The enhancement of engineering materials by surface modification has extended the operational envelope for many structures in terms of resistance to corrosion, wear, fatigue, and other forms of surface degradation and has made feasible the introduction of novel materials into high-performance applications. Surface engineering, using thermal-spray coatings, represents a pragmatic and highly costeffective means of satisfying stringent design criteria (e.g., in aerospace applications), operating under extreme environments (e.g., high temperatures, wear, and corrosion), and introducing a multiplicity of functions (e.g., thermal barriers, biomedical implants, and electronic multilayers). Thermal-spray technology as a family of processes is distinguished by its ability to deposit overlays of metals, ceramics, polymers, and composites of these materials in layers of substantial thickness (e.g., >25 µm) for engineering applications, often with equipment that can operate in the atmosphere and can be portable for use in the field. While traditional applications of thermal-spray coatings have addressed issues of surface protection, there is a growing activity in the use of the basic technology concepts for producing engineered functional surfaces and devices that offer the materials engineer a new scale of construction between thin films and macroscopic structures.

The silicon at the heart of highperformance integrated circuits (ICs) is the most precisely engineered material in mass production today. Crystalline silicon, purified to the level required for manufacturing modern, high-performance microelectronics (better than parts-per-billion purity), is arguably the most perfect material known. It is produced in large quantities and very economically. Continuing progress in basic silicon materials science has been central to this achievement. At the same time, access to such a perfect material and detailed knowledge of silicon's properties have provided the experimentalist with excellent opportunities to devise rather elegant experiments in whichthe parameters can be controlled very precisely. Silicon has similarly provided condensed-matter theorists with a nearly ideal test bed for new theoretical approaches. This issue of MRS Bulletin is dedicated to the materials physics that has both enabled the development of such a highly perfect material and benefited from its availability.

Thermal spray is a continuous, directed, melt-spray process in which particles (e.g., 1–50 μm in diameter) of virtually any material are melted and accelerated to high velocities, through either a combustion flame or a dc or rf nontransferred thermal-plasma arc. The molten or semimolten droplets impinge on a substrate and rapidly solidify to form a thin “splat.” The deposit is built up by successive impingement and interbonding among the splats. The splats accumulate into a wellbonded deposit, generally > 10 μm thick.

The microstructure and properties of thermally sprayed deposits depend critically on the thermal- and kinetic-energy histories of the particles entrained in the hot-gas jet. At impact, the particle temperature, molten fraction, size, velocity, and chemistry, along with substrate temperature and surface characteristics, control the morphology of individual particle splats. These factors control the adhesion, strength, microstructure, and porosity of a coating and influence the residual-stress state. In order to produce higher-quality coatings and expand the use of this versatile family of technologies, the ability to model and measure particle behavior is essential.

In thermal spraying, coatings are formed by particles flattening and piling up on the substrate. At impact, the sudden deceleration of the particle causes a pressure buildup at the particle-surface interface; the high pressure inside the particle forces melted material to flow laterally and ductile material to deform. The particle spreads outward from the point of impact and forms a “splat.” The arresting of spreading results from the conversion of particle kinetic energy into work of viscous deformation and surface energy. Solidification constraint (when the solidification front is advancing from the substrate surface fast enough to interact with the liquid during spreading) and mechanical constraint (due to the roughness of the substrate surface) can interfere with the flattening process.

In the manufacture of solid-oxide fuel cells (SOFCs), thermal-plasma technology, especially plasma spraying, offers several advantages, the most important of which is the ability to achieve considerably higher deposition rates than those that can be obtained with such approaches as physical or chemical vapor deposition (PVD or CVD). This article describes an international collaborative research project partially funded by NEDO (New Energy and Industrial Technology Development Organization, Japan) and carried out at five research laboratories around the world: at the University of Tokyo and National Institute for Research in Inorganic Materials (NIRIM) in Japan, the University of Minnesota in the United States, the University of Sherbrooke in Canada, and DLR—Stuttgart in Germany. The objective of the project was to conduct a comparative study on the possible use of four thermal-plasma spraying techniques for the integrated fabrication of SOFCs.

Ceramic thermal-barrier coatings (TBCs) have received increasing attention for gasturbine engine applications. The advantages of using TBCs include increased fuel efficiency by allowing higher gas temperatures and improved durability and reliability from lower component temperatures. As illustrated in Figure 1, TBCs can provide effective heat insulation to engine components, thus allowing higher operating temperatures and reduced cooling requirements. Atypical two-layer TBC system consists of a porous ZrO2-Y2O3 ceramic top coat and an oxidation-resistant metallic bond coat. These TBC systems can be applied to the metal substrate either by plasma spray or by electron-beam physical vapor deposition (EB-PVD) techniques.

Thermal-spraying of polymers can be traced back to the 1940s, when polyethylene (PE) was first produced by E.I. du Pont de Nemours & Company. Early work with flame-spraying guns was unsuccessful because the equipment, designed for spraying metals, produced a flame that was both too hot and too short to melt the PE without degradation. The technology has advanced considerably in the past 20 years. Flame-spray equipment and hardware accessories have been designed by numerous companies (PFS Thermoplastics, Alamo, Eutectic + Castolin, etc.) specifically for polymeric materials. The new equipment provides either a lower flame temperature or uses special cooling shrouds to cool the center of the flame, providing a longer residence time at a lower temperature. Many plastics can now be completely melted in-flight and allow heat-sensitive components to be coated with plastic.