ICMCTF 2024 Session MA3-1-MoM: Hard and Nanostructured Coatings I

We explore transition metal nitride and carbide compounds and multilayers using a combination of epitaxial layer growth, first-principles calculations, and measurements of lattice parameters and mechanical properties. Rock-salt structure nitrides are both mechanically and thermodynamically stable for group 3 transition metals. However, increasing the valence electron concentration by moving towards the right in the periodic table increases the strength of metal-metal bonds leading to a brittle-to-ductile transition and enhanced toughness, but also decreases the vacancy formation energy on both cation and anion sublattices, resulting in vacancy-stabilized compounds like cubic WN with a dramatically reduced elastic modulus, and new thermodynamically stable phases like a 5-fold coordinated base-centered monoclinic stoichiometric MoN. Epitaxial WC_x layers exhibit a cubic phase that is stabilized by carbon vacancies but phase competition involving hexagonal and orthorhombic W₂C and amorphous carbon lead to an epitaxial breakdown. Epitaxial MoC_x shows a similar phase competition between cubic δ-MoC_x(111) and hexagonal β-Mo₂C(0001) as a function of CH₄ content in the processing gas. In contrast, epitaxial TiC_x is phase pure over a large composition range as the cubic phase is stabilized by the entropy of random C-vacancies for x < 1. However, carbon interstitials and small clusters are energetically unfavorable leading to amorphous C segregation for x > 1, as detected by photoelectron and Raman spectroscopies. This causes a decrease in the elastic modulus and hardness from 462 and 31 GPa for stoichiometric TiC to 201 and 13.5 GPa for x = 1.8. Epitaxial TiC_1-xN_x (001) layers show a nearly composition independent elastic modulus but a hardness that decreases approximately linearly from 31 to 21 GPa with increasing x = 0.0-1.0. TiN-TiC multilayers exhibit a 5-30% superlattice hardening effect, reaching 34 GPa for an epitaxial layer with a 6 nm lattice period.

Titanium carbonitride coatings were investigated for use in electrochemical cells. Here, contact resistance should be minimal while maintaining mechanical strength and a fairly good corrosion resistance. Similar to titanium carbides, an increased carbon content leads to the formation of an amorphous carbon (a-C) phase resulting in nanocrystalline grains of titanium carbonitride surrounded by an a-C matrix. Fine-tuning the microstructure of titanium carbonitride films contact resistance can minimise contact resistance as it is largely determined by surface properties such as hardness and formability, which in turn vary with the amount and structure of the a-C phase. Depending on the carbon source used during the sputter process, the microstructure of the deposited films changes. The aim of this work is to compare the formation of a-C during sputter deposition using two different carbon sources: graphite and methane. Films were either deposited by co-sputtering from a Ti- and a graphite target under N₂ flow or by sputtering solely from a Ti-target under N₂ and CH₄ gas flow. For each process films of different carbon content were deposited and analysed using XRD, XPS, SEM, and Raman spectroscopy. Additionally, properties such as hardness, resistivity, and contact resistance were also investigated. Results show that the carbon concentration of the films varies from 10-24 at.%. Using XPS and XRD, it was found that the films contain NaCl-type Ti(C,N) and an amorphous carbon (a-C) phase. For different carbon concentrations Ti(C,N) shows a varying lattice parameter between 4.26 to 4.32 Å. Furthermore, an increasing overall carbon content causes an increased amount of a-C phase, which has a significant effect on the properties of the films. Comparing the a-C content of films with a similar overall carbon content suggests that carbon is more effectively incorporated in Ti(C,N) grains when using methane as a carbon source. The hardness of the films varied between 12 and 35 GPa and it was found to be dependent on the carbon content, where a lower carbon content corresponded to a reduced hardness. The peak hardness of 35 GPa was found for the film with the highest carbon content deposited using methane as a carbon source. In terms of contact resistance, the lowest values (below 10mΩ) were found for titanium carbonitride coatings with small amounts of a-C, which outperformed both titanium carbide and nitride reference coatings.

High residual stress is one of the critical problems encountered in hard coatings deposited by physical vapor deposition. In industrial applications, introducing a metal interlayer, such as Ti and Cr, is a common practice to relieve the residual stress, thereby enhancing the adhesion and life span of the hard coatings. Our previous studies [1,2] proposed an energy-balance physical model to evaluate the stress and energy relief efficiency of metal interlayers in hard coatings. The assumption of the model was that the relief of stored elastic energy (G_S) in the hard coating and the bending energy of the Si substrate are balanced by the plastic work done by the metal interlayer. The results showed that the energy relief efficiency of the metal interlayer depends on the plastic behavior of the metal, where Zr interlayer has a lower extent of stress relief in ZrN/Zr coating than Ti interlayer in ZrN/Ti coating [1], because the strength coefficient (k) of Zr is higher than that of Ti. In addition, the results indicated that the plastic deformation capability of the metal interlayers is not fully consumed, and the plastic deformation of the metal interlayer is localized within a narrow layer nearby the coating/interlayer interface, which was named as effective deformation thickness (EDT). The EDT was used to depict the energy relief extent in the bilayer coatings [2]. However, whether the energy-balance model and the concept of EDT could be applied in a multilayer structure is still unclear. In this study, the alternated four layered TiN/Ti/TiN/Ti coating was selected as a model architecture to investigate the stress relief and stress distribution of multilayer hard coatings. TiN coating with Ti interlayer was deposited on Si substrate using dc unbalanced magnetron sputtering, where the TiN film and Ti interlayer were alternately stacked to produce structure, TiN, TiN/Ti, TiN/Ti/TiN and TiN/Ti/TiN/Ti. The overall stress and the layered stress of the coatings were measured using the laser curvature method and the average X-ray strain (AXS) combined with nanoindentation methods [3-5], respectively. By using the energy-balance model and the concept of EDT [1,2], the stress and energy relief efficiency of the multilayer architecture could be estimated.

[1] J.-H. Huang et al., Surf. Coat. Technol., 434 (2022) 128224.
[2] J.-H. Huang et al., J. Vac. Sci. Technol. A, 41 (2023) 023104.
[3] C.-H. Ma et al., Thin Solid Films, 418 (2002) 73.
[4] A.-N. Wang et al., Surf. Coat. Technol., 262 (2015) 40.
[5] A.-N. Wang et al., Surf. Coat. Technol., 280 (2015) 43.