Some Olympic gold medals have become bald...How many ways are there to gild a silver medal?

Produced by: Science Popularization China

Produced by: Zhang Hao (Osaka University)

Producer: Computer Network Information Center, Chinese Academy of Sciences

On the evening of August 23, Zhu Xueying, the women's trampoline champion at the Tokyo Olympics, asked on a social media platform, "Can your medals... also be peeled off?"

The gold medal in question has a peeling area clearly visible on the upper left (Photo source: Zhu Xueying's Weibo)

The picture shows the gold medal she won at the Tokyo Olympics. There is a dime-sized area on the medal that is "bald", which is very obvious to the naked eye. Zhu Xueying explained that she did not scratch the gold medal on purpose, but found a small piece missing from the medal and thought it was dirty. She rubbed it with her hand but it didn't change, so she scratched it again, and unexpectedly the skin peeled off.

Today we will not discuss too much about the reasons why the gold medal has a "bald skin", but mainly talk about the production process of the Tokyo Olympic gold medal.

Medals are made from recycled materials from thousands of households

According to public information from the Tokyo Olympic Organizing Committee, the three metals of gold, silver and copper used in the Tokyo Olympic medals were all extracted from discarded mobile phones and other electronic devices.

From April 2017 to March 2019, the Tokyo Olympic Organizing Committee launched a campaign called "Made from Urban Mines, Everyone's Medals" across Japan. This activity recycled nearly 80,000 tons of small household appliances. In the end, the project obtained 32 kilograms of gold, 3,500 kilograms of silver, and 2,200 kilograms of bronze, meeting the demand for 5,000 medals for the Olympic and Paralympic Games.

Used mobile phones (Photo source: twenty20.com)

The recovered metals are purified and refined, then reduced to their elemental raw material state, and then mechanically cut into pieces close to the size of the final medal.

Afterwards, the cold pressing method was used to form the image of the goddess of victory Nike and the Olympic rings on both sides of the medal.

The Olympic silver medal is made of pure silver, the bronze medal is made of bronze (95% copper, 5% tin), and the gold medal is made of silver as a base with a layer of gold plated on the surface of the base. The weight of this gold-plated layer is 6 grams. The difference in medal structure also leads to different processes for silver, bronze and gold medals.

Silver and bronze medals can be shipped as finished products after a little heat treatment and surface polishing after the cold pressing process. However, gold medals are special and require another plating process on the base.

Crafts made from waste electronic components (Photo source: twenty20.com)

How many ways are there to gild a gold medal?

There are many technologies for allowing one layer of metal to adhere to another surface. The most widely used in industry are electroplating and chemical plating. In addition, magnetron sputtering is also a common method.

Electroplating is the use of electric current to drive metal ions in the solution to adhere to the surface where they are desired to be deposited. Before plating, the substrate needs to go through multiple cleaning processes to remove impurities such as oil and oxides on the surface.

Before entering the last electroplating tank, the surface of the substrate metal should be very clean. During this process, the fresh substrate atoms exposed on the surface are highly active and urgently need to combine with other substances to reduce the surface energy, because only in this way can they reach a more stable state. At this time, the current pushes the heterogeneous metal ions in the solution to the surface of the substrate, where they are reduced to a single substance and can also be tightly combined with the substrate metal. In layman's terms, the electroplating process can be understood as letting two metals grow together.

The biggest constraint of electroplating is that the substrate must be conductive. Secondly, the coating/substrate combinations that can be achieved by electroplating are actually limited, and many metals cannot be plated by electroplating. In addition, many metals can only be plated on specific metal surfaces that are compatible with them. If the substrate is changed, the effect will be greatly reduced.

Chrome-plated tableware can maintain its shine for a long time (Image source: twenty20.com)

Chemical plating is not limited to the material of the substrate. Chemical plating is also a commonly used metal plating method in industry. As long as the substrate is immersed in a solution containing the plating element ions, under specific reaction conditions, the ions will be deposited on the surface of the substrate. This is different from electroplating. Chemical plating does not require current drive.

For example, when we insert an iron rod into a copper sulfate solution, copper ions will precipitate onto the iron surface, while iron will enter the solution in the form of ions. After a period of time, a uniform copper coating will form on the surface of the iron rod.

In addition, the silver mirror reaction can also be regarded as a chemical plating method.

The biggest advantage of chemical plating is that its substrate does not need to be conductive to be plated. For example, plastic, glass and even silicon wafers can be plated by chemical plating to form a dense coating. For example, if you want to electroplate a bright chrome decorative layer on the surface of plastic, you can only use chemical plating to plate a layer of metal on the plastic surface first, so that electroplating can be carried out on this basis.

Nickel-plated parts processed by chemical plating (Image source: AliceLr)

Many types of coatings can be produced by either chemical plating or electroplating. For example, silver and gold coatings can be produced by both methods. However, there are still great differences in the properties of chemical plating and electroplating, and the specific process conditions will also greatly affect the properties of the coating.

Compared with chemical plating, the biggest advantage of electroplating is its high efficiency and rapid coating growth. However, chemical plating must control the reaction speed and cannot be too fast, otherwise various coating defects will occur.

Integrated circuit boards often need to be plated with silver or gold, but integrated circuit boards are generally made of resin and cannot conduct electricity. Therefore, the factory will first use chemical plating to plate a layer of palladium in the areas where plating is required on the circuit board. This layer of palladium may be only a few dozen nanometers thick. They are tightly arranged and have high strength, and can provide a conductor layer for the subsequent electroplating process. Therefore, we often call this layer of palladium a seed layer. With the seed layer, the subsequent process of electroplating gold or silver can be carried out.

By the way, why are large amounts of gold, silver and copper materials needed on circuit boards?

First of all, gold can play a good anti-oxidation role. During high-temperature welding, it can be well infiltrated with liquid solder to obtain high-quality solder joints.

Silver, as a metal with extremely high electrical and thermal conductivity, is often used as wires and connectors in circuit boards.

Copper is relatively cheap and has good electrical and thermal conductivity, so it is often used as the substrate for the conductive part of the circuit board. This is why the large number of used mobile phones recycled by the Tokyo Olympics can be refined into precious metals needed for medal manufacturing.

Circuit boards contain a lot of gold, silver and copper (Image source: twenty20.com)

Magnetron sputtering is the only choice for producing high-purity coatings. The principle of magnetron sputtering is different from electroplating and chemical plating. Magnetron sputtering uses a high-voltage electric field to ionize the atoms on the surface of the target material and bombard the surface of the substrate to be plated in the form of plasma, thereby forming a coating that is tightly attached to the substrate.

If we want to plate gold on a silver plate, we use gold as the target. Compared with electroplating, the advantage of magnetron sputtering is that the purity is very high. The metal targets are all high-purity metals, and this process is carried out in a high vacuum chamber, which almost does not introduce impurities.

But electroplating is different. The plating tank environment of electroplating is complex, which is a mixed system of various inorganic salts and organic substances. The plating layer often contains a large number of inclusions. These inclusions will be mixed into the plating layer as the plating process proceeds and cannot be removed.

Magnetron sputtering has another advantage: unlike electroplating, there are many infeasible combinations. Basically, as long as the material can be made into a target material, it can be well plated on the substrate, and the conditions for the combination of metal categories are not so harsh.

Of course, the reaction conditions are the opposite. Magnetron sputtering requires a high vacuum environment, and the cost is obviously much higher than electroplating. The electroplating tank can be tens of meters long and several meters wide, while the chamber of magnetron sputtering is limited by the vacuum degree and it is difficult to be so large. Even the chamber of industrial-grade equipment may only be as big as the drum of your washing machine.

Magnetron sputtering. The purple light in the picture is the plasma above the target (Image source: taken by the author)

Why do I think the gold medals at the Tokyo Olympics were made using "electroplating" technology?

Since there are at least three methods, which method did the Tokyo Olympics use to gild the silver medal?

After analysis, I think it should be "electroplating".

Why do you say that?

The gold plating on the gold medals of this Olympics is 6 grams (in line with the specifications of the Olympic Committee for gold medals). The density of gold is 19.32 grams per cubic centimeter, so we can deduce that the total volume of the gold plating is 0.31 cubic centimeters. The size of this medal is 8.5 centimeters in diameter, and the total area of ​​the two surfaces of the medal is 113.43 square centimeters. Therefore, the average thickness of the gold plating is about 27 microns.

This number may not seem large, but it actually exceeds the processing limit of chemical plating and magnetron sputtering, because the coating growth rate of these two methods is only about 1 micron per hour. Electroplating of 27 micron coating can be completed in just tens of minutes, which has an advantage in production efficiency, and the cost is the lowest compared to the other two methods.

After the "gold medal bald skin incident", the author realized through the above analysis and calculations that the coating process used for the gold medals of the Tokyo Olympics can only be electroplating.

This is also supported by evidence in the promotional video released by the Olympic Committee.

The electroplating process of the gold medal (note the three current contacts) (Image source: IOC official Youtube)

Whether it is electroplating, chemical plating or magnetron sputtering, they are all mature metal coating technologies that humans have used for many years. As long as the process flow is strong, poor coating will generally not occur. In most cases, the joints of the coatings have extremely high strength. Even if the interface breaks, it often occurs on the side close to a certain layer of metal, rather than peeling along the entire interface.

The electroplating of gold on the surface of silver is one of the oldest electroplating methods. On the one hand, gold is a rare and precious metal with stable properties and tends to be reduced in electrolyte solutions. On the other hand, silver and gold have similar crystal structures and similar atomic sizes, so they can "dissolve like in like".

In fact, the composition percentage of gold-silver alloy can range from 1% to 100%, a phenomenon called infinite solid solution. In this sense, gold plating on the surface of silver is not a difficult process technology. However, since it is a process, there must be issues such as the rationality of parameter settings and yield.

Regardless of the cause of this incident, as a person working in the manufacturing industry, I want to say to myself and my colleagues that only by improving quality awareness can we provide products that stand the test of time.