•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.
The use of composite edible films made from biopolymers has attracted interest as a way to reduce pollution and recycling problems; however, the relation between barrier, mechanical and structural ...properties of the films have been scarcely studied. The aim of this work was to evaluate composite zein–chitosan edible films by applying common nanotechnology tools and to relate the results to zein concentration and film structural changes. Rougher, more elastic, and less hard film structures with better water vapour barrier properties were obtained using larger zein concentrations. Raman spectroscopy exhibited unexpected interactions, as indicated by the disappearance of the thiol groups of cysteine in the zein films and the appearance of OSO and COS groups in the blended materials in conjunction with the disappearance of zein ɛ-amino and NH2 functional groups in the zein film samples, thereby confirming changes in the blended film structure. Zein concentration presented linear correlations with water vapour permeability (R=−0.978) and film roughness (R=0.929). The composite films presented better barrier and mechanical properties than single ingredient films. This information shows the benefit of using protein–polysaccharide blends to prepare edible films.
•A nitriding pretreatment increases the adhesion of AlCrN coating on 4140 steel.•The adhesive failure mode of AlCrN is changed to cohesive by the nitrides layer.•The COF of AlCrN is reduced to 50% ...due to the nitriding pretreatment.•The specific wear rate of AlCrN is greatly reduced by the nitriding pretreatment.
In this work, a gas nitriding pretreatment (iron nitrides layer) formed on the surface of 4140 steel is proposed to improve the adhesion and wear resistance of the AlCrN coatings deposited by arc cathodic PVD. AlCrN, FexN and AlCrN/FexN (duplex) systems were formed with total thicknesses of 3, 10 and 13 µm, respectively. Using spherical instrumented indentation, hardness more than 22 GPa were determined for the AlCrN-based coatings, meanwhile 10 GPa for the iron nitrides. The adhesion of the coatings were evaluated by progressive load scratch tests (PLST) where the duplex coating reduced the CoF and changed the failure mode from adhesive to cohesive for the AlCrN coating. The AlCrN/FexN coating exhibited the lowest specific wear rate and the severity of damage was reduced in multipass scratch wear tests (MPST). The duplex system presented the best adhesion and wear resistance.
Titanium nitride (TiN) coatings with a Cr metallic interlayer were deposited on AISI 316L stainless steel substrates using D. C. unbalanced magnetron sputtering, changing substrate temperature and ...gas mixture. The effect of the sliding motion in wear tests was studied, comparing multipass (unidirectional) and reciprocating (bidirectional) sliding modes. In addition, microstructure, mechanical properties and adhesion were analyzed. The structural characterization was performed by scanning electronic microscopy (SEM) and grazing incidence X-ray diffraction (GIXRD). Moreover, hardness of the coating was measured by spherical nanoindentation, and thermal residual stresses were estimated. The scratch tests were conducted using a steel ball of 1 mm of diameter. To study the effect of sliding motion, the wear tests were carried out with an alumina ball of 3 mm of diameter, in both sliding modes the same linear test geometry, a stroke length of 3 mm and loads of 0.5 and 1 N were used. TiN coatings thicknesses ranged from 1.1 to 1.8 μm, growing in preferred orientations (200) and (111). It was found that increasing the nitrogen content during the deposition cause a decrease in the adhesion of the coating. In the sliding wear tests, the coefficient of friction (COF) values ranged from 0.12 and 0.22. The effect of sliding motion in COF was more evident in the tests performed with 0.5 N load, reciprocating exhibited higher values than multipass. The specific wear coefficients k varied from 6.9 × 10−4 to 1.88 × 10−3 mm3 · N−1m−1 for all tests, the highest value was obtained in a reciprocating test.
•Thermal residual stresses from −0.11 to −0.29 GPa were estimated.•Adhesion decreased by increasing nitrogen content.•Tribological behavior is studied by multipass and reciprocating sliding motion.•With 0.5 N test, lower COF was obtained in multipass than in reciprocating.
•Boriding reduced friction by up to 30% compared to heat-treatment in dry tests.•In dry tests, the borided steel showed mild wear and the heat-treated steel showed severe wear.•Grease lubrication ...reduced friction similarly in the borided and heat-treated steel.•The wear resistance of the borided steel improved by about 10 times using grease.
This work presents new results about the dry and grease-lubricated wear resistance of borided AISI 52100 steel. Boriding was conducted at 850 °C for 1 h. The reciprocating wear tests were conducted with an alumina ball of 6 mm of diameter, with normal load of 20 N and sliding distances of 100, 150 and 200 m; a lithium grease was employed for lubrication. In comparison with heat-treated AISI 52100 steel, boriding reduced the coefficient of friction by 15–30% in dry conditions, although both stabilized similarly in grease-lubricated tests. In addition, the results showed that, for the longer sliding distance, boriding increased the wear resistance up to 8 times in dry conditions and up to 10 times using grease for lubrication. The implementation of boriding significantly improved the tribological performance of AISI 52100 steel in both tests conditions.
In this work, an experimental-numerical evaluation of the standing contact fatigue testing of a nitrided AISI 316L steel was developed. The nitride layers were formed at the surface of an AISI 316L ...steel by a salt bath nitriding process at a temperature of 580 °C for 1, 3 and 5 h of exposure time, obtaining three different layer thicknesses. In order to know the mechanical response and the different mechanisms of damage associated with the standing contact fatigue test, Hertzian tests were performed on a MTS machine by cyclic loading of a sphere on a flat surface formed by the layer/substrate system. The standing contact fatigue test was developed through two main stages. First, the critical loads for each treatment condition were determined by monotonic tests, where the appearance of circular cracks was considered as the failure criterion. Subsequently, cyclic subcritical loads were applied at a frequency of 5 Hz. A numerical model based on the finite element method was developed to evaluate the stress field generated in the system by cyclic contact loads. The results indicate that the thinnest thickness of nitride layer exhibits better resistance to standing contact fatigue.
•Nitride layers were evaluated through standing contact fatigue.•The stress field associated with contact damage modes in the nitride layers was obtained by the finite element method.•Cohesive damage was a function of both the range of the maximum principal stress and the amplitude of the radial distance.
The objective of this study was to evaluate the effect of the presence of calcium oxalate (CaOx) crystals on the micromechanical properties of sclerenchyma tissue from the pecan nutshell (Carya ...illinoinensis). The microstructure of the cross-section nutshell was examined using light microscopy (LM) and atomic force microscopy (AFM). Using an instrumented indentation system, indentation tests with maximum loads of 500 mN were made on the biological material where the variables studied were the number of crystals present in the evaluated area and the size of individual crystals. Microscopic analysis revealed that the pecan nutshell consists of sclerenchyma tissue with multiple CaOx crystals randomly distributed throughout the material, exhibiting prismatic shapes and various sizes. The results of the indentation tests showed that the examined areas where there were crystals (1, 2 or up to 3) presented values of hardness and elastic modulus significantly higher (P < 0.05) compared to the sclerenchyma (without crystals). Likewise, there were no significant differences (P > 0.05) between the values of the micromechanical properties of the areas evaluated as a function of the number of crystals. On the other hand, it was observed that the size of the crystals did not show a direct correlation with the mechanical properties evaluated as expected. In conclusion, the biomineralization phenomenon is a successful strategy designed by nature to improve the rigidity of the pecan nutshell, where the CaOx crystals strengthen the structure by increasing the micromechanical properties.
•The effect of CaOx crystals on the micromechanical properties of the sclerenchyma from the pecan nutshell were evaluated by indentation tests.•The CaOx crystals increased the hardness (≈20%) and elastic modulus (≈13%) of the sclerenchyma tissue locally.•There were no significant differences between the values of the micromechanical properties evaluated as a function of the number of crystals.•The size of the crystals did not show a direct correlation with the mechanical properties.•CaOx crystals strengthen the structure of the pecan nutshell, demonstrating that they fulfill a mechanical function.
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