In this study, the fracture toughness of the Fe2B layer was estimated using the Berkovich indentation technique. Boron diffusion at the surface of the AISI 1018 steel was conducted using a ...powder-pack boriding process at a temperature of 1273K with 6h of exposure. The mechanical characterization of the Fe2B layer was performed on three different distances from the surface using a range of indentation loads (10 to 500mN) for each distance. The behavior of the hardness as a function of the indentation load showed the presence of the indentation size effect (ISE) at the different distances from the surface, in which the apparent or real hardness was estimated according to the concept of the Nix and Gao model. Finally, two indentation-cracking models were used to estimate the fracture toughness of the Fe2B layer on the different distances from the surface; the results were ranged from 1.5 to 4.2 MPa m1/2, which denoted the brittleness and the influence of the anisotropic nature of the boride layer.
•The fracture toughness along the depth of the Fe2B layer was evaluated.•The apparent hardness of the boride layer was established by the GND theory.•The fracture resistance of the Fe2B layer is a function of the indentation distance.
Apart from its dietary use, Opuntia ficus-indica (OFI), also known as nopal, is widely used in diverse non-food areas given its multiple properties and health benefits. In Mexico City, each year ...de-thorning process of nopal generates around 40,000 tons of waste. This waste contains a huge amount of hemicellulose and cellulose that can be used as a new biodegradable nanocomposite. Therefore, the aim of this work was to purify nopal thorns to obtain cellulose nanoparticles by high impact milling or high-energy ball milling. Confocal laser scanning microscopy (CLSM) showed that thorns have two main polysaccharides arranged in parallel fibers: cellulose and lignin. Size of the obtained cellulose nanoparticles were ranged from 24 to 122 nm. Crystallinity, as well as the type of cellulose obtained, were analyzed by means of XRD to evaluate its potential use as nanocomposite.
•Strain hardening exponents n of borided and nitrided layers were calculated.•Boriding time had a negligible effect on strain hardening exponent.•A longer boriding time decreased the plastic work of ...indentation.•A short boriding time presented better standing contact fatigue performance.•The nitride layer withstands mechanical loading without spallations.
In this study, boride and nitride layers formed on H13 steel were assessed by cyclic spherical contacts. Boriding was conducted at 800 °C for 1 and 5 h, and nitriding at 580 °C for 1 h. The hardness and strain hardening exponents n of the layers were estimated by spherical indentations (5 µm radius). The standing contact fatigue (SCF) performance was investigated by cyclic spherical contacts (Al2O3, 3 mm diameter), first applying monotonic load tests from 300 to 1000 N to establish the critical loads considering the onset of certain cracks for each layer; later, between 102 and 105 cycles applying different subcritical loads. The results revealed strain hardening exponents of nB1h = 0.226, nB5h = 0.230 and nN1h = 0.241. In SCF, a critical load of 600 N in both layers was found. In cyclic tests, the thicker boride layer presented larger cracking and moderate spallations, attributed to its mechanical properties; the thinner layer exhibited a better SCF resistance. Regarding nitriding, well-defined circumferential cracks without spallations were observed.
•Porosity of calcified chicken eggshell was examined by AFM and image processing.•The palisade layer exhibits the highest degree of porosity and largest thickness.•Bubble pores in all layers have ...mainly circular shapes and similar sizes.•A gradient of porosity along the cross-section of calcified eggshell was observed.
In this work, the porosity of the layers of calcified chicken eggshell (vertical crystal layer VCL, palisade layer PL and mammillary layer ML) was evaluated using atomic force microscopy (AFM) and image processing (IP). AFM topographic images were obtained from different locations for each layer and along the cross-section of calcified eggshell. Roughness parameters, surface area values, pore size and shape, surface porosity, area occupied by pores and pore density were obtained from AFM and IP. It was observed that the thickest layer (PL) exhibited the highest degree of porosity (surface porosity = 2.75 ± 1.68%, pore density = 162 ± 60 pores/μm2) when compared to the other two layers. In general, the pores located in all layers (“bubble pores”) had circular shape and similar sizes. Measurements revealed a porosity gradient along the cross-section which varied with position, i.e., increasing surface porosity from the VCL towards the region of the PL closer to the ML, and decreasing surface porosity from this location towards the ML innermost surface. This suggests that the calcified eggshell has a sandwich-like structure where porosity may influence gas exchange and mechanical properties. The combination of AFM with IP presented here provides a simple and precise method to study porosity in calcified chicken eggshell, and this methodology could be used to examine other types of porous biological materials.
In the present work a comprehensive characterization of the hierarchical architecture of the walnut shell (Juglans regia L.) was carried out using scanning electron microscopy (SEM), atomic force ...microscopy (AFM) and confocal laser scanning microscopy (CLSM). Furthermore, micromechanical properties (hardness, HIT and elastic modulus, EIT) of plant tissues were evaluated at cell wall level by applying the instrumented indentation technique (IIT). The complex architecture of the material was described in terms of four hierarchical levels (HL): endocarp (H1), plant tissues (H2), plant cells (H3) and cell wall (H4). Our findings revealed that the walnut shell consists of a multilayer structure (sclerenchyma tissue, ST; interface tissue, IT; porous tissue, PT; and flattened parenchyma tissue, FPT), where differences in the microstructure and composition of plant tissues generate parallel gradients along the cross-section. The indentation tests showed a functional gradient with a sandwich-like configuration, i.e., a lightweight and soft layer (PT, HIT = 0.04 GPa) is located between two dense and hard layers (ST, HIT = 0.33 GPa; FPT, HIT = 0.28 GPa); where additionally there is an interface between ST and PT (IT, HIT = 0.16 GPa). This configuration is a successful strategy designed by nature to improve the protection of the kernel by increasing the strength of the shell. Therefore, the walnut shell can be considered as a functionally graded material (FGM), which can be used as bioinspiration for the design of new functional synthetic materials. In addition, we proposed some structure-property-function relationships in the whole walnut shell and in each of the plant tissues.
•The hierarchical architecture and micromechanical properties of the walnut shell were characterized.•Parallel gradients (microstructure, composition, mechanical properties) through the cross-section of the biological material were observed.•The walnut shell exhibits a sandwich-like configuration.•Structure-property-function relationships were proposed in the whole walnut shell and in each of the plant tissues.•The walnut shell can be considered as a functionally graded material (FGM).
The morphology and micromechanical properties of the mineral crystals embedded in the pecan nutshell (Carya illinoinensis) were characterized. Qualitative and quantitative morphological analyses ...carried out revealed that the crystals were comprised of calcium oxalate (CaOx) and have a wide range of sizes, with prismatic shapes, distributed heterogeneously in the sclerenchyma tissue. From indentation tests, it was found that CaOx crystals are stiffer structures compared to stone cells (sclerenchyma tissue), showing hardness and elastic modulus values of 0.53 ± 0.19 GPa and 9.4 ± 1.80 GPa, respectively. Additionally, the values of fracture toughness (0.08 ± 0.02 MPa m0.5) and the brittleness index (9336 m−0.5) revealed that these types of structures are extremely brittle. The results obtained suggest that the main function of the CaOx crystals is to provide structural support to tissue. The presented methodology demonstrates the potential of the instrumented indentation technique (IIT) for in situ micromechanical characterization of mineral crystals located in plant tissues.
Diagram of the morphological and micromechanical study of the CaOx crystals embedded in the pecan nutshell. Display omitted
•The morphology and micromechanical properties of the crystals embedded in pecan nutshells were studied.•The crystals are comprised of calcium oxalate (CaOx).•CaOx crystals are heterogeneously distributed and have prismatic shapes.•CaOx crystals are very stiff and extremely brittle structures.•The indentation technique is suitable for the in situ characterization of CaOx crystals.
Some mechanical properties for cobalt boride (CoB and Co
2
B) coatings were obtained using the Vickers depth-sensing indentation technique. The coatings were developed on the surface of a CoCrMo ...alloy using the powder-pack boriding process at temperatures between 1223 and 1273 K using different exposure times for each temperature. Vickers indentations were conducted at constant distances from the surface using loads ranging from 15 to 450 mN. For the entire set of experimental conditions, the behavior of the indentation properties was examined as a function of the indentation loads. Universal expressions were used to determine the apparent or real hardness, the indentation Young’s modulus, and fracture toughness of the CoB and Co
2
B coatings. The results indicated that the CoB and Co
2
B coatings exhibited an apparent hardness of 20 and 17 GPa, respectively, in which the fracture toughness of the cobalt boride coatings only varied slightly in the set of experimental conditions proposed in this work.
In this study, erosion tests were conducted to evaluate the behavior of AISI 420 against walnut particles. This organic material was selected to evaluate its performance as abrasive particle and know ...if this “soft abrasive” could really cause any damage on the material surface. The interest on walnut shell grit arose because this is extremely durable, angular and can be reused several times in many applications. Walnut producers have indicated that this can remove matter from surfaces without scratching or pitting underlying material. However, in this work, wear mechanisms similar to those observed when hard abrasive particles are used for erosion testing, were identified. SEM photographs showed micro-cutting (scratches) and micro-ploughing actions and particles embedded on the surface of AISI 420. In fact, this latter mechanism affected considerably the erosion rates at all incident angles. Finally, AFM technique and surface profilometry were employed to obtain the wear scar profiles to show the degradation of the surfaces by the impact and sliding actions of the walnut particles.
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