Enhanced Strength and Ductility in Magnesium Matrix Composites Reinforced by a High Volume Fraction of Nano- and Submicron-Sized SiC Particles Produced by Mechanical Milling and Hot Extrusion

In the present study, Mg nanocomposites with a high volume fraction (10 vol %) of SiC particles were fabricated by two approaches: mechanical milling and mixing, followed by the powder consolidation steps, including isostatic cold pressing, sintering, and extrusion. A uniform distribution of the high content SiC particles in a fully dense Mg matrix with ultrafine microstructure was successfully achieved in the mechanically milled composites. The effect of nano- and submicron-sized SiC particles on the microstructure and mechanical properties of the nanocomposites was evaluated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), and X-ray diffractometry (XRD) were used to characterize microstructures of the milled and mixed composites. Mechanical behavior of the Mg composites was studied under nanoindentation and compressive loading to understand the effects the microstructural modification on the strength and ductility of the Mg/SiC composites. The mechanical properties of the composites showed a significant difference regarding the size and distribution of SiC particles in the Mg matrix. The enhanced strength and superior ductility achieved in the mechanically milled Mg composites are mainly ascribed to the effective load transfer between matrix and SiC particles, grain refinement of the matrix, and strengthening effects of the nano- and submicron-sized SiC particles.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Development and Characterization of Mg-SiC Nanocomposite Powders Synthesized by Mechanical Milling

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Magnesium powder in micron scale and various volume fractions of SiC particles with an average diameter of 50 nm were co-milled by a high energy planetary ball mill for up to 25 h to produce Mg-SiC nanocomposite powders. The milled Mg-SiC nanocomposite powders were characterized by scanning electron microscopy (SEM) and laser particle size analysis (PSA) to study morphological evolutions. Furthermore, XRD, TEM, EDAX and SEM analyses were performed to investigate the microstructure of the magnesium matrix and distribution of SiC-reinforcement. It was shown that with addition of and increase in SiC nanoparticle content, finer particles with narrower size distribution are obtained after mechanical milling. The morphology of these particles also became more equiaxed at shorter milling times. The microstructural observation revealed that the milling process ensured uniform distribution of SiC nanoparticles in the magnesium matrix even with a high volume fraction, up to 10 vol%

Corrosion protection of Mg-SiC nanocomposite through plasma electrolytic oxidation coating process

Understanding the role of nanoparticles in magnesium (Mg)-based materials and protective coating provides valuable information to achieve an optimized combination of mechanical and corrosion protection properties of Mg nanocomposites. The present study investigates the effects of SiC nanoparticles on the corrosion behavior and structure of Mg-SiC composites substrates coated by plasma electrolytic oxidation (PEO). Moreover, the influence of different volume fractions of SiCn up to 10% on corrosion behavior and galvanic reactions between Mg matrix and SiC particles was also investigated. The morphology, distribution of the phases, and the microstructure of the coating were characterized by SEM, EDAX, X-ray photoelectron spectroscopy, and XRD. The corrosion resistance of the samples was determined through dynamic polarization and electrochemical impedance spectroscopy tests before and after PEO coating treatment. The results indicate that the Mg nanocomposite with 1 vol% SiCn (M1Sn) coated by PEO coating shows higher corrosion resistance than the samples with a higher percentage of SiCn, as well as the sample without SiCn particles.publishedVersio

An investigation on non-isothermal crystallization behavior and morphology of polyamide 6/ poly(ethylene-co-1-butene)-graft-maleic anhydride/organoclay nanocomposites

Nanocomposites based on polyamide 6 (PA6) and poly(ethylene-co-1-butene)-graft-maleic anhydride (EB-g- MAH) blends have been prepared via melt mixing. The effect of blend ratio and organoclay concentration on the crystallization and melting behavior of specimens were studied. Three types of commercial organo-modified clay (Cloisite 30B, Cloisite 15A and Cloisite 20A) were employed to assess the importance of the nanoclay polarity and gallery distance. The crystallization behavior was investigated using differential scanning calorimetry (DSC) and wide angle X-ray diffraction spectroscopy (WAXD). The strong interactions between amine end groups of PA6 and maleic anhydride groups of EB-g-MAH led to complete inhibition of EB-g-MAH crystallization according to the DSC results. A transformation from the α form to the γ form crystals of PA6, induced by both organoclays and EB-g-MAH, was monitored by WAXD and DSC. Small angle X-ray scattering (SAXS) was used to evaluate the morphology of nanocomposites. Moreover, transmission electron microscopy (TEM) was conducted to determine the location of organoclays and indicated that the organoclays mainly present in the PA6 matrix and rarely distribute in the EB-g-MAH phase in the case of low polarity organoclays. It was also evidenced that the organoclay with the most affinity to PA6 (Cloisite 30B) had the largest effect on the thermal and crystallization behavior of this phase in the blend

Effect of Smear Layer on the Push-Out Bond Strength of Two Different Compositions of White Mineral Trioxide Aggregate

Introduction: The aim of this in vitro study was to evaluate the effect of smear layer on the push-out bond strength of white mineral trioxide aggregate (WMTA) with and without disodium hydrogen phosphate (Na2HPO4). Materials and Methods: Dentin discs with standard cavities were obtained from extracted human single-rooted teeth and divided to 4 groups (n=15) according to the irrigation regimen and the canal filling material. In groups 1 and 3, canals were irrigated with normal saline; in groups 2 and 4, irrigation method included sodium hypochlorite (NaOCl) and then ethylenediaminetetra-acetic acid (EDTA). The canals were filled with WMTA in first and second groups and with WMTA+Na2HPO4; in groups 3 and 4. The samples were wrapped in wet gauze and incubated in 37°C for 3 days. The push-out bond strength was then measured by means of the Universal Testing Machine and the failure modes were examined under stereomicroscope at 40× magnification. Tow-way ANOVA was used to evaluate the effect of material type and smear layer removal. Post hoc Tukey test was used for the two-by-two comparison of the groups. Results: The greatest and lowest mean±standard deviation for push-out bond strength were observed in groups 4 (4.54±1.14 MPa) and 1 (1.44±0.96 MPa), respectively. The effect of removing the smear layer on the push-out bond strength of WMTA+Na2HPO4 was significant (P=0.01), but not for WMTA (P=0.52). Interestingly, there was significant difference between groups 1, 3 and 2, 4 (P<0.05). The failure mode for all experimental groups was of mixed type. Conclusion: Under circumstances of this in vitro study, removal of smear layer increases push-out bond strength when Na2HPO4 is added to WMTA

Effect of Smear Layer on the Push-Out Bond Strength of Two Endodontic Biomaterials to Radicular Dentin

Introduction: This in vitro study was designed to evaluate the effect of smear layer removal on push-out bond strength of white mineral trioxide aggregate (WMTA) and calcium-enriched mixture cement (CEM). Materials and Methods: Dentin discs with 3 mm thicknesses were divided into 4 groups (n=15): group 1: irrigation of the canal with normal saline and filling with WMTA; group 2: irrigation with sodium hypochlorite (NaOCl) and ethylenediaminetetraacetic acid (EDTA), and then filling with WMTA; group 3: same as group 1 but the lumens were filled with CEM; group 4: same as group 2 but the lumens filled with CEM. The samples were incubated at 37°C for 3 days after wrapping in gauze pieces moistened with distilled water. The push-out bond strengths were then measured by the universal testing machine and the failure modes were examined under a stereomicroscope at 40× magnification. Data were analyzed using two-way ANOVA and post-hoc Tukey’s test for bond strength. Statistical significance was set at P<0.05. Results: The greatest and lowest mean standard deviation for push-out bond strength were observed for groups 4 (3.13±1.46 MPa) and group3 (1.44±0.96 MPa), respectively. The effect of smear layer removal on push-out bond strength of CEM was significant (P=0.01), however, it was not significant for WMTA (P=0.52). The failure mode for all the groups was of mixed type. Conclusion: Under the limitations of this study, smear layer removal is recommended for CEM in order to gain higher push-out strength

Effect of hot isostatic pressing on densification, microstructure and nanoindentation behaviour of Mg–SiC nanocomposites

The production of fully dense nanocomposites with a homogeneous distribution of nanoparticles through powder metallurgy (PM) techniques is challenging. Additionally to mechanical milling, pressing and sintering, a final consolidation process is needed to fully densify the nanocomposite. Hot isostatic pressing (HIP) is a promising alternative method to other hot forming processes to eliminate porosity in these PM parts. In contrast to hot extrusion, for instance, isotropic properties are achieved, and textures, as they are usually observed in Mg after uniaxial deformation, are avoided. Here, we evaluate the effect of HIP on the densification, microstructure and (nano)hardness of Mg–SiC nanocomposites. Even though density increased indeed, we observed no increase in the mechanical properties, due to significant heterogeneity in the microstructure. SiC-free regions with a higher grain size developed. Local nanohardness measurements of the HIPed Mg nanocomposite revealed that these regions had a significantly lower nanohardness than the SiC-containing regions. Under consideration of mechanisms reported to be active in Mg in the pressure and temperature regime we used, we conclude that grain growth is the most likely mechanism leading to the microstructure observed after HIP. This is driven by the thermodynamic pressure to decrease the grain boundary energy and facilitated by a slightly inhomogeneous distribution of SiC nanoparticles in the sintered nanocomposite

Effect of Synthetic Tissue Fluid on Microleakage of Grey and White Mineral Trioxide Aggregate as Root-End Filling Materials : An in vitro study

Objectives: The success of endodontic surgery has been shown to depend partly on the apical seal. Grey mineral trioxide aggregate (GMTA) produces hydroxyapatite twice as often as white mineral trioxide aggregate (WMTA) when suspended in a phosphate buffered saline (PBS) solution. The aim of this in vitro study was to compare the microleakage phenomenon of gray and white mineral trioxide aggregates as root-end filling materials after immersion in synthetic tissue fluid (STF). Methods: 55 single-rooted extracted maxillary anterior human teeth were divided into two experimental groups of 20 teeth each, plus 3 groups of 5 teeth each as two negative and one positive control groups. The root canals were cleaned, shaped, and laterally compacted with gutta-percha. The root ends were resected and 3 mm deep cavities were prepared. The root-end preparations were filled with GMTA or WMTA in the experimental groups. Leakage was determined using a dye penetration method. Data were analysed using analysis of variance (ANOVA) at the 0.05 level of significance. Results: The mean dye leakage was 0.40 ± 0.1 mm for GMTA and 0.50±0.1 mm for WMTA groups, respectively. There was no significant difference between the two experimental groups (P = 0.14). Conclusion: Despite the different properties and behaviours of GMTA and WMTA in STF, there were no significant differences in microleakage when using GMTA or WMTA.