Thermal Spraying for Power Generation Components

Coatings constitute an intrinsic part of the power generation hardware. Thousands of patents, papers and conference presentations address new coating types, new hardware and software, new process developments, new chemical compositions. A huge unpublished knowledge is stored in manufacturers ′Know How′. However, sometimes coatings are still considered as an "art" and there are fair reasons for that. The thermal spray is still not a ′plug and play′ tool and the product quality largely depends on the deep understanding of process physics and hardware features, accumulated experience, engineer?s intuition and operator′s training.
This book now deals with questions that are essential for a good performance of this "art":Is there a given process stability?What is the ratio of deterministic and stochastic in the coating process?Is there an inherent process capability for a given specification which cannot be improved?What is the right preventive maintenance strategy?Is there a chance to end up with coating process capabilities in the order of other manufacturing processes?What can be predicted and designed a–priori by physical modeling and off–line programming and what can be achieved by trials and errors only?
This book is not a pure scientific book. It is of most value for the engineer involved in design, processing and application of thermally sprayed coatings: To understand the capability and limitation of thermal spraying, to understand deposition efficiency—and the importance of maintenance and spare parts for quick change over of worn equipment, to use offline programming and real equipment in an optimum mix to end up with stable processes in production after shortest development time and in the end to achieve the final target in production: Process stability at minimum total cost.

Spis treści:
Preface.
The Authors of this Book.
1 Int roduction.
1.1 Requirements for Materials and Coatings in Powerplants.
1.2 Examples of Coatings in Gas Turbines.
1.3 Defi nition of Thermal Spraying (THSP).
1.4 Thermal–Spraying Systems.
1.5 Coatings for Power–Generation Components.
1.6 The Complete Manufacturing and Coating Process.
1.7 Coating–Process Development.
1.8 Tasks for “Target” Readers.
2 Practical Experience Today.
2.1 Coating Processes.
2.2 Basics of Thermal Spraying.
2.3 Feedstock.
2.3.1 Wire.
2.3.2 Powder.
2.3.2.1 Powder Types.
2.3.2.2 Powder–Production Processes and Morphologies.
2.3.2.3 Powder Characterization.
2.3.2.4 Powders for Power–Generation Applications.
2.4 Thermal–Spraying Equipment.
2.4.1 Example of a Low–Pressure Plasma–Coating System.
2.4.2 Flame and Arc Spray Torches.
2.4.3 HVOF Process.
2.4.3.1 Comparison of HVOF Fuels.
2.4.3.2 A Brief Overview of the Major Existing HVOF Systems.
2.4.3.3 Possible Improvements of HVOF Systems.
2.4.4 Plasma Process.
2.4.4.1 A Brief Overview of Plasma Torches.
2.4.4.2 Possible Improvements of Plasma Systems.
2.5 Work Flow and Important Coating Hardware.
2.5.1 Powder Preparation and Powder–Delivery System.
2.5.1.1 Powder Preparation.
2.5.1.2 Powder Delivery and Injection System.
2.5.1.3 Powder Injection and Plasma/Hot Gas Jet.
2.5.1.4 Injector Plugging and “Spitting”.
2.5.1.5 Powder Buildup at the Front Nozzle Wall.
2.5.2 Cooling System.
2.5.3 Power–Supply System.
2.5.4 Gas Supply and Distribution System.
2.5.5 Manipulation Systems.
2.5.6 Fixtures and Masking.
2.6 Examples of Coated Power–Generation Components.
2.7 Production Experience.
2.7.1 Surface Preparation.
2.7.1.1 Internal Plasma and Transferred Arc.
2.7.2 Process and Systems.
2.7.2.1 The Programming of the Coating Process.
2.7.3 Finishing.
2.7.4 Repair of Turbine Parts.
2.7.4.1 Coating Removal, Stripping.
2.7.4.2 Restoration of the Base Materials.
2.7.4.3 Refurbishing, Recoating.
2.8 Commercial.
2.8.1 General.
2.8.2 Surface Preparation.
2.8.3 Coating Equipment.
2.8.4 Finishing.
3 Quality and Process Capability.
3.1 Quality Assurance.
3.2 Sources of Process Variations.
3.2.1 Special Causes of Coating–Process Variation.
3.2.2 Stochastic Nature of a Spray Process.
3.2.2.1 Arc and Jet Pulsations.
3.2.2.2 Powder–Size Distribution.
3.2.2.3 Powder Injection.
3.2.2.4 Powder Shape.
3.2.2.5 Particle Bonding.
3.2.2.6 Gun and Component Motion and Positioning.
3.2.3 Drifting.
3.2.4 Stability of the Quality Control.
3.3 Process Capability and Stable Process.
3.3.1 Defi nition of Process Capability.
3.3.2 Defi nition of a Stable Coating Process.
3.3.3 Operational Window.
3.3.4 What Process Capability is Required?
3.3.5 Additional Factors that Affect the Process Capability.
3.3.6 Case Study: Achievable Process Capability.
3.3.6.1 Part Complexity.
3.3.6.2 Mutual Position of the Gun and Component Fixtures.
3.3.6.3 Powder Quality.
3.3.6.4 Torch Pulsations and Drifting.
3.3.6.5 Instability of the Quality–Control Process.
3.3.6.6 Surface Preparation and the Part Temperature.
3.3.6.7 Conditions of the Powder–Injection System.
3.3.6.8 Process Capability.
3.4 Maintenance.
4 Theory and Physical Trends.
4.1 Coating Formation from Separate Particles: Particle Impact, Spreading and Bonding.
4.2 Physics of Plasma Torches.
4.2.1 Plasma Properties.
4.2.2 Gas Dynamics of Plasma Torch.
4.2.3 Energy Balance of the Plasma Gun.
4.2.4 Major Trends.
4.2.4.1 Variation of the Gun Power; the Gas Flow Rates and Composition Unchanged.
4.2.4.2 Variation of the Plasma Composition at the Same Specifi c Plasma Enthalpy.
4.2.4.3 Variation of the Plasma Flow Rate at Unchanged Gun Power an d Gas Composition.
4.2.4.4 Effect of Nozzle Diameter.
4.2.5 Plasma Swirl.
4.3 Structure of Plasma Jets.
4.3.1 APS Jet.
4.3.2 Structure of LPPS Jet.
4.4 Particles in Plasma.
4.4.1 Particles at APS.
4.4.2 Particle at LPPS.
4.4.2.1 Particle Acceleration and Heating in the LPPS Free Jet.
4.4.2.2 Particle Acceleration and Heating Inside the Nozzle.
4.5 Spray Footprint (Spray Pattern).
4.6 Infl uence of Particles on Plasma Flow.
4.7 Substrate Surface Temperature.
4.8 Formation of the Coating Layer.
4.9 Use of Different Plasma Gases.
4.10 Some Distinguishing Features of HVOF Physics.
5 Offl ine Simulation of a Thermal–Spray Process.
5.1 Simulation in Production.
5.2 Physical Background of Simulation Package.
5.2.1 Viscoplasticity Model of a Splat and Particle Bonding.
5.2.2 Thermodynamic and Transport Properties of Argon/Hydrogen Mixtures.
5.2.3 Modeling of the Plasma Gun.
5.2.4 Modeling of the Plasma Jets.
5.2.4.1 APS Jet.
5.2.4.2 LPPS Jet.
5.2.5 Acceleration and Heating of Particles in Plasma.
5.2.6 Surface Thermal Conditions.
5.3 Spray Pattern.
5.3.1 Calibration of the Bonding Model and Sensitivity of a Spray Pattern to the Process Parameters, Spray Angle and Bonding Model.
5.3.2 Coating Porosity and Roughness.
5.4 Modeling of Turbine Blades.
5.5 Coating Thickness Optimization and Stochastic Modeling Tools.
5.6 Simulation of HVOF Process.
5.7 Use of Offl ine Simulation in Coating Development.
5.7.1 Application Areas of Modeling in the Coating Process.
5.7.1.1 Coating Defi nition and Design for Coating.
5.7.1.2 Coating–Process Development.
5.7.1.3 Part Development.
5.7.1.4 Physical Modeling and Offl ine Simulation as Process–Diagnostic Tools.
5.7.1.5 Simulation as a Numerical Experiment.
5.7.1.6 When the Offl ine Simulation Should Be Used.
6 Standards and Training.
6.1 Standards, Codes.
6.1.1 Introduction to