Thermal spray applications

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. 

Chromium carbide is important in powder preparations designed for thermal spray applications of corrosion and wear-resistant coatings on tool and machine parts. Lower carbon carbides of chromium are important in hardfacing rods and electrodes for weld-applied overlays on machine wear surfaces. However, these carbides are usually formed in situ from Cr and C in the rod and not added as preformed carbides. The properties of Cr3C2 are listed in Table 2. 

The addition of aluminum to zinc is beneficial in improving its corrosion resistance and has resulted in development of coatings with aluminum contents between 5 and 55 %, like Galfan and Galvalume for hot dip and Zn-15 % Al alloy for thermal spray applications. In these coatings, the initial... 

Acid plant process gas ducting is often constructed of metallized carbon steel where temperatures exceed 400 °C. Metallizing refers to the thermal spray application of a thin (0.25-0.31 mm) layer of aluminum to the inside surface of the carbon steel duct. The aluminum prevents the oxidation of the carbon steel which would cause it to corrode at an accelerated rate if it were not protected. 

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 ... 

Chemical vapor deposition competes directly with other coating processes which, in many cases, are more suitable for the application under consideration. These competing processes comprise the physical vapor deposition (PVD) processes of evaporation, sputtering, and ion plating, as well as the molten-material process of thermal spray and the liquid-phase process of solgel. A short description of each process follows. For greater detail, the listed references should be consulted. 

There are job shops and applicators in the thermal spraying business and the annual value was about 1 x 109 in 1995. Up to 65% of the total company sales represented materials prices from l- 25/kg. Equipment prices for spray technology and the number of units sold per annum in the mid-1990s are given in Table 3. 

Thermal-aerosol applicators are employed to a considerable extent in the outdoor application of adulticides (sprays designed for killing adult mosquitoes). Well over 2,(XX),000 gallons of oil are used annually in these applicators. 

II. MARKET IMPACT — APPLICATIONS OF THERMAL SPRAY AS A SUBSTITUTE FOR PLATING... 

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. 

The selection of an alloy for a specific application is based on the cost and the corrosion resistance of the alloy in the environment of interest. It is also possible to subject the chosen alloy to a process by which the corrosion resistance of the selected material can be improved within the acceptable limits. Some of the corrosion prevention and protection strategies with respect to the aluminum-based alloys are (i) design (ii) alloy selection and joint sealants (iii) aluminum thermal spraying anodic coatings (iv) inhibitors (v) conversion and organic coatings and (vi) cathodic protection. 

Karthikeyan, J. (2009) Bioceramic composite coatings by cold spray, in Thermal Spray 2009 Expanding Thermal Spray Performance to New Markets and Applications, ASM International, pp. 391 -396. 

Gadow, R., Killinger, A., and Stiegler, N. (2010) Hydroxyapatite coatings for biomedical applications deposited by different thermal spray techniques. Surf. Coat. Technol., 205 (4), 1157 — 1164. 

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. 

Deshpande, S., Kulkarni, A., Sampath, S., and Herman, H. (2004) Application of image analysis for characterization of porosity in thermal spray coatings and correlation with small angle neutron scattering. Surf. Coat. Technol., 187, 6-16. 

Non-destructive adhesion testing of plasma-sprayed coatings using ultrasounds and laser shocks, in Thermal Spray 2004 Advances in Technology and Applications, Proceeding of the ITSC, Osaka, Japan, May 10-12, 2004 (eds C.C. Berndt, K.A. Khor, and E. Lugscheider) ASM International. 

Surface engineering by thermal spray technologies has attracted increasing attention over the last decades. This book provides the physical background needed to understand the technique and critically assesses its advantages and disadvantages for present and emerging applications. 

The mechanical properties of sintered apatites has limited their application to low stress areas in the body. To overcome this difficulty, apatites are applied as coatings on the surface of metallic implants where high loads on the implant are expected. Various coating options are available including thermal spraying, sputter deposition, pulsed laser deposition, sol-gel deposition, electrophoretic coating, electrodeposition, and biomimetic deposition. These are discussed in turn. 

Ben-Nissan B, Green DD, Kannangara GSK, Chai CS, Milev A (2001) P NMR studies of diethyl phosphite derived nanociystalline hydroxylapatite. J Sol-Gel Sci Technol 21 27-37 Berndt CC, Haddad GN, Farmer AJD, Gross KA (1990) Thermal spraying for bioceramic applications— A review. Materials Forum 14 161-173... 

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