ICMCTF 2025 Session MA3-1-TuM: Hard and Nanostructured Coatings I

The quaternary Ti_1-x-yAl_xTa_yN system has been shown to possess superior thin film properties compared to Ti_1-xAl_xN. In addition to the impact of Ta content, the sputtering technique significantly influences the structural and mechanical characteristics of these films. In this study, high power impulse magnetron sputtering (HiPIMS) was employed to deposit dense, tough, and hard Ti_1-x-yAl_xTa_yN thin films from composite targets, which were then compared to TiAlN thin films. The effects of Ta addition were explored both experimentally and through simulations. VirtualCoater® was used to simulate thin film growth and properties, providing insight into the role of Ta in the densification process and the relationship between target composition and film composition. Subsequently, the mechanical, structural, and thermal properties of the films were experimentally examined, highlighting the significant benefits of Ta addition.

The observed enhancements are attributed to: (1) increased hardness due to film densification facilitated by intense Ta bombardment, (2) stabilization of the cubic structure at elevated temperatures, and (3) superior thermal resistance resulting from the formation of a mixed (Ti_xAl_yTa_z) oxide monolayer, as opposed to the Al₂O₃/TiO₂ bilayer observed in TiAlN-based coatings, as confirmed by XPS depth profiling.

Finally, dry cutting tool tests demonstrated a substantial increase in tool life and improved surface finish of the machined parts.

Currently, the coating of microtools plays a critical role in precision manufacturing, as tool performance and longevity are heavily influenced by the properties and quality of the coating employed. In this context, HiPIMS technology provides hard coatings with a smooth finishing, a low defect density, and homogeneous coverage of 3D intricate parts – an essential advantage when coating tools with diameters smaller than a millimeter – thus representing an ideal choice for these applications.

AlTiN-based coatings doped with silicon (AlTiSiN) and boron (AlTiBN) have been developed to meet the specific demands of micro-tool applications, such as enhanced wear resistance, high thermal stability, and low friction in extreme operating environments. Properties such as hardness, adhesion, and residual stress were tailored and correlated to HiPIMS process parameters. In addition to mechanical properties analysis, tool performance was evaluated through machining tests, selecting Hardened Steel (HRC60) and Ti6AlV4 alloy as case materials. Both the finishing of the machined parts, and the wear suffered by the tool were analyzed.

The results highlight the potential of HiPIMS-deposited AlTiN-based coatings to significantly extend tool life and improve machining quality in precision manufacturing. Furthermore, this study provides insights into the trade-offs between boron and silicon doping, offering practical guidelines for selecting the most appropriate coating for specific micro-tool applications.

Our findings underline the versatility of HiPIMS technology in tailoring thin-film properties for high-performance applications, demonstrating its growing relevance in the field of advanced microtools.

Keywords: hard coatings, nitrides, sputtering, HiPIMS.

The High-Power Impulse Magnetron Sputtering (HiPIMS) technique enables the deposition of coatings with high hardness, low defect densities, and uniform, conformal coverage over complex 3D geometries, meeting the strict tolerances required in micromachining applications. High-speed machining (HSM) applications demand not only high hardness but also sufficient fracture toughness K_IC, which is critical for maintaining the structural integrity of both bulk and coated engineering components [1]. However, these mechanical properties are often antagonistic, particularly in materials capable of plastic deformation, where high hardness typically correlates with lower fracture toughness [2].

This study aims to systematically evaluate the fracture toughness and machining performance of AlTiN- and TiN-based coatings doped with Si, deposited by HiPIMS using different process parameters. Micro-fracture toughness was assessed on freestanding films using single cantilever bending tests, effectively minimizing residual stress and substrate interactions to obtain precise K_IC​ values. Crack formation at the cutting edge of a 0.4 mm diameter microdrill was observed using FIB, while the composition of the coatings was determined by GDOES. Additionally, XRD was employed to analyze grain growth texture and peak shifts, enabling the evaluation of biaxial stresses. The AlTiSiN coatings exhibited a hardness of 35 GPa, while TiSiN coatings reached 40 GPa. Fracture toughness ranged from 1.78 MPa·√m to 2.2 MPa·√m, depending on the HiPIMS parameters used. In micromachining tests on the TiAl6V4 alloy, the coatings allowed continuous micro-milling to be extended from 40 minutes to over one hour. These findings link toughness values, stress reduction, and crack formation at the cutting edges with tool durability in machining applications.

[1]. BARTOSIK, M., et al. Fracture toughness of Ti-Si-N thin films. International Journal of Refractory Metals and Hard Materials, 2018, vol. 72, p. 78-82.

[2]. HAHN, Rainer, et al. Superlattice effect for enhanced fracture toughness of hard coatings. Scripta Materialia, 2016, vol. 124, p. 67-70.

Transition metal nitrides (TMeNs) are widely used in protective hard coatings for cutting tools due to their superior mechanical and tribological properties, and high thermal stability. Among the TMeNs, TiZrN possesses higher hardness, better thermal stability, and greater corrosion resistance, compared with their counterpart binary TMeNs, and therefore TiZrN becomes a promising hard coating material and attracts attention from both academia and industry. The purposes of this study is to investigate the effects of duty cycle (D-series) and working pressure (P-series) on the structure and mechanical properties of TiZrN thin films. TiZrN coatings were deposited on Si substrate using high-power pulsed magnetron and unbalanced magnetron co-sputtering techniques (HPPMS/UBMS). The dc-UBMS was for sputtering zirconium, while HPPMS was for sputtering titanium. The duty cycle of HPPMS was adjusted from 100% to 5%, and at a duty cycle of 5%, the working pressure was varied from 3 to 10 mTorr. The results of X-ray diffraction (XRD) showed that the film structure varied with the two process parameters. As the duty cycle decreased, the preferred orientation of the TiZrN thin films changed from random to (200), with increasing peak power density finally reaching 0.3 kW/cm², indicating that the increase in energy facilitated the formation of the (200) texture. Residual stress increased with decreasing duty cycle, except in the D5 sample. However, the hardness, resistivity, and roughness of D-series samples remained unaffected by changing duty cycle. With increasing working pressure, the preferred orientation of the TiZrN coatings switched from (200) to (111). The cross-sectional observation of P-series samples by scanning electron microscopy revealed that the microstructure of the coatings changed from dense to a loosely packed columnar structure for the specimen deposited at the highest working pressure of 10 mTorr, where high roughness, high electrical resistivity, and low residual stress were also measured.

Keywords: TiZrN; high power pulse magnetron sputtering (HPPMS); duty cycle; working pressure

Many transition-metal carbides, nitrides, and oxides are inherently non-stoichiometric compounds, characterised by broad homogeneity ranges in their phase diagrams. Deviations from stoichiometry, defined as the ratio of non-metal to metal atoms (x), can drastically affect properties. While substoichiometric compounds (x<1) have been widely studied, superstoichiometric compounds (x>1) remain largely unexplored.

Here we present our finding on superstoichiometric (Al,Cr)N_x coatings sputter-deposited from an Al₆₀Cr₄₀ target at power densities reaching 840 W/cm². Experimental and computational analyses reveal that excess nitrogen predominantly occupies interstitial lattice sites. Upon surpassing a critical concentration (x≈1.06), grain renucleation rates increase, disrupting columnar growth and altering the preferential orientation from (111) to (220). The coatings exhibit a single-phase, face-centred cubic structure, a dense microstructure, and reduced surface roughness compared to benchmark coatings produced by cathodic arc evaporation.

Remarkably, hardness, fracture toughness, and wear resistance equal or exceed those of the benchmark coatings. Our findings highlight the advantages of superstoichiometric (Al,Cr)N_x as effective wear-resistant materials for advanced engineering applications, while also suggesting broader implications for the utilisation of superstoichiometric nitrides across various industries.

Understanding the relationship between thermally induced decomposition in metastable material systems and their mechanical properties is critical for designing thin films with improved wear resistance and thermal stability. Our previous work revealed that achieving improved solubility of B in fcc-TiN requires a deviation from the TiN-TiB tie line toward Ti-rich compounds, facilitated by the formation of vacancies in the non-metal sublattice. This deviation was achieved by non-reactive co-sputtering of a Ti target alongside TiN and TiB₂ targets, allowing full incorporation of 8.9 at.% B into the fcc-TiN lattice. In contrast, co-sputtering TiN and TiB₂ yielded compositions along the TiN-TiB₂ tie line, with excess B forming amorphous grain boundary phases [1]. In this study, we systematically annealed (1) a single-phase fcc-Ti–B–N coating with a composition of Ti_1.08B_0.18N_0.74 and (2) a Ti–B–N coating with amorphous B-rich grain boundary phases with a Ti_1.01B_0.21N_0.78 stoichiometry. Annealing was performed at 700°C, 800°C, 900°C, 1000°C, 1200°C and 1400°C for 10 minutes. Both coatings retained high hardness of 30±1 GPa up to 1200°C. The results of micro-cantilever bending tests indicate an inverse relationship between fracture toughness (K_IC) and annealing temperature (T_a). The Ti_1.01B_0.21N_0.78 coating exhibits a K_IC of 2.1 ± 0.1 MPa·m^0.5, which increases to 3.8 ± 0.3 MPa·m^0.5 upon increasing T_a to 1000°C, but K_IC for the Ti_1.08B_0.18N_0.74 sample was observed to decrease from 2.7 ± 0.1 MPa·m^0.5 in the as-deposited state to 2.3 ± 0.1 MPa·m^0.5 at T_a=1000°C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses confirm that K_IC improves only when metastable Ti–B–N decomposes into co-existing thermodynamically more stable fcc-TiN and hcp-TiB₂ phases without the formation of additional hcp-Ti precipitates. These findings highlight the critical influence of compositional and structural control on the thermal and mechanical stability of Ti–B–N thin films, providing new pathways for their use in high-performance applications.