More than 25 years ago, plasma spraying was established as a commercial process, but only recently have serious attempts been reported to establish a solid scientific base for this technology.
The first part of this paper refers to the basic processes involved in plasma spraying, including plasma generation, plasma jet formation, particle injection, particle heat and momentum transfer, and particle deposition. In the second part, recent results obtained in this laboratory are summarized. These results are mainly concerned with the effects of vortex flow on particle motion and on air entrainment during atmospheric spraying.
1. Introduction
Plasma spraying has been around for more than 25 years. In spite of this fact, the science base for this technQlogy is still poorly established and for certain aspects virtually non-existent.
In the fifties, plasma torches were developed to test materials at high enthalpies for simulated re-entry vehicles. And in the late fifties and early sixties the first serious attempts were reported using plasma torches for spraying of primarily refractory materials. Today, we know that almost any material can be used for plasma spraying on almost any type of substrate. This flexibility is probably one of the major reasons for the rapid expansion of this technology.
Applications for plasma spraying include corrosion-, temperature-, and abrasion-resistant coatings and production of monolithic and near net shapes which at the same time take advantage of the rapid solidification process. Powders of glassy metals can be plasma sprayed without changing their amorphous characteristics. Very recently high temperature superconductive materials have been deposited by the plasma spray process.