Greger Håkansson

Oxidation of metastable single‐phase polycrystalline Ti0.5Al0.5N films: Kinetics and mechanisms

Metastable single‐phase, NaCl‐structure, polycrystalline Ti0.5Al0.5N alloy films have been shown to exhibit much better high‐temperature (750–900 °C) oxidation resistance than polycrystalline TiN films grown under similar conditions. The Ti0.5Al0.5N alloys, ≂3 μm thick, were deposited at temperatures between 400 and 500 °C on stainless‐steel substrates by dc magnetron sputter deposition in mixed Ar+N2 discharges with an applied negative substrate bias Vs of either 0 or 150 V. Oxidation in pure O2 initially occurred at a rate that varied parabolically with time. The oxide overlayers consisted of two partially crystalline sublayers, the upper one Al‐rich and the lower one Ti‐rich, with no measurable N concentrations in either. Inert‐marker transport experiments showed that oxidation proceeded by the simultaneous outward diffusion of Al to the oxide/vapor interface and inward diffusion of O to the oxide/nitride interface. The oxidation rate constant K increased with oxidation temperature Tox at a rate much h...

Microstructure and physical properties of polycrystalline metastable Ti0.5Al0.5N alloys grown by d.c. magnetron sputter deposition

Metastable single-phase polycrystalline Ti0.5Al0.5N alloy films, approximately 3 μm thick, have been grown on stainless steel substrates by d.c. magnetron sputter deposition from a TiAl target in mixed Ar-N2 discharges. The deposition temperature was between 400 and 500 °C and the applied negative substrate bias Vs was varied from 0 to 250 V. Electron microprobe analyses, using pure elemental standards, showed that the film composition was independent of Vs to within the experimental accuracy, ±3 at.%. All films were found, based upon X-ray diffraction (XRD) and transmission electron microscopy (TEM), to have a B1 NaCl structure (AlN crystallizes in the wurtzite structure) with a (111) preferred orientation. The lattice parameter a0 of films grown at Vs < 80 V was 0.4176 nm and cross-sectional TEM showed that such films had a columnar structure exhibiting a high intercolumn, as well as intragrain, porosity. Increasing Vs above 80 V, however, resulted in a rapid increase in both a0 and the width of the XRD peaks, while the average grain size decreased from 105 nm at Vs=0 to about 35 nm. There was also a corresponding decrease in the void density at high substrate biases, the columnar structure was interrupted and the dislocation density increased. All these effects were associated with the introduction and/or agglomeration of ion-irradiation- induced defects in the films. The microindentation hardness of the films increased with increasing substrate bias and ranged from 1000 kgf mm−2 at Vs = 0 to 4200 kgf mm−2 at Vs = 250 V.

Microstructures of TiN films grown by various physical vapour deposition techniques

Abstract A study has been made of TiN coatings deposited on steel substrates by five commercially available physical vapour deposition (PVD) methods; low voltage electron beam evaporation, triode high voltage electron beam evaporation, random-arc evaporation, steered-arc evaporation and magnetron sputtering. The microstructure and substrate-film interfacial microchemistry of the films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (EDX), cross-sectional transmission electron microscopy (XTEM) and scanning transmission electron microscopy (STEM) combined with EDX analyses of XTEM samples. The XRD analyses showed that all the films were in a state of compressive stress with interplanar distances as much as 1.7% higher than reference bulk values. SEM examination revealed only minor variations in surface roughness among the samples except for the arc-evaporated films which contained large droplets and craters resulting from the detachment of droplets. The number density and average sizes of droplets and craters were lower in the steered-arc sample than in the random-arc sample. XTEM analyses showed that all the films had columnar structures with clearly defined substrate-film interfacial layers. The films appeared dense except for the magnetron-sputtered sample which exhibited intercolumnar porosity. STEM-EDX analyses showed large variations in the microchemistry of the substrate-film interfacial regions which consisted, depending on the sample, of renucleated near-surface substrate grains, intentionally (or, in at least one case, unintentionally) introduced foreign material or gas-bubble-like inclusions. However, the microchemistry of these interfacial regions was, in most cases, understandable on the basis of the substrate pretreatment and/or choice of film growth parameters.

Microstructure, stress and mechanical properties of arc-evaporated Cr–C–N coatings

Abstract The relationships between coating microstructure and properties in the Cr–C–N system have been investigated as a function of composition and post-deposition annealing. Coatings of varying compositions were grown using arc-evaporation, by varying the reactive gas flow ratio fR=f(C2H4)/f(N2) from 0 to 0.2, and were found to consist primarily of the cubic δ-Cr(C,N) phase. Changes in both the unstressed lattice parameter, ao, and X-ray diffraction background intensity indicate that both the carbon concentration within the δ-phase and amorphous/crystalline content increases with fR. Increasing fR also decreases the magnitude of the compressive biaxial residual stress, from approximately 6 to 1 GPa, while increasing both the inhomogeneous stress and thermal stability. The elastic modulus and hardness of as-deposited coatings were determined from nanoindentation to be 320 and 23 GPa, respectively, for moderate carbon concentrations (fR≤0.05). Concurrent variations in microstructure and hardness with post-deposition annealing indicate that the as-deposited hardness is significantly enhanced by the microstructure, primarily by lattice defects and related stresses (microstresses) rather than average stresses (macrostresses).

Microstructural investigation of droplets in arc-evaporated TiN films

Abstract The microstructure and composition of macro particles (droplets) in TiN films deposited by arc evaporation on cemented-carbide substrates were investigated using a combination of scanning electron microscopy, cross-sectional transmission electron microscopy (TEM) including lattice resolution TEM, X-ray diffraction (XRD), energy-dispersive X-ray analysis and electron energy loss spectroscopy. The apparent surface number density of droplets, with diameters of 0.1–10 μm, was found to be about 10 7 cm −2 . Droplets were incorporated in the film at various distances from the substrate surface. In between the droplets, the TiN films exhibited a dense columnar microstructure. On top of the incorporated droplets, the TiN films grew in a pronounced columnar structure with a column diameter close to the droplet diameter. The core of the droplets consisted of equiaxed grains of an α-Ti superstructure containing approximately 3–5 at.% N whereas the rim of the droplets had increasing nitrogen content up to 50 at.% XRD showed evidence for the presence of Ti 2 N possibly at the rim of the droplets. Beneath each droplet a large voided region was observed with the shape of a flattened torus as a consequence of droplets being incorporated in the solid state and subsequent shadowing of the Ti flux to the film.

Transmission electron microscopy studies of microstructural evolution, defect structure, and phase transitions in polycrystalline and epitaxial Ti1-xAlxN and TiN films grown by reactive magnetron sputter deposition

Abstract The microstructural evolution, as revealed by plan-view and cross-sectional transmission electron microscopy of polycrystalline NaCl-structure Ti 1− x Al x N ( x ≤0.5) and TiN films deposited by reactive magnetron sputter deposition is similar. At low temperatures ( T s ≤500° C ) and in the absence of ion bombardment, the film are underdense and exhibit a pronounced columnar morphology. The addition of low energy ( E i ⪅200 eV )ion irradiation during deposition using incident ion-to-neutral ratios J i J n ≤1 in mixed Ar N 2 plasmas results in film densification, defect incorporation and a more equiaxed grain structure. At higher incident flux ratios ( J i J n ⩾4 ) , low-energy ion irradiation provides increased apparent adatom mobility and, hence, larger grain size. The epitaxial temperature for μm-thick films of both Ti 1− x Al x N ( x ≤0.5) and TiN, deposited in pure N 2 discharges, on Mg(100) substrates, is ≈ 500°C. The density of {111} dislocation loops n d , the primary defects, was found to depend both on T s and E i . For example n d was decreased by several orders of magnitude using low-energy ion irradiation. However, the use of higher-energy (≥ 300 eV) ion irradiation gave rise to the formation of Ar and/or N 2 gas bubbles due to precipitation of trapped ions. Plastic deformation in the films took place through glide of edge dislocations. Phase transition reaction paths were determined for Ti 1− x Al x N as a function of x and T s . In the case of epitaxial Ti 0.5 Al 0.5 N, increasing T s resulted in surface-initiated spinodal decomposition during growth in the range T s = 540−560 °C with the formation of compositionally modulated NaCl-structure plateles along [100] as a precursor to bulk phase separation of wurtzite-structure AlN and NaCl-structure TiN at higher T s . In the case of Ti 1− x Al x N grown on oxidized silicon at T s =500 ° C , increasing x resulted in single-phase NaCl-structure alloys for 0⩽ x ⪅0.4, two-phase TiN and AlN-structure for 0.4 x ⪅0.9, and single-phase AlN for x >0.9. Pseudomorphic forces associated with growth on MgO(001) substrates extended the stability range for the NaCl-structure phase to x ⩾0.5 at T s °C.

Microstructural evolution during tempering of arc-evaporated Cr–N coatings

Cr-N coatings were arc-deposited at 50 and 300 V. The changes in the coating microstructure and phase content during tempering were monitored. As a result, the phase stability and activation energi ...

Properties of TiN and CrN coatings deposited at low temperature using reactive arc-evaporation

Abstract PVD TiN and CrN coatings were deposited at both low (≈200°C) and standard temperatures (≈400 °C) using reactive arc- evaporation. Variations in microstructure (obtained by TEM and XRD), morphology (studied using SEM) and chemical composition (determined by both EDS and GD-OES) were correlated to mechanical (residual stress state, microhardness) and tribological properties (scratch response, abrasive wear resistance). The results show a significant increase in hardness and residual compressive stress for the low-temperature TiN coating as compared with the standard-temperature coating. This was attributed to a drastic decrease in grain size and an increased compressive microstrain for the low-temperature coating. Moreover, the critical load and abrasive wear resistance of the low- temperature TiN coating had decreased as compared with the standard-temperature coating, although the hardness was significantly higher. This was due to the poor quality of the low-temperature TiN coating in combination with the high residual compressive stress. It is further shown that mechanical and tribological properties of the low-temperature CrN coating were comparable with the standard-temperature CrN coating, even though there was a difference in microstructure and chemistry.

Microstructure–property relationships in arc-evaporated Cr–N coatings

Chromium nitride (Cr-N) coatings have received increased attention for tribological applications due to their favorable properties including wear resistance, toughness and oxidation resistance. The ...

Growth of TaC thin films by reactive direct current magnetron sputtering: Composition and structure

Single‐phase TaC films were grown by reactive dc magnetron sputtering onto high speed steel substrates kept at 650 °C (0.22 Tm ). The films were grown in both Ar/CH4 and Xe/CH4 mixtures using target‐to‐substrate distances, dts, of 6 and 15 cm. For all growth conditions, plasma‐probe measurements were carried out. As‐deposited films were analyzed using Auger electron spectroscopy (AES), x‐ray diffraction (XRD), and transmission electron microscopy (TEM). The C content of the films, as measured with AES, was found to increase linearly with increasing CH4 partial pressure in all cases. However, both the slope of these curves and the CH4 pressure needed in order to obtain a certain C content in the films were different depending on both dts and the sputtering gas. These results are explained with arguments based on cracking of the CH4 molecules into more reactive CHn radicals in the plasma and a different gas scattering of these radicals compared to the sputtered Ta atoms. Also, the structure of the deposited...