Are you saying the ESD process does not produce a suitable bond with the metallic substrate it is applied to?

NO, ESD positively, definitely, absolutely etc., etc., develops a metallurgical bond with the substrate by melting the surface and alloying/ boding with it

How does ESD compare with the 'exploding wire technique', used for decades to deposit hard metal on bare machined aluminum cylinder bores on Japanese motorcycles?

Mike Halloran
Pembroke Pines, FL, USA

I think the bond produced between a metal substrate and a cermet coating applied by ESD (or a similar process) is not the metallurgical result you describe. A zirconium, tungsten carbide, or titanium diboride coating applied to a metal substrate by ESD would not "alloy" with the metal. Instead it would result in a mechanical bond, similar to something like diffusion bonding.

@tbelna, Materials science provides scientific evidence that even in sintering of cermets such as silicon carbide, diffusion of grains to other grains boundries creates a bond which we call "diffusion bond" which is a "metallurgical bond" On a lighter side having played football in the old world, where we actually use out FOOT to keep kicking the ball, I found it hilarious that in the New World it is called by some other name "SOCCER". In the land of the free and the home of the braves, the appellation "Football" is usurped for a game where the foot was used only twice to progress the ball towards the goal. It is mostly push and shove and throw.

Therefore, if you insist that it is a mechanical bond, then have it your way. A rose by any other name smells just as sweet (if you do not have an allergy to rose). The Materials Science Engineers call any bulk bond by a process such as melting, diffusion or even surface migration, etc., a metallurgical bond if the atoms in the bond are held by the interatomic force and do not fall apart by mere attrition. In the galvanizing of steel a metallurgical bond is created by the reaction of zinc atoms with iron atoms. In ESD the carbides and borides of W,Ti,Cr, etc dissociate at the localized high temperature created in the electric micro-arc, and get incorporated with the substrate elements. We call this a metallurgical bond. But if you want to emulate calling a legitimate football as soccer, and wish to "think" that a diffusion bond is a mechanical bond, have it your way, We live in a free country with the freedom of expression. But the wear resistance of ESD coatings with cermets is an established scientific fact based on hard experimental evidence.
"Goodwill towards all and Malice to none"

@MikeHalloran. Not being familiar with the 'exploding wire technique', a quick look at some info on the Internet gave the following relevant information to give you a scientific reply. For the exploding wire technique, the basic components needed for the exploding wire method are a thin conductive wire and a capacitor. The wire is typically gold, aluminum, iron or platinum, and is usually less than 0.5mm in diameter. One can read the rest on the Internet.

The ESD uses an "electrode", which is more a "rod" than a wire. The wire would perhaps melt and spatter too much and may give a very rough coating. I have not used a wire but cermet and metal rods with 1/8 inch diameter to 1/2 inch diameter. The other characteristics are similar to the 'exploding wire technique'. I do not have the exact parameters but it is also a rapid charging and discharge of a capacitor creating localized very high temperatures. I also have observed a plasma around the electrode; but it is from the ionization of air or the gas surrounding the electrode. The deposited materials is in the form of imperfect spheres which form a bond with the substrate and with each other. There are many good publications on ESD especially from the Russian scientists and some from the US Energy Research laboratories. They are better "Pundits" than "yours truly" who is basically a practicing scientist and a "technician".
"Goodwill to all and malice towards none"

Ramkiyengar- Thanks for the reply. I'm not a specialist in metallurgy, but I am definitely interested in the subject. I did a bit of reading on the ESD process. The papers I read do use the term "alloying" to describe the ESD coating process, which I found a bit confusing. From chemistry class I recall an alloy is a solid solution or compound of elements where at least one is a metal.

Diffusion bonding is a solid state joining process where atoms of the contacting surfaces intermingle over time if subjected to the right combination of pressure, temperature, and atmosphere. However, the depth of the diffusion zone can be quite limited when dissimilar materials are being joined. While I believe the material within the diffusion zone can technically be described as an alloy, I also think there might be a better term that could be used.

I've done some work with friction welding, which is a solid state process. And one thing I recall looking at micro-sections taken from the welds is that it is very hard to see any grain boundary around the weld zone.

Below is a cross section of a carbon steel surface that was ESD coated with a dissimilar material. You can clearly see a well defined boundary between the coating and substrate. This is not what I would describe as "alloying", but that is the term the author uses. To me the interface looks more mechanical than metallurgical in nature, but I'm no expert.



Interesting discussion and I've learned a bit about ESD processes.