Target poisoning during CrN deposition by mixed high power impulse magnetron sputtering and unbalanced magnetron sputtering technique

Target poisoning phenomenon in reactive sputtering is well-known and has been studied in depth over the years. There is a clear agreement that this effect has a strong link on the quality, composition, properties and pronouncedly on the deposition rate of PVD coatings. With the introduction of IPVD techniques such as the relatively novel High Power Impulse Magnetron Sputtering (HIPIMS), which have highly ionized plasmas of the depositing species (metal and gas ions), target poisoning phenomenon is highly contested and thus has been left wide open for discussion. Particularly there have been contradicting reports on the presence of prominent hysteresis curves for reactive sputtering by HIPIMS. More work is needed to understand it which in turn will enable reader to simplify the coating deposition utilizing HIPIMS. This work focuses on the study of Chromium (Cr) targets when operated reactively in argon + nitrogen atmosphere and in different ionizing conditions, namely (a) pure HIPIMS (b) HIPIMS combined with UBM (Unbalanced Magnetron Sputtering) and (c) pure UBM. Nitrogen flow rate was varied (5 sccm to 300 sccm) whereas the average power on target was maintained around 8kW. Target resistance vs N2 flow rate curves for these conditions have been plotted in order to analyze the poisoning effect. When only one UBM target was operating target poisoning effect was prominent between the flow rates of 80 and 170 sccm. However it appeared reduced and in nearly same flow rate ranges (90 and 186 sccm) when only one HIPIMS target was operating. When 4 UBM targets were operated, target poisoning effect was evident however expectedly moved to higher flow rates (175 sccm and above) whereas appeared diminished when 2 UBM and 2 HIPIMS were running simultaneously. Further, to analyze the effect of actual target conditions (poisoning) on deposition rate and on the properties of the films deposited, commercially widely used Chromium nitride (CrN) coatings were deposited in mixed HIPIMS and UBM plasma and at 5 different flow rates of nitrogen. Detail characterization results of these coatings have been presented in the paper which will assist the reader in deposition parameter selection. Keywords : HIPIMS, UBM, CrN, Nitrogen flow rate, target poisoning, PVD coating

Effect of substrate bias voltage on defect generation and their influence on corrosion and tribological properties of HIPIMS deposited CrN/NbN coatings

Substrate bias voltage is one of the most influential deposition parameter for physical vapour deposition processes as it can directly control the adatom mobility during coating growth. It influences the hardness, roughness as well as the microstructure of the coatings. Thus, bias voltage could also affect the defect formation during the coating deposition. High Power Impulse Magnetron Sputtering (HIPIMS) has been proven useful in producing void free and arc droplet free dense coatings. However, such coatings can still suffer from some defects associated with external factors (independent of deposition technique), such as substrate irregularities and the flakes coming from the chamber components. In order to study the effects of bias voltage (Ub) on the defect formation during HIPIMS process, four sets of CrN/NbN coatings were deposited at Ub = - 40 V, - 65 V, - 100 V and - 150 V. Microscopic studies revealed that with the increase in bias voltage the coatings morphology was altered and the percentage of surface area covered by optically visible defects was increased from 3.13 % to 4.30 %. The defects on the coatings deposited at Ub = - 100 V and - 150 V led to preferential corrosive attack resulting in a sharp increase in corrosion current density during Potentiodynamic polarisation experiments. Room temperature pin-on-disc tribological tests exhibited the influence of defects on the wear behaviour; however, the coefficient of friction (µ) values were mainly influenced by the nature of the oxides formed during the tests. Coating microstructure and bilayer thickness, along with the coating defects determined the coefficient of wear (Kc) values. This study revealed that the coating deposited at Ub = - 65 V had the highest wear resistance (Kc = 2.68 × 10-15 m3N-1m-1) and the lowest friction (µ = 0.48)

Corrosion behaviour of post-deposition polished droplets-embedded arc evaporated and droplets-free HIPIMS/DCMS coatings

In this study, the effect of metal rich core of the droplets on the corrosion properties of TiN, CrN and ZrN arc evaporated nitride coatings has been investigated and the corrosion properties of such coatings have been compared with droplet free, highly dense coatings grown by combined high power impulse magnetron sputtering (HIPIMS) and direct current magnetron sputtering (DCMS) technique. An industrial size Hauzer HTC 1000-4 system enabled with HIPIMS technology was used for the deposition of combined HIPIMS/DCMS coatings. The corrosion behaviour of the coatings was studied by potentiodynamic polarisation test (ACM instruments Gill AC potentiostat, -1V to +1V) using 3.5% NaCl solution. Initially, as-deposited arc evaporated coatings with an exposed surface area of 1 cm2 were subjected to corrosion. Then, the coatings were gently polished to expose the metal rich core of the droplets. Subsequently, fresh un-corroded area of the polished coating was subjected to corrosion with previously corroded area masked. It has been found that mechanical polishing considerably deteriorated the corrosion performance of arc coatings by forming more than one galvanic couple between the two parts (metal rich, nitrogen rich) of the same droplet itself or between the metal rich part of the and the adjoining coating/exposed substrate. It has been further demonstrated that the droplet free highly dense HIPIMS/DCMS coatings exhibited superior corrosion resistance as compared to the arc-evaporated coatings. Raman analysis was used to study the constituents of the corrosion products. Scanning electron microscopy (SEM, planar view) was used to examine the as-deposited and corroded coating surfaces to define morphological differences. Energy dispersive X-ray (EDX) analysis was done to study the composition of the coatings. Cross-section SEM and ball cratering techniques (CSEM calo wear tester) were used to measure the thickness of the coatings

ZrN coatings deposited by high power impulse magnetron sputtering and cathodic arc techniques.

Zirconium nitride (ZrN) coatings were deposited on 1 micron finish High Speed Steel (HSS) and 316L Stainless Steel (SS) test coupons. Cathodic Arc (CA) and HIPIMS (High Power Impulse Magnetron Sputtering) + UBM (Unbalanced Magnetron Sputtering) techniques were utilised to deposit coatings. CA plasmas are known to be rich in metal and gas ions of the depositing species as well as macro-particles (droplets) emitted from the arc sports. Combining HIPIMS technique with UBM in the same deposition process facilitated increased ion bombardment on the depositing species during coating growth maintaining high deposition rate. Prior to coating deposition, substrates were pretreated with Zr + rich plasma, for both arc deposited and HIPIMS deposited coatings, which led to a very high scratch adhesion value (LC2) of 100 N. Characterisation results revealed the overall thickness of the coatings in the range of 2.5 µm with hardness in the range of 30-40 GPa depending on the deposition technique. Cross-sectional Transmission Electron Microscopy (TEM), tribological experiments such as dry sliding wear tests and corrosion studies have been utilised to study the effects of ion bombardment on the structure and properties of these coatings. In all the cases HIPIMS assisted UBM deposited coating fared equal or better than the arc deposited coatings, the reasons being discussed in this paper. Thus H+U coatings provide a good alternative to arc deposited where smooth, dense coatings are required and macro droplets cannot be tolerated

Effect of chamber pressure on defect generation and their influence on corrosion and tribological properties of HIPIMS deposited CrN/NbN Coatings

It has been reported that compared to state-of-the-art technologies, High Power Impulse Magnetron Sputtering produces very dense and droplet free coatings due to the high plasma density and ionisation rate. However, thorough investigation of the coating morphology by Scanning Electron Microscopy, optical microscopy and other surface analysis methods revealed the existence of various types of coating defects. This study reports the influence of chamber pressure in particular on defect formation in CrN/NbN nanoscale multilayer coatings. The coating series was deposited using combined HIPIMS/UBM technique while varying the total chamber pressure from 0.2 Pa to 1 Pa. Four types of defects were identified, namely, nodular, open void, cone-like and pinhole. Defect density calculations showed that the coating produced at the lowest pressure, 0.2 Pa, had the lowest defect density of 0.84%. As expected coating corrosion properties improved linearly with decreasing defect density. Potentiodynamic polarisation corrosion studies revealed that in the potential range of - 300 mV to + 300 mV, the current density decreased with decreasing defect density (from 5.96% to 0.84%). In contrast, pin-on-disk tribology tests at room temperature demonstrated that the tribological properties of the coatings deposited at different chamber pressures were dependent on the crystallographic orientations and on the nature of the oxides formed at the tribological contact. Coatings with (200) crystallographic orientation had lower wear rates ( 1.6×10-15 m3N-1m-1) whereas coating with (111) crystallographic orientation had the highest wear rate (2.6×10-15 m3N-1m-1). Friction properties were influenced by the tribolayer formed during the tribological tests. However, for the coatings deposited at same chamber pressure of 0.35 Pa but with different defect densities (due to the difference in chamber cleanliness), the friction behaviour was directly influenced by the coating defects. The friction co-efficient (μ) decreased by a factor of two from 0.48 to 0.25 when the defect density decreased from 3.18% to 1.37%

Long-term behaviour of Nb and Cr nitrides nanostructured coatings under steam at 650°C. Mechanistic considerations.

There is an increasing demand for steam power plants to operate in super-critical conditions i.e. temperatures in excess of 600°C. Under these conditions creep resistant ferritic steels oxidize and therefore require coatings in order to last. Physical vapor deposition and especially High Power Impulse Magnetron Sputtering deposited CrN/NbN nano-scale multilayer coatings with a 2.45 Cr/Nb ratio showed excellent performance when exposed to 650 °C in pure steam environment up to 2,000 h. However the role of Nb in offering protection is unclear. In order to study the long term behaviour of this type of coatings as well as to determine the influence of Nb on their oxidation resistance, a CrN/NbN coating with a 1.16 Cr/Nb ratio was studied for 12,650 h. The coating is hard, well adhered and resistant to environmental corrosion, which are properties required in particular for coatings to be applied on turbine blades. The coating also protects P92 from steam oxidation at 650º C, however coating growth defects influence significantly the oxidation resistance. The long-time exposure allowed to study the protection/ degradation mechanisms provided by this type of ceramic coatings. It was found that oxide nodules grow due to the presence of coating defect originated from substrate defects. Moreover, the higher Nb CrN/NbN coating slowly oxidizes, consuming the coating to a large extent after 12,650 h. As a result, protective oxides containing Cr and Nb are developed, remaining well attached to the substrate for at least the test duration, and preventing further substrate oxidation by steam. Interestingly, thin voids present in the as deposited coating self-heal by forming Cr rich oxides, which block steam to reach the substrate

Investigation of High Power Impulse Magnetron Sputtering deposited nanoscale CrN/NbN multilayer coating for tribocorrosion resistance

Recycling equipment (waste/sea water/chemicals) need high tribocorrosion resistance. In this work High Power Impulse Magnetron Sputtering technique deposited nanoscale CrN/NbN multilayer coating for tribocorrosion resistance is explored. Sliding-wear experiments were conducted on CrN/NbN coated High Speed Steel (HSS) test coupons with an alumina (Al2O3) ball as a counterpart in a corrosive environment (3.5% NaCl solution) under potentiodynamic and potentiostatic conditions. Results reveal that coated substrates exhibited (by a factor of 3) lower corrosion currents and high tribo-corrosion resistance (Kc = 2 × 10−15 m3N−1m−1) as compared to uncoated HSS specimens. The alumina counterpart exhibited negligible wear in all the tests. Superior adhesion and dense microstructure consisting of flat and well-defined hard nitride nanolayers leads to stable friction coefficients and retain the unique nanoscale layer-by-layer wear mechanism without delamination. Effect of corrosion on friction coefficients, wear mechanisms and vice versa has been presented

Growth and Characterization of p-Type and n-Type Sb2Se3 for Use in Thin-Film Photovoltaic Solar Cell Devices

In this study, a two-electrode electrodeposition technique was employed to grow thin films of antimony selenide (Sb2Se3) on glass/fluorine-doped tin oxide (FTO) substrates. The highest quality thin films were consistently obtained within the range of 1600 mV to 1950 mV. Subsequent electrodeposition experiments were conducted at discrete voltages to produce various layers of thin films. Photoelectrochemical cell (PEC) measurements were performed to characterize the semiconductor material layers, leading to the identification of both p-Type and n-Type conductivity types. Optical absorption spectroscopic analysis revealed energy bandgap values ranging from 1.10 eV to 1.90 eV for AD-deposited Sb2Se3 samples and 1.08 eV to 1.68 eV for heat-treated Sb2Se3 samples, confirming the semiconducting nature of the Sb2Se3 material. Additionally, other characterization techniques, including X-ray diffraction analysis, reveal that the AD-deposited layers are almost amorphous, and heat treatment shows that the material is within the orthorhombic crystalline system. Heat-treated layers grown at ~1740 mV showed highly crystalline material with a bandgap nearing the bulk bandgap of Sb2Se3. Raman spectroscopy identified vibrational modes specific to the Sb2Se3 phase, further confirming its crystallinity. To explore the thin-film morphology, Scanning Electron Microscopy (SEM) was employed, revealing uniformly deposited material composed of grains of varying sizes at different voltages. Energy Dispersive X-ray analysis (EDX) confirmed the presence of antimony and selenium in the material layers

Cavitation erosion performance of CrAlYN/CrN nanoscale multilayer coatings deposited on Ti6Al4V by HIPIMS

Water droplet erosion (WDE) protection of Ti6Al4V turbofan blades is of paramount importance to the aviation industry. A novel CrAlYN/CrN nanoscale multilayer coating deposited by the HIPIMS technique was evaluated as a potential candidate for this application. Literature suggests a strong correlation in performance ranking under WDE and cavitation erosion (CE) tests. Hence, the WDE performance of the CrAlYN/CrN coating on Ti6Al4V was investigated with an ultrasonic cavitation device. The results show excellent adhesion and superior erosion resistance of the CrAlYN/CrN coating (erosion rate lower by a factor of 14 compared to the bare Ti6Al4V substrate) and compared to coatings reported in the literature with spallation as their main erosion mechanism. Cross-sectional FIB studies revealed formation of substrate cracks underneath the coating when CE generated stresses exceeded the fatigue strength of the Ti6Al4V alloy. The interfaces of the nanoscale multilayers protected the substrate by forming an effective barrier against shock waves, delayed fatigue crack formation, deflected and arrested any cracks formed impeding the overall coating damage. The research shows that the CE resistance is influenced by the coating's texture and elastic properties, (Young's modulus). The paper discusses the erosion mechanisms of the coating and the excellent CE protection it offers