The effect of the shape and size of the pores on the mechanical properties of porous HAP-based bioceramics

In this study, the influence of the shape and size of the pores on the mechanical properties of the obtained porous HAP-based bioceramics was investigated. The porous HAP-based bioceramics were obtained starting from spherical calcium hydroxyapatite powder, obtained by hydrothermal syntheses. The number of shapeless inter-agglomerate pores decreased and amount of spherical intra-agglomerate pores increased on increasing the sintering temperature from 1100 °C to 1250 °C. The shape of pores also changed with thermal treatment of specimens; the small pores remained spherical while the larger pores became more spherical in shape, as was proved by image analysis. A three-dimensional, finite element unit cell model was applied to evaluate the influence of pore shape on the mechanical strength of HAP ceramics. By analyzing the effect of the shape of pores to the fracture toughness of sintered porous HAP bioceramics, it was observed that the more spherical the pores were, the tougher became the bioceramics. After sintering at 1250 °C for 2 h, measured toughness was 1.31 MPa m^1/2, which is a relatively high value for this type of bioceramics.     

A new method to extract elastic modulus of brittle materials from Berkovich indentation

This work describes a method to reliably measure the elastic modulus of brittle materials from Berkovich indents under the continuous stiffness mode. It involves depositing a metallic film at the surface of the tested material to absorb the inelastic damage caused by the penetrating indenter. The Zhou-Prorok model was employed and rearranged to decouple the film and substrate contributions. This converted it into a hyperbolic form that approached an asymptote as the indenter approaches the film/substrate interface. This asymptote was described by a simple linear approximation whose slope directly revealed the substrate’s elastic modulus. The method enabled the elastic behavior of the brittle ceramic to be assessed without the indenter penetrating it. In fact, this method can assess properties of an unknown film/substrate composite as long as one has an estimate of the film thickness and the film can appreciably plastically deform.     

Mechanical properties of BaCe0.65Zr0.2Y0.15O3-δ – Ce0.85Gd0.15O2-δ dual-phase proton-conducting material with emphasis on micro-pillar splitting

BaCe_0.65Zr_0.2Y_0.15O_3-δ – Ce_0.85Gd_0.15O_2-δ (BCZ20Y15-GDC15) dual-phase material revealed potential for H₂ production technologies due to its exceptional H₂ permeation and chemical resistance. In this article, mechanical properties of BCZ20Y15-GDC15 dual-phase material were investigated to evaluate the mechanical behavior and develop strategies to warrant structural stability. Elastic modulus, hardness and fracture toughness values were studied using different indentation-based methods. The fracture experiments at different length-scales both revealed that the introduction of GDC15 makes the material tougher, facilitating the further design of robust and reliable components.     

Mechanical properties of BaCe0.65Zr0.2Y0.15O3-δ proton-conducting material determined using different nanoindentation methods

Proton-conducting membranes have great potential for applications in proton conducting membrane reactors for the production of commodity chemicals or synthetic fuels as well as for use in solid oxide fuel cells. However, to ensure the long-term structural stability under operation relevant conditions, the mechanical properties of the membrane materials need to be characterized. BaCe_0.65Zr_0.2Y_0.15O_3-δ is of particular interest due to its proven functional properties. In this research work, the mechanical properties of BaCe_0.65Zr_0.2Y_0.15O_3-δ were determined on different length scales using different methods including impulse excitation, indentation testing, and micro-pillar splitting. A detailed microstructural analysis of pillars revealed that irregular results are caused by pores causing crack deflection and complex crack patterns.     

Improved properties of hydroxyapatite–carbon nanotube biocomposite: Mechanical,in vitro bioactivity and biological studies

The present work describes a simple shear mixing technique for developing a hydroxyapatite (HAp)–carbon nanotube (CNT) nanocomposite and the effect of reinforcement on the physical, mechanical, in vitro bioactivity and biological properties of HAp. XRD and FTIR confirmed that the main phase of the composites is HAp. HRTEM images demonstrated the formation of a two-dimensional nanocomposite structure, whereas FESEM images indicated the formation of nanosized HAp grains featuring sporadically distributed CNT molecules. No major phase changes in HAp were observed with up to 5% added CNT. However, adding more than 1% CNTs caused an increase in internal crystal strain and increased substitution of CO₃²⁻ for OH⁻ and PO₄³⁻ groups in pure HAp. The average crystallite size increased from ~46 nm to ~100 nm with only 0.5% added CNT, remained nearly unaffected up to 2% CNTs thereafter and suddenly decreased at 5% CNTs (~61 nm). The FESEM and HRTEM images clearly showed the attachment of MWCNT chains on HAp grains, which directly affected the samples' fracture toughness and flexural strength. Of the samples, 1% showed maximum values of K_1C, whereas 5% showed maximum values of HV and three-point bending flexural strength. The in vitro bioactivity indicated increased apatite formation on the sample surface up to 1% CNTs after 24 weeks. However, adding 2% and 5% CNTs resulted in a manifold increase in apatite formation up to 12 weeks, after which dissolution increased up to 24 weeks, possibly due to increased substitution of CO₃²⁻ for OH⁻ and PO₄³⁻ groups. This result is confirmed by the FTIR studies. For all added CNT contents, all samples exhibited high haemocompatibility. However, there was a compromise between the observed mechanical properties and in vitro bioactivity studied up to 24 weeks, and care must be taken before selecting any final application of the nanocomposites.     

Effect of impurities on the microhardness of borax decahydrate

Microhardness of borax decahydrate single crystals grown from pure and impure borax solutions was measured. It was found that the microhardness of borax decahydrate crystals showed clear load dependence. The results obtained from Meyer Law indicated that Meyer index was found to be less than 2. The microhardness measurements were also evaluated according to the PSR (proportional specimen resistance) model to analyze normal ISE and measured microhardness values were converted to load-independent microhardness. The variation of these load independent microhardness values with different types and concentrations of impurities was determined and found that the microhardness of borax decahydrate crystals was strongly affected by ionic impurities. Based on microhardness measurements it was observed that radial and lateral cracks occurred during the measurements. The dependence of these crack lengths on applied load and fracture toughness was investigated by some empirical equations. Crystal surface stresses were evaluated using an empirical equation and it was determined that tensile stresses were effective on the microhardness of borax decahydrate crystals.     

Effects of bismuth oxide on the sinterability of hydroxyapatite

The sinterability of Bi₂O₃-doped hydroxyapatite (HA) has been studied and compared with the undoped HA. Varying amounts of Bi₂O₃ ranging from 0.05 wt% to 1.0 wt% were mixed with the HA. The study revealed that most sintered samples composed of the HA phase except for compacts containing 0.3, 0.5 and 1.0 wt% Bi₂O₃ and when sintered above 1100 °C, 1000 °C and 950 °C, respectively. In general, the addition of 0.5 wt% Bi₂O₃ was identified as the optimum amount to promote densification as well as to improve the mechanical properties of sintered HA at low temperature of 1000 °C. Throughout the sintering regime, the highest value of relative bulk density of 98.7% was obtained for 0.5 wt% Bi₂O₃-doped HA when sintered at 1000 °C. A maximum Young's modulus of 119.2 GPa was measured for 0.1 wt% Bi₂O₃-doped HA when sintered at 1150 °C. Additionally, the ceramic was able to achieve highest hardness of 6.08 GPa and fracture toughness of 1.21 MPa m^1/2 at sintering temperature of 1000 °C.     

Effects of modulation ratio on the microstructure,mechanical and tribological properties of WB2/Cr multilayer films deposited by magnetron sputtering

WB₂/Cr multilayer films with different modulation ratios (λ = 1, 3, 5, 7, 12, and 20) were deposited by a combination of direct-current and pulse direct-current magnetron sputtering, and the number of bilayers was fixed at ten. The effect of the modulation ratio on the microstructure, mechanical and tribological properties of the multilayer films was investigated in detail. X-ray diffraction demonstrates that a preferred orientation of WB₂ (101) and Cr (110) exists, and WB₂ (101) dominates the film's growth with increasing of modulation ratio. The TEM results show that the multilayer films consist of nanograins dispersed in an amorphous matrix in WB₂ layers and polycrystalline grains in Cr layers. The hardness increases with the increasing modulation ratio, and the maximum hardness (31.1 GPa) is obtained at λ = 20. The indentation toughness presents an opposite changing trend, and the maximum indentation toughness (1.264 MPa m^1/2) is obtained in S1 at λ = 1 which conforms to the rule of mixture due to the relatively thick bilayer thickness (Λ = 160–192 nm). The wear mechanism is investigated, and the results suggest that the multilayer film with λ = 7 possesses the best wear resistance (2.06 × 10^?7 mm³/Nm), benefiting from the balance of hardness and indentation toughness.     

Multi-component oxide lens glass with ultra-high mechanical properties inspired by the high-entropy concept

Lenses with high transmittance, high refractive index and excellent scratch resistance are in urgent need for cameras in high-end smartphones, and existing resin lenses exhibits intrinsic upper limits in those properties. Glass, with its naturally high refractive index and mechanical properties, is considered an ideal candidate for high-end lenses, however, due to the lack of targeted design, the intrinsic hardness, modulus and fracture toughness of conventional glass are low and its scratch resistance is inadequate. Here, we combined multi-compositional design and structural modulation of high mechanical lens glass with high-entropy concept, and successfully prepared multi-component 31.6RO-4.1Y₂O₃-23.7TiO₂-7.4ZrO₂-33.2Al₂O₃ (R = Ba, Sr, Ca) glasses with ultra-high hardness (11.06 GPa), modulus (147.6 GPa) and indentation fracture toughness (1.334 MPa m^0.5). The excellently comprehensive properties of the glass are attributed to the synergistic effect of multiple high dissociation energy and high field strength oxides, which leads to the movement of low-coordinated Al^[4] to the higher-coordinated Al^[5]/Al^[6].     

Experimental investigation on criterion of three-dimensional mixed-mode fracture for concrete

An investigation was carried out to establish a criterion for three-dimensional (3-D) mixed-mode fracture of concrete. Experiments on 3-D mixed-mode fracture were conducted to obtain the critical load P_cr. The stress intensity factor of the specimens per unit load with different combinations of K_I,, K_II and K_III was calculated by the mixed hybrid finite element method. Based on the stress intensity factor per unit load and the critical load, 25 groups of critical stress intensity factors were calculated, from which a criterion for the 3-D mixed-mode fracture of concrete was derived.