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Coatings and surface treatments
Deposition by thermal spraying
Industrial applications
G. LE MAGNAN
Directeur Général
CRITT MDTS, Charleville Mézières (F)
THERMALLY SPRAYED COATINGS
  • Flame spraying
  • Electric arc deposition
  • Plasma spraying
  • FLAME SPRAYING

    Principle

  • Use of high energy provided by the combustion of a thermal spraying gas
  • Varying temperature from 2 800°C (oxygen/propane)to 3 150°C (oxyacetylene)
  • Materials used: Tm < 2 800°C, non-subliming
  • POWDER FLAME SPRAYING

  • Oxygen-gas torch equipped with a powder injection facility
  • Low cost and ease of application
  • Maintenance – Machinability
    – Abrasion or corrosion problems
  • Materials used – Fusible Ni or Co based, Co + Cr, B, Si
    – Ni or Co based + carbides
  • Equipments – Flame torch with integrated powder reservoir
    – Flame torch with separate powder reservoir
  • Particle velocities : 30 m/s
  • Torch – substrate distance : 100 – 200 mm
  • Coating thicknesses : 100 µm
  • Deposition rate : 2 – 3 kg/hour
  • Remelting possible after deposition (improved adhesion)
  • WIRE FLAME SPRAYING

  • Two part gun – Torch
    – Wire feed
  • Identical gases – Acetylene, propane, tetrane
  • Wire feed – Air turbine (lower price and weight)
    – Electric motor (constant couple)
  • Particle velocities – 150 m/s
  • Torch – substrate distance – 100 – 200 mm
  • Coating thicknesses – Several tenths of mm to several mm
  • Deposition rate – 1 kg/hour (ceramics) to 30 kg/hour(Zn)
  • APPLICATIONS

  • Metals : Zn, Al, Cu, Sn, Pb, Ni, Mo, steels, stainless steels
  • Zn, Cu, Ni or Sn based alloys
  • Ceramics in the form of a wire of a bar : NiAl, NiCrBSi, Al2O3, Al2O3-TiO2, Cr2O3, ZrO2-CaO
  • EXAMPLES

  • Mo coating on a synchrotron ring
  • Coatings of NiCrBSi containing carbides on rollers in steel making
  • AlZn coating on metallic workpieces
  • Al2O3 coatings on inductors
  • HYPERSONIC SPRAYING

  • Gas exit speed Mach 1 < V < Mach 2 – Non continuous hypersonic spraying
    – Continuous hypersonic spraying
  • NON CONTINUOUS HYPERSONIC SPRAYING
  • Simultaneous introduction of the powder and combustion gas (O2 + C2H2) into a tube called the detonation cannon
  • Frequency : 8 ignitions/seconde
  • Ejection speed : 950 m/s
  • Characteristics – Dense coating, porosity < 1 %
    – Adhesion ≈ 80 MPa
    – Surface Roughness : Ra ± 3 µm
    – Deposition rate : 3 – 5 kg/hour
    – Maximum thickness : 0,2 / 0,3 mm
  • Materials – Carbides of W or Cr with Ni or Co binder
    – Al2O3
    – Cr2O3
  • CONTINUOUS HYPERSONIC SPRAYING

  • HVOF process (High velocity oxygene fuel)
  • HVAF process (High velocity air fuel)
  • HVOF = O2 + C3H4 or propylene or tetrene or H2
    HVAF = kerosene + compressed air

  • Coating : excellent quality
  • Porosity < 2 %
  • Good adhesion : 70 – 100 MPa
  • Low surface roughness
  • Coating materials – Ni, Co, Inconel
    – Carbides of W and Cr
    – Oxides (with O2/H2 flame)
  • Gas consumption : 1 000 l/min
  • Gas velocity : 2 100 m/s
  • Deposition rate : 9 kg/hour
  • ELECTRIC WIRE ARC DEPOSITION

  • Principle – FGeneration of an electric arc (6 000°C) between two wires. An air jet atomises the molten metal.
  • Equipment – Direct current powder generator : 20 – 40 V – 100 – 500 A
  • Characteristics – Spray velocity : 250 m/s
    – Coating thicknesses : 0,2 – 3 mm
    – Adhesion : 20 – 40 MPa
    – Deposition rate : 5 – 30 kg/hour
    – Porosity : 2 – 5 %
  • LIMITS OF THE PROCESS

  • Materials must be conductors
  • Oxidation which may be reduced by using a neutral gas (nitrogen)
  • FUTURE DEVELOPMENTS

  • Development of wires which are sheathed to aid melting
  • Simultaneous deposition of two wires of different composition
  • PLASMA SPRAYING

  • Principle – Fourth state of matter
    – solid –> liquid –> gas –> plasma
  • APPLICATIONS
    1- Wear resistant coatings

    In this group materials have :

  • Good abrasion resistance (Mo, Cr, Ni-Cr-B-Si alloys, Al2O3+TiO2, Cr3C2+NiAl, Al2O3, WC)
  • Good fatigue resistance (steels, Cr3C2, Ni-B-Cr alloys, Al2O3+TiO2, Al2O3+Cr2O3, WC+Co, Co-Cr-W-Si alloys)
  • Good erosion resistance (Ni-Cr-B alloys, Al2O3+TiO2, WC-Co, CaZrO3, Cr3C2+NiAl)
  • 2- Heat resistant coatings

  • Protection against oxidising atmospheres (Al, Ta, Ni-Cr alloys, Cr2O3)
  • Protection against corrosive atmospheres (Al, Ta, NiCr, Ni-B-Si-Mo-WC)
  • Erosion resistance at temperatures below 850°C (Nb, Co-Cr-W, Al2O3-ZrO2, MgZrO3)
  • Thermal barrier coatings (Al2O3-ZrO2, MgZrO3, oxyde+NiAl)
  • Protection against molten metal attack (W, MgZrO3, ZrSiO4, TiN, ZrNi, TiB2, ZrB2, Nb, Mo)
  • 3- Corrosion resistant coatings

  • In the chemical and foot industries and for applications involving water immersion (Al, Zn, TaCR, Cr2O3, CrB2, WSi2)
  • In saline or polluted environments (Al, Zn, Cr, Ta, Cr2O3)
  • 4- Coatings with electrical properties

  • Radiofrequency masks (Al, Zn, Cu, Sn)
  • Dielectric coatings (Al2O3)
  • Coatings with good conductivity (Al, Cu)
  • 5- Coatings for mould repair

  • Non ferrous metals (Ti and Ti alloys, Al and Al alloys (Al-Mg, NiAl, Al-Si), Cu and bronze alloys, Co and Co-Cr-W-Si alloys, Ni and Ni alloys (Ni-B-Si, NiAl, NiCu))
  • Ferrous metals (steels, Cr steels, austenitic steels, low C steels, cast iron).
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