Synthesis of Ni(Cr)–hBN Nanocomposite by Mechanical Alloying and Its Thermodynamical Evaluation

Abstract

In the present study, the feasibility of fabricating an Ni(Cr)/hBN self-lubricating nanocomposite by mechanical alloying technique was studied. To achieve this goal, a Ni–20 wt % Cr powders mixture was first milled up to 40 h to produce the nanostructured Ni(Cr) solid solution. hBN particles were subsequently added to Ni(Cr) and further mixed for 3 h. X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) were performed to study phase transformations and microstructural evolutions during the milling process. The results showed that the dissolution of Cr in Ni started after 5 h and completed after 30 h of mechanical alloying. TEM studies also showed that the 40 h milled Ni(Cr) solid solution had the average grain size of 50 nm. Miedema’s semi-empirical thermodynamic model was used to predict the most stable phases in the Ni–Cr binary system. The results showed that the Gibbs free energy change of solid solution formation (–8.44 kJ/mol) was much lower than that of the amorphous phase (–1.3 kJ/mol) in the chemical composition of Ni–20 wt % Cr, thereby confirming the experimental results. The formation of a fine lamellar structure during the milling process was found to be the main reason for the solid solution formation. The nanostructured Ni(Cr) solid solution showed suitable thermal stability at 800°C. It was also observed that hBN particles were homogenously distributed in the as-fabricated Ni(Cr) after 3 h of mixing. The results of the present study also revealed that the mechanical alloying process could be regarded as a high-potential technique for the fabrication of metal-based self-lubricating composites.