Studio per lo sviluppo di nuovi rivestimenti ceramico compositi impiegabili in sistemi abradibili

Una strategia per l’incremento delle prestazioni delle turbine a gas consiste nella riduzione del trafilamentodei gas nella zona ad alta temperatura della turbomacchina grazie alla riduzione della distanza tra lasommità delle palette e la copertura protettiva della cassa della turbina. A tal fine sono stati studiati sistemiabradibili realizzati mediante diverse tecniche. Sono stati realizzati sistemi compositi compositi a baseCoNiCrAlY/Al2O3 mediante tecnica Air Plasma Spray (APS), a base NiCrAlY/graphite mediante tecnica LaserCladding e barriere termiche spesse e porose. Le resistenze all’ossidazione e alla fatica termica dei rivestimentisono state valutate per mezzo di prove di ossidazione isoterma e ciclica. I rivestimenti compositiCoNiCrAlY/Al2O3 e NiCrAlY/graphite dopo 1000 ore a 1100°C non hanno mostrato rilevanti modificazionimicrostrutturali. La resistenza all’ossidazione dei nuovi rivestimenti compositi soddisfa le specifiche deiproduttori, gli “Original Equipment Manufacturer” (OEM). Le barriere termiche spesse e porose superano i testdi fatica termica imposti dalle procedure degli OEM. La durezza di tali rivestimenti suggerisce il loro uso incombinazione con palette dotate di rivestimento abrasivo sull’estremità. Le barriere termiche hanno mostratobuone caratteristiche di abradibilità

Untersuchung der chemischen Reaktionen von energetischem Sauerstoff mit Graphit, B_4C sowie bor- und siliziumhaltigen Kohlenstoffmaterialien

One of the major problems in plasma-wall-interaction is the impurity control of the fusion plasma. In this task the use of carbon as plasma facing material was a major step in reducing the metal and oxygen impurity concentration in the plasma. The remaining impurities were then carbon and oxygen, the last one being responsible for the high carbon contamination of the plasma due to chemical erosion in form of CO and CO_2. It has been found that the use of boron/carbon (B/C) and silicon/carbon (Si/C) materials has significantly reduced the oxygen contamination in the plasma due to a gettering effect and therefore led to an enhanced plasma performance. For a better understanding of the underlying reaction mechanisms irradiation experiments have been performed to investigate the chemical erosion, the retention behaviour, the thermal desorption and the energy distribution of the reaction products during the bombardment of different pure carbon as well as B/C- and Si/C-materials with energetic oxygen ions depending on target temperature. The experiments were performed with a mass separated "1"8O_2"+ ion beam of 2-10 keV in the temperature range between room temperature and 1800 K. The reaction products were detected by means of mass spectroscopy either with residual gas analysis or by direct detection without hitting the wall in a ''line-of-sight'' quadrupol mass spectrometer. The energy of the reaction products was measured by means of ''time-of-flight'' experiments. (orig./MM) overthermal component with a maximum at 0.12 eV whereas the CO_2 molecules are only thermally released. In comparison to pure graphite the B/C- and Si/C-materials have shown an enhanced retention capacity and a reduced chemical erosion yield due to energetic oxygen up to temperatures of 1000 K. The thermal desorption of the retained oxygen occures mainly in form of different boron oxides (BO, BO_2, B_2O_2 and B_2O_3) for temperatures above 1100 K. Besides CO and CO_2 these boron oxides are also released during irradiation of the B/C materials even at room temperature. CO and CO_2 show nearly the same energy distribution as from pure graphite. At room temperature all boron oxides are emitted only with overthermal energies with maxima in the range of 0.06-0.14 eV whereas at higher temperatures and additional thermal component appears. The behaviour of the chemical erosion of the B/C materials is in good agreement with the observed reduction of the oxygen contamination in fusion devices in which such materials are used for plasma facing components. This is mainly due to the high gettering of oxygen and the reduced chemical erosion yield. The appearence of molecules with overthermal energies leads to a higher penetration of the chemical erosion products into the plasma edge and therefore influences the ionization and transport process of those molecules in the plasma. (orig.)SIGLEAvailable from TIB Hannover: RA 831(2995) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman