Page 8 - дисертація Головчук

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its cavities between the particles are filled with air, which intensively oxidizes the

melt drops inside the PW and at their ends during the spraying of coating.

By phase analysis it was established that the matrix phase of the coating

structure made of 50Cr6Mn2MoSi PW was martensite with a small amount of Fe 3O 4

iron oxide while the coating of 250Cr21WVMnSi PW showed residual austenite

with a small amount of martensite, (Cr 2O 3 + Fe 3O 4) chromium and iron oxides. The

rapid crystallization of drops on the steel base provided the formation of a large

amount of residual austenite in the coating structure. The matrix phase of the coating

made of 50CrNi25MnSi PW was martensite, which contained nanosize inclusions

of complex-alloyed FeCrB boride and a small amount of Fe 3O 4 iron oxide. In this

case, the rapid crystallization of drops led to the formation of a large number of

boride crystallization centers, which, due to the lack of time, managed to grow only

to nanosizes.

The porosity of the coatings of all alloying systems increases with the lamellae

thickness in their structure, due to the use of PW of larger diameter. This pattern is

caused by an increase of the drop size that formed the coating. Drops of larger

diameter during the impact on the surface are sprayed much more intensely, which

leads to the formation of microcavities between the lamellae.

It was found that the average microhardness of coatings (martensitic class and

boride-containing martensitic one) made of 50Cr6Mn2MoSi PW

and 50CrNi25MnSi PW increases with lamellae thickness in their structure, which

was especially evident when PW of larger diameter were used. This was explained

by the evaporation and burnout of alloying elements from drops, especially carbon,

during their spraying. As the diameter of the drops increased, the amount of burned

out carbon decreased. And the higher carbon content in the martensitic matrix phase

of the coatings caused the increase in microhardness.

At the same time, the larger diameter of 250Cr21WVMnSi PW coatings

(austenite of carbide class) were used, the opposite tendency of lamellar thickness

influence on microhardness was revealed. This results from phase composition of

these coatings, in particular, the presence of a significant amount of residual