The changing face of design using engineering materials and coating systems for advanced aero-engine applications is discussed by Keith Harrison of the Surface Engineering Division at IOM3 and Mike Hicks of Rolls-Royce, UK.
Since the inception of the gas turbine, materials have been at the forefront of enabling technologies, something that is unlikely to change in the foreseeable future.
In early engine designs, coating technologies were seen as remedies to addressing design shortfalls and introduced into operation through a ‘build and test’ philosophy. Today, operating conditions, economic and environmental factors are compelling materials scientists to adopt the approach of robust design of material systems, combined with advanced manufacturing technologies, to meet future challenges. In this context, modelling techniques will become increasingly important to optimise materials and manufacturing processes and reduce development costs and cycle times. The benefits in embracing this move from reactive to predictive design are illustrated in the diagram above.
Modern aero-engine materials are therefore very different from those of 50 years ago when steels dominated. In their place have emerged two major alloy classes, the nickel-based superalloys and titanium alloys. Steels now only remain in applications such as bearings and shafts where their high strength and surface hardenability are advantageous. Aluminium alloys have virtually disappeared from the engine.
In parallel with these alloy developments, increasing reliance is being placed on coatings and surface treatments to achieve the reliability and performance retention targets of engine components. Inherent to this is the need to give surface treatments adequate consideration at the design stage rather than viewing them as remedies for problems arising in engine operation. It is also important to consider the design and behaviour of the whole coating system (i.e. coating plus substrate and environment) rather than optimising each element in isolation.
The last decade has seen the launch of many new engine programmes to support the rapid growth in commercial airline traffic. Each of these programmes has provided opportunities and challenges to introduce new materials into the engine core.
The development of aero-engine materials against this changing business scene is illustrated by the increasing complexity of new material solutions and how engineers are seeking to tailor the properties of a component to fulfil different requirements in specific engine locations. This increased complexity also requires more of a ‘systems approach’. A gated review process is used to determine the readiness levels of emerging technologies.
Until recently, the competitive edge in the aero-engine market was gained by improved performance and reductions in cost of ownership. However, the market is now essentially mature with improvements in engine performance becoming more difficult to achieve. With reduced scope for technological advancement, other opportunities are being sought to achieve product differentiation and a competitive advantage. A major consideration is to be the first to produce an engine for the market at the lowest cost and best suited specification to the customer.
A key aspect in achieving these goals is the use of generic designs, using proven technology aligned to a rigorous assessment of materials and manufacturing technology readiness levels, and optimum timing of when a particular development is mature enough to be included in a committed engine project. The product development process is a four-stage lifecycle that covers all activities from the generation of initial concept through to entry into service and beyond into its service life.
At each stage of the product development process, the capability that needs to be acquired, internally or externally through the supply chain, is identified and used to determine future technology requirements combined within the technical specification. Prioritisation of the technologies takes place by comparing the costs associated with acquiring the technology with the benefits that the product will deliver to the customer.
For example, a new turbine blade in an advanced material with improved cooling and application of an aerofoil thermal barrier coating will enhance engine thrust weight, reduce fuel consumption and improve life. The benefits in terms of fuel burn, payload and operating cost can be assessed together with predicted sales to establish whether the return on investment satisfies acceptable criteria. It is also necessary during this stage of the process to carry out continuous competitive assessments of both products and technology. In the former, this is expressed by leads and lags in years.
Throughout this process, good design ensures that the product, at whole engine, sub-system and component level, can be manufactured consistently to a high standard of quality within the specified cost and supported in service in line with customer expectations. By focusing effort on defining the manufacturing process concurrently with the product provides protection against excessive production costs downstream, in the same way that development costs are minimised.
Once full-scale production has begun it will be possible, although to a lesser extent, to further reduce costs by continuous improvement activities, which drive the product and processes to maturity with the aim of improving productivity and quality. One key aspect is verifying component performance against design intent (customer requirement) in the product development process.
Given the great strides in component design since the 1960s, the life of the high pressure turbine blade identified in the chart p19, middle, will be around five to six years in operation, or around five million miles. Thus mathematical models able to predict design behaviours across the entire range of parameters are under development to reduce the time required to introduce new material systems into service.
In the compressor, particularly in military engines operating in desert conditions, coatings are being applied to blading to withstand the erosive effects of sand particles and hence to retain engine performance. The availability of effective erosion resistant coatings to Rolls-Royce is seen to be strategic to future compressor designs and, during project planning, the company began work on advanced erosion resistant coatings under a BRITE/EURAM project. European funding from this scheme provided support to industry, academia and research organisations for pre-competitive collaborative and co-operative research in materials, design and manufacturing technologies.
At the end of the four-year study, a number of coatings had been developed, capable of meeting current European specifications for erosion performance. These systems, multilayer ceramic/ceramic or metal/ceramic combinations, offer significant improvements over those initially based on titanium nitride coatings employed in the tooling industry to improve wear performance. Furthermore, they can be optimised by tailoring the number and thickness of the individual layers together with their chemical composition.
These coatings are now being progressively introduced into engine projects. A further deliverable from this work has been a Monte Carlo-based computer model that can reliably predict materials performance under a wide range of erosion conditions.
Perhaps the best example of surface engineering being integrated into the design process can be found in the high and intermediate pressure turbines where nickel-based alloys have
dominated. In this application, aerofoils operate under the most arduous conditions of temperature and stress during service, while operating in a gas stream at temperatures in excess of the melting point of the alloy. Operation in this environment puts severe demands on both the mechanical properties and environmental stability of the blade system, and success is only possible through close interaction of design, materials and manufacturing technologies. Major advances in cooling efficiency bring complexity to the manufacturing technology and require significant effort to control costs within the product development process.
Since the inception of the jet engine, the need to increase thrust has necessitated the development of improved material systems with higher temperature capability (see graph of Increases in turbine entry temperature).
In the early years, material development alone was responsible for the increase in turbine entry temperature (TET). With time, to permit further increases in TET, the development of superalloys with improved mechanical properties, particularly creep and fatigue characteristics, has generally been achieved at the expense of the corrosion/oxidation performance, with the consequences that component life became corrosion limited. This necessitates coating systems to restore adequate environmental protection.
All engine companies use single crystal alloys in the most demanding high pressure turbine stages and the continued drive for more temperature capability has resulted in successive generations of alloy with ever more exotic alloying additions. The latest fourth and fifth generation alloys employ ruthenium as well as rhenium, which was introduced in increasing amounts in the second and third generations.
In the early 1960s, problems were experienced with premature blade failures as a result of environmental degradation and, for the first time, surface coatings were considered in the design process. These simple aluminide-type coatings provided a cheap, cost effective solution for blades operating in aggressive conditions significantly extending component life.
During the next 20 years improvements in turbine material capability, internal cooling designs and higher thrust requirements, lead to a situation where protective aluminide coatings did not achieve adequate component life. This situation was swiftly resolved by application of a platinum aluminide coating, which, at the time, was under development within the coating acquisition programme. In this instance, platinum was directly plated onto the base material and subsequently subjected to an aluminising cycle to form a ß – [PtNi] Al phase with superior oxidation resistance to simple nickel aluminides. MCrAlY overlay coatings have also been employed, where M represents Ni and/or Co.
The latest blades also use thermal barrier coatings (TBCs). These coatings are low thermal conductivity ceramics that prevent the flow of heat from the gas stream to the underlying blade. This maximises the benefit obtained from the complex blade cooling and offers a potential increase in operating temperature of over 100ºC. The majority of TBCs are based on yttria-stabilised zirconia, with the most recent developments containing additional elements to further reduce thermal conductivity. Coatings are applied by electron beam physical vapour deposition to develop the columnar grain microstructure necessary to resist thermal and mechanical strains. The coating system must be very durable since any breach would expose the blade to gas stream temperatures that can not be endured by the metal alone. Bond coats are therefore an essential technology and the subject of much current research.
Likewise, component life prediction and assessment are critical in the safe and economical use of these systems.
The total effect of all the advances in blade cooling technology, coupled with alloy and coating development, has been to increase turbine entry temperatures by around 700ºC since the original wrought alloys.