Coating processes – Coating by vapor – gas – or smoke – Mixture of vapors or gases utilized
Reexamination Certificate
1999-02-26
2001-01-23
Beck, Shrive (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Mixture of vapors or gases utilized
C427S578000, C427S376200, C428S688000, C428S698000
Reexamination Certificate
active
06177136
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for the coating of substrates with a silicon-containing protective layer by chemical vapour deposition (CVD) coating.
2. Description of the Prior Art
In order to protect high-temperature components of carbon from oxidation it is known to provide the component with a quartz glass (SiO
2
) layer by CVD using silanes. The amorphous quartz glass layer passes into the crystalline state (cristobalite) at a temperature above approximately 1,100° C. This so-called transition of quartz leads to cracks in the coating which lead to rapid oxidation of the carbon component, in particular following cooling and re-heating of the component, that is to say under conditions of thermal shock stress.
Although cracking can be checked by applying further layers, for example of silicon carbide, such a sequence of different layers is associated with a corresponding number of process steps and is hence costly and time-consuming. Moreover, a quartz glass layer applied to a metal substrate by CVD leads to spalling under conditions of mechanical and thermal shock stress.
The object of the invention is to provide substrates of different types in simple manner with a firmly adhering protective layer which also withstands high thermal shock stresses.
This is achieved according to the invention as decribed below.
SUMMARY OF THE INVENTION
In accordance with the invention, a compound of the following structure (1):
in which
R
1
is an alkyl group having 1 to 4 carbon atoms, and
R
2
is hydrogen or an alkyl group having 1 to 4 carbon atoms is used as the starting compound for the CVD process for the coating of the substrate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
R
1
is preferably a methyl radical and R
2
hydrogen. That is to say, tri(dimethylaminosilyl)-amino-di(dimethylamino) borane which has the following structural formula (2):
is preferably used.
A thermal CVD process, in particular an LPCVD (low-pressure CVD) process, is preferably employed as the CVD process. However, other CVD processes, in particular plasma CVD, may also be used according to the invention in place of the thermal CVD process.
The apparatus for the thermal CVD process preferably has a pressure-tight supply vessel which contains the liquid starting compound according to the formula (1) or (2) and is under pressure from an inert gas, for example argon. The liquid starting compound is supplied by way of a flow measuring device to a mixing means into which an inert gas, for example nitrogen, flows at the same time by way of a corresponding gas flow measuring device, as a result of which there is formed from the liquid starting compound in the mixing means an aerosol which is vaporised without residue in a heated vaporiser. The vapour is supplied to one end of a coating oven of preferably tubular construction, into which the substrate to be coated or a plurality of substrates are arranged above one another and/or in sequential manner. A vacuum pump is connected to the other end of the tubular oven.
When tri(dimethylaminosilyl)-amino-di(dimethylamino) borane is used as the starting compound, the vaporiser temperature is, for example, from 40° C. to 150° C., preferably 75° C. to 100° C. The negative pressure in the coating oven can be from 10
−1
to 10
−5
; it is normally around 10
−2
to 10
−3
bar.
The deposition conditions can be maintained in precise manner using the CVD apparatus which is described, thus enabling layers having highly reproducible properties to be obtained.
In the coating oven the substrate is heated to a temperature of from 400° C. to 1800° C., in particular 650° C. to 1400° C. This enables a layer to be produced which remains amorphous at up to 1900° C.
The layer produced by the process according to the invention contains the following elements (wherein this term also includes the elements bonded together): silicon, nitrogen, boron and carbon. The elements Si, N and B may be present, as in the starting compound, in a molar ratio of 1:1:1. The layer may also contain, in addition to the elements Si, N, B and C, organic residues formed from the starting compound which influence the properties of the layer. If these organic residues are to be removed from the layer, the substrate is heated to a correspondingly high temperature. The CVD deposition in the oven may, however, also take place with the substrate at a temperature which is rather lower, and a thermal post-treatment be carried out at from 600° C. to 1800° C. in an oven, in order to expel the organic residues.
The Si—N—B—C layer thus formed is in particular suitable for protecting metal parts. The metal parts may be of, for example, steel or a titanium alloy.
The protective layer applied to the metal part by the process according to the invention is distinguished by high adhesion. This is in particular extremely successful if the metal part is coated in the unpolished state, for instance has a peak-to-valley height of more than 5 #m.
The layer which is applied to the metal part by the process according to the invention has high wear resistance and lubricating properties in addition to the high adhesion. The former properties can be influenced by the content of organic residues deriving from the alkyl groups of the starting substrate.
Owing to the excellent tribological properties of the ceramic protective layer created according to the invention, the process according to the invention may be used, for example, for coating metal parts in automotive construction.
In addition to the coating of metal substrates, the process according to the invention is above all also eminently suitable for the coating of carbon substrates or of composite substrates prepared from carbon and silicon, which are exposed to high temperature, in particular to high thermal shock stress, for example in space travel, for example for jet engine nozzles. It is furthermore suitable for the coating of ceramic substrates.
If the carbon, silicised carbon or ceramic components coated with the Si—N—B—C protective coating by the process according to the invention are heated to temperatures of, for example, from 900° C. to 1800° C., in particular 1200° C. to 1600° C., in an oxygen-containing atmosphere such as, for example, air, the Si at the surface of the protective coating oxidises to SiO
x
, that is to say SiO and SiO
2
.
The latter oxidation may take place as a result of post-treating the coated substrate in an oven, or during use of the component at high temperatures in the atmosphere.
The SiO
x
which is formed at the surface of the component has a relatively low melting point owing to the boron. Consequently, the protective layer in the surface region melts even when the temperature is relatively low, and the melt seals any cracks which may arise in the underlying region of the protective layer, as a result of which oxygen penetration to the substrate of carbon or of carbon/silicon is prevented. That is to say, the SiO
x
content in the surface of the protective layer is effective at high temperatures, while the Si—N—B—C phase of the remaining protective layer, that is to say the base layer, protects the component from oxidation at low temperatures. The same is true of the nitrogen-containing substrates.
As a result of the process according to the invention, a protective layer is created which protects the coated component from oxidation at up to approximately 1900° C. in reliable manner even under conditions of thermal shock stress in accordance with the processes hitherto.
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D
Baldus Hans-Peter
Benien Hannelore
Haltrich Marc
Meistring Rolf
Passing Gerd
Bayer AG
Beck Shrive
Chen Bret
Connolly Bove & Lodge & Hutz LLP
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