CRC Thickness
03-12-2009, 03:58 PM,
#1
CRC Thickness
Dear all,

What is the maximum CRC coating thickness in practise?

Thanks in advance

HONG
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03-13-2009, 03:37 PM,
#2
RE: CRC Thickness
Hi HONG

Chromium carbide/nickel chromium coating thickness limitations will depend very much on specific powder used, process equipment type, parameters and substrate geometry.

Typically (flat or OD's), plasma sprayed will be 0.010" - 0.025" (0.25 - 0.4 mm), HVOF will be 0.025" - 0.050" (0.4 - 1.25 mm) with maybe a few going thicker. Generally, coarser grade powders will have higher thickness limit than fine powders.
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04-15-2009, 09:32 PM, (This post was last modified: 04-15-2009, 09:36 PM by mehongbo.)
#3
RE: CRC Thickness-A new question
Dear Gordon,
I first of all appreciate the input, Gordon.
But a new question for this old thread

Why hvof should go thicker than aps? Does it generally apply to metallic coating as well (understand depends on type of material)? Although aps introduce more thermal stress to coating and base metal, much higher % porosity plus oxide should somewhat level off this stress. On the other hand, hvof at least semi-melts the powdered material which actually results in not too significantly lower thermal stress comapred to aps. Much higher density from the hvof high dynamic engery is another plus to limit the stress relief. Could you explain?

Thanks
HongBo

(03-13-2009, 03:37 PM)Gordon Wrote: Hi HONG

Chromium carbide/nickel chromium coating thickness limitations will depend very much on specific powder used, process equipment type, parameters and substrate geometry.

Typically (flat or OD's), plasma sprayed will be 0.010" - 0.025" (0.25 - 0.4 mm), HVOF will be 0.025" - 0.050" (0.4 - 1.25 mm) with maybe a few going thicker. Generally, coarser grade powders will have higher thickness limit than fine powders.

Sorry to not complete
.......Then the overall internal stress in hvof should be higher than in aps, thus lower thickness.
Thanks
Hong

(04-15-2009, 09:32 PM)mehongbo Wrote: Dear Gordon,
I first of all appreciate the input, Gordon.
But a new question for this old thread

Why hvof should go thicker than aps? Does it generally apply to metallic coating as well (understand depends on type of material)? Although aps introduce more thermal stress to coating and base metal, much higher % porosity plus oxide should somewhat level off this stress. On the other hand, hvof at least semi-melts the powdered material which actually results in not too significantly lower thermal stress comapred to aps. Much higher density from the hvof high dynamic engery is another plus to limit the stress relief. Could you explain?

Thanks
HongBo

(03-13-2009, 03:37 PM)Gordon Wrote: Hi HONG

Chromium carbide/nickel chromium coating thickness limitations will depend very much on specific powder used, process equipment type, parameters and substrate geometry.

Typically (flat or OD's), plasma sprayed will be 0.010" - 0.025" (0.25 - 0.4 mm), HVOF will be 0.025" - 0.050" (0.4 - 1.25 mm) with maybe a few going thicker. Generally, coarser grade powders will have higher thickness limit than fine powders.
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04-16-2009, 01:21 AM,
#4
RE: CRC Thickness
Hi Hong

Quote:Why hvof should go thicker than aps? Does it generally apply to metallic coating as well (understand depends on type of material)? Although aps introduce more thermal stress to coating and base metal, much higher % porosity plus oxide should somewhat level off this stress. On the other hand, hvof at least semi-melts the powdered material which actually results in not too significantly lower thermal stress comapred to aps. Much higher density from the hvof high dynamic engery is another plus to limit the stress relief. Could you explain?

As you probably know coating thickness limitations are very much dictated by the levels of residual stress left in the coatings after spraying. Typically, tensile stresses build up in many coatings due to shrinkage and at a certain thickness these stresses can overcome the strength of the bond or even coating strength and fail.

Generally, coatings produced from coarser grade powders will have higher thickness limit than those from fine powders. This I believe is due to the coarser powders producing more porous open structure coatings. The formation of porosity tends to absorb some of these stresses. In fact some porosity is caused by particle shrinkage on cooling, so this probably one mechanism by which stresses are reduced.

Now this may first appear at odds with why HVOF coatings (in general terms) have higher thickness limitations than plasma spray. Well, I think this is primarily due to the higher particle impact energies and lower particle temperatures. Your comment (if I'm understanding correctly)
Quote:On the other hand, hvof at least semi-melts the powdered material which actually results in not too significantly lower thermal stress comapred to aps.
I would say this significantly reduces tensile stresses forming in the first place. Another effect, not too often mentioned, is the peening effect from solid unmolten particles inducing compressive stresses that counter the tensile stresses. Overall, HVOF coating tends to be a little more "solid state" in its formation than plasma - more cold working and production of compressive stress to counter the shrinkage tensile stress. Cold spray or cold gas-dynamic spraying process coatings will take this a step further, may be to the point where we get concerned about too much compressive stress Happy0193
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04-16-2009, 02:30 PM,
#5
RE: CRC Thickness
(04-16-2009, 01:21 AM)Gordon Wrote: Hi Hong

Quote:Why hvof should go thicker than aps? Does it generally apply to metallic coating as well (understand depends on type of material)? Although aps introduce more thermal stress to coating and base metal, much higher % porosity plus oxide should somewhat level off this stress. On the other hand, hvof at least semi-melts the powdered material which actually results in not too significantly lower thermal stress comapred to aps. Much higher density from the hvof high dynamic engery is another plus to limit the stress relief. Could you explain?

As you probably know coating thickness limitations are very much dictated by the levels of residual stress left in the coatings after spraying. Typically, tensile stresses build up in many coatings due to shrinkage and at a certain thickness these stresses can overcome the strength of the bond or even coating strength and fail.

Generally, coatings produced from coarser grade powders will have higher thickness limit than those from fine powders. This I believe is due to the coarser powders producing more porous open structure coatings. The formation of porosity tends to absorb some of these stresses. In fact some porosity is caused by particle shrinkage on cooling, so this probably one mechanism by which stresses are reduced.

Now this may first appear at odds with why HVOF coatings (in general terms) have higher thickness limitations than plasma spray. Well, I think this is primarily due to the higher particle impact energies and lower particle temperatures. Your comment (if I'm understanding correctly)
Quote:On the other hand, hvof at least semi-melts the powdered material which actually results in not too significantly lower thermal stress comapred to aps.
I would say this significantly reduces tensile stresses forming in the first place. Another effect, not too often mentioned, is the peening effect from solid unmolten particles inducing compressive stresses that counter the tensile stresses. Overall, HVOF coating tends to be a little more "solid state" in its formation than plasma - more cold working and production of compressive stress to counter the shrinkage tensile stress. Cold spray or cold gas-dynamic spraying process coatings will take this a step further, may be to the point where we get concerned about too much compressive stress Happy0193

Thanks, Gordon.
I try to summarize the comments:
1. in aps coating, tensile stress (much better than using thermal stress Ashamed0002) is higher than hvof coating.
2. in aps coating, porosity is the major factor to counter the tensile stress.
3. in hvof coating, higher-particle-peeing effect induced compressive stress is the major factor to counter the tensile stress.
4. in comparasion, overall residual stress of aps coating is higher than that of hvof coating.

I did not do any test. But No. 3 has convinced me of your previous input.
Nice forum

Hong
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