Advantages of nanocomposite coatings deposited by high power pulse magnetron sputtering technology

Recent advantages in PVD coatings for cutting tools enable high speed and dry machining with superior cutting parameters in commercial manufacturing sectors. For this reason hard coatings with high oxidation resistance and thermal stability are used for economically justifiable machining. In this regard nc-(Ti,Al)N/a-Si₃N₄ films were sputtered on tungsten carbide cutting tools and WC/Co samples by using the high power pulse magnetron sputtering (HPPMS) technology. Coating composition, microstructure and applied properties were investigated by using X-ray diffraction, scanning electron microscope and nanoindentation. The hardness value was about 29 GPa for a Si content of 3.3 at.%. The grain size was about 6 nm. As this study focuses on the thickness uniformity of the coatings, SEM pictures of the cross-section have been taken around the cutting edge to determine the deposition rate and the film growth. The coatings morphology has been compared to middle frequency and direct current sputtered nanocomposite (Ti,Al,Si)N films. The results demonstrate the enhanced HPPMS coatings properties, including a denser structure, a smoother surface and a favourable thickness uniformity.     

Multilayered gradient CrN/ZrN coatings synthesized by magnetron sputtering

The multilayer gradient CrN/ZrN coatings were synthesized by a dual cathode DC magnetron sputtering.The influence of different species of reaction gases and partial pressures on structure and mechanical properties was investigated using XRD, AES, XPS, and nanoindentation. The results show that N2-NH3 mixture process gas is of benefit to the synthesis of superhard multilayered gradient CrN/ZrN coatings. The presence of the preferred orientations of CrN(111), (200) and ZrN (111), (220) in the structure is a main reason for superhardness of multilayered gradient coatings.     

Discharge physics of high power impulse magnetron sputtering

High power impulse magnetron sputtering (HIPIMS) is pulsed sputtering where the peak power exceeds the time-averaged power by typically two orders of magnitude. The peak power density, averaged over the target area, can reach or exceed 10⁷ W/m², leading to plasma conditions that make ionization of the sputtered atoms very likely. A brief review of HIPIMS operation is given in a tutorial manner, illustrated by some original data related to the self-sputtering of niobium in argon and krypton. Emphasis is put on the current–voltage–time relationships near the threshold of self-sputtering runaway. The great variety of current pulse shapes delivers clues on the very strong gas rarefaction, self-sputtering runaway conditions, and the stopping of runaway due to the evolution of atom ionization and ion return probabilities as the gas plasma is replaced by metal plasma. The discussions are completed by considering instabilities and the special case of “gasless” self-sputtering.     

Control of reactive high power impulse magnetron sputtering processes

High power impulse magnetron sputtering (HIPIMS) is a technologically important physical vapour deposition (PVD) process that is able to provide a highly ionised flux of sputtered species. It is thought to be particularly important for applications where there is a need to coat 3D features (e.g. vias and trenches in semiconductor applications). HIPIMS may have other added benefits, as compared to DC or medium frequency AC/pulse-DC magnetron sputtering, related to better coating structure-property relationship control through self-species (sputtered metal) plasma/ion assistance.Many of the technologically important thin films (e.g. transparent conductive oxides, permeation barrier coatings, etc.) are sputtered from metal targets in a reactive gas atmosphere, usually Ar + O₂ or N₂, to ensure industrially relevant coating deposition rates. Enhanced structure-property relationship control of these thin film materials is highly desirable; hence, it also is desirable to use HIPIMS in a reactive deposition mode. Preliminary trials of reactive HIPIMS however have indicated that the control of this process using conventional means, such as conventional plasma emission monitoring (PEM) is difficult. Thus, the application of reactive HIPIMS is rather limited and the potential benefits are not realised, especially in the areas where precise process control and long term stability in a reactive environment are required.In this paper reactive HIPIMS process (Ti in Ar/O₂ atmosphere) is investigated and various control options are evaluated. The application of a recently developed PEM based reactive HIPIMS control method is reported. Performance of the developed technique is compared to that of the conventional PEM, Penning-PEM and λ-sensor based methods. It is shown that conventional PEM is impractical to control reactive HIPIMS, while the constant reactive gas flow method does not lead to a stable deposition process. The new PEM based process control technology was shown to provide precise control and stable operation of reactive HIPIMS discharges anywhere within the hysteresis loop. It was also found to be superior when compared to oxygen partial pressure control based techniques.     

On the film density using high power impulse magnetron sputtering

The influence on thin film density using high power impulse magnetron sputtering (HiPIMS) has been investigated for eight different target materials (Al, Ti, Cr, Cu, Zr, Ag, Ta, and Pt). The density values as well as deposition rates have been compared to results obtained from thin films grown by direct current magnetron sputtering (DCMS) under the same experimental conditions. Overall, it was found that the HiPIMS deposited coatings were approximately 5-15% denser compared to the DCMS deposited coatings. This could be attributed to the increased metal ion bombardment commonly seen in HiPIMS discharges, which also was verified using a global plasma model to assess the degree of ionization of sputtered metal. One key feature is that the momentum transfer between the growing film and the incoming metal ions is very efficient due to the equal mass of film and bombarding species, leading to a less pronounced columnar microstructure. As expected the deposition rates were found to be lower for HiPIMS compared to DCMS. For several materials this decrease is not as pronounced as previously reported in the literature, which is shown in the case of Ta, Pt, and Ag with rate_HiPIMS/rate_DCMS ~ 70-85%, while still achieving denser coatings.     

Characterization of nitride coatings on high density graphite deposited by magnetron sputtering

Cathode deposit consolidation operation after electrorefining of spent metallic fuels of fast breeder reactors involves melting of U and Pu at 1300 °C after distillation of occluded chloride salt and Cd, in graphite crucibles. Nitride coatings possessing high hardness, melting point and thermodynamic stability against reactive materials and molten LiCl-KCl salts have greater potential for coating the graphite crucibles. In the present study nanocrystalline TiN, ZrN and Ti-Si-N coatings were deposited on high density graphite disc and crucible samples by DC/RF magnetron sputtering. The coated samples were characterized by SEM, GIXRD and AFM. The results indicated that coatings with uniform thickness of 3 to 6 µm were deposited on high density graphite which adheres well to the substrate. The surface morphology of TiN, ZrN and Ti-Si-N coatings examined by SEM and AFM showed the presence of spherical nanoparticles of nitrides getting agglomerated into clusters. Characterisation of nitride coated crucibles was carried out before and after uranium melting by induction heating to simulate cathode processor crucible conditions. TiN and Ti-Si-N coating appears to offer better stability, ease of ingot release and coating adhesion. The paper highlights the results of the present investigation.     

非平衡磁控溅射Ti掺杂类金刚石薄膜的基本特性

利用非平衡磁控溅射技术在镜面抛光的SCM415渗碳淬火钢基片上沉积了无掺杂类金刚石(DIE)薄膜和不同含量Ti掺杂类金刚石(Ti-DIE)薄膜.利用AFM、SEM、TEM对薄膜的微观结构与形貌进行了观察,利用纳米硬度计、摩擦磨损试验仪及纳米划痕仪测试了薄膜的显微硬度、摩擦系数及薄基结合强度.结果表明:随着Ti的掺杂,薄膜硬度先迅速降低,然后保持不变,在Ti含量为25at%时薄膜硬度出现回升,膜基结合强度随Ti的掺杂呈单调增强趋势.与无掺杂类金刚石薄膜相比,掺杂Ti后薄膜表面微观凸凹增多,摩擦系数增大.对于Ti-DIE薄膜来说,随着Ti掺杂量的增加,摩擦系数出现减小的趋势.其原因在于Ti掺杂量的增加使Ti-DLC薄膜变得更加致密,同时Ti的掺杂还有利于弥补基体表面的凸凹缺陷,使薄膜变得更平滑.     

Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering

MoS_x-based films were deposited using magnetron sputtering from a pure MoS₂ target. Alloying was accomplished by “co-deposition” from separate targets onto substrates having a two-fold rotation. An additional experiment had also a Cr⁺-ion etch for surface preparation, followed by a Cr adhesion layer, made using a Cr target mounted on a cathodic arc evaporation source. MoS_x and Al- and Ti-alloyed MoS_x coatings have been deposited onto high speed steel (HSS) and glass substrates for corrosion investigations.The coatings were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, hardness and adhesion measurements. The corrosion behaviour of the samples was electrochemically measured by open-circuit-potential (OCP) measurements and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7). Additionally the MoS_x-based coatings on HSS have been exposed to salt spray tests. The corrosion investigations revealed that the addition of Al and Ti to MoS_x shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoS_x coating a little bit less noble. In agreement with the OCP measurements, the corrosion potential E_corr in potentiodynamic corrosion tests was the highest for non-alloyed MoS_x coatings on HSS substrates. After the potentiodynamic corrosion tests, a strong corrosive attack could be observed for all coated samples. In salt spray tests the lowest number of corrosion pits was found for the MoS_x-Al (Cr⁺) coating on HSS.     

Droplet-free TiC nanocrystal-containing diamond-like carbon coatings deposited by combined cathodic arc MF magnetron sputtering

Droplet-free TiC-containing diamond-like carbon (DLC) nanocomposite coatings were deposited by using a combined cathodic arc middle-frequency magnetron sputtering system using C₂H₂ as working gas. X-ray photoelectron spectroscopy and transmission electron microscopy showed that with increasing flow rate of C₂H₂, the structure of nanocomposite coatings changed from TiC nanograin-containing to graphite nanograin-containing DLC. Wear experiment showed that the friction coefficient of TiC-DLC nanocomposite coatings decreased with increasing C₂H₂. A low friction coefficient of 0.07 was obtained at 480 sccm C₂H₂.     

Corrosion behaviors of Cr-Al-N coatings deposited by reactive magnetron sputtering

CrN and Cr-Al-N coatings were deposited by reactive magnetron sputtering on the glass substrate, and their corrosion behavior was studied. The electrochemical tests using both DC （polarization curves） and AC techniques （EIS） were carried out on Potentiostal/Galvanstat （EG＆G） in 3.5% （mass fraction） NaCl solution. After immersed into NaCl solution for 1 h, the mass of the CrN coating keeps constant with the time continuing. This can be explained by the passivation of the coating. The comparison between the corrosion potential （φcom） of the Cr-Al-N coatings with different aluminum contents reveals that the corrosion potentials of the aluminum contain coatings are nobler than that of the CrN coatings. This means that the addition of aluminum shifts the corrosion potential to more positive potential value. Among these coatings, CrN in NaCl solution exhibits the worst corrosion resistance, while the corrosion resistance of Cr0.63Al0.37N in NaCl solution is the best. The polarization data and EIS data suggest that addition of aluminum can improve the corrosion resistance of CrN coating.     

Microstructure control of CrNx films during high power impulse magnetron sputtering

The microstructure and composition of CrN_x (0 ≤ x≤ 1) films grown by reactive high power pulsed magnetron sputtering (HIPIMS or HPPMS) have been studied as a function of the process parameters: N₂-to-Ar discharge gas ratio, (f_N2/Ar), negative substrate bias (V_s), pulsing frequency, and energy per pulse. The film stoichiometry is found to be determined by the composition of the material flux incident upon the substrate during the active phase of the discharge with no nitrogen uptake between the high power pulses. Scanning electron microscopy investigations reveal that for 0 < f_N2/Ar < 0.15 and 150 V bias, a columnar film growth is suppressed in favor of nano-sized grain structure. The phenomenon is ascribed to the high flux of doubly charged Cr ions and appears to be a unique feature of HIPIMS. The microstructure of column-less films for 100 V ≤ V_s ≤ 150 V is dominated by the CrN and hexagonal β-Cr₂N phases and shows a high sensitivity to V_s. As the amplitude of V_s decreases to 40 V and self-biased condition, the film morphology evolves to a dense columnar structure. This is accompanied by an increase in the average surface roughness from 0.25 nm to 2.4 nm. CrN_x samples grown at f_N2/Ar ≥ 0.3 are columnar and show high compressive stress levels ranging from −7.1 GPa at f_N2/Ar = 0.3 to −9.6 GPa at f_N2/Ar = 1. The power-normalized deposition rate decreases with increasing pulse energy, independent of f_N2/Ar. This effect is found to be closely related to the increased ion content in the plasma as determined by optical emission spectroscopy. The HIPIMS deposition rate normalized to DC rate decreases linearly with increasing relative ion content in the plasma, independent of f_N2/Ar and pulsing frequency, in agreement with the so-called target-pathways model. Increasing frequency leads to a finer grain structure and a partial suppression of the columnar growth, which is attributed to the corresponding increase of the time-averaged mean energy of film-forming ions arriving at the substrate.     

Microstructure and tribology of TiC(Ag)/a-C:H nanocomposite coatings deposited by unbalanced magnetron sputtering

TiC(Ag)/a-C:H nanocomposite coatings with various Ag concentrations were fabricated on Si p(100) substrates. The composition and structure of as-deposited nanocomposite coatings were systemically investigated, and the friction and wear behaviors were also evaluated under the ambient, high temperature and high vacuum, respectively. Results show that the TiC nanocrystallites were formed in the amorphous hydrogenated carbon matrix near the substrate. The co-dopant Ag possessed nanocrystalline structure in the as-fabricated coatings whilst it formed Ag clusters (10–50 nm) on the surface. Furthermore, the introduction of Ag caused a significant reduction in the residual compressive stress without considerable decrease of the hardness and improved the adhesive strength of nanocomposite coatings. Tested as-deposited and after annealed at 500 °C coatings, the TiC(Ag)/a-C:H coatings showed a reduction of friction coefficients and wear rates with increment of Ag concentration. Under high vacuum condition, the TiC(Ag)/a-C:H coatings presented superlow friction behavior where the friction coefficient was reduced from 0.01 to 0.005 and lifetime increased from 0 to 1500 cycles. The significant improvement in tribological properties was mainly attributed to the low shear strength of Ag clusters on the surface as well as Ag diffusion to surface and wear track of coatings. The superior friction and wear behaviors of TiC(Ag)/a-C:H coatings make them good candidates as solid lubrication materials in space and aircraft applications.     

Synthesis and properties of Cr-Al-Si-N films deposited by hybrid coating system with high power impulse magnetron sputtering (HIPIMS) and DC pulse sputtering

The CrN and Cr-Al-Si-N films were deposited on Si wafer and SUS 304 substrates by a hybrid coating system with high power impulse magnetron sputtering (HIPIMS) and a DC pulse sputtering using Cr and AlSi targets under N2/Ar atmosphere.By varying the sputtering current of the AlSi target in the range of 0-2.5 A,both the Al and Si contents in the films increased gradually from 0 to 19.1% and 11.1% (mole fraction),respectively.The influences of the AlSi cathode DC pulse current on the microstructure,phase constituents,mechanical properties,and oxidation behaviors of the Cr-Al-Si-N films were investigated systematically.The results indicate that the as-deposited Cr-Al-Si-N films possess the typical nanocomposite structure,namely the face centered cubic (Cr,Al)N nano-crystallites are embedded in the amorphous Si3N4 matrix.With increasing the Al and Si contents,the hardness of the film first increases from 20.8 GPa for the CrN film to the peak value of 29.4 GPa for the Cr0.23Al0.14Si0.07 N film,and then decreases gradually.In the meanwhile,the Cr0.23Al0.14Si0.07N film also possesses excellent high-temperature oxidation resistance that is much better than that of the CrN film at 900 or 1000 °C.     

Growth characteristics of MoS2 coatings prepared by unbalanced bipolar DC magnetron sputtering

MoS2 coatings were prepared by unbalanced bipolar DC magnetron sputtering under different argon pressures and for different deposition times, and the structure and morphology of MoS2 coatings were determined and observed respectively by X-ray diffractometry and scanning electron microscopy. The results show that at lower argon pressures of 0.15 Pa and 0.40 Pa, MoS2 coatings are formed with the （002） basal plane parallel to the surface, whereas the coating deposited at the argon pressure above 0.60 Pa has the （002） basal plane perpendicular to the surface. Two stages can be classified for the formation of MoS2 coating. At the initial stage of coating formation, the （002） basal plane with S-Mo-S layer structure grows on the substrate whatever the argon pressure is. And then the coating under 0.40 Pa argon pressure still grows with （002） laminate structure, but the coatings under 0.88 Pa and 1.60 Pa argon pressures turn to grow with the mixed basal and edge orientations. The morphology and structure of MoS2 coatings are highly related to their growth rate and the energy of sputtered particles.     

Structural,mechanical and electrical-contact properties of nanocrystalline-NbC/amorphous-C coatings deposited by magnetron sputtering

Niobium-carbide nanocomposite coatings with a carbon content varying from 43 to 64 at.% were deposited by dual DC magnetron sputtering. X-ray diffraction, x-ray photoelectron spectroscopy and electron microscopy showed that all coatings consisted of nanometer sized NbC grains embedded in a matrix of amorphous carbon. Mechanical properties and electrical resistivity showed a strong dependency on the amount of amorphous carbon (a-C) and NbC grain size in the coating. The highest hardness (23 GPa), elastic modulus (295 GPa) and the lowest resistivity (260 μΩ cm) were measured for the coating with about 15% of a-C phase. Contact resistance measurements using a crossed cylinder set-up showed lowest contact resistance for the coating containing 33% a-C (140 μΩ at a contact force of 100 N), which is comparable to a Ag reference (45 μΩ at a contact force of 100 N). Comparison with TiC-based nanocomposites studied under similar conditions showed that the NbC system has less tendency to form a-C and that lowest contact resistance is obtained at comparable amounts of a-C phase in both material systems (33% for NbC compared to 35% for TiC). With these good electrical contact properties, the NbC nanocomposites can be considered as a potential material for electrical contact applications.     

The structure and properties of VN-VCN-VC coatings deposited by a high energy ion assisted magnetron sputtering method

In the last years the scientific interest for transition metal compounds, used for protective and functional coatings, was extended and included, besides Ti, other elements such as Zr, V, and Hf as potential candidates. Due to their promising mechanical properties, and resistance to high temperature, V compounds received an increasing attention from scientists and end-users.The present work is dedicated to the study of the characteristics and the properties of the vanadium based multilayer hard coatings deposited by a magnetron sputtering process, assisted by a high voltage pulse discharge. Different multilayer coatings (VC/VN, VN/VC, VN/VCN/VC) were obtained and comparatively analyzed relative to vanadium nitride and vanadium carbide monolayers. Coatings with microhardness up to 3700 HV 0.05 and thickness up to 20 μm have been produced.Elemental and phase composition, as well as tribological characteristics (wear factor and friction coefficient) of the coatings were investigated. The chemical and structural investigations were performed by GDOES (Glow Discharge Optical Emission Spectrometry), EDX (Energy Dispersive X-ray analysis), XRD (X-ray Diffraction) and SEM (Scanning Electron Microscopy), while microhardness measurements and wear test were used to assess the coatings characteristics. The friction coefficient and the wear resistance of the coatings were evaluated by using pin on disc tests in unlubricated conditions.     

Exploring the potential of high power impulse magnetron sputtering for growth of diamond-like carbon films

Amorphous carbon films are deposited employing high power impulse magnetron sputtering (HiPIMS) at pulsing frequencies of 250 Hz and 1 kHz. Films are also deposited by direct current magnetron sputtering (dcMS), for reference. In both HiPIMS and dcMS cases, unipolar pulsed negative bias voltages up to 150 V are applied to the substrate to tune the energy of the positively charged ions that bombard the growing film. Plasma analysis reveals that HiPIMS leads to generation of a larger number of ions with larger average energies, as compared to dcMS. At the same time, the plasma composition is not affected, with Ar⁺ ions being the dominant ionized species at all deposition conditions. Analysis of the film properties shows that HiPIMS allows for growth of amorphous carbon films with sp³ bond fraction up to 45% and density up to 2.2 g cm^− 3. The corresponding values achieved by dcMS are 30% and 2.05 g cm^− 3, respectively. The larger fraction of sp³ bonds and mass density found in films grown by HiPIMS are explained in light of the more intense ion irradiation provided by the HiPIMS discharge as compared to the dcMS one.     

Deposition of wear resistant coatings based on diamond like carbon by unbalanced magnetron sputtering

Amorphous hydrogenerated carbon films containing a small amount of metal (Me:C-H) have been deposited by closed field unbalanced magnetron sputter ion plating. The films have graded film compositions to optimise the adhesion to the substrates, and multilayer TiC/Ti:C-H films have also been deposited. The films have excellent properties: very high measured microhardness (more than 4000 H_v), excellent adhesion (L_c 115–125 N), coefficients of friction against WC of less than 0.2, and volumetric wear rates one fifth that of titanium nitride. The coating procedure is ideally suited to the system used, and many applications for the films are already realised.     

Microstructure and properties of low friction TiC---C nanocomposite coatings deposited by magnetron sputtering

The objective of nanocomposite coatings combining hard and lubricant phases is the development of advanced multi-functional protective thin films showing abrasion resistance, and simultaneously, low friction. Up to now, no clear relation between constitution, microstructural properties and performance of such nanocomposite coatings based on dry lubricants like carbon or MoS₂ has been evaluated. Deposition techniques, constitution, properties and performance of magnetron-sputtered nanocomposite coatings in the TiC---C system are presented. The Vickers hardness could be optimized to values of polycrystalline TiC thin films, and at the same time, low friction coefficients against steel, similar to diamond-like amorphous carbon, could be realized. The mechanical properties and the tribological behavior of these thin films are related to the chemical composition and the microstructure of these advanced materials, characterized by electron microprobe analysis, Auger electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy.     

Microstructure, mechanical and tribological properties of CrN/W2N multilayer films deposited by DC magnetron sputtering

CrN/W₂N multilayer films with various bilayer periods of 15-85 nm were deposited on high speed steel (W18Cr4V) substrates by means of DC closed field unbalanced magnetron sputtering. The morphology and microstructure of the multilayer films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The mechanical and tribological properties were evaluated using a nanoindentor, Rockwell and scratch tests and a conventional ball-on-disk tribometer, respectively. There were some transverse grains at the layer interface and the interface between the CrN and W₂N layers was not so sharp owing to atom diffusion through the interface. In the bilayer period range, the microhardness, elastic modulus and adhesive strength of the CrN/W₂N multilayer films increased with the decrease of bilayer period. The CrN/W₂N multilayer film with a bilayer period of 15 nm showed the highest hardness (29.2 GPa), elastic modulus (376 GPa) and best adhesion strength, it also had the highest wear resistance and lowest friction coefficient.