Growth and properties of single transition metal nitride-carbide superlattices

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Authors
Azoff-Slifstein, Moishe
Issue Date
2024-08
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Electronic thesis
Thesis
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en_US
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Materials engineering
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Transition metal nitrides (TMNs) and carbides (TMCs) are desirable materials for applications such as hard coatings, diffusion barriers, and optical coatings due to their high intrinsic mechanical strength, hardness, and thermal and chemical stability. Superlattices, nanoscale laminate composites consisting of alternating layers of two materials, allow for mechanical property enhancements through local strain variations and shear modulus mismatch which result in interfacial dislocation pinning at specific bilayer periods. Nanoscale engineering of TMNs and TMCs into superlattices increases the appeal within the application of hard, wear resistant coatings without significant alterations to existing fabrication techniques. I have studied three sets of single transition metal nitride/carbide superlattices, including TiN/TiC, MoN/MoC, and VN/VC, in order to investigate their hardness and elastic modulus as a function of bilayer period and the effect of material selection on the mechanical properties of superlattices. Further investigations develop a fundamental understanding of the effects of lattice constant and shear modulus mismatch on the potential enhancements of superlattice mechanical properties beyond intrinsic material values. Additionally, I have studied three sets of TiN/TiC superlattice series grown with different crystalline quality and growth orientations to better understand the effect that these properties have on superlattice hardening.Rocksalt structured TiN(001)/TiC0.5(001) superlattices with bilayer periods  from 1.5 to 30 nm are grown on MgO(001) substrates via DC reactive magnetron sputtering in alternating Ar/N2 and Ar/CH4 gas mixtures at 1100 C. Microstructural characterization confirm epitaxial TiN(001) and TiC0.5x(001) demonstrating cube-on-cube epitaxy with the MgO(001) substrate, such that TiN/TiC[100] || MgO[100]. Measured XRD data shows an average superlattice relaxed lattice constant ao = 4.25 Å. The TiN(001)/TiC0.5(001) superlattices demonstrate superlattice enhancement at  = 6 nm of H = 34 GPa, 1.4 the hardness of pure TiN and 1.1 that of pure TiC. Elastic modulus also has an enhancement at the same bilayer period with E = 750 GPa increasing 1.7 above that of pure TiN and TiC. The enhancements are attributed to shear modulus mismatch and lattice constant mismatch of 1.3 % forming coherency strains, both of which inhibit dislocation motion at superlattice interfaces and improve the mechanical properties. Additional rocksalt structured TiN/TiC superlattice series are grown on MgO(001) and Al2O3(0001) with increased carbon content. Microstructural characterization of TiN/TiC on MgO(001) confirms the presence of both (001) and (111) orientations, with a preferred (001) orientation. Hardness initially increases with bilayer period, reaching a maximum at  = 7.5 nm of H = 17.9 GPa before decreasing with further increasing bilayer period. TiN/TiC on Al2O3(0001) demonstrates strong (111) growth and exhibits no increase in hardness above the intrinsic values of the separate materials. Overall, polycrystalline TiN/TiC superlattices have a reduced hardness and modulus relative to epitaxial samples, and the (001) orientations have increased mechanical properties compared to (111) orientations. 1.2 m thick molybdenum nitride/carbide superlattices with bilayer period  = 1.5 to 30 nm are grown on MgO(001) substrates by DC reactive magnetron sputtering at 800 ℃. XRD confirms rock-salt structured MoN/MoC on MgO(001) with approximately random grain orientations. Mechanical property measurements yield a maximum of H = 12 GPa and E = 255 GPa at  = 15 nm. The measured superlattice hardness is 471% and 50% larger than that of measured pure MoN and MoC, respectively. The superlattice hardening is the result of an 85 GPa shear modulus mismatch between MoN and MoC, as well as a 3.2 % lattice mismatch which generates stress fields and prevents dislocation motion. Vanadium nitride/carbide superlattices 1.5 m thick are also grown on MgO(001) substrates with bilayer period  = 1.9 to 30 nm at Ts = 1000 ℃. XRD measurements confirm that the VN/VC films exhibit an epitaxial rock-salt structured matrix, with cube-on-cube epitaxy with the MgO(001) substrate, such that VN/VC[100] || MgO[100]. Results of XRD on VN/VC superlattices show a measured relaxed lattice constant ao = 4.158 Å. Additionally, XRD,RSM, and SEM indicate that the samples are predominantly epitaxial with misoriented grains present. Reciprocal space mapping indicates increasing in-plane compressive strain as a function of increasing bilayer period. XPS and EDS measurements confirm stoichiometric VC but reduced nitrogen content. The VN/VC superlattice films indicate a decreased hardness with increasing bilayer period, from H = 16.4 to 11.1 GPa, and elastic modulus E = 260 GPa that is constant as a function of bilayer period. Although the material system does have a shear modulus mismatch of 67 GPA, the superlattice hardness values do not exceed those of pure VN or VC likely due to a low lattice mismatch (1.0 %) which suppresses the typical mechanical property enhancement exhibited by superlattices.
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August2024
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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