Hi Harvey
Sound of silence
well I suppose understandable in the light that any company spending time, effort and cost on development may not want to give up their secrets.
The specification may be unrealistically tight, but I assume some are managing.
I suppose you are at the point of wondering whether further development is going to be worthwhile and cost effective for this job. Well if you are going to persevere, I can only make a few comments, which probably you already appreciate from your trials anyway:
Powders - I know you are restricted to a few approved powders only, but choosing the one with the tightest particle size distribution and maybe one with lower high end particle size amount may help.
Powder injection - very critical. Injecting powder in through the side of the plasma jet is quite complex. In an ideal world the powder particles would be all exactly the same size and shape, but they are not. The plasma jet has a classifying effect - smaller particles penetrate less than the larger. Larger particles with more momentum may pass right through the plasma jet too quickly, not picking up enough heat and velocity, some ending up as unmelts in the coating. Setting up the injection parameters is really getting the best balance in conditions for the majority of the particles, inevitably that means some particles may be under or over-heated, lack velocity etc. and end up effecting the quality of the coating. Important to remember, when changing plasma conditions, changes to powder injection may be necessary also.
Powder feed rate - It may be worth trying a lower feed rate than recommended, sometimes quality gains can be made at the expense of production through-put.
Plasma parameters - again very critical. Getting right balance of plasma gas flows, plasma gas mix and current. Generally, if unmelts are excessive (assuming powder, injection and feed rate are optimal) then the larger particles need more heat. This can be achieved by lowering the plasma velocity to get more dwell time for particles and/or by increasing heating potential of plasma. This again is complex and not as straight forward as you may first think.
Without going into other parameter considerations, you can see how complex development gets with a mind boggling number of possible parameter permutations.