Nanoindentation reveals mechanical properties within thermally sprayed hydroxyapatite coatings

Nanoindentation offers a unique capability to assess the mechanical properties of polished cross-sections of thermal spray coatings. This study set out to investigate the suitability of nanoindentation to extend the analysis of the cross-section to multiple points for a more detailed insight into variations through the thickness of the coating. Hydroxyapatite (HAp) was classified to three different particle sizes (20-40, 40-60 and 60-80 μm) and then thermally sprayed to produce a structurally homogeneous coating. Micro-Raman Spectroscopy (MRS) revealed dehydroxylation of the powder during the traverse in the thermal zone. Multiple indentations with a separation of 4 μm showed a transition within the thickness of the coating. Use of smaller particles led to higher hardness and elastic modulus compared to powder containing larger particles. The mechanical property profile provides valuable information for optimizing the processing of such coatings and feedback for the design of coating properties.     

Microstructural characteristics and mechanical properties of carbon nanotube reinforced aluminum alloy composites produced by ball milling

The influence of milling time on the structure, morphology and thermal stability of multi-walled carbon nanotubes (MWCNTs) reinforced EN AW6082 aluminum alloy powders has been studied. After structural and microstructural characterization of the mechanically milled powders micro- and nano-hardness of the composite powder particles were evaluated. The morphological and X-ray diffraction studies on the milled powders revealed that the carbon nanotubes (CNTs) were uniformly distributed and embedded within the aluminum matrix. No reaction products were detected even after long milling up to 50 h. Nanotubes became shorter in length as they fractured under the impact and shearing action during the milling process. A high hardness of about 436 ± 52 HV is achieved for the milled powders, due to the addition of MWCNTs, after milling for 50 h. The increased elastic modulus and nanohardness can be attributed to the finer grain size evolved during high energy ball milling and to the uniform distribution of hard CNTs in the Al-alloy matrix. The hardness values of the composite as well as the matrix alloy compares well with that predicted by the Hall–Petch relationship.     

Surface characterisation of DC plasma electrolytic oxidation treated 6082 aluminium alloy: Effect of current density and electrolyte concentration

Plasma electrolytic oxidation (PEO) is a specialised but well-developed process which has found applications in aerospace, oil/gas, textile, chemical, electrical and biomedical sectors. A novel range of coatings having technologically attractive physical and chemical properties (e.g. wear- and corrosion-resistance) can be produced by suitable control of the electrolyte as well as electrical parameters of the PEO process. Oxide ceramic films, 3 to 40 μm thick, were produced on 6082 aluminium alloy by DC PEO using 5 to 20 A/dm² current density in KOH electrolyte with varied concentration (0.5 to 2.0 g/l). Phase analysis (composition and crystallite size) was carried out using X-ray diffraction and TEM techniques. Residual stresses associated with the crystalline coating phase (α-Al₂O₃) were evaluated using the X-ray diffraction Sin²ψ method. Nanoindentation studies were conducted to evaluate the hardness and elastic modulus. SEM, SPM and TEM techniques were utilised to study surface as well as cross-sectional morphology and nano features of the PEO coatings. Correlations between internal stress and coating thickness, surface morphology and phase composition are discussed. It was found that, depending on the current density and electrolyte concentration used, internal direct and shear stresses in DC PEO alumina coatings ranged from − 302 ± 19 MPa to − 714 ± 22 MPa and − 25 ± 12 MPa to − 345 ± 27 MPa, respectively. Regimes of PEO treatment favourable for the production of thicker coatings with minimal stress level, dense morphology and relatively high content of α-Al₂O₃ phase are identified.     

Shear banding deformation in Cu/Ta nano-multilayers

Nanoscale Cu/Ta multilayers with individual layer thickness ranging from 3 to 70 nm were deformed under nanoindentation at room temperature. Shear bands can be observed only when individual layer thickness is reduced to 9 nm or below, indicating formation of shear bands in the Cu/Ta multilayers is layer thickness dependent. By observing the cross sectional transmission electron microscope images of the indentation fabricated through focused ion beam technique, shear banding deformation causing a unique layer-morphology with prevalent mismatched laminate structure has been reported for the first time. By capturing and analyzing a series of typical indentation-induced deformed microstructures, a new physical mechanism of shear banding behavior in metallic nano-multilayers is suggested.     

Transient viscoelasticity study of tobacco mosaic virus/Ba2+ superlattice

Recently, we reported a new method to synthesize the rod-like tobacco mosaic virus (TMV) superlattice. To explore its potentials in nanolattice templating and tissue scaffolding, this work focused the viscoelasticity of the superlattice with a novel transient method via atomic force microscopy (AFM). For measuring viscoelasticity, in contrast to previous methods that assessed the oscillating response, the method proposed in this work enabled us to determine the transient response (creep or relaxation) of micro/nanobiomaterials. The mathematical model and numerical process were elaborated to extract the viscoelastic properties from the indentation data. The adhesion between the AFM tip and the sample was included in the indentation model. Through the functional equation method, the elastic solution for the indentation model was extended to the viscoelastic solution so that the time dependent force vs. displacement relation could be attained. To simplify the solving of the differential equation, a standard solid model was modified to obtain the elastic and viscoelastic components of the sample. The viscoelastic responses with different mechanical stimuli and the dynamic properties were also investigated.     

Dislocation luminescence in GaN single crystals under nanoindentation

This work presents an experimental study on the dislocation luminescence in GaN by nanoindentation, cathodoluminescence, and Raman. The dislocation luminescence peaking at 3.12 eV exhibits a series of special properties in the cathodoluminescence measurements, and it completely disappears after annealing at 500°C. Raman spectroscopy shows evidence for existence of vacancies in the indented region. A comprehensive investigation encompassing cathodoluminescence, Raman, and annealing experiments allow the assignment of dislocation luminescence to conduction-band-acceptor transition involving Ga vacancies. The nanoscale plasticity of GaN can be better understood by considering the dislocation luminescence mechanism.     

High temperature nanoindentation: The state of the art and future challenges

Nanoindentation measurement capabilities at elevated temperatures have developed considerably over the last two decades. Commercially available systems can now perform stable indentation testing at temperatures up to ∼800 °C with thermal drift levels similar to those present at room temperature. The thermal management and measurement techniques necessary to achieve this are discussed here, with particular emphasis on systems featuring independent heating of both the indenter and the sample. To enable measurements at temperatures where oxidation of the indenter and/or sample are a concern, vacuum nanoindentation techniques have also been developed. A natural extension of testing in vacuo is elevated temperature nanoindentation in situ in the scanning electron microscope, and the additional requirements for and benefits of this are discussed. Finally, several new emerging testing techniques are introduced: thermal cycling/fatigue, interfacial thermal resistance measurement and small scale transient plasticity measurements.     

Evaluation of rhenium carbide as a prospective material for hard coating

The literature reveals that interstitial alloys based on rhenium as a precursor might be extremely hard, becoming suitable to be used as hard coatings. In this work, we have produced rhenium carbide (ReC_x) films by the reactive pulsed laser deposition method. Nanoindentation has been performed to estimate hardness. The maximum hardness value for ReC_x films resulted to be 22.5 GPa. We found no evidence that ReC_x films have hardness, or plasticity, higher than competitive hard coating materials. Our results and the fact that rhenium is expensive and scarce, suggest that preceding reports are overoptimistic on the prospective use of rhenium carbide as hard coatings.     

(3-Glycidoxypropyl)-terminated silsesquioxane impact on nanomechanical properties of polyimide coatings exposed to atomic oxygen

Based on (3-glycidoxypropyl)silsesquioxane (GSSO) derived from the hydrolytic condensation of (3-glycidoxypropyl)trimethoxysilane (GPMS), GSSO-containing polyimide hybrid films were prepared using the sol-gel process and spin coating. Nanoindentation tests were carried out to study the influence of GPMS in the hybrid films on nanomechanical properties before and after exposure to atomic oxygen (AO) environments. The results show that hybrid films are three to four times harder than the usual plastic substrates. Compared with the unexposed AO samples, the elastic modulus (E) and the hardness (H) of the pristine Kapton AO-exposed samples significantly decreased, whereas the hardness of AO-exposed hybrid films increased slightly. This difference in nanomechanical properties is attributed to the chemical and the microstructural sample changes.