Patent Application
20070225150
The invention relates to sintered ceramic materials comprising a crystalline phase and an intergranular phase and especially to silicon nitride and sialon-based ceramics but also ceramics based on other hard phases, such as silicon carbide. A method for manufacturing such ceramics is based on selecting additives as sintering agents which improve the sintering process and the resulting intergranular glass phase.
Si3N4 and SiAlON based ceramic materials have been intensively investigated during the last decades due to their superior mechanical properties with good thermal stability and excellent thermo-shock properties. These properties have a wide range of applications and used such as ceramic cutting tools, ceramic bearings, ceramic substrate, space industry, and continues to receive attention in the automotive component market. As compared with carbide-based materials, or steel materials, silicon nitride generally offers the potential of relatively high heat resistance and chemical stability, relatively low density, good mechanical properties such as hardness and toughness, and good electrical insulation characteristics. To illustrate the advantages, in the context of the cutting tool industry, these properties can combine in whole or in part to allow operations to proceed at higher speeds and temperatures, with resulting potential cost savings. The potential market for the above properties indicates use in other applications, such as, extrusion dies and automotive components, turbocharger components, swirl chambers, and engine valve.
Single-phase of Si3N4 is a high covalent compound and exist in 2 hexagonal polymorphic crystalline forms α- and β-Si3N4, β-Si3N4 being stable than the α form. The structure of α- and β-Si3N4 is build up from basic SiN4 tetrahedra joined in three-dimensional network by sharing corners, with common nitrogen to the three tetrahedral sites. Either structure can be generated from the other by a 180° rotation of 2 basal planes. The α- to β-Si3N4 transition is usually by a solution-precipitation reaction of Si3N4 and molten glass. The strong covalent bonds of Si3N4 produce some mechanical and engineering properties for these materials such as: low thermal expansion coefficient, which results to good thermal shock resistance, high strength which results to high toughness, greater Young's modulus than some metals, thermal stability, up to 1800° C., which is temperature when Si3N4 starts to decompose. The weak point of this material is difficulties of self-diffusion and production of Si3N4 into a dense body by classical method of ceramic processing technology. This problem can be helped to a large extent by using sintering additives, glass-formers and formation of sialons by substituting silicon and nitrogen with aluminium and oxygen.
Nitrogen rich sialon phases have been extensively studied in connection with the development of high performance ceramics, especially in α- and β-sialon systems [T. Ekström and M. Nygren, J. Am. Ceram. Soc., 75, 259 (1992)]. The structure of α-Si3N4 was established using single crystal X-ray diffraction (XRD) data and film methods [R. Marchand et al, Acta Cryst. B25, 2157 (1969)] and more accurate atomic positions were obtained in later single crystal XRD studies [I. Kohatsu et al, Mat. Res. Bul., 9, 917 (1974) and K. Kato et al., J. Am. Ceram. Soc., 58, 90 (1975)]. Structural changes of α-Si3N4 with temperature, below 900 C have also been investigated using neutron powder diffraction data [M. Billy et al., Mat. Res. Bul., 18, 921 (1983)]. The α-Si3N4 crystallises in the space group P31c with the unit cell parameters a=7.7523(2), c=5.6198(2) Å, V=292.5 Å3 [Powder Diffraction File 41-0360, International Centre for Diffraction Data, Newtown Square, Pa.] and unit cell content Si12N16.
The α-sialons are solid solutions that have a filled α-Si3N4 type structure. There are two substitution mechanisms. First, silicon and nitrogen can be substituted simultaneously by aluminium and oxygen. Second, the structure has two large, closed cavities per unit cell that can accommodate additional cations of metals, M=Li, Mg, Ca, Y, and Rare Earth (RE) elements. A general formula for α-sialons can thus be written as MxSi12−(m+n)Al(m+n)OnN16−n, where x (=m/v)≦2, and v is the average valency of the M cation. For all of the known α-sialon compositions the m/(m+n) ratio is found to be below 0.67. Examples of reported α-sialon phases are Y.5 (Si9.75 Al2.25) (N15.25 O.75) and Ca.67 (Si10A12) (N15.3 O.7) [F. Izumi et al., Journal of Materials Science, 19, 3115 (1984)]. This is likely due to the synthesis approach that is usually used with metal oxides or carbonates of M=Li, Mg, Ca, Y, and RE as additives used either as substitution in α-sialon crystal structure or as glass-formers and sintering additives. The addition of M in the form of oxides incorporates oxygen atoms in the sialon system.
In the Swedish patent application SE 0300056-9 is described a method for obtaining nitrogen rich glass phases by using non oxide additives. The mechanical properties of the nitrogen rich glass phases have been reported to be improved with the increased nitrogen content.
Even though α-sialon phases have been used in many different commercial applications, specially as single phase ceramics or together with other compounds in composite ceramics, and despite an intensive scientific investigations and developments in this field there has been crucial limitations in the chemical compositions of the crystalline α-sialon phases as well as in the intergranular glassy phase found in the ceramic bodies produced.
One of the important uses of sialon-based ceramics or silicon nitride or silicon carbide is their high temperature applications. The most important parameter for high temperature applications is the inter-granular glass phase in the ceramics. By increasing the nitrogen content and thereby getting better mechanical properties and higher glass transition temperatures the obtained ceramics show much better high temperature stability with respect to chemical stability as well as mechanical stability.
This invention presents a new group of sintered ceramic materials, such as silicon nitride, silicon carbide or sialon-based ceramics and a new method of producing those.
The new ceramics comprises nitrogen rich intergranular glass phases. The sialon-based ceramics can also comprise alpha-sialon phases with high nitrogen content.
The nitrogen rich intergranular glass phases obtained during the production of the sintered ceramic materials improve the properties of the sintered ceramic bodies in several aspects. The liquid phase obtained during the synthesis provides elongated crystallites having good reinforcement properties. The superior mechanical properties of the nitrogen rich glass phases provide ceramics with improved hardness and improved fracture toughness. The increased glass transition temperature by increasing nitrogen concentration of the glass phases gives ceramics with better creep resistance and therefore improved high temperature properties. The glass phase can have a nitrogen content with a N:O ratio higher than 35:65 and the glass transition temperature can be above 950° C.
The method for manufacturing the sintered ceramic materials includes synthesis at temperatures generally in the range of 1500-1800° C. in nitrogen atmosphere using a starting powder together with sintering agents comprising additives such as Li, Na, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Th, Pa or U. The additives are used as non oxide precursors such as pure metal, nitrides or hydrides or other sources that transforms to nitrides or metallic state in the nitrogen atmosphere at elevated temperatures used during the synthesis. The above mentioned metals can also be added as oxides or carbonates if used together with graphite in nitrogen atmosphere in order to form the metal nitride through a carbothermal reduction. The densification can be obtained by using a hot pressing synthesis, a gas-pressure synthesis or at ambient pressure.
This unique process provides a possibility to incorporate additives in synthesis of silicon nitride, silicon carbide or sialon-based ceramics without simultaneous incorporation of oxygen atoms. The materials obtained by this process have been found to posses good mechanical properties such as high Vickers hardness values typically above 18.0 Gpa and values as high as 22.3 Gpa and fracture toughness values typically above 5.0 MPa·m1/2 and values as high as 7.8 MPa·m1/2.
Another important aspect of the invention is that sintered ceramic materials comprising new α-sialon compositions can be obtained were the aluminium content in the crystalline phase is fully or partially balanced by addition of the stabilising metals rather than the exchange of nitrogen by oxygen.
An embodiment of the invention includes a synthesis method to prepare ceramic materials comprising α-sialons with new compositions. By using non oxide additives or by reducing an additive in oxide form in the synthesis process, the concentration of oxygen in the α-sialon can be significantly reduced. The α-sialon compositions can be described by the formula MxSi12−(m+n)Al(m+n)OnN16−n, where x (=m/v)≦2, and v is the average valency of the M cation. The new compositions obtained by this method have one of the following elements M, or combinations thereof, in the cavity of the α-sialon structure: Li, Na, Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Th, Pa or U. At the same time the ratio of m/(m+n) must be higher than 0.7, or higher than 0.8 or 0.9.
For the sintering process the liquid phase is important as well as the solidified liquid which forms an intergranular glass phase. According to the invention the glass phase can be formed with significantly higher nitrogen content. This is possible since the precursors used in the synthesis as additives are non oxide materials, or a mixture of precursors which transforms to a nitride during the synthesis in nitrogen atmosphere, and therefore it allows much higher nitrogen incorporation due to the metal nitrides that are formed which are very reactive and act as glass modifiers.
The synthesis processes according to the invention allows for production of highly densified silicon nitride, silicon carbide or sialon based ceramics. The densification is promoted by higher concentrations of additives. The additives are important components in the process of forming the liquid phase, which is essential for the recrystallisation of the crystalline silicon nitride or sialon phases. The densification can be obtained by using a hot pressing synthesis, a gas-pressure synthesis or synthesis at ambient pressure.
The sintered ceramic material can be used as powder samples, sintered ceramic bodies or thin films in different applications such as ceramic cutting tools, ceramic ball bearings, ceramic gas turbines, ceramic body implants, wear resistant ceramics, magneto-optical applications, substrates for electronics and luminescent materials. The sintered ceramic material can comprise other materials such as whiskers or other reinforcement structures or be applied with surface layers of other materials.
Synthesis Procedures Used for the Below Mentioned Examples:
The silicon nitride, silicon carbide and sialon ceramics were obtained with other crystalline and amorphous glass phases by using pressure less synthesis in a graphite furnace, radio frequency induction furnace or a hot pressing synthesis using a uni-axial pressure of 32 Mpa. The synthesis atmosphere used was nitrogen independent of the furnace used. The precursors used in every specific synthesis were carefully ground and pressed to pellets, before placing in the furnace. In those cases were nitrides, hydrides or pure metals of additives such as Mg, Ca, Sr, Y or rare earths were used, contacts with air was avoided in order to avoid oxidation of those precursors.
The hot-pressed samples were prepared under a uni-axial pressure of 32 Mpa at 1750° C. during 4 hours in flowing nitrogen atmosphere. The samples synthesised in the graphite furnace or the radio frequency furnace was prepared at 1750° C. during 4 hours in flowing nitrogen atmosphere, using ambient gas pressure.
Summary of the Properties of the Sialons Series ACN
Summary of the Properties of the Sialons Series ACH
Summary of the Properties of the Sialons Series AYH
Summary of the Properties of the Sialons Series BCN
Summary of the Properties of the Sialons Series BCH
Summary of the Properties of the Sialons Series BYH
Summary of the Properties of the Sialons Series CCH
