JPS61183178A - Method of joining silicon nitride ceramic to metal - 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 [Technical Field of the Invention] The present invention relates to a method for joining silicon nitride ceramics and a metal, and more particularly, to a method for joining silicon nitride ceramics and a metal that provides high joint strength even at high temperatures.

[Technical background of the invention and its problems] Ceramic materials have recently been used in a wide range of fields such as structural materials and functional materials due to their excellent properties. In many cases, the parts are made of ceramic alone, but
In order to use ceramics in more fields, it is necessary that they can be bonded with metals and made into composite materials. In this case, sufficient bonding strength is required to use the bonded body as a structural component, while continuity at the bonding interface is required to use it as a functional component.

However, since ceramics and metals have different atomic bonding states, when joining metals and ceramics, their chemical properties such as reactivity, and physical properties such as thermal expansion coefficient and electrical conductivity are significantly different. Therefore, it is difficult to properly wet both members and perform a reliable metallurgical bond.

In particular, silicon nitride ceramics, which have high chemical stability among ceramics, are difficult to bond with metals because they have poor wettability with metals.

Under these circumstances, a method of bonding silicon nitride ceramics and metal using active metals such as Ti and Zr and utilizing a reaction at the interface between the two has been developed. This method is usually carried out using an alloy in which metals whose eutectic melting point is considerably low, such as a silver-copper alloy, is used to obtain good bonding strength.

However, in the method using the active metal described above, the composition of the metal used is limited to one with a low melting point, so the resulting joined body has a large The property of high-temperature strength will not be utilized, that is, the useful properties of silicon nitride ceramics will be diminished.

[Object of the Invention] An object of the present invention is to provide a method for bonding silicon nitride ceramics and metal, which solves the above-mentioned problems and provides high bonding strength even at high temperatures.

[Summary of the Invention] In order to achieve the above-mentioned object, the present inventors have developed conditions for bonding silicon nitride ceramics and metals, each of which has significantly different physical properties, to achieve a highly reliable bonded state even at high temperatures. After considering the following requirements, we found that: (1) the silicon nitride ceramic and the metal to be joined must be chemically bonded; (2) the joint should use a brazing filler metal that is significantly affected by high temperatures. (3) If the joint area is large, the structure must be such that thermal stress does not occur at the joint.

Therefore, in order to satisfy the conditions (1) and (2), the present inventors first determined that metallization chemically bonded to the silicon nitride ceramic surface using a metal that is highly stable even under high temperatures. The metal to be bonded may be chemically bonded to the silicon nitride ceramic via the metallized layer. Regarding (3), a thermal stress suppressing layer may be formed between the ceramic and the metal to be bonded. After conducting various studies based on this idea, we discovered that under certain conditions, the surface of silicon nitride ceramics dissociates into silicon and nitrogen.

Therefore, based on this fact, the present inventors placed the metal to be metallized on the surface of silicon nitride ceramics and heat-treated the entire surface under vacuum, and as expected, the silicon nitride on the surface of the silicon nitride ceramics became silicon The dissociated silicon reacts with the metal to be metallized to form a certain molten reaction product, which wets the surface of the silicon nitride ceramic, resulting in the formation of silicon nitride. We have discovered that a chemically bonded metallized layer is formed on the ceramic surface. Therefore, if the metallized layer and the metal to be bonded are brought into contact and diffusion bonding is performed under predetermined conditions, the metallized layer and the bonding metal will be easily bonded. Furthermore, in order to prevent the occurrence of thermal stress between the metal to be joined and the ceramic, it is possible to use the metallized layer as a thermal stress suppressing layer or to interpose a material that alleviates the difference in thermal expansion coefficient between the ceramic and the metal to be joined. The present invention was completed based on the knowledge that the method was suitable.

That is, the method for joining silicon nitride ceramic and metal of the present invention includes disposing a metal or alloy highly reactive with silicon on the surface of silicon nitride ceramic, heating the entire ceramic and the metal or alloy under vacuum, forming a metallized layer on the silicon nitride ceramic surface by dissociating the silicon nitride ceramic surface into nitrogen and silicon and forming a reaction product between the silicon generated by the dissociation and the metal or alloy; It is characterized by the steps of placing the metal to be bonded on the metallized layer and heating the whole while applying pressure.

First, the silicon nitride ceramic to be metalized in the present invention is not particularly limited, and may contain a sintering aid other than silicon nitride.

The present invention is fully effective even on high-purity silicon nitride ceramics that do not contain sintering aids or the like.

Next, the metal or alloy that is highly reactive with silicon is a metal or alloy that is made of a transition element and has a melting point higher than the melting point of at least one of the reaction products with silicon. Preferably, it is a metal that forms a eutectic with silicon and is a metal of the same type as the metal to be bonded.

Such metals or alloys are heated during the heat treatment described below.
Although the metal or alloy itself does not melt, the reaction product with silicon that is dissociated on the surface becomes molten and wets the surface of the silicon nitride ceramic, so that it firmly adheres to the surface.

If the metal or alloy used does not exhibit the above-mentioned melting point characteristics, for example, if the metal or alloy itself becomes molten at the temperature at which the reaction product becomes molten, the molten metal or alloy will not melt on the silicon nitride ceramic surface. Since silicon covers the metal or alloy, diffusion of silicon into the metal or alloy is inhibited, and alloying between silicon and the metal does not proceed, and as a result, wetting phenomenon with the ceramic surface does not occur.

A preferred metal applicable to the present invention is Fe.

Metals such as Go, Ni, Pt, and alloys thereof 1 For example, alloys containing at least 50% by weight of at least one of Fe, Go, Ni, and Pt, specific examples include: Silicon steel, carbon steel 9 alloy steel.

Examples include special steel, nickel-chromium alloy, nickel-aluminum alloy, nickel-titanium alloy.

The metals or alloys mentioned above are powders.

It may be used in either a foil or wire form, or a combination of these forms may be used.

The metal or alloy to be joined is not particularly limited, but is preferably a metal of the same type as the metal or alloy to be metalized. Furthermore, since molybdenum and silicon nitride have similar coefficients of thermal expansion, when this metal is used as a metal to be bonded, regardless of the size of the bonding area,
There is no need to provide a thermal stress suppression layer, which will be described later.

Next, the method of the present invention will be explained. First, a method applied when the area to be bonded is small will be described. First, the metal or alloy described above is placed on the surface of the silicon nitride ceramic. At this time, if the metal used is in the form of powder, it is made into a paste using an appropriate solvent.
It is best to apply this to the ceramic surface.

Next, the ceramic with metal arranged on its surface is placed in a vacuum furnace. Then, the entire ceramic is heated after the pressure inside the furnace is reduced to a vacuum state, or the entire ceramic is heat-treated while the pressure inside the furnace is reduced.

The degree of vacuum applied at this time is from normal pressure to 10-” Torr.
Low/medium vacuum of 10-3 orr or less, high/ultra-high vacuum of 10-3 orr or less, but preferably 1O-3 to 1o
It is a high vacuum of near Torr.

Here, there is a correlation between the degree of vacuum and the heating temperature described below. In other words, in normal cases, the degree of vacuum is increased (the pressure inside the furnace is decreased)
The temperature at which silicon nitride on the surface of silicon nitride ceramics dissociates into silicon and nitrogen (dissociation temperature) depends not only on the heating temperature but also on the pressure inside the furnace. Based on that.

In the heat treatment operation, first, it is preferable to start heating at a temperature equal to or lower than the above-mentioned dissociation temperature so that silicon nitride on the surface of the silicon nitride ceramic dissociates into nitrogen and silicon under a predetermined vacuum. The entire ceramic is heated to a maximum heating temperature, which will be described later, and maintained at this temperature for a predetermined period of time.

The maximum heating temperature is set to a temperature north of the melting point of the above-mentioned reaction product and below the melting point of the metal or alloy itself.

If the maximum heating temperature is less than the melting point of one reaction product, the reaction product will not become molten and will not wet the ceramic surface, and if the temperature is even lower, no reaction between silicon and metal will occur. Conversely, if the heating temperature is higher than the melting point of the metal, the metal itself becomes molten and inhibits dissociation of the silicon nitride ceramic surface.

The specific values and ranges of the melting point of the above-mentioned reaction products, the dissociation temperature of the ceramic surface, and the maximum heating temperature cannot be determined unconditionally because they depend on the metal used and the degree of vacuum applied, but the maximum heating temperature is 1200 It is preferable that the temperature is at least ℃.

In this way, a metallized layer is formed on the surface of the silicon nitride ceramic. The metallized layer is a uniform alloy layer made of the metal and the silicide of the metal, and is chemically bonded and firmly attached to the silicon nitride ceramic surface.

Next, the metal to be bonded is placed on the formed metallized layer, and these, the ceramic, the metallized layer, and the metal to be bonded are all loaded into a hot press, for example, and
Heat and pressure treatment is performed for a predetermined period of time to diffusion bond the metallized layer and the metal to be bonded.

The atmosphere to be applied at this time is 1, for example, argon, hydrogen,
Inert or reducing gases such as nitrogen and helium are preferred. Note that a vacuum may be used.

The heating temperature during the diffusion bonding process is set to a temperature lower than the melting point of the metals to be bonded. Preferably, the temperature is set lower than the melting point of the metal or alloy that is highly reactive with silicon. The applied pressure is between 1 and 5000 kg/cm, preferably between 10 and 1000 kg/d.

At the contact interface between the metallized layer and the metal to be bonded by the heating/pressure treatment described in E, atoms in the metallized layer and atoms in the metal to be bonded undergo interdiffusion, and as a result,
The silicon nitride ceramic and the metal to be bonded are bonded via the metallized layer, and the bonded portion has high strength.

On the other hand, when the area to be bonded is large, a step of providing a thermal stress suppressing layer is added to the above method.

Methods for providing the thermal stress suppression layer include a method using a metallized layer and a method in which another material is interposed between the metallized layer and the metal to be bonded.

When using a metallized layer, it is preferable not to form the metallized layer on the entire surface of the ceramic, but to form it partially.

It is preferable to form the metallized layer in spots.

When interposing other materials, there are two methods: forming a layer whose coefficient of thermal expansion gradually changes along the thickness direction as a thermal stress suppression layer between the ceramic and the metal to be bonded; There is a method in which a material whose coefficient is between that of the ceramic and the metal or a composite material made of two or more substances with different thermal expansion coefficients is interposed between the ceramic and the metal to be bonded as a thermal stress suppression layer. be.

In the case of forming the former layer, 1. For example, a thin layer is formed on the metallized layer using a material whose thermal expansion coefficient is close to that of silicon nitride, for example, by a vapor deposition method, and then the thermal expansion coefficient approaches that of the metal to be bonded. A method is adopted in which layers of a multilayer structure with varying coefficients of thermal expansion are formed by changing materials.

In the latter case, metals with a low coefficient of thermal expansion,
One example is a method in which a composite material consisting of a nonmetal and a metal, such as cermet, is used.

If the above-mentioned thermal stress suppression layer is interposed between the silicon nitride ceramic and the metal to be bonded, after the bonding process is completed, the bonding part will be bonded to the ceramics during the process of cooling from the bonding temperature. Residual stress due to the difference in thermal expansion with metal can be reduced, and cracks and the like can be prevented from occurring at the joint.

[Embodiments of the invention] Example silicon nitride ceramic flat plate (25sm×25+s■
) on the surface of the Ni under 325 mesh (Tyler sieve)
The powder was applied in a spotted pattern to a thickness of 30 l using ethyl alcohol as a medium and the plate was placed in a vacuum oven.

After setting the inside of the vacuum furnace to I x 104 Tarr, 90
Ceramic flat plate as a whole at a heating rate of 0'O/hr.
It was heated to 250°C and held at this temperature for 10 minutes.

After cooling, the surface of the ceramic plate was observed.1
A metallized layer with a spotted pattern was formed.

When the metallized layer was analyzed by X-ray diffraction, it was confirmed that the metallized layer was a mixed crystal of Ni(α) and Ni3S2.

This is thought to be caused by the following reaction.

Si3N4 → 3Si+2N2 Si+old → Ni(α) + Ni5Si2 reaction product (
Since the melting point of Ni(α) + N15Si2) is 1152° C., which is lower than the heating holding temperature (1250° C.), it is thought that this reaction product becomes liquid on the surface of the silicon nitride ceramic and wets the surface.

Next, on the metallized layer there is a Xi gold alloy
A board of Lua 38 (25 m + w x 25 m) was placed on top and the whole was loaded into a hot press. After heating to 1200℃ with argon gas atmosphere, 200kg/
A pressure of d was applied and held in that state for 30 minutes.

After cooling, the cross section of the joint between the silicon nitride ceramic and the Inconel Fuco 8 alloy was examined in detail using a scanning electron microscope equipped with an analyzer, and the presence of a reaction layer was observed at the interface between the silicon nitride ceramic and the metallized layer. A chemical bond was confirmed between them. Furthermore, it was confirmed that at the interface between the Inconel Fuco 8 alloy and the metallized layer, the boundary between the two was unclear and the atoms of both were well diffused into each other.

Note that the bonding between silicon nitride ceramics and Inconel Fuco 8 alloy is achieved only through one dotted metallized layer, and it is difficult to bond the entire surface, so even if a large thermal change is caused to the bonded area, However, no structural defects such as cracks were observed, and a good bonding state was maintained.

The bonding strength between the silicon nitride ceramic and the Inconel Fuco 8 alloy was 150 kg/- or more at 850°C, which was sufficient for practical use.

[Effect of the invention] As is clear from the above description, the method of the present invention.

Silicon nitride ceramics and metal can be bonded with high bonding strength even at high temperatures. In addition, if a thermal stress suppression layer is provided between silicon nitride ceramics and the metal to be bonded, it is possible to suppress the occurrence of thermal stress regardless of the size of the bonding area. However, it has great industrial value because metals can be joined depending on the purpose.

Claims (1)

[Claims] 1. A metal or alloy that is highly reactive with silicon is placed on the surface of silicon nitride ceramics, and the entire ceramic and metal or alloy are heated under vacuum, and the surface of the silicon nitride ceramics is heated with nitrogen. a step of forming a metallized layer on the surface of the silicon nitride ceramic by dissociating it with silicon and forming a reaction product between silicon and the metal or alloy produced by the dissociation; and a step of forming a metallized layer on the surface of the silicon nitride ceramic; A method for joining silicon nitride ceramics and metal, characterized by the following steps: placing metal and heating the entire body while applying pressure. 2. The method according to claim 1, wherein the metallized layer is partially formed on the surface of the ceramic. 3. Between the metallized layer and the metal to be bonded, from the metallized layer to the metal to be bonded, the thermal expansion coefficient approximates the thermal expansion coefficient of the silicon nitride ceramic. 2. The method of claim 1, wherein there is interposed a layer of material whose coefficient of thermal expansion approximates that of the metal. 4. A composite material or a metal whose thermal expansion coefficient as a whole is intermediate between the silicon nitride ceramic and the metal to be bonded is interposed between the metallized layer and the metal to be bonded. The method described in Section 1.