MECHANICAL PROPERTIES

The mechanical qualities of hard chrome are basically its hardness and wear resistance. As a hard chrome coating only improves the surface of the treated workpieces, they have the same mechanical strength as their untreated counterparts. In other words, the process of hard chrome plating does not reduce the mechanical strength of the underlying metal to such an extent that it no longer has the quality required for its designated purpose.

Furthermore, from a hard chrome plating viewpoint, the thermal treatments to which the workpieces have been subjected prior to chrome plating are of major importance. Thus, if such treatments have created abnormal internal stresses that may result in the formation of fatigue cracks, hard chrome plating will enhance such cracks rather than reducing them, thereby reducing the useful life of that part.

HARDNESS OF HARD CHROME

The elevated hardness of this coating (approx. 70 Rc = 1100 Vickers) is a major reason for its use. In contrast to standard thermal treatments, hard chrome plating does not affect the structure of the base metal because it is applied at temperatures below 60 ºC.

MOHS SCALE
Standard Mineral
Hardness
Talc
1
Gypsum
2
Calcite
3
Fluorite
4
Apatite
5
Feldspar
6
Quartz
7
Topaz
8
Corundum
9
Diamond
10
 
MOHS SCALE
Metals Hardness
Lead 1,5
Tin 1,8
Cadmium 2
Zinc 2,5
Gold 2,5
Silver 2,7
Aluminium 2,9
Copper 3
Nickel 3,5
Platinum 4,3
Iron 4,5
Cobalt 5,5
Tungsten 7,5
Chromium 9

This table shows that hard chrome can scratch glass.

 HARDNESS OF ELECTRONIC COATINGS

(Corresponding to the Brinell scale)

The Brinell hardnesses are extrapolated.

brinell

Special hardness meters (MICRO HARDNESS TESTERS) with weak loads that do not penetrate the chromium layer are used to measure the hardness of HARD CHROME.

GOOD COEFFICIENT OF FRICTION

As hard chrome has a very low coefficient of friction, hard chrome plated workpieces subjected to such forces also have a lower friction coefficient, thus increasing their service life and the performance of the machinery for which they were designed.

It should be noted however that hard chrome coatings have a very low strength and therefore are poorly resistant to deformation due to impacts or vibrations. As such, they cannot be used for workpieces which suffer high load impacts.

As hard chrome is currently used to coat sliding workpieces, it is worth noting that this material gives excellent results when in contact with:

- Anti-friction materials
- Lead bronzes
- Fine cast iron

And gives good results when in contact with:

- Mild or semi-hard steels, rubber, plastic materials.

In contrast, hard chrome cannot be used in contact with:

- Hard steels
- Phosphor bronzes
- Light alloys
- Chromium
 
TABLE OF FRICTION COEFFICIENTS FOR HARD CHROME
Metal Couples.  At rest In movement Comments
Chrome with chrome 0,14 0,12 High risk of seizure
Chrome on anti-friction 0,15 0,13
Chrome on steel 0,17 0,16
Steel on anti-friction 0,25 0,2
Anti-friction on anti-fr. 0,54 0,19
Steel on steel 0,3 0,2
 
TABLE OF FRICTIONS FOR HARD CHROME
Guide or bearing material  Lubrication conditions Working conditions Results
Anti-friction - - Excellent
Fine cast iron - - Excellent
Lead bronze - - Good
Rubber Aqueos - Good
Plastics Aqueos - Good
Mild steel Abundant Low speed Good
Semi-hard steel Abundant Low speed Good
Hard steel Abundant Low speed Good
Hard steel Abundant High speed Delicate
Hard steel Abundant High pressure Delicate
Light alloy - - Delicate
Phosphor bonze - - Poor
Chromium - - Poor
 
There does not appear to be any clear link between hardness and wear resistance, although there is some degree of interdependence resulting from the characteristic binding strength of hard chrome, the roughness of the coating and its treatment.

However, as our general understanding of the processes involved in wear is not yet fully developed, there is currently no universal control method. Presently the TABER method tends to be the most commonly used, although it cannot be applied in practice due to its complexity. Based on our experience, we can advise you in most aspects of wear, abrasion and wetting.

CORROSION RESISTANCE

Corrosion is defined as the gradual destruction of the surface of metallic or non-metallic materials. It occurs as a result of chemical (gases or molten metals) or electrochemical processes (salts, acids, bases).

During deposition, the hard chrome layer generates strong internal stresses within its structure that result in a cracked coating. However, it has no effect on its binding to the base metal. An increased thickness overcomes this problem, resulting in the absence of deep cracks that penetrate to the base material. In the worst of cases, a few micropores remain, although these also disappear at greater thicknesses. Although it is possible to produce a crack-free hard chrome coating in certain bath conditions, the resulting chrome loses some of its key characteristics, such as hardness.

We perform double-layer chrome plating, in other words an initial crack-free coating (soft, with a varying roughness) followed by a classic hard chrome coating to ensure that the final cracked layer does not penetrate to the underlying metal, thereby considerably delaying oxidation of the treated workpieces.

If the base metal contains micropores or metallic inclusions, the defects are propagated in the layer of chrome meaning that correct deposition is impossible. Generally speaking, in the absence of highly demanding technical reasons, simple or conventional chrome plating tends to be performed as it is sufficient and cheaper.

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