EP1920088B1 - Method for producing a composite body with an electrodeposited coating under internal compressive stress - Google Patents

Description

Technical area

The invention relates to a method for producing a composite body with a pressure-resistant voltage-loaded galvanic coating.

State of the art

The provision of electrodeposited layers of bodies with a technical surface is mostly for the purpose of corrosion protection, wear protection or to achieve optical or decorative properties. Particularly in the case of components and machine components, such galvanic surface finishes contribute significantly to increasing the service life of the individual components.

Typical applications are for example chrome-plated cylinder liners of diesel engines, chromed conveyor pipes for concrete mixtures, pump components that are corrosive stressed, or forming or deep drawing tools that are galvanically coated for the purpose of wear and adhesion protection, for example, with hard chrome.

For this purpose, in a conventional manner mostly made of metal base body coated in a galvanic deposition process with, for example, chromium or nickel, whereby corresponding surface-coated composite arise. In particular, the deposition of brittle-hard galvanic layers such as "hard chrome" on component surfaces, while effective to protect the components from wear, but formed during the coating usually micro and macro tensile stresses within the layer material, which exceed the material strength of the electroplating layer at least locally, so that cracks develop in the layer. The micro- and macrocracks resulting from these micro- and macro-tensile stresses, as well as residual residual stresses remaining in the electroplating layer, can significantly reduce the strength and wear resistance of the layer. As crack propagation progresses, which is assisted by mechanical stress, the cracks can reach the component surface and the base material of the component. Due to the stress exaggeration at the crack tip of a forming crack, an external stress, which is uncritical in an uncoated component, causes a cracked growth in the base material in a coated component, and ultimately a failure of the component. Cracks that extend through the entire layer thickness also reduce the corrosion resistance of the entire composite body.

The avoidance of cracks and tensile residual stresses in galvanic coatings has so far been sought only through experiments to optimize the galvanic deposition process as such. This approach is successful for thin layers or for individual areas in the construction of thicker layers, but no reliably applicable process techniques are known to protect the above electroplating layers with a reliable robustness against the effect of production-related cracks.

document GB 477 295 A discloses a method for producing a composite body comprising the steps of: providing a composite body by electroplating a brittle chromium or nickel layer onto a metallic base body, performing a mechanical surface treatment.

document US 2003/041924 A1 discloses a method for producing a composite body comprising the steps of: providing a composite body by electroplating a brittle-hard chromium layer on a base body, a metal body, performing a chemical surface treatment.

document GB 1 492 503 A discloses a method for producing a composite body comprising the steps of: providing a composite body by electroplating a brittle chromium layer on a base body, a metal body, steel in D5, performing a thermal surface treatment.

None of the cited documents discloses preliminary experiments.

Presentation of the invention

The invention is therefore based on the object to provide measures with which it is possible to produce galvanically coated components that contain no surface-open, manufacturing cracks in the coating and at least in part of the coating have high compressive stresses, the existing crack at least partially enclose. Basically, the Strength and corrosion resistance of electroplated components can be significantly improved.

The solution of the problem underlying the invention is specified in claim 1. The concept of the invention advantageously further features can be found in the dependent claims and in the following description.

• Zunächst wird ein mit einer galvanischen Schicht versehener Verbundkörper, der einen Basiskörper aufweist und der einem galvanischen Beschichtungsverfahren unterzogen worden ist, bereitgestellt. Der Beschichtungsvorgang erfolgt im Rahmen einer an sich bekannten Weise durch galvanische Materialabscheidung, bei der vorzugsweise ein aus metallischem Werkstoff bestehender Basiskörper ganz oder teilweise mit einer Hartgalvanikschicht, vorzugsweise einer Hartchromschicht oder Nickelschicht überzogen wird. Unmittelbar im Anschluss an den Beschichtungsvorgang erfolgt eine Oberflächenbehandlung der galvanischen Schicht durch einen auf die galvanische Schicht gerichteten mechanischen Energieeintrag mit an die Beschaffenheit des Basiskörpers sowie der darauf abgeschiedenen galvanischen Schicht angepassten Behandlungsparametern derart, dass ausschließlich lokale plastische Deformationen in der galvanischen Schicht eingebracht werden.

Um die Einstellung der für den Erfolg des Verfahrens notwendigen Behandlungsparameter vorzunehmen, sind erfindungsgemäß zwei Vorversuche durchzuführen:

• Bspw. an einer Platte, aus dem zu behandelnden Verbundkörper wird die Abhängigkeit der Druckfließgrenze und der Sprödbruchgrenze der auf dem Basiskörper aufgebrachten galvanischen Schicht von der Werkzeuggeometrie jener Werkzeuge, mit denen die galvanische Schicht lokal deformiert wird, ermittelt, woraus eine erforderliche Werkzeuggeometrie, die erforderliche und maximal zulässige Kraft auf das Werkzeug sowie die Härte des mit der zu behandelnden galvanischen Schicht in Kontakt gelangenden Werkzeugbereichs bestimmt werden.

According to the solution, a composite body with a pressure-resistant, galvanically loaded, electroplated coating is produced by the combination of the following method steps:

• First, a composite-coated composite body having a base body and subjected to a galvanic coating process is provided. The coating process takes place in a manner known per se by electrodeposition, in which preferably a base body consisting of metallic material is completely or partially coated with a hard electroplated layer, preferably a hard chrome layer or nickel layer. Immediately following the coating process, the galvanic layer is surface-treated by a mechanical energy input directed onto the galvanic layer with treatment parameters adapted to the nature of the base body and the galvanic layer deposited thereon such that only local plastic deformations are introduced into the galvanic layer.

In order to carry out the setting of the treatment parameters necessary for the success of the method, two preliminary tests are to be carried out according to the invention:

• For example. on a plate, from the composite body to be treated, the dependence of the Druckfließgrenze and brittle fracture limit of the applied on the base body galvanic layer of the tool geometry of those tools with which the galvanic layer is locally deformed determined, resulting in a required tool geometry, the required and maximum permissible force on the tool as well as the hardness of the tool area coming into contact with the galvanic layer to be treated are determined.

In a second step, as part of a multiple ball pressure test, it is determined which number of repeated indentations per contact surface is permissible, without damaging the layer surface, but at the same time plastically deforming the layer surface. In this way, the allowable coverage level, i. set the number of tool impressions per contact surface.

The preferred tool material is a material having at least the same hardness as the galvanic layer to be treated itself. As a tool shape, a smooth as possible, rounded shape is selected for the contact area between the tool and the galvanic layer. This preliminary test results in the required tool geometry and the required and maximum permissible force on the tool.

After determining the treatment parameters described above, the surface to be treated surface of the layer of brittle-hard coating material, for example in the context of a shot peening process treated.

The striking on the galvanic layer spherical tool elements can be thrown in the context of a blasting system by compressed air or by means of a blower on the layer surface with adjustable kinetic impulse, so that each point of the layer surface to be treated one or more times.

The mechanical method is based in particular on the fact that the layer surface is locally plastically deformed with a suitable tool and in the layer compressive residual stresses are generated. Essential in the implementation of the mechanical surface treatment is that at the same time produced in the production of plastic surface deformations damage in the form of brittle fracture processes on the galvanic layer surface whose strength-reducing effect is greater than the strength-increasing effect caused by the "plasticization" compressive stresses.

The above-mentioned requirement is achieved by limiting the plastic deformation to laterally narrowly limited layer surface areas and secondly by the fact that the tool with which the Layer surface of the workpiece to be treated comes into contact, having a certain contour in the region of the contact surface, which is generally considered to be not sharp-edged.

If a material-specific limit value, which is dependent on the shape of the tool, for the contact surface between the tool and the layer surface and for the indentation depth of the tool in the galvanic layer is not exceeded, the desired effect, a deliberately introduced plastic deformation, can be achieved by a repeated, laterally offset local surface treatment , can be achieved surface covering.

The above-described requirement is met in particular by the fact that the tool has a suitable geometry, which is preferably round in shape and does not exceed a critical tool diameter dependent on the material of the galvanic layer to be processed. In the case of a ball, as used in shot peening experiments come and the use of hard chrome as a layer material have proven suitable ball diameter of a maximum of 6 mm. The above-mentioned critical diameter, which is decisive for the dimensioning of the sphere, also defines the narrowly limited surface area within which the plastic deformation must take place on the layer surface.

In addition to the geometrical dimensioning of the tools with which the galvanic layer surface is machined, and in this case also hammers, nails and rollers should be mentioned as an alternative to shot peening, especially in the shot peening process, the kinetic momentum acting on the layer surface plays an important role. The geometry of the tool and the pulse setting, with which the tool strikes against the layer surface to be processed, are to be adjusted such that the desired plastic deformation takes place before the brittle fracture preferably not to take place, ie the height of the pulse input is to be such that the Extent of any occurring harmful brittle fracture is limited so much that the positive influence of plastic deformation on the strength of the galvanic layer outweighs.

By solving the galvanic layer with residual compressive stresses in the way, for example, by an already discussed shot blasting process and thus achievable closing by itself, as a result of the generated on the base body electrodeposited layer cracks by the generated plastic deformation near-surface areas of the layer are often occurring and known disadvantages of galvanic layers with respect to the reduction of the component strength and an insufficient corrosion resistance effectively mitigated or even completely eliminated. At the same time, the strength and wear resistance of the layer itself are improved. This may possibly be dispensed with a solidification treatment of the base material from which the base body. The coating process according to the solution also provides additional degrees of freedom in the choice of coating parameters, especially as the risk of cracking due to existing tensile residual stresses within the galvanic layer is less important due to its effective reduction.

Brief description of the invention

Fig. 1a, b

Diagrammbeispiele für den tiefenabhängigen Eigenspannungsverlauf innerhalb einer galvanischen Schicht ohne (a) und mit (b) lösungsgemäßer Behandlung.

The invention will now be described by way of example without limitation of the general inventive idea by means of embodiments with reference to the drawings. Show it:

Fig. 1a, b

Diagram examples for the depth-dependent residual stress curve within a galvanic layer without (a) and with (b) solution according to treatment.

Ways to carry out the invention, industrial usability

On a flat sample consisting of a metallic material, preferably steel, a hard chrome layer has been deposited with a layer thickness of 0.3 mm by means of a galvanic material deposition.

Residual stress tests were performed on the sample to check for residual stresses within the hard chrome layer. The measurement results shown in the diagrams represent residual stress depths which have been measured transversely and along the sample or layer extension.

In the case of the diagram in FIG. 1a For example, an untreated hard chrome layer has been measured in accordance with the prior art. The unfavorable tensile residual stresses of about 200 MPa in the layer down to the interface of hard chrome layer / base body in 0.3 mm depth can be seen.

The diagram according to Fig. 1b shows the residual stress profile in a hard chrome layer, which has been deposited in a comparable manner on the base body, according to the case in Fig. 1a but after a shot peening treatment of the hard chrome layer. Evident are the favorable, extremely high residual compressive stresses of up to 2000 MPa in most of the layer.

It could be shown that hard chromium coated base bodies treated in accordance with the solution show no cracks or extensions of already existing cracks in the hard chrome layer under Hertzian stress (ball pressure test), since only a plastic deformation in the chromium layer could be achieved. In contrast, in the case of composites produced according to the state of the art crack formation in the chromium layer could be achieved by ball impression without any problems.

Claims (3)

1. A method for producing a composite body with an electrodeposited coating under internal compressive stress,
   characterised by the following method steps:

   - preparing a composite body which is provided with an electrodeposited layer and has a base body which has been subjected to an electrodeposited coating process,

   - carrying out a surface treatment of the electrodeposited chromium layer by the introduction of mechanical energy directed at the electrodeposited layer, with treatment parameters adapted to the nature of the base body and the electrodeposited layer deposited thereon in such a way that local plastic deformations are introduced into the electrodeposited chromium layer, wherein the treatment parameters are established in a preliminary trial as follows:

   - in a first step, the dependence of the compression creep limit and the brittle fracture limit of the electrodeposited chromium layer deposited on the base body is determined from the tool geometry of the tools acting on the electrodeposited chromium layer in the course of the surface treatment, from which a required tool geometry, the required and maximum permissible force on the tool and the hardness of the tool region coming into contact with the electrodeposited chromium layer to be treated are determined,

   - in a second step, a determination is made of the number of permissible repeated ball indentations per contact area that are required in order to deform plastically the electrodeposited layer, but without damaging the electrodeposited layer, and

   - that a shot blasting process, hammering, nailing or rolling is used as a surface treatment.
2. The method according to claim 1,
   characterised in that the chromium layer is a brittle-hard hard chromium layer.
3. The method according to claim 1 or 2,
   characterised in that the base body is a metallic body or at least one electrically conductive surface.

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