Youre quite correct when you say that preheat will increase the bond strength, its just not normally required.
Cant recall the figures for zinc off the top of my head, and im too lazy to check (ill also forget between now and monday), but for thermal spray aluminium (TSA) that we use on offshore jobs, the spec is usually minimum 5Mpa (5 mega pascals) which youll achieve no problems with a flame spray gun (oxy/acet/air, with the wire fed into the back of the gun, and melted in a constant stream, to be thrown at the job) with occasional 9Mpa min (which is where things get interesting, especially on stainless steel, as youll normally be 7-8 without preheat, and have to start thinking about preheat which brings its own school of issues.....
Arc spray coatings (think mig welder, but with two wires instead of one and an earth, they come together at a head, and continously melt, then are proppelled onto the job by the positive pressure pocket of air behind the arc point, giving you a nice stream of well melted particles, thrown at higher speed onto the job) are typically quoted as 12-15Mpa without preheat, which is normally fine, but Arc spraying soft wires is a faff, so we dont normally bother (its also a case of fitness for purpose, for a big flat area, arc spray is king, but for smaller more intricate stuff, its akin to painting a doorframe with a yard broom or similar)
Oh, and yes, when i say freezes on impact, solidifes is exactly what i mean! The difference in temperature (the delta) between the base material and the spray particles will have a great effect on bond strength, the smaller the delta, the more time the particles will have to 'splat' into the prepared surface, which gives them a better bond.
Im in serious danger of waffling incessently here, so im going to pause here, but any more questions, please feel free to ask, I dont pretend to be the worlds greatest expert in therml spray stuff, but ive been doing it nearly 20yrs now, and what we do, i know well.
