Hard-chrome plated layer

Details Hard-chrome plated layer Hard-chrome plated layer

2001-03-07

2003-01-07

Koehler, Robert R. (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C029S888074, C205S109000, C205S113000, C205S243000, C205S283000, C428S610000, C428S613000, C428S614000, C428S636000, C428S908800, C428S926000, C428S935000

Reexamination Certificate

active

06503642

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to an electrodeposited hard-chromium coat, particularly for a piston ring, which is substantially formed of an electrolyte containing hexavalent chromium, wherein there are cracks in the coat and diamond particles are embedded in these cracks.
b) Description of the Related Art
Electrodeposited hard-chromium coats have been known from the prior art for a long time and are used, for example, as surface coating in shock absorber pistons, hydraulic parts, piston rings and printing rollers.
Although electrodeposition of chromium still requires a relatively large amount of energy, electrodeposited chromium is very economical in terms of utilization of resources, since virtually 100% of the chromium electrolyte can also be deposited as a chromium coat; this is why chromium electroplating is still frequently used today.
European Patent EP 0 217 126 describes an electrodeposited hard-chromium coat of the type mentioned above with a network of cracks extending through the entire thickness of the coat, wherein solids particles are embedded in these cracks. A chromium coat of this type is produced by microcrack-forming chromium plating baths, known per se, preferably chromic acid baths, with solids particles dispersed therein. During chromium plating, the workpiece to be chromium-plated is first cathode-connected so that a chromium coat with microcracks is formed, and the workpiece is then anode-connected so that the microcracks expand to the desired gap width and the cracks fill with solids particles. This is followed by cathode switching again, so that the solids particles are encapsulated and enclosed by the closing of the cracks. This cyclical reversal of current can be repeated a number of times, if required, wherein the chromium plating parameters can be varied in accordance with the application such that the desired crack width, crack density and crack filling takes place, possibly with different solids particles filling.
European Patent EP 0 668 375 B1 discloses a process for producing a hard chromium composite coating on a substrate comprising a disperse phase and is particularly suited for mechanical components that are subjected to high-temperature friction. This process comprises the step of electrodeposition of at least one hard-chromium coat in a chromium plating bath of the type which forms microcracks and which contains a suspension of a predetermined concentration of particles of a given size of a nonmetal which is insoluble in the bath. At the same time, in the course of said deposition step, the substrate is permanently maintained at cathode potential and a pulsating cathode current which cycles over time between a minimum value and maximum value is supplied in order to achieve a chromium coat comprising a matrix with microcracks having a given distribution and a disperse phase comprising said nonmetal particles, some of which are enclosed in the microcracks while some are embedded directly in the matrix. The chromium plating bath is based on chromic acid and contains predominantly hexavalent chromium in solution. A coat which is produced by this process and which has a relatively low hydrogen content is also described in this European patent.
Further, European Patent Application EP 0 841 413 A1 discloses a piston ring with a nitrided coat over its entire surface, wherein a chromium composite layer is formed on its surfaces. This coat has a network of cracks formed at its outer surface and internally. Particles of Si
3
N
4
are enclosed in these cracks; the average size of the Si
3
N
4
particles is 0.8 to 3 &mgr;m and the dispersion ratio of these particles in the electrolyte is 3 to 15 percent by volume. Improved resistance to frictional wear and corrosion are achieved with a surface coating of this kind.
Another known piston ring which is described in European Patent Application EP 0 841 414 A1 differs from that known from EP 0 841 413 A1 in that round aluminum particles are enclosed in the cracks, wherein the average particle size is between 0.7 and 10 &mgr;m and the dispersion ratio of the round aluminum particles in the electrolyte is 3 to 15 percent by volume.
Finally, the German Offenlegungsschrift, or Laid Open Application, DE 197 45 811 A1 describes an electrodeposited hard-chromium coat with a network of cracks extending partially or completely through the coat thickness and solids particles which are embedded and encapsulated in the cracks. The electrodeposited hard-chromium coat comprises at least two layers of hard chromium, at least one of which is deposited by pulsating DC current, so that the chromium has different forms of crystallization. The hard chromium can be alloyed, in addition, with the metals tungsten, vanadium and/or molybdenum.
OBJECT AND SUMMARY OF THE INVENTION
Proceeding from this known prior art, it is the primary object of the invention to provide an electrodeposited hard-chromium coat having improved physical characteristics such as improved resistance to wear and resistance to corrosion in particular.
This object is met by an electrodeposited hard-chromium coat of the generic type in which the diamond particles have a size ranging from 0.25 to 0.5 &mgr;m.
However, the indicated diamond particle size does not mean that all particles must necessarily have the same size; rather, they can have different sizes and must merely be within the range of 0.25 to 0.5 &mgr;m.
The electrodeposited chromium coat according to the invention is formed substantially from an electrolyte containing hexavalent chromium. In contrast to a chromium formed of trivalent electrolyte, the chromium formed of hexavalent electrolyte has more lattice imperfections because the chromium formed from a hexavalent electrolyte contains, in addition to the body-centered cubic chromium, more hexagonal chromium hydride, which is a result of the extensive hydrogen formation during electrodeposition. This results in a greater number of lattice defects and therefore also in an even greater hardness of the deposited chromium.
The hard-chromium coat according to the invention need not necessarily be pure chromium. On the contrary, alloying of the chromium, especially with the metals molybdenum, vanadium and tungsten, can be advantageous for certain applications.
Surprisingly, through the use of diamond particles ranging in size from 0.25 to 0.5 &mgr;m, coats can be achieved with even better properties compared with particle sizes used according to the known the prior art.
Coatings of electrodeposited hard-chromium coats with Al
2
O
3
particles which range in size from 2 to 5 &mgr;m and are embedded in the network of cracks were formerly commonly used for piston rings. These coats formerly showed the best characteristics with respect to wear resistance and corrosion resistance.
Tests with diamond particles met with little success previously because only electrodeposited hard-chromium coats which have inferior characteristics compared with coats formed with aluminum oxide particles and which are also substantially more expensive could be achieved using diamond particles with sizes from 2 to 5 &mgr;m as is conventional for aluminum oxide particles.
In a test under engine conditions, piston rings provided with a coating according to the invention were used in a 6-cylinder turbo diesel engine under full load for 85 hours. Results showed that with cylinder wear of approximately 0.17 &mgr;m/1000 km, which is approximately the same as in formerly used electrodeposited chromium coats with aluminum oxide particles, ring wear was reduced by half with the electrodeposited hard-chromium coat according to the invention, other conditions remaining the same, namely, ring wear of only 0.2 &mgr;m/1000 km occurred compared with 0.5 &mgr;m/1000 km using a conventional electrodeposited hard-chromium coat with aluminum oxide particles as piston ring coating.
In addition, it was shown in a simulated test for corrosion resistance that a diamond-embedded chromium coat with particle sizes ranging from 0.25 to 0.5 &mgr;m (as r

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