JPH02250938A - Electrically conductive cermet composition excellent for corrosion and erosion resistance and making thereof, and making of ignition plug or glow plug using it - Google Patents

Abstract

PURPOSE: To improve the resistance of an electrical conductive cermet composition to high temp. corrosion and electrical erosion by incorporating specific amounts of partially oxidized metal grains having specific oxygen/metal into an alumina-base ceramics.

CONSTITUTION: This electrical conductive cermet composition is a composition which contains alumina-base ceramics and 36-60wt.% of partially oxidized metal grains providing sufficient electric conductivity required of the composition and in which the proportion of oxygen to metal in the above grains is regulated to 0.3-10wt.%. This composition has remarkably improved high temp. corrosion resistance and spark discharge erosion resistance. This composition can be obtained by mixing components of the electrical conductive cermet composition or precursors thereof, a compaction additive, and a sintering additive in prescribed proportions, compacting the resultant mixture, and then sintering the resultant green compact at high temp., and the partial oxidation of the metal grains is performed before the mixing stage or during the sintering stage.

COPYRIGHT: (C)1990,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to ignition or heating devices, in particular spark plugs or plasma discharge plugs for gasoline engines and glow plugs for diesel engines or external combustion engines (for example Stirling engines). The present invention relates to a conductive cermet composition that can be used for manufacturing, and also to a method for manufacturing the same and a method for manufacturing a spark plug or glow plug using the same.

In this specification, conductive cermets and conductive ceramics are distinguished as follows. That is, a conductive cermet is a binary composition in which a particulate conductive component (for example, particles of metal powder) and an insulating ceramic material are mixed. In contrast, conductive ceramics consist of conductive minerals. These conductive minerals include certain metal carbides, nitrides, borides, silicides, and oxides, such as ZrC, WC, SiC, TiN, Mo2N, and TaN.
, NbN, 5CB2. TiB2. MoB, ZrB,
(Y, Ca) (Cr, Mn)03. (La.

5r) (Cr, Mn)03. It is a single substance such as or a mixture with other minerals.

Conductive cermets require less severe sintering conditions than conductive ceramics, and can provide a variety of electrical properties (
They are technically and economically advantageous in many applications because of their good compositional control during the component interrogation step) and, in some cases, because of the low cost of the starting materials.

[Conventional technology]

A large number of conductive ceramics and conductive cermets useful in the field of ignition and heating are known in the art. Some of these compositions are mixtures of insulating and conductive ceramic or metal particles and can therefore be thought of as a midpoint between conductive cermets and conductive ceramics. .

For example, the ignition spark plug disclosed in JP-A-150,579/1980 (automotive parts) has a center electrode made of Al20a, TiO2, 'riC, CrzOa,
NbC, WC, Sic, TaC.

It is made of a cermet composition containing insulating or conductive ceramics such as Mo5iz and metal particles such as Cr, Co, Fe, AI, W, and Mo.

In the ignition spark plug disclosed in JP-A-44391/1986 (NGK), a part of the center electrode is A 1
203. T i 02. Cr20s, Z to
2. S to, Y2O3, Lazy3. Nio.

Ceramics selected from Cab, TiC, SsC, B4C, CrzC2°WC, TiN, AIN, BN, and MO5+2, and Cr, Co, and Mo.

It is made of a cermet obtained by sintering a composition containing metal particles selected from Mn, Pt, and Pd.

The conductive cermet composition described in JP-A-44393/1986 (NGK) is somewhat similar to the one in the above publication, but since the metal particles are anisotropically distributed, it is difficult to form a ceramic matrix. The concentration of metal particles inside changes in a predetermined pattern depending on the direction.

No. 4,427,915 also describes conductive cermet compositions for ignition spark plugs. These compositions are very similar to those described above, but
The metal particles further include noble metals such as A g *Ru+Rh and Au.

In addition to the above publications, EP-
A-171,153 (General Motors), US-
A-4,475,029 and 4,633.064 (
Both Nippondenso), US-A-4,528,121 (
Hitachi), US-A-4,205,363 (CARBOR
UNDUM).

[Problem to be solved by the invention]

Although the electrically conductive cermets and electrically conductive ceramics of the above-mentioned prior art for making spark plug electrodes or incandescent bodies have advantages, it is desirable to further improve their performance, especially their resistance to hot corrosion and electrical erosion due to spark discharges. is desired.

[Means to solve the problem]

The above problem is solved according to the present invention by containing an alumina-based ceramic and 30 to 5 Qwt% of metal particles, the metal particles being partially oxidized, and the weight ratio of oxygen to metal in the particles being The problem is solved by a conductive cermet composition with excellent corrosion resistance and erosion resistance of 0.3 to 10%.

The metal oxide of the metal particles is preferably in the form of a film attached to at least a portion of the surface of the particles.

It is desirable that the conductive cermet composition of the present invention further contains 2 to 20 wt% of a silicate base glassy phase. Moreover, the above glassy phase includes SiO2, MgO, Cab
, Nano, Fe1Os+T iOz, Z n O,
P b 02. and B20. It is desirable to contain at least one oxide of the following.

Further, it is desirable that the alumina-based ceramic is at least partially made of mullite represented by the formula 3A1203.2SiOz.

The conductive cermet composition of the present invention comprises an alumina-based ceramic and a 30-
6Qwt% of metal particles, and has significantly improved high temperature corrosion resistance and spark discharge erosion resistance. The main improvement of the present invention over the prior art is that the metal particles described above have an oxygen/metal weight ratio of 0.3 to 10%.
(that is, 0.003 to 0.1).

The exact reasons why the presence of partially oxidized metal particles in the compositions of the invention significantly improves performance are not yet understood in detail (and are not necessary to clarify the scope of the invention). When a metal oxide is attached to the metal of the particle, preferably in the form of an oxide film or layer on at least a portion of the particle surface, the strength of the cermet increases.
It has been found that the resistance to wear and tear caused by friction, corrosion, and electrical erosion is significantly improved. That is, particulate metal oxides, especially in the case of chromium, have a ceramic phase (A
1203), which significantly lowers the discontinuity barrier between the metal particles and the ceramic phase. This oxide layer, which adheres to the metal particles and is interposed between the metal particles and the ceramic matrix, therefore increases the compatibility of the cermet as a whole and provides suitable reinforcement and good mechanical properties.

The particle size or mesh size of the metal powder of the cermet composition of the present invention may be about 0.5 to 50 μm, and 1
A particle size of around μm is desirable. Suitable metals for making this powder are, of course, the conventionally known metals commonly used for making cermets, such as Cr, Ni, C
o, Fe, Mo, W, Ti, etc.

In order to improve the compactness and sintered state of the compositions of the invention, the ceramic phase can be supplemented with other minerals, especially 1-2 Qwt.
% of a glassy silicate phase. In addition to containing 3iCh, this glassy phase contains MgO, Cab
, 20. to BaO, Na 20 +. F e20s
+ T j 02. Z r Oa, Z nO,
It optionally contains one or more selected from Pboa, B2O3, and other oxides conventionally used to make glass. In one aspect of the present invention,
The ceramic of the cermet composition is at least partially mullite, i.e. having the chemical formula 3A1*Oa.2Si02
It consists of an aluminosilicate mixture represented by

In the method of manufacturing the cermet composition of the present invention,
Preparation is first carried out by adding forming additives and sintering additives to a mixture of each component (precursor compound) of the cermet. The resulting mixture is molded into a "green" body that can be safely handled and heated without mechanical abrasion. This "green" body is then heated to a high temperature (for example, around 1200-1600°C). Sintering into an article made of the desired electrically conductive cermet composition.

There are several methods for forming oxides on the surface of metal particles. The simplest method is to use pre-oxidized metal particles as the cermet component. Another method is
This involves oxidizing the particle surface during sintering. In the latter case, the oxygen required for this oxidation can be obtained either by additionally adding an oxidant to the preforming mixture, which is later decomposed by the heat during sintering to release its oxygen, or by containing oxygen or an oxygen-containing gas. It is supplied by sintering in an atmosphere of

Conventional methods are used to form green bodies made of or containing the cermet compositions of the present invention. For example, a pressure (isotropic or anisotropic) of about 1 to several T/cm2 is applied to a mold containing the above-mentioned blended mixture. Dip coating methods can also be used if it is necessary to deposit a layer of conductive cermet on a substrate. In this case, the substrate is immersed in a slurry containing cermet components for a predetermined period of time, taken out, dried, and then sintered.

Well-known conventional sintering methods can also be used to sinter the shaped bodies. For example, the green body can be heated in a furnace under a compositionally controlled atmosphere, such as a neutral gas (argon, helium) or hydrogen, nitrogen, or a mixture thereof. If it is desired to surface oxidize the metal particles of the composition during sintering, sintering is performed in an atmosphere having an oxygen partial pressure. The oxygen partial pressure can be controlled by introducing a gas that releases oxygen, for example a 20/80 (v/v) H2/8°0 mixture.

This oxidation can also be carried out independently of the sintering step at a lower temperature, for example around 500-1200°C.

If the thermal decomposition of the oxidizing agent added to the formulation mixture causes surface oxidation of the metal grains of the cermet, this oxidizing agent
It can be selected from organic oxidants which thermally decompose at the sintering temperature or mineral oxidants which release oxygen during sintering. Examples of oxidizing agents include metal oxides, metal hydroxides (e.g. A
I (OH) 3, Fe (OH) s, Na2
0-), or salts (e.g. nitrates, oxalates,
carbonates, titanates, etc.). The weight proportion of the above oxidant in the formulation mixture basically depends on the degree of oxidation of the metal particles required, but is generally about 0.5-20w.
It is in the range of t%.

When preparing the cermet of the present invention using a metallic powder that has been previously oxidized (hereinafter abbreviated as "pre-oxidized"), this pre-oxidized powder can be obtained by various oxidation methods. For example, to surface oxidize particles of chromium powder, the chromium powder can be contacted with a heated oxygen stream or a dichromate solution. Generally, the method of surface oxidizing metal powder is well known, so a more detailed explanation will not be given here. The degree of oxidation of the metal particles used in the present invention, that is, the weight ratio of oxygen in the metal particles is 0.
It is desirable that it is 3 to 20%.

In the following, the invention will be explained in more detail by means of examples with reference to the accompanying drawings.

〔Example〕

Example 1 (Invention) The following ingredients were kneaded together with 400 g of alumina peas having a diameter of 12 mm in an alumina ball mill for 24 hours.

Kneading component 1 Alumina powder 59.5g (particle size: approx. 1 μm 0 approx. 80%
-40.5 g of chromium powder (temperature: 300°C, oxygen content: about 1~1%)
1.5 g of fish oil (dispersant) 50 g of butanol (solvent) 8 g of camphor (binder) The resulting mixture was dried and sieved through a 300 μm screen. This powder was compression molded into a disk (diameter 15 mm, thickness 3 mm) under a pressure of 1° IT/cm 2 .

After sintering the disk at 1550℃ for 2 hours, it was made into a 2×2×5mm
Cut into small blocks.

In order to test the electrical properties and durability of the cermet composition, the above rod-shaped sample was fixed to the center electrode of an ignition spark plug with conductive silver epoxy adhesive and the distance between the electrodes = 1.
mm), and in this state, spark discharge (plasma discharge, 1000 VDC, 1 Joule/spark) was performed on the rod-shaped sample 15,000 times. Next, the rod-shaped sample was placed in a solvent (
After removal, washing and drying with (for example, trichlorethylene), the weight loss was measured. Next, the above weight loss was converted into volume loss using the actual cermet density of 4.15 g/am3 measured in a conventional manner. Table 1 shows the volume loss caused by Spark Electrician Lotion. The values in the table are expressed as percentages of the volume loss of conventional nickel alloys of conventional ignition spark plugs tested under the same conditions.

To test hot corrosion resistance (e.g. to approximate hot corrosion in a combustion chamber), 0.2% (v/V) or 3
The bar sample was heated at 1000° C. for 24 hours in an atmosphere obtained by burning methane containing g/m3 H2S. The test results are also shown in Table 1 in terms of relative weight gain. This weight increase is due to metal oxidation.

Example 2 (Comparative Example) The same operation as in Example 1 was performed. However, in the preparation ingredients, unoxidized chromium powder of the same particle size was used instead of pre-oxidized chromium powder. The same tests as in Example 1 were conducted on cermet samples obtained by sintering this comparative composition, and the results are summarized in Table 1.

Example 3 (Comparative Example) The same operation as in the above example was carried out using unoxidized chromium powder. However, the silicate glass phase is not used, and A1 containing a portion of MgO (4 wt%) is used to replace it.
．． 03 was used.

That is, the other components were the same as in Example 1, the operating conditions were the same as in the previous examples (Examples 1 and 2), and the following components were different.

Alumina 61°9g Pure chromium powder 38.1g Mg0 0.25g With the above composition, sintering was performed at 1550°C in the same manner as on each side above,
The resulting cermet sample was tested as in Example 1.

The results are summarized in Table 1.

Example 4 (Invention) The cermet composition of Example 1 was used to produce an ignition spark plug. This spark plug is schematically shown in FIG.

This spark plug has a conventional metal shell 1 integrated with a ground electrode 4, and includes an insulating member 2 (made of vitrified alumina ceramics) and the following elements within the hollow shaft core of this member. That is, the terminal rod 9
, a conductive gasket 7, an interference suppressor resistor 8, and a central electrode 6 made of a cermet composition according to the invention. This electrode 6 is obtained by molding and sintering in a conventional manner under the conditions described in Example 1. The several plug parts described above were assembled according to conventional spark plug production methods.

Testing of the spark plug described above in an automobile engine has shown that it has a significantly longer service life than a standard comparison plug with a nickel alloy center electrode and a comparison plug with a conductive cermet center electrode according to the prior art. confirmed.

As a variant, it is also possible to weld or braze to the ground electrode 4 of the plug shown in FIG. 1 a small rod or cap 5 made of the same cermet composition as the central electrode. In this case, the service life of the plug will be even longer.

Example 5 (Invention) A glow plug for a diesel engine was produced using the same cermet composition as in Example 1. FIG. 2 schematically shows such a plug.

This plug has a metal outer shell 12 provided with a threaded portion 13 (as in the case of the plug in Example 4), and this shell is a hollow insulating member formed by two members 14 and 15 axially abutted against each other. surrounding. Insulating member 1
A metal terminal rod 16 in the hollow part 4 contacts the inner annular part of the part 15 and is retained within the part 14 by a large cross-section overhanging bottom part 17. The hollow cylindrical insulating member 15 is coated on its outer and inner surfaces with a cermet layer 18 having a composition according to the invention. As shown in FIG. 2, this layer 18 seamlessly covers the entire free surface of member 15, including the outer terminal surface 19 of insulator 15, forming a continuous electrical resistor. Both ends of this electrical resistor are electrically connected to the metal sheath 12 at the bottom 17 of the rod 16 and its outer periphery 20, respectively.

When a suitable voltage is applied between the sheath 12 (ground terminal) and the terminal rod 16 (plug terminal), a current flows through the layer 18, heating it to an incandescent state. Due to the high temperature corrosion resistance of the cermet composition used to form this layer 18, the use of this glow plug in a diesel engine significantly increases its operating life.

Preferably, a dip coating method is used to coat the surface of the hollow insulator 15 with the layers 18, 19, 20. In the formulation of Example 1, the following dispersants and binders are used in place of the solvent. That is, trichlorethylene:ethanol=1
= 50 g of a mixture of 1 (solvent), 5-10% of polyethylene butyral, polyethylene glycol: dioctyl phthalate) = 6% of a mixture of 1:1, and 0.5-1%
(7) FLUORAD (manufactured by 3M) (surfactant). The amount of solvent can vary and depends on the viscosity imparted to the slurry and ultimately on the thickness of the layer coated on the cermet 18.19.20. Under the above-mentioned normal conditions, a layer having a thickness of about 100 to 300 μm after drying is formed by one immersion treatment. Of course, if the coating thickness is desired to be thicker than this, the dipping treatment may be repeated after drying the first layer. The layer deposited by the dipping process is dried in air (hot or room temperature) to evaporate the solvent and then sintered at about 1550° C. under the conditions described in Example 1. Assemble the glow plug parts as usual. That is, the rod 16 is inserted into the insulated member 14 and the bottom 17 of the rod is pressed against the member 15 (the member 15 has previously been provided with a layer of sintered cermet) to ensure that the cermet layer and the bottom 17 are in good contact with each other. A soft conductive gasket, such as an annealed copper washer, can be used to make electrical contact (for better contact), and the resulting assembly is fitted integrally within the shell 12.

Example 6 (Invention) A cermet composition was prepared by kneading the following ingredients in a ball mill for 24 hours using 200 g of alumina balls.

Mullite powder 28.5g oxidized (5%
) chromium powder 21.5g fish oil (dispersant)
0.75 g 1:2 mixture of tert-butanol and petroleum ether (solvent) 50 g Camphor (molding binder) 4.0 g The above formulation was shaped into disks as in Example 1, and the resulting green disks were Sintering was performed at 1480°C for 2 hours in argon. The sintered compositions were tested exactly as on each side. Results first
Shown in the table.

Example 7 (Invention) The same operation as in Example 6 was performed. However, they differ in the following points.

Talc powder (4SiCh・3M g O・H
2C) was added, and the sintering temperature was set at 1450°C. The test results are summarized in Table 1.

Example 8 (Comparative Example) The same operation as in Example 7 was performed. However, the only difference is that pure chromium powder is used instead of oxidized Cr powder. The results are summarized in Table 1.

Table 1 (Genmuji: Invention, #: Comparative example (vehicle weight) Value for standard nickel alloy (20rn
Relative value with 100% erosion (volume loss of 0.33 mm' for a sample of m').

The results in Table 1 show that the use of oxidized metal powder in the conductive cermet or the additional presence of a glassy phase improves the electrical erosion properties and corrosion resistance.

FIG. 3 shows the microstructure of the cermet of Example 7 according to the present invention, and FIG. 4 shows the microstructure of the cermet of Example 8, which is a comparative example.

In the cermet of the present invention, the metal grains are tightly incorporated into the ceramic phase, and no clear boundary line is observed.

In contrast, in the comparative sample of Example 8, a clear boundary line is observed between the metal grains and the ceramic phase.

Example 9 (Invention) The following ingredients were mixed into 250 m using 400 g of zircon balls.
A cermet composition was prepared by kneading in one jar for 48 hours.

A1.0 powder (same as Example 1): 61.9 g Chromium powder (particle size ~1 μm, containing approximately 3.5 wt% Cr1es): 40.47 g Fish oil: 1.5 g Tertiary butanol/petroleum ether (9/ 3): 0 g Camphor: 8 g The resulting mixture was dried by evaporation under reduced pressure and then sieved through a 300 μm screen.

Next, it was formed into the shape of the spark plug electrode described in Example 4 and sintered at 1560°C.

A spark plug using the obtained cermet as the center electrode was installed in an RVI Bujo engine with 98ROM fuel, 0
．． A test was conducted using 15gPb/Il. Table 2 shows the amount of erosion (mm) of the electrode versus time (hr).

A comparative sample was prepared in the same manner as above. However, the amount of chromium oxide in the chromium powder used was zero or negligible (POUDNET chromium powder). The results are also shown in Table 2.

Table 2 A cermet electrode was prepared as in Example 9 using the following formulation composition.

A11o3 (same particle size as Example 1): 53.36g Chromium powder (3.5% Oa as Cr203): 46.64g The above formulation (as in the previous example) 1.5g fish oil 1 tertiary BuOH/petroleum ether 24 with 200 g of zircon balls in the presence of 50 g of a 1:3 mixture and 8 g of camphor
Kneaded for hours. The resulting mixture was treated as in Examples 1 and 8, then under milder conditions than in Example 8 (lower temperature, shorter time).
Sintered (denoted by sample symbol 3°502).

Separately, a cermet composition was produced using the same blending ratio and operating conditions as above, but using chromium powder with an oxygen (Cr203) content of 5 wt%. This sample is indicated by symbol 602.

Both samples were tested as in Example 9. The results are shown in Table 3.

Table 3 Electrode erosion amount (mm)

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view schematically showing an ignition spark plug for a gasoline engine in which some of the parts are made of conductive cermet according to the invention, and FIG. FIG. 3 is a cross-sectional view schematically showing a glow plug for a diesel engine made of the conductive cermet according to the present invention, FIG. 3 is a micrograph showing the metallographic structure of the cermet composition according to the present invention, and FIG. 2 is a micrograph showing the metal structure of a conventional cermet composition for comparison. 1: Metal shell, 2: Insulating member, 4 Nigerland electrode, 5: Rod or cap, 6: Center electrode, 7: Conductive gasket, 8: Interference suppressor resistor (interference suppressor resistor), 9: Terminal rod , 12: Metal outer shell, 13: Threaded portion, 14.15: Insulating member, 16: Metal terminal rod, 17: Overhanging bottom, 18: Cermet layer, 19: External terminal surface.

Claims (1)

[Claims] 1. It contains an alumina-based ceramic and 30 to 60 wt% of metal particles, the metal particles are partially oxidized, and the weight ratio of oxygen to metal in the particles is 0.
A conductive cermet composition with excellent corrosion resistance and erosion resistance of 3 to 10%. 2. The conductive cermet composition of claim 1, wherein the metal oxide of the particles is in the form of a coating attached to at least a portion of the surface of the particles. The conductive cermet composition according to claim 2, further comprising 3.2 to 20 wt% of a silicate base glassy phase. 4. The glassy phase includes SiO_2, MgO, and CaO.
, Na_2O, Fe_2O_3, TiO_2, ZnO,
The conductive cermet composition according to claim 3, containing at least one oxide of PbO_2 and B_2O_3. 5. The ceramic is at least partially of the formula 3Al
The conductive cermet composition according to claim 2, comprising mullite represented by _2O_3.2SiO_2. 6. The following steps (1) to (3): (1) mixing the components of the conductive cermet composition or their precursors with a forming additive and a sintering additive at a predetermined mixing ratio; (2) the obtained forming the blended mixture into a green compact; and (3) sintering the green compact at a high temperature to form a desired cermet composition, and partially oxidizing the metal particles in the mixing process. Process (1)
Claim 1 carried out before or during the above sintering step (3).
A method for manufacturing the conductive cermet composition described. 7. 7. The method of claim 6, wherein partial oxidation of the metal particles is carried out during the sintering step, and the oxidation is carried out by oxygen released by decomposition of the oxidant of the mixture during heating at the sintering temperature. 8. 7. The method of claim 6, wherein partial oxidation of the metal particles is performed during the sintering step, the sintering is performed in an oxidizing atmosphere, and the oxidation is performed by an oxidizing reaction of the oxidizing atmosphere. 9. A method for manufacturing a spark or plasma discharge spark plug for a gasoline engine, comprising using the conductive cermet composition according to claim 1 and forming at least a portion of one or more electrodes of the spark plug by the method according to claim 6. 10. Using the conductive cermet composition according to claim 1,
A method for manufacturing a glow plug for a diesel engine, comprising forming a plug portion acting as a glow element by dip-coating an insulating ceramic attractant with a layer of cermet having the proportions set forth in claim 6, and then sintering said layer.