What are Ceramic Matrix Composites?
Ceramic Matrix Composites (CMCs) are the materials that are made up of ceramic fibres reinforced with particulates, cloth or fibres to form a ceramic fibre-reinforced material.
Traditional ceramics easily get fractured on the application of load, while the ceramic matrix composites show greater crack resistance.
Some ceramic matrix composites show higher strength as well as high temperature stability.
How do CMCs differ from conventional ceramics?
Ceramic Matrix Composites (CMCs) differ from conventional ceramics in following ways.
- CMCs shows higher fracture toughness
- They are highly resistant to thermal shock
- CMCs shows improved dynamical load capability
How are CMCs named?
The name of CMCs includes the type of fiber and type of matrix.
It is represented by;
Type of fiber/type of matrix
- C/C stands for carbon fiber reinforced carbon and
- C/SiC stands for carbon fiber reinforced silicon carbide
Sometimes the CMC manufacturing process is also mentioned along with the name.
For example, if C/Si is manufactured by a liquid polymer infiltration process, then it is named as LPI-C/Si.
Examples of Ceramics Matrix Composites materials
Some common examples of ceramic matrix composites (CMCs) materials are mentioned below.
- Continuous SiC fibre reinforced glass-ceramics
- Carbon-Carbon composites
- Zirconia-toughened and SiC whisker toughened alumina
Ceramic Matrix Composites processing methods (How are CMCs made?)
Various ceramic matrix composites processing methods (or ceramic matrix composites manufacturing processes) are listed below.
- Powder processing
- Slurry infiltration
- Liquid infiltration
- Chemical vapor infiltration
- Directed metal oxidation
Let’s now discuss each of these CMC processing methods one by one.
#1) Powder processing
You can see the Powder processing method for CMCs in the above chart.
Powder processing method is generally used to manufacture discontinuously reinforced CMCs.
It is difficult to process long discontinuous as it breaks into short fibres during the mixing phase and consolidation phase.
#2) Slurry infiltration
The slurry infiltration process for ceramic matrix composites is shown in the above diagram.
In slurry infiltration process, the matrix is in the form of slurry or liquid which infiltrates the fibre and this results in a composite.
The slurry used in this process contains following things;
- Matrix powder
- Liquid carrier (alcohol or water)
- An organic binder
The quality of composite part depends upon various parameters like;
- Type of binder and amount of binder
- Particle size distribution
- Powder content
- Carrier medium, etc
During this process, the infiltration can be improved by adding some wetting agents in the liquid slurry.
Also the size of matrix powder should be less than the diameter of the fibre to get proper impregnation.
The liquid carrier is then allowed to evaporate after the infiltration process.
After this, the organic binder is burned out and the consolidation process is carried out by hot pressing which gives desired ceramic matrix composites.
#3) Liquid infiltration
The schematic diagram for liquid infiltration process is shown in the above image.
The entire process of liquid infiltration is similar to the resin transfer molding process used for polymers.
In the liquid infiltration process, there are three major issues that one should look after.
- Chemical reactivity: As the liquid infiltration process is carried out at high temperatures, the matrix and the reinforcement may react with each other and cause unnecessary reaction which weakens the bonding between matrix phase and reinforcement phase.
- Melt viscosity: The viscosity of the melted ceramics is higher than that of the metals, and because of this it is difficult for ceramic infiltrant to infiltrate into the fibrous ceramic preform.
- Wettability of reinforcement: The wetting of infiltrant may not be done perfectly which results in improper bonding which may lead to failure of components at the interface of reinforcement and the matrix.
#4) Chemical vapor infiltration
In chemical vapour infiltration process, the porous preform is prepared and it is placed in the reactor to act as a reinforcement phase.
After this, the reactant gases are supplied to the reactor which diffuses into the preform.
During this process, the decomposition of the reactants fills the space between the fibers.
The diameter of the fibers increases as the reaction progresses (you can see this in the diagram).
#5) Directed metal oxidation
The schematic diagram of directed metal oxidation process for CMCs processing is shown above.
Directed metal oxidation process involves a metal whose oxidation would take place in a particular direction to create a reaction product.
The steps involved in the indirect metal oxidation process are mentioned below.
- Step 1: Preparation of preform
- Step 2: Growth barrier
- Step 3: Matrix infiltration
First of all, the preform is prepared and then the growth barrier is placed on the top of the preform to stop the growth of the matrix material (as shown in diagram). During the matrix infiltration stage, the molten alloy is subjected to direct ed oxidation that results in formation of the desired reaction product on the surface of the molten metal and it grows outward. The oxidised metal enters into the preform and then it forms the matrix inside the preform.
Applications/Examples of Ceramic matrix composites (CMCs)
Examples (or Applications) of Ceramic matrix composites are listed below.
- Automobiles: Ceramic components and CMCs are lightweight and so they can be used in automobiles. Because of the light weight of the ceramic components, the fuel consumption of the automobiles will be less.
- Heat exchangers: CMCs are cost effective and have high temperature capability, so they are suitable for its use in heat exchangers. Ceramics and CMCs also eliminate the use of cooling systems in heat engines as they can resist the high temperature.
- Aerospace and military: As the CMCs have light weight and high strength at high temperatures, they are applicable in military and aerospace applications.
- Bearings: CMCs have anti corrosion properties and have the ability to operate with less lubrication, due to which it can be used in bearings of missiles.
- Dies and tool bits: Ceramics and CMCs are used in dies and tool bits, due to its higher strength.
- Other Examples: Various other applications/examples of ceramic matrix composites includes:
- Grinding wheel,
- Ceramic brakes, etc.