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

Six strategies to produce application tailored nanoscale multilayer structured PVD coatings by conventional and High Power Impulse Magnetron Sputtering (HIPIMS)

The application field of functional coatings produced by Physical Vapor Deposition, (PVD) is ever expanding. Consequently the demand for novel materials exhibiting extraordinary properties is growing intensively. The paper summarises the experience and the expertise gained at Sheffield Hallam University in UK through work dedicated to the development of such advanced materials spanning more than two decades. It discusses six strategies to produce PVD coatings where the main three steps followed in any coating development process namely material selection, structure selection and finally coating deposition method consideration have been application informed. All described coatings however, utilise nanoscale multilayer structure and have been deposited by Direct Current Unbalanced Magnetron Sputtering (DCMS) or the novel High Power Impulse Magnetron Sputtering (HIPIMS) techniques

Lubricated sliding wear mechanism of chromium-doped graphite-like carbon coating

The current research aims to discuss the tribological behaviour of Chromium-doped graphite-like carbon coatings and suggest a wear mechanism under both dry (in air) and boundary lubricated sliding condition based on phase composition of the wear product generated in wear track during pin-on-disc experiments. As expected, the friction coefficient reduces from 0.22 to 0.12 due to addition of lubricant. Raman analysis indicates that wear mechanism is oxidative in dry sliding condition whereas it is chemically reactive in the presence of lubricant. It is speculated that the key-factor of reduced friction and wear coefficient in lubricated condition is the formation of CrCl3 due to tribochemical reaction between coating and oil. CrCl3 has graphite-like layered structure; therefore it acts like solid lubricant

Isothermal and dynamic oxidation behaviour of Mo−W doped carbon-based coating

The oxidation behaviour of Mo−W doped carbon-based coating (Mo−W−C) is investigated in elevated temperature (400°C−1000°C). Strong metallurgical bond between Mo−W−C coating and substrate prevents any sort of delamination during heat-treatment. Isothermal oxidation tests show initial growth of metal oxides at 500°C, however graphitic nature of the as-deposited coating is preserved. The oxidation progresses with further rise in temperature and the substrate is eventually exposed at 700°C. The performance of Mo−W−C coating is compared with a state-of-the-art coating, which shows preliminary oxidation at 400°C and local delamination of the coating at 500°C leading to substrate exposure. The graphitisation starts at 400°C and the diamond-like structure is completely converted into the graphite-like structure at 500°C. Dynamic oxidation behaviour of both the coatings is investigated using Thermo-gravimetric analysis carried out with a slow heating rate of 1°C/min from ambient temperature to 1000°C. Mo−W−C coating resists oxidation up to ~800°C whereas delamination of coating is observed beyond ~380°C. In summary, Mo−W−C coating provides improved oxidation resistance at elevated temperature compared to coating

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

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

Defect growth in multilayer chromium nitride/niobium nitride coatings produced by combined high power impulse magnetron sputtering and unbalance magnetron sputtering technique

In recent years, high power impulse magnetron sputtering (HIPIMS) has caught the attention of users due to its ability to produce dense coatings. However, microscopic studies have shown that HIPIMS deposited coatings can suffer from some surface imperfections even though the overall number of defects can be significantly lower compared to, for example, arc deposited coatings of similar thicknesses. Defects can degrade the coating performance thus any kind of defect is undesirable. To better understand the nature of these imperfections and the science of their formation, a series of Chromium Nitride/Niobium Nitride (CrN/NbN) coatings were deposited using HIPIMS technique combined with unbalanced magnetron sputtering (UBM) by varying deposition times (t = 15 to 120 minutes). All other deposition parameters were kept constant in order to deposit these coatings with a consistent deposition rate and stoichiometry. In addition, coatings were deposited using pure UBM technique to compare the defects generated by these two different physical vapour deposition approaches. High-resolution scanning electron microscopy images revealed that HIPIMS/UBM and pure UBM CrN/NbN coatings have similar types of defects which could be categorised as: nodular, open void, cone-like and pinhole. Interestingly, there was no evidence of droplet formation in HIPIMS/UBM deposited coatings. The defect density calculation indicated that the defect density of HIPIMS/UBM coatings increased (from 0.48 to 3.18%) with the coating thickness. A coating produced in a relatively clean chamber had a lower defect density. Potentiodynamic polarisation experiments showed that the fluctuation in corrosion currents in HIPIMS/UBM coatings reduced with the coating thickness. This indicated that though visible on the surface, most of these defects did not penetrate thorough the whole thickness of the coating

Effect of the degree of high power impulse magnetron sputtering utilisation on the structure and properties of TiN films

TiN films were deposited using high power impulse magnetron sputtering (HIPIMS) enabled four cathode industrial size coating system equipped with HIPIMS power supplies. The standard version of this system allows control over the ion bombardment during coating growth by varying the strength of the electromagnetic field of the unbalancing coils and bias voltage applied to the substrate. The coatings were produced in different coating growth conditions achieved in combined HIPIMS — direct current (dc) unbalanced magnetron sputtering (HIPIMS/UBM) processes where HIPIMS was used as an additional tool to manipulate the ionisation degree in the plasma. Four cathode combinations were explored with increasing contribution of HIPIMS namely 4UBM (pure UBM), 1HIPIMS + 3UBM, 2HIPIMS + 2UBM and 2HIPIMS (pure HIPIMS) to deposit TiN coatings. Optical emission spectroscopy (OES) measurements were carried out to examine the plasma generated by the various combinations of HIPIMS and UBM cathodes. The micro-structural study was done by scanning electron microscopy (SEM). X-ray diffraction (XRD) technique was used to calculate the residual stress and texture parameter. It has been revealed that the residual stress can be controlled in a wide range from − 0.22 GPa to − 11.67 GPa by intelligent selection of the degree of HIPIMS utilisation, strength of the electromagnetic field of the unbalancing coils and the bias voltage applied to the substrate while maintaining the stoichiometry of the coatings. The effect of the degree of HIPIMS utilisation on the microstructure, texture and residual stress is discussed. Combining HIPIMS with dc-UBM sputtering is also seen as an effective tool for improving the productivity of the deposition process