Hot news - thermal management 10 years on

TWI Bulletin, November - December 2004

Advanced thermal protection systems for combustor cans

John Fernie studied for a BSc in Metallurgy and Microstructural Engineering, followed by a PhD in Physics. He has nearly 20 years experience in ceramics and has worked in industry and at TWI. A European Engineer, John is currently manager of the Ceramics group at TWI and is responsible for the development and commercialisation of several ceramic based innovations.

The potential of hot-wall combustors for achieving increased efficiencies and lower emissions, has been recognised for a long time, but the formidable materials problems have been a severe deterrent. As John Fernie explains, thermal protection systems are crucial to the success of these programmes. This piece introduces the technology and recent results from the Ceramics Group at TWI.

Gas turbines are a versatile, cost effective source of electricity, mechanical power and propulsion. At the heart of the engine is the combustor. The combustion section contains the combustion chambers, igniter plugs, and fuel nozzle or fuel injectors, and is located directly between the compressor and turbine sections. It is designed to burn a fuel-air mixture and to deliver combusted gases to the turbine.

The temperature of the gases at this point is known as the CDT (Combustor Delivery Temperature) and increasing this temperature has a direct benefit on the efficiency of the engine. A schematic diagram of a gas turbine combustor is shown in Fig.1.

The air used for burning is known as primary air. However, some air (secondary air) is re-directed by holes and louvres in the combustion chamber liner. This secondary air cools the combustor wall to below 900°C (which promotes the production of CO), and quenches the stoichiometeric combustion flame to below 1600°C (which reduces the formation of nitrogen oxides, NOx).

The basic concepts used today have changed little over the years. However, the drive to improve engine combustion efficiency while reducing emissions has dictated that the operating temperatures within gas turbine engines have increased significantly over the last 30 years. This has led to extensive developments in materials, manufacturing techniques and design.

The state of the art

Research into metallic structures has led to significant advances in combustor can technology, with state-of-the-art nickel based superalloys having maximum operating temperatures of 800-850°C. Today's CDT are around 1400°C; accomplished using thermal barrier coatings (TBCs) and high levels of cooling gas (delivered via intricately machined channels). This combination prevents excessive creep and melting of the combustion chamber. However, the drive for higher temperatures continues to be a major goal for the industry.

Increasing engine temperatures compensated for by increased cooling actually results in poorer performance. This is because the cooling gas is provided from the compressor stage which results in losses in efficiency and consequently higher fuel consumption. It also increases harmful emissions of NOx and CO into the environment. A solution is to increase the performance of the TBC.

Thermal barrier coatings

The TBC concept involves placing a thermal insulating layer between the combustor and the hot gas heat source in order to reduce the heat transfer to the component. The required properties of the TBC include;

• Low thermal conductivity
• Resistance to corrosion and erosion
• Good fracture toughness
• Matched thermal expansion to the substrate
• High melting point
• Good thermal shock resistance
• Chemical stability
• High flame reflectivity

Since the 1960s, plasma sprayed zirconia TBCs have been used extensively as the top-coat on combustion chamber walls. This uses air plasma sprayed yttria-stabilised zirconia, APS YSZ, and offers outstanding mechanical, chemical and thermal properties. Typical structures are shown in Fig.2a. Unfortunately, the material shows insufficient phase stability and accelerated sintering at temperatures above 1200°C.

Experience has demonstrated that the thickness of TBC that provides optimum adherence and durability is approximately 0.25mm ceramic with a 0.1mm bondcoat. Thicker TBCs have exhibited reduced lifetimes due to increased residual stresses induced during the spraying operation.

Further increases in the thrust to weight ratio of advanced engines will require even higher CDT, approximately 1500°C. These higher temperatures can only be achieved through uneconomical advanced cooling (taking more air from the compressor) or by the introduction of alternative technology.

An alternative technology, which has come to the fore, is electron beam plasma vapour deposition (EBPVD). During EBPVD, a high energy EB melts and evaporates a ceramic source (yttria stabilised zirconia is again the material of choice) in a vacuum chamber, and preheated substrates are positioned in the vapour cloud.

Highly columnar microstructures are formed and aerodynamically smooth surfaces obtained. Due to the columnar structure ( Fig.2b), the lifetime of the TBC is prolonged and the damage tolerance improved.

There are pros and cons to each process/structure; APS has a lower thermal conductivity, whilst EBPVD has a higher strain tolerance. Current research includes attempts to tailor the microstructure of the TBC to incorporate the benefits of both processes (in a zig-zag).

In the short term, the goal is to produce improved TBCs on Ni alloys and this will be via developments in zirconia coatings (probably by EBPVD). Zirconia is particularly well suited not only because it has a low thermal conductivity, but also its CTE (co-efficient of thermal expansion) is a close match to the can materials ie Ni-alloys.

However, there is ongoing research to replace Ni-alloys with ceramic-matrix-composites materials. The alternative can materials, based on ceramic-matrix-composites, have much lower values for CTE and require different barrier systems.

Ceramics as combustor can materials

In an attempt to increase operating temperatures, effort has been aimed at replacing the complete combustor unit with ceramic parts. Monolithic (single phase) ceramics having suitable temperature capability, including materials such as silicon nitride (Si ₃ N ₄ ) and silicon carbide (SiC) etc., have all been considered. However, the key limitations of low fracture toughness, K _IC and the difficulty in designing for ceramics have severely restricted their application.

These limitations have been recognised since the 1980s and since that time, work on ceramics matrix composites (CMCs) has represented the best possibility of success. CMC technology relies on the presence of secondary phases. Ceramics can be reinforced using particulates, platelets and whiskers but it is the use of continuous fibres that has borne the greatest benefit. These act in various ways: crack deflection and fibre pull-out, for example to absorb the energy of a crack - and to induce what is sometimes referred to as 'graceful failure'. Graceful failure in these systems is to a large degree influenced by the interface between the fibre and the matrix.

At present there are two candidate CMC systems; SiC/SiC and oxide/oxide. These systems are under investigation the world over, however, it is the nature of the fibres and the control of the interface with the matrix which currently restricts their usage. Both systems suffer from high temperature degradation above 1200°C, and in both cases a coating is required for environmental or thermal protection.

For both systems it is important to develop a TBC that is thermally insulating, has a matched CTE to the substrate and is capable of being joined.

TWI activities in thermal management

The Ceramics Group at TWI has been active in the development of thermal protection systems for more than 10 years (via several single client and collaborative programmes).

This work has investigated a number of potential systems including ceramic fibreboard and foams. However, a recent innovation within the group has developed an extremely refractory, thermally insulating material - TeMuS. This material is capable of being applied as a coating or fabricated as a structural material and has a thermal expansion very close to that of the oxide/oxide system. Together with in-house developed joining technology (described later)TeMuS may offer a potential solution to the issue of the thermal protection of CMCs in gas turbine applications. Figure 3 shows a schematic of the proposed system.

TeMuS is a low-density, thermal insulation material based on ceramic hollow spheres bonded together using a spinel matrix. TeMuS was originally designed as a high temperature insulation material for a variety of applications including gas turbines and furnace insulation.

The material is dimensionally and thermally stable at temperatures up to and greater than 1800°C (~2073K). The thermal properties have been shown to be excellent. The properties of TeMuS (including thermal test data) are outlined in Table 1, and compared to various other TBC systems. The data in the table has been acquired to ASTM standards in an independent lab.

Table 1 Typical properties of TeMuS compared to conventional TBCs

However, the insulating nature of TeMuS has been demonstrated in a series of in-house tests. One face of a TeMuS tile is heated and the temperature drop ( ie cold face temp) measured. A typical result for a 5mm thick tile is shown in Fig.4. The hot face was 1600°C and the temp drop across 5mm was nearly 700°C.

The bulk of the TeMuS material comprises bubble alumina (a low-density thin walled hollow sphere) that has a low thermal conductivity. This bulk is bonded together using a highly refractory spinel matrix. Spinel is a magnesium aluminate (MgO.Al ₂ O ₃ ) based ceramic material, which is capable of temperatures greater than 1800°C. In the current innovation this is formed by the decomposition of a pre-ceramic binder to form magnesia and subsequent reaction with alumina to form the final product. This allows a dense matrix to be achieved at temperatures and times below those required for conventional sintering.

The actual manufacturing route for TeMuS is very simple. The bubble alumina and other powder constituents are thoroughly mixed and the pre-ceramic binder added which is mixed into slurry. This is cast to the required shape, dried and then cured. A schematic representation of TeMuS is given in Fig.5.

TeMuS has been joined to alumina ceramics using novel technology based on glass-ceramics. Joining ceramics using glass-ceramics is not new in itself - but the TWI technology termed active glass incorporates further additions of metal oxides that on heating migrate to the surface to form a reactive layer and bond. A typical microstructure is shown in Fig.6 and bond strengths have been measured in excess of 90MPa - well above the strength of any oxide composite.

The use of active glass technology offers potential in other materials systems and applications. This is because the CTE of glass-ceramics can, to a large extent, be tailored by changing the composition. Such changes can be made without affecting the active nature of the material. This may be an advantage in the low CTE, SiC/SiC systems.

Economic benefits

Hot-wall combustors not only offer the potential for very low emissions and a means of achieving very high turbine-inlet temperatures. The major incentive for use of ceramics in turbines is the possibility of operating the engine atturbine inlet temperatures up to about 1370°C.

In turn, an increase in wall temperature of the engine from 1100°C to 1450°C, represents a 2.5% increase in engine efficiency and a reduction in fuel consumption of 1.5% (figures based on a 5MW engine). The total cost saving on fuel alone will be measured in billions of Euros per annum.

Other potential advantages include reduced engine size and weight, reduced exhaust emissions and the capability to burn alternative fuels.

Intellectual property

The TeMuS thermal barrier technology is subject to a patent application by TWI (WO03104164). Much of the TWI work was performed by Paul Jackson, Clement Jones and Alan Taylor.