Pulsed laser deposition of (TaNbHfTiZr)C high-entropy carbide layers with 38 GPa hardness

High-entropy carbides (HECs) have been identified as high performance materials with attractive properties including superior penetration hardness in comparison to binary carbides. Traditional fabrication techniques for HEC materials often focus on creating bulk material with limited tunability. The growth of HEC coatings with the versatile thin-film growth method pulsed laser deposition (PLD), however, remains unexplored. Here, we fabricate thin films of the refractory high-entropy carbide (TaNbHfTiZr)C on Al₂O₃(0001) and Si(100) substrates using PLD. PLD allows to optimize the composition and structure of the HEC layers to achieve single-phase, oriented carbide layers via systematic variations of the laser fluence, inert gas background pressure, and deposition temperature. Upon growth at room temperature, the carbide is found to coexist with a secondary phase of elemental carbon, which likely acts as embedding layer for carbide particles. The surface content of elemental carbon drops strongly when increasing the Ne background pressure while the carbon attributed to the carbide lattice responds less to pressure. At high growth temperatures (850 °C), little to no elemental carbon is observed and the carbide phase shows a significantly smaller carbon deficiency (MC_0.92). This change in composition is concomitant with an increased crystallinity and the emergence of an orientational preference with respect to the Al₂O₃(0001) substrate. The layers with optimized crystallinity (ultra-high vacuum, 850 °C) achieve a hardness of 38.3 ± 1.9 GPa, which is close to the highest bulk values for HECs and demonstrates the promise of PLD-grown HECs for hard coatings.