WC/Co Coating Microstructure

[kriskros]

Hello All

Again - old subject in the forum but new for me. We are recently working on develop HVOF coating with customer approved powders - Metco 5847 and Praxair WC-731-1. Tensile, bond strength, microstructure and micro hardness required. We couldn't get required min 950HV0,3 hardness with Praxair powder in any configuration, so right we are testing Metco 5847. We get tensile and micro hardness, porosity also good but "clusters" in coating appears, which customer is not happy about. What are those clusters might be and how can we eliminate it from coating.

Thank You very much for help.



Admin Edit - This is a new thread split from http://www.gordonengland.co.uk/sef/Threa...rs-by-HVOF and for continuity a few quotes from earlier posts:

Hi,

I cant use Plasma.. Their product specification clearly specifies HVOF!! So option of using another process is ruled out

My colleagues suggested that the higher hardness range is due to over heating of the sprayed particles. So to counter this i will have reduce the fuel and oxygen input. Am i goin in the right direction here??!

Also, if i am to reduce it...How much shud i reduce it...1-2% or should it be as high as 5-10%

Whatever the case, I have no doubts that i will have to conduct all tests again!

Hi K09

My colleagues suggested that the higher hardness range is due to over heating of the sprayed particles.

This is certainly a possibility. Production of metastable phases can harden and make the supporting matrix more brittle. Increased hardness and reduced ductility of the CoCr matrix can show itself sometimes (not always) as a harder coating as a whole, this condition tends to have negative effects on durability and wear resistance of the coating.

Do you have any (other than hardness) indications towards over-heating and production of unwanted phases? Hardness alone is not a good indicator.

So to counter this i will have reduce the fuel and oxygen input. Am i goin in the right direction here??!

Also, if i am to reduce it...How much shud i reduce it...1-2% or should it be as high as 5-10%

This is difficult to answer, as I don't know your base line conditions. Also, you need to be sure that the higher coating hardness (according to your customer specification) that you are achieving is due to particle over-heating and is not just the natural results of good process/coating.

You could manipulate parameters to give softer coatings and make people happy that specifications are being met, but without knowing exactly what is going on inside the coating, you could end up with poorer coating. Just seems possibly a retrograde step.

Hi Gordon,

Can microstructure images help in making any conclusions in this matter?

I have a report which includes results of Microstructure, Bend test, Bond Strength and Hardness test. All results are positive. The testing facility infact, in a telephone conversation also commented that the coating appeared better than Detonation spray coatings!

If microstructure can help, i will forward you a copy of the image for analysis

Hi K09

It can be difficult to see the more subtle effects in normal metallography. In more extreme cases, primary WC phases may appear reduced and matrix increased. A very qualitative method I have used is to etch samples with a modified Murakami's reagent (basically a diluted form). The ideal coating, one with no other phases than WC and CoCr matrix will show no or little etching effects. Unwanted phases tend to be vigorously etched. It is not a pretty etch and can spoil samples if you need to archive or process further, so I would advise making a separate specimen/mount for this purpose or at least leaving this test to the very last.

[Gordon]

Hi Kriskros

The clusters could be from contamination (poorly cleaned powder feeder, dirty spray environment, nozzle spits etc.). Overcome by better house keeping.

More likely I think, a natural variation in composition between individual particles within your powder. The cluster areas being Co rich and lacking carbides. This is fairly common in these types of coating. Not a lot can be done other than trying another powder with a more homogeneous particle composition.

[kriskros]

Hi Kriskros

The clusters could be from contamination (poorly cleaned powder feeder, dirty spray environment, nozzle spits etc.). Overcome by better house keeping.

More likely I think, a natural variation in composition between individual particles within your powder. The cluster areas being Co rich and lacking carbides. This is fairly common in these types of coating. Not a lot can be done other than trying another powder with a more homogeneous particle composition.

Hello Gordon

Thank You for reply. As I wrote before we can use Praxair powder instead of Metco but we cannot achieve required hardness.
As for Co clusters we had only local indications when we coat with 65kW power. When we are going down with power to 52-58kW to make parameters more let's say more economic (fuel usage) we get more clusters.

What other than Murakamis reagent can be used to reveal carbides?

[Gordon]

Hi Kriskros

What other than Murakamis reagent can be used to reveal carbides?

I would not use Murakamis reagent to reveal carbides. Looking at your photo, polishing relief effect looks very good for revealing carbides

A very qualitative method I have used is to etch samples with a modified Murakami's reagent (basically a diluted form). The ideal coating, one with no other phases than WC and CoCr matrix will show no or little etching effects. Unwanted phases tend to be vigorously etched. It is not a pretty etch and can spoil samples if you need to archive or process further, so I would advise making a separate specimen/mount for this purpose or at least leaving this test to the very last.

[kriskros]

I would not use Murakamis reagent to reveal carbides. Looking at your photo, polishing relief effect looks very good for revealing carbides

Yes - in this case I have used SiO2 suspension for final polishing which pH causes a slight etching effect. But I want to make sure that we have right microstructure. I followed your advice, purchased Murakami's reagent and etch for about 20s. Below images that i have received. Can you help me with identification of phases/constituents? I would be very grateful

[Gordon]

Hi kriskros

In most cases what we are after is a coating that is made up of the same phases as the powder - WC in a relatively pure ductile Co matrix.

During heating a point can be reached where WC and Co will go into solution with one-another and in more extreme conditions carbon will be lost to oxidation. On rapid cooling during coating formation any solid solution of WC and Co will tend not to revert back to separate phases of WC and Co but form metastable phases, which can make the coating binder phases brittle and reduce the amount of WC.

The very dark areas of your etch indicate areas of metastable phases (in extreme cases it will also etch tungsten if decarburisation/oxidation has been severe. It should not effect WC particles or pure Co phases. As you can see the etch is not pretty and gives a lot of staining. You will need to experiment with the etch to get a feel for it. I use to dilute the etch with water to make it less aggressive. A very good WC/Co coating will show very little etching and indicates that the coating was applied with minimal heating to the particles.

[Gordon]

Hi kriskros

I think I will spit our posts from "Micro Hardness (Vickers) by HVOF" thread to form a separate thread titled "WC/Co Coating Microstructure" as I think we have gone a little off-topic and it will be lost in that large thread.

[kriskros]

Hi kriskros

I think I will spit our posts from "Micro Hardness (Vickers) by HVOF" thread to form a separate thread titled "WC/Co Coating Microstructure" as I think we have gone a little off-topic and it will be lost in that large thread.

Hello Gordon

Thank You for fast and detail respond. I will go on with etching -see what I will managed to get.
As for spraying parameters - at this moment we spray with 62kW with feed rate 20g/min. What do you think about this ratios?

[Rafal]

Hi Kriskros

The clusters could be from contamination (poorly cleaned powder feeder, dirty spray environment, nozzle spits etc.). Overcome by better house keeping.

More likely I think, a natural variation in composition between individual particles within your powder. The cluster areas being Co rich and lacking carbides. This is fairly common in these types of coating. Not a lot can be done other than trying another powder with a more homogeneous particle composition.

Hello Gordon

Thank You for reply. As I wrote before we can use Praxair powder instead of Metco but we cannot achieve required hardness.
As for Co clusters we had only local indications when we coat with 65kW power. When we are going down with power to 52-58kW to make parameters more let's say more economic (fuel usage) we get more clusters.

What other than Murakamis reagent can be used to reveal carbides?

Hi Kriskros,

I'm not sure what spray gun you are using if it's radial or axial powder injection. But one more thing worth checking would be your carrier gas settings, perhaps you are experiencing under or over-injection leading to powder sticking in the nozzle and eventually causing spiting of material with highly decomposed WC.
I would suggest inspecting your spray geometry and checking condition of injectors.

Kind regards

[jonini]

Hi Kriskros,

For you to get more hardness on wc-731-1 by HVOF, try to adjust your gun distance 15" away from the sample, then do the micro sample preparation also try to review/change your grinding/polishing technique I think this is one of the reason why you are having poor structure. As I used to do micro sample on this powder I'm getting <1% porosity, clusters are not seen and the hardness I'm getting is around 1100-1250 HV0.30.

HOPE THIS HELP,
Jonini

[karlo]

Hi Jonini,

What preparation method are you using? I often find the 1micron stage causes "pluck out" of carbide particles that can sometimes be mistaken for porosity.

Karlo

[jonini]

Hi Jonini,

What preparation method are you using? I often find the 1micron stage causes "pluck out" of carbide particles that can sometimes be mistaken for porosity.

Karlo

Hi Karlo,

Yes we will get pluck out, if you will only use the grinding/polishing machine as your final stage, try this after using the most fine grit or even diamond suspension, use diamond belts (adhesive at the back) having 6,3,0.5microns (sorry its not 1micron) place it on a glass or something flat w/c water flows and have it polish it manually (push and pull direction), you will noticed the difference and that the so called pluck out will be lessen. Give me info if this one works.

Hope this help,
Jonini

[karlo]

Thanks Jonini,
I am achieving great results if i just miss out the 1 micron stage but sometimes some scratches aren't removed when going from the 3micron to the 0.5micron. I am using semi auto prep (for repeatability).

prep is:

diamond grinding disk then 9 micron, 3micron, 1micron and 0.5 final polish.

[sreenuvundela]

Hi

My suggestion is better to ask powder manufacturers (Praxair and Metco) about suitable process parameters on HVOF Gun to coat their powders and they forward technical data sheet with coating characteristics.

Please carry EDS and Phase analysis of coating so that you can know what elements (or) Phases are present at clusters. once you know what is there in cluster then it will be very easy to eliminate the same.


Regards,

SREENIVAS