Peter Mechnich

Reaction-bonding behavior of mullite ceramics with Y2O3 addition

Abstract Mullite ceramics were fabricated by a reaction-bonding technique from powder mixtures of Al 2 O 3 and Si, with an Y 2 O 3 addition. The mullitization and densification behavior was investigated as a function of Y 2 O 3 content, heating rate, processing temperature and holding time as well as mullite seeds. It has been shown that mullite formation occurs by nucleation and growth within an aluminosilicate glass, but lattice and grain-boundary diffusion becomes important at the later stage of mullite growth. The addition of Y 2 O 3 can decrease the glass viscosity and thus enhance the mullitization reaction, while the incorporation of mullite seeds may reduce the grain size and consequently allow pore elimination under conditions of shorter-range diffusion. After heating at 1400°C for 5 h or 1450°C for 2 h, an almost fully dense state was achieved for 15 mol% Y 2 O 3 -doped and 5 mol% mullite-seeded specimens.

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Abstract Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of α-Al 2 O 3 and quartz, with an Y 2 O 3 addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y 2 O 3 content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y 2 O 3 -doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.

Effect of cyclic infiltrations on microstructure and mechanical behavior of porous mullite/mullite composites

Abstract A porous mullite fiber/mullite matrix composite was cyclically infiltrated in an AlCl 3 solution to introduce alumina into the pore space. It is shown that the matrix porosity decreases with the number of infiltration cycles. Moreover, the microstructural observations indicate that the distribution of residual pores within the infiltrated composites is heterogeneous, with a lower porosity in the surface region relative to the interior region. Beyond 4 cycles of infiltration, surface embrittlement occurs due to a significant enhancement of the interparticle bonds and the fiber/matrix interfaces in the surface region, leading to a notable reduction in fracture energy.

Mullitization and densification of Y2O3-doped reaction-bonded mullite ceramics with different compositions

Mullite ceramics were fabricated by a reaction-bonding technique from the powder mixtures of Al2O3 and Si with an Y2O3 addition. The effects of the initial powder composition on the mullitization and densification behaviors were investigated. It has been shown that mullitization and densification can be simultaneously enhanced by a low-viscosity yttrious silicate glass. After a 2 h heating at 1400 °C, all the specimens exhibited a dense microstructure. Moreover, mullite grains were observed to have an elongated morphology, while the yttrious silicate glass was found to crystallize into Y2Si2O7 at the later stages of mullite growth.

Y2SiO5 environmental barrier coatings for niobium silicide based materials

Abstract The oxidation behaviour of Nb silicide based alloys, considered as potential ultra-high temperature material for gas turbine engine applications, can be improved by chromia–silica forming coatings with M7Si6 structure. To stabilise the protective oxide scales against water vapour corrosion in combustion atmospheres, environmental barrier coatings (EBCs) of yttrium silicate were deposited on an FeB modified M7Si6 based bond coat using magnetron sputtering. The 15–20 μm thick ceramic top coats with an approximate chemical composition of 25Y–13Si–62O (at.-%) were dense and amorphous after deposition. They were annealed in vacuum to get a crystalline structure with the predominant phase Y2SiO5. Samples with this EBC system were tested in rapidly flowing water vapour at temperatures between 1100 and 1300°C for up to 16 h. At 1100 and 1200°C, significantly reduced mass gains were determined for samples with yttrium silicate top coat in comparison to those coated only with FeB containing layers. The EBC partially transformed into the Y2Si2O7 phase and exhibited microporosity. At 1300°C, the yttrium silicate layer decomposed forming yttrium oxides.

Development and Test of Oxide/Oxide Ceramic Matrix Composites Combustor Liner Demonstrators for Aero-engines

Ceramic matrix composites (CMC) offer the potential of increased service temperatures and are thus an interesting alternative to conventional combustor alloys. Tubular combustor liner demonstrators made of an oxide/oxide CMC were developed for a lean combustor in a future aero-engine in the medium thrust range and tested at engine conditions. During the design various aspects like protective coating, thermo-mechanical design, development of a failure model for the CMC as well as design and test of an attachment system were taken into account. The tests of the two liners were conducted at conditions up to 80% take-off. A new protective coating was tested successfully with a coating thickness of up to t=1 mm. Different inspection criteria were derived in order to detect crack initiation at an early stage for a validation of the failure model. With the help of detailed pre- and post-test computer tomography scans to account for the micro structure of the CMC the findings of the failure model were in reasonable agreement with the test results.

The CaSO4 phase in fully infiltrated electron-beam physical vapour deposited yttria stabilized zirconia top coats from engine hardware

Abstract Thermal barrier coating (TBC) recession mechanisms caused by CaSO4 and Fe –Ti – rich CaO– MgO–Al2O3 –SiO2 (CMAS) (FTCMAS) particles on an in-service electron-beam physical vapour deposited (EB-PVD) yttria stabilized zirconia (YSZ) coated high-pressure turbine blade have been studied using analytical electron microscopy and concomitant laboratory experiments. A vapour phase deposition process is proposed for CaSO4 in order to rationalize the unique microstructure of pure CaSO4 infiltration through the full thickness of the YSZ top coat prior to deposition FTCMAS particles. While no CaSO4/YSZ interaction is observed in the bottom sections of the coating, distinct reactive interfaces develop at the surface due to YSZ interaction with both, CaSO4 and FTCMAS particles at working temperatures. Upon partial dissolution of the YSZ column tips the CaSO4/YSZ interface results in CaZrO3 formation, while the FTCMAS/YSZ interface consists of a thin double layer of CaZrO3 and a garnet-type Ca3Zr2(Fe, Al, Si)3O12 phase, also known as the mineral kimzeyite. Despite CaSO4 infiltration and an accumulated service life time of 17,000 h the top coat was still found to be intact. Bond-coat rumpling at the less efficiently cooled regions close to the trailing edge gives rise to strong surface undulations. This effect is particularly pronounced on the pressure surface and yields a crest/trough topography. The well adherent CaZrO3 layer from the CaSO4/YSZ interface may have a similar impact on TBC spallation as a glassy CMAS-type overlay.

DEVELOPMENT AND TEST OF OXIDE/OXIDE CMC COMBUSTOR LINER DEMONSTRATORS FOR AERO ENGINES

Ceramic matrix composites (CMC) offer the potential of increased service temperatures and are thus an interesting alternative to conventional combustor alloys. Tubular combustor liner demonstrators made of an oxide/oxide CMC were developed for a lean combustor in a future aero-engine in the medium thrust range and tested at engine conditions. During the design various aspects like protective coating, thermo-mechanical design, development of a failure model for the CMC as well as design and test of an attachment system were taken into account. The tests of the two liners were conducted at conditions up to 80% take-off. A new protective coating was tested successfully with a coating thickness of up to t=1 mm. Different inspection criteria were derived in order to detect crack initiation at an early stage for a validation of the failure model. With the help of detailed pre- and post-test computer tomography scans to account for the micro structure of the CMC the findings of the failure model were in reasonable agreement with the test results.