CH676525A5 - - Google Patents

Description

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CH 676 525 A5

Description

The present invention relates to compositions of electrically conductive cermets intended for the manufacture of ignition and heating devices, in particular spark or plasma discharge candles for petrol engines or incandescent candles for diesel or external combustion engines.

The difference between electroconductive ceramics and electroconductive cermets should be noted here. Electroconductive ceramics are mineral compounds of the carbide, nitride, boride, silicon-hard type or electrically conductive oxides, for example ZrC; WC; SiC; A; M02N; TaN; NbN; SÇB2; TÌB2; MoB; ZrB; (Y, Ca) (Cr, Mn) O3; (La, Sr) (Cr, Mn) O3; Doped BaTiOa. Electroconductive cermets are binary compounds combining an insulating ceramic and an electroconductive element (for example, a metallic powder).

Compared to electroconductive ceramics, cermets have certain technical and economic advantages, in particular a great flexibility of control of the electrical properties (through the formulation) as well as a sintering temperature and costs of raw materials which can, in certain case, be lower than that of electrically conductive ceramics.

The state of the art already includes numerous documents relating to compositions of electrically conductive cermets and mixed compositions containing both insulating and conductive ceramics and metallic particles, such compositions being applicable to spark plugs.

Thus, published patent application JP-A150 579/1980 (JIDOSHA BUHIN) describes spark plugs in which the central electrode consists of cermet compositions containing insulating or conductive ceramics such as AI2O3, TÌO2, TiC , CfeOs, NbC, WC, SiC, TaC, M0SÌ2 and metallic particles, for example Cr, Co, Fe, Al, W, Mo and other similar metals.

Document JP-A 44 391/1986 (NGK) discloses a spark plug whose central electrode consists partly of cermets obtained by sintering compositions containing a ceramic chosen from AI2O3, TÌO2, CteOs, Zr02, SÌO2 , Y2O3, La20s, NiO, CaO; TiC, SiC, B4C, Cr2C2, WC, UN, AIN, BN, and M0SÌ2, as well as metallic particles chosen from Cr, Co, Mo, Mn, Pt and Pd.

- Document Jp-A 44 393/1986 (NGK) discloses electroconductive cermet compositions similar to those described above but in which the metallic particles are distributed anisotropically, that is to say that their concentration in the matter varies, directionally, according to a certain profile.

Document US-A 4,427,915 (NGK) also describes compositions of electrically conductive cermets intended for spark plugs very similar to the above compositions, the choice of metals in particles, however, still including noble metals such as Ag, Ru , Rh, Au and the like.

In addition to the documents cited above, one can also mention the existence of the following documents concerning spark plugs in which electrically conductive cermets and ceramics are involved: EP-A 171 153 (GENERAL MOTORS); US-A 4,475,029 and 4,633,064 (NIPPODENSO); US-A 4,528,121 (HITACHI); US-A 4,205,363 (CARBORUNDUM).

Despite the advantages of known cermet and ceramic compositions for the manufacture of electrodes and incandescent bodies of spark plugs, it was desirable to further improve some of their properties, in particular their resistance to hot corrosion and their resistance to erosion by electric shock.

The present inventors have developed cermet compositions defined in claim 1, which constitute an important step in the desired direction.

Indeed, the presence of oxide of the metal of the particles, preferably in the form of a film or film covering at least a part of the surface of these, considerably improves the behavior of the cermet, in particular with respect to the corrosion and erosion phenomena. The oxide of the metal of the particles has, in particular, in the case of chromium, a property of solubility in the ceramic phase (AI2O3) and the presence of this oxide film attached to the metallic grains and interposed between them and the ceramic matrix provides the whole with improved cohesion and thus strengthens the mechanical resistance of the cermet.

The particle size of the metal powders of the present cermet compositions can be of the order of 0.5 to 50 (im, a particle size close to 1 μm being preferable. The metals taken into account are those used generally in cermets, in particular those described in the state of the art; mention may be made, for example, of Cr, Ni, Co, Fe, Mo, W, Ti, etc.

To improve the densification and the sintering conditions of the present compositions, a silicate glassy phase can be added to the ceramic phase, in a proportion of 1 to 20% by weight. This glassy phase contains SÌO2 and, if desired, one or more oxides chosen from MgO, CaO, BaO, Na20, K2O, F62O3, TiO2, Z1O2, ZnO, Pb02, B2O3, and other oxides usually used in the manufacture of glasses. In one embodiment, the ceramic of the cermet consists, at least in part, of Mullite, that is to say an alumina silicate 3A12O3.2SÌO2.

The process for manufacturing cermet compositions according to the invention consists in first preparing by mixing a formulation essentially comprising the components of the cermet (or of the precursors thereof) as well as molding and sintering additives. Then we form or mold this mixture

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CH676 525 A5

so as to produce an article or object molded in "green" capable of then being handled and heated without deterioration. Then the “green” molded article is then sintered at high temperature (for example around 1200-1600 ° C.) so as to produce the article consisting of the desired cermet composition.

As regards the formation of the oxide on the surface of the metallic particles, one can proceed in various ways; either pre-oxidized particles are used, or surface oxidation is carried out during sintering. In the latter case, the oxygen necessary for this oxidation can come either from an additional oxidizing ingredient incorporated into the mixture of the formulation before molding and which decomposes on sintering heat by oxidizing the metal particles, or from the atmosphere. in which the sintering is carried out, the latter then containing oxygen.

The techniques used for forming or molding articles or objects made up or comprising cermets according to the invention are traditional. Thus, the mixture of the formulation can be placed in a mold which is subjected to a pressure (isostatic or other) of the order of 1 to several T / cm2. One can also operate by dipping in a suspension of the formulation in a liquid, in particular when one wishes to produce objects comprising a layer of cermet composition.

For the sintering of molded objects, traditional techniques known to those skilled in the art are also used, for example heating the object in an oven under a controlled atmosphere, for example a neutral gas (argon, helium) or under nitrogen or hydrogen. When it is desired to cause the surface oxidation of the metal grains of the composition during sintering, the latter is carried out under an atmosphere containing a partial oxygen pressure which can be adjusted by adding a gaseous mixture H2 / H2O 20-80% ( GO/). This oxidation can also be carried out at a temperature lower than that of sintering, in particular around 500-1200 ° C.

When the metal grains are oxidized by thermal decomposition of an oxidizing agent incorporated in the mixture, one can choose this one among the organic oxidizing agents (which volatilize at the sintering temperature) or the mineral oxidizing agents which release l oxygen during sintering. As oxidants, there may be mentioned metal oxides, hydroxides and peroxides, for example AI (OH) 3, Fe (OH) 3, Na2O2 as well as salts, for example nitrates, Oxalates, carbonates, etc. The proportion by weight such oxidants in the formulations vary, preferably between 0.5 and 20% by weight and essentially depends on the rate of oxidation that one wants to obtain. When using preoxidized metal powders in cermet formulations, these can be obtained by the usual oxidation means. For example, to surface oxidize the particles of a chromium powder, it can be circulated in a stream of hot oxygen or in a bichromate solution. In general, the techniques for surface oxidation of metal powders are well known to those skilled in the art. The oxidation rate of the metal powders used in the present invention is favorably between 0.3 and 20%.

The examples which follow, for the understanding of which reference will be made to the appended drawing, illustrate the invention in more detail.

FIG. 1 schematically represents a spark plug for gasoline engines, certain components of which are produced in cermets of composition according to the invention.

FIG. 2 schematically represents an incandescent candle for diesel engines, certain components of which are produced by means of composition of cermets according to the invention.

Example 1

In a alumina ball mill containing 400 g of alumina balls 12 mm in diameter, 59.5 g of alumina powder (particle size approximately 1 μm) containing 10% by weight of a vitreous phase was ground at about 80% of SÌO2, the remainder being MgO, CaO and NaaO, 40.5 g of chromium powder (previously oxidized in an oxygen stream at 300 ° and containing, by weight, about 1-3% of oxygen ), 1.5 g of fish oil (dispersant), 50 g of butanol (solvent) and 8 g of camphor (binder).

The mixture was then dried and sieved through a 300 µm sieve; then the powder was molded into discs (diameter 15 mm, height 3 mm) under a pressure of 1.1 T / cm2.

These discs were sintered for 2 h at 1550 ° C. under argon, after which they were cut (by means of a diamond saw) into bars or blocks of 2 × 2 × 5 mm. For the measurements, a cermet rod according to the invention was fixed by means of an epoxy silver glue (conductive glue) to the central electrode of a spark plug (interelectrode distance 1 mm) and this pellet was subjected to a succession of 150,000 sparks (plasma discharge 1000 VDC; 1 joule per spark). After which the tablet was peeled off (by dissolving the adhesive in a suitable solvent, for example trichlorethylene), the latter was dried and the weight loss was determined by weighing. This weight measurement was converted into a volume value taking into account the actual density of the cermet, (4.15 g / cm 3) measured by the usual means. The result of the loss of volume by EDM is shown in Table 1 below expressed in% relative to that of a nickel alloy used for the electrodes of a conventional spark plug.

For the resistance to thermal corrosion measurements, the pellets were subjected to 24 h heating (1000 ° C.) under an atmosphere obtained by combustion of methane at 0.2% H2S (V / V), that is to say - say 3 g / m3. The results are also shown in Table 1 in terms of relative increase in weight, this increase coming from oxidation of the metal.

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CH676525 Ä5

Example 2 (comparative)

The manipulations described in Example 1 were repeated, replacing, in the formulation, the oxidized chromium powder with the ordinary chromium powder (non-oxidized). The cermet samples obtained by this control formulation were tested identically and the results also appear in Table 1 in terms of relative increase in weight, this increase coming from unoxidation of the metal.

Example s (comparative)

The manipulations of the preceding examples were repeated using a non-oxidized chromium powder, and omitting the glassy phase which was replaced by an additional quantity of Al2O3 and a portion of MgO. Thus, in addition to the other ingredients and the operating conditions identical to those of Examples 1 and 2, the following were used:

Alumina 61.9 g Pure Cr powder 38.1 g MgO 0.25 g

The cermet compositions sintered at 1550 ° as before were tested as indicated in Example 1. The results are shown in Table 1.

Example 4

Using a cermet composition from Example 1, a spark plug was produced according to the diagrammatic representation of FIG. 1.

This candle comprises an integral traditional metal casing 1 with an external electrode 4, an insulating body 2 (made of vitrified alumina ceramic) and, in an axial hollow part of this insulating body, the following elements: a contact rod 9, a conductive joint 7, a deworming resistor 8 and a central electrode 6 produced in a cermet according to the invention. This electrode 6 was obtained by molding and sintering according to the usual means under the conditions described in Example 1. The various elements of the spark plug were assembled according to the usual techniques for manufacturing spark plugs.

This type of candle, used in an automobile engine, has demonstrated a significantly greater longevity than a standard candle of the same construction in which the central electrode was either in the usual nickel alloy or in cermet depending on the state. of technique.

Alternatively, the ground electrode 4 of the spark plug, shown in Figure 1 may be provided by welding or brazing a pellet 5 with a cermet composition identical to that of the central electrode. In this case, the life of the candle is still greatly extended.

Example 5

An incandescent candle intended for diesel engines was produced, using a formulation identical to that of Example 1. Such a candle is represented schematically alafig 2.

This candle comprises a metal casing 12 comprising (like the candle of Example 4, moreover) a threaded zone 13, this casing surrounding a hollow insulating body composed of two axially contiguous parts 14 and 15. The hollow part of the insulator 14 contains a metal rod 16 in contact with the internal annular zone of the part 15 and retained in the part 14 by its base 17, enlarged, provided with a shoulder. The hollow cylindrical insulator 15 is covered, both on its internal lateral face and on its external lateral face, with a layer of cermet 18 according to the invention. This layer is extended, without transition, through the external end face 19 of the insulator 15 as indicated in the drawing and therefore constitutes a resistor whose respective ends are in electrical contact, on the one hand 18 with the base 17 of the rod 16 and, on the other hand the other 20 (the outer peripheral zone of the layer 18), with the metal envelope 12. By applying a suitable potential between this envelope 12 (ground pole) and the rod 16 (pole of the candle), an electric current is obtained in the layer 18 which brings it to incandescence. Due to the high resistance to thermal corrosion of the cermet composition used to make this layer 18, the present incandescent candle demonstrates considerable longevity when used on a diesel engine.

To carry out the deposition of the layer 18,19,20 on the hollow insulating body 15, it is preferably carried out by dipping. The formulation of Example 1 is used containing, by weight, instead of the solvents, dispersants and binders indicated: 50 g of a 1: 1 trichlorethylene-ethanol mixture (solvent); 5-10% of polyvinyl butyral (binder); 6% of a 1: 1 mixture of polyethylene glycol-dioctyl phthalate and 0.5-1% of FLUORAD (3M) (Surfactant). The quantity of the solvent depends on the viscosity which it is desired to give to the dispersion and, by 4

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CH676 525 A5

so much, the thickness that we want to give to the layer 18,19,20. Under the usual conditions indicated above, a quenching operation provides a layer of approximately 100-300 µm after drying. One can, of course, repeat the quenching operation to obtain thicker layers. After the layer has dried, it is sintered at 1550 ° C. under the conditions described in Example 1.

For the assembly of the components of the candle, one proceeds traditionally, in the sense that one introduces the rod 16 in the insulating body 14, one presses the base 17 of the rod against the body 15 provided with its layer of sintered conductive cermet so as to make contact between the latter and said base 17 (an electroconductive joint can be used if desired, for example a soft copper ring), and the whole is crimped in the casing 12.

Examples

A cermet formulation was prepared by grinding for 24 h with 200 g of beads, as in the previous Examples, the following ingredients:

Mullite powder

28.5g

Oxidized chromium powder (5%)

21.5g

Fish oil (dispersant)

0.75 g

Fert.butanol-petroleum ether mixture

50g

(1/2) (solvent)

Camphor (molding binder)

4.0g

Using this formulation, disks were molded as in Example 1 and sintered for 2 h at 1480 ° C under argon. The same tests and measurements were then carried out as in the previous Examples.

(see Table 1)

Example 7

The manipulation of Example 6 was repeated, with the following differences: addition to the formulation of 0.85 g of talcum powder (4SiO2.3MgO.H2O); sintering temperature 1450 ° C. The test results are shown in Table 1.

Example s (comparative)

The manipulations of Example 7 were repeated, the only difference being the replacement of the oxidized chromium powder with pure chromium powder. For the results see Table 1.

Table 1

Electrical properties and resistance to corrosion and erosion of the cermet compositions described in Examples 1-3 and 6-8

Example (**) Density Resistivity Comparative resistance (*) to

(g / cm3) (Q.cm) erosion (% V corrosion (%)

1 (T)

4.15

0.6

0

7.9

2 (C)

4.07

2.9

86

7.9

3 (C)

4.37

49

137

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6 (T)

3.76

3.4

23

-

7 (T)

3.72

2.2

13

7.9

8 (C)

3.4

0.15

173

-

(*) Compared to a standard nickel alloy sample considered to have 100% erosion (0.38 mm3 loss for a 20 mm3 bar).

(**) T a test example; O = comparative example.

It can be seen from the test results collected in Table 1 that the use of an oxidized metal powder, as well as the presence of a glassy phase, improves the properties of resistance to electroerion and thermal corrosion. Furthermore, microscopic examination of the structure of the cermet of Example 7 as well as that of the cermet of Example 8 shows that in the cermet according to the invention, the grains

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CH676 525A5

The tiles are intimately incorporated into the ceramic material, without any significant transition, whereas in the control sample of Example 8, this transition is clearly visible.

Claims (10)

Claims 1. Composition of electroconductive cermet having good corrosion and / or erosion resistance, consisting of a ceramic phase based on alumina and 30-60% by weight of metallic particles, characterized in that the particles metals are in the partially oxidized state, the oxygen / metal weight ratio thereof being from 0.3 to 10%. 2. Composition according to claim 1, characterized in that said oxide forms a film on at least part of the surface of the metal particles. 3. Composition according to claim 2, characterized in that it contains, in addition, from 2 to 20% by weight of a glassy phase based on silicates. 4. Composition according to Claim 3, characterized in that the vitreous phase contains SiOfe and at least one of the following oxides: MgO, CaO, NaaO, Fe203, HO2, Z1O2, ZnO, Pb02, B2O3. 5. Composition according to claim 2, characterized in that the ceramic phase consists, at least partially, of mullite, that is to say 3AI2O3.2SÌO2. 6. Method for the manufacture of electroconductive cermet compositions according to claim 1, characterized in that (1) a formulation is prepared by mixing containing the elements of the composition or their precursors as well as molding and sintering additives; (2) the mixture is formed or molded so as to produce green molded articles or parts; and (3) these green articles are sintered at high temperature so as to obtain the desired cermet composition, the partial oxidation of the metallic particles being carried out either before said mixing or during sintering. 7. The method of claim 6, wherein the partial oxidation of the metal particles is carried out during sintering, characterized in that it is caused by an oxidant, incorporated in the mixture, which decomposes at the sintering temperature and releases its oxygen. 8. The method of claim 6, wherein the partial oxidation of metal particles is carried out during sintering, characterized in that it is carried out in an oxidizing atmosphere so as to achieve, thanks to this atmosphere, said oxidation partial. 9. Use of cermet compositions according to claim 1, for the manufacture of spark plugs or plasma discharge candles for petrol engines, characterized in that at least one of the electrodes thereof is produced according to the following steps: (1) a formulation is prepared by mixing containing the elements of the composition or their precursors as well as molding and sintering additives; (2) the mixture is formed or molded so as to produce said electrode; and (3) this electrode is sintered at high temperature. 10. Use of cermet compositions according to claim 1, for the manufacture of incandescent candles for diesel engines, characterized in that the portion of the latter to be brought to incandescence is produced by covering, by dipping, an insulating ceramic core. a layer of said composition, and then sintered it at high temperature. 6