Method and Process Development of Advanced Atmospheric Plasma Spraying for Thermal Barrier Coatings

Method and Process Development

of Advanced Atmospheric Plasma Spraying

for Thermal Barrier Coatings

Sebastian Mihm,Thomas Duda,Heiko Gruner,Georg Thomas,and Birger Dzur

(Submitted September 2,2011;in revised form December 28,2011)

Over the last few years,global economic growth has triggered a dramatic increase in the demand for resources,resulting in steady rise in prices for energy and raw materials.In the gas turbine manufacturing sector,process optimizations of cost-intensive production steps involve a heightened potential of savings and form the basis for securing future competitive advantages in the market.In this context,the atmospheric plasma spraying (APS)process for thermal barrier coatings (TBC)has been optimized.A constraint for the optimization of the APS coating process is the use of the existing coating equipment.Furthermore,the current coating quality and characteristics must not change so as to avoid new quali-fication and http://m.sodocs.net/doc/878124e55727a5e9846a61a2.html ing experience in APS and empirically gained data,the process optimization plan included the variation of e.g.the plasma gas composition and flow-rate,the electrical power,the arrangement and angle of the powder injectors in relation to the plasma jet,the grain size distribution of the spray powder and the plasma torch movement procedures such as spray distance,offset and iteration.In particular,plasma properties (enthalpy,velocity and temperature),powder injection conditions (injection point,injection speed,grain size and distribution)and the coating lamination (coating pattern and spraying distance)are examined.The optimized process and resulting coating were compared to the current situation using several diagnostic methods.The improved process significantly reduces costs and achieves the requirement of comparable coating quality.Furthermore,a contribution was made towards better comprehension of the APS of ceramics and the definition of a better method for future process developments.

Keywords

atmospheric plasma spray (APS),gas turbines,particle plasma interaction,residual stress deter-mination,thermal barrier coatings (TBCs),torch design,yttria stabilized zirconia (YSZ)

1.Introduction

Plasma coated thermal barrier coatings (TBC)have been established successfully in the gas turbine manufac-turing business since the seventies (Ref 1).In the hot gas

path section of gas turbines TBCs fulfill the function of thermal insulation,thereby lowering the temperature of the metallic portion of the part.Firing temperatures in the combustion chamber above 1300°C and limited long-term operation temperatures of approx.950°C for the metallic materials resulted in high demands on the coating and cooling systems for blades,vanes and combustor parts.Typical TBC are multi-layer systems based on a duplex structure,a dense metallic bond coat layer (material:MCrAlY,M-Ni and/or Co)and a porous ceramic top coat layer (material:YSZ,yttria-stabilized zirconia),shown in Fig.1.

The dense MCrAlY coating protects the base material against corrosion/oxidation and—provides adhesion for the ceramic top coat.The porous ceramic top coat—in concert with the external and internal component cooling—functions as a thermal barrier.Contrary to the dense MCrAlY coating,a defined porosity of the YSZ coating is necessary to compensate for strain differences and to reduce thermal conductivity.These specific requirements pose a challenge to the technology in producing such coating systems.

In addition to ensuring coating quality,the manufac-turing costs are more and more the focus of current developments.The production of porous YSZ coatings is done by atmospheric plasma spraying (APS).In using this technology,the plasma torch construction is one limiting factor for process improvements.For example,voltage

This article is an invited paper selected from presentations at the 2011International Thermal Spray Conference and has been expanded from the original presentation.It is simultaneously published in Thermal Spray 2011:Proceedings of the International Thermal Spray Conference ,Hamburg,Germany,September 27-29,2011,Basil R.Marple,Arvind Agarwal,Margaret M.Hyland,Yuk-Chiu Lau,Chang-Jiu Li,Rogerio S.

Lima,and Andre

´McDonald,Ed.,ASM International,Materials Park,OH,2011.

Sebastian Mihm and Thomas Duda ,Department TMRR,ALSTOM (Switzerland)Ltd.,Zentralstrasse 42,5242Birr,Switzerland;Heiko Gruner and Georg Thomas ,Medicoat AG,

Gewerbe Nord,5506Ma

¨genwil,Switzerland;and Birger Dzur ,Technische Universita

¨t Ilmenau,PF 100565,98684Ilmenau,Germany.Contact e-mail:Sebastian.Mihm@http://m.sodocs.net/doc/878124e55727a5e9846a61a2.html .

JTTEE5

Method and Process Development of Advanced Atmospheric Plasma Spraying for Thermal Barrier Coatings

21:400–408

DOI:10.1007/s11666-012-9745-2

1059-9630/$19.00ÓASM International

400—Volume 21(3-4)June 2012Journal of Thermal Spray Technology

P e e r -R e v i e w e d

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