Dr.-Ing. Klaus Schneider is a Consultant in materials, manufacturing and logistics for the power generation industry, after being in charge of research projects at ALSTOM, Switzerland. He has more than 30 years of experience in this business.

Thermal Spraying for Power Generation Components

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

John Wiley & Sons

Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
All right reserved.
ISBN: 978-3-527-31337-2

Chapter One

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 production 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 specific 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 cost

Operation cost

Long-term reliability

Availability 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 simplification 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: Steel, low alloyed up to high chromium steels

Nickel alloys

Cobalt alloys

Copper 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, strains

Operational temperatures

Temperature changes

Environment, atmosphere Design 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 mental 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 find 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-temperature 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.

Basically there are three types of components:

Large single structural components like casings

Multiple medium-sized components with plane or slightly curved surfaces, like combustor parts

Multiple complex-shaped components, like turbine vanes and blades

The following example of a stationary gas turbine illustrates the situation (Fig. 1):

The air intake is a steel construction most probably painted with a zinc-rich paint. The compressor blades and vanes 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 and turbine 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 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 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.

1.3 Definition of Thermal Spraying (THSP)

We will use the definition of thermal spraying as given by ASM):

“A group of processes in which finely 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”.

Another detailed description is found in the US patent classification.

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

Processes wherein a gaseous flame 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 flame spraying and is included in this and indented subclasses.

(2) Electric-arc metal spraying is properly classified in this and indented subclasses.

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

(4) Thermal spraying is properly classified 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 classified according to the energy source needed to heat and accelerate the particles. In the European standards EN 657 as well as in the equivalent international standard ISO 14917 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.

1.5 Coatings for Power-Generation Components

What is specific 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 components of industrial structures in order to protect them against corrosive attack or wear. The first applications go back to the year 1909. A Swiss patent was applied for by Dr. M. U. Schoop for using flame-spray techniques.

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

Mechanical stresses, loadings, strains

Operational temperatures

Temperature changes

Environment, atmosphere, chemical attacks

Design lifetime (times and cycles)

Expected safe operation times

Cost

The metallurgist translates these requirements into:

Coating chemistry

Coating microstructure, e.g. phases, oxides, grain size, porosity

Coating thickness

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

Investment cost

Operation cost

Long-term reliability

Availability and scheduled, short maintenance

The specifications for manufacturing and purchasing will address:

Repeatable manufacturing process with defined process parameters

Defined coating material, e.g. powder specification

Required coating thickness and tolerance

Required coating microstructure

Allowable coating defects and microstructure

Defined coating substrate interface and tolerances of bonding defects

Defined 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 specifications 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 refurbishment of a component. Let us take the example of a turbine blade. Figure 3 shows a typical manufacturing chain for a new component.

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 fired to provide stability during casting at temperatures in the order of 1500C. 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, finishing 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 find 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):

[ILLUSTRATION OMITTED]

All three influencing parameters have a specific effect on the coating quality. The spraying equipment provides the coating thickness and microstructure, fixture and masking influence 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 reflect 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 influencing 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 influence the coating quality in a specific way. In addition, some of the parameters are not independent. They will influence each other.

Another look at the coating process from a shop floor 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.

The excellent review on plasma spraying 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 influence 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 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 find 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.

(Continues…)

Excerpted from Thermal Spraying for Power Generation Componentsby Klaus Erich Schneider Vladimir Belashchenko Marian Dratwinski Stephan Siegmann Alexander Zagorski Copyright © 2007 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Excerpted by permission.
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