Hard chrome coating

A hard chrome layer is a chrome layer applied with the help of electroplating technology , which serves to protect against wear and corrosion . The characteristic property is the thickness of the layer, which is selected depending on the expected material load. With a usual layer thickness between 20 μm and 500 μm, it is significantly thicker than a bright chrome plating.

properties

The hardness of pure chrome is between 1450 and 2050 HV . Electrolytically deposited chromium layers are in practice with 600–1200 HV, well below these values. They are ideal for wear protection.

Hard chrome layer with a clear network of cracks perpendicular to the surface under the scanning electron microscope. The layer was galvanically applied to an aluminum alloy with a layer thickness of around 50 μm.

The applied chrome can develop matt-brittle, shiny-hard or milky-soft and thus have different properties. The most common variant in practice is the shiny, hard chrome layer, the properties of which are listed in this article.

The very low coefficient of sliding friction is between 0.06 and 0.22, depending on the friction surface, which is why hydraulic cylinders are often provided with a hard chrome layer.

The wettability of the chromium layer is also of technical interest, which is heavily dependent on the crack formation typical of chromium. Lubricants can be distributed very well over this network of cracks and adhere well to the surface. Chromium applied with a shiny hard surface has a very small grain size of less than 0.1 µm. The grains grow perpendicular to the surface of the substrate. Internal stresses arise during the deposition. These tensile stresses exceed the cohesive forces of the chrome, which is why there are also vertical cracks in the chrome layer.

The crack formation results in hard chrome layers having a lower density compared to pure chrome . The density of the hard chrome layers is 6.9 while the density for pure chrome is 7.14 . ${\ displaystyle {\ frac {g} {cm ^ {3}}}}$ ${\ displaystyle {\ frac {g} {cm ^ {3}}}}$

With milky and matt coatings, cracking does not occur, the grain size is up to 100 µm.

Manufacturing process

The chromium is deposited on the material to be chromed by means of electrolytic deposition from the chromium electrolyte . Substrates can be metals or plastics. The properties of the coating are determined by current density and temperature during deposition.

Low current densities of approx. 10–60 and temperatures in the range of 30–60 ° C ensure a shiny precipitate. Higher temperatures are required for a milky precipitation, while an increase in the current density leads to a dull precipitation. ${\ displaystyle {\ frac {A} {dm ^ {2}}}}$

application

Because of their high hardness, hard chrome layers are primarily used in wear protection. They can also be used to protect against corrosion, as chromium forms a dense and low-defect oxide layer when it comes into contact with air and thus changes to a passive state. For this, however, a layer thickness of at least 50 μm is required, which excludes corrosion protection for decorative bright chrome plating.

The high hardness and the low coefficient of friction of the hard chrome layer allow it to be used, for example, in the areas listed below.

Areas of application for the hard chrome layer
Branch of industry	Application area
mechanical engineering	Compression molds Printing cylinder Tools Teaching / measuring tools Bearing shafts
Automotive industry	Engine cylinder piston Cylinder liners Crankshafts Camshafts Piston rings
Printing industry	Rollers Pump shafts

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h Dr. Heinz W. Dettner and Dr. Johannes Elze: Handbook of electroplating . Ed .: Dr. Heinz W. Dettner and Dr. Johannes Elze. 1st edition. tape 2 . Carl Hanser Verlag, Munich 1966, p. 148-172 .
↑ Charlotte Schade and Herbert Käszmann: Electroplated Chromium - A Look at the Mechanical Properties. March 9, 2013, accessed February 6, 2018 .
↑ Günther A. Lausmann and Jürgen N. Unruh: The galvanic chrome plating . 2nd Edition. tape 35 . Eugen G. Leuze Verlag, Bad Saulgau 2006, ISBN 3-87480-216-7 , p. 258-302 .
^ ^A ^b N. N. Greenwood and A. Earnshaw: Chemistry of the elements . Eds: NN Greenwood and A. Earnshaw. 1st edition. 1988, ISBN 3-527-26169-9 , pp. 1291-1292 .
↑ Erhard Hornbogen and Hans Warlimont: Metals - Structure and properties of metals and alloys . 6th edition. Springer Wieweg, Berlin / Heidelberg 2016, ISBN 978-3-662-47951-3 , p. 346 .

[Handbuch-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h Dr. Heinz W. Dettner and Dr. Johannes Elze: Handbook of electroplating . Ed .: Dr. Heinz W. Dettner and Dr. Johannes Elze. 1st edition. tape 2 . Carl Hanser Verlag, Munich 1966, p. 148-172 .

[2] Charlotte Schade and Herbert Käszmann: Electroplated Chromium - A Look at the Mechanical Properties. March 9, 2013, accessed February 6, 2018 .

[3] Günther A. Lausmann and Jürgen N. Unruh: The galvanic chrome plating . 2nd Edition. tape 35 . Eugen G. Leuze Verlag, Bad Saulgau 2006, ISBN 3-87480-216-7 , p. 258-302 .

[:0-4] A ^b N. N. Greenwood and A. Earnshaw: Chemistry of the elements . Eds: NN Greenwood and A. Earnshaw. 1st edition. 1988, ISBN 3-527-26169-9 , pp. 1291-1292 .

[5] Erhard Hornbogen and Hans Warlimont: Metals - Structure and properties of metals and alloys . 6th edition. Springer Wieweg, Berlin / Heidelberg 2016, ISBN 978-3-662-47951-3 , p. 346 .