RESUMEN
Tailoring mechanical properties of transition metal carbides by substituting carbon with nitrogen atoms is a highly interesting approach, as thereby the bonding state changes towards a more metallic like character and thus ductility can be increased. Based on ab initio calculations we could prove experimentally, that up to a nitrogen content of about 68% on the non-metallic sublattice, Ta-C-N crystals prevail a face centered cubic structure for sputter deposited thin films. The cubic structure is partly stabilized by non-metallic as well as Ta vacancies - the latter are decisive for nitrogen rich compositions. With increasing nitrogen content, the originally super-hard fcc-TaC0.71 thin films soften from 40 GPa to 26 GPa for TaC0.33N0.67, accompanied by a decrease of the indentation modulus. With increasing nitrogen on the non-metallic sublattice (hence, decreasing C) the damage tolerance of Ta-C based coatings increases, when characterized after the Pugh and Pettifor criteria. Consequently, varying the non-metallic sublattice population allows for an effective tuning and designing of intrinsic coating properties.
RESUMEN
The demand to discover new materials is scientifically as well as industrially a continuously present topic, covering all different fields of application. The recent scientific work on thin film materials has shown, that especially for nitride-based protective coatings, computationally-driven understanding and modelling serves as a reliable trend-giver and can be used for target-oriented experiments. In this study, semi-automated density functional theory (DFT) calculations were used, to sweep across transition metal diborides in order to characterize their structure, phase stability and mechanical properties. We show that early transition metal diborides (TiB2, VB2, etc.) tend to be chemically more stable in the AlB2 structure type, whereas late transition metal diborides (WB2, ReB2, etc.) are preferably stabilized in the W2B5-x structure type. Closely related, we could prove that point defects such as vacancies significantly influence the phase stability and even can reverse the preference for the AlB2 or W2B5-x structure. Furthermore, investigations on the brittle-ductile behavior of the various diborides reveal, that the metastable structures are more ductile than their stable counterparts (WB2, TcB2, etc.). To design thin film materials, e.g. ternary or layered systems, this study is important for application oriented coating development to focus experimental studies on the most perspective systems.
RESUMEN
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 Ti1-xAlxN 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 Ti1-xAlxN 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 Ti1-xAlxN as compared with their as-deposited counterpart. The ability of Ti1-xAlxN 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.
RESUMEN
While it is well-known that supersaturated cubic-structured Ti1-x Al x N can be prepared by physical vapor deposition, the impact of point defects on formation process and cubic to wurtzite transition is largely unexplored. Irrespective of point defects, ab initio calculations correctly predict the Al concentration of the cubic to wurtzite transition. By means of density functional theory we show that vacancies on metal and/or non-metal sites only slightly affect the cubic to wurtzite transition region, whereas they clearly affect the physical properties.
RESUMEN
Exploiting the mechanisms responsible for the exceptional properties of aluminum based nitride coatings, we apply ab initio calculations to develop a recipe for designing functional thin film materials based on ternary diborides. The combination of binary diborides, preferring different structure types, results in supersaturated metastable ternary systems with potential for phase transformation induced effects. For the exemplary cases of M x W1 - x B2 (with M = Al, Ti, V) we show by detailed ab initio calculations that the respective ternary solid solutions are likely to be experimentally accessible by modern depositions techniques.
RESUMEN
The complex structure of Ta2O5 led to the development of various structural models. Among them, superstructures represent the most stable configurations. However, their formation requires kinetic activity and long-range ordering processes, which are hardly present during physical vapor deposition. Based on nano-beam X-ray diffraction and concomitant ab initio studies, a new metastable orthorhombic basic structure is introduced for Ta2O5 with lattice parameters a = 6.425 Å, b = 3.769 Å and c = 7.706 Å. The unit cell containing only 14 atoms, i.e. two formula unit blocks in the c direction, is characterized by periodically alternating the occupied oxygen site between two possible positions in succeeding 002-planes. This structure can be described by the space group 53 (Pncm) with four Wyckoff positions, and exhibits an energy of formation of -3.209 eV atom-1. Among all the reported basic structures, its energy of formation is closest to those of superstructures. Furthermore, this model exhibits a 2.5 eV band gap, which is closer to experimental data than the band gap of any other basic-structure model. The sputtered Ta2O5 films develop only a superstructure if annealed at temperatures >800 °C in air or vacuum. Based on these results and the conveniently small unit cell size, it is proposed that the basic-structure model described here is an ideal candidate for both structure and electronic state descriptions of orthorhombic Ta2O5 materials.
RESUMEN
The influence of reactive and non-reactive sputtering on structure, mechanical properties, and thermal stability of Zr1 - xAlxN thin films during annealing to 1500 °C is investigated in detail. Reactive sputtering of a Zr0.6Al0.4 target leads to the formation of Zr0.66Al0.34N thin films, mainly composed of supersaturated cubic (c) Zr1 - xAlxN with small fractions of (semi-)coherent wurtzite (w) AlN domains. Upon annealing, the formation of cubic Zr-rich domains and growth of the (semi-)coherent w-AlN domains indicate spinodal-like decomposition. Loss of coherency can only be observed for annealing temperatures above 1150 °C. Following these decomposition processes, the hardness remains at the as-deposited value of ~ 29 GPa with annealing up to 1100 °C. Using a ceramic (ZrN)0.6(AlN)0.4 target and sputtering in Ar atmosphere allows preparing c-Zr0.68Al0.32N coatings with a well-defined crystalline single-phase cubic structure combined with higher hardnesses of ~ 31 GPa. Due to the absence of (semi-)coherent w-AlN domains in the as-deposited state, which could act as nucleation sites, the decomposition process of c-Zr1 - xAlxN is retarded. Only after annealing at 1270 °C, the formation of incoherent w-AlN can be detected. Hence, their hardness remains very high with ~ 33 GPa even after annealing at 1200 °C. The study highlights the importance of controlling the deposition process to prepare well-defined coatings with high mechanical properties and thermal stability.
RESUMEN
Protective coatings for high temperature applications, as present e.g. during cutting and milling operations, require excellent mechanical and thermal properties during work load. The Ti(1 - x)Al(x)N system is industrially well acknowledged as it covers some of these requirements, and even exhibits increasing hardness with increasing temperature in its cubic modification, known as age hardening. The thermally activated diffusion at high temperatures however enables for the formation of wurtzite AlN, which causes a rapid reduction of mechanical properties in Ti(1 - x)Al(x)N coatings. The present work investigates the possibility to increase the formation temperature of w-AlN due to Hf alloying up to 10 at.% at the metal sublattice of Ti(1 - x)Al(x)N films. Ab initio predictions on the phase stability and decomposition products of quaternary Ti(1 - x - y)Al(x)Hf(y)N alloys, as well as the ternary Ti(1 - x)Al(x)N, Hf(1 - x)Al(x)N and Ti(1 - z)Hf(z)N systems, facilitate the interpretation of the experimental findings. Vacuum annealing treatments from 600 to 1100 °C indicate that the isostructural decomposition, which is responsible for age hardening, of the Ti(1 - x - y)Al(x)Hf(y)N films starts at lower temperatures than the ternary Ti(1 - x)Al(x)N coating. However, the formation of a dual phase structure of c-Ti(1 - z)Hf(z)N (with z = y/(1 - x)) and w-AlN is shifted to ~ 200 °C higher temperatures, thus retaining a film hardness of ~ 40 GPa up to ~ 1100 °C, while the Hf free films reach the respective hardness maximum of ~ 38 GPa already at ~ 900 °C. Additional annealing experiments at 850 and 950 °C for 20 h indicate a substantial improvement of the oxidation resistance with increasing amount of Hf in Ti(1 - x - y)Al(x)Hf(y)N.
RESUMEN
Transition metal nitrides like CrN and TiN are widely used in automotive applications due to their high hardness and wear resistance. Recently, we showed that a multilayer architecture of CrN and TiN, deposited using the hybrid-high power impulse magnetron sputtering (HIPIMS) and direct current magnetron sputtering (DCMS)-HIPIMS/DCMS deposition technique, results in coatings which indicate not only increased mechanical and tribological properties but also friction coefficients in the range of diamond-like-carbon coatings when tested at RT and ambient air conditions. The modulated pulsed power (MPP) deposition technique was used to replace the HIPIMS powered cathode within this study to allow for a higher deposition rate, which is based on the complex MPP pulse configuration. Our results on MPP/DCMS deposited CrN/TiN multilayer coatings indicate excellent mechanical and tribological properties, comparable to those obtained for HIPIMS/DCMS. Hardness values are around 25 GPa with wear rates in the range of 2 × 10-16 Nm/m3 and a coefficient of friction around 0.05 when preparing a superlattice structure. The low friction values can directly be correlated to the relative humidity in the ambient air during dry sliding testing. A minimum relative humidity of 13% is necessary to guarantee such low friction values, as confirmed by repeated tests, which are even obtained after vacuum annealing to 700 °C. Our results demonstrate that the co-sputtering of high metal ion sputtering techniques and conventional DC sputtering opens a new field of applications for CrN/TiN coatings as high wear resistance and low friction coatings.
