What Are The Common Quality Problems of Tungsten Carbide Coating

During the preparation and use of tungsten carbide coating, the following types of quality problems often occur:

1. Poor combination of coating and substrate

1, manifestation: the coating appears peeling, peeling phenomenon, in the impact of external force, friction or thermal stress, the coating is easy to separate from the substrate surface.

2. Reason analysis:

Improper surface pretreatment: there are impurities such as oil, oxide and rust on the surface of the substrate, which are not cleaned up, or the degree of sandblasting is not enough, resulting in insufficient mechanical biting force between the coating and the substrate, affecting the binding strength.

Large difference in thermal expansion coefficient: the thermal expansion coefficient of tungsten carbide coating and the base material is very different. In the heating and cooling stage of the spraying process, due to the different degrees of thermal expansion and contraction, large thermal stress is generated, thus destroying the combination of coating and base material.

Unreasonable spraying process parameters: such as improper distance between the spray gun and the workpiece, too fast or too slow spraying speed, insufficient powder heating, etc., will make the coating and the matrix are not firmly combined.

Second, the coating hardness does not meet the requirements

1. Manifestation: the coating hardness is lower or higher than the design requirements. Lack of hardness will lead to poor wear resistance of the coating, easy to be worn; If the hardness is too high, it may increase the brittleness of the coating, decrease the impact resistance, and easy to crack.

2. Reason analysis:

Powder quality problems: tungsten carbide powder purity is not enough, particle size distribution is unreasonable or contains impurities, will affect the hardness of the coating. For example, excessive impurities in the powder may hinder the normal growth and dense accumulation of tungsten carbide crystals, reducing the hardness of the coating.

Spraying process parameters: Different spraying process parameters have a significant impact on coating hardness. For example, when flame spraying, the flame temperature and gun moving speed are inappropriate, which may lead to insufficient powder melting, loose coating structure and reduced hardness; In plasma spraying, if the plasma energy is too high, the tungsten carbide may be decomposed or over-sintered, changing the coating structure and affecting the hardness.

Improper post-treatment: The heat treatment process after spraying is incorrect, such as the heat treatment temperature is too high or the time is too long, which may change the coating organization and change the hardness.

Third, there are pores in the coating

1. Manifestation: There are holes inside or on the surface of the coating, which can be found by metallographic microscope or non-destructive testing means. The existence of pores will reduce the density of the coating and affect the properties of the coating.

Reason analysis:

2, powder factor: tungsten carbide powder moisture, containing water, water evaporation in the spray process to form pores; The powder is agglomerated or the particle size is not uniform, and the agglomerated powder cannot be fully dispersed and melted during spraying, and the air is wrapped inside to form pores.

Spraying process: the powder feed speed is too fast, the powder can not be fully melted in the spray flame flow and is sprayed to the surface of the matrix, forming pores after accumulation; The distance between the spray gun and the workpiece is too far, and the powder cools too fast during flight and fails to melt completely, which also leads to the formation of pores. In addition, the spraying environment contains more dust or impurities, which may also form pores when mixed into the coating.

Crack in coating

3. Manifestation: Cracks visible to the naked eye or visible with the help of a microscope appear on the surface of the coating, cracks may run through the coating or exist only in local areas of the coating.

Reason analysis:

4, thermal stress concentration: in the spraying process, the coating is rapidly heated and cooled, resulting in large thermal stress. If the coefficient of thermal expansion between the coating and the substrate is very different, or the thickness of the coating is not uniform, and the thermal stress distribution is uneven, it is easy to crack in the stress concentration.

5, coating brittleness: due to poor powder quality, improper spraying process and other reasons, resulting in the internal structure of the coating is not ideal, brittle increase. Under the influence of external force or thermal cycling, the coating can not withstand the stress change, resulting in cracks.

Coating thickness problem: Too thick coating will increase the accumulation of internal stress, reduce the deformation resistance of the coating, easy to crack.

Fourth, the coating thickness is not uniform

1, the form of performance: the thickness of the coating in different positions on the substrate surface is obviously different, some places are too thick, some places are too thin, affecting the overall protective effect of the coating.

2. Reason analysis:

Spray gun operation problems: the distance and Angle between the spray gun and the workpiece surface, as well as the moving speed of the spray gun are unstable, resulting in uneven deposition of powder on the substrate surface. For example, the area where the spray gun is closer to the workpiece has more powder deposition and thicker coating; Farther away, the powder deposition is less and the coating is thinner.

The shape of the workpiece is complex: for the workpiece with a complex shape, such as a groove, corner and other structures, the powder is affected by the air flow and the geometry of the workpiece during the spray process, and the deposition situation is different in different parts, which is easy to cause the coating thickness is not uniform.