Optimisation Of The HVOF Thermal Spray Process For
Optimisation of the HVOF Thermal Spray ProcessFor Coating, Forming and Repair of ComponentsbyJit Cheh Tan, BSc (Eng)Ph.D1997
Optimisation of the HVOF Thermal Spray ProcessFor Coating, Forming and Repair of ComponentsbyJit Cheh Tan, BSc (Eng)A thesis submitted in fulfilment of the requirement for the degree ofDoctor of PhilosophySupervisors:Professor M.S.J. HashmiDr L. LooneyDublin City UniversitySchool of Mechanical & Manufacturing EngineeringSeptember 1997
DECLARATIONI hereby certify that this material, which I now subm it for assessment on theprogramme o f study leading to the award of Doctor o f Philosophy is entirely myown work and has not been taken from the work o f others save and to the extendthat such work has been cited and acknowledged within the text o f my work.S ig n ed :I.D . N o .: 9 3 7 0 0 5 7 1D a t e 8 * S e p te m b e r 1 9 9 7I
ACKNOWLEDGEMENTSI would like to express my sincere thanks to Professor M.S.J. Hashmi and DrLisa Looney for their continued support and supervision throughout the course o f theproject.I would also like to thank Mr Martin Johnson for his co-operation andassistance in preparing all the experimental specimens. I must also thank Mr LiamDominican and Mr Ian Hopper from the workshop for their help at various stages ofthis work,I would like to thank my girlfriend, Janet , for her love, support andencouragement during these long years. And last, but not least, a very special thanksto my parents and my family for their constant love and support.II
Table of ContentsDeclarationIAcknowledgementsIITable o f ContentsIIIAbstractVIICHAPTER 1INTRODUCTION1CHAPTER 2LITERATURE SURVEY42.1 Introduction to Surface Engineering42.2 Overview Of Coating Technologies62.3 Thermal Spraying102.3.1 Wire Spraying102.3.2 Electric Arc Spraying122.3.3 Plasma Spraying12Atmospheric Plasma Spraying (APS)12Vacuum Plasma Spraying (VPS)142.3.4 Flame Thermal Spraying16Pulse Combustion HVOF (Detonation Gun) Process18Continuous Combustion HVOF Process182.4 The HVOF Process202.4.1 Combustion and Gas Dynamic o f the HVOF System202.4.2 Advantages and Disadvantages o f the HVOF System262.4.3 Characteristics o f HVOF Coatings262.4.4 Comparison o f HVOF and Plasma Thermal Spraying282.4.5 Future Potential and Markets292.5 Thermally Sprayed Coatings312.5.1 Composition o f Thermal Sprayed Coatings342.5.2 Residual Stress342.5.3 Bond Strength352.5.4 Hardness372.5.5 Ani sotropy in Thermally Sprayed Coatings37HI
2.6 Spray Forming382.6.1 Current Conventional Sinterforming Processes forParticulate Materials382.6.2 History o f Spray Forming Techniques382.6.3 Thermal Spray Forming o f Solid Components392.6.4 The HVOF Forming Process392.6.5 Osprey Forming42CHAPTER 3 EXPERIMENTAL EQUIPMENT AND PROCEDURES3.1 HVOF Thermal Spraying System44443.1.1 Diamond Jet Gun443.1.2 Powder Feed Unit493.1.3 Gas Flow Meter Unit503.1.4 Gas Regulator and Manifolds513.1.5 Air Control Unit523.2 Procedure for HVOF Spraying533.2.1 Spraying Substrate and Surface Preparation533.2.2 Spraying Process543.2.3 Post Spray Treatment Process553.3 Thermal Spray Safety Measures573.3.1 Gas Cylinder Use573.3.2 "Diamond Jet Equipment" Safety583.3.3 Metal Dust583.3.4 Eye Protection593.3.5 Reduction o f Noise Hazard593.3.6 Personal Protection593.3.7 Reduction o f Respiratory Hazards603.3.8 General HVOF Gun Operational Precautions60IV
CHAPTER 4 COATING PROPERTIES AND MEASUREMENT METHODS614.1 Coating Thickness Control and M easurement Method614.2 Hardness Measurement634.3 Porosity Measurement654.4 Adhesion Bond Strength Measurement664.5 Optical Microscope704.6 Residual Stress Measurement704.6.1 X-Ray Diffraction Stress Determination714.6.2 The Hole Drilling Method784.6.2.1 THE HOLE DRILLING PROCEDURE WITHRS-200 MILLING GUIDE794.6.2.2 Strain Gauges and Strain Indicator804.6.2.3 Drilling Samples834.6.2.4 Data and Calculation844.7 Three Point Bend Test86CHAPTER 5 EXPERIMENTAL WORK AND RESULTS5.1 Fabrication o f Free Standing Components88885.1.1 Fabrication o f Free Standing Solid Components885.1.2 Characterisation o f Free Standing Components955.2 HVOF Sprayed coatings1075.2.1 Experimental M atrix - Coatings1095.2.2 Results:1105.3 Repair o f damaged components using the HVOF Process1405.3.1 Machinability o f repaired components1445.3.2 The optimisation o f HVOF repair process1525.4 Statistical Analysis o f Results186V
CHAPTER 6 CONCLUSIONS AND RECOMMEMDATIONS1966.1 Conclusion:1966.2 Recommendations for future pendix A207Appendix B209Appendix C210Appendix D217VI
Abstract:Optimisation of the HVOF Thermal Spray Process for Coating, Forming andRepair o f Componentsby Jit Cheh TanBSc (Eng)The High Velocity Oxy-Fuel (HVOF) Thermal Spraying technique has beenwidely adopted in many industries due to its flexibility, and cost effectiveness inproducing superior quality o f coating. The demand o f high-technology industries andthe availability o f new advanced materials have generated major advances in thisfield. The HVOF thermal spray process has been utilised in many industries to applycoatings on components to protect against wear, heat and corrosion, and also to buildup worn components. This spraying technology is not limited to coating substrates butalso encompasses the manufacture o f net shaped component from materials which aresometimes difficult to form by conventional methods.A knowledge o f coating properties, testing and evaluation methods is essentialin order to apply coating technology to a specific application. While sprayingparameters and substrate surface preparations directly impact the coating properties, itis equally important to know the spraying technique required to deposit coatinghaving these properties and the processing parameters which have to be applied.The thesis reports the development and optimisation o f the HVOF thermalspray process for coating, forming and repair o f components. A die was designed tomanufacture free standing WC-Co inserts, and a similar technique was then followedto fabricate free standing annular rings and solid discs. The effects o f sprayingparameters on the components properties such as residual stresses and hardness wereinvestigated and limitations identified.Experiments to assess the coatings properties involved the combinations o fthree spraying powders, (1) Austenitic stainless steel (2) WC-Co and (3) Tool steelmatch powder on stainless steel 316L andD2 tool steel substrates. Investigations werecarried out on the effect o f spraying distance, sprayed coating thickness and pre-sprayheat treatment on coating properties including hardness, bond strength and residualVII
stress. Results reveal that there are strong correlations between the bonding strength,coating thickness and residual stress in coatings. The tensile residual stresses coupledwith increasing coating thickness cause the degradation o f bond strength withincreasing coating thickness.Optimisation of the repair o f damaged components using the HVOF techniqueinvolved the use o f similar combinations o f powder and substrate materials. Testswere carried out to identify the adhesion strength o f the repaired material sprayedunder various conditions which were varied, including (1) repair thickness (2) pre repair and post-repair heat treatment (3) repair wall angle and (4) substrate surfacepreparation. In addition, the finish machining possibility o f these repairedcomponents was evaluated.VIII
CHAPTER 1 INTRODUCTIONThe recognition that the vast majority o f engineering components can potentially degradeor catastrophically fail in service because o f such surface related phenomena as wear,corrosion and fatigue, led in the early 1980’s to the rapid development o f surfaceengineering. Surface engineering involves, amongst other processes, the application o ftraditional and innovative coating technologies to engineering components and materials,in order to produce a composite material with properties unattainable in either the base orsurface materials.O f all the advanced coating techniques, the thermal spraying process is one o f the mostsuccessful and versatile because o f the very wide range o f coating materials and substratesthat can be processed, e.g. from tape recording heads, to print rollers, and bridgestructures. The High Velocity Oxy-Fuel (HVOF) process is one o f the most popularthermal spray technologies and has been utilised in many industries because o f itsflexibility, and the superior quality o f coatings produced compared to other thermalspraying techniques.The demands o f high-technology industries, and the availability o f new particulatematerials have generated major advances in this field. The need for selection o f matchingsubstrate and spraying materials, and the determination o f optimised spraying parametersand substrate surface preparation methods are the main problems associated with theHVOF spraying process.The main objective o f this research is to investigate and optimise spray parameters for anumber o f different applications o f the technique, namely:1. Fabrication o f solid WC components using the HVOF process2. Reduction o f residual stresses in HVOF formed and coated components3. Repair o f damaged components using the HVOF process.l
The work programme o f this project is outlined in the following schematic diagram.HVOF ProcessSpray FormingCoatingsPowder Materials:SubstratesRepair WorkStainless SteelStainless SteelStainless SteelTungsten CarbideTungsten CarbideTungsten CarbideMatch Tool SteelMatch Tool SteelStainless SteelStainless SteelTungsten CarbideTungsten CarbideTool SteelTool SteelNitrided Tool SteelNitrided Tool SteelExperimental1) Residual Stress1) Residual Stress1) Pull TestMethods:-XRD-XRD2) Bend Test-Hole Drilling3) Machining Processes2) Hardness4) Investigation under3) Pull TestMicroscopeThe remainder section o f this thesis is divided into a number o f chapters. Chapter 2, theliterature review, gives a general introduction to surface engineering focusing mainly onflame thermal spraying technology. The spray forming process, its history background arediscussed. This chapter also details the characteristics o f the High Velocity Oxy-fuel(HVOF ) process and coatings.2
The experimental equipment and procedures o f the HVOF thermal spraying system arepresented in chapter 3. It includes brief descriptions o f individual units within the wholeHVOF system, and the safety aspects o f this system.Measurement methods are described in chapter 4. These include detailed descriptions o fresidual stress measurement on coatings and free standing components using both the Xray diffraction and Hole drilling methods. Data, results and calculations are also included.Coating adhesion strength and bend test methods are also detailed in this chapter.Chapter 5 presents the experimental results, including results o f free standing componentsfabricated using the HVOF process. The effects o f substrate pre-spray heat treatment onthe residual stresses o f the components sprayed are analysed and discussed. The effects o fpre-spray substrate surface temperature, substrate surface treatment and post-spray heattreatment on coating bond strength are also discussed. This chapter also includes theresults o f a study carried out on the effect o f component defect angle on the adhesion andbend strengths o f repairs. Tests were also carried out on the effect o f different substratesurface preparations on the adhesion strength o f the sprayed coatings.Statistical analysis o f the results are also presented in this chapter.Conclusions for the present work and suggestion for further work are included in Chapter3
CHAPTER 2 LITERATURE SURVEY2.1 Introduction to Surface EngineeringSurface engineering in today’s engineering world embraces the design, evaluation andperformance in service o f a total system including a substrate through the interface to acoating [1], It is a branch o f science that deals with methods for achieving desired surfacerequirements and assessing surface behaviour in service for engineering components [2], Thebehaviour o f a material is greatly dependent upon the surface o f the material, the shape o f thecontact surface, the environment and the operating conditions.Surface properties for certain engineering applications can be selected on the basis o f asubjective judgement, i.e. colour or texture for decoration. However, surfaces not only definethe outer limits o f bodies, they are also called upon to perform a variety o f engineeringfunctions, possibly completely different from those required o f bulk materials. Modern processenvironments which contribute to wear o f machine tools in industry can be very complex,usually involving a combination o f chemical and physical degradation. Surface properties o fthe components used in a particular working environment have to be designed in accordanceto that working environment. Various surface properties that are relevant to the behaviour o fengineering components are shown in Figure 1.The surface properties o f the materials o f a component may change noticeably as a result o fthe environment in which it operates. The outer surface o f bulk material is known to consist o fseveral zones having different physical and chemical characteristics particular to bulk materialitself [3]. The construction o f a metal surface is shown schematically in Figure 2. Above theworked layer, there is a region o f amorphous or microcrystalline material called the BilbyLayer, resulting from the surface melting and flowing during work hardening. Above this is anoxide layer, the formation o f this layer depending on the environment and surface oxidationmechanism. Outermost is a layer o f absórbate, which is generally a layer o f water vapour orhydrocarbon from the surroundings which may condense and become physically adhered tothe surface.4
Sur faceNatureShapeOthersInteraction linessWavinessSegregationCohesive EnergyContaminationPhysisorptionPoint ImperfectionWork HardeningChemisorptionDislocationCompound FormationGrain BoundariesCorrosionConductivitySurface EnergyHardnessFigure 1 Various surface propertiesFigure 2 Schematic representation o f metal surface [3],5
2.2 Overview O f Coating TechnologiesA coating may be defined as a near surface region with properties different from the bulkmaterial it is deposited on. Thus the material system (coating and substrate) form a compositewhere one set o f properties are obtained from the bulk substrate and another from the coatingitself. In short, the complex coating-substrate combination fulfils the desired coating propertyrequirement. Figure 3 illustrates some o f the inter related properties o f the complex systemincluding processing and environment which may be controlled within specified limits toensure that the overall engineering requirements o f the system are fulfilled.A coating process involves the selection o f deposition material, the transport o f the materialand the accumulation o f the material on the substrate. These steps can be completely separatefrom each other, or may be superimposed on each other depending upon the process used.The methods of depositing coating materials can be grouped into three distinct types viz.Vapour (gaseous) Phase, Liquid Phase and Molten or Semi-molten Phase. Figure 4 shows thewide variety o f surface coating techniques in use. Figure 5 details the classification o f variousflame spraying processes.The selection o f a particular deposition process depends on several factors, including:1) The material to be deposited,2) rate o f deposition required,3) limitations imposed by the substrate (eg. maximum allowable deposition temperature),4) adhesion o f the deposited material to substrate,5) process energy,6) purity o f target material since this will influence the impurity content in the film,7) apparatus required and availability o f same,8) cost, and9) ecology considerations6
Process TypeWorking PressureType O f EnvironmentBiasPhase DistributionSubstrate TemperatureIonisation GradeGas And MaterialHardnessWear ResistanceThermal ExpansionPhase etic PropertiesElectrical Properties *------ SUBSTRATECompositionProduction TypeGeometryPhase DistributionThermal ExpansionElectrical PropertiesM agnetic PropertiesXi Ü I4-------- ' APPLICATIONCorrosionAbrasionAdhesionOxidationBuild Up of LayersEnvironmentAestheticDimensional M ismatchFigure 3 The inter-relationship o f coating, substrate, process and application7
SITIONf a c in g ;!rCHEMICAL VAPOURDEPOSITION (CVD)iPHYSICAL VAPOUR PHYSICAL-CHEMICALDEPOSITION (PVD) VAPOUR DEPOSITION1) CONVENTIONAL CVD2) LOW PRESSURE CVD3) LASER INDUCED CVD4) ELECTRON ASSISTED CVDI. ATOMISED LIQUIDSPRAYBRUSH, PAD ANDROLLER S: ELECTROCHEMICALDEPOSITION /:4. CHEMICAL DEPOSITION5. DIP PROCESS6 SCREENING ANDl it h o g r a p h y ;;;7. INTERMETALUCCOMPOUND8. FLUIDEZED BEDTHERMAL SPRAYING j welding J11. OXY-DUEL GAS FLAME2. ELECTRIC ARC3. PLASMA ARC1.2.3.4.5.9 SPARK HARDENINGJO. SOL GELII. SPIN ON1-------------- ,GLiAlDDIN1tELCTRIC ARCr1-----------— I---------SPRAY AND FUSEDPLASMAARC1CONTINUOUS COMBUSTIONWELDBRAZEEXPLOSIVEMECHANICAL METHODDIFFUSION BONDING1FLAME1PULSED COMBUSTION1. JET KOTE2. DIAMOND JET3. TOP GUNFigure 4 Classifications o f various coating techniques8
T herm al SprayingElectric ArcW ire ArcPlasma ArcContinuous CombustionPulse CombustionFlameJet KoteDiamond JetTop GunFigure 5 Classification o f various flame thermal spraying9
2.3 Thermal SprayingThermal spraying is the generic name for a family o f coating processes in which a coatingmaterial is heated rapidly in a hot gaseous medium, and simultaneously projected at highvelocity onto a prepared substrate surface where it builds up to produce the desired coating. Ithas a long history beginning with work carried out in the late nineteenth century . The earliestcommercial application is attributed to Schoop in Switzerland who by 1910 [4] had developeddevices for melting tin or zinc and projecting the molten metal with compressed air. By themid-1920s metal spraying had found use in at least 15 countries [4], In the last three decadesthé demands o f high technology industries, eg. the aerospace industry, have lead to majoradvances in this field. New materials used in these industries require higher energy to processthem and this challenge has been met with considerable success. It is now possible to sprayvirtually any material provided that it melts (or becomes substantially molten) withoutsignificant degradation during a short residence in a heat source. A further improvement o fcoating properties, in particular the reduction o f coating porosity, is being attempted by theuse o f new methods for the post-treatment o f thermally sprayed coatings, ultrasoniccompression, hot isostatic pressing and shot penning or hammering. Thus, as will be describedlater, most metals, alloys, many ceramics, cermets and even plastics can be thermally sprayed.There are numerous thermal processes which can be classified under specific headings, andthese are now considered in some detail.2.3.1 Wire SprayingAlthough this process was the first to be developed from the pioneering work by Schoop, itstill maintains a prominent position in surface technology. Wire-spraying 'guns' operating onthe same basic principle are marketed by various manufacturers [5], A wire, typically 3-5 mmin diameter, is fed by a variable-speed motor or air turbine through the centre o f a multi-jetcombustion flame (Figure 6) . The tip o f the wire melts and an annular gas jet (usually o fcompressed air) strips molten particles from it, and propels them to the substrate at a velocityo f approximately 100m/s. The fuel gas used is generally acetylene, although for some metalspropane or hydrogen is preferred (e.g. copper reacts with
thermal spray technologies and has been utilised in many industries because of its flexibility, and the superior quality of coatings produced compared to other thermal . literature review, gives a general introduction to surface engineering focusing mainly on flame thermal spraying technology. The s
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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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