The present invention relates to a process and an apparatus for the coating or modification of surfaces.
Gaseous phase layer depositions or also CVD (Chemical Vapour Deposition) processes are known for the coating of surfaces with layers as thin as possible in the range from 0.5 nm to 50 μm, in order to alter for example the corrosion resistance of the material to be coated or the adhesive properties of the surface. In gaseous phase layer deposition a gaseous phase is excited in a regulated manner, physico-chemical processes occur within the gaseous phase, and as a result thin layers are formed on substrates. The temperatures in these processes are as a rule between 200 and 2000° C. Depending on the nature of the energy source one speaks of thermal, plasma-activated, photon-activated or laser-activated gaseous phase layer depositions. The gaseous components are normally led together with a carrier gas at pressures between 10 mbar and 1 bar through a reaction chamber, in which the physico-chemical processes take place and in which the solid-state components that are thereby formed are deposited as a thin layer on substrates. The volatile byproducts are removed together with the carrier gas.
In gaseous phase layer depositions, in which so-called cold plasmas are used as the energy source, the process is carried out at low pressures that lie significantly below the atmospheric pressure of about 1 bar.
From DE 102 36 430 a thermal process for low-temperature gaseous phase deposition is known, in which the coating process takes place within a reaction chamber. The reaction chamber is connected to a pump that serves to regulate the pressure and to remove byproducts and excess gases from the reaction chamber.
In addition processes are also known in which a so-called thermal plasma is used for the coating. In these processes, which are also termed plasma spraying processes, the operation may be carried out under atmospheric pressure, which means that a special pressure chamber is therefore not necessary. In the plasma spraying process a gas is heated in an arc discharge of high output, namely 5 to 100 kW, to a high temperature of 400 to 10,000 K in order to accelerate metal or ceramics particles to velocities of above 100 m/sec, which are introduced into the discharge and form molten droplets in the hot gaseous stream. The hot molten droplets solidify when they strike the surface to be coated and form there the desired layer. The layer thickness in plasma spraying processes is between 100 μm and a few mm.
From DE 199 55 880 a process is known for the plasma-activated deposition of layers in so-called barrier or corona discharges at atmospheric pressure. In this case a corona discharge is ignited between one or more electrodes and the substrate, which must be metallically conductive. The coating is thereby deposited on the surface of the substrate. Reference should also be made to DE 195 15 069 and DE 195 05 449.
From Japanese Journal of Applied Physics, Vol. 21, No. 4, 1982, L183 to L185 a gaseous phase deposition of diamond particles is known, in which the activation of a methane-hydrogen mixture necessary for the deposition is produced by means of a hot filament. The deposition for the coating takes place in a reduced pressure chamber, in which the pressure during the coating is between about 1/100 and 1/10 bar and the flow rate is ca. 10 ml/min. The reduced pressure chamber is also heated by a furnace situated outside the chamber. In WO 00/47795 a gaseous phase deposition with the aid of a hot filament in a reaction chamber is likewise described; the substrate is in this case subjected to a pressure of about 40 Torr. Reference should also be made to EP 0637639, U.S. Pat. No. 5,256,206, EP 0561511, U.S. Pat. No. 5,079,038 and JP 2092895.
From DE 198 07 086 a process is known for coating surfaces of a substrate, in which a first gaseous phase is brought into the plasma state by means of an electrical field and forms a plasma jet. A second gaseous phase that contains one or more precursors, an aerosol and/or a pulverulent solid is then introduced into this plasma jet, and particle species suitable for the layer deposition are formed by physico-chemical reactions between the plasma-activated gaseous phase and the admixed gaseous phase. These particle species are transported for the layer deposition by the plasma jet to the substrate to be coated and form a layer on the latter. The process can be carried out under atmospheric pressure. High voltage electrodes are necessary in order to generate the electrical field that is required to convert the gaseous phase into the plasma state. The voltage for generating the electrical field is for example 12 kV and has a sinusoidal frequency of 20 kHz.
The coating processes known from the prior art—insofar as they are at all suitable for forming thin layers—are disadvantageous insofar as a high investment in apparatus is necessary to carry out the processes. The majority of the aforementioned processes, and more especially the hot-filament processes, require a pressure-tight reaction chamber. This means that a corresponding pumping technique is required in order to be able to adjust the necessary pressure conditions in the reaction chamber. In addition investment costs are involved in receivers and containers. A further disadvantage arising from the need for a reaction chamber is that the substrate whose surface is to be coated has to be arranged in the reaction chamber. This in turn means that such a process must in principle be carried out discontinuously if substrate batches are to be coated one after the other. There is also the fact that the size of the reaction chamber must match the size of the substrate. The high expenditure on apparatus also means that the execution of the process is very labour-intensive unless a cost-intensive automation is carried out in a complex coating plant.
Although the process that is described in DE 198 07 086 does not require a closed reaction chamber, nevertheless here too the expenditure on apparatus is still high. In particular, the generation of an electrical field requires the provision of a high voltage source. Apart from the high voltage source that according to the aforementioned printed specification has to be provided as generator, suitable high voltage electrodes also have to be provided. In order to achieve a strong electrical field a high voltage must exist between the electrodes, which should be as close together as possible. The formation of an undesirably hot discharge or arc discharge between the electrodes, which can lead to the destruction of the electrodes and substrate, can be avoided if the flow of the gaseous phase that is to be converted into the plasma state is matched to the time-variable voltage at the electrodes. In this connection a dielectric or a plurality of dielectrics may in addition also be provided between the electrodes and the plasma.
The provision of the electrical field is thus complicated and difficult. The production of the apparatus, especially in the region of the electrodes, requires a high degree of accuracy. In addition there is the fact that in the implementation of the process the electrical voltage for generating the electrical field must be accurately matched to the gaseous stream guided between them. It must also be borne in mind that the use of high voltage is likewise governed by special safety requirements, which necessitate appropriate safety measures.