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.     

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.     

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.     

Formation of highly porous ceramic based on silicon nitride

The formation of the structure of a porous ceramic based on Si₃N₄ has been investigated. It has been found that the structure can be controlled over a wide range of porosities. Materials based on a consisting of a single fraction silicon nitride of grain size 3–5 μm, with the addition of a fine-grained pore agent have most uniform, hard, and developed porous structure. A comparative evaluation of the properties of material based on SiC and Si₃N₄ showed that silicon nitride materials of the same porosity are stronger and that small micropores can be formed in them. That is of fundamental importance in the development of materials—catalyst carriers for chemical production and various kinds of filtering devices. With the results of the investigations general technological recommendations can be made for producing ceramics with specific structures.     

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.     

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.     

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.     

Development of nonoxide ceramics based on silicon carbide and nitride

Research on nonoxide ceramics based on silicon carbide and nitride is reviewed along with related technological developments. The role of I. N. Frantsevich in initiating the development of such materials is shown. Three main stages in the development of the ceramics are distinguished. The relationship between the physical properties and applications of ceramics based on SiC and Si₃N₄.Institute of Materials Sciences, Ukranian Academy of Sciences, Kiev. Translated from Poroshkovaya Matallurgiya, Nos. 7/8(380), pp. 24–32, July–August, 1995.     

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.     

Characterization of Tribological and Mechanical Properties of the Si3N4 Coating Fabricated by Duplex Surface Treatment of Pack Siliconizing and Plasma Nitriding on AISI D2 Tool Steel

Silicon nitride (Si₃N₄) coating was deposited on AISI D2 tool steel through employing duplex surface treatments—pack siliconizing followed by plasma nitriding. Pack cementation was performed at 650 °C, 800 °C, and 950 °C for 2 and 3 hours by using various mixtures to realize the silicon coating. X-ray diffraction analyses and scanning electron microscopy observations were employed for demonstrating the optimal process conditions leading to high coating adhesion, uniform thickness, and composition. The optimized conditions belonging to siliconizing were employed to produce samples to be further processed via plasma nitriding. This treatment was performed with a gas mixture of 75 pct H₂-25 pct N₂, at the temperature of 550 °C for 7 hours. The results showed that different nitride phases such as Si₃N₄-β, Si₃N₄-γ, Fe₄N, and Fe₃N can be recognized as coatings reinforcements. It was demonstrated that the described composite coating procedure allowed to obtain a remarkable increase in hardness (80 pct higher with respect to the substrate) and wear resistance (30 pct decrease of weight loss) of the tool steel.

     

Interaction of ferrosilicon with nitrogen in the presence of zircon and ilmenite concentrates in the process of self-propagating high-temperature synthesis

The equilibria in ferrosilicon-zircon-nitrogen and ferrosilicon-ilmenite-nitrogen systems are calculated. Nitriding of these compositions is investigated experimentally and the experimental and calculated data are compared. It is established that the combustion in the ferrosilicon-additive-nitrogen systems can be implemented with a zircon additive no larger than 60% and that of ilmenite no larger than 40%. The composition of the final product of the mixture with zircon includes Si₄N₄, Fe, Si₂N₂O, and ZrO₂; that with ilmenite includes Si₃N₄, Fe, Si₂N₂O, and TiN. The absence of ZrN in the former case contradicts the results of the calculation and the presence of TiN in the latter case corresponds to them.     

Some Characteristic Features of Formation of Ceramics Based on Composite Powders SiC ― Si3N4 ― Si2N2O

We have studied the characteristic features of synthesis of composite powders SiC Si₃N₄ Si₂N₂O. We have investigated processes involving hot pressing of these powders without activating additives and a protective atmosphere. We consider the mechanical properties of ceramics obtained on the basis of these composite powders.     

Structuring and Mechanical Properties of Composite Ceramics SiC(C) ― Si3N4, Sintered under High Pressure

We have studied the kinetics of high-pressure sintering of a composite SiC(C) ― Si₃N₄ powder of a certain phase composition. We consider structuring and mechanical properties of the ceramics obtained on the basis of this powder.     

Dielectric characteristics of sintered silicon nitride materials

We have studied and summarized data on structure formation and dielectric characteristics of Si₃N₄-based materials obtained by hot pressing and activated sintering. We have measured the dielectric characteristics in the frequency range 1 kHz to 10 MHz. We have established that the level of the dielectric characteristics of the materials is significantly affected by the content of highly dispersed Si₃N₄ powder obtained by plasmochemical synthesis in the original mix. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(412), pp. 22–26, March–April, 2000.     

Structuring and Mechanical Properties of Composite Ceramics SiC(C) ― Si3N4, Sintered under High Pressure

We have studied the kinetics of high-pressure sintering of a composite SiC(C) ― Si₃N₄ powder of a certain phase composition. We consider structuring and mechanical properties of the ceramics obtained on the basis of this powder.

     

Phase reaction and diffusion path of the SiC/Ti system

Bonding of SiC to SiC was conducted using Ti foil at bonding temperatures from 1373 to 1773 K in vacuum. The total diffusion path between SiC and Ti was investigated in detail at 1673 K using Ti foil with a thickness of 50 μm. At a bonding time of 0.3 ks, TiC at the Ti side and a mixture of Ti₅Si₃C _x and TiC at the SiC side were formed, yielding the structure sequence of β-Ti/Ti+TiC/Ti₅Si₃C _x +TiC/SiC. Furthermore, at the bonding time of 0.9 ks, a Ti₅Si₃C _x layer phase appeared between SiC and the mixture of Ti₅Si₃C _x and TiC. Upon the formation of Ti₃SiC₂ (T phase) after the bonding time of 3.6 ks, the complete diffusion path was observed as follows: β-Ti/Ti+TiC/Ti₅Si₃C _x +TiC/Ti₅Si₃C _x /Ti₃SiC₂/SiC. The activation energies for growth of TiC, Ti₅Si₃C _x , and Ti₃SiC₂ were 194, 242, and 358 kJ/mol, respectively.     

High-temperature reaction between silicon carbide and nitrogen under pressure

The effects of temperature and nitrogen pressure are studied on the SiC Si₃N₄ transformation of silicon carbide powders of various phase compositions, specific surface areas, and contents of mixtures. It is shown that the degree of transformation increases with nitrogen pressure up to 10 MPa and that, in all temperature and pressure ranges of nitrogen, it is higher for bulk free powder than the preliminarily compacted material. In 30–60 min, a complete transformation of SiC into Si₃N₄ occurs under 10 MPa nitrogen pressure and at 1650–1750°C temperature.Institute of Inorganic Chemistry, Slovakian Academy of Sciences. Institute of Superhard Materials, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, No. 4(364), pp. 1–6, April, 1993.     

Preparation of materials in the system Al2O3 (with additions)-TiB2 and study of their electrical resistance

Materials in the system AI₂O₃ (with additions of ZrO₂, SiO₂, CeO₂, Si₃N₄, AIN)-TiB, containing titanium boride concentrations up to 70 wt. % were prepared by sintering in carbon monoxide (T = 1873 K, t = 3 h). The relative density of these materials did not exceed 69% of theoretical, with a minimum at approximately 38% TiB₂ on the density vs. composition curve. The specific electrical resistance of the materials decreased from 3.410⁹ to 82 Ω cm with increasing titanium boride concentration. An abrupt decrease in electrical resistance occurred in the range 20-30% titanium boride.     

High-temperature interactions of refractory metal matrices with selected ceramic reinforcements

Interdiffusion and reactions occurring at high temperatures between refractory metals (Nb and Ta) and ceramic materials (SiC and A1₂O₃) have been investigated. Diffusion couples were fabricated by depositing Nb and Ta films of ~l-μm thickness onto polished ceramic substrates. These diffusion couples were vacuum annealed at high temperatures for various times. Interfacial reactions were evaluated using optical metallography, Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Kinetic studies in the 800 °C to 1200 °C temperature range for the Nb/SiC system indicated that Nb₂C initially forms, followed by the more stable NbC_xSi_y phase. In some instances, layered structures containing the phases NbC, Nb₂C, and NbC_xSi_y, were observed. The activation energies of formation for the NbC_x and NbC_xSi_y, phases were determined from these measurements. Results from the Ta/SiC system were found to be similar to those from the Nb/SiC system. In both Nb/Al₂O₃ and Ta/Al₂O₃ diffusion couples, annealing for up to 4 hours in the 1100 °C to 1200 °C range did not result in any significant reactions. These results suggest that A1₂O₃ may be a promising diffusion barrier between Nb and Ta metal matrices and SiC ceramic reinforcements. formerly with Lockheed Research and Development Division, is Senior Member of Technical Staff, Sandia National Laboratories, Albuquerque, NM 87185