Thermal spray coating

Thermal spray coating- study of different thermal spray coating processes, components and steps in spray coating.

1. SPRAY COATINGS Submitted by Jabin Mathew Benjamin 13MY04 Dept. Of Metallurgical Engineering 12-Mar-14 1 Dept. of Metallurgical Engineering

2. Need For Surface Hardening Produce surfaces that wear only a little , resistant to tarnishing and corrosion, 12-Mar-14 2 Dept. of Metallurgical Engineering

3. Thermal Spray Coating A group of coating processes where the coating is deposited on a prepared substrate by applying a stream of particles, metallic or nonmetallic, which flatten more or less forming platelets, called splats, with several layers of these splats forming the coating Any material on almost any substrate 12-Mar-14 3 Dept. of Metallurgical Engineering

4. Spray Coatings Use either axial or radial consumable injection in a high energy flow resulting from combustion or high-velocity gas streams. Coating thickness 50 μm to a few mm Bonding Substrate surface undercuts simple roughening 12-Mar-14 4 Dept. of Metallurgical Engineering

5. Components An energetic gas flow An appropriate gun Devices for feeding, accelerating, heating, and directing the flow of a thermal spray material toward the substrate. Feedstock P owder , wire, rod, or cord. Fed at a velocity allowing the spray gun to melt them Auxiliary gas feed To accelerate atomized molten material into the spray gun Controlled atmosphere or a soft vacuum In air, coating oxidation occurs, increasing with the temperature 12-Mar-14 5 Dept. of Metallurgical Engineering

6. Substrate Should not be degraded by heat Substrate should be roughened for good adhesion Abrasive blasting – Aluminium Oxide 2.5 µ m roughness optimum Difficult to spray hardened steels Thin sections prone to distortion during blasting and heating 12-Mar-14 6 Dept. of Metallurgical Engineering

7. Fig: Components of spray coating system 12-Mar-14 7 Dept. of Metallurgical Engineering

8. Steps In Spray Coating Substrate preparation Generation of the energetic gas flow Particle or wire or rod or cord injection Energetic gas particle or droplet interaction Coating formation 12-Mar-14 8 Dept. of Metallurgical Engineering

9. Thermal Spray Hardening Gas Combustion Oxy fuel process using Wire feed Powder feed Rod feed Jet / powder feed Detonation Gun process Arc process Plasma arc with powder feed Arc spray with wire feed 12-Mar-14 9 Dept. of Metallurgical Engineering

10. Wire Processes Wire from reel fed to oxyacetylene flame Metal droplets atomized by air jets Atomized metal spray coats substrate Gun to substrate distance 10 – 25mm Commonly sprayed materials Zn, Al for corrosion resistance Bronzes for wear resistance 12-Mar-14 10 Dept. of Metallurgical Engineering

11. Wear application: 1.25mm Corrosion resistance: 25µm Max thickness : 6mm Deposition rate: 93m 2 per hour per 25µm Flame temperature: 2760 C 12-Mar-14 11 Dept. of Metallurgical Engineering

12. Powder Spray Powder feed instead of wire Oxyacetylene torch modified for powder feed No high pressure air to assist atomization; low deposition rate Lower bond strength and higher porosity Easy method for materials that cannot be made into wire 12-Mar-14 12 Dept. of Metallurgical Engineering

13.  More sophisticated equipment uses compressed air.  Increased atomization.  Higher deposition rate and bond strength.  Flame temperature: 2500 C  Coatings Carbides High alloy steels Ceramics 12-Mar-14 13 Dept. of Metallurgical Engineering

14. Rod Consumable Ceramics cannot form flexible wire Coatings made of powder; too friable Newly designed ones use solid rod of ceramic Impact velocity: 2.8m/s Rod consumables Al 2 O 3 Cr 2 O 3 Ceramic mixtures 12-Mar-14 14 Dept. of Metallurgical Engineering

15. Detonation Gun (D- Gun) Powder fed under small gas pressure Explosive mixture of O 2 and acetylene detonated using spark Temperature: 3870 C Detonation: 4 to 8 times per sec; 730 m/s N 2 gas for flushing detonated gas Coating thickness: 75 to 125µm Noisy process; done in soundproof room For Carbides Ceramics High bond strength and coating density Good surface finish 12-Mar-14 15 Dept. of Metallurgical Engineering

16. Combustion Jet Or High Velocity Oxygen Fuel (HVOF) Process Continuous gas combustion jet: heat source and carrier O 2 and fuel gas like propylene, H 2 Consumable sprayed as powder to center of jet stream Temperature: 2980 C and velocity: 1370 m/s 45kg per hour deposition rate Consumable: Tungsten carbide, cobalt High bond strength High cost and safety issues involved 12-Mar-14 16 Dept. of Metallurgical Engineering

17. Electric Arc Spraying Uses electric arc as heating source Uses two consumable wires: higher deposition rate Wires on motor driven feed rolls and insulated from each other meet at tip of torch After energizing the torch, wires on contact produce arc Arc melts metal and air jet carries it to substrate Wires as large as 1.5mm Spraying soft materials for corrosion resistance; Zn, Al 12-Mar-14 17 Dept. of Metallurgical Engineering

18. Plasma Arc Deposition Consumable powder melted and atomized in plasma Tungsten electrodes and Ar gas Temperature: 28000 o C 12-Mar-14 18 Dept. of Metallurgical Engineering

19. Fig: Paper machine roll coated by NiCrBSi using two powder flame guns (Courtesy of Castoline ) 12-Mar-14 19 Dept. of Metallurgical Engineering

20. Fig: (a ) PTA-coated tooth of excavator with Ni base coating + WC (25 kg/h) and (b) cross section of the coating (courtesy of Castolin ) 12-Mar-14 20 Dept. of Metallurgical Engineering

21. Comparison Properties Electro/ electroless plating CVD PVD Thermal spray Equipment cost Low Moderate Moderate to high Moderate to high Operating cost Low Low to moderate Moderate to high Low to high Coating thickness 10 μ m–mm 10 μ m–mm Very thin to moderate 50 μ m–mm Adherence Moderate mechanical to good chemical bond good chemical to excellent diffusion Bond Moderate mechanical to good Chemical Bond Good mechanical bond Coating materials Metals Metals, ceramics, Polymers Metals, ceramics, polymers Metals, cermets, ceramics, polymers Surface finish Moderately coarse to glossy Smooth to glossy Smooth to glossy Coarse to Smooth (0.12 µm to 0.5 µm) 12-Mar-14 21 Dept. of Metallurgical Engineering

22. Coating Evaluation Destructive testing: Tensile shear tests Ends of two strips of desired substrate are sprayed with desired consumable Coated ends epoxied together Uncoated ends put to tensile tester and pulled to failure If epoxy fails and the coating is intact Good coating Bond strength as “greater than X”, X- tensile strength of epoxy 82 to 138 MPa nominal Non-destructive testing Visual inspection Porosities, impurities, cracks UT inspection, Thermal imaging 12-Mar-14 22 Dept. of Metallurgical Engineering

23. Comparison Between Spray Processes Wire gun Heavy deposits: upto 0.100 inch For steels, brass, bronze Powder module Minor shop repairs: upto 0.030 inch For nickel base alloys Rod feed Wear resistant coatings: upto 0.020 inch For ceramics D-gun Premier coatings: upto 0.010 inch Of hardfacing alloys, carbides Densest coating Electric arc Rebuilding large areas with steels: upto 0.100 inch For Al and Zn Plasma arc Applying hardfacing alloys, repairs: upto 0.015 inch For metals and ceramics 12-Mar-14 23 Dept. of Metallurgical Engineering

24. Applications of Spray Coating Wear-resistant coatings against abrasion, erosion Corrosion-resistant coatings Heat resistant coatings Thermal insulation or conduction coatings Electromagnetic shielding Medical coatings 12-Mar-14 24 Dept. of Metallurgical Engineering

25. Do’s And Don’t’s Of Thermal Spray Coating Do’s Apply coating to undercuts to avoid end chipping Hold gun normal while spraying Plug keyways when coating Don’t’s Coat end of parts subject to chipping Coat faces subject to impact Spray at an angle < 60 Coat cutting edges 12-Mar-14 25 Dept. of Metallurgical Engineering

26. Reference Cartier M, Handbook of surface treatments and coatings. ASME Press, New York, NY, 2003 Davis JR, Handbook of thermal spray technology. ASM International, Materials Park , OH, 2004 Chattopadhyay R (2001 ), Surface wear. ASM International, Materials Park, OH Kenneth G. Budinski , Surface Engineering for Wear Resistance, Prentice Hall Inc., 1988, Pg: 221-240. ASM Handbook volume 4, Heat Treatment, 1991. 12-Mar-14 26 Dept. of Metallurgical Engineering