What effect can the residual stress of a thermal spray coating have on the coating

Thermal spraying is a very good way of anti-corrosion treatment, with the improvement of China's scientific and technological level, at present, China also attaches great importance to anti-corrosion treatment of steel components, and now more is used thermal spraying anti-corrosion treatment, because this layer can isolate the steel matrix from the external environment, so as to play the role of anti-corrosion, so what are the advantages of thermal spraying anti-corrosion treatment? Thermal spraying anti-corrosion treatment is only a method of anti-corrosion treatment, but this method is more commonly used, thermal spraying anti-corrosion treatment can achieve the protection of steel, can reduce the maintenance of steel, and can also be based on the thickness of this coating out of its protection life, the main thing is that this anti-corrosion treatment can reduce environmental pollution, the cost of the later stage is much smaller than other coating methods, so for users will of course choose this method. ...

1) Coating structure characteristics:

1) Layered structure of coating: It is formed by a large number of saucer-shaped particles parallel to each other;

2) The porous structure of the coating: the time of particle collision, deformation and condensation is extremely short;

3) the presence of oxide inclusions in the coating: their amount depends on the heat source, material and spraying conditions;

4) Anisotropy of coatings: layered structure anisotropy;

5) Coating residual stress;

6) The structure of the coating is an accumulation layer of a series of sheet materials separated by fine oxides and pores;

7) After the coating is properly treated, its structure will change. For example, the remelting of the coating eliminates oxide inclusions and porosity in the coating, the layered structure becomes a homogeneous structure, and the bonding state of the coating to the substrate surface is also changed.

2) The binding mechanism of the coating.

The bonding of the coating includes the bonding of the coating to the surface of the substrate and the bonding of particles to particles in the coating. The bonding strength of the former is called the bonding force; The binding strength of the latter is called cohesion.

1) Mechanical bonding: When the spraying particles in the molten state collide with the basic surface, the deformed particles and the concave and convex rough surface of the substrate surface mechanically occlusive, this combination is called the "anchor effect", for example, when plasma or oxyacetylene sprays ceramic materials, the combination of coating and substrate belongs to mechanical bonding.

2) Physical bonding: The adhesion of the coating to the surface of the substrate is caused by the van der Waals force (a weak electrical force of attraction between neutral molecules or atoms).

3) Chemical or micrometallurgical bonding: When the surface of the matrix is melted by high-temperature particles and reacts with them to form intermetallic compounds, the combination of its coating and the surface of the matrix is called chemical bonding. When the sprayed particles diffuse with the atoms on the surface of the matrix, it is called micrometallurgical bonding.

In general, the bonding of the coating to the substrate surface is dominated by mechanical bonding.

Perhaps a more accurate description of this phenomenon is that the residual stress in the coating decreases as the layer thickness increases.

The reason is that this stress is due to the difference in the coefficient of thermal expansion and elastic modulus of the substrate and the coating, and in order to maintain the bond between the two materials at the cross-section after the temperature change, the substrate and the coating must each produce a force (or pressure) and a pressure (or tension), and the absolute value of the two forces is equal. Internal stress is this force divided by the cross-sectional area of the material.

The cross-sectional area of the substrate is much larger than that of the coating, so the internal stress of the coating is very high and gradually decreases with the increase of coating thickness.

What is the thermal spray coating structure, so the performance of the coating is directional.

Thermal spraying technology: repair the wear and strain of the main shaft, crankshaft and all shafts of various equipment journals, bearing gears, oil seal gears, keyways and other defects. Anti-cavitation and anti-wear treatment of "boiler four tubes" (water wall tube, superheater tube, preheater tube and economizer tube) spraying protection, circulating vulcanized bed boiler, membrane wall thermal spraying protection, fan blades, wire drawing tower wheels, wire pickle cylinders, wind guide blades of water turbines, labyrinth rings of water turbine blades and other components.

Ceramic-coated piston rod and hydraulic cylinder for large hydraulic cylinders and complete systems for position measurement, ceramic coating of reciprocating pump plungers in chemical pumps, surface spraying of mechanical seal rings and bushings, spraying of ceramic butterfly valve sealing surfaces instead of inlay ring structures, spraying ceramics of high-parameter ball valves, surface thermal spraying protection of key components of equipment used in oil and gas exploration and drilling processes such as drill bits, shafts, bushings, grouting pumps, etc. In the plastics industry, plastic extruder screws, plastic shredder nozzles, plastic film production rollers. In the metallurgical industry, advanced coatings such as continuous annealing furnace rolls, tensioning rollers and deflector rollers, self-cleaning furnace rollers, sinking rollers for hot-dip galvanizing, and stabilizing rollers.

Surface strengthening coating of hot-rolled seamless pipe head, copper alloy hot extrusion die strengthening coating. In the chemical fiber industry, all kinds of groove rollers, spindle cups, drafting rollers, guide wire rollers, surface ceramic coatings, anti-corrosion and anti-wear protection on the surface of papermaking dryers, glazing calender singles, pulp vacuum suction box boards, cast iron printing roller surface spraying protection in the printing industry, ceramic anilox rollers, corona rollers.

Characteristics of coating microstructure :

Formed by a large number of parallel saucer-shaped particles bonded to each other, the time for particle collision, deformation and condensation is extremely short, and the amount of oxide inclusions in the coating depends on the heat source, material and spraying conditions. Laminar structure to anisotropic, the structure of the coating is an accumulation of a series of flake materials separated by fine oxides and pores, and the structure of the coating changes after appropriate treatment.

Bonding Mechanism of Coating:

In general, the bonding of the coating to the substrate surface is dominated by mechanical bonding. The combination of coating includes the combination of coating and substrate surface and the combination of particles and particles in the coating, when the spraying particles in the mechanical molten state collide with the basic surface, the deformed particles and the concave-convex rough surface of the substrate surface mechanically occlude, for example, when plasma or oxyacetylene sprays ceramic materials, the combination of coating and substrate belongs to mechanical bonding. Physical Bonding:

The adhesion of the coating to the surface of the substrate is determined by the van der Waals force.

Thermal spraying inevitably has defects such as unmelted spherical particles, incomplete inter-particle sintering, porosity, microcracks and low interfacial bonding strength in the prepared coating, which seriously affects the performance of the coating, so the thermal spraying tool coating needs post-treatment, and the commonly used processes include flame remelting, laser remelting and furnace remelting.

HVOF spraying wear-resistant coating is supersonic spraying tungsten carbide coating, with high hardness, high bonding strength, good wear resistance, taking Beijing Naimer company as an example, using JP8000 equipment spraying, tungsten carbide coating thickness can reach more than 1mm.

There are a lot of dense ones, generally it should be a problem on the surface of the workpiece (the sprayed product), there are some slight dust or product chips on the surface of the workpiece, which are mixed in the coating during the spraying process, resulting in black spots (very many, dense).

What material do you spray on?