THERMAL SPRAYING

Process characteristics:                    high-end process

Process temperature:                         hot

Process speed:                                   slow

Processable additive materials:       powder
                                                            (metals/carbides/oxides)

Process gases:                                   argon, hydrogen, helium,
                                                             nitrogen

In plasma spraying, the spray material, in powder form, is melted by a plasma jet in or outside the spray torch 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 torch 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 using a specially shaped nozzle attachment.

Applications include aerospace (e.g. turbine blades and inlet surfaces), medical technology (implants), thermal insulation coatings

Process characteristics:                    high-end process

Process temperature:                         hot

Process speed:                                   slow

Processable additive materials:       powder
                                                            (metals/carbides/oxides)

Process gases:                                   argon, hydrogen, helium,
                                                             nitrogen

Plasma spraying processes are usually operated at normal atmospheres and are thus known as Atmospheric Plasma Spraying (APS).  In addition, when handling oxidation-critical materials, the process can also be used in a vacuum chamber as vacuum plasma spraying (VPS) or low-pressure plasma spraying (LPPS). In vacuum plasma spraying, the vacuum chamber is first vacuumized down to approx. 10-2 mbar, or better still 10-3 mbar, to remove residual gas impurities. The pressure is then increased to a range of approx. 20 to 800 mbar by introducing an inert gas. During spraying, the chamber pressure is kept constant using a pump system.

On the one hand, this process group enables the processing of materials with an extremely high affinity for oxygen or nitrogen (refractory metals, MCrAIY alloys) while avoiding oxidation processes as far as possible and thus achieving high phase stability and purity. On the other hand, the production of very low-porosity coatings with high adhesion is possible. This results from the higher particle velocity due to the low pressure and the possibilities for component cleaning and preheating or even activation during spraying using a so-called transferred arc. For this purpose, another voltage source is connected between the plasma nozzle and the potential-free component holder. The cleaning, preheating or power booster effect is achieved by reversing the polarity accordingly.

In addition, the spraying process can be lowered to a range of less than 1mbar. Voraussetzung hierfür ist ein ausreichenddimensionierter Pumpenstand. Prerequisite for this is a sufficiently dimensioned pump stand. At about 1000A, a plasma cloud forms from the spray jet. This is then referred to as the thin-film process. However, only the finest powder particles in the range < 5 µm can be processed in this case.

Applications include the production of very low-porosity coatings with high adhesion (e.g. for coating gas turbine blades and vanes)

Process characteristics:                    high-end process

Process temperature:                         hot

Process speed:                                   slow

Processable additive materials:       powder
                                                            (metals/carbides)

Process gases:                                   propane, propene, ethylene, 
                                                             acetylene, hydrogen

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 oxy-fuel 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.
 

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

Process characteristics:                    cost-effective entry process

Process temperature:                         hot

Process speed:                                   slow

Processable additive materials:       wire
                                                            (metals/carbides)

Process gases:                                  acetylene, nitrogen, 
                                                            compressed air

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.

Applications include shift forks, synchronizer rings or piston rings in the automotive industry (with molybdenum).

Process characteristics:                    cost-effective entry process

Process temperature:                         hot

Process speed:                                   slow

Processable additive materials:       powder
                                                            (metals/carbides)

Process gases:                                  acetylene, oxygen, nitrogen, 
                                                            argon

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, etc.

Process characteristics:                    cost-effective process 
                                                             for robust coatings

Process temperature:                         hot

Process speed:                                   fast

Processable additive materials:       wire (metals)
Process gases:                                  argon, nitrogen, 
                                                            compressed air

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.

Special application for the internal coating of rotationally symmetrical components

Process characteristics:                    high-end process 
Process temperature:                         hot

Process speed:                                   slow

Processable additive materials:       wire (metals)
Process gases:                                  argon, hydrogen, nitrogen 
                                                            compressed air

By means of high-voltage ignition, an electric arc is generated with currents in the range of 60 to 150 A. The wire spray material is melted by an electric arc and the argon/hydrogen plasma.

The molten wire end is atomized using  compressed air and accelerated towards the substrate as molten particles. This subsequently leads to a lamellar layer structure with defined porosity on impact with the pretreated, roughened cylinder surface.

Economical production of cylinder running surfaces in aluminum cylinder crankcases in large-scale industrial use and in the repair sector; coating of rotationally symmetrical components, such as inner walls of tubes (diameters realized to date: 65 - 250 mm)