Oxidation inhibitor coating

The invention relates to an oxidation inhibitor coating for high-melting metals selected from the group of molybdenum, tungsten, tantalum and niobium and/or alloys thereof. The oxidation inhibitor coating comprises 1 to 14% by weight boron, 0.1 to 4% by weight carbon and the balance silicon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an oxidation inhibitor coating, and more 
particularly to an oxidation inhibitor coating applied to a substrate 
comprising a high-melting metal selected from the group molybdenum, 
tungsten, tantalum, niobium and their alloys, or composites thereof, 
wherein the coating comprises silicon as well as 1 to 14% by weight boron. 
2. Description of the Related Art 
High-melting metals have the properties of retaining their strength up to 
highest temperatures. However, a problem is the fact that such metals and 
alloys have only low resistance to oxidation when subjected to high air 
temperatures of over 400.degree. C., or to other oxidizing media. 
For the purpose of reducing such high susceptibility to oxidation, it is 
known to apply suitable protective coatings to the surface of the 
high-melting metals. Particularly, the application of coatings based on 
silicon, which, by a diffusion annealing treatment, jointly form with the 
high-melting metal a silicide, has often been used for this purpose. When 
high-melting metals coated in such a way are exposed to an 
oxygen-containing atmosphere at high temperatures, an oxide layer forms on 
the surface of the silicide, such layer of oxide acting as a protective 
coating against further oxidation. If coating of pure silicon is applied 
to the high-melting metal, the oxide layer on the layer of silicide is 
SiO.sub.2. However, pure SiO.sub.2 forms relatively slowly and has a high 
melting point, so that such a coating has poor crack-healing properties 
especially when the high-melting metal is used at temperatures below 
1200.degree. C., and consequently is an inadequate protection against 
oxidation in many cases. 
For the above-mentioned reasons, the use of modified coatings, especially 
of coatings based on two materials such as SiC, SiB, SiGe, SiMn, SiTi, 
SiCr, but also based on three materials such as SiCrAl, SiTiAl, SiCrB, 
SiCrTi and SiCrFe, has gained acceptance in practical life. The use of 
modified coatings based on silicon has the advantage that the silicide 
coatings form lower-melting oxide mixtures as compared to pure SiO.sub.2, 
so that such coatings have good crack-healing properties and protect the 
surface of the high-melting metal across a wide temperature range. The 
antioxidation coatings can be applied by all sorts of different coating 
methods such as plasma spraying, electrophoresis, melt flow electrolysis, 
melt immersion methods, CVD- or PVD-method, by applying a slurry of the 
desired powder mixture to the surface of the high-melting metal (slurry 
coating), or by curing the high-melting metal in a powder mixture with 
activator (pack cementation). Thereafter, in case of the low-temperature 
coating methods, a diffusion annealing process is carried out for forming 
the layers of silicide, at temperatures between 1200.degree. C. and 
1600.degree. C., under protective gas or in a high vacuum. In connection 
with high-temperature coating methods (melt flow electrolysis, melt 
immersion method, CVD-process, pack cementation, and plasma spraying as 
well, as a rule), layers with adequate thickness are precipitated, so that 
the layers of silicide can form in the course of oxidation during use, 
without permitting oxygen to penetrate the coating to any greater extent. 
A drawback of such known oxidation inhibitor coatings, however, is that 
their adhesion is often not very good, and that they, furthermore, show a 
certain porosity and unevenness. 
SUMMARY OF THE INVENTION 
An object of the present invention, is therefore to create an oxidation 
inhibitor coating for high-melting metals that has enhanced adherence of 
the coating, uniformity and tightness, and therefore offers distinctly 
improved protection against oxidation versus oxidation inhibitor coatings 
of the type known heretofore. 
According to the present invention, this and other objects of the invention 
are accomplished by providing an oxidation inhibitor coating comprising 
0.1 to 4% by weight carbon in addition to boron and silicon. 
The foregoing specific objects and advantages of the invention are 
illustrative of those that can be achieved by the present invention and 
are not intended to be exhaustive or limiting of the possible advantages 
which can be realized. Thus, these and other objects and advantages of 
this invention will be apparent from the description herein or can be 
learned from practicing this invention, both as embodied herein or as 
modified in view of any variations which may be apparent to those skilled 
in the art. Accordingly, the present invention resides in the novel parts, 
constructions, arrangements, combinations and improvements herein shown 
and described. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It was found in this connection that an oxidation inhibitor coating 
comprising 5 to 12% by weight boron and 0.5 to 3% by weight carbon, the 
balance silicon, produces particularly good results. 
The oxidation inhibitor coating according to the invention has been tested 
with excellent results both for massive substrates consisting of 
high-melting metals, and intermediate layers consisting of these 
materials. 
It came as a complete surprise, and to an extent that could not be 
expected, that such small components of carbon in the oxidation inhibitor 
coating could lead to improvements in the resistance to oxidation which, 
versus pure boron-silicon coatings, may reach the factor 2 under certain 
application conditions. The carbon added for producing the protective 
coating serves not only as an alloying element, but also as an activator 
which, in connection with the high-temperature coating, removes 
diffusion-inhibiting oxygen in the form of CO or CO.sub.2 in the course of 
the heat treatment, or also during the first time of use in an oxidizing 
atmosphere. This was reflected by the fact that the carbon content in the 
heat-treated oxidation inhibitor coating, or in an oxidation inhibitor 
coating that had already been in use for a brief time at an elevated 
temperature, was lower by a factor of up to 10 than the amount of carbon 
applied originally. Thin carbon component, which is initially reduced, 
then stabilizes, and then remains largely constant until the oxidation 
inhibitor coating fails. 
The special oxidation-inhibiting effect of the carbon was in no way 
foreseeable because a person of ordinary skill in the art primarily had to 
expect carburization of the substrate material on account of the carbon. 
The thicknesses of the oxidation inhibitor coating according to the 
invention that are of interest in practical application are in the range 
between 50 .mu.m and 500 .mu.m. Coating thicknesses between 100 .mu.m and 
300 .mu.m have been successfully used in connection with a particularly 
preferred embodiment of the oxidation inhibitor coating. 
Basically, oxidation inhibitor coatings according to the invention can be 
produced by all known coating methods. However, atmospheric plasma 
spraying and the slurry method have been found to be particularly 
advantageous coating methods. 
The invention is further explained in the following examples.

EXAMPLE 1 
Cylindrical test specimens with 10 to 25 mm diameter and 50 to 250 mm 
length made of molybdenum were sandblasted on their surfaces and all sharp 
edges were rounded. A powder mixture of 880 g silicon powder, 100 g boron 
powder and 20 g carbon powder was mixed for 30 minutes in a tumbling 
mixer. Subsequently, a slurry was prepared in the tumbling mixer by adding 
560 ml colorless nitro-lacquer dissolved in 140 ml nitro-dilution, and 
four-hour homogenizing of the mixture in the mixer. The test specimens 
were coated by spraying them with the slurry. Following 24 hours of air 
drying, the test specimens were subjected to protective gas annealing 
(H.sub.2, 1 bar) for 2 hours at 1370.degree. C., which completely removed 
the lacquer components of the slurry. Thereafter, poorly adhering slurry 
residues were removed from the test specimens, the specimens were visually 
inspected for cracks or peeling spots, and newly coated when necessary. 
Test specimens coated in this manner had coating thicknesses in the range 
of 50 to 100 .mu.m. For testing the resistance to oxidation, the coated 
test specimens were annealed in air at 1200.degree. C., whereby it was 
found that the average useful time until failure of the oxidation 
inhibitor coating came to 3000 hours. For comparison purposes, test 
specimens were coated in the same way with a slurry of the same 
composition, but without carbon components, and also tested in air at 
1200.degree. C. It was found that with test specimens (without carbon 
components) coated in this way, the average useful life came to only about 
2000 hours. 
EXAMPLE 2 
Plate-like test specimens with the dimensions 300 mm.times.200 mm.times.6 
mm made of molybdenum were sandblasted on their surfaces, and all edges 
and corners were rounded. Subsequently, the test specimens were coated by 
atmospheric plasma spray coating. The spray powder used was prepared as 
follows: 8.8 kg silicon powder, 1.0 kg boron powder and 0.2 kg carbon 
powder was mixed, subsequently sintered for 3.5 hours under hydrogen at 
1350.degree. C. to 1380.degree. C., and a powder fraction with a grain 
size in the range of 36 to 120 .mu.m was obtained by screening the 
mixture. Plasma spraying as such was carried out with the usual 
adjustments to an average coating thickness of 250 to 300 .mu.m, which was 
obtained after a number of spraying operations. With annealing of the test 
specimens at 1400.degree. C. in air, an average useful life of 300 hours 
was achieved. 
EXAMPLE 3 
Plate-like specimens as described in Example 2, but consisting of tungsten, 
were coated with the same spray powder and under the same conditions as 
described in Example 2. With annealing of the tungsten specimens coated in 
this manner, at 1400.degree. C. in air, an average useful life of 200 
hours was achieved. 
Although illustrative preferred embodiments have been described herein in 
detail, it should be noted and will be appreciated by those skilled in the 
art that numerous variations may be made within the scope of this 
invention without departing from the principle of this invention and 
without sacrificing its chief advantages. The terms and expressions have 
been used herein as terms of description and not terms of limitation. 
There is no intention to use the terms or expressions to exclude any 
equivalents of features shown and described or portions thereof and this 
invention should be defined in accordance with the claims which follow.