Ceramic-carbon composites designed for piston group of combustion engines
Andrzej Posmyk, Henryk Wistuba, Paweł Falkowski
Quarterly No. 2, 2011 pages 97-101
DOI:
keywords: porous ceramic, composite, glassy carbon, sliding, cylinder liner
abstract The paper presents the production technology of ceramic-carbon composites designed for cylinder inserts of piston devices. Porous oxide ceramics with an assumpted porosity (20, 30 and 35%) obtained by the method of sintering grains of ceramic powder was used as a composite matrix. In the method, the sintering process is carried out in such a way that the ceramic powder particles form durable bonds (they sinter) in a properly prepared shaped sample but at the same time they do not form a dense polycrystalline material. The composite was obtained by introducing a glassy carbon precursor into the open pores of the ceramic and was then subjected to pyrolysis in the atmosphere of argon. As a result of the conducted technological tests, diverse contents of glassy carbon in the ceramic matrix were obtained. The influence of the open porosity of an oxide ceramic matrix upon the tribological properties of the fabricated composite sliding against a cast iron piston ring in the conditions of friction in air was examined. The material is destined for cylinder inserts in machines and piston devices e.g. combustion engines, air compressors and pneumatic servo-motors. The material was obtained in three consecutive stages i.e. ceramic samples with a given porosity were obtained by the gel casting method, saturation of the porous samples with a carbon precursor, and finally pyrolysis of carbon precursor introduced into the pores of oxide ceramics. The obtained ceramic-glassy carbon composite (CGC) features low thermal conductivity close to conductivity of oxide ceramics , a high wear resistance and friction coefficient which allows sliding in the conditions of limited lubrication. The fabricated material was subjected to tribological tests on a pin-on-disc stand sliding against a cast iron pin in the conditions of friction in air. The friction coefficient of the examined contact largely depends upon the ceramic matrix porosity. The lowest value (µ ≈ 0.3, at p = 0.8 MPa, v = 2.5 m/s, s = 5000 m) was obtained in the contact with 30% matrix porosity. The results are the basis for further investigations on optimal chemical composition and manufacturing processes in order to reach the required utility properties. Composites manufactured upon an oxide ceramic matrix with a 30% porosity feature the best tribological properties. Such properties are achieved due to a sufficient amount of glassy carbon present in the oxide ceramics pores. Glassy carbon is marked by low shear resistance and high ceramic matrix hardness therefore low values of friction forces are possible to be reached.