Metal Spraying

Metal spraying is the application of heated metal to a surface to form a coating. Thermal spraying procedures are coating processes that include spraying melted (or heated) materials onto a surface. Heat is applied to the "feedstock" (coating precursor) via electrical (plasma or arc) or chemical techniques (combustion flame).

(Credit: Adrian Wilson  / McFade Photography)

When compared to other coating processes such as electroplating, physical and chemical vapor deposition, thermal spraying can provide thick coatings (approximate thickness range is 20 micrometers to several mm, depending on the process and feedstock) over a large area at a high deposition rate. Metals, alloys, ceramics, polymers, and composites are among the coating materials accessible for thermal spraying.

They are fed as powder or wire, heated to a molten or semimolten state, and accelerated as micrometer-sized particles towards substrates.

Thermal spraying is typically powered by combustion or an electrical arc discharge. Coatings are formed as a result of the accumulation of numerous sprayed particles. The surface may not heat up significantly, allowing flammable substances to be coated.

Porosity, oxide content, macro and micro-hardness, bond strength, and surface roughness are commonly used to assess coating quality. Coating quality often improves as particle velocities rise.

Thermal spraying is classified into several types:

• Plasma spraying
• Detonation spraying
• Wire arc spraying
• Flame spraying
• High-velocity oxy-fuel coating spraying (HVOF)
• Warm spraying
• Cold spraying

Particle velocities are generally modest (150 m/s) in traditional (invented between 1910 and 1920) but still extensively used methods such as flame spraying and wire arc spraying, and raw materials must be molten to be deposited. Plasma spraying, invented in the 1970s, used a high-temperature plasma jet generated by arc discharge with typical temperatures over 15000 K, allowing it to spray refractory materials such as oxides, molybdenum, and so on.

Pre-treatment :

Surface adhesion is purely mechanical, thus a solid key devoid of grease or other impurities is essential. As a result, rigorous cleaning and pre-treatment of the surface to be coated is critical. Surface roughening is often accomplished through grit blasting with dry corundum. Other media, such as cold iron, steel grit, or SiC, are also employed in some applications.

All objects are grit blasted with abrasive grit to create a surface roughness of 100-300µin. Other critical aspects besides grit type include particle size, particle shape, blast angle, pressure, and grit media purity. Substrate materials that can endure blasting methods to roughen the surface, with a surface hardness of around 55 Rockwell C or below, are suitable. To prepare substrates with a higher hardness, special processing processes are necessary.

Some of the available metal spray materials include:

• Aluminium coatings
• Bronze coatings
• Coatings of pure copper
• Carbon and low alloy steel
• Lead based coatings (equivalent to white metal)
• Molybdenum coatings

Plasma Spray Process

The Plasma Spray Process involves spraying molten or heat softened material onto a surface to create a coating. Powdered material is injected into a high-temperature plasma flame, where it is rapidly heated and propelled to a high velocity.

The heated substance strikes the surface of the substrate and immediately cools, forming a coating. When done correctly, this plasma spray process is referred to as a "cold process" (relative to the substrate material being coated) because the substrate temperature can be kept low during processing, preventing damage, metallurgical changes, and distortion to the substrate material.

Flame Wire Spraying

A common gas spraying method is flame wire spraying, in which wires are sequentially fed into a gas flame, such as oxy-acetylene or oxy-propane, become molten, and then sprayed with compressed air to form coatings.

Characteristics :

• Using lightweight equipment, suitable for on-site construction
• Little to no thermal distortion of base metals
• Adjustable thickness range (0.1~10 micrometers)
• A wide choice of materials
• High wear resistance of coatings

All kinds of metals that can be made into wires, including zinc, aluminum, carbon steel, stainless steel, and molybdenum, can be applied to this spraying technique. Thickness range is wide, from 0.1 up to 10 micrometers.

Flame Rod Spraying

This process uses an oxide ceramic rod as a coating material. The ceramic rod is heated and molten in an oxy-acetylene flame, then finely split by compressed air and solidified on a base metal, forming a coating. This approach is appropriate for corrosion-protection spraying or wear-control spraying because it incorporates ceramics, which enhances heat resistance around hot spots.

Characteristics :

• Sprays completely molten particles only
• Provides excellent coupling of particles and forms highly cohesive coatings
• Three or four times faster than powder spraying
• Less affected by heat than plasma spraying
• Limited choice of coating materials (limited to oxide ceramics)

Flame Powder Spraying

A powder substance is delivered into a spray gun through a powder pump and melted before being heated in a high-temperature flame to generate a coating. There are primarily three powder feed methods: directly connecting the powder pump to the spray gun, utilizing a separate powder pump, and feeding into the spray gun using an inert gas (e.g., nitrogen).

Characteristics :

• A wide choice of materials, including metals, alloys, plastics, and ceramics
• Long melting time in a flame provides the excellent adhesive strength of materials (90% efficiency for self-fluxing alloys)
• Very low noise level
• Uses compressed gases for the acceleration and cooling of particles
• Using a light spray gun allows manual spraying

HVOF Flame Spraying

This approach creates coatings by utilizing the intense spray impact produced by higher spray velocity. It is separated into HVOF (High Velocity Oxy-Fuel), which uses a mixture of fuel and oxygen, and HVAF (High Velocity Air-Fuel), which uses a mixture of fuel and air. It operates on the premise that

• Combustion occurs inside a tube
• A combustion gas is sprayed at high velocity through a nozzle
• Powder is fed into the sprayed gas and gets heated, molten, and sprayed at high velocity, finally forming a coating.

Characteristics :

• Two or four times faster spraying
• Can form delicate coatings
• Oxidation occurs less
• Requires finer powders
• Can cause clogging in the spray gun
• Generates ultrasound
• Work needs to get done inside an isolated soundproof room

Arc Spraying

In this standard electric spraying, two metal wires form an electrical arc discharge, become molten, and are fed at a rate that matches the melting velocity. The melted metals are then finely separated by compressed air and sequentially molded on a base metal to form a coating. When compared to flame spraying approaches, it performs admirably. Because a coating material can be totally molten at high temperatures, it has excellent adhesion to a base material. However, coating materials are only available in electrically conductive varieties.

Characteristics :

• Fast spraying (two to four times faster than flame spraying)
• High adhesive strength and coating strength
• Using different types of materials, can form alloy coatings
• Low operating costs

Detonation Thermal Spraying Process

The detonation cannon is essentially a long water-cooled barrel with gas and powder inlet valves. Oxygen and fuel (most commonly acetylene) are supplied into the barrel, along with a charge of powder. The gas mixture is ignited with a spark, and the resulting detonation warms and drives the powder down the barrel to supersonic velocity. After each detonation, the barrel is purged with nitrogen. This process occurs numerous times each second. When heated powder particles collide with the substrate, their tremendous kinetic energy causes a dense and robust coating to form.