Nanoindentation with spherical tips of single crystals of YBCO textured by the Bridgman technique: Determination of indentation stress–strain curves

The mechanical properties of superconductor ceramics are of interest in the manufacture of superconducting devices. The current trend is to produce smaller devices (using, e.g., thin films), and the correct characterization of small volumes of material is critical. Nanoindentation is used to assess mechanical parameters, and several studies determine hardness and Young's modulus by sharp indentation. However, studies on the elasto-plastic transition with spherical indentation are scare. Here we used, spherical diamond tip indenter experiments to explore the elasto-plastic transition and to measure the yield strength of the orthorhombic phase of YBa₂Cu₃O_7?δ (YBCO or Y-123) at room temperature. The study was carried out for a range of monodomains on the (1 0 1)-plane for Bridgman samples. Inspection of the load–unload curves for penetration depths lower than 200 nm allows for observation of the elasto-plastic transitions. Focused ion beam (FIB) trenches showed no cracking due to the indentation, although oxygenation cracks were apparent. The mean pressure for the onset of elasto-plastic deformation is 3.5 GPa, and the elastic modulus, E, calculated using the Hertzian equations is 123.5 ± 3.4 GPa.     

Relationship between mechanical properties and coefficient of friction of sputtered a-C/Cu composite thin films

The article reports on properties of a-C films containing different amount of Cu. Films were sputtered by unbalanced magnetron from a graphite target with Cu fixing ring in argon under different deposition conditions. Relationships between the structure, mechanical properties, macrostress σ and coefficient of friction (CoF) μ of a-C/Cu films sputtered on Si substrates were investigated in detail. Besides, a special attention was concentrated on investigation of the effect of a deposition rate a_D of the a-C/Cu film on its hardness H and macrostress σ. Four main issues were found: (1) the addition of Cu into a-C film strongly influences its structure and mechanical properties, i.e. the hardness H, effective Young's modulus E⁎ macrostress σ and CoF, and makes it possible to form electrically conductive films; here E⁎ =  E / (1 − ν²), E is the Young's modulus, and ν is the Poisson's ratio, (2) the hardness H and compressive macrostress σ of the a-C/Cu film decrease with increasing a_D due to decreasing of total energy E_T delivered to the film during its growth, (3) hard a-C/Cu films with low value of CoF (μ ≈ 0.1) can be sputtered at high deposition rates a_D ranging from ~ 10 to ~ 80 nm/min, and (4) CoF decreases with increasing (i) hardness H and (ii) resistance of film to plastic deformation characterized by the ratio H³/E⁎² but only in the case when compressive macrostress σ is low.     

Examination of nanocrystalline TiC/amorphous C deposited thin films

The relationship between structural, chemical and mechanical properties of nanocrystalline TiC/amorphous C (TiC/a:C) thin films was studied. Thin films were deposited by DC magnetron sputtering on oxidized silicon (Si/SiO₂) substrates in argon at 25 °C and 0.25 Pa. The input power of the carbon target was kept at constant value of 150 W while the input power of the titanium target was varied between 15 and 50 W.It was found that all thin films consist of a few nanosized columnar TiC crystallites embedded in carbon matrix. The average size of TiC crystallites and the thickness of the carbon matrix have been found to correlate with Ti content in the films. The mechanical properties of the films have been strictly dependent on their structure. The highest values of the nanohardness (∼66 GPa) and Young's modulus (∼401 GPa) were observed for the film with the highest TiC content which was prepared at the largest input power of Ti target.     

Monochromatic photoluminescence obtained from embedded ZnO nanodots in an ultrahard diamond-like carbon matrix

At room temperature, we observe the self assembly of nanoclusters in an amorphous matrix using a vacuum deposition technique. Self-assembled ZnO nanoclusters embedded in hard diamond-like amorphous carbon thin films, deposited by high vacuum Filtered Cathodic Vacuum Arc (FCVA) technique at room temperature without post-processing, have been observed. A selective self assembly of metal and oxygen ions in a 3-element plasma was observed. XPS distinctly showed presence of ZnO and DLC-mixture in 5, 7 and 10 at.% Zn (in target) films while maintaining high sp³ content. This in turn improved the Young's modulus value of the ZnO nanoclusters embedded in DLC film (~ 220 GPa) compared to bulk ZnO (~ 110 GPa). Films with ZnO detected were observed to exhibit absorption edge at 377 nm monochromatic UV light emissions. This corresponded to a band gap value of about 3.30 eV. The emission with greatest intensity (after normalization) was detected from 10 at.% Zn (in target) film where presence of ZnO nanoclusters (~ 40 nm) in DLC matrix were confirmed by TEM. This showed that well-defined crystalline ZnO nanoclusters contributed to strong PL signal. Strong monochromatic emissions detected hinted that no defect states were present.     

Grafting of epoxy chains onto graphene oxide for epoxy composites with improved mechanical and thermal properties

Epoxy composites filled with both graphene oxide (GO) and diglycidyl ether of bisphenol-A functionalized GO (DGEBA–f–GO) sheets were prepared at different filler loading levels. The correlations between surface modification, morphology, dispersion/exfoliation and interfacial interaction of sheets and the corresponding mechanical and thermal properties of the composites were systematically investigated. The surface functionalization of DGEBA layer was found to effectively improve the compatibility and dispersion of GO sheets in epoxy matrix. The tensile test indicated that the DGEBA–f–GO/epoxy composites showed higher tensile modulus and strength than either the neat epoxy or the GO/epoxy composites. For epoxy composite with 0.25 wt% DGEBA–f–GO, the tensile modulus and strength increased from 3.15 ± 0.11 to 3.56 ± 0.08 GPa (∼13%) and 52.98 ± 5.82 to 92.94 ± 5.03 MPa (∼75%), respectively, compared to the neat epoxy resin. Furthermore, enhanced quasi-static fracture toughness (K_IC) was measured in case of the surface functionalization. The GO and DGEBA–f–GO at 0.25 wt% loading produced ∼26% and ∼41% improvements in K_IC values of epoxy composites, respectively. Fracture surface analysis revealed improved interfacial interaction between DGEBA–f–GO and matrix. Moreover, increased glass transition temperature and thermal stability of the DGEBA–f–GO/epoxy composites were also observed in the dynamic mechanical properties and thermo-gravimetric analysis compared to those of the GO/epoxy composites.     

Fracture surface and correlation of buckling force with aspect ratio of C60 crystalline whiskers

The structure and mechanical properties of crystalline whiskers with an average diameter of 600 nm, composed of C₆₀ molecules were studied by transmission electron microscopy combined with nanonewton force measurements used in atomic force microscopy. C₆₀ nanowhiskers with a body-centered tetragonal structure are compressed along their long axis. Young's modulus of the C₆₀ nanowhiskers was estimated to be 28 ± 5 GPa. The buckling stress was correlated to the aspect ratio of length to diameter of the C₆₀ nanowhiskers. The (100) surface was the principal fracture surface of the C₆₀ nanowhiskers.     

Low temperature elastic properties of chemically reduced and CVD-grown graphene thin films

We have measured internal friction and shear modulus of both reduced graphene oxide and chemical-vapor deposited graphene films measuring as thin as 5 nm. Graphene oxide films were deposited from solutions by spin-coating, and graphene films were synthesized by chemical-vapor deposition (CVD) on Ni thin films. In both cases, these films were transferred from their host substrate into a water bath, then re-deposited onto to a high-Q single crystal silicon mechanical double-paddle oscillator. A minimal thickness dependence of both internal friction and shear modulus was found within the experimental uncertainty for reduced graphene oxide films varying thickness from 5 to 90 nm. The internal friction of all films exhibits a temperature independent plateau below 10 K. The values of the plateaus are similar for both the reduced graphene oxide films and CVD graphene films, and they are as high as the universal “glassy range” where the tunneling states dominated internal friction of amorphous solids lies. This result shows that from a mechanical loss point of view, both graphene oxide and CVD graphene films have high and similar level of disorder. Raman measurements performed on the same samples show higher structure order in CVD graphene films than in graphene oxide films. Our results suggest that internal friction probes different sources of disorder from those by Raman, and the disorder is not directly related to the existence of C–O binding in the graphene oxide films. The shear modulus averages 53 GPa after subtracting Young's modulus component from the vibration mode used in experiments.     

Synthesis of polylactide/clay composites using structurally different kaolinites and kaolinite nanotubes

Polymer/clay nanocomposites receive much attention due to their interesting mechanical and thermal properties. Currently, the vast majority of plastics are made from petroleum-based synthetic polymers that do not degrade in a natural environment and their disposal poses a serious problem. An environmentally-conscious alternative is to design polymer nanocomposites that are biodegradable.In the present work the synthesis and properties of novel polymer/clay nanocomposites based on biodegradable polymer-polylactide (PLA) were investigated. Kaolinite nanotubes obtained by an intercalation/deintercalation method as well as platey kaolinites of different structural orders were used as fillers. Mechanical properties of composites (tensile strength (S_U) and Young's modulus (E)) were measured. The surface of the formed polymer derivatives was examined by AFM (Atomic Force Microscopy). The structural characterization was carried out using infrared spectroscopy (IR). Composites surface wettability was studied by measuring the water contact angle.The mechanical tests revealed that both S_U and E values increased significantly after addition of the nano-filler in comparison to the pure PLA. Regardless of the filler content the increase of S_U and E values was higher in the case of the nanotubular kaolinite. In particular, a threefold increase of the E value was noticed. For the most homogeneous kaolinite nanotubes/PLA nanocomposite S_U increased from ~ 29 MPa (pure PLA) to ~ 43 MPa, while E increased from ~ 0.7 GPa (pure PLA) to ~ 2.3 GPa. These mechanical parameters were comparable with the ones measured for polypropylene (S_U = 40 MPa; E = 1.5–2.0 GPa) and polystyrene (S_U = 40 MPa; E = 3.0–3.5 GPa). Differential IR spectra of the nanocomposites indicated an interaction of kaolinites inner surface hydroxyls with PLA which was confirmed by an intensity decrease of a band at ~ 3690 cm^? 1. The presence of highly dispersed nanotubular kaolinite particles in the polymer matrix which contributed to the improvement of PLA mechanical properties was observed using AFM. The contact angle measurements showed that the addition of kaolinites led to changes of wettability, yet the synthesized materials still possessed hydrophilic surfaces.     

Elastic behaviour of zirconium titanate bulk material at room and high temperature

Zirconium titanate (ZrTiO₄) is a well known compound in the field of electroceramics, however, its potential for structural applications has never been analysed. Moreover, it is compatible with zirconia, thus, zirconium titanate–zirconia composites might have potential for structural applications in oxidizing atmospheres. Nevertheless, there are currently no data about elastic properties of zirconium titanate materials in the literature. In view of the importance of these properties for the structural integrity of components subjected to high temperature and mechanical strains, an attempt was done in this work to determine the elastic properties of ZrTiO₄, both at room and high temperature. Young's modulus (161 ± 4 GPa), shear modulus (61 ± 1 GPa) and Poisson's ratio (0.32 ± 0.01) values at room temperature have been estimated for a fully dense single phase ZrTiO₄ material from experimental data of sintered single phase ZrTiO₄ materials with different porosities (6–19%). Values for room temperature Young's modulus are in agreement with those obtained by nanoindentation. Young's modulus up to 1400 °C shows an unusual dependence on temperature with no significant variation up to 500 °C an extremely low decrease from 500 to 1000 °C (≈0.02–0.03% every 100 °C) followed by a larger decrease that can be attributed to grain boundary sliding up to 1400 °C.     

Deposition and characterization of diamond-like carbon films by microwave resonator microplasma at one atmosphere

Diamond-like carbon (DLC) films have been deposited at atmospheric pressure by microwave-induced microplasma for the first time. Typical precursor gas mixtures are 250 ppm of C₂H₂ in atmospheric pressure He. Chemically resistant DLC films result if the Si (100) or glass substrate is in close contact with the microplasma, typically at a standoff distance of 0.26 mm. The films deposited under this condition have been characterized by various spectroscopic techniques. The presence of sp³ CH bonds and ‘D’ and ‘G’ bands were observed from FTIR and Raman spectroscopy, respectively. The surface morphology has been derived from SEM and AFM and shows columnar growth with column diameters of approximately 100 nm. Likely due to the low energy of ions striking the surface, the hardness and Young's modulus for the films were found to be 1.5 ± 0.3 GPa and 60 ± 15 GPa respectively with a film thickness of 2 μm. The hypothesis that a high flux of low energy ions can replace energetic ion bombardment is examined by probing the plasma. Rapid deposition rates of 4–7 μm per minute suggest that the method may be scalable to continuous coating systems.     

Microstructure,hardness and toughness of boron carbide thin films deposited by pulse dc magnetron sputtering

Boron carbide thin films were deposited on (100) silicon substrates at ambient temperature via. pulse dc magnetron sputtering. Various frequency and duty cycles were applied to the hot-pressed B₄C target in order to understand their influence on the structure and mechanical properties of the B₄C films. X-ray Energy dispersive spectrum, Raman spectroscopy and Transmission electronic microscopy were used to characterize the composition and microstructure of the films. Nanoindenter was employed to measure the hardness and modulus. The film toughness was evaluated by a microindentation method. The results show that both pulse frequency and duty cycle significantly affect the B/C atomic ratio and then hardness and modulus in the boron carbide films. However, the amorphous structure of the films was maintained when the frequency and duty cycle changed. The maximum hardness of 29 GPa and modulus of 247 GPa combined with relative high toughness (3.3 MPa m^1/2) were achieved under 50 kHz frequency and 30% duty cycle. In addition, there was no evidence to prove that the graphite phase existed in the B₄C films although exceeded C concentration was detected.     

High-temperature hot-pressing of titanium carbide–graphite hetero-modulus ceramics

Hetero-modulus ceramic–ceramic composite materials (HMC) present the combination of ceramic matrix with high Young's modulus and the inclusions of a dispersed phase with low Young's modulus. The densification of 60–100 vol% TiC–0–40 vol% C (graphite) HMC during hot-pressing of powder compositions at 2200–2400 °C and applied pressures of 8–16 MPa is studied. The general theory of bulk-viscous flow for porous body is developed to describe obtained experimental data. This relation as well as Kovalchenko's equation for the evolution of relative density for non-uniform deformation of porous body, incorporating different stages of creep, are used to determine the activation energy and the exponent constant of hot-pressing process, which values are 270 ± 30 kJ mol⁻¹ and 3 ± 0.3, respectively. The obtained experimental data are discussed on the basis of extensive and detailed analysis of the previous studies on diffusion and diffusion-related phenomena in the Ti–C system.     

Thermomechanical properties of Y-substituted SrTiO3 used as re-oxidation stable anode substrate material

Re-oxidation robustness is important to warrant a reliable operation of anode-supported solid oxide fuel cell systems. The current work concentrates on the mechanical properties of re-oxidation stable Y-substituted SrTiO₃ ceramic for the use as anode substrate material. Room temperature micro-indentation yielded Young's modulus and hardness of 160 and 7 GPa, respectively, whereas the temperature-dependent modulus was measured with a resonance-based method up to ∼950 °C. The effective Young's modulus as a function of porosity was measured at room temperature and compared with fracture strength data. The fracture toughness was assessed using a combination of pre-indentation cracks and bending test. Creep rates were measured at 800 and 900 °C in a 3-point bending configuration. Post-test fractographic analysis performed using stereo, confocal and scanning electron microscopy, revealed important information on fracture origins and critical defects in the material. A methodology to assess the mechanical properties of porous materials is suggested.     

Pyrochlore structure and dielectric properties of bismuth zinc niobate thin films prepared by RF sputtering

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) thin films with thickness from 60 nm to 200 nm were prepared by radio-frequency magnetron sputtering and post-annealed from 550 °C to 650 °C. The x-ray diffraction results indicated that the BZN thin films possessed a cubic pyrochlore phase. The BZN thin films exhibited thickness-independent dielectric properties with dielectric constant of ~180 and low loss tangent less than 1% at 10 kHz as the film thickness decreased to 60 nm. The BZN thin films with thickness of 200 nm and post-annealed at 650 °C had a tunability of 32.7% at a DC bias field of 1.5 MV/cm. The results suggest that the BZN thin films have promising applications on the embedded capacitors, tunable devices and energy storage devices.     

Mechanical behavior and ultrasonic non-destructive characterization of elastic properties of cordierite-based ceramics

The structural and morphological evolutions of cordierite-based ceramics produced from stevensite/andalusite mixture sintered from 1150 to 1350 °C were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical behavior was investigated by three-point bending and Brazilian tests. The elastic properties were evaluated using ultrasonic non-destructive testing (NDT). XRD results revealed that the main crystalline phase formed at 1300 and 1350 °C was cordierite with traces of mullite. A linear-elastic behavior followed by brittle fracture was observed in three-point bending test with the presence of multiple discontinuities. Flexural and diametral compression strength values of cordierite sintered at 1300 °C were 39.4±4 and 21.8±2 MPa, respectively. The elastic properties such as Young's modulus (38.7–45.1 GPa), shear modulus (17.90–19.42 GPa) and Poisson ratio (0.08–1.6) of cordierite-based ceramics produced at 1350 and 1300 °C were also determined.     

Thermal annealing behaviour of alloyed DLC films on steel: Determination and modelling of mechanical properties

A shortcoming of diamond-like carbon (DLC) films is the poor stability of their microstructure and properties at elevated temperatures. In this study, the effect of annealing on the stability of DLC films alloyed with silicon and deposited on steel is investigated. A comprehensive study of the mechanical properties is carried out by a novel method combining normal indentations with micro- and macroindentors assisted by finite element calculations of the indentation. The mechanical properties of the layers are correlated to structural changes in the film and to interface reactions.While it has become a common practice to determine hardness and the Young's modulus of thin films by nanoindentation and to calculate residual stresses from the bending of the film/substrate system, evaluation of the interface toughness, which is a measure of adhesion, and of the film rupture strength is less straightforward. Here, Hertzian-type ring cracks are generated in the film by nanoindentation of the film/substrate system with spherical diamond tips. From the critical load for crack generation the film rupture strength is deduced using finite element calculations. Similarly, Rockwell C hardness tests in combination with calculations are performed to measure the interface toughness.Applying these methods to DLC films on steel, it has been found that the Young's modulus decreases with increasing silicon content and the residual stress drops below 1 GPa. The rupture strength approaches its theoretical limit of E/10. Annealing at 500 °C reduces the adhesion energy significantly. The variation of mechanical properties can be attributed to structural changes in the film as investigated by Raman spectroscopy.     

Alumina/zirconia composites toughened by the addition of graphene flakes

The effect of added graphene flakes on the mechanical properties of a composite containing 20 wt% Al₂O₃ and 80 wt% ZrO₂ (stab. 3 mol% Y₂O₃) was studied. To obtain samples, a commercial ceramic powder produced by Tosoh (Japan), and graphene oxide (GO) made at the Institute of Electronic Materials Technology (Poland) were used. The obtained composites were based on aqueous mixtures of both components. After drying, they were sintered in an uniaxial pressure (HP) furnace. The composites contained from 0% to 3% of GO by weight. Results showed the influence of GO content i.e. fracture toughness has a maximum for 0.02% GO (increase by 42% in comparison to GO-free matrix) and afterwards decreased, strength decreased in the whole GO content range. Young's modulus and Vickers hardness remained constant up to 0.2% GO, and then decreased.     

Mechanical measurements of ultra-thin amorphous carbon membranes using scanning atomic force microscopy

The elastic modulus of ultra-thin amorphous carbon films was investigated by integrating atomic force microscopy (AFM) imaging in contact mode with finite element analysis (FEA). Carbon films with thicknesses of ～10 nm and less were deposited on mica by electron beam evaporation and transferred onto perforated substrates for mechanical characterization. The deformation of these ultra-thin membranes was measured by recording topography images at different normal loads using contact mode AFM. The obtained force-distance relationship at the center of membranes was analyzed to evaluate both the Young’s modulus and pre-stress by FEA. From these measurements, Young’s moduli of 178.9 ± 32.3, 193.4 ± 20.0, and 211.1 ± 44.9 GPa were obtained for 3.7 ± 0.08, 6.8 ± 0.12, and 10.4 ± 0.17 nm thick membranes, respectively. Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were used for characterizing the chemical and structural properties of the films, including the content of sp² and sp³ hybridized carbon atoms.     

Effect of graphene addition on the mechanical and electrical properties of Al2O3-SiCw ceramics

This paper presents a study on graphene-reinforced Al₂O₃-SiCw ceramic composites and the relationship between graphene oxide (GO) loading and the resulting mechanical and electrical properties. Well-dispersed ceramic-GO powders were fabricated using a colloidal processing route. Dense composites were obtained via spark plasma sintering, a technique that has the ability to reduce GO to graphene in situ during the sintering process. The mechanical properties of the sintered composites were investigated. The composite with only a small amount of graphene (0.5 vol.%) showed the highest flexural strength (904 ± 56 MPa), fracture toughness (10.6 ± 0.3 MPa·m^1/2) and hardness (22 ± 0.8 GPa) with an extremely good dispersion of graphene within the ceramic matrix. In addition to these exceptional mechanical properties, the sintered composites also showed high electrical conductivity, which allows the compacts to be machined using electrical discharge machining and thus facilitates the fabrication of ceramic components with sophisticated shapes while reducing machining costs.     

Ceramic TiC/a:C protective nanocomposite coatings: Structure and composition versus mechanical properties and tribology

The relationship between the structure, elemental composition, mechanical and tribological properties of TiC/amorphous carbon (TiC/a:C) nanocomposite thin films was investigated. TiC/a:C thin film of different compositions were sputtered by DC magnetron sputtering at room temperature. In order to prepare the thin films with various morphology only the sputtering power of Ti source was modified besides constant power of C source. The elemental composition of the deposited films and structural investigations confirmed the inverse changes of the a:C and titanium carbide (TiC) phases. The thickness of the amorphous carbon matrix decreased from 10 nm to 1–2 nm simultaneously with the increasing Ti content from 6 at% to 47 at%. The highest hardness (H) of ~26 GPa and modulus of elasticity (E) of ~220 GPa with friction coefficient of 0.268 was observed in case of the film prepared at ~38 at% Ti content which consisted of 4–10 nm width TiC columns separated by 2–3 nm thin a:C layers. The H3/E2 ratio was ~0.4 GPa that predicts high resistance to plastic deformation of the TiC based nanocomposites beside excellent wear-resistant properties (H/E=0.12).