Principles and Processes of Thermal Spraying Technology？

Thermal spraying technology involves heating coating materials to a molten or semi-molten state using various heat sources. These materials are then atomized and accelerated by high-speed gas to impact the substrate surface, forming a coating with properties compared to the original substrate material, thereby enhancing the surface performance of the workpiece. Thermal spray coatings exhibit wear resistance, corrosion resistance, high-temperature resistance, and thermal insulation properties. They can also repair worn or corroded parts, making them widely applicable in aerospace, machinery manufacturing, and petrochemical industries.

1. Characteristics of Thermal Spraying Technology

1. Versatile Coating Materials: Coatings made from various materials can be prepared on different substrates. Metals, ceramics, metal ceramics, and engineering plastics can all be used as thermal spraying materials. The substrate temperature is typically low, ranging from 30 to 200°C, which minimizes deformation and reduces the heat-affected zone.
2. Flexible Operation: The technology allows for the spraying of objects of various specifications and shapes, making it particularly suitable for large-area coatings and field operations.
3. Wide Coating Thickness Range: Coatings can be produced in a thickness range from a few microns to several millimeters, with easy control. The spraying efficiency is high and cost-effective, with production rates ranging from several kilograms to tens of kilograms per hour.

2. Types of Thermal Spraying Processes

2.1 Flame Spraying Process
Flame spraying encompasses both wire flame spraying and powder flame spraying. It generally uses the combustion of oxygen and acetylene gases to provide heat for melting the spraying material. Compressed gas atomizes and accelerates the spraying material, which then deposits onto the substrate to form a coating. The advantages of flame spraying include low equipment investment, ease of operation, portability for on-site work, no electrical requirements, and high deposition efficiency. It remains a preferred choice for spraying pure molybdenum coatings.

2.2 Arc Spraying Process
Arc spraying involves two insulated linear electrodes supplied with a direct current voltage of 18-40V. A wire feed mechanism forwards the wires, and when they approach each other, an arc is generated at the tips, melting them. Compressed air then atomizes the molten droplets, forming a spray jet that deposits on the workpiece surface. Arc spraying is limited to conductive metal wires and is primarily used for applying aluminum-zinc anti-corrosion coatings and stainless steel coatings for the repair and surface enhancement of large components.

2.3 High-Temperature Coating Spraying
Thermal spraying technology can also be used to improve the high-temperature oxidation resistance of mechanical components. For example, a supersonic flame-sprayed Co-Cr-Ni coating serves as a wear-resistant coating below 900°C and is a primary high-temperature protective coating for continuous annealing furnace bottoms in the metallurgical industry.

2.4 Functional Coating Spraying
Thermal spraying technology finds wide applications in the electrical industry, such as spraying shielding coatings to eliminate electromagnetic and radio wave interference while mitigating electrostatic discharge sparks. Zinc coatings from arc spraying can enhance attenuation at high energy levels.

2.5 Shaping through Spraying
The use of thermal spraying for manufacturing mechanical parts has rapidly developed in recent years. Techniques such as arc spraying can be used to create molds for stamping plastic and leather products, while plasma spraying can produce ceramic or refractory metal nozzles.

3. Recent Advances in Thermal Spraying Technology

With continuous improvements and refinements in thermal spraying technology, its application fields are expanding. Modern plasma spraying technologies utilize computer control systems, significantly reducing workload and error rates. High-power plasma spraying technology has emerged, with deposition efficiencies improving, and spraying power reaching 200 kW. Recently, reactive spraying technology has been developed, synthesizing special component coatings in situ through chemical reactions between the components of the spraying material or between the spraying material and the spraying gas. The materials used in thermal spraying have also evolved, with high-temperature self-propagating technology enabling the production of new spraying powders, including ceramic powders like TiC, TiB₂, and ZrB₂.

Future developments in thermal spraying technology are likely to focus on the following areas:

1. Developing new spraying materials, with nanomaterials becoming a key focus.
2. Exploring new applications for thermal spraying technology, such as integrating thermal spraying with heat treatment techniques.
3. Promoting the application of computers in thermal spraying to achieve programmatic operation.