The discovery of high temperature superconductivity (HTSC) in oxide compounds has confronted materials scientists with many challenging problems. These include the preparation of ceramic samples with critical current density of about 106 A/cm2 at 77 K and sufficient mechanical strength for large-scale electrotechnical and magnetic applications and the preparation of epitaxial thin films of high structural perfection for electronic devices.

The main interest in the growth of single crystals is for the study of physical phenomena, which will help achieve a theoretical understanding of HTSC. Theorists still do not agree on the fundamental mechanisms of HTSC, and there is a need for good data on relatively defect-free materials in order to test the many models. In addition, the study of the role of defects like twins, grain boundaries, and dislocations in single crystals is important for understanding such parameters as the critical current density. The study of HTSC with single crystals is also expected to be helpful for finding optimum materials for the various applications and hopefully achieving higher values of the superconducting transition temperature Tc than the current maximum of about 125 K. It seems unlikely at present that single crystals will be used in commercial devices, but this possibility cannot be ruled out as crystal size and quality improve.

Over the past two decades a vast number of publications have emerged from laboratories all over the world, describing the application of plasmas for preparing and processing materials. MRS symposia, scientific journals and books, and complete conference series are solely devoted to this specific topic.

Modern VLSI integrated circuits, for instance, would simply not exist without sophisticated plasma etching techniques. But highly reactive, partly ionized and dissociated, quasi-neutral gases—plasmas—are not only useful for etching purposes, i.e., the removal of materials. They are also very valuable tools for the deposition of materials with unique structures and compositions at lower temperatures than for conventional thermally induced chemical vapor deposition processes. Backed by intensive research activities and more than a decade of practical experiences, plasma deposition technologies are now penetrating a number of industrial manufacturing processes.

Plasmas can be classified into two basic categories — non-isothermal, and isothermal or thermal plasmas.

Within the high electric fields applied for non-isothermal plasma generation at reduced pressure, free electrons are accelerated to energies that correspond to several thousand degrees in the case of thermal activation. The neutral species in the gas phase and the heavy ions are either not influenced by the fields or cannot follow changing fields fast enough. Their temperature stays low, resulting in a difference between electron and gas temperature. In these nonequilibrium plasmas, the collisions of high energy electrons and gas molecules result in dissociation processes that would only occur at very high temperatures of more than 5,000 K in the case of thermal equilibrium. Therefore, non-isothermal plasmas allow the preparation of materials and compositions that are difficult to obtain using thermally activated, conventional CVD. Due to the initiation of chemical reaction by collisions with “hot” electrons rather than hot gas molecules, the processing temperature can, in many cases, be kept lower than in conventional deposition processes.

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.