Plasma Spraying In The Development Of Thermal Spraying Industry

       

With the development of science and
technology, there are more and more kinds of surface modification technologies,
such as surface coating, ion implantation and ion extraction. These methods can
improve the surface hardness, fracture toughness and bending strength of
ceramic materials, reduce the friction coefficient, and improve the wear
resistance, chemical properties, high temperature oxidation resistance and
mechanical properties. Plasma spraying is a new plasma spraying method which is
developed to overcome the shortcomings of conventional atmospheric plasma
spraying. The working principle of plasma spraying is: the main gas and
secondary gas are input from the back of the spray gun, the main gas flow
between the tungsten electrode and the primary nozzle is small, and the
secondary gas with large flow is ejected through the secondary nozzle through
the gas swirl ring. Plasma spraying coating has the following characteristics:
uniform coating of high melting point material, dense and hard coating, small porosity,
high bonding strength of ceramic coating of high melting point material.

Plasma spraying is a new type of
multi-purpose precision spraying method developed after flame spraying. It has
the following characteristics: (1) ultra high temperature characteristics, easy
to spray high melting point materials. ② The coating is compact and has high
adhesion strength. ③ Because inert gas is used as working gas, the spraying
material is not easy to oxidize.

Plasma spraying process is carried out by
using plasma arc. Ion arc is compression arc. Compared with free arc, its arc
column is thin, current density is high, gas ionization degree is high, so it
has the characteristics of high temperature, energy concentration and arc
stability.

There are three forms of plasma arc
according to different methods of connection

① Non transfer arc: refers to the plasma
arc generated between cathode and nozzle. In this case, the positive electrode
is connected to the nozzle, the workpiece is not charged, and an arc is
generated between the cathode and the inner wall of the nozzle. The working gas
is heated through the arc between the cathode and the nozzle, causing total or
partial ionization, and then the nozzle ejects to form a plasma flame (or
plasma jet).

This kind of plasma arc is used in plasma
spraying.

② Transfer arc: the plasma arc transferred
from the spray gun to the machined part. In this case, the nozzle is not
connected to the power supply, and the workpiece is connected to the positive
electrode. The arc flies between the cathode and anode (workpiece) of the spray
gun. The working gas is sent around the arc and then ejected from the nozzle.

Plasma cutting, plasma arc welding and
plasma arc smelting use this kind of plasma arc.

③ Combined arc: the non transfer arc
ignites the transfer arc and heats the metal powder. The transfer arc heats the
workpiece to produce a molten pool on its surface. In this case, the nozzle and
the workpiece are connected to the positive electrode.