Higher Microhardness Values Research Articles

Physical characteristics of ceramic/glass-polymer based CAD/CAM materials: Effect of finishing and polishing techniques.

PURPOSEThe aim of this study was to compare the effect of different finishing and polishing techniques on water absorption, water solubility, and microhardness of ceramic or glass-polymer based computer-aided design and computer-aided manufacturing (CAD/CAM) materials following thermocycling.MATERIALS AND METHODS150 disc-shaped specimens were prepared from three different hybrid materials and divided into five subgroups according to the applied surface polishing techniques. All specimens were subjected up to #4000 grit SiC paper grinding. No additional polishing has been done to the control group (Group I). Other polishing procedures were as follows: Group II: two-stage diamond impregnated polishing discs; Group III: yellow colored rubber based silicone discs; Group IV: diamond polishing paste; and Group V: Aluminum oxide polishing discs. Subsequently, 5000-cycles of thermocycling were applied. The analyses were conducted after 24 hours, 7 days, and 30 days of water immersion. Water absorption and water solubility results were analyzed by two-way ANOVA and Tukey post-hoc tests. Besides, microhardness data were compared by Kruskal-Wallis and Mann-Whitney U tests (P<.05).RESULTSSurface polishing procedures had significant effects on water absorption and solubility and surface microhardness of resin ceramics (P<.05). Group IV exhibited the lowest water absorption and the highest microhardness values (P<.05). Immersion periods had no effect on the microhardness of hybrid ceramic materials (P>.05).CONCLUSIONSurface finishing and polishing procedures might negatively affect physical properties of hybrid ceramic materials. Nevertheless, immersion periods do not affect the microhardness of the materials. Final polishing by using diamond polishing paste can be recommended for all CAD/CAM materials.

A comparative evaluation of the microhardness of glass ionomer cements modified with chitosan and chlorhexidine: A 1-year in vitro study

Aims and Objectives: Enhancement of the therapeutic properties of glass ionomer cements with antimicrobials, without alterations in its existing physical properties is of current research interest. One such innate property is microhardness. The aim of this study was to compare the microhardness of chlorhexidine-modified glass ionomer cement (CHX GIC) and chitosan-modified glass ionomer cement (CHT GIC) with an unmodified glass ionomer cement (FUJI IX GIC) and to study the variations in their microhardness when stored for a year. Materials and Methods: In an experimental analytic study, Fuji IX GIC liquid was modified with chitosan to form 10% vol/vol of CHT GIC. Fuji IX GIC powder was modified with chlorhexidine diacetate powder to form 1% wt/wt CHX GIC. Fuji IX GIC was used as control. Forty cylindrical samples each of the control and experimental cements were prepared, embedded in resin and stored in distilled water. Vickers microhardness testing was performed at stipulated time periods. The mean and standard deviation of each group were calculated. The data were analyzed using analysis of variance and post hoc Tukey test for multiple comparisons. Results: Materials showed significant difference in microhardness (P <0.000) at all time periods chosen for the study. FUJI IX GIC had the highest microhardness values at all time periods. CHT GIC showed the lowest microhardness and CHX GIC showed excellent stability over time. Conclusion: Although the therapeutic effects of modified GICs are proven, these additives can significantly and negatively alter the existing microhardness of the parent material.

The effects of sodium hexametaphosphate combined with other remineralizing agents on the staining and microhardness of early enamel caries: An in vitro modified pH-cycling model

This study was conducted to determine the effects of sodium hexametaphosphate (SHMP) combined with other remineralizing agents on the staining and microhardness of early enamel caries. in This in vitro study The enamel buccal surfaces of 70 bovine incisors were classified into seven study groups (n = 10). Remineralizing agents were employed alone and in combination with SHMP in different groups, including: (1) 8% SHMP, (2) 2% sodium fluoride, (3) 2% sodium fluoride + SHMP, (4) Remin Pro®, (5) Remin Pro®+SHMP, (6) MI Paste Plus, and (7) MI Paste Plus + SHMP. A modified pH-cycling technique was used to reconstruct the dynamics of caries. Colorimetric and microhardness analyses were conducted before demineralization (T1), after caries formation (T2), and after the remineralizing treatment (T3). The data were analyzed by the one-way analysis of variance and the repeated measurement analysis (P > 0.05). After remineralizing cycles, the experimental groups treated with either SHMP alone or in combination with other materials showed less significant changes in the three variables of color (Δa, Δb, and ΔL) and the overall color change (ΔE). The enamel caries treated with Remin Pro® presented the highest color change, while Remin Pro®+ SHMP resulted in the least changes. The mean value of microhardness after remineralization improved significantly in all groups, except in the MI Paste Plus + SHMP group that showed the lowest value. In contrast, the highest microhardness value was recorded for Remin Pro®, being comparable to that of the sound teeth (P > 0.05). SHMP, either alone or combined with remineralizing agents, created the least staining. Remineralizing materials alone showed higher surface hardness, while sodium fluoride alone showed higher surface hardness when combined with SHMP.

Influence of water aging on surface hardness of low-shrinkage light-cured composite resins

This study evaluated the microhardness of four types of low-shrinkage composite resins and two types of universal composite resins with either 12 or 16 J/cm2 light power energy. Three disks were made for each group (n = 3) for a total of 36 specimens. The specimens were prepared by condensing the composite resin into a circular copper mold (diameter: 6 mm; height: 2 mm) and polymerizing with 700 mW/cm2 light power density. The microhardness values of the resin specimens were measured using a Vickers hardness tester after different storage durations. Z250 and Clearfil Majesty Posterior composites showed softer subsurfaces when comparing the 24 h samples for all six types. Conversely, Kalore GC and Admira (AD) composites showed harder subsurfaces during the 24 h samples. All the composite resins showed significant differences (P < 0.05) in microhardness values at one of the aging times when they were polymerized with either 12 or 16 J/cm2 light power energy. Composite resin AD had a higher microhardness value after polymerization with 12 J/cm2 than with 16 J/cm2. The results indicated that low-shrinkage composite resins have better subsurface characteristics, and the light power energy of 16 J/cm2 is better for the polymerization of most composites.

Aging of WE43 magnesium alloy after mechanical crushing and subsequent high pressure torsion

Present work aims at investigation of the consequences of mechanical crushing prior to high pressure torsion (HPT) of the Mg-Y-Nd-Zr (WE43) alloy. Specifically the presence and size of the effect on the aging properties compared to the initially solid state and subsequent HPT are studied. For this, the WE43 alloy was mechanically crushed into particles of 0.5 –1 mm size. Than the obtained powder was formed to pellets and deformed at a pressure of 6 GPa for 10 revolutions with 1 rpm rotation speed. Thermal stability of the HPT processed alloy microstructure was studied by monitoring its microhardness and aging. Mechanical crushing and subsequent HPT processing at room temperature results in significant strengthening of magnesium alloy WE43. It was found that strengthening induced by HPT sustained to 200°C. The strength of the HPT processed alloy was additionally improved by subsequent aging. Extraordinarily high maximum value of microhardness of 1557 ± 25 MPa was reached. We suppose that crushing prior to high pressure torsion creates additional defects induced by the surfaces of individual powder particles during HPT. Additionally the surfaces of individual powder particles can act as segregation centers for rare earth elements. That decreases electrical resistivity due to lower precipitate dissolution and lower solid solution supersaturation.

Effects of Gas Nitriding on Fatigue and Crack Initiation of Ti6Al4V produced by Selective Laser Melting

Selective laser melting (SLM) is an additive manufacturing technique which permits fabrication of three dimensional parts by selectively melting consecutive layers of metallic powder. This allows the production of parts with high geometrical complexity. Titanium alloy Ti-6Al-4V (Ti64) is widely used in industry due to its high strength-to-mass ratio, corrosion resistance and biocompatibility. SLM increases the application range of Ti64 because of its flexibility for prototyping any part and its low material waste. Nitriding is a diffusion-based thermo-chemical treatment for interstitial hardening of the surface of Ti64 alloy products. This study characterized the fatigue behaviour of SLM-produced Ti64 nitrided and annealed bars in as-built and machined surface conditions. The surface of the SLM-produced Ti64 parts after gas nitriding showed high values of micro-hardness up to 550 HV just below the surface. Fatigue testing was performed to assess the materials fatigue strength and fractographic imaging was used to examine fracture surface and nitride layer characteristics. Nitriding was found to reduce the fatigue strength of the samples to a similar level irrespective of being in the as-built or machined condition. The effect of nitriding on crack initiation and growth at various stress levels under fatigue loading was investigated.

Wear Resistance and Morphological Characterization of High-power Laser Clad Coatings on Ti-6Al-4V Alloy

Ti-Si binary coatings were cladded on Ti-6Al-4V alloy surface for improvement morphological orientation, microstructure and surface properties. The coatings were developed at 2.1 kW power, laser scanning speed of 1.2, 1.6 and 2.0 m/min, spot diameter of 3 mm and 0.5 overlap ratio. An optical microscope was used to magnify and analyse the microstructural arrangement and a microhardness indenter used to measure microhardness at 100gf for 10 seconds. The indent spacing was maintained at 100 µm between adjacent indents. The phase content of the coatings was analyzed with XRD technique. The coating wear resistance was tested at 10N force loading for 7 minutes using a TRB tribometer and characterized using SEM. Clad coatings were observed to exhibit refinement in microstructure with an increase in laser scanning speed where transformation occurred from equiaxed grains with localized smaller grains to refined grain structure. The highest degree of refinement was observed at 2.0 m/min scanning speed. It was also found that the microhardness property was improved by 2.44, 2.60 and 2.64 times the microhardness of Ti-6Al-4Vat 342.3 HV0.1. It is understood that high microhardness values favor improved wear resistance behavior. XRD analysis on the coatings revealed presence of Ti5Si3, Ti, Si and TiSi2 phases where Ti and Si are in matrix capacity and Ti5Si3 and TiSi2 present as intermetallic phases. The reported improvement in microhardness was attributed to the presence of Ti5Si3 and TiSi2. The action of the laser scanning speed was found to influence the clad height where an increase in laser scanning speed resulted in a decrease in clad height in the order 685.9, 491.4 and 291.5 µm for 1.2 m/min, 1.6 m/min and 2.0 m/min respectively. Wear test indicated that the coatings displayed an improved wear resistance by evidence of little material removal exhibited, compared to the substrate material which was dilapidated by severe adhesive wear and further aggravated by plastic deformation.

TRIBOLOGICAL BEHAVIOR OF THIN NANO TUNGSTEN CARBIDE FILM DEPOSITED ON 316L STAINLESS STEEL SURFACE

This study presents the tribological behavior of austenitic 316L Stainless Steel (SS) coated with nano Tungsten Carbide (WC). The nano WC particles were prepared by mechano chemical method. The tungsten and toluene have been ball milled for 40[Formula: see text]h led to the synthesis of WC nano particles. An average particles size of 48[Formula: see text]nm was achieved. The prepared nano WC particles were deposited on 316L SS substrate as a thin film using DC magnetron sputtering process. The thickness of the nano WC coating was 5[Formula: see text][Formula: see text]m. The synthesized nano WC particles and the thin nano WC film are characterized using Scanning Electron Microscope (SEM), X-ray Diffraction (XRD) and Energy Dispersive X-ray Analysis (EDAX) technique. Vickers microhardness test was conducted to evaluate the microhardness of the thin film. A high microhardness value of 2242 HV[Formula: see text] was observed. The coated specimens are subjected to wear test using pin on disc setup and the tribological parameters such as friction and wear are analyzed. The results were compared with uncoated 316L SS specimen and micro WC particles coated 316L SS. The nano WC coated 316L SS possess high hardness and better wear resistance when compared with 316L SS and micro WC coated 316L SS specimen.

Multilayered ZrN/CrN coatings with enhanced thermal and mechanical properties

Multilayered ZrN/CrN coatings were produced by the cathodic arc physical vapor deposition (CA-PVD). Morphology, element distribution, structural and thermal properties were investigated. Microhardness and nanoindentation tests were conducted to study the mechanical properties of the coatings. The experimental results of scanning electron microscopy (SEM) revealed the cellular microrelief of the surface and well-defined multilayered architecture of the coatings. Energy dispersive spectroscopy (EDS) provided the chemical characterization of the coatings and revealed the formation of stoichiometric composition in coatings. X-ray diffraction (XRD) studies showed that (200) and (111) plane reflections of ZrN and Cr2N phases, correspondingly, were with maximum intensity. Transmission electron microscopy (TEM) analysis revealed that the films comprise of nanometric equiaxed grains with average sizes from 12.8 to 15.1 nm for ZrN layers and from 14.5 to 28.1 nm for CrN layers. The high-temperature heat treatment caused exothermic and endothermic reactions, which were mainly attributed to the improvements or disordering of the coatings' structure, consequently. The highest values of microhardness (4966HV0.025) and nanohardness (24.58 GPa) were obtained for the sample with a bilayer thickness of 732 nm, the average crystallite size of 13.3 nm and nitrogen content of 49 at%. Experimental results indicated that deposited composites can be effectively used in the production of industrial tools, implements for cutting etc.

Effect of bonding time on microstructure and mechanical properties of diffusion brazed IN-939

The applicability of the diffusion brazing technique for joining IN-939 superalloy using a Ni-Cr-B interlayer was assessed. The phases formed during the brazing process in the isothermally solidified zone (ISZ), athermally solidified zone (ASZ) and diffusion affected zone (DAZ) were studied. An inverse relationship between the bonding time and the width of the ASZ was perceived. The analytical model predicted the holding time of ∼36 min for completion of the isothermal solidification. This was corroborated by the experimental observations. The solid-state diffusion found to be a more effective transportation phenomenon than the base metal dissolution in increasing the concentration of solute elements in the ISZ. The formation of Cr-rich boride precipitates in DAZ was determined as the main contributing factor to the high microhardness values in this region. The shear strength of the joints raised by increasing the bonding time and reached the value of 655 MPa in the bonding time of 60 min implying a remarkable joint efficiency of ∼85%. Chromium was detected as the most influential element on the shear strength of the joints. Dimple-like and semi-cleavage morphologies were observed in the fracture surfaces of the bonds with complete and partial isothermal solidification, respectively.

Microstructure and oxidation behavior of the CrMoNbTaV high-entropy alloy

Abstract

Spark plasma sintered Ti6Al4V-ZrO2 bio-composites: optimization of ball milling and turbula mixing of powders and their subsequent sintering parameters

The use of Ti6Al4V as a biomaterial has been limited due to the dissociation of V and Al ions into the surrounding environment which can cause inflammations, cell damage and toxicity. To prohibit these undesirable challenges, the addition of ZrO2 into Ti6Al4V was proposed because ZrO2 is biocompatible, it has good wear and corrosion resistant properties, it has low affinity to bacterial colonization and it has high fracture toughness. The aim of this work is to determine the optimum powder synthesising parameters by Tubular mixer and ball mill for fabrication of sintered composites. Powder mixtures were prepared by 49 rpm, 72 rpm and 74rpm for 2, 4 and 8 hours followed by spark plasma sintering at 1100°C at heating rate of 100°C/min for 10 min and pressure of 50 MPa. from the results obtained it was discovered that Tubular mixing of 90 Vol% Ti6Al4V and 10 Vol% ZrO2 at 72 rpm and ball milling of 90 Vol% Ti6Al4V and 10 Vol% ZrO2 at 74rpm speed have proved to produce homogeneous admixed powders, which can further produce fully dense SPS compacts with higher microhardness values.

The influence of Nano-TiO2 and Nano-Al2O3 Particles in Silicate Based Electrolytes on Microstructure and Mechanical Properties of Micro Arc Coated Ti6Al4V Alloy

Ceramic oxide coatings were fabricated on Ti6Al4V alloy by micro arc oxidation (MAO) in silicate electrolytes, with nano-TiO2 and nano-Al2O3 and without nano-additives. Effects of nano additives on the structural and mechanical properties of the MAO coatings were analysed. The MAO coatings were investigated by scanning electron microscopy (SEM), Energy-Dispersive spectroscopy (EDS) micro-hardness and linear wear test. Results show that the surface morphology and tribological properties of MAO coatings are greatly influenced by nano additives in electrolytes. After the coating treatment the surface hardness values increased from 300 HV0.1 to 635 HV0.1. The hardness tests show that nano-Al2O3 additition coating has higher microhardness values than those without any additives. The results indicated that TiO2 and Al2O3 nanoparticles into the coatings make harder and denser surface and increase the wear resistance. The wear resistance of the nano-TiO2 and nano-Al2O3 additives coatings was enhanced about 5 times than without nano additives coating.

Microstructural and Mechanical Studies of T91/T22 Welded Joints

Comparative studies have been performed to decide an appropriate combination of welding process and filler material by virtue of microstructural evolution, micro-hardness studies, tensile strength and fractographic analysis. Manual arc welding and tungsten inert gas welding processes are used along with different filler materials to manufacture T91/T22 welded joints. Studies with the purpose of comparison and evaluation of different zones of the weldments have been carried out. The highest value of micro-hardness observed on the T91 HAZ of the weldments may be attributed to martensitic structure of the region. The fracture morphology of both the weldments obtained from T22 BM has revealed the ductile fracture. Comparatively higher tensile strength (578 MPa) of T91/T22, GTAW combination is noticed by virtue of lower heat input. The better performance of T91/T22, GTAW weldment can be quoted on the basis of better joint integrity, tensile strength and ductility (26.4%).

Microstructural features of friction stir welded dissimilar Aluminium alloys AA2219-AA7475

High strength, good corrosion resistance, light weight make aluminium alloys a material of choice in many industrial sectors like aerospace, marine etc. Problems associated with welding of these alloys by fusion welding processes restricted their use in various industries. Friction stir welding (FSW), a clean solid-state joining process, easily overcomes various difficulties encountered during conventional fusion welding processes. In the present work, the effect of rotational speed (710 rpm, 900 rpm and 1120 rpm) on micro-hardness distribution and microstructure of FSWed dissimilar aluminium alloy joints were analyzed. Plates of AA7475-T761 and AA2219-O having thickness of 2.5 mm were welded by fixing AA7475 on retreating side (RS) and AA2219 on advancing side (AS). Welded joints were characterized by Vickers micro-hardness testing, scanning electron microscopy (SEM) and optical microscopy (OM). Results revealed that rotational speed significantly affects the micro-hardness due to increase in grain size, coarsening and dissolution of strengthening precipitates and re-precipitation. Higher micro-hardness values were observed in stir zone due to grain refinement and re-precipitation. Minimum micro-hardness value was observed at the TMAZ/HAZ of advancing side due to thermal softening.

The Influence of Milling and Spark Plasma Sintering on the Microstructure and Properties of the Al7075 Alloy.

The compact samples of an Al7075 alloy were prepared by a combination of gas atomization, high energy milling, and spark plasma sintering. The predominantly cellular morphology observed in gas atomized powder particles was completely changed by mechanical milling. The continuous-like intermetallic phases present along intercellular boundaries were destroyed; nevertheless, a small amount of Mg(Zn,Cu,Al)2 phase was observed also in the milled powder. Milling resulted in a severe plastic deformation of the material and led to a reduction of grain size from several µm into the nanocrystalline region. The combination of these microstructural characteristics resulted in abnormally high microhardness values exceeding 300 HV. Consolidation through spark plasma sintering (SPS) resulted in bulk samples with negligible porosity. The heat exposition during SPS led to precipitation of intermetallic phases from the non-equilibrium microstructure of both gas atomized and milled powders. SPS of the milled powder resulted in a recrystallization of the severely deformed structure. An ultra-fine grained structure (grain size close to 500 nm) with grains divided primarily by high-angle boundaries was formed. A simultaneous release of stored deformation energy and an increase in the grain size caused a drop of microhardness to values close to 150 HV. This value was retained even after annealing at 425 °C.

Effect of the Laser Scan Rate on the Microstructure, Magnetic Properties, and Microhardness of Selective Laser-Melted FeSiB

FeSiB alloys have been fabricated by selective laser melting (SLM) using a laser scan rate of 0.1–1.5 m/s, laser power of 90 W, scan line spacing of 40 μ m, and layer thickness of 50 μ m. X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, atomic force microscopy, profilometer, microdurometer, and vibrating sample magnetometer have been used to investigate the structural, microstructural, roughness, microhardness, and magnetic property changes during the laser melting process. The produced samples exhibit a nanocomposite structure consisting of nanocrystalline α-Fe0.95Si0.05 and Fe2B phases embedded into an amorphous e-FeSi-type matrix. The selective laser-melted samples show high density, low surface roughness, higher microhardness values (1654–2273 Hv), and soft magnetic properties (Ms = 188.6–199 emu/g and Hc = 43.8–73 Oe).

Thermal Stability of Microstructure and Microhardness of Heterophase BCC-Alloys After Torsional Deformation on Bridgman Anvils

The results of investigations of thermal stability of microstructure and microhardness of alloys of the V–4Ti–4Cr and Mo–47Re systems, subjected to torsional deformation by high quasi-hydrostatic pressure at room temperature, are reported. It is shown that submicrocrystalline and nanocrystalline states, and the respective high values of microhardness, persist up to the upper bound (~0.4 Тmelt) of the temperature interval of their recovery and polygonization in a single-phase state. The main factors ensuring thermal stability of highlydefective states in heterophase alloys are discussed.

Fabrication of Ti-25Ni-25W Alloy by Mechanical Alloying and Spark Plasma Sintering for Biomedical Applications

This research work is carried out to successfully manufacture a modern range of β-type Ti-25Ni-25W alloy by mechanical alloying and spark plasma sintering (SPS) approach. In this research work, the impact of pressure and sintering temperature on the mechanical properties and microstructure of the sintered alloy was studied. The field-emission scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to inspect the surface topography and elemental composition of the sintered Ti-25Ni-25W alloy. The Vickers hardness tester was used to calculate the micro-hardness of the manufactured alloy. Meanwhile the sintering process of Ti-25Ni-25W alloy, it is detected that the complicated reactions between the W and alloying elements takes place. Biocompatible phases (W2C0.84, Ni2Ti, and Ni3C) are developed with the help of these complicated reactions. These biocompatible phases are favorable for formation of apatite and also enhances the bioactivity of the alloy for osseointegration. The maximum value of micro-hardness shown by manufactured Ti-25Ni-25W alloy is 617 HV. The manufactured Ti-25Ni-25W alloy exhibits very high micro-hardness value as compared to the alloys described in literature. The observations are drawn from the current research work is that the β-type Ti alloy (Ti-25Ni-25W) is recommended as a material for biomedical applications

Effect of acidic pH on microhardness and microstructure of theraCal LC, endosequence, mineral trioxide aggregate, and biodentine when used as root repair material.

Introduction:The aim of this study was to investigate the microhardness and microstructural features of newer tricalcium silicate materials: TheraCal LC, mineral trioxide aggregate (MTA), biodentine (BD), and Endosequence Root Repair Material (ERRM) putty, after exposure to acidic environments in comparison with distilled water.Materials and Methods:A total of 80 extracted single-rooted premolars were collected. All the selected specimens were sectioned vertically, and cavities were prepared on the root surface. Specimens were divided into four groups of 20 each, i.e., Group 1: (n = 15) MTA (ProRoot, Dentsply Tulsa Dental, Tulsa, OK, USA), Group 2: (n = 15) BD (Septodont, France), Group 3: (n = 15) ERRM putty (Brasseler, USA), and Group 4: (n = 15) TheraCal LC (Bisco Inc Schaumburg). Materials were placed into prepared cavities. About 10 specimens per each group were exposed to butyric acid buffered at a pH level of 5.5 for 7 days at 37c, and 10 specimens from each group were exposed to distilled water serving as a control group. The surface microhardness was measured after exposure to either acid or distilled water. Scanning electron microscope was used to observe the internal microstructure morphology. Two-way analysis of variance was applied to evaluate the Knoop microhardness value (KHN).Results:Results showed that the microhardness values of the materials were significantly higher in the neutral environment of butyric acid at pH 7.4 when compared to those in the acidic condition of pH 5.4 for all groups (P < 0.001). TheraCal LC had higher microhardness values than BD, MTA, ERRM putty at 5.5 pH levels (P < 0.001).Conclusion:The microhardness values of TheraCal LC, BD, ERRM Putty, and MTA were reduced in an acidic environment, which resulted in these materials having more porous and less crystalline microstructures. TheraCal LC seems the most suitable material for application to an area of inflammation where a low pH value may exist.