Award Title: Mn+1AXn Phase Solid Solutions: Unique Opportunities at Engineering Bulk and Surface Properties

Michel W. Barsoum, Drexel University, DMR 0503711

Researchers at Rowan U. used a combinatorial method to synthesize thin epitaxial films of (Cr,V)2AlC and (Cr,V)2GeC solid solutions. These materials are candidates for application as thin film protective coatings as a result of their low synthesis temperatures and oxidation resistance.

Previously it was shown that epitaxial V4AlC3 films can be synthesized on TiC seed-layers. During combinatorial synthesis of (V,Cr)2AlC a new phase (V1-xCrx)4AlC3, where x = 0.35-0.8 was found. The upper right figure is an XRD pattern of a film in the group. This is surprising since Cr4AlC3 is not known to exist in bulk form, yet it obviously forms as a solid solution phase with a Cr concentration as high as 80%. For wear resistant coatings, the friction coefficients, µ, should be low. In (V,Cr)2GeC solid solution thin films, low µ’s, that were independent of composition, were measured.

The thermal expansions, of a large number of MAX phases was measured using dilatometry and high-temperature x-ray diffraction. The lower right figure shows the average appears to be dependent mostly on the M-group element. Additional analysis of the XRD data finds the anisotropy in thermal expansion is mostly a function of the A-group element.

X-ray diffraction pattern of (V0.61, Cr0.39)4AlC3 thin film showing (001) growth. Minor secondary phase of (V,Cr)C was also observed.

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Average thermal expansion as a function of M-group element for MAX phase materials (solid markers). Open markers represent the thermal expansion of the respective binary carbide.

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Award Title: Mn+1AXn Phase Solid Solutions: Unique Opportunities at Engineering Bulk and Surface Properties

Michel W. Barsoum, Drexel University, DMR 0503711 Synthesis of New Materials:

As a result of their layered structure, elastic properties and high conductivities, MAX-phase materials are candidates for protective coating and electronic applications. These require their synthesis in the form of thin film materials. We successfully synthesized epitaxial films by means of combinatorial synthesis: (V,Cr)2AlC and (V,Cr)2GeC along with a new phase of (V1-xCrx)4AlC3. These are “new” materials since this is their first synthesis in thin-film form.

Preparation of Future Scientists - Combinatorial Synthesis Research

During the 2008-09 academic year Rowan undergraduate students participated in the combinatorial film synthesis research effort. The group consisted of students funded by the grant, as well as some taking research for course/program credit. Each student performed research on one system either (V,Cr)2AlC or (V,Cr)2GeC, from synthesis to material characterization. During this time the research team was able to synthesize complete solid solutions for both systems and discovered a new phase of (V1-xCrx)4AlC3. Another important find is that the solid solutions have lower surface friction than those of the end member.

Included among the educational activities were research presentations given by students to the entire group. Students also gave presentations for scientific symposia such as STEM (Science, Technology, Engineering, and Mathematics) held at Rowan and the annual APS meetings. By the end of their research program all students were proficient in performing measurements using x-ray diffraction, scanning electron microscopy, atomic force microscopy, profilometry, electrical conductivity measurements, and making their own sputtered films. This type of training and experience prepares them for graduate school or industry.

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The c-lattice constant calculated from x-ray diffraction results as a function of Cr concentration shows linear behavior for the solid solutions of (V,Cr)2AlC (top) and (V,Cr)2GeC (bottom). Solid and dashed lines represent guides for the eye for thin film and PDF values. The variations in calculated values can be attributed to deviations in carbon composition.