Preheating silicon bars for arc spraying

[banquo]

We're trying to spray bars of silicon with steel alloys (don't ask.. ) and are having adhesion problems. On the first pass, the coating looks OK, but on later passes it begins to blister, and it's clear there is poor adhesion. There's no gap between passes, so it gets hotter as time progresses.
We blast the surface with abrasive before attempting to spray, so there is some surface roughness.
I'm not directly involved in the project, but being the resident Jack of all Trades (and master of none..) have been asked to comment.
I've suggested that the blistering is an indication that the spray didn't adhere in the first place, and is becoming dislodged due to thermal expansion on subsequent passes.
I've suggested preheating (as hot as they can get it) before spraying commences. I'm not sure of the principle of bonding to something like silicon. It alloys readily with most metals, but has an extremely high thermal conductivity, so I'm guessing it's sucking the heat away, and acting as a heat sink.

Does that make any sense, and would we expect preheating to be required for this kind of duty?

Comments aprreciated!

[Gordon]

Hi banquo

to the Surface Engineering Forum.

First, can't remember ever spraying onto silicon substrates, so my comments are not from first hand experience

A few questions:

Quote:On the first pass, the coating looks OK, but on later passes it begins to blister, and it's clear there is poor adhesion. There's no gap between passes, so it gets hotter as time progresses.

At what coating thickness does coating fail?
What coating thickness are you trying to achieve?
How hot does coating/substrate get after time?

When you say steel alloy coating, can you be more specific? High carbon steels and high chromium steels tend to be low shrink, less stressful and have greater thickness tolerance than low carbon steels and austenitic stainless steels.

Have you tried using bond coats like nickel/aluminium or molybdenum?

Quote:I've suggested preheating (as hot as they can get it) before spraying commences. I'm not sure of the principle of bonding to something like silicon. It alloys readily with most metals, but has an extremely high thermal conductivity, so I'm guessing it's sucking the heat away, and acting as a heat sink.

Does that make any sense, and would we expect preheating to be required for this kind of duty?

Silicon I believe has a relatively low thermal coefficient of expansion. I think a moderate preheat around 100 C would be beneficial (as long as surface is not being oxidised) while during spraying process keep coating/substrate temperature as constant as possible.

If all this fails then yes I would experiment with different preheat temperatures and spraying temperatures.

[banquo]

Cringeing apologies for the long delay in replying...
I missed the e-mail notification.. and then lost the link - you know how it goes...

Anyhow, the problem seems to have been resolved, now that the person who actually knew how to do the job returned. I can't be very specific about what we're doing, because it's experimental..

Thanks very much for the reply.

(11-25-2009 03:25 PM)Gordon Wrote:  Hi banquo

to the Surface Engineering Forum.

First, can't remember ever spraying onto silicon substrates, so my comments are not from first hand experience

A few questions:

Quote:On the first pass, the coating looks OK, but on later passes it begins to blister, and it's clear there is poor adhesion. There's no gap between passes, so it gets hotter as time progresses.

At what coating thickness does coating fail?
What coating thickness are you trying to achieve?
How hot does coating/substrate get after time?

When you say steel alloy coating, can you be more specific? High carbon steels and high chromium steels tend to be low shrink, less stressful and have greater thickness tolerance than low carbon steels and austenitic stainless steels.

Have you tried using bond coats like nickel/aluminium or molybdenum?

Quote:I've suggested preheating (as hot as they can get it) before spraying commences. I'm not sure of the principle of bonding to something like silicon. It alloys readily with most metals, but has an extremely high thermal conductivity, so I'm guessing it's sucking the heat away, and acting as a heat sink.

Does that make any sense, and would we expect preheating to be required for this kind of duty?

Silicon I believe has a relatively low thermal coefficient of expansion. I think a moderate preheat around 100 C would be beneficial (as long as surface is not being oxidised) while during spraying process keep coating/substrate temperature as constant as possible.

If all this fails then yes I would experiment with different preheat temperatures and spraying temperatures.