porosity of coatings

Posted by Alexei Goncharov ( on 09:47:11 31/01/06

Dear Mr. Gordon,
In my last message 11.01.06 I touch on an question of adhesion of coatings (1). Now I present my point of view on porosity (2). I will be very grateful to everybody for any comments.
2. Succeeding your idea that the main factor of porosity of coatings is rapid solidification of melting material of particles I d like to propose tow more major ways of realization of that factor. In addition to mentioned in your review low impact energy and shadowing effects it has to be regarded: high concentration of spraying material (A) and wide range of size of the particles (B).
A. Clustering of the particles catches bubbles of environmental gas at the substrate and heats them during solidification. Formation of cavity or gas leakage through the coating just build up will be the result of heat exchange. The gas leakage leads to creation of open pores which will reproduce by the same manner crossing the thickness of the coating. Layering of coatings can be regarded as a consequence of open porosity if the substrate is sprayed by several passage of the torch (gun) along the component. The gas filled open pores acts in this case as air damper hindering to valuable contact of solidifying material and sprayed before. It worth to notice here that contamination of the surface plays the same role as a gas being in open pores. Layering will increase if the coating is significantly cooled during the period between tow passages. Obviously it happens because of the gas increase inside the coating. Some years ago I suggested the method of thickness growing (in the case, for instance, if thickness is not enough for grinding) which consists of three phases: cleaning, heating and additional spraying. The destination of each phase is clear keeping in mind said above.
B. During interaction of supersonic gas flow with the substrate its turnover from normal move to parallel to the surface is realized in shock wave standing near by. The vector of particles velocity changes from semi normal direction to that inclined to the substrate. The less is the size of particles, the more is an angular displacement. So, in polydispersal divergent flow the situation when the products of burning (or products of detonation) with small particles inside streamlines the surface with solidified massive particles is quite realistic. As a result of streamlining the growth of inhomoginities takes place. They achieve the height 0.1mm for alumina and 0.3mm for titania (0.5mm total thickness) detonation coatings on plane surface. Mentioned above shadowing effects will occurs on the inhomoginities giving arches, niches and terraces which can develop into pores in further spraying. Best regards,
Alexei Goncharov

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