Brief Introduction of Plasma Spray Coating
Plasma spraying is a method that utilizes a non-transferred plasma arc as the heat source for coating. The compressed arc needed for spraying is generated by a plasma spray gun. In the spray gun, the cathode (electrode) is typically made of tungsten or tungsten alloys, or other high-melting-point materials. The anode, also known as the nozzle, is commonly made of pure copper or copper with tungsten lined. The cylindrical cathode tip and the concentric funnel-shaped anode configuration constitute the arc chamber. When the working gas is introduced tangentially or axially through the arc in the anode, electrons emitted from the cathode interact with gas molecules or atoms, causing gas dissociation or ionization. The working gas is heated in the arc zone and expands radially and axially, accelerating out of the anode to form a plasma jet.
The ignition of the plasma arc usually relies on high-frequency discharge sparks. Powdered spray materials are carried and injected into the jet by an inert gas. The jet accelerates and heats the powder to a molten or semi-molten state, which is then sprayed onto the surface of the substrate. The high-temperature particles in flight undergo deformation upon impact, followed by rapid solidification and cooling, forming rapidly quenched thin flakes that adhere to the treated substrate surface. The continuous accumulation of these rapidly quenched flakes, also known as flattened particles, results in a layered structure coating. The substrate for spraying can be any solid material, not necessarily a conductor.
In the body of a direct current plasma arc spray gun, the mechanical compression, thermal compression, and self-magnetic compression effects of a water-cooled anode result in a core temperature of the plasma jet reaching up to 32,000°C. The speed exceeds 2,000 m/s, and the energy density approaches 105 W/mm², second only to laser and electron beams. The temperature distribution of the plasma flame is shown in Figure 6-10. The plasma arc jet exhibits energy characteristics of high temperature, high enthalpy, and high speed. Even the most refractory materials, such as tantalum carbide with a melting point of 3,875°C, can be melted and sprayed into shape in the plasma jet.
The method of introducing spray materials into the jet depends on the design of the plasma gun, including external powder feed, internal powder feed, and axial powder feed methods. The final thickness of the coating (mostly in the range of 0.10 to 0.50 mm) is determined by factors such as powder mass flow rate, gun movement speed, and the number of spraying passes. During plasma spraying, the amount of heat absorbed by the substrate is minimal, and materials as delicate as paper can serve as the spraying substrate. Typically, there is no need to preheat the workpiece, acting as the substrate, before spraying, although in some cases, preheating to below 200°C may be done if required. During plasma spraying, the workpiece usually undergoes minimal metal phase changes and exhibits little deformation.
Comparisons Between Plasma Spray Coating and Other Methods
A performance comparison between plasma spraying and other spraying methods is provided in the below table:
Classifications of Plasma Spray Coating
The plasma spraying method, already applied in engineering, can be broadly categorized into different types. Below is the classification:
Plasma spray coating can be classified based on various factors, including the type of feedstock material, the application, and the specific characteristics of the coating. Here are some common classifications:
1. Feedstock Material:
Ceramic Plasma Spray Coatings: These coatings are composed of ceramic materials such as alumina, zirconia, and chromium oxide. They are often used for applications requiring wear resistance, thermal barrier properties, or corrosion resistance.
Metallic Plasma Spray Coatings: Metallic coatings are created using materials like nickel, aluminum, and alloys. These coatings are utilized for applications where improved wear resistance, corrosion protection, or electrical conductivity is needed.
Composite Plasma Spray Coatings: These coatings are a combination of ceramic and metallic materials, offering a mix of properties from both types. They are designed to provide enhanced performance in specific applications.
Thermal Barrier Coatings (TBCs): Designed to protect components from high-temperature environments, TBCs are often applied to turbine blades, combustion chambers, and other parts in gas turbines.
Wear-Resistant Coatings: These coatings are used to improve the wear resistance of components subjected to abrasive or erosive conditions, such as industrial pump components or cutting tools.
Corrosion-Resistant Coatings: Plasma spray coatings can provide corrosion protection for a variety of materials in harsh environments, extending the lifespan of components.
Biomedical Coatings: Plasma spray is employed to deposit biocompatible coatings on medical implants to enhance their performance and integration with the human body.
Porosity Level: Coatings can be classified based on their porosity, which influences properties like thermal insulation and permeability.
Density: Some coatings are classified by their density, as higher-density coatings may exhibit improved mechanical properties.
4. Powder Size and Morphology:
Fine or Nanostructured Coatings: Some applications require coatings with fine or nano-sized particles to achieve specific properties or surface finishes.
5. Intended Purpose:
Protective Coatings: Used to protect surfaces from environmental factors such as corrosion, wear, and high temperatures.
Functional Coatings: Engineered to provide specific functionalities, such as electrical conductivity, thermal insulation, or biocompatibility.
These classifications help describe the diverse range of plasma spray coatings and their applications across various industries.
Today we have gone through some knowledge about the plasma spray working process, principles, and plasma spray classifications. In the next few days, we will introduce more about plasma spray classification in another aspect.