Effect of TiO2 addition on thermal and mechanical properties of Y-Si-Al-O-N glasses

Y-Si-Al-O-N glasses are intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of oxynitride glass phases determine the sintering/shrinkage behaviour. Several investigations on oxynitride glass formation and properties have shown that addition of nitrogen increases glass transition and softening temperatures, viscosity, elastic modulus and hardness. In the present study, effect of TiO₂ addition on thermal and mechanical properties of Y-Si-Al-O-N glasses is investigated since the most typical Si₃N₄ ceramics for bearing applications are fabricated using a Si₃N₄-Y₂O₃-Al₂O₃-TiO₂-AlN system. Addition of TiO₂ is effective in preparing Y-Si-Al-O-N glasses with lower glass transition temperatures and with higher hardness.     

Influences of rare-earth oxide additives on the formation and properties of porous Si2N2O ceramic

Here we prepared porous silicon oxynitride (Si₂N₂O) ceramics by reaction sintering of SiO₂ and Si₃N₄ using five different rare-earth oxides (RE₂O₃, RE = Lu, Yb, Y, Sm, and La) as sintering aids. The influences of RE₂O₃ on the formation, densification, microstructure, and mechanical properties of Si₂N₂O ceramics have been investigated in detail. The results have indicated that with the increase in RE ionic radius, the formation temperature of Si₂N₂O decreases, and the densification process could be promoted by RE₂O₃ with larger RE³⁺ ionic radius. In addition, microstructures and mechanical properties are highly dependent on the RE₂O₃ additives. With the increase in RE³⁺ ionic radius, Si₂N₂O changes from platelike crystals to elongated crystals. The samples doped with La₂O₃ and Sm₂O₃ with elongated crystals exhibit higher flexural strength and higher Vickers hardness.     

Microstructural relationship with fracture toughness of undoped and rare earths (Y, La) doped Al2O3–ZrO2 ceramic composites

Ceramic composites in undoped Al₂O₃–5 wt% ZrO₂ (AZ) and doped with rare earth elements Y, La separately and simultaneously were prepared by solid state sintering process. These composites were characterized for microstructural investigation and determination of phase formation to draw a possible relationship between these characterization results with the fracture toughness measured by single-edge precracked beam (SEPB) test method using three-point bend test. The fracture toughnesses of Y and Y + La doped AZ are found to be higher for samples sintered at 1700 °C for long soaking times, than that of La doped and undoped AZ composites. It is predicted from the XRD and EDS analyses that the phases of Zr_0.88Y_0.12O_1.94 and Zr_0.935Y_0.065O_1.968 are formed at or near the intergranular region and therefore the higher fracture toughness of Y and Y + La doped AZ composites compared to undoped AZ and La doped AZ composite for samples sintered at 1700 °C for long soaking times, is attributed to these intergranular phases.     

Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

Microstructure and mechanical property of silicon nitride (Si₃N₄) ceramic are strongly dependent on the selection of sintering additives. When rare‐earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si₃N₄ grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α‐β phase transformation and (ii) segregation behavior at the interface were studied for Si₃N₄ ceramics sintered using mixture of La₂O₃ and Lu₂O₃ as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α‐β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β‐Si₃N₄/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth.     

Effect of chemical composition of intergranular glass on superplastic compressive deformation of β-silicon nitride

The effect of chemical composition of Y₂O₃–Al₂O₃–SiO₂-based intergranular glass on superplastic deformation of β-Si₃N₄ was studied by compression tests at 1873 K. All hot isostatically pressed Si₃N₄ materials had essentially the same microstructure and the same amount of glass phase, which was different in composition only. The relation between flow stress and glass composition qualitatively corresponded to the effect of chemical composition on viscosity of Y₂O₃–Al₂O₃–SiO₂ glass. However, the flow stress was not proportional to the viscosity of Y₂O₃–Al₂O₃–SiO₂ glass, probably because the composition of intergranular glass phase had changed by dissolving Si₃N₄. The strain hardening (increase of flow stress with deformation) was dependent on the chemical composition of intergranular glass. Actually, the apparent strain hardening was not proportional to the strain but was proportional to time. The crystallization of Si₂N₂O was also proportional to time, and was dependent on the chemical composition of the intergranular glass in a similar way to the strain hardening. Thus, it was suggested that the crystallization of Si₂N₂O reduced the amount of the intergranular glass, thereby increasing flow stress.     

Crystal structure of rare-earth silicon-oxynitride J-phases,Ln4Si2O7N2

Rare-earth silicon-oxynitride J-phases, Ln₄Si₂O₇N₂ (Ln=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), were prepared by the N₂ gas-pressured sintering method at 1 MPa of N₂ and 1500–1700 °C. The Rietveld analysis was carried out for X-ray powder diffraction data measured at room temperature. The crystal structures of Ln₄Si₂O₇N₂ were refined with the structure model of La₄Si₂O₇N₂ for Ln=La, Pr, Nd, and Sm, and with that of Lu₄Si₂O₇N₂ for Ln=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The refined monoclinic unit-cell parameters (lengths a, b, c, angles β, and volume V) increased linearly in their two series of Ln with increasing ionic radii of rare-earth atoms. Discontinuities of the unit-cell parameters were found between the two Ln series.     

Silicon Nitride—Grain Boundary Oxynitride Glass Interfaces: Deductions From Glass Bulk Properties

Oxynitride glasses exist as grain boundary phases in Si₃N₄ ceramics. This paper provides an overview of oxynitride glasses outlining effects of composition on properties. A review of the effects of grain boundary glass chemistry on fracture resistance of silicon nitride is given. A knowledge of overall additive compositions and their quantities in Si₃N₄ combined with measured properties of bulk glasses allows residual stresses in the interfacial glasses to be calculated. Increase in Y:Al ratio leads to higher thermal expansion mismatch and higher residual stresses in intergranular glasses. Values are in good agreement with those obtained using micromechanical finite element analysis.     

The wettability of Y–Al–Si–O–N oxynitride glasses and its application in silicon nitride joining

In the present work, first, the wettability of Y–Al–Si–O–N oxynitride glasses on Si₃N₄ substrates was investigated. It was found that the wettability of the glass depended on the ratios of Y₂O₃/Al₂O₃, i.e. when the ratio of Y₂O₃/Al₂O₃ increased, the wettability of corresponding glass on Si₃N₄ substrate improved. Based on the wettability work, Si₃N₄ ceramics can be successfully joined using glass with the best wettability. It was proved that a proper joining temperature is important for sound joints; a lower temperature would result in incomplete contact of the glass brazing layer with Si₃N₄ while a temperature higher than 1600° would cause separation of Si₃N₄ joints by complete drainage of brazing glass into bulk Si₃N₄ ceramics. In the range of experiment time, to prolong brazing time is of benefit to the shear strength.     

Effect of CaO Addition on Compressive Deformation of Silicon Nitride Ceramic with Y‐Mg‐Si‐O‐N Glassy System

Creep behavior of Si₃N₄ polycrystals containing Y₂O₃‐MgO‐SiO₂ glass phase (with and without Calcium oxide [CaO] additive) was studied by compression tests between 1500 and 1700°C. We studied the effect of CaO additive on flow stress, microstructural evolutions, and thermal stability of the intergranular glass phase during deformation. While the addition of CaO did not affect grain size, the flow stress decreased with the amount of CaO. This result suggested that the addition of CaO reduced the viscosity of intergranular glass phase. The addition of CaO further improved the thermal stability of the glass phase by suppressing the evaporation at elevated temperatures.     

Enhanced thermal conductivity in Si3N4 ceramic by addition of a small amount of carbon

In order to fabricate Si₃N₄ ceramic with enhanced thermal conductivity, 93 mol%α-Si₃N₄-2 mol%Yb₂O₃-5 mol%MgO powder mixture was doped with 5 mol% carbon, and sintered firstly at 1500 °C for 8 h and subsequently at 1900 °C for 12 h under 1 MPa nitrogen pressure. During the first-step sintering, the carbothermal reduction process significantly reduced the oxygen content and increased the N/O ratio of intergranular secondary phase, resulting in the precipitation of Yb₂Si₄O₇N₂ crystalline phase, higher β-Si₃N₄ content and larger rod-like β-Si₃N₄ grains in the semi-finished Si₃N₄ sample. After the second-step sintering, the final dense Si₃N₄ product acquired coarser elongated grains, lower lattice oxygen content, tighter Si₃N₄-Si₃N₄ interfaces and more devitrified intergranular phase due to the further carbothermal reduction of oxynitride secondary phase. Consequently, the addition of carbon enabled Si₃N₄ ceramic to gain a significant increase of ∼25.5% in thermal conductivity from 102 to 128 W∙m⁻¹ K⁻¹.     

Fatigue Crack Growth Behavior of Silicon Nitride: Roles of Grain Aspect Ratio and Intergranular Film Composition

The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE = Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO‐RE₂O₃ and results were compared with a commercial Al₂O₃‐Y₂O₃‐doped material. Decreasing stress intensity range (ΔK) fatigue tests were conducted using compact‐tension specimens to measure steady‐state fatigue crack growth rates. Specimens doped with MgO‐RE₂O₃ additives showed a significantly higher resistance to crack growth than those with Al₂O₃‐Y₂O₃ additives and this difference was attributed to the much higher grain aspect ratio for the MgO‐RE₂O₃‐doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R‐curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO‐RE₂O₃‐doped silicon nitrides displayed similar steady‐state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.     

Texture,microstructures, and mechanical properties of AlN‐based ceramics with Si3N4–Y2O3 additives

Textured AlN‐based ceramics with improved mechanical properties were prepared by hot pressing using Si₃N₄ and Y₂O₃ as additives. The introduction of Si₃N₄–Y₂O₃ into AlN matrix led to the formation of secondary Y₃AlSi₂O₇N₂ and fiber‐like 2H^δ AlN‐polytypoid phases, the partial texture of all crystalline phases, and the fracture mode change from intergranular to transgranular. Consequently, Vickers hardness, fracture toughness and flexural strength of AlN‐based ceramics by the replacement of Y₂O₃ by Si₃N₄–Y₂O₃ increased significantly from 10.4±0.3 GPa, 2.4±0.3 MPa m^½ and 333.3±10.3 MPa to 14.2±0.4 GPa, 3.4±0.1 MPa m^½ and 389.5±45.5 MPa, respectively.     

Local Fracture Toughness of Si3N4 Ceramics Measured using Single‐Edge Notched Microcantilever Beam Specimens

Local fracture toughness gives us useful and important information to understand and improve mechanical properties of bulk ceramics. In this study, the local fracture toughness of silicon nitride (Si₃N₄) ceramics was directly measured using single‐edge notched microcantilever beam specimens prepared by the focused ion beam technique. The measured fracture toughness of grain boundary of the Si₃N₄ ceramics is higher than the fracture toughness of SiAlON glass, which exists in the grain boundaries of Si₃N₄ ceramics. It is also shown that the fracture toughness of grain boundary depends on the rare earth oxide added as a sintering aid, which is expected in terms of the difference in the grain‐boundary structure. The fracture toughness of a single β‐Si₃N₄ grains is higher than the grain‐boundary fracture toughness. It was also higher than the value estimated from ab initio calculations and surface energy, which means that any dissipative energy should be included in the fracture toughness of a grain in spite of the brittle fracture in Si₃N₄. The fracture toughness of polycrystals of Si₃N₄ ceramics measured using single‐edge notched microcantilever beam specimens is intermediate between those of grains and grain boundaries, and it agrees with the estimated initial value of the Rcurve, K_I0, in Si₃N₄ ceramics.     

Enhanced thermal conductivity in Si3N4 ceramic with the addition of Y2Si4N6C

Si₃N₄ ceramic was densified at 1900°C for 12 hours under 1 MPa nitrogen pressure, using MgO and self‐synthesized Y₂Si₄N₆C as sintering aids. The microstructures and thermal conductivity of as‐sintered bulk were systematically investigated, in comparison to the counterpart doped with Y₂O₃‐MgO additives. Y₂Si₄N₆C addition induced a higher nitrogen/oxygen atomic ratio in the secondary phase by introducing nitrogen and promoting the elimination of SiO₂, resulting in enlarged grains, reduced lattice oxygen content, increased Si₃N₄‐Si₃N₄ contiguity and more crystallized intergranular phase in the densified Si₃N₄ specimen. Consequently, the substitution of Y₂O₃ by Y₂Si₄N₆C led to a great increase in ~30.4% in thermal conductivity from 92 to 120 W m^?1 K^?1 for Si₃N₄ ceramic.     

A novel approach for processing CaAlSiON glass-ceramics by spark plasma sintering: Mechanical and electrical properties

Lithium containing glassy materials can be used as solid electrolytes or electrode materials for lithium-ion batteries due to their high energy density. Conventional melt-quenched Ca₁₁Al₁₄Si₁₆O₄₉N₁₀ glass powder containing 24 e/o N, doped with Li-ions (1, 3, and 6 wt. %) and sintered by spark plasma sintering technique (SPS) was studied. The benefits of using SPS to produce glass-ceramics are rapid heating rates compared to conventional consolidation techniques and tuning of properties, adjusting the temperature, holding time (closed to Tg temperature), heating rate (solidification), and pressure (densification) profile during the heat treatment using SPS. Pure glass and glass-ceramic were obtained under identical SPS conditions and compared with pristine oxynitride and soda-lime-silicate (float) glasses. XRD and SEM analysis confirmed that increasing the amount of Li increases the crystallinity in the glass matrix. Nano-indentation analysis showed a decreased hardness and reduced elastic modulus values with the addition of Li-ions. The direct current conductivity increases with the addition of Li due to the high mobility of Li-ions. However, the float glass sample doped with 6 wt.% of Li exhibits even higher values of D.C. conductivity, than the analogously doped Ca₁₁Al₁₄Si₁₆O₄₉N₁₀ glass. The magnitude of activation energy (more than 1 eV) is typical for an ion hopping mechanism and the D.C. conduction mechanism is dominated by Li⁺ hopping.     

Load‐deflection behavior of fracture toughness testing of ceramics by SEVNB method

This paper aims to explore the load‐deflection behavior of fracture toughness testing of ceramics by single‐edge V‐notched beam (SEVNB) method. The fracture toughness of Si₃N₄, 3Y‐TZP, SiC, and 8Y‐FSZ ceramics were measured by SEVNB method and single‐edge notched beam (SENB) method, respectively. The load‐deflection behavior varies with R‐curve behavior of the ceramics, the test methods and the loading conditions. Through comparative analysis, the results show that the actual fracture toughness of ceramics by SEVNB method can be determined by maximum flexure load and the notch length at loading rate of 0.05 mm/min in air. The obtained actual fracture toughness values of Si₃N₄, 3Y‐TZP, SiC, and 8Y‐FSZ ceramics are 5.2 ± 0.21, 4.5 ± 0.12, 3.2 ± 0.15, and 1.6 ± 0.07 MPa ^. m^1/2, respectively.     

Re-examination of phase relations between solid solutions in R4Al2O9-R4Si2N2O7-R4Si2O10 (R=Y,Gd, Yb) systems

Samples in Si–Al-R-O-N (R = Y, Gd, Yb) systems were prepared by solid-state reactions using R₂O₃, Al₂O₃, SiO₂ and Si₃N₄ powders as starting materials. X-ray diffraction was done to investigate RAM-J(R) solid solutions [RAM = R₄Al₂O₉, J(R) = R₄Si₂N₂O₇] formation and their equilibrium with RSO (R₄Si₂O₁₀). Phase relations between RAM, J(R) and RSO at 1700 °C were summarized in a phase diagram. It was determined that a limited solid solution of RAM and RSO could be formed along RAM-RSO tie-line, while RAM and J(R) form a continuous solid solution along RAM-J(R) tie-line. In RAM-J(R)-RSO ternary systems, the RAM-J(R) tie-lines were extended towards the RSO corner to form a continuous solid solution area of JRAMss (R = Y, Gd, Yb). The established phase relations in the Si–Al-R-O-N (R = Y, Gd, Yb) systems may facilitate compositional selections for developing JRAMss as monolithic ceramics or for SiC/Si₃N₄ based composites using the solid-solutions as a second refractory phase.     

Stability of intergranular phases in hot-pressed Si3N4 studied with mechanical spectroscopy and in-situ high-temperature XRD

The impulse excitation technique (IET) and high temperature X-ray diffraction (HTXRD) were used to investigate the intergranular glass phase and its crystallisation behaviour in four hot-pressed silicon nitrides. The internal friction or damping peak height measured with IET near the glass transition temperature, T_g, is used as a qualitative indicator for the amount of residual intergranular amorphous phase after sintering. Silicon nitride powder was hot-pressed with different sintering additives. The silicon nitride containing 4 wt.% Al₂O₃ does not reveal an internal friction peak at T_g, i.e. it does not contain a significant amount of intergranular glass phase. Three other silicon nitrides, containing either 8 wt.% Y₂O₃, 6 wt.% Y₂O₃+2 wt.% Al₂O₃, or 2 wt.% Y₂O₃+4 wt.% Al₂O₃+2 wt.% TiN, do show an internal friction peak near T_g. This “T_g-peak” is nearly unaffected by heating up to 1400 °C in the silicon nitride with Y₂O₃+Al₂O₃+TiN sintering aids, whereas the amount of intergranular glass in the ceramics containing either Y₂O₃+Al₂O₃ or Y₂O₃ as a sintering aid is strongly reduced by subsequent heating. As observed from HTXRD, the onset temperature of crystallisation of the intergranular glass in the ceramic containing Y₂O₃+Al₂O₃ sintering aids is about 1100 °C, with the formation of Y–N-apatite (Y₂₀N₄Si₁₂O₄₈) and O-sialon (Al₀.₀₄Si₁.₉₆N₁.₉₆O₁.₀₄). The O-sialon phase in the yttria and alumina containing ceramics, formed either during sintering or during heat treatment, is not stable at elevated temperatures and dissolves in the intergranular glass phase between 1300 and 1400 °C. The O-sialon phase in the ceramic without Y₂O₃ sintering additive, however, is thermally stable. The presence of Ti⁴⁺ ions in the intergranular glass phase is suggested to inhibit its crystallisation, resulting in a stable high temperature damping behaviour.     

Reaction Synthesis and Mechanical Properties of Lu4Si2O7N2

Crystallized Lu–Si–O–N phases were believed to be the grain‐boundary (GB) phases that might provide Si₃N₄ with excellent high‐temperature mechanical properties. However, little is known about the intrinsic properties, as well as the synthesis, of the Lu–Si–O–N ceramics. This work reveals the reaction paths of heating Lu₂O₃, SiO₂, and Si₃N₄ powder mixtures (with the stoichiometry of 4:0.96:1) from room temperature to 1600°C. Thereafter, dense Lu₄Si₂O₇N₂ samples are synthesized by in situ reaction/hot‐pressing method, and the mechanical properties at room temperature and elevated temperatures are reported for the first time. The Lu₄Si₂O₇N₂ samples show significant high‐temperature mechanical properties, such as the elastic stiffness remains 77% from room temperature to 1500°C; and bending strength keeps 93% from room temperature to 1400°C. The present results shine a light on Lu₄Si₂O₇N₂ as a promising target GB phase for the optimization of high‐temperature mechanical properties of Si₃N₄.     

Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.