Pushing Carbon Graphite’s Boundaries with Enhanced Oxidation Resistance
There is a growing need for materials that are able to endure extreme temperatures for extended periods of time.
As engineers continually push the limits of what is possible, the materials used in their designs must adapt to meet new, even more demanding requirements. Highly-engineered pieces of equipment, ranging from furnaces to jet engines, must now operate with improved efficiencies at higher temperatures, often with extended duty cycles and lifespans. As a direct result, there is a growing need for materials that are able to endure extreme temperatures for extended periods of time.
This is especially applicable to moving components which experience rubbing during operation. Seals and bearings that go into ovens, rockets, and turbines can see temperatures above 1000 F, all while rubbing against some type of counter face. This type of extended exposure wreaks havoc on most materials. However, over the years, mechanical carbon graphite has been engineered and developed to handle these extreme application scenarios.
Technological Background: Why Choose Carbon Graphite Materials?
Before we get into how carbon graphite has been developed to maximize temperature capabilities and enhance wear resistance, it’s important to understand why we choose carbon-based materials. The reason is found on the molecular level.
Carbon can exist in three forms: diamond, graphite, and amorphous carbon. In short, these three forms are different because their carbon atoms have different amounts of bonded valence electrons. Changing the amount of bonded valence electrons drastically affects material properties. Comparing a diamond to amorphous carbon illustrates this point well.
Diamond: Four bonded valence electrons. Highly ordered lattice structure.
Graphite: Three bonded valence electrons. Strong graphene layers bound together via weak van der Waals forces.
Amorphous Carbon: Two bonded valence electrons. Strong “entanglement” of carbons.
For the purposes of industrial materials applications, we are mainly concerned with carbon graphite, which is a combination of amorphous carbon and graphite. In carbon graphite, the amorphous carbon provides a strong matrix that binds with the graphite. Although the amorphous carbon provides strength, the graphite is what provides the material’s most important property—self-lubrication.
The low-friction property is directly attributed to graphite’s molecular structure. Graphite is the crystalline layered form of carbon. Individual layers of graphite are bound by incredibly strong covalent planar forces. These layers are able to “slide” with respect to each other, since they are only bound together via weak van der Waals forces. This is analogous to sliding a deck of cards across a table. The cards slide over one another easily and, once sprawled out over the table, form a relatively slippery surface. However, if you take a single card and try ripping it apart by pulling in the planar direction on either end, you are going to have a difficult time.
On top of its self-lubricity, carbon graphite has a host of other material properties which make it incredibly desirable:
- Carbon graphite is very chemically inert, allowing it to be exposed to a wide variety of chemicals without showing signs of corrosion.
- It is a great conductor of electricity, so it can be used in applications where an electrical current must be transferred between a moving and a stationary component (i.e. motor brushes, electrical contacts, etc.).
- It is thermally conductive, so it can dissipate heat very effectively.
- It is also very heat resistant, which makes carbon graphite the material of choice in “rubbing” applications where other forms of lubrication are simply burnt off due to excessive heat.
Visual Representation of Carbon Graphite on a Microscopic Level: The active sites on the carbon matrix get attacked by O2 at temperatures of 650°F or greater.
As a result of the carbonization process the carbon graphite goes through during production, its base form has a network of interconnected porosity. Carbon graphite manufacturers are able to take advantage of this porosity by impregnating the material with metals, resins, salts, etc. to enhance specific material properties, depending on the final application of the material. Some examples of common impregnations include:
- Copper impregnation to increase strength.
- Silver impregnation to increase electrical conductivity.
- Carbon impregnation to enhance chemical resistance.
Visual Representation of Graphite on a Microscopic Level: Graphite has fewer active sites at which oxygen can react with carbons, reducing the oxidation rate.
Developing Carbon Graphite Materials for High Heat Applications
Carbon graphite manufacturers are constantly working towards the development of grades with improved material properties. This development is guided by the needs of the end user. In relatively recent years, there has been great demand for a higher temperature rated material in almost all industries that utilize carbon graphite, ranging from aerospace to oil and gas production. This message has been received loud and clear by materials manufacturers, who have directed a large portion of their R&D towards offering a high temp solution.
When carbon graphite is exposed to extreme heat, there is one major contributor to material degradation: oxidation. Oxidation (reaction shown below) occurs at temperatures greater than 650˚F and can cause catastrophic damage to a system:
C + O2 + Heat → CO2
As this reaction occurs, carbon atoms in the carbon graphite matrix react with oxygen molecules. This causes the material to experience mass loss as carbon atoms bind to free oxygen and are released as carbon dioxide. Heat is a necessary catalyst in this reaction—without heat, there wouldn’t be enough activation energy for the carbon and oxygen to react with each other. Over time, this measureable weight loss results in the deterioration of physical properties such as strength and wear resistance.
Visual Representation of Graphite Impregnated with Oxidation Inhibitor on a Microscopic Level: Decreasing the surface area of the graphite grains exposed to oxygen via impregnation yields optimal oxidation resistance.
When conducting oxidation tests, engineers typically use a 5% weight loss as the benchmark for the point at which oxidation has diminished the carbon properties to below the minimum specifications for the material. Depending on the material grade, temperature, and the amount of oxygen present in the application, a part can reach this point in hours, years, or perhaps it won’t reach this point at all.
There are ways to fight oxidation. The main method used to do so is to reduce the amount of active sites at which carbon atoms are available to react with oxygen. As mentioned earlier, a typical carbon graphite component consists of grains of graphite within a carbonaceous matrix. The layered graphite grains have relatively few active sites available for oxidation to occur. The carbon surrounding these grains, however, has a less ordered structure which has many more active sites exposed to oxygen. As a result, increasing the amount of graphite and decreasing the amount of amorphous carbon in a part significantly improves the material’s oxidation resistance. This is achieved through graphitization, or the conversion of carbon to graphite through the introduction of extreme heat over time.
Although graphitization has considerable effects when it comes to enhancing oxidation resistance, oxidation rates can be even further reduced through impregnating the graphite’s porosity with an oxidation inhibitor. The oxidation inhibitor chemically bonds to the active sites of the graphite matrix and acts as a barrier between the carbon and oxygen atoms.
When impregnated with an oxidation inhibitor, graphite shows dramatically improved oxidation resistance, to the point where the material can withstand excursions to 1000 F and higher. For seals and bearings in ovens, engines, and turbines where hot, high pressure gases are present, this property is essential. The fact that graphite retains its self-lubricating properties at these temperature extremes makes it unique. In high temperature applications where any type of rubbing occurs, there is no other material that can outperform graphite.
Jet engines can utilize multiple carbon graphite components in their complex design.
High Temperature Graphite in Use Today
Newly developed high temperature graphite grades have already found a place in the modern engineering world. A considerable amount of high temperature graphite is currently sold to the aerospace industry, specifically to jet engine manufacturers. These engines have turbines which must operate at peak efficiency, all while handling dizzying rotational speeds and extreme temperatures.
High temperature graphite can not only withstand these conditions, but it enhances efficiency through the effective dissipation of heat (as mentioned earlier, graphite has excellent thermal conductivity). This is just one of many high temperature graphite applications that is currently in production.
Driven by demand across many different industries, research and development of carbon graphite oxidation resistance has resulted in a profoundly improved material compared to what was previously available. This has enabled engineers to exceed restrictions which have to this point hindered technological advances. The need to overcome these limitations is the driving factor behind this rapid development in recent years—development that will only accelerate in years to come.