Thermal spray bonding

A ceramic bond coat is almost exclusively used to modify a surface for adhesive bonding. The ceramics include, but are not limited to, alumina, zirconia, titania, spinels, carbides and combinations of these materials." The thermally sprayed bond coatings provide a rough surface on the order of 8 p,m R that has a three-dimensional morphology. Arc-plasma is traditionally used for deposition of the ceramics for these applications. 

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... 

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. 

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. 

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. 

Kurzweg, H., Heimann, R.B., Troczynski, T., (1998), Adhesion of thermally sprayed hydroxyapatite-bond-coat systems measured by a novel peel test , J. Mat. Sci. Med., 9(1), 9-16. 

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. 

Alternative deposited coatings involve thermal spray, such as plasma spray or two-wire arc [60]. Substrate alloy effects are minimal and coating chemistry and morphology can be designed for specific applications. Corrosion protection and good adhesive bond strength can be provided and maintained [61-64],... 

Figure 10.29 Bond strength of thermal spray coatings as a function of blasting quality for different types of grit and sand.

Numerous processing techniques are used to deposit and bond Ca-P powders to substrates, in-clusing plasma and thermal spraying (de Groot et al., 1987 Koch et al., 1990), ion-beam and other sputter-deposition techniques (Ong et al., 1S>91 Wolke et al., 1994), electrophoretic deposition (Ducheyne et al., 1986, 1990), sintering (de Groot, 1983 Ducheyne et al., 1986, 1990), sol-gel techniques (Chai et al., 1998), pulsed laser deposition (Garcia et al., 1998), and chemical vapor deposition (Gao et al., 1999). 

Further detail may be found in articles on Pre-treatment of metals prior to painting, Pre-treatment of copper, Thermal spray processes and Rubber to metal bonding - pre-treatments. 

In Thermal spray processes the nature of the process was described here, applications to adhesion are presented. Thermal spray coatings for applications involving adhesion are in three general areas, metallic bond or anchor coating, ceramic bond coat, and sprayed polymer as the adhesive. In many cases, the surface modification by the deposition of a thermally sprayed coating is an alternative to environmentally hazardous methods. The thermal spray process is a dry process in which the effluent is collected in a dry cartridge system and usually recycled. 

East Camino Undercarriage, Placerville eastbound, zinc arc-sprayed on deck with striped pattern to ensure concrete bond to asphalt overlay 12.5 cm of 225 (im zinc 12.5 cm gap, 300 m zinc westbound, thermally sprayed 500 p,m zinc on soffit, 150 m. ... 

Distinguishing between pores, cavities, and pull-outs can be difficult in the preparation of thermally sprayed coatings, as well. Cavities remains between the solidified particles of the spray material. Occluded gases may also be present, and particles that are only partially bonded to one another may be broken apart by thermal stresses. These laminar composites are discussed in Section 4.4. However, it is only possible to make conditional recommendations for the preparation of the sprayed coating being examined in a given case. Success depends on experience and on the execution of the preparation steps in a manner that is appropriate for the given material. 

Electric arc and plasma-arc thermal spraying are used when higher temperatures are required for improved bonding and coating density, or to liquefy higher melting point coating materials. 

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