Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal or alloy coating on...
Reexamination Certificate
1999-09-21
2001-11-20
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal or alloy coating on...
C205S102000, C205S111000, C205S112000, C205S179000
Reexamination Certificate
active
06319385
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a process for electrochemically (galvanically) applying a surface coating. In particular, the invention relates to a process for depositing a surface coating on a component as described in my earlier U.S. Pat. No. 5,415,761.
Surface structures described therein are obtained by chemical etching after coating or by mechanical machining such as grinding or sand-blasting. A hard-chromium layer is then applied to the thus created surface structure. The various processing steps required in the production of such components are elaborate and they require complex process technology. The final cost is essentially determined by the mechanical or chemical processing steps necessary for generating the structure.
In structuring metal coating layers, use is also made of elaborate and very difficult-to-control dispersion-deposition processes in which a specific surface structure is obtained through organic or inorganic foreign substances which are included, for example, in a chromium layer and/or which block the growth of the chromium layer during the deposition process. The result is a rough surface. The foreign substances are present in the form of dispergate in the electrolyte.
U.S. Pat. Nos. 4,468,293 and 4,515,671 to Polan et al. relate to a process for electrochemical coating in which a pulse-like current is used for nucleation. A first portion of the pulse has a relatively high current for less than 0.1 sec. and the second portion of the pulse has a much lower current. If a suitable current density is used, the resulting nuclei form a dendritic structure. It is thus possible in one working operation to generate rough dendritically structure surfaces. The current density is understood to be the mean current density at the cathode surface.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a process for applying a surface coating, which overcomes the above-mentioned disadvantages of the heretofore-known processes of this general type. More specifically, the object is to propose an improved process for the electrochemical application of structured metal layers—such that no further mechanical or chemical aftertreatment is necessary and which allows the generation of diverse structured metal layers. It is a further object to provide an apparatus for implementing the process.
With the foregoing and other objects in view there is provided, in accordance with the invention, an improved process for electrochemically depositing a surface coating, wherein a component is subjected to a galvanic bath and the surface layer is deposited on the component with a structured outer surface topography, by forming a plurality of island formations of deposition material on a surface of the component by providing an electrical pulse with a source of electrical energy, and causing a growth of the deposition material on the plurality of island formations for forming the structured outer surface topography with a follow-up pulse of the source of electrical energy. The improvement comprises the step of increasing a strength of the electrical pulse (e.g. current or voltage) during the step of forming the plurality of island formations in a plurality of steps.
The structured layer is applied directly by galvanic means to the object that is to be coated. For this purpose, the object must have an electrically conductive surface which, usually, has been ground in order to provide a smooth substrate base for the structured layer. Prior to the coating process, the object is cleaned and degreased according to conventional electro-deposition practice. The object is immersed as the cathode into a galvanic bath in which there is also an anode. The distance between the anode and the cathode is usually in the range between 1 and 40 cm.
The following are preferably used as electrolyte: chromium electrolytes, particularly sulfuric chromium electrolytes, nitrosulfuric chromium electrolytes or alloying electrolytes.
A process voltage may be applied between anode and cathode and the flowing current causes a coating of material on the object to be coated, which is used as the cathode. The invention proposes that positive current steps be applied in order to form islands. The process of structure generation consists of a nucleation phase (seed creation) and an island growth phase. First of all, in the nucleation phase, process voltage and process current are increased in a plurality of steps from a starting value to a structure current density with in each case a predeterminable change in the current density of 1 to 6 mA/cm
2
per step. The starting value is 0 mA/cm
2
, but it may also be higher if the nucleation phase directly follows a preceding galvanic process phase and the current is not lowered in between to zero. The time between two current-density increases is approximately 0.1 to 30 seconds. Most frequently, intervals between steps of approximately 7 seconds are employed. New nuclei are formed with each current step. In contrast to pulse-current coating, the process current in this case does not fall back to zero after each positive step, but it is further increased with each current step. This makes it possible, in particular, for more roundly and more uniformly shaped nuclei or bodies to be deposited on the object than is possible with the known pulse-current processes. The current steps are applied to the bath in such a number until a structured layer consisting of deposits of individual or adjacent, approximately spherical or dendritic bodies (islands) is obtained on the surface of the object.
Preferably, a structured-layer thickness of 4 &mgr;m to 10 &mgr;m is desired with the nucleation phase. Usually, this necessitates between 10 and 240 current steps, particularly good results being obtained with 50 to 60 steps.
The current density obtained after completion of the last current step is the structure current density. Reaching the structure current density largely signals the completion of the nucleation phase, the actual formation of the structure. The configuration of the resulting structure is dependent on many parameters, above all on the selected structure current density, the number, magnitude and time interval of the current steps, the immersion bath temperature, and the electrolyte used. The current density per step as well as the time between two current-density increases can be changed during the nucleation phase. Depending on the nature of the current function, it is possible to produce different surface structures which are mainly characterized by different peak-to-valley heights. The ideal process parameters can be established simply by empirical means. In general terms, it may be stated that a higher bath temperature and a higher acid content of the electrolyte entails employing a greater structure current density.
Usually, the structure current density is two to three times the current density used in the case of normal direct-current coating. Direct-current coating employs current densities in the range from 15 to 60 mAcm
2
, the value of the current density being dependent on the electrolyte and on the bath temperature. In the case of structure coating, current densities in the range from 30 to 180 mA/cm
2
are possible.
Next comes the nucleus-growth phase, also referred to as the island growth formation. A process current with a current density in the range from 80% to 120% of the structure current density is thereby applied during a predetermined ramp period. An approximately uniform current flows during the ramp period; this leads to the growth of the structure produced on the object. Depending on the duration of the ramp period, the structure layer may be more or less heavily pronounced. Growth takes place faster at the highest points of the structure layer than at the low points between the island nuclei deposited in the nucleation phase. This results, initially, in a further increase in the roughness during the nucleus-growth phase. The ramp period is usually in a range from 1 to 600 secon
Gorgos Kathryn
Greenberg Laurence A.
Heidelberger Druckmaschinen AG
Leader William T.
Lerner Herbert L.
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