I seem to get many emails asking "What are cold & hot processes in the thermal spray?" I thought this would be a good and important topic for discussion, not only in respect to thermal spray but also generally to all surface engineering processes.

A few more common questions:

"A plasma running at 15000 K, how is this a cold process?"
"We use hot zinc spray, but now told it is a cold process - confused"
"Why is cold spray process included as thermal spray process?

Interested in hearing your views

Indeed a very commonly asked question that causes some considerable confusion in the industry by new and old users.

We've always explained it as a cold process because whilst the particles may be molten, the actual amount of heat energy in the particle is tiny in comparison the the mass of the substrate. Therefore, when the particle hits the substrate, the heat energy in the particle dissipates extremely quickly (nanoseconds to use the current buzzwords).

We have a picture of the founder of the process, Dr Schoop, spraying his hand with flame spray and we assume, a low melt material such as zinc or tin. PLEASE DON'T TRY THIS UNLESS YOU KNOW EXACTLY WHAT YOU ARE DOING. USING OTHER MATERIALS WILL HURT AND MAY CAUSE PHYSICAL INJURY. We also often spray cardboard business cards as a demonstration that it is a cold process. We've also just done some trials spraying expanded polystyrene, with some surprising success using arc spray.

I will be very interested to see how this post progresses. It's great to have other views put forward in a forum.

My way of thinking is that its purely related to the part/substrate being treated. If part/substrate properties are not changed by heating effects call it a cold process.

Thanks to Stuart and Thermalman for their good responses.

Anymore views or comments out there