ICMCTF2012 Session BP: Symposium B Poster Session

Protective hard coatings make use of the outstanding mechanical and thermal stability of early transition metal nitrides (TMN) and their alloys with Al. Modern applications require sophisticated designs of the protective thin films in which a multiscale modelling plays an important role. For example, stress management of the thin film is a crucial factor affecting its high temperature stability. This is an ideal task for the finite elements method where elastic properties of the studied materials are the necessary input.

The scatter of published elastic constants for the systems of interest is enormous. For example, for TaN one can find the value of C₁₁ ranging from 680 GPa to 880 GPa. This of course raises questions about reliability not only of the first principle calculations but also the subsequent larger-scale modelling using these values as inputs as well as how compatible are the values published in various papers.

In this paper we critically revise the computational methodology of second and third order elastic constants calculations focusing on a large family of binary TMN (TM=Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta) and AlN in their cubic (B1, NaCl prototype) modification. Apart from the single crystal properties, also estimates for polycrystalline samples will be discussed which are more appropriate for modelling of the thin films. In addition, we will present description of the single crystal and polycrystalline elastic behaviour of the ternary Zr_1–xAl_xN system.

1. Jia-Hong Huang, Tzu-Chun Lin, Ge-Ping Yu, Surf. Coat. Technol., 206(2011)107.

Abstract

An optimization study on the parameters of Titanium Nitride thin film coating on Al7075-T6 alloy, using magnetron sputtering technique is presented. The effects of the film thickness, temperature, DC bias voltage, rate of nitrogen and DC power on the adhesion and microstructure of the coated samples are investigated. Each sample is coated at two steps. Firstly samples are coated with pure Titanium and secondly they are coated with titanium nitride which this effort is increased the adhesion of TiN to surface profoundly. The coating properties probed in this work included the surface thickness, morphology of deposited coating, adhesion between the coating and substrate, and surface characterization using a field emission scanning electron microscope (FESEM) with energy dispersive X- ray (EDX), X-ray diffraction (XRD). SEM analyses showed that all the films had columnar and dense structure with clearly defined substrate-film interfacial layers. Scratch result showed, the adhesion between substrate and thin film was increased with increasing the DC power from 300W to 500W. The result also showed an increase; in dc bias voltage, the adhesion was increased from 25V to 75V, but with more increasing (100V) the adhesion was decreased.

EPMA analysis of tool-work interface showed that the belag was formed on the coating surface during cutting, which was Mn-Si-O or Al-O. Formation of the Mn-Si-O based belag could help increasing the cutting speed compared to formation of an Al-O based belag, while Al-O based belage was performed as protective layer against wear resistance. The belag formation was related to the chemical composition of work materials, chemical composition of the coatings and cutting temperature.

The as-deposited laminated Ta–Zr coatings exhibited nanocrystalline or amorphous states, depending on the chemical compositions. As annealed in oxygen containing atmospheres, Zr oxidized preferentially. The hardness increased as the oxygen content in the coating increased, due to the formation of ZrO₂. To behave as a protective coating applied in high temperature with an appropriate hardness, such as coatings on glass molding dies, the coating need to endure annealing treatment in an oxygen containing atmosphere at 600 ^oC. In this study, the periods of laminated coatings were controlled by rotating speeds of the substrate holder. T he annealing treatments were conducted at 600 ^oC under atmospheres of 20 and 50 ppm O₂-N₂, respectively. The variations in crystalline structure, hardness, surface roughness and chemical composition profiles in depth after various annealing times were investigated. The lifetime of a protective Ta–Zr coating was justified by exposed the coating in a 50 ppm O₂-N₂ environment to achieved a high hardness of 10 GPa and then annealed in a 20 ppm O₂-N₂ atmosphere for long times.

In order to fulfill the requirements in industrial applications, hard coatings possessing both high hardness and good corrosion resistance are demanded. Ti-Si-N is one of the coatings that have been extensively studied in recent years. Most studies on Ti-Si-N coatings were focused on the hardening mechanisms due to structure evolution, composition variation, or impurity effect; however, few studies have been performed on the relation between residual stress and thickness variation. In this study, a series of Ti-Si-N coatings were deposited on AISI D2 tool steel without interlayer using unbalanced magnetron sputtering at different deposition durations. The coating thickness of all specimens was larger than 1μm. The purpose of this study was to investigate the mechanical properties and corrosion resistance of the Ti-Si-N coatings. X-ray diffraction results showed that the thick Ti-Si-N coatings were composed of nanocrystalline TiN and amorphous Si₃N₄. Coating thickness was obtained from focused ion beam (FIB) measurement. X-ray photoelectron spectroscopy (XPS) was used to characterize the bonding state and compositions of the coatings. The hardness of the coatings was measured by nanoindentation. The residual stress was determined by modified sin²ψ x-ray diffraction and laser curvature methods to explore the residual stress state and stress distribution in the coatings. The adhesion of Ti-Si-N coatings was evaluated by scratch test. The influence of the coating thickness on corrosion resistance was evaluated by potentiodynamic scan and salt spray test. From the corrosion results, the thick Ti-Si-N coatings could effectively prevent the D2 steel from corrosion. The hardness of the coatings was related to the compositions and texture. The coating thickness was correlated to the residual stress, where a critical stress was found for the spallation of the coatings.

Nanocrystalline TiZrN films were produced using unbalanced magnetron sputtering. The aim of this study was to investigate the effect of nitrogen flow rate on the microstructure and properties of the TiZrN films with nitrogen flow controlled form 1.3 to 2.5 sccm. Thin films of TiZrN were deposited by magnetron unbalanced sputtering on our previous optimum coating conditions for TiZrN. The results of the variation of nitrogen flow rate did not significantly affect the film thickness. The major effects of the nitrogen flow rate were on the texture coefficient, N/(Ti, Zr) ratio, hardness, and resistivity of the TiZrN films. The texture coefficients could be calculated from the integrated intensity of the corresponding XRD peaks. The results indicated that (111) plane was the dominant preferred orientation for all TiZrN specimens. Two other peaks (200) and (220) of TiZrN films could be observed for the specimen at higher nitrogen flow rate. The N/(Ti, Zr) ratio of the TiZrN films increased with respect to the nitrogen flow rate, but increased slowly as nitrogen flow rate further increased. Hardness of TiZrN films first increased to its maximum of 33.1GPa and then decreased as nitrogen flow rate further increased. The result showed that film hardness was not directly dependent on the (111) preferred orientation. The hardness of TiZrN thin films was due to solid solution strengthening and nanograin boundary sliding mechanism. The electrical resistivity and packing density of thin film related to the lattice defects. Similar to hardness, packing density also increased to a maximum at a critical nitrogen flow rate and the lowest resistivity corresponded to the highest packing density.

The quest of ultraperformant, hard protective coatings [1] is of major technological importance due to their applications in cutting tools, automotive and space industry, among others. Within this framework materials that exhibit hardness above 40 GPa are considered as superhard. Supehard materials are usually nanostructured, like the well known nc-TiN/a-Si₃N₄ [1]. Recently it was possible to grow the ternary compound d-Ti_xTa_1-xN [2] and its hardness has been measured to be as high as 42 GPa [3]; the later makes it a strong candidate for being a member of the group of superhard materials.

In this work, we investigate the effects of composition and microstructure of d-Ti_xTa_1-xN coatings on their mechanical performance. The d-Ti_xTa_1-xN coatings have been grown by dual-cathode magnetron sputtering (DCMS) and dual ion beam sputtering (DIBS). The DIBS-grown samples exhibit globular grain morphology and similar grain sizes for all values of x. On the contrary, the DCMS-grown samples exhibit strong variations of their microstructure, from globular to strongly columnar, with varying x, as identified by X-ray Diffraction and Electron Microscopy analyses. Therefore, by comparing their mechanical performance we can discriminate the effects of composition and microstructure. The mechanical testing has been performed in terms of hardness measurements by nanoindentation, tribological testing by lateral force measurements and wear testing by ball on disk measurements. According to ab-initio calculations of the elastic moduli, the pure d-TaN was expected to be the hardest member of the d-Ti_xTa_1-xN family. However, the experimentally produced d-TaN coatings are defective and underdense [4,5]. Here we prove that alloying TiN with TaN improves the mechanical performance by stabilizing the rocksalt structure for Ta-rich ternary d-Ti_xTa_1-xN coatings, which are harder than their constituents (TiN, TaN). Indeed, the Ta-rich d-Ti_xTa_1-xN coatings were superior in most aspects of mechanical testing.

[1] S. Veprek, J. Vac. Sci. Technol. A17, 2401 (1999).

[2] L.E. Koutsokeras, G. Abadias, Ch.E. Lekka, G.M. Matenoglou, D.F. Anagnostopoulos, G.A. Evangelakis, P. Patsalas, Appl. Phys. Lett. 93, 011904 (2008).

[3] G. Abadias, L.E. Koutsokeras, S.N. Dub, G.N. Tolmachova, A. Debelle, T. Sauvage, and P. Villechaise

J. Vac. Sci. Technol. A 28, 541 (2010).

[4] C.-S. Shin, D. Gall, P. Desjardins, A. Vailionis, H. Kim, I. Petrov, J.E. Greene, M. Odén, Appl. Phys. Lett. 75, 3808 (1999).

In this work we investigate the origin of paramagnetic centers in hard graphite-like amorphous carbon. The films were deposited by 1) plasma enhanced chemical vapor deposition (PECVD) using the decomposition of methane (CH₄) and 2) sputtering, using an argon ion gun to sputter a pure graphite target. High concentration of sp² films was obtained adopting high bias in the PECVD technique. These films have small band gap (0.5-1.0eV) and hardness of approximately 15 GPa. The films prepared by the sputtering technique have sp² concentration of approximately 90 %, zero energy band gap and hardness of 20-30 GPa. Electron spin resonance (ESR) was performed at the X-band (9.4 GHz) microwave frequency using 100 kHz field modulation. A non-saturating power of 5 mW was adopted. The ESR measurements revealed an unexpected low density of paramagnetic centers, ascribed to conduction electrons with a g-value of about 2.003. These results are compared with data reported for amorphous carbon films deposited by different techniques and with different sp² concentrations.

Many researches of thin film preparation and application for nitride films were reported. Nitride films have fascinate properties such as high hardness, wear proof, electrical properties and so on. On the other hand, boron nitride is one of the artificial material which doesn’t exist naturally. Cubic boron nitride has high hardness and thermal conductivity next to diamond. Investigation was carried out on the effects of sputtering gases on the preparation of BN films using RF sputtering and preparation of BN films on the CVD diamond substrates.

Boron nitride was prepared using RF reactive sputtering. Si and CVD diamond were used as the substrates. Mixture of Ar-N₂ and Ar-NH₃ were used as reactive sputtering gases. The ratio of Ar:N₂ and Ar:NH₃ were 1:1, 1:3 and 1:5. Sputtering time was fixed to 1 h. CVD diamond substrates were synthesized using microwave plasma CVD. To obtain CVD diamond substrates, CH₄-H₂ was used as a reaction gases for diamond synthesis. CH₄ flow rate was 1 SCCM, H₂ flow rate was 100 SCCM and pressure was 5.3 kPa, microwave power was 400 W, respectively. Reaction time was fixed to 5 h. Surface of deposits was observed using SEM. Deposits were e stimated by XPS and Raman spectroscopy.

As a result of SEM observation of the film on the Si substrates, surface morphologies of sputtered films were smooth. From XPS measurement, the B-N bond was observed in XPS spectra of each samples. Hardness of the film was increased and friction coefficient was decreased with increasing of N₂ ratio in Ar-N₂ sputtering gas for the Si substrate. As a result of SEM observation of the film on the CVD diamond substrates, BN film on CVD diamond crystals was recognized in each condition. In addition, diamond peak at 1333cm^-1 and the amorphous boron nitride peak at 1550 cm^-1 were observed in Raman spectra for Ar-N₂ sputtering gas. From XPS measurement, B-N bond was obtained in XPS spectra of each samples.

As a conclusion, high hardness BN films could be obtained in not so slow deposition rate using Ar-NH₃ sputtering gas. Moreover preparation of the BN film on the CVD diamond substrates could be performed.

Crystalline alumina PVD-coatings offer high potential for different applications in which high chemical inertness, high hot hardness and high oxidation resistance is important. Especially the metastable γ-phase is topic of many researches because, in comparison to the stable a-phase, it can be deposited by means of MSIP (Magnetron Sputter Ion Plating) at relatively low process temperatures below 650 °C and it is more fine-grained than α-Al₂O₃. At high temperatures γ-Al₂O₃ transforms into α-Al₂O₃, which limits the application temperature. But until now it is not clearly investigated, up to which temperatures γ‑Al₂O₃ thin films are stable and which mechanisms influence the stability. In the following paper the influence of doping with Silicon is investigated. DSC (differential scanning calorimetry) as well as XRD (X-Ray diffractometry) measurements show that adding 4 at-% Silicon leads to a high amount of amorphous phase in the as deposited coating and a decreasing in hardness from 18 to 10 GPa. Nevertheless formation of the a-phase is retarded to temperatures above 1200 °C while for the undoped coating the α-phase is formed at temperatures of 1100 °C. For the Si-doped coating a transformation via the θ-phase was detected which is not seen for the undoped coating. In combination with a (Ti,Al)N-interlayer, which is necessary to provide a sufficient adhesion, Silicon improves compound properties after thermal exposure. This was proven by scratchtests and impact testing before and after annealing the samples at 900 °C.

Hard coatings based on transition metal nitrides, carbides or borides gained increasing importance. The properties of these hard coatings have been enhanced by adding with other elements such as Al, Si, Cr, V, Mo, etc. In this study, structural, mechanical and tribological properties of TiTaBN coatings obtained with Ta incorporation into TiBN were investigated. TiTaBN films were deposited on D2 steel and glass substrates using pulsed-dc closed field unbalanced magnetron sputtering (CFUBMS) with Taguchi L₉ (3⁴) experimental method. Morphology and structure of coatings were analyzed by SEM, EDS, XRD, and XPS. The hardness of TiTaBN coatings were determined with microhardness tester. The friction and wear properties of the coatings were analyzed at different test athmospheres (humid air, distillated water, dry nitrogen and synthetic oil) by using pin-on-disc. Consequently, TiTaBN coatings deposited at the R8 coating parameters exhibited dense and a columnar structure. These coatings deposited at the R8 coating parameters contains TiN, TaN, TiB₂ and BN phases and these coatings having ultra hardness (52.55GPa) demonstrated very high wear resistance at the different test athmospheres.

Metal transition nitrides alloyed with C, Si or Al, have a wide range of applications, mainly in the metal-mechanical field as hard and low friction coatings. It is worth mentioning that the properties of these materials depend strongly on the alloying concentration and therefore a lot of work has been devoted in the last years to investigate deposition techniques capable to form such ternary nitrides with controlled composition. In a previous work, we proposed the so-called Reactive Crossed Beam Pulsed Laser Deposition (RCBPLD) technique as an alternative to produce ternary compounds with controlled composition; particularly, this technique was applied successfully to deposit Ti-C-N thin films controlling the C content. In this work, it is reported the use of the RCBPLD to prepare nanostructured TiSiN thin films. With this experimental configuration, the amount of silicon incorporated in the film is controlled in an easy way varying the mean Si⁺ kinetic energy. The film structure, mechanical properties, composition and surface morphology were investigated as a function of the Si⁺ kinetic energy. These properties were studied using the following characterization techniques: Raman spectroscopy, X-ray diffraction, nanoindentation, X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. It was found that the Si content, which was varied from 1.6 to 3.5 at.%, depends, approximately linearly, on the silicon ion kinetic energy. Ti-Si-N films with hardness as high as 34.0 GPa, which is suitable for many mechanical applications, were obtained. The hardness was strongly affected by the silicon ion energy, and there exists an optimal energy value, and consequently a certain value of Si content at which the maximum hardness is reached. These results show that the properties of the deposited material are controlled partially by the Si⁺ kinetic used for thin film growing.

TiB₂ thin films have been studied extensively due to their outstanding properties (e.g. high hardness, high thermochemical stability), making them highly attractive for many applications in erosive, abrasive, corrosive and/or high-temperature environments. Despite their excellent properties, the usability and commercialisation of TiB₂ films has been mainly hindered due to their brittleness and limited film-substrate adhesion, caused by the primarily strong covalent bonding in the hexagonal B network and the high compressive stresses developed in deposited films on various substrates. An effective route for improving adhesion and toughness in hard ceramic films is the introduction of ductile metal additions (e.g. Al) or layers, therefore modifying their bonding type and structure.

In this study, Ti_xAl_yB₂ thin films (0.88≤ x ≤1.04; 0.12≤ y ≤0.31), with Al contents between 4.1-9.4 at.%, were deposited onto Si (100) and AISI 316 stainless steel substrates by simultaneous co-sputtering from TiAl and TiB₂ targets in an argon discharge at 170˚C. The coating stoichiometry, relative phase composition, nanostructure, density and mechanical properties were determined using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Laser Acoustic Surface Waves (LAwave), in combination with nanoindentation measurements. It is shown that Al substitutes for Ti in stoichiometric closed-packed hexagonal nanocrystalline thin films with 4.1 at.% Al, having an average grain size of 2-3 nm. As the Al concentration is increased, the crystallinity, average TiB₂ grain size, density, hardness and elastic modulus decreases from ~2-3 to 1 nm, 4.2 to 3.8 g/cm³, 32.9 to 20.6 GPa and 335 to 250 GPa, respectively, while the film adhesion increases. Density and elastic modulus from ab initio calculations are compared to experimental results. The elastic modulus values measured by nanoindentation are lower than the calculated ab initio data, attributable to the nanocrystalline nature of the deposited films.

This work develops (AlCrTaTiZr)NC_y multi-component carbo-nitride films, with the incorporations of quinary metallic elements, nitrogen and carbon, as protective hard coatings by co-sputtering of AlCrTaTiZr alloy and graphite in an Ar/N₂ mixed atmosphere with the application of different substrate biases. All the (AlCrTaTiZr)NC_y films deposited at different conditions exhibited a simple face-centered cubic structure. As the applied substrate bias and graphite-target power increased, the deposited (AlCrTaTiZr)NC_y coatings transformed from a large columnar structure with a (111) preferred orientation to a nanocrystalline or even near-amorphous structure. With increasing substrate bias and graphite-target power, the hardness and H/E ratio of the coatings increased from 18 GPa and 0.07, to much higher values of about 32 GPa and 0.12, respectively, attributed to the densification of the coatings, the introduction of covalent-like carbide bonds, the refinement of grains and the formation of nanocomposite structure. Because of the severe lattice distortions in the multi-component coatings caused by the addition of differently-sized atoms, the dominant deformation mechanism of the coatings was found to be the formation of stacking faults, rather than complete dislocations only.

It is well known that TiAlN films have been preferred in cutting tools for their mechanical properties and good oxidation resistance. However, many studies have reported that they oxidize over 800 ºC. The addition of another element into matrix is one of the techniques to improve the properties. In our previous researches, we reported on the effect of yttrium addition to TiAlN films and investigated the mechanical and thermal properties. It was concluded that the films with the yttrium content 2 at.% showed the highest hardness and excellent oxidation resistance up to 900ºC.

In this study, we synthesized Ti₄₉Al₄₉Y₂N films changing substrate temperatures at 100ºC, 200ºC, 300ºC, 400ºC and bias-voltages at 50 V, 100 V, 200 V, 300 V and investigated their oxidation resistance and mechanical properties. The films were deposited on Si, WC-Co, and SUS304 substrates by the cathodic arc ion plating (AIP) method. The hardness and adhesion were analyzed by a micro-Vickers hardness tester and Rockwell hardness tester, respectively. For the evaluation of the oxidation resistance, X-ray diffraction (XRD) and glow discharge optical emission spectrometry (GDOES) were performed to identify oxide layers of the films annealing at 900ºC for 1 hour in air.

Bismuth-Iron-Oxides (BFO) can be grown in five different phases, which can have very important multiferroic, magneto-optical and optoelectronic properties, making them attractive for different technological applications. The synthesis of these materials in the form of thin films has become quite difficult, being the laser ablation technique one of the most suitable. In the present work, it is proposed the simultaneous ablation of two targets (Bi and Fe₂O₃) in a reactive atmosphere (containing O₂), in order to deposit BFO thin films with different compositions. Prior to deposition, the plasma parameters, such as, mean kinetic ion energy (Ep), plasma density (Np) and the type of excited species, were studied, in an attempt to correlate these parameters with the properties of the deposited BFO thin films. Deposition of the thin films was carried out at room temperature and the working pressure was varied in the range between 10 and 50 mTorr with an Ar/O₂ =80/20 gas mixture. The iron oxide (FO) plasma parameters were kept constant at Ep (FO) = 100 eV and Np(FO) = 2 x 10¹³ cm^-3, whilst the bismuth plasma parameters were varied in the range between 30 and 300 eV for the Ep(Bi) and 8 x 10¹¹ – 9 x 10¹³ cm^-3 for the Np(Bi). The optical emission spectroscopy (OES) showed that the most abundant excited species present in the plasmas were neutral Fe and neutral, once and three times ionized Bi. The deposited samples were characterized by Raman spectroscopy, X-Ray diffraction, EDS and RBS. The properties of the films are presented as a function of the plasma parameters.

Manganese Phosphate is an Industrial coating used to reduce friction and improve lubrication in sliding components. In this study, the tribology behavior of TIN (which as produce both PVD and CVD ) Manganese Phosphate with Molybdenum disulphide (MoS₂) coated AISID2 steels was investigated. The Surface morphology of manganese phosphate coatings was examined by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) .The wear tests were performed in a pin on disk apparatus as per ASTM G-99 Standard. The wear resistances of the coated steel were evaluated through pin on disc test using a sliding velocity of 3.3 m/s under Constant load of 50 N and controlled condition of temperature and humidity. The Coefficient of friction and wear loss were evaluated. The temperature rise after 15 min and 30 min were recorded for each load. Wear pattern of TIN, Manganese Phosphate with Molybdenum disulphide (MoS₂) coated pins were captured using Scanning Electron Microscope (SEM). Based on the results of the wear test the manganese phosphate with Molybdenum disulphide (MoS₂) coating exhibited the lowest coefficient of friction and the lowest wear loss under 50 N load.

Diamond like carbon (DLC) coating has grown up to be a common way to improve the surface properties of dies and punches in dry stamping, by its high wear resistance and low friction coefficient. Aiming at its applications to an oxide-glass stamping mold-dies or a protective coating for MEMS/NEMS, high thermal stability and elasticity are much required. In fact, the stability above the glass-transition temperature of oxide glass and nano-scaled dimensional guaranty are needed in the mold-stamping process of collective lens for solar panel. Nano-columnar DLC coating is suitable for this application because of the high durability and metallic doping is effective to improve the thermal stability at elevated temperature.

Nanoindentation is an effective technique for determining mechanical properties of bulk materials and thin films. Prevailing measurement uncertainties in nanoindentations by Vickers or Berkovich diamond pyramids are commonly caused by manufacturing imperfections of the indenter’s side angles and tip sharpness. Moreover, the tip geometry changes during the indenter lifetime, due to diamond wear.

The present paper deals with a fast method for estimating diamond indenters’ tip nano and micro geometry. On one hand, this method is based on a combination of nanoindentations on Si(100) used as reference material and on the other hand, on FEM supported calculations of Martens hardness. The hardness calculations are conducted using equivalent indenter tip geometry with geometrical characteristics which may vary for a specific set of parameters, describing the real indenter with manufacturing imperfections. These parameters are varied in successive iterations until the calculated hardness converges with that of the reference material.

For a quick determination of these parameters, the software package “TIDE” (TΙp Deviations Estimation) has been developed. “TIDE” is based on numerous FEM supported simulation’s results of nanoindentations onto the reference material varying the indenter tip geometry. By this software package, a quick prediction of nanoindenters’ tip equivalent geometry is facilitated, also for anticipating changes due to wear of the diamond over time.

KEYWORDS: Nanoindentation, tip, imperfections, wear

Microstructure analysis has attracted high interest in hard coating developments as often the microstructure has a decisive impact on the hardness of materials [1]. For example, the isostructural spinodal decomposition results in hardness enhancement in TiAlN. Modeling microstructure evaluation via diffusion requires not only energetic, mechanical but also kinetic parameters of the materials. Here we present results on vacancy, divacancy formation and migration in Ti_0.5Al_0.5N alloy using first principles density functional theory calculations. We pay special attention to the analysis of the impact of local environments.

[1] P. H. Mayrhofer, C. Mitterer, L. Hultman and H. Clemens, Prog. Mat. Sci. 51, 1032 (2005).

Growth of multicomponent nitride thin films is typically performed under kinetically limited conditions. Thus, the study of surface diffusion is one key to understanding behavior such as texture development and clustering during crystal growth. An earlier study of diffusion on TiN(001) and (111) surfaces [1] has shown that there is a large difference in diffusivity of adatoms on different surface directions, which greatly influences the preferred growth direction. We present results from first-principle studies using the nudged elastic band technique to calculate the energy barriers on low index crystal surfaces. By introducing different metal atoms and clusters in the surface of TiN and calculating the changes in the surface energy barriers, we show the effects on surface diffusion due to the change in the local environment. For example we show that the introduction of configurational disorder in Ti_0.5Al_0.5N(001) slow down the Ti adatom diffusion as compared to the pure TiN(001) case.

[1] D. Gall, S. Kodambaka, M. A. Wall, I. Petrov, and J. E. Greene, J. Appl. Phys. 93, 9086 (2003)

This study utilized electroless nickel (EN) plating and cathodic arc evaporation (CAE) technologies to deposit the protective coatings onto ductile iron substrates. Polarization corrosion tests were performed in 3.5% sodium chloride. The erosion tests were also carried out using Al2O3 particles (~177 mm in size and Mohr 7 scale) of about 5 g, and then surface morphologies of the eroded specimens were observed. To further understand the coating effects on both the corrosive and erosive behaviors of ductile iron, coating structure, morphology, and adhesion were analyzed using XRD, SEM, and Rockwell C indentation, respectively. The results showed that EN coating exhibited an amorphous structure, while TiAlZrN had a multilayered type. With regard to both the corrosion resistance and erosion resistance, the TiAlZrN/EN duplex coated specimens performed better than did the uncoated and monolithic EN or TiAlZrN ones.

In this work an AISI F51 duplex stainless steel was DC-Plasma nitrided (PN) at 400^oC, during 20 hours in a 75% N₂ + 25% H₂ atmosphere. A modulated plasma nitrided layer formed on the specimen´s surface: the nitrided layer observed on the ferritic regions was 3 µm thick, while the nitrided layer formed on the austenitic regions of the microstructure was ~2 µm thick. Very fine martensite needles were observed on the ferritic regions, while expanded austenite layer (Υ_N) formed on the austenitic regions. The nitrogen content of the nitrided layer was estimated from X-ray diffraction measurements and WDX as being ~3.4 to 4.4 wt % N, leading to colossal supersaturation and strong hardening of the surface, up to 1350 HV. The 400 ºC plasma nitrided layer did not impair the corrosion resistance of the duplex stainless steel. These results are discussed based on the hypothesis that, during nitriding, ferrite transforms at first to austenite and then to expanded austenite, due to nitrogen pickup. The expanded austenite formed on ferrite regions transforms to martensite, under stresses developed during the formation of expanded austenite in the neighboring austenite grains.

Keywords: Plasma nitriding, Duplex stainless steel, Expanded austenite, Martensite

Residual stress on thin films can be significantly high, sometimes as large as several gigapascals and they can be either compressive or tensile. For the super-hard coatings, such as metal nitrides or carbides, the hardness and the compressive stress are strongly correlated; in such a way that the high hardness values are usually obtained for highly strained films. This correlation imposes some limitations in both the thickness of the films and their use for high temperature applications. In this work, we have produced nanocomposite thin films of tantalum nitride (TaN) and niobium nitride (NbN) nanocrystals embedded in amorphous silicon nitride (SiNx) phase. The films were deposited using two magnetrons (pure metal and silicon) and the silicon content on the films was varied by increasing the radio frequency power applied to the Si-target, while the other deposition conditions remained fixed. In both cases, the results showed that the hardness increased as the Si content increased from 0 to 5-6 at%, but further Si incorporation resulted in a decrease in the hardness.

The aim of the present work was to study the possible correlations between the residual stress, the silicon content and the hardness of the films. The residual stress was determined the lattice strain method using X-ray diffraction. The sin^2(psi) method was used to determine the stress tensor. The results showed that the films present a triaxial stress state with compressive components in the plane normal to the growth direction and shear components. From this analysis it was demonstrated that even for the nanocomposite films, the maximum hardness films also showed the highest stresses. Stress in the direction x and y are compressive and equivalent in magnitude so that is not a change or stress gradient in the direction perpendicular plane to the film growth. On the other hand, the stress in the z direction (perpendicular to the growth of the film) was tensile.

We concluded that the described method commonly used to study bulk materials can be used to calculate the stress tensor in hard nanocomposite coatings. The results obtained indicated that the hardness was directly related to the stress; larger hardness values were obtained for the samples with the higher stress.

Acknowledgements: We wish to acknowledge the financial support from DGAPA-UNAM IN103910. G. Ramírez acknowledges CONACYT for his PhD scholarship.

Tantalum is a refractory metal with low electrical resistivity. Interest in the electrical properties of Ta thin films have been stimulated by its potential applications in electronic devices. Bulk Ta has a bcc structure, known as the α-phase. However, during thin film growth, the metastable tetragonal β-phase is commonly formed, especially on Si substrates. β-Ta has a higher resistivity, is also harder and more brittle than α-Ta. Role of impurities, defect concentration and stress state have often been reported. Nonetheless, the preferential formation of the β-phase onto Si substrates is not fully understood.

To address the impact of growth conditions on physical properties of thin sputtered Ta films, we present results obtained by combining highly sensitive in-situ stress measurements by the substrate curvature technique and ex-situ structural investigations (XRD and HRTEM). Thin films of Ta were grown at room temperature on an amorphous Si layer, or on Si substrates, using magnetron sputter-deposition under Ar atmosphere. A structural change occurring in the nanometric range (~ 3-4 nm) is clearly identified, from both the real-time stress evolutions and ex-situ structural characterizations, which is attributed to a polymorphic crystallization of an amorphous film, initially stabilized by a minimization of the interface energy. The amorphous to β-crystalline transformation is accompanied by the development of an intrinsic growth stress, with a steady-state component ranging from a tensile state to a compressive one as a function of processing parameters (Ar working pressure : 0.1 – 0.8 Pa, substrate bias voltage : 0 – 100V). Finally, these results suggest that the nucleation into the β-phase would be favoured by similar specific volumes of the two metastable amorphous-Ta and β-Ta phases, whereas the β-phase growth at larger thicknesses would be explained by the weak difference between the Gibbs energies of α-Ta and β-Ta.

Thermo-reactive deposition/diffusion (TRD) process is a method used to prepare hard, wear-resistant coatings of carbides, nitrides, or carbonitrides on steels. In this study, carbide coating was tried to deposit on T10 steels by duplex treatment. The steel substrate was immersed in a molten salt bath consisting of vanadium then in a molten salt bath consisting of titanium at 1000 ^oC. The obtained coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). The results showed that the coating obtained from the duplex treatment was composed of two distinct layers. The outer layer was titanium carbide and the inner layer was vanadium carbide. The substrate /vanadium carbide coating interface and the vanadium carbide coating/ titanium carbide coating interface is distinct and without transition zone. The micro-hardness, scratch and pin-on-disk wear tests were conducted to evaluate the mechanical properties. The results showed the hardness of the duplex coating is higher than the vanadium carbide single layer. And the duplex coating exhibited excellent adhesive strength and outstanding wear resistance.

Enhanced glow discharge plasma immersion ion implantation (EGD-PIII) conducted using a small pointed hollow anode and large tube cathode has certain advantages over conventional plasma immersion ion implantation (PIII). In EGD-PIII, the plasma is produced by self glow discharge induced by the negative high voltage applied to the sample. The plasma distribution measured by Langmuir probe measurements discloses that the electron density is quite uniform in the vicinity of the negatively biased substrate. Although the impact energy and ion implantation fluence have been demonstrated to be better in EGD-PIII than those in conventional PIII, lateral non-uniformity in the ion fluence is observed during hydrogen implantation into a silicon wafer and the ion focusing effect depends on the plasma density. An insulated tube placed between the chamber and gas inlet is employed to increase the interaction path for electrons and neutrals, and theoretical and experimental studies reveal that the insulated tube can enhance ionization of plasma gases with low ionization efficiency such as hydrogen. However, the implantation current is observed to increase sharply at a certain pressure when the plasma gas consists of diatomic molecules. In this work, we experimentally investigate the implantation current characteristics in EGD-PIII. The plasma potential is measured to investigate the discharge phenomenon and X-ray photoelectron spectroscopy (XPS) is conducted to corroborate the findings.