Carl J. Boehlert

Hydroxyapatite coatings containing graphene oxide and palladium were hydrothermally deposited on a β-type Ti30Zr5Mo alloy in order to establish how the organization of graphene oxide and the incorporation of palladium influence the structural, electrochemical, and biological performance of the coating system. Six compositions—hydroxyapatite, hydroxyapatite with 1 wt% palladium, hydroxyapatite with 2 wt% palladium, hydroxyapatite/graphene oxide, and the corresponding graphene oxide composites containing 1 and 2 wt% palladium—were compared using X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy, potentiodynamic polarization, cell-density analysis with A7R5 cells, and bioluminescence imaging of Staphylococcus aureus Xen36. The apatite framework was retained in every composition, and Raman spectroscopy confirmed that the carbon phase derived from graphene oxide survived hydrothermal processing. X-ray photoelectron spectroscopy of the graphene oxide coating containing 2 wt% palladium revealed oxygenated carbon environments and predominantly oxidized, oxygen-coordinated palladium species rather than metallic palladium. Electrochemical behavior depended strongly on composition. The hydroxyapatite coating containing 2 wt% palladium showed the lowest corrosion current density, 1.46 ± 0.02 µA cm⁻² (0.0127 ± 0.0002 mm year⁻¹), and a mean total resistance of 15.73 ± 0.13 kΩ cm², close to that of the unmodified hydroxyapatite coating (1.95 ± 0.04 µA cm⁻²; 16.52 ± 0.40 kΩ cm²). The hydroxyapatite/graphene oxide coating showed the weakest barrier behavior, with a corrosion current density of 4.21 ± 0.07 µA cm⁻² (0.0366 ± 0.0006 mm year⁻¹) and a mean total resistance of only 8.50 ± 0.15 kΩ cm². Microscopic and electrochemical evidence indicated that the expected barrier effect of graphene oxide was lost because the graphene oxide-containing layer was structurally heterogeneous. This layer contained folded, sheet-like domains and pathways that remained accessible to the electrolyte, and restacking or aggregation of the sheets induced by calcium ions is proposed as a plausible origin of this morphology. Adding 1 wt% palladium partially restored coating coverage and raised the mean total resistance to 14.97 ± 0.20 kΩ cm² (2.15 ± 0.04 µA cm⁻²), whereas 2 wt% palladium lowered the interfacial charge-transfer resistance and reduced the mean total resistance to 11.73 ± 0.30 kΩ cm² (2.91 ± 0.06 µA cm⁻²), even though the outer part of the coating remained intact. All coatings supported higher short-term A7R5 cell densities than the uncoated substrate, and the highest value, 330.2% of the control, was obtained for the graphene oxide coating containing 2 wt% palladium. The same graphene-oxide/palladium coatings, however, produced considerably stronger bioluminescent responses from Staphylococcus aureus than the remaining compositions. Overall, the hydroxyapatite coating containing 2 wt% palladium offered the most balanced combination of electrochemical and biological performance. The results show that the behavior of these coatings is governed by the organization of graphene oxide, by the interfacial chemistry associated with palladium, by the pathways available to the electrolyte, and by buried charge-transfer processes, rather than by nominal additive content alone. Their architecture is characterized by folded sheet-like domains and accessible electrolyte pathways, with Ca²⁺-mediated restacking/aggregation proposed as a plausible origin. Incorporation of 1 wt% Pd partially restored coating coverage and increased the mean total resistance to 14.97 ± 0.20 kΩ cm², whereas increasing Pd to 2 wt% reduced the interfacial charge-transfer resistance and decreased the mean total resistance to 11.73 ± 0.30 kΩ cm² despite retention of the outer-coating barrier. All coatings supported higher short-term A7R5 cell densities than the uncoated substrate, with HA/GO-2Pd reaching 330.2% of the control. However, the GO/Pd coatings produced substantially higher Xen36 bioluminescent responses than the remaining compositions. Overall, HA-2Pd provided the most balanced electrochemical and biological performance, while the results demonstrate that the behavior of HA/GO/Pd coatings is governed by GO organization, Pd-associated interfacial chemistry, electrolyte-access pathways, and buried interfacial charge-transfer processes rather than nominal additive content alone.

This review discusses acoustoplastic treatment as a modern surface engineering technique using ultrasonic vibrations to enhance the material properties including elongation-to-failure, strength, corrosion resistance, and fatigue life. The reduction of an object's resistance to plastic deformation upon subjecting it to intense mechanical vibrations at ultrasonic frequency (most often 20–40 kHz) is known as acoustoplasticity. This review covers the mechanisms of ultrasonic softening (including grain boundary growth and dislocation behavior) as well as applications of the ultrasonic-aided methods including hybrid processes, additive manufacturing, and surface processing. Process-specific outcomes are quantified for a range of materials (aluminum, steel, titanium, etc.), and they highlight future avenues for energy-efficient and scalable surface treatments. Existing constraints are also elucidated, and directions for future research and implementation of this technology are discussed.

Advanced biomaterials are required for bone reconstruction to overcome the mechanical and biological limitations of conventional titanium alloys. In this study, hydroxyapatite–chitosan–palladium (HA-Cs-Pd) composite coatings were successfully synthesized on metastable β-Ti12Cr alloy surfaces using a hydrothermal method. The coatings were characterized using XRD, Raman, FT-IR, XPS, and FE-SEM/EDS analyses, while their electrochemical, wettability, antibacterial, and cellular response properties were systematically evaluated. The results showed that chitosan incorporation improved coating compactness and corrosion resistance by enhancing barrier properties, whereas Pd addition influenced crystal growth behavior and enhanced antibacterial activity against Pseudomonas aeruginosa. In vitro biological analyses demonstrated favorable cell viability and surface adhesion behavior on the coated samples. Among the investigated systems, the HA–Cs–wt.%2Pd coating exhibited the most balanced overall performance. The findings suggest that HA–Cs–Pd composite coatings may provide an effective surface modification strategy for orthopedic and dental implant applications.

Slip bands (SBs), as a fundamental manifestation of plasticity, play a crucial role in governing the macroscopic mechanical behaviors of metals. Understanding SB evolution and its underlying slip system (SS) is essential. The widely adopted assumption that only one SS operates within one SB underlies many SB-based investigations. In this work, we experimentally resolved and quantitatively analyzed bi-basal slip in a Mg alloy under compression for the first time. Bi-basal slip is defined here as activation of two basal SSs within an individual SB and/or across parallel SBs in the same grain. High-resolution digital image correlation (HRDIC) was used to obtain nanoscale full-field deformation components and the displacement field, enabling quantification and tracking of the Burgers vector components for individual SBs or SB segments. Three representative scenarios of bi-basal slip were identified: Bi-basal slip across parallel SBs, uniformly distributed bi-basal slip in an individual SB, and non-uniformly distributed bi-basal slip in an individual SB. Slip transfer across grain boundaries can introduce Burgers vectors different from those nucleated in the grain interior, which was identified as an important driver for the bi-basal slip. On the other hand, bi-basal slips not associated with slip transfer were also observed, indicating activation of intragranular dislocation sources with different Burgers vectors in one SB. The potential effects of bi-basal slip on slip localization and deformation compatibility were discussed. This work provides new insights into nanoscale SB evolution and offers a quantitative analysis framework for investigating multiple SSs within an individual SB for hexagonal close-packed metals.