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Proceedings of a Workshop: Potential Applications of Concentrated Solar Energy
to alter the bulk. This gives the ability to independently optimize the bulk and surface properties; for instance, a surface region may need to be hard or corrosion resistant, while mechanical toughness is required of the bulk. In the first talk of Session 3, William D. Sproul of Northwestern University, discussed several current surface modification technologies.
Surface Heating with Light
Since visible light is absorbed in about the first 30 nm in most metals and opaque materials, intense light fluxes can be used to selectively heat the surface region. Techniques which use light to heat surfaces can be classified according to the interaction time and energy density. In general, the processes which have a short interaction time (10-4-10-10 sec.) have a high energy density (104 -1010 W/m2). Short interaction time processes can achieve extremely high heating and cooling rates (1012 K/sec.), and thus can be used to produce nonequilibrium phases such as metallic glasses. Laser glazing of a previously applied thin film can heal pinholes and defects as well as promote adhesion and densification. Slower interaction time processes can be used to anneal or harden the surface region or melt a previously applied powder or film. Powder melting can produce a dense well-adhered film. These films can be several mils to over a hundred mils thick. One of the primary disadvantages is that the surface tends to be quite rough and requires a postmachining process. Lasers can be used to ablate material to pattern the surface on a very fine scale. Printing roles are patterned in this manner to achieve optimum inking characteristics.
Thermal Spray/Plasma Arc
In the techniques of thermal spray or plasma arc, powder is fed into a heat source (either a flame or electric arc) where it is melted and then accelerated as molten droplets into the surface where it condenses into a film. This is a relatively inexpensive and rapid process for forming a surface film, but the films which result have a high density of several types of defects, including unmelted particles, voids, and oxidized particles. The process is extremely complex, with many parameters that affect the final film properties in a complicated manner. Nonetheless, this is a widely used process, particularly in the aerospace industry where a typical gas turbine engine has 15 pounds of thermal spray coatings.
Chemical Vapor Deposition
Chemical vapor deposition (CVD) uses thermal energy to decompose precursor molecules which results in deposition of a film. This technique is widely used in the semiconductor and metallurgical coatings industries to produce Si, SiO2, TiN, and Al2O3, among others. of particular interest is photo-assisted CVD where light is used to not only heat the surface region, but to photolytically assist in breaking the precursor molecule bonds. Principal disadvantages are the high temperatures required and the toxicity of the precursor materials. Advantages are nondirectionality of deposition, which results in uniform coatings on complex shaped parts, and low cost.
Physical Vapor Deposition
In physical vapor deposition, films are produced by physical transport of atoms from source to substrate. Evaporation techniques use resistance or electron beam heated hearths to evaporate the source atoms, while sputter deposition forms films from atoms ejected from the source by bombardment by energetic particles, usually inert gas ions. Both techniques need a vacuum environment, and the film properties are strongly affected by the energy and surface mobility of the arriving species.