

In summary, based on the limited available studies it would appear that

a silicon nitride optimized for bearing application would be HIP'ed, have minimal

grain boundary phases, minimal porosity, and a minimal inclusion content.

Influences of such features as grain size, L/D, effect of additive chemistry,

and alpha/beta phase ratios require investigation.

References

[1] J. J. McLaughlin, "Alternate Ceramic Roller Bearing Program" Final Report,

Contract N00019-81-C-0284 , November 1983.

[2] J. W. Lucek, "Rolling Wear of Silicon Nitride Bearing Materials," ASME

Paper No. 90-GT-165, June 1990.

[3] W . F . Mandler and C. F. Musolff, "Rolling Contact Fatigue Testing of

Silicon Nitride," Paper No. 85-C-86, presented at Am. Ceram. Soc. Annual

Meeting, Chicago, March 1986.

122
Role of

Composition, Microstructureand Processing

on the

Performance of Ceramic Rolling Element Bearings

R. Nathan Katz

Worcester Polytechnic Institute

Presented at NIST

17 April 1991

• Ceramic Rolling Element Bearings Have

Demonstrated Unique Capabilitiesfor Several

Niche Applications and Have Considerable

Promise In Many More Areas

• To Broaden Their Applications Base We Must

Find Ways to I Cost and t Reliability

• What Is The Role of Compositional,

Microstructuraland Process Variables In Attaining

The Above Goal?

123
DESIRED PROPERTIES FOR

BEARING MATERIALS

Fracture Toughness, K| c High > 5 MN m-3/2

Hardness High ;>1200 kg mm-2

Elastic Modulus Low <210 GPa

Density Low < 4 Mg m-3

Bend Strength High > 700 MPa

Corrosion Resistance High

Upper Use Temperature /High > 800°C

Failure Mode "Steel-Like" Small Spalls

Spallation

PROPERTIES OF HIGH PERFORMANCE

STRUCTURAL CERAMICS VERSUS M-50 STEEL

Property

Silicon . NitrideSilicon Carbide Alumina Zironcia M-50 Steel

Description HIP>d

NBD-200

Sintered Fully Dense

Sintered

Sintered Transformation- ToughenedWrought

Ingot

Fracture Toughness, Ki c /MN nr3'2

5-6 4 5 8-10 12-16

Hardness, H

/kg mm*2

~1 800-2000 ~2800 ~2000 ~1300 ~800

Elastic Modulus, E

• /GPa

310 410 385 205 210

Density, o/Mg m"3 3-2 3-1 4; 56 8

Modulus of Rupture /MPa750 450 550 600-900 NA

Corrosion Resistance High High High High Moderate

Upper Use Temperature

/•C

1100 1400 1000+ 800-900 325

Failure Mode Spading Fracture Fracture Spoiling Spalling

124
COMPOSITION

One "Family" of High Performance Ceramics vs. Another

(i.e. Si

3 N4 vs. SiC)

• Compositional Variations Within A Given Family

© No Studies Of The Effect of Composition on

Rolling Contact Bearing Performance

- i.e No Studies Where:

. Grain Size

. Process Method

. Grain Morphology

Were Kept Constant and Composition Varied

• Similarly No Studies Where Microstructure Was

Systematically Varied

• Experience in the Development

NC 132 - NBD 100 - NBD 200

Provides Some Information on the

Effect Processing

125
Comparison of the RCF Behavior of Several Ceramics

Ave Cycles

Material to Failure Failure Mode

Si3 N4

- NC-132

(l%MgO)

6.9 X 107 "small" spalls

Si3N„ - Sintered & HIP

(Y,P3 + A1A)

1.5 X 10s "line" spalls

PSZ (Zr02) 1.6 X 10" "large" spalls

SiC - HIP 1.3 X 104 "large, line" spalls

Si3N„ - Sintered & HIP

(AY4)

1.0 X 10

7 no spalls on rod

(ball spalls)

Si3 N4 - HIP

(1% MgO)

6.9 X 10

7 "small" spalls

• Hertz Stress = 860,000 psi

9 Materials of varying processes and developmental maturities

Source: JJ. McLaughlin

Effect of Composition on Si 3 N1 Rolling Element Dearintzs

Material Process H L,0 Weibull Wear

(GPa) (10

f'cyc) m (mMO 10) 7% Y203 + 5% A12 03 (SIALON)

Sintered 13.5 1.66 1.09 - 1.6

5% Y203 + 2% AI203

Sintered

&

HIP

15.6 336 1.39 - 0.25

1% MgO Hot

Pressed

16.4 0.58 0.59 - 0.1

1% MgO HIP

(low WC)

16.5 > 10.1 2.03 < 0.1

© All RCF Tests @ 6.4 GPa

O No Information on Grain Size,

Morphology Starting Powder, etc. • Wear Measured @ 200 km Element Travel

Source: J. Lucek

126
RCF Evaluation of Three Si^N, Materials

Material Hertz Weibull Parameters Porosity

(Ksi) Bio* B50* m Peak Max

M-50 steel 797 3.4 11.6 1.55

NC-132 893 74.5 109.6 4.88 2-3 fim 6-7 jim

SN-A 500 1.23 3.09 2.04 6-7 tim 6-7 Jim SN-B 507 0.007 0.14 0.63 2-4 fim 35+ Mm

• Materials A & B - Sintered Si3 N4

, no other data provided • + = X106 stress cycles

Source: W.F. Mandler & C.F. Mosolff Cummins Engine Co. Presented @ 88'h Annual Meeting

of the American Ceramic Society

Paper 85-C-86 (1986)

Effect of Processing on RCF of SijN, with 1% MgQ

Process H L10 Weibull

(GPa) (10

6 cycles) m

Hot Pressed 16.4 0.58 0.59

HIP 18.1 4.08 1.26

HIP

(low W.C.)

16.5 > 10.1 2.03

• Grain Size « constant, but in H.P.

is anisotropic • All tests at 6.4 GPa

Source: J. Lucek

ASME 90-GT-165

(June, 1990)

127
Ceramics for high performance rolling element bearings 7

TABLE 1 Rolling Four-ball Fatigue Life Test Data on 17-5 mm CVM M-50 Steel and Silicon

Nitride Balls Finished by Various Methods

Spindle ball Spindle Max. Hertz No. oj CVM Test life dptnUlC Dull speed stress M-50 steel 110* revs Irpm IGPa (ksi) support ball Ojicf issi

set failures

M-50 5 200 4-7 (680) 0 20-2 Spalled 0 3-2 Spalled

1 260 Spalled 0 no Spalled 0 71 Spalled 0 11-7 Spalled

As-received 5 200 "4 7 (680) 2 117-3 Spalled

silicon nitride 10000 5-5 (800) ! 120 Spalled (NC-132) 10000 5-5 (800) 0 666 Spalled 10000 5-5 (800) 0 24-6 Spalled 10000 5-5 (800) 1 41-5 Spalled 10000 5-5 (800) 2 183 Spalled

Diamond lapped 10000 5-5 (800) 1 190 8 Intact

silicon nitride 1 183-6 Intact (NC-132) 3 1830 Intact

1 182-4 Intact

3 1020 Spalled

3 183-6 Intact

THE KEY PROPERTY OF FULLY

DENSE S13N4 FOR BEARINGS

THE FAILURE MODE - SPALL FORMATION

NOT CATASTROPHIC FAILURE

THIS IS THE SAME FAILURE MODE AS HIGHPERFORMANCE BEARING STEELS

128
Mechanism Of Failure For SLN, In RCF

• Tensile Stress at the Boundary of the

Wear Track -» Circumferential Cracks

@ Edge of Wear Track Just Beyond the

Contact Ellipse

• These Cracks Grow Slowly Until a Spall

Forms - Due to Crack Linkage and

Elimination of Unfractured Area Beneath

the Spall

Crack Growth Mechanisms That Could Affect RCF Performance

9 Static Fatigue

Nature and Composition of Grain Boundary Phases

9 Mechanical Fatigue

Grain Size, Grain Morphology, Grain Boundary Phase

Composition and Nature, Stress Amplitude

# Thermal Fatigue

Thermal Physical Properties, Strength Determining

Micro-Structural Features

• Stress Induced Phased Transformations

May be Important for Transformation Toughened Materials

129
Factors Expected to Influence Wear of Ceramics in RCF

© Hardness and Fracture Toughness

(i.e.

Wear a

LoadQ—• (EiH ) p

Distance V • H*

where: m & n » 0.5

Q & P M 0.8 - 1.2)

© Surface Chemistry - Tribochemical Wear

(i.e. Si02 Formation)

• Surface Quality, Voids, Defects, etc.

Su ggested Strategy to Predict RCF Behavior

Since in Most Ceramics it is Difficult if not Impossible

to Independently Vary Grain Size, Morphology, Composition

and Processing:

© Evaluate: Mechanical Fatigue

Static Fatigue

Wear

Etc.

for Candidate Bearing Ceramics

© Correlate These Results with RCF Performance

• If Correlations Exist, Use These Laboratory Tests

as Prediction Tests and in Materials Optimization

Studies

130
Effects of Machining and Finishing on Performance

Michael N. Gardos

Hughes Aircraft Company

El Segundo, CA 90245

and

Julian R. Pratt

Spheric Ltd.

W. Sussex, England

Technical Issues of Precision HIP-Si3 N/, Ball Polishing

For the first time in the history of tribology, production ball grinding

and polishing apparatus were used as tribometers . The volume attrition rates

of near net- shape ceramic balls and the sphericity of the final bearing balls

were used to examine the premises that (a) only homogeneous Si 3 NA

, with isotropic

structural integrity, can be converted into precision balls , and (b) the abrasive

(polishing) wear resistance of the ceramics are inversely proportional to their

respective (KIc

0 - 75 x R^ 05 ) factors (the Evans -Wilshaw wear relationship).

Under the auspices of the 1986-1990 DARPA/Hughes Tribological Fundamentals of

Solid Lubricated Ceramics Program (Part I),* Spheric Ltd. ground and polished

one hot-pressed (HP) and five hot- isostatically pressed (HIP) Si 3 N4 ceramics into

0.5 in. dia bearing balls under identical preparatory conditions, after having

determined the hardness and fracture toughness of the ball stocks by Vickers

indentation and the crack- tip-extention- indicated Kx c measurements. The removal

rates of the respective ceramics were periodically monitored until the desired

ball diameter was reached. The log of the volumetric wear rates (y) were then

plotted as a function of each x =(Klc

0,75 x H^ 05 ) -1

, with a high correlation

coefficient of 0.99.

The data confirmed both hypotheses: (a) only isotropic HIP-Si3 NA can be

polished into precision-spherical bearing balls, and (b) the Evans-Wilshaw wear

factor is an excellent predictor of the abrasive (polishing) wear of the ball

ceramics. Some of the end-products are slated for hybrid bearing tests in LOX

applications

.

Socio-Political and Economic Issues Controlling the Technology Insertion of

Ceramic Balls in Hybrid Bearings

Inspite of the extensive data-base showing (a) the commercial availability

of at least one highly reproducible, excellent quality near-net shape ball stock

* M. N. Gardos, "Determination of the Tribological Fundamentals of Solid

Lubricated Ceramics, Volume 1: Summary; Volumes 2 and 3: Appendices,"

WRDC-TR-90-4096 , Hughes Aircraft Company, El Segundo, CA, November

1990 (in press)

.

131
fabricated from HIP-Si3 NA and (b) the ability to turn the spherical preforms into

super-high-grade, low wear rate ceramic balls, a firm capable of producing such

balls is facing the following major obstacles of technology insertion:

• There is no standard ceramic material (or materials) everyone agrees

upon;

• There is no standard ball-polishing process (ceramic ball fabrication

is more an art than a science)

;

• The bearing industry is conservative with no vision (it has no wish to

change, i.e., to use anything but all-steel bearings);

• One must find a progressive technologist at each bearing manufacturer

to champion the cause of precision hybrid bearings (ceramic balls with

steel races)

;

• Not everyone needs a Grade 1 (ultrahigh precision) ball (every bearing

customer may need ceramic balls conforming to different specifications)

.

Once the industry recognizes the almost unlimited potential of hybrid

bearings, in time HIP-Si3 NA balls and steel races will replace 50 or 60 percent

of all-steel bearings currently in use.

i

132
NIST/DARPA Ceramic Bearing Technology

Workshop

April 17-18, 1991

i

1-1 EFFECTS i lu i J OFvl MACHINING I invllllilllu Mills AND FINTSHTNG 1 1111 Jn InU

ON PERFORMANCE

Mike Gardos (HUGHES)

AND

Julian Pratt (SPHERIC)

SPHERICS ROLE IN THE

DARPA/HUGHES "FUNDAMENTALS" PROGRAM

NRL

I 1

| AFOSR |

I .

I

DARPA

| AFWAL |

I I

1

TA k T, INC.

WRDC/MLBT

HUGHES \ J /

PROGRAM

DIRECTION SEM

TRIBOMETRY

TESTER 2A AND

EXPERIMENTS

LUBRICIOUS

OXIDES AND FLUORIDES

CERAMIC WEAR

EQUATIONS

SDI TECH. TRANSFER

AEROSPACE

CORP.

GEORGIA TECH

CALTECH

SORETRIB (FRANCE)

UNIV. OF

ILLINOIS

GEORGIA TECH

(LUB. OXIDES)

CERCOM

ICI/EBONEX

CSEM

(Switzerland;

lub. OXIDES

UNIV. OF ARIZONA

MTI

CRYSTALLUME

MARCHEM

INTERCAL

122

SPHERIC (ENGLAND)

CERBEC

BABCOCK

AND

WILCOX

ESK

CARBORUNDUM

SAPHIRWERK

(SWITZERLAND)

MICROSCOPY RESEARCH

LABS. INC. CSEM

'Switzerland; 'revetest

133
t^s is a- "meos.— iHArRitOfi-, &or thas is also Hoot)

*£>A-U-S ARE. PousHEb!.'! '

-mese rack are.

PoUSHIMfi- PLATES, AU&

T>lAMOMI> - HV0«t>CA«6e*J

MUC.HI S

WEAR OF CERAMICS AS FUNCTIONS

OF Kic AND H

(THEORY AND LITERATURE DATA)

»*C | I.I „mt-Co

/•/ AljOj.TIC

'( WC (2.7 *r*lCo

•—• *tc H1U

*ic

Fig 7 -Wear reslatanco of the tested materials plotted aa a function ot (closed data points) or K^H"1 (open data points). IBaldoni.et al, ASLE Trans., 29, p.347, 19861

Evans SWIIshaw, Acta Metall.,24,1976:

V = WEAR VOLUME;

K Ic

= FRACTURE TOUGHNESS;

H = HARDNESS

;

P = NORMAL LOAD;

N = NO. OF ABRASIVE PARTICLES;

d = SLIDING DISTANCE.

134
THE APPLICABILITY OF THE

EVANS-WILSHAW WEAR RELATIONSHIP

w

<

as

EdO ~

ru X "> U B

-JO

>

BABCOCK CStOOM

HIP

CEROOM HP _^ NORTON

^-TOSHIBA

UBE

0 0018 0.0019 0.0020 0.0021 0.0022 0.0023 0.0024 0 0025 0.0026

l/(TOUGHNESS\75 • Hv\5)

y = - 5.55e-l2 + 3.28e-9x RA2 = 0.990

135
Table 16. BaJI/grinding/polishing data for precision bearing balls fabricated from selected HP/HIP-Si3N4 materials.

-

rcorrvi/uiD Utrt^l^v^n \

i hj B&W NORTON UBE

SPHERIC DATA *< ' CBALL GRADE^r

0.510500 QJ 0 .5 1 9500

(1_0_^ Cv OTAHI UIA 0.509350 0.51 2260 0.522047 0.5 1 771 7 END DIA In. 0.500900 0.501 530 0.5004 50 0.500900 0.499409 0.500236

TIME hours 6 1

1

' 9 12 60 63 HARDNESS Hv (5kg0 Kg/mm*2 1655 1390 1570 1620 1545 1610 TOUGHNESS MPa m»1/2 4.70 5.60 5.82 5.00 6.93 6.97

DIA REDUC. RATE in./hr 0.001600 0.001633 0.000677 O.D00946 0.000377 0.000277 HV j TOUGHNESS 777B.5 77B4.0 9137.4 91000.0 10706.9 11221.7 VOL CHANGE RATE in."3/hr 0.000642 0.000666 0. 000350 0.0003B1 0.000154 0.000112 WHv »1/2 0.002561 0.026622 0.025237 0.023440 0.A5441 0.024922

l/TOUGHNESS'3/4 0.313275 0.274700 0.266874 0.299069 0.234126 0.233117

1/(TOUGH»3/4)(Hv'l/2] 0. 007700 0.00736B 0.006735 0.007010 0.005956 0.005809

CONVERTED TO SI UNrTS

START DIA m 0.012967 0.013195 0.012912 0.01301 1 0.013260 0.013150 END DIA m 0.012723 0.012739 0.012711 0,012723 0.012665 0.012706

TIME sec 21600 39600 32400 43200 216000 226800

VOL CHANGE RATE m"3/sec 2.92E-12 3.04E-12 1.60E-12 1.74E-12 7.03E-13 5.14E-13 HARDNESS Hv MPa 16230.59 13631.73 15396.99 1784B.74 15151.82 15789.27 TOUGHNESS MPa m*1/2 4.70 5.60 5.62 5.00 6.93 6.97 1/(TOUGH-3/4|(Hv*1/2) .002459 .002353 .002151 .002239 .0D1902 .001855

PROBLEMS WITH Hv /Ki c AS WEAR-DETERMINING

8

PARAMETERS

• HARDNESS IS NOT A FUNDAMENTAL PROPERTY; IT DEPENDS ON BOTH THE ELASTIC AND PLASTIC

RESPONSE OF THE MATERIAL.

• K Ic (THE CRITICAL STRESS INTENSITY FACTOR, WHICH IS A MEASURE OF FRACTURE TOUGHNESS)

DEPENDS HIGHLY ON

* THE RESIDUAL STRESSES IN THE CERAMIC; AND

* THE SIZE/TYPE/FREQUENCY OF SURFACE FLAWS LEFT BEHIND BY THE GRINDING/ POLISHING PROCESS.

• ARE K Ic AND H

v ENOUGH TO DEFINE "BEARING WORTHINESS" OF A CERAMIC?

(IS RCF RESISTANCE 0 1 X 10 6 PSI/R.T. IMPORTANT?)

t WHAT HAPPENS IN HIGH TEMPERATURE AIR?

136
FIRST LESSON

SraOUC ENGINEERING UMITfO

• NO STANDARD MATERIAL

10

SECOND LESSON

IMURIC ENGINEERING lUUTtD

• NO STANDARD MATERIAL

• NO STANDARD MANUFACTURING PROCESS

137
NMP = 1824 7.8 rp» 83/87^91 83 39: 51 7'321n tlaster

fHBUC CHGIHEUING UMITtD

LESSONS LEARNED

(a reminder) tmnw DMUNiniNO umitio

• NO STANDARD MATERIAL

• NO STANDARD MANUFACTURING PROCESS

• NO VISION, A CONSERVATIVE INDUSTRY

• NO WISH TO CHANGE

138
THE FIFTH ELEMENT

smuic DKiHconxc umitio

• EVERY BEARING CUSTOMER MAY NEED A

DIFFERENT SPECIFICATION BALL

139

Requirements and Issues in QC and NDE of Ceramic Bearings

B . T. Khuri-Yakub

E.L. Ginzton Lab.

Stanford University

Stanford, CA 94305

In the last fifteen years, several ultrasonic techniques have been

developed for the non- destructive evaluation of dense ceramics. Techniques for

the detection of both bulk and surface defects have been established with

detection limits in the micron range. A number of these techniques will be

reviewed to show their viability. For instance, we will show how it is possible

to use long wavelength scattering to determine the stress intensity factor, and

hence the fracture stress, of a part due to the presence of a surface crack.

We will also show how very high frequency (100-500 MHz) ultrasonic waves can be

launched in hot pressed ceramics in order to investigate the presence of small

bulk defects.

More recently, we have adapted one such technique to the problem of

inspecting ceramic bearing balls. A medium frequency (100 MHz) acoustic

microscope capable of measuring amplitude and phase has been used to scan ceramic

bearing balls with spalls, surface cracks, and shallow depressions. We

demonstrated the ability to detect defects a few microns in size in hot

isostatically pressed silicon nitride. We also made a rotation scanner capable

of rotating bearings under a static acoustic microscope lens.

Many ultrasonic techniques have been developed to successfully inspect

ceramic materials. The problem of inspecting ceramic bearings can be addressed

with an adaptation of some of the old techniques, along with the development of

a new method for global sorting or inspection of the bearings for defects in both

the bulk and the surface

.

141

Requirements and Issues in QC and NDE

of

Ceramic Bearings

B.T.Khuri-Yakub

E.L.Ginzton Lab.

Stanford University

Stanford, CA 94305

143
OULTINE

1- Introduction

2- NDE requirements of ceramic bearings

3- Comparison of available technologies

4- Review of ultrasonic ceramic NDE techniques

5- Ultrasonic ceramic bearings NDE

6- Conclusions

NDE Requirements of Ceramic Bearings

1- Bulk density variations.

2- Bulk cracks.

3- Inclusions: large grains

voids

Fe, Si, other densifying agents

4- Surface defects: gouges

vertical cracks

narrow, shallow depressions

144
Table VI. The State of the Art In Ceramic NDE

NOE Technique and

Reference Material Defect Type

Defect

Size (urn) Current Limitations

Microfocus X-Ray with Image Enhancement [26]HP Si3N4

Inclusions:

• High Density

• Low Density

-25

250

• Sensitivity

• Material Thickness

Greater than 0.250 in.

RB Si3N4Inclusions:

• High Density

• Low Density

125 250

• Low Density Inclusions

High Definition X-Ray

Microradiography [25]

RB Si3N4 Pores and

Surface Defects

20 (Film) 100 (Image

Intensifier)

• Non-Film Imaging

• Sample Thickness

Conventional and

Microfocus X-Radiographic Techniques [27]

Green and

Sintered SiC and

Si3N4

Seeded Voids 115(100%

Detection) 80 (< 50 %

Detection)

• Sample Thickness

• Requires Fine Grained Rim and Special Developing Methods

Dye Penetrants [26] HP Si3N4

Siliconized SiC

Sintered SiC

Surface -250

250-500

• Sensitivity

• Surface Conditions

Scanning Laser

Acoustic Microscope (SLAM)

[26]

HP Si3N4

Injection Molded RB Si3N4

Surface

Subsurface

Laminations

100

100

• Undefined

• Material Porosity

Notes: RB - Reaction-Bonded; HP » Hot-Pressed. Other less successful techniques include

Vibration Analysis, Acoustic Emission, Microwaves, and Optical Holographic Interferometry [25]. The

format of this table is adapted from a table in [26]; the specific entries are collected from a variety of sources, as cited above.

145
Table VI. The State of the Art in Ceramic NDE (continued)

NDE Technique and

Reference Material Defect Type

Defect

Size (urn) Current Limitations

Laser Photoacoustic Spectroscopy .[26]RB Si3N4

Surface Near Surface

50-100 100

• Undefined

Ultrasonics

(45 MHz) [26]

HP S13N4 Inclusions:

• High Density

• Low Density

-50

-50

• Component Geometry

RB SI3N4 Inclusions:

• High Density

• Low Density

125

125

• Component Geometry

• Attenuation (RB Si3N4)

Ultrasonics

(230 MHZ) [26]

HP Si3N4 Inclusions:

• Hign Density

• Low Density

25

• Contact Techniques

Ultrasonic Surface Wave Pulse-Echo Techniques (100 MHz)

[3]

HP Si3N4

Surface:

• Planar

• Neck Region of Turbine Blade

25

50

• Sensitivity to

Variations in the Water Coupling Length

Ultrasonic Surface Wave Pulse-Echo Techniques (< 10 MHz)

[3,28]

HP Si3N4Surface 50-250 • Contact Techniques

Ultrasonic Surface Wave Pulse-Echo Techniques (20 MHz

and 50 MHz) [29]

AlphaSintered SICSurface Cracks and Pores

50 • Defect Type

Characterization

Notes: RB - Reaction-Bonded; HP - Hot-Pressed. Other less successful techniques include

Vibration Analysis, Acoustic Emission, Microwaves, and Optical Holographic Interferometry [25]. The

format of this table is adapted from a table in [26]: the specific entries are collected from a variety of sources, as cited above.

146
Companson of Available Technologies

1- X-Ray computed tomography

2- Microrocus X-Ray

3- Thermal imaging

4- Visual optical inspection

5- Fluorescent penetrant

6- Nuclear magnetic resonance

7- Ultrasonic: Low frequency

High frequency

Scanning acoustic microscopy

BULK ULTRASONIC TECHNIQUES

1 - C-Scan imaging at 1 0- 1 00 MHz

a- Flaw detection

b- Velocity mapping

c- Porosity mapping

2- A-scan at 100-500 MHz

a- Haw detection

b- Velocity mapping

c- Porosity mapping

d- Attenuation measurement

147
Totally raflactadwavas

Spadman surfaca —Tranamttad wavas

•Acoustic focal plan*

inaJda of spadman

Surtaea

rellectad wavas

0 -.

Input signals

caaIL

Gata signal

Tima

500 - WC

oI 600 -

CRACK

0 I i 1 i I i I i L_

0 200 400 600 800

DOMINANT OEFECT DIMENSION (^tm)

148
MODES OF OPERATION OF C-SCAN

Pulse with sub-surface gate.

Pulse with gate at back wall

Pulse with gate at back wall, measure time delay.

Pulse defocus to isolate SAW signal.

Pulse with sub-surface multiple gates.

Pulse with sub-surface gate and phase ( 1 80 degrees) measurement ability.

-TVTRANSDUCER

SAPPHIRE

MATERIAL

UNDER TEST

R = 20 cm

2a * .1 cm

F = 172 N S 39 LbS.

149
/

rRANSDUCER

/-WEDGE

*R

CRACK

// ^CERAMIC^ * ft //'

150
SURFACE ULTRASONIC TECHNIQUES

1- Low frequency 1-10 MHz

a- Single crack sizing and SIF determination

2- Medium frequency 10-100 MHz

a- Single scratch sizing and SIF determination

b- Machining damage evaluation

3- High frequency

a- Scanning acoustic microscopy

151
1 1 1 REVERSIBLE ^1 FRACTURE

CRACK OPENING o---*.

300

STRESS

(MPa) 200

1fjf ^-2nd UNLOAD

j(jT AND RELOAD

-

100 1st UNLOAD 11

AND RELOAD M

0 li 1 1 0 5U IUU lOU

ACOUSTIC CRACK DEPTH ( Mm)

200

koB

Horizontal surface displacement in the backscattered Rayleigh wave vs. reflection coefficient for a subsurface

crack.

152
153
0.7

VOID

0.6

0.5

0.4

0.3

0.2

0.1

^VVVWww^

1 10 20

ka

Fig. 3a. The scattering cross-section a from a void i n Si 3N4 as a function of ka .

VOIO

[ I

Fig. 4a. Relative scattering cross-section from

a void as a function of time.

154
Fe

Fig. 3hT. The scattering cross-section o for

Si 3N

4 as a function of ka .

155
Typical Grinding Lavage Data froSilicon Nitride Samples

Sample Range of Maximum Range of Typical

Description Predicted Depths Separations Befor Scanned Cracks tween Significant

(um) Defects (urn)

5 urn cut

undressed

wheel 180

grit 22-40 120-260

10 urn cut

dressed

wheel 180

grit 25-45 150-200

DEFECT

SIGNAL

CORNER

V 90«H

TRANSDUCER"SAMPLE GEOMETRY FOR

SINGLE CRACK DATA

156
V \ BULK TRANSDUCER

\\ LONGITUDINAL //

/

WAVE /// ACOUSTIC ENERGY \V\ /// LEAKING SACK INTO WATER \ \ WATER

•-WW*- ! CRACK

Si 3 N 4 SAW "

157
Different Kinds of Defects of Ceramics Bearing Balls

158
Synthesizer

(300-500MHz)

300 MHz

Phase Controled H RF Circuits

300MHz Reference

RF Switch Power Amp

L-P Filter Amp. Limiter

S/H A/D IBM PC-AT

COMPUTER

Water

Bearing Bad

Block Diagram of the Amplitude Phase

Measurement system

159
Why use SAM ?

• Operation for both ceramics and metals.

. The ability to detect not only the surface

damage but also subsurface cracks and

bulk defects.

• Higher sensitivity to detect very shallow

surface depressions than conventional

optical microscopes; more tolerant to

vibrations than confocal scanning optical

microscopes.

Amplitude & Phase Measurement

Electronics

A/D

7854

Scope

D/A

Computer

(IBM PC-AT)

855C

Programble

Controller

Block Diagram of a SAM Bearing Ball Inspection System

160
Performance of phase and

amplitude measurement

system

Noise Level Amplitude(p-p) phase

Electronics 0.7% ±0.5°

Rotation Scanner 2% ±4°

x-y Scanner 4% ±7.5°

The high noise level is caused by the

vibration of the scanning system which is not acceptable for ball bearing inspection.

Solution -- Differential phase

measurement:

Use mixed mode transducer to generate both longitudinal and shear waves

in fused quartz buffer rod.

Develop a differential phase

measurement system to detect the phase

difference between the two beams.

161
Y-Cut LiNb03

Fused Quartz

Water

Performance of simple phase

measurement system :

Noise Level Amplitude phase

Electronics 0.7% 1°

Rotation Scanner 2% 8°

x-y Scanner 4% 15°

The high noise level is caused by the vibration

of the scanning system which is not acceptable for ball bearing inspection.

Performance of differential

phase measurement system :

Rotation Scanner 2% 0.8°

162
f=116MHz F#(for L-wave)=.8

UJQ

t

-10-

Amplitude

UJ<

I

a

• Phase

2000

X (urn)

Ceramic Ball, Radius=l/8"

Scan Area= 1mm x 1mm,

lOO-i

Focused on Top

80-

60-

4 0-

20-

0-

•

( ) 20 40 60 80 100

163
100

Ceramic Ball, Radius=l/8"

Scan Area= 1mm x 1mm,

Defocused 0.40 mm

80

60

4 0- if , ,; *?v

20-

0^

3 20 40 60 80 100

Ceramic Ball, Radius=l/8"

Scan Area= 1mm x 1mm,

Defocused 0.10 mm

164
1,4 DINITROCUBANE

Ceramic Ball, Radius=l/8"

Scan Area= 1mm x 1mm,

Defocused 0.25 mm

165
IOOt

Ceramic Ball, Radius=l/8"

Scan Area= 1mm x 1mm,

Defocused 0.35 mm

8 0-

60-

4 0

20

0

y*v?~» - ^^W^Bt'Wt*^ jff:-Jr. Mm

6 20 40 60 80 100

IOOt

Ceramic Ball, Radius=l/8"

Scan Area= 1mm x 1mm,

Defocused 0.25 mm

80-]

60-

40

20-

0-

( 20 4 0 60 8 0 1 00

166
CONCLUSIONS

1- Ultrasonic NDE shows most promise.

2- NEEDS: a- A global method of inspection.

b- A green ceramic inspection

method.

c- A method for single defect

evaluation.

167

Ductile Regime Grinding of Brittle Materials

Thomas A. Dow, Steven C. Fawcett, and Ronald 0. Scattergood

Precision Engineering Center

North Caroline State University

Raleigh, NC 27695

Over the past several years, many researchers have studied the process of

material removal in brittle materials including diamond turning and fixed

abrasive grinding. A regime of material removal called ductile regime turning

and grinding has been defined for which there is no apparent damage to the

finished surface. The operating conditions and material properties (modulus E,

fracture toughness Kc

, and hardness H) which define the limits of this regime

can be written in terms of a critical depth of cut expressed as:

2 * E

'

This model can be used to predict the relative performance of different

ceramic materials.

It has been observed that the appearance of a fracture free surface does

not mean the material was removed exclusively by plastic or ductile processes.

In fact, it has been shown that for diamond turning of brittle materials such

as silicon and germanium, much of the material removal is by brittle fracture

and only the material closest to the final surface is removed by plastic flow.

The chip producing mechanism is said to be in the ductile regime if the

dimensions of the chip do not exceed the critical depth over the finished surface

of the workpiece. Experimental measurements have corroborated this model and

demonstrated the relationship between the uncut chip thickness and the damage

to the surface for the geometry of plunge grinding and diamond turning with a round nose tool. However the case of the contour grinding is more complicated

and has lead to a computer simulation of the chip geometry and prediction of the

resulting damage.

The results of the model prediction can be compared to the actual roughness

achieved in contour grinding of SiC. The measured surface roughness and its

increase with feed rate as well as the calculated roughness and the theoretical

finish assuming infinite speed of the grinding wheel (Equation (1)). The

predicted surface finish closely matches the measured surface roughness to

provide a strong correlation between the simulation and actual ground parts.

Since the surface finish results from several factors (machine vibrations,

brittle fracture sites, geometric relationships, relative rotational speeds,

etc.), the measured value could vary greatly from the simulated finish. However,

in this particular experiment, the parameters were selected such that the maximum

chip thickness was less than the critical value and little damage was observed

on the final surface. Therefore, the geometrical features created by the

169
interaction of the grinding wheel and the part were the main cause of the surface

roughness

.

The geometric chip model can be used to evaluate the contribution of the

material, geometry of the wheel, and the operating conditions on the surface

damage. Damage, in the form of fractures, will occur at the location where the

chip thickness exceeds a critical value. These cracks will penetrate some

distance into the material. If this penetration is less than the thickness of

the material removed on subsequent passes of the wheel, a damage -free surface

will result. The 3-D shape of the chip illustrates the variation in thickness

in the cutting direction as well as the feed direction. By using the thickness

of the chip over the finished surface, the amount of fracturing on the finished

surface can be estimated from 1-D plunge grinding experiments. Because the

grinding wheel makes a specific number of revolutions for each revolution of the

workpiece, the uncut chip thickness will decrease as the grinding wheel moves

toward the center of the part. For this reason, the values are plotted versus

part radius . The predicted percent fracture appears to give an accurate

description of the fracturing that remains in the surface after the grinding

operation is completed.

170
DUCTILE REGIME GRINDING OF BRITTLE

MATERIALS

Tom Dow

Steve Fawcett

Ron Scattergood

Precision Engineering Center

North Carolina State University

Raleigh, NC

Workpiece Grinding Wheel

Schematic of the plunge grinding apparatus.

Depth of cut per

171
9iU

s

C3u

fa

c

u

CU

100

80

60

40

20 h

HIP SiC

ill * 10 100

Depth of Cut (nm)

1000

r-rincreasing grinding depth

172
where

dc = Critical depth of cut for ductile response

(3 = Factor involving wheel, diamond, and coolant

Kc = Fracture toughness for small cracks

E = Youngs modulus

H = Hardness for small penetration depths

173
IMAUE ANALYSIS SYSTEM

Macintosh Computer Interrupted lest cut samples

Software packages: Video Micro Scaler Inverted microscope

I'rismview & Slatview N. w | t |, Nomarski fillers (inside Macintosh) \.

Scion frame grabber board

(Inside Macintosh)

IMAGE ANALYSIS OF SHOULDER CUTS

(Interrupted cutting tests)

it

:

j %

I

1I

k

T1'K''.« -mil

174
DTM Grinding Spindle

Z Axis

Motor

Vacuum Chuck

Air Bearing

X Axis

Grinding

Chip Formation Geometry

Cutting

direction

Subsequent grinding wheel revolutions

175
Chip Ccyiuvtry

176
Crossfeed (jim/rev)

< 50

) 2.2 4.4 6.6 8.8 11.0

I 1 ' i 1

(ran) 40

\ —*~Nose Radius

\

~~~° Crossfeed

30

20

RMS 10

* 1 . L i 1 1 0

CI 200 400 600 800 1000

Wheel Nose Radius (urn)

177
178
MACHINING OF CERAMICS FOR BEARING

APPLICATIONS

• High-precision machine tools

• "Ductile-regime" conditions

• Machine variables and wheel parameters

• Mechanisms, process models, and control strategies

• Applications to precision mechanical as well as optical and

electronic surfaces

179

Design and Life Prediction Issues for Ceramic Bearings

Crawford Meeks

AVCON, Inc.

Northridge, CA 91324

The development of ceramic bearing technology offers many benefits for DOD

applications from advanced aircraft gas turbines, to inertial guidance

instruments, to ultra-precision pointing and tracking gimbals.

Development of advanced bearings utilizing ceramic materials and transfer

lubricants is a highly heuristic process. The materials technologist and bearing

designer must work in concert to be effective. Even the best materials system

will not perform effectively in a poorly designed rolling element bearing.

The bearing designer aids the materials technologist by establishing goals

for materials performance such as friction coefficients, traction coefficients,

strength and modulus of elasticity requirements. The designer, through creative

synergy, aims to devise design solutions to thermal expansion mismatches, and

wear rates that result in finite limits on potential life. Examples of such

design processes abound in the ceramic bearing field from composite ball cages

of metal with transfer film inserts to compliant mounting schemes for race-toshaft and race- to-housing interfaces. Non- symmetrical ball cage pocket

arrangements, and deliberately unbalanced ball cages are examples of the designer

aiding in the assimilation of advanced materials technology into the bearing

design process.

Because of the complexity of rolling element bearings, computer methods

are necessary for just analyzing stress versus load relationships. For example,

just determining the load, working contact angle, and stress of each ball in a

bearing requires solution of five complex simultaneous equations N number of

times (here N is the number of balls in the bearing) . In critical performance

applications and high speed bearing design, the computer becomes the only means

for predicting bearing component behavior.

Over a period of several years, a comprehensive ball bearing analysis tool

"BABERDYN" has been developed. This computer program is based on the use of

"smart" algorithms to achieve high accuracy and computational efficiency. These

algorithms were developed by writing the equations of motion of the balls and

ball cage in a rotating coordinate frame. This results in very simple one and

two term equations that are numerically integrated to obtain velocities and

displacements from the basic Newtonian acceleration equations. The results are

then transformed to an inertially fixed coordinate system through transformation

matrices. This method both improves computational accuracy and computational

efficiency. In fact these algorithms are so efficient, that a personal computer

(PC) versions of the program now yields useful analytical results in over five

shaft revolutions in run times of about one half hour.

BABERDYN has been proven valid and accurate through comparison of results

with empirical data on ball cage motions and through demonstration of problem

181
solving ability by elimination of existing failures or poor performance in real

bearing applications.

Besides the process of design synthesis through computerized analysis of

rolling element bearing design and defining of performance goals for trobological

materials systems, computerized design tools can be useful in predicting life

and reliability of bearing/materials systems. By identifying the potential

failure modes of bearings and testing for the critical values of materials system

operation such as wear rates, and fatigue spalling functions, regression analysis

can be used to develop algorithms for predicting bearing long term performance

characteristics and life can be analyzed.

The key potential failure modes in ceramic bearings are:

o ball wear

o raceway wear

o ball cage wear

o fatigue

o insufficient (or excessive) lubricant transfer to the races or balls.

By identifying these failure modes and performing tribo -materials tests

on specimens and real bearings, data can be developed for predicting the severity

of such degradation effects and to predict the longevity of bearings.

A comprehensive program is needed that will:

1. Expand and improve on existing computerized analysis tools.

2. Establish a research/development program in which the designer/

analyst, and tribologist work in close concert to establish materials

"windows" of performance to guide the tribological research.

3. Create a team effort in which the designer/analyst apply

tribomaterials systems to new bearing designs and iterate the designs

based on results of bearing environmental and performance tests.

4. Modify, and improve upon, existing regression analysis tools for

predicting bearing life factors based on ongoing research that

characterizes ceramic materials systems (wear rate, fatigue, etc.)

and incorporate these into existing computer models.

Such a comprehensive, cooperative program integrating the efforts of

research tribologists , bearing designers and computer analysts will maximize the

potential for success and permit giant strides in improvement of performance of

advanced ceramic bearing systems.

182
DESIGN AND LIFE PREDICTION ISSUES

for

CERAMIC BEARINGS

by

CRAWFORD MEEKS

AVCON-Advanced Controls Technology, Inc. 19151 Parthenia Street. Unit G

Northridge, CA 91324

(818)886-0250

DEVELOPMENT OF ADVANCED CERAMIC

BEARINGS IS A HIGHLY HEURISTIC PROCESS

AVCON

SYSTEM

REQUIREMENTS

DESIGN ANALYST

PREDICTIONS:

' LIFE

* TORQUE

• TORQUE NOISE

' RELIABILITY

MATERIALS PERFORMANCE

GOALS

BEARING DESIGN

TESTING

TRIBOLOGIST

DATA

DATA

DATA

183
DEVELOPMENT OF HIGH DN, HIGH TEMPERATURE

BEARINGS REQUIRES OPTIMUM DESIGN AS WELL

AS ADVANCE TRIBO-MATERIALS SYSTEMS AVCON

IMPORTANT DESIGN VARIABLES IN ROLLING ELEMENT

RFARINfi^- DcnnI o

.

BALL BEARINGS ROLLER BEARINGS

0 BALL DIAMETER 0 ROLLER DIAMETER O CONTACT ANGLE 0 ROLLER CONE ANGLE

O PITCH DIAMETER 0 PITCH DIAMETER O RACEWAY CURVATURE o GUIDE FLANGE HEIGHT

(INNER and OUTER)

O NUMBER OF BALLS o ROLLER-CAGE POCKET CLEARANCE

(LONGITUDINAL & LATERAL)

0 SHOULDER HEIGHT 0 ROLLER-CAGE TO RACE LAND CLEARANCE

0 BALL-CAGE POCKET o DENSITY OF ROLLERS, ROLLER-CAGE CLEARANCE

0 DENSITY OF BALLS, 0 PRELOAD (OR DIAMETRAL CLEARANCE)

BALL-CAGE 0 PRELOAD o SYMMETRY

0 SYMMETRY o TYPE OF ROLLER CAGE DESIGN

0 MACHINED

0 FORMED

0 TRANSFER FILM INSERTS 0 FACE-TO-FACE OR o FACE-TO-FACE OR

BACK-TO-BACK BACK-TO-BACK O TYPE OF BALL-CAGE DESIGN

o CYLINDER POCKET

0 CYLINDER CONE POCKET

o RACE GUIDED

0 CROWN OR SOLID RAIL

SEVERAL FACTORS CONTRIBUTE TO

LOADS AND STRESSES IN WEAR AFFECTED

ZONE IN ROLLING ELEMENT BEARINGS AVCON

EXTERNAL LOADS

O RADIAL

O AXIAL

O MOMENT

O UNBALANCE

O "FRAME" VIBRATION

INTERNAL LOADS

O CENTRIFUGAL

O GYROSCOPIC

O DYNAMIC COLLISION OF CAGE AND ROLLING

ELEMENTS

O BALL/ROLLER-TO-RACE SKIDDING FORCES

184
DESIGN IS A HEURESTIC PROCESS

OF SYNTHESIS

O ANALYSIS AND OPTIMIZATION OF KEY DESIGN

VARIABLES

O OPTIMIZATION OF COMPLEX INTERDEPENDANT

FUNCTIONS SUCH AS:

BALL-CAGE CLEARANCES (POCKET and LAND)

TRACTION COEFFICIENT

MASS PROPERTIES OF BALL-CAGE

O APPLICATION OF EXPERIENCE, JUDGEMENT FROM

REAL BEARING TESTS

O INTEGRATION WITH SHAFT, HOUSING

o FITS

o EXPANSION MISMATCH

o PRELOADING SCHEME (SOFT or HARD)

O INTUITION ! !

SCHEMATIC OF BALL BEARING AND SEPARATOR

DYNAMIC MODEL "BABERDYN"

A¥COi

V. V, V

Z.Z.Z

X. x.x

SIX DEGREE OF FREEDOM MODEL OF SEPARATOR

NOMENCLATURE: N = NORMAL BALL/RACE LOAD

I = TRACTION COEFFICIENT

Xb/ft = SUP VELOCITY OF BALL ON RACE

FS = SEPARATOR IMPACT FORCE

K = BALL POCKET STIFFNESS C = BALL POCKET DAMPING

COEFFICIENT Mb = BALL MASS

F| o

= TRACTION/SLIP FORCES

OF INNER/OUTER RACE CONTACT

III. BALL. RACE

IMPACT FORCES ON SEPARATOR SUMMED TO CALCULATE SEPARATOR MOTIONS

COMPUTES BALL/RACE DISPLACEMENTS STRESS AND NORMAL LOAD OF EACH BALL DUE TO

EXTERNAL LOADS

II. COMPUTES SEPARATOR - BALL/RACE IMPACTS -— F| = l(N.Xb/Rj, <l

Xb = I IFS. K. C. Mb. F|. Fol

Fo = MN.Xb/R0

•)

185
THE SELECTION OF A ROTATING COORDINATE

SYSTEM FOR WRITING EQUATIONS GREATLY H SIMPLIFIES MATHEMATICAL INTEGRATION BUT| AVCOIM

PRESERVES RIGOROUS SOLUTION H

SEPARATOR NOMINAL SPIN RATE |w B/s> uB/s = IjujU-yl + Uod * r\] 2

WHERE; . COS0

[11

121

USING SMALL ANGLE APPROXIMATIONS. THE BALL- SEPARATOR TO SEPARATOR-FIXED TRANSFORMATION IS:

-«z 1 "x

»v -»x

1

THE MATRIX [Bj) DEFINES THE POSITION OF THE Ith POCKET

IN THE SEPARATOR AS: COS 0, SIN i/i; 0

[Bj] = ' -SIN iP; COS V, 0

O 0 1 THE TOTAL FORCE ON THE SEPARATOR IS:

(3)

141

F= Z|BjlT A Fj

i

AND. THE TOTAL MOMENT ON THE SEPARATOR IS: M= IlB^AMj

I 1

I )

NOMENCLATURE:

AF,= TOTAL IMCREMENTAL FORCE VECTOR

A = TOTAL IMCREMENTAL MOMENT VECTOR

rB

= RADIUS OF BALL

tp = PITCH RADIUS OF BALL ORBIT

P= CONTACT ANGLE

«X. *y. flj. = SMALL ANGLES (Xs Ys Zs

IS OBTAINED BY

ROTATING X Y Z THROUGH THESE ANGLES)

lli; = ANGULAR POSITION OF THE l-th BALL

Uj = ROTATION RATE OF INNER RACE w = ROTATION RATE OF OUTER RACE

VERY SIMPLE VECTOR EQUATIONS ARE INTEGRATED TO SOLVE FOR FORCES AND ACCELERATIONS OF BEARING COMPONENTS

186
OUTPUTS OF BABERDYN COMPUTER PROGRAM AVCON

• CAGE DISPLACEMENTS: Xe ,YC ,Zt ,ttY ,6^

• CAGE VELOCITIES AND ACCELERATIONS

• H ENERGY DISSIPATION IN BALL POCKETS

AND CAGE LANDS AND BALL/RACE SKIDDING

• BALL POCKET NORMAL FORCES

• BALL/RACE SLIP VELOCITIES AND ACCELERATIONS

• TORQUE, TORQUE NOISE

• TORQUE DUE TO GEOMETRIC IMPERFECTIONS

OPTIMIZATION OF BALL-CAGE FOR MINIMUM

BALL-POCKET WEAR AVCON

430 n to OOO H

CONTACT »H6U " 30'

Flo, 14-OpllmlMllon ot bill Mpirilor lo/ minimum b»lt-pockel WEAR

187
FOLLOWING DESIGN GUIDELINES DEVELOPED

BY BABERDYN INCREASED 600°F, 40,000 BEARING

LIFE FROM 3 MINUTES TO 38 MINUTES AVCON

Figure a. SeiMubncating polyimide composite separators

after Bearing Test No. 2, prepared from cylindrical 3D graphite liber weaves ( AC-1 /T50C2).

188
AXIAL PROBES 16C00BPM

AXJAL LOAD 1000 LBS WLCT Oil - JOW

01 01 03 04 OB

I

tin;

07 08

TIV1E |S£C)K*0 "

jj

r

1 1 r OB 10

3

AVCOIM

BALL SEPARATOR

MOTIONS —

COMPARISON OF

STRIP CHART

MEASUREMENTS

ANDCOMPUTER

PREDICTIONS

I J .Li SB335Bi AXIAL PROBES

10000 HPM

AXIAL LOAD UO IDS = INLET oa - JCO»» AVCON

BALL SEPARATOR

MOTIONS —

COMPARISON OF

STRIP CHART

MEASUREMENTS

ANDCOMPUTER

PREDICTIONS

189
1 w axial pnoeES

1 AXIAL LOAD * 100 US

| inui on - xct

i

1 1 I

1 i

' r

0 01 01 01

I

I

' ' I' '

' I I

' 04 Ob 06 01 OB

hi-tt iStCiBlO 1

1 sm

i—i

-r10 ti IJ

AVCON

BALL SEPARATOR

MOTIONS —

COMPARISON

OF STRIP CHART

MEASUREMENTS

AND COMPUTER

PREDICTIONS

AVCON

BALL SEPARATOR

MOTIONS —

COMPARISON OF

STRIP CHART

MEASUREMENTS

AND COMPUTER

PREDICTIONS

0 01 01 03 04 06 OB 07 08 09 10 1.1 12

TiMf (StOXIO 1

190
AVCON

BALL SEPARATOR

MOTIONS —

COMPARISON OF

STRIP CHART

MEASUREMENTS

AND COMPUTER

PREDICTIONS

AVCON

-O006 -0010

-00)6

BALL SEPARATOR

MOTIONS —

COMPARISON OF

STRIP CHART

MEASUREMENTS

AND COMPUTER

PREDICTIONS

191
AVCOW

BALL SEPARATOR

MOTIONS —

COMPARISON OF

STRIP CHART

MEASUREMENTS

AND COMPUTER

PREDICTIONS

AVCON

BALL SEPARATOR

MOTIONS —

COMPARISON OF

STRIP CHART

MEASUREMENTS

AND COMPUTER

PREDICTIONS

192
j AXIAL MIOSES J 3000 WM

p AXIAL LOAD - 1000 US

Win OIL - 200*

AVCON

BALL SEPARATOR

MOTIONS —

COMPARISON

OF STRIP CHART

MEASUREMENTS

ANDCOMPUTER

PREDICTIONS

AVCON

BALL SEPARATOR

MOTIONS —

COMPARISON

OF STRIP CHART

MEASUREMENTS

AND COMPUTER

PREDICTIONS

193
BABERDYN VALIDITY HAS BEEN VERIFIED

EXPERIMENTALLY AVCOIM

1. CORRELATION OF FORCE PREDICTIONS WITH

CANTILEVER BEAM INSTRUMENTED BALL-CAGE

2. CORRELATION OF CAGE MOTION PREDICTIONS AND

PROXIMETER MEASUREMENTS OF CAGE MOTION

(BOTH AIRFORCE AND COMSAT)

3. GGIII GYRO BEARING CAGE FAILURES ELIMINATED

4. CMG WHEEL BEARING GROAN ELIMINATED

5. IT FOLLOWS THAT IF CAGE MOTION PREDICTIONS

ARE CORRECT, THEN, PREDICTIONS OF COLLISION

FORCES, BALL SLIP, TORQUE NOISE AND HEATING

RATES ARE VALID SINCE CAGE MOTIONS ARE

DETERMINED BY INTEGRATING COLLISION FORCES

AND DIFFERENTIATING RESULTS FOR VELOCITIES

AND DISPLACEMENTS

6. THE COMPUTER PROGRAM WAS SHOWN TO BE VERY

COST EFFECTIVE BY PREDICTING AN AVERAGE OF

OVER 7 SHAFT REVOLUTIONS IN TEN CASES

ANALYZED FOR AN AVERAGE OF LESS THAN 16

MINUTES OF CPU TIME

PERFORMANCE AND LIFE PREDICTION

ARE ANALYTICAL PROCESSES AVCON

LAB TESTING OF

WEAR RATES,

TRACTION, FRICTION

REGRESSION

ANALYSIS TO

DEVELOP

THEORETICAL

MODEL

COMPARISON

OF PREDICTIONS

AND TEST

RESULTS

FORMULATION OF

PERFORMANCE

PREDICTION

ALGORITHMS

194
THE FIRST STEP IN PREDICTING BEARING

LIFE IS TO IDENTIFY FAILURE MODES AVCOIM

FAILURE MODES OF "SOLID" LUBRICATED CERAMIC

BEARINGS:

O WEAR OF RACES

O WEAR OF BALLS OR ROLLERS

O WEAR OF CAGE

O TOO HIGH OR TOO LOW FILM TRANSFER

O FATIGUE

195
BALL-CAGE POCKET WEAR MAY BE DETERMINED

FROM BEARING TESTS OR FRICTION/WEAR TEST

MACHINES AVCON

V = wear scar volume (cm 1

) a = half length of elliptical wear scar (cm)

b = half width of elliptical wear scar (cm)

h = depth of wear scar (cm)

SCHEMATIC OF TRANSFER LUBRICANT SYSTEM

OF CERAMIC BEARING AVCON

EQUILIBRIUM CONDITION:

T - i n

* m^/pj - m^fit x i h WHERE T • FIL" THICKNESS ICM|

.. - INITIAL FILM THICKNESS ICMI - u

- LIBRICANT TRANSFERRED BALL TO RACE [CM] mtf - LUBRICANT WORN OFF RACEWAY 1GM|

m,,, • LUBRICANT TR ANSFt RREO F ROM SEPARATOR TO BALL (GM)

ft - RACEWAY WEAR (CM)

p • OENSlTY (GM/CM3!

* - AREA OF BALL WEAR CONTACT 2DNE (CM2I

196
REGRESSION ANALYSIS OF RACEWAY

WEAR MUST ACCOUNT FOR DIMINISHING

PRELOAD VS WEAR EFFECTS

AVCON

FROM LUNDGERG AND PALMGREN, THE CLASSICAL

APPROACH TO BEARING FATIGUE LIFE IS:

tn i = 0 1053 {£) [23]

where

91 = reliability for time (

e = Weibull slope -10/9 for ball bearings

I = operating time in hours

tio = Fatigue life (hours) for 0.90 reliability associated with the corresponding load stress condition

This equation can be extended to allow for decreasing

preload in the following way (2):

(^lo)equivalcn, = < 2 ^TT [ 241 1= 1 (l|0),

where

(CD), = percent of the total running time in question to be spent at a given load/stress condition

n = chosen number of increments to allow for chancing the stress condiiions

ROLLING ELEMENT BEARING LIFE PREDICTION

AVCON

1. IDENTIFY FAILURE MODES

2. TEST FOR WEAR RATE IN LAB TESTS

(A) BALL/RACEWAY

(B) BALL-CAGE POCKET

(C) BALL-CAGE TO LAND

3. MODIFY CLASSICAL* FATIGUE MODELS

ANALYTICALLY FOR CERAMIC MATERIALS SYSTEMS

4. DETERMINE LOADING CONDITIONS FROM

ANALYTICAL DYNAMIC MODELS

5. COMBINE EFFECTS FOR OVERALL BEARING SYSTEM

LIFE, RELIABILITY PREDICTION

* BASED ON OIL LUBRICATED, 521 00 BEARING STEEL

TESTS

197
GIANT STEPS FORWARD IN CERAMIC BEARING

TECHNOLOGY CAN BE REALIZED

AVCOIM

A COMPREHENSIVE, COOPERATIVE PROGRAM THAT INTEGRATES THE

EFFORTS OF THE TRIBOLOGIST, BEARING DESIGNER AND COMPUTER

ANALYST WILL MAXIMIZE THE PROBABILITY OF SUCCESSFUL HIGH

TEMPERATURE, HIGH DN BEARINGS.

SPECIFICALLY:

1, EXPAND AND IMPROVE ON EXISTING COMPUTERIZED ANALYSIS

TOOLS,

2. ESTABLISH A RESEARCH/DEVELOPMENT PROGRAM IN WHICH THE

DESIGNER/ANALYST AND TRIBOLOGIST WORK IN CLOSE CONCERT

TO ESTABLISH MATERIALS "WINDOWS" OF PERFORMANCE TO

GUIDE THE TRIBOLOGICAL RESEARCH,

3. CREATE A TEAM EFFORT IN WHICH THE DESIGNER/ANALYST APPLY

TRIBOMATERIALS SYSTEMS TO NEW BEARING DESIGNS AND

ITERATE THE DESIGNS BASED ON RESULTS OF BEARING

ENVIRONMENTAL AND PERFORMANCE TESTS.

4. MODIFY, AND IMPROVE UPON, EXISTING REGRESSION ANALYSIS

TOOLS FOR PREDICTING BEARING LIFE FACTORS BASED ON

ONGOING RESEARCH THAT CHARACTERIZES CERAMIC MATERIALS

SYSTEMS (WEAR RATE, FATIGUE, ETC.) AND INCORPORATE THESE

INTO EXISTING COMPUTER MODELS.

198 i
Tribology Issues Related to Machining and Performance

of Ceramic Bearings

Said Jahanmir

Tribology Group

National Institute of Standards and Technology

Gaithersburg, MD 20899

This presentation will provide a brief summary of recent research at NIST

related to machining and tribological performance of ceramic rolling element

bearings. The specific topics to be covered include: the effect of contact load

and temperature on wear, boundary and solid lubrication of silicon nitride, selflubricating composites for bearing cages, and chemically assisted ductile regime

grinding.

The goal of our project on wear of ceramics is to elucidate the fundamental

mechanisms of wear of different ceramic tribomaterials tested under various

conditions of load and temperature. The testing program on two materials (AD998 alumina and NBD-100 silicon nitride) was recently completed. The results

have been assembled in wear transition diagrams , which are two-dimensional plots

showing different regimes of tribological behavior separated by solid lines

depicting the transitions between the regimes. For example, the tribological

behavior of alumina can be divided into four regimes: lubrication with films

formed by tribochemical reactions with water vapor, mild wear by plowing, severe

wear by fracture, and glass -phase lubrication at high temperatures. The

tribochemical reaction, plowing, and fracture regimes are also observed for

silicon nitride . As the temperature is increased, selective oxidation of various

second phases in silicon nitride is also observed, and at higher temperatures

oxidation of the primary phase is found to contribute to the wear process.

Preliminary results on silicon carbide (Hexoloy SA) have shown that, similar to

silicon nitride , tribochemical reaction, fracture, and oxidation control the wear

process of this material. A linear elastic fracture mechanics model was used

to analyze the contact fracture process. The model has confirmed that

propagation of cracks from pre-existing flaws controls the fracture process.

Furthermore, the model substantiated the strong role of friction coefficient that

has been observed experimentally. Future research should evaluate the effect

of microstructure and composition on the transitions. This information could

lead to the development of ceramic tribomaterials with a larger load- carrying

capacity and improved reliability.

Transition from mild to severe wear by fracture is highly sensitive to the

coefficient of friction, because of the effect of friction on the tensile

component of stress at the trailing edge of the contact circle. Therefore, even

a small reduction in the coefficient of friction can result in a large increase

in transition load, and a reduction in the rate of wear. The coefficient of

friction can be reduced by hydrocarbon lubricants and polar additives. Although

limited data have been published, the details of the boundary lubrication of

silicon nitride are not known. Particularly, the adsorption mechanism of polar

compounds on ceramic surfaces and the role of chemical structure on the

199
adsorption process need to be analyzed. This type of information can be used

to develop effective lubricants for marginally lubricated ceramic bearings.

Hydrocarbon liquid lubricants can not be used at elevated temperatures or

under vacuum environments due to severe degradation and evaporation. Solid

lubricants must be used under these conditions. One of the problems is the

delivery of the solid lubricant to the contact. In rolling element bearings

solid lubrication may be provided by a self -lubricating composite cage containing

the solid lubricant as a second phase. Our research in this project has been

concentrated on understanding the mechanisms of wear and transfer film formation

in both metal-matrix and ceramic-matrix composites. Our recent results have

confirmed that in these materials lubrication is achieved by the formation of

a transfer film, which contains materials from both the composite and the

counterface, as well as tribochemical reaction products. Therefore, a thorough

understanding on the formation of the transfer film and the relationship between

the micros true ture of the composite and mechanical properties of the transfer

film are needed to optimize the microstructure and composition of selflubricating materials.

The performance and reliability of ceramic rolling element bearings are

controlled by such factors as microstructure and composition, surface roughness

and integrity, and contact conditions. Machining and final finishing of ceramic

components can have a strong influence on reliability and performance because

of the damage produced during these operations. Formation of defects and

residual stresses due to machining can be detrimental and promote early failure

by fatigue and spalling. In addition to the effect of damage on performance,

machining contributes to the high cost of ceramic bearings. Therefore, it is

imperative to find innovative and cost-effective methods to machine ceramic

bearing components that are free of machining generated defects. Our machining

program is focused on four topics: chemically assisted machining, damage

characterization and sensor development, optimization of the diamond grinding

process, and intelligent machining systems. Our efforts on chemically assisted

ductile regime grinding is one of the projects focused on finding new ways to

machine damage-free high-precision surfaces. Recent results have shown that

ductile regime grinding of advanced ceramics, such as silicon carbide, is

feasible. The concept of ductile regime grinding is based on the idea that at

very small depths of cut the material can be removed by plastic deformation

rather than fracture, which is normally observed for brittle materials. In our

program, we are investigating the effect of chemical environment on the process

of chip formation, with the goal of improving the grinding process through

selection of proper chemical compounds for addition to the grinding fluid.

200
TRIBOLOGY ISSUES RELATED TO MACHINING

AND

PERFORMANCE OF CERAMIC BEARINGS

Said Jahanmir

National Institute of Standards and Technology

April, 17,1991

201
0 Sliding Wear and effect of contact conditions

0 Boundary and solid lubrication

0 Self-lubricating composites

O Chemically assisted machining

Project Goals:

Determine the effect of contact

condition and micro structure on wear mechanisms of advanced

ceramics

Assemble data in simple diagrams

for use by design and materials

engineers

Develop mechanics models to

predict the effect of parameters

on severity of wear

202
HIGH TEMPERATURE TRIBOMETER

P

LOAD: 0.5 —15 kg

SPEED: 1 mm/sec. — 10 mm/sec.

TEMPERATURE: 21 C — 1000*C

STROKE LENGTH: 0 — 60 mm

ENVIRONMENT: AIR

SURFACE PROFILE

250/i.m

r

25fxm

6 kg, 200

8C, 120 min.

250/im

- r

25/xm

6 kg, 400

C C, 120 min.

203
WEAR TRANSITION DIAGRAM OF SILICON NITRIDE

1 -I 1 —L-+

0 200 400 600 800 1000

TEMPERATURE ( C)

204
12

11 -

10

0

e

7

e

o

ALPHA SiC

i

1

•

1

, —

,

I

1

K=10B-i | fr~f\ OO I nUiA6 i K=10B-3

f=0.80

j K=10B-i

j I —U.UU |K>10B-3

.r==u.7o

1 1 m

K<10B-<J ~*| o

f=0.22 1

1

1 T

0.2 0.4 0.0 (Thousands)

TEMPERATURE (DEO C)

0.8

Possible Fracture Initiation Modes

1) Shear and microplastic deformation below

the contact.

2) Tensile microcracks behind the contact.

205
CRACK /

P (critical load)

S.S.CHIANG AND A.G. EVANS MODEL

1000

a

<

o

a:o

a.

9c

100

10::

yield

«»

<3b_

CD

fracture

CD 00 CD

o mild wear

severe wear +

200 400 600

TEMPERATURE (°C)

—i

800 1000

206
20

a

CL

CD

>X

ot

O: ball (AOS95)

A: disc (A0998)

200 400 600 800

TEMPERATURE (*C)

—I—•

1000 1200

o

a.

u

5£

1--

« >

t:;* —A

• k—i -t-

*

i > " 0:bon(A0995)

A: disc (AD998)

y.—i i

i i—

1 » 1

1 i 1

200 400 600 800

TEMPERATURE (t)

1000 1200

207
208
10000-r

9

O

_J

3

O

p

a:o

1000ir

100,r

V\ 0.0

\ 0.2

0.4

O.fi

0.8

1—1 1, 1

0.9

1 i 200 400 600

TEMPERATURE('C)

800 1000

209
FRICTION COEFFICIENT OF SI,N« ON SI,N«

1—

Stable

Values

SUdlng Distance (m)

Friction coefficient of SISN4 sliding against Itself at low speeds

(t mm/a) In various environments plotted as a function of sliding

distance. The load la 9.81 M.

210
10-'»

i ii i i—i

1 i i

i r

V

JL JLJL J till I 3 4 0 110 20 40 00 100 200 400

Sliding Mine* (m)

-Logarithmic plot of ths wtir rate of KsN* sliding against Itself at low velocity in various environments.

Approaches to High Temperature Ceramic Components

Ring/Liners - "Once Through" Fluid System

Valves/Guides Self Lubricating Materials

Rolling Contact Bearings Self Lubricating Cage

Brakes Composite Materials

211
PURPO SE OF NIST PROJECT

DEVELOP THE KNOWLEDGE BASE FOR

LOWER WEAR, SELF -LUBRICATING

COMPOSITES FOR GAGES, GEARS,

BEARINGS AND SEALS.

CAGE -LUBRICATED SOLID LUBRICATED BEARINGS

Technical Advantages

• Low cost, low maintenance

• Simple, self-contained system

• High temperature and vacuum capability

• Oil-free applications

Technical Taaues

• Understand wear and transfer mechanisms

for cage/ball and ball/race contacts.

• Optimize cage materials and bearing

designs for low wear and long life.

212
CAGE MATERIALS vs MoS2 COATED 440C

(33N, 0.14 m/t, 1900 m, arjon)

a

i

g4

PSu

0

s

-3

7a

70

m.

10

>1

04

n 84

11 c

9

14 1211

am O08 0U2 ais as

gold

gold ini i plate NICKEL MOLYBDENUH

TITANIUM

BRONZE COPPER (ROD) 9002-lOMtiSa BOCu-201 .13. 10 AMICON PTFE

I I P0LYIM1DE SP3

I? P0LV1MIDE SPI

13 POLVIMIDE SP2I

IU nmoii)

024 028

Aver»|e Prlotlon Coemolent

Hohman + Aoro-1 O Aero-2 HCT

WEAR MODEL

/////>/ ft r / '/(/>/'{" V • 7

213
0.6

0.0

AI203

Si3N4

[]

10 20 30 40 50 60 70 80 90 100

Area Percent Graphite

c

oo

c

o

'-*-> o

u_

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

- o——O M50 ball/Zr02

flat

- •— * M50 ball/Zr02-Gr (20 v*) flat

o^_———

" o

> ^8^^-^ • ^^^^

- o

i i —-—i 0 100 200 300

Temperature °C

400

214
• Cost-Effectiveness - The Primary

Impediment to Utilization of

Advanced Ceramics

• Machining/Finishing - A Significant

Portion of Component Cost

Component Reliability - A Function

of Quality of Finished Surface

Industrial Needs

o Methods of Rapid Material Removal

with Minimal Damage

o In-situ and Post-machining NDE

Techniques for Damage Analysis

o Assessment of Surface Damage

Effects on Strength and

Performance

215
NT ST PROGRAM ON RAT>TD MACHINING

Proj ects

(1) Rapid Chemical Assisted

Machi ni ng

(2) Damage Characterization and

Sensor Development

(3) Optimization of Diamond

Grinding Process

(4) Intelligent Machining System

Mechanisms of Material Removal

a) Ductile

Chip

Format i on

Plastic

Deformation

Res i dual

Stress

b) Brittle

Fracture

Cracks and

Plastic

Deformati on

Res i dual

Stress

216
Typical Results on Si3N4

for Nano - Indentor

100 i

VtTTK (on)

jjajr* - iitaaai BOsm rtf.- rrj. *f 10

M -

0 -| 1 1 1 1 1 1 1 1 1 1 1 1 1

0 9J *A OJ SU I Li 14

DIRI (am)

217
NIST Precision Machining Research Facility

SiC Ductile Grinding Program, FY '91

• CVD SiC and siliconized SiC

- resin bond wheels

- optimize finish/sub-surface damage

- low damage microgrinding vs ductile regime grinding

• Metal bond wheels (for lower wear)

- dressing and truing for nm class run-out

- performance vs resin bond

• Chemical effects

- enhanced transition depths?

- slow speed scratch tester, single grit and

machining trials

CONCLUDING REMARKS

0 Ceramic rolling element bearings

must be designed with minimum sliding

0 Lubrication is critical to bearing

performance

0 Combination of self-lubricating

cages and solid lubricated races

is recommended

0 Reliable models are needed for failure

prediction

O Chemically assisted ductile regime

grinding can provide damage-free

precision bearing components

218
Appendix A

NIST/DARPA

CERAMIC BEARING TECHNOLOGY WORKSHOP

April 17-18, 1991

Sheraton- Potomac Hotel

Rockville, MD

April 17

8:30 am Introductory Remarks

Dr. Said Jahanmir, NIST, Workshop Chair

8:35 Welcome and Overview of Materials Science and Engineering Lab.

Dr. Lyle Schwartz, NIST

8:45 Workshop Agenda

Dr. Bill Coblenz, DARPA

9:00 Advantages of Ceramic and Ceramic Hybrid Bearings for Military

Applications

Dr. James Dill, Mechanical Technology, Inc.

9:30 Payoffs and Challenges in Utilization of Ceramic Bearings in

Spacecraft Mechanisms

Dr. Paul Fleischauer, Aerospace Corp.

10:00 Coffee Break

10:15 Surface and Subsurface Navy Applications and Requirements

Dr. Gerry Phillips, NSRDC

10:45 Instrument Bearing Requirements and Issues

Bob Westerholm, Litton Guidance

11:15 High Temperature Applications and Challenges

Dr. Lewis Sibley, Tribology Systems, Inc.

11:45 LUNCH

1:00 pm Effects of Composition, Microstructure , and Processing on Ceramic

Bearing Performance

Prof. Bob Katz, Worcester Polytech

1:30 Effects of Machining and Finishing on Performance

Dr. Mike Gardos , Hughes, and Julian Pratt, Spheric, Inc.

219
Requirements and Issues in Quality Control and NDE of Ceramic

Bearings

Prof. Pierre Khuri-Yakub, Stanford University

Coffee Break

Ductile Regime Grinding of Ceramic Bearings

Prof. Tom Dow, North Carolina State University

Design and Life Prediction Issues for Ceramic Bearings

Crawford Meeks, AVCON, Inc.

Tribology Issues Related to Machining and Performance of Ceramic

Bearings

Dr. Said Jahanmir, NIST

Discussion Groups Convene

o Ceramic Processing and Blank Fabrication

Dr. Jim Hannoosh, CERBEC , & Dr. Subhas Malghan, NIST

o Machining, Quality Control, and NDE

Dr. K. Subramaninan, Norton, & Dr. Grady White, NIST

o Design, Performance Testing, and Life Prediction

Dr. James Dill, Mechanical Technology Inc. , & Marshall Peterson, NIST

Happy Hour (Cash Bar)

Dinner

Invited Speaker: Dr. John Alic, Senior Associate, Office of

Technology Assessment, "U.S. Manufacturing: An Agenda for

Competitiveness"

April 18

Discussion Groups (continue)

Coffee Break

Recommended Research (presentations by Group Chairs)

Discussions and Concluding Remarks

S. Jahanmir and W. Coblenz

Lunch/Adj ourn

220
Appendix B

Final Participants List

Jim Aeschliman

MPB Corp.

P.O. Box 547

Keene, NH 03431-0547

John Alic

U.S. Congress

Office of Technical Assessment

Washington, DC 20510

Dave Almy

UES, Inc.

4401 Dayton-Xenia Road

Dayton, OH 45432-1894

Brian Bergsten

U.S. Air Force

WL/POSL

WPAFB, OH 45433

Duncan A. Boyce

Raytheon

50 Apple Hill Drive

Tewksbury, MA 01876

Harold Burrier

The Timken Co.

1835 Dueber Ave. , SW

Canton, OH 44706

Bernard J . Busovne

Garrett Ceramic

19800 S. Van Ness Ave.

Torrance, CA 90501

Y. P. Chiu

The Torrington Co.

59 Field St.

Torrington, CT 06790

William J. Chmura

The Torrington Co.

59 Field St.

Torrington, CT 06790

William S. Coblenz

DARPA/DSO

1400 Wilson Blvd.

Arlington, VA 22209

John H. Coleman

INSACO, Inc.

P.O. Box 9006

Quakertown, PA 18951

Thomas L. Daugherty

David Taylor Research Center

Annapolis, MD 21402

James F. Dill

Mechanical Technology

968 Albany -Shaker Road

Latham, NY 12110

Sokka Doraivelu

UES, Inc.

4401 Dayton-Xenia Road

Dayton, OH 45432-1894

Thomas A. Dow

Precision Engineering Center

North Carolina State Univ.

Box 7918

Raleigh, NC 27695-7918

William A. Ellingson

Argonne National Laboratory

9700 South Cass Ave.

Mail Stop 212

Argonne, IL 60439

Andre Ezis

Cercom, Inc.

1960 Watson Way

Vista, CA 92083

Robert H. Feest

The Barden Corp.

200 Park Ave.

Danbury, CT 06810

Traugott Fischer

Stevens Institute of

Technology

Materials Department

Hoboken, NJ 07030

221
Paul D. Fleischauer

The Aerospace Corp

.

P.O. Box 92957

Los Angeles, CA 90009-2957

Rosendo Fuquen

The Timken Co.

1835 Duebei Ave.

Canton, OH 44706

Michael N. Gardos

Huges Aircraft Co.

P.O. Box 902

E1/F150

El Segundo, CA 90245

George Gazza

U.S. Army Materials Tech. Laboratory

SLCMT-EMC

Watertown, MA 02172

Robert B. Gilbert

Allied Signal Aerospace Co.

Garrett Engine Division

111 S. 34th St., P.O. Box 5217

Phoenix, AZ 85010

Willard W. Goodwin

New Hampshire Ball Bearing

Route 202 South

Peterborough, NH 03458

Clark Griffiths

Splits Ballbearing

Highway Four

Lebanon, NH 03766

John Halloran

University of Michigan

Department of Materials

Science & Engineering

Ann Arbor, MI 48109

James G. Hannoosh

CERBEC

10 Airport Park Road

East Granby, CT 06026

Roy Hansen

Litton Systems

5500 Canoga Ave.

Woodland Hills, CA 91365

Andrew C . Harvey

Foster-Miller, Inc.

350 Second Ave

.

Waltham, MA 02154

Allen Hopper

Battelle Corp.

505 King Ave.

Columbus, OH 43201

George W. Hosang

Sundstrand Power Systems

4400 Ruffin Road

P.O. Box 85757

San Diego, CA 92186-5757

Lewis K. Ives

National Institute of

Standards & Technology

Bldg. 220, Rm. A215

Gaithersburg, MD 20899

Steven R. Johnson

General Electric

1 Neumann Way

MD A328

Cincinnati, OH 45215-6301

Robert Katz

Worcester Polytec. Institute

Dept. of Mechanical

Engineering

100 Institute Rd.

Worcester, MA 01609

John E. Keba

Rockwell International

Rocketdyne Division

6633 Canoga Ave.

Mail Stop 1A32

Canoga Park, CA 91303

B. T. Khuri-Yakub

Stanford University

3 Encina Hall

Stanford, CA 94305

E . Kingsbury

C. S. Draper Laboratory

Mail Stop 42

555 Technology Square

Cambridge, MA 02139

222
Gerald P. KoHutovic

United Technologies

Pratt & Whitney Aircraft

400 Main St.

East Hartford, CT 06108

Bruce Kramer

National Science Foundation

1800 G St. , N.W.

Washington, DC 20550

Frank M. Kustas

Martin Marietta

P.O. Box 179

Mail Stop - B3085

Denver, CO 80201

Philip G. Laferriere

Aviation Applied

Technology Directorate

U.S. Army Aviation

ATTN: SAVRT-TY-ATP

Ft. Eustis, VA 23604-5577

Jorn Larsen-Basse

National Science Foundation

1800 G St. , N.W.

Washington, DC 20550

John Law

Teledynes CAE

1330 Laskey Road

Toledo, OH 43612

Subhas Malghan

National Institute of

Standards & Technology

Bldg. 223, Rm. A256

Gaithersburg, MD 20899

Francis Marchand

C.S. Draper Lab.

555 Technology Square

Cambridge, MA 02139

Bobby D. McConnell

TriboTech Consultants

43 Monterey Road

Dayton, OH 45419-2566

Karl D. Mecklenburg

U.S. Air Force

Materials Lab.

594 Dorado Drive

Fairborn, OH 45324

Crawford Meeks

AVCON

19151 Parthenia St.

Unit G

Northridge, CA 91324

Raymond Middleton

U.S. Army Materials and

Technology Laboratory

405 Arsenal St.

Watertown, MA 02172

T. W. Mohr

The Timken Co.

1835 Dueber Ave.

Canton, OH 44706

Yngve Naerheim

Rockwell International

Science Center

1049 Camino Dos Rios

P.O. Box 1085

Thousand Oaks, CA 91360

Jim O'Donnell

Naval Air Propulsion Center

P.O. Box 7176

Code PE32:I0D

Trenton, NJ 08628

Barry O'Dwyer

Miniature Precision Bearings

Precision Park

P.O. Box 547

Keene, NH 03431

Mark Parish

CPS Superconductor

155 Fortune Blvd.

Milford, MA 01757

Marshall Peterson

National Institute of

Standards & Technology

Bldg. 220, Rm. A215

Gaithersburg, MD 20899

223
E. Pfaffenberger

General Motors Corp.

Allison Gas Turbine Division

P.O. Box 420

Speed Code S-48

Indianapolis, IN 46206-0420

Gerry Phillips

David Taylor Research Center

Code 2832

Annapolis, MD 21402

Robert S. Polvani

National Institute of

Standards & Technology

Bldg. 220, Rm. A107

Gaithersburg, MD 20899

Julian R. Pratt

Spheric Engineering Ltd.

Fleming Way

Crawley

West Sussex, RHIO 2SQ

England

K. S. Ramesh

Battelle Memorial Institute

505 King Ave.

Columbus, OH 43201

Mark Rhoads

General Electrical

Aircraft Engineering

One Neumann Way

Mail Stop M85

Cincinnati, OH 45215-6301

William Ruff

National Institute of

Standards & Technology

Bldg. 220, Rm. A215

Gaithersburg, MD 20899

Lyle Schwartz

National Institute of

Standards & Technology

Bldg. 223, Rm. B309

Gaithersburg, MD 20899

Lewis Sibley

Tribology Systems, Inc.

225 A. Plank Ave.

Paoli, PA 19301

Tony Sides

NAVSEA

Washington, DC 20362

Frederick Slaney

Textron- Lycoming

550 Main St.

Stratford, CT 06497

K . Subramanian

Norton Co.

1 New Bond St.

Worcester, MA 01606

Bob Westerholm

Litton Systems

5500 Canoga Ave.

Woodland Hills, CA 91365

Grady White

National Institute of

Standards & Technology

Bldg. 223, Rm. A256

Gaithersburg, MD 20899

-& U.S. GOVERNMENT PRINTING OFFICE: 1991 — 311-891/ 40502
NIST-114A

(REV. 3-90)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NUMBER

NIST/SP-824

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE

November 1991

TITLE AND SUBTITLE

Ceramic Bearing Technology: Proceedings of the NIST/DARPA Workshop

on Ceramic Bearing Technology, April 17-18, 1991

AUTHOR(S)

Said Jahanmir (Editor)

PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURG, MD 20899

7. CONTRACT/GRANT NUMBER

B. TYPE OF REPORT AND PERIOD COVERED

Final

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Defense Advanced Projects Agency

National Institute of Standards and Technology

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBLIOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

The objectives of the workshop were to assess the status of ceramic bearing

technology, and identify the key research topics needed to expand the range

of applications for ceramic bearings. A total of eleven invited

presentations were given at the workshop which was attended by seventy-five

representatives from industry, government and universities. The

presentations and subsequent discussions covered present and potential

future applications of ceramic bearings, and topics related to processing,

machining, quality control, design, testing and performance evaluation.

The report includes short abstracts and the viewgraphs used in the

presentations, summary of the discussions, and a list of recommendations

for future research.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

bearing design; ceramic bearings; ceramic machining; ceramic processing; friction

and wear; non- destructive evaluation; silicon nitride; surface fatigue; tribology

13. AVAILABILITY

X

UNLIMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD. VA 22161.

14. NUMBER OF PRINTED PAGES

219

15. PRICE

ELECTRONIC FORM
•


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treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive

in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NIST under the sponsorship of other government agencies.

Voluntary Product Standards — Developed under procedures published by the Department of

Commerce in Part 10, Title 15, of the Code of Federal Regulations. The standards establish

nationally recognized requirements for products, and provide all concerned interests with a basis

for common understanding of the characteristics of the products. NIST administers this program

as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series — Practical information, based on NIST research and experience,

covering areas of interest to the consumer. Easily understandable language and illustrations

provide useful background knowledge for shopping in today's technological marketplace.

Order the above NIST publications from: Superintendent of Documents, Government Printing Office,

Washington, DC 20402.

Order the following NIST publications— FIPS and NISTIRs—from the National Technical Information

Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB) — Publications in this series

collectively constitute the Federal Information Processing Standards Register. The Register serves

as the official source of information in the Federal Government regarding standards issued by

NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717 (38 FR 12315,

dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NIST Interagency Reports (NISTIR)— A special series of interim or final reports on work

performed by NIST for outside sponsors (both government and non-government). In general,

initial distribution is handled by the sponsor; public distribution is by the National Technical

Information Service, Springfield, VA 22161, in paper copy or microfiche form.
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