Carbon/carbon composites (C/C) are the ideal solution for today’s fast-moving heat-treatment industries. Among the vast array of benefits, C/C provides the perfect answer for businesses looking for a material with high mechanical strength, great thermal conductivity, prolonged life, energy savings and overall reduction in running costs.
The purpose of this article is to look at the benefits of C/C composites and the two main types available to business users.
C/C Composite Overview
Carbon-fiber-reinforced carbon composites, commonly referred to as carbon-carbon or C/C, is an advanced material that’s made of thin carbon fibers and a carbon-matrix binder, creating a compound or composite of highly durable materials.
The basic manufacturing technique of C/C composites involves carbon fibers and a matrix going through several layers of impregnation and heat cycles. Once these steps are complete, graphitization is the final process, and this leaves you with a C/C composite material.
C/C composite development dates back to the 1960s, where it was developed for the nose cones on ballistic missiles. A material needed to be able to cope with the extreme temperatures produced at hypersonic speeds and or atmospheric re-entry. C/C is also extensively used as a friction material for aviation and automotive applications.
In the last decade, C/C has seen an increased use in modern industries that specialize in heat treatment, furnace manufacturing, PV and sintering. Vacuum and gas furnaces are typically (and were historically) utilized within these industries because they operate in high-temperature environments.
C/C Composites and Traditional Heat Materials
Due to its ability to heat in a higher-temperature environment than historical furnaces, C/C composites are now regarded as the best-performing materials for high-temperature industries, especially when compared to traditional heat-resistant materials such as graphite, ceramics, steel and iron. C/C composites have several advantages, which include: long life cycles, excellent heating and cooling times, extremely lightweight (1/5th the weight of iron), resistance to chemical attack and low thermal expansion (Fig. 1).
While C/C is deemed as possibly the ideal material for hot-zone environments, graphite is still the dominant material, and this is driven by the relatively low cost. C/C has always been considered an expensive material. Even today, this is still a common misconception and has prevented businesses from pursing the material as a replacement for existing technology. This fallacy is derived from opinions based on the cost of the material from over 30 years ago, when it wasn’t uncommon to find C/C selling for $3,000 per kg/m3. Today, however, we are seeing prices lower than $300. And with this in mind, it’s easy to see why C/C continues to be a competitive alternative to premium-grade graphite, which is only marginally lower in terms of cost.
When choosing a material for high-temperature applications, ROI (return on investment) is something not always considered. For certain applications, the user will search for the lowest price while not considering the long-term effects on their increased overall costs and the impact it can have on productivity. Materials that do not offer the same levels of performance as C/C can have several negative effects. These include more maintenance and decreased batch cycles as well as furnaces that will require more downtime and an increase in energy consumption. C/C offers an excellent
ROI, reducing operating and energy costs while increasing output.
When looking at increasing productivity, a key factor is the strength-to-weight ratio, which has an important role to play when looking to reduce costs and increase output. A good example is to compare graphite or steel trays to a C/C tray (Fig. 2). For graphite or steel materials to achieve the same desired strength as C/C, their size needs to be considerably larger (thus heavier) in order to have a similar strength ratio as C/C.
The downside of increasing the size and weight of the tray is that it reduces the physical volume available within a furnace while it increases the weight and heating times. This limits the amount of trays that can be loaded and stacked at any one time while heating and cooling rates can increase, which further increases energy consumption.
C/C composites do not suffer from these inherent issues. C/C composites have a far higher mechanical strength in simple terms, which means you can manufacture lighter, stronger trays that are smaller in size while still holding the same volume of product when compared to graphite and steel trays.
With a reduced physical size and weight of C/C applications, there are positive effects on overall productivity. Output is increased and long-term costs reduced due to C/Cs high-performing strength and thermal conductivity.
Long- and Chopped-Fiber C/C Composites
The most commonly used C/C composites for high-temperature environments are plain-woven long carbon fiber (long CF). Typically, this starts as a woven carbon cloth.
Short, chopped fiber C/C composite is the popular alternative due to its excellent machining qualities and high strength. Chopped C/C fibers are multi-directional and vary in length. This type of C/C material is also called random weave, but in most cases it’s referred to as chopped fiber (Fig. 3).
Chopped-fiber C/C is fairly young compared to long-fiber C/C. It was first developed by a Dr. Nakagawa in Japan as a friction material for high-performing automotive, motorsport clutches. They required a friction material that could be machined into complex shapes and would retain high interlaminar strength while at the same time offer a low wear rate.
It was soon discovered that chopped fiber was the ideal material to be used in high-temperature environments, especially those that operated in a vacuum. Due to the high interlaminar strength and density, we find chopped-fiber C/C increasingly popular in the heat-treatment industry. It is also used for automotive and aerospace braking systems.
C/C in the Modern Heat-Treatment Industry
When looking for a C/C composite, there are there are two key areas that should be focused on – interlaminar shear strength (ILSS) and density. As a rule, the higher the density, the longer the life span.
There are other areas you may also need to consider when evaluating your individual specifications, depending on how and where the C/C composite would be used in an application. Overall, ILSS and density are the priorities, especially if the material is going to be machined into complex shapes and structures.
Density is the first area most users focus on. With higher density, the material is stronger and the life span longer. The average density for long-fiber C/C is around 1.3-1.4 g/cm3, which can be increased to around 1.5-1.6 by further stages of impregnation and baking. To achieve a higher density, you would, on average, need to do 10 cycles of baking and impregnation. The uniformed structure of long fiber limits the amount of open pores for the matrix to penetrate, which is why we see several cycles during the bake and impregnation stages to make sure the matrix fully bonds to the fiber. Even so, there is a limit to the maximum density of long fiber.
Chopped fiber does not suffer from the same issues during production. If we look at PC70 as an example of chopped-fiber C/C composite, the arrangement of the fibers is non-uniform. The random structure of the fibers allows it be more porous, granting easier impregnation of the matrix. After one impregnation and bake cycle, the density is 1.7 g/cm3.
ILSS also plays a key role in determining the overall strength, and this is especially applicable when machining C/C. ILSS describes the force that is needed to cause cracking between layers of fibers, which leads to delamination. Fibers arranged in a plain-weave stacked fashion (2D) are prone to cracking due to the layers not being bridged by other fibers. A good example of when this happens is during machining. Precision parts like bolts (Fig. 4) or threaded studs are prone to delamination because the cutting process effectively pulls the layers apart.
3D structures, like chopped fiber, do not suffer from this issue. Chopped fiber effectively covers a 360-degree bonded area at the X and Y axes, while there is also an increased bond on the Z axis. The stronger bonds found between each layer of chopped fiber greatly increase the ILSS.
Interlaminar strength of 2D plain-woven long-fiber C/C composite ranges between 5.2 and 13.2 MPa. The interlaminar strength of chopped-fiber C/C composite (e.g., PC70) is 17 MPa.
The high density and ILSS, coupled with the other benefits of C/C composites, gives chopped fiber a unique advantage over other commonly used materials in high-temperature environments. Chopped fiber offers very similar machining characteristics to isotropic graphite. Another benefit is the durability when loading and unloading a furnace. Chopped fiber is very durable against shock and accidental knocks.
Even though short fiber has several advantages in terms of its strength and excellent machining qualities, it does have a couple of drawbacks. Elastic modulus is generally higher in long-fiber C/C because the carbon fibers are bundled in a way to raise the volume. Chopped fiber has the opposite impact because the fibers are not bundled. This makes it harder to compete with long fiber when trying to raise the carbon-fiber volume. Because of this, it can be easier to manufacture certain structural items with long-fiber C/C composites. These include tubes, U and L profiles.
There is another point concerning the complexity of manufacturing chopped-fiber C/C composites. They do require a different approach compared to long-CF composites. This reduces the number of options when choosing a manufacturer for C/C composite materials.
C/C composites offer far more flexibility and overall savings for their users than traditional materials used in hot-zone environments. If you couple this with chopped-fiber composites – which offer excellent strength, cost savings and machining capabilities on par with graphite –
C/C offers boundless possibilities.
Although C/C composites are suitable for a variety of industries, users in a hot-zone environment need to consider their overall requirements before making the investment. IH