The main objective of this paper is to help on the clarification of the lack of consensus in the bibliographic data concerning the tribological behaviour of Si 3 N 4 -BN composites. Unlubricated sliding tests by pin-on-disc were carried out with three grades of composite materials with 10, 18 and 25 vol.% of BN. The addition of BN to the Si 3 N 4 matrix resulted in a slight reduction of the friction coefficient, which decreased from 0.82 for monolithic Si 3 N 4 to 0.67 for Si 3 N 4 -10%BN materials. Wear coefficients (K) were above 10 −5 mm 3 N −1 m −1 for all materials tested and increased sharply with increases in BN volume fraction greater than 10%, e.g. K ∼ 10 −3 mm 3 N −1 m −1 for Si 3 N 4 -25%BN. The lowest values of friction and wear coefficients were obtained when the composites were tested with the BN platelets oriented parallel to the sliding direction. The morphological study of the worn surfaces revealed generalised brittle intergranular microcracking at the dispersoid/matrix interface as the main wear mechanism. Under the experimental conditions of this study, the formation of stable protective layers of the soft lubricious oxide H 3 BO 3 , or the solid lubricant BN·H 2 O, was not observed.

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Silicon nitride (Si 3 N 4) and silicon nitride/silicon carbide (Si 3 N 4 /30 wt.% SiC) seal rings were tested as self-mated pairs and dissimilar sealing systems against grey cast iron. The tribological experiments were conducted in a ring-on-ring tribometer at V ≈ 4 m s −1 of linear speed, in the range of 0.3-1 kN of applied load, under a pressure of 2 × 10 5 Pa of a mixture of 20 vol.% of hydrogen peroxide in deionised water, which gives an effective pressure (P) between 0.2 and 1.3 MPa. The homologous tribosystems are ineffective to seal due to catastrophic abrasive wear. The system with better performance is the dissimilar pair Si 3 N 4 /grey cast iron, that presented wear coefficient values of K = 4.2 × 10 −8 mm 3 N −1 m −1 for the ceramic ring and K = 1.3 × 10 −7 mm 3 N −1 m −1 for the metallic ring, in tests driven for about 60 h and 890 km, with full sealing. An amorphous passivating film of silica protects the ceramic surface. This system still presents an excellent combination K × PV ≈ 0.1 m h −1 , this product being a measure of the surface loss of a machine component for a given time of service.