In this article, we will be discussing some major things you need to know about thermal barrier coating. We will discuss from what exactly the process is to what it entails, its types and uses, and so much more.
Therefore, if you wish to understand or learn more about thermal barrier coating, keep reading this article. We hope at the end of the day, you will have enough basis idea about this topic.
TBCs – What exactly is it?
Thermal barrier coatings refer to advanced systems that are used to coat metallic surfaces. The metallic surfaces that are usually coated with TBCs often operate in elevated temperatures. As such, they require advanced TBCs to ensure they are properly protected.
TBCs often insulate the surfaces they are used on from prolonged and large heat loads. This way, a substantial temperature difference will be created between the coated surface and the alloys. Therefore, a high operating temperature is obtained while the structural component’s thermal exposure is greatly limited.
These coatings reduce thermal’s fatigue and oxidation, therefore, extending the life of the surface they coat.
TBCs material requirements are somewhat similar to heat shields material requirements. However, with heat shields, emissivity is an important factor. The need to develop advanced and new TBCs only keeps increasing by the day. This is because the demand for engines that are not just efficient but can also run at higher temperatures and are also durable is constantly on the rise.
Types of TBCs
The following are some of the common types of TBCs:
1. Mullite
This is a silica and alumina compound and its formula is 3AI203-2SiO2. Its density is low and it also has several great mechanical properties, has reduced thermal conductivity, increased thermal stability, and is resistant to oxidation and corrosion.
But, it is plagued by volume contraction and crystallization; this often leads to delamination and cracking. You can visit here if you wish to learn more about delamination. Due to this, it is best suited for diesel engines since they usually have relatively low surface temperatures and the temperature variations throughout the coating are usually large.
2. YSZ
This type of thermal barrier coating is quite popular as it is widely used and studied. Its popularity can be attributed to its excellent performance in main applications like gas turbines and diesel engines.
Also, it is among the very few oxides that the technology then known as plasma spraying can be used to deposit in thick films.
When it comes to properties, its thermal conductivity and shock resistance are quite low, while its thermal expansion is high. But, because of its phase instability, its operating limit is quite low at 1200°C. Finally, because of oxygen transparency, it can corrode.
3. Ce02 + YSZ
Ceria (Ce02) thermal expansion coefficient is quite higher than that of YSZ. Its thermal conductivity is also lower.
Therefore, adding Ce02 to YSZ coating significantly increases the coating’s performance, mostly when it comes to its resistance to thermal shock.
However, there are some bad effects to this mix. The major ones are that the coating is made less porous and the hardness is reduced.
Uses of TBCs
TBCs have the following uses:
1. Aviation
There has been an interest in increasing gas turbines efficiency for aviation purposes. This has caused more research to be done in the area of high combustion temperatures. This is because turbine competence is greatly influenced by combustion temperature.
The higher the combustion temperature, the more improved the machine’s thermodynamic efficiency becomes. In aviation, TBCs are used to protect nickel-based superalloys in the aviation turbines from thermal cycling and melting.
2. Automotive
Ceramic TBCs are fast becoming the go-to when it comes to automotive uses. They are specially made to cut down heat loss in the engine’s exhaust system parts like the turbocharger casings, tailpipes, exhaust manifolds, downpipes, and exhaust headers.
A common name for this practice is “exhaust heat management” you can visit https://en.wikipedia.org/wiki/Exhaust_heat_management to learn more about the process. Also, when TBCs are used under a car’s bonnet, they can reduce the engine’s bay temperature. This ultimately reduces the air intake temperature.
Although these coatings are directly applied to the engine’s exhaust system metallic parts, technological advances have brought a change. Now, these coatings can be applied to composite materials using a plasma spray. Therefore, finding ceramic-coated parts in modern car engines is now commonplace.
Processing
TBCs can be produced in several ways including:
- Direct vapor deposition
- Electrostatic spray-assisted vapor deposition
- Air plasma spray
- Electron beam physical vapor deposition
- High velocity oxygen fuel
Also, an active research field is the improvement of advanced processing methods and coatings. A great example of this is solution precursor plasma spray. This process has been used to make some TBCs that have low thermal conductivities and they achieve this without having to give up their durability.
Conclusion
In this article, some important aspects of thermal barrier coatings like types, uses, and methods of processing have been discussed.
