Tungsten

Using a metal to apply a protective coating onto the surface of another metal workpiece (or one made of other materials, such as ceramics or plastic) is a time-tested manufacturing process that has numerous industrial benefits. While “standard” metals, such as nickel, zinc, tin or copper, or precious metals such as gold, silver or platinum and their alloys are normally used to apply the coating, it can also be conducted with a category of materials known as refractory metals that possess their own set of beneficial properties.

Tungsten is one refractory metal that, while difficult to work with, offers several key benefits when used in specific metal finishing situations.

What Are Refractory Metals?

The refractory metals consist of a group of five materials: tungsten, molybdenum, tantalum, niobium and rhenium. These metals share a number of common characteristics, to varying degrees, including:

• Extremely high melting point — particularly tungsten, molybdenum and tantalum — making them valuable for processing molten metals and minerals.
• Ability to retain their strength at extreme temperatures: many tungsten-coated products can exhibit twice the tensile strength of iron at room temperature.
• Unsurpassed resistance to the destructive forces of corrosion: tungsten coatings can stymie the spread of both red and white rusts that can cause the premature demise of metal parts.
• Excellent ability to conduct electricity and heat: tungsten and other refractory metals are frequently used in electronics manufacturing applications and as heat sinks.
• Extreme hardness: many cutting tools, for instance, are made of tungsten carbide.
• High density/specific gravity: The heavy weight of tungsten enables it to be used for ballasts.

While the refractory metals share many of the same properties, each offers its own set of unique characteristics that distinguish it from the others. In this article, we will take a close look at tungsten, and, more specifically, the benefits of plating with tungsten.

A Brief History of Tungsten

Tungsten is a hard, lustrous, silvery-white metal that was discovered in 1779 by Irish chemist Peter Woulfe. Tungsten was originally named “wolfram,” the word from which its chemical symbol, “W” is derived. Some countries still refer to tungsten as wolfram. In 1781, a Swedish chemist named Karl Wilhelm Scheele isolated tungsten as an element and gave it its current name. The Swedish words “tung” and “sten” mean “heavy” and “stone” — an apt description for this solid, dense metal material.

Tungsten remained a relatively obscure element until the mid-1840s, when researcher Robert Oxland was granted a patent for the manufacturing of tungstic acid and sodium tungstate for industrial use. A decade later, Oxland took out a second patent for the production of an iron-tungsten alloy that serves as the basis for the modern, high-speed steel materials that are manufactured today.

Perhaps the most notable development regarding tungsten occurred about a half-century later, when it was first used in the manufacturing of lightbulb filaments. This remains a prominent tungsten application to this day.

Key Tungsten Properties and Characteristics

Like all refractory metals, tungsten exhibits a number of properties and characteristics that make it extremely beneficial for product manufacturing and other industrial purposes. Among its most notable characteristics is its melting point of 6,170° F (3,410° C), which is the highest of any known metal. This makes tungsten a preferred choice for use in manufacturing applications involving extreme temperatures.

While tungsten is able to retain its strength at elevated temperatures, it tends to develop volatile oxide films at temperatures greater than 538° F. Consequently, it’s necessary to take steps such as coating, creating a vacuum or using it in a protective environment during high-temperature applications.

Tungsten also has a very high tensile strength (3,000° F or 1,650° C), even with tungsten wires featuring extremely small diameters. Additionally, tungsten is one of the densest metals available and is extremely ductile, which enables it to be drawn out into a thin wire form that is used in many manufacturing applications.

As mentioned earlier, the “tung” in tungsten means “heavy.” Just how heavy is tungsten? One cubic inch of tungsten weighs more than two-thirds of a pound. Only platinum, iridium, osmium and rhenium metals weigh more.

Tungsten’s superior corrosion resistance makes it suitable for use in severe environments. Tungsten offers good resistance to atmospheric corrosion and moisture at ambient temperatures, as well as nitric, sulfuric and hydrofluoric acids at room temperatures. Additionally, tungsten is capable of resisting liquid metals such as mercury, sodium, magnesium and hydrogen. However, tungsten does react with carbon monoxide, carbon dioxide, sulfur and nitrogen dioxide at elevated temperatures.

Other useful properties of tungsten include the ability to conduct electricity more effectively than other standard metals, such as nickel and iron, and high thermal neutron absorption cross-section.

Exploring the Many Uses of Tungsten

The first widespread industrial application of tungsten metal was as a ductile incandescent lamp filament used in fluorescent lamps, lightbulbs and vacuum tubes — a process that was developed in the early 20^th century by Dr. William D. Coolidge of General Electric. This innovation built upon the evacuated glass bulb technology first developed by Thomas Edison in the latter part of the 19^th century. Dr. Coolidge’s production of ductile tungsten enabled the use of a tungsten wire that could be drawn down to a diameter of approximately one-sixth of a human hair.

When alloyed with carbon to form tungsten carbide, tungsten serves as the building material for products where hardness and wear resistance are of the utmost importance. Examples include cutting tools such as drills, knives, circular saws and various turning and milling tools used in industries such as petroleum, woodworking, mining and metalworking.

Tungsten’s combination of hardness and heat resistance makes it extremely valuable in the production of heavy metal alloys such as high-speed steel. According to Chemicool, the alloys used to produce high-speed steel contain a tungsten content of up to 18 percent. Applications for these tungsten-based materials include the manufacturing of rocket nozzles, various automotive parts and components, radiation shielding and a wide range of wear-resistant coatings.

Tungsten is also useful in various chemical manufacturing applications. For instance, tungsten sulfide is an effective high-temperature lubricant and acts as a catalyst in hydrodesulfurization processes. Tungsten oxides are widely used in the manufacturing of ceramic glazes. Tungsten-calcium and tungsten-magnesium can be found in fluorescent lighting products, while also adding strength to various catalysts that can prolong catalyst life.

Due to its ability to retain its strength at extreme temperatures, tungsten is widely used in high-temperature welding, heating and electrical applications. A prominent industrial manufacturing technique is tungsten inert gas (TIG) welding, an arc welding methodology featuring non-consumable tungsten electrodes. Because of its electrical conductivity, tungsten is also used in the manufacturing of integrated circuits and other electronic parts and components. Finally, tungsten’s heavy weight makes it suitable for use as ballast and counterweights.

About Plating Onto Tungsten Base Materials

All of the attractive properties of tungsten (hardness, corrosion resistance, ability to retain its strength at elevated temperatures, etc.) that make it so useful in product manufacturing also make it equally valuable in metal finishing applications, particularly electroplating. However, as with all refractory metals, tungsten in its pure form is a highly reactive material, which makes plating with tungsten an extremely challenging process.

Now, let’s take a closer look at electroplating, as well as some specific tungsten plating processes.

Tungsten Plating Services

Electroplating is a metal finishing technique that uses an electric current to reduce dissolved metal ions for the purpose of forming a metal coating on an electrode. The process entails the submersion of the workpiece, referred to as the substrate, into a specially formulated electrolyte solution, otherwise known as the plating bath. Because an electric current is used to facilitate the deposition of the coating, electroplating is often referred to as electrodeposition. Because of its high degree of reactivity, it is extremely difficult to plate a substrate with tungsten on its own. It is normally co-deposited with nickel or other metals.

Traditionally, plating with tungsten using an aqueous (water-based) plating bath did not yield acceptable results for a number of reasons:

• The tendency for the coating process to cease after the initial strike (flash deposit)
• High oxide content in the deposit
• Heavily pitted deposits
• Coatings exhibiting poor mechanical properties
• Limited effectiveness when employing co-deposition techniques with other metals
• Low cathode efficiency

However, a plating technique developed more than a half-century ago for the manufacturing of rocket nozzles was able to overcome these obstacles. Specifically, this method involves the electroplating of tungsten from nonaqueous organic electrolytic Lewis acid solutions. This highly specialized tungsten plating process relies on the use of a tungsten halide solution that renders the tungsten into an inactive state.

Brush Electroplating On Tungsten Surfaces

UPDATE: (8/24/2021) SPC no longer offers brush plating services. This section is for educational purposes only.

A more recent tungsten plating innovation is the development of an effective brush electroplating technique. While closely related to standard electroplating, brush plating differs in that, instead of substrate immersion, it involves the application of the coating with a brush saturated with the electrolyte solution.

The brush is typically comprised of stainless steel wrapped in a cloth material and connected to a low-voltage DC power source. The workpiece is then connected to the negative output. Because this process enables a more effective coating application on targeted areas of the substrate, brush plating is often referred to as selective or spot plating.

As far as the disadvantages of brush plating, it can be much more labor-intensive than standard electroplating, as it requires much more operator involvement. It can also be much more difficult to achieve the desired plating thickness for coating requirements exceeding 0.7 mm. Also, traditional electroplating tends to be more cost-effective when coating an entire part, instead of specific, targeted areas.

The tungsten brush plating process actually utilizes a nickel-tungsten alloy consisting of approximately 40-percent tungsten and 60-percent nickel by weight. This produces a hard coating that substantially improves the surface properties of the substrate by increasing hardness and wear resistance. The high tungsten content also provides the element of thermal stability. Exposing the coating to high temperatures for a short period of time can further increase its hardness.

Electroless Tungsten Plating Services

Electroless plating dates back to the 1940s and now represents a significant segment of the metal finishing marketplace. The key difference between electro- and electroless plating is that the latter does not require an external power source and makes use of only one electrode as opposed to two. Instead of electrodeposition, coating occurs via an autocatalytic reaction.

Electroless plating is regarded as a simpler, cleaner process than electroplating. Advantages include the ability to achieve a more uniform coating thickness, as the process enables the plating solution to reach deep, recessed areas on the surface of the substrate. It’s also much easier to control overall coating thickness and prevent the excess accumulation of the coating around edges and in corners. Electroless plated coatings tend to be less porous and offer better protection against corrosion. Finally, the lack of the need for electricity and the more simplified plating setup can make electroless plating more cost-effective in the long run.

Tungsten is not amenable to the electroless plating process on its own with the technology that is currently available. However, tungsten electroless plating is achievable via the deposition of tungsten-carbide, an extremely hard, diamond-like metal alloy. This technique requires the use of an electroless plating solution that includes a nickel-phosphorous alloy, which acts as a binder that drives the build of thickness and incorporates the particles of tungsten-carbide as a trapped material within the metal-crystalline growth.

Plating onto Tungsten Surfaces

Although difficult to achieve, it is also possible to plate onto a tungsten substrate in some situations for the purpose of enhancing corrosion protection. While it is virtually impossible to plate onto tungsten alone, effective techniques are available for electrodeposition onto a tungsten carbide workpiece. Nickel is the coating that tends to work best with tungsten; however, the part should be put through a post-coating embrittlement relief process to ensure sufficient, long-lasting coating adhesion.

The Importance of Choosing the Right Plating Company

Only a handful of metal finishing companies have the experience and expertise necessary to successfully plate tungsten onto other metals. Sharrett’s Plating Company has developed a highly effective tungsten plating process that can deliver consistent results for companies in industries such as aerospace, automotive and electronics. We also have the ability to customize our tungsten plating technique to meet the requirements and specifications of your industrial operation. In addition, we have the capability to plate onto tungsten and other refractory metals.

When you choose SPC for your tungsten plating needs, you also work with a company that makes quality our primary focus. We are an ISO-certified metal finishing operation that has earned the reputation as an early adopter of business improvement processes that we implement throughout every area of our organization. Our team of engineers, scientists, production personnel and other metal finishing professionals are committed to one goal: achieving total satisfaction for every customer, regardless of project size or scope.

Additional Resources:

• Electroless Nickel Plating Tungsten