Problems to coat small spheres with APS

[Victor Rubio]

Hello colleagues,

We have problems to coating some spheres (Co-Cr-Mo material) with METCO 131VF (alumina-titania powder), we need build a system where we mount the spheres and then apply the coating, but we dont know:

The rotational frecuency RPM to sphere and traverse speed which the robot should be moved? because to sphere diameters is so small (0.75") and we need apply 200 micrometers thick.

How affect exposition time plasma on sphere?

Somebody know how much material remove the grid blasting process,if i use a alumina mesh 40?

i never saw a problem like this, i see in others thread where talk about coating on cylindrical forms but never on spheres, and with that diameter (very small).

I see in other thread with the name " WC-Co16 coating problem enquiry" the administrator suggest 5 points or steps for solution the problem with rotating pieces (cylindrical only) but i wonder how can apply that method or steps on my problem.

This is the link of the thread mentioned above:

http://www.gordonengland.co.uk/sef/Threa...ht=WC-Co16

i am open for all suggestions or comments.

my regards from mexico friends.

[sreenuvundela]

Hi Rubio

First I suggest you to consider powder flame spray gun like sulzermetco 5P (OR) 6P Gun with forced cooling over Plasma Spray system because the heat input is relatively low and that helps you to carry coating operation little bit easy.

The material removal in the grit blasting process depends on parameters such as Blasting pressure, Girt size( only this parameter mentioned by you) and mainly on the hardness of substrate. In general only 20 microns dimensional difference will be there before and after grit blasting and we can not ensure that it will be always minus (-) side because as you know the blasting will create peaks and valleys.

Regarding exposing time, you have to monitor substrate temperature such that it should not exceed more than 150 degree centigrade.

For your application, you have to prefer bond coat with Ni base so that it can take care of coefficient of thermal expansion differences between substrate and coating material and you have arrange very effective force cooling setup during coating.

About coating powder, i recommend you to go for higher percentage of Titanium oxide such as 60Alumina with 40 Titania.

In this thread, i feel that 200 microns thickness is too high, you have to try for 100 microns with 50 microns bond coat first and after successful trails with said thicknesses then you can aim for required thickness.


Good luck for the same.


Regards,

SREENIVAS

[Victor Rubio]

(03-13-2014 10:15 AM)sreenuvundela Wrote:  Hi Rubio

First I suggest you to consider powder flame spray gun like sulzermetco 5P (OR) 6P Gun with forced cooling over Plasma Spray system because the heat input is relatively low and that helps you to carry coating operation little bit easy.

The material removal in the grit blasting process depends on parameters such as Blasting pressure, Girt size( only this parameter mentioned by you) and mainly on the hardness of substrate. In general only 20 microns dimensional difference will be there before and after grit blasting and we can not ensure that it will be always minus (-) side because as you know the blasting will create peaks and valleys.

Regarding exposing time, you have to monitor substrate temperature such that it should not exceed more than 150 degree centigrade.

For your application, you have to prefer bond coat with Ni base so that it can take care of coefficient of thermal expansion differences between substrate and coating material and you have arrange very effective force cooling setup during coating.

About coating powder, i recommend you to go for higher percentage of Titanium oxide such as 60Alumina with 40 Titania.

In this thread, i feel that 200 microns thickness is too high, you have to try for 100 microns with 50 microns bond coat first and after successful trails with said thicknesses then you can aim for required thickness.


Good luck for the same.


Regards,

SREENIVAS

Thank you so much for reply the post Sreenivas.
I am studying a master degree and is necesary for me take some parameters of other research, in specific a previous research of a collegue, where he was cover the same alloy (on a flat disc) with the same powder and use 9MB gun, with spray rate to 23 g/min, and i need take the majority of parameter used for him, but i will coat spheres.

In the grit-blasting, the pressure used for him was 20-40 psi, with an angle 45°, and alumina mesh 60. Really appreciate the estimation proporcionated for you (20 microns or less) it is so useful for me.

My collegue applied the 200 microns on the discs, without use the bond coat, and obtained good results, i need use only biocompatibles materials, like metco131VF and BioDur CCM plus (Co-Cr-Mo) due my research focus in that materials, but if i have some problems maybe take in account your advice about Ni base, thank you so much..

My principal question is about the surface speed on the sphere and traverse speed on the gun, because i need know how RPM need to apply to sphere for obtain a good coatings, with the same properties like my collegue

Other question is about the spray rate, if I choice a thickness to 20 microns per pass, how many g/min need to apply in the spray rate, there a relation, or mathematical formula?

Well, if you can help me with this, it will be amazing, but with your reply i am very very greatful.

Regards.

Víctor Rubio.

[sreenuvundela]

Hi

I thought that it is industrial application. Any way, i will tell you how any coating will be developed on any geometry.

1st Step: You have to take process parameters from powder manufacturer and you have to prepare one Flat sample by keeping coating gun stationary and carry characterization of the sample to check whether microstructure, coating hardness and porosity with in required limits. if yes, your process parameters are Good enough to start coating. Kindly note that powder folw rate also will be given by powder manufacturer.

2nd Step: By maintaining above good process parameters you have to set surface speed such that you have to get coating thickness in the range of 15 to 20 micorns. To know suitable surface speed, you have to do by trial and error method. For this, you have to coat one layer and you have to measure coating thickness and band width (or) coating spot diameter which will help you to calculate coating Gun Speed.

3rd Step: After knowing optimum surface speed (mm/min), you have to calculate RPM of cylinder by using formula (surface speed (velocity) = (Pi X Dia X RPM / 60)), you will know RPM to be set for Cylinder.


For Sphere the diameter varies from centre so surface speed and Gun speed also should vary in the same way.

I hope i guided you to get solution for your problem.

Regards,

SREENIVAS

[Victor Rubio]

(03-14-2014 09:03 AM)sreenuvundela Wrote:  Hi

I thought that it is industrial application. Any way, i will tell you how any coating will be developed on any geometry.

1st Step: You have to take process parameters from powder manufacturer and you have to prepare one Flat sample by keeping coating gun stationary and carry characterization of the sample to check whether microstructure, coating hardness and porosity with in required limits. if yes, your process parameters are Good enough to start coating. Kindly note that powder folw rate also will be given by powder manufacturer.

2nd Step: By maintaining above good process parameters you have to set surface speed such that you have to get coating thickness in the range of 15 to 20 micorns. To know suitable surface speed, you have to do by trial and error method. For this, you have to coat one layer and you have to measure coating thickness and band width (or) coating spot diameter which will help you to calculate coating Gun Speed.

In this step, do you refer 15 to 20 microns per pass, right ? Then you suggest me coat one layer and measure the thickness, for example set to 60 RPM and in 1 second i have a revolution, and then i coat for one second and then measure the thickness, and depends that, i will propose a number to RPM for achieve the desired thickness (200 microns), or my example is not corect?

In the first step, i have the parameters used to coat flat discs, in fact have the gun velocity is 2 m/s for flat surface with the disc without move, only moving the gun. this gun velocity maybe useful for the surface speed?, i mean convert the linear velocity (2 m/s) to rotation speed, without move the gun, only rotate the sphere. Do you suggest me something like that or other way to do it?

I am very greatful for your replies, because you are the second to help in that forum, really i thought receive more help here, but with your advices is enough help. Thank you so much SREENIVAS.

Regards from México,

Víctor Rubio
[/quote]

[djewell]

A simple way to think about coating a sphere is to approximate it as a series of flat discs of equal thickness but varying diameters stacked together. The more discs you have, the closer approximation of a sphere you can make. Then, you start with the smallest diameter and determine your rotational and traverse speeds. As you traverse across the discs, the diameters of your theoretical discs change, so you slow down on each one a little bit based on the ratio of the diameters.

For example, if the diameter of the first disc was 5 and the second disc was 6, then the the traverse speed across the second disc should be 5/6 of the speed across the first. Calculate the ratios of each disc so you can create your spray program with changing speeds.


  Ball approximation.TIF (Size: 52.07 KB / Downloads: 147)

[Victor Rubio]

(03-14-2014 09:48 PM)djewell Wrote:  A simple way to think about coating a sphere is to approximate it as a series of flat discs of equal thickness but varying diameters stacked together. The more discs you have, the closer approximation of a sphere you can make. Then, you start with the smallest diameter and determine your rotational and traverse speeds. As you traverse across the discs, the diameters of your theoretical discs change, so you slow down on each one a little bit based on the ratio of the diameters.

For example, if the diameter of the first disc was 5 and the second disc was 6, then the the traverse speed across the second disc should be 5/6 of the speed across the first. Calculate the ratios of each disc so you can create your spray program with changing speeds.

Thank you so much djewell, actually i am consider seriously don´t move the gun, because my sphere have a diameter 19.05 mm but i need coat like a belt above the principal diameter (only 17.63 mm to new diameter). maybe i only coat half sphere maximum, but i only need a belt with a 6 mm width. 3 mm width is my spray foot print.

I show you in the attached image the belt i need coat.

If you have a new suggestion after see the picture, i will be very greatful. I reiterated my gratitude.

[djewell]

If you are only trying to spray a band around part of the sphere, then you can treat it as a cylinder. The difference in thickness will not be worth discussing. I think you will find that the effect of the smaller diameter will be offset by the change in deposit efficiency due to the change in angle of incidence.

[Victor Rubio]

(03-17-2014 01:56 PM)djewell Wrote:  If you are only trying to spray a band around part of the sphere, then you can treat it as a cylinder. The difference in thickness will not be worth discussing. I think you will find that the effect of the smaller diameter will be offset by the change in deposit efficiency due to the change in angle of incidence.

Yes is the principal target, because the sphere after will use to tribological tests in ball on disc tribometer, and not need all sphere coated.

Really my first interest in this problem is ensure the uniformity in the coating band around the sphere, and that is the reason for which i need know the surface speed, but if you tell me can use the treat it as a cylinder is enough for me, and i will take this how my start point. Actually i have the "Thermal spray RPM & traverse speed calculator" attached in this forum, do you believe that file is useful for calculate the surface speed in my problem?

I have calculations for the gun, i need move the gun such that the gun and surface band are perpendicular, for ensure a good deposit efficiency and less porous coating and other issues caused for the spray angle.

Thank you so much djewell for your answer.

[Victor Rubio]

Hi friends,

I have a powder METCO 131VF, it has a coverage of 1.98 m2/hr to 0.1mm, i need coat a diameter of 17.6314 mm with surface area of 582.2691 mm2 on a sphere (Like a band around the sphere, attached a picture). I need obtain 200 microns of thickness with APS process.

How can i do an experiment for achieve the desired thickness varying the surface speed and spray rate only?

For example in the METCO 131VF technical bulletin suggest 23 gr/min to spray rate,

Is posible, modify that spray rate to dicrease or increase the spray time?

I obtained a surfcae speed of 1354 RPM using the “Thermal spray RPM & traverse speed calculator" sheet.

Somebody can say me any suggest or advice for do my experiment?

the other parameters (like current, voltage, and spray distance, etc.) i obteained to the powder technical bulletin.

A lot of thanks for your time.

Regards to mexico

Víctor Rubio