Process for the preparation of a sintered ceramic spark plug
Process for the preparation of a ceramic wherein: 50% to 75% by weight of a compound intended to form a conductive phase, and 25% to 50% by weight of one or more materials allowing the formation of insulating phases composed of silicon nitride and modified silicon oxynitride are mixed in a receptacle containing a liquid after a thermal treatment leading to sintering of the ceramic; this mixture is subjected to grinding, drying and sieving operations; this mixture is pressed; and this mixture is sintered so as to obtain a ceramic with a porosity of between 0 and 30%. The sintering is carried out under a pressure of at least 50 bar of a gas which is inert towards the constituents of the mixture. Ceramic thus obtained and spark plug comprising it.
1. Process for the preparation of a ceramic wherein:
50% to 75% by weight of a compound intended to form a conductive phase, and 25% to 50% by weight of at least material allowing the formation of insulating phases composed of silicon nitride and modified silicon oxynitride are mixed in a receptacle containing a liquid after a thermal treatment leading to sintering of the ceramic;
this mixture is subjected to grinding, drying and sieving operations;
this mixture is pressed;
and this mixture is sintered so as to obtain a ceramic with a porosity of between 0 and 30%,
wherein the sintering is carried out under a pressure of at least 50 bar of a gas which is inert towards the constituents of the mixture.
2. Process according to claim 1 , wherein the said inert gas is nitrogen.
3. Process according to claim 2 , wherein the pressure is at least 80 bar.
4. Process according to claim 3 , wherein the mixture is made of compounds and sintering is carried out so as to obtain a free silicon nitride content of 15 to 22% in the final ceramic.
5. Process according to claim 2 , wherein the conductive phase is selected from SiC and MoSi 2 and mixtures thereof.
6. Process according to claim 5 , wherein the mixture is made of compounds and sintering is carried out so as to obtain a free silicon nitride content of 15 to 22% in the final ceramic.
7. Process according to claim 2 , wherein the mixture is made of compounds and sintering is carried out so as to obtain a free silicon nitride content of 15 to 22% in the final ceramic.
8. Process according to claim 1 , wherein the pressure is at least 80 bar.
9. Process according to claim 8 , wherein the mixture is made of compounds and sintering is carried out so as to obtain a free silicon nitride content of 15 to 22% in the final ceramic.
10. Process according to claim 1 , wherein the conductive phase is selected from SiC and MoSi 2 and mixtures thereof.
11. Process according to claim 10 , wherein the mixture is made of compounds and sintering is carried out so as to obtain a free silicon nitride content of 15 to 22% in the final ceramic.
12. Process according to claim 1 , wherein the mixture is made of compounds and sintering is carried out so as to obtain a free silicon nitride content of 15 to 22% in the final ceramic.
13. Process according to claim 1 , wherein the porosity of the ceramic obtained is between 0% and 15%.
14. Process according to claim 1 , wherein 3% to 10% by weight, based on the total mass of the compounds intended to form the conductive and insulating phases, of at least one compound selected from the group consisting of binding and plasticizing organic compounds, are added to this mixture and in that after pressing and before sintering a binder-removing operation is carried out.
15. Process according to claim 14 , wherein grinding is carried out in two stages, in that the addition of the plasticizer takes place between the two stages, and in that the second grinding stage is less energetic than the first.
16. Process according to claim 1 , wherein the said at least one material allowing the formation of the insulating phases is selected from the group consisting of: Si 3 N 4 , Al 2 O 3 , MgO, CaO and the rare-earth oxides such as Y 2 O 3 .