Stoichiometric TiN Research Articles

Structural and corrosion behaviour of stoichiometric and substoichiometric TiN thin films

This paper reports the structural and electrochemical behaviour of TiN x thin films prepared by d.c. reactive magnetron sputtering. X-Ray diffraction showed the development of the hexagonal α-Ti phase, with strong [002] orientation, for low nitrogen contents. For nitrogen contents of 20 and 30 at.%, the ε-Ti 2 N phase appears with [200] orientation. With further increasing the nitrogen content, the δ-TiN phase becomes dominant. Composition and the resulting changes in microstructure (crystalline phases and the lattice distortion induced by the growth conditions) are the two main parameters that seem to rule coating properties. Results of potentiodynamic polarisation tests showed that all films have a high corrosion resistance reflected by corrosion current densities below 0.7 μA/cm 2. Also, electrochemical impedance spectroscopy tests corroborated the results obtained in the polarisation tests, showing that films containing low percentages of nitrogen (less than 8%) reveal the best corrosion resistance. Further increases in nitrogen content lead to a decrease in the corrosion resistance. An exception to this behaviour was found for the film with 30 at.% N. This sample presents an excellent corrosion resistance, which in fact, increases with the immersion time. Higher nitrogen contents (52 and 55 at.%) promote a relative increase in the corrosion resistance when compared with 50 at.% films. This behaviour might be explained by the particular microstructural characteristics of the films.

Growth, surface morphology, and electrical resistivity of fully strained substoichiometric epitaxial TiNx (0.67⩽x<1.0) layers on MgO(001)

We have grown single-crystal NaCl-structure δ-TiNx layers with x ranging from 0.67 to 1.00 on MgO(001) at 700 °C by ultra-high-vacuum reactive magnetron sputtering of Ti in mixed Ar/N2 discharges in order to investigate microstructural evolution and the physical properties of TiNx as a function of the N vacancy concentration. High-resolution x-ray diffraction and transmission electron microscopy results show that all layers grow with a cube-on-cube epitaxial relationship to the substrate, (001)TiN∥(001)MgO and [100]TiN∥[100]MgO. The relaxed lattice parameter ao(x) decreases linearly from 4.240 Å with x=1.00 to 4.226 Å with x=0.67. Stoichiometric TiN(001) layers are fully relaxed at the growth temperature while layers with 0.67⩽x⩽0.92 are fully coherent with their substrates. Surface morphologies vary dramatically with x. TiNx(001) layers with x=0.67–0.82 have very flat surfaces arising from large cation surface diffusion lengths approaching values corresponding to step flow. However, the surfaces of the TiN0.92(001) and TiN1.00(001) layers, which were grown at higher N2 partial pressures, consist of a periodic two-domain ripple structure along the 〈110〉 directions due to kinetic roughening associated with lower cation surface mobilities resulting from higher steady state N coverages. TiN1.0(001) layers grown in pure N2 exhibit growth mounds that are predominantly square with edges aligned along the 〈110〉 directions. The room-temperature resistivity, 13 μΩ cm with x=1.00, increases from 52 μΩ cm for TiNx(001) layers with x=0.92 to 192 μΩ cm with x=0.67, due primarily to increased carrier scattering from N vacancies.

Corrosion protection of magnetron sputtered TiN coatings deposited on high strength aluminium alloys

TiN x coatings were deposited on aluminium AA7075-T6 substrates by reactive DC magnetron sputtering. The main objective was to improve the impermeability to corrosive media by controlling the morphology of the coatings. It was found that by increasing the bias voltage during deposition to −150 V the normally arising columnar structure is reduced due to enhanced defect and nucleation site formation. In consequence these coatings exhibit a reduced porosity and therefore fewer possible paths for corrosive media like aqueous sodium chloride solution to reach the interface substrate/coating. A further effective improvement of the corrosion behaviour compared to stoichiometric TiN coatings was achieved by decreasing the nitrogen content in the TiN x layers below 20 at.% N. This has been confirmed by potentiodynamic measurements as well as salt spray tests. It is proposed that this further improvement is due to the formation of a thicker and denser surface oxide layer as soon as the coating is exposed to air.

Corrosion protection of magnetron sputtered TiN coatings deposited on high strength aluminium alloys

TiNx coatings were deposited on aluminium AA7075-T6 substrates by reactive DC magnetron sputtering. The main objective was to improve the impermeability to corrosive media by controlling the morphology of the coatings. It was found that by increasing the bias voltage during deposition to −150 V the normally arising columnar structure is reduced due to enhanced defect and nucleation site formation. In consequence these coatings exhibit a reduced porosity and therefore fewer possible paths for corrosive media like aqueous sodium chloride solution to reach the interface substrate/coating. A further effective improvement of the corrosion behaviour compared to stoichiometric TiN coatings was achieved by decreasing the nitrogen content in the TiNx layers below 20 at.% N. This has been confirmed by potentiodynamic measurements as well as salt spray tests. It is proposed that this further improvement is due to the formation of a thicker and denser surface oxide layer as soon as the coating is exposed to air.

Deposition pressure dependence of internal stress in TiN films deposited by filtered cathodic vacuum arc

TiN films were deposited by an off-plane double bend filtered cathodic vacuum arc technique. The composition, structure, and surface morphology of the films were characterized by x-ray photoelectron spectroscopy, x-ray diffraction, and atomic force microscopy, respectively. Internal stress was determined by a substrate bending method. The influence of the deposition pressure on the composition, structure, and internal stress of the films was studied systematically. At a deposition pressure of 1×10−5 Torr, the films are composed of single α-TiN0.30 phase with fairly low internal stress, and the atomic ratio of N to Ti is 0.32. As the deposition pressure increases to 5×10−5 Torr, the N/Ti ratio increases to 0.56, and the films are composed of a mixture of hexagonal α-TiN0.30 and cubic TiN0.90. The formation of TiN0.90 phase and the mismatch of these two kinds of phases contribute to a dramatic increase of internal stress in the films. The increase of deposition pressure to 2×10−4 Torr results in the formation of stoichiometric TiN films with single TiN phase, which corresponds to slightly lower internal stress. However, a further increase of deposition pressure results in a continuous increase in the N/Ti ratio and the formation of overstoichiometric films. The incorporation of excess N atoms in the films accounts for the further increase of internal stress.

XPS, AFM and nanoindentation studies of Ti 1− xAl xN films synthesized by reactive unbalanced magnetron sputtering

A combined investigation of chemical analysis and bonding states in Ti 1− x Al x N films sputter-deposited on Si(1 0 0) in an Ar–N 2 gas mixture by X-ray photoelectron spectroscopy and of surface morphology and mechanical properties by atomic force microscopy (AFM) and nanoindentation measurements is reported. It was found that a linear increase in the Al concentration of the films was observed with increasing Al target current up to 7 A, while the reverse trend was seen for the Ti concentration. The Al and Ti in the films presented in a form of stoichiometric TiN and AlN at different atomic concentration of Al. Several types of chemical states, such as Al 2O 3, TiN x O y , Ti 2O 3 and TiO 2 have been identified. However, no unbound Al and Ti atomic species were detected in films. By applying the height–height correction functions to the measured AFM images, a steady growth roughness exponent α=0.94±0.03 was determined for all the Ti 1− x Al x N films. The value of α is consistent with growth model predictions incorporating surface diffusion. It was also found that the improved mechanical properties of Ti 1− x Al x N films with the addition of Al into TiN matrix were attributed to their densified microstructure with development of fine grain size and reduced surface roughness. The effect of aluminium in stabilizing the Ti–Al–N structure was also elucidated and explained on the basis of the adatom mobility and the surface diffusion of atoms.

Structure–property relationships in single- and dual-phase nanocrystalline hard coatings

The structure of hard coatings deposited by PVD (physical vapor deposition) strongly depends on the growth parameters. In this work, the influence of microstructure and chemical composition on mechanical properties was investigated in detail for several nanocrystalline hard coatings. Single-phase TiN, CrN and TiB 2 as well as dual-phase CrN–Cr 2N, TiN–TiB 2 and TiC–TiB 2 coatings were prepared by reactive or non-reactive unbalanced dc magnetron sputtering, respectively. The hardness of the coatings investigated will be discussed with respect to the average grain size and residual biaxial stresses. By optimizing both nanostructure and biaxial stresses of stoichiometric TiN coatings, their microhardness ( H) could be improved from 33 to 56 GPa, whereas the reduced Young's modulus ( E *) changed from 402 to 480 GPa. Thus, the ratio H 3/ E *2 which is representative for the resistance to plastic deformation could be increased from 0.222 to 0.806 GPa. For Cr–N coatings, H 3/ E *2 values up to 0.521 GPa were obtained, depending on their chemical composition and nanostructure. The nanocrystalline dual-phase coatings TiN–TiB 2 and TiC–TiB 2 even yielded H 3/ E *2 values up to 1.332 and 1.575 GPa, respectively. Comparing the different single- and dual-phase coatings investigated, the establishment of correlations between structure and mechanical properties enables the development of advanced coatings with high hardness and high resistance against plastic deformation. In addition, this work shows the tremendous importance of a controlled deposition process for optimizing coating properties.

Optical and electrical properties of TiN/n-GaN contacts in correlation with their structural properties

The optical and electrical properties of TiN contacts on Si-doped GaN were investigated in correlation with their structural properties. Stoichiometric TiN films were directly deposited on 2.5 µm thick n-GaN by dc reactive magnetron sputtering at room temperature, while the stoichiometry and the structural characteristics of the TiN films were determined by in situ spectroscopic ellipsometry (SE). SE was also used for characterization of the GaN surface and for chemical etching of gallium oxide. Current–voltage measurements showed an ohmic behaviour for the as-deposited and annealed TiN/GaN samples. The specific contact resistivity was found to be 4.5 × 10−3 Ω cm2 for the as-deposited TiN film, becoming as low as 5.9 × 10−4 Ω cm2 after annealing at 400 °C. Further thermal treatment over 500 °C resulted in significant TiN oxidation and poor adhesion of the TiN film on the GaN, leading to an increase in specific contact resistivity. Transmission electron microscopy revealed structural and interfacial contact changes after high thermal treatment.

On the computation of the absolute hardness of thin solid films

A study has been conducted in order to evaluate the descriptive capability of a modified form of the models advanced by Jönsson and Hogmark, Burnett and Rickerby, Chicot and Lesage and Korsunsky et al., regarding the composite hardness measurements carried out on different substrate–film systems. The modification that has been introduced involves the incorporation of the indentation size effect on both the substrate and film hardness through the power relationship, for the description of the change in hardness with the indentation diagonal, earlier proposed by Meyer. The modified models were first evaluated employing the data already published by Chicot et al., for Ti and TiC films formed on the surface of a chromium steel of a high carbon content. Subsequently, they were tested employing the data obtained experimentally with three under stoichiometric TiN x coatings, deposited by PVD magnetron sputtering on 316L stainless steel. It has been determined that the composite hardness measured on such systems can be satisfactorily described by means of any of the four models analyzed. However, it has also been found that the model proposed by Korsunsky and co-workers not only gives rise to the minimum quadratic difference between most of the measured and predicted values of the composite hardness, but also provides useful information regarding the film toughness. An optimization procedure is outlined which can be used in order to compute unambiguously and simultaneously all the constants involved in any of the four modified models, taking into consideration all the experimental measurements. The models advanced by Burnett and Rickerby, Chicot and Lesage and Korsunsky et al. have been found to predict a smooth increase in hardness as the indentation diagonal decreases, outside the range of experimental measurements. On the contrary, it has been shown that the model advanced by Jönsson and Hogmark breaks down when tested outside the range of experimental values of hardness, particularly at low indentation diagonal values.

Effect of ion bombardment and substrate orientation on structure and properties of titanium nitride films deposited by unbalanced magnetron sputtering

The effect of substrate orientation and ion bombardment during the growth on the structure and properties of TiN films deposited by reactive unbalanced magnetron sputtering has been reported. Films deposited at a nitrogen partial pressure of 5×10−5 mbar and a current density of 2.50 mA cm−2 were golden yellow in color, characteristic of stoichiometric TiN. The effect of Si(100) and Si(111) substrates on the TiN film along with the substrate bias has been investigated. With an increase in the substrate bias on Si(111) substrate, TiN(111) is the most preferred orientation. On a Si(100) substrate with an increase in the substrate bias, TiN(220) orientation has been observed. The influence of the substrate on the growth of TiN films has been explained in terms of surface energy. The variation of grain size, resistivity, and the internal stress of TiN films as the function of substrate bias have also been investigated.

Wear behavior of PACVD tin coatings deposited onto tool steels

During cold forming processes like deep drawing of steel sheets, tools are subjected to various loads. During forming, Hertzian contact pressures and frictional forces cause fatigue and adhesion, respectively. Abrasion may take place if wear particles are entrapped in the contact zone. Therefore, the aim of this work is to evaluate TiN coatings deposited by plasma assisted chemical vapor deposition (PACVD) onto tool steels with respect to adhesion, fatigue wear properties. Stoichiometric TiN coatings with different morphologies were deposited with and without plasma-nitriding of the steel substrate and characterized by X-ray diffraction (XRD) and scanning electron microsopy. The adhesion of the coatings was investigated using the VDI indention test and a surface fatigue test, where cyclic loads up to 80 kN are applied to the tool surface by cylindrical indenters. Adhesive wear of the coatings have been evaluated using a ball-on-disc tribometer. Abrasion resistance was characterized using a small-scale abrasive wear test.

Effect of coated edge geometry on internal stress distribution in multilayered coatings

This paper presents results of computation internal stresses in several types of hard coatings deposited on rounded edges whose radii are of the same order as the coating thickness. The model of a coated substrate is a straight edge consisting of two planar surfaces with varying opening angle α joined by cylindrically shaped surface. The internal stresses are computed by the finite element method. Four different combinations of TiTiN multilayers are studied and especially the peak values of internal stresses in the interface are compared. It is shown that in multi-layer sandwich coatings, the maximum stress values are lower, nevertheless, the most important role has the ratio coating thickness/edge radius. For this reason, in order to suppress the stress in the coating-substrate interface in the coated edge, a special care should be denoted to the edge smoothing before the deposition process. It is also shown that the optimum coating model is a bilayer coating consisting of a gradual Ti–TiN interlayer and outer layer of stoichiometric TiN.

Preparation and characterization of TiN films by electron cyclotron resonance (ECR) sputtering for diffusion barrier applications

An electron cyclotron resonance (ECR) plasma system has been used to deposit TiN films by reactive sputtering process. The effect of magnetic mirror field on the current–voltage characteristics of the cylindrical sputtering cathode has been studied. Stoichiometric TiN films with minimum stress and specific resistivity of 82 μΩ cm and surface roughness of 3 Å have been deposited at a substrate temperature of 350°C. The films showed strong (2 0 0) orientation on Si(1 0 0), Si(1 1 1) and glass substrates. The use of TiN as a barrier layer in copper metalisation has been investigated. No detectable diffusion was observed up to a temperature of 700°C for a film thickness of 600 Å. The resistivity data is in conformity with the RBS depth profiling. The quality of the films has been explained in terms of improved microstructure and packing density as a result of high-density ion bombardment.

Interfacial dislocations in TiN/GaN thin films

Thin films of stoichiometric cubic TiN for ohmic contact formation aredirectly deposited, by rf magnetron sputtering at room temperature, on(0001) surfaces of GaN epilayers grown on c-plane sapphire. Chemicaletching and in situ dry etching of the free GaN surface allows anepitaxial growth of the deposited TiN films as revealed by high resolutionelectron microscopy observations. Taking into account the experimentallydetermined orientation relationship of the two structures, (0001) GaN∥(111) TiN, [112̄0] GaN∥[11̄0] TiN, familiesof dislocations are expected in the interface plane to accommodate the misfit(~5.8%) with a spacing of 4.5 nm. The mathematical formulation of thecircuit mapping technique is used to analyse the misfit dislocation content onHREM images since it allows the exact determination of the Burgers vector.This gives a result ½[011̄] TiN or (1/3)[12̄10] GaN.A demistep of height cGaN/2 is observed at the TiN/GaN interface and fromthere a (112̄) twin emanates into the TiN layer. It appears that suchdemisteps have an important role in mediating the columnar growth of the TiNmaterial.

Formation of titanium nitrides via wet reaction ball milling

A new mechanochemical route to synthesize TiN has been developed by ball milling elemental titanium powders with the organic compound pyrazine in a benzene solution for periods of up to 336 h. The structures of the milled powders were examined by means of X-ray diffractometry. Titanium nitrides were formed directly during the milling. Unlike the dry ball-milling process, the present wet milling resulted in the formation of an intermediate titanium nitride Ti 2N which has never been found in previous studies using dry milling. With increasing milling time, Ti 2N was observed to gradually transform into the stoichiometric compound TiN. Upon heating, the Ti 2N formed during milling was completely transformed to TiN. The intrinsic mechanisms of the solid–liquid reactions are discussed.

Plasma source low-energy ion-enhanced deposition of thin films

Plasma source low-energy ion-enhanced deposition is a new approach for the deposition of thin films on steel and alloys with specific advantages over ion beam-enhanced deposition (IBED) and the complex combined plasma source ion implantation (PSII) and IBED (PSII-IBED). It may be used to develop a thin film process using the plasma-based low-energy ion implantation including plasma source ion nitriding and plasma source ion carburizing. A deposition apparatus based on an electron cyclotron resonance (ECR) microwave plasma source, a magnetron sputtering target and a pulsed low-voltage power supply from 0.5 to 2 kV has been developed. The magnetron sputter deposition of Ti metal films and plasma source low-energy nitrogen ion implantation under a high nitrogen ion implantation dose rate of 0.65–1.20 mA/cm 2 are used alternately in order to synthesize the stoichiometric TiN films on A3 mild steel and M2 high-speed steel at a deposition rate of 50–110 nm/min. The Knoop microhardness of the 1.2–1.6 μm thick TiN films on the M2 steel is in the range of 18.0–20.0 GPa (0.05 N load). The critical loads of the TiN films on the M2 steel measured by the scratch test are 22–35 N corresponding to the formation of an intermixed layer of about 40 nm thick between the film and the steel substrate. The corrosion resistance of the TiN films in 0.5 mol/l H 2SO 4 solution is evaluated by performing electrochemical polarization measurements. Their superior corrosion resistance is a result of the dense fine-grained microstructure and is independent of the preferential orientation of the films.

The synthesis of TiN by ball-milling – a neutron diffraction study

Titanium nitride of FCC B1 structure forms readily on milling Ti in nitrogen and ammonia environments. Rietveld refinements of time-of-flight neutron diffraction patterns show that stoichiometric TiN is produced on milling Ti in nitrogen, whereas the off-stoichiometric nitride TiN 0.7, associated with the presence of hydrogen, is formed on milling in ammonia. The transformation of Ti to the nitride proceeds more rapidly in ammonia due to the catalytic effect of hydrogen.

The effect of substrate temperature and biasing on the mechanical properties and structure of sputtered titanium nitride thin films

The mechanical properties of titanium nitride (TiNx) thin films have been investigated using depth sensing nanoindentation tests. The effects of substrate temperature (Ts) and of substrate biasing (Vb) on the mechanical properties and the microstructure of the TiNx films were studied. Ts and Vb have strong effect on the film's microstructural characteristics such as density, grain size and orientation. It was found that deposition at high Ts and Vb promotes the growth of (002) oriented films with density close to the bulk density of stoichiometric TiN, indicating the absence of voids and the growth of stoichiometric TiN. The film hardness and elastic modulus were measured using the continuous stiffness measurements technique. It was found that there exists a direct correlation between the film's mechanical properties and microstructure. The films that exhibit the best mechanical performance are those grown along the (002) orientation and being denser and stoichiometric.

Direct gaseous nitridation of the Ti–6Al–4V alloy by nitrogen

The gaseous nitridation of the Ti–6Al–4V alloy in a furnace with nitrogen improves strongly the superficial microhardness of the alloy from 405 to 2133 HV0.3. For temperatures between 1175 and 1300°C, it leads to a very complex structure composed (in cross-section, from the surface to the heart) of several layers: (i) stoichiometric TiN, (ii) sub-stoichiometric δ-TiN0.7 (without vanadium and aluminium), (iii) a thin film of alloy enriched in vanadium and aluminium, (iv) an α-(Ti, N) phase, containing aluminium (from TiAl0.12N0.25 to TiAl0.12N0.12) and (v) the Ti–6Al–4V alloy with needles of the α-(Ti, N) phase together with aluminium and traces of nitrogen (less than TiAl0.12N0.05). The reaction, which is not influenced by the pressure of nitrogen, involves a gaseous diffusion through the superficial layers and is governed by the volumic diffusion of nitrogen in the α-phase, giving parabolic kinetics with the rate law: where Δm/S is the gain of mass per surface unit (g cm−2), R the gas constant and T the temperature (K).

The microstructure and electrical properties of directly deposited TiN ohmic contacts to Gallium Nitride.

When the stoichiometric TiN was deposited directly on GaN, we obtained columnar TiN grains of 5-20 nm section which cross the whole film thickness and are rotated mostly around the [111] axis. The conventional epitaxial relationship is obtained and no amorphous patches are observed at the interface. The deposition of TiN on Si doped GaN layers lead to the formation of an ohmic contact, whereas we obtain a rectifying contact on p type layers.