TiN/NbN Nanoscale Multilayer Coatings Deposited by High Power Impulse Magnetron Sputtering to Protect Medical-Grade CoCrMo Alloys

Abstract

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.

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

“…In this experiment no cracks were observed to form at the corners of the indent, which is clear example for the high fracture toughness of the superlattice structured coatings. The result confirms the observations from our previous experiments with Vickers indentations using several orders of magnitude higher indentation loads of 30 kgf where no radial crack formation along the edges of the indent were found (Sugumaran et al, 2021).…”

“…Another important observation is that despite of the sizeable plastic deformation of the substrate material under the indentation, the NbN base layer/substrate bonding has remained uncompromised. Due to the application of the HIPIMS surface pre-treatment, as described in section 2.1, no lateral cracks or delamination were observed to form in this critical zone in the coating, which demonstrates the strong coating adhesion as previously reported based on scratch test results (Sugumaran et al, 2021).…”

“…For the relatively high elastic modulus ratio of the current hard film/soft substrate combination in the range of E coating /E substrate = 1.81 indicating that the substrate does not provide the necessary load bearing support for the brittle thin film, and so the fracture toughness of the coating becomes one of the lifetime defining properties for the coating. However, despite of the relatively high elastic moduli ratio our previous research showed that TiN/NbN-CoCrMo system has surprisingly high impact load resistance and high fracture toughness, which was attributed to the high coating integrity and exceptionally strong coatingsubstrate adhesion bonding (Sugumaran et al, 2021). In the current investigation, the emphasis was put on the more detailed understanding on the role of the coating architecture behind the enhanced fracture toughness of the superlattice TiN/NbN films when deposited on medical grade CoMoCr alloys.…”

“…Considering the above guidelines, TiN/NbN coating utilising nanoscale multilayer/superlattice structure was deposited by High Power Impulse Magnetron Sputtering technology to protect medical grade CoCrMo alloys for medical implant applications. The tribological, corrosion and barrier properties against metal ion leaching in Hank's solution of this coating were described in detail elsewhere (Sugumaran et al, 2021). However, the coating's fine microstructure on nanoscale and their load bearing capacity and the response of the nanolayered stack of ceramic TiN and NbN under localised load and the subsequent crack formation mechanism due to such load have not been reported.…”

A Self-Flux-Biased NanoSQUID with Four NbN-TiN-NbN Nanobridge Josephson Junctions

“…It is used in industry for surface hardening and corrosion protection, as well as for decorative Au-like coatings [17]. Multilayers of NbN/TiN have enhanced mechanical, anticorrosion, and superconducting properties when compared with single-layer NbN and TiN films [18][19][20]. MoRe is another corrosion-resistant superconducting material that has been used for the preparation of nanoSQUIDs [21,22].…”