Thermal spraying technology has a long history, and its origin can be traced back to the beginning of this century. In the early days, people only compressed the molten metal to form a liquid flow, and then sprayed the surface of the object to be sprayed to form a membrane structure. Among them, the difficulties of the technical core mainly lie in the spraying temperature, droplet dissolution and impact speed on the sprayed object surface. Throughout the development history of thermal spraying technology, these three points are taken as the basis to move forward.
Temperature and speed depend on different heat sources and equipment structures. In a sense, the higher the temperature and speed, the more conducive to the formation of excellent coatings, which leads to the competition and coordination of temperature and speed in the whole process of technological development. The selectability of a wide range of spraying materials is another advantage of thermal spraying technology. It can make the working surface of different equipment "turn iron into gold, wear helmets and wear armor". It is these three elements and many other controllable influencing factors that make thermal spraying a truly unique technology with superposition effect. It can design a variety of modified surfaces and obtain a wide range of applications from general mechanical maintenance to high-tech fields such as aerospace and bioengineering.
1、 Spraying material
Spraying materials have different forms such as powder, wire, belt and rod. Their components are metals, alloys, ceramics, cermets and plastics. Powder materials occupy an important position, with more than 100 kinds. Wire and strip are mostly metal or alloy (composite wire still contains ceramics or plastics); There are only a dozen kinds of bars, mostly oxide ceramics.
2、 Spraying method
In order to provide different heat sources, it can be divided into combustion method and electrothermal method. The former includes combustion flame spraying, explosive spraying and high speed flame spraying (HVOF); The latter includes arc spraying and plasma spraying (also divided into atmospheric pressure plasma spraying, low pressure plasma spraying and water stable plasma spraying). The spraying temperature (strictly speaking, the temperature when the droplet strikes the substrate surface) and the velocity of the droplet hitting the substrate surface have an important impact on the coating.
3、 Technical characteristics of thermal spraying
1. The base material is not limited, it can be metal or non-metal, and can be sprayed on various base materials;
2. There are a wide range of coating materials that can be sprayed. Thermal spraying technology can be used to spray almost all solid engineering materials, such as cemented carbide, ceramics, metal, graphite, etc;
3. During spraying, the temperature rise of the base material is small without stress and deformation;
4. The operation process is flexible and convenient, not limited by the shape of the workpiece, and the construction is convenient;
5. The coating thickness can range from 0.01 to a few millimeters;
6. The coating has various properties, which can form coatings with various special functions such as wear resistance, corrosion resistance, heat insulation, oxidation resistance, insulation, conductivity and radiation protection;
7. Strong adaptability and good economic benefits.
4、 Formation and evaluation of coatings
After the spraying material is sprayed by the spraying equipment with some form of heat source, its flight time is only a few thousandths of a second or less before reaching the sprayed substrate surface. In such a short time, it is heated, melted or semi melted to form small and dispersed droplets, which rush to the surface of the substrate and are beaten into flat laminated pieces. The previously generated flat pieces are covered by the latecomers, and soon a covering layer composed of many flat rolls is formed, that is, the spraying layer. The higher the heat source temperature, the greater the droplet impact velocity and the denser the coating. The coating properties are related to many factors.
5、 Coating function
When an engineering problem is raised, first of all, the working conditions of the sprayed part of the workpiece to be constructed (usually the working surface of equipment or facilities) shall be determined, and the coating function is the main basis for determining the spraying process and materials, and whether it is allowed economically.
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