Decrease In Microhardness Research Articles

Effects of micrometre-sized graphite flake to reinforce the performances of poly(lactic acid) thermoplastic biocomposites

Biocomposites of poly(lactic acid) (PLA) and micrometre-sized graphite (GP) flake powder with 0–30 wt% GP contents have been prepared using extrusion moulding followed by compression moulding. The pure PLA and PLA-GP composites (PGC) have been examined by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy (RS), X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), transmission electron microscopy (TEM), mechanical and micromechanical testing, differential thermal analysis (DTA) and thermogravimetric analysis (TGA). FTIR spectra confirm the physical bond formation between GP and PLA. RS distinguishes the D-band spectra of pure PLA and PGC. XRD shows a partially crystalline structure in the PLA. SEM and TEM exhibit a clear dispersion of GP particles in PLA matrix at lower loading and aggregates at higher loading. With an increase of filler content, the tensile and flexural strengths decrease, but the Young’s and tangent moduli are observed to increase by 58% and 77%, respectively. These increments represent an increase in the stiffness of the materials and are found to be consistent with the theoretical values. A decrease in microhardness with increase in filler content is also observed. Both the DTA and TGA reveal an increased thermal stability of the composites.

Effect on the mechanical properties of human and bovine dentine of intracanal medicaments and irrigants.

Evidence is lacking concerning the suitability of using bovine dentine in endodontic research. This study compared the mechanical properties of human and bovine root dentine using endodontic medicaments and irrigants. Standardized human and bovine dentine bars were allocated to six experimental groups (n = 15): calcium hydroxide paste (Calasept® Plus); Odontopaste® ; 0.5% and 1% NaOCl gels; 1% and 4% NaOCl solutions. The bars were exposed to the materials for 7 days and then immediately subjected to a three-point bend test and Vickers microhardness test. Data were analysed using analysis of variance with Fisher's pairwise comparisons, with P < 0.05. Human dentine displayed a significantly higher modulus of elasticity (P = 0.001), higher microhardness (P < 0.001), and lower flexural strength (P = 0.004) compared with bovine dentine. Calcium hydroxide and Odontopaste® did not result in a significant change to the mechanical properties of human dentine. In human dentine, 0.5% NaOCl gel caused a significant decrease in flexural strength (P < 0.001) and microhardness (P = 0.02). Bovine dentine was a suitable substrate relative to human dentine. The use of a 0.5% or 1% NaOCl gel as an intracanal medicament is not supported by this study.

Effect of Shot Peening on Microstructure and Properties of 34CrMo4 Alloy

Plastic deformation can induce surface modification, such as shot peening (SP) on workpiece surface is the hot issue of recent scientific research. SP is the efficient way to improve mechanical behavior of specimens by inducing sever plastic deformation on their surface. Nevertheless, this surface treatment induced complex microstructural evolutions such as grain refinement, will enhance the corrosion resistance of specimens. In this work, the microstructure and properties of 34CrMo4 alloy of before and after SP for 20 min have been investigated. The evolution of microstructure and properties were analyzed from the surface and cross-section. The microstructure morphology at the different depth was determined by optical microscope. The results show grain size is increasing with the depth, and the microhardness and compressive residual stress decrease gradually. In terms of corrosion resistance, the 50 μm depth specimen has the best property than other depth, which the potential and corrosion current density are-0.484 V and-5.72 Acm-2, respectively. The maximum polarization resistance is 2055 Ωcm2 by capacitive arc radius of electrochemical impedance spectroscopy.

The Investigation of Microhardness of Grey Cast Iron Dendritic Structure

The dependence of the dendritic crystals of primary austenite microhardness on indentation load within the range from 0,01 to 2 N is investigated. Tests are carried out on specimens with a volume fraction of a dendritic component of 15%, 30% and 60%. It is found that at any volume of dendritic fraction microhardness increases with an increase in load from 0,01 to 0,1 N and then, in the process of a further indentation load growth, decreases a little. Decrease in microhardness in the range of loadings from 0,1 N to 2 N can be accounted for by growth of an impression size and a spread of plastically deformed volume to a dendritic crystal surface.

Effects of a potentially erosive antiasthmatic medicine on the enamel and dentin of primary teeth: An in situ study.

This study evaluates, in situ, the effect of an anti-asthmatic medicine on the enamel and dentin of primary teeth. Twenty-eight specimens of enamel and dentin were prepared, selected, sterilized, and stored. Microhardness, microscopic morphology, and initial surface roughness were evaluated. Fourteen volunteers wore palatal devices containing two dentin specimens and two enamel specimens. Volunteers underwent surface treatment three times a day for 5 days, with the dripping of 5 ml of salbutamol sulfate for 1 min, only on specimens from one side of the device. At the end of the experiment the microscopic morphology, roughness, surface loss, and microhardness were ascertained. For enamel surfaces exposed to the medicine, erosion was observed in the microscopy images, along with a significant increase in roughness (p = .0325) and tissue loss (p = .03251) and a significant decrease in microhardness (p = .0325). For the dentin surfaces, erosion was observed in the microscopy images, but there was no significant increase in roughness (p = .593) or tissue loss (p = .285) nor any decrease in microhardness (p = .1088). In conclusion, the salbutamol sulfate had an in situ erosive effect on primary teeth enamel and this effect was observed morphologically on primary dentin. RESEARCH HIGHLIGHTS: Asthma is considered a global health problem, and its prevalence has increased in many countries, especially among children, as well as dental erosion. This study has clinical relevance because provides data on the erosive effect of a commonly prescribed anti asthmatic drug on deciduous teeth.

Thermal Stability of Structure and Properties of the Surface Layer of Instrumental Steel Alloyed with Zirconium and Silicon Atoms under the Action of Compression Plasma Flows

The phase and elemental composition and microhardness of instrumental steel U9 with zirconium and silicon coatings subjected to compression plasma flows and air thermal annealing are investigated. It is found that plasma impact leads to the formation of a surface layer with the thickness of up to ~8.5 μm alloyed with zirconium and silicon atoms and containing Fe2Zr intermetallic. Formation on the surface of the oxide γ-ZrO2 and carbonitride Zr(C, N) as a result of interaction with the residual atmosphere of the vacuum chamber is found. Change in the phase composition and dispersion of the structure leads to a twofold increase in microhardness. The alloyed layer retains the stability of the structure and phase composition (excluding polymorphic transition in ZrO2) up to 400°C. Annealing at 600°C leads to the internal oxidation accompanied by formation of a surface iron oxide scale and penetration of the oxygen atoms to the whole depth. The increase in the annealing temperature leads to the decrease in microhardness throughout the alloyed layer.

Microstructural characterization and wear properties of silver and gold nanoparticle doped K-Mg-Al-Si-O-F glass-ceramics

In ascertaining the effects of silver (Ag) and gold (Au) nanoparticles on crystallization of boro-alumino-silicate system; the K2O-MgO-Al2O3-SiO2-B2O3-F glasses doped with/without 0.2 wt% Ag- and Au- content were melt-quenched at 1550 °C. Doping of nanoparticles considerably increased the glass-transition temperature and softening point but decreased the thermal expansion. A sharp crystallization exotherm in differential scanning calorimetry (DSC) is observed at 750 °C ( ± 1 °C) for glass without nanoparticle and that broadened to 800–855 °C when contains nanoparticle. Opaque glass-ceramics were derived from the glasses by controlled heat-treatment at 1050 °C with predominant crystalline phase fluorophlogopite (KMg3AlSi3O10F2) mica. Traces of Ag- and Au- particles were also identified from X-ray diffraction (XRD) technique. The activation energy (Ec) of crystallization (344 ± 17 kJ/mol) is decreased to 233 ( ± 12) and 307 ( ± 15) kJ/mol (Kissinger method) on doping with Ag- and Au- nanoparticles, respectively. Compact microstructure (FESEM) composed of rock like and plate-like mica crystals are developed in base glass-ceramic and that gets restructured to interlocked type morphology in presence of Ag- nanoparticle. Significant microstructural change induced by nanoparticle addition caused the decrease in microhardness (4.31–4.66 GPa) and increase in thermal expansion. Friction and wear testing under reciprocative sliding (using WC-Co ball) exposed that the average coefficient of friction (COF) is 0.60 ± 0.2 for all glass-ceramics at 20 N load and 10 Hz frequency. At a lower load of 5 N, the average COF value is increased from 0.69 to 0.92 on use of Au-nanoparticle. A Similar trend was also observed at 10 N load as COF increased from 0.62 to 0.78.

Influence of Ultrapulsed CO2 Laser, before Application of Different Types of Fluoride, on the Increase of Microhardness of Enamel In Vitro.

Objective To evaluate the influence of ultrapulsed CO2 laser in combination with commercial fluoride products in order to verify the increase of microhardness of artificial enamel caries lesions. Materials and Methods Bovine enamel specimens were prepared, and artificial enamel caries lesions were created. Teeth were randomly divided into 5 groups (n=10): treated with laser (L), laser + neutral fluoride gel 2% (LNF), laser + acidulated phosphate fluoride gel 1.23% (LAFG), laser + acidulated fluoride mousse 1.23% (LAFM), and laser + fluoride varnish 5% (LFV). Microhardness was evaluated at baseline, after caries induction, after CO2 laser irradiation + fluoride treatment in the 1st week, and after fluoride treatment at 3rd and 5th week. Results There was a decrease in microhardness in all groups after artificial enamel caries lesion formation; no increase in microhardness was found in the first and third weeks in all groups (p > 0.05). In the fifth week, an increase in microhardness occurred in all groups (p < 0.05). Conclusion Although CO2 laser irradiation in combination with different commercial fluoride products was capable of increasing microhardness on enamel caries lesions in bovine tooth enamel it is necessary to confirm these results by testing the isolated effect of fluoride on enamel surface microhardness. Also, although microhardness was higher in the fluoride varnish group than in the other groups in the fifth week it is not possible to discard the best effect of fluoride varnish treatment on absence of artifacts that may occur with the other fluoride treatments. Clinical Relevance In order to prove that CO2 laser may contribute to an increase in microhardness when applied to enamel lesions in combination with different commercial fluoride products it is necessary to conduct additional studies. Also, higher microhardness of fluoride varnish group should be carefully considered.

Microstructure and properties of heat treated 1Cr17Ni4MoB steel fabricated by laser melting deposition

A 1Cr17Ni4MoB steel was fabricated on the 40CriNiMoA surface by laser melting deposition (LMD), and then underwent heat treatment. The microstructure evolution and properties of laser deposited 1Cr17Ni4MoB steel and that underannealing temperatures of 500 °C, 700 °C, 900 °C for 1 h were characterized, especially their interface properties. The strength mechanism of laser deposited steel was analyzed. The results show that the microstructure of deposition is comprised of martensite dendrite, interdendritic carbon-boride (M2B) and retained austenite. The overlap of layer reduces cracks and coarsening equiaxed grains are observed between layers for the epitaxial grown from unmelted dendrite. The microhardness of deposition reaches up to approximately 600 HV, which is attributed to martensitic transformation, solution strengthening, carbon-boride and fine dendrites. With the increase of annealing temperature, martensite decomposes into ferrite and precipitated (Fe,Cr)23C6 along martensite lath, resulting in the decrease of microhardness. Meanwhile, lamellar eutectic borides go through fusing and spherification. The appearance of retained austenite was explained by carbon partitioning. Composition analysis shows that carbon segregate in plane grain and C-depletion area was observed in substrate. The increase of annealing temperature facilitates carbon diffusion from substrate, and carbon content exceeds deposition with the annealing temperature of over 700 °C.

Investigations on Microstructure and Properties of 16 wt% Cr-3 wt% B-0.6 wt% C-1 wt% Mn-Fe Alloy

In this paper, a wear-resistant alloy with the chemical composition of 16 wt% Cr-3 wt% B-0.6 wt% C-1 wt% Mn-Fe, in which M2B was the antifriction skeleton, was prepared in a medium-frequency induction furnace. The microstructure and mechanical properties were experimentally investigated. The results show that the microstructure of the Fe-Cr-B alloy was composed of lath martensite and clavate, reticular, and clustering borides (M2B). After the thermal treatment, the morphology, chemical composition, and volume fraction of the M2B did not change significantly. Because of the reduction in element saturation, secondary borides M23(B,C)6 precipitated from the matrix, which resulted in a decrease in matrix microhardness. As a result, the bulk hardness and abrasive resistance of the alloy accordingly decreased, and the impact toughness inversely increased. According to the results of XRD, electronic probe microanalyzer (EPMA), and TEM, the chemical formula of M2B was FeCr0.89Mn0.14(B,C), which resulted in a body-centered tetragonal (BCT) structure. The chemical formula of the M23(B,C)6 was Fe17.97Cr4.13Mn1.14 (B,C)6, which resulted in a face-centered cubic (FCC) structure.

Temperature-Time Superposition Effect on Compressive Properties of AZ31B Magnesium Composite Foams

Magnesium composite foams with 10 vol. % of hollow ceramic microspheres (CMs) were prepared by modified melt foaming method. Specimens with homogeneous pore structures were subjected to various heating temperature (150, 250, 320, 400, and 500 °C, respectively) and enduring times (1, 2, 4, 6, and 24 h, respectively). Evolution of microstructure and mechanical properties of the samples, before and after the heating processes were examined by applying X-ray diffraction technique (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and quasi-static uniaxial compression test. The results showed that as heating temperature and enduring time increasing, β-Mg17Al12 phases gradually dissolved, resulting in a solid-solution strengthening effect. Meanwhile, internal stress relaxation in the matrix leads to the decrease of yield strength and micro hardness of the specimens. When compared with the unheated foams, the treated specimens possessed lower micro-hardness, yield strength, and energy absorption capacity due to the dissolution of β-Mg17Al12 phases and the release of internal stress. However, higher strain hardening exponents for almost all of the treated composite foams were observed and the reasons were discussed. It is proposed that more factors should be taken into account when using heated composite foams in practical applications.

Dry sliding friction and wear behaviour of hybrid AA7075/Si3N4/Gr self lubricating composites

The present investigation aims to study the effect of graphite on the friction and wear behaviour of AA7075/Si3N4 composites. Four different sample compositions have been fabricated by stir casting technique. The percentage of silicon nitride has been fixed as 8 wt% and the percentage of graphite has been varied from 0–6 wt%. The cast composites have been tested for microhardness and density prior to the tribological testing. Unidirectional friction and wear testing has been carried out for all compositions under five different loading conditions. Moreover, in order to elucidate the effect of speed on the tribological behaviour testing at 4 m s−1 and 7 m s−1 has also been carried out. SEM analysis has also been carried out for morphological analysis of the worn surfaces. A decrease in microhardness and density of the composites is observed with an increase in graphite wt% in the base alloy. The addition of graphite led to a linear decrease in the COF. However, the wear loss exhibited a decreasing trend till 4 wt% graphite and thereafter it increased. The increase in the normal load led to an increase in COF as well as wear loss. The increase in the sliding speed led to an increase in the wear loss, however the COF exhibited a decreasing trend with an increase in speed. The SEM images revealed that the addition of graphite resulted in a decrease in the abrasive wear. Mild delamination and smoother surfaces were observed for higher content of graphite. The fabricated self lubricating composite could serve various anti-wear and antifriction applications particularly in automotive sector.

Microstructure, conductivity and mechanical properties of calcia stabilized zirconia ceramics obtained from nanosized precursor and reduced graphene oxide doped precursor powders

In the work 12CaO-88ZrO2 (12CSZ, mol%) ceramics was manufactured both from nanopowder, obtained via cryochemical technique, and composite precursor 12CSZ + 0.25 wt% rGO (reduced graphene oxide). Via SEM, XRD and Raman spectroscopy the detailed investigation of the effect of the precursor type and intermediate processing on the microstructure and electrical conductivity of ceramics was carried out. It was shown that rGO is completely removed during the annealing at 1550 °C for 3 h in air with no effect on the high ionic conductivity of ceramics. The use of nanosized powder and the additional processing step results in vacuum dense solid electrolytes characterized by well-formed cubic zirconia based solid solution, thin discontinuous grain boundaries and rather high ionic conductivity. The addition of rGO leads to slight microhardness (HV) decrease comparing to ceramics manufactured from the nanosized precursor. As a result, a new technique for zirconia based solid electrolytes having both high electrical conductivity at high temperatures and sufficient mechanical properties was suggested.

Magnetic Hysteresis Loop Technique as a NDE Tool for the Evaluation of Microstructure and Mechanical Properties of 2.25Cr–1Mo Steel

This study intends to understand the effect of short-term overheating on microstructure modifications, and variation in mechanical and magnetic properties of boiler tubes. It is based on the hypothesis that short-term overheating on boiler tubes leads to microstructural changes degrading their mechanical properties, thus resulting in their failure. As part of this study, fresh 2.25Cr–1Mo boiler tube samples were heat treated in the range of 700–950 $$^{\circ }\hbox {C}$$ . Magnetic hysteresis loop (MHL) measurements were carried for the non-destructive evaluation (NDE) of the mechanical properties that get altered due to microstructural modifications. For comparison, MHL was also carried out on a service-exposed boiler tube, which had failed due to short-term overheating. The magnetic parameters were correlated with the change in microstructure and micro-hardness of the samples. A decrease in coercivity, remanence, maximum induction and micro hardness were found at the lower soaking temperatures, due to: easy magnetic domain wall and dislocation motions for the stress relaxation; annihilation and recovery of dislocations; increase in inter carbide distance; and the decrease in the number density of carbides for the coarsening of carbides. A subsequent increase in coercivity, remanence and maximum inductions along with hardness are due to the nucleation and growth of fresh bainitic phase, obstructing the magnetic domain wall and dislocation motion. A drastic decrease in coercivity for the service-exposed tube is due to the transformation of MX type carbides to $$\hbox {M}_{23}\hbox {C}_{6}$$ type and accumulation of such carbides at the grain boundaries along with the decrease in number density of the carbides. The presence of scale has less effect on the coercivity, but its demagnetizing effect largely decreases the remanence and maximum induction. The results of the study show that the MHL can be a suitable NDE technique for the evaluation of change in microstructure and degradation of mechanical properties in power plant steels.

Microstructure and Mechanical Characterization of a Dissimilar Friction-Stir-Welded CuCrZr/CuNiCrSi Butt Joint

Dissimilar CuNiCrSi and CuCrZr butt joints were successfully frictionstirwelded at constant welding speed of 150 mm/min and rotational speed of 1400 rpm with the CuCrZr alloy or the CuNiCrSi alloy located on the advancing side (AS). The microstructure and mechanical properties of joints were investigated. When the CuCrZr alloy was located on the AS, the area of retreating material in the nugget zone was a little bigger. The Cr solute-rich particles were found in the nugget zone on CuCrZr side (CuCrZr-NZ) while a larger density of solute-rich particles identified as the concentration of Cr and Si element was found in the nugget zone on CuNiCrSi side (CuNiCrSi-NZ). The Cr precipitates and δ-Ni2Si precipitates were found in the base metal on CuNiCrSi side (CuNiCrSi-BM) but only Cr precipitates can be observed in the base metal on CuCrZr side (CuCrZr-BM). Precipitates were totally dissolved into Cu matrix in both CuCrZr-NZ and CuNiCrSi-NZ, which led to a sharp decrease in both micro-hardness and tensile strength from BM to NZ. When the CuNiCrSi was located on the AS, the tensile testing results showed the fracture occurred at the CuCrZr-NZ, while the fracture was found at the mixed zone of CuNiCrSi-NZ and CuCrZr-NZ for the other case.

Structure and Crystallographic Texture Changes of Ferritic Martensitic Steel Resulting from Thermal Creep and Ageing Tests

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The concentration of calcium phosphate spheres on micro hardness of dentin

Calcium phosphate spheres was synthesized by hydrothermal method and used to make anti-sensitivity Toothpaste. The toothpaste was used to apply on dentin surface daily. The calcium phosphate facilitated more remineralization effect as the applied time was 7 days compared to fewer days. The new remineralized layer leads to approximately 22% decrease of micro hardness of dentin.

Microstructural Evolution of NiCrBSi Coatings Fabricated by Stationary Local Induction Cladding

The development of induction cladding has been restricted by the complicated geometric characteristics of workpieces and the large heat-affected zone in the cladded workpieces. In this paper, three-dimensional continual local induction cladding (3D-CLIC) was proposed as a potential process to clad coating over a substrate with curved surface, and a stationary local induction cladding (SLIC) experiment was conducted as an exploratory study of 3D-CLIC. The microstructures and microhardness in the coatings were measured by SEM, EDS, XRD and microsclerometer, respectively. The results indicate that the coating is metallurgically bonded with the substrate without any defects. A compositional gradient exists in the diffusion transfer belt (DTB), and it decreases with the increase in induction heating time. The coating is mainly composed of (Fe, Ni), CrB, M7C3, Ni3B, Ni3Si and M23C6 (M = Cr, Ni, Fe). Among the carbides, M7C3 presents several morphologies and M23C6 is always attached to the DTB. A special phenomenon of texture was found in the SLIC coatings. The preferred orientation in (200) crystal plane or the restrained orientation in (111) (200) crystal plane becomes more obvious as the scanning speed increases. The maximum average microhardness is 721 HV when the coating is heated for 5 s. The wear loss of different samples increases with increasing induction heating time. The longer heating time would result in higher dilution in the SLIC coatings due to the complete mixing with the substrate, thus leading to the decrease in microhardness and wear loss.

Joining of 3D-printed AlSi10Mg by friction stir welding

Friction stir welding is a solid-state welding technology capable of joining metal parts without melting. The microstructure of the material evolved during the process from columnar grain along the thermal gradient in the melt pool to fine equiaxed grains. A significant decrease in microhardness in the stir zone was observed with the lowest hardness at approximately 3 mm from the weld centre. The decrease in the microhardness is mainly attributed to the dissolution of hardening precipitates in the aluminium matrix. Defects in the weld were observed due to insufficient heat input. Heat input could be increased with the increase in rotational speed of the welding tool, with some improvements in strength.

Observations on {332} twinning-induced softening in Ti-Nb Gum metal

Thermomechanical-cycling processes were applied to a metastable Ti-23Nb-0.7Ta-2Zr-1.0O (at%) alloy to control a volume fraction of mechanical twins, resulting from activation of twinning-induced plasticity effects. Analysis of the microstructure features designed using electron backscattering and X-ray diffraction revealed the deformation bands induced by the cycling process were characterized as only {332}< 113 >β twinning. It was also observed that the fraction of twins was significantly increased with increasing the number of the cycles in the process, resulting in a pronounced softening effect. To shed a light on the relevant micro-scale features (e.g., geometric orientation and stress concentrations) responsible for the microstructure evolution and mechanical response, the local Schmid factor of grains and geometrically-necessary dislocation densities were evaluated from experimentally observed results, and correlated with deformation twinning. A good correlation between the local geometric Schmid factor and softening behavior, due to texture evolution within twins was established, as well as the effect of local stress concentrations on microstructure evolution of this Ti-Nb Gum metal. Significant decrease in micro-hardness (approximately 18% from untwinned structure) was achieved by highly twinned structure, and a possible deformation mechanism was established to enhance ductility without evident elastic properties variation in metastable β Ti alloys.