Forming of silicon nitride by the HIP process

Journal of Rare Earths, 2012

This paper presented the microstructure, mechanical properties and oxidation behavior of silicon nitride obtained by both conventional sintering and sintering followed by hot isostatic pressing (HIP). Silicon nitride with additives such as 5 wt.% Al 2 O 3 and 5 wt.% Ln 2 O 3 (Ln= La, La concentrate, Gd or La+Gd) were studied. The results revealed that Gd 2 O 3 additions increased the formation of elongated grains of ȕ-Si 3 N 4 , the fracture toughness and oxidation resistance. La 2 O 3 additions led to higher densification and hardness values, while addition of La 2 O 3 concentrate promoted the formation of materials with intermediate properties, compared to the other studied compositions. Hot isostatic pressing increased the hardness but decreased the fracture toughness of the material, mainly because it allowed residual pores to close and also reduced the average aspect ratio of ȕ-Si 3 N 4 grains.

Journal of the European Ceramic Society, 1990

Three HIP-ed silicon nitride materials were studied, two of which were produced from KemaNord powders and the third one produced from UBE silicon nitride powder. The materials were alloyed with 2"5 wt% Y203. Besides this one of the KemaNord materials also contained 1 wt% Al20 3. Fracture strength and fracture toughness were determined as a function of temperature (20-1500°C). The KemaNord materials' susceptibility to slow crack growth was studied by using two different cross-head speeds when fracture testing the specimens. An extensive fractography was made to determine the fracture initiating flaw. The differences in behaviour between the materials can be explained by differences in composition, impurity content and/or grain size. On a ktudik trois types de nitrure de silicium pressk isostatiquement h chaud" deux btaient blaborks d partir de poudres KemaNord et un ?l partir de poudre UBE. On a allik ces poudres avec 2"5% massiques de 11203. Une des poudres KemaNord contenait bgalement 1% massique de AI20 3. On a mesurO leur rksistance ?t la rupture et leur tknacitk en fonction de la tempOrature (20-1500°C). On a ktudik la sensibilitk des matbriaux KemaNord d la propagation lente des .fissures en utilisant deux vitesses diffbrentes lors de l'essai de fissuration. Les dkfauts provoquant la rupture ont ktk rbvklbs par des analyses Jractographiques poussbes. Les diffOrences de comportement entre les matkriaux peuvent ~tre expliqubes par leurs compositions, leur teneurs en impuretbs et/ou la taille de leurs grains. Es wurden drei durch heiflisostatisches Pressen hergestellte Siliziumnitridwerkstoffe verglichen. Zwei der Siliziumnitridausgangspulver stammten yon Kema Nord und das Dritte wurde yon UBE hergestellt. Den Ausgangspulvern wurden je 2"5Gew.% Y203 zugegeben. Daneben enthielt eines der KemaNord Pulver auflerdem 1 Gew.% A120 3. Die Bruchfestigkeit und die Bruchziihigkeit wurden in Abhiingigkeit yon der Temperatur (20-1500°C) ermittelt. An den KemaNord Materialien wurde das langsame Riftwachstum wiihrend der Festigkeitstests mit Hilfe yon zwei Querhauptgeschwindigkeiten untersucht. Um den bruchausl6senden Rift zu bestimmen, wurden ausgedehnte Untersuchungen der Bruchfliichen durchgefiihrt. Das unterschiedliche Verhalten der einzelnen Materialien kann auf Unterschiede in der Zusammensetzung, den Anteil an Verunreinigungen und/oder die Korngr61~e zuriickgefiihrt werden.

2018

Sintered reaction bonded silicon nitride ceramic have high hardness, high wear resistance, good thermal insulator, electrical insulator, nonmagnetic, oxidation resistant, prone to thermal shock, chemically stable and high dielectric properties due to these properties sintered reaction bonded. We have two types of sintered reaction bonded silicon nitride. One type has as-received Si powder (4.3μm) and other one has planetary ball milled powder (0.43). We fix three sintering temperature 1850, 1900, 1950 °C in both type. We will determine the grain size and effect of sintering on grain size of bimodal microstructure from different methods but Inspector Matrox 2.1 software give most suitable grain size. Grain size decreases on increasing sintering temperature. We use Vickers microhardness at 200g load for 15 sec., and hardness decreases on increasing grain size for SA and SP both. KeyWordsSRBSN, Bimodal micrtostructure, Grain Size, Image Analyzer, Inspector Matrox 2.1 software, Vicker M...

Ceramics International, 2018

The effect of nanosized oxidized silicon nitride powder particles on the microstructural and mechanical properties of hot isostatic pressed silicon nitride was studied. The starting <alpha> Si 3 N 4 powders were oxidized for 10 and 20 h at 1000 ˚C in ambient air environment. An amorphous oxide layer was observed on the surface of Si 3 N 4 powder particles using high resolution transmission electron microscopy (HRTEM) whose thickness increased with the oxidation time. The powders were hot isostatic pressed at 1500 ˚C and 1700 ˚C in nitrogen gas environment under 20 Mpa for 3h. Complete <alpha> to <beta> transformation was observed in Si 3 N 4 samples sintered at 1700 C under 20 MPa for 3 h compared to Si 3 N 4 samples sintered at 1500 ˚C. Silicon oxynitride (Si 2 N 2 O) was found in sintered samples whose amount increased with the oxidation time. The <beta> phase decreased with the increase of Si 2 N 2 O phase in the sintered samples. The flexural strength of samples sintered at 1700 ˚C is higher due to the higher amount of <beta>-Si 3 N 4 phase than for samples sintered at 1500 ˚C. Decreasing flexural strength was observed with the oxidation time. Higher amount of oxygen in the base powders resulted in a higher Si 2 N 2 O content in the sintered Si 3 N 4 .

Journal of Materials Science, 1973

Grain morphology, distribution of impurities and inclusions, phases and dislocation structures in two grades of hot-pressed Si3N4 were investigated by means of replica and thin foil transmission microscopy, and by X-ray diffraction, microprobe and Auger analyses. High concentrations of impurities, specifically Ca, were detected at the grain boundaries. Fe-W-Si particles were seen within the grain. Non-densified Si3N4 inclusions were found to be detrimental to the strength. Possible correlations among strength, densification data and distribution of elements and phases are discussed.

Ceramics International, 2006

Composite materials based on Si 3 N 4 were synthesized by introducing previously prepared b-Si 3 N 4 seeds into a mixture of a-Si 3 N 4 and 10 wt% of sintering additive. The densification process during pressureless sintering was studied in the presence of CeO 2 additive, which is less frequently used as sintering aid to Si 3 N 4 . The effect of seed concentration and sintering time on the densification and fracture toughness was followed. The results showed that the addition of seeds retarded densification. On the other hand, fracture toughness passes through maximum at the content of 3 wt% seeds. #

Journal of Materials …, 2008

Processing plays an important role in determining the microstructure of silicon nitrides which in turn influences the mechanical properties, such as hardness and toughness. Sintering aids are an important processing parameter. The influence of the chemistry of sintering aids on properties of silicon nitrides is a well-explored subject. Here the size of sintering aids used and its impact on microstructure and mechanical properties is explored. Specifically the use of nano-scale versus micron-scale sintering aids is examined. Microstructures and mechanical properties for silicon nitrides hot-pressed with nano-sized sintering aids are compared with a reference silicon nitride hot-pressed with micron-sized sintering aids. Hardness and fracture toughness are determined at room temperature using hardness indents. Grain size and aspect ratio distributions are determined for the two silicon nitrides and their impact on mechanical properties are examined. Toughening behavior is studied by experimentally determining R-curves. Both toughness and toughening (R-curve) behavior are shown to improve with the use of nano-scale sintering aids.

Journal of the American Ceramic Society, 1990

Densifying silicon nitride with a YSiAlON glass additive produced 99% dense materials by pressureless sintering. Subsequent heat-treating led to nearly complete crystallization of the amorphous intergranular phase. Transmission electron microscopy revealed that for heat treatments at 1350"C, only &Y2Si207 was crystallized at the grain boundaries. At a higher temperature of 1450"C, primarily YSiOIN and Y4Si207N2 in addition to small amounts of Y2SiOs were present. Al existed only in high concentrations in residual amorphous phases, and in solid solution with Si3N4 and some crystalline grain-boundary phases. In fourpoint flexure tests materials retained up to 73% of their strengths, with strengths of up to 426 MPa, at 1300°C. Highstrength retention was due to nearly complete crystallization of the intergranular phase, as well as to the high refractoriness of residual amorphous phases. [

Ceramics International

Composite materials based on Si 3 N 4 were synthesized by introducing previously prepared b-Si 3 N 4 seeds into a mixture of a-Si 3 N 4 and 10 wt% of sintering additive. The densification process during pressureless sintering was studied in the presence of CeO 2 additive, which is less frequently used as sintering aid to Si 3 N 4 . The effect of seed concentration and sintering time on the densification and fracture toughness was followed. The results showed that the addition of seeds retarded densification. On the other hand, fracture toughness passes through maximum at the content of 3 wt% seeds. #

Acta Materialia, 1998