Reasons for spalling, delamination and porosity of thermal spray coatings

       The coating peels off from the substrate, most of which occur during machining or use. Considering the spraying process, the main reasons for peeling are:



    (1) The oxides, oil stains, rust spots, dust or moisture on the surface of the workpiece are not removed;

    (2) The preheating temperature of the workpiece is insufficient;

    (3) The bottom layer is too thin and is not firmly combined with the workpiece substrate;

    (4) The coating is too thick and uneven, resulting in a large internal stress of the coating. When the internal stress of the coating exceeds the strength of the coating itself, the coating will crack;

    (5) When spraying the inner hole, the cooling shrinkage of the coating is easy to cause peeling, so spray as thin as possible to reduce the shrinkage rate during cooling. In addition, cooling too fast after spraying can also lead to peeling of the coating.

    Reasons for stratification:

    The interlayer separation of the coating is caused by the fact that the time interval of each spraying is too long, and the two layers before and after the coating are not firmly combined due to the adsorption of gas and dust or the decrease in temperature on the first sprayed coating; If the coating is too thick and the internal stress is too large, it can also lead to interlayer separation.

    Causes of looseness:

    When the equipment is fixed, the causes of loose coating are: the spraying distance is too large, the kinetic energy of powder particles is small, the plasticity is poor, and the oxidation is serious; the powder feeding amount is too large; the heat energy of the flame is not concentrated; the gas contains excessive moisture ; The injection angle is too large to deviate from the vertical direction, etc.