Mechanical and tribological properties of Nd- and Yb-SiAlON composites sintered by hot isostatic pressing

Abstract

Single phase α-, β- and two phases α/β-sialon composites were sintered with both Nd 2O 3 and Yb 2O 3 by hot isostatic pressing (HIP). The mechanical and tribological properties of these composite ceramics have been investigated with respect to their application in direct fuel injection pumps for gasoline engines. Scanning electron microscopy (SEM) was used to analyse the fracture surfaces of sialons following sintering and their worn surfaces after the tribological tests. Microstructural evolution and its change with composition as well as the influence of the microstructural changes on the mechanical and tribological properties have been reported. It has been found that increasing α-sialon phase fraction of the Nd-sialons increases the hardness due to the harder nature of this phase, whereas no significant change in the indentation fracture toughness was observed. Increasing Nd content in these sialons resulted in no significant change in both of the hardness and fracture toughness values. On the other hand, single phase Yb-α-sialons showed a slight decrease in the hardness and a gradual increase in the fracture toughness with increasing Yb content. In a comparative way, it can be seen that Yb-doping gives slightly higher hardness and toughness than Nd-doping. This has been explained by the difference between the cationic radii of Nd 3+ and Yb 3+ and the corresponding ionic field strength. Increased ionic field strength of the Yb 3+ in Yb-sialons resulted in an improvement in their mechanical properties. Tribological tests were performed using a laboratory tribometer with ball-on-plate geometry in reciprocating sliding contact under lubrication with isooctane. Based on the friction and wear data, a relative ranking of the different wear pairs of Nd- and Yb-sialon plates together with a commercial Si 3N 4 plate (used as a reference material) against Si 3N 4 ball was established. Sliding pairs of Yb-doped α-sialon ceramics showed higher friction coefficient and lower wear resistance compared to Nd-doped β- and α/β-sialon composites and the commercial Si 3N 4 ceramic. The lowest friction and the highest wear resistance were measured for the Nd-doped sialon with an α:β-ratio of 88:12. SEM micrographs of the worn surfaces revealed that the main wear mechanisms were microabrasion and tribochemical reactions that resulted in the formation of a thin tribolayer composed of compact wear debris in the tribocontact.