2016_Mikula_Acta_Mat.pdf

Acta Materialia, 2016

Obtaining high hardness combined with enhanced toughness represents one of the current challenges in material design of hard ceramic protective coatings. In this work, we combine experimental and ab initio density functional theory (DFT) analysis of the mechanical properties of Ti-Al-Nb-N coatings to validate the results of previous theoretical investigations predicting enhanced toughness in TiAlN-based systems highly alloyed (>25 at. %) with nitrides of pentavalent VB group elements Nb, Ta, and V. As-deposited Ti 1x-y Al x Nb y N coatings (y ¼ 0 ÷ 0.61) exhibit single phase cubic sodium chloride (B1) structure identified as TiAl(Nb)N solid solutions. The highest hardness,~32.5 ± 2 GPa, and the highest Young's modulus, 442 GPa, are obtained in Nb-free Ti 0.46 Al 0.54 N exhibiting pronounced 111 growth-orientation. Additions of Nb in the coatings promote texture evolution toward 200. Nanoindentation measurements demonstrate that alloying TiAlN with NbN yields significantly decreased elastic stiffness, from 442 to~358 ÷ 389 GPa, while the hardness remains approximately constant (between 28 ± 2 and 31 ± 3 GPa) for all Nb contents. DFT calculations and electronic structure analyses reveal that alloying dramatically reduces shear resistances due to enhanced d-d second-neighbor metallic bonding while retaining strong metal-N bonds which change from being primarily ionic (TiAlN) to more covalent (TiAlNbN) in nature. Overall, Nb substitutions are found to improve ductility of TiAlN-based alloys at the cost of slight losses in hardness, equating to enhanced toughness.

Thin Solid Films, 2012

Improved toughness in hard and superhard thin films is a primary requirement for present day ceramic hard coatings, known to be prone to brittle failure during in-use conditions. We use density functional theory calculations to investigate a number of (TiAl)1−xMxN thin films in the B1 structure, with 0.06≤x≤0.75, obtained by alloying TiAlN with M=V, Nb, Ta, Mo and W. Results show significant ductility enhancements, hence increased toughness, in these compounds. Importantly, these thin films are also predicted to be superhard, with similar or increased hardness values, compared to Ti0.5Al0.5 N. For (TiAl)1−xWxN the results are experimentally confirmed. The ductility increase originates in the enhanced occupancy of d-t2g metallic states, induced by the valence electrons of substitutional elements (V, Nb, Ta, Mo, W). This effect is more pronounced with increasing valence electron concentration, and, upon shearing, leads to the formation of a layered electronic structure in the compound material, consisting of alternating layers of high and low charge density in the metallic sublattice, which in turn, allows a selective response to normal and shear stresses.

Scientific Reports, 2017

Hard coatings used to protect engineering components from external loads and harsh environments should ideally be strong and tough. Here we study the fracture toughness, K IC , of Ti 1−x Al x N upon annealing by employing micro-fracture experiments on freestanding films. We found that K IC increases by about 11% when annealing the samples at 900 °C, because the decomposition of the supersaturated matrix leads to the formation of nanometer-sized domains, precipitation of hexagonal-structured B4 AlN (with their significantly larger specific volume), formation of stacking faults, and nano-twins. In contrast, for TiN, where no decomposition processes and formation of nanometer-sized domains can be initiated by an annealing treatment, the fracture toughness K IC remains roughly constant when annealed above the film deposition temperature. As the increase in K IC found for Ti 1−x Al x N upon annealing is within statistical errors, we carried out complementary cube corner nanoindentation experiments, which clearly show reduced (or even impeded) crack formation for annealed Ti 1−x Al x N as compared with their as-deposited counterpart. The ability of Ti 1−x Al x N to maintain and even increase the fracture toughness up to high temperatures in combination with the concomitant age hardening effects and excellent oxidation resistance contributes to the success of this type of coatings. Hard coatings are applied to protect tool and component surfaces as well as entire devices in harsh environments and/or demanding application conditions. The coatings are usually ceramic materials, which are known for their beneficial properties such as high hardness and wear resistance, high melting temperatures, high-temperature strength, chemical inertness and oxidation resistance. However, these materials often possess a relatively low (fracture) toughness. A certain degree of toughness, however, is crucial for the reliability and safe operation of critical components. Various strategies have been applied to enhance the fracture toughness of bulk materials 1 and hard coatings 2,3. Since the pioneer works in the nineteen eighties 4,5 , Ti 1−x Al x N has evolved to one of the most widely used and industrial relevant hard coating systems 6. Age hardening effects are (besides enhanced oxidation resistance 7 and resistance against wear 4,5 compared to TiN) considered to be the major basis for its industrial success. At temperatures typical for cutting tools operation, supersaturated face-centered cubic Ti 1−x Al x N isostructurally decomposes into nanometer-sized AlN-rich and TiN-rich domains. This is due to spinodal decomposition causing self-hardening effects 8-11. Nonetheless, the influence of its characteristic thermally activated decomposition and the resulting self-organized nanostructure on the fracture toughness is yet to be studied. The present work revolves around the hypothesis that (besides the well-known self-hardening effects 9) also the fracture toughness of Ti 1−x Al x N coatings increases at elevated temperatures. Potential fracture toughness enhancing mechanisms in the self-organized nanostructure of B1 AlN-rich and TiN-rich domains 8-11 are based on: coherency strains, spatially fluctuating elastic properties, and stress-induced phase transformation toughening from cubic to hexagonal AlN phases under volume expansion at the tip of a propagating crack similar to Yttrium-stabilized zirconia bulk ceramics 12 or Zr-Al-N based nanoscale multilayers 13. We will also see that the B4 AlN phase formation can play a key role for the fracture toughness evolution of Ti 1−x Al x N. By using high-resolution transmission electron microscopy (HRTEM), we observed severely distorted B4 AlN with multiple stacking faults and indications of nano-twins. Twinning represents a mechanism capable of simultaneously enhancing strength and ductility in materials 14 .

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2017

Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/propagation leading to brittle failure during application, is a primary industrial requirement. In this work, experimental methods supported by ab initio density functional theory (DFT) calculations and electronic structure analyses are used to investigate the mechanical behavior of magnetron sputtered Ti-Al-Ta-N hard coatings. The as-deposited Ti1-x-yAlxTayN (y = 0–0.60) films exhibit a single phase cubic sodium chloride (B1) structure identified as TiAl(Ta)N solid solutions. While the hardness H of Ti0.46Al0.54N (32.5 ± 2 GPa) is not significantly affected by alloying with TaN (H of the quaternary nitrides varies between 26 ± 2 and 35 ± 4 GPa), the elastic stiffness monotonically decreases from 442 to 354 GPa with increasing Ta contents, which indicates improved toughness in TiAlTaN. Consistent with the experimental findings, the DFT results show that Ta substitutions in TiAlN reduce t...

Vacuum, 2019

Ti-TiAl 3 in-situ composites containing different percentages of Nb and B were effectively produced from Ti, Al, Nb and B powders by electric current assisted sintering (ECAS) technique which is a powder metallurgy processing method. Samples are sintered for 90 s with 2000 A current. The effect of B and Nb on the hardness, fracture toughness and wear resistance of samples were studied. The microstructure properties of the sintered samples were analysed with scanning electron microscopes (SEM), the phases in the samples were determined with XRD and their hardness and fracture toughness values were measured with a Vickers hardness tester with a load of 0.98 N and 98 N respectively. The highest fracture toughness value has been obtained with wt %10 Nb addition as 5.23 MPa m 1/2 , whereas the highest hardness was determined as 965 HV for wt%5 B reinforced in situ-Ti-TiAl 3 composite. Best wear resistance was obtained in the 47.5Ti-47.5Al-5B sample. While Nb additive had a negatory effect on wear resistance, additive B had a positive effect on wear resistance.

Surface and Coatings Technology, 2007

Superhard nanostructured coatings, prepared by plasma-assisted chemical vapour deposition (PACVD) and physical vapour deposition (PAPVD) techniques, such as vacuum arc evaporation and magnetron sputtering, are receiving increasing attention due to their potential applications for wear protection. In this study nanocomposite (TiAl)B x N y (0.09 ≤ x ≤ 1.35; 1.07 ≤ y ≤ 2.30) coatings, consisting of nanocrystalline (Ti,Al)N and amorphous BN, were deposited onto Si (100), AISI 316 stainless steel and AISI M2 tool steel substrates by co-evaporation of Ti and hot isostatically pressed (HIPped) Ti-Al-B-N material from a thermionically enhanced twin crucible electron-beam (EB) evaporation source in an Ar plasma at 450°C. The coating stoichiometry, relative phase composition, nanostructure and mechanical properties were determined using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), in combination with nanoindentation measurements. Aluminium (∼10 at.% in coatings) was found to substitute for titanium in the cubic TiN based structure. (Ti,Al)B 0.14 N 1.12 and (Ti,Al)B 0.45 N 1.37 coatings with average (Ti,Al)N grain sizes of 5-6 nm and either ∼70, or ∼90, mol% (Ti,Al)N showed hardness and elastic modulus values of ∼40 and ∼340 GPa, respectively. (Ti,Al)B 0.14 N 1.12 coatings retained their 'as-deposited' mechanical properties for more than 90 months at room temperature in air, comparing results gathered from eight different nanoindentation systems. During vacuum annealing, all coatings examined exhibited structural stability to temperatures in excess of 900°C, and revealed a moderate, but significant, increase in hardness. For (Ti,Al)B 0.14 N 1.12 coatings the hardness increased from ∼40 to ∼45 GPa.

BHM Berg- und Hüttenmännische Monatshefte, 2008

The use of γ-TiAl based alloys in high temperature applications requires an effective protection against oxidation. Therefore, two coatings, Al 2 Au and Cr 0.45 Al 0.53 Y 0.02 N (CrAlYN), are tested for their oxidation protection and influence on the mechanical properties of a γ-TiAl based alloy (Ti-47Al-2Cr-0.2Si). Thermal exposure in air reveals that both coatings significantly improve the oxidation resistance. The specific mass gain after oxidation for 672 h at 800 °C is 0.12 mg/cm 2 for CrAlYN coated and 0.79 mg/cm 2 for Al 2 Au coated, but 8.9 mg/cm 2 for the uncoated γ-TiAl. The plastic strain in the γ-TiAl outer fiber during four-point-bending tests is reduced from 0.52 % to 0.31 and 0.20 % by the deposition of Al 2 Au and CrAlYN, respectively. After oxidation at 800 °C for 168 h, uncoated and Al 2 Au coated γ-TiAl fail without plastic deformation due to the formed oxide layers and interdiffusion zones. Contrary, CrAlYN deposited γ-TiAl still exhibits plastic strain of 0.097 and 0.122 % after oxidation at 800 °C for 168 and 672 h. Based on these results it can be concluded that CrAlYN can effectively retard oxidation and interdiffusion of γ-TiAl. Hence, the commonly observed loss of mechanical properties by oxidation and interdiffusion can be avoided.

Journal of Alloys and Compounds, 2018

The electronic structures and mechanical properties of Nbedoped TiAl 2 intermetallic compounds have been investigated using the firsteprinciples method based on the density functional theoretical framework. The calculated results indicate that the structural symmetry of orthorhombic TiAl 2 has been changed after the Al or Ti atoms are replaced by the Nb atoms. The formation energy of the systems in which Al atom replaced by Nb is lower than that of Ti atom replaced by Nb. Accordingly, the Nb atoms are more likely to occupy the sites of Al atoms to form a stable structure when the Nb atoms are introduced into orthorhombic TiAl 2 compound. Additionally, the formation energy of the doped systems increases with the increase of Nb atom percentage. Nb doping weakens the covalent bonding between Ti and Al atoms and enhances the metal bonding between them. The band structures of Nbedoped TiAl 2 systems also indicate that they all have metallic conductivities, which is beneficial to decrease the brittleness of orthorhombic TiAl 2 intermetallic compounds at room temperature. Compared with those systems in which Ti atoms replaced by Nb atoms, the ductility of the Nbedoped TiAl 2 system in which Al atoms are replaced by Nb atoms has been improved better. It is attributed to the density of state near Fermi energy level being increased after Al atoms being replaced by Nb atoms. The fracture strength of Nbedoped TiAl 2 systems is both better than that of the purely TiAl 2 , especially in the system of Ti atoms replaced by Nb atoms.

Transactions of Nonferrous Metals Society of China, 2019

The γ-TiAl based Ti−Al−Mn−Nb alloys with different Nb additions were fabricated by selective laser melting (SLM) on the TC4 substrate. The effects of Nb content on microstructure and properties of the alloys were investigated. The results reveal that the alloys consist of γ-TiAl phase with tetragonal lattice structure and α 2-Ti 3 Al phase with hcp lattice structure, and show a sequential structure change from near full dendrite to near lamellar structure with the increase of Nb addition. Owing to the higher Nb content in γ-TiAl phase and the formation of near lamellar structure, the alloy with 7.0 at.% Nb addition has the best combination of properties among the studied alloys, namely, not only a high hardness of HV 2000, a high strength of 1390 MPa and a plastic deformation of about 24.5%, but also good tribological properties and high-temperature oxidation resistance.

Journal of Inorganic and Organometallic Polymers and Materials, 2017