High-Temperature Neutron Diffraction of the AlN–Al2O3–Y2O3 System

The importance of aluminum nitride (AlN) stems from its application in microelectronics as a substrate material due to high thermal conductivity, high electrical resistance, mechanical strength and hardness, thermal durability, and chemical stability. Yttria (Y₂O₃) is the best additive for AlN sintering. AlN densifies by a liquid-phase mechanism, where the surface oxide, Al₂O₃, reacts with Y₂O₃ to form an Y-Al-O-N liquid that promotes particle rearrangement and densification. Construction of the phase relations in this multicomponent system is essential for optimizing the properties of AlN. The ternary phase diagram of the AlN–Al₂O₃–Y₂O₃ was developed by Gibbs energy minimization using interpolation procedures based on modeling the binary subsystems. This paper aims at testing the resultant understanding experimentally at selected compositions using in situ high-temperature neutron diffractometry. These experimental results agree with the thermodynamic calculations of AlN–Al₂O₃–Y₂O₃. The ternary phase diagram has been constructed for the first time in this work. High-temperature neutron diffractometry has permitted real time measurement of the reactions involved in this ternary system, especially to determine the temperature range for each reaction, which would have been difficult to establish by other means.     

Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3

The eutectic composition between Y₄Al₂O₉ and Y₂O₃ was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y₂O₃ and 29.5 mol% Al₂O₃). Based on the current results and other published data, the pseudobinary system between Al₂O₃ and Y₂O₃ is revised.     

Neutron Diffraction Study of Ferroelasticity in a 3 mol% Y2O3–ZrO2

In situ neutron diffraction patterns were recorded from a 3Y-TZP sample during a complete loading–unloading cycle at compressive loads up to 2.3 GPa. The macroscopic stress–strain diagram shows elastic behavior to 1.7 GPa followed by volume conserving plastic strains of ∼1.6 × 10⁻³. There were no signs of t → m transformation in the neutron diffraction patterns, and intensity changes in the pattern show that the plasticity is due to ferroelastic switching of tetragonal zirconia crystals. Quantification of the degree of switching gives good agreement with the macroscopic strains. The ferroelastic switching is completely reversed by a process akin to creep relaxation on unloading. Lattice parameters, elastic constants, and structural changes as a function of load are also discussed.     

HRTEM Investigations of New AlN Polytypoids in the High-AlN Region of the AlN–Al2O3–Y2O3 System

The AlN polytypoid phases in selected materials in the AlN–Al₂O₃–Y₂O₃ system has been investigated using TEM methods. Two new AlN polytypoid phases, 24H and 39R, were identified in the pseudobinary AlN–Al₂O₃ system. The 39R phase existed as single grains while 24H was observed only between subblocks of 33R and 39R. The new polytypoids are built on the same structural models that are previously reported for the other polytypoids in the same system, consisting of arrays of planar (oxygen-containing) and corrugated inversion domain boundaries (IDBs). Only one type of interface structure was observed for the planar IDBs of the new polytypoids.     

Deformation of Monoclinic ZrO2 Polycrystals and Y2O3-Stabilized Tetragonal ZrO2 Polycrystals below the Monoclinic–Tetragonal Transition Temperature

Ultrafine-grained monoclinic ZrO₂ polycrystals (MZP) and 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (3Y-TZP) were obtained by hot isostatic pressing (HIP). Both MZP and TZP were "high-purity" materials with impurities less than 0.1 wt%. The deformation behavior was studied at 1373 K, which was lower than the monoclinic ↔ tetragonal transition temperature. The stress exponent of 3Y-TZP with grain size of 63 nm was 3 in the higher stress region, and increased from 3 to 4 with decreasing stress. The deformation of MZP was characterized by a stress exponent of 2.5 over a wide stress range. The strain rate of 3Y-TZP was slower than that of MZP by 1 order of magnitude. It was suggested that either the doped yttrium or the difference in the crystal structure affected the diffusion coefficients of ZrO₂.     

Influence of Plasma Spray Parameters on Formation and Morphology of ZrO2–8 wt% Y2O3 Deposits

Spray prints of thermal spray coatings were created on glass slides for air-plasma-sprayed 8-wt%-yttria-partially-stabilized zirconia (YSZ) deposits. The spray parameters such as carrier gas flow rate, standoff distance, and torch power were systematically changed to investigate the influence of these parameters on the YSZ deposit characteristics. The deposit properties such as deposition efficiency (DE), substrate coverage, deposit thickness, and roughness were measured. The deposits sprayed with a 3.5–4.0 L/min carrier gas flow rate at an 80 mm standoff distance exhibited higher values of DE within the range of studied process parameters. The DE increased as much as 25% by varying the carrier gas flow rate from 2.0 to 4.0 L/min. The deposits sprayed at a higher standoff distance and low torch power gave poor deposit characteristics. The deposit characteristics were compared with the in-flight particle parameters and revealed that the deposit characteristics strongly depended on the in-flight particle temperature. Using the in-flight particle properties, the flattening ratio and the splat thickness were calculated. The average size of particles adhering to the substrate was found to drastically change with a change of process conditions, being much less than the average size of the starting powder.     

Consolidation of Eutectic Powder of Al2O3–GdAlO3

A eutectic solid of Al₂O₃–GdAlO₃ was prepared by arc discharge and crushed to a eutectic powder of 3–125 μm. The powder was consolidated by the spark plasma system (SPS). The consolidated powder duplicated the eutectic structure: Al₂O₃ and GdAlO₃ were joined to each crystal. There were no flaws such as cracks or pores in the eutectic composite. The bending strength was half that of the eutectic composite prepared by unidirectional solidification.     

Influence of Plasma Spray Parameters on In-Flight Characteristics of ZrO2–8 wt% Y2O3 Ceramic Particles

Yttria-partially-stabilized zirconia was atmospherically plasma sprayed by systematically varying the process conditions including carrier gas flow rate, torch power, standoff distance, and Ar/H₂ ratio in the plasma gas mixture. The in-flight particle parameters such as temperature, velocity, number, and size were determined using a commercially available diagnostic system. The particle parameters were controlled by the particle trajectory in the plume and plasma jet characteristics. The average temperature and the velocity of particles, which reached their maximum at an intermediate carrier gas flow rate of 3.5 L/min, varied as much as 6% and 25%, respectively, with a 75% variation in the carrier gas flow rate by going from the lowest to the intermediate rates. The average temperature and the velocity of particles were lower for a lower torch power, a higher Ar/H₂ ratio, and a larger standoff distance. It was necessary to obtain data on particle populations larger than 1000 for statistically reliable and reproducible information from the diagnostic system.     

Ionic Mobilities and Association Energies from an Analysis of Electrical Impedance of ZrO2–Y2O3 Alloys

The dc conductivities of ZrO₂–Y₂O₃ ceramic alloys (in the range 3–12 mol% of Y₂O₃) have been obtained from ac impedance measurements at temperatures between 250° and 370°C. The Almond–West ac conductivity model has been applied to evaluate hopping rates in this system. The migration enthalpies were evaluated and shown to increase with yttria concentration, but all values determined were shown to be lower than the corresponding activation enthalpies for conductivity. The association enthalpies thus calculated were shown to be very small in 3 mol% Y₂O₃–ZrO₃ and to increase with yttria concentration until the yttria contents were high enough to form fully stabilized cubic zirconia. For these samples the association enthalpies are about 0.19 eV, and no longer sensitive to yttria content. The low hopping rate at high yttria concentration might be attributed to low entropy in the system, which might be attributed to the formation of vacancy clusters and/or an ordering of the structure.     

Phase Distribution and Residual Stresses in Melt-Grown Al2O3-ZrO2(Y2O3) Eutectics

Al₂O₃-ZrO₂ eutectics containing 0 to 12.2 mol% Y₂O₃ (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y₂O₃, while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model.     

Properties of (Y)ZrO2–Al2O3 and (Y)ZrO2–Al2O3–(Ti or Si)C Composites

The effect of Al₂O₃ and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al₂O₃–(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al₂O₃ to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO₂ and TiC therefore must have inhibited the t - → m -ZrO₂ martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO₂ and Al₂O₃. In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

Mechanical Properties and Oxidation Behavior of Al2O3–AlN–TiN Composites

The study examines the effect which the composition of hot-pressed electroconductive ceramics has on their structure, mechanical properties, and oxidation behavior, for ceramics of the type AIN–Al₂O₃–42 wt% TiN, differing in the AIN/Al₂O₃ ratio. The results are physico-mechanical property data, including density, hardness, strength, fracture toughness, and wear resistance. A correlation was found between the wear resistance and fracture toughness. The analysis of oxidation products revealed the formation of α-Al₂O₃ and rutile in the temperature range from 600° to 1100°C and aluminum titanate above 1200°C. The spallation of the oxide layer caused low oxidation resistance of Al₂O₃-rich composites above 1250°C. The oxidation of composites was compared with the oxidation of pure TiN. The relationship is discussed between material properties, composition, phases, and processing parameters.     

Degradation Mechanisms of ZrO2–8 wt% Y2O3/Ni-22Cr-10Al-1Y Thermal Barrier Coatings

Duplex ZrO₂–8 wt% Y₂O₃/Ni-22Cr-10Al-1Y thermal barrier coatings (TBCs) on Mar-M247 superalloy were tested under different operating conditions within the temperature range 1000° to 1150°C. Results of experiments in this study show that oxidation of bond coatings is the dominant TBC degradation mechanism whereas the operationally induced stresses exert a conjugate effect. The mechanisms of sintering and phase transformation of top coatings do not contribute to failure of TBCs within the temperature range studied. NiO and Ni(Cr,Al)₂O₄ grown on the surfaces of the bond coatings seem to accelerate spalling of the top coatings along a top coating/bond coating out-grown oxide interface. However, it is also concluded that the lifetime of TBCs is not directly related to a critical specific weight gain under thermal cycling conditions.     

Change in Oxygen Content of Y2O3-Nd2O3-Doped Silicon Nitride During Firing

The oxygen content of silicon nitride with 1 mol% Y₂O₃—Nd₂O₃ additive was measured after firing to determine the compositional change during gas-pressure sintering. Oxygen content decreases from 2.5 to 0.94 wt% during firing for 4 h at 1900°C and 10-MPa pressure in N₂. This decrease in oxygen results from the release of SiO gas generated by a thermaldecomposition reaction between Si₃N₄ and SiO₂. The resultant sintered silicon nitride material contains less than 1 wt% oxygen.     

Crack Resistance and Fatigue of Transforming Ceramics: I, Materials in the ZrO2–Y2O3–Al2O3 System

Crack resistance characteristics and fatigue properties have been studied in four types of Y₂O₃–TZP ceramics including one containing Al₂O₃. The largely linear-elastic behavior connected with the very small transformation zone (<5 μm) explains the absence of any resistance-curve behavior and the flaw-controlled strength. The crack resistance shows high sensitivity to environment-induced subcritical crack growth. This influence is also operative in both types of fatigue experiments, i.e., under static and cyclic stresses, leading to reduced fatigue thresholds compared with K _IC. While for static conditions a benefit is observed from enhanced t-m ZrO₂ transformation, cyclic stresses provoke an additional fatigue effect. However, if the cyclic stresses are restricted to subthreshold values, cyclic stress-induced effects in the process zone provide an improvement of the materials being visible as a strengthening effect.     

Phase Relations in the System Al2O3–B2O3–Nd2O3

The phase relations at a temperature below "subsolidus" in the system Al₂O₃–B₂O₃–Nd₂O₃ are reported. Specimens were prepared from various compositions of Al₂O₃, B₂O₃, and Nd₂O₃ of purity 99.5%, 99.99%, and 99.9%, respectively, and fired at 1100°C. There are six binary compounds and one ternary compound in this system. The ternary compound, NdAl₃(BO₃)₄ (NAB), has a phase transition at 950°C ± 15°C. The high-temperature form of NAB has a second harmonic generation (SHG) efficiency of KH₂PO₄ (KDP) of the order of magnitude of the form which has been used as a good self-activated laser material, and the low-temperature form of NAB has no SHG efficiency.     

Phase Composition, Oxygen Content, and Thermal Conductivity of AIN(Y2O3) Ceramics

An indirect method to determine the oxygen dissolved in AIN is devised for AIN(Y₂O₃) ceramics and then related to thermal conductivity. Dissolved oxygen is determined by first constructing the AIN-rich corner of the AIN—Y₂O₃—Al₂O₃ phase diagram (isothermal section). This is achieved by (1) measuring the total oxygen content and subtracting from it the oxygen in the Y₂O₃, resulting in a virtual alumina content; (2) placing the sample composition on the diagram; (3) determining the phases present by XRD for each sample; and (4) drawing phase boundaries which best agree with the phases present. The intersection of these tie lines through the sample location with the AIN—Al₂O₃ axis then gives the particular Al₂O₃ oxygen content dissolved in the AIN lattice. For the system AIN—Y₂O₃—Al₂O₃, it is shown that it is indeed this fraction of the total oxygen content that has a decisive limiting influence on thermal conductivity of dense, polyphase AIN ceramics.