New applications and new product qualities by RF plasma spraying

摘要

DC plasma spraying with its products has gained a high technical importance. Despite the relatively long time of the use of this type of coating process and the performed improvements and modifications in the past it has still been keeping some disadvantages which are inherent largely to its principle and the way of its function and which hence can not be completely overcome. To these drawbacks belong particularly the narrow hot core of the plasma jet with very strong radial and longitudinal gradients of temperature and velocity, the more or less radial powder feed with respect to the jet axis and for certain cases the short interaction time between the powdery spray material and the plasma. In general, limited deposition efficiencies particularly with high melting materials and the requirement to choose narrow grain size fractions of fine powders, mostly below 50 μm, are main consequences. With the recently appeared availability of technically reliable RF plasma torches whose basic development can be traced back to about 40 years ago [1], some of the disadvantages of the DC spray method cited already above are no longer existing or can be avoided to a great extent. Their design and their way of plasma generation open up the potential for new processes, coatings and product qualities which enriches and extends the plasma technology. However, RF plasma spraying will not be a replacement of the DC spraying, rather a supplement. Of course, there are some fields of competition.