Single-phase [NaCl]-structure Ti_1 − xZr_xN thin films (0 < x < 1) have been deposited using cathodic arc plasma deposition. The films were investigated using X-ray diffraction (XRD), transmission electron microscopy, differential scanning calorimetry (DSC), and nanoindentation. Density functional theory calculations on phase stabilities show that the pseudo-binary TiN–ZrN system exhibits a miscibility gap, extending over 0 ≤ x ≤ 0.99 at 1000 °C, with respect to phase transformation from a solid solution into a two-phase mixture of [NaCl]-structure TiN and ZrN components. The films were found to retain their as-deposited single-phase structure during post-deposition annealing at 600 °C (18 h), 700 °C (12 h), 1100 and 1200 °C (2 h), and for as long as 195 h at 600 °C. DSC revealed no heat flow during annealing, similar to TiN, and only the x = 0.53 film exhibited a slight increase in XRD peak broadening after annealing at 1200 °C, consistent with spinodal decomposition. This effective thermal stability of the alloys is explained by the combination of a limited driving force for phase transformation and an insufficient atom diffusivity. In terms of mechanical properties, films with composition deepest within the miscibility gap showed a hardness of ∼ 30 GPa after annealing at 1100–1200 °C; a value only moderately lower than in the as-deposited condition. The principal hardening mechanism for the Ti_1 − xZr_xN films is proposed to be solid-solution hardening through local lattice strain fields originating from difference in atomic radius of Ti and Zr. The material system is thus promising for cutting tool applications.

The structure, mechanical properties, and machining performance of arc evaporated Ti–Al–O–N coatings have been investigated for an Al_0.66Ti_0.34 target composition and O2/(O2+N2) gas flow-ratio varied between 0 to 24%. The coating structure was analysed using SEM, EDX, XRD, XPS, TEM, and STEM. Mechanical properties were analysed using nanoindentation and the deformation behaviour was analysed by probing the nanoindentation craters. The coatings performances in cutting tests were evaluated in a turning application in low carbon steel (DIN Ck45). It is shown that the addition of oxygen into the arc deposition process leads to the formation of a dual layer structure. It consists of an initial cubic NaCl-structure solid solution phase formed closest to the substrate, containing up to 35 at.% oxygen (O/O+N), followed by steady-state growth of a nanocomposite compound layer comprised of Al₂O₃, AlN, TiN, and Ti(O,N). The addition of oxygen increases the ductility of the coatings, which improves the performances in cutting tests. At high levels of oxygen, (>13 at.%), however, the performance is dramaticallyreduced as a result of increased crater wear.

The mechanical properties and machining performance of Ti_1−xAl_xN-coated cutting tools have been investigated. Processing by arc evaporation using cathodes with a range of compositions was performed to obtain coatings with compositions x=0, x=0.25, x=0.33, x=0.50, x=0.66 and x=0.74. As-deposited coatings with x≤0.66 had metastable cubic structures, whereas x=0.74 yielded two-phase coatings consisting of cubic and hexagonal structures. The as-deposited and isothermally annealed coatings were characterised by nanoindentation, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Cutting tests revealing tool wear mechanisms were also performed. Results show that the Al content, x, promotes a (200) preferred crystallographic orientation and has a large influence on the hardness of as-deposited coatings. The high hardness (∼37 GPa) and texture of the as-deposited Ti_1−xAl_xN coatings are retained for annealing temperatures up to 950 °C, which indicates a superior stability of this system compared to TiN and Ti(C,N) coatings. We propose that competing mechanisms are responsible for the effectively constant hardness: softening by residual stress relaxation through lattice defect annihilation is balanced by hardening from formation of a coherent nanocomposite structure of c-TiN and c-AlN domains by spinodal decomposition. This example of secondary-phase transformation (age-) hardening is proposed as a new route for advanced surface engineering, and for the development of future generation hard coatings.

We have produced pure thin-film single-crystal Ti₂AlN(0001), a member of the M_n+1AX_n class of materials. The method used was UHV dc reactive magnetron sputtering from a 2Ti:Al compound target in a mixed Ar–N₂ discharge onto (111) oriented MgO substrates. X-ray diffraction and transmission electron microscopy were used to establish the hexagonal crystal structure with c and a lattice parameters of 13.6 and 3.07 Å, respectively. The hardness H, and elastic modulus E, as determined by nanoindentation measurements, were found to be 16.1±1 GPa and 270±20 GPa, respectively. A room-temperature resistivity for the films of 39 μΩ cm was obtained.

The work presented herein is about characterizing phase transformations in cathodic arc plasma-deposited Ti_1-xA1_xN and Ti_1-zZr_zN thin films for cutting tool applications, and to investigate how the films' mechanical properties are affected by such transformations during thermal annealing. Post-deposition analyses were carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM), nanoindentation, four-point probe sheet resistance, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and Rutherford backscattering spectrometry (RBS). For Ti_1-xA1_xN, residual stresses relax through annihilation of depositioninduced lattice defects in the 500-900°C regime. Stress relaxation is a multiple process with activation energies of 2.0-2.9 eV. At ~900°C, phase transformation from the as-deposited metastable single-phase [NaCl] structure into the thermodynamically stable [NaCl]-TiN and [wurtzite]-A1N proceeds through spinodal decomposition, during which [NaCl]-TiN and [NaCl]-A1N domains form from the [NaCl]-Ti_1-x,A1_xN matrix. Activation energies for the transformation process of 2.9-3.5 eV indicate grain boundary and defect-assisted segregation of Ti and A1. The films age harden during transformation, with an increase in film hardness from the as-deposited condition of ~35 GPa to ~36-37 GPa following post-deposition annealing at 900°C, while pure TiN softens to ~20 GPa. Hardening originates from coherency strains due to lattice-mismatch between [NaCl]-structure TiN and AIN domains formed during initial stages of spinodal decomposition. Ti_1-xA1_xN-coated cutting tools can therefore be said to 'adapt' to the high temperatures and cutting forces encountered during in-service machining operations. For Ti_1-zZr_zN, calculations on phase stabilities using density-functional theory (OFT) show that the pseudo-binary system exhibits a miscibility gap. Thus, there is a driving force for transformation from the as-deposited metastable single-phase [NaCl] structure into [NaCl]-structure TiN and ZrN components. For such compositions, an essentially retained film hardness after post-deposition annealing at 1100-1200°C has been observed. The principal hardening mechanism for this particular nitride thin film system is proposed to be solid-solution hardening through localized lattice strain fields originating from difference in atomic radius of Ti and Zr. Finally, single-crystal Ti₂A1N thin films belonging to the so-called MAX-phase class of materials have been successfully synthesized by reactive magnetron sputtering. The results are promising for the prospects of synthesizing a range of MAX-phase nitride materials as single-crystal thin films and polycrystalline coatings.

Age hardening by spinodal decomposition in ceramic thin film systems is reviewed. This is a new concept for advanced surface engineering with applications for wear-resistant coatings in machining processes like high-speed and dry cutting. The reactive arc deposition method with relatively low substrate temperatures is employed to produce supersaturated solid solutions of the material by ion-bombardment-induced mixing of atoms and kinetic limitation to reduce thermodynamically-driven segregation during synthesis. It is shown using electron microscopy, X-ray diffraction, and nanoindentation techniques that Ti1-xAlxN (0 less than or equal to x less than or equal to 1) coatings with compositions in the miscibility gap undergo spinodal decomposition during annealing at temperatures between similar to900 degreesC and 1100 degreesC. As a result, coherent cubic-phase nanometer-size domains form that cause an increase in hardness. These intermediate metastable domains transform into their stable phases TiN and hexagonal wurtzite-structure AlN during further thermal treatment. The findings are corroborated by Ab initio calculations of phase stability and molar volume for competing phases. Activation energies for the processes indicate defect-assisted segregation of Ti and Al. It is inferred that the success of Ti1-xAlxN coatings is not only based on its superior oxidation resistance, but also on its ability for self-adaptation to the thermal load applied during cutting by age hardening. The findings and experimental approach have implications also for other ternary and multinary ceramic systems including the group-III nitride alloys.

The phenomenon of age hardening could be evidenced in thin film applications. A model system, Ti_1-xAl_xN was chosen as such coatings are known for their excellent wear resistance enabling improved machining processes like high-speed and dry cutting. Here, we show unambiguously that metastable Ti_1-xAl_xN coatings initially undergo spinodal decomposition into coherent cubic-phase nanometer-size domains, causing an increase in hardness at elevated temperatures. These intermediate metastable domains transform into their stable phases TiN and AlN during further thermal treatment. Activation energies for the processes indicate defect-assisted segregation of Ti and Al. The findings are corroborated by ab initio calculations. A long-standing discussion on the thermal stability of this important class of ceramics is thus resolved.

We report the stress relaxation behavior of arc-evaporated TiC_xN_1−x thin films during isothermal annealing between 350 and 900°C. Films with x=0, 0,15 and 0,45 each having an initial compressive intrinsic stress σ_int = -5.4 GPa were deposited by varying the substrate bias V_s and the gas composition. Annealing above the deposition temperature leads to a steep decrease in the magnitude of σ_int to a saturation stress value, which is a function of the annealing temperature. The corresponding apparent activation energies for stress relaxation are E_a=2.4, 2.9, and 3.1 eV, for x=0, 0,15 and 0,45 respectively. TiC_0.45N_0.55 films with a lower initial stress σ_int = -3 GPa obtained using a high substrate bias, show a higher activation energy E_a=4.2 eV.In all the films, stress relaxation is accompanied by a decrease in defect density indicated by the decreased width of X-ray diffraction peaks and decreased strain contrast in transmission electron micrographs. Correlation of these results with film hardness and microstructure measurements indicates that the stress relaxation is a result of point-defect annihilation taking place both during short-lived metal-ion surface collision cascades during deposition, and during post-deposition annealing by thermally activated processes. The difference in E_a for the films of the same composition deposited at different V_s suggests the existence of different types of point-defect configurations and recombination mechanisms.