Effects of sintering temperature on the microstructure, mechanical, tribological and thermophysical properties of GNPs/IN738 composite

Abstract

While IN738 Ni-based superalloy is a high strength alloy, it is feasible to improve its properties at the bulk level by reinforcing with graphene nanoplatelets (GNPs), taking advantage of the superior mechanical, tribological and thermal properties using the spark plasma sintering technique. In the present study, the influence of spark plasma sintering temperature range between 900 and 1100 C on the microstructure, mechanical, tribological and thermophysical properties of GNPs/IN738 composite is assessed. The dispersion of GNPs reinforcement and alloying metals to form composite powder is conducted using a turbular mixer and low-frequency planetary ball milling, followed by spark plasma sintering. The relative density of the sintered samples assessed following Archimedes' method indicates increasing densification with the increasing sintering temperature from 94.7% (900 C) to 98.5% (1100 CÞ. The microstructure assessed via SEM, XRD and Raman spectroscopy indicates the formation of precipitate gamma, intermetallic gamma prime, solid solution and GNPs strengthening phases. Thus, the mechanical (micro/nano hardness and Young's modulus), tribological (wear rate and coefficient of friction), and thermophysical (thermal diffusivity, thermal conductivity, and specific heat capacity) properties increased with the increasing sintering temperature. The microhardness increased from 354HV (900 C) to 469HV (1100 C), nanohardness from 8 GPa (900 C) to 17 GPa (900 C), and Young's modulus from 190 GPa (900 C) to 291 GPa (1100 C). The wear rate reduced with an increase in sintering temperature for the three loads of 5, 10 and 20N. The thermophysical properties assessed from 25 to 600 C show the formation of few inflection points as the temperature increases, which is attributed to the dissolution and rearrangement of precipitate gamma prime and Cr in solid solution phases. Similarly, the small increment in the thermal diffusivity is equally associated with the smoother phonon transition at the GNPs/ matrix interface.