Diagnostic Tools For HVOF Process Optimization
ESPOO 2005This publication is available fromVTT INFORMATION SERVICEP.O.Box 2000FI–02044 VTT, FinlandPhone internat. 358 20 722 4404Fax 358 20 722 4374ISBN 951– 38– 6677– 7 (soft back ed.)ISSN 1235– 0621 (soft back ed.)ISBN 951– 38– 6678– 5 (URL: http://www.vtt.fi/inf/pdf/)ISSN 1455– 0849 (URL: http://www.vtt.fi/inf/pdf/)Erja TurunenDenna publikation säljs avVTT INFORMATIONSTJÄNSTPB 200002044 VTTTel. 020 722 4404Fax 020 722 4374Diagnostic tools for HVOF process optimizationTätä julkaisua myyVTT TIETOPALVELUPL 200002044 VTTPuh. 020 722 4404Faksi 020 722 4374VTT PUBLICATIONS 583In the thermal spray process the coating is built up from lamellas formedby rapid solidification of the melted or semimelted droplets attached to thesubstrate. A typical structure for the coating is a pancake-like lamellarstructure, where the flattening stage and adhesion between the lamellas,together with the coating material itself, define the main properties of thecoating. High velocity processes especially HVOF (High velocity oxy-fuel)spraying are the most potential methods for producing a good adherentcoating with low porosity.From a scientific point of view, particle velocity and particletemperature, together with substrate temperature, are the main parametersaffecting the deposit formation. They determine the deposit build-upprocess and deposit properties.The aim of this work was to show the workability of diagnostic tools inthe HVOF process. The focus was on first order process mapping, includingon-line diagnostics and single splat studies. The main focus was on theHVOF spraying of alumina. The target was to obtain a systematicunderstanding of the influence of the process conditions on themicrostructure development in HVOF alumina coatings. The study aimed toproduce information for a first order process map, and was carried out ata much deeper level than previously reported. The obtained data wasapplied for nanostructured alumina composite coatings, and the effect ofthe process conditions was compared on the obtained coatingmicrostructure and properties. Also quasicrystalline materials were studiedby using same methods.It was shown that diagnostic results can be correlated with the coatingmicrostructure and coating properties in HVOF spraying. It was alsodemonstrated that the coating properties and coating quality can beimproved by optimizing and carefully selecting the spray parameters.VTT PUBLICATIONS 583Erja TurunenDiagnostic tools for HVOF processoptimization
VTT PUBLICATIONS 583Diagnostic tools for HVOF processoptimizationErja TurunenVTT Industrial SystemsDissertation for the degree of Doctor of Science in Technology to bepresented with due permission of the Department of Materials Science andEngineering, for public examination and debate in Auditorium 1 (Vuorimiehentie 2 A)at Helsinki University of Technology, Espoo, Finland) on the16th of December, 2005, at 12 noon.
ISBN 951–38–6677–7 (soft back ed.)ISSN 1235–0621 (soft back ed.)ISBN 951–38–6678–5 (URL: http://www.vtt.fi/inf/pdf/)ISSN 1455–0849 (URL: http://www.vtt.fi/inf/pdf/)Copyright VTT Technical Research Centre of Finland 2005JULKAISIJA – UTGIVARE – PUBLISHERVTT, Vuorimiehentie 5, PL 2000, 02044 VTTpuh. vaihde 020 722 111, faksi 020 722 4374VTT, Bergsmansvägen 5, PB 2000, 02044 VTTtel. växel 020 722 111, fax 020 722 4374VTT Technical Research Centre of Finland, Vuorimiehentie 5, P.O.Box 2000, FI–02044 VTT, Finlandphone internat. 358 20 722 111, fax 358 20 722 4374VTT Tuotteet ja tuotanto, Metallimiehenkuja 8, PL 1703, 02044 VTTpuh. vaihde 020 722 111, faksi 020 722 7069VTT Industriella System, Metallmansgränden 8, PB 1703, 02044 VTTtel. växel 020 722 111, fax 020 722 7069VTT Industrial Systems, Metallimiehenkuja 8, P.O.Box 1703, FI–02044 VTT, Finlandphone internat. 358 20 722 111, fax 358 20 722 7069Otamedia Oy, Espoo 2005
Turunen, Erja. Diagnostic tools for HVOF process optimization. Espoo 2005. VTT Publications 583.66 p. app. 92 p.Keywordsthermal spraying, HVOF, high velocity oxi-fuels, process optimizaticdiagnostics, single splat studies, surface coatings, alumina, quasicrystals,nanofractionsAbstractIn the thermal spray process the coating is built up from lamellas formed byrapid solidification of the melted or semi-melted droplets attached to thesubstrate. A typical structure for the coating is a pancake-like lamellar structure,where the flattening stage and adhesion between the lamellas, together with thecoating material itself, define the main properties of the coating. Thermal spraycoatings are often applied for better corrosion and wear resistance. Therefore,low porosity and good adhesion are desired properties for the coating. Highvelocity processes – especially HVOF (High velocity oxy-fuel) spraying – arethe most potential methods for producing a good adherent coating with lowporosity.From a scientific point of view, particle velocity and particle temperature,together with substrate temperature, are the main parameters affecting thedeposit formation. They determine the deposit build-up process and depositproperties. Particle velocity and temperature affect the deposit efficiency as wellas the microstructure.The aim of this work was to show the workability of diagnostic tools in theHVOF process. The focus was on first order process mapping, including on-linediagnostics and single splat studies. Nanocrystalline alumina composites andquasicrystals were selected, two materials that are complex to spray. With bothmaterials the melting state of the particles must be well optimized in order toproduce dense, well-adhered coating without unwanted changes in coating phasestructure.The main focus was on the HVOF spraying of alumina. The target was to obtaina systematic understanding of the influence of the process conditions on the3
microstructure development in HVOF alumina coatings. Conventional limits ofgas ratios and flows were exceeded to obtain a wide velocity-temperature range.The study aimed to produce information for a first order process map, and wascarried out at a much deeper level than previously reported. Propylene andhydrogen as fuel gases were compared, and other variables, such as total gasflow rate, fuel gas/oxygen ratio, and standoff distance were also varied. Theobtained data was applied for nanostructured alumina composite coatings, andthe effect of the process conditions was compared on the obtained coatingmicrostructure and properties.On-line diagnostic measurements, in which particle temperatures and velocitiesin the flame can be measured, were performed. The main work was carried outfor alumina by using a DPV-2000 system. Two clear regions of differenttemperature and velocity arise from the use of different fuel gases. Single splatstudies correlated well with the obtained coating properties, and a first orderprocess map for alumina was created showing the window for the sprayparameters producing best coating quality plotted against coating hardness andabrasive wear resistance.It was shown that diagnostic results can be correlated with the coatingmicrostructure and coating properties in HVOF spraying. It was alsodemonstrated that the coating properties and coating quality can be improved byoptimizing and carefully selecting the spray parameters.4
PrefaceThe research work for this thesis was carried out in the Surface Engineeringgroup at VTT, the Technical Research Center of Finland, from 2000 to 2005.The work related to Publication IV was partly carried out during the 6 weeksresearch period I spent at Stony Brook University, USA, in the summer of 2002.The work was then continued in Finland, as well as finalizing the publication.The thesis was supervised by Professor Simo-Pekka Hannula, to whom I wouldlike to express my sincere gratitude for his guidance, support and fruitfuldiscussions around the issues related to this thesis.My sincere thanks also go to my co-workers at VTT. My closest co-workers,Tommi Varis, Kimmo Ruusuvuori and Maria Oksa, have all been verysupportive and understanding during my most hectic period when I waspreparing this work. Markku Lindberg, Alpo Viitanen and Seija Kivi get specialthanks for their technical support in making coatings and samples. Jari Keskinen,Pertti Lintunen, Teppo Fält and Tom E. Gustafsson have also all an unique rolein this co-operation. I also want to thank my group leader, Jari Koskinen, for allhis support and late evening discussions, which increased my motivation evenmore. I want to give special thanks to all the other members of my group forcreating such a good and motivating atmosphere to work in.Acknowledgement is also due to Professor Sanjay Sampath and his group atStony Brook University, USA, for their co-operation and the resources madeavailable to me. Special greetings go to Jonathan Gutleber and Anirudha Vaidyafor those long hours in the spray booth together.The financial support provided by VTT, the National Technology Agency(Tekes), and Finnish industry is gratefully acknowledged.I also want to thank my family and friends for all their support, encouragementand care over these years. And, last but not least, I thank Elina for her positiveattitude and patience towards my work.Espoo, November 2005Erja Turunen5
ContentsAbstract. 3Preface . 5List of included publications. 8Author’s contribution. 9List of symbols. 101. Introduction. 111.1 Thermal spraying. 111.1.1 HVOF Spraying . 121.1.2 Microstructure of thermal spray coatings. 141.2 Tools for process optimization . 151.2.1 On-line diagnostic methods. 161.2.2 Splat studies . 181.2.3 HVOF process optimization. 201.3 Coating materials and properties . 221.3.1 Alumina. 231.3.2 Nanostructured alumina . 241.3.3 Quasicrystals . 241.4 Aim of the research . 262. Experimental methods . 282.1 Materials . 282.2 Thermal spray test setup . 292.3 On-line diagnostics. 292.4 Single splat studies . 302.5 Coating deposition. 312.6 Characterization. 323. Results. 343.1 Powder characterization . 343.1.1 Alumina. 343.1.2 Quasicrystals . 346
3.2Process optimization. 363.2.1 On-line diagnostics. 363.2.2 Single splat studies. 393.2.2.1Alumina . 393.2.2.2Quasicrystals. 413.2.3 Coating characterization and properties. 434. Discussion. 515. Summary and conclusions . 55References. 56AppendicesPublications I–VIAppendix I of this publication is not included in the PDF version.Please order the printed version to get the complete publication(http://www.vtt.fi/inf/pdf/)7
List of included publicationsThis thesis consists of a summary of the main results and six appendedpublications I–VI.ITurunen, E., Varis, T., Vierimaa, K. & Hannula, S.-P. Spray parameteroptimisation and tribilogical properties of thermally sprayed quasicrystallineand partially quasicrystalline coatings. Proc. Estonian Acad. Sci. Eng., 9, 4(2003), pp. 293–303.IIHuttunen-Saarivirta, E., Turunen, E. & Kallio, M. MicrostructuralCharacterisation of Thermally Sprayed Quasicrystalline Al-Co-Fe-CrCoatings. Journal of Alloys and Compounds, 354, 1–2 (2003), pp. 269–280.III Huttunen-Saarivirta, E., Turunen, E. & Kallio, M. Influence of Cr Alloyingon the Microstructure of Thermally Sprayed Quasicrystalline Al-Cu-FeCoatings. Intermetallics 11 (2003), pp. 879–891.IV Turunen, E., Varis, T., Hannula, S.-P., Vaidya, A., Kulkarni, A., Gutleber, J.,Sampath, S. & Herman, H. On the role of particle state and depositionprocedure on mechanical, tribological and dielectric response of highvelocity oxy-fuel sprayed alumina coatings. Materials Science andEngineering. Accepted for publication, in print (2005).VTurunen, E., Varis, T., Gustafsson, T.E., Keskinen, J., Fält, T. & Hannula, S.-P.Parameter optimization of HVOF sprayed nanostructured alumina andalumina-nickel composite coatings. Surface and Coatings Technology.Accepted for publication, in print (2005).VI Turunen, E., Varis, T. Gustafsson, T. E., Keskinen, J., Lintunen, P., Fält, T.& Hannula, S.-P. Process optimization and performance of nanoreinforcedHVOF-sprayed ceramic coatings. Proceedings of 16th International PlanseeSeminar, May 30–June 3, 2005, Reutte, Austria. Pp. 422–433.8
Author’s contributionI was the main author of the Publications I, IV–VI. I prepared the test matrixesand schedules, participated in the spray and diagnostic tests, studied thediagnostic data and compared them with the splat results and coatingperformance data. Co-authors were essential in the following tasks: T. Varis wasin charge of the thermal spray studies at VTT, and J. Gutleber and A. Vaidya atStony Brook University, USA; T. E. Gustafsson made the SEM studies and T.Fält the nanoindentation studies; J. Keskinen produced special nanostructuredceramic powders; and Professor Hannula supervised this work.E. Saarivirta-Huttunen made separated, wide microscopy studies for quasicrystallinecoatings and was the main author of Publications II and III. For thosepublications, E.T was in charge of thermal spray coating manufacturing, processoptimization and coating properties characterization, including microstructureand Pin-on-Disc testing. The button test was carried out by K. Vierimaa ofMetso Corp.9
List of symbolsHVOFHigh velocity oxy-fuelAPSAtmospheric plasma sprayCTECoefficient of thermal expansionXRDX-ray diffractionSWSprayWatch on-line diagnostic equipmentCCDCharge-coupled deviceIRinfraredα, γSymbols for different alumina phasesSEMScanning electron microscopeTEMTransmission electron cient of frictionPoDPin-on-discQCQuasicrystalm/smeters per secondkWkilowatts CCelsius degreekg/hkilograms per hourµmmicrometernmnanometerl/minlitres per minute diameter10
1. Introduction1.1 Thermal sprayingThermal spraying is a general term to describe all methods in which the coatingis formed from melted or semi-melted droplets. In thermal spraying the materialis in the form of powder, wire or rod and is fed into the flame produced by aspray gun, where it melts and the formed droplets are accelerated towards thesubstrate to be coated. The thermal and kinetic energy of the flame can beproduced either with burning mixtures of fuel gas and oxygen, or by using anelectrical power source. Based on the energy source, thermal spray methods canbe divided into a few main groups: plasma spray methods, flame spray methods,high velocity oxy-fuel methods, electrical arc methods, and, as the latesttechnology, cold gas methods.1, 2, 3In thermal spraying the coating is built up from the lamellas formed by rapidsolidification of the melted or semi-melted droplets attached to the substrate. Atypical structure for the coating is a pancake-like lamellar structure, where theflattening degree and adhesion between the lamellas, together with the coatingmaterial itself, define the main properties of the coating. The adhesion andporosity of the coating is mainly defined by the particle melting behavior and thevelocity when attaching to the surface. In addition, due to the fast cooling rate ofthe particles, some special features, such as residual stresses and the metastablephases can be observed in the thermally sprayed coatings.1, 2, 3Thermal spray coatings are often applied for better corrosion and wearresistance. Therefore, low porosity and good adhesion are desired properties forthe coating. High velocity processes – especially HVOF (High velocity oxy-fuel)spraying – are the most potential methods for producing a good adherent coatingwith low porosity. In HVOF spraying heat is produced by burning mixtures ofoxygen and fuel gas, mainly hydrogen, kerosene, propane, propylene, natural gasor acetylene. Due to the special nozzle design, a jet with supersonic speed isproduced. Another commonly used method is APS (Atmospheric plasma spray),where the energy is based on the plasma produced by ionizing an inert gas,typically a mixture of argon and hydrogen or helium, between the anode and thecathode in the spray gun. Due to the high energetic ionized plasma, thetemperature of the plasma flame is very high.11
The main difference between HVOF and APS is the relationship between thekinetic and thermal energy of the process described by the particle velocity andthe flame temperature. Typical ranges of these parameters for each of theprocess are given in Table 1.Table 1. Characteristic features of HVOF and APS processes.3Spraying methodParticle velocity (m/s)Flametemperature ( C)HVOF500–700 3 000APS150–400 8000–12000The ability to produce dense coatings with low amount of phase transformationsand oxidation is the main feature of the HVOF process. This is due to the shortdwell time of the particles in a relatively cold flame. It is widely used to producecermet and metal coatings. The HVOF process has also demonstrated an abilityto deposit dense ceramic coatings, such as alumina.4, 5, 6, 7Due to the high process temperature, APS which enables good melting of theceramic particles is often used to produce a ceramic coating.The use of thermal spray coatings has traditionally been based on extending thelife of the component. However, thermal spray coatings have increasingly beenconsidered “prime reliant” and such
Diagnostic tools for HVOF process optimization Erja Turunen VTT Industrial Systems Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Materials Science and Engineering, for public examination and debate in Auditorium 1 (Vuorimiehentie 2 A)
CHAPTER 1 INTRODUCTION 1.1 Introduction 1 CHAPTER 2 LITERATURE REVIEW 2.1 Introduction 4 2.2 Overview of Coating Techniques 4 2.3 Thermal Spray Techniques 7 2.3.1 HVOF Thermal Spray Process 9 2.3.2 HVOF Gun Design 11 2.4 The HVOF Process 14 2.4.1 Combustion and Gas Dynamics of t
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In this work the HVOF coating process for producing nanocrystalline Al 2O 3- and Al 2O 3–Ni-coatings is described. Focus is on the process control, lamellae microstructure, lamellae interaction and their effect on mechanical coating properties, such as hardness, wear resistance and fracture toughness. 2. Experimental procedure 2.1. Spray .
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