Abstract: In a sliding element having first and second members in friction and/or sealing contact, the first member s made by sintering a ceramic mixture based on silicon nitride, the content of the silicon nitride phase and/or of the silicon nitride mixed crystal phase being at least 60 weight-percent and the second counter-element member being formed of a ceramic material based on aluminum oxide.

Abstract: A machine component is disclosed composed of 3.0 to 25 weight percent of zirconium oxide and/or hafnium oxide which are present in the interior of the machine component at room temperature predominantly in the metastable tetragonal modification and at the surface of the machine component in the monoclinic modification, said machine component having a profile bearing ratio of about 10 to 40% and an center line average surface roughness R.sub.a in the contact area of less than 0.3 .mu.m. The machine components of the present invention are leakproof, exhibit good thermal shock durability and exhibit good frictional engagement characteristics for long periods of time.