In order to study the influence of pre-existing dislocations on the pop-in phenomenon in magnesium oxide, an original experimental protocol, based on low-rate cleavage, has been developed to ...introduce a controlled dislocation density in MgO single crystals. Nanoindentation tests have been performed using a spherical indenter and the pop-in load has been measured as a function of the pre-existing dislocation density. The dislocation structures have been then characterized individually by chemical etching and atomic force microscopy observations. A double etching method has allowed distinguishing the pre-existing dislocation behaviour in the indenter stress field and the dislocation nucleated below the indenter during the pop-in. These experiments show that the pre-existing dislocations lower the pop-in load and promote the dislocation nucleation below the contact area. However, the analysis of the dislocation structures nucleated during the pop-in shows no direct relation between the pre-existing dislocations and those nucleated during the pop-in.
Chromium nitrides were deposited by RF reactive magnetron sputtering from a Cr target on high carbon steel substrates XC100 (1.17 wt% carbon) in a N2 and Ar gas mixture. In order to investigate the ...formation of chromium nitrides, carbide and carbonitride compounds were subjected to vacuum annealing treatment for 1 h at various temperatures ranging from 700 to 1000 °C. The samples were characterized by EDS, XPS, XRD, SEM, nanoindentation and tribometry. The results showed the emergence of Cr2N and CrN during the early stages of annealing and the appearance of chromium carbonitride phases only at 900 °C. The (111) preferred orientation of the fcc CrN phase was changed to (002) at 900 °C in parallel with the appearance of chromium carbides. Nanoindentation tests revealed a gradual increase of the Young's modulus from 198 to 264 GPa when increasing the annealing temperature, while the hardness showed a maximum value (H = 22.4 GPa) at 900 °C. The low friction coefficient of the CrCN coating against a 100Cr6 ball was approximately 0.42 at 900 °C. The enhancement of mechanical and tribological properties was attributed to the stronger bonding CrC at the CrN/XC100 interfaces as confirmed by XPS results.
•CrCN coatings were deposited on steel by RF reactive magnetron sputtering.•Microstructure of the coatings was strongly affected by the annealing treatment.•High temperature promotes the diffusion of carbon from substrate towards the film.•Thermal durability and high abrasive wear resistance with CrC and CrN bonds.
We report the mechanical behavior and deformation mechanisms in sapphire (Al2O3) studied using ~1µm diameter micro-pillar compression experiments performed in situ in a scanning electron microscope ...(SEM) at room temperature. Four crystallographic orientations: 〈10–10〉, 〈1–210〉, 〈0001〉 and 〈−1012〉 corresponding to M, A, C and R planes respectively, were studied. The residual deformations of the four planes were analyzed using Schmid law and geometric observations. Several deformation mechanisms such as cracking and plasticity were observed in our tests, and their probability of occurrence were strongly orientation dependent. Thus, for the same pillar size, pillars oriented along 〈1−210〉 are more prone to deform plastically than those 〈10–10〉 oriented. For pillars exhibiting plasticity, the resolved shear stress achieved just before the load drop was close to one thirtieth of the shear modulus and was consistent with heterogeneous nucleation mechanisms.
This paper presents a comparative study between DC magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HIPIMS) of Al–Si–N. Coatings were synthesised through co-sputtering of Al ...and Si in a mixed Ar/N2. One set of DCMS experiments and one set of hybrid HIPIMS (Al-target)/DCMS (Si-target) experiments were conducted. It was found that a higher partial pressure of N2 was necessary to obtain fully nitrided material using the HIPIMS process. The Si content of the samples was varied between 0 and 16at.%. All processes were characterised using optical emission spectroscopy (OES) as well as energy-resolved mass spectrometry (E-MS). The obtained coatings were characterised using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), as well as UV–Vis spectroscopy and nanoindentation.
The HIPIMS processes were found to provide a highly activated growth environment, with Al+ comprising 87% of the total ion flux, compared to 0.6% for the DCMS case, where Ar+ was found to be the dominating species comprising 90% of the total ion flux. Coatings from both HIPIMS and DCMS processes were found to form nanocomposites of a solid solution phase (Al1-xSix)N and most likely a SiNy phase, as shown by XRD and XPS analyses. Compared to coatings from DCMS, samples deposited with HIPIMS had a slightly more textured AlN-phase with smaller grains, as well as smoother and, denser morphology as observed by SEM.
In agreement with previous studies, the coatings had a high transparency in the visual and near IR range; an optical band gap (E04) between 4.6 and 5.2eV and a refractive index between 1.9 and 2.1 was observed. The ternary coatings studied here were found to be hard with the HIPIMS coatings (combined average 22±3GPa) being harder than their DCMS counterpart (combined average 17±1GPa). A maximum hardness of 27GPa was observed for the sample deposited with HIPIMS and a Si-content of 7at.%.
•Al–Si–N coatings were successfully deposited using DCMS and HIPIMS techniques.•Both processes and growth conditions were characterised using OES and E-MS tools.•Both HIPMS and DCMS gave nanocomposite coatings with similar optical performance.•The here deposited HIPIMS coatings were found to be denser and harder than the DCMS coatings.•The observed differences can be explained through the different growth conditions.
In this work was prepared α-Al2O3 alloys from laboratory aluminum oxide powder that was milled for different periods of time and sintered at a temperature of 1450 °C. The difference between the ...prepared samples was studied using several experimental measurement techniques, including X-ray diffraction, scanning electron microscopy and measurement of physical and mechanical properties. Moreover, the effect of milling time on the formation and sintering of alpha-alumina, by milling the mixture at different times using high energy crushing technique was studied. An influence of milling time on density, open spaces and microstructure of the samples was analyzed. The obtained results showed that longer milling duration led to alloys with higher hardness (H) and modulus of elasticity (E). This improvement is due to lower porosity and corresponding higher density at high temperatures. A noticeable decrease in the size of the particles with the increase of the milling time led to an increase in the lattice parameter accompanied by a decrease in defects and ionic voids. The percentage of pores reached 0.04 % within 24 h of grinding after it was approximately 0.20 %, while the density reached 96 % after the same highest grinding time. Tests showed that the value of friction coefficient decreases, while it increases with the increase in the applied pressure force and this was confirmed by SEM images of the samples. the main factor to reduce friction is the increase in grinding time, regardless of the value of the applied load. The results showed that the Al2O3 alloy applied to it with a load of 2 N and milled for 24 h had a minimum value of 1.94 µm3 wear volumes and a wear rate of 1.33 (µm3∙N−1∙µm−1). The sample milled for 24 h showed the best result, characterized by the lowest wear size, specific wear rate and the highest hardness with extraordinary density of 96 %, which is important in the field of biomaterials applications.
The influence of milling time on the tribological behavior of a Co–Cr–Mo alloy designed for biomedical applications, synthesized via mechanical alloying is investigated. Elemental Co, Cr and Mo ...powders are milled using different milling times (2, 6, 12 and 18 h) in a high-energy ball mill. The resulting powders were subjected to cold uniaxial and hot isostatic pressing respectively, followed by sintering to obtain cylindrical samples, which were evaluated for their structural, mechanical and the wear behavior. Results showed that the grain and crystallite sizes of the powders decreased with increasing milling time, reaching low values of <10 μm and 32 μm respectively, at higher milling times. Furthermore, the wear rates and the coefficients of friction were lower, at higher milling times due to high densities (96%), and higher elasto-plastic resistance, as presented by the H/E and H3/E2 values of 0.026 and 0.0021 GPa, respectively. Increased milling time enables the refinement of grains and reduction in porosity in the Co–Cr–Mo alloy, which in turn increases the alloy's elasto-plastic resistance and enhances its wear resistance.
Implant-related follow up complications resulting from poor implant integration, delamination, chipping, mechanical instability, inflammation or graft-vs-host reaction may lead to low patient ...tolerance, prolonged care and sometimes leading to a second surgery. Hence, there is an urgent need for developing biomaterials which will help to overcome the above compatibility problems. Ti based alloys have been widely used for biomedical applications, due to their excellent properties, such as low modulus, high biocompatibility and high corrosion resistance. In order to further improve the physical, mechanical and tribological properties of these alloys, microstructural modification is often required. Hence, this study aims to develop and evaluate the structural and tribological behavior of Hot Isostatic Pressed (HIPed) and sintered Ti-6Al-7Nb samples containing niobium, which is less toxic and less expensive as compared to the usual alloying element, vanadium (Ti-6Al-4 V). The Ti-6Al-7Nb alloys were fabricated by using nanoparticle powders milled for different durations (2, 6, 12 and 18 h) to evaluate the effect of milling time on the morphological and structural properties. Friction and wear tests were carried out on the (HIPed) and finally sintered Ti-6Al-7Nb alloy samples, to evaluate their tribological properties under different applied loads (2, 8 and 16 N), with an alumina α-Al2O3 ball as a counter face using an oscillating tribometer. The physical characterization of the nanopowders formed using different milling times indicated that the particle and crystallite size continually decreased with increasing milling time, while the microstrain increased. It is observed that the friction coefficient and wear rate for the samples prepared by powders milled for 18 h and tested under 2 N were lowest with values of 0.25 and 1.51 × 10−2 μm3∙N-1 μm-1, respectively compared to other milled samples. This improvement in tribological properties is attributed to the grain refinement at high milling times. The antibacterial evaluation of the fabricated alloys showed an improvement in antibacterial performance of the samples milled at 18 h compared to the other milling times.
The development of instrumented nanoindentation consists of non-destructive tests applied to miniature volumes of material (PMMA). The present research focuses on the factors explaining the variation ...in the trends of the mechanical properties studied. The evolution of Young's modulus (E) and contact hardness (H) with depth (h) and indentation force (P) shows the existence of an inflection point (2.77 nm) at low penetrations which separates two zones with the first increasing trend and the second decreasing. Explained respectively by the surface hardening induced by the preparation of the material surface and the existence of a surface hardness gradient denoted by the indentation size effect (ISE) observed at very low depths. In addition, on detection of a critical penetration depth below which the effect of the surface on the nanohardness dominates, the variation in the penetration charge is of the order of 9.71 nm. The differences in results of E and H between the dynamic and static modes are of the order of 8.46% and 6.44% inducing an overestimation of 35 MPa in value of E and an underestimation of 1.23 MPa in value of H. They tend to affect the expected nanoscale precision of the indentation to determine the nanomechanical properties of PMMA.
Abstract
Nanoindentation is a technique commonly used to measure the mechanical properties of thin films even at depths less than 1
μ
m. In fact, the characterization is based on the study of the ...load/displacement curves resulting from the nanoindentation test. We aim to use the backtracking search optimization algorithm (BSA) to improve the extraction of
P(h)
curves performed by nanoindentation on galvannealed Fe-Zn coating in terms of precision and dispersion. Indeed, the originality of this study is not limited only to the
P(h)
curve extraction methods, but also to its application in modeling the reverse approach for the case of Fe-Zn coating deposited on a Dual-Phase DP600 steel substrate. Indeed, the BSA approach showed more precision (with respect to the determined mean value) and less dispersion (magnitude of error around the identified mean value) compared to the Least-squares approach. The average error with the BSA and LS methods is respectively 0.89% and 3.16% for the yield stress
σ
Y
and 3.17% and 7.93% for the strain hardening exponent (n). This reduced the error variability in the prediction of the constitutive law to 72% and 60% for (
σ
Y
) and
n
, respectively. Thus, we solved the problems of accessibility, uniqueness of the solution, precision (+10%) and dispersion (−85%) of the required prediction models for the Fe-Zn coating behavior.
This study aims to examine the effect of replacing vanadium by niobium and iron on the tribological behavior of hot-isostatic-pressed titanium alloy (Ti–6Al–4V) biomaterial, using a ball-on-disk-type ...oscillating tribometer, under wet conditions using physiological solution in accordance with the ISO7148 standards. The tests were carried out under a normal load of 6 N, with an AISI 52100 grade steel ball as a counter face. The morphological changes and structural evolution of the nanoparticle powders using different milling times (2, 6, 12 and 18 h) were studied. The morphological characterization indicated that the particle and crystallite size continuously decrease with increasing milling time to reach the lowest value of 4 nm at 18-h milling. The friction coefficient and wear rate were lower in the samples milled at 18 h (0.226, 0.297 and 0.423; and 0.66 × 10
−2
, 0.87 × 10
−2
and 1.51 × 10
−2
µm
3
N
−1
µm
−1
) for Ti–6Al–4Fe, Ti–6Al–7Nb and Ti–6Al–4V, respectively. This improvement in friction and wear resistance is attributed to the grain refinement at 18-h milling. The Ti–6Al–4Fe samples showed good tribological performance for all milling times.
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