HYSOP’s goal is to develop solutions for manufacturing lightweight high temperature (HT) turbine components and to design new coating systems (protection against oxidation, water vapour and CMAS).
Nb/Nb5Si3 and Si3N4/MoSi2 composites are lightweight HT materials (density < 6.5~7 and < 5.6 g/cm3, respectively) with application potential above 1300°C making them candidates for advanced aero-engine components, allowing reduction of fuel consumption, CO2 emissions and cooling air needs, hence a further increase in efficiency and reduction in engine weight.
Though remarkable HT mechanical properties have been achieved (strength, creep), especially in the FP6 ULTMAT project, short/medium term application cannot be envisaged since improved oxidation resistance and optimised microstructures for enhanced mechanical properties are required.
The partners (engine manufacturer, research centres, universities) will join their expertise to reach following objectives:
The project started with the setting of quantified technical goals for the manufactured materials and coating systems, as well as of common test conditions. The hybrid components which will be manufactured have been designed, corresponding to real, but simplified, engine components (blade, vane, seal segment), and the CAD files issued.
The development of protective coatings, based on the results delivered at the end of the ULTMAT project (FP6, 2004-2008) but also on new coating concepts, has led to significant performances in both isothermal and cyclic oxidation tests, which are now being extended to the harshest conditions, i.e. up to 1300°C in dry or wet air or with a CMAS deposit. The adequacy of these coatings to serve as bond coats for environmental/thermal barrier coatings (E/TBC) has been evaluated. Two zirconia-based EB-PVD coatings, yttria partially stabilized zirconia and gadolinium zirconate, as well as thin magnetron sputtered yttrium silicate, were deposited on coated samples and their preliminary testing has started; some coating systems have already shown fairly good lifetimes under cyclic conditions at 1100°C and 1200°C, reaching the maximum exposure times, 1000 and 500 one-hour cycles, respectively.
To allow the start of the activities related to the innovative manufacturing techniques, based on PM, a single Nb-Si alloy composition, which can be optimised in a second step, has been defined, and gas-atomised (GA) powders have been made available for the project (several tens of kg). In parallel, several Si3N4-MoSi2 “ceramic” material compositions have been defined, which differ in Si3N4 / MoSi2 mass ratio, nature and content of additives, etc. The corresponding powder mixtures have been prepared and compacted using gas pressure sintering, PIM (powder injection moulding) and spark plasma sintering. The characterisation of the compacted samples is in progress in terms of density, microstructure, α/β Si3N4 ratio, toughness bending strength and oxidation resistance…) has led to the selection of three compositions for further investigations.
The fraction of fine particles, suited for PIM, in the GA Nb-Si powder being relatively low, the mechanical milling (MM) of coarser particles has been studied and the process parameters optimised from a technical but also economic point of view (final particle size distribution, powder yield, milling duration, contamination…). Production of powders by mechanical alloying (MA) is also considered in the project and optimised processing conditions have been defined.
Because of the late availability of the powder batches, the PIM development could start only on small powder batches, but is now dealing with kg-size batches. Process conditions are now defined (feedstock preparation, mould injection, debinding and sintering to the final shape) for the GA and MA Nb-Si powders and the Si3N4-MoSi2 powders.
HIPing operating parameters have been defined for simple shapes with coarser and finer GA and MA powders, leading to low porosity levels (less than ca. 0.5%) and a limited reaction of the powder with the can.
HIPed and PIMed samples were compared for their microstructure after heat treatment at 1300°C to 1500°C and the effect of iron contamination from balls and vial was investigated. The mechanical characterisation programme has started with compression tests (up to 1000°C in a first step), toughness tests at room temperature, while creep tests are under preparation.
First complex shapes (blade, seal segment, vanes) have been produced by HIPing with a low level of defects while the mould filling for production of a PIM component was optimised by computer simulation, thanks to the previous determination of feedstock properties (viscosity, etc.). New HIPed parts and a first PIMed component will be produced in the next weeks.
The additive manufacturing technique to be used in the project for parts made of Nb-Si alloy, to be built on or joined to nickel base superalloys, has been changed from direct laser fabrication (DLF) to selected laser melting (SLM, using a powder bed). A set of processing parameters leading to an acceptable porosity level could not be obtained, so that this process has been discarded. Finally, processing conditions (braze nature, form and composition, surface preparation, brazing cycle, etc.) are studied for Nb-Si/superalloy and ceramic/superalloy joints, in accordance with the design of the hybrid components.
After a slow start for some activities due to the late availability of materials and powders, the HYSOP project is now on good tracks with already encouraging results, but with still technological hurdles to overcome. For some of the preliminary outcomes, communications at conferences have been made and publications are under preparation.