Environmental Engineering Reference
In-Depth Information
entire unit is maintained at a low pressure (50-70 mbar). This technique not only
allows longer jet length and higher particle velocity accompanied by heating of
the substrate to 1073-1173 K, but unwanted gas-metal reactions are also
avoided, producing coatings with a high degree of density and good adhesion.
The greater processing flexibility and closer compositional control of LPPS have
permitted deposition of coatings with desired compositions and microstructures
that cannot be achieved by electron beam physical vapor deposition (EPPVD).
The most corrosion-resistant overlay coatings of current use for gas turbine foils
that rely on the formation of protective Al
2
O
3
scales are the MCrAlY (having a
nominal composition in wt% of 18% Cr, 23% Co, 12% Al, 0.5% Y, and balance
Ni) coatings with minor element contents and microstructural homogeneity, and
are produced by LPPS. The current generation of LPPS coatings of the type
Ni-Co-Cr-Al-Hf-Si-Y protect-single crystal airfoils in production of JT9D and
PW2037 engines.
Another technique of recent origin is laser-assisted spraying, whereby the coat-
ing powder is blown from the side into a high-power laser beam, which heats it
to the melting point so that the added elements are either alloyed with the sub-
strate or embedded as solid particles in the molten surface.
Vapor Deposition and Related Techniques.
Physical vapor deposition (PVD)
consists of evaporating the elements required to form the coating, typically by
directing an electron beam onto the substrate, in a high-vacuum chamber and
allowing the elements to condense on it, which may be preheated to improve the
adhesion and is usually rotated to improve the uniformity of the coating [75].
Accordingly, application of coatings to the interiors of holes or into hidden cavi-
ties is difficult, and since only thermal energy is involved and no vacuum glow
discharge is used, the adhesion of the coating is also poor. Ion plating is a related
approach where, by increasing the gas pressure (
1 MPa) in the deposition
chamber and creating a glow discharge, the energy of the ionized gas atoms,
which are usually argon, can be used to clean the component surface by sputtering
for improved adhesion of the coating.
Chemical vapor deposition (CVD) involves volatilization of a molecular spe-
cies containing the element or elements required for the coating; the molecules
subsequently decompose onto the component surface, depositing the element/
elements. Thus, the coating is obtained either by thermal decomposition (pyroly-
sis) or chemical reaction in the gaseous phase. A typical example of thermal
decomposition of the gaseous phase is
1473 K
TiI
4(g)
→
Ti
(deposit)
2I
2(g)
(6.56)
Similar decomposition of the gaseous phase by reduction reaction with hydro-
gen or a metallic vapor can be illustrated as
