Nowadays, industrial parts are often subjected to harsh conditions that result in wear, erosion, abrasion, and corrosion. To prevent this from happening, they need protective coatings. While many coating methods exist, thermal spray coating is one of the most popular, chosen for its speed and versatility.
In this article, we will explore how the different spray coating technologies work. We will also explain why surface preparation is essential prior to thermal spraying and how to do it. In doing so, we will explore innovative ways to prepare surfaces through laser technology, which improves precision and helps reduce dependency on consumables.
- What is Thermal Spray Coating?
- Thermal Spray Coating Processes
- How to Prepare a Surface for Thermal Spray Coating
Thermal spray coating is a process that sprays coating materials using the pressure of a high-heat gas. The material then bonds to the surface, forming a coating that grants properties such as corrosion protection, thermal resistance, environmental protection, lubricity, wear resistance, and electrical/thermal conductivity.
Coating materials usually come in powder or wire form through a feeder and may include metals, alloys, ceramics, cermets, carbides, plastics, or composites. The spray gun, also known as spray torch, is the main tool used to perform the operation. It can be operated manually or by robot arm.
Thermal spray systems are widely used in engineering for aerospace, mechanical, marine, and automotive applications. They are also used for electronics, biomedical as well as several other applications.
Several criteria are used to evaluate the quality of the coating, such as bond strength, porosity, oxidation, hardness, and roughness. The coating’s thickness can be as small as 20 microns, or it can be several millimeters thick.
There are various thermal spray processes. While they may look similar, they differ on several points, such as:
- The method used to generate heat
- The temperatures and velocities reached by the gas stream and the coating material
- The coating materials that can be used
- The type of bonding that takes places
Let’s look at the different types of spraying processes to understand the difference.
Plasma spraying makes use of a plasma gun to spray the coating material onto the substrate. A powder feedstock is introduced into a plasma jet, where it is subjected to the high temperature of the flame (typically between 10,000–15,000°C). The heat turns the powder into high-velocity droplets traveling at up to 800 m/s that adhere to the substrate, where they cool off and solidify to form a protective coating.
Detonation spraying is a process that creates explosions several times a second to heat up and send the powder feedstock through a detonation barrel at supersonic speed (≈3,500 m/s). A spark is used to detonate a mixture of oxygen and fuel (typically acetylene), causing the temperature inside the barrel to reach ≈4,000°C. The substrate is deformed on impact, causing it to wrap around the sprayed particles and form a strong mechanical bond.
Wire Arc Spraying
Wire arc spraying, also known as twin wire arc spraying, creates an electrical discharge that arcs between two wires, one positive charged and the other negatively charged. The electric arc can reach 5000°C and is formed as the two wires meet in the spray gun. As the high heat melts the wires, a gas stream is used to spray the molten metal, creating droplets that interlock one on top of another on the substrate. Sprayed particles reach low speeds (between 60–300 m/s) compared to other methods.
Flame spraying ignites a combination of oxygen and fuel (typically propane or acetylene) to produce a flame that heats and melts the feedstock (coating material). The feedstock is either a metal wire, or a ceramic or metal powder. The flame’s temperature is around 3000°C, which melts the material and converts it into fine droplets that build up on the substrate to form a coating layer.
The first video shows flame spraying using powder feedstock. Here’s another video that shows flame spraying using wire feedstock:
High Velocity Oxy-Fuel Spraying (HVOF Coating)
High velocity oxy-fuel spraying ignites a mixture of oxygen and fuel (typically kerosene or hydrogen) in a combustion chamber. The gas mixture reaches around 3000°C, and high pressure ejects it through a nozzle at about 1,000 m/s. The feedstock is injected into the high velocity gas stream. Typical powder feedstocks include tungsten carbide, which can be mixed with other powdered materials such as cobalt-chrome, nickel, or chromium. Other powder feedstocks include chromium carbide and stainless steel.
High Velocity Air Fuel Spraying (HVAF Coating)
High velocity air fuel spraying injects powder into a gas jet that consists of compressed air and fuel (typically propane, propylene, or natural gas). It is brought to a temperature of about 2000°C, which is above the melting temperature of the coating material, but below its evaporation temperature. HVAF can spray any powder materials, except ceramics. Sprayed particles can reach a velocity of 1200 m/s. Metals sprayed using HVAF produce almost no oxides compared to HVOF.
Cold spraying is different from other thermal spray processes because sprayed materials are not melted. An inert gas (such as nitrogen or helium) is preheated electrically between 0–900°C. The coating material is then accelerated through the high-pressure gas stream, reaching supersonic speeds of up to 1,200 m/s. The process relies on kinetic energy to deform and heat the sprayed particles upon impact, making them adhere to the surface. Unlike other thermal spray processes, there is no heat affected zone and no oxidation of sprayed materials.
Spray and Fuse
Spray and fuse is a thermal spray process (such as HVOF or HVAF) followed by an additional step that fuses the coating with the substrate. Fusing is done using an oxygen-acetylene torch or a vacuum furnace, which operates at temperatures between 950–1250°C. This creates a metallurgical bond similar to welding instead of an adhesive bond, resulting in stronger coatings without any porosities.
To ensure the quality of the coating, one of the most important steps is surface preparation. A well-prepared surface is essential to obtain a good bonding quality between the substrate and the coating.
Two treatments are essential to prepare surfaces for thermal sprays:
- Surface cleaning: Contaminants located on the surface of the substrate can interfere with the bonding mechanism. Oils, oxides, and other surface contaminants must be removed before the coating process.
- Surface roughening: Coatings do not adhere well to smooth surfaces. To increase adhesion, surfaces need to be roughened so that when droplets (or “splats”) interlock with the substrate, they form a strong mechanical bond.
To perform these treatments, several technologies are used:
- Grit blasting, or abrasive blasting, is well established when it comes to thermal spray coating. Manufacturers use it to remove contaminants and roughen the surface all at once. The operation is performed in a restricted area where operators wear PPE.
- Masking tapes are used to prevent specific areas on the substrate from being spray coated or grit blasted. Heat-resistant and abrasion-resistant tapes are typically applied manually. Masking is frequently the cause of errors or imprecisions.
- Lasers are versatile tools that widen possibilities for spray coating applications and help reduce the number of consumables. Unlike grit blasting, lasers can clean and/or texture precise locations on the substrate. Lasers can also eliminate the need for masking tapes, or correct masking errors on high-precision parts.
If you are manufacturing parts that are spray treated and think you could benefit from laser surface preparation, Laserax experts can help you.