Pulsed Direct Current Magnetron Sputtering Research Articles

Layered Al2O3-SiO2 and Al2O3-Ta2O5 thin-film composites for high dielectric strength, deposited by pulsed direct current and radio frequency magnetron sputtering

Multilayer thin films have the potential to act as high dielectric strength insulation for wire and microelectronics. In this study, films consisting of 2, 4 or 8 layers, composed of Al2O3 with SiO2 or Ta2O5, were prepared via pulsed direct current and radio frequency magnetron sputtering to a thickness of between 152 and 236 nm. The dielectric strengths of all films exceeded the 310 Vμm−1 achieved for PDC Al2O3. Maximum dielectric strengths were obtained for four layer composites; Al2O3-SiO2-Al2O3-SiO2 (466 Vμm−1) and Al2O3-Ta2O5-Al2O3-Ta2O5 (513 Vμm−1), each containing two PDC-Al2O3 and two RF-SiO2/Ta2O5 layers. Whilst the average dielectric strength was higher in the Ta2O5 composites, they suffered from higher leakage prior to breakdown with ca. 6.5 nA compared to ca. 0.1 nA for SiO2 composites. The mechanical properties of the composites were poorer due to increased intrinsic coating stress. Samples exhibited complete interfacial delamination with maximum coating adhesion strengths of 22 and 25 MPa. The variance resulted from larger coefficient of thermal expansion for Ta2O5 compared to SiO2. Sputtered composites of Al2O3 and either SiO2 or Ta2O5 had high breakdown strength with reasonable adhesion and could be suitable for insulating copper conductors in the aerospace and automotive industries.

The relationship between carrier mobility and bicrystalline grains boundary of rutile TiO2 film

The rutile titanium dioxide (TiO2) has attracted enormous interest in sensor field owing to its stability at high temperature. But less is known about electron transfer efficiency of rutile TiO2, which is mainly influenced by the electron mobility. The aim of this study is to investigate the relationship between carrier mobility and bicrystalline grains boundary of rutile TiO2 film. In this study, TiO2 film was deposited by the direct current pulsed magnetron sputtering technology with sputtering power from 150 to 900 W. Then the TiO2 films were annealed at [Formula: see text] in air atmosphere. By this way, the rutile TiO2 films were obtained. Special attention was paid to the carrier mobility of rutile TiO2 films. The Hall measurement results showed that with the increase of sputtering power from 150 to 900 W, the carrier mobility of rutile TiO2 films increased from 0.75 to [Formula: see text], gradually. Taking the TEM and SAED results together, our data indicated that the carrier mobility of rutile TiO2 film was influenced by the bicrystalline grains boundary. The large angle bicrystalline grain boundary seriously deteriorates the carrier mobility of rutile TiO2 film. On the contrary, the low angle bicrystalline grain boundary has little influence on the carrier mobility of rutile TiO2 film.

Understanding the deformation and cracking behavior of Cr-based coatings deposited by hybrid direct current and high power pulse magnetron sputtering: From nitrides to oxynitrides

The increasing demand for corrosion and wear reduction in modern tribological systems places high requirements on hard coatings deposited by physical vapor deposition (PVD) technology in numerous technical applications e.g., plastic processing, die casting or machining. One possible approach to withstanding the excessive wear is coating of tools with PVD hard coatings of the system Cr-Al-O-N. Each of the constituent elements can provide the coating with particular characteristics in point of its performance under different loading conditions. Therefore, the ongoing improvement in modern surface engineering requires comprehensive insights into the elastic-plastic deformation and cracking behavior of such coatings. For this purpose, several modifications and extensions of established experiments are required to overcome the limitations involved in characterization of thin hard coatings e.g., as a result of the μm ranges. In the presented work, three coatings CrN, (Cr,Al)N and (Cr,Al)ON deposited on quenched and tempered AISI 420 steel substrate were investigated. All coating systems were deposited through a hybrid technology, consisting of direct current and high power pulse magnetron sputtering (dcMS/HPPMS). The deformation and cracking behavior of the coatings and coating/substrate compounds were studied through application of static loadings by nanoindentation and Rockwell tests as well as dynamic loading conditions using nanoscratch tests. Complementary quantitative investigations were performed by means of depth profiling using confocal laser scanning microscopy (CLSM). Qualitative analyses of Rockwell imprints and nanoscratch tracks were conducted through scanning electron microscopy (SEM). Based on the results, precise analyses of nanoindentation force-displacement curves can improve the understanding about the possible crack formation in the coatings. Compared to (the binary) CrN, the ternary coating system (Cr,Al)N exhibits more promising characteristics in point of elastic-plastic deformation behavior and crack resistance. The crack resistance decreases through incorporation of oxygen into the ternary nitride and deposition of quaternary oxynitrides (Cr,Al)ON. Such character of oxynitrides can be overseen taking into consideration only the static loadings and therefore, applying of the both static and dynamic loading conditions are required to gain precise knowledge on the deformation and cracking behavior of thin hard coatings.

Reactive Magnetron Sputter Deposition of Copper on TiO2 Support for Photoreduction of CO2 to CH4

In this work, nanocrystalline Cu/TiO2 catalysts have been synthesized by using pulsed direct current (DC) reactive magnetron sputtering of Cu targets in an Ar atmosphere onto P25-TiO2 support. The oscillating bowl was used to make the uniform coating on the substrate. The Cu doping content was varied by adjusting the coating time. The thus-obtained catalysts were characterized by using the X-ray diffraction (XRD), UV-Vis spectroscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The photocatalytic activities of all catalysts were studied via the photocatalytic reduction of CO2 and H2O to CH4 under UV irradiation, and compared with the pure TiO2 support and conventional-impregnation-made Cu/TiO2. The results showed that the photocatalytic performance of sputtering-made Cu/TiO2 catalyst was much better than the pure TiO2 support. Therefore, reactive magnetron sputtering was a promising technique for deposition of metal onto the support and use as the catalytic process.

Roughness of Deposited Carbon-Based Coatings and Its Statistical Characteristics at Nano and Microscales

Topography of surfaces may influence many processes in tribology including friction and adhesion. Its influence is usually taken into account in various statistical models of rough surfaces. Most of these models are based on an explicit or implicit assumption of normality of the asperity heights or similar assumptions that involve Gaussian distributions. Recently it has been shown that the height distribution of surfaces prepared by grinding are not Gaussian at both nano and micro-scales, while topography of epoxy resin replicas of polishing papers having nominal asperity sizes up to several micrometers, was Gaussian. Here we study roughness of carbon-based coatings deposited by direct current pulsed magnetron sputtering with and without substrate bias voltage at micro and nano-scale. Hardness measured using a Berkovich indenter tip gave 43 (biased) and 14 (non-biased) GPa, respectively. First the heights of the nano-asperities were determined by AFM (Atomic Force Microscopy). Then the heights of the micro-asperities were measured by a profilometer (a stylus). Finally the same regions measured by stylus were again studied by AFM. Standard statistical parameters of surfaces are determined at each scale. It has been also shown that the stylus measurements did not cause plastic deformations of the harder (biased) sample because the distributions of heights at nano-scale were the same. Using the experimental data obtained, the assumption of the normal distribution for the roughness heights has been studied by application of various modern tests of normality. Measurements of surfaces by stylus and by AFM with the 117 nm steps showed that the surfaces satisfy the assumption of normality of the heights. However, further studies with the 10 nm AFM steps showed that the roughness of the non-biased sample is not normal. Hence, the applicability of the standard statistical models of adhesive contact between rough solids to the non-biased sample may be questionable.

Activity and durability of electrocatalytic layers with low platinum loading prepared by magnetron sputtering onto gas diffusion electrodes

This article presents development of magnetron sputtering technology for catalytic Pt coating production. Low Pt-loaded gas diffusion electrodes were prepared by a single-step magnetron sputtering in different regimes. Pt and Pt-carbon films (95–97 wt% Pt) were deposited directly onto the gas diffusion layers (GDLs): carbon paper (Sigracet® 39 BC) and carbon cloths (GDL-CT, ELAT® LT 1400 W). A bipolar pulsed direct current (DC) magnetron sputtering with pulsed frequency 100 kHz was used as the method for catalysts deposition. Total Pt loading in the thin films varied in the range 0.04–0.13 mg*cm−2. Obtained catalysts onto GDLs were investigated by scanning/transmission electron microscope (S/TEM), X-ray Absorption Near Edge Structure (XANES) and potentiodynamic methods. The deposited catalysts had a high electrochemical surface area (ECSA) and stability determined by the durability stress test (DST) method. The highest ECSA was obtained with Pt target in the pulsed (100 kHz) sputtering mode. This ECSA values were rather stable during 3000 cycles of stress-tests versus 500 cycles for Pt deposited by polyol method on the same gas diffusion layers.

Microstructure, optical and electrical properties of thin films of gallium-phosphorus-titanium alloys synthesized by asymmetric bipolar pulsed direct current magnetron sputtering

Thin films of gallium-phosphorus-titanium (Ga-P-Ti) alloys were prepared on glass substrates at 573 K by an asymmetric bipolar pulsed direct current sputtering technique using an argon atmosphere and targets made from gallium phosphide (GaP) powder and metallic titanium (Ti), at the surface ratios of 8:1, 5:1, 2:1 and 1:1 GaP to Ti (GaP:Ti) on the sputtered area. Examination by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscope indicated that the as-deposited films from the sputtering targets having GaP:Ti ratios of 8:1, 5:1, and 2:1 were polycrystalline with the cubic zinc-blende crystal structure having GaP as the host material, i.e., Ti-doped GaP. Elemental compositions of the film obtained from the target at a GaP:Ti ratio of 5:1 closely resembled the theoretically predicted intermediate band compound Ga4P3Ti. It was projected that the Ga4P3Ti compound could be fabricated by co-sputtering of GaP and Ti from a single target having the surface area ratio GaP:Ti of 3.5:1. Optical transmission and reflection spectra, temperature dependence of electrical resistivity, and light response of the electrical resistivity showed semiconductor-like behavior for the films obtained from the targets with the GaP:Ti of 8:1 and 5:1, and were metal-like for those deposited from the other targets. Optical band gaps determined from the transmission spectrum of the semiconducting films by Tauc's expression for indirect transition were 1.2–1.5 eV. The results of the study could provide an alternative route for fabricating the intermediate band material based on the Ga-P-Ti system.

The Effects of Annealing Temperature on the Structural Properties of ZrB2 Films Deposited via Pulsed DC Magnetron Sputtering

Zirconium diboride (ZrB2) thin films were deposited on a Si(100) substrate using pulsed direct current (dc) magnetron sputtering and then annealed in high vacuum. In addition, we discussed the effects of the vacuum annealing temperature in the range of 750 to 870 °C with flowing N2 on the physical properties of ZrB2 films. The structural properties of ZrB2 films were investigated with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The XRD patterns indicated that the ZrB2 films annealed at various temperatures exhibited a highly preferred orientation along the [0001] direction and that the residual stress could be relaxed by increasing the annealing temperature at 870 °C in a vacuum. The surface morphology was smooth, and the surface roughness slightly decreased with increasing annealing temperature. Cross-sectional TEM images of the ZrB2/Si(100) film annealed at 870 °C reveals the films were highly oriented in the direction of the c-axis of the Si substrate and the film structure was nearly stoichiometric in composition. The XPS results show the film surfaces slightly contain oxygen, which corresponds to the binding energy of Zr–O. Therefore, the obtained ZrB2 film seems to be quite suitable as a buffer layer for III-nitride growth.

Structural and Electrocatalytic Properties of Platinum and Platinum-Carbon Layers Obtained by Magnetron-Ion Sputtering

This article is devoted to further development of magnetron sputtering technology for catalysts and catalysts layer production for fuel cells and other electrochemical devices. Platinum-carbon films with Pt content up to 95–97 wt % were deposited using different sputtering regimes—DC (direct current) sputtering with and without application of a pulse negative bias voltage to the titanium substrate and also bipolar pulse sputtering with frequency of 10 kHz and 100 kHz. Composite platinum carbon targets were used for sputtering. Characteristics of platinum-carbon films were compared with those of platinum films deposited using the same regimes. The main methods of investigation were scanning transmission electron microscopy (STEM) with energy dispersive X-ray spectroscopy; potentiostatic and potentiodynamic methods. The catalytic activity of platinum-carbon films increased with platinum content and at a platinum concentration of 95–97 wt % became higher than that of platinum films sputtered in the same regimes. It was proposed that carbon atoms deposited on the substrate limited the mobility of the deposited platinum species and inhibited Pt cluster growth. Platinum-carbon films produced by pulsed DC magnetron sputtering with pulsed frequency 100 kHz consisted of narrow Pt columns with dome nanotops forming a well-developed surface. The porosity and specific surface of these columnar nanopillar films were higher compared with those of pure platinum films deposited under the same conditions. Moreover, the platinum-carbon films deposited using a bipolar pulse regime with a frequency of 100 kHz had the highest specific surface, porosity (30%) and catalytic activity in hydrogen and oxygen evolution due to a high ion current density and reduced pulse duration which inhibited the growth of large platinum globules.

Effect of magnetron sputtered anode functional layer on the anode-supported solid oxide fuel cell performance

Nickel oxide-yttria stabilized zirconia (NiO-YSZ) thin films were reactively sputter-deposited by pulsed direct current magnetron sputtering from the Ni and ZrY targets onto heated commercial NiO-YSZ substrates. The microstructure and composition of the deposited films were investigated with regard to application as thin anode functional layers (AFLs) for solid oxide fuel cells (SOFCs). The pore size, microstructure and phase composition of both as-deposited and annealed at 1200 °C for 2 h AFLs were studied by scanning electron microscopy and X-ray diffractometry and controlled by changing the deposition process parameters. The results show that annealing in air at 1200 °C is required to improve structural homogeneity of the films. NiO-YSZ films have pores and grains of several hundred nanometers in size after reduction in hydrogen. Adhesion of deposited films was evaluated by scratch test. Anode-supported solid oxide fuel cells with the magnetron sputtered anode functional layer, YSZ electrolyte and La0.6Sr0.4Co0.2Fe0.8O3/Ce0.9Gd0.1O2 (LSCF/CGO) cathode were fabricated and tested. Influence of thin anode functional layer on performance of anode-supported SOFCs was studied. It was shown that electrochemical properties of the single fuel cells depend on the NiO volume content in the NiO-YSZ anode functional layer. Microstructural changes of NiO-YSZ layers after nickel reduction-oxidation (redox) cycling were studied. After nine redox cycles at 750 °C in partial oxidation conditions, the cell with the anode NiO-YSZ layer showed stable open circuit voltage values with the power density decrease by 11% only.

Influence of HPPMS on Hybrid dcMS/HPPMS (Cr,Al)N Processes

Due to their beneficial properties, chromium-based nitride hard coatings deposited by physical vapor deposition (PVD) are applied as protective coatings in many technical applications. In the deposition of these coatings, hybrid dcMS/HPPMS processes consisting of direct current and high power pulse magnetron sputtering can be used in order to benefit from both processes, dcMS and HPPMS. While dcMS provides a considerably higher deposition rate, coatings deposited by HPPMS usually exhibit a significantly higher hardness and show a denser morphology and a smoother surface. The overall aim of this work is to achieve an improved understanding of the hybrid process dcMS/HPPMS. Different nitride (Cr,Al)N coatings were investigated. These were deposited by industrial-like coating processes using an industrial coating unit equipped with six cathodes on the quenched and tempered tool steel AISI 420 (X42Cr13, 1.2083). The aim of the investigations was to analyze the influence of HPPMS on the hybrid dcMS/HPPMS process within the coating plasma at the substrate side and with regard to the deposited coatings. In a first step, the (Cr,Al)N coating plasma was analyzed with a fine spatial resolution along the rotation line of the substrates using dcMS, HPPMS and hybrid dcMS/HPPMS processes. Changes in the plasma composition were investigated from the substrate position using optical emission spectroscopy (OES). In a second step, (Cr,Al)N coatings were deposited using the same process parameters. The coatings were analyzed regarding morphology and deposition rate using scanning electron microscopy (SEM). Furthermore, the contents of Al and Cr in the coatings were analyzed by energy dispersive X-ray spectroscopy (EDS). By comparing the measurements, the influence of HPPMS on the plasma within the hybrid dcMS/HPPMS process was analyzed and compared to the resulting coating properties morphology, deposition rate and chemical composition. This new experimental methodology was introduced before using a large-volume coating unit, but modified with only two cathodes. Within the scope of this work, it was for the first time transferred to an industrial coating unit with six cathodes, different targets and applied under industrial-like process conditions.

Influence of ZnO-Based Sub-Layers on the Growth of Hydrogen Doped Indium Oxide

Because of its high mobility, hydrogen-doped indium oxide (In2O3:H) has a high potential as front contact in thin film or Si heterojunction solar cells. Certain growth conditions are required to process high-mobility In2O3:H. In Cu(In,Ga)Se2 (CIGS) devices, the growth of In2O3:H is influenced by the existing sublayers. Therefore, the intention of the present study is to investigate these influences on In2O3:H, deposited by pulsed direct current magnetron sputtering onto bare glass substrates or Zn(O,S) and ZnO layers, as they are commonly used in CIGS solar cells. The amorphous Zn(O,S) and the crystalline ZnO films were deposited by radio frequency sputtering onto planar glass as well as rough CIGS samples. On the basis of X-ray diffraction and transmission electron microscopy measurements, the structure of the as-grown In2O3:H films was evaluated. X-ray diffraction patterns show amorphous growth on glass and Zn(O,S) and a higher crystallinity of In2O3:H films grown on ZnO layers. The preferred orientatio...

Improved electrical properties of pulsed DC magnetron sputtered hydrogen doped indium oxide after annealing in air

Its high electron mobility makes hydrogen doped indium oxide (IOH) a promising transparent conductive oxide for photovoltaic and other applications. We report on a deposition process for IOH that is attractive for industrial application, using large area pulsed direct current (DC) magnetron sputtering and annealing in air. Structural, optical and electrical properties as well as stability of films annealed in air were assessed and compared to vacuum annealed films. Critical factors were found to be water and oxygen partial pressures during the sputter process, which, when chosen inappropriately, can cause a low density film structure, enhancing diffusion of atmospheric species into the indium oxide layer during crystallization in air. This leads to a lower charge carrier density of air annealed films compared to vacuum annealed films. We introduce a bi-layer stack to reduce diffusion processes during air annealing and thus improve the charge carrier density without adversely affecting the electron mobility or optical absorption.

TiN Films Deposited on Uranium by High Power Pulsed Magnetron Sputtering under Low Temperature.

Depleted uranium (DU) is oxidized readily due to its chemical activities, which limits its applications in nuclear industry. TiN film has been applied widely due to its good mechanical properties and its excellent corrosion resistance. In this work, TiN protection films were deposited on DU by direct current magnetron sputtering (DCMS) and high power pulsed magnetron sputtering (HPPMS), respectively. The surface morphology and microstructures were investigated by atomic force microscope (AFM), scanning electron microscopy (SEM), and grazing incidence X-ray diffraction (GIXRD). The hardness and Young’s modulus were determined by nano-Indenter. The wear behavior and adhesion was analyzed by pin-on-disc tests and scratch adhesion tests and the corrosion resistance was evaluated by electrochemical measurements. The results show that the TiN films that were deposited by HPPMS outperformed TiN film deposited by DCMS, with improvements on surface roughness, mechanical properties, wear behavior, adhesion strength, and corrosion resistance, thanks to its much denser columnar grain growth structure and preferred orientation of (111) plane with the lowest strain energy. Besides, the process of Ti interlayer deposition by HPPMS can enhance the film properties to an extent as compared to DCMS, which is attributed to the enhanced ion bombardment during the HPPMS.

A micro-electromechanical systems based vibration energy harvester with aluminum nitride piezoelectric thin film deposited by pulsed direct-current magnetron sputtering

Piezoelectric vibration-based energy harvesting is a promising alternative for the durable and reliable power supplies for the low-powered wireless sensor networks nodes. However, improving the piezoelectric vibration-based energy harvester’s energy conversion efficiency related closely on the structure parameters and materials become significant and urgent. In this paper, a micro-electromechanical systems (MEMS) cantilever-based aluminum nitride (AlN) vibration energy harvester is presented, modeled, optimized and fabricated. The c-axis oriented AlN piezo thin film is deposited by the pulsed direct-current magnetron sputtering with the optimized process parameters. Furthermore, the coupled distributed parameter model is derived in detailed, verified by using the ANSYS finite element analysis, and applied to the optimization of structural parameters and load resistance. Finally, the prototype of the device is fabricated by the standard bulk-silicon technology. As a result, at 1g and 210.85 Hz, the maximum output root mean square (RMS) voltage can reach to 4.66 V, the output average power and output average power density of the prototype is up to 56.4 μW and 854.55 μW/(cm3·g2) at a load resistance of 146.6 kΩ, respectively. The experimental results agree well with theoretical analysis. The derived CDP model has an important and effective guiding role in cantilever based piezoelectric vibration energy harvester for structural design and performance optimization, especially for the device with long tip mass. The prototype has wide application prospects in the fields of the wireless sensor network node such as the structural health monitoring system.

Dielectric breakdown of alumina thin films produced by pulsed direct current magnetron sputtering

Alumina films (~2 μm thick) were deposited with a mixed Cu/Al interlayer onto copper. Direct current (DC)/Pulsed DC (PDC) magnetron sputtering techniques were independently compared for reactive alumina sputtering. In DC sputtered films, elemental aluminium of 9.2 at.% and nano-crystallites were present within the x-ray amorphous matrix, resulting from target arcing. Defects lead to premature dielectric breakdown/increased current leakage. PDC sputtering improved film quality by removing crystallites, metallic clusters and through thickness cracking. Time dependent dielectric breakdown (TDDB) measurements were carried out using conductive atomic force microscopy identified an improvement in dielectric strength (166 to 310 V μm−1) when switching from DC to PDC deposition power. TDDB suggested that at high applied field the dominant pre-breakdown conduction mechanism was Fowler-Nordheim tunnelling in DC films. Tensile pull-off adhesion ranged from 56 to 72 MPa and was highest following incorporation of an Cu/Al blended interfacial layer. Scratch testing indicated various cracking/buckling failures.

Cu and Pt clusters deposition on TiO2 powders by DC magnetron sputtering for photocatalytic hydrogen production

Titanium dioxide-based photocatalyst powders were obtained by deposition of copper and/or platinum clusters by means of pulsed direct current magnetron sputtering with different deposition times and plasma composition during Cu/Pt sputtering. A top-down configuration was employed with the sputtering source facing the powder holder mounted on a shaker mechanism, which continuously mixed the powder during the sputtering process. HRTEM analyses revealed the presence of well dispersed, subnanometric sized metal clusters, even for long deposition times, while XRPD analysis showed no modification of the TiO2 crystal structure upon metal deposition. The so obtained powders were tested as photocatalysts in methanol photo-steam reforming for hydrogen production. The presence of Pt clusters increased the photoactivity with respect to that of bare TiO2. The plasma composition during Cu sputtering was found to strongly affect the photoactivity of the obtained materials, Cu alone deposited as co-catalyst in an Ar-only atmosphere imparting better photoactivity than Cu sputtered in Ar/O2. When the deposition of Cu clusters was coupled with the deposition of Pt clusters, an additive effect of the two metals in increasing TiO2 photoactivity was observed if Cu clusters were sputtered in the absence of oxygen.

Tribological studies on self-lubricating (Cr,Al)N+Mo:S coatings at elevated temperature

Due to the high material utilization and the associated resource and energy efficiency, production processes from the field of cold forging of steel are of great importance. At present, environmentally harmful lubricants have to be used to ensure process stability as well as low wear and friction. Due to environmental, economic and legislative aspects, there is an increased research potential to reduce or to completely substitute lubricants. To achieve the goal of lubricant free dry cold forging of steel, physical vapor deposition (PVD) coatings with self-lubricating properties are applied on forming tools. Promising for this application are PVD coatings on the basis of a (Cr,Al)N hard phase with embedded molybdenum and sulfur which simultaneously meet the requirements of high wear resistance and friction reduction. In the presented work, three self-lubricating coatings (Cr,Al)N+Mo:S deposited with varying bias voltage were investigated. The hybrid PVD technology, consisting of direct current and high power pulse magnetron sputtering dcMS/HPPMS, was used for the coating deposition in an industrial coating unit. Two different steels AISI D2 (DIN 1.2379) and AISI M2 (DIN 1.3343) were used as substrate materials. The influences of substrate material and heat treatments at T=250°C were investigated. The samples were analyzed with respect to the universal hardness HU and modulus of indentation EIT using nanoindentation. To determine the interfacial adhesion of the compound coating/substrate, Rockwell indentation and scratch tests were carried out. For the analysis of the tribological behavior, unhardened AISI 5115 (DIN 1.7131) was used as counterpart material. The influence of the elevated temperature on the friction and wear behavior was investigated by Pin-on Disk (PoD) at T=250°C and T=23°C. The wear of the basic bodies as well as the counterparts were analyzed by confocal laserscanning microscopy. The investigations have shown that the high temperature has a significant influence on the friction and wear behavior. The wear rate of the counterparts decreases by up to a factor greater than x=100 compared to the test at T=23°C. Furthermore, the wear rates of the basic body of the uncoated reference specimen increase rapidly at tribological stress at high temperatures. The examined coatings exhibit high wear resistance at elevated temperatures. The contact surface was studied by Raman spectroscopy after the tribological tests. PoD tests have shown, that the applied coatings led to a significantly reduction in friction most likely due to the tribo-chemically induced formation of self-lubricating MoS2 reaction layers.

Thermal stability of nanocomposite Mo2BC hard coatings deposited by magnetron sputtering

The investigation of hard coatings under thermal load is crucial in order to obtain information on the thermal stability and possible changes of microstructure and mechanical properties. In addition, advanced heating studies may also provide feedback for the grain growth mechanism occurring during annealing and thus, help to predict optimum post-growth annealing conditions for producing high-performance hard coatings. Here, we investigate the thermal response of Mo2BC, deposited by bipolar pulsed direct current magnetron sputtering in an industrial chamber on a silicon substrate at a substrate temperature of 380 °C. Ex-situ and in-situ X-ray diffraction and transmission electron microscopy studies are performed at elevated temperatures to track changes in the structure. Whereas the as-deposited nanocomposite coating exhibits small spherical nanocrystals (1.2 nm in diameter) embedded in an amorphous matrix, a fully crystalline structure, mainly consisting of elongated and interconnected crystals with lengths of up to 1 μm, is obtained at elevated annealing temperatures. Hardness and Young's modulus increase by ~8% and ~47%, respectively, compared to the as-deposited coating. Delamination from the silicon substrate only occurs at temperatures above 840 °C. Thus, our detailed study of the micro- and nanostructure evolution upon thermal annealing suggests that heat treatments below 840 °C are a suitable method to improve the crystallinity and mechanical properties of nanocomposite Mo2BC coatings.

Fracture toughness of Mo2BC thin films: Intrinsic toughness versus system toughening

The fracture behaviour and microstructure evolution of sputtered Mo2BC films as a function of their deposition temperature is studied. Bipolar pulsed direct current magnetron sputtering was used to deposit Mo2BC thin films onto Si (100) wafers at substrate temperatures ranging from 380 to 630 °C. Microstructural characterization by transmission electron microscopy revealed that increasing the deposition temperature induces larger and more elongated grains, and a higher degree of crystallinity, transitioning from a partially amorphous to a fully crystalline film. The intrinsic fracture toughness of the Mo2BC films was studied by focussed ion beam milled micro-cantilever bending tests. A mild dependency of the intrinsic fracture toughness on the substrate deposition temperature was found. Fractograph analysis showed that the fracture behaviour was dominated by intergranular fracture or by fracture within the amorphous regions. Additionally, nanoindentation based fracture toughness measurements were used to probe the fracture behaviour of the Mo2BC/Si system, where residual stresses define the ‘apparent’ fracture toughness of the system. Depending on the substrate deposition temperature either compressive or tensile residual stresses developed in the films. This causes a relative change in the system toughness by up to one order of magnitude. The fracture experiments clearly reveal that notched cantilevers provide intrinsic toughness values of a material, while nanoindentation probes the toughness of the entire coating-substrate system. The combination of both techniques provides valuable design information for enhancing fracture resistance of Mo2BC films.