The concept of plasma is central to many scientific and engineering disciplines—from the design of neon advertisement lights to fusion physics. Plasmas vary from low density, slight states of ionization (outer space) to dense, thermal plasmas (for extractive metallurgy). And plasmas are prominent in a wide range of deposition processes — from nonthermal plasma-activated processes to thermal plasmas, which have features of flames and which can spray-deposit an enormous variety of materials. The latter technique, arc plasma spraying (or simply, plasma spraying) is evolving rapidly as a way to deposit thick films (>30 μm) and also freestanding forms.
This article will review the technology of plasma spraying and how various scientific disciplines are contributing to both an understanding and improvement of this complex process.
The plasma gun dates back to the 1950s, when it was introduced for the deposition of alloys and ceramics. Due to its high temperature flame it was quickly discovered that plasmas could be used for depositing refractory oxides as rocket nozzle liners or to fabricate missile nose cones. In the latter technique, the oxide (e.g., zirconia-based ceramics, spinel) was sprayed onto a mandrel and the deposited material was later removed as a free-standing form.
The technique's versatility has attracted considerable industrial attention. Modern high performance machinery is commonly subjected to extremes of temperature and mechanical stress, to levels beyond the capabilities of present-day materials. It is becoming increasingly common to form coatings on such material surfaces to protect against high temperature corrosive media and to enhance mechanical wear and erosion resistance. Several thousand parts within an aircraft gas turbine engine have protective coatings, many of them plasma sprayed. In fact, plasma spraying has emerged as a major means to apply a wide range of materials on diverse substrates. The process can be readily carried out in air or in environmental chambers and requires very little substrate surface preparation. The rate of deposit buildup is rapid and the costs are sufficiently low to enable widening applications for an ever increasing variety of industries.