Unique performance of thermal barrier coatings made of yttria-stabilized zirconia at extreme temperatures (>1500°C)

Yttria-stabilized zirconia (YSZ) has been for several decades the state of the art material for thermal barrier coating (TBC) applications in gas turbines. Although the material has unique properties, further efficiency improvement by increasing the temperature is limited due to its maximum temperature capability of about 1200°C. Above this temperature the deposited metastable tetragonal (t´) phase undergoes a detrimental phase transformation as well as enhanced sintering. Both processes promote the failure of the coatings at elevated temperatures and this early failure has been frequently observed in gradient tests. In this paper, we now experimentally shown for the first time that under typical cycling conditions not the time at elevated temperatures leads to the reduced lifetime but the transient cooling rates. If cooling rates were reduced to 10K/s, TBC systems could be operated in a burner rig at a surface temperature well above 1500°C without showing a lifetime reduction. The explanation of these astonishing findings is given by the evaluation of energy release rate peaks during fast transient cooling in combination with the phase evolution during cooling with the used cooling rates.     

Valid KIC Determination via In-Test Subcritical Precracking of Chevron-Notched Bend Bars

The initiation of the crack in a chevronnotched bend bar was studied. Ideal testing conditions depend strongly on specimen preparation and material; valid K_IC values do not always result. An in-test precracking technique was developed for ceramics at room and elevated temperatures. A yttria-partially-stabilized zirconia ceramic was tested, and the results showed that ideal testing conditions were obtained, which guaranteed valid K_IC determination.     

Effect of Alumina Addition on the Distribution of Intergranular-Liquid Phase during Sintering of 15-mol%-Calcia-Stabilized Zirconia

In a 15-mol%-CaO-stabilized zirconia (CSZ) specimen, the grain boundaries at the surface region exhibited a lower electrical resistance than those at the inner region. However, when up to 1.5 mol% Al₂O₃ was added, the grain boundaries at the surface showed a higher resistance. These results were explained in terms of redistribution of the intergranular-liquid phase during CSZ sintering.     

Observation of ceramic cracking during quenching

It is difficult to observe the thermal shock cracking in real time, so the measurement of the crack after thermal shock is considered as an alternative method. This paper proposes a new experimental method which can exhibit the thermal shock cracking in real time by water quenching of translucent ceramic and high‐speed imaging. The crack propagation is captured, and the crack growth rate is calculated. The results confirm the previous theoretical predictions of crack propagation under thermal shock. This paper expands the research on understanding the failure mechanisms of ceramic materials in thermal shock.     

Catalytic Effect of CeO2‐Stabilized ZrO2 Ceramics with Strong Shock‐Heated Mono‐ and Di‐Atomic Gases

The reducibility of synthesized ceria‐stabilized zirconia (CSZ) with strong shock‐heated test gases is investigated. Free piston‐driven shock tube operating at hypersonic speed at Mach number of 6–8 has been used to heat the ultrahigh pure test gases like Ar to ~12800 K, N₂ to ~7960 K, and O₂ to ~5500 K at a medium reflected shock pressure (5.0–7.4 MPa) for a short duration of 1–2 ms test time. Under this extreme thermodynamic condition, test gases undergo real gas effects. The structural and spectroscopic investigations of CSZ (Ce₂Zr₂O₈) after interaction with shock‐heated argon gas show pyrochlore structure of Ce₂Zr₂O_7?δ which is observed to be black in color. In presence of shock‐heated N₂ gas, CSZ remains in fluorite structure by changing its color to pale green as nitrogen atoms fill oxygen vacancies. After O₂ interaction with the shock wave, CSZ remains pale yellow but the X‐ray diffraction pattern shows the presence of monoclinic ZrO₂ due to phase separation. During reduction process, Ce⁴⁺ has been reduced to Ce³⁺ which is an unusual effect. In this study, the catalytic and surface recombination effects of CSZ due to shock‐induced compression in millisecond timescale are presented.     

Ultimate Strengths of Partially Stabilized Zirconia Ceramics

Previous results for ultimate tensile fracture strengths of partially stabilized zirconia (PSZ) containing preexisting finite cracks are reformulated in terms of a new transformation-strengthening parameter. This allows a simplified method for estimating the strengths of bulk PSZ containing arbitrarysized, preexisting critical cracks.     

Thermal Shock Behavior of Duplex Ceramics

The thermal-stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, Δ T _c , is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal-stress-induced microcracking within pressure zones and crack initiation at pressure zone–matrix interfacial defects develop before Δ T _c is reached. The effects are accompanied by a gradual reduction in strength. At Δ T _c , critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with the K ^R -curve behavior.     

Sintering of Partially Stabilized Zirconia by Microwave Heating Using ZnO–MnO2–Al2O3 Plates in a Domestic Microwave Oven

Partially stabilized zirconia (PSZ) powders were fully densified by microwave heating using a domestic microwave oven. Pressed powder compacts of PSZ were sandwiched between two ZnO–MnO₂–Al₂O₃ ceramic plates and put into the microwave oven. In the first step, PSZ green pellets were heated by self-heating of ZnO–MnO₂–Al₂O₃ ceramics (1000°C). In the second step, the heated PSZ pellets absorbed microwave energy and self-heated up to a higher temperature (1250°C), leading to densification. The density of PSZ obtained by heating in the microwave oven for 16 min was 5.7 g/cm³, which was approximately equal to the density of bodies sintered at 1300°C for 4 h or 1400°C for 16 min by the conventional method. The average grain size of the sample obtained by this method was larger than the average grain size of samples sintered by the conventional method with a similar heating process.     

部分稳定氧化锆陶瓷的凝胶注模成型工艺

用流变学的方法研究了不同条件,如:固相含量、分散剂加入量、烧结助剂、增塑剂等对碱性部分稳定氧化锆(partially stabilized zirconia,PSZ)悬浮体的流变性的影响.结果表明:分散剂含量对悬浮液的流变性能有明显影响,当PSZ固相体积含量为55%时,分散剂加入量(占固相含量的质量分数)应为0.4%.当固相体积含量为50%～56%时,氧化锆碱性料浆呈现剪切变稀行为,具有较低的黏度(在剪切速率为10 s-1时,低于50mPa·s).氧化锆陶瓷碱性料浆(pH>7)在低的剪切速率(<100s-1)时,表现为剪切变稀.凝胶注模法生产的PSZ陶瓷坯体的内部结构是均匀的.     

Neutron Diffraction Studies of Phase Transformations between Tetragonal and Orthorhombic Zirconia in Magnesia-Partially-Stabilized Zirconia

Neutron powder diffraction and conventional dilatometry have been used to investigate the tetragonal-to-orthorhombic phase transformation and the orthorhombic-to-tetragonal reversion in a high-toughness magnesia-partially-stabilized zirconia. For this material, the onset temperature on cooling for the tetragonal-to-orthorhombic transformation (determined by dilatometry) was 192 K, and the reversion on subsequent heating occurred between 500 and 620 K. Neutron diffraction patterns were recorded at temperatures down to 19 K then up to 664 K, and analyzed by the multiphase Rietveld method to determine the amounts of different phases as well as their lattice parameters and unit-cell volumes. It is notable that, at its maximum, the orthorhombic phase amounted to 45% of the sample by weight. Length changes were measured, using pushrod dilatometers, in the temperature range 80 to 700 K. Length changes calculated from the neutron diffraction determinations of the proportions and unit-cell volumes of the different phases are in very good agreement with the directly measured values.     

Morphology and Phase Stability of Nitrogen–Partially Stabilized Zirconia (N-PSZ)

The surface layer of yttria-doped tetragonal zirconia materials that have been heat-treated with zirconium nitride was observed to consist of a nitrogen-rich cubic matrix with nitrogen-poor tetragonal precipitates. The precipitates had a thin, oblate-lens shape, similar to those observed in magnesia–partially stabilized zirconia. Because of the fast diffusion of N⁴⁻ ions, the precipitates grew rather large, up to ∼5 μm in length, and remained stabilized in the tetragonal form at room temperature. Because the nitrided layer grew in the two-phase field, the size and distribution of the precipitates each was very irregular. The nitrogen content was observed to determine the proportion of cubic and tetragonal phases in the same way as in conventional cation-stabilized partially stabilized zirconia. A ternary phase diagram for the zirconium(yttrium)–nitrogen–oxygen system was suggested to explain the concentration gradient in the cubic matrix and the phase distribution of the nitrided layer.     

Grinding Temperature Measurements in Magnesia-Partially-Stabilized Zirconia Using Infrared Spectrometry

Results of temperature measurements by analysis of the thermal emission spectra generated during grinding and subsequently transmitted through partially stabilized zirconia workpieces are presented. Portions of emitted visible and near-infrared spectra were collected with spectrometers. Source temperatures were determined by fitting the scaled spectrometer output spectra to blackbody curves. Simulations showed that the effective temperatures determined by this method will be strongly biased toward hot-spot (flash) temperatures, which are expected to occur at the grinding grit–workpiece interface. Hot-spot temperatures on the order of 3000 K were obtained for grinding with both SiC and diamond wheels. These high temperatures modify the grinding process and the phase content of grinding chips.     

Microstructure and Mechanical Properties of Fine-Grained Magnesia-Partially-Stabilized Zirconia Containing Titanium Carbide Particles

Fine-grained Mg-PSZ has been fabricated by adding TiC particles. The average cubic grain size was smaller by more than an order of magnitude than without TiC when the TiC content was over 2.5 vol%. Pressurelessly sintered specimens contained numbers of relatively large pores while hot-pressed ones were fully dense. For hot-pressed specimens, addition of TiC particles did not affect the growth behavior of tetragonal precipitates during annealing. With increasing TiC content, the bend strength of hot-pressed specimens increased while the fracture toughness decreased. The bend strength and the fracture toughness of fine-grained Mg-PSZ containing 5 vol% TiC were 980 MPa and 8.2 MPa·m^1/2, respectively.     

Effects of Processing Parameters on the Deposition of Yttria Partially Stabilized Zirconia Coating During Suspension Plasma Spray

The process–microstructure relationship in suspension plasma spray (SPS) of yttria partially stabilized zirconia (YSZ) has been studied experimentally. An ethanol‐based suspension with a powder loading of 25 wt% was plasma sprayed with radial injection under four different plasma conditions, to examine the effects of plasma gas composition (Ar/He ratio), secondary gas (Ar/He and Ar/H₂), and the nozzle diameter of the plasma gun. The suspension feeding rate was optimized firstly and coatings were prepared for microstructural observation. Capturing of in‐flight particles into water as well as collection of splats formed on heated flat metal substrates were utilized in order to better understand the more complicated intermediate process steps in SPS. It was found that a plasma jet with higher momentum allowed a higher suspension flow rate and both columnar and deep vertically cracked structure could be created depending on the plasma parameters as well as the substrate surface roughness.     

R-Curve Behavior and Thermal Shock Resistance of Ceramics

The influence of R -curve behavior of ceramics on the strength degradation associated with thermal shock is explored. Of particular significance for this interdependence is the observed nonlinear stress-strain behavior of materials that exhibit minimal strength degradation under severe thermal shock conditions. These two features, R -curve behavior and nonlinear behavior, are incorporated into a fracture mechanics analysis to provide a framework with which to understand severe thermal shock of ceramics. This analysis enables the estimation of the crack growth due to thermal shocking and also the anticipated strength degradation. The influence of specimen size is also addressed, and it is shown that greater strength degradation is anticipated with increasing specimen size. Experimental results for an alumina-zirconia composite material are presented to support the simple analysis.     

Durability of YSZ coated Ti2AlC in 1300°C high velocity burner rig tests

A thermal barrier coating (TBC) system survived 500 hours in aggressive, 1300°C burner rig testing. The yttria-stabilized zirconia (7YSZ) TBC was plasma sprayed on an oxidation-resistant Ti₂AlC-type MAX phase and tested in a jet fuel burner at 100 m/s, using 5 hours cycles. No coating spallation or surface recession was observed; Al₂O₃-scale growth produced a slight 2.4 mg/cm² mass gain. The coating surface exhibited craze-cracked colonies of [111]_flourite textured columns, with no visible moisture attack. The 20 μm alumina scale remained intact under the YSZ face, about twice that producing failure for TBC/superalloy systems. TiO₂ nodules, initially formed on the uncoated backside, were removed, and Al₂O₃ was etched through volatile hydroxides formed in water vapor (~10%). Overall, the test indicated exceptional stability of the YSZ/Al₂O₃/Ti₂AlC system under turbine conditions due in large part to close thermal expansion matching.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.