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Hydraulics

The field of hydraulics encompasses a wide range of components such as cylinders, pistons, valves, and pumps. These components are often subjected to harsh operating conditions including high pressures, abrasive particles in the hydraulic fluid, and corrosive environments. Thermal spray technology provides a means to enhance the performance and durability of hydraulic components.

thermal spray techniques used in the hydraulics industry

Flame Spraying

    • Principle: 

Utilizes the heat generated by the combustion of fuel gases (such as acetylene, propane, etc.) mixed with oxygen to melt the coating material, which is then propelled by compressed air onto the surface of the hydraulic component.

    • Advantages: 

The equipment is relatively simple and inexpensive, making it suitable for small-scale operations and on-site repairs. It can spray a variety of materials, including metals, alloys, and some ceramics, providing good corrosion resistance and moderate wear resistance. For example, in some small hydraulic systems, flame spraying zinc or aluminum coatings can be used for basic anti-corrosion protection.

    • Limitations: 

The resulting coating may have relatively lower density and hardness compared to other techniques, and the bonding strength with the substrate may not be as high.

High-Velocity Oxygen-Fuel Spraying (HVOF)

    • Principle: 

Combines oxygen and a fuel gas (such as kerosene) to produce a high-temperature, high-velocity combustion jet. The coating powder is introduced into this jet, where it is rapidly heated and accelerated to a high velocity before being deposited onto the workpiece surface.

    • Advantages:

Produces coatings with high density, excellent hardness, and good wear resistance. It can achieve a smooth and uniform coating surface, which is beneficial for components requiring precise dimensions and low surface roughness, such as hydraulic cylinders and pistons. The coatings formed by HVOF have a strong bonding with the substrate, which can withstand high pressures and mechanical stresses, significantly prolonging the service life of hydraulic components.

    • Limitations: 

The equipment is more complex and expensive, and the operating costs are relatively high due to the need for high-purity oxygen and specialized fuel gases.

Plasma Spraying

    • Principle:

 Generates a plasma arc by ionizing a gas (such as argon, nitrogen, or a mixture of gases) using an electric arc. The plasma arc provides extremely high temperatures, melting the coating powder and propelling it towards the substrate at high speed to form a coating.

    • Advantages:

 Allows for the deposition of a wide range of materials, including high-melting-point ceramics, metal alloys, and cermets. It can produce coatings with high density, low porosity, and excellent wear resistance, corrosion resistance, and thermal insulation properties. Plasma spraying is particularly suitable for hydraulic components that require high-performance coatings to resist harsh operating conditions, such as high-temperature, corrosive fluids, or abrasive particles.

    • Limitations: 

The equipment is costly, requires a high level of technical expertise to operate, and has relatively high energy consumption. The coating process may also introduce some residual stresses in the coating, which need to be carefully controlled to avoid coating failure.

Arc Spraying

    • Principle: 

Uses an electric arc formed between two continuously fed metal wires as the heat source to melt the wires. The molten metal is then atomized by a high-speed gas stream and sprayed onto the surface of the hydraulic component to form a coating.

    • Advantages:

 It is a highly efficient spraying method, capable of quickly depositing thick coatings, which is suitable for large-area protection and components with high production requirements. The coating has good bonding strength with the substrate, mainly through mechanical and metallurgical bonding, providing reliable protection against wear and corrosion. Arc spraying can use a variety of metal wires as coating materials, such as aluminum, zinc, stainless steel, nickel-chromium alloy, etc., allowing for customization based on the specific needs of different hydraulic components.

    • Limitations: 

The coating quality may be slightly inferior to plasma spraying in terms of density and surface finish. Additionally, the process may generate more heat compared to some other techniques, which could potentially affect the substrate material if not properly controlled.

Cold Spray

    • Principle: 

In cold spray, the coating material is in a solid state and is accelerated to a high velocity using a supersonic gas jet. When the particles impact the substrate, they plastically deform and bond to the surface, forming a coating.

    • Advantages:

 Since the process does not involve melting the coating material, it can avoid some of the problems associated with high-temperature spraying, such as oxidation, phase transformation, and thermal stress. Cold spray is suitable for depositing materials that are sensitive to high temperatures, such as certain metals and alloys. It can also be used to repair damaged surfaces or add thin layers of coating without significantly affecting the substrate’s properties.

    • Limitations: 

The equipment for cold spray is relatively specialized and expensive. The process requires high gas pressures and velocities, which may limit its application to certain types of hydraulic components. Additionally, the choice of coating materials is somewhat restricted compared to other thermal spray techniques.

How to choose the appropriate thermal spraying technology in the hydraulic industry?

Based on the working environment of components

    • Wear environment


High wear conditions (such as heavy-duty hydraulic equipment): 

If hydraulic components work under high-pressure and high-frequency friction environments, like the pistons and cylinder walls of heavy-duty hydraulic presses, technologies capable of forming high-hardness wear-resistant coatings should be prioritized. High-Velocity Oxygen-Fuel Spray (HVOF) is a good choice as it can spray hard materials such as tungsten carbide-cobalt (WC-Co) to form coatings with a hardness of up to 1200 – 1600HV, effectively resisting abrasive wear and adhesive wear.

Wear caused by particulate matter (such as hydraulic fluid containing impurities): 

For components of hydraulic valves and pumps, when the hydraulic fluid contains particulate impurities and is prone to cause erosive wear, plasma spraying of ceramic coatings (such as alumina (Al₂O₃) or zirconia (ZrO₂)) is more suitable. The high hardness of ceramic coatings can resist the scouring of particles and reduce component wear.

    • Corrosion environment

External corrosion (such as in humid or coastal environments):

 For hydraulic components exposed to the external environment, such as outdoor hydraulic cylinders and pipelines, flame spraying of zinc or aluminum coatings is an economical and effective method. This kind of coating protects the substrate through the sacrificial anode method and can provide good anti-corrosion effects in humid or salt-fog environments.

Internal corrosion (such as due to chemical substances in hydraulic oil):

 When components inside the hydraulic system are corroded by chemical substances in the hydraulic oil or possible water ingress, plasma spraying of nickel-chromium (Ni-Cr) alloy coatings is more appropriate. It can form a barrier on the component surface to prevent corrosive media from contacting the substrate and reduce the corrosion risk.

    • Environment with lubrication requirements:

 In environments where it is necessary to reduce friction and improve lubrication performance, such as precision hydraulic systems with high requirements for energy efficiency, cold spray or plasma spray of molybdenum disulfide (MoS₂) coatings are good choices. These coatings can form solid lubricant films, reduce the friction coefficient between moving parts, and ensure that hydraulic components can slide smoothly even under high pressure.

Considering the shape and size of components

    • Large components (such as large hydraulic cylinders): 

For large-area hydraulic components, arc spraying is an efficient technology. It can quickly form coatings on large-area surfaces and can use a variety of metal wire materials as spraying materials. The bonding force between the coating and the substrate is also relatively strong, making it suitable for anti-corrosion and wear-resistant protection of large components.

    • Complex-shaped components (such as precision valves): 

If the components have complex shapes, such as hydraulic valves with fine internal structures, plasma spraying is a more appropriate choice. Plasma spraying can precisely control the direction and speed of sprayed particles, enabling the coating to evenly cover the surfaces of complex-shaped components, ensuring that each part can be well protected. Moreover, it can precisely control the thickness and surface roughness of the coating.

Based on cost factors

    • Equipment cost


In the case of limited budget: 

Flame spraying equipment is simple and has a relatively low price. For small hydraulic enterprises or application scenarios where the requirements for coating performance are not particularly high, flame spraying equipment is an affordable choice. It can meet basic needs such as anti-corrosion, wear resistance, and surface repair.

In the case of high-performance requirements: 

Although plasma spraying equipment and High-Velocity Oxygen-Fuel Spray equipment have relatively high prices, they can prepare high-performance coatings. For key hydraulic components, such as high-precision hydraulic pumps and valves, the investment in such equipment is worthwhile because high-quality coatings can significantly improve the performance and service life of the components.

    • Operating cost


Large-scale application scenarios: 

The operating costs of flame spraying and arc spraying mainly include the costs of fuel, electrode materials, and compressed air, which are relatively low. In large-scale hydraulic component protection projects, such as the anti-corrosion of a large number of hydraulic cylinders, the operating cost advantage of arc spraying is obvious, and it can reduce costs while ensuring the quality of the coating.

High-performance coating application scenarios: 

Plasma spraying and High-Velocity Oxygen-Fuel Spray need to consume a large amount of electrical energy and expensive spraying powder materials, so the operating costs are relatively high. However, for those hydraulic components that require high-performance coatings to ensure long service life and high performance, such as hydraulic components in the aerospace field, the high cost investment can be compensated by extending the service life of the components and improving the reliability of the system.

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