Characterization of microstructure and high-temperature wear behavior of pack-borided Co-based Haynes 25 superalloy

Abstract

In this study, the effect of thermochemical boriding on the microstructural properties and high-temperature wear resistance of cobalt-based Haynes 25 superalloy was investigated. Haynes 25 alloy was subjected to a pack-boriding process at 850, 950, and 1050 °C for 4 h using a mixture of 90% B4C (boron carbide) and 10% NaBF4 (sodium tetrafluoroborate). Optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray (XRD) analysis, microhardness, and high-temperature wear tests were performed for the characterization of boride layers obtained after the boriding process. Adhesion wear tests were carried out in a ball-on disc wear device with a 20 N load against an alumina (Al2O3) ball and a sliding distance of 250 m at both room temperature and 500 °C. Due to the high hardness and self-lubricating properties of the boride layers obtained, the borided samples exhibited a higher wear resistance than the untreated sample at both test temperatures, i.e. 18 times higher at room temperature and 4 times higher at 500 °C. The decrease in the wear performance of the boride layers at high temperatures is due to the fact that the wear mechanism turns into fracture-type wear besides oxidative wear at 500 °C. On the other hand, the wear mechanism of the untreated Haynes 25 alloy at room temperature is mixed type (abrasive, oxidative, and plastic deformation), while at 500 °C it turns into an oxidative and fracture type wear mechanism. Oxidative supported fracture type wear mechanism was observed in borided samples both at room temperature and at 500 °C, but the oxidative effect was considerably higher at 500 °C than at room temperature.