Technical Characteristics Of Thermal Spraying

       

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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