UTC 2002
`General Electric v. United Technologies
`IPR2016-01289
`
`1
`
`

`
`Longman Scientific & Technical
`Longman Group UK Limited
`Longrnan House, Burnt Mill, Harlow
`Essex CMZO ZJE, England
`and Associated Companies throughout the world.
`
`Copublished in the United States with
`John Wiley & Sons lno., 605 Third Avenue, New York, NY 10158
`
`© Longman Group UK Limited 1989
`
`Ali rights reserved; no part of this publication may be
`reproduced, stored in a retrieval system, or transmitted in
`any form or by any means, electronic, mechanical,
`photocopying, recording, or otherwise, without the prior
`written permission of the Pubiishers, or a licence
`permitting restricted copying in the United Kingdom issued
`by the Copyright Licensing Agency Ltd, 33-34 Alfred Place, London, WC1 E 7DP
`
`First published in 1989
`
`British Library Cataloguing in Publication Data
`Hocking, M.G.
`Metallic and ceramic coatings: production, high
`temperature properties and applications.
`1. High temperature corrosion resistant coatings
`I. Title Ii. Vasantasree, V. 111. Sidky, RS.
`667‘.9
`
`ISBN El-SEE-E33135-5
`
`Library of Congress Cataloguing in Publication Data avail-abie
`
`Printed and Bound in Great Britairi
`at the Bath Press, Avon
`
`2
`
`

`
`1 “
`
`l 2 2 4 5 5 6 6 6 8 9
`
`High Temperature Coatings ~ The Background
`introduction
`
`High Temperature Materials
`1.2.1 lvietailic Materials
`1.2.2. Ceramic Materials
`1.2.3. Composite Materials
`1.2.4. Rapidly Solidlfied Materials
`
`High Temperature Systems
`1.3.1. General
`
`1.3.2. Coal, Oil & Fluidized—Bed Systems
`1.3.3. Gas Turbine & Diesel Engine Systems
`1.3.4. Nuclear Power Systems
`
`Contents
`
`Foreword
`Preface
`
`Chapter 1
`1 .1 .
`
`1.2.
`
`Chapter 2
`2.1.
`
`High Temperature Coating Systems
`General
`
`2.2.
`
`Metallic Coating Systems
`2.2.1. Diffusion Coatings
`2.2.2. Overlay Coatings
`2.2.3. Other Metailic Coatings
`2.2.4. Metal Coatings on Ceramics
`
`Ceramic Coating Systems
`2.3.1. General
`
`2.3.2. Oxidation Resistant CVD Coatings
`2.3.3. Hard Overlay Coatings
`2.3.4. Rubbing Seals
`2.3.5. Thermal Barrier Coatings
`2.3.6. Erosion Resistant Coatings
`2.3.7. Diffusion Barrier Coatings
`2.3.8. Ceramic Coatings on Ceramics
`2.3.9. Desirable Ceramic Coating Properties
`Composite Coating Systems
`
`3
`
`

`
`2.5. Amorphous Coatings
`
`2.6. Coating Processes
`2.8.1. General
`
`2.6.2. Coatings & Coating Process Classification
`List of Acronyms
`
`Physical Vapour Deposition (PVD)
`introduction
`
`2.7.
`
`3.1.
`
`3.2. Evaporation Process
`3.2.1. The Basic Process
`
`3.2.2. Apparatus
`3.2.3. Evaporation Sources
`3.2.4. Evaporation Techniques
`
`Sputtering Process
`3.3.1, The Process in General
`
`3.3.2. Sputtering Methods
`
`lon Plating & Ion Implantation
`3.4.1.
`loo Platting
`3.4.2.
`ion implantation
`
`Chapter 4 Chemicai Vapour Deposition (CVD)
`4.1.
`Introduction
`4.1.1. General
`4.1.2. The Status of Information on CVD
`4.1.3. Earlier Literature on CVD
`4.1.4. Variations in CVD Process
`
`The CVD Process
`4.2.1. General
`4.2.2. CVD Terminology & Equipment
`4.2.3. CVD Control Parameters
`
`CVD Reactor
`4.31. The Fteactant Supply System
`4.3.2. The Deposition System
`4.3.3. The Recycle/Disposal System
`CVD Reactors: Line Diagrams
`
`The Fundamentals of CVD & CVD Technology
`4.5.1. CVD Thermodynamics
`4.5.2. CVD Kinetics
`4.5.3. CVD & Mass Transport
`CVD Reactions: A Selection
`
`CVD Products & Process Routes — A Cross Section
`4.7.1. Products & Routes
`_ 4.7.2. CVD Process Optimization
`
`Plasma Assisted CVD (PACVD); Plasma Enhanced
`CVD (PECVD)
`introduction
`4.8.1.
`4.8.2. The PACVD Technique
`4.8.3. PACVD Production Technology
`4.8.4. PACVD Materials
`
`4
`
`

`
`4.9.
`
`Laser — CVD
`
`4.9.1. Laser Crystals
`
`Chapter 5
`
`5.1.
`
`5.2.
`
`Chapter 6
`
`6.1.
`
`8.2.
`
`Coatings by Pack, Siurry, Sol-=Gel, Hot-Dip,
`Eiectrochernicai & Chemical Methods
`introduction
`
`The Pack Coating
`5.2.1. The Technique
`5.2.2. Specific Pack Examples
`5.2.3. Vacuum Pack Process
`
`5.2.4. Vacuum Slip-Pack Process
`5.2.5. Pressure Pulse Pack 8r CCRS Pack Methods
`5.2.6. Features of the Pack Process
`
`Slurry Coating Technique
`5.3.1. The Primary Method
`5.3.2. Double Slurry & Fioli Slurry Methods
`5.3.3. The Reaction Sinter Process
`
`The Sol-Gel Coating Technique
`5.4.1. General
`5.4.2. The Sol—Gel Method
`
`Hot Dip Coating Process
`
`Electrochemical & Chemical Coating Methods
`5.6.1. General
`
`5.6.2. Electropiating & Chemical Coating Processes
`5.6.3. Composite Deposition Technology
`5.6.4. Aqeuous Electroplating
`5.6.5. Specialised Applications of Electrodeposition
`5.6.6. Chemical Coating or Electroless Coating
`5.6.7. Fused Salt Electroplating or Metalliding
`5.6.8. Precious Metal Plating
`
`Coatings by Laser Surface Treatment, Rapid
`Solidification Processing, Spraying, Welding,
`Ciadding Sr Diffusion Methods
`In This Chapter
`
`Laser Coating Technology
`6.2.1. Laser Characteristics
`6.2.2. Features of a Laser Device
`
`6.2.3. Laser Surface Treatment (LST)
`6.2.4. Laser Assisted Coating Technology
`
`Rapid Solidification Processing (RSP)
`6.3.1. RSP Materials
`
`Droplet Transfer Coatings by Spraying
`
`Coating by Plasma Spraying
`6.5.1. Fundamental Aspects
`6.5.2. Atmospheric Pressure Plasma Spraying
`6.5.3. New Deveiopments in Plasma Spraying
`6.5.4. Low Pressure & Vacuum Plasma Spraying
`6.5.5. Thermal Spraying — Aspects of Coating
`Production
`
`5
`
`

`
`A 6.6 Other Droplet Transfer Coating Methods
`6.6.1. Coating by Detonation Gun (D—Gun)
`6.6.2. Flame Spraying
`6.6.8. Electric Arc Spraying
`6.6.4. Wire Explosion Spraying
`6.6.5. Liquid Metal Spraying
`
`6.7. Coating by Welding
`6.7.1. General
`
`6.7.2. Weld Surfacing
`6.7.8. Weld Surfacing Parameters
`6.7.4. Weld Surfacing Methods
`6.7.5. Weld Coated Finish
`
`6.8. Clad Surfacing
`6.8.1. Rolling & Extrusion
`6.8.2. Explosive Cladding
`
`6.9. Diffusion Bonding
`8.9.1. Primary Characteristics of Diffusion Bonding
`6.9.2. Bonding Mechanism
`6.9.3. Rose of lnterlayers in Diffusion Bonding
`6.9.4. Diffusion Bonded Materials
`6.9.5. Hot isostatic Press Bonding — (HIP)
`6.9.6. Electro—ii/lagnetic Impact Bonding (El\!llB)
`
`Chapter 7 Physical & Mechanicai Properties of Coatings
`7.1.
`introduction
`
`7.2. Evolution & Microstructure of Coatings
`7.2.1. General
`7.2.2. The Role of the Substrate
`7.2.3. Gas Pressure
`
`7.2.4. CVD Deposit Microstructure
`7.2.5. Pack & Plasma Deposit lvlicrostructure
`7.3. Thermal Expansion, Thermal Cycling
`
`7.4. Thermal Conductivity, Thermal Barrier Coatings
`7.4.1. Thermal Barrier Coatings (TBC)
`lnterditfusion
`
`.5.
`X‘
`
`_a:*‘\. Adhesion
`§7.6.1. General
`j7.6.2. Substrate Cleanliness vs Adhesion & Bonding
`i.7.7. ,5 Bonding
`V” 7.7.1. General
`7.7.2. Bond Strength
`
`7.8.
`
`internal Stresses, Strain & Fatigue Lite
`7.8.1. General
`
`7.8.2. Film Thickness Effect & lon—lmpect Stresses
`7.8.3. Stress Models
`7.8.4. Stress Relief Measures
`
`7.8.5. Strain & Fatigue
`7.9. Strength
`
`7.10. Ductility, Creep
`
`6
`
`

`
`Hardness vs Wear & Erosion Resistance
`7.11.1. Basic Aspects
`7.11.2. Hardness & Wear
`
`7.11.3. Hardness vs Application
`7.11.4. High Temperature Hardness & Hardfacing
`7.11.5. Rubbing Seals
`7.11.6. Hardness & Erosion Resistance
`7.11.7. Hard Coatings — Route Record
`7.11.8. Wear vs Multi—Component & Multi—Layered
`Coatings
`
`Chemical Properties of Coatings
`introduction
`
`Coating/Substrate/Environment vs Degradation
`8.2.1. General
`
`8.2.2. Effect of Ion implantation on Degradation
`8.2.3. Degradation Affected by Problem Areas in
`Substrate/Coating Compatibility
`
`Environmentai Conditions for Coating Degradation
`8.3.1. Fiuidized-Bed Combustors ~ Coal Gasifiers
`8.3.2. The Gas Turbine Environment
`
`8.3.3. Nuclear Technology & Tribology
`Environments
`
`Chemical Degradation Mechanisms
`8.4.1. General
`
`8.4.2. Case 1: Bivalent Metal + Oxygen = Metal
`Oxide
`
`8.4.8. Case 2: Multiple Elements + Oxygen :2
`Multiple Oxides
`8.4.4. Case 3: Multiple Elements + Multiple
`Reactants r lvlultiple Products
`8.4.5. Case 4: Cyclic Oxidation
`Oxidation & High Temperature Corrosion
`Hot Corrosion
`
`8.8.1. Factors at the initiation Stage of Hot Corrosion
`
`Carburization/Oxidation Degradation
`8.7.1. Carburization in the Presence of Sulphidation
`
`The Liquid Phase Effect
`8.8.1. Liquid Phase from the Environment
`8.8.2. Liquid Phase from Alloy + Gas Reactions
`8.8.3. The Effect of the Liquid Phase
`
`Fluxing Mechanisms in Hot Corrosion
`8.9.1. The Na2SO4 Model
`8.9.2. Acidic Fluxing
`8.9.3. Basic Fluxing
`8.9.4. Studies in Melt Systems
`
`Hot Corrosion in S2 & S02 Environments
`8.10.1. Goat Gasitier Conditions
`8.10.2. S2, S0,; Effect in Gas Turbine Conditions
`Erosion — Corrosion
`
`7
`
`

`
`8.12.
`
`Degradation of Alurninide Coatings
`8.12.1. Group 1. Aluminides
`8.12.2. Aluminide Case History: Selected Work
`8.12.3. Chrome — Aluminide Coating Degradation
`
`8.15.
`8.16.
`
`Chapter 9
`9 .1 .
`9.2.
`
`9.3.
`
`9.4.
`9.5.
`9.6.
`9.7.
`9.8.
`9.9.
`
`9.10.
`
`9.11.
`
`Chapter 10
`
`10.1.
`10.2.
`
`10.3.
`
`Degradation of l\/lCrAl Systems
`8.13.1. Multiple Additive Effect on MCrAl
`. Degradation
`
`Degradation of Silicide Coatings
`8.14.1. Silicide in Steels
`8.14.2. Silicide on Retractory Metals
`8.14.3. Silicide Degradation in Gas Turbine Alloys
`Degradation of Thermal Barrier Coatings
`Burner Rig Degradation Data
`
`Characterization, Repair & Functions oi Coatings
`General
`
`Modeling & Simulation
`
`Non-Destructive Testing (N DT) Methods
`9.3.1. Acoustic Methods
`9.3.2. Thermal Methods
`9.3.3. Dye Penetration
`9.3.4. Eddy Current Method
`Adhesion
`Tests on Mechanical Properties
`Structural Characterization
`Spectroscopy
`Degradation Characterization
`Pack Coating Characterization
`
`Coating Repair & Joining Techniques
`9.10.1. Repair Techniques
`9.10.2. Coated Component Joining Techniques
`Functions of High Temperature Coatings
`
`Strategic Metal Conservation, Coatings
`Achievements 8: Future
`-
`Introduction
`Coatings to Conserve Strategic Metals
`
`Present Achievements & Future Requirements of
`Coatings
`10.3.1. Aerospace
`10.3.2. Combustion Engines,
`10.8.3. Energy Conversion
`10.3.4.
`industriai (Petrochemical, glass making,
`machine tools)
`
`High Temperature Coatings: Present & Future
`Applications
`10.4.1. Generai Properties Assessment
`10.4.2. Future Work in Coatings for Gas Turbines
`10.4.3. Coatings for Supersonic & Aerospace
`Hardware
`
`8
`
`

`
`10.4.4. Coatings for Diesel Engines
`10.4.5. Coatings for Coal Gasitiers
`10.4.6. Coatings for Nuclear Reactors
`10.4.7. Coatings for Power Plant Systems
`10.4.8. Coatings & Solar Power
`
`Coating Production Methods: Future Work
`10.5.1. Evaporation
`10.5.2. Sputtering
`10.5.3.
`ion implantation
`10.5.4. PVD in General
`
`10.5.5. Chemical Vapour Deposition
`10.5.6. The Fused Slurry Process
`10.5.7. Laser Assisted Coating
`10.5.8. Spraying Methods
`10.5.9. Wire Explosion
`10.5.10 Sol-Gel Ceramic Coatings
`10.5.11. Diffusion Bonded Coatings
`10.5.12. Amorphous Alloys
`10.5.13. Wear Resistant Coatings
`10.5.14. Erosion Resistant Coatings
`10.5.15. Miscellaneous
`10.5.16. Repair Si Joining Methods
`106. Coating Properties— Further Work
`107. Testing Methods ~ Further Work
`
`Bibliography
`References
`Index
`
`9
`
`

`
`Foreword
`
`Coatings enable the attributes of two or more materials [the substrate and
`the coatings(s)] to be combined to form a composite having characteristics
`not readily or economically available in a monolithic material. Examples are
`tribological properties and high strength coupled with corrosion resistance.
`In the high temperature field economic and technical pressures to achieve
`extended lives and greater reliability plus a need to conserve certain
`relatively scarce (and hence expensive) or strategic alloying elements, have
`dictated increasing recource to coatings. The evolution of stronger, more
`creep resistant alloys to enhance thermal efficiency resulted in loss of
`oxidation and corrosion resistance. The need for surface stabilization led to
`the growing need for coatings and rapid developments in the field of surface
`engineering. Operation of materials at progressively higher temperatures
`close to the melting points of conventional alloys, Where adequate cooling
`leads to loss of efficiency, has led to the development of ceramic thermal
`barrier coatings. Coatings may also be essential in high temperature
`materials strengthed by fibres, to achieve compatibility between the matrix
`and reinforcement.
`As in most fields of technology, the field of high temperatures has also
`witnessed the practical and sometimes serendipitous application of materials
`in advance of theoretical ‘understanding of service problems. Between 'l950
`and 1970, very active scientific contributions were made to the development
`E11’1d‘llSC of high temperature materials and understanding of their response
`to their operating environment. The eighties saw a shift in emphasis to
`conservation of strategic materials and studies of ways and means of
`rninitnising loss and damage to expensive components with attendant loss of
`operating time and revenue. Although the literature on coatings can be
`traced back to the era of electro~deposition in general, and aluminising of
`iron in the early forties, much of the information on coatings relevant to
`achievement of optimum high temperature performance — production,
`
`10
`
`

`
`xiii
`
`control and replacement economics ~ has accumulated significantly in the
`seventies and to date. This book surveys the means of production of
`coatings, and their application over the broad range of high temperatures,
`above 400°C. The coating techniques discussed however, are actually used
`on substrates employed over entire groups of temperature ranges, from
`cryogenic to very high temperature. Thus the methods presented are
`selectively adapted in practice, for use as and on semi-conductor materials
`both in the regular and the rapidly growing areas of VLSI—comp0nents and
`surface mounting technology, as well as on materials used in power and
`energy generation and tribology.
`The authors are to be congratulated on their diligence and scholarship in
`bringing together such a comprehensive survey in the important field of
`surface engineering in high temperature technology.
`
`J.F.G. Conde
`
`Senior Consultant, lately
`Head, Metallurgy & Ceramics Division
`Admiralty Research Establishment,
`PEMOD, Holton Heath, Poole, Dorset, UK.
`
`11
`
`

`
`ereiaele
`
`Definite limits are being approached, or have been reached, in single
`materials development. A solution is to produce. a composite material which
`combines the best properties of different metals or ceramics. This may be a
`coating of one material on another. Hybrid materials bring their own
`problems, especially :in property matching at the interface, and coatings
`technology has thus emerged as a challenging field both for the fundamental
`and applied research Worker.
`This book was written to fill a longexisting need for a comprehensive
`source of information on metallic and ceramic coatings with all basic
`information and data presented in one compendium. Coating production
`methods as well as their application in various environrnents are discussed,
`with emphasis directed towards their high teinperature properties. The first
`chapter gives the background, and the range of high temperature application
`of coatings is indicated in Chapter 2. Chapters 3-6 survey the several
`techniques available for the production of coatings of various kinds and
`thicknesses on both metallic and ceramic substrates. Coatings properties are
`surveyed in the next two chapters — physical properties in Chapter 7 and
`chemical behaviour in Chapter 8. The influence of mechanical and thermal
`factors are interwoven in both these chapters. Coating testing and inspection
`methods: NDT, microscopy and spectroscopy and coating repair are
`discussed in Chapter 9 along with a tabulation of coatings function in the
`various high temperature areas. Chapter 10 surveys the success record of
`high temperature coating application, its role in saving strategic metals
`together with the scope of coating technology and further areas for
`investigation.
`A survey book of this nature aims to be a comprehensive source of an
`cnorrnous amount of information. The need for brevity restricts descriptive
`discussions. A general. bibliography is given which provides materi al for
`further reading on much of the basic inforrnation given here. A separate list
`
`12
`
`

`
`XV
`
`is provided of more than 2000 references referred to in the text. These and
`related papers of the same authors were used in reviewing the specific topics
`concerned. Further references to the work of individual authors can be
`
`accessed through the references listed. The literature output in the fields of
`fuel—cell and serni—conductor technology has been prolific, and their pace of
`expansion needs to be justified with a survey exclusively devoted to them.
`References in this area have been kept peripheral and a number of diagrams
`and tables have been edited and redrawn to suit the requirements of this
`book. The authors would like to extend a collective ackiiowledgement to all
`those whose contribution and expertise have made this review possible.
`Tables, figures, specific information and or data adapted for this book are
`acknowleged with a reference to the authors by the usual norm of scientific
`publication. Thanks are also due to Elsevier Sequoia and to Dr. G. Perugini
`for granting permission for the reproduction of figures 2.4 to 2.7 and 8.30.
`If this book proves useful to industrialists and practicing engineers who
`seek statc—0f-the—art information, provides the junior and senior academic
`population with good information on the sources and background in both
`theoretical and practical fields in metallic and ceramic coatings and surface
`modification, and offers ideas and relevant lines of investigation to
`investigators who look for areas of further work, then the authors’ efforts
`have been Worthwhile.
`
`Our special thanks are due to Mr J.F.G. Conde for writing a foreword to
`the book, and to all our colleagues in the Departments of Materials and of
`Mechanical Engineering at Imperial College, for their help and C.0—operation
`at various stages of its preparation.
`
`The Authors
`
`February 1989
`
`13
`
`

`
` cHAeTEs1
`iernperature coatings ~— the
`c background
`
`.1.
`
`INTEDDBCION
`
`=major challenge in technological development is to continue to
`eet requirements for new materials for use in progressively
`ore stringent conditions. Usually one or more of a material‘s
`roperties are incompatible with the conditions prevailing in the
`.,operating environment. In the material-environment configuration,
`"the surface of a component is a vital parameter in determining
`ts optimum usefulness. This is the basis for the development of
`oating technology.
`
`ubstrate in order to shield the component from an aggressive
`nvironment. The system is invariably hybrid, whether it has been
`chieved by means of a surface modification of the component
`substrate itself or one or more other materials have been applied
`s?a coating to the component surface. in either case, surface
`
`V ating system and it includes surface modification although it
`ay be more accurate to designate the latter as a surface modi-
`fied system.
`
`“e need for coating systems in the field of high temperature
`arose when improved performance criteria could not be met adequa~
`ly in spite of new materials with superior physical, mechanical
`“d metallurgical properties. Operating efficiency and production
`economy had to be considered and an improvement sought.
`
`14
`
`

`
`HIGH TEMPERATURE COATINGS * THE BACKGROUND
`
`In this chapter a very brief account is given of high temperature
`materials and systems where coatings have been necessary for
`enhancing component endurance and thereby the system function.
`
`ls2. HIGH TEHPERATURE MATERIALS
`
`1.2.1. METALLIC MATERIALS:
`
`High temperature metallic materials have developed rapidly beyond
`the early series of conventional ferrous alloys consisting of
`steels and stainless steels of various compositions, physical,
`thermal and mechanical history and microstructure. Chromium addi~
`tions, up to 25 wt.% have been used and contribute to an improve-
`ment in high temperature strength and oxidation resistance 0
`steels. Rupture strength increases were realised up to 340 MN m"
`with 15 Wt&% Cr, and a hig er temperature capability at 25 wt.%Cr
`at the cost of 100 MN m‘
`drop in rupture strength. Ferrous
`alloy development is well documented and Fe—base alloys are
`satisfactory up to l0O0°C under most oxidising conditions.
`Seve~
`ral high strength nickel— and cobalt—based alloys were developed
`in order to meet more complex high temperature corrosion pheno-
`mena.Superalloys » viz.Nimonics,lnco—alloy series, MAR— and
`Rene— series of cobalt alloys etc” came in the course of deve—
`lopment of gas turbine alloys. Chromium content has been varied
`over a wide range from lO—50 wt.%,
`in the Fe-, Ni-, Co— based
`alloys but it is the effect on properties by all other additives
`viz. Al, Ti, Si, Mo, Nb, Hf, Y, Ce, Zr, W etc., that resulted in
`the considerable expansion of superalloy uses.
`
`Superalloy components were prepared as cast and wrought, first by
`air melting and later by special vacuum melting procedures.
`Directional solidification, single crystal material preparation,
`the powder metallurgy route of fabrication (PM alloys), mechani-
`cal alloying (MAP alloys), dispersion strengthening and oxide
`dispersion strengthening (D8 and ODS) have been the various other
`process routes resorted to in improving superalloy performance.
`High temperature creep, strength and corrosion resistance have
`been improved but the melting points of these alloys are still in
`the order of 1250-l30OOC. Material improvements between 1940-1985
`have yielded a 300 degrees (C)
`increase in turbine entry tempera-
`ture, while the benefits of cooling have given an added advantage
`of operation up to about l320°C (Byworth 1986; Alexander & Driver
`1986). The role of alloying additives particularly those with
`sup. above 140000 in increasing the temperature range of super~
`alloys has been examined (Jena & Chaturvedi 1984; Fleischer
`1988).
`But a further
`increase in operating temperatures solely
`on the basis of superalloy component fabrication seems unlikely.
`Coatings are necessary,
`
`Progress in the development of creep resistant refractory alloys,
`viz. Mo—, Nb—, Ta— and Webased alloys has been explored for space
`
`2
`
`15
`
`

`
`HIGH TEMPERATURE COATINGS - THE BACKGROUND
`
`unuclear applications (Wadsworth et al 1988). Elements with high
`ielting points such as C, Cr, Mo, Nb, V, W, Zr etc., fail due to
`ixcessive oxidation» Volatile oxide products and molten oxide
`’ gmation are two other undesirable features (Table 1:1). Alloys
`
`y oxidation. They are also brittle and component welding has
`een a problem
`n gen r l,
`,
`ic. Metals withwbcc struct re ha
`ittle transition temperature (DBTT) giving poor mechanical
`Vroperties at lower temperatures, resulting in cracking during
`hermal cycling. Additives modify DBTT, eg. Re added to M0, W and
`“. Alternatives to Re and extensions of this beneficial effect
`re desirable, as are mechanistic studies of the phenomenon.
`
`TABLE 1:1
`
`HIGH TEMPERAEEBE EATERIAL CONSTITUENTS: %LTING PORQTS
`
`Element Melting Point,°C
`
`Element Melting Point,°C
`
`A1
`
`Mn
`
`Si
`
`C1"
`
`1840
`
`Cr,Pt,Si can form volatile oxides under certain condi-
`tions;
`C, W & Mo form gaseous oxides; Mo, Nb, Ta, W,
`& Zr undergo excessive oxidation; V forms molten oxides
`at >650°C; Ti dissolves oxygen.(Partington 1961)
`
`esistance heating elements function longer when coated, e.g.
`iuminized superalloy by surface oxidation self~proteotion. Other
`kamples of oxidation protection are 18-8 stainless steels plasma
`prayed with Nifil. _S.,,.i and plasma
`oated Nicr on steel used in annealing mil1s.High temperature
`etallic materials have to be assessed further but together with
`urface modification as a feature.
`
`16
`
`

`
`HIGH TEMPERATURE COATINGS - THE BACKGROUND
`
`1.2.2. CERAMIC MATERIALS:
`
`Ceramics appear at first an explorable alternative as high tempe~
`rature materials because they have much higher melting points.
`Their brittleness is the principal drawback, and they are diffi-
`cult to shape. Ceramics cannot function under conditions of
`mechanical and thermal shock (or cycling). Their high notch
`sensitivity renders them vulnerable to fracture under impact
`conditions. However,
`they have excellent corrosion resistance and
`low thermal conductivity. Ceramics are good candidates for ther—
`mal barrier materials. It is a two—fold problem to develop cera-
`mics as coatings:
`to find the most suitable types of coating
`systems and to establish a satisfactory method of coating conso-
`lidation.
`
`in that fracture occurs due to
`Ceramics are brittle (inductile)
`lack of significant plastic deformation (ductility) to absorb
`stresses by increasing the energy required for fracture propaga-
`tion (Godfrey l983/4).
`If a stress rate is high, e.g. explosive
`shock, it may exceed the dislocation propagation rate in a metal;
`the metal ductility then becomes irrelevant and it shows brittle
`behaviour;
`in this special case a ceramic may be stronger. But
`in general use,
`the non—ductile fracture behaviour of ceramics
`limits their use at high loads.
`
`The low ductility of ceramics can be partly compensated for by a
`large modulus of elasticity,
`freedom of the microstructure from
`weak phases and a grain shape and size which maximises the work
`of fracture. Phase transformation toughening is also an area
`worth exploring. Big improvements in fracture toughness and
`microstructure free of holes and flaws has been achieved by hot
`pressing Si3N4 and sic and adding a coating of a lower density
`reaction bonded surface layer containing Si
`(Kirchener & Seretsky
`1974 .
`
`( 3
`
`impact on ceramics is destructive but in situations
`Large object
`where only very small projectiles are possible, e.g. sand or dust
`erosion in an industrial compressor, hard ceramics perform much
`better than metals. Hot pressed silicon nitride eroded 5
`times
`less than a superalloy in a dust erosion conditions test (Napier
`& Metcalfe l977L
`
`ln general, above 1000°C ceramics have better strength, creep
`and oxidation resistance than superalloys.
`This temperature is
`considerably lower if comparison is made with ordinary alloys.
`Alumina has a high thermal expansion and thus undergoes thermal
`stress cracking; zirconia is stabilized by additives to reduce a
`destructive phase change at lOOD°C, but thermal cycling above
`lO00°C may still cause problems. It also has a high thermal
`expansion but its low thermal conductivity and very high tough-
`ness and strength below lO00°C make it suitable for diesel
`engines, which operate in a maximum temperature zone of 800°C.
`A
`zirconia coating life depends on its tetragonal phase content,
`and methods of zirconia stabilization for diesel engine coatings
`
`4
`
`17
`
`

`
`HIGH TEMPERATURE COATINGS - THE BACKGROUND
`
`fhave been discussed (Kvernes l983L
`
`igic and silicon nitride have very low thermal expansion. At high
`emperature, oxidation evolves CO or N2 which causes porosity in
`he protective layer on the ceramic. Above l500°C oxidation
`becomes severe. Production methods such as hot isostatic pressing
`jgxp), pyrolytic deposition and use of fugitive sinterers, e.g.
`Al in SiC, offer scope for further research. Sintering without
`”re5sing is feasible for Sialon, and materials comparable with
`ct pressed Si3N4 are obtainable (Godtrey l983/4L
`
`;.2. 3., COMPOSITE HATERIALS:
`
`The challenge for new materials has reached an exciting stage
`"With the development of composites. Dispersion strengthened
`"lloys, e.g. TD—NiCr, are the forerunners of the composite alloy
`group. Metal matrix composites {MMC) have advanced considerably
`' “om aerospace applications into light alloy technology, particu-
`arly the aluminium alloy series. Powder, particulates and fibres
`have been incorporated, mostly via the powder metallurgy route.
`“omposites in matrices of Mg and Ti have also been tried but to a
`ore limited extent. Ni- and Co- base alloys with incorporated
`oxides have been fabricated via the mechanical alloying route.
`
`Although MMC materials have been available for the last two
`decades they are yet to be tested to their full potential for
`high temperature behaviour and as coating materials (Harris 1988;
`tacey 1988). The development of carbon—carbon composites started
`n 1958 but
`intense research did not begin until the space
`shuttle project gathered momentum (Buckley l988). Ceramic coa—
`tings for carbon composites was a logical consequent material
`odification in the process of optimising the application of the
`omposite and combat its vulnerability to oxidation (Strife &
`y Sheehan l988L Composite coatings and composite/multilayer coa-
`ings have shown a considerable range and potential for develop-
`ent and application in the fields of high temperature and space
`’chnolo9Y (Wadsworth et al 1988; Das & Davis 1988; Lewis l987L
`
`ZA. RAPIQLY SOLIDIFIED MATERIALS:
`
`apid solidification processing (RSP) of metallic materials has
`Aened a totally new facet of alloy technology. The new materials
` e first called metallic glasses implying that their structure
`yyamorphous. Materials prepared by RSP are mostly limited to
`arrow strips.
`The technique can be adapted to incorporate
`_ppersed phases to consolidate the property advantages offered
`’ MMC, and the conjoint product promises to provide a superior
`urface modification system yet to be fully explored (Hancock
`_
`; Metallurgia report,l987; Waterman l987; Easterling,
`l988L
`ore details are given in section 1.6 of this chapter.
`
`18
`
`

`
`HIGH ‘TEMPERATURE COATINGS - THE BACKGROUND
`
`1.3.. HIGH TEM? SYSTEMS.
`
`l.3.,l., GENERAL:
`
`Power and energy systems where corrosion at high temperature is a
`severe problem are considered in this section. Component failure
`is often due to the synergistic action of several features in
`operation at high temperature conditions - the temperature and
`time scale of actual operation,
`local high temperature changes,
`the material used,
`the fuel employed,
`the prevailing environment
`and changes in it, mechanical and thermal stress cycles in the
`course of operation as well as shut down and rest periods. The
`insidious nature of high temperature corrosion lies in the fact
`that it causes a small section of the component to fail but the
`consequential costs for repair and re—instal1ation stages are
`very high.
`
`(1985) gives a comprehensive survey of a number of other high
`Lai
`temperature systems where molten salts, molten glass, hot metal
`and halogens comprise the corrosive media. Most of them use the
`high temperature alloys discussed here. A more detailed discus-
`sion on material degradation is given in a later chapter. Fig.1-1
`indicates the prevailing conditions for a number of high tempera-
`ture systems discussed below (Natesan,
`l985L
`
`}m3.2. COAL, OIL & FLU1DIZED~BED SYSTEMS:
`
`Combined cycle electricity generation from coal gasification
`offers a potentially large increase in overall efficiency com-
`pared to the more conventional coal-fired and oil-fired boiler
`systems. Coal composition varies widely resulting in a wide range
`of chemical composition of both the gasifiable material as well
`as the ash and erodent products. Temperature regimes can extend
`from 350°C for pressurised steam to fluid ash products at l695°C
`in combustion beds. Heat extractors operate at around 565°C in
`general
`in the UK. A heat flux prevails;
`the evaporator tubes
`operate at a metal surface temperature of 4500 while the super-
`heater tubes go up to 650°C. Corrosion rates of 100-500 nm/h and
`in extreme cases >1000 nm/h have been recorded (Meadowcroft,
`1987). The corrosive medium is aggressive to both Fe and Ni but
`not Cr.
`
`The gasification process operates at much higher temperatures
`than boiler units. The physical conditions can thus be severe,
`with hot, abrasive-erosive particles and ash impingement. The
`chemical environment is pffdominanfigy reducing with oxygen poten-
`tials (pO2) as low as 10
`to l
`at g bed temperature of 850-
`8700Cs3Sulphur potentials of 10’
`to 10” , and carbon activity of
`3x10"
`(sometimes approaching unit activity), have also been
`recorded (Weber,1986). Coal gasification atmospheres may contain
`CO
`and H20,
`each l0%,
`H28 0.5% and balance N2.
`90% of the
`
`6
`
`19
`
`

`
`HIGH TEMPERATURE COATINGS - THE BACKGROUND
`
`Temperature,
`
`C
`
`logpS2,atm
`
`800
`
`Temperature,
`' 500
`
`C
`
`-15
`
`tog p03 ,atm
`
`1: Atmospheric fluidized—bed combustors & Pressurized fEu§dized»bed
`combustors turbines;
`2: Gas~cooled Reactor;
`
`3: OH refining processes; 4: Coal gasification
`
`Fig. 7-1: Prevaient Suiphur & Oxygen Partial Pressures over
`400 ~ 1100
`C in Various High Temperature systems
`
`(Natesan 1985)
`
`oride content in coal emerges as 80 vol. ppm I-ICJ. during comb-
`ion (Meadowcr0ft,l987).
`
`ack by alkali sulphate phases stabilised with high localized
`entials of 503 have been noted in coal—fired boilers. A large
`’_nt of Na2SO4 is the predominant corrodent
`in the combustion
`eiof black liquor boilers. Sulphur, alkali metals and vanadium
`{e catastrophic attack in oil~fired boilers. Sulphidation and
`dation by the action of H23 and CO is the most common mode of
`ack. chlorides promote low melting eutectics with sulphates,
`_ h affect low pressure boilers (100 bar) operating at 370°C.
`Aoratory simulations have not