From jet engines to power generation, gas turbines are the engine at the heart of transportation and power. The global gas turbine market was estimated to be worth $18.14 billion in 2017 with an expected annual growth rate higher than 3% over the next 5 years; the growing demand for power worldwide being one key factor contributing to the growth of the market .
The gas turbine engine is also a prominent piece of engineering, with blades turning at high speeds and temperatures reaching 1500ºC, the most advanced materials and technologies need to be employed.
A gas turbine is essentially an internal combustion engine. Air is firstly compressed and brought to a combustion chamber, where fuel is sprayed into the air and ignited generating a high-temperature and high-pressure gas. This gas enters a turbine, where its energy is transformed into a rotational movement that can be used to, for example, provide thrust or generate electricity.
Chapter 1: The history of gas turbines
The history of gas turbines dates back to the late XIX century when the firsts experimental gas turbines were conceived. Seeking to increase the efficiency of the available steam turbine engines and to reduce its operational costs the technology quickly developed into the first operational jet engine by Hans von Ohain in 1937.
During these inventions, it became evident that highly resistant materials were essential for harnessing the potential of these engines. Since then, the developments in gas turbines and new materials have been hand in hand. These class of materials are called “superalloys” and are known for their high mechanical strength, low deformation at high temperatures and good resistance to corrosion or oxidation.
Nickel-based superalloys are the highest exponent of these alloys, their development virtually exploded in the 50s and 60s, and they are still being used up until today.
Chapter 2: Current situation
The efficiency of gas turbines is the hurdle with which we, engineers and scientists working in the field, deal every day. Higher efficiency rates directly translate into a profit increase, and perhaps more importantly, a reduction of gas emissions.
Fundamentally, the efficiency of gas turbines is limited by its operating temperature, which in turn is limited by the capabilities of the materials used. Currently, Nickel-based superalloys are used to reach maximum temperatures of 1150°C. The advancements in coatings and cooling systems have enabled these materials to operate at temperatures as high as 1500°C. However, the inefficiency losses increase with the use of the coatings and complicated cooling systems.
Improvement of Ni-based superalloys is becoming increasingly hard, and it seems they are already reaching their full potential. Therefore, new materials capable of sustaining higher temperatures are expected to be the future of gas turbines.
Additionally, weight reductions are also an important area of improvement. In recent years, developments in Titanium-based alloys led to the replacement of some Ni-based parts in jet engines . This area of development is particularly interesting for the aerospace industry, where the weight of an aircraft is a key factor in its overall performance.
Chapter 3: The future
In the field of high-temperature alloys, new exotic combinations have been researched for over 20 years and are now reaching their last steps of development. Metal alloys with intermetallic phases seem to be potential candidates, in particular, silicides of refractory metals are calling a lot of attention.
These alloys can be considered some kind of composite structure, one of the constituents provides outstanding resistance to oxidation but poor mechanical properties while the other one provides the required mechanical properties but lacks oxidation resistance. By combining the two parts, a constructive combination is created resulting in a material with the best properties of its parts.
Coating technology is another factor that is continuously being improved and adapted to new alloys. They provide an excellent protection to the underlying material making them a great complement. Ceramic materials are also expected to be introduced in the future; they are ideal thanks to their resistance to high temperatures and light-weight. Although ceramics are traditionally known for being particularly brittle, new advancements on Ceramic Matrix Composites (CMCs) have provided extraordinarily high toughness .
Additionally, the disruptive technology of additive manufacturing (AM) is set to have a significant impact on the industry. AM offers the opportunity of fabricating more complex parts than conventional manufacturing thanks to a layer-by-layer build-up. The technology has evolved in recent years considerably reducing its costs, and it is expected to continue in the same way. Furthermore, processing of certain alloys using AM can lead to enhanced mechanical properties. An example of how the technology is already starting to reshape the sector is the achievement by Siemens to make a full-load test of a gas turbine with AM built blades .
New materials have always been hand in hand with gas turbine development. Nowadays, technology is advancing faster than ever before bringing new possibilities for the researchers involved with improving these engines.
In the near future, we will most likely see new alloys coming to form part of turbine engines. Together with innovative manufacturing techniques, a revolution in the sector is seemingly coming ahead.
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