Metallizing Thermal Spray

The metallizing technique for applying metal coatings consists of thermally spraying one type of metal onto a metallic substrate, most often steel, or reinforced concrete to provide cathodic protection to the rebar material as discussed in Chap. 13. The technique is accomplished in several ways. Table 14.6 shows the main metallic materials that have been used for the production of spray coatings and Table 14.7 summarizes the limits and applicability of each technique. 

Coatings can be sprayed from rod or wire stock or from powdered materials. The material (e.g., wire) is fed into a flame, where it is melted. The molten stock is then stripped from the end of the wire and atomized by a high-velocity stream of compressed air or other 

Aluminum Highly resistant to heat, hot water, and corrosive gases. Excellent heat distribution and reflection. 

Brass Machines well, takes a good finish. 

Bronze Excellent wear resistance exceptional machineability dense coatings (especially Al, Bronze). 

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Fifth National Conference on Thermal Spray, June 7—11, 1993, Anaheim, Calif, American Society for Metals, Metals Park, Ohio, 1993. 

Coating metal surfaces with water-impermeable barriers (such as paint, epoxies, thermal sprays, grease, and oil)... 

Bailey, J. C., Porter, F. C. and Round, M., Metal Spraying of Zinc and Aluminium in the UK , Proc. 12th Int. Thermal Spraying Conf, Paper 8 (1989)... 

To avoid this phase change, zirconia is stabilized in the cubic phase by the addition of a small amount of a divalent or trivalent oxide of cubic symmetry, such as MgO, CaO, or Y2O3. The additive oxide cation enters the crystal lattice and increases the ionic character of the metal-oxygen bonds. The cubic phase is not thermodynamically stable below approximately 1400°C for MgO additions, 1140°C for CaO additions, and below 750°C for Y2O3 additions. However, the diffusion rates for the cations are so low at Xhtstsubsolidus temperatures that the cubic phase can easily be quenched and retained as a metastable phase. Zirconia is commercially applied by thermal spray. It is also readily produced by CVD, mostly on an experimental basis. Its characteristics and properties are summarized in Table 11.8. 

Protecting a surface from corrosion by coating can be accomplished by a number of well-established processes which include paints, metal plating (with zinc or cadmium), diffusion, thermal spraying, and, more recently, vapor deposition processes. Of these physical vapor deposition (PVD) is used extensively in corrosion protection. Typical applications are ... 

Thermal spray is a well-established, relatively low-cost, industrial processes which is used widely for the deposition of metals and compounds, including the refractory carbides and nitrides. Examples are coatings of tungsten carbide with a cobalt binder which are of major industrial importance. PI... 

Most metals and many refractory compounds can be thermal-sprayed. Applications include coating of gas-turbine components for aircraft and industrial use, components of steam turbines and diesel engines, components for the oil and gas industry, paper and pulp industry, and chemical processing industry. 

Meial Mairix Composites. Silicon carbide particles are contributing to easy-to-cast metal-matrix composites (MMCs). When compared with their non-reinforced counterparts, the SiCp/Al components are more wear resistant, stiffer, and stronger, accompanied by improved thermal stability. Additional advantages include lower density and lower cost. Nearly all prior aluminum MMCs required labor-intensive methods, such as powder metallurgy, diffusion bonding, squeeze casting, or thermal spraying. 

Spherical particles proved to be superior in several applications owing to their favorable properties. Thus, they are used in thermal spraying for their excellent flowabil-ity, in powder metallurgy because of their excellent reproducibility in manufacturing parts with controlled porosity and as a filler material, as well. Metal microspheres can be easily produced by melt atomization. Similar method in the case of ceramics is impractical. Micron-sized ceramic particles, however, can be smelted by thermal plasmas that provide exceptional conditions for spheroidization due to its high temperature. In terms of purity and residence time of the particles in the hot temperature core, RF plasmas provide better conditions as compared to arc plasmas. 

Chadwick JK, Wilson HK, White MA. 1997. An investigation of occupational metal exposure in thermal spraying processes. Sci Total Environ 199 115-124. 

Nanocrystalline materials have received extensive attention since they show unique mechanical, electronic and chemical properties. As the particle size approaches the nanoscale, the number of atoms in the grain boundaries increases, leading to dramatic effects on the physical properties and on the catalytic activity of the bulk material. Nowadays, there is a wide variety of methods for the preparation of nanocrystalline metals such as thermal spraying, sputter deposition, vapor deposition and electrodeposition. The electrodeposition process is commercially attractive since it can be performed at room temperature and the experimental set-up is less demanding. Furthermore, the particle size can be adjusted over a wide range by controlling the experimental parameters such as overvoltage, current density, composition, and temperature (see Chapter 8). 

An ideal bond coat should have several characteristics that include uncompromised biocompatibility, good adhesion to both the metal substrate and the osseoconductive top coat, and a well-defined melting point to allow application of thermal spray technology. The adhesive function of the bond coat has been loosely compared to the action of a double-sided adhesive tape. 

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