Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si₃N₄, Y₂O₃, Al₂O₃, AlN and TiO₂ was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol^–1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. -cristobalite and Y₂O₃·2SiO₂ were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of -cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.Concurrent with Kanagawa Academy of Science and Technology.     

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.     

Oxidation behavior of hot isostatically pressed silicon nitride containing Y2O3

Oxidation in the presence of air and water vapor at high temperatures wasstudied for Si₃N₄ ceramics containing Y₂O₃ and Al₂O₃ as sintering aids.The test environments for this study consisted of air with 0, 1.2,and 6.4 v/o H₂O at temperatures from 1000°C to 1350°C. Theoxidation exposure times were up to 500 hours. The presence of water vaporenhances oxidation and crystallization of the oxidation phases. Weight losswas observed for the oxidation in air or dry air because of Na contaminationduring the fabrication processing. The effect of applied stress on the growthof oxide scale is minimal, however, the applied stress resulted in deeperpenetration of oxygen and pit formation in the oxide phase.     

Si3N4-ZrO2 composites with small Al2O3 and Y2O3 additions prepared by HIP

Si₃N₄-ZrO₂ composites have been prepared by hot isostatic pressing at 1550 and 1750 °C, using both unstabilized ZrO₂ and ZrO₂ stabilized with 3 mol% Y₂O₃. The composites were formed with a zirconia addition of 0, 5, 10, 15 and 20 wt%, with respect to the silicon nitride, together with 0–4 wt% Al₂O₃ and 0–6 wt% Y₂O₃. Composites prepared at 1550 °C contained substantial amounts of unreacted -Si₃N₄, and full density was achieved only when 1 wt% Al₂O₃ or 4 wt % Y₂O₃ had been added. These materials were generally harder and more brittle than those densified at the higher temperature. When the ZrO₂ starting powder was stabilized by Y₂O₃, fully dense Si₃N₄-ZrO₂ composites could be prepared at 1750 °C even without other oxide additives. Densification at 1750 °C resulted in the highest fracture toughness values. Several groups of materials densified at 1750 °C showed a good combination of Vickers hardness (HV10) and indentation fracture toughness; around 1450 kg mm^–2 and 4.5 MPam^1/2, respectively. Examples of such materials were either Si₃N₄ formed with an addition of 2–6 wt% Y₂O₃ or Si₃N₄-ZrO₂ composites with a simultaneous addition of 2–6 wt%Y₂O₃ and 2–4 wt% Al₂O₃.     

Sintering process of Y2O3-added Si3N4

The sintering process of Y₂O₃-added Si₃N₄ has been investigated by dilatometry and microstructural observations. Densification was promoted above 1440 ° C by the formation of eutectic melts in the Y₂O₃-SiO₂-Si₃N₄ triangle. However, the dilatometric curves indicated no shrinkage corresponding to the rearrangement process, despite liquid-phase sintering. The kinetic order for The Initial-stage sintering was 0.47 to 0.49. These values indicated that the phase-boundary reaction was rate controlling. The apparent activation energy (323 kJ mol^–1) was smaller than the dissociation energy for the Si-N bond (435 kJ mol^–1). ESR data and lattice strain indicated that the disordered crystalline structure of the Si₃N₄ starting powder promoted the reaction of Si₃N₄ with eutectic melts. After a period of initial-stage sintering, the formation of fibrous -Si₃N₄ grains resulted in interlocked structures to interrupt the densification.     

Sintering process of Y2O3 and Al2O3-doped Si3N4

The sintering process of Y₂O₃- and Al₂O₃-doped Si₃N₄ has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of -Si₃N₄ in the initial process. The densification once terminated after the second process. The / transformation of Si₃N₄ and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of -Si₃N₄, where the fibrous grains grew remarkably. The kinetic order for the densification of -Si₃N₄ indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol^–1 (<1450 ° C) and 193 kJ mol^–1 (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.     

Hot isostatic pressing of Si3N4 with Y2O3 additions

The effect of Y₂O₃ additive on the properties of hot isostatically pressed silicon nitride was studied. The influence of small additions of Y₂O₃ on the densification of silicon nitride was investigated. The density and elastic moduli of the product increase with increasing of the Y₂O₃ additions. The hot isostatically pressed pure silicon nitride consists of -Si₃N₄, -Si₃N₄and Si₂N₂O; phase content of the hot isostatically pressed silicon nitride with 10 wt % Y₂O₃addition consists of -Si₃N₄, yttrium silicate and Y₂Si₃O₃N₄. The effect of the outgassing of the specimens prior to hot isostatical pressing on the properties of the final material is discussed.     

Effect of CeO2, MgO and Y2O3 additions on the sinterability of a milled Si3N3 with 14.5 wt% SiO2

Specimens of milled -Si₃N₄ with 0 to 5.07 equivalent per cent of CeO₂, MgO or Y₂O₃ additions were pressureless sintered at 1650 to 1820° C for 4 h in static nitrogen at 34.5 kPa (5 psi) gauge pressure and while covered with a mixture of Si₃N₄+SiO₂ powders. The density — per cent addition — temperature plots showed maxima which, for all three additives, occurred between 1.2 and 2.5 equivalent per cent. Maximum densities resulted on sintering in the 1765 to 1820° C range and were 99.6 per cent of theoretical with 2.5 equivalent per cent CeO₂, 98.5 per cent of theoretical with 1.24 to 1.87 equivalent per cent MgO, and 99.2 per cent of theoretical with 2.5 equivalent per cent Y₂O₃. Also, densities 94 per cent of theoretical were obtained with as little as 0.62 equivalent per cent additive (1.0 MgO, 2.11 CeO₂ or 1.85 Y₂O₃, in wt%). X-ray diffraction showed that the materials were predominantly -Si₃N₄ with some or no Si₂N₂O. Scanning electron photomicrographs showed microstructures of elongated grains with aspect ratios of about 5, with all additives.     

Pressureless sintered SiAlON with low amounts of sintering aid

-SiAlONs of compositions Si_2.6Al_0.393Y_0.007O_0.4N_3.6 and Si_2.6Al_0.384Y_0.014O_0.4N_3.6 were pressureless sintered from mixtures of Y₂O₃ and separately milled -Si₃N₄, AlN, and SiO₂. On sintering, the carbon content of these SiAlONs was reduced to negligible levels and their oxygen content was also proportionately reduced, presumably due to reaction of carbon with SiO₂. These SiAlONs had densities in excess of 98% of theoretical, four-point bend strengths of 460 and 155 MN m^–2 at r.t. and 1400° C, respectively, and 1400° C oxidation rates lower than those reported in the literature for hot-pressed Si₃N₄ and for a similar but stronger SiAlON with 2.5 wt % Y₂O₃. These results indicate that increasing the Y₂O₃ content of SiAlONs increases their strength but decreases their oxidation resistance.     

High-temperature oxidation of silicon nitride-based ceramics by water vapour

Hot-pressed Si₃N₄, sintered Si₃N₄ and three kinds of sialon with different compositions were oxidized in dry air and wet nitrogen gas atmospheres at 1100 to 1350° C and 1.5 to 20 kPa water vapour pressure. All samples were oxidized by both dry air and water vapour at high temperature, and formed oxide films consisting of SiO₂, Y₂Si₂O₇ and Y4A1209. The oxidation rate was in the order sialon > sintered Si₃N₄ > hot-pressed Si3N4. The oxidation rate of sialon increased with increasing Y₂O₃ content, and oxidation kinetics obeyed the usual parabolic law. The oxidation rates in dry air and wet nitrogen were almost the same: the rate in wet nitrogen was unaffected by water vapour pressure above 1.5 kPa. The activation energy was about 800 kJ mol^–1.     

Stability of Si3N4-Al2O3-ZrO2 composites in oxygen environments

Oxidation of dense Si₃N₄-Al₂O₃-ZrO₂ and Si₃N₄-Al₂O₃ compacts, at 873–1773 K and 98 KPa air atmosphere, results in two different parabolic oxidation regimes. Oxygen diffusion is likely to be the governing step at low temperature (T<1623 K (H= 100kJ mol^–1)), whereas at T> 1623 K (H = 800kJ mol^–1) metal cation diffusion through the grain boundary phase appears limiting. The excellent stability in oxygen environments of the Si₃N₄-Al₂O₃-ZrO₂ composites compared to other ZrO₂-Si₃N₄ materials derives from (i) absence of easy-to-oxidize Zr-O-N phases; (ii) reduced amount of grain boundary phase, and possibly (iii) decreased solubilization rate of the nitride phases in the high viscous oxide film.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Effects of oxidation on intergranular phases in silicon nitride ceramics

The effects of oxidation on changes in the secondary phases of two Si₃N₄ ceramics were investigated by transmission electron microscopy. The Si₃N₄ materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb₂O₃+0.5 vol% Al₂O₃, containing a Yb₂Si₂O₇ crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si₃N₄ material, sintered with 5 wt% Y₂O₃+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y₂Si₂O₇ accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.     

Study on oxidation behavior of (Nd,Y)- and (Nd,Yb)-α-Sialon

The oxidation behavior of multi-cation -Sialons containing Nd and Y or Yb has been investigated for the compositions (Nd_0.18Y_0.18)Si_10.38Al_1.62O_0.54N_15.46 and (Nd_0.18Yb_0.18)Si_10.38Al_1.62O_0.54N_15.46 respectively in the temperature range of 1200°C to 1400°C in air. The grains of silicate containing Nd and Y as well as Nd and Yb were observed in preferred orientation on the surface of the materials oxidized at 1200°C or 1300°C for 20 h for (Nd,Y)- and (Nd,Yb)--Sialon respectively. By increase of oxidation temperature from 1300°C to 1400°C, bubble, which was caused by softening of silicate oxidation layer, occurred and glassy phase then appeared obviously. The phases formed on the surfaces of multi-cation -Sialons during the oxidation were also discussed in this paper.     

Surface oxidation kinetics of Si3N4-4%Y2O3 powders studied by Bremsstrahlung-excited Auger spectroscopy

Samples of silicon nitride powder containing 4.0% Y₂O₃ in weight were heated in air at temperatures between 900 and 1000 °C. The average SiO₂ layer thickness on the Si₃N₄ powder particles, as a function of time at a particular temperature, was measured by Bremsstrahlung-excited Auger electron spectroscopy. Oxidation was found to follow a linear rate law with an activation energy of 56±1.5 kcal mol^–1. The yttrium level measured by X-ray photoelectron spectroscopy was also found to decrease as a function of the oxide layer thickness. This suggests that there is a reaction between the Si₃N₄ and Y₂O₃ particles which results in the formation of an yttrium-rich phase at the interface between the surface SiO₂ layer and the underlying Si₃N₄ particle.     

Creep behaviour of Si3N4/Y2O3/Al2O3/AIN alloys

The compression creep behaviour of pressureless sintered Y₂O₃/Al₂O₃/AIN-doped Si₃N₄ was studied between 1473 and 1673 K, under stresses ranging from 100–300 MPa. Strain rate versus stress and temperature analysis give a stress exponent n1 and an activation energy Q=860 kJ mol^–1. Microstructural change was investigated by transmission electron microscopy. The observed strain whorls, the stress exponent and the activation energy are indicative of a solution-diffusion-precipitation accommodated grain-boundary sliding where the diffusion through the glass is rate controlling.     

Oxidation of sintered silicon nitride

The influence of oxidation at 1200 °C in air for up to 1000 h on the mechanical properties of two Si₃N₄-Y₂O₃-Al₂O₃ materials with different Y₂O₃/Al₂O₃ ratios, Material A (Si₃N₄-13.9 wt% Y₂O₃-4.5 wt% Al₂O₃) and Material B (Si₃N₄-6.0 wt% Y₂O₃-12.4 wt% Al₂O₃), was investigated. The oxidation significantly improves the high-temperature strength and fracture toughness of both materials, but more for Material A. After oxidation, Material A at 1300 °C retains 93% of its room-temperature strength and 87% higher than that before the oxidation. The oxidation has a different effect on the room-temperature K _IC for the two materials. The room-temperature Weibull modulus of Material A decreased by more than half while the 1200 °C Weibull modulus decreased slightly after oxidation. The annealing treatment prior to oxidation had no effect on the high-temperature strengths of the materials after oxidation. The effect of oxidation on mechanical properties is discussed in terms of the microstructure change of the materials.     

Pressureless sintering of Y2O3-CeO2-doped sialons

Various sialon materials have been prepared by pressureless sintering at 1775 and 1825 °C using Y₂O₃ and/or Ce0₂ as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y₂O₃/CeO₂: 100/0, 75/25, 50/50, 25/75, 0/100 corresponding to 6.0 and 9.25 wt% for the pure Y₂O₃ and pure CeO₂, respectively. Only one of the compositional series reached full density at 1775 °C with cerium replacing yttrium, whereas at 1825 °C all compositional series except one became dense. The samples sintered showed that yttrium but not cerium stabilizes the sialon phase in these ceramics. The dense cerium-sialon ceramics sintered at 1825 °C have as good hardness and indentation fracture toughness as the corresponding yttrium-sialon ceramics, or even higher for the sialon type of materials. For the mixed - sialon materials the hardness decreased as the amount of a sialon phase decreased by increasing cerium-doping.     

Effect of matrix liquid phase on interphase formation in SiC fibre-reinforced Si2N2O-Al2O3-CaO composites

Matrix compositions based on Si₂N₂O, with Al₂O₃ and CaO additions, were used to hot press Nicalon SiC fibre-reinforced composites at 1600 °C. With both CaO and Al₂O₃ additions, eutectic melting formed an appreciable volume of liquid phase during hot pressing, which remained as a stable glassy phase in the cooled composites. This liquid phase fostered formation of 240 nm thick carbon-rich interphases between the fibres and the matrix. These interphases showed relatively low interfacial shear strength and resulted in composites which showed non-catastrophic, notch-independent fracture. Matrices using either Al₂O₃ or CaO did not form adequate liquid phase to form coarse interphases, and fracture was catastrophic in nature. Post-heat treatment of the composites at 1000 °C showed peripheral oxidation (removal of the carbon content of the interphase) indicating limited protection afforded when glassy phase was present in the matrix. Controlled cooling in the hot press did not cause the liquid regions to devitrify.     

Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y2O3

The effect of water vapor on the mechanical behavior of Si₃N₄ ceramics was studied. Strength measurements by flexural dynamic fatigue tests were made at temperatures from 1038°C to 1350°C and at actuator speeds of 8.4 × 10^–2 and 8.4 × 10^–5 mm/s (200 and 0.2 MPa/s). Step stress rupture tests were also performed at 1288°C and 1150°C. Water vapor had a beneficial effect on the flexural strength due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases. Time-to-failure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores were the primary mechanism for slow crack growth for NT 164 Si₃N₄.