Surface speed

[Panf]

Hello, Forum!
Sorry for my English!!!

I have question. Influence surface speed during HVOF, APS and Flame Spray on quality coatings (bond strength, porosity, residual stresses...). How choose surface speed for each technique?

Thank you.

[derek]

Panf,

We have APS and air spray system, we are using about 250 surface feet/min speed.

[Gordon]

Hi Panf

to the Surface Engineering Forum.

Quote:Posted by Panf - Today 10:38 AM
I have question. Influence surface speed during HVOF, APS and Flame Spray on quality coatings (bond strength, porosity, residual stresses...). How choose surface speed for each technique?

This is a good question

Quote:Posted by derek - Today 11:31 AM
We have APS and air spray system, we are using about 250 surface feet/min speed.

This is roughly the sort of surface speed required, particularly for plasma and HVOF processes. It also tends to be maximum capability of most gun traversing equipment.

The most notable advantage of the thermal spray process over most other surface engineering coating processes is that it is a "cold process" in respect to the substrate. May people when first introduced to thermal spray are usually very surprised and find it hard to understand how this is a cold process. The process may use plasma at 15000 degree C and spray molten high temperature materials, how can that be cold? The process is normally used in a manner where very thin layers of coating are rapidly deposited in stages, so that substrate heating can be controlled and kept very low. Analogous to passing ones hand through a hot flame, the quicker the hand moves, the less likely you will be burnt. This allows for temperature sensitive substrate to be coated with materials that have a very much higher melting point (ceramics on aluminium or plastics). Sorry for rambling on, I just wanted to highlight the importance of surface speed.

The relative surface speed between the gun and substrate and the speed or step distance in the transverse direction are very important parameters. Along with powder feed rate, these parameters dictate the thickness of material deposited and exposure to hot gases on the surface per pass of the gun.

With most thermal spray processes, the main heat input is from the deposited coating rather than the hot gas stream (possible exception being HVOF). So when setting surface speed and traverse speed or step distance we need to account for coating material properties and deposit rates. The shape, size and mass of the substrate, along with thermal conductivity and effects of any auxiliary cooling processes also need consideration.

Usually my first consideration when selecting suitable surface speeds and other parameters, is the thickness of coating material to deposit per pass of the spray gun. Generally, flame and arc spray processes tend to be a little less critical than plasma or HVOF with around 0.001" to 0.004" (25 to 100 microns) thickness per pass giving good results. Plasma and HVOF, particularly when ceramic or cermet (WC/Co) type coatings are being sprayed, should always be kept below 0.001" (25 microns) per pass and ideally much lower.

I select surface speeds that give me the desired thickness per pass (trial and error or from experience) and allows me control over coating/substrate temperature during spraying, rather than just accepting a general recommended surface speed, which may not take account of the many variable factors involved.

The effects of excessive low surface speed and high thickness per pass on coating integrity is mainly heat and stress related and can result in cracking, layering faults and delamination.

[Panf]

Dear Gordon!
I glad that you understand me. Thanks for you reply.
Few question more:
1. thickness of metall coatings (NiCrBSi or FeCrNi...) must be <25 micron per pass too?
2. what you recomended for surface cooling (air, water or what)?
3. Efficiency during spraying with high surface speed decreased or not?

Thank you.

[Gordon]

Hi Panf

1. I assume you are referring to HVOF metal coatings. Generally, these tend not to be as critical as cermets and would choose around 25 microns per pass.

2. Should additional cooling be required first consider air. If air blast cooling is not sufficient, then consider carbon dioxide cooling systems. I have used water spray mists, and in certain conditions it can be very effective. Water is very risky to use though, and generally best avoided.

3. I have thought that at very high surface speeds you might expect drop off in deposit efficiency, but I have found no noticeable evidence of this effect.

[Panf]

Thank you very much, Gordon.
I hope you advices help me.

[Vladimir]

Panf Wrote:Hello, Forum!
Sorry for my English!!!

I have question. Influence surface speed during HVOF, APS and Flame Spray on quality coatings (bond strength, porosity, residual stresses...). How choose surface speed for each technique?

Thank you.

Hallo Panf,
my experiences with HVOF is: surface speed 30-40 m/s

[Panf]

Really???
surface speed 30...40 m/s it's 1800...2400 m/min!!! for detail ?100mm W=7600 RPM....... - unreal!!!
correct me if I wrong...

[Vladimir]

Panf Wrote:Really???
surface speed 30...40 m/s it's 1800...2400 m/min!!! for detail ?100mm W=7600 RPM....... - unreal!!!
correct me if I wrong...

Sorry!!! 30-40m/min.

[Gordon]

The highest surface speed I think I've ever used was around 10 m/s ("single pass" plasma spray r&d). That certainly stretched the capabilities of the handling equipment and gave concerns regarding health and safety. Imagine a solid lump of steel 500 mm by 300 mm diameter revolving at 600 rpm, thats a lot of energy stored, potentially very hazardous if it becomes unbalanced or set free. Typically 0.5 - 5 m/s is more common and I think Vladimir may be in error with his units or decimal place. If not, I would be interested to know more about his process set-up.

[Gordon]

Hi Vladimir

Your last post just beat me to it

[Vladimir]

Gordon Wrote:Hi Vladimir

Your last post just beat me to it

Hi Gordon

I corrected my first mistake (30-40m/s - its wrong, I know),but 30-40m/min its 0,5-0,66m/s - and this is OK for HVOF.

In accordance to Technical TAFA Bulletin:
"RECOMMENDED FEEDS AND SPEEDS FOR SPRAYING WITH THE JP-5000
Note: these are recommended minimum feeds and speeds - some substrates or coatings may require different settings. Some operators may feel more comfortable with their own settings. This does not guarantee coating quality or the lack of substrate overheating.
In general, a good average substrate surface velocity is: 100 ? 150 SFPM (31 ? 46 m/min)"

Vladimir

[Gordon]

Hi Vladimir

My smilie was not aimed at your mistake (easily made), only that your correction message put my message a little out of context.

Those sort of speeds are very often given as general guidance, but in my experience many applications are best done with much higher speeds. Spraying of carbides and ceramics or coating substrates with a limited heat sink or are heat sensitive are good examples. I've seen far too many problems where unsuitable speeds and feeds have been used (many using so called recommended settings). "Let's think out of the box," could be applicable here (sorry for using management-speak buzzwords). Usually, though we are limited by the capability of our handling equipment and have to compromise. Sometimes lowering powder feed rates (deposition rates) can be a useful trick, if the desired speeds can not be achieved. Please read my first post in this thread for the reasoning.

[bhellman]

Dear Friends.
We made a engineering diploma work on the influence of surface speed (actually passing speed). If one consider the difference in the actual passing velocity between various methods, the outcome is that the work peace surface velocity is minor compared to the particle velocity for any method. Of course one can imagine that spray methods with "low" particle velocity, can tolerate lower passing speeds, OK.
The diploma work showed that the HVOF-process can take at least up to
300 m/min of surface speed without any quality problems, in the contrary the coatings where much cleaner than on lower speeds.
I remember, in my beginning in this business, 10 to 30 m/min was considered the accurate values.
My feeling is that more important are factors like (for rotating objects):
- the right feed (rotation and pitch) to get the optimum lap and impact point.
- to be aware that the spray impact point temperature is OK.
- to have the optimum feed/revolution, 50 to 60 % overlapping.
- to have a optimized ventilation or local directed suction.
bhe

[hvofhamid]

Panf Wrote:Hello, Forum!
Sorry for my English!!!

I have question. Influence surface speed during HVOF, APS and Flame Spray on quality coatings (bond strength, porosity, residual stresses...). How choose surface speed for each technique?

Thank you.

of course you have to calculate your speed owing the diameter you coat and rotation speed

[bhellman]

bhellman Wrote:Dear Friends.
We made a engineering diploma work on the influence of surface speed (actually passing speed). If one consider the difference in the actual passing velocity between various methods, the outcome is that the work peace surface velocity is minor compared to the particle velocity for any method. Of course one can imagine that spray methods with "low" particle velocity, can tolerate lower passing speeds, OK.
The diploma work showed that the HVOF-process can take at least up to
300 m/min of surface speed without any quality problems, in the contrary the coatings where much cleaner than on lower speeds.
I remember, in my beginning in this business, 10 to 30 m/min was considered the accurate values.
My feeling is that more important are factors like (for rotating objects):
- the right feed (rotation and pitch) to get the optimum lap and impact point.
- to be aware that the spray impact point temperature is OK.
- to have the optimum feed/revolution, 50 to 60 % overlapping.
- to have a optimized ventilation or local directed suction.
bhe

Still read by comments on this subject. Even if not absorbing the way of thinking, please consider its validity.
br bhe

[hvofhamid]

bhellman Wrote:

bhellman Wrote:Dear Friends.
We made a engineering diploma work on the influence of surface speed (actually passing speed). If one consider the difference in the actual passing velocity between various methods, the outcome is that the work peace surface velocity is minor compared to the particle velocity for any method. Of course one can imagine that spray methods with "low" particle velocity, can tolerate lower passing speeds, OK.
The diploma work showed that the HVOF-process can take at least up to
300 m/min of surface speed without any quality problems, in the contrary the coatings where much cleaner than on lower speeds.
I remember, in my beginning in this business, 10 to 30 m/min was considered the accurate values.
My feeling is that more important are factors like (for rotating objects):
- the right feed (rotation and pitch) to get the optimum lap and impact point.
- to be aware that the spray impact point temperature is OK.
- to have the optimum feed/revolution, 50 to 60 % overlapping.
- to have a optimized ventilation or local directed suction.
bhe

Still read by comments on this subject. Even if not absorbing the way of thinking, please consider its validity.
br bhe

hello everybody
before start working in spray coating ,I worked in several posts of mechanical fabrication,when I started coating we had lot of problems because we haven t the right speed and the right rotation.we only could coat succefuly one diameter .then i realize if i can use the diferent conditions(rotation,speed)owing diameters and calculate the speed coating conditions ,may be we can solve the problem.since then we are working and have no problem.I hope my english will be understood and happy new year to all of you
we can coat all diameters with diferent speed and we have no problems

[Gordon]

More info related to this thread here
with a copy below.

Howied Wrote:Hi all,

Anyone here having any idea about RR?s coating application?[/url]

Anyway, I am spraying with METCO 73F-NS-2 to MSRR9507/1, which is WC-16Co powder.

I found that there is micro cracking among coating; I suspect that it is because deposit efficiency is 0.00075? per pass. May be too higher.

1, anyone can tell me the proper surface speed and traverse speed for this coating?
2, how to reduce micro cracking if you are facing this problem?
3, how to adjust to reduce deposit efficiency?

Gordon Wrote:Hi Howied

I take it that the amount of microcracking or shrinkage cracks in your coatings are deemed as unacceptable by RR standards. Microcracking is acceptable up to a certain level.

What (I assume plasma spray) process/parameter/coating type?

Deposit efficiency from a thermal spray point of view is really the %wt of your powder that ends up in your coating. I assume you mean deposit rate/thickness per pass/cycle of the spray gun.

Quote:1, anyone can tell me the proper surface speed and traverse speed for this coating?

My way of looking at surface speed and traverse rates on rotating parts:

Determine spray foot print width (probably ~4mm). Set traverse rate in conjunction with surface speed (via rpm, diameter dependant) to give desired deposit thickness per pass while traversing a spray foot print width minus say 20% per revolution.(minimum coating overlap, while maintaining even coating deposition with no barber pole effect). Note the traverse rate determines the deposit thickness per pass, while the rotational speed/surface speed influences the traverse distance travelled per revolution.

I think a deposit thickness per pass for this type of coating is best at ~ 0.0002"

Localised over heating of the coating due to insufficient cooling or too heavy deposit per pass or over heating powder particles during spraying (incorrect plasma parameters) causing oxidation/decomposition of WC primary particles and embrittlement of Co matrix are probable reasons for increased microcracking or macrocracking.

Quote:2, how to reduce micro cracking if you are facing this problem?

Make sure you are using recommended parameters, try to keep coating/substrate temperatures as constant as possible throughout spraying (low deposit thickness per pass will help here). Make sure your well within the recommended coating thickness limitation (~0.015" for this type of coating).

Quote:3, how to adjust to reduce deposit efficiency?

I think I covered that above if I understand correctly.

Hope that helps.

fhyuan Wrote:I also think it is right the thickness per pass of the spray gun is the deposit rate.

The deposit rate should be mostly dependent on traverse rate and the surface speed is set to resolve the barber pole mark and to achieve the improved microstructure of the coatings.

Gordon Wrote:you mean deposit rate/thickness per pass/cycle of the spray gun.

My way of looking at surface speed and traverse rates on rotating arts:

I think a deposit thickness per pass for this type of coating is best at ~ 0.0002"

I think I covered that above if I understand correctly.

Hope that helps.

Gordon Wrote:Hi fhyuan

to the Surface Engineering Forum.

Thanks for your comments, it made me realise i had not written part of my previous post very well:

Quote:My way of looking at surface speed and traverse rates on rotating parts:

1. Determine spray foot print width (probably ~4mm)
2. Set surface speed (via rpm, diameter dependant) to achieve desired thickness per pass.
3. Set traverse rate to give spray foot print width minus say 20% per revolution.(minimum coating overlap, while maintaining even coating deposition with no barber pole effect).

I will edit the post so it reads:

Quote:My way of looking at surface speed and traverse rates on rotating parts:

Determine spray foot print width (probably ~4mm). Set traverse rate in conjunction with surface speed (via rpm, diameter dependant) to give desired deposit thickness per pass while traversing a spray foot print width minus say 20% per revolution.(minimum coating overlap, while maintaining even coating deposition with no barber pole effect). Note the traverse rate determines the deposit thickness per pass, while the rotational speed/surface speed influences the traverse distance travelled per revolution.

Fergus Wrote:Hi

Setting up speeds and feeds this way sounds logical, but where do we start . Is there a way of working this out or a formula to give us a starting point.

thanks

Fergus

fhyuan Wrote:Fergus, I think Gordon has given the above answer for your question. First test to have the foot print width, then calculate the relationship between the traver rate and the surface speed (RPM).

Fergus Wrote:Hi

Setting up speeds and feeds this way sounds logical, but where do we start . Is there a way of working this out or a formula to give us a starting point.

thanks

Fergus

Gordon Wrote:Hi Fergus, Howied and all,

Fergus Wrote:Hi

Setting up speeds and feeds this way sounds logical, but where do we start . Is there a way of working this out or a formula to give us a starting point.

thanks

Fergus

I think most would start from a point from past experience, but you can theoretically calculate this using information supplied by powder/equipment supplier/manufacturer.

This only covers rotating cylindrical work pieces. For other types you will need to modify method.

1. Calculate coating area (3.14DL)
2. Use COVERAGE (sqft/hr/0.001" or sqm/hr/0.1mm) found hopefully in powder/equipment bulletin/manuals for your particular coating to calculate SPRAY TIME
3. Divide desired coating thickness by the thickness per pass (for carbides and ceramics I would aim for ~ 0.0002" or 5 micrometres per pass) to get number of passes required
4. Part length x No. of passes divided by the spray time will give you TRAVERSE SPEED
5. Traverse speed divided by traverse distance per revolution (this distance should be slightly smaller than your spray footprint) to get rotational frequency RPM.

The theoretical calculations do not always work exactly in practise, but should give a good starting point for fine tuning.

Example: 6" diameter x 12" long cylinder coated with Metco 73F-NS-2 (P73F-10 spec) to 0.010" thick.
1. 3.14 x 6 x 12 = 226 sqin or 1.57 sqft coating area
2. Coverage from Metco 73F-NS-2 tech bulletin says 135 sqft/hr for 0.001" coating thickness.
for 0.010" coating thickness thats 13,5 sqft/hr
for 1.57 sqft thats (1.57/13.5) gives us 0.116 hrs or 7 minutes spray time.
3. 0.010/0.0002 = 50 passes
4. 12" x 50/7 mins = 86"/min or 7.17 ft/min traverse speed
5. Lets say 0.16" traverse per rev 86/0.16 = 537 RPM
6. So that gives us a starting point of 537 RPM (850 SFPM) and a traverse rate of 86"/min using 50 passes and spray time around 7 minutes.

There is a point particularly with high spray rate/high deposit efficient materials on smaller diameters where the part revolution frequency RPM becomes difficult to manage. We can only compromise in these situations, by excepting higher deposits per pass or reducing spray rates. In many situations carbide and ceramic coatings are applied much thicker per pass than I personally would like, but have proved adequate for purpose (I just know they are not as good as they could be ).

Beware my maths may not be that good

[ahmadsahid]

Dear,

Can you give the reference of above formula?

[Gordon]

Hi ahmadsahid

to the Surface Engineering Forum.

(05-25-2009 02:17 PM)ahmadsahid Wrote:  Dear,

Can you give the reference of above formula?

Not sure what you mean, but if it is data like powder required, coverage, density, coating weight etc., then you will need reference to the specific material/process technical data sheets or contact your supplier. Failing that you will need to conduct your own actual deposition tests (which would be the more accurate way).

One point that may not have been made clearly in above posts, is that any over-spraying (times when gun is not actually spraying on intended area to be coated) must also be considered in calculations.