Electrolysis: processes – compositions used therein – and methods – Electroforming or composition therefor – Powder – flakes – or colloidal particles
Reexamination Certificate
2001-06-14
2004-02-24
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electroforming or composition therefor
Powder, flakes, or colloidal particles
C205S176000, C205S178000, C205S184000, C205S224000, C427S212000, C427S585000
Reexamination Certificate
active
06695960
ABSTRACT:
The present invention relates to a process for producing a metal alloy powder containing nickel and/or cobalt, chromium, aluminium and yttrium in the form of a &ggr; phase and a &bgr; phase dispersed in the &ggr; phase, capable of forming from the &bgr; phase, by exposure to air at elevated temperatures, an adhering superficial alumina barrier.
It is known to protect metal parts operating at high temperatures, in particular aeronautical turbine blades, against corrosion and/or oxidation by applying various coatings obtained by diffusion (mainly based on nickel aluminide &bgr;-NiAl, if necessary modified by additional elements). These coatings have numerous advantages but are restricted as regards composition. For certain applications the “ideal” coating may have a chemical composition that is impossible to obtain by diffusion. This is why the projection application of alloys such as those described by the qualitative formula MCrAlY (where M=Ni and/or Co and/or Fe) has been studied by means of physical deposition techniques. These techniques enable so-called “active” elements such as yttrium, hafnium, tantalum and zirconium to be added in trace amounts (up to 2%) to the deposited coating.
A typical example of an MCrAlY coating consists of a nickel-based alloy containing 20% of cobalt powder, enabling the following reaction to be avoided:
&ggr;-Ni+&bgr;-NiAl→&ggr;′-Ni
3
Al+&agr;-Cr,
which is possible above 1,000° C., 20 to 25% of chromium in order to reinforce the resistance to type I corrosion, 6 to 8% of aluminium (aluminoforming compound) and about 0.5% of yttrium, which reinforces the adherence of the alumina layer to the aluminoforming alloy. Its general microstructure is that of a two-phase alloy containing precipitated &bgr;-NiAL (aluminoforming phase) in a &ggr; matrix. According to the conditions of use, other elements may be added and/or the above concentrations may be altered. For example, if the coating is intended to be used to prevent type II hot corrosion in the presence of vanadium, the concentration of chromium may exceed 30% by weight. Numerous MCrAlY compositions are commercially available, the most commonly used being those known under the names AMDRY 997 (NiCoCrAlYTa) and AMDRY 995 (CoNiCrAlYTa).
A particularly interesting feature of these protective alloys is the possibility of adding active elements thereto. The addition, in small amounts (of the order of 1 atomic % or less), of elements such as Y at Hf improves very significantly the adhesion of Cr
2
O
3
or Al
2
O
3
layers to the alloys in question. Due to this, the protective effect of the oxide layer is preserved over prolonged periods, particularly under thermal cycling conditions.
Among the physical deposition techniques noted above for obtaining MCrAlY coatings, there may be mentioned in particular hot projection and more especially plasma projection, in which the material to be deposited is introduced, by means of a carrier gas, into the jet of a plasma torch in the form of powder granules 20 to 100 &mgr;m in diameter. After having melted, the droplets of the material that has liquefied are projected at high speed onto the surface of the substrate. The plasma flame is produced by the very rapid expansion in a nozzle anode of a plasma-forming gas (Ar+10% H
2
for example) ionised during its passage through an arc chamber. Any material available in powder form that can be melted without decomposing or evaporating can thus be deposited on the surface of a substrate. This projection deposition may take place either at atmospheric pressure (in air or in a neutral atmosphere) or under reduced pressure. In all cases the coating is formed at a high rate, typically at a rate of 100 &mgr;m/minute. This deposition technique is extremely directional and is thus difficult to employ with parts that are of complex shape.
It is mainly plasma projection under reduced pressure that is used for the deposition of MCrAlY type alloys. The projection device is installed in an enclosure that is subjected to a high reduced pressure (P=0.05 bar). This enables oxidation of the projected alloy particles to be avoided, increases the velocity of the gases in the plasma jet and elongates the flame, which in turn increases the impact velocity of the molten particles and, consequently, reduces the porosity. Finally, it should be noted that this technique permits an initial ionic pickling to be carried out by polarising the surface of the substrate, which improves the adhesion of the coating to the substrate. The deposits obtained are adherent and slightly brittle and may be very thick (several millimetres thick). After projection of the particles, the MCrAlY coatings are diffused during a thermal treatment in vacuo. They are however rough and require a post-operative machining followed by a tribo-finishing.
There may also be mentioned a high velocity flame projection technique carried out by reacting a fuel (hydrocarbon and/or hydrogen) and a combustion-supporting medium (air reconstituted from a mixture of nitrogen and oxygen, or pure oxygen).
Another category of physical deposition techniques is that of vapour phase physical deposition, which involves triode cathodic sputtering and evaporation in an electron beam.
For the triode cathodic sputtering a three-electrode system polarised to a value of several kilovolts and placed in an enclosure subjected to a vacuum of about 10
−2
Pa enables extremely adherent MCrAlY alloys that are non-porous and less directional than in the case of plasma projection to be deposited at a rate between 5 and 25 &mgr;m/hour.
In order to effect evaporation in an electron beam in an enclosure maintained under a vacuum harder than 10
−4
Pa, an electron beam is focussed on the surface of the material to be deposited contained in a cooled metal crucible. A continuous ingot feed system enables the level of the liquid bath and the deposition conditions to be maintained constant. The emitted vapours condense on the substrate arranged opposite the liquid bath. This substrate is maintained at a sufficiently high temperature so as to minimise the inherent defects in the columnar growth of the deposit. These defects are subsequently eliminated by shock blasting followed by a thermal diffusion treatment and elimination of stresses. The deposition rates may reach values of up to 25 &mgr;m/min. This technique is described in U.S. Pat. No. 5,698,273 A, and was mainly developed for the deposition of MCrAlY on turbine machinery blades. Its widespread use is however limited on account of the associated investment and operating costs. Furthermore, this process is extremely directional and does not enable certain coating compositions to be easily obtained (e.g. in the case of alloys containing elements with widely differing vapour pressures).
On the other hand this process enables a combined protective +thermal barrier coating to be produced (zirconium oxide partially stabilised with yttrium oxide (ZrO
2
+8% Y
2
O
3
)), this combined coating having better mechanical properties and a better resistance to thermal shock than coatings obtained by plasma projection.
Electrolytic deposition of the alloy MCrAlY(Ta) is impossible since it would involve the combined deposition in aqueous medium (water is no longer stable beyond −1 V with respect to a normal hydrogen electrode) of nickel (EO=−0.44 V), cobalt (EO =−0.28 V), chromium (EO=−0.744 V), aluminium (EO=−1.662 V), yttrium (EO=−2.372 V) and, possibly, tantalum (EO=−0.750 V). In order to obtain an MCrAlY deposit electrochemically it would thus be necessary to produce a composite deposit comprising on the one hand nickel and/or cobalt, and on the other hand particles of CrAlY, and then to effect the diffusion of the composite consisting of electrolytic deposit+particles+substrate at high temperature (typically 2 hours at 1100° C.). Examples of implementation of the above are described in U.S. Pat. Nos. 4,305, 792 A, 4,81
Bacos Marie Pierre
Josso Pierre
Christie Parker & Hale LLP
Onera (Office National d' Etudes et de Recherchers Aerospat
Phasge Arun S,.
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