Combinatorial investigation of nanolaminate ternary carbide thin films
Authors:   , Scabarozi Theodore H. , Barsoum Michel W. 
Published: July 2021

MAX phases have shown a combination of metal- and ceramic-like properties making them candidates in aerospace and other high-performance applications currently dominated by superalloys. The MAX phases follow the general formula M_[n+1]AX_n, where M is an early transition metal, A is an A-group element, X is C and/or N, and n = 1 to 3. The hexagonal structure consists of edge sharing M6X octahedra interleaved with Agroup element layers. This layered structure, referred to as nanolaminate, leads to the unique and interesting properties. With more than fifty MAX phases already identified, an almost unlimited number of solid solution possibilities exist. Combinatorial methods provide a technique which allows for a large number of thin film samples to be generated with minimal time and expense. In this research, investigations of thin film MAX phase ternary carbides synthesized by magnetron sputtering in the temperature range of RT-1000 °C are reported. The ultimate goal was to synthesize MAX phase thin film solid solutions by a combinatorial method in an attempt to identify enhanced properties. The M₂AC MAX phases that formed in the following systems, Ti-Nb-Al-C, V-Cr-Al-C, V-Cr-Ge-C, were examined. In all solutions, only mixing of the M elements was investigated. All textured films grew epitaxially (c-axis) on c-sapphire substrates or deposited binary carbide buffer layers. The lowest synthesis temperature resulting in textured growth was for V₂AlC at 600 °C, however; formation of nanocrystalline Cr₂AlC was observed at 550 °C as indicated by Raman spectroscopy. High temperature X-ray diffraction of amorphous Cr-Al-C and Cr-Ge-C films showed textured growth of the MAX phase occurred around 650 °C, and 725 °C, respectively. All combinatorial studies were performed at 850 °C with (Ti_[1-x]Nb_x)₂AlC films grown on TiC buffer layers while (V_[1-x]Cr_x)₂AlC and (V_[1-x]Cr_x)2GeC grown directly on sapphire. Complete solubility across the entire range of x was observed for all systems. Additionally, new thin film phases of VAlC₂, V₄AlC₃, Nb₅Al₃C_x, Cr₅Ge₃C_x, (Ti_[1-x]Nb_x)₃AlC₂, (Ti_[1-x]Nb_x)₄AlC₃, and (V_[1-x]Cr_x)₄AlC₃ were discovered. The M-element impacts many different properties of MAX phase films. The surface of most films were rough, some containing large hexagonal crystals. Yet, this work has demonstrated that the surface roughness can be tuned using elemental substitutions on the M-sites. While friction testing found all films to have relatively low coefficients of friction (<0.12), this too was found to be influenced by the M-element. Raman spectroscopy of (Ti_[1-x]Nb_x)₂AlC films indicates possible stiffening around x = 0.75 explicitly demonstrating the role of the M-element in this solid solution. All films were good electrical conductors with metal-like conduction down to 2K with magnitude and temperature dependence of the resistance tunable through composition. The Hall coefficient and magnetoresistance were also controlled by M-element substitution. While dramatic changes have not been observed, it is clear that the application of combinatorial methods has improved the understanding of the role of the M-element in determining the properties of MAX phases improving the ability to use the materials for thin film applications and beyond.