CVD Coatings

In the CVD coating process, the tools are heated in a sealed reactor with gaseous hydrogen at atmospheric or lower pressure volatile compounds are added to the hydrogen to supply the metallic and nonmetaUic constituents of the coating. For example, TiC coatings are produced by reaction of TiCl vapors with methane (CH and hydrogen (H2) at 900 to 1100°C. The reaction is... 

If the rf source is applied to the analysis of conducting bulk samples its figures of merit are very similar to those of the dc source [4.208]. This is also shown by comparative depth-profile analyses of commercial coatings an steel [4.209, 4.210]. The capability of the rf source is, however, unsurpassed in the analysis of poorly or nonconducting materials, e.g. anodic alumina films [4.211], chemical vapor deposition (CVD)-coated tool steels [4.212], composite materials such as ceramic coated steel [4.213], coated glass surfaces [4.214], and polymer coatings [4.209, 4.215, 4.216]. These coatings are used for automotive body parts and consist of a number of distinct polymer layers on a metallic substrate. The total thickness of the paint layers is typically more than 100 pm. An example of a quantitative depth profile on prepainted metal-coated steel is shown as in Fig. 4.39. 

The requirements placed on the performance and reliability of CVD coatings are continuously upgraded. For one thing, this means the need for an ever increasing degree of purity of the precursor materials since impurities are the maj or source of defects in the deposit. The purity of a gas is expressed in terms of nines, for instance, six nines, meaning a gas that is 99.9999% pure, which is now a common requirement. It is also expressed in ppm (parts per million) or ppb (parts per billion) of impurity content. 

Niobium carbide, also known as columbium carbide, is a important refractory material with a high melting point. It is used as a CVD coating mostly on an experimental basis. Niobium carbide has two phases Nb2C and the monocarbide NbC. The latter is the only phase of industrial importance and the only one reviewed here. Its characteristics and properties are summarized in Table 9.5. 

Titanium nitride (TiN) is an important industrial material used extensively as a CVD coating. Its characteristics and properties are summarized in Table 10.7. 

Hot- and cold-mirror CVD coatings are used in projectors to maintain the film gate at low temperature and avoid damaging the film. They are also used increasingly in tungsten-halogen lamps. 

CVD coatings are used extensively in applications requiring resistance to wear and corrosion, often over a wide range of temperature. As mentioned in Ch. 1, these coatings and their substrates can be considered as composites which provide unique combinations of properties. 

The CVD coating materials for wear and corrosion resistance consist mostly of carbides and nitrides and, to a lesser degree, borides. Table 17.1 compares the relative properties of these materials. 

Relevant Wear and Corrosion Properties of CVD Coating Materials (at 25°C)... 

The function of the carbon coating is to contain the byproducts of the fission reaction, thereby reducing the shielding requirements. It also protects the nuclear fuel from embrittlement and corrosive attack and from hydrolysis during subsequent processing steps. CVD coatings of alumina deposited at 1000°C and beryllia deposited at 1400°C have also been studied for that purpose.P l... 

Williams, B. E., Stiglich, J., and Kaplan, R, CVD Coated Powder Composites, Part I Powder Processing and Characterization, Ultramet, in Proc. IMS Ann. Meeting, New Orleans, LA (Feb. 1991)... 

These high demands are not yet fulfilled by any available fiber coating. Only a C-coated SiC-fiber (NL 607) from Nippon Carbon is commercially available. To meet the above demands, a multilayer fiber coating is necessary. In a joint effort with ABB Heidelberg and TU Chemnitz, a CVD C-coating on C- and SiC-fibers and a C/SiC double CVD coating on C-fibers was developed and tested (Fig. 3). 

CVD Coatings. As in PVDj the structure of the deposited material depends on the temperature and supersaturation, roughly as pictured in Figure 8 (12). In the case of CVD, however, the effective supersaturation, ie, the local effective concentration in the gas phase of the materials to be deposited, relative to its equilibrium concentration, depends not only on concentration, but on temperature. The reaction is thermally activated. Because the effective supersaturation for thermally activated reactions increases with temperature, the opposing tendencies can lead in some cases to a reversal of the sequence of crystalline forms listed in Figure 8, as temperature is increased (12). 

Figure 2.8 CVD coatings on cemented carbide substrates, (a) Single layer TiC coating, 8 pm thick (b) Multilayer TiC/TiCN/TiN coating, —10 xm total thickness, and (c) TiCN coating supporting multiple alternating coating layers of A1203 and TiN. (Adapted from Ref.

LahannJ, Klee D, Pluester W, Hoecker H, Bioactive immobilization of r-hirudin on CVD-coated metallic implant devices. Biomaterials 2001 22(8) 817-826. 

Microstructural examination of billets A and B by SEM revealed the major difference between them was in the CVD layer deposited on the fibers before additional densification steps were performed. In billet A the layer appeared to be porous (see Figure 6), whereas billet B exhibited a uniformly dense CVD coating. The remaining portions of the matrix are derived from the liquid pitch impregnations, and these looked similar for A and B. It is possible that the lower shear strength of billet A can be attributed to the more porous and probably weaker CVD layer. 

Figure 6. Composite A interfilament matrix with porous CVD coating the filament.

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