Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si₃N₄−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m²/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si₃N₄, β-Si₃N₄, β-SiC, and X-ray amorphous Si₃N₄. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Salaspils. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 7–12, January–February, 1999.     

Properties and structure features of nanocomposite powders based on SiC

Features of the synthesis of SiC−Si₃N₄−Si₂N₂O composite powders are studied. The characteristics of the powders are examined on the basis of x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The structure features and mechanical properties of a ceramics formed on the basis of the synthesized powders are also studied. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Traslated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 12–16, July–August, 1999.     

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.     

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.     

Composites with a functional gradient in the systems Si3N4−Al2O3−Y2O3−TiC

Composites with a functional gradient in the system Si₃N₄−Al₂O₃−Y₂O₃−TiC were made by laminating and sintering ceramic films obtained by tape casting. The films had high contents of TiC and Al₂O₃ and were of different thicknesses. Materials with a high density and high fracture toughness (K_1c≈9.3 MPa·m^1/2) were obtained. Warsaw Polytechnic Institute. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 1–7, July–August, 1999.     

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.     

Sintered ceramics based on boron nitride and carbide

The formation of BN-B₄C composite materials by sintering in nitrogen is investigated. Structural, mechanical, and chemical characteristics of these materials are examined. Excellent dielectric properties, thermal and chemical stability, and erosion resistance in high-intensity laser beams enable high-temperature application of BN-B₄C composite materials. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 58–63, 2007.     

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.     

Combustion synthesis of α-Si3N4-(MgO, Y2O3) composites

The possibility of obtaining composite powders of α-Si₃N₄-Y₂O₃ and α-Si₃N₄-MgO by self-propagating high-temperature synthesis (SHS) is studied. It is established that the α → β phase transformation starts at temperatures lower than the melting points of the corresponding eutectics. Metastable composite powders based on α-Si₃N₄ with a high rate of phase transition are obtained. The composite powders (α-Si₃N₄-Y₂O₃, α-Si₃N₄-MgO) are used in hot pressing technology. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 10–14, 2007.     

Nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings for hard-alloy cutting tools

Nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings with various contents of chromium and nitrogen are obtained by the magnetron sputtering of multiphase composite targets. Their structure and phase composition are investigated by X-ray phase analysis, transmission and scanning electron microscopy, X-ray photoelectron spectroscopy, and optical emission glow-discharge spectroscopy. The Ti-Cr-B-N and Ti-Cr-Si-C-N coatings are based on the fcc phase with texture (100) and crystallite size <25 nm. The Si₃N₄-based hexagonal phase was also revealed in the Ti-Cr-Si-C-N coatings. An investigation into the properties of coatings with the use of methods of nanoindentation, scratch testing, and by performing tribological tests showed that they have a hardness of up to 30 GPa, an adhesion strength no lower than 35 N, and their friction coefficient falls in the range of 0.35–0.57. Coatings also possess high thermal stability, resistance to oxidation, and corrosion stability in a 1N H₂SO₄ solution. The data obtained in tests of hard-alloy cutting tools indicate that the deposition of nanostructured Ti-Cr-B-N and Ti-Cr-Si-C-N coatings increases its resistance by a factor of 11–17.     

Self-propagating high-temperature synthesis of composite targets based on titanium carbonitride,silicide, and aluminide for ion-plasma deposition of multifunctional coatings

The macrokinetic features of combustion of the mixtures in the Ti-Al-Si₃N₄-C system calculated for the formation of compact ceramic materials (CCMs), the composition of which is described by the general formula X(TiAl₃) + (100 − X)(0.448TiC_0.5 + 0.552(Ti₅Si₃ + 4AlN) with mixture parameter X = 10–50%, are investigated. Compact CCM samples with the main structural components in the form of TiC _x N _y grains and binding phases TiAl3 and Ti5Si3 are fabricated by the technology of forced self-propagating high-temperature synthesis (SHS) compaction. An increase in X promotes the formation of the M _n +1AX _n phase with the composition Ti₃SiC₂ in the synthesis products. Complex investigations into the physicomechanical properties of the obtained ceramics are performed. Based on their results, the inverse dependence of the density and hardness of compact materials on parameter X is established. Tests of the samples for oxidation resistance showed that the obtained CCMs based on titanium carbonitride, silicide, and aluminide possess excellent resistance to high-temperature oxidation, and their oxidation rate in air at t = 900°C for 30 h does not exceed 7.8 × 10⁻⁵ g/(m² s).     

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.     

Resistive Properties of the Sintered Ceramic Composite Si3N4 - SiC

A technology has been developed for activated sintering of resistive Si₃N₄ - SiC ceramic composite. The microstructure, electrophysical properties and strength of the materials obtained have beenstudied over a wide range of concentration.     

Making and structural features of Ti-N-B and Ti-N-C nanocomposites

The paper examines the high-pressure sintering of nanograined ceramic polycrystals based on TiN-TiB₂ and TiC_0.5N_0.5 refractory compounds. Using the optimum pressure (up to 4 GPa) allows keeping the initial nanostate (TiN-TiB₂ and TiC_0.5N_0.5) and obtaining high-density ceramics with enhanced mechanical properties. An x-ray structural analysis is used to examine how the TiN-TiB₂ and TiC_0.5N_0,5 crystalline structure evolves during temperature-pressure treatment, which produces new ceramic materials. Based on the properties of the polycrystalline materials obtained, the temperature-time mode for the consolidation of initial nanopowders is determined to ensure favorable parameters of sintered nanograined ceramics. __________ Translated from Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 62–72, 2008.     

Development of a new CuNiTiB brazing alloy for joining Si3N4 to Si3N4

Joining Si₃N₄ to Si₃N₄ was carried out initially with a Cu₃₄Ni₂₇Ti₃₉ brazing alloy prepared by double melting under a vacuum condition. However, the strength of the joints was not as high as expected. The causes were studied. Based on the results of the analysis, a CuNiTiB brazing filler metal was designed. The Si₃N₄/Si₃N₄ joints were then brazed with this new brazing alloy in the paste form, and joints with a three-point bend strength of 338.8 MPa at room temperature were obtained. The interfacial reactions of the joint are also discussed. With the rapidly solidified foils of the brazing alloy, the bend strength of the Si₃N₄/Si₃N₄ joints under the same brazing conditions is raised to 402 MPa at room temperature. The Si₃N₄/Si₃N₄ joints brazed with this newly developed brazing alloy exhibit a rather high and steady bend strength (about 406 MPa) at 723 K.     

The effect of production conditions on the structure formation and on the electrical conductivity of hotpressed materials in the system nitride-silicon carbide

Conclusions n materials of the system Si₃N₄-SiC are hot-pressed, there occur processes of carbidization of Si₃N₄ and of reduction of the surface film of SiO₂ accompanied by weight loss and increased concentration of SiC. The degree of such physicochemical transformations is well correlated with the magnitude of the specific weight losses. The electrical conductivity of the investigated materials is basically determined by the content of the conducting phase SiC in the initial charge; however, when the concentration of the introduced silicon carbide is low, then the amount of secondary SiC forming in hot pressing plays a considerable role.Translated from Poroshkovaya Metallurgiya, No. 2(290), pp. 51–54, February, 1987.     

Wear resistance of silicon carbide and nitride based ceramic materials

Various factors that affect the nature of wear in SiC and Si₃N4₂ based ceramics have been analyzed. It is shown that adhesion, mechanochemical and diffusion interactions in the contact zone and wear due to fatigue, thermal stresses and abrasion are the predominant factors. Ceramics based on SiC and Si₃N₄ are shown to have excellent wear resistance. Poreless silicon nitride materials that have good chemical stability, heat and crack resistance appear promising as ceramic—metal friction couples and for metal machining. Silicon carbide based poreless materials are efficient ceramic—ceramic friction couples and for service under severe hydro and gas abrasive media attack.Translated from Poroshkovaya Metallurgiya, No. 5, pp. 3–8, May, 1993.     

Reaction of boron-containing materials with titanium during self-propagating high-temperature synthesis

We have studied the contact reaction of titanium, chromium, and tungsten borides with titanium. We have established that the materials react according to an exothermic solid-phase reaction mechanism with formation of titanium monoboride and diboride. The reaction is limited by diffusion of boron atoms through the layer of titanium monoboride formed. This makes it possible to successfully use reaction to effectively control the process for obtaining composite materials in the Ti-B₄C system under selfpropagating high-temperature synthesis conditions. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 67–72, March–April, 2006.     

Reaction of a molten mixture of titanium,aluminum, and silicon oxides with hot-pressed silicon nitride

The formation temperature of a liquid phase and the solidification temperature of a molten mixture of Al₂O₃-TiO₂-SiO₂ oxides on a silicon nitride substrate are determined. Data are obtained for the change in kinetics. It is established that the intensity of interaction of molten Al₂O₃-TiO₂-SiO₂ with silicon nitride depends on the oxide mixture composition. With heating there are two possibilities: improvement and worsening of Si₃N₄ crystallite wetting with a liquid phase as well as solidification of the melt. The temperature range where a liquid phase exists for actual materials is about 15°C, which markedly worsens the process of structure formation with Si₃N₄ during sintering.Translated from Poroshkovaya M etallurgiya, No. 5, pp. 39–44, May, 1993.     

Microstructure and thermal stability of coated Si3N4 and SiC

Microstructure of coatings obtained by treating Si₃N₄ and SiC in Cr powders at 1273–1623 K has been studied employing XRD, SEM, AES and TEM. In accordance with thermodynamic calculations and kinetic consideration, the coatings have layered structures and contain metal-rich silicides and metal-rich nitrides or carbides. The microstructure of the coatings has been found to depend on the treatment conditions. The kinetics of the coatings growth obeys a parabolic growth law, the activation energies being close to the activation energies for self-diffusion of the corresponding metals. Thermal stability of the coated and uncoated Si₃N₄ and SiC in Fe-, Ni- and Co-based matrices has been studied and the coatings have been found to considerably improve the stability of Si₃N₄- and SiC-metal interfaces.