Wind turbines, nuclear power plants or sealing of agitator shafts in vaccine production are just some of the numerous fields of application for mechanical seals. The requirements placed on these seals and the materials used in them are as varied as their areas of applications. The state of the art is the use of silicon carbide (SiC) sliding parts, which are often additionally coated with diamond. In very demanding applications, diamond coatings can also wear out within a year. For this reason, SiC-diamond composites have been investigated as sliding material.

For these tests, a large number of components were manufactured by the Fraunhofer IKTS using an inlay technology in a conventional SiC ring. The ceramics were then subjected to mechanical processing to meet the high requirements of a mechanical seal. Tests were carried out in a ring-on-ring tribometer setup under different conditions and in comparison with different SiC ceramics as well as diamond-coated ceramics:

Test setup 1:      Distilled-water lubrication, up to 700 N normal force, varying sliding surface speeds
Test setup 2:      Varying lubrication situations, sliding surface pressure 2 bar and speed 2 m/s

Under these conditions, the composite performed as well as normal SiC materials in terms of coefficient of friction and showed significantly better wear resistance as measured by 3D white light interferometry. The wear results are comparable to those of diamond coatings (approximately 2 µm wear depth after an experiment with test setup 2), while the coefficient of friction is at least twice as high (0,06).

Tests on mechanical seal test rigs with pressures up to 100 bar and temperatures up to 100 °C showed no measurable wear. Concerns about degradation of the SiC phase were not confirmed. Even in tests of more than 500 hours under moderate test conditions, neither the diamond showed increased wear, nor could increased SiC leaching be observed.

Overall, the diamond-SiC composite has shown to have high potential as a sliding material both on classical tribometer test rigs in a wide variety of ramp tests and in different lubricant situations, as well as on application-related mechanical seal test rigs. 

This work was funded by the German BMWK.