Highest Mean Microhardness Research Articles

Silver nanoparticles versus chitosan nanoparticles effects on demineralized enamel

BackgroundTo compare the impacts of different remineralizing agents on demineralized enamel, we focused on chitosan nanoparticles (ChiNPs) and silver nanoparticles (AgNPs).MethodsThis study was conducted on 40 extracted human premolars with artificially induced demineralization using demineralizing solution. Prior to the beginning of the experimental procedures, the samples were preserved in artificial saliva solution. The nanoparticles were characterized by transmission electron microscopy (TEM) and teeth were divided into four equal groups: Group A was utilized as a control group (no demineralization) and received no treatment. Group B was subjected to demineralization with no treatment. Group C was subjected to demineralization and then treated with ChiNPs. Group D was subjected to demineralization and then treated with AgNPs. The teeth were evaluated for microhardness. The enamel surfaces of all the samples were analysed by scanning electron microscopy (SEM) for morphological changes and energy dispersive X-ray analysis (EDX) for elemental analysis.ResultsThe third and fourth groups had the highest mean microhardness and calcium (Ca) and phosphorous (P) contents. SEM of these two groups revealed relative restoration of homogenous remineralized enamel surface architecture with minimal micropores.ConclusionChitosan nanoparticles (NPs) and silver NPs help restore the enamel surface architecture and mineral content. Therefore, chitosan NPs and AgNPs would be beneficial for remineralizing enamel.

The evolution of bainite and mechanical properties of direct laser deposition 12CrNi2 alloy steel at different laser power

In this paper, 12CrNi2 alloy steel samples were successfully fabricated by direct laser deposition (DLD) technology. The evolution of bainite in the DLD 12CrNi2 alloy steel process at different laser power of 1800 W, 2000 W and 2200 W and mechanical properties of as-deposited samples were investigated. The results showed that the middle-upper microstructure of the samples fabricated at different laser power transformed from lath bainite (LB) to granular bainite (GB) with increasing laser power. In addition, granular bainite was divided into blocky granular bainite (GB1) and lath-like granular bainite (GB2) according to different formation mechanisms, and the amount of GB1 increased and GB2 decreased in the range of laser power from 2000 W to 2200 W. No preferred texture was observed in the EBSD maps due to the complex heat flux direction which was resulted in the reciprocating scanning strategy. With the increase of laser power, the proportion of high-angle grain boundaries increased from 33.1% to 46.4% and that of low-angle grain boundaries decreased from 66.9% to 53.6%. The sample fabricated at 2000 W had the highest mean microhardness (331 HV0.2) and the best combination of ultimate tensile strength (757 MPa) and elongation (9.1%). However, the sample fabricated at 2200 W had the best impact toughness (aku = 100.0 J/cm2) because it contained a large amount of GB1 with dispersive and spherical-like island structures. This study provides the theoretical and experimental basis for the design of laser power and controllability of microstructure and properties in the DLD alloy steel process.

Characterization Investigations of a Al-5 wt.% Si/2 wt% (NbB<sub>2</sub>, NbC) Hybrid Composite Fabricated via Mechanically Alloying and Sequentially Milling (Mechanical Alloying + Cryomilling)

In this study, microstructural and mechanical properties of a Al-5 wt.% Si/2 wt% (NbB2, NbC) composite synthesized by sequentially milling (mechanical alloying (MA) and/or cryogenic milling (CM)) were investigated. Nb2O5, B2O3 and C powder blends were milled using high energy milling for 5 h and annealed at 1400 oC for 12 h to produce NbB2-NbC hybrid powders. The NbB2-NbC hybrid powders were mixed with the matrix Al-5 wt.% Si powders to constitute the Al-5 wt.% Si/2 wt.% (NbB2-NbC) powders blends which were mechanically alloyed (MA'd) for 4 h using SpexTM Mixer/Mill, cryo-milled for 10 min in a SpexTM 6870 Freezer/Mill and finally MA’d for 1h in SpexTM Mixer/Mill again. As-blended, MA’d and cryomilled powders were compacted in a hydraulic press with a uniaxial pressure of 450 MPa. Compacted samples were sintered at 570°C for 2 h under Ar gas atmosphere. Microstructural characterizations of the as-blended/MA'd powders and the sintered composites were performed using X-ray diffractometry (XRD) and scanning electron microscopy (SEM) techniques. Density and microhardness measurements and sliding wear tests were performed on the sintered composite samples. Sequentially milled and sintered Al-5 wt.% Si-2 wt.% (NbB2-NbC) samples had the highest mean microhardness value (2.29 ± 24.98 GPa) and the lowest wear volume loss (0.038 mm3).