• Schneider, Johannes
• Holzer, S.
• Abo-Naf, S. M.
• Hoffmann, M. J.
• Gahr, K. H. Zum
• Dulias, U.

Abstract

Single phase alpha-, beta- and two phases alpha/beta-sialon compositeswere sintered with both Nd2O3 and Yb2O3 by hot isostatic pressing(HIP). The mechanical and tribological properties of these compositeceramics have been investigated with respect to their application indirect fuel injection pumps for gasoline engines. Scanning electronmicroscopy (SEM) was used to analyse the fracture surfaces of sialonsfollowing sintering and their worn surfaces after the tribologicaltests. Microstructural evolution and its change with composition aswell as the influence of the microstructural changes on the mechanicaland tribological properties have been reported. It has been found thatincreasing alpha-sialon phase fraction of the Nd-sialons increases thehardness due to the harder nature of this phase, whereas no significantchange in the indentation fracture toughness was observed. IncreasingNd content in these sialons resulted in no significant change in bothof the hardness and fracture toughness values. On the other hand,single phase Yb-alpha-sialons showed a slight decrease in the hardnessand a gradual increase in the fracture toughness with increasing Ybcontent. In a comparative way, it can be seen that Yb-doping givesslightly higher hardness and toughness than Nd-doping. This has beenexplained by the difference between the cationic radii of Nd3+ and Yb3+and the corresponding ionic field strength. Increased ionic fieldstrength of the Yb3+ in Yb-sialons resulted in an improvement in theirmechanical properties. Tribological tests were performed using alaboratory tribometer with ball-on-plate geometry in reciprocatingsliding contact under lubrication with isooctane. Based on the frictionand wear data, a relative ranking of the different wear pairs of Nd-and Yb-sialon plates together with a commercial Si3N4 plate (used as areference material) against Si3N4 ball was established. Sliding pairsof Yb-doped alpha-sialon ceramics showed higher friction coefficientand lower wear resistance compared to Nd-doped beta- andalpha/beta-sialon composites and the commercial Si3N4 ceramic. Thelowest friction and the highest wear resistance were measured for theNd-doped sialon with an alpha/beta-ratio of 88:12. SEM micrographs ofthe worn surfaces revealed that the main wear mechanisms weremicroabrasion and tribochemical reactions that resulted in theformation of a thin tribolayer composed of compact wear debris in thetribocontact. (c) 2006 Elsevier B.V. All rights reserved.

Topics

• surface
• phase
• scanning electron microscopy
• wear resistance
• strength
• composite
• hardness
• ceramic
• fracture toughness
• hot isostatic pressing
• sintering