The Importance of the Fluorine Effect on the Oxidation of Intermetallic Titanium Aluminides

Due to the low Al activity within technical titanium aluminides and the similar thermodynamic stabilities of Al- and Ti-oxide these alloys always form a mixed oxide scale at elevated temperatures consisting of TiO2, Al2O3 and also nitrides if the exposure takes place in air. This mixed scale does not provide any oxidation protection especially under thermocyclic load or in water vapor containing environments. Thus accelerated oxidation occurs. Alloying of additional elements such as Nb improves the oxidation behavior if the additions stay within a certain concentration range but such additions cannot suppress non-protective mixed scale formation. Coatings are another way to protect these materials but several obstacles and new degradation mechanisms exist such as delamination e.g. due to CTE mismatch or development of brittle intermetallic phases due to interdiffusion. Therefore, other suitable protective measures have to be undertaken to make sure a protective oxide scale will develop. The so called halogen effect is a very promising way to change the oxidation mechanism from mixed scale formation to alumina formation. After optimized halogen treatment the alumina layer is very protective up to several thousand hours even under thermocyclic load and in atmospheres containing water vapor or SO2

Investigation of the oxidation resistance of ZrB2-based monoliths using polymer-derived Si(Zr,B)CN as sintering aid

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Oxidation resistance of ZrB₂‐based monoliths using polymer‐derived Si(Zr,B)CN as sintering aid

The focus of the present work is the investigation of the influence of polymer‐derived ceramics, used as sintering aids for preparing ZrB₂‐based monoliths, on their high‐temperature oxidation behavior. For the preparation of the monoliths, ZrB₂ powder was coated with polymer‐derived SiCN, SiZrCN, or SiZrBCN and subsequently densified via hot‐pressing at temperatures as low as 1800°C. To investigate the oxidation kinetics, thermogravimetric analysis (TGA) was performed at 1300°C in synthetic air with exposure times of 50 and 100 h. A detailed study of the materials oxide scale and subsurface microstructure was conducted using optical microscopy, electron probe microanalysis, scanning electron microscopy, and X‐ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations using the CALPHAD method, which led to a better understanding of the oxidation mechanism. In comparison to the literature data of ZrB₂–SiC, the results show improved oxidation resistance for all three investigated materials. The formation of gaseous species during oxidation, in particular CO, CO₂, B₂O₃, and SiO, within the oxide scale of the monoliths was rationalized via CALPHAD calculations and used to explain the oxidation behavior and kinetics and also the formation of bubbles in the subsurface region of the oxidized specimens

Novel Cr/Si-Slurry Diffusion Coatings for High Temperatures

Surface enrichment in Al, Si, and Cr can greatly improve high temperature oxidation resistance of many alloys. Al, Si, and Cr coatings are commonly applied via simple slurries or more complex pack cementation processes. Due to the high melting point of Cr, the deposition of Cr-based diffusion coatings by the slurry technique has proved challenging, and to date, Cr has mostly been applied by pack cementation. Here, a novel Cr-Si coating process via the slurry technique is described which has been developed and then demonstrated on two Ni-based superalloys, Rene 80 and Inconel 740H. The addition of Si to the slurry lowers the melting point via a Cr-Si eutectic and enables the formation of a liquid phase during heat treatment. Through this Cr-Si slurry coating process diffusion layers enriched by Cr and Si of about 150 µm were achieved. Oxidation behavior was studied through isothermal exposures at 900 °C for 1000 h in lab air. Uncoated Rene 80 and IN740H both showed formation of a Ti-containing Cr2O3 scale below a thin TiO2 top layer. Underneath the external scale a zone of internally oxidized Al grew over the exposure time and reduced the load-bearing cross-section progressively. In comparison, the Cr/Si-coated samples did not show internal Al oxidation, but a slow-growing Si-rich oxide film underneath the external Cr2O3 scale. This subscale represents an additional oxygen diffusion barrier. Thus, the weight gain during exposure for the coated samples was significantly lower than for the uncoated materials

Oxidation resistance of ZrB₂-based monoliths using polymer-derived Si(Zr,B)CN as sintering aid

The focus of the present work is the investigation of the influence of polymer-derived ceramics, used as sintering aids for preparing ZrB₂-based monoliths, on their high-temperature oxidation behavior. For the preparation of the monoliths, ZrB₂ powder was coated with polymer-derived SiCN, SiZrCN, or SiZrBCN and subsequently densified via hot-pressing at temperatures as low as 1800°C. To investigate the oxidation kinetics, thermogravimetric analysis (TGA) was performed at 1300°C in synthetic air with exposure times of 50 and 100 h. A detailed study of the materials oxide scale and subsurface microstructure was conducted using optical microscopy, electron probe microanalysis, scanning electron microscopy, and X-ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations using the CALPHAD method, which led to a better understanding of the oxidation mechanism. In comparison to the literature data of ZrB₂–SiC, the results show improved oxidation resistance for all three investigated materials. The formation of gaseous species during oxidation, in particular CO, CO₂, B₂O₃, and SiO, within the oxide scale of the monoliths was rationalized via CALPHAD calculations and used to explain the oxidation behavior and kinetics and also the formation of bubbles in the subsurface region of the oxidized specimens

Oxidation resistance of ZrB 2 ‐based monoliths using polymer‐derived Si(Zr,B)CN as sintering aid

The focus of the present work is the investigation of the influence of polymer-derived ceramics, used as sintering aids for preparing ZrB2-based monoliths, on their high-temperature oxidation behavior. For the preparation of the monoliths, ZrB2 powder was coated with polymer-derived SiCN, SiZrCN, or SiZrBCN and subsequently densified via hot-pressing at temperatures as low as 1800 degrees C. To investigate the oxidation kinetics, thermogravimetric analysis (TGA) was performed at 1300 degrees C in synthetic air with exposure times of 50 and 100 h. A detailed study of the materials oxide scale and subsurface microstructure was conducted using optical microscopy, electron probe microanalysis, scanning electron microscopy, and X-ray diffraction. The experimental findings were compared to thermodynamic equilibrium calculations using the CALPHAD method, which led to a better understanding of the oxidation mechanism. In comparison to the literature data of ZrB2-SiC, the results show improved oxidation resistance for all three investigated materials. The formation of gaseous species during oxidation, in particular CO, CO2, B2O3, and SiO, within the oxide scale of the monoliths was rationalized via CALPHAD calculations and used to explain the oxidation behavior and kinetics and also the formation of bubbles in the subsurface region of the oxidized specimens

Monolithic ZrB2‐based UHTCs using polymer‐derived Si(Zr,B)CN as sintering aid

In the present work, dense ZrB2-based monoliths were produced via hot-pressing of ZrB2 powders coated with amorphous polymer-derived SiCN, SiZrCN, or SiZrBCN at relatively moderate temperatures (i.e., 1800celcius). Thus, ZrB2 particles were embedded in polymer-derived ceramic shell were realized by coating their surface with the corresponding polymeric precursors (polysilazane, Zr-modified polysilazane, Zr- and B-modified polysilazane, as for SiCN, SiZrCN, and SiZrBCN; respectively), followed by cross-linking at 200celcius and pyrolysis at 1100celcius in Ar atmosphere. The obtained monolithic samples were carefully characterized concerning their phase composition and microstructure. Typically, the densified samples exhibit a homogeneous microstructure consisting of ZrB2 grains and a multi-phasic polymer-derived ceramic grain boundary phase (beta-SiC, t-BCN, ZrO2, and ZrC). The prepared monoliths exhibited fair hardness values, that is, 12.5, 14.2, and 13.3 GPa for ZrB2/SiCN, ZrB2/SiZrCN, and ZrB2/SiZrBCN