Development of nanostructured PVD coatings for total knee replacement joints using HIPIMS.

The aim of this study was to develop thin film coatings for total knee replacement joints using high power impulse magnetron sputtering (HIPIMS). An industrial size four cathode magnetron sputtering system equipped with direct current (DC) and HIPIMS power supplies was used for this purpose. Initially, Plasma diagnostics were carried out using optical emission spectroscopy (OES) while sputtering Ti target in Ar + N2 atmosphere by utilizing various HIP IMS/conventional DCMS (henceforth UBM) source combinations by varying the process parameters such as coil current and N2 flow. Then, single layer titanium nitride (TiN) coating was deposited by varying the degree of HIPIMS utilisation and the process parameters such as bias voltage and coil current to thoroughly understand the effect of degree of HIPIMS utilisation on the microstructure, residual stress, texture, mechanical, tribological and corrosion properties of such coatings. The degree of HIPIMS utilisation was altered by increasing the number of HIPIMS targets used for the deposition. Four different source combinations were used for this purpose, as follows: 4 cathodes in conventional DCMS mode to deposit pure UBM coating, 1 HIPIMS + 3UBM and 2HIPIMS + 2UBM cathodes to deposit combined HIPIMS/UBM coatings and 2HIPIMS cathodes to deposit pure HIPIMS coatings. TiN/NbN, TiCN/NbCN and CrN/NbN multilayer coatings were deposited on CoCr alloy test buttons along with other (HSS, SS and Si) substrates since our intended application is on total knee replacement joints made of CoCr alloy. The knowledge gained by investigating the TiN (Ar + N[2]) plasma and the properties of TiN was used to determine the process parameters for depositing the multilayer coatings. X- ray diffraction (XRD) technique was used for calculating the texture, residual stress and bilayer thickness of the coatings. Nanoindentation method was used to determine the nano hardness of the coatings. The adhesion strength of the coatings was estimated by scratch and Rockwell indentation tests. Pin on disc method was used for the tribological studies such as coefficient of friction and coefficient of wear. Surface roughness measurements were carried out using a surface profiler. Microstructural characterisation of the coatings was carried out using scanning electron microscope (SEM) and transmission electron microscope (TEM). Potentiodynamic polarisation method was utilised to study the corrosion performance of the coatings. Raman spectroscopy was used to study the constituents of the corrosion products and evaluate the corrosion damage. OES measurements revealed that the degree of metal ions (Ti[1+]) increased with increasing degree of HIPIMS utilisation. The hardness, tribological and corrosion properties of TiN coatings improved with increasing degree of HIPIMS utilisation. TiN and multilayer coatings deposited by HIPIMS exhibited a smooth columnar microstructure without any voided region along the column boundaries. TiN/NbN, TiCN/NbCN and CrN/NbN multilayer coatings deposited on CoCr alloy, HSS and SS test buttons exhibited superior mechanical, tribological and corrosion properties as compared to the underlying substrate

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

Dry sliding wear mechanisms of HIPIMS plasma nitrided CoCrMo alloy for medical implant applications

Unlike the state-of-the-art plasma nitriding technologies where a low ionisation glow discharge is used, nitriding of CoCrMo alloy was carried out in an industrial size vacuum coater, utilising novel high ionisation, high power impulse magnetron sputtering plasma discharge mixed with unbalanced magnetron sputtering discharge. Three different nitriding voltages were used for the nitriding process. To generate more ions, two pairs of Cr and Nb targets were also operated at low power in N2:H2 gas atmosphere. Dry sliding pin on disc tests were performed at room temperature to measure the coefficient of friction and to estimate the wear coefficient. In the case of samples nitrided at lower nitriding bias voltages such as -700 V and -900 V, the oxidative mild wear mechanism was identified as the dominant wear mechanism. For higher voltage of -1100 V, despite the formation of an oxide based tribolayer severe abrasion was identified as the predominant wear mechanism due to the operation of three body contact wear involving high hardness wear particles. Among the three nitrided samples, the sample nitrided at -900 V was identified as the best with the lowest wear coefficient of Kc = 1.11×10-15 m3N-1m-1 and Knoop microhardness of 2230 HK0.010

Microstructure and load bearing capacity of TiN/NbN superlattice coatings deposited on medical grade CoCrMo Alloy by HIPIMS

In recent years significant progress has been made in the application of various ceramic, namely MeN functional coatings to engineer the surfaces of medical implants utilising metal-on-metal (MoM) articulation. This article reports on the load bearing capacity and structural response of TiN/NbN superlattice coatings deposited on medical grade CoCrMo alloy substrate under the application of localised load and the subsequent crack formation mechanism. The coatings have been deposited by mixed High Power Impulse Magnetron Sputtering-Unbalanced Magnetron Sputtering (HIPIMS-UBM) process. In the case of TiN/NbN coating deposited on CoCrMo substrate where Ecoating/Esubstrate is as high as 1.81 indicating that the substrate does not provide the necessary load bearing support for the brittle thin film, the utilisation of the Berkovich indentation technique proved to be a potent approach to study coating material as well as structural response to applied concentrated load. FIB/SEM analyses of the indented coatings revealed that in the hard-on-soft material systems cracks will initiate due to sub-coating substrate deformation and then propagate towards the coating surface. The FIB/SEM and low magnification XTEM analysis showed that an exceptionally strong TiN/NbN coating substrate adhesion bonding was achieved due to the utilisation of the HIPIMS pre-treatment. High resolution XTEM analyses revealed for the first time that during the indentation a collective rotation and alignment of the individual layers of the superlattice stack takes place without compromising coatings integrity which is clear evidence for the exeptionally high coating fracture toughness. The high toughness of the superlattice structured TiN/NbN coatings combined with their exceptionally high adhesion on madical grade CoCrMo ranks them as a strong candidate for medical implant applications

Correlation between the microstructure and corrosion performance of the HIPIMS nitrided bio-grade CoCrMo alloy

Corrosion performance of CoCrMo alloy (F75) plasma nitrided with High-Power Impulse Magnetron Sputtering (HIPIMS) technique was thoroughly investigated. Open Circuit Potential (OCP) measurements and potentiodynamic polarisation tests exhibited a strong correlation between the transmuting microstructure (as a result of varying nitriding voltage from −700 V to −1100 V) and its corrosion performance. A significant improvement in the ECorr values was noticed (around −590 mV for untreated as compared to −158.17 mV for −1000 V) when analysed against 3.5% wt. NaCl solution. Similarly, results against Hank's solution also exhibited a significant increase in ECorr values (around −776 mV for untreated as compared to −259 mV for −1000 mV). Irrespective of the nitriding voltage, HIPIMS nitriding led to a significant improvement in the corrosion resistance of the alloy. For nitriding voltages −700 V and −900 V, a diffusion based S phase layer played a significant role in imparting corrosion resistance. On the contrary, precipitation of chromium-based nitrides (CrN and Cr2N), observed in samples nitrided at relatively higher voltages of −1000 V and −1100 V, resulted in its relative deterioration. A preferential dissolution of the grains and its grain boundaries, along with a sluggish dissolution of the grains and metal carbides appeared to be the dominant corrosion mechanism for the nitrided alloys. Specimens nitrided at −700 V and −900 V displayed the best corrosion resistance, which was deemed to be derived from the right combination of a thicker S phase layer and the compound layer consisting of M2–3N and M4N phases

Improving the Quality of Friction Stir Welds in Aluminium Alloys

The Stationary Shoulder Friction Stir Welding (SS-FSW) technique benefits from reduced heat input, improved mechanical properties and surface finish of the weld, avoiding the need for post weld processing. Coatings on the tool probe and the shoulder for welding of aggressive Aluminium alloys have rarely been successful. Such coatings must be well adherent and inert. In this study, coated tools were used for SS-FSW of AA6082-T6 alloy. Performance of a nanoscale multilayer TiAlN/VN coating deposited by High Power Impulse Magnetron Sputtering (HIPIMS) technology was compared with amorphous Diamond Like Carbon (a-C:H) by Plasma Assisted Chemical Vapour Deposition (PACVD), AlTiN deposited by arc evaporation and TiBCN along with TiB2 produced by Chemical Vapour Deposition (CVD) methods. The TiAlN/VN coating was found to have low affinity to aluminium, acceptable coefficient of friction and provided excellent weld quality by inhibiting intermixing between the tool and workpiece materials resulting in a significant reduction in tool wear

TiN/NbN nanoscale multilayer coatings deposited by High Power Impulse Magnetron Sputtering to protect medical grade CoCrMo alloys

This study describes the performance of nanoscale multilayer TiN/NbN coatings deposited on CoCrMo medical-grade alloys by utilising novel mixed high power impulse magnetron sputtering (HIPIMS) and direct current unbalanced magnetron sputtering (UBM) technique in an industrial size vacuum coater. Scanning electron microscopy analysis showed that these coatings were extremely dense without any intercolumnar voids. The coating exhibited high hardness of 28 GPa, as well as low friction and wear coefficient of 0.7 and 1.4 × 10−14 m3·N−1·m−1, respectively, as compared to the bare material. Scratch tests revealed superior coating to substrate adhesion due to the HIPIMS etching prior to coating deposition. Energy-dispersive X-ray analysis of the wear debris generated during the impact test together with focused ion beam cross-section analysis in different locations of the impact crater revealed the coating failure mechanism and further confirmed the excellent coating to substrate bonding strength. Potentiodynamic polarisation tests in NaCl and Hank’s solutions revealed the clear passivation behaviour, several orders of magnitude lower corrosion currents, and high pitting potentials of the coating, which guarantee excellent protection to the base alloy in such aggressive environments. Inductively coupled plasma mass spectrometry analysis of Hank’s solution containing corrosion debris of the coated sample revealed that the leaching of harmful metal ions from the base material was reduced to below the detection limit of the technique, thus demonstrating the high barrier properties 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

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

Development of superlattice CrNNbN coatings for joint replacements deposited by High Power Impulse Magnetron Sputtering

The demand for reliable coating on medical implants is ever growing. In this research, enhanced performance of medical implants was achieved by a CrN/NbN coating utilising nanoscale multilayer/superlattice structure. The advantages of the novel High Power Impulse Magnetron Sputtering technology, namely its unique highly ionised plasma were exploited to deposit dense and strongly adherent coatings on Co-Cr implants. TEM analyses revealed coating superlattice structure with bi-layer thickness of 3.5 nm. CrN/NbN deposited on Co-Cr samples showed exceptionally high adhesion, critical load values of LC2= 50 N in scratch adhesion tests. Nanoindentation tests showed high hardness of 34 GPa and Young's modulus of 447 GPa. Low coefficient of friction (µ) 0.49 and coating wear coefficient (KC) = 4.94 x 10-16 m3N-1m-1 were recorded in dry sliding tests. Metal ion release studies showed a reduction in Co, Cr and Mo release at physiological and elevated temperatures, (70 oC) to almost undetectable levels (<1 ppb). Rotating beam fatigue testing showed a significant increase in fatigue strength from 349±59 MPa (uncoated) to 539±59 MPa (coated). In vitro biological testing has been performed in order to assess the safety of the coating in biological environment, cytotoxicity, genotoxicity and sensitisation testing have been performed, all showing no adverse effects. Keywords: Orthopaedic implant, High Power Impulse Magnetron Sputtering, Superlattice coating, Corrosion, Biocompatibility