Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-12-03
2004-10-05
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C091S307000, C091S309000, C427S255120, C427S255290, C427S255700, C428S216000, C428S332000, C428S469000, C428S472000
Reexamination Certificate
active
06800383
ABSTRACT:
TECHNOLOGY FIELD
The invention is related to the technology of the deposition of composite surface systems possessing high resistance to wear, erosion and chemicals. More specifically, the invention is related to the technology of the deposition of coatings containing tungsten carbides and mixtures of them with each other and with tungsten or free carbon.
Superhard erosion and corrosion resistant coatings, including those containing tungsten carbides, are widely used in manufacturing various articles of tools in present-day mechanical engineering. Such coatings possess high resistance to erosion, chemicals and wear, and thus substantially increase the life of mechanical engineering products and of tools operated under demanding conditions.
PRIOR ART
Patent GB 2 179 678 describes a surface composite system with high resistance to wear and erosion consisting of a mixture of tungsten (for plasticity) and tungsten carbide W
2
C (for hardness). These hard coatings made from a fine-grain mixture of tungsten carbide with metallic tungsten were obtained by means of physical vapour deposition (PVD) by spraying tungsten and carbon from separate sources. The tungsten and carbon are condensed on different-type substrates to form the said alloys of tungsten with tungsten carbide.
However, the rate of synthesis of tungsten carbides is very low, and internal stresses in the coatings increase sharply as the tungsten-carbon layer grows, resulting in delamination of the coatings. For this reason, it is impossible to produce sufficiently thick coatings by the PVD method. Furthermore, the physical vapour deposition method is intrinsically inapplicable for deposition of coatings on items of complex shape due to the impossibility of depositing the coatings on the parts of the item shadowed relative to the incident beam.
The chemical vapour deposition process (CVD) eliminates these disadvantages. The CVD process can be used to deposit wear and erosion resistant coatings on substrates and on items of complex shape.
In a typical CVD process for the deposition of composite coatings, the substrate is heated in the reaction chamber, and the previously mixed gas reagents are then introduced into this chamber. By varying the composition of the reaction mixture and of the parameters of the process (temperature of the substrate, composition of the reaction mixture, flow rate, total pressure in the reaction mixture, temperature of the gases supplied, etc.), it is possible to obtain a variety of coatings.
Among the CVD methods of tungsten carbide coating deposition, only the fluoride method makes it possible to form tungsten carbides of high quality at a low temperature. For this purpose, one may use thermal decomposition of a mixture of tungsten hexafluoride, hydrogen and carbon-containing gas in the CVD process.
Various reagents were used as carbon-containing gases, e.g. dimethylether, amines, propylene, etc., with the aid of which one may synthesise tungsten carbide of one or two compositions.
For example, the thermal decomposition of dimethylether (DME) (EP 0 328 084 B1) results in the formation of the mixture W+W
3
C; W+W
2
C+W
3
C; W+W
2
C in the form of bilaminar coatings. The internal tungsten layer of the coating is obtained from the as mixture WF
6
(0.3 l/min), H
2
(3 l/min), Ar (4.0 l/min) at 460° C. The external layer containing a mixture of tungsten with W
3
C is obtained from a mixture of WF
6
(0.3 l/min), H
2
(3 l/min) and DME (0.4 l/min) at 460° C. at a total pressure of 40 torr. The external coating W+W
2
C is obtained from a mixture of WF
6
(0.3 l/min) and DME (0.55 l/min) at 460° C. at a total pressure of 40 torr. The external coating W+W
2
C is obtained from a mixture of WF
6
(0.3 l/min), Ar (4.5 l/min) and DME (0.85 l/min) at 460° C. and a total pressure of 40 torr.
Patent JP 9113527 A 19910204 describes how tungsten carbide WC is obtained from a gaseous mixture of WF
6
, H
2
and amines with an atomic ratio of C to N equal to 1:20 and H to W equal to 1:15 at 400-900° C. The patent cites the production of WC from the mixture WF
6
:trimethylamine:H
2
=1:2:3 (the atomic ratios are C/W=6.0, H/W=6.0). The flow rate is 120 cm
3
/min at 800° C. and the total pressure is equal to atmospheric. A 70 &mgr;m layer forms in 30 minutes.
Patent JP 8857301 A 19880310 describes how a W
3
C coating on an aluminium substrate is obtained from a gaseous mixture of WF
6
, H
2
and aromatic hydrocarbon with atomic ratios C/W equal to 2-10 and H/C exceeding 3 at temperature 250-500° C.
Patent JP 84280063 A 19841228 describes how a W
2
C coating on a graphite substrate is obtained from a gaseous mixture of WF
6
, C
3
H
6
and H
2
with inert gas. The preferred regime:mixture WF
6
:H
2
=1:3-1:15 with an admixture of C
3
H
6
in the reaction mixture with molar ratio 0.01-0.3; the temperature of the substrate is 350-600° C.
Patent JP 84204563 A 19840929 describes how a W
2
C coating is obtained from a gaseous mixture of WF
6
, H
2
(molar ratio WF
6
:H
2
=1:3-1.15) and cyclopropane with molar ratio in the mixture 0.01-0.3 at a substrate temperature of 350-600° C. The example cited is the manufacturing of a W
2
C coating on a copper substrate from the mixture WF
6
: 40, H
2
: 320, Ar: 40, C
3
H
8
: 10 cm
3
/min at 500° C. with a growth rate of 3.3 &mgr;m/min.
EP A 0 305 917 describes how super-hard fine-grain non-columnar laminar tungsten-carbon alloys are obtained by chemical vapour deposition. The described alloys contain carbide phases consisting of W
2
C or W
3
C or mixtures of them with each other. It is demonstrated that these tungsten carbon alloys, when deposited on certain types of substrate, have a net of very fine micro-cracks all over the deposit. Coatings made from these alloys have inadequate resistance to wear and erosion.
EP 0 411 646 A1 describes a multilayer coating containing alternating layers of tungsten and a mixture of tungsten with tungsten carbides in the form of W
2
C, W
3
C or a mixture of them. It is demonstrated that such a coating increases the resistance of the material to wear and erosion. It is known, however, that the maximum composition effect is observed for layers with a distinct boundary between them. This is obviously not the case for the conjunction of layers of tungsten and the mixture of tungsten with tungsten carbide which is characteristic of this patent.
SUBSTANCE OF THE INVENTION
It follows from the patents cited above that different reagents and different technologies are used for the production of different types of tungsten carbides. In this connection, the main aim of this invention is to develop a universal technology making it possible to obtain all the known carbides, mixtures of them and also new carbides.
Furthermore, the problem of increasing the hardness of tungsten carbide coatings remains very important, because such key parameters as strength and wear resistance are related specifically to hardness.
A solution to these and other problems is provided by this invention, due to the development of a new method for the production of tungsten carbides and mixtures of them. The main distinguishing feature of the method is the preliminary thermal activation of the hydrocarbons used in the CVD process. The synthesis of a tungsten carbide layer of a certain composition depends on an activation temperature that varies from 500 to 850° C., on a total pressure in the reactor that varies from 2 to 150 kPa, and on the partial pressure of the hydrocarbon reagent.
Preliminary activation of the hydrocarbons results in the formation of the necessary concentration of hydrocarbon radicals and their associates with fluorine in the gaseous phase over a wide range. The proposed method makes it possible to alloy the carbides and/or mixtures of them with fluorine and fluoride-carbon compositions. Fluorine, as the most active chemical element, strengthens the interatomic bonds when it penetrates into the carbide lattice. It is the strengthening of the interatomic bonds in the carbide which produces the increase in
Kuzman Vladimir P.
Lakhotkin Jury V.
Hardide Limited
Sheridan & Ross P.C.
Turner Archene
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