interface oxide
03-09-2009, 07:32 PM,
#1
interface oxide
Hello everybody, I am codep. I'm a new member of the surface engineering forum.
Nowadays I have some trouble with my test coupons which are rejected due to thin continuous oxide layer between the coating and substrate. I made several test coupons belong to rene 80, t400 and silveralloy, and all of them were rejected by same reason.
I made some arrangements on grit blasting but nothing changed. I think this is not related with spray parameters, just related with surface preparation, preheating or cooling air. I am getting suspicious of shop air used for auxiliary cooling. Does any contaminant in air like water or oil cause continuous thin oxide layer between the substrate and coating?

Or does any faulty or improper specimen preparation for metalographic evaluation cause interface oxides?

Note: I used metco 3mb and 9mb for rene 80 and t400, Metco 14E wire gun for silveralloy
Reply
03-09-2009, 08:36 PM,
#2
RE: interface oxide
Hi codep

Sign0016 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.Happy0193).

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.
Reply
03-09-2009, 11:05 PM,
#3
RE: interface oxide
The oxide layer is only (interface oxide) between the substrate and coating, And other than the interface oxides, all criteria are accepted. Just the problem is interface oxides. And I have this problem with different types of coating material (rene 80, t-400 and silveralloy). I am checking the substrate temperature during the coating process and it is not more than 100 C.
I sprayed t-400 with Ni-Al (metco 450) bond coat and without bond coat, nothing changed.
The auxiliary cooling air are on during preheating, and next trial I will turn them off while preheating, after starting of spraying I will turn them on.
Reply
03-10-2009, 10:28 AM,
#4
RE: interface oxide
(03-09-2009, 11:05 PM)codep Wrote: The oxide layer is only (interface oxide) between the substrate and coating, And other than the interface oxides, all criteria are accepted. Just the problem is interface oxides. And I have this problem with different types of coating material (rene 80, t-400 and silveralloy). I am checking the substrate temperature during the coating process and it is not more than 100 C.
I sprayed t-400 with Ni-Al (metco 450) bond coat and without bond coat, nothing changed.
The auxiliary cooling air are on during preheating, and next trial I will turn them off while preheating, after starting of spraying I will turn them on.

Can we have any microphaphy of this?
Reply
03-11-2009, 09:19 PM, (This post was last modified: 03-11-2009, 09:31 PM by codep.)
#5
RE: interface oxide
[/quote]

Can we have any microphaphy of this?
[/quote]

Here are some photos of T400 and rene 80 at different magnifications

   
   
   
   
   
[/quote]

Here are some photos of T400 and rene 80 at different magnifications

[/quote]

   

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?
Reply
03-12-2009, 02:53 PM,
#6
RE: interface oxide

Can we have any microphaphy of this?
[/quote]

Here are some photos of T400 and rene 80 at different magnifications


[/quote]

Here are some photos of T400 and rene 80 at different magnifications

[/quote]


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?
[/quote]

Hello


Ok I see, this is not easy to said. It can be bad adherence or pollution or oxydation.
Do you have good surface preparation? Do you blow the surface after grit blasting?
Do you know your adhesive strength?

You can try to add air jet on the torch if you have not, or try to preheat substrate a lot before spray.

Polishing more than 10 minutes is not a problem.
Reply
03-12-2009, 08:26 PM,
#7
RE: interface oxide
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.
Reply
03-13-2009, 10:02 AM,
#8
RE: interface oxide
(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.
Reply
03-14-2009, 09:11 AM,
#9
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
Reply
03-16-2009, 03:00 PM,
#10
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 Happy0193). 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 Shy
Reply
03-18-2009, 06:11 AM,
#11
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 Happy0193). 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 Shy

Thanks a lot, GordonSmile
Reply




Possibly Related Threads…
Thread Author Replies Views Last Post
  NiCrAlY interface separation edrazee 10 1,122 03-05-2020, 01:10 AM
Last Post: edrazee
  interface problem marzal 14 7,220 09-25-2015, 11:00 AM
Last Post: Joris Kraak
  interface contamination Brent 5 6,105 04-06-2012, 06:04 PM
Last Post: Brent
  How to decrease interface contamination Zhiguo 2 3,962 02-24-2012, 07:04 AM
Last Post: Zhiguo
  Aluminum oxide+titanium oxide schumi 11 11,371 04-19-2011, 11:15 PM
Last Post: loriolo
  Surface treatement for high-friction interface with human hair davidHMC 2 4,324 06-22-2009, 07:47 PM
Last Post: Gordon
  Interface separation in Copper Nickel Indium coating Donald 10 15,232 04-26-2009, 02:30 AM
Last Post: Donald
  Interface separation: Process or Bakelite? gejohn 2 5,430 02-05-2008, 12:18 PM
Last Post: Gordon



Users browsing this thread: 1 Guest(s)





Surface Engineering Forum Sponsor - Alphatek Hyperformance Coatings Ltd