Method for depositing a layer on the inside of cavities of a work piece

Disclosed herein is a process for the chemical deposition of a material layer on the inside of cavities of a work piece by conducting a gas stream in a substantially laminar flow along a heated work piece wherein deposition occurs from the gaseous phase, at a pressure of less than 10.sup.4 Pascal (N/m.sup.2).

BACKGROUND OF THE INVENTION 
This invention relates to a method for depositing a layer on the inside of 
cavities of a work piece and particularly, to the chemical deposition of 
such a layer from the gaseous phase. 
In the construction of chemical apparatus, work pieces are often required 
which have a very high resistance to corrosive media such as acids. 
Although glass or plastic meet this criterion, if such work pieces are 
provided for use at high pressures, such materials can not be employed for 
such apparatus or for piping systems, and metallic materials such a 
Hostalloy C or tantalum must be used instead. However, these materials are 
very expensive and are difficult to work. To reduce the manufacturing 
costs, thought has been given to the possibility of making such 
liquid-carrying or gas-carrying piping systems of steel parts which are 
plated on the inside with a corrosion protection layer, for example, of 
tantalum. Plating such hollow parts, which often have different inside 
diameters and, for example, several separately disposed inside canals and 
openings, can usually be accomplished only by precipitation from the 
gaseous phase (CVD deposition), since materials such as tantalum, which 
are suitable for corrosion protection, cannot be electrodeposited. Such 
CVD deposition methods are known, for example, from U.S. Pat. No. 
3,127,641, in which a method for the deposition of tungsten on the inside 
of a copper tube is described. According to another known method for the 
inside coating of work pieces, a work piece to be plated is brought into a 
suitable susceptor, and the deposition is accomplished by means of a CVD 
method using electric induction heating for heating the work piece. 
Further information on the chemical gaseous-phase deposition of different 
materials, such as Ta, W, Mn, Si, Ti, TiC, CrC is given in the journal 
"Thin Solid Films", 24 (1974), pages 157 to 164. 
Inside coating of complicated and difficult-to-manufacture parts, such as, 
for example, multi-path valves and pump housings which have more than two 
openings and several internal canals, has not been possible to date with 
satisfactory results, since the deposited layers exhibited uneven 
thickness and, in part, also open places. For this reason such parts have 
been made of solid material, for example, solid tantalum. Such fabrication 
is very expensive, and, in addition, it must further be taken into 
consideration that certain work pieces can be made of steel but not, for 
example, of a material such as tantalum. 
SUMMARY OF THE INVENTION 
It is an object of this invention to describe a method for depositing a 
layer on the inside of cavities of a work piece, in which the deposition 
is accomplished by means of chemical precipitation from the gaseous phase 
(CVD method), whereby layers can be obtained which have uniform thickness 
and the corrosion resistance of which is comparable to that of the solid 
material. 
This problem is solved, according to the present invention, by the 
provision of a method for depositing a material layer on the inside of 
cavities of a work piece by chemical precipitation from the gaseous phase 
(CVD deposition) wherein the parts of the work piece to be provided with 
the layer are heated and a gas stream containing the components of the 
material to be deposited is conducted along the surface of the work piece 
parts in a substantially laminar flow pattern, the deposition of the 
material being conducted at a pressure less than about 10.sup.4 Pascal 
(N/m.sup.2). 
Investigations which were carried out in connection with the invention have 
shown that in depositing the layering material from the gaseous phase, the 
prevailing gas pressure is not, as is often assumed, without influence on 
the deposited layer, but that the quality of the deposited layer can be 
improved if the deposition process is carried out at a gas pressure which 
is much lower than atmospheric pressure. The cause thereof can be 
considered to be that the gas or the gas mixture from which the 
precipitation takes place should flow past the parts to be coated in as 
laminar a manner as possible, or that the flow should be adjusted so that 
turbulence that occurs separates rather than develop as stationary eddies. 
The result of the development of stationary eddies is that in the volume 
in question, the reaction gas is depleted of the material to be 
precipitated and that as a result, the deposition in this area takes place 
with a smaller layer thickness than at the remaining parts of the work 
piece. The development of laminar flow, however, is promoted if the 
precipitation is carried out at a pressure reduced from atmospheric 
pressure. According to one preferred embodiment of the invention, the flow 
of the gas or gas mixture used for the deposition can further be 
influenced favorably by conducting the gas stream along the surfaces to be 
coated by means of a flow profiler fitted to the shape of the work piece. 
With appropriate design such a flow profiler causes the gas to flow 
uniformly past all parts of the work piece to be coated, so that overall, 
a uniform deposition of the layer is obtained. Since at the recited gas 
pressure, reduced below atmospheric pressure, the flow cross section of 
the reaction gas can vary by more than a factor of 10, it is only 
necessary that such a profiler roughly fit the same shape of the work 
piece. Thus, work pieces with changes of the inside diameter, for example, 
from 5 cm to 1 mm can still be coated with a coherent layer. For work 
pieces with several internal canals, the flow profiler is designed so that 
the flow resistance for the gas discharge from the individual internal 
canals is always of approximately the same magnitude. It is thereby 
achieved that equal quantities of reaction gas mixture are transported 
through each opening. With appropriate design of profilers it is likewise 
possible, according to the method of this invention, to also plate the end 
face of flange connections or the outside of threads.

DETAILED DESCRIPTION OF THE INVENTION 
As an example of the present invention there is described the coating of a 
work piece of steel with a tantalum layer. The work piece 1 is located in 
a quartz vessel 2 that can be evacuated. This quartz vessel is connected 
to a vacuum pump 6 via a flange system 3 with valves 4 connected thereto 
and via a cooling trap 5. In the vessel, flow profilers 7 are arranged 
around the work piece. The vessel 2 is surrounded by an induction coil 8, 
by which the work piece is heated inductively. At the other end face of 
the vessel is located the inlet for the reaction gas. The reaction 
##STR1## 
is used for making the tantalum layer. The hydrogen required for the 
reaction is taken from a pressure bottle 9 and conducted via an oxygen 
absorber 10, a control valve 11 and a cooling trap 12 to a supply tank 13, 
which contains TaCl.sub.5. This supply tank 13 can be heated, so that the 
solid tantalum chloride can be kept at a temperature between about 
90.degree. and 130.degree. C. At this temperature, the vapor pressure of 
the tantalum chloride is sufficiently high so that the hydrogen is 
enriched with a sufficient quantity of gaseous TaCl.sub.5. This reaction 
gas is now conducted to the work piece by way of a heated piping system 
(T.apprxeq.150.degree. C.). For the precipitation, the work piece is 
heated by means of the induction coil 8 to a temperature between about 
900.degree. and 1150.degree. C. At the heated points of the work piece, a 
solid tantalum layer is precipitated in accordance with the reaction 
##STR2## 
During the precipitation, a pressure of about 3.times.10.sup.3 Pa 
(Pascal,=N/m.sup.2) is maintained in the vessel 2 by means of the vacuum 
pump 6. At a reaction temperature of, for example, 1000.degree. C. for the 
deposition, the throughput of hydrogen is, for example, 1 liter/min, and 
the temperature in the supply tank 13 is kept at about 95.degree. to 
100.degree. C. The temperature of the work piece can be determined, for 
example, by means of a pyrometer 18. 
FIG. 2 shows diagrammatically how a work piece 1, which has several 
internal canals 101, 102 and 103 with varying inside diameters, is 
provided with a corrosion-resistant layer, e.g., a tantalum layer. The 
work piece 1 is fastened in a holder 20. At the outlet openings of the 
internal canals 101, 102 and 103, flow profilers 7 are pushed on. The gas 
is fed in by way of tube 21 which surrounds the flow profiler which is 
arranged at the gas inlet opening of the work piece. The flow profilers 7 
cause a uniformly distributed gas flow along the surfaces to be coated of 
the internal canals 101, 102 and 103 of the work piece. After it has 
flowed through the work piece, the reaction gas is suctioned off via the 
flange connection 23. 
Besides the deposition of metal, other materials can be deposited by means 
of the method according to the invention, for example SiO.sub.2, through 
an appropriately developed CVD process, using, for example, the reaction