Introduction

Thermal spraying is a well established technology for applying wear and corrosion resistant coatings in many key industrial sectors, including aerospace, automotive, power generation, petrochemical and offshore. In recent years, improvements to equipment and material quality have enhanced the technical credibility of the thermal spraying processes, leading to a significant growth in new markets, e.g., biomedical, dielectric and electronic coatings.

As a consequence, there are many options open to the spray coating supplier in terms of thermal spraying equipment, coating materials and gas selection, but these are often dependent on the environment to which the coating is subjected.

The Thermal Spraying Gas Selector offers a significant enhancement to customer support. It offers detailed advice on the following, in a form that can be accessed by a user with even a limited technical background:

		Materials selection
		Process selection
		Gas selection and flow rates
		Deposit efficiency
		Process economics and comparative costs

It will be possible to use Thermal Spraying Gas Selector in a variety of applications:

		To provide technical knowledge to non-technical personnel, such as salespersons answering phone enquiries
		To standardise answers to technical enquiries by different groups within Air Products

Introduction to Thermal Spraying

Thermal spraying processes have been widely used for many years throughout all the major engineering industry sectors for component protection and reclamation. Recent equipment and process developments have improved the quality and expanded the potential application range for thermally sprayed coatings. The main benefits and features of thermal spraying as a coating process are summarised below.

		Comprehensive choice of coating materials: metals, alloys, ceramics, cermets and carbides.
		Thick coatings can be applied at high deposition rates.
		Coatings are mechanically bonded to the substrate—can often spray coating materials which are metallurgically incompatible with the substrate, e.g., materials with a higher melting point than the substrate.
		Components can be sprayed with little or no pre- or post-heat treatment, and component distortion is minimal.
		Parts can be rebuilt quickly and at low cost, and usually at a fraction of the price of a replacement.
		By using a premium material for the thermal spray coating, coated components can outlive new parts.
		Thermal spray coatings may be applied both manually and automatically.

Adhesion
With the lower energy processes of flame and arc spraying, adhesion to the substrate is considered to be largely mechanical and is dependent on the workpiece being very clean and suitably rough. Roughening is carried out by grit blasting and occasionally, rough machining.

With the higher energy processes, bond strengths are higher because of the higher impact velocities. Adhesion is improved through disruption of oxide layers on the substrate and embedding of the particles into the substrate. There is some evidence to suggest that a certain proportion of diffusion bonding takes place. Surface preparation by cleaning and grit blasting is extremely important.

Equipment costs
Flame and arc spraying require relatively low capital investment and are portable—they are quite often applied in open workshops and on-site. The higher energy processes require significant capital investment, are generally installed in spraying booths and often operated mechanically or automatically.

Consumables
Consumables used for thermal spraying are available in two basic forms: wire and powder.

Wire consumables are principally used in arc spraying, which requires a continuous, electrically conducting consumable. Most wires are solid in form, and based on aluminium, zinc or steel compositions. In common with welding consumables, cored wires (containing ceramic or alloying material) are now available. Some flame spraying systems also use wire consumables, principally for spraying zinc or aluminium.

Powder consumables are available for flame, plasma and HVOF spraying. Materials sprayed include plastics, metals, carbides and ceramics, and many compositions can be sprayed by all four processes. The optimum powder specification depends on the process, and includes consideration of particle size and distribution, morphology and manufacturing route.

Flame Spraying

Flame spraying is the oldest of the thermal spraying processes. A wide variety of materials can be sprayed by this process, and the vast majority of components are sprayed manually. Flame spraying uses the heat of combustion of a fuel gas (usually acetylene or propane) and oxygen mixture to melt the coating material, which can be fed into the spraying gun in two forms, either powder or solid wire/rod. The two consumable types give rise to the two process variants: powder flame spraying and wire flame spraying.

In the case of the powder flame spraying process, powder is fed directly into the flame by a stream of compressed air or inert gas (argon or nitrogen). Alternatively, in some basic systems, powder is drawn into the flame with air by a venturi effect, which is sustained by the fuel gas flow. It is important that the powder is heated sufficiently as it passes through the flame. The carrier gas feeds powder into the centre of an annular combustion flame where it is heated and propelled towards the substrate. A second outer annular gas nozzle feeds a stream of compressed air around the combustion flame, which accelerates the spray particles towards the substrate and focuses the flame.

In the wire flame spraying process, the wire feed rate and flame settings must be balanced to produce continuous melting of the wire and a fine particulate spray. The annular compressed air flow atomises and accelerates the particles towards the substrate.

Examples of applications
Corrosion protection of structures and components (e.g., bridges, offshore platforms, L.P.G. bottles) with aluminium or zinc coatings. Aluminium is more expensive, but has resistance to acidic gaseous atmospheres (e.g., associated with the products of fossil fuel combustion), as well as neutral solutions, e.g., salt water. Zinc has resistance to alkaline corrosion.

Reclamation of worn shafts, particularly of bearing areas with 13% chromium stainless steel or bronzes. The coatings produced are quite porous and lubricants can be absorbed into the coating, enhancing the performance of the bearing.

Spraying of self fluxing hard facing alloys e.g., NiCrBSi alloys to improve wear resistance at very high temperature. These alloys are generally fused to the substrate by heating (by oxyacetylene) to over 900°C to give a fully dense, metallurgically bonded coating. A good application example is glass plungers, where the abrasive medium is molten silica glass at temperatures approaching 700°C.

Arc Spraying
Arc spraying is the highest productivity thermal spraying process. A DC electric arc is struck between two continuous consumable wire electrodes which form the spray material. Compressed gas (usually air) atomises the molten spray material into fine droplets and propels them towards the substrate. The process is simple to operate and can be used either manually or in an automated manner. It is possible to spray a wide range of metals, alloys and metal matrix composites (MMCs) in wire form. In addition, a limited range of cermet coatings (with tungsten carbide) can also be sprayed in cored wire form, where the hard ceramic phase is packed into a metal sheath as a fine powder.

The combination of high arc temperature (6000 K) and particle velocities in excess of 100 m.sec-1 gives arc sprayed coatings superior bond strengths and lower porosity levels when compared with flame sprayed coatings. However, the use of compressed air for droplet atomization and propulsion gives rise to high coating oxide content.

Examples of applications
Arc spraying has the highest deposition rate of the thermal spraying processes and can be used to spray large areas or large numbers of components on repetitive production line applications.

		Spraying of large structures e.g., bridges, lamp posts and offshore structures with corrosion resistant zinc and aluminium coatings.
		Reclamation of engineering components (journals, bearings, shafts) with steels and bronzes.
		Spraying of electronic component housings with conductive coatings (copper, zinc and aluminium) for shielding from electromagnetic interference.
		Manufacture of moulds for plastic components (plant pots) using alloys of low melting point (Sn/Zn mixtures).

Plasma Spraying

The plasma spraying process uses a DC electric arc to generate a stream of high temperature ionised plasma gas, which acts as the spraying heat source. The arc is struck between two non-consumable electrodes, a tungsten cathode and a copper anode within the torch. The torch is fed with a continuous flow of inert gas, which is ionised by the DC arc, and is compressed and accelerated by the torch nozzle so that it issues from the torch as a high velocity (in excess of 2000 m/sec), high temperature (12000–16000 K) plasma jet. The coating material, in powder form, is carried in an inert gas stream into the plasma jet where it is heated and propelled towards the substrate. Because of the high temperature and high thermal energy of the plasma jet, materials with high melting points can be sprayed.

Plasma spraying produces a high quality coating by a combination of a high temperature, high energy heat source, a relatively inert spraying medium and high particle velocities, typically 200–300 m.sec-1. However, inevitably some air becomes entrained in the spray stream and some oxidation of the spray material may occur. The surrounding atmosphere also cools and slows the spray stream.

		Plasma spraying is widely applied in the production of high quality sprayed coatings.
		Spraying of seal ring grooves in the compressor area of aeroengine turbines with tungsten carbide/cobalt to resist fretting wear.
		Spraying of zirconia-based thermal barrier coatings (TBCs) onto turbine combustion chambers.
		Spraying of wear resistant alumina and chromium oxide ceramic onto printing rolls for subsequent laser and diamond engraving/etching.
		Spraying of molybdenum alloys onto diesel engine piston rings.

High Velocity Oxyfuel (HVOF) Spraying

The most recent addition to the thermal spraying family, high velocity oxyfuel spraying has become established as an alternative to the proprietary, detonation (D-GUN) flame spraying and the lower velocity, air plasma spraying processes for depositing wear resistant tungsten carbide-cobalt coatings. HVOF spraying differs from conventional flame spraying in that the combustion process is internal, and the gas flow fates and delivery pressures are much higher than those in the atmospheric burning flame spraying processes. The combination of high fuel gas and oxygen flow rates and high pressure in the combustion chamber leads to the generation of a supersonic flame with characteristic shock diamonds. Flame speeds of 2000ms-1 and particle velocities of 600–800ms-1 are claimed by HVOF equipment suppliers. A range of gaseous fuels is currently used, including propylene, propane, hydrogen and acetylene.

Although similar in principle, potentially significant details, such as powder feed position, gas flow rates and oxygen to fuel ratio, are apparent between each system. The HVOF process produces exceptionally high quality cermet coatings (e.g., WC-Co), but it is now also used to produce coatings of metals, alloys and ceramics. Not all HVOF systems are capable of producing coatings from higher melting point materials, e.g., refractory metals and ceramics. The capability of the gun is dependent upon the range of fuel gases used and the combustion chamber design.

A liquid fuel (kerosene) HVOF system, has just been launched, which is capable of much higher deposition rates than the conventional gas-fuelled units.

Examples of Applications HVOF spraying is a very recent process development, yet the high quality of the coatings produced at competitive cost has already seen its introduction in a number of very significant industries. Potential applications overlap with plasma and D-gun spraying, particularly for WC-Co coatings.

		Tungsten carbide-cobalt coatings for fretting wear resistance on aeroengine turbine components.
		Wear resistant cobalt alloys onto fluid control valve seating areas.
		Tungsten carbide-cobalt coatings on gate valves.
		Various coatings for printing rolls, including copper, alumina, chromia.
		NiCrBSi coatings (unfused) for glass plungers.
		NiCr coatings for high temperature oxidation/corrosion resistance.
		Alumina and alumina-titania dielectric coatings.
		Biocompatible hydroxylapatite coatings for prostheses.

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