Posted by Alexei Goncharov (195.96.79.2) 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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