WHAT IS THERMAL SPRAYING?

Boosting the productivity and capacity of technical installations and machinery leads to increased demands on components. A key to protecting surfaces against such high stresses is thermal spraying.

In recent years, thermal spraying has become increasingly important, both in the manufacturing of new parts and in the repair of existing parts.

Thermal spraying is a surface coating technology which combines a variety of positive characteristics. Its special features testify to this:

• Numerous combinations of base material and coating material are possible.
• Shortages of raw materials, and resulting high prices, are forcing industry to use high-grade materials specifically for the production of high-quality surfaces which possess properties the base materials do not have.
• The flexibility of thermal spraying means that high-grade worn parts can be repaired in a variety of ways. Low repair costs and relatively short downtimes represent major advantages in relation to other refurbishing methods.
• The term "thermal spraying" covers a range of spray processes. They are classified according to the type of spray material, type of operation or type of energy source, as defined in the standard EN ISO 14917.

Differentiation of the Thermal Spray Processes

By virtue of their process-related properties, the individual thermal spray processes do not compete for applications, but instead complement each other. In order to produce spray coatings, all thermal spray processes require two types of energy: Thermal energy and kinetic energy.

The energy sources currently in use are the oxy-fuel-gas flame, the electric arc, the plasma jet, the laser beam and gas heated to approx. 600°C. Thermal energy is needed to melt or fuse the spray material. Kinetic energy, coupled to the particle velocity, influences the coating density, the bond strength of the coating itself and the bond strength of the coating to the base material. The kinetic energy in the different thermal spray processes varies greatly and also depends on the coating material and particle size.

Flame Spraying with Wire or Rod

Flame Spraying with Powder

Flame Spraying with Plastics

High Velocity Oxy-Fuel Flame Spraying (HVOF)

Detonation Flame Spraying

Plasma Spraying

Laser cladding

Arc Spraying

Cold Spraying

PTA

In wire or rod flame spraying, the spray material is continuously melted in the centre of an oxy-acetylene flame. With the aid of an atomizing gas such as compressed air or nitrogen, the droplet-shaped spray particles are discharged from the melting zone and propelled onto the prepared workpiece surface.

Flame spraying with wire is a widely applied method with a very high coating quality standard. In the automotive industry, for example, several hundred tons of molybdenum, per year, are used to coat gear selector forks, synchronizing rings or piston rings.

Flame Spraying with Wire
(1) Acetylene / oxygen  (2) Wire or rod  (3) Torch nozzle
(4) Oxy-acetylene flame and spray particles  (5) Workpiece

In powder flame spraying, the spray material in powder form is melted or fused in an oxy-acetylene flame and propelled onto the prepared workpiece surface with the aid of expanding combustion gases.

If necessary, an additional gas (e.g. nitrogen) can be used to accelerate the powder particles. The range of spray powders available is enormous, comprising well over 350 different types.

Powders are classified as self-fluxing and self-adhering. Self-fluxing powders normally require additional thermal post-treatment. In most cases, this "fusing" step is carried out using oxy-acetylene torches, which are extremely well-suited to this task.

The adhesion of the spray coating to the base material is greatly enhanced by the heat treatment, rendering it impervious to gases and liquids.

Applications include shaft sleeves, roll-table rollers, bearing seats, ventilating fans, extruder screw rotors.

Flame spraying with powder (1) Acetylene / oxygen  (2) Powder hopper  (3) Torch nozzle (4) Carrier gas + powder  (5) Oxy-acetylene flame and spray particles (6) Workpiece	Fusing with oxy-acetylene flame

In flame spraying with plastics, the plastic coating material does not come into direct contact with the oxyacetylene flame. A powder-feed nozzle is located in the centre of the flame spray gun. This is surrounded by two ring-shaped nozzle outlets, the inner ring being for air or an inert gas and the outer ring for the thermal energy source, i.e. the oxy-acetylene flame.

The plastic coating material is there-fore not melted directly by the flame, but by the heated air and radiation heat. The mobility of flame spraying with plastics, e.g. its use on-site, makes it increasingly versatile in its application.

Applications include every kind of railing, feed-through pipes in walls, drinking-water tanks, garden furniture, swimming-pool markings, and the coating of recycled plastic components.
Coating a vessel for the chemical industry using flame spraying with plastics.

Flame spraying with plastics (1) Acetylene / oxygen  (2) Plastic granulate  (3) Torch nozzle (4) Air blanket  (5) Oxy-acetylene flame  (6) Melted plastic (7) Workpiece	Flame Spraying with Plastics of a tank for the chemical industry

High velocity oxy-fuel spraying involves a continuous gas combustion under high pressure in a combustion chamber. The spray material, in powder form, is fed into the central axis of the chamber. The high pressure of the oxyfuel gas mixture produced in the combustion chamber - and in the expansion nozzle which is usually located down-stream of the chamber — in turn produces the desired high flow velocity in the gas jet. In this way, the spray particles are accelerated to high velocities, leading to exceptionally dense spray coatings with excellent adhesion. Due to the sufficient but moderate heat input, the spray material undergoes only slight metallurgical changes as a result of the spray process, e.g. minimal formation of mixed carbides. With this method, extremely thin coatings with a high dimensional accuracy can be produced.

The fuel gases which can be used are propane, propylene, ethene, acetylene and hydrogen.

Applications include sliding surfaces of steam irons, rollers for the photo-graphic industry, machine parts for the petrochemical and chemical industry, e.g. pumps, slides, ball valves, mechanical sealings, Kaplan blades, every kind of anti-wear protection, also in connection with anti-corrosion protection, electrically insulating coatings (oxides).

HVOF spraying (1) Oxy-fuel  (2) Powder + carrier gas (3) Torch nozzle with or without water cooling (4) Oxy-fuel flame and spray particles  (5) Workpiece

Shock diamonds in high velocity oxy-fuel spraying

Detonation flame spraying is an intermittent spray process. The so-called detonation gun consists of a discharge pipe with a combustion chamber at one end. A mixture of acetylene, oxygen and spray powder is fed into the chamber and detonated using a spark. The shock wave produced in the pipe accelerates the spray particles. These are then heated at the front of the flame and propelled at high speed in a focused jet onto the prepared workpiece surface. After each detonation, the combustion chamber and the pipe are purged with nitrogen. The very high quality standard of these spray coatings generally justify the higher costs involved in this process.

Applications include pump plungers in gas compressors and pumps, rotors in steam turbines, gas compressors or expansion turbines, and in papermaking machinery, the rolls used in wet areas of the production process and calendar rolls.

Detonation flame spraying
(1) Acetylene  (2) Oxygen  (3) Nitrogen   (4) Spray powder
(5) Detonator  (6) Water-cooled discharge pipe  (7) Workpiece

In plasma spraying, the spray material, in powder form, is melted by a plasma jet in or outside the spray gun and propelled onto the workpiece surface. The plasma is produced by an arc which is constricted and burns in argon, helium, nitrogen, hydrogen or their mixtures. This causes the gases to dissociate and ionize; they attain high discharge velocities and, on recombination, transfer their thermal energy to the spray particles.

The arc is not transferred, i.e. it burns inside the spray gun between a centred electrode (cathode) and the water-cooled spray nozzle forming the anode. The process is applied in a normal atmosphere, in a shroud gas stream, i.e. inert atmosphere (e.g. argon), in a vacuum and under water. A high-velocity plasma can also be produced by means of a specially shaped nozzle attachment.

Applications include the aerospace industry (e.g. turbine blades and abradable surfaces), medical technology (implants) and thermal barrier coatings.

Plasma spraying (1) Rare gas  (2) Cooling water  (3) Direct current  (4) Powdered spray material (5) Cathode  (6) Anode  (7) Workpiece	Plasma spraying of a paper roll

In laser cladding, a powdered spray material is fed into a laser beam via a suitable powder nozzle. By means of laser radiation, both the powder and a minimal proportion of the base material surface (micro-zone) are melted and the spray material and the base material are metallurgically bonded. A shroud gas serves to protect the melt pool.

One application for laser cladding is the partial coating of stamping, bending and cutting tools.

Laser Cladding (1) Laser beam (2) Shroud gas (3) Powder (4) Workpiece

In arc spraying, two similar or different types of spray material in wire form are melted off in an arc and propelled onto the prepared workpiece surface by means of an atomizing gas, e.g. compressed air. Arc spraying is a high-performance wire spraying process in which only electrically conductive coating materials can be used, however.

When using nitrogen, argon or nitrogen / oxygen mixtures as the atomizing gas, oxidation of the materials can largely be prevented, respectively, specific coating properties can be achieved.

Applications include large-area coating of vessels, anti-corrosion protection, bond coatings, cylinder liners, etc.

Arc spraying
(1) Atomizing gas  (2) Wire-feed control  (3) Torch head
(4) Electrically conductive wire  (5) Workpiece

In cold spraying, the kinetic energy, i.e. the particle velocity, is increased and the thermal energy reduced. In this way it is possible to produce spray coatings which are virtually free of oxides.
Cold spraying

This new development became known under the name CGDM (Cold Gas Dynamic Spray Method).

By means of a gas jet heated to approx. 600 °C at a corresponding pressure, the spray material is accelerated to > 1000 m/s and brought to the surface to be coated as a continuous spray jet. The particle jet can be focused on cross-sections of 1.5 x 2.5 up to 7 x 12 mm. The deposition rate is 3 to 15 kg/h.

Laboratory investigations show that cold spray coatings have extremely high bond strengths and are exceptionally dense. Whereas with traditional thermal spray processes, the powder in the spray process must be heated to above its melting temperature, the cold spray process requires a powder temperature of only a few hundred degrees. The oxidation of the spray material and the oxide content of the sprayed coating are therefore reduced considerably. Coated substrates reveal no material changes due to thermal influence.

Applications include automobile industry, anti-corrosion protection and electronics, for example.

Cold Spraying
(1) Carrier gas  (2) Process gas  (3) de-Laval nozzle
(4) Supersonic gas stream and spray particles  (5) Workpiece

PTA - Plasma Transferred Arc Surfacing with Powder

In the PTA process, the surface of the workpiece is surface melted. A high-density plasma arc serves as the heat source and the metal powder as the surfacing material. The arc is formed between a non-consumable electrode and the workpiece. The plasma is generated in a plasma gas (e.g. argon, helium or argon-helium mixtures) between the central tungsten electrode (-) and the water-cooled anode block (+) in the transferred arc. The powder is supplied to the torch by means of a carrier gas, heated in the plasma jet and deposited on the workpiece surface where it melts completely in the melt pool on the substrate.

The entire process takes place in the atmosphere of a shroud gas (e.g. argon or an argon-hydrogen mixture).

The PTA process facilitates a minimal mixing of base and coating material (5 - 10 %), a small heat-affected zone, a high deposition rate (up to 20 kg/h), a true metallurgical bond between the substrate and the coating - and thus extremely dense coatings - and the flexible use of alloys.

The surfacing powders most frequently used can be classified as nickel-base, cobalt-base and iron-base alloys.

Applications include the coating of a wide variety of base materials, e.g. low-alloyed steel, stainless steel, cast iron, bronze, nickel-base super-alloys.

PTA - Plasma Transferred Arc (1) Direct current (2) Plasma gas (3) Carrier gas + powder (4) Shroud gas (5) Anode (6) Cathode (7) Water cooling (8) Workpiece

Branches of Industry

• Medical instruments
• Power plant construction
• Chemical plant construction
• Plastics processing industry
• Pump industry
• Ferrous and non-ferrous metal production
• Electronics industry
• Apparatus construction
• Smelteries
• Foundries
• Steel production
• Steel drawing works
• Aerospace industry
• Automotive industry
  - Cars
  - Shipbuilding
  - Agricultural machinery
• Petroleum refining
• Mining
• Energy and water supply
• Mechanical engineering
• Paper industry
• Printing trades (presses)
• Glass industry
• Household and kitchen appliances
• Medical technology

Artificial hip implant

Flame spraying of the legendary Mercedes 300 SL to protect against corrosion.

  Pump housing

Axle flange of a truck

Gauge for the manufacture of hollow glass

Plasma-sprayed frying pan

Flame spraying of a Kaplan turbine

Fusing of a roll-table roller

Applications

• Anti-wear coatings
• Anti-corrosion coatings
• Thermal barriers
• Anti-friction coatings
• Particle erosion
• Granular abrasion
• Electrical conductivity
• Electrical resistance
• High-temperature protection
• Salvaging of scrapped parts - Bearing coatings
• Chemical exposure
• Oxidizing atmospheres
• Resistance to galling

Advantages of Thermal Spraying

• Any material can be coated
• Any material can be applied as a coating
• The material to be coated is not thermally altered
• Parts of any size and geometry can be coated
• Thermal spraying lends itself to automation
• Flexible operation
• Very high reproducibility
• High dimensional accuracy
• High quality standard
• Several elements can be included and combined in the spray coating (e.g. Cr, Ni, carbides, etc.)

Words and pictures from the brochure
LINSPRAY® – Gases and know-how for Thermal Spraying
by courtesy of
Linde GmbH, Unterschleissheim (D)

Last updated: 23.08.22

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