Monolithic ZrO 2-8mol% YO 1.5 (YSZ) with t' tetragonal phase was found to have high indentation fracture toughness of ~91Jm -2. The effect of transformation of t' phase by cubic zirconia (c) precipitation on fracture toughness was studied. Fracture toughness decreased rapidly at the onset of precipitation, by a factor of ~3 at 1150°C after 12h and ~2.6 at 1250°C after 24h. Ferroelasticity is the toughening mechanism at 1150°C and up to 96h at 1250°C. Ferroelastic toughening was analyzed in terms of tetragonality of the major phase as well as microstructural changes such as coarsening, increasing cubic phase fraction and residual stresses. Transformation toughening was observed after 192h at 1250°C, with an increase in toughness by a factor of ~2.5. The results show that the toughness of initially t' YSZ thermal barrier coatings would evolve during service due to phase transformations. The toughening mechanism may change from ferroelasticity to transformation toughening with TBC aging. © 2012 Elsevier B.V.

Ashutosh S. Gandhi

Cubic phase

CUBIC ZIRCONIA

Ferroelastic toughening

FERROELASTICITY

INDENTATION FRACTURE TOUGHNESS

Microstructural changes

Tetragonal phase

Tetragonality

TOUGHENING MECHANISMS

Transformation toughening

Thermal barrier coatings

Toughness

Yttria stabilized zirconia

Zirconia

Phase Transitions

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IIT Madras

Materials Science and Engineering A

The effect of high-temperature exposure on the t′ phase transformation and indentation toughness in zirconia with 8 mol% REO1.5 (RE = Gd and Y) was studied. Dense compacts prepared by spark plasma sintering were subjected to thermal exposure at 1150 °C up to 96 h. As-processed materials comprising of the t′ phase exhibited high indentation toughness: GdSZ 117 ± 8 Jm−2 and YSZ 98 ± 9 Jm−2. Upon thermal exposure, toughness decreased initially and then increased gradually. After 96 h, GdSZ showed a higher toughness compared to YSZ. Ferroelasticity remains the dominant mechanism, although onset of transformation toughening was observed in GdSZ by 96 h. Two tetragonal phases appeared in GdSZ. The minor tetragonal phase is likely an artifact of coherency strains between the cubic precipitates and tetragonal matrix. The matrix tetragonal was depleted in GdO1.5 with time. Consequent increase in tetragonality led to enhanced ferroelastic toughening. The transformations in YSZ occurred much slower than in GdSZ; hence, the toughness recovery was slower as well. © 2017, Springer Science+Business Media New York.

Journal of Materials Science

Toughness evolution in Gd- and Y-stabilized zirconia thermal barrier materials upon high-temperature exposure

In this study, the low temperature synthesis of dense titanium diboride (TiB2) compacts by reaction spark plasma sintering (RSPS) of ball milled mixtures of titanium and boron powders is presented. The influence of milling time and sintering temperature on the in-situ reactive synthesis, densification, grain size and mechanical properties of TiB2 was investigated. TiB2 compacts with relative density above 99% have been obtained at a significantly low sintering temperature of 800 °C. X-ray diffraction analysis confirmed the formation of TiB2 along with a small amount of secondary TiB phase. The effect of TiB on the grain size and the hardness and fracture toughness was investigated. TiB2 compacts processed were found to have a nanohardness &gt; 26 GPa and an elastic modulus &gt; 570 GPa and indentation fracture toughness more than 3.30 MP a.m1/2. It is shown that RSPS can be an attractive route for synthesis of ultrahigh temperature ceramics at low temperatures. © 2014 Elsevier Ltd.

International Journal of Refractory Metals and Hard Materials

Low temperature synthesis of dense TiB2 compacts by reaction spark plasma sintering

Dense pellet of rare-earth oxide (5YbO1.5+1.5GdO1.5+1.5NdO1.5) stabilized zirconia, a candidate thermal barrier composition, was prepared by spark plasma sintering. Isothermal annealing was carried out at 1250°C up to 192h. X-ray diffraction and Raman spectroscopy were used to characterize the phases. Cubic precipitates formed in the initial tetragonal phase from 6h, with volume fraction of 22.31% after 192h. 15.35vol.% of monoclinic phase was observed in XRD at 192h along with cubic and two tetragonal phases. Indentation toughness increased by 49% from 155±24Jm-2 at 0h to 231±71Jm-2 at 6h of annealing and it decreased towards the initial value upon further annealing to 192h. Ferroelastic toughening dominates until transformation toughening ensues at 192h. The toughness is consistently higher than for singly doped, sintered t' zirconia pellets that are reported in the literature. © 2015 Elsevier Ltd.

Journal of the European Ceramic Society

Phase and fracture toughness evolution during isothermal annealing of spark plasma sintered zirconia co-doped with Yb, Gd and Nd oxides

Degradation due to molten salt attack is one of the failure mechanisms of thermal barrier coatings. Thermochemical attack of the salt mixture Na2SO4-30mol% NaVO3 on ZrO2-8mol% YO1.5 (8YSZ) at 950°C was studied by two types of experiments. Sintered compacts were exposed to 25mgcm-2 salt dosage for up to 96h. In the other set of experiments, 10-35wt.% 8YSZ powder was mixed with the salts to study the dissolution of 8YSZ in the molten salt. The role of volatile losses was also examined. The results show that more than 25wt.% 8YSZ dissolves in the sulphate-vanadate melt at 950°C, followed by slow reactions to form YVO4 and NaYV2O7 at 950°C. The unreacted Y2O3 and monoclinic ZrO2 precipitate out separately during rapid cooling (∼300°C/min). Slow cooling at ∼3°C/min leads to the formation of ZrOS apart from ZrO2 and Y2O3. © 2013 Elsevier Ltd.

Molten salt attack on t' yttria-stabilised zirconia by dissolution and precipitation

Thermal Barrier coatings (TBC's) protect the gas turbine blades at high temperature exposure. The t′ phase is metastable and slowly transforms to the high-temperature equilibrium state consisting of tetragonal (t) and cubic (c) during high temperature exposure. A comparative study of the phase stability and fracture toughness of monolithic Y, Yb and Gd stabilised zirconia with composition 8 mol%MO1.5 was studied. Powders were prepared by co-precipitation method with crystallite size ∼15 nm. Pressureless sintering, Spark plasma sintering (SPS) at 1250°C for 10 min was carried out to produce compacts with &gt;96% relative density for fracture toughness measurements XRD and Raman studies revealed the the initial phase was t′. After sintering, no spontaneous monoclinic phase formed during cooling. The fracture toughness of the sintered pellets of different composition was measured. The results were analysed in terms of the effect of phase constitution on fracture toughness. The role of ferroelastic toughening in these materials was explored. © 2011 Indian Institute of Metals.

Transactions of the Indian Institute of Metals

Fracture toughness of t' ZrO2 stabilised with MO1.5 (M =Y, Yb &amp; Gd) for thermal barrier application

The mechanisms whereby intermixed zones of alumina and zirconia are formed at the interface between the metallic bond coat and the ceramic top coat (yttria-stabilized zirconia) in thermal barrier coating systems have been investigated. The results lead to the following mechanism for the formation of the zones. The predominant mechanism for intermixed zone formation involves formation of a metastable alumina polymorph (θ or γ) during TBC deposition, with a significant amount of zirconia dissolved in it. The outward growth also begins to incorporate zirconia particles, which initiates the formation of the intermixed zone. Upon thermal exposure, the metastable TGO continues to grow outward, extending the intermixed zone, and eventually transforms to the equilibrium α-Al2O3. The transformation to α-Al2O3 results in an increase in the volume fraction of zirconia in the intermixed zone as it is rejected from solution. Once the α-Al2O3 appears, subsequent TGO growth produces a columnar zone of the TGO without a second phase. When α-alumina was preformed on the bond coat, prior to TBC deposition, no intermixed zone was formed for Pt-modified aluminide bond coats. © THE MINERALS, METALS &amp; MATERIALS SOCIETY and ASM INTERNATIONAL 2007.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Development of intermixed zones of alumina/zirconia in thermal barrier coating systems

Izvestiya Rossiiskoi Akademii Nauk. Seriya Geograficheskaya.

Journal of the American Ceramic Society

Phase Stability of t′-Zirconia-Based Thermal Barrier Coatings: Mechanistic Insights

Acta Materialia

Anisotropic TGO rumpling in EB-PVD thermal barrier coatings under in-phase thermomechanical loading

Effect of phase transformations on the fracture toughness of t' yttria stabilized zirconia

Journal	Materials Science and Engineering A
ISSN	09215093
Open Access	No