Yes, the -325 mesh powder would have a much greater total surface area than the -140mesh particles. Again, we’re referring to the total surface area of all the particles in each of those two identical-volume boxes, one filled with the coarser -140 mesh powder, and the other box filled with the much finer, smaller -325 mesh powder.
My point is, that if you are applying a given quantity “X” of BFM paste or powder to a particular joint to be brazed, then, if that quantity “X” consists of -140 mesh powder, the total surface area of powder in that paste that will be exposed to the oxygen in the brazing atmosphere will be much less than the total surface area of powder in a similar volume of -325 mesh BFM paste/powder.
Consequently, if, for any reason, your brazing atmosphere is “marginal”, i.e., has too high a dewpoint, or a high leak-up rate in a vacuum-furnace, etc., then, because of the excess amount of moisture (thus oxygen) in such a marginal-atmosphere, you will probably find that finer mesh powders, with their much greater total surface area exposed to all that excess oxygen, will tend to resist melting properly, and perhaps merely “ball-up” into lumps rather than flow out nicely. I’ve seen this happen, whereas a much coarser powder in the same environment flowed out okay because so much less of that coarse powder’s total volume had any oxides.
This phenomenon has led to an interesting quality-control test that you can use to verify the quality of your brazing atmosphere. Put two different mesh-size powders (of the same BFM alloy) on a sheet (keep the two small piles of BFM powder well separated from each other), and run them in one of your regular brazing production runs. After the brazing cycle, compare the flowing characteristics of the two small piles of powder. They should both have flowed out nicely. BUT, if the -325 mesh powder tends to ball-up on the sheet while the -140-mesh powder flows out okay, then your furnace atmosphere is becoming marginally poor, and this test can be a quick way to catch that before it actually hurts some of your production parts.
Next Month: We’ll look at the answers to our brazing quiz from last month.