III

Klaus Erich Schneider, Vladimir Belashchenko,Marian Dratwinski, Stephan Siegmann,Alexander Zagorski

Thermal Spraying forPower Generation Components

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K. E. Schneider, V. Belashchenko, M. Dratwinski, S. Siegmann, A. ZagorskiThermal Spraying for Power Generation Components

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1308vch00.indd II1308vch00.indd II 18.08.2006 14:00:5018.08.2006 14:00:50

III

Klaus Erich Schneider, Vladimir Belashchenko,Marian Dratwinski, Stephan Siegmann,Alexander Zagorski

Thermal Spraying forPower Generation Components

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IV

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library

Bibliographic information published bythe Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche National-bibliografi e; detailed bibliographic data are available in the Internet at http://dnb.d-nb.de.

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form by photoprinting, microfi lm, or any other means nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifi cally marked as such, are not to be considered unprotected by law.

Composition Manuela Treindl, LaaberPrinting Strauss GmbH, MrlenbachBinding Litges & Dopf Buchbinderei GmbH, Heppenheim

Printed in the Federal Republic of GermanyPrinted on acid-free paper

ISBN-13: 978-3-527-31337-2ISBN-10: 3-527-31337-0

Authors

Klaus Erich SchneiderKuessaberg, Germanye-mail: KlausErich.Schneider@t-online.de

Vladimir BelashchenkoConcord, NH, USAe-mail: belashchenko@comast.net

Marian DratwinskiStein, Switzerlande-mail: dratwinski@bluewin.ch

Stephan SiegmannEMPAThun, Switzerlande-mail: stephan.siegmann@empa.ch

Alexander ZagorskiALSTOMBaden, Switzerlande-mail: alexander.zagorski@power.alstom.com

CoverSimulated Spray Pattern, ALSTOM

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V

Preface

Coatings constitute an intrinsic part of the power-generation hardware. Thousands of patents, papers and conference presentations address new coating types, new hardware and software, new process developments, new chemical compositions. A huge unpublished knowledge is stored in manufacturers know-how.

However, sometimes coatings are still considered as an art and there are fair reasons for that. The thermal spray is still not a plug and play tool and the product quality largely depends on the deep understanding of process physics and hardware features, accumulated experience, engineers intuition and operators training.

This book now deals with questions that are essential for a good performance of this art:

Is there a given process stability? What is the ratio of deterministic and stochastic in the coating process?Is there an inherent process capability for a given specifi cation that cannot be improved?What is the right preventive maintenance strategy?Is there a chance to end up with coating-process capabilities in the order of other manufacturing processes?What can be predicted and designed a priori by physical modeling and offl ine programming and what can be achieved by trial and error only?What can be done to describe and control quality?

This book is not a pure scientifi c book. It is of most value for the engineer involved in design, processing and application of thermally sprayed coatings:

To understand the capability and limitations of thermal spraying, to understand deposition effi ciency and the importance of maintenance and spare parts for quick changeover of worn equipment, to use offl ine programming and real equipment in an optimum mix to end up with stable processes in production after the shortest development time and in the end to achieve the fi nal target in production:

Process stability at minimum total cost

Klaus Schneider

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VI Preface

Acknowledgement

The authors would like to thank the following companies and institutions for supplying valuable material published and unpublished for this book.

ALSTOM, Sulzer Metco, Turbocoating, CENIT, EMPA Material Science and Technology, Praxair, HC Starck, Siemens, ASM, Elsevier Publ., Stellite Coatings, Progressive Technologies, National Research Council Canada.

And personal acknowledgements to F. Stadelmaier (TACR), P. Ryan, P. Holmes, J. E. Bertilsson (ALSTOM), A. Scrivani (Turbocoating), A. Sickinger (ASA, California, USA), K. Matty (former AETC).

In particular, I would like to express my gratitude to the management and my colleagues at ALSTOM for the assistance and valuable discussions during all the years that enabled me to start this book. The production experience with offl ine programming and monitoring was only possible together with the erection and start-up of the ALSTOM coating shop in Birr, Switzerland.

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VII

The Authors of this Book

Klaus E. Schneider received a degree in Physics and Materials Science and a PhD in Materials Science and Technology from the University of Erlangen, Germany. He has three decades of experience in manufacturing and materials technology in power and turbine engineering, mechanical engineering. During his professional carreer at BBC, ABB, ALSTOM Mannheim, Germany, and Baden, Switzerland (19742004), he worked in several leading positions in materials, supply management and manufacturing. He was responsible for national and international R&D programmes and for erecting new manufacturing facilities. Since 2004 he is active as a consultant for materials and manu-facturing technology.

Vladimir Belashchenko has a PhD in Physics and Chemistry of Plasma Technology and a ScD in Materials Science. He has over 30 years of experience in research, development and implementation of thermal spray equipment, materials and technologies. In 1992, he obtained the ASM International Award, in 2004 the R&D 100 Award.

Marian Dratwinski is a process development engineer with a very wide range of technical knowledge and experiences. In his current post, he is responsible for Coating Applications Development at Sulzer Metco AG in Switzerland.

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VIII

Stephan Siegmann received his degree in Physics from the Uni-versity of Basel, Switzerland. After completing the PhD in the fi eld of Thermal Spraying, he was working as Vice Manager Research at MGC Plasma Company at Muttenz, Switzerland, in the fi eld of waste treatment by thermal plasma at 1.2 MW. In the year 1994 he changed back to his former fi eld of Thermal Spraying and built up a position at the Swiss Federal Institute for Materials Science and Technology (EMPA), where he is now responsible for all Thermal Spray activities.

Alexander Zagorski received his degree in Mechanical Engineering from the Novosibirsk State Technical University and his PhD in Hydromechanics and Plasma from the Institute of Theoretical and Applied Mechanics in Novosibirsk, Russia. After having worked in Research and Development for eighteen years, he is now the Expert Engineer at the ALSTOM Customer Service Development in Switzerland.

The Authors of this Book

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IX

Contents

Preface V

The Authors of this Book VII

1 Introduction 11.1 Requirements for Materials and Coatings in Powerplants 11.2 Examples of Coatings in Gas Turbines 21.3 Defi nition of Thermal Spraying (THSP) 51.4 Thermal-Spraying Systems 51.5 Coatings for Power-Generation Components 61.6 The Complete Manufacturing and Coating Process 71.7 Coating-Process Development 121.8 Tasks for Target Readers 15

2 Practical Experience Today 172.1 Coating Processes 172.2 Basics of Thermal Spraying 212.3 Feedstock 232.3.1 Wire 232.3.2 Powder 242.3.2.1 Powder Types 242.3.2.2 Powder-Production Processes and Morphologies 272.3.2.3 Powder Characterization 332.3.2.4 Powders for Power-Generation Applications 362.4 Thermal-Spraying Equipment 402.4.1 Example of a Low-Pressure Plasma-Coating System 412.4.2 Flame and Arc Spray Torches 432.4.3 HVOF Process 452.4.3.1 Comparison of HVOF Fuels 472.4.3.2 A Brief Overview of the Major Existing HVOF Systems 482.4.3.3 Possible Improvements of HVOF Systems 512.4.4 Plasma Process 542.4.4.1 A Brief Overview of Plasma Torches 58

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X Contents

2.4.4.2 Possible Improvements of Plasma Systems 632.5 Work Flow and Important Coating Hardware 652.5.1 Powder Preparation and Powder-Delivery System 682.5.1.1 Powder Preparation 682.5.1.2 Powder Delivery and Injection System 682.5.1.3 Powder Injection and Plasma/Hot Gas Jet 732.5.1.4 Injector Plugging and Spitting 752.5.1.5 Powder Buildup at the Front Nozzle Wall 772.5.2 Cooling System 772.5.3 Power-Supply System 792.5.4 Gas Supply and Distribution System 802.5.5 Manipulation Systems 812.5.6 Fixtures and Masking 832.6 Examples of Coated Power-Generation Components 842.7 Production Experience 862.7.1 Surface Preparation 872.7.1.1 Internal Plasma and Transferred Arc 892.7.2 Process and Systems 912.7.2.1 The Programming of the Coating Process 942.7.3 Finishing 952.7.4 Repair of Turbine Parts 952.7.4.1 Coating Removal, Stripping 972.7.4.2 Restoration of the Base Materials 982.7.4.3 Refurbishing, Recoating 982.8 Commercial 992.8.1 General 992.8.2 Surface Preparation 1032.8.3 Coating Equipment 1032.8.4 Finishing 104

3 Quality and Process Capability 1053.1 Quality Assurance 1053.2 Sources of Process Variations 1053.2.1 Special Causes of Coating-Process Variation 1073.2.2 Stochastic Nature of a Spray Process 1083.2.2.1 Arc and Jet Pulsations 1083.2.2.2 Powder-Size Distribution 1093.2.2.3 Powder Injection 1103.2.2.4 Powder Shape 1103.2.2.5 Particle Bonding 1103.2.2.6 Gun and Component Motion and Positioning 1103.2.3 Drifting 1113.2.4 Stability of the Quality Control 1123.3 Process Capability and Stable Process 1153.3.1 Defi nition of Process Capability 115

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XIContents

3.3.2 Defi nition of a Stable Coating Process 1173.3.3 Operational Window 1183.3.4 What Process Capability is Required? 1223.3.5 Additional Factors that Affect the Process Capability 1243.3.6 Case Study: Achievable Process Capability 1253.3.6.1 Part Complexity 1253.3.6.2 Mutual Position of the Gun and Component Fixtures 1253.3.6.3 Powder Quality 1253.3.6.4 Torch Pulsations and Drifting 1263.3.6.5 Instability of the Quality-Control Process 1283.3.6.6 Surface Preparation and the Part Temperature 1283.3.6.7 Conditions of the Powder-Injection System 1293.3.6.8 Process Capability 1293.4 Maintenance 130

4 Theory and Physical Trends 1334.1 Coating Formation from Separate Particles: Particle Impact,

Spreading and Bonding 1334.2 Physics of Plasma Torches 1384.2.1 Plasma Properties 1394.2.2 Gas Dynamics of Plasma Torch 1454.2.3 Energy Balance of the Plasma Gun 1474.2.4 Major Trends 1494.2.4.1 Variation of the Gun Power; the Gas Flow Rates and Composition

Unchanged 1494.2.4.2 Variation of the Plasma Composition at the Same Specifi c Plasma

Enthalpy 1494.2.4.3 Variation of the Plasma Flow Rate at Unchanged Gun Power

and Gas Composition 1504.2.4.4 Effect of Nozzle Diameter 1514.2.5 Plasma Swirl 1514.3 Structure of Plasma Jets 1514.3.1 APS Jet 1514.3.2 Structure of LPPS Jet 1534.4 Particles in Plasma 1554.4.1 Particles at APS 1564.4.2 Particle at LPPS 1584.4.2.1 Particle Acceleration and Heating in the LPPS Free Jet 1584.4.2.2 Particle Acceleration and Heating Inside the Nozzle 1604.5 Spray Footprint (Spray Pattern) 1614.6 Infl uence of Particles on Plasma Flow 1644.7 Substrate Surface Temperature 1654.8 Formation of the Coating Layer 1674.9 Use of Different Plasma Gases 1684.10 Some Distinguishing Features of HVOF Physics 169

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XII Contents

5 Offl ine Simulation of a Thermal-Spray Process 1715.1 Simulation in Production 1715.2 Physical Background of Simulation Package 1755.2.1 Viscoplasticity Model of a Splat and Particle Bonding 1755.2.2 Thermodynamic and Transport Properties of Argon/Hydrogen

Mixtures 1765.2.3 Modeling of the Plasma Gun 1765.2.4 Modeling of the Plasma Jets 1765.2.4.1 APS Jet 1775.2.4.2 LPPS Jet 1775.2.5 Acceleration and Heating of Particles in Plasma 1795.2.6 Surface Thermal Conditions 1805.3 Spray Pattern 1825.3.1 Calibration of the Bonding Model and Sensitivity of a Spray Pattern

to the Process Parameters, Spray Angle and Bonding Model 1825.3.2 Coating Porosity and Roughness 1855.4 Modeling of Turbine Blades 1875.5 Coating Thickness Optimization and Stochastic Modeling Tools 1895.6 Simulation of HVOF Process 1955.7 Use of Offl ine Simulation in Coating Development 1995.7.1 Application Areas of Modeling in the Coating Process 1995.7.1.1 Coating Defi nition and Design for Coating 1995.7.1.2 Coating-Process Development 1995.7.1.3 Part Development 2005.7.1.4 Physical Modeling and Offl ine Simulation as Process-Diagnostic

Tools 2015.7.1.5 Simulation as a Numerical Experiment 2015.7.1.6 When the Offl ine Simulation Should Be Used 202

6 Standards and Training 2056.1 Standards, Codes 2056.1.1 Introduction to Standards 2056.1.2 Quality Requirements for Thermally Sprayed Structures and

Coating Shops 2066.1.3 Qualifi cation and Education of Spraying Personnel 2096.2 Special Case: Spraying for Power-Generation Components 2116.2.1 Coating-Process Development 2126.2.2 Coating Production 2136.2.3 General Requirements for Coating-Shop Personnel 213

7 Monitoring, Shopfl oor Experience and Manufacturing Process Development 215

7.1 Monitoring, Sensing 2157.1.1 Introduction of Monitoring 2157.1.2 Particle-Monitoring Devices 217

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XIII

7.1.3 Infl uence of Spray Parameters on Particle Speed and Temperature 218

7.1.4 Infl uence of Particle Velocity and Temperature on Microstructure 219

7.2 How to Use Monitoring for Process Control 2227.2.1 Monitoring, Sensing from a Job Shop Point of View 2227.2.2 Vision for Future Coating Control and Monitoring 2247.3 Manufacturing Coating Development 2287.3.1 Coating Development Process 2297.3.2 Coating Defi nition and Coating Specifi cation;

Design for Coating 2307.3.3 Process Development 2337.3.3.1 Powder Selection 2347.3.3.2 Torch Parameters 2347.3.3.3 Spray Pattern and Standoff Distance 2347.3.3.4 Coating Mono-Layer; Powder Feed Rate and Traverse Gun Speed 2357.3.3.5 Spray Trials and Coating Qualifi cation 2357.3.3.6 Sensitivity Checks 2367.3.4 Part Development 2367.3.4.1 Coating Program 2367.3.4.2 Process Qualifi cation and Preserial Release 2377.3.5 Serial Release 239

8 Outlook, Summary 2418.1 Thermal Spray Torches 2428.2 Future Offl ine Programming and Monitoring in Process

Development and Production 244

References 245

Subject Index 261

Contents

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XIV

Disclaimer

While every precaution has been taken in the preparation of this book, the publisher and the authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein.

As far as the authors of this book specify products of third parties they merely provide a description pertaining to this book. They do not want to promote or advertise any product or are liable for specifi c qualities of these products. In no event the authors are liable for damages suffered or personal injury including every kind of damages, especially consequential damages, arising out of the use or the inability to use these products. The same applies to the facts and information taken from foreign authority. The authors do not guarantee that the provided facts and information is state of the art, correct, complete or the quality thereof.

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1

1 Introduction

1.1 Requirements for Materials and Coatings in Powerplants

We do not want to write another book on thermal spraying, plasma spraying, HVOF (see Section 2.4.3) and other spraying processes. We will not repeat what is already written in excellent books, reviews, and journals. Many general descriptions of thermal spraying can be found today in the Internet on web pages of equipment suppliers, material and gas suppliers, coating shops and research facilities. Our intentions are to show some ways how to achieve a stable reliable coating produc-tion for power-generation equipment within reasonable time and at optimum cost. We will address how to identify problems and mistakes in advance. We will show how to minimize development effort and to improve product quality.

First, we will try to simplify and summarize the topic of this book:

Electric power generation today and in the future is using and will use steam turbines, gas turbines and turbogenerators, steel tubing and heat exchangers and boilers. Components consist of many parts that are welded, brazed or assembled. Each part has a specifi c function within the powerplant. The original equipment manufacturers (OEMs) and the powerplant customers like utilities or other power producers consider as the most important parameters of a powerplant:

Investment costOperation costLong-term reliabilityAvailability and scheduled, short maintenance

These parameters translate into requirements for components like material cost, optimized fuel cost, high operation temperatures and long operation times without in-operation control possibilities.

Today, all powerplant hardware is coated wherever no affordable and reliable structural material can be found that resists the operation environment.

For simplifi cation we start with the view of a metallurgist:Metallurgists select materials for specific applications or for a variety of

applications. A powerplant is basically built from metals. The structural materials and functional materials are metals and metallic alloys:

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1 Introduction2

Steel, low alloyed up to high chromium steelsNickel alloysCobalt alloysCopper and brass

In some rare cases titanium alloys are used. This picture is completely different from aero engines where the weight of a part is important. In power-generation parts weight is only important as material cost and for rotating parts if weight causes mechanical stresses.

Designers select materials for operational conditions like:

Mechanical stresses, loadings, strainsOperational temperaturesTemperature changesEnvironment, atmosphereDesign lifetime (times and cycles)Expected safe operation times

and of course for cost reasons.If a material class is not able to withstand the operational temperatures cooling

is required by available cooling media that are mainly air, steam, or water. In closed-cycle cooling other media like hydrogen are being used.

In many cases a division of material properties for a variety of tasks is required. Base metal has to have the required strength. Coatings withstand the environ men-tal attack or add additional properties like wear resistance. In cooled components thermal-barrier coatings reduce the temperature gradient within the structural material. The designer selects the structural material and the coating by iterating the loading, component thicknesses and cost.

1.2 Examples of Coatings in Gas Turbines

We promised to address powerplant components. However, when we look more closely we fi nd the following situation:

In steam turbines thermal-spray coatings are not in standard use. In certain cases erosion damages are solved by replacing missing material by a thermal spray over lay of erosion-resistant material containing tungsen carbide or chromium carbide.

Large-scale application is found in boilers where the tubes are coated by wire spray. For example, FeCrAl and FeCrAlY coatings are used generally as high-tempe rature oxidation protection to resist corrosive gases in boiler atmospheres.

The more complex applications are found in gas turbines, especially at higher temperatures. Therefore we will concentrate on examples from industrial gas turbines.

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31.2 Examples of Coatings in Gas Turbines

Basically there are three types of components:

Large single structural components like casingsMultiple medium-sized components with plane or slightly curved surfaces, like combustor partsMultiple complex-shaped components, like turbine vanes and blades

The following example of a stationary gas turbine illustrates the situation (Fig. 1):The air intake (1) is a steel construction most probably painted with a zinc-rich

paint. The compressor blades and vanes (2) are made out of Cr steels where in certain operation regimes aqueous corrosion, pitting corrosion might end up in corrosion fatigue or stress corrosion conditions. Here the OEM will decide to use higher alloyed steels, titanium alloys or protection of the parts by coating. For clearance-control purposes the counterparts of the rotating compressor blades might be coated with so-called abradable coatings.

The hot section parts in the combustor (3) and turbine (4) are made out of nickel- or cobalt-based alloys. In some cases ceramics are used. If oxidation and hot corrosion becomes important coatings are also used. In some cases for air-cooled components the cooling is assisted by ceramic thermal-barrier coatings that reduce the operational temperature of the structural material the part is made of. The exhaust (5 and 6) again is made out of zinc-plated or zinc-sprayed steel. Rotor and stator casings are steel components, sometimes coated. For certain operation conditions nickel-based alloys are used for rotor disks.

Wherever parts are rubbing against each other in operation or in order to control gaps between components wear-resistant coatings or so-called abradables are being used.

Years ago it was already noted that in aero engine components up to 80% of all components are coated by thermal spraying. Today, in stationary gas turbines probably 50% of components are coated. In earlier days galvanic processes like

Fig. 1 Siemens Westinghouse gas turbine (courtesy of Siemens).

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1 Introduction4

chrome plating, chemical vapor deposition methods (CVD) or pack processes (explained later in Section 2.1) had been used. Today many of them are replaced by thermal-spray processes.

Table 1 shows examples of coated components, materials, coatings and the basic requirements for the coating application. Details of coating compositions and requirement for feedstock will be found later in the Section 2.2.

Table 1 Components of stationary gas turbines, ther base and coating materials.

Component Base metal Coating Coating process

Coating requirement

Air intake Steel Zinc, epoxy Painting Oxidation, aqueous corrosion, erosion

Compressor blading

12%, high Cr steel, TiAl6V4

Aluminum, ceramic, PTFE

Painting Aqueous corrosion, erosion, stress corro-sion, corrosion fatigue

Compressor leakage control

12%, high Cr steel, TiAl6V4

Abradables: metal matrix, solid lubricant, and polyester

Plasma spraying

Leakage reduction

Assembled structures

Steel castings, Ni base castings and sheets

CrC, WC + Ni,Co

Wire spray, APS, HVOF

Wear, friction welding

Casings Low alloyed castings

NiCrxx, NiAlxx HVOF Oxidation

Combustor parts

Ni, Co superalloy, Ni-based sheet

NiAl, MCrAlY APS, HVOF Hot corrosion, oxidation, bond coat

Cooled combustor parts

Ni, Co superalloy, Ni-based sheet

ZrO2 8Y2O3 1 1) APS, HVOF Thermal barrier,

surface temperature reduction

Gas turbine blades and vanes

Ni, Co superalloy Cr, Al CVD, Aluminizing, Chromizing

Hot corrosion, oxidation, bond coat

M(Ni,Co)CrAlY (+ Re,Ta.)

LPPS, HVOF (+2nd process)

Hot corrosion, oxidation, bond coat

PtAl Galvanic Pt Aluminizing

AlSi Slurry painting + sintering

ZrO2 8Y2O3 APS, HVOF Thermal barrier, surface temperature reduction

1) Yttria Stabilized Zirconia (YSZ).

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5

1.3 Defi nition of Thermal Spraying (THSP)

We will use the defi nition of thermal spraying as given by ASM2):A group of processes in which fi nely divided metallic or nonmetallic surfacing

materials are deposited in a molten or semimolten condition on a substrate to form a spray deposit.

The surfacing material may be in the form of powder, rod, cord, or wire [1].Another detailed description is found in the US patent classifi cation [2].

Subclass 446 sprays coating utilizing fl ame or plasma heat (e.g., fl ame spraying, etc.):

Processes wherein (1) a gaseous fl ame is used to heat and project a coating material toward a substrate or (2) a coating material is converted to or engulfed by a highly ionized gas composed of ions, electrons and neutral particles in which the positive ions and negative electrons are roughly equal in number, and projected on to a substrate

In addition, the following notes are included:

(1) Torch spraying is considered a form of fl ame spraying and is included in this and indented subclasses.

(2) Electric-arc metal spraying is properly classifi ed in this and indented sub-classes.

(3) Explosive or detonation spray vaporization, wherein the vaporized coating is applied in the form of a spray is properly classifi ed in this and indented subclasses.

(4) Thermal spraying is properly classifi ed in this and indented subclasses.

In short: Thermal spraying are all coating processes that coat surfaces with heated particles that are deposited by a high enthalpy kinetic gas stream. The feedstock used could be wire (if the material can be drawn as wire) or powder.

1.4 Thermal-Spraying Systems

Thermal-spray equipment can be classifi ed according to the energy source needed to heat and accelerate the particles. In the European standards EN 657 [3] as well as in the equivalent international standard ISO 14917 [4] the different systems are described. A typical overview of thermal-spraying processes is shown in Fig. 2. For power-generation components thermal spraying by gas and electric arc discharge spraying are applied.

1.4 Thermal-Spraying Systems

2) ASM = American Society of Materials.

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1 Introduction6

1.5 Coatings for Power-Generation Components

What is specifi c in coatings, especially in thermal spraying for power-generation components? Why do we need another book on the subject thermal spraying? There are so many excellent reviews around. When looking for thermal spraying in the Internet search engines like Google will show millions of web pages.

Thermal spraying has been used for decades for applying coatings on compo-nents of industrial structures in order to protect them against corrosive attack or wear. The fi rst applications go back to the year 1909. A Swiss patent was applied for by Dr. M. U. Schoop for using fl ame-spray techniques [5].

In order to answer the question what is specifi c in coatings for power-generation components? let us start with the design requirements shown earlier and apply them to coatings:

Mechanical stresses, loadings, strainsOperational temperaturesTemperature changesEnvironment, atmosphere, chemical attacksDesign lifetime (times and cycles)Expected safe operation timesCost

Fig. 2 Overview of the different thermal-spray processes in analogy to EN 657 [3].

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7

The metallurgist translates these requirements into:

Coating chemistryCoating microstructure, e.g. phases, oxides, grain size, porosityCoating thickness

For production and purchasing people these requirements have to be put into specifi cations for manufacturing and purchasing. The specifi cation and the corresponding quality-assurance procedure have to ensure that the coating will meet the requirements of the powerplant operator:

Investment costOperation costLong-term reliabilityAvailability and scheduled, short maintenance

The specifi cations for manufacturing and purchasing will address:

Repeatable manufacturing process with defi ned process parametersDefi ned coating material, e.g. powder specifi cationRequired coating thickness and toleranceRequired coating microstructureAllowable coating defects and microstructureDefi ned coating substrate interface and tolerances of bonding defects3)

Defi ned coating surface, e.g. roughness, oxide layer, residual stress and tolerances

The answer to the question why this book is written is:We found a lack in combination of several disciplines that make a reliable,

affordable coating. Only the teamwork of design, manufacturing and supplier is able to provide the right product.

We will show as a thread running through this book that only the intelligent combination of process physics, accumulated experience and operator training can supply coatings with the required quality.

Finally, by complying with such manufacturing and purchasing specifi cations the OEM or the overhaul shop will guarantee the reliable operation of the coated part in powerplant service.

1.6 The Complete Manufacturing and Coating Process

Coating never is a standalone process within manufacturing, repair or refurbish-ment of a component. Let us take the example of a turbine blade. Figure 3 shows a typical manufacturing chain for a new component.

3) Bonding defects are details in the interface coating substrate that are not allowed according to specifi cation.

1.6 The Complete Manufacturing and Coating Process

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1 Introduction8

Before the investment casting takes place alloy has to be procured. Ceramic cores shaping the interior of the cooled blade have to be injected and fi red to provide stability during casting at temperatures in the order of 1500 C. Wax is injected around the core and a shell mold is applied. By removing the wax the cavity in the shape of the cooled blade is formed.

Vacuum casting, fi nishing and heat treatment provide an airfoil that later will be coated. Other processes like machining, electro discharge machining (EDM) will follow before coating. It is evident that certain processes have to take place before coating and others will follow the coating process. The latter processes have to be done in such a way that the coating is not damaged by these operations. The coating process is not independent of the other processes.

In more detail every coating process consists of 3 steps:

Surface preparation Coating application Finishing/post treatment

All thermal-spraying processes require these 3 steps as well. When concentrating on the coating application we fi nd the following situation: Coating by thermal spraying can be divided into 3 topics shown in Fig. 4 as the example of low-pressure plasma spraying (LPPS):

Fig. 3 Gas turbine blade manufacturing process.

Fig. 4 Major parameters of infl uence of plasma spraying (courtesy of ASA).

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9

All three infl uencing parameters have a specifi c effect on the coating quality. The spraying equipment provides the coating thickness and microstructure, fi xture and masking infl uence the coating thickness distribution. The powder forms the coating microstructure by chemical composition and grain-size distribution. Of course, this representation may be rather schematic and does not refl ect the whole complexity of internal structures and cross-links between the topics.

Details of process and system are given in Fig. 5.It can be clearly seen that the number of infl uencing parameters increases.

There are not only the spraying equipment and handling system together with the control equipment that determine the coating quality. There are the outside factors such as gases, electrical power and cooling water that enter the system. All these parameters can be controlled within the production facility. However, the powder quality is controlled by the powder supplier.

A more detailed view of additional parameters is given in Fig. 6. It shows that gas supply, power source, controller and cooling features represent important factors for coating quality.

When analyzing the coating process many process parameters (without powder material) can be found.

A system analysis divides each parameter into more subparameters. Each of the subparameters will infl uence the coating quality in a specifi c way. In addition, some of the parameters are not independent. They will infl uence each other.

Another look at the coating process from a shop fl oor perspective, i.e. from practical experience is given in Fig. 7.

Even more parameters are shown that can be adjusted or occur during coating production.

All the examples show that there are a high number of parameters to be considered in order to produce a high-quality coating in serial production.

Fig. 5 LPPS process and system (courtesy of Sulzer Metco).

1.6 The Complete Manufacturing and Coating Process

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1 Introduction10

Fig.

6 S

yste

m a

naly

sis

for

plas

ma

spra

ying

(co

urte

sy o

f ALS

TOM

).

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11

The excellent review on plasma spraying [6] estimates that 50 to 60 parameters have to be considered.

When looking through the literature and conferences one gets the feeling that everything is addressed and already resolved. Many technical universities seem to have an activity in plasma spraying or thermal spraying in order to evaluate spraying parameters and their infl uence on coating properties.

However, experience in production and procurement of powerplant equipment shows that always the same or new mistakes are made. Unknown coating defects

Fig. 7 Factors infl uencing the thermal-spraying process.

1.6 The Complete Manufacturing and Coating Process

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1 Introduction12

arise. Changes in personnel result in a new learning curve. Deviation of established working parameters results in changes in coating quality and in a number of improvement actions.

1.7 Coating-Process Development

The basic principle for coating of power-generation parts is:When a new coating process is to be established a process development has to

take place. This process development has to result in reliable, stable production. The main task is to fi nd the operational window, i.e. the manufacturing regime where small deviations in process parameters have negligible effect on the product quality.

A factorial test matrix will result in a huge number of tests required, which is already restricted by the fact what kind of power-generation parts have to be coated. Either they are single pieces, like one casing per turbine, or when they come in larger quantities like turbine blades they are very expensive easily summing up to thousands of Euro per destroyed part.

Table 2 shows an example of a coating-development matrix for coatings for a turbine blade and the expected correlation with the coating specifi cation.

Let us use an example: take Table 2 and make crosses in each fi eld where an experiment is required.

If a turbine blade has to be coated by all three processes like APS, HVOF and LPPS, it becomes evident how many tests are required. In addition the table shows how important in process control4) is. This is especially necessary because in many cases there are no nondestructive methods available for controlling online the coating quality.

Process development must result in a repeatable stable manufacturing and quality-assurance process. Every coating process shows a scatter in quality results specifi ed in the coating requirements. The results can be measured by applying a 6-sigma routine and determining process capabilities. A 6-sigma process assures that only 34 defects per million [7] are allowed. A 4-sigma process exhibits already 6210 defects per million.

Just to show an example: Assume a gas turbine with 400 coated blades. If the coating is a 4-sigma process then you will fi nd 23 blades in the turbine with defective coating. The coating life determines the maintenance interval of the whole powerplants. Therefore the process capability is important and has to be measured.

This process capability requires a good interaction between design and manu-factur ing. The result is a product that can be manufactured by a defi ned and released manufacturing process.

4) In process control means controlling the established process parameters during coating.

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13

Tabl

e 2

Coa

ting-

deve

lopm

ent m

atri

x.

Coa

ting

par

amet

ers

Equ

ipm

ent

In p

roce

ss c

ontr

olIn

fl ue

nce

AP

SLV

PS

HV

OF

AP

SLV

PS

HV

OF

Pre

coat

ing

Surf

ace

qual

ity

Bon

din

gC

lean

lines

sB

ondi

ng

Rou

ghn

ess

Bon

din

gO

xida

tion

Bon

din

gP

reh

eati

ng

Por

osit

yC

oolin

gP

oros

ity

Tran

sfer

red

arc

clea

nin

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ng

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yin

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eter

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pow

erT

hic

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ng

qual

ity

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qual

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r fe

ed r

ate

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ess

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tin

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rayi

ng

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ess

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tin

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abili

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tin

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qual

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tD

iagn

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knes

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ng

qual

ity

Stab

ility

Gas

fl ow

Stab

ility

Por

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yV

acu

um

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tin

g qu

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elat

ive

mov

emen

tSp

eed

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ickn

ess

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osit

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An

gle

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ost

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t tre

atm

ent

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t tre

atm

ent

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tin

g qu

alit

ySu

rfac

e tr

eatm

ent

Rou

ghn

ess

Cos

t

1.7 Coating-Process Development

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