Industrial Applications of Thermal Spraying Technology

Abstract

At its early stages of development, thermal spray technology was mostly used for the repair, rebuilding, retrofitting, and for surface protection against corrosion, erosion and wear. The wider acceptance of the technology for industrial-scale production has started in the late eighties and early nineties, with applications limited to high added-value components in the aeronautic and nuclear industry. Over the two past decades, a wide range of industrial-scale surface modification processes became available. The choice of a specific coating and/or thermal spray process, for a given service condition, depends, however, on the expectation of the user and the cost that could be tolerated for the application. This chapter presents the advantages and limitations of the different spray processes. Then the different coating applications are described, with coatings resistant to wear, corrosion and oxidation, providing thermal protection, clearance control, good bonding, electrical and electronic properties, free standing spray-formed parts, medical applications, replacement of hard chromium… potential applications. These applications are then presented according to the industrial users: aerospace, land-based turbines, automotive, electrical and electronic industries, corrosion applications for land-based and marine applications, medical engineering, ceramic and glass manufacturing, printing, pulp and paper, metal processing, petroleum and chemical industries, electrical utilities, textile and plastic, polymers, reclamation… The development of thermal sprayed coatings in the different countries is then discussed, the last part of the chapter being about the economic analysis of the different spray processes.

These are presently accepted for applications ranging from tribological and wear resistant applications including lubricity and low-friction surfaces, to resistance to corrosion and/or oxidation, thermal protection, freestanding components, electrical and optical components, electromagnetic shielding, electrical insulation, abradable seals, biomedical applications, superconducting oxides, components with coefficient of thermal expansion tailored to service conditions, magnetic coatings, solid oxide fuel cells, replacement of hard chromium, as well as ornamental applications.. This affected, in turn, the selection of the material to be applied for the coating, and the spray process to be used. The coating design process is often complicated, by the fact that in practice components are not always devoted to a single requirement such as wear or corrosion or electrical insulation or thermal insulation. In most cases, coatings must resist to different combined needs: for example, wear is often linked to corrosion.

Appendices

Appendix: Use of the Different Spray Materials

In this appendix only basic information about the main sprayed materials and the most frequently sprayed materials are presented. For more detailed information the reader must consult the different powder and wire suppliers. It must also be kept in mind that the coating resulting from the spray process has properties (thermal and electrical conductivities for example) different from those of powder or wire properties. Such differences result from the increase of the oxide content, the pores generated during spraying, the real contacts between layered splats, … Properties also depend on the way the sprayed material has been manufactured. In the following X-20Y-10Z means 20 wt% of Y, 10 wt% of Z, the balance being X.

18.1.1 A.1 Metals

Aluminum Al (99 wt%) T _m = 660 °C:

• Good corrosion resistance in industrial atmospheric conditions

• Good electrical and thermal conductivity

• Soft and ductile → repair Al alloys

• Non-magnetic (electromagnetic shielding)

• Sprayed by flame (powder or wire), wire arc, plasma, and cold spray

Aluminum base Al–Si (95/5 or 12 wt%)

• Salvage of parts made of Al, Mg, and their alloys: excellent finish

• Sprayed by flame (powder or wire), wire arc, plasma, and cold spray

Cobalt based powders (Pure cobalt T _m = 1,495 °C):

• Stellite^® (Co–xCr–yW–zC)

• Against galling, cavitation

• For metal friction, high angle erosion, high temperature hardness, good resistance to abrasion, rather good wettability

• Very good oxidation resistance

• Replace WC in high temperature applications

• Repair of Cobalt-based parts

• Sprayed by HVOF, flame, PTA, Plasma

Triballoy^® (Co–xCr–yMo–zSi) or (Co–xCr–yMo–zSi–tNi)

• Against galling for metal to metal friction

• High temperature hardness

• Good resistance to corrosion and oxidation

• Very good wear properties from room temperature to 860 °C

• Sprayed by HVOF, flame, PTA, Plasma

Co–25.5Cr–10.5Ni–7.5W–0.5C

• High abrasive, sliding fretting and cavitation wear resistance up to 800–850 °C

• Good oxidation resistance

• Behave well between 540 and 840 °C

• Sprayed by HVOF, flame, PTA, plasma

Co–28Mo–8Cr–2Si

• Up to 760 °C low coefficient of friction

• Good corrosion resistance

• Sprayed by HVOF, flame, PTA, plasma

Co–28Mo–17Cr–3Si

• Excellent sliding wears resistance

• Good hot corrosion resistance and moderate oxidation resistance up to 800 °C

• Sprayed by HVOF, flame, PTA, plasma

Copper Cu (99 wt%) (T _m = 1,084 °C)

• Very good electrical and thermal conductivities

• Good resistance to inks (paper and printing)

• Used to repair Cu base alloys

• Non-magnetic (electromagnetic shielding)

• Sprayed by flame (powder or wire), wire arc, plasma, HVOF, and cold spray

Copper-based powder: Cu–36Ni–5In (T _m = 1,150 °C):

• Very dense coatings with good resistance to galling and fretting

• Sprayed by flame (powder or wire), wire arc, plasma, HVOF, and cold spray

Aluminum Bronze Cu–9.5Al–1Fe:

• For pumps (against cavitation)

• Piston guides (soft bearing surfaces)

• Shifter forks and compressor air seals (friction)

• Strength and hardness twice that of other bronzes

• Sprayed by flame (powder or wire), wire arc, plasma, HVOF, and cold spray

Supra bronze: Cu–40Zn–0.8Sn–0.75Fe–0.24Mn

• For metallizing work

• Sprayed by flame (powder or wire), wire arc, plasma, HVOF, and cold spray

Iron-based powders (Pure iron T _m = 1,538 °C)

Low carbon steels (C < 0.25 wt%):

• An inexpensive low carbon steel powder

• Corrosion resistant → repair + good wear resistance in lubricated service

• May contain martensitic phases

• Produces machinable coatings

• Sprayed by flame, wire arc, plasma, HVOF, and cold spray

High carbon steels (C > 0.8 wt%)

• Reclamation

• Wear and erosion resistance

• Sprayed by flame, wire arc, plasma, HVOF

Stainless steels (SS) Fe-13 or 14Cr–1Ni:

• Good resistance to wear and corrosion

• Best all purpose (SS)

• Sprayed by flame, wire arc, plasma, HVOF

Stainless steels (SS) Fe–18Cr–8Mg–5Ni:

• Reclamation

• Corrosion protection

• Low shrinkage and good machinability

• Sprayed by flame, wire arc, plasma, HVOF

Stainless steels (SS) Fe–17Cr–12NI–2.5Mo–1Si–0.1C (ASI 316):

• Corrosion protection

• Dimensional restoration

• Cavitation and low temperature erosion resistance

• Sprayed by wire or powder flame, wire arc, HVOF

Cored Wires (wire arc sprayed):

• Some of them (Fe–Cr–P–C–…) form amorphous phases upon spraying

• Rather good resistance to corrosion (for example, with H₂SO₄)

• Good resistance to abrasion

Molybdenum Mo (Pure molybdenum T _m = 2,623 °C)

• Self bonding to most metallic surfaces, especially steels

• Natural lubricity and high hardness: good wear properties

• Maximum service temperature 316 °C

• Salvage and build-up of Ni base alloy components

• High density coatings

• Fretting resistant

• Used for pump parts, diesel engine fuel, injectors, piston rings, synchronized ring, press fits, valves, gears, cam followers, …

• Sprayed by HVOF, flame (powder or wire), wire arc, plasma

Self-fluxing alloys: Mo+25 (Ni–Cr–B–Si–Fe)

• High wear resistance, low friction coefficient

• Against steel, good scuff resistance

• Can be used for hard facing, hard bearing surfaces

• Against abrasion

Nickel Ni (99.5 wt%) (Pure nickel T _m = 1,455 °C)

• Good bonding to steel

• Good corrosion and oxidation resistance up to 980 °C

• Resist heat and prevent scaling of carbon and low alloy steels in hot atmospheres

• Salvage and build-up of Ni base alloys

• Easily machined

• Sprayed by HVOF, flame (powder or wire), wire arc, plasma

Ni–20Cr:

• Good surface appearance

• Good machinability

• Protective coatings against oxidizing gases at high temperature (up to 980 °C)

• Electrical conductors

• Surfacing

• Other different types of NiCr (Cr 10–17 wt%) but also addition of Fe, Mo with specific applications (see suppliers guides); for example:

Ni–16Cr–8Fe

• Machinable “stainless” coatings for salvage and build-up applications on corrosion resistant steels

• Sprayed by HVOF, flame (powder or wire), wire arc, plasma

Ni–18Cr–6Al (composite or clad):

• Good oxidation resistance

• Good machinability

• Bonding and surfacing layers

• Self-bonds to most metallic surfaces

• Sprayed by HVOF, flame (powder or wire), wire arc, plasma

Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe:

• Wear and corrosion protection

• Coatings contain some amounts of glassy phases (due to addition of refractory metals and metalloids)

• Sprayed by HVOF, flame (powder or wire), wire arc, plasma

Ni–5Al (clad):

• Good hardness and refractoriness (formation of nickel aluminide)

• Oxidation and abrasion resistant

• Adhere very well to smooth substrates

• Bond coat

• Resizing of over machined parts or of worn-out parts

• Possible: Al 20 wt% (clad) → self-bonds to most metal surfaces

• Sprayed by HVOF, flame (powder or wire), wire arc, plasma

Nickel-based self-fluxing alloys

• Ni–10Cr–2.5B–2.5Fe–2.5Si–0.15C

• The only one producing machinable-fused coating

• Resistance to abrasive wear, fretting, cavitation, and erosion up to 840 °C

• Ni–17Cr–4Fe–4Si–3.5B–1C

• Dense coating good corrosion resistance

• Ni–17Cr–4Fe–4Si–3.5B–1C

• Dense, hard, oxide free coating

• Piston rings, cylinder liners, utility exhaust fan

Superalloys: M.Cr.Al.Y with M = Ni, Co, Fe, Ni–Co

• Composition optimized: for substrate compatibility and environmental resistance

• Protection against corrosion and oxidation in high temperature applications:

• Aero gas turbines

• Marine turbines

• Stationary gas turbines

• Design criteria—Avoid phase transformation during engine start up and shut down, Avoid brittle phases (ex μ, σ, αCr)

• Limit brittleness increasing with oxide content

• Form a stable and adherent oxide scale of Al₂O₃ (addition of active elements: 0.5 % wt Y)

• Increase corrosion resistance (by increasing Cr content)

• Control thermal expansion coefficient: increasing with Cr and decreasing with Al

• Keep ductile coatings (ductile to brittle temperature influenced by Cr and Al contents)

• Many compositions exist

• Sprayed by HVOF, plasma (if possible under soft-vacuum to limit oxidation)

18.1.2 A.2 Ceramics (Oxides)

For most oxides the thermal expansion coefficient is low compared to that of most metals and it has to be taken into account because they are often used at high temperature.

Alumina Al₂O₃ (T _m = 2,050 °C)

• Main problem: Starting from α phase, melting and fast cooling (spraying) result in γ phase. Unfortunately around 1,000 °C γ phase transforms into α phase with a volume increase of about 4 % resulting in coating peeling off. Thus coatings should be used below 900 °C

• They are not too sensitive to oxygen losses

• They have a good resistance to abrasive, sliding and friction wear up to approximately 800 °C

• Poor resistance to shock or impact loading

• High dielectric strength, good electrical insulating coatings

• Reactive with molten salts

• Porous coatings

• Sprayed mainly by plasma, sometimes by HVOF (particles below 22 μm in diameter) and also by flame (rods or cords)

Titanium dioxide TiO₂ (T _m = 1,843 °C)

• Very sensitive to oxygen losses resulting in strong modifications of coating properties: color from white to black and especially electrical properties

• Loss and gain of oxygen reversible

• Very good wettability

• Excellent surface finish

• Excellent adhesion

• Rather low porosity

• Sprayed by plasma, HVOF, and also by flame (rods or cords)

Alumina–Titanium dioxide Al₂O₃–xTiO₂

• TiO₂ in the range 2–50 wt% (most common: 3–13–40 wt%):

  • Lowers Al₂O₃ coatings porosity.

Al₂O₃–3TiO₂

• Can be used with most acids and alkalis

• Good for abrasion, erosion, and sliding wear

• Maximum service temperature 840 °C

• Less brittle but lower dielectric strength than pure Al₂O₃ coatings

• Sprayed by plasma

Al₂O₃–13TiO₂

• Applications similar to those of Al₂O₃–3TiO₂ but lower hardness and dielectric strength and less resistance to chemical attack

• Maximum service temperature 540 °C

• Sprayed by plasma

Al₂O₃–40TiO₂

• Formation of Al₂TiO₅

• Softer and less resistant to chemicals

• Excellent finishing properties

• Sprayed by plasma

• Other compositions are used, for example, with SiO₂ below 2 wt%

Chromium Oxide Cr₂O₃ (T _m = 2,435 °C)

• Its stoichiometry depends strongly upon spray conditions (high oxygen pressure needed), and sub-stoichiometric coatings have a metallic behavior with a poor corrosion resistance

• Coatings have high hardness (1900–2000 HV_5N)

• Excellent wear resistance

• Low porosity

• Excellent finish

• Used on sliding surfaces

• Good wear resistance

• Insoluble in acids, alkalis, and alcohol

• Maximum service temperature 540 °C

• Excellent engraving properties

• Sprayed by plasma, HVOF, and also by flame (rods or cords)

Cr₂O₃–3TiO₂–5SiO₂

• TiO₂ limits oxygen losses

• Resist better than Cr₂O₃ to impacts

• High wear and corrosion resistance

Hydroxyapatite Ca₅ (PO₄)₃ OH

• For coating medical and dental implants

• Biocompatible and bioactive (main constituent of bones)

• Sprayed by plasma

Zirconia, ZrO₂

• Interesting mechanical properties

• Good wear resistance

• Low thermal conductivity (1–5 W/m K)

• Three phases: monoclinic (m), tetragonal (t), and cubic (c). Upon cooling around 1,000 °C, cubic or tetragonal phases transform into monoclinic with volume increase of about 10 % and coating peeling off. Thus only totally (c phase) or partially (t′ non-transformable t phase) stabilized zirconia can be sprayed.

  Most used stabilizers are CaO, MgO, Y₂O₃, CeO₂. CaO and MgO are rather cheap but the maximum service temperature is about 500 °C. Very good results are obtained with Y₂O₃; with about 8 wt% t′ phase is obtained, while with13 wt% c phase is obtained, as with 24–25 wt% CeO₂. The maximum service temperature (about 1,350 °C at the best) depends not only on the phase (best results with c phase) but also on the way the powder is manufactured (see Sect. 11.1.2.9) best results being obtained when zirconia and stabilizer particles are very small and uniformly distributed.

• Coatings have excellent thermal shock resistance and good oxidation and corrosion resistance.

• They are mainly used as thermal barriers.

• Sprayed by plasma and also by flame (rods or cords)

Zircon ZrSiO₄ (infusible)

• Dissociated during spraying (65 % ZrO₂, 35 % SiO₂)

• Not wetted by liquid metals: used for casting

• Good resistance to liquid glass.

• Good resistance to combustion gases

18.1.3 A.3 Cermets

They are made of a metal matrix, to achieve a good toughness, where are imbedded ceramic particles either of oxides or carbides for the hardness and wear resistance. Here again the manufacturing process plays a key role, for example, sintered particles behaving very differently than blended ones.

With oxides. In most cases they are blends. For example

Al₂O₃–30(Ni–20Al)

• Denser coatings than pure ceramic, more abrasion, and shock resistant, hard, and smooth

• Addition of alumina particles modifies the electrical resistance of the metal matrix

MCrAlY + Al₂O₃ (<3 μm)

• Hardness increases with Al₂O₃ content

• Electrical resistance decreases with Al₂O₃ %

With carbides (the most used)

Most used ones are:

WC, Cr₃C₂, and also sometimes TiC

• If all of them have melting temperatures over 1,900 °C (for example, T _m = 2,870 °C for WC) they are relatively sensitive to oxidation

• WC oxidation starts at 500–600 °C and oxidation produces W₂C decomposing into W over 1300 °C

Cr₃C₂

• Is not the only chromium carbide: Cr₇C₃ (T _m = 1,782 °C), Cr₂₃C₆ (T _m = 1,518 °C). Cr₃C₂ is mostly used in spraying and its oxidation starts at 800–900 °C, however, Cr₂₃C₆ has an excellent wear resistance.

TiC

• Has a unique cubic phase (T _m = 3,170 °C), which oxidation starts at 800–900 °C.

• At last carbides dissolve more or less in the liquid metal or alloy matrix, the dissolution increasing with temperature over the matrix melting temperature. The metal matrix lowers the wear resistance of carbides but increases resistance to mechanical or thermal shock. Phase changes of metal matrix must be avoided during the service (for example, that of Co occurs at about 480 °C). To conclude chemical changes occur during spraying especially in air atmosphere: oxidation, decomposition, and dilution. Thus microstructural properties of sprayed cermets depend strongly on spray conditions (VPS, IPS, APS, HVOF, HVAF, …), particle morphology and manufacturing process, and ceramic mean grain size. Only a few examples are given below:

WC–8Co

• Dense, hard wear-resistant coating

• Sprayed by plasma, HVOF, HVAF

WC–12Co

• Excellent low-temperature wear resistance

• Sprayed by plasma, HVOF, HVAF

WC–17Co

• Higher Co level improves toughness and fretting resistance

• Cannot be used in corrosive media

Cr₃C₂–25(Ni–20Cr)

• Oxidation resistant up to 900 °C

• Good corrosion resistance

• Excellent for high-temperature cavitation, abrasion, and sliding wear

Cr₃C₂–7(Ni–20Cr)

• Very good resistance to high temperature fretting and wear (higher carbide content increases hardness)

18.1.4 A.4 Abradables

They are designed to wear preferentially upon contact with mating part in order to automatically establish clearance. They comprise a metal matrix and non-metallic filler such as graphite, polyester, polymide, boron nitride, and friable material, the role of filler being to weaken the matrix integrity. The metal matrix is made of Ni, Al, Cu, Co bases, and superalloys.

The main difference of the base material is related to service temperature.

• Al–Si with C: up to 315–425 °C

• Al–Si with polyester: up to 350 °C

• The filler content can be varied

• Both are used for the compressor section of jet engines

• Co–polyester–BN

• They are used up to 700 °C

• Cu: Aluminum bronze alloy–polyester or Cu–14polyester–8Al–1Fe–5Binder

• Maximum service temperature: 650 °C

• Ni–graphite

• They are used up to 480 °C

• Also self-lubricating

• MCrAlY–polyester and/or BN

• Temperatures up to 1,200–1,300 °C

Nomenclature

Ah:

Amount to be amortized per hour of equipment work (€/h or USD/h)

Ap:

Amortization cost per kilogram of powder used (€/kg or USD/kg)

Fr:

Process gas flow rate per hour (m³/h)

Nh:

The number of hours devoted per year to spray the specific coating

Ny:

Number of years of the amortization period

p :

Pressure (Pa)

Pc:

Deposited powder cost per kilogram (€/kg or USD/kg)

Pcc:

Cost of each component (electrodes, nozzle, O-ring, …) (€ or USD)

Pcp:

Deposited powder cost per hour (€/h or USD/h)

Pe:

Cost of the equipment (€ or USD)

Pee:

Cost of 100 kW (€ or USD)

Pen:

Energy cost per deposited powder kilogram (€/kg or USD/kg)

Pep:

Cost of components related to the deposition of powder kilogram (€/kg or USD/kg)

Pg:

Gas price per 100 m³ (€ or USD)

Pgp:

Gas price per kilogram deposited (€/kg or USD/kg)

Pp:

Energy cost per deposited powder kilogram (€/kg or USD/kg)

Pt:

Plasma torch power (kW)

qp:

Powder quantity necessary for each part (kg)

Qp:

Quantity of powder sprayed per hour (kg/h)

Sc:

Surface to be coated (m²)

tc:

Mean life time of each component (electrodes, nozzle, O-ring, …) (h)

tp:

Time necessary to spray the part (h)

v :

Velocity (m/s)

wc:

Thickness of the coating including the overspray before machining (mm)

η _e :

Percentage corresponding to effective spray due to loss at holes and edges (%)

η _p :

Powder or wire deposition efficiency

ρ _p :

Feed stock specific mass (kg/m³)