High Velocity Oxy-Fuel (HVOF) Thermal Spray Deposition Of .

High Velocity Oxy-Fuel (HVOF) Thermal Spray Depositionof Functionally Graded CoatingsA Thesis Submitted to theFaculty o f Engineering and Computing,School o f Mechanical and Manufacturing Engineering,Dublin City UniversityFor the Degree o f Doctor o f PhilosophyByMahbub Hasan, B.Sc. Eng.Materials Processing Research Centre andNational Centre for Plasma Science and TechnologyDublin City UniversityResearch SupervisorsDr. Joseph Stokes (BA, BAI, Ph.D., MIEI)Dr. Lisa Looney (BA, BAI, Ph.D., CEng., MIEI)Professor M. S. J. Hashmi (Ph.D., D.Sc., CEng., FIMechE., FIEI, MASME)J a n u a ry 2 0 0 5

DECLARATIONI hereby certify that this material, which I now submit for assessment on theprogramme of study leading to the award o f Doctor of Philosophy, is entirely myown w ork and has not been taken from the work o f others save and to the extentthat such work has not been cited and acknowledged within the text of my work.Signed:Date: 25/01/05(Mahbub Hasan)Student I.D. No.: 99146215I

ACKNOW LEDGEM ENTThere are many individuals who have assisted me during my present work. I would liketo thank you all.My first vote o f thanks must go to Dr. Joseph Stokes for his unceasing enthusiasm,interest, constructive criticism and practical hand on assistance with the HVOF thermalspray system and for putting up with me over the years. His expertise, availability todiscuss ideas and willingness to give his knowledge were instrumental in thecompletion o f this thesis. I owe him much gratitude.I would like to express my sincere thanks to Dr. Lisa Looney, for whom I have thegreatest respect and admiration. Her guidance and supervision were invaluable. I amextremely grateful for all her advises and suggestions towards solving the problem. I amprivileged to have worked with her.I will be forever indebted to Professor M. S. J. Hashmni who not only funded myproject but also supported and supervised me unstintingly. Without his support andencouragement this research would not have been done.I would like to acknowledge our technicians Mr. Michael Tyrrell, Liam Domnican,Chris Crouch, Keith Hickey, Michael May, Alan Meehan and Jim Barry for theirtechnical help and discussions at various stages o f the work. Special thanks to Mr.Michael Tyrrell for his regular support during my work.I am grateful to Professor Mohiuddin Ahmed (BUET) for initially selecting me from theDepartment o f Materials and Metallurgical Engineering (BUET) for doing research inDCU and assisting me in coming to Ireland.My most sincere gratitude is extended to my family, especially my mother, belovedfather and elder brother (who died in a mine blast in Georgia) who have given theirutmost support throughout my life so far. They have inspired me whole-heartedly sincemy childhood to progress in my educational career leading to Ph.D.II

I wish to thank all o f my fellow postgraduate students for their support and friendshipduring my study in DCU. My thanks also go to the Bangladeshi Community (speciallyJulfikar Haider, my housemate for over four years) in Ireland who provided me a lot o fsupport and fun during my life in Dublin.Ill

DEDICATIONDEDICATED TOMY PARENTS(LATE DR. AHMAD HUSAIN & MAHBUBA HUSAIN)ANDELDER BROTHER(LATE COLONEL MUHAMMAD HUSSAIN)IV

ABSTRACTHigh Velocity Oxy-Fuel (HVOF) Thermal Spray Deposition ofFunctionally Graded CoatingsMahbub HasanThe present study investigates an innovative modification o f a HVOF (High VelocityOxy-Fuel) thermal spray process to produce functionally graded thick coatings. In orderto deposit thick coatings, certain problems have to be overcome. More specifically theseproblems include minimizing residual stresses, which cause shape distortion in assprayed components. Residual stresses in coatings also lead to adhesion loss,interlaminar debonding, cracking or buckling and are particularly high where there is alarge property difference between the coating and the substrate. Graded coatings enablegradual variation o f the coating composition and/or microstructure, which offers thepossibility o f reducing residual stress build-up in coatings.In order to spray such a coating, modification to a commercial powder feed hopper wasrequired to enable it to deposit two powders simultaneously. This allows deposition ofdifferent layers o f coating with changing chemical compositions, without interruptingthe spraying process. Various concepts for this modification were identified and onedesign was selected, having been validated through use o f a process model, which wasdeveloped using ANSYS Finite Element Analysis. The model simulates the flow ofnitrogen gas and powder through the system, and verified the supply o f mixedcomposition powders. Based on this information a multi-powder feed unit wasmanufactured, commissioned and calibrated. Multi-layer coatings o f aluminium andtool-steel were sprayed onto aluminium substrates. The chemical composition ofdifferent layers of a five layer graded coating was determined using energy dispersiveX-ray spectroscopy (EDS) to confirm functionality.Subsequently, various controlled parameters o f the HVOF spraying process werestudied for this type o f coating using 33 factorial design o f experiments. Results wereanalysed in terms o f surface stress to deposition thickness ratio. The best combinationo f spray parameters identified for deposition o f the mixed coating resembles thoserecommended for aluminium powder alone. It is proposed that this arises from thethermal properties o f the constituent powders.Different types o f aluminium/tool-steel functionally graded coatings were thendeposited using the optimised set o f spray parameters, and considered using Clyne’sanalytical method o f stress analysis and Vickers hardness testing method. Coatingscomposed o f thicker layers resulted in much higher residual stress, but also improvedhardness compared to thinner samples. It was found that if 5 layers o f graded materialare sprayed, and the residual stress compared to that o f a traditional single layer (of thesame thickness), an approximately 48 % reduction can be achieved. However thisbenefit is mitigated somewhat by the fact that applying these multi-layers reduces thehardness to by approximately 16 % compared to the traditional single layered deposit.Therefore an engineer must compromise between the stress and hardness whendesigning a functionally graded coating-substrate system.V

TABLE OF stractTable of ContentsList of FiguresList of TablesCHAPTER 11.1CHAPTER 2IHIVVVIKXIVINTRODUCTIONIntroduction1LITERATURE REVIEW2.1Introduction42.2Overview of Coating Techniques42.3Thermal Spray Techniques2.3.1 HVOF Thermal Spray Process2.3.2 HVOF Gun Design79112.4The HVOF Process2.4.1 Combustion and Gas Dynamics o f theHVOF System2.4.2 Advantages o f the HVOF Coating2.4.3 Disadvantages o f the HVOF System1414Thermally Sprayed Coatings2.5.1 Input Powder Production2.5.2 Coating Deposition, Solidification andBuild-Up2.5.3 Residual Stress2.5.4 Coating Structure and Properties18181921272.6Functionally Graded Materials (FGM)2.6.1 Constructive Processes2.6.2 Transport-Based Processes2930322.7Functionally Graded Coatings2.7.1 Different Techniques Producing FunctionallyGraded Coatings35352.5VI1617

2.7.2Characteristics and Properties o f FunctionallyGraded Coatings2.7.3 Applications o f Functionally Graded CoatingsCHAPTER 34447EXPERIMENTAL WORK & Thermal Spraying SystemGas supply and flow meter unitPowder feed unitDiamond Jet (DJ) gunSupport 3.3.6of a Dual Powder Feed SystemDesign ConceptsRating ChartAdvantages and DisadvantagesDescription o f Chosen Concept DeviceNitrogen Gas-Powder Flow ModelDesign Calibration and Test626369707174813.4HVOF Spraying Procedure3.4.13.4.283Surface PreparationSpraying Process83833.5Optimisation of Spray Parameters863.6Coating Characterization Techniques3.6.1 Microscopy3.6.2 Energy Dispersive X-Ray Spectroscopy (EDS)3.6.3 X-Ray Diffraction Phase Characterization3.6.4 Measurement o f Mechanical Properties3.6.5 Measurement o f Residual StressCHAPTER 48888959697104RESULTS & 2.31131141161394.3of SimulationInitial TestsFinal SimulationEffect o f Gravity and Change o f Dimension o fthe Gas-Powder Carrying Tubes and Pick-UpShaft4.2.4 Conclusion o f the ResultsCalibration Tests4.3.1 Powder Flow Bench TestsVII151153153

4.4CHAPTER 54.3.2 In Situ Flow Tests159Optimisation of Spray Parameters4.4.1 Chemical Composition of Different Layers ofa Graded Coating4.4.2 Microstructure and Phase Identification4.4.3 Measurement o f Young’s Modulus andPoisson’s RatioMeasuremento f Residual Stress4.4.4161161Variation of Residual Stress4.5.1 Variation o f Residual Stress with DepositThickness4.5.2 Variation o f Residual Stress with Numbero f Layers4.5.3 Effect on Hardness189189Comparison Between Stress Measurements196165169172192193CONCLUSIONS &RECOMMENDATIONS5.15.2ConclusionsRecommendations for Future W ork198201PUBLICATIONS ARISING FROM THIS ixAppendixAppendixAppendixABCDEDifferent Parts Involving Concept FourANSYS ResultsResults o f Aluminium Powder Flow Bench TestsResults o f Tool-Steel Powder Flow Bench TestsStress Distribution ProfileVIIIA1A12A20A23A26

LIST OF ure2.12.22.32.42.52.62.7Figure 2.8Figure 2.9Figure 2.10Figure 2.11Figure 2.12FigureFigureFigureFigure2.132.142.152.16Figure 2.17Figure 2.18Figure 03.21Coating deposition techniquesDevelopment o f the Thermal Spray TechnologySchematic of cross-section o f a Diamond Jet spray gunSchematic o f a throat combustion burner HVOF gunSchematic o f a chamber combustion burner HVOF gunTheoretical flame temperature against oxygen/fuel ratioCross-section o f a columnar structure (single lamella) formed aftersolidificationSchematic of quenching stressesChange o f state o f substrate and particle during coating depositionQualitative quenching stress development in aluminium/tool-steelfunctionally graded coatingSchematic of cooling stressesQualitative cooling stress development in aluminium/tool-steelfunctionally graded coatingSchematic section of a spray depositSchematic o f the solid-state powder consolidation processFunctionally graded coating of material A and BSchematic of a single torch and dual feeder system for theproduction of functionally graded coatingsInjection o f the ceramic and organic powders in the hottest andcolder part o f the flame respectivelySchematic of the production o f graded coatings using pre-mixedpowders and a single torchSchematic o f the production o f FGC using the slurry dipping process681112131620The HVOF thermal spray systemThe gas flow m eter unitThe powder feed unitSchematic cross-section o f the hopper assembly on the DJ powderfeed unitDifferent parts o f the Diamond Jet gunCross-section o f assembled Diamond Jet gunSchematic o f the traverse unit and carbon dioxide cooling systemSchematic of graded coatings; (a) undesired layered, (b) desiredheterogeneousSchematic o f the control system and powder feed hopperFlow diagram o f the second proposed systemSchematic diagram o f concept twoFlow diagram o f the third proposed systemSectional assembly drawing o f the proposed designed partsSectional assembly drawing o f the designed parts along withthe previous hopperPhotograph o f dual powder feed unitGeometry o f the powder and nitrogen gas flow tubesSchematic of applied boundary conditionsSchematic o f a scanning electron microscope (SEM)Schematic o f an energy dispersive X-ray sprectroscopy (EDS)Schematic of an eddy current gaugeThe cantilever approach for measuring the Young’s modulus andPoisson’s 666676869727677949599101