• Journal of the Korean institute of surface engineering
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• v.47 no.5
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• pp.257-262
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• 2014

In this paper, the behavior of hydrogen embrittlement of 680MPa DP sheet steel according to hydrogen charging conditions in acid and alkali electrolytes atmosphere was investigated. At this time, 0.5 M $H_2SO_4$ and 0.5M NaOH was used for electrolytes atmosphere and the effect on embrittlemnet of 680MPa DP sheet steel according to current density and charging time was evaluated by the change of subsurface microhardness in DP specimens chared hydrogen. As a result of this experiment, the microhardness of the layer directly below the surface was increased more than the microhardness of the subsurface zone in both electrolytes cases, but the change of the subsurface microhardness in both electrolytes was more affected by the increase of charging time than the increase of current density. The microhardness of subsurface zone in 0.5 M $H_2SO_4$ acid electrolyte was increased more than the microhardness in 0.5M NaOH alkali electrolyte. It was supposed that acid atmosphere was more sensitive to hydrogen embrittlement than alkali atmosphere on electrolyte atmosphere of hydrogen charge.

• Restorative Dentistry and Endodontics
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• v.41 no.1
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• pp.12-21
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• 2016

Objectives: To evaluate the effects of three acids on the microhardness of set mineral trioxide aggregate (MTA) and root dentin, and cytotoxicity on murine macrophage. Materials and Methods: OrthoMTA (BioMTA) was mixed and packed into the human root dentin blocks of 1.5 mm diameter and 5 mm height. Four groups, each of ten roots, were exposed to 10% citric acid (CA), 5% glycolic acid (GA), 17% ethylenediaminetetraacetic acid (EDTA), and saline for five minutes after setting of the OrthoMTA. Vickers surface microhardness of set MTA and dentin was measured before and after exposure to solutions, and compared between groups using one-way ANOVA with Tukey test. The microhardness value of each group was analyzed using student t test. Acid-treated OrthoMTA and dentin was examined by scanning electron microscope (SEM). Cell viability of tested solutions was assessed using WST-8 assay and murine macrophage. Results: Three test solutions reduced microhardness of dentin. 17% EDTA demonstrated severe dentinal erosion, significantly reduced the dentinal microhardness compared to 10% CA (p = 0.034) or 5% GA (p = 0.006). 10% CA or 5% GA significantly reduced the surface microhardness of set MTA compared to 17% EDTA and saline (p < 0.001). Acid-treated OrthoMTA demonstrated microporous structure with destruction of globular crystal. EDTA exhibited significantly more cellular toxicity than the other acidic solutions at diluted concentrations (0.2, 0.5, 1.0%). Conclusions: Tested acidic solutions reduced microhardness of root dentin. Five minute's application of 10% CA and 5% GA significantly reduced the microhardness of set OrthoMTA with lower cellular cytotoxicity compared to 17% EDTA.

• Journal of the korean academy of Pediatric Dentistry
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• v.24 no.1
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• pp.325-336
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• 1997

The purpose of this study was to evaluate the compensation effect of exposure duration increase to decreased light intensity of visible-light curing unit. The specimen with 2mm thickness was made of Restorative $Z-100^{TM}$ (A2 shade, 3M Dental Products, U.S.A.) and cured with $Optilux^{TM}$ (Demetron Research Co. U.S.A.). The light intensity was controlled to 420 $mW/cm^2$, 540 $mW/cm^2$, 630 $mW/cm^2$ and curing time, also, controlled to 40, 60, 80 seconds. Cured specimen was stored in a light-proof container for 24 hours to post-irradation was completed. Microhardness of top and bottom surface of specimen were measured to evaluate the depth of cure. The obtained results were as follows: 1. The microhardness of top and bottom surface of the composite resin specimen was increased significantly as light intensity and exposure time was increased (P<0.01). 2. Light intensity was more correlated with bottom microhardness(${\gamma}{\geq}$0.438) than top microhardness(${\gamma}{\geq}$0.213), and exposure time was more correlated with top microhardness (${\gamma}{\geq}$0.424) than bottom microhardness(${\gamma}{\geq}$0.335). 3. The regressive equation was obtained in this study as follows : $H=0.07{\times}D+0.012{\times}I+76$ (H : Microhardness(KHN), D : Exposure time, I : Light intensity)

• Journal of Periodontal and Implant Science
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• v.25 no.3
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• pp.614-622
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• 1995

Root caries is very frequently developed on exposed root surface after periodontal surgical treatment. In order to determine the anti-caries effect of Nd : YAG laser irradiation on periodontally exposed root surface, 40 mandibular molar teeth that had been extracted due to excessive periodontal destruction were used as the experimental teeth. All teeth were treated by the same procedure as conventional periodontal root treatment, ie thorough scaling, root planing and root conditioning with tetracycline HCl(100mg/ml, 5min.). Within middle one third of root, mesial half surface(20) or distal half surface(20) was randomly irradiated at various power of 1.0W, 2.0W, 3.0W and 4.0W for 60 seconds by non-contact(5mm) delivery of a pulsed Nd : YAG laser(EN.EL.EN060, Italy). The microhardness was measured by Vikers microhardness tester(Wilson, USA) at 2mm/second of jog speed under 100gm load. The difference of microhardness between irradiated side and non-irradiated side was statistically analyzed ANOVA and Duncan's method. Following results were obtained ; 1. The microhardness(Knoop hardness number) was significantly higher in laser irradiated surface than non-irradiated surface(p<0.05). 2. There was no significant difference in microhardness between experimental groups classified by different laser power(p>0.1). The results suggest that Nd : YAG laser irradiation on exposed root suface after periodontal therapy may inhibit the root caries development by enhancing surface microhardness.

• Restorative Dentistry and Endodontics
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• v.40 no.2
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• pp.104-112
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• 2015

Objectives: This study was performed to investigate the effects of different intracanal medicaments on chemical structure and microhardness of dentin. Materials and Methods: Fifty human dentin discs were obtained from intact third molars and randomly assigned into two control groups and three treatment groups. The first control group received no treatment. The second control group (no medicament group) was irrigated with sodium hypochlorite (NaOCl), stored in humid environment for four weeks and then irrigated with ethylenediaminetetraacetic acid (EDTA). The three treatment groups were irrigated with NaOCl, treated for four weeks with either 1 g/mL triple antibiotic paste (TAP), 1 mg/mL methylcellulose-based triple antibiotic paste (DTAP), or calcium hydroxide [$Ca(OH)_2$] and finally irrigated with EDTA. After treatment, one half of each dentin disc was subjected to Vickers microhardness (n = 10 per group) and the other half was used to evaluate the chemical structure (phosphate/amide I ratio) of treated dentin utilizing attenuated total reflection Fourier transform infrared spectroscopy (n = 5 per group). One-way ANOVA followed by Fisher's least significant difference were used for statistical analyses. Results: Dentin discs treated with different intracanal medicaments and those treated with NaOCl + EDTA showed significant reduction in microhardness (p < 0.0001) and phosphate/amide I ratio (p < 0.05) compared to no treatment control dentin. Furthermore, dentin discs treated with TAP had significantly lower microhardness (p < 0.0001) and phosphate/amide I ratio (p < 0.0001) compared to all other groups. Conclusions: The use of DTAP or $Ca(OH)_2$ medicaments during endodontic regeneration may cause significantly less microhardness reduction and superficial demineralization of dentin compared to the use of TAP.

• Journal of the korean academy of Pediatric Dentistry
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• v.37 no.4
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• pp.438-444
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• 2010

Giomer is a recently developed light-cured resin-based material. This study compared compressive strength and microhardness of composite resin, giomer and compomer after 5000 times of thermocycling at $5^{\circ}C$ and $55^{\circ}C$. The following results were obtained. 1. Composite resin resulted in the highest compressive strengths both before and after thermocycling, followed by giomer and compomer. There were statistically significant differences between composite resin and giomer/compomer (p<0.05), but no statistically significant differences between giomer and compomer. 2. Both before and after thermocycling, microhardness values appeared in the order of composite resin, giomer and compomer with statistically significant differences in microhardness of composite resin, giomer and compomer (p<0.05). 3. After thermocycling, microhardness of composite resin, giomer and compomer decreased with a statistically significant difference (p<0.05). In conclusion, giomer demonstrates higher microhardness than compomer, but lower compressive strength and microhardness than composite resin. In addition, the decrease in microhardness and compressive strength after thermocycling proves the necessity for a thorough understanding in mechanical properties of restoration materials prior to their clinical application.

• Journal of the korean academy of Pediatric Dentistry
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• v.47 no.3
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• pp.320-326
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• 2020

The aim of this study was to compare compressive strength and microhardness of recently introduced alkasite restorative materials with glass ionomer cement and flowable composite resin. For each material, 20 samples were prepared respectively for compressive strength and Vickers microhardness test. The compressive strength was measured with universal testing machine at crosshead speed of 1 mm/min. And microhardness was measured using Vickers Micro hardness testing machine under 500 g load and 10 seconds dwelling time at 1 hour, 1 day, 7 days, 14 days, 21 days and 35 days. The compressive strength was highest in composite resin, followed by alkasite, and glass ionomer cement. In microhardness test, composite resin, which had no change throughout experimental periods, showed highest microhardness in 1 hour, 1 day, and 7 days measurement. The glass ionomer cement showed increase in microhardness for 7 days and no difference was found with composite resin after 14 days measurement. For alkasite, maximum microhardness was measured on 14 days, but showed gradual decrease.

• Restorative Dentistry and Endodontics
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• v.25 no.1
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• pp.17-40
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• 2000

Resin cements are used for cementing indirect esthetic restorations such as resin or porcelain inlays. Because of its limitations in curing of purely light cured resin cements due to attenuation of the curing light by intervening materials, dual cured resin cements are recommended for cementing restorations. The physical properties of resin cements are greatly influenced by the extent to which a resin cures and the degree of cure is an important factor in the success of the inlay. The purpose of this study was to evaluate the influence of porcelain thickness and exposure time on the polymerization of resin cements by measuring the microhardness and the degree of conversion, to investigate the nature of the correlation between two methods mentioned above, and to determine the exposure time needed to harden resin cements through various thickness of porcelain. The degree of resin cure was evaluated by the measurements of microhardness [Vickers Hardness Number(VHN)] and degree of conversion(DC), as determined by Fourier Transform Infrared Spectroscopy(FTIR) on one light cured composite resin [Z-100(Z)] and three dual cured resin cements [Duo cement(D), 3M Resin cement(R), and Dual cement(DA)] which were cured under porcelain discs thickness of 0mm, 1mm, 2mm, 3mm with light exposure time of 40sec, 80sec, 120sec, and regression analysis was performed to determine the correlation between VHN and DC. In addition, to determine the exposure time needed to harden resin cements under various thickness of porcelain discs, the changes of the intensity of light attenuated by 1mm, 2mm, and 3mm thickness of porcelain discs were measured using the curing radiometer. The results were obtained as follows ; 1. The values of microhardness and the degree of conversion of resin cements without intervening porcelain discs were 31~109VHN and 51~63%, respectively. In the microhardness Z was the highest, followed by R, D, DA. In the degree of conversion, D and DA was significantly greater than Z and R(p<0.05). 2. The microhardness and the degree of conversion of the resin cements decreased with increasing thickness of porcelain discs, and increased with increasing exposure time, D and R showed great variation with inlay thickness and exposure time, whereas, DA showed a little variation. 3. The intensity of light through 1mm, 2mm, and 3mm porcelain inlays decreased by 0.43, 0.25, and 0.14 times compared to direct illumination, and the respective needed exposure times are 53 sec, 70 sec, and 93 sec. In D and R, 40 sec of light irradiation through 2mm porcelain disc and 80 sec of light irradiation through 3mm porcelain disc were not enough to complete curing. 4. The microhardness and the degree of conversion of the resin cements showed a positive correlationship(R=0.791~0.965) in the order of R, D, Z, DA. As the thickness of porcelain discs increased, the decreasing pattern of microhardness was different from that of the degree of conversion, however.

• Journal of Technologic Dentistry
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• v.33 no.2
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• pp.147-156
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• 2011

Purpose: The author performed experiments on the microhardness and the marginal fitness of composite resins after polymerizing 4 kinds of composite resins on MOD standard specimens. Methods: For this study, in order to compare the microhardness and the marginal fitness of 4 kinds of composite resins. Results: The results are as follows. 1. In case of the microhardness, Tescera scored the highest among the four kinds of composite resins. 2. In case of the marginal fitness, Premise Indirect scored the highest. However, there was no statistically significant difference, all staying in clinically practical range. 3. Considering the above results, among the four kinds of composite resins, the author advises to use Tescera during teeth restoration which scored the highest in the micro- hardness. Conclusion: Conclusion: All of the composite marginal fitness were within the clinically useful range.

• The Journal of the Korea Contents Association
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• v.13 no.12
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• pp.893-899
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• 2013

This study was to determine the efficacy in the remineralization on premary teeth following the application of toothpaste with and without sodium fluoride. All tooth were checked using microhardness tester and the calcium (Ca) and phosphorous (P) concentration by electron probe microanalyzer (EPMA) analysis for the 4 weeks. The collected data was analysed using the statistical software program SPSS, 20.0. The applied enamel of premary teeth with 0.243% fluoridated toothpaste resulted in significant differences in the microhardness values (p>0.05). The Ca and P concentrations of applied premary teeth with fluoridated toothpaste were also significant difference in norm wt% (p>0.05). As a results, a increasing the surface microhardness of the premary teeth and providing a remineralizing effect by increased Ca and P concentrations.