Atmospheric Plasma Spray Hossein Movla1,* with significant contributions from Jafar Fathi2, Sirous Khorram3, Mohammad Ali Mohammadi2,4, Arash Nikniazi1, Foozieh Sohrabi2 And With Special Thanks to Mohammad Soltanpour 1 Department of Solid State Physics, Faculty of Physics, The University of Tabriz, Tabriz 2 Department of Atomic and Molecular Physics, Faculty of Physics, The University of Tabriz, Tabriz 3 Plasma lab, Research Institute for Applied Physics and Astronomy (RIAPA), The University of Tabriz 4 Sahand Plasma Focus Lab, Research Institute for Applied Physics and Astronomy (RIAPA), The University of Tabriz, Tabriz Atmospheric Plasma Spray

What is Plasma? Fourth state of matter Gas-like Consists of neutral atoms/molecules, ions, and electrons High energy, highly reactive substance Electrons have 10,000 K worth of energy Activates surfaces What is Plasma?

Gas discharge plasma Electric field causes acceleration of electrons electron-impact ionization current to flow through gas

Electrical properties of plasma Negative and positive charges balance each other Plasma is electrically neutral. Property known as quasi-neutrality Plasma is electrically conductive Electrical conductivity comparable to that of metals at room temperature Electrical properties of plasma

Thermal and nonthermal plasma Thermal and nonthermal plasmas If Te ≈ Tion then we have a thermal plasma If Te » Tion then plasma is nonthermal or nonequilibrium or cold plasma In a thermal plasma, there is Local Thermodynamic Equilibrium (LTE) LTE requires… Te ≈ Tion Excitation equilibrium Chemical equilibrium Thermal and nonthermal plasma

Natural and man-made plasma Natural plasma Lightning strikes, high pressure, high luminosity Aurora borealis, low pressure, low luminosity Man-made plasma Flames, low ionisation level Glow discharge, low pressure 1 eV = 7740 K Natural and man-made plasma Source: Boulos et al. “Thermal Plasmas Fundamentals and Applications V. 1, 1994, Vol. 1”

Atmospheric Plasma Non-equilibrium plasma Electrons at 10-20,000 ºC Ions, molecules at 60-80 ºC Relatively uniform energy distribution Does not require special vacuum chambers like low pressure plasma Atmospheric Plasma

Physics of the Vacuum Arc – The Arc Discharge D.C. Discharges Corona High V, Low I At sharp point Glow Discharge V ~ 100’s V, I ~mA’s Cathode fall 150-550 V, depends on gas and cathode material Arc 10’s of volts, A-kA Cathode spots Physics of the Vacuum Arc – The Arc Discharge

Gas discharge plasma At atmospheric pressure: Breakdown voltage is very high Breakdown mechanism is often streamer breakdown (spatially nonuniform) breakdown voltage Paschen curves Breakdown voltage increases with the product of gas pressure & electrode separation Gas discharge plasma Schütze et al. 1998

Composition of a plasma gas Gas mixtures - Air Dissociation N2 ↔ N + N O2 ↔ O + O Recombination N + O ↔ NO N + O2 ↔ N2 Ionization NO ↔ NO+ + e- N2 ↔ N2+ + e- N ↔ N+ + e- N+ ↔ N++ + e- O2 ↔ O2+ + e- O ↔ O+ + e- O+ ↔ O++ + e- Ar ↔ Ar+ + e- Ar+ ↔ Ar++ + e- Composition of a plasma gas Gas mixtures - Air

Difference between Glow and Arc – cathode electron emission process ‘individual’ secondary emission of electrons by: Ions (depends on ionization energy, not kinetic energy) Excited Atoms Photons Not enough! Multiplication in avalanche near cathode Need high cathode drop (100’s of V’s) Used in sputtering to accelerate bombarding ions into ‘target’ cathode Arc Collective electron emission Current at cathode concentrated into cathode spots Combination of thermionic and field emission of electrons Can get sufficient electron current Low cathode voltage drop (10’s of V’s) High temp. in cathode spot gives high local evaporation rate – used in vacuum arc deposition Difference between Glow and Arc – cathode electron emission process

Generating a thermal plasma High intensity arcs Free burning arcs, e.g. steel furnaces Wall stabilized arcs, e.g. arc lamp (lab studies) Convection-stabilized arcs, e.g. hot button plasma torches Magnetically stabilized arcs, e.g. plasma torches with magnetic field Thermal RF discharge Capacitive coupling: HF electrical field Inductive coupling: Time-varying magnetic field Microwave discharge The discharge is part of the MW circuit. Impacts plasma configuration and volume Low pressure operation results in deviation from equilibrium More recently, stable higher pressure plasma possible Generating a thermal plasma

Commercial Plasma Applications Lighting Steel Melting Furnaces Cutting and Welding Thermal spray Atmospheric plasma spray (APS) Vacuum plasma spray (VPS) Chemical Vapour deposition (CVD) Physical Vapour deposition (PVD) Waste Recycling Commercial Plasma Applications

Why Coating? high temperature resistance, high corrosion resistance, chemical inertness, high hardness, high wear, abrasion and erosion resistance, creep resistance, good adhesion, high fracture toughness, good thermal conductivity, thermal shock resistance, smooth surface/low porosity, and thermal fatigue cracking resistance. Why Coating?

Wear and Corrosion-resistant Coatings Pure Carbides (TiC, ZrC, HfC, NbC, TaC, WC) Cemented Carbides, and Tungsten Carbide/Cobalt Coatings (WC/Co) Titanium Carbide-based Coatings (TiC-TaC-NbC, TizAlC) Chromium Carbide-based Coatings (CrZrAlC) Boride-based Coatings (CrB2, BN) Oxide Coatings Alumina-based Coatings (A12O3/TiO2, A12O3/ZrO2) Chromia-based Coatings (Cr2O3) Metallic Coatings Refractory Metal Coatings (Mo, Ti and W) Superalloy Coatings (NiCoCrAlY) Diamond Coatings Wear and Corrosion-resistant Coatings

Coating Methods Chemical coatings Sol-gel coatings Chemical Vapor deposition (CVD) Physical Vapor deposition (PVD) Vacuum Evaporation technique Sputtering methods Laser ablation Thermal Spraying Coating Methods

Ceramic Coatings in the Industrial Environment 39% were produced by physical vapor deposition techniques (PVD) 26% by chemical vapor deposition (CVD) 23% by thermal spraying 12% by wet processing including sol-gel technique Ceramic Coatings in the Industrial Environment annual average growth rate of 12% US $3 billion by the year 2000 US $6.5 billion by the year 2009 individual annual growth rates are: engines (28%), marine equipment (18%), chemical processing (15%), military (11%)', and construction (11%).

Total projected sales = $4.1 billion (1995) 80% Coating Total projected sales = $4.1 billion (1995) 7.8 % Carbides 5.2% Nitrides 4.2% Oxides 2.8% Other

three basic methods Flame spraying Arc spraying and Plasma spraying. There are three basic methods of creating and delivering the necessary molten metal, spray, and these are ： Flame spraying Arc spraying and Plasma spraying. three basic methods

flame spraying Arc spraying Temperature limited by internal heat of gasses Oxyacetylene torch ( T = 2700 ºC) Detonation gun (D-gun) ( T = 3200 ºC) Jet Kote system ( T = 3000 ºC) Hypervelocity oxyfuel gun ( T = 3000 ºC) Arc spraying Temperature unlimited, controlled by energy input Electric arc wire-spraying Air plasma spraying (APS) ( T = 15000 ºC) Inductive plasma (IPS) ( T > 15000 ºC) Reduced pressure ('vacuum') plasma ( T > 15000 ºC) RF-Plasma spraying ( T > 15000 ºC) Low pressure laser spraying ( LPLS ) ( T = 10000 ºC)

Advantages of Plasma Systems Plasma acts as a resistive heating element that cannot melt and fail Torch operates with most gases – not a combustion process Temperature unlimited (T > 15000 ºC) High temperatures, presence of ions, free electrons and UV allow for highly efficient coating The high intensity plasma heat allows for designing compact systems Vacuum not applied No by-products (reduced by-products) High efficient and Cost-effective Advantages of Plasma Systems

Types of plasma torches Source: Camacho, S.L., “Industrial-worthy plasma torches: state of the art”, Pure and Appl. Chem, Vol 60, No. 5, pp 619-632, 1988

Plasma Spray Gun

The objective of the our work are: Choosing the best type of Plasma Spray Solving the technological problems of Plasma spray production Producing protective coatings using the method of gas-dynamic spraying, improving technological effectiveness of spraying Reducing the cost of materials for spraying Manufatoring a lab-scale plasma spray The objective of the our work are:

Plasma Disadvantages/Challenges New technology perception Control of NOx emissions from air torch Developing markets for slag Scale-up risks Limitation of size of commercially available torches Markets’ risks Plasma Disadvantages/Challenges

Future Work Using a high-power rectifier (36 KW) Coating of multiple materials by changing the deposition parameters Processing of oxygen sensitive materials due to processing in controlled atmosphere. Robotic control Use of reactive gases such as hydrogen to reduce impurities such as carbon and oxygen. Techniques to enable the VPS of the ultra-fine powders are currently being evaluated. Modeling the processes in this system Manufacturing an industrial-scale of plasma spray Future Work

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