JPS605075A - Manufacture of silicon nitride sintered body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a silicon nitride burner having high toughness and high strength. Sintered silicon nitride (S i 8N) has excellent strength and wear resistance at high temperatures, has a small coefficient of thermal expansion, and is chemically stable. It is attracting attention as a material for high-temperature applications in gas turbines, radiant tubes, and ponds. Since silicon nitride powder bed itself is a substance with poor sintering properties, it is generally necessary to mix a sintering aid in the production of sintered bodies, and the composition of the aid has so far been studied. Many efforts have been made to promote sinterability and improve high-temperature strength through ingenuity. For example, yttrium oxide (Y2O2), magnesia (MgO), Alemina (A#203), etc. have been used as effective auxiliaries. Are known.

However, in general, the bonds between the constituent atoms of ceramics are mainly covalent bonds or ionic bonds (usually a hybrid of these bonds), so while they have high elastic modulus and high strength, they have a complex crystal structure and are prone to spatial gaps. It has many structures. For this reason, a major drawback of general ceramics is that, unlike metals, dislocations cannot move at low temperatures, and they exhibit a behavior called brittleness. Regarding this brittleness, for example, if a heterogeneous phase is formed within the sintered body, when a crack propagates due to external stress, the fracture energy is absorbed by the dispersed heterogeneous phase, resulting in a decrease in fracture toughness. There is also the view that this will improve the brittleness, and various studies have been conducted on the composition of auxiliary components for the purpose of improving brittleness.

However, with regard to silicon nitride heat exchangers, sufficient results have not been obtained so far in terms of toughness, and the fracture toughness value (Klo) does not exceed 5 MN.
The reality is that it generally remains at about 8-4 M N =m-3/'''.

The present invention has been made in view of the above, in order to improve the fracture toughness value and increase the strength of a silicon nitride heat exchanger body.

The method for producing a silicon nitride sintering body of the present invention includes silicon nitride powder, a sintering aid containing a heavy earth oxide of 5 to 28% by weight, and a zirconium oxide or a partially stabilized 7 to 30% by weight zirconium oxide. A mixture containing zirconium oxide is molded and sintered. In addition, in the production method of the present invention, tungsten carbide (wc) is optionally added in a weight ratio of 2 to 20% as a sintering aid together with each of the above mentioned aids.

The rare earth oxide blended as a sintering aid has the role of promoting sintering and improving the strength of the sintered body.

In order to obtain this effect sufficiently, at least 5 parts by weight is required. Preferably, it is 7 or more. As the rare earth oxide, yttrium oxide or an oxide belonging to the lanthanide series rare earth oxide group may be arbitrarily selected and used, but from the viewpoint of sintering promotion effect and mechanical properties such as strength, oxide (Y2O2), -ytane oxide (La203), neodymium oxide (Nc120
3), praseodymium oxide (Pr601, ), or a mixture of any two or three of these are preferably used.

Zirconium oxide (Z r 02 ) or partially stabilized zirconium oxide used in combination with the rare earth oxide not only promotes sintering but also improves the fracture toughness of the sintered body. In order to fully exhibit this effect, at least 7 parts by weight is required, more preferably 9 parts by weight or more. -) Ruconium oxide and partially stabilized zirconium oxide can be considered to be equivalent substances in terms of improving fracture toughness and promoting sintering. Both may be used in combination.

Further, tungsten carbide is blended as desired for improving sinterability, improving toughness, etc., and the blending amount for this purpose needs to be 2 parts by weight or more.

As the blending ratio of each of the sintering aids increases, the effect of each addition increases, but even if too much is blended, the increase in effect will be small compared to the blending amount, and in addition, the effect of silicon nitride in the mixture will increase. As the relative proportion of the base powder decreases, its characteristics as a silicon nitride heat-generating body are weakened. Therefore, the northern limit of rare earth oxide should be 288 parts by weight, zirconium oxide or partially stabilized zirconium oxide should be 300 parts by weight, and tungsten carbide should have a range of 20 parts by weight. Good results can be obtained by incorporating 200 parts by weight of zirconium oxide or partially stabilized zirconium oxide and up to 12 parts by weight LIb of tungsten carbide.

Silicon nitride powder, the main raw material, has an α-type and a β-type crystal structure, and as is well known, from the viewpoint of sintered body strength, it is necessary to promote crystallization of the grain boundary layer within the sintered body. The α type is advantageous, and preferably powders with a gelatinization rate of about 90 or higher are used.

According to the present invention, the desired sintering process is achieved by blending silicon nitride powder with the required amounts of each of the above-mentioned auxiliary agents and, if necessary, adding an appropriate molding auxiliary agent. Obtain a body. "Forming and sintering" means that shaping and sintering are done in one step, depending on the process being applied, for example, hot press method or hot isostatic pressure sintering method (II I P method). This includes cases where molding and sintering are carried out as separate steps, as in the case of pressureless sintering. Both processes are carried out under normal conditions; for example, in the hot press method, the mixture is filled into a mold of the desired shape, and an appropriate pressure is applied (for example, 200 to 400 ml).
00#r〆-) and sintering temperature (e.g. 1600-18
Sintering is achieved at 50°C). In addition, in the pressureless sintering method,
Appropriate amount of a suitable molding aid (e.g. methylcellulose.0
1-2.01. The mixed mixture is molded into a desired shape by an appropriate molding method, such as -shaft press, rubber press, injection molding, or other method, and then sintered at an appropriate sintering temperature and atmospheric pressure under an inert atmosphere such as nitrogen gas. For example, 1600~
Sintering is completed at 1850°C and 1~1 Ok (j I'/crd).

Next, examples of the present invention will be described.

Example [A] Hora 1-press method silicon nitride powder (gelatinization rate 95, average particle size 0.6μ77
A sintering aid was added to 1), and a sintered body (40mm x 20+++m x 6mm
) was obtained.

[B] Pressure-sintered silicon nitride powder (gelatinization rate 95qb, average particle size O6μ71
A sintering aid and a 0.54 methyl cellulose aqueous solution as a forming aid are added to 1) (10 CC for 30 g of silicon nitride powder), mixed, and after being molded into a disk body using a -axial press method, By the pressure sintering method, a disk-shaped sintered body (diameter 50 wn
m) was obtained.

The fracture toughness value (K□.) and bending strength of each of the sintered bodies obtained by the above-mentioned sintering methods were measured. The fracture toughness value is measured using Knoop-Indentation-
8trcogtb method. The bending strength test is 3
The bending was performed using a three-point bending method (span distance 30+++ m) using a specimen measuring mm x 8 mm x 40 mm.

The formulation of the sintering aid and the test results are shown in Table 1.

In the table, "A" in the "manufacturing method" column means a hot press method, and rBJ means a pressureless sintering method. Incense (1) to (8) are invention examples,
(101) and (102) are comparative examples using conventional general additive formulations.

As shown in the table, the sintered body obtained by the present invention is
Regardless of the molding and sintering method, it has a high fracture toughness value that far exceeds conventional standards, and its strength also exceeds that of conventional materials made using the same molding and sintering methods.

As previously mentioned, the silicon nitride heat exchanger obtained by the present invention has high fracture toughness and high strength that are significantly superior to conventional materials, making it suitable for various structural members, tools, etc. Stable surface can guarantee durability.

Agent: Patent attorney Shinhachibe Miyazaki

Claims (2)

[Claims] (1) A mixture in which a silicon nitride powder bed is blended with a 5-28 weight class upper class oxide and a 7-30 weight class zirconium oxide and/or partially stabilized zirconium oxide as a sintering aid. A method for producing a high-toughness, high-strength silicon nitride heat exchanger body, which is characterized by molding and sintering. (2) The method for producing a silicon nitride burner according to item (1) above, wherein the rare earth oxide is one or a mixture of two or more of yttrium oxide, lanthanum oxide, or praseodymium oxide.