The effect of TiO₂ additions on CaO–MgO–Al₂O₃–SiO₂ (CMAS) crystallization behavior from the melt

Abstract

Titania (TiO2) was introduced into a model calcium‐magnesium aluminosilicate (CMAS) glass in additions of 5‐20 wt%. The crystallization behavior of the mixtures was characterized over a series of temperature profiles and compared to that of CMAS alone. X‐ray diffraction, differential scanning calorimetry, light and scanning electron microscopy, and energy dispersive spectroscopy were used to characterize glass and crystalline products. Titania additions in the amount of approximately 12.5‐20 wt% aided in the formation of CaTiO3 from melts equilibrated at either 1300 or 1500°C and cooled at 10°C/min. Holding CMAS + TiO2 (TiO2 ≥ 10 wt%) at 900°C after cooling from 1300/1500°C resulted in the formation of additional crystalline phases including melilite, paqueite, and diopside. Implications for CMAS interactions with thermal and environmental barrier coatings are discussed.

Developments in Thermodynamic Models of Deposit-Induced Corrosion of High-Temperature Coatings

“…For instance, ytterbium disilicate is preferred to yttrium disilicate for EBC applications owing to undesirable polymorphic transitions during processing in the latter. There has also been interest to develop EBCs based on REO 1.5 -TiO 2 systems, 105 and TBCs containing TiO 2 and TaO 2.5 . [106][107][108][109] While there have been some experimental studies to assess how each of these novel coating candidates reacts with CMFAS-type deposits, 22,105,108 the lack of thermodynamic models for the relevant Yb-, Ta-, and Ti-oxide subsystems complicates the implementation of models for these systems.…”

“…There has also been interest to develop EBCs based on REO 1.5 -TiO 2 systems, 105 and TBCs containing TiO 2 and TaO 2.5 . [106][107][108][109] While there have been some experimental studies to assess how each of these novel coating candidates reacts with CMFAS-type deposits, 22,105,108 the lack of thermodynamic models for the relevant Yb-, Ta-, and Ti-oxide subsystems complicates the implementation of models for these systems. There is a need to further basic scientific investigations to determine the key phase relationships and thermodynamic properties necessary to enable future extensions of the thermodynamic databases to encompass these systems.…”

Evaluation of environmental barrier coatings: A review

“…The columnar porous structure, which is typically adopted in TBC with good thermal shielding and stress tolerance, is not suitable anymore. This is the main difference in microstructure between TBC and EBCs 48 . It raises several questions: Dense multilayer EBCs set a much more critical requirement in CTE matching between EBCs and substrates as well as among EBCs’ layers.…”

Calcia–magnesia–alumina–silicate (CMAS) attack mechanisms and roadmap towards Sandphobic thermal and environmental barrier coatings

“…Solutes such as aluminium and titanium-containing compounds have been utilised in this approach. Specifically Ti 4+ ions are utilised when incorporating TiO 2 [86,118], and Al 3+ ions are used when incorporating Al 2 O 3 [81,86,119,120]. Drexler et al [119] investigate 7YSZ+Al+Ti materials in the form of bulk ceramic pellets with 15% porosity.…”

“…Other studies have proposed a more direct role of TiO 2 if used in sufficient quantity [118]. Webster et al [118] showed that incorporating TiO 2 in quantities over 12.5 wt-% into a CMAS glass powder induces the formation of CaTiO 3 (Figure 20) as a CMAS crystallisation product in the temperature range of 1300–1500°C.…”