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

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

Investigating worn surfaces of nanoscale TiAlN/VN multilayer coating using FIB and TEM

TiAlN/VN multilayer coatings exhibit excellent dry sliding wear resistance and low friction coefficient, believed to be associated with the formation of tribo-films comprising Magnéli phases such as V2O5. In order to investigate this hypothesis, dry sliding wear of TiAlN/VN coatings was undertaken against Al2O3. Focused ion beam was used to generate site-specific TEM specimens. A thin (2-20nm) tribo-film was observed at the worn surface, with occasional 'roll-like' wear debris (φ 5-40nm). Both were amorphous and contained the same Ti, Al and V ratio as the coating, but with the nitrogen largely replaced by oxygen. No evidence of Magnéli phases was found. © 2006 IOP Publishing Ltd

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

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

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)

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

Wear and friction of TiAlN/VN coatings against Al2O3 in air at room and elevated temperatures

TiAlN/VN multilayer coatings exhibit excellent dry sliding wear resistance and low friction coefficient, reported to be associated with the formation of self-lubricating V2O5. To investigate this hypothesis, dry sliding ball-on-disc wear tests of TiAlN/VN coatings on flat stainless steel substrates were undertaken against Al2O3 at 25 C, 300 C and 635 C in air. The coating exhibited increased wear rate with temperature. The friction coefficient was 0.53 at 25 C, which increased to 1.03 at 300 C and decreased to 0.46 at 635 C. Detailed investigation of the worn surfaces was undertaken using site-specific transmission electron microscopy (TEM) via focused ion beam (FIB) microscopy, along with Fourier transform infrared (FTIR) and Raman spectroscopy. Microstructure and tribo-induced chemical reactions at these temperatures were correlated with the coating’s wear and friction behaviour. The friction behaviour at room temperature is attributed to the presence of a thin hydrated tribofilm and the presence of V2O5 at high temperature