Thermal spraying

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. 

Powders for spraying. A range of powder compositions have been examined for use in plasma spraying. Amorphous calcium phosphate (Liu et al. 1994), biologically derived 

Implant materials for coating. Prosthetic materials coated with HAp include titanium, Ti-6A1-4V, stainless steel, Co-Cr-Mo, and alumina (Jiang and Shi 1998). These materials are roughened by grit blasting for a mechanical interlock between the melted component of the particle and the substrate. The Ti-6A1-4V and Cr-Co-Mo alloys are the most common. Ideally, the elastic modulus and co-efficient of thermal expansion of the substrate and the coating material will be matched to minimize any residual stresses at the interface. Hydroxylapatite (E = 100 GPa and a = 12 x 10 °C (Perdok et al. 1987)) is 

Cr-Co alloys are known for their high strength and wear resistance. Despite these benefits, there is a concern about the adverse biocompatibility of chromium and cobalt ions, whereas other studies have indicated that chromium promotes the crystallization of HAp (Wakamura et al. 1997) and, therefore, might be an important contributing factor leading to good fixation in bone. 

Surface preparation and bonding. Diffusion has been proposed to occur between titanium and HAp to produce a calcium titanate (de Groot 1987). Sintering studies between titanium and HAp have confirmed this diffusion (Lacout et al. 1984, Chai et al. 1993, Knepper et al. 1998) with another study suggesting incorporation of titanium into the HAp structure (Weng et al. 1994). Titanium oxide (Ti02) is an essential component of this diffusion reaction, and improved bond strength on oxidized substrates has been reported (Ueda et al. 2000). 

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Thermal spray processes can be used to give coatings of chromium carbide or nickel chromium for erosion resistance, copper nickel indium for fretting resistance, tungsten carbide cobalt for wear and abrasion resistance, and even aluminum siHcon polyester mixtures for abradabiHty. 

Surface cleanliness of the substrate is important for all thermal spray processes. Degreasing, which formerly often reHed on freons or chlorinated... 

Ziac foil coated with a conductive, pressure-sensitive adhesive is used for repair of other ziac coatings or for imparting corrosion resistance at field sites. The 0.08-mm ziac tape or sheet has a 0.025-mm coaductive adhesive. The laminate is cut to size and pressed tightly to activate the adhesive. Conductive tape can be wrapped around pipe, especially around welds or connections. The corrosion resistance of this material is iatermediate between galvanized or thermally sprayed coatings and zinc-filled paints (21,50). 

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

The coating has to adhere to the substrate. In older thermal spray technology, the bonding may be mechanical as a result of the interlocking between... 

Newer high velocity thermal spray coating processes produce coatings in compression rather than tension because of the shot peening effect of the supersonic particles on impact. This has permitted coating as thick as 12,500 p.m without delamination as compared to older processes limited to 1,250 p.m. The reduced residence time of particles at temperature minimises decomposition of carbides present in conventional d-c plasma. This improves wear and hardness (qv) properties. 

The selection of a particular deposition process depends on the material to be deposited and its availabiUty rate of deposition limitations imposed by the substrate, eg, maximum deposition temperature adhesion of deposit to substrate throwing power apparatus required cost and ecological considerations. Criteria for CVD, electro deposition, and thermal spraying are given in Table 2 (13). 

Characteristic Evaporation Ion plating Sputtering Chemical vapor deposition Electro- dep 0 sitio n Thermal spraying... 

There are job shops and apphcators in the thermal spraying business and the annual value was about 1 x 10 in 1995. Up to 65% of the total company sales represented materials prices from 1— 25 /kg. Equipment prices for spray technology and the number of units sold per annum in the mid-1990s are given in Table 3. 

ASM International, Materials Park, Ohio. Miscellaneous thermal spray conference proceedings. 

Chromium carbide is important in powder preparations designed for thermal spray apphcations of corrosion and wear-resistant coatings on tool and machine parts. Lower carbon carbides of chromium are important in hardfacing tods and electrodes for weld-apphed ovedays 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 Ci2C2 are hsted in Table 2. 

J. W. Holmes and B. H. Pilsner, Thermal Spray, Proceedings of the National Thermal Spray Conference 1987, ASM, 1988, p. 259. 

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

Improvement has also been made to inerease the wear resistanee of some transition pieees in the aft end or pieture frame area. Cobalt-base hard eoatings applied by thermal spray have been tested in field maehines and the best spray has been shown to improve the wear life of sealing eomponents by more than four times. 

Platinum coatings may also be thermally sprayed or sputtered onto the titanium, to provide uniform well-bonded coatings. Titanium rod may also be spiral wound with platinum wire. However, the use of these techniques is limited. 

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)... 

Childs, J. N., Zinc sprayed coalings in a liquid cargo tanker ship, Proc. 8th Int. Thermal Spray. Conf., American Welding Society, pp. 449-455 (1976)... 

Nagaska el ai., Weathering Test on Zinc, Aluminium and Zinc-Aluminium Alloy Sprayed Coating for 5000 Days, Proc. lOth Int. Thermal Spray. Corf., DVS, Diisseldorf, Poster report (1983)... 

Oeshsle, S. J. and Childs, J. N., Thermal Sprayed Coatings, in Steel Structures Painting Council Manual, Vol. 1, pp. 456-464 (1982)... 

Shaw, B. A. and Moran, P. J., Characterization of the Corrosion Behaviour of Zinc-Aluminium Thermal Spray Coatings , Corrosion 85, Boston, March 1985, NACE, Houston. Paper 212 (1985)... 

James, D. H., Thermal Spraying by the Electric Arc Process , Metallurgist and Materials Technologist, IS, 85-90 (1983)... 

Harris, S. J., Green, P. D. and Cobb, R. C., Thermally Sprayed Al-Zn-In-Sn Alloys in 3rd Int. Conf. on Advances in Surface Engineering for Corrosion and Wear Resistance, Newcastle-upon-Tyne, 1-10(1992)... 

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 ... 

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