Alloy Oxidation - Environmental Degradation of Metals - page 401

ments did not meet with success. Self-healing SiO 2 layers formed on silicide
coatings offer a much better oxidation-resistant barrier than Al 2 O 3 layers devel-
oped on aluminide coatings. The protectiveness of silicide coatings is mainly due
to (1) formation of glassy SiO 2 scales, (2) low diffusivity of molecular oxygen
in these scales, (3) high flexibility of SiO 2 in being able to form modified glasses
or silicates as a result of uptake of elements from the substrate or environment,
(4) self-healing properties of SiO 2 , (5) adjustment of thermal expansion by the
use of additive elements, and (6) inertness of silicon against sulfidation [79].
Silicide coatings are normally produced by the diffusion of silicon into the
refractory metal substrates having acceptable mechanical properties, thus forming
MoSi 2 or WSi 2 , which on oxidation at high temperature gives rise to the formation
of a thin, protective, glassy layer of SiO 2 . This layer exhibits protective properties
to as high a temperature as 1973 K, which is close to the melting point (1998
K) of crystobalite-modified SiO 2 . Such silicide coatings are not preferred for
protecting nickel-based alloys, mainly due to brittleness of the coating and the
rapid rate of Si diffusion into the alloy substrate during high-temperature expo-
sures. However, attempts to modify the standard M-Cr-Al-Y coating systems with
silicon additions up to about 2.7% on nickel-based superalloys have demonstrated
encouraging results with respect to oxidation and hot corrosion resistance [79].
Moreover, the addition of boron markedly improves the spalling resistance of
the SiO 2 scales due to formation of SiO 2 -B 2 O 3 glass scale with a higher thermal
expansion coefficient and a lower softening temperature [79]. Even though high-
temperature capabilities of silicide coatings are excellent, the useful upper tem-
perature limit of their applications in oxidizing environments are determined by
refractoriness of the coating components, their rates of conversion to oxide, the
necessity that the oxide be silica, the rate and site of vapor phase material loss,
and the rate of diffusional reactions between coating and substrate [80]. The
refractoriness of the coating depends on the melting point of the initial material
and the product of reactions between the substrate and environment. The silicide
coatings formed on refractory metals are primarily composed of a layer of the
most silicon-rich intermetallic in the binary system, which gets converted to lower
silicides by solid-state diffusion and silica during their high-temperature exposure
to oxidizing environments as depicted in Fig. 6.56a and b [80]. Nevertheless,
MoSi 2 and WSi 2 are the desired intermetallics to be formed on Mo and W during
the coating process, although during their service conditions they are converted
to lower silicides by diffusion processes. In the case of Mo, in addition to MoSi 2 ,
lower silicides, such as Mo 5 Si 3 and Mo 3 Si, can also form during performance of
the coating by inward diffusion of silicon toward the substrate metal [2]. In Fig.
6.56a, Mo 3 si phase is not observable for the limited exposure time of the coating.
Similar silicide phases also occur in tungsten. The oxidation resistance of these
silicides decreases in the order MoSi 2

Mo 3 Si, i.e., with decreasing
silicon content in the silicide [81]. The two lower silicides may eventually show

Mo 5 Si 3