Durable hard coating containing silicon nitride

1. A slip for producing a durable hard coating on a substrate comprising:

a) a suspension of silicon nitride particles in an amount of 30% by weight to 60% by weight and

b) a binder comprising a nanocomposite comprising a mixture of nanosize solid particles and/or precursors of nanosize solid particles in an organically modified inorganic polycondensate or a precursor thereof from production via sol-gel process;

wherein the slip does not comprise a release agent selected from the group consisting of graphite, graphite compounds, sulphides, selenides, tellurides, metal sulphides, metal selenides, metal tellurides, metal-free phthalocyanines, indanthrene dyes, borax, lead oxide, zinc oxide, mica, talc, cadmium chloride, lead iodide, cobalt chloride and silver sulfate.

2. The slip according to claim 1 , wherein the organically modified inorganic polycondensate or precursor thereof is an organically modified inorganic polysiloxane or precursor thereof.

3. The slip according to claim 1 , wherein the nanosize solid particles are metal oxide particles.

4. The slip according to claim 1 , wherein the nanosize solid particles are selected from among SiO 2 , TiO 2 , ZrO 2 , Al 2 O 3 , AlOOH, Y 2 O 3 , CeO 2 , SnO 2 , iron oxides, and Ta 2 O 5 , or among precursors of these nanosize solid particles which are converted by means of the sol-gel process into solid particles.

5. The slip according to claim 1 , wherein SiO 2 particles and/or precursors of SiO 2 particles which are converted by means of the sol-gel process into nanosize SiO 2 particles are present as nanosize solid particles.

6. The slip according to claim 1 , wherein the nanosize solid particles have been surface-modified with a surface modifier having a molecular weight of less than 1500.

7. The slip according to claim 1 , wherein the nanosize solid particles have been modified with a surface modifier containing an anhydride group, acid amino group, amino group, SiOH group, hydrolysable radicals of silanes and/or a β-dicarbonyl group.

8. The slip according claim 1 , wherein the nanocomposite is obtainable by the sol-gel process by reacting nanosize solid particles with one or more silanes of the general formula (I):

R x SiA (4-x)   (I)

where the radicals A are identical or different and are hydroxyl groups or hydrolysable groups, the radicals R are identical or different and are nonhydrolysable groups and x is 0, 1, 2, or 3, with at least 50 mol % of the silanes having x≧1.

9. The slip according to claim 1 , wherein the binder is obtainable by the sol-gel process by reacting one or more silanes of the general formula (I):

R x SiA (4-x)   (I)

where the radicals A are identical or different and are hydroxyl groups or hydrolysable groups, the radicals R are identical or different and are nonhydrolysable groups and x is 0, 1, 2, or 3, with at least 50 mol % of the silanes having x≧1.

10. A shaped body comprising a substrate having a durable hard coating, wherein the hard coating has been produced from a slip according to claim 1 .

11. A shaped body according to claim 10 , wherein the substrate comprises quartz, graphite, ceramic, SiO 2 (fused silica) or an iron alloy.

12. A shaped body according to claim 10 which is a melting crucible having a substrate composed of quartz, graphite or ceramic for silicon melts.

13. A process for producing a shaped body according to claim 10 , comprising the steps of:

applying the slip to the substrate by using a single or repeat doctor blade coating, dipping, flooding, spin coating, spraying, brushing or painting,

forming a durable hard coating on the substrate by hardening the applied slip.