Characteristics Of Explosive Spraying And Supersonic Spraying

       

Explosive spraying is to inject the powder
into the spray gun and introduce oxygen acetylene mixture to ignite the
ignition gas to produce 3300 ℃ high temperature. The powder is heated and
sprayed on the surface of the workpiece at more than twice the speed of sound
to form a diameter of ~ 20 mm and a diameter of ~ 8 mm μ M thick coating with
high bonding strength and high density. The whole coating is formed by
overlapping small circular flakes.

(1) When 0.25mn thick WC is sprayed on the
damping table of ferroalloy fan blade of aeroengine, the service life is
increased by 10 times

(2) A layer of Cr3C2 with thickness of
0.12mm is sprayed on the locating clasp of combustion chamber, and the service
life is increased by 7 times.

(3) At present, explosive spraying is
mainly used to prepare high-quality wear-resistant coating.

Supersonic flame spraying (HVOF) is to
introduce fuel gas (hydrogen, propane, propylene or MAPP) and combustion aids
into the combustion chamber in a certain proportion for mixing and explosive
combustion. The high temperature gas enters the copper nozzle through four
inclined nozzles in the upper combustion head of the combustion chamber, and
the powder is quantitatively sent into the high temperature gas along the
carbide central sleeve in the combustion head by the powder feeding gas (AR or
N2), The nozzle is brought out by high temperature and high speed gas, the
coating is formed on the workpiece by high speed nozzle, and the whole spray
gun is cooled by circulating water.

Characteristics of thermal spraying
technology: the application of thermal spraying technology has developed from
the preparation of decorative coatings to the preparation of various functional
coatings, such as wear-resistant, anti-corrosion, oxidation-resistant, heat insulation,
electrical conductivity, insulation, friction reduction, lubrication, radiation
protection and so on. Thermal spraying can be used for both repair and
manufacture. Because the performance of the coating material is better than
that of the substrate, the product quality can be greatly improved by coating
the surface of the parts with the coating material.

Selection of coating materials:

(1) It is better for the thermal spraying
material to have a wide liquid region, which can keep the droplet of the
coating material in liquid phase for a long time.

(2) Good thermal stability, no sublimation
and no decomposition in high temperature flame. If the material is easy to
decompose at the temperature of thermal spraying, it is not suitable for
thermal spraying.

(3) The spraying material should have good
wettability with the substrate in the molten state, so as to ensure a good
bonding performance between the coating and the substrate.

(4) It has the similar coefficient of
thermal expansion with the base material, so as to prevent the larger thermal
stress due to the large difference of the coefficient of thermal expansion.

Coating formation process:

(1) The spray material is heated to a
molten state.

(2) When the spray material is atomized
into tiny droplets and impinges on the surface of the substrate at high speed,
the greater the kinetic energy of the particles impinging on the substrate and
the greater the impact deformation, the better the coating bonding.

(3) The molten high-speed particles deform
after impacting the surface of the substrate and condense to form a coating.

Thermal spraying has become an important
method for the preparation of wear-resistant coating, which is widely used in
various fields of national economy. The porous layered structure of the coating
makes its wear-resistant service behavior significantly different from that of
the same kind of block. The most important feature is the influence of load.
Under low stress service conditions, the thermal spraying coating shows good
wear resistance, but when the stress is higher than a certain level, due to the
accelerated propagation of cracks along the interface of deposited particles,
the wear is accelerated, especially under the conditions of erosion,
cavitation, fretting, fatigue and abrasive wear, the potential of the coating
material can only play 10% - 30%. Therefore, it is an important way to make
full use of the material potential, improve the service efficiency of thermal
spraying coating and save resources and energy to establish the basic research
on the systematic relationship between the coating structure, performance,
service conditions and service efficiency, and to establish the database to
guide the design and use of wear-resistant coating.