Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1998-04-20
2001-03-13
Speer, Timothy M. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S627000, C428S641000, C428S655000, C428S663000, C428S664000
Reexamination Certificate
active
06200691
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to coatings for refractory metals and, in particular, to a coating system incorporating novel coefficient of thermal expansion (CTE) matching and diffusion barrier layers.
BACKGROUND OF THE INVENTION
The use of refractory metals, such as molybdenum and tantalum, has been investigated for a number of applications due to the high temperature mechanical properties and high melting points of such metals. The principal disadvantage of the use of refractory metals in high temperature applications is the poor resistance of refractory metals to oxidation. Molybdenum, in particular, is subject to severe oxidation in high temperature environments.
In order to reduce oxidation, researchers have investigated the use of a variety of coating systems on refractory metal substrates. In particular, certain silicides of refractory metals, such as molybdenum disilicide, have been identified as promising coating system components for oxidation resistance. Unfortunately, in such coating systems, silicon tends to diffuse from the oxidation resistance coating into the substrate resulting in formation of less oxidation resistant sub-silicides and impaired mechanical properties, e.g., loss of ductility. Some research has addressed the possibility of synthesizing a nitrided composite film as a potential diffusion barrier layer, based on encouraging diffusion barrier characteristics reported for a nitrided molybdenum disilicide film. However, research is continuing and it is apparent that substantial challenges remain with respect to fully realizing the potential benefits of refractory metals for a variety of high temperature applications.
SUMMARY OF THE INVENTION
The present invention relates to a refractory metal coating system including a novel diffusion barrier layer that can be used in conjunction with a CTE matching layer. It has been recognized that a significant problem associated with oxidation resistance coating systems for high temperature, refractory metal applications is the potential CTE mismatch between the substrate and the oxidation resistance coating layer. This CTE mismatch can result in interface stresses and possible faults as a result of differential expansion/contraction due to temperature variations associated with high temperature applications. The present invention addresses this potential CTE mismatch as well as potential coating material diffusion concerns to provide an improved refractory metal coating system.
According to one aspect of the present invention, a CTE matching layer is provided to address CTE mismatch concerns. In particular, the resulting coated product includes a substrate of refractory metal such as molybdenum, an oxidation resistance layer that may be formed from, for example, molybdenum disilicide, and a CTE matching layer disposed between the substrate and the oxidation resistance layer having a first CTE at a first portion thereof adjacent to the substrate and a second CTE at a second portion thereof adjacent to the oxidation resistance layer. Preferably, the first CTE substantially matches the CTE of the substrate or any intervening coating layer such as a diffusion barrier layer. The second CTE preferably matches that of the oxidation resistance layer.
According to another aspect of the present invention, a novel barrier layer is provided as part of a coating system for a refractory metal. The coating system includes an oxidation resistance coating and, preferably, a CTE matching layer in addition to the diffusion barrier layer. The barrier layer, which is disposed between the oxidation resistance coating and the refractory metal substrate, inhibits diffusion of material from the oxidation resistance coating and/or CTE matching layer into the substrate. A preferred barrier layer is a composite material including a carbon containing component such as SiC. The barrier layer may, for example, be a material in the Mo—Si—C—N quarternary system.
In one embodiment, an oxidation resistance coating system is provided for a molybdenum substrate. The coating system includes an oxidation resistance layer formed on the molybdenum substrate, a CTE matching layer disposed between the oxidation resistance layer and the molybdenum substrate, and a barrier layer disposed between the CTE matching layer and the molybdenum substrate. The oxidation resistance layer is preferably formed from a silicide of molybdenum such as MoSi
2
. A self-healing silica scale forms on this layer during oxidation and provides a diffusion barrier for oxygen. The oxidation resistance layer may be doped, e.g., with boron and/or germanium to lower the viscosity of the resulting glassy oxide. The CTE matching layer has a graded composition and CTE across its depth such that the CTE at an upper portion of the CTE matching is substantially the same as that of the oxidation resistance layer and the CTE at a lower portion of the CTE matching layer is substantially the same as the substrate or is adjusted slightly from the CTE of the substrate in view of the intervening barrier layer. In this regard, a preferred CTE matching layer is a composite coating formed from MoSi
2
+X SiC where X varies as a function of depth from substantially zero at the MoSi
2
interface to a value selected so that the CTE of the composite coating substantially matches that of the molybdenum substrate and/or the barrier layer at the barrier layer interface. A composite of MoSi
2
with 1.96 moles of SiC (i.e., approximately 50 percent by weight) has a similar CTE to Mo, but this value of X may be varied somewhat due to the presence of the diffusion barrier layer. The diffusion barrier layer, in this embodiment of the invention, inhibits diffusion of carbon and silicon into the substrate, and is preferably a coating in the Mo—Si—C—N quaternary system, i.e., MoSi
x
C
y
N
z
. A suitable barrier layer can be conveniently formed by sputtering the MoSi
2
+X SiC composite coating material in a nitrogen ambient, where the nitrogen pressure during deposition is selected, in conjunction with the other deposition process parameters, to yield appropriate diffision barrier layer characteristics.
This coating system can be fabricated according to the present invention by: 1) forming a composite Mo Si
2
+X SiC sputtering target by reaction synthesis using powders of Mo, Si and &bgr;-SiC and hot pressing, 2) RF sputtering the composite target in a nitrogen ambient to form the barrier layer, 3) continuing to RF sputter the composite target in a vacuum or inert ambient to form the CTE matching layer, 4) applying a layer of doped MoSi
2
, e.g., by plasma spraying, and 5) annealing the composite films (before or after application of the oxidation resistant coating), e.g., at 1000° C. for 30 minutes in a vacuum furnace. Analysis has shown that the films from the composite target are amorphous as deposited. Upon annealing, the CTE layer crystallizes but the barrier layer remains amorphous and is effective in minimizing diffusion of silicon and carbon into the molybdenum substrate.
The present invention thus provides a refractory metal coating system that reduces oxidation and therefore yields a coated product suitable for various high temperature refractory metal applications in the glass manufacturing, nuclear energy, aircraft and space technology industries. The coating system also reduces potential damaging stresses due to CTE mismatch between the substrate and the oxidation resistance coating material, and reduces diffusion of coating materials into the substrate which could otherwise compromise the mechanical properties of the coated refractory metal product.
REFERENCES:
patent: 5300322 (1994-04-01), Lowden
patent: 5304342 (1994-04-01), Hall et al.
patent: 5472487 (1995-12-01), Chin et al.
patent: 5874175 (1999-02-01), Li
S. Govindarajan, J.J. Moore, T.R. Ohno and J. Disam, “Development of a Si/C Diffusion Barrier Layer Based on the Mo-Si-C-N System,” Cover Sheet for ICMCTF97 Conference, Paper No. A1.03. (No Month/Date).
S. Govindarajan, J.J. Moore, D.J. Aldrich
Govindarajan Shrinivas
Moore John J.
Colorado School of Mines
Holme Roberts & Owen LLP
Speer Timothy M.
Young Bryant
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