Gold plating is much used in order to ennoble ornamental objects and, in this case, sufficiently protective thin coatings are generally used to give special colours to surfaces. Gold plating, however, is also used in other industries, such as those relating to printed circuit boards, electrical contacts, semiconductors, rectifiers, reflectors, vacuum tubes, waveguides, electronic components, parts of reactors, satellites.
Gold thicknesses range from 0.1 μm for decorative-protective purposes, to 1000 μm for electroformed objects; their hardness has ranges from 65 to 89 kp/mm² but it can also reach 400 kp/mm² for electroplating combined with other metals. For pressure gold plating, a hardness of about 90 kp/mm² is usually sufficient.
Tribological aspects of gold and its alloys, namely the chemical, physical, and mechanical properties of gold-plated surfaces in mutual contact and rubbing against each other, are of great importance in all equipment affected by friction, wear, and lubrication. In these cases, the nature of the substrate (base metal) and any layers deposited below or above gold are also relevant.
An extremely soft and ductile metal, as well as highly resistant to corrosion, gold can be deposited via a considerable number of solutions, and then it may show very different properties. On the basis of these properties, gold plating baths can be grouped as follows:
- Cyanide baths, that give rise to very tender pure gold deposits (VPH 40), mostly used for decorative purposes.
- Usually cyanide baths, from which the gold is deposited as an alloy (especially Au/Cu, Au/Ni, Au/Ag), in colours from white to red, with hardness up to 250 vph and with excellent resistance to wear, especially if it contains Sn or Sb.
- Acidic baths (pH 4-5,5) that provide virtually pure deposits (traces of Co), transparent, with hardness 110-115 VPH. These baths have low cathode efficiency and often give rise to tensioned deposits.
- Baths known as «Neutral or acid fine gold processes», based on citrates, phosphates, citrates-phosphates (pH 6.2 to 6.8), which give rise to mostly non-porous deposits, with hardness 100 110 VPH.
- Baths of acidic type of recent development, that give deposits with a particular structure (hardness 180 VPH) if cold-worked, of the cyanide-free alkaline type, that give glossy and ductile deposits with hardness of 100-250 Vickers.
Gold deposits are usually preferred hard, since it is commonly believed that a hard deposit resists better to wear than a tender deposit; however, there is no relationship between hardness and resistance to wear and/or abrasion. On the contrary, it has been demonstrated that a gold deposit with hardness 200-250 HV wears out more quickly than one with hardness 100-150 HV. Very hard deposits (450 VHN) are mostly fragile and stressed, they crack easily, have decreased resistance to corrosion and tarnishing, prevent reaching of high temperatures, and, after some time, cannot be welded properly.
The deposits obtained with conventional baths are obviously harder than annealed gold (hardness ≥ 28 kg/mm²); with organic additives (“hardeners”) a hardness of 130 kg/mm² can be obtained; the presence of other metals increases the hardness variously: with 0.5% Sb, the hardness is 203 kg/mm², with 1% Ag it is 115 kg/mm², with 1.8% Ni it is 150-180 kg/mm².
The hardness of a deposit depends, in any case, by its structure and by the nature and proportion of impurities or additives (“hardeners”) it contains; ultimately, by the nature of the bath and its conduction parameters.
This has been amply demonstrated by S. E. CRAIG and his collaborators, who studied 14 different deposits of pure gold (24 k) obtained from both fresh and aged baths, determining its density, electrical resistivity, structure (X-ray diffraction and electron microscopy scan), and concluded that purity, microstructure, and bulk density are based on the composition of the bath and its aging.
As mentioned above, there is no direct relationship between hardness and wear resistance, this being linked to the structure of the deposit (e.g. in the case of gold-copper alloys), its surface roughness, and its “toughness”, or what in metallurgy is called working power of a metal and defined as the product of tensile strength by the stretching. According to FLÜHMANN, mechanically tough coatings, with good resistance to wear and corrosion and good electrical and thermal conductivity, are ternary alloys of gold with relatively high content of two non-noble metals.