Making More Efficient Engines
Ceramic thermal barrier coatings are critical to improved engine performance
If you have ever flown on a jet aircraft or used electricity generated from natural gas, you have benefited from gas turbine technology. The metallic components of gas turbines are protected from the extreme combustion temperatures in part by high performance ceramics known as thermal barrier coatings (TBCs), Figure 1. For the last 30 years, the current industry-standard TBC composition has been 7-8wt% (weight percent) Yttria (YO1.5) Stabilized Zirconia (ZrO2), commonly known as 7YSZ. Both power generation and aviation industries aim to increase the operating temperatures of gas turbines to reap the economic and environmental benefits of improved engine efficiency. Unfortunately 7YSZ has inherent temperature limitations that prevent it from performing well at high (>1300°C) operating temperatures. This has motivated researchers to search for novel TBCs with higher temperature capabilities.
CeO2-, TiO2-stabilized ZrO2 (CeTiSZ) shows promise at 1350°C
Researchers at UC Santa Barbara have investigated several systems as alternatives to 7YSZ. The greatest challenge for novel TBC systems is developing materials that are phase stable (i.e. don’t undergo harmful phase changes at high temperatures) and have adequate toughness. Often TBCs that are phase stable have low toughness, while TBCs that have adequate toughness are susceptible to phase transformations that degrade performance. In a recent study (referenced below), TBCs in the CeO2-TiO2-ZrO2 system exhibit both phase stability and adequate toughness up to at least 1350°C. In this system the tetragonal phase does not transform, which is critical to maintaining high toughness. Experiments indicate that the new CeTiSZ system has toughness values that are higher than 7YSZ and increase with aging at 1350°C.
Research is ongoing to investigate the performance of CeTiSZ at even higher temperatures
While the results at 1350°C are promising, turbine manufacturers are now interested in developing TBCs that perform well at 1500°C so that they can increase operating temperatures (and thus efficiency) from today’s current levels. Therefore, before CeTiSZ is adopted by industry, its performance at higher temperatures must be investigated. If the system’s performance at higher temperatures is proven to be resistant to phase changes, it may enable gas turbines to be operated more efficiently, saving energy, fuel costs, and lowering carbon emissions.
Author: Chandra Macauley, November 2013
Materials Department, UC Santa Barbara
To read the full paper, click here.
Jessica A. Krogstad, Maren Lepple, Carlos G. Levi. Opportunities for improved TBC durability in the CeO2-TiO2-ZrO2 system. Surface and Coatings Technology. 221 (2013) 44-52