What materials are suitable for coating preparation using plasma spraying? What are the pre-treatment requirements for different substrates?

What materials are suitable for coating preparation using plasma spraying? What are the pre-treatment requirements for different substrates?

Coating materials suitable for plasma spraying and pre-treatment requirements for different substrates

1、 Types of coating materials suitable for plasma spraying

Metal and alloy materials

Pure metals: including aluminum, copper, nickel, titanium, etc., suitable for preparing conductive coatings, impermeable coatings, or sacrificial anode coatings. For example, aluminum coatings are commonly used for cathodic protection of steel components, copper coatings can serve as conductive layers for electronic components, and titanium coatings are used for inner wall protection of chemical equipment due to their excellent corrosion resistance. Pure metal powder has a low melting point (660-1668 ℃), and the arc power should be controlled between 30-50kW during spraying to avoid excessive melting and particle splashing.

Alloy materials: Nickel based alloys (such as Ni Cr, Ni Al, NiCrAlY) are the most widely used, and Ni Cr alloy coatings can withstand oxidation temperatures up to 800 ℃, making them suitable for high-temperature component protection; Co based alloy (such as Co-Cr-W) coating has outstanding wear resistance and is used for wear-resistant parts such as engine valves and bearings; Iron based alloys (such as stainless steel) have lower coating costs and are suitable for structural component protection in moderately corrosive environments. The particle size of alloy powder is usually controlled between 20-60 μ m, and the sphericity is ≥ 80% to ensure fluidity.

ceramic materials

Oxide ceramics: Aluminum oxide (Al ₂ O ∝) coating has high hardness (HV800-1200) and good insulation properties, and is used as a wear-resistant insulation coating for molds and bearings; Zirconia (ZrO ₂) is widely used as a thermal barrier coating for gas turbine blades due to its low thermal conductivity (0.1-2W/(m · K)) and high temperature stability. When in use, Y ₂ O ∝ stable phase needs to be added to prevent high-temperature phase transformation cracking; Chromium oxide (Cr ₂ O3) coating has excellent corrosion resistance and wear resistance, and is suitable for surface protection of hydraulic piston rods, printing rollers, etc. Oxide ceramics have a high melting point (1500-2700 ℃), and the spraying power needs to be increased to 40-60kW, with a spraying distance controlled between 120-180mm.

Carbide and nitride ceramics: Tungsten carbide (WC Co) coating is the preferred choice in the field of wear resistance, with a hardness of HV1200-1800, used for strong wear parts such as excavator bucket teeth and shield machine cutting tools; Chromium carbide (Cr ∝ C ₂ - NiCr) coating is resistant to high temperature oxidation at 800 ℃ and suitable for high-temperature wear-resistant scenarios (such as the lining of garbage incinerators); Titanium nitride (TiN) coating is golden yellow in color, combining decorative and wear-resistant properties, and is used for surface strengthening of cutting tools and molds. This type of material requires the use of agglomerated sintered powder with a particle size of 30-80 μ m to ensure sufficient melting during spraying.

composite material

Metal ceramic composite: such as Ni Al/Al ₂ O3, Co/WC composite coatings, combining the toughness of metals and the wear resistance of ceramics, suitable for working conditions with both impact loads and wear (such as crusher hammers). By adjusting the ratio of metal to ceramic (usually 30% -50% metal phase), the overall performance of the coating can be controlled.

Metal non-metal composites: including metal based graphite coatings (such as Cu Graphite), utilizing the self-lubricating properties of graphite for high-temperature sliding components (such as bearing cages); Metal based glass coatings have excellent sealing and corrosion resistance, and are used for sealing surfaces such as pipeline joints. Composite powders need to be prepared using mechanical mixing or coating processes to ensure uniform composition.

functional materials

Thermal barrier coating material: With ZrO ₂ - Y ₂ O3 as the core and NiCrAlY bonding layer, a double-layer structure is formed, which can reduce the substrate temperature by 100-300 ℃ on the surface of high-temperature components and extend the service life.

Biomedical materials: Hydroxyapatite (HA) coating has biocompatibility and can promote bone tissue growth and fusion when sprayed on the surface of titanium alloy artificial bone; The titanium alloy coating serves as a surface modification layer for the implant to enhance its adhesion with human tissue.

Conductive and insulating coating materials: Metal coatings such as copper and silver are used for conductive applications, while aluminum oxide and beryllium oxide coatings are used as insulation layers for insulating components of motors and electronic devices.

2、 Pre treatment requirements for different substrates

Metal substrates (steel, aluminum alloy, titanium alloy, etc.)

Surface cleaning: Firstly, degreasing is carried out using organic solvents such as alcohol and acetone to remove surface oil stains; For substrates with oxide layers (such as hot-rolled steel), acid washing (hydrochloric acid or sulfuric acid solution) or laser cleaning should be used to remove the oxide scale. After acid washing, rinse with clean water and dry to ensure that there is no residual acid solution on the surface (pH 6-7). Stainless steel substrates can be subjected to electrolytic polishing to remove passivation films and enhance coating adhesion.

Coarsening treatment: Sandblasting treatment is mainly used, and 80-120 mesh white corundum or brown corundum sand is selected as the steel substrate. The sandblasting pressure is 0.4-0.6MPa, and the roughness is controlled at Ra3.2-6.3 μ m; Soft metal substrates such as aluminum alloys and titanium alloys require a reduction in sandblasting pressure (0.2-0.4 MPa) and the use of finer sand particles (120-180 mesh) to avoid excessive plastic deformation on the substrate surface. The roughness should be controlled at Ra2.5-5 μ m. Complex shaped substrates can be chemically etched (such as using phosphoric acid chromic acid etching solution for aluminum alloys) instead of sandblasting to form uniform micro concave convex structures.

Pre treatment and post-treatment: After roughening, spraying should be carried out within 4 hours to avoid secondary oxidation of the surface; If the storage time exceeds 4 hours, the surface needs to be blown with compressed air again to remove floating dust, and if necessary, a second light sandblasting (pressure 0.1-0.2 MPa) should be performed to activate the surface.

Ceramic substrates (alumina ceramics, silicon nitride ceramics, etc.)

Surface cleaning: Due to the presence of adsorbed water and pollutants on ceramic surfaces, ultrasonic cleaning (using neutral detergent as the cleaning agent) is required. The cleaning time is 20-30 minutes and the temperature is 50-60 ℃ to remove oil stains and impurities; For sintered ceramics, if there is a glaze layer on the surface, it needs to be removed by polishing with a diamond grinding wheel to expose a rough surface.

Coarsening and activation: Ceramics have high hardness and limited sandblasting effect. Laser roughening (power 50-100W) is commonly used to form micrometer level grooves on the surface, or a combination of sandblasting and coating transition layer is used. The transition layer is made of materials compatible with both ceramics and spray materials (such as NiCr alloy), with a thickness of 50-100 μ m, to enhance interfacial bonding.

Temperature control: Ceramic substrates have low thermal conductivity, and after pretreatment, they need to be preheated to 100-200 ℃ (to avoid cracking due to excessive temperature difference with sprayed particles), with a preheating rate of ≤ 5 ℃/min to prevent thermal shock damage to the substrate.

Non metallic substrates (engineering plastics, composite materials, etc.)

Surface cleaning: Degrease plastic substrates (such as PE and PVC) with isopropanol to avoid swelling caused by strong solvents; Carbon fiber composite materials require a soft bristled brush combined with compressed air to remove surface fiber dust, and if necessary, low-temperature plasma cleaning (power 50-100W, time 5-10 minutes) to improve surface activity.

Coarsening and modification: The surface hardness of plastic is low, and nylon sand or glass microspheres (particle size 50-100 μ m) should be selected for sandblasting, with a pressure of 0.1-0.2 MPa and a roughness controlled at Ra1.6-3.2 μ m; Alternatively, polar groups can be introduced through chemical etching (such as etching polytetrafluoroethylene with sodium naphthalene solution) to enhance coating adhesion. Carbon fiber composite materials can be polished with sandpaper (80-120 mesh) to expose the fiber-reinforced phase and form mechanical anchoring.

Special pretreatment: Due to the poor heat resistance of non-metallic substrates (such as most plastics with heat resistance<150 ℃), the heat input during spraying needs to be controlled after pretreatment. Low temperature plasma spraying technology (arc power<30kW, spraying distance>200mm) or water cooling on the back of the substrate is used to ensure that the substrate temperature does not exceed its glass transition temperature.

Heterogeneous material composite substrate

For metal ceramic composite substrates (such as metal matrix ceramic coating repair parts), it is necessary to focus on treating the interface transition zone, removing the residual part of the original coating, roughening the exposed part of the metal matrix with sandblasting, and laser etching the residual part of the ceramic to form a stepped transition structure, enhancing the bonding between the new coating and the substrate.

When pretreating metal plastic composite substrates, it is necessary to select processes based on the characteristics of the two materials separately. Conventional sandblasting is used for the metal part, and low-temperature plasma cleaning and mild roughening are used for the plastic part to ensure that the pre-treatment effect matches in different areas.