Micro Hardness (Vickers) by HVOF

[Alexangel1226]

Gordon,

This is Alex. First, i would like to comment on the new forum interface, much professional and better.

Regarding my question on microhardness sprayed by HVOF. I would like to know what are the factors that affect the microhardness of a HVOF coating. If my mirco metallorgraphy has pass the photo standard, what would possibly be the factors which fails the vickers hardness test. Would increasing the density of coating help? or oxide concentration? as oxide will increase the coating hardness(Rockwell).

To increase the density or microhardness, what should be the key parameters to adjust or look into? stand-off distance? HVOF fuel or O2 gas mass flow? or turntable speed?

Pls advice.Thanks

Regards,
Alex

[Alexangel1226]

Gordon,

For yur additional info, i am spraying T800 coating using HVOF, as it is a very fine powder, does powder type affect the spraying to achieve the microhardness? And further to the microhardness problem, oxide overmax condition is another problem which i face. Thanks in advance for your help.

Regards,
Alex

[Gordon]

Hi Alex and welcome to the Surface Engineering Forum.

Coating density and oxide content will effect hardness and the powder properties will also be a very significant factor. Generally, gas flow and/or oxygen/fuel ratio is probably the key parameter adjustment, followed by powder spray rate.

Are you trying to produce a standard type coating following recommended procedures or something special? If you are having problem with a standard coating, I would investigate why before attempting any parameter changes. Could you could provide us with a little more info on your problem.

[Alexangel1226]

Gordon,

Thanks for replying. Yes, I am spraying to comply to the aviation coating standard. It is quite stringent. In fact I have managed to established a set of parameter which could pass the metallorgraphy evaluation, hardness test as well as tensile test.

The problem happened just recently that had puzzled me. However, the probelm hads been resolved. Just to share with you the detail. I have performed a thorough check on the equipments and finally found out that the hopper pressure was not stable during the spraying.

Thus, the compactness of the coating build-up layers was affected and result in powder deposition rate inconsistency. This has probably led to a microhardness failure at certain area when the indentations were performed at different layers of the cross sectional coating.

The inconsistency of the feedrate due to hopper instablity, has also affected the oxide concentration and evenness. At some point when the hopper is feeding less powder, the powder tend to be overmelted or burnt and caused more oxidation while at some point of time the powder was fed at higher rate.

After we rectify the hopper problem, I used the same parameter and spray a coupon. The spraying was smooth and the result passed all result as expected. This round it has explained the logic of the phenomena.

Anyway, the challenges of HVOF process still lie ahead, tight control of the process, best practices, equipments, ..etc are the key to the solutions.

Thanks and regards,
Alex

[Gordon]

Hi Alex

Good to hear you have solved your problem. It is satisfying when you can find the source of a problem that makes logical sense. Powder feeding problems, barrel/air cap fouling, dropped and damage hardware, and propane/propylene gas supply problems, account for many of the problems I've encountered. Usually a good operator will quickly notice and rectify these problems. All to often I have seen people with similar problems, but rather than patiently look for faults in their system, start changing parameters and lead themselves up the garden path

[k09]

Hi,

Well, I know this thread is very old... but I have a relevant query.

According to metco spec sheet, WC (86-10-4) has microhardness of 1200 to 1400HV 0.3. I have made some samples and I found the microhardness to be 1300 to 1400 HV 0.3 which is fine but, the component that we are going to coat has a microhardness limit of 1300HV 0.3..

I have checked all the parts earlier and did not find any irregularities with the hardware. Assuming the system is functioning as it should be..kindly suggest how I can get the hardness in the required range by tweaking the oxygen fuel ratio.

Also, Is it true that the temperature of the component being sprayed also has an effect on the resulting hardness? for e.g. if we coating untill the component temperature reaches 180degree C, will this affect the hardness. Although I do not intend to but just needed to know. Generally, we donot let the temperature exceed 120 degree C and the deposition per pass is in the range of 10 to 25 microns/pass.

Regards
K09

[Gordon]

Hi K09

First, I must ask is 1300 HV/0.3 limit a maximum or minimum? Does it refer to an average of many readings or just one?

Strange one as 1300 HV/0.3 is pretty much the typical hardness for this type of coating!

I would not at this stage modify parameters to alter hardness, until you understand reasoning behind the limit.

[k09]

Hi Gordon,

Let me explain,

- The Metco Powder spec-sheet says that the microhardness range for WC(86-10-4) coating is 1200 to 1400 HV

- All samples sprayed by us had microhardness in the range 1350 to 1400 HV (based on atleast 20 Readings)

- Microhardness requirement on the component to be coated with WC(86-10-4) is 900(min) to 1300(max) HV (As per component specifications)

even though the difference is just 100 units and I am also pretty sure that this wont cause any operational interference of any sort but since the quality norms are very stringent, I have to get the hardness level in the range of 1200 to 1300HV. They say even 1320 is not acceptable!!

So, what should I do to get the hardness values in the required range?! Also, changing the powder is not an option for them..

Regards

K09

[drramc]

Hi,
If the spec. is 1200 to 1300 HV you have to be within the limit.Metco's specification has a wide limits and the manufacturer love to produce products with wider property range so as to cover bigger application areas with less stringent quality control.You can ask the manufacturer to give you a product in the hardness range you want , and this being within their specfied range,they can do the required adjustment in composition for your product.As a leading manufacturer , we used to do this.
ram chattopadhyay
ram chattopadhyay

[Gordon]

Hi K09

What other coating quality and process requirements are specified by your customer?

Tuning the process to give lower hardness coatings may also reflect on other coating properties.

You could plasma spray with relatively low energy and achieve the spec as described Now which coating would you prefer?

[k09]

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!

[Gordon]

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.

[k09]

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

[Gordon]

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]

Moved posts from kriskros and Gordon to form new thread "WC/Co Coating Microstructure" as it was going a little off-topic.