Improvement in the Mechanical Properties of Silicon Nitride Ceramic in High-Temperature Deformation Processes

Studies have been made on the changes in structure and properties of sintered materials: Si₃N₄ - 5 mass% Y₂O₃ - 2 mass% Al₂O₃, Si₃N₄ - 5 mass% Y₂O₃ - 5 mass% Al₂O₃, and Si₃N₄ - 40 mass% TiN on deformation in a high-pressure chamber of toroid type (pressure 4–5 GPa, temperature 1000–1600 °C), and also by direct extrusion with degrees of reduction of 55 and 72% (temperature 1750–1850 °C, pressure on the plunger 20–30 MPa). After pressure-chamber treatment, the materials have elevated mechanical characteristics: HV₁₀ ≈ 16.7 GPa, K_Ic up to 8.4 MPa · m^1/2 for the system Si₃N₄ - Y₂O₃ - Al₂O₃; and HV₁₀ ≈ 16.9 GPa, K_Ic up to 9.4 MPa · m^1/2 for Si₃N₄ - TiN. A structure feature is the small size of the coherent-scattering regions: 51 nm for Si₃N₄ and 65 nm for TiN in the system Si₃N₄ - TiN, and 33 nm for specimens in the system Si₃N₄ - Y₂O₃ - Al₂O₃. High-temperature extrusion results in a structure with β-Si₃N₄ grains elongated along the deformation direction. The anisotropic structure has K_Ic values in directions perpendicular to and parallel to the direction of extrusion of 11.5–12.0 MPa · m^1/2 and 7.5–7.8 MPa · m^1/2, respectively. The hardness after extrusion becomes 16.0 GPa.     

A Thermodynamic Model for Representation Reaction Abilities of Structural Units in Full Composition Range of Fe–Si Binary Melts Based on the Atom–Molecule Coexistence Theory

A thermodynamic model for calculating the mass action concentrations of structural units in Fe–Si binary melts based on the atom–molecule coexistence theory, i.e., the AMCT–N_i model, has been developed and verified through comparing with the reported activities of both Si and Fe in the full composition range of Fe–Si binary melts at temperatures of 1693, 1773, 1873, and 1973 K from the literature. The calculated mass action concentration N_Si of free Si or N_Fe of free Fe in the full composition range of Fe–Si binary melts has a good 1:1 corresponding relationship with the reported activity a_R,Si of Si or a_R,Fe of Fe relative to pure liquid Si(l) or Fe(l) as standard state. The calculated mass action concentration N_Si of free Si has a good corresponding relationship with the calculated activity a_%,Si of Si referred to 1 mass% of Si as standard state as well as the calculated activity a_H,Si of Si relative to the hypothetical pure liquid Si(l) as standard state. The calculated activity a_%,Si or a_H,Si of Si is much greater than the calculated mass action concentration N_Si of free Si in Fe–Si binary melts. The reaction abilities of both Si and Fe show a competitive or coupling relationship in Fe–Si binary melts at the above‐mentioned four temperatures. The calculated mass action concentrations N_i of six structural units as Fe, Si, Fe₂Si, Fe₅Si₃, FeSi, and FeSi₂ cannot show the linear relationship with the calculated equilibrium mole numbers n_i in 100‐g Fe–Si binary melts simultaneously. A spindle‐type relationship between the calculated mass action concentration N_i and the calculated equilibrium mole number n_i of FeSi and FeSi₂ in Fe–Si binary melts has been found.     

Physico-Chemical Investigation of Component Interactions of the Si ― Al ― O ― N ― Ti System in the Region Si3N4 ― AlN ― Al2O3 ― SiO2 ― TiN ― TiO2. Part 1. Subsystems Si3N4 ― Al2O3, Si3N4 ― TiN,and Al2O3 ― TiN

Using the methods of differential thermal and x-ray diffraction analysis an investigation was made of component reactions in the Si Al O N Ti system, particularly between the compounds Si ₃N₄ Al₂O₃, Si₃N₄ TiN, and Al₂O TiN under conditions approximating those used in the hot pressing of composites. It was established that in the reaction of Si₃N₄ with Al₂O₃, -sialon, SiO₂, AlN, and the intermediate reaction products (mullite and X-phase) are formed. In the reaction of Si₃N₄ with TiN, as a result of the decomposition of Si₃N₄ at 1650-1900°C titanium disilicide is produced, which forms eutectics with free silicon and residual TiN at 1320 and 1280°C, respectively. The reaction of Al₂O₃ with TiN similarly leads to the formation of a eutectic between Al₂O₃ and spinel at 1850°C. The presence of eutectic liquids in the specimens after sintering promotes densification of the material, and improves certain of its mechanical properties.     

Physicochemical Investigation of the Reaction of Components of the Si — Al — O — N — Ti System in the Si3N4 — Al2O3 — AlN — SiO2 — TiN — TiO2 Region. Part 2. Subsystems Si3N4 — AlN,TiN — AlN,Al2O3 — AlN

The reaction of components of the Si — Al — O — N — Ti system in its elements Si ₃N₄ — AlN, TiN — AlN, and Al ₂O ₃ — AlN was investigated by differential thermal and x-ray diffraction analysis. It was established that upon hot pressing mixtures of Si₃N ₄ and AlN (up to 1950°C) free silicon is formed by the decomposition of Si ₃N₄, which reacts with oxygen present as an impurity to form SiO. When TiN reacts with AlN a phase with the spinel structure (Al₂₃O ₂₇N ₅), which can form only in the presence of excess oxygen, appears in addition to the initial components. Spinel is also produced by the reaction of Al ₂O₃ with AlN. In this case a eutectic between Al ₂₃O₂₇N₅ and Al₂O₃ is observed.     

Desilicating Zircon with Plasma Heating and Phase Equilibrium Analyses

Desilicated zirconia has a great variety of applications. The maximum SiO₂ content in desilicated zirconia is <7 mass%. The present techniques to produce zirconia are always accompanied by many drawbacks. So the authors have developed a new plasma process. Desilicated zirconia (88.6 ~ 96.9 mass% ZrO₂) and magnesia‐stabilized zirconia (> 91.54 mass% ZrO₂, < 5.39 mass% MgO) were produced successfully from zircon using a 150 kW plasma rotating furnace. The effects of time and carbon content on the desilication degree were investigated. The mixture treated in the plasma furnace included condensed phases Zr, ZrC, ZrN, ZrO₂, C, Si, SiC, SiO₂ and gaseous phases SiO, O₂, N₂, CO. The established phase equilibrium diagrams of the Si‐C‐N‐O and Zr‐C‐N‐O systems suggest that the formation of Si₃N₄ is thermodynamically impossible, and the formation of ZrC and ZrN is thermodynamically possible in the central high‐temperature region of the plasma furnace. Experimental results supported the analyses.     

Self-joining of Si3N4 using metal interlayers

This work focuses on various aspects of diffusion bonding of Ti-foil and Nb-foil interlayers during the self-joining of Si₃N₄. Joints were diffusion joined by hot-uniaxial pressing at temperatures ranging from 1200 °C to 1600 °C using different holding times. The microstructural characterization of the resulting interfaces was carried out by scanning electron microscopy, electron-probe microanalysis (EPMA), and X-ray diffraction. The results showed that Si₃N₄ could not be bonded to Ti at temperatures lower than 1400 °C; however joining was successful at higher temperatures. On the other hand, Si₃N₄ was solid-state bonded to Nb at temperatures ranging from 1200 °C to 1600 °C. Joining occurred by the formation of a reactive interface on the metal side of the joint. Ti₅Si₃, TiSi, and TiN were detected at the Si₃N₄/Ti interface, and Nb₅Si₃ and NbSi₂ at the Si₃N₄/Nb interface, resulting from high-temperature reaction between Ti or Nb and Si₃N₄. Four-point bending testing gave a maximum joint strength of 147 MPa for Si₃N₄/Ti/Si₃N₄ samples hot pressed at 1500 °C and 120 minutes. However, strong joints were obtained above 1450 °C (>100 MPa). These results indicated that there is a strong relationship between the thickness of the interface and the mechanical strength of the preceding joints. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

In-Situ Reactive Synthesis of Si2N2O Ceramics and Its Properties

Si₂N₂O is considered as a new great potential structural/functional material in place of Si₃N₄ for high-temperature applications. In the present work, Si₂N₂O ceramics were in-situ reactive synthesized by a nitridizing powder mixture of Si and SiO₂ using an optimized two-step sintering process according to thermodynamic analyses. The results showed that the purity of Si₂N₂O in the produced ceramics increased with an increase in final sintering temperature, while the shrinkage and Vickers hardness decreased. After final sintering above 1923?K (1650?°C), pure nanograined Si₂N₂O ceramics can be obtained. Flexural strength and fracture toughness both showed peak values at 1873?K (1600?°C). The reaction mechanism was proposed and then the difference of the produced ceramics was discussed.     

Nitrogen Ceramics: Liquid Phase Sintering

Abstract

The role of a minor silicate eutectic liquid phase as a transport medium in sintering hot–pressed silicon nitride (β Si₃N₄) ceramics was identified in the 1970s. A similar mechanism is applicable to hot–pressed Si–Al–O–N ceramic alloys which offer an advantage in control of the final liquid volume and hence in superior high temperature mechanical properties. By increasing the liquid volume it is possible to densify ceramic alloys without application of pressure at the sintering temperature and hence to fabricate components of complex shape. The Lucas Syalon ceramics typify the new range of pressureless–sintered ceramics based on the β Si₃N₄ structure. They are fabricated from the ultrafine compound powders α Si₃N₄, SiO₂, Al₂O₃, Y₂O₃, and a polytypoid phase (a substitute for A1N). The ceramics consist of submicrometre solid solution crystals of general composition Si_3?xAl_xO_xN_4?x(x < 1) within a minor matrix phase which may be either a glassy Y–Si–Al oxynitride or be crystallized to form yttrogarnet. Analysis of matrix glass compositions shows them to be residues of liquids near to a ternary eutectic in the Y₂O₃–SiO₂–Al₂O₃ system which is well below the sintering temperature of ～ 1800°C. Sintering models, based on particle rearrangement due to dissolution of the major α Si₃N₄ component in the eutectic liquid and its reprecipitation as a β Si₃N₄ solid solution, are discussed. Properties and current applications of Syalon ceramics are surveyed briefly. PM/0266     

A generalized approach to the flood-knapp structure based model for binary liquid silicates: Application and update for the PbO-SiO2 System

The nonideal activity of a metal oxide in a molten binary silicate system is described by treating the liquid as an ideal solution and by considering the formation of a few complexes. Application of this approach to the binary system PbO-SiO₂ shows that the experimentally determined activity of PbO(l) can be modeled by considering the lead silicate melt as an ideal solution of Pb²⁺ and O²⁻,SiO ₄ ⁴⁻ , Si₂O ₇ ⁶⁻ , Si₁₂O ₃₇ ²⁶⁻ , and Si₄O ₁₀ ⁴⁻ . The calculated Gibbs free energy values for the formation of the anionic complexes from O²⁻ and SiO ₄ ⁴⁻ are: ΔGℴ(Si₂O ₇ ⁶⁻ )/J · mol⁻¹ = 38977 − 30.909(T/K); ΔGℴ(Si₁₂O ₃₇ ²⁶⁻ )/J · mol⁻¹ = 200158 − 121.813(T/K); Δℴ(Si₄O ₁₀ ⁴⁻ )/J · mol⁻¹ = 104627 − 28.094(T/K). Values of Gibbs free energy of formation of the solid phases PbO, Pb₄Si0₆, Pb₂SiO₄, PbSiO₃, and SiO₂ which, together with the melt model data, give the best fit to experimental phase relations in the system PbO-SiO₂ were calculated. These values are all in good agreement with literature data.     

Microstructure of silicon nitride ceramics sintered with rare-earth oxides

Microstructures of Si₃N₄ with crystallized grain boundaries of rare-earth silicates were investigated. β-Si₃N₄ grains in samples sintered with Yb₂O₃ were more elongated than those sintered with Dy₂O₃. Amorphous thin layers between crystallized grain boundaries of silicate and Si₃N₄ grains were recognized for both specimens sintered with Dy₂O₃ and with Yb₂O₃. In the sample sintered with Dy₂O₃, very clean grain boundaries between Si₃N₄ grains which did not contain the heavy elements were frequently recognized, while samples with Yb₂O₃ had an amorphous grain boundary phase containing Yb. Unusual structures with distorted lattice images in regions rich in Si and O were discovered in crystalline Dy-silicate phases.     

Calculation of the standard heat capacities and entropies of crystalline silicon oxides

The calculation procedure developed earlier is used to calculate the standard heat capacities and entropies of crystalline silicon oxides Si₂O, SiO, Si₃O₄, Si₂O₃, and SiO₂. Calculation equations are derived to determine these parameters for silicon oxides of any composition, including nonstoichiometric oxides.     

Effect of Manganese on the Formation of Fe-rich Phases in Electromagnetic Stirred Hypereutectic Al-22Si Alloy with 2 % Fe

The effect of Mn on the microstructure of electromagnetic stirred hypereutectic Al-22Si-2Fe (% w/w) alloys was studied. The results show that the alloy with a Mn/Fe ratio zero, contained plentiful α-Al₄FeSi₂ phases existing as mainly intermetallic compounds in the solidified microstructures by electromagnetic stirring (EMS) process. With EMS process, the alloy with 0.61 % Mn contained acicular β-Al₅(Fe, Mn)Si, δ-Al₄(Fe, Mn)Si₂ and blocky α-Al₁₅(Fe, Mn)₃Si₂ phases in the solidification microstructure of the stirred A1 alloy. As the Mn/Fe ratio increased to 1, intermetallic compounds were mainly in the form of blocky and fine α-Al₁₅(Fe, Mn)₃Si₂ phases in the microstructure. The intermetallic compounds were examined with an optical microscope, scanning electron microscope, and X-ray diffraction. The acicular δ-Al₄(Fe, Mn)Si₂ and blocky α-Al₁₅(Fe, Mn)₃Si₂ phases were also analyzed by transmission electron microscopy.     

Making low-porosity materials with improved thermal resistance on the basis of corundum with added ZrO2, Si3N4, AlN, and SiO2

Added ZrO₂, Si₃N₄, AlN, and SiO₂ affect the consolidation of aluminum oxide on sintering in nitrogen, vacuum, and air, as well as affecting the thermal resistance of the materials. The relative density of material based on Al₂O₃ is largely dependent on the type and amount of additive, the component ratio, and the sintering medium. High density and thermal stability occur in materials formed by sintering in nitrogen from aluminum oxide containing ZrO₂, Si₃N₄ and SiO₂. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3-4(400), pp. 48–52, March–April, 1998.     

Interdiffusivities and mass transfer coefficients of NaF gas

To clarify the vaporization phenomena of sodium fluoride (NaF), the interdiffusivities of NaF in N₂, Ar, and He were measured by thermogravimetric analysis from 1300 to 1770 K. At 1540 K, the interdiffusivity of NaF in N₂ (2.46 cm²/s) was slightly larger than that in Ar (2.40 cm²/s), while the interdiffusivity of NaF in He (6.00 cm²/s) was significantly larger than those in the N₂ and Ar systems. The Chapman-Enskog equation was used to calculate the interdiffusivities of NaF in N₂, Ar, and He. Calculated results were in good agreement with observations for nitrogen and argon; however, calculated results for the NaF-He system were significantly higher than those observed. Overall experimental mass transfer coefficients were described by a Ranz-Marshall type correlation based upon the Nusselt number (Nu), the Reynolds number (Re), and the Schmidt number (Sc): Nu=7.0+0.28 (Re)⁴(Sc)² in which the viscosities in the NaF-N₂, Ar, or He system were calculated from the Lennard-Jones parameters developed in this study.     

Phase Diagrams of the Zr–Si–RE (RE=La and Er) Ternary Systems at 773?K (500?°C)

The isothermal sections of the phase diagram of the Zr–Si–RE (RE=La and Er) systems at 773 K (500 °C) have been investigated using X-ray power diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM) with the aid of metallographic analysis. The existences of 10 binary compounds, namely ZrSi₂, α-ZrSi, α-Zr₅Si₄, Zr₃Si₂, Zr₂Si, RESi₂, RESi_2–x , RESi, RE₅Si₄, and RE₅Si₃ have been confirmed in the Zr–Si–RE (RE=La and Er) systems, respectively. As for the reported binary compound RE₃Si₂, only La₃Si₂ has been observed in the Zr–Si–La system, whereas Er₃Si₂ was not found. No binary compound was found in the Zr–RE binary systems, and no ternary compound was found in the current ternary systems. None of the phases in Zr–Si–La system reveals a remarkable solid solution at 773 K (500 °C). However, the maximum solid solubility of Zr in Er, Er₅Si₃, Er₅Si₄, ErSi, ErSi_1.67, and ErSi₂ is determined to be approximately 12.0 at. pct, 2.4 at. pct, 3.0 at. pct, 3.3 at. pct, 2.2 at. pct, and 1.8 at. pct, respectively. The maximum solid solubility of Er in ErSi₂ is approximately 1.8 at. pct. No remarkable solid solubility of the elements in any of the other phases has been observed.     

Reaction of Composite Ceramic Materials with Corrosionally Active Media

The oxidation of composite powders and monolithic AlN SiC, AlN SiC TiB₂, and AlN SiC ZrB₂ ceramics in air up to 1600°C was studied by the methods of thermogravimetric, differential thermal, x-ray diffraction, and electron-probe microanalysis. The exceptionally high corrosion resistance of these materials was established. The corrosion resistance and possibility of using structural ceramics of the systems TiB₂ AlN, TiB₂ TiN, and TiC_0.5N_0.5 in sea water was demonstrated. The toxicity of Si₃N₄, AlN, BN, and TiN powders was analyzed on the basis of their reactions with biochemical media. It was proven that TiN based materials are highly stable in the oral cavity.     

Fabrication and characteristics of AA6061/Si3N4p composite by the pressureless infiltration technique

The tensile properties and microstructures of AA6061/Si₃N₄ particle composites fabricated by pressureless infiltration under a nitrogen atmosphere were analyzed. In addition, the control AA6061 without Si₃N₄ particles fabricated by the same method was investigated to separate the effect of Si₃N₄ particle addition. It was found that AlN particle layers formed on the surface of Al particles in the powder bed, which replaced the Mg₃N₂ coated layers through the following reaction: Mg₃N₂ + 2Al → 2AlN + 3Mg. Thus, the spontaneous infiltration results from a great enhancement of wetting via the formation of Mg₃N₂ by the reaction of Mg vapor and nitrogen gas. The increased tensile strength and 0.2 pct offset yield strength in the control AA6061 were largely due to fine AlN particles formed by the aforementioned in situ reactions, as compared to commercial AA6061. In the composite reinforced with Si₃N₄ particles, of course, the AlN was also formed through the following additional reaction at the Si₃N₄ particle/Al melt interfaces: Si₃N₄ + 4Al → 4AlN + 3Si. However, this AlN may not contribute to the increase in strength because its formation is compensated by the consumption of Si₃N₄ particles. Consequently, the strength increase of the composite fabricated by the present method is attributed to the fine AlN particles formed in situ, as well as the fine reinforcing Si₃N₄ particles, as compared to commercial AA6061.     

Production of silicon nitride by the acid enrichment of products of combustion of ferrosilicon in nitrogen

Production of silicon nitride by acid enrichment of products of interaction between ferrosilicon and gaseous nitrogen under conditions of self-propagating high-temperature synthesis (SHS) is studied. The effect of the nature of acid, its concentration, agitation of solution, and process temperature is determined. The reaction of the Si₃N₄ + Fe composite powder and the hydrochloric acid solution is found to have a stage behavior. The apparent activation energy of iron passing into the solution is determined. The purity of the produced Si₃N₄ powder is demonstrated to depend on a degree of nitration of SHS products. The chemical and phase composition of the powder and its specific surface are determined.     

Formation of silicon nitride from silica

Conclusions The formation of Si₃N₄, during the carbothermic reduction of SiO₂ in the presence of small Fe additions combined with nitridation is a complex process involving several solid-solid, gas-solid, gas-liquid-solid, and gas-gas reaction stages. The catalytic action of iron manifests itself in the formation of an intermediate compound — iron carbosilicide. The product of the nitridation of FeSi_xC_y is-Si₃N₄, and that of the nitridation of silicon, -Si₃N₄.Translated from Poroshkovaya Metallurgiya, No. 7(247), pp. 13–17, July, 1983.     

Formation of a Corrosion-Resistant Layer on Steel by Reaction with a Si3N 4 ― Al2O3 Composite under the Conditions of Light-Thermal Treatment

The formation of a surface layer on low-alloyed steel during light-thermal treatment with a composite material based on Si₃N₄ Al₂O₃ was investigated. The working surfaces were studied using metallographic, x-ray diffraction, and electron-probe microanalysis. It was found that the corrosion-resistant phases Al₂SiO₅, (Fe, Cr)₂O₃, (Cr, Al) ₂O₃, and NiCrO₄ formed in the alloyed layer, increasing its microhardness by 2-5 times and its corrosion resistance in sea water by more than two orders of magnitude.