Preventing Erosion Resistance in Gas Turbine

While India's defense expenditure continues to grow annually, its 2019 budget was set at $61 billion, a modest amount when compared to the $261 billion spent by China.

This budget deficit both concerns and challenges India's combat readiness in regions that see high levels of conflict, such as the Sino-Indian border. Possessing fewer weapons is most definitely a problem, especially if these weapons are in poor condition. This could lead to a real tragedy for India.

Turbofan Jet Engine.

Turbofan Jet Engine. Image Credit: Ducom

Tribology is well known for its ability to improve the functional state of weapons. When tribology-enabled, wear-resistant materials are embedded into a machine’s components, they are able to reduce its power consumption while increasing the service life of its components.

NAL has worked with the Indian defense department for a long time. It is currently focused on the development of new technologies aimed at lowering costs and time associated with the design, prototyping, manufacturing and maintenance of aerospace systems.

A key example is the GTMAP (gas turbine materials and processes) national initiative from the Indian defense department. This program aims to develop expertise in materials and manufacturing, thus enabling the development of robust gas turbine technologies for the Indian Air Force.

Fretting wear of turbine blades due to radial strain and vibration.

Fretting wear of turbine blades due to radial strain and vibration. Image Credit: Ducom 

One such emerging program in the field of GTMAP is working to develop and deploy hard face coatings that are designed to improve the functional state of gas turbines. These coatings are expected to endure severe fretting and erosion in both the compressor and high pressure turbine parts of an engine.

Why Erosion Resistance in Gas Turbines is a Huge Concern

It is impossible to avoid solid particle erosion (SPE), which is triggered by airborne particles such as sand, dust and fly ashes. Impact and ingestion of these particles can damage components such as rotor blades and stator vanes.

An appropriate surface finish is therefore vital for aerodynamic efficiency. Erosion causes surface profile deterioration, which leads to reduced engine power, compressor stalls and, in some extreme cases, full blade failures.

Innovative Solutions from NAL to Address Poor Erosion Resistance

Ideally, a coating should decentralize any stress caused by the erodent, while simultaneously arresting crack propagation and coating delamination. Single phase coatings such as TiN and TiAlN display high levels of erosion resistance for glancing incidence, but these show poorer erosion resistance at regular incidence angles because of their low toughness and the high stress of applications.

To address these issues, second generation coatings were developed which possessed a suitable combination of soft (metallic) multi-layered and hard (ceramic) design components.

NAL has created ultra-thin (3-4 nm) metal/ceramic bilayers that have ultra-small grain sizes. This introduces many interfaces for energy dissipation while balancing ductility with hardness.

The use of exceptionally smooth coatings possessing a surface roughness of Ra = 35 nm, which were deployed using magnetron sputtering, ensured that aerodynamic properties were not affected. A thin coating of < 10 microns added just 5 mg/cm2 mass to the compressor blade, allowing for minimal mass penalty.

The Evaluation of Coatings

The coated substrates displayed considerable improvements in erosion performance after being tested with a Ducom Air Jet Erosion Tester, using ASTM G76 standard parameters at 400 °C.

The erosion tester offered a high level of flexibility, allowing a range of impingement angles - from 15 degree to 90 degree – to be tested. It also accommodated temperatures up to 1000 °C and speeds from 30 to 150 m/sec, useful in mapping erosion characteristics across a broad envelope which was relevant for the compressor blades.

The company was also able to validate a series of erosion mechanisms as a function of impingement angle.1,2,3

The optimized Ti/TiN coating’s erosion resistance was found to be 74, 13 and 12 times higher than the bare Ti6Al4V substrate, when working with erodent speeds 30, 60 and 100 m/s, respectively.

The optimized films’ erosion resistance performance was unchanged after 100 hours annealing in air at 400 °C. The Ti/TiN coating’s corrosion resistance performance was found to be 10 times better than the Ti6Al4V substrate in a 3.5% NaCl solution. Using these tests, it was possible to identify the coatings which provided the best erosion resistance.

Selection of erosion resistant material in the lab (at coupon level) using Ducom Air Jet Erosion Tester and its deployment on to a turbine blade for testing in the field (component level).

Selection of erosion resistant material in the lab (at coupon level) using Ducom Air Jet Erosion Tester and its deployment on to a turbine blade for testing in the field (component level). Image Credit: Ducom

Potential Challenges for the Development of Gas Turbine Materials

While Ti based alloys and Ni based superalloys are conventionally utilized in gas turbine applications, ceramic matrix composite could offer a real alternative due to its significantly higher temperature stability (up to 400 °F).

These materials still require improvements in terms of their manufacturing capability, but CSIR-NAL has major programs in place, working with materials and process developments around SiC-SiC composites.

Seawater environments introduce both erosion and both corrosion and erosion induced deterioration in fighter jets. This corrosion is primarily related to combustion products in high-pressure turbines. While coatings were tested up to 100 m/sec in this example, instrumentation capabilities should ideally permit test speeds of 300-400 m/sec.

References

  1. ‘Nanolayered multilayer Ti/TiN coatings: Role of bi-layer thickness and annealing on solid particle erosion behaviour at elevated temperature’, Surface and Coating Technology (2019), vol 357, pp 204-211
  2. ‘Ultra-thin multilayered erosion resistant Ti/TiN coatings with stress absorbing layers’, Applied Surface Science (2019), vol 478, pp 872-881
  3. ‘Solid particle erosion and corrosion resistance performance of nanolayered multilayered Ti/TiN and TiAl/TiAlN coatings deposited on Ti6Al4V substrates’, Surface and Coating Technology (2020), vol 387

This information has been sourced, reviewed and adapted from materials provided by Ducom. 

For more information on this source, please visit Ducom.

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