Metal treatment – Process of modifying or maintaining internal physical... – Treating loose metal powder – particle or flake
Reexamination Certificate
2000-01-27
2001-07-03
Mai, Ngoclan (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Treating loose metal powder, particle or flake
Reexamination Certificate
active
06254704
ABSTRACT:
BACKGROUND
Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. In a plasma type of thermal spray gun, a high temperature stream of plasma gas heated by an arc is used to melt and propel powder particles. Other types of thermal spray guns include a combustion spray gun in which powder is entrained and heated in a combustion flame, such as a high velocity, oxygen-fuel (HVOF) gun.
One type of thermal spray powder is formed of chromium carbide and nickel chromium alloy. The carbide does not melt well and would be too brittle alone in a coating, so the alloy, typically nickel with 20% by weight chromium, is incorporated in each powder particle to provide a matrix. Chromium carbide and nickel chromium alloy are selected for high temperature, corrosive and oxidizing environments such as in a gas turbine engine, up to about 815° C.
There are three forms of chromium carbide, Cr
3
C
2
, Cr
7
C
3
and Cr
23
C
6
according to a standard phase diagram. The first, Cr
3
C
2
, is most wear resistant and stable, melting at 1811° C. The second melts at 1766° C. The third, Cr
23
C
6
, is least wear resistant and stable, melting at 1576° C. The first and second form have orthorhombic structure, and the third form is cubic.
Present commercially available powders of chromium carbide with nickel-chromium commonly are produced by blending, or by chemical or mechanical cladding of the alloy onto grains of the carbide, or by mixing, sintering and crushing. Such methods are relatively expensive and effect particles with relatively large grains of carbide. During spraying these grains are exposed to oxidizing conditions which decarborize the carbide and introduce oxides into the coatings. Also the larger grains in coatings can cause scuffing of mating surfaces.
A group of chromium carbide powders were introduced recently by Praxair Surface Technologies, Indianapolis, Ind., according to a brochure “CAT Powders—Introducing A Whole New Breed of CrC—NiCr Powder Technology” (undated). These are CRC-410 (70CrC-30 NiCr), CRC-425 (60CrC-40 NiCr) and CRC-415 (35CrC-65 NiCr). The present inventors obtained an x-ray diffraction analysis of these powders which showed the carbide to be in the form of Cr
23
C
6
, and a chemical analysis which determined a ratio (by weight) of chromium to carbon in the powders to be 22.2 for powders designated CRC-410-1 and CRC-425-1, and 37.6 for CRC-415-1.
SUMMARY
An object of the invention is to provide a novel thermal spray powder of chromium carbide and nickel-chromium, the powder having reduced cost and producing thermal sprayed coatings having high temperature properties comparable to or better than coatings from conventional powders of similar composition.
The foregoing and other objects are achieved by a thermal spray powder having a size essentially between 10 &mgr;m and 125 &mgr;m, with each powder particle consisting essentially of nickel, chromium and carbon. The chromium consists of a first portion and a second portion, the nickel being alloyed with the first portion in an alloy matrix. The second portion and the carbon are combined into chromium carbide substantially as Cr
3
C
2
or Cr
7
C
3
or a combination thereof, with the chromium carbide being in the form of precipitates essentially between 0.1 &mgr;m and 5 &mgr;m distributed substantially uniformly in the alloy matrix. The chromium should have a ratio by weight to the carbon between 6 and 12.
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L. Russo and Dorfmann, “A Structural Evaluation of HVOF Sprayed NiCr-Cr3C2 Coatings”, Proceedings of Thermal Spraying, May 22-26, 1995, Kobe, Japan (High Temp. Soc. of Japan).*
Brochure- “Cat Powders- Introducing a Whole New Breed of CrC-NiCr Powder Technology”, Praxair Surface Technologies (undated).*
Reardon, J.D. et al: “Plasma- and Vacuum—Plasma- Sprayed chromium carbide composite coatings”, Thin Solid Films (1981) pp. 345-351.*
Mor, F. et al: “Tribological behavior of different HVOF spray cartridge”. Adv. Powder Metall Part Matter (1996) (vol. 5) pp. 18/55-18/68.
Dorfman Mitchell R.
Laul Komal
Somoskey, Jr. Ronald Eugene
Mai Ngoclan
Schaefer, Esq. Ira J.
Sulzer Metco (US) Inc.
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