Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Patent
1985-11-25
1987-08-18
Newsome, John H.
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
427423, B05D 108
Patent
active
046876782
DESCRIPTION:
BRIEF SUMMARY
In the field of gas turbines the development is characterized by increased engine temperatures. This development has made it necessary to change the composition of for instance nickel base alloys towards lower contents of oxidation resistant elements like chromium and higher contents of high temperature strengthening .gamma.'-forming elements like aluminium. The resistance against high temperature corrosion in the low chromium nickel base alloys has then been maintained by coating the components for increased oxidation resistance. The most common type of coating has been nickel aluminide with additions of chromium, silicon and sometimes platinum. The coating is obtained by forming an aluminium layer on the base material by chemical vapour deposition, and forming the nickel aluminide by a subsequent diffusion heat treatment.
A later development has been to build up "overlay coatings" by physical vapour deposition, plasma spraying or vacuum plasma spraying. These types of coatings are often called MCrAlY:s after the elements in the composition, where M can be Fe, Ni, Co or NiCo.
The expression MCrAlY only refers to the chemical composition, not to thermodynamical phase composition of the coatings. FeCrAlY has a ferritic body centered cubic (bcc) crystal structure which is ductile, the others a face centered (fcc) intermetallic cubic structure which is brittle in comparison.
Of the above mentioned methods of deposition, physical vapour deposition is generally considered to be the most expensive method and ordinary plasma spraying the cheapest. Ordinary plasma spraying has up to now not been used so frequently as other methods in spite of the cost factor, because the oxides formed are considered to be detrimental to the properties of the coating. This has been one of the reasons behind the development of the vacuum plasma process intended to give an oxide free coating.
Of the coating compositions mentioned above, FeCrAlY is known since the 1930:s under the designation "Kanthal", the others have been developed later on.
The present invention, which is of interest for aircraft engines and gas turbines, differs from conventional coating in the way that instead of trying to avoid oxides more or less unintentionally formed during coating and considered detrimental, a coating is intentionally formed consisting of a mixture of oxide- and metal phase particles, which by subsequent treatments is turned to a coating with properties equal or superior to those of a pure metallic coating with the same metal phase composition both with regard to hot corrosion and to heat conducting properties. The characteristics of the invention are evident from the attached patent claims. Rig tests as shown in FIG. 3 confirm that the object of the invention has been reached. The tests also confirm that the low alloy cost plasma sprayed FeCrAlY under these circumstances is quite comparable if not superior to the high alloy cost vacuum plasma sprayed CoCrAlY. As the bodycentered cubic FeCrAlY-coating is more ductile that the facecentered intermetallic cubic coatings, it can also serve as underlay coating for ceramic coatings with the advantage that the coefficient of expansion is more than 30% lower than for a face centered cubic coating and nearer the coefficient of expansion for ceramics. The ductility of FeCrAlY is also an advantage with regard to resistance against thermal fatigue in the matrix-coating-ceramic interfaces.
Coatings on high temperature alloys are slowly consumed by diffusion of metal atoms from the interior matrix-coating interface inwards and outwards and from oxygen and sulphur from the exterior atmosphere inwards. The efficiency of a coating can be judged by the time it takes until the coating shows signs of penetration as shown in FIG. 3.
The life requirements vary among other things with the times between engine overhauls, which can be 200-600 hrs for military jet engines up to 3000 hrs for civil jet engines and even longer for stationary gas turbines.
The diffusion of metal atoms from a nickel base alloy into an ove
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patent: 4198442 (1980-04-01), Gupta et al.
patent: 4248940 (1981-02-01), Goward et al.
patent: 4275090 (1981-06-01), McComas et al.
patent: 4411936 (1983-10-01), Schrewelius
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