• Journal of Oral Medicine and Pain
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• v.21 no.2
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• pp.207-224
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• 1996

This study was designed to determine the most effective concentration of fluoride and energy density of laser irradiation for the anticarious effect. For this study surface hardness in enamel was measured before and after irradiation with pulsed Nd;YAG laser and the topical application of fluoride. Of the permanent mandibular anterior hovine teeth, healthy, carious free ones were used. Three hundred specimens were made. Specimens within 25~45 Vickers hardness numbers were assigned to 20 control and experimental stoops ; each containing 15 specimens. After forming artificial carious lesions, 10 J/$\textrm{cm}^2$, 20 J/$\textrm{cm}^2$, and 30 J/$\textrm{cm}^2$ energies were irradiated on the enamel surface of each experimental group. Also NaF, NH4F, Elmex gel(amine fluoride) and APF gel fluoride compounds were applied topically. Next, all the specimens were placed into the pH circulatory procedures for eight days. Vickers hardness numbers were measured using a microhardness tester. Surface changes of the enamel were observed using an scanning electron microscope. The comparative ana1ysis yielded the following results : 1. The reduction of surface hardness of the enamel surface was less in all groups with fluoride application than in the group without fluoride application. 2. The APF gel croup with 10 J/$\textrm{cm}^2$ irradiation showed the lowest reduction of surface hardness. 3. The reduction of surface hardness of the enamel surface in the group of laser irradiation without fluoride application not showed any significant difference according to the energy density of the laser. 4. Under the scanning electron microscope, in enamel irradiated with 10J/$\textrm{cm}^2$ showed appearance similar to acid etching surface. In enamel irradiated with 20 J/$\textrm{cm}^2$, line enamel crack was detected. In enamel irradiated with 30j/$\textrm{cm}^2$, severe enamel crack and fusion of enamel were detected. These results suggest that one could obtain the best anticariogenic effects without damage to teeth in the group of application of APF gel after laser irradiation with 10 J/$\textrm{cm}^2$.

• Journal of Technologic Dentistry
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• v.31 no.4
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• pp.9-15
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• 2009

The purpose of this study was to observe the change of mechanical property and microstructure in porcelain fused to gold alloy by heat treatment. PFG alloys are composed with Au-Pd-Ag alloy of the additional elements with indium, tin and copper. Specimens were tested in hardness using vicker,s micro-hardness tester and the surface micro structural changes were analysed by SEM and EDS. The results were as fellows: 1. The vickers hardness showed highest in Au-Pd-Ag alloy of the additional element with tin. 2. By hardening-oxiding result, the vicker,s hardness increased in additional element with tin but there was no significant difference in additional elements with indium and copper. 3. The surface oxide layer of Au-Pd-Ag alloy with added indium and tin increased but there was small change in additional element with copper. 4. The elements of indium and tin increased with increasing heat treatment in the surface alloy.

• Advances in materials Research
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• v.7 no.1
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• pp.17-28
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• 2018

Niobium is a candidate base for new alloys devoted to applications at especially elevated temperatures. Elaborating and shaping niobium-based alloys by conventional foundry may lead to mechanically interesting microstructures. In this work a series of charges constituted of pure elements were subjected to high frequency induction melting in cold crucible to try obtaining cast highly refractory Nb-xCr and Nb-xCr-0.4 wt.%Calloys(x=27, 34 and 37 wt.%). Melting and solidification were successfully achieved. The as-cast microstructures of the obtained alloys were characterized by electron microscopy and X-ray diffraction and their hardness were specified by Vickers macro-indentation. The obtained as-cast microstructures are composed of a body centered cubic (bcc) niobium dendritic matrix and of an interdendritic eutectic compound involving the bcc Nb phase and a $NbCr_2$ Laves phase. The obtained alloys are hard to cut and particularly brittle at room temperature. Hardness is of a high level (higher than 600Hv) and is directly driven by the chromium content or the amount of {bcc Nb - $NbCr_2$} eutectic compound. Adding 0.4 wt.% of carbon did not lead to carbides but tends to increase hardness.

• Journal of Technologic Dentistry
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• v.25 no.1
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• pp.63-70
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• 2003

The microstructure and hardness of a porcelain fused Au-Pt-Cu-In alloy was investigated using optical microscopy, secondary electron microscopy, electron probe microanalyzer, transmission electron microscope, and vickers hardness. The hardness of the heat-treated Au-Pt-Cu-In quartenary alloy reached a maximum value in 30 min at 550$^{\circ}C$ in the range of 150 to 950$^{\circ}C$. In the aged Au-Pt-Cu-0.5In alloy at 550$^{\circ}C$, the hardness of the alloy rapidly increased until 30min with increasing aging time and after that it was remained nearly constant value. Based on above results, glazing and final aging of the porcelain fused Au-Pt-Cu-0.5In alloy were performed at 920 and 550$^{\circ}C$, respectively. The hardness of Au-Pt-Cu-0.5In alloy glazed at 920$^{\circ}C$ was 90 Hv and that of the alloy aged for 30 min at 550$^{\circ}C$ increased to 160 Hv. This indicates that a ceramic-metal crown with high strength can be manufactured using the glazing at 920$^{\circ}C$ and followed final aging at 550$^{\circ}C$ for 30 min.

• Journal of the Korean institute of surface engineering
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• v.38 no.4
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• pp.156-162
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• 2005

Nickel-iron nanocrystalline alloys with different compositions and grain sizes were fabricated by electro-plating for MEMS devices. The iron content of the deposits was changed by varying the nickel/iron ion ratio in the electrolyte. X-ray diffraction (XRD) analysis was applied for measuring the strength of the texture and grain size of the deposits. The nickel/iron atom ratio of the deposits was analyzed by EDS. The hardness of the alloys was evaluated by Vickers hardness indenter. The internal stress of the deposits was measured by Thin Film Stress Measurement using Stoney's formula. Surface morphology and roughness were investigated by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The results of this study revealed that at a grain size of approximately $17\~24$nm the hardness, internal stress and roughness depend strongly on the iron content. With increasing the iron content, the hardness and internal stress of the deposits increased. An excellent correlation between the increase in the internal stress and the loss of (200) texture was found.

• Korean Journal of Materials Research
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• v.13 no.9
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• pp.606-612
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• 2003

Surface and mechanical properties of thin films with submicron thickness was characterized by nanoindentation with Berkovich and Vickers tips, and scanning probe microscopy. Nanoindention was made in a depth range of 15 to 200 nm from the surface by applying tiny force in a range from 150 to $9,000 \mu$N. Stiffness, contact area, hardness, and elastic modulus were determined from the force-displacement curve obtained. Reliability was first tested by using fused quartz, a standard sample. Elastic modulus and hardness values of fused quartz measured were the same as those reported in the literature within two percent of error. Mechanical properties of ITO thin film were characterized in a depth range of 15∼200nm. As indentation depth increased, elastic modulus and hardness decreased by substrate effect. Ion beam deposited DLC thin films were indented in a depth range of 40∼50 nm. The results showed that the DLC thin film using benzene and bias voltage 0∼-50 V has elastic modulus and hardness value of 132 and 18 GPa respectively. Pure DLC thin films showed roughnesses lower than 0.25 nm, but silicon-added DLC thin films showed much higher roughness values, and the wavy surface morphology.

• Journal of the Korean institute of surface engineering
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• v.43 no.2
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• pp.86-90
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• 2010

In this study, we conducted heat treatment on 14K, 18K yellow gold alloy at various temperature conditions for improving their hardness and moderating their surface defects. Also after the heat treatment we used EPMA (Electron Probe Micro Analyzer), XRF (x-ray Fluorescence spectroscopy) for qualitative analysis and OM (optical microscope), SEM (scanning electron microscope) to investigate the changes of surface grain boundary. We used Vickers hardness tester to verify the changes of hardness. After the heat treatment, 14K, 18K gold alloys showed improved hardness and moderated surface defects at specific temperatures and duration.

• Journal of the Korean Society for Heat Treatment
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• v.36 no.4
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• pp.206-214
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• 2023

Tungsten carbide has many industrial applications due to its high electrical and thermal conductivity, high melting temperature, high hardness and good chemical stability. Because tungsten carbide is difficult to sinter, it is sintered with nickel or cobalt as a binder and is currently used in nozzles, cutting tools, and molds. Alumina is reported to be a viable binder for tungsten carbide due to its higher oxidation resistance and lower cost than nickel and cobalt. The ultrafine tungsten carbide-graphene-alumina composites were rapidly sintered in a high frequency induction heating active sintering unit. The microstructure and mechanical properties (fracture toughness and hardness) of the composites were investigated and analyzed by Vickers hardness tester and electron microscope. Since the high-frequency induction heating sintering method enables high-speed sintering, ultrafine composites can be prepared by preventing grain growth. In the tungsten carbide-graphene-alumina composites, the grain size of tungsten carbide increased with the amount of alumina participation. The hardness and fracture toughness of the tungsten carbide-5% graphene- x% alumina (x = 0, 5, 10,15) composites were 5.1, 8.6, 8.6, and 8.4 MPa-m^1/2 and 2384, 2168, 2165, and 2102 kg/mm², respectively. The fracture toughness increased without a significant decrease in hardness. Sinterability was improved by adding alumina to tungsten carbide-graphene.

• Korean Journal of Materials Research
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• v.33 no.7
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• pp.309-314
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• 2023

This study aimed to address the limitations of traditional plasma nitriding methods by implementing a short-term plasma oxy-nitriding treatment on the surface of AISI 420 martensitic stainless steel. This treatment involved the sequential formation of nitride and oxide layers, to enhance surface hardness and corrosion resistance, respectively. The process resulted in the formation of a 20 ㎛-thick nitride layer and a 3 ㎛-thick oxide layer on the steel surface. Initially, the hardness increased by 2.2 times after nitriding, followed by a subsequent decrease of approximately 31 % after oxidation. While the nitriding process reduced corrosion resistance, the subsequent oxidation process led to the formation of a passive oxide film, effectively resolving this issue. The pitting corrosion of the oxide passive film started at 82.6 mVssc, providing better corrosion resistance characteristics than the nitride layer. Consequently, the trade-off between surface hardness and corrosion resistance in plasma oxy-nitrided AISI 420 martensitic stainless steel is anticipated to be recognized as an innovative and comprehensive surface treatment process for biomedical components.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1503-1512
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• 2013

In this study, we proposed an indentation evaluation method for fracture toughness using cohesive finite element simulations. First, we examined the effect of material properties (yield strain, Poisson's ratio, and elastic modulus) on crack size during Vickers indentation and then generated a regression formula that explains the relations among fracture toughness, indentation load, and crack size. We also proposed another indentation formula for fracture toughness evaluation using the contact size a and E/H (H: hardness). Finally, we examined the relation between the crack size and the indenter shapes. Based on this, we can generate from the formula obtained using the Vickers indenter a formula for an indenter of different shapes. Using the proposed method, fracture toughness is directly estimated from indentation data.