03-09-2009, 08:36 PM,
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Gordon
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RE: interface oxide
Hi codep
 to the Surface Engineering Forum.
I'm not really sure I understand your meaning of "thin continuous oxide layer between the coating and substrate".
Surface preparation should really only effect grit/dust substrate interface contamination or the lack of interface cohesion.
Over heating during preheating and spraying, contaminated air etc are certainly not going to help the coating and may in fact produce a very thin continuous oxide film at the interface, but I would be surprised if your laboratory metallography could resolve or see this layer (unless you are really cooking it to produce obvious blue/black scale.  ).
If it is coating oxide at the interface, then this will be spraying parameter based and if causing a problem I would expect the whole coating to be effected not just the interface.
I suspect the problem to be an apparent/perceived gap or lack of cohesion between substrate and coating seen during metallography. Assuming metallographer interpretation is correct, then surface preparation, surface contamination are probably the first things to investigate, but possibly not the only ones.
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03-11-2009, 09:19 PM,
(This post was last modified: 03-11-2009, 09:31 PM by codep.)
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codep
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RE: interface oxide
[/quote]
Can we have any microphaphy of this?
[/quote]
Here are some photos of T400 and rene 80 at different magnifications
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Here are some photos of T400 and rene 80 at different magnifications
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Also more photos from T400 @ 100X, 200X and 500X
Exactly I don't know what is the dark layer between the substrate and coating. It seems like oxide layer.
One of my colleague advised me not to polish more than 10 minutes during the specimen preparation for metalographic evaluation. Polishing more than 10 minutes can cause interface separation?
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03-14-2009, 09:11 AM,
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codep
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RE: interface oxide
(03-13-2009, 10:02 AM)landemarre Wrote: (03-12-2009, 08:26 PM)codep Wrote: We take a lot of care on blasting, and we blow the surface with shop air.
We have air jets on the plasma gun but especially we are not using them. We
prefer auxiliary air. Because the air jets near the flame makes lots of oxide and cracks between the layers in the coating.
Also we preheat the parts before spraying, but after preheating the operator may open the auxiliary air too early.
Ok so you have not enough preheat substrate before spray, you could try open auxiliary air only after first pass.
Hello Landemarre;
What should be the surface temperature of part/coupon during preheating to achieve the required (or maximum) cohesion by eliminating the interface oxide
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03-16-2009, 03:00 PM,
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Gordon
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RE: interface oxide
Hi Codep
Quote:What should be the surface temperature of part/coupon during preheating to achieve the required (or maximum) cohesion by eliminating the interface oxide
"The higher the substrate temperature, the higher the bond strength" would be true if not complicated by surface de-activation/oxidation/differential coefficients of expansion. So we need to compromise. The first benefit of preheat is to avoid the problem of water condensation on substrate. Pre-expansion of the substrate can sometimes help counteract tensile stress build-up in coating. Bonding is more efficient at higher temperatures, unless hindered by oxidation. Generally, for ferrous and nickel based substrates a preheat of 90-120 C is normally recommended. Substrates like aluminium and magnesium alloys should not be directly preheated with flame or plasma on area to be sprayed, but low or moderate indirect preheat can be beneficial to avoid condensation. Ideally, temperature for coating and substrate during spaying should be as constant as possible (typically 200 C max), particularly avoiding cyclic extremes. Now there are always a few cases where much higher or lower temperature can be used with good effect, but for most the above is good practice.
Preheating will not really effect the apparent (visual by normal metallography) interface oxide. Substrate oxide films will be too thin to resolve easily. The actual oxides that you see at the interface are just the natural oxides produced from the coating, or alumina grit left over from blasting. Preheating and temperature control during spraying though may have some effect on the cohesion properties along the interface, but will show as bonding faults ie gap between substrate and coating.
Metallographic specimen preparation technique can have a big effect on what is seen under the microscope. Poor techniques, over polishing etc. can make things look a lot worse than they really are (in a few cases it can make things look better than they really are  ). If you are getting good and in specification tensile bond strength test results using same procedure, then your interface properties can't be too bad
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03-18-2009, 06:11 AM,
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codep
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RE: interface oxide
(03-16-2009, 03:00 PM)Gordon Wrote: Hi Codep
Quote:What should be the surface temperature of part/coupon during preheating to achieve the required (or maximum) cohesion by eliminating the interface oxide
"The higher the substrate temperature, the higher the bond strength" would be true if not complicated by surface de-activation/oxidation/differential coefficients of expansion. So we need to compromise. The first benefit of preheat is to avoid the problem of water condensation on substrate. Pre-expansion of the substrate can sometimes help counteract tensile stress build-up in coating. Bonding is more efficient at higher temperatures, unless hindered by oxidation. Generally, for ferrous and nickel based substrates a preheat of 90-120 C is normally recommended. Substrates like aluminium and magnesium alloys should not be directly preheated with flame or plasma on area to be sprayed, but low or moderate indirect preheat can be beneficial to avoid condensation. Ideally, temperature for coating and substrate during spaying should be as constant as possible (typically 200 C max), particularly avoiding cyclic extremes. Now there are always a few cases where much higher or lower temperature can be used with good effect, but for most the above is good practice.
Preheating will not really effect the apparent (visual by normal metallography) interface oxide. Substrate oxide films will be too thin to resolve easily. The actual oxides that you see at the interface are just the natural oxides produced from the coating, or alumina grit left over from blasting. Preheating and temperature control during spraying though may have some effect on the cohesion properties along the interface, but will show as bonding faults ie gap between substrate and coating.
Metallographic specimen preparation technique can have a big effect on what is seen under the microscope. Poor techniques, over polishing etc. can make things look a lot worse than they really are (in a few cases it can make things look better than they really are ). If you are getting good and in specification tensile bond strength test results using same procedure, then your interface properties can't be too bad 
Thanks a lot, Gordon
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