• Restorative Dentistry and Endodontics
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• 제46권4호
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• pp.49.1-49.8
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• 2021

Objectives: To evaluate the fracture incidence of Reciproc R25 instruments (VDW) used during non-surgical root canal retreatments performed by students in a postgraduate endodontic program. Materials and Methods: From the analysis of clinical record cards and periapical radiographs of root canal retreatments performed by postgraduate students using the Reciproc R25, a total of 1,016 teeth (2,544 root canals) were selected. The instruments were discarded after a single use. The general incidence of instrument fractures and its frequency was analyzed considering the group of teeth and the root thirds where the fractures occurred. Statistical analysis was performed using the χ² test (p < 0.01). Results: Seven instruments were separated during the procedures. The percentage of fracture in relation to the number of instrumented canals was 0.27% and 0.68% in relation to the number of instrumented teeth. Four fractures occurred in maxillary molars, 1 in a mandibular molar, 1 in a mandibular premolar and 1 in a maxillary incisor. A greater number of fractures was observed in molars when compared with the number of fractures observed in the other dental groups (p < 0.01). Considering all of the instrument fractures, 71.43% were located in the apical third and 28.57% in the middle third (p < 0.01). One instrument fragment was removed, one bypassed, while in 5 cases, the instrument fragment remained inside the root canal. Conclusions: The use of Reciproc R25 instruments in root canal retreatments carried out by postgraduate students was associated with a low incidence of fractures.

• Restorative Dentistry and Endodontics
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• 제46권2호
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• pp.16.1-16.11
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• 2021

Objectives: The aim of this study was to evaluate the shaping ability of the TruShape and Reciproc Blue systems and the apical extrusion of debris after root canal instrumentation. The ProTaper Universal system was used as a reference for comparison. Materials and Methods: Thirty-three mandibular premolars with a single canal were scanned using micro-computed tomography and were matched into 3 groups (n = 11) according to the instrumentation system: TruShape, Reciproc Blue and ProTaper Universal. The teeth were accessed and mounted in an apparatus with agarose gel, which simulated apical resistance provided by the periapical tissue and enabled the collection of apically extruded debris. During root canal preparation, 2.5% sodium hypochlorite was used as an irrigant. The samples were scanned again after instrumentation. The percentage of unprepared area, removed dentin, and volume of apically extruded debris were analyzed. The data were analyzed using 1-way analysis of variance and the Tukey test for multiple comparisons at a 5% significance level. Results: No significant differences in the percentage of unprepared area were observed among the systems (p > 0.05). ProTaper Universal presented a higher percentage of dentin removal than the TruShape and Reciproc Blue systems (p < 0.05). The systems produced similar volumes of apically extruded debris (p > 0.05). Conclusions: All systems caused apically extruded debris, without any significant differences among them. TruShape, Reciproc Blue, and ProTaper Universal presented similar percentages of unprepared area after root canal instrumentation; however, ProTaper Universal was associated with higher dentin removal than the other systems.

• Restorative Dentistry and Endodontics
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• 제45권4호
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• pp.51.1-51.10
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• 2020

Objectives: No studies have yet assessed vibration characteristics according to endodontic file length. Accordingly, the objective of the present study was to examine the vibration characteristics according to nickel-titanium file length and to compare these characteristics between different file systems. Materials and Methods: A total of 45 root canal models were divided into 3 experimental groups (n = 15 each) based on the file system used (ProTaper Gold [PTG], ProTaper Next, or WaveOne Gold [WOG]). Each experimental group was further divided into 3 subgroups according to file length (21, 25, or 31 mm). An electric motor (X-SMART PLUS) was used in the experiment. For each file system, vibrations generated when using a size 25 file were measured and used to calculate the average vibration acceleration. The differences in vibrations were analyzed using 1-way analysis of variance and the Scheffé post hoc test with a confidence interval of 95%. Results: In the PTG file system, significantly lower vibration acceleration was observed when using a 21-mm file than when using a 31-mm file. In the WOG file system, significantly stronger vibration acceleration was observed when using a 31-mm file than when using 21- or 25-mm files. Regardless of the file length, the WOG group exhibited significantly stronger vibration acceleration than the other 2 experimental groups. Conclusions: In clinical practice, choosing a file with the shortest length possible could help reduce vibrations. Additionally, consideration should be given to vibrations that could be generated when using WOG files with reciprocating motion.

• Wind and Structures
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• 제37권4호
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• pp.289-302
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• 2023

Insulating glass units (IGUs) have been widely used in buildings in recent years due to their superior thermal insulation performance. However, because of the panel reciprocating motion and fatigue deterioration of sealants under long-term wind loads, many IGUs have the problem of early failure of watertight properties in real usage. This study aimed to propose a statistical method for wind-induced deflection of IGU panels during the whole life service period, for further precise analysis of the accumulated fatigue damage at the sealed part of the edge bond. By the estimation of the wind occurrence regularity based on wind pressure return period, the events of each wind speed interval during the whole life were obtained for the IGUs at 50m height in Beijing, which are in good agreement with the measured data. Also, the wind-induced deflection analysis method of IGUs based on the formula of airspace coefficient was proposed and verified as an improvement of the original stiffness distribution method with the average relative error compared to the test being about 3% or less. Combining the two methods above, the deformation of the outer and inner panes under wind loads during 30 years was precisely calculated, and the deflection and stress state at selected locations were obtained finally. The results show that the compression displacement at the secondary sealant under the maximum wind pressure is close to 0.3mm (strain 2.5%), and the IGUs are in tens of thousands of times the low amplitude tensile-compression cycle and several times to dozens of times the relatively high amplitude tensile-compression cycle environment. The approach proposed in this paper provides a basis for subsequent studies on the durability of IGUs and the wind-resistant behaviors of curtain wall structures.

• 한국산학기술학회논문지
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• 제21권6호
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• pp.204-211
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• 2020

경질 크롬 도금은 도금과정에서 유독성 물질인 Cr⁺⁶을 배출시키기 때문에 환경오염과 폐암을 유발하고 있어 최근 고속화염 용사법 (HVOF)을 이용한 코팅 방법이 대체 방법으로 각광받고 있다. 본 연구에서는 HVOF 방법을 이용하여 경질 크롬 도금 방법을 대체하기 위한 WC-CoCr과 WC-CrC-Ni 서멧 코팅을 제조하였으며, 제조된 코팅층의 물리적/화학적 특성인 미세경도, 기공도, 결정상 및 미세구조를 경도기, 광학현미경, X-선 회절 (XRD), 주사전자현미경(SEM) 및 EDS를 이용하여 분석하였다. 코팅층의 마모 및 마찰 거동 분석을 위해 왕복 슬라이딩 마모 테스트 방법을 이용하여 25 ℃, 250 ℃, 450 ℃의 온도에서 실시하였으며, 두 코팅층의 마모/마찰 특성을 비교 평가하여, 코팅층의 내마모 성능이 미세구조와 금속기지 결합제 간의 연관성이 있음을 확인하였다. 결과적으로 미세구조가 균일하고, 기공도가 낮을수록 내마모성이 향상되고, 금속기지 결합제가 많을수록 우수한 내마모성을 나타내는 것으로 확인되었다. 즉, 균일한 미세구조와 과량의 Co, Cr 금속기지 결합제, 낮은 기공도로 인해 온도와 테스트 온도와 관계없이 WC-CoCr 코팅이 WC-CrC-Ni 코팅에 비해 내마모성이 우수한 것으로 나타났다.

• Tribology and Lubricants
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• 제31권3호
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• pp.95-101
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• 2015

Carbon nanotubes (CNTs) are widely used in polymer composites as filler materials to enhance various characteristics of the composites because of their remarkable mechanical, electrical, and thermal properties. In this study, we investigate the effects of MWCNTs on the electrical and wear characteristics of high-impact polystyrene (HIPS) composites, and compare the results with the effects of carbon black (CB). The HIPS composites are classified as Bare-HIPS, MWCNT-HIPS composites containing 2, 3, 4, and 5 wt% MWCNTs, and CB-HIPS containing 17 wt% CB. Electrical characteristics are evaluated by measuring the surface resistance using a 4-point probe. Wear characteristics are evaluated using the reciprocating wear test, and a chrome steel ball with a curvature of 6.3 mm is used as the counterpart. The results show that the addition of MWCNTs or CB can improve the electrical and wear characteristics of HIPS composites. In the case of MWCNT-HIPS composites, surface resistance, friction coefficient, and specific wear rate decrease as the concentrations of MWCNTs increase. Moreover, the addition of MWCNTs is more effective in improving the electrical and wear characteristics of HIPS composites compared to the addition of CB. To fabricate the HIPS composite with appropriate electrical and wear characteristics, more than 4 wt% MWCNTs is added to HIPS.

• 열처리공학회지
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• 제25권1호
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• pp.6-13
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• 2012

HVOF thermal spray coating of 80%WC-CoFe powder is one of the most promising candidate for the replacement of the traditional hard chrome plating and hard ceramics coating because of the environmental problem of the very toxic $Cr^{6+}$ known as carcinogen by chrome plating and the brittleness of ceramics coatings. 80%WC-CoFe powder was coated by HVOF thermal spraying for the study of durability improvement of the high speed spindle such as air bearing spindle. The coating procedure was designed by the Taguchi program, including 4 parameters of hydrogen and oxygen flow rates, powder feed rate and spray distance. The surface properties of the 80%WC-CoFe powder coating were investigated roughness, hardness and porosity. The optimal condition for thermal spray has been ensured by the relationship between the spary parameters and the hardness of the coatings. The optimal coating process obtained by Taguchi program is the process of oxygen flow rate 34 FRM, hydrogen flow rate 57 FRM, powder feed rate 35 g/min and spray distance 8 inch. The coating cross-sectional structure was observed scanning electron microscope before chemical etching. Estimation of coating porosity was performed using metallugical image analysis. The Friction and wear behaviors of HVOF WC-CoFe coating prepared by OCP are investigated by reciprocating sliding wear test at $25^{\circ}C$ and $450^{\circ}C$. Friction coefficients (FC) of coating decreases as sliding surface temperature increases from $25^{\circ}C$ to $450^{\circ}C$.

• 대한기계학회논문집A
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• 제37권11호
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• pp.1349-1354
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• 2013

고온의 가스유로를 조절하는 개폐식(On-Off)밸브의 구동축과 라이너 사이에 위치하여 구동장치로의 고온 가스유입을 방지하는 기밀 소재에 대하여 연구하였다. 기밀 소재로 사용된 그라파이트(Graphite)는 구동장치에 의한 구동축의 지속적인 움직임으로 인해 마모가 발생한다. 본 논문은 고온가스 조절 밸브 내의 기밀 소재로 사용된 그라파이트(Graphite, HK-6)에 대한 상 고온 마모 특성을 평가하였다. 구동축의 소재인 레늄합금(W-25Re)에 대하여 그라파이트의 마모 시험을 수행하고 마찰계수와 비마모율을 비교하였다. 상온과 실제 작동 환경인 고온 $485^{\circ}C$에서 미끄럼속도와 접촉하중의 변화에 따른 마찰계수와 비마모율을 평가하였다. 마모표면의 SEM 분석을 통하여 상 고온 마모표면에 관찰되는 third body 를 확인하고 이로 인한 윤활효과를 고찰하였다.

• 대한금속재료학회지
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• 제46권6호
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• pp.351-356
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• 2008

Thermally sprayed tungsten carbide-based powder coatings are being widely used for a variety of wear resistance applications. The coating deposited by high velocity processes such as high velocity oxy-fuel (HVOF) thermal spraying is known to provide improved wear resistant property. In this study, optimal coating process (OCP) is obtained by the study of coating properties such as surface hardness, porosity, surface roughness and microstructure of 9 coatings prepared by Taguchi program for 3 levels of four spray parameters. The Friction and wear behaviors of HVOF WC-CoCr coating prepared by OCP, electrolytic hard chrome (EHC) plating and Inconel718 (In718) are investigated by reciprocating sliding wear test at $25^{\circ}C$, $450^{\circ}C$. Friction coefficients (FC) of all of the 3 samples are decreased as increasing sliding surface temperature from $25^{\circ}C$ to $450^{\circ}C$. FC of WC-CoCr decreases as increasing the surface temperature from $0.33{\pm}0.02$ at $25^{\circ}C$ to $0.26{\pm}0.02$ at $450^{\circ}C$, showing the lowest FC among the 3 samples. Wear trace (WT) and wear depth (WD) of WC-CoCr are smaller than those of EHC and In718 both at $25^{\circ}C$ and $450^{\circ}C$. These show that WC-CoCr is highly recommendable for protective coating on In718 and other metal components.

• 구강회복응용과학지
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• 제28권4호
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• pp.371-383
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• 2012

성공적인 근관치료를 위하여 원래 근관 형태를 따르는 근관 성형 과정이 중요하다. 그러나 대부분의 근관은 만곡되어 있고, 근관 성형 기구는 직선의 금속을 기본으로 만들어지기 때문에 근관 성형 후 만곡 근관이 원래 근관 형태를 벗어나는 오류가 나타날 수 있다. 이런 오류들로 인하여 근관 치료가 실패할 수 있으므로 근관 성형시 원래 만곡형태를 유지할 수 있는 다양한 기술과 방법들이 소개되고 있다. 본 연구의 목적은 만곡 근관 형태의 레진 모형을 이용하여 기구의 형태와 사용방법이 다른 세 가지 Ni-Ti 파일을 사용하여 Ni-Ti 파일 종류에 따른 만곡 근관에 대한 성형 후 형태 유지능을 비교하는 것이다. 직경과 경사도가 ISO 15, 0.02인 30개의 레진모형을 사용하였다. 그룹 1은 Profile을 이용하여 #20/.06까지 근관 성형하였고, 그룹 2는 Mtwo를 이용하여 #35/.04까지 근관 성형하였으며, 그룹 3은 PathFile을 이용하여 glide path를 만들어준 다음 WaveOne 파일 중 Primary을 사용하여 근관 성형하였다. 기구 사용 전과 후의 레진모형을 스캔하여 디지털 이미지로 만들었다. 각 이미지를 중첩시킨 다음 Matlab r2010b 소프트웨어를 이용하여 곡률반경의 비(CRr)를 계산하였다. 이를 바탕으로 Ni-Ti file 간 성형 후 형태 유지능을 비교하기 위하여 1-way ANOVA를 이용하여 세 그룹의 CRr의 차이에 대한 유의성 검정을 시행하였다.(P<.05) 또한 continuous rotary system과 single file system간의 형태 유지능을 비교하기 위하여 independent t-test를 이용하였다.(P<.05) 곡률반경의 비(CRr)를 이용하여 Ni-Ti 파일 종류에 따른 만곡 근관의 형태 유지능을 비교한 결과 Ni-Ti file 간에는 통계적으로 유의한 차이가 있었다.(P<0.05) ProFile과 WaveOne은 Mtwo에 비해서 만곡 근관에서의 형태 유지능이 우수하게 나타났다.Ni-Ti 기구 사용 방법에 따른 만곡 근관 형태 유지능을 비교한 결과 continuous rotary system과 single file system 간에는 통계적으로 유의한 차이를 보이지 않았다.(P>0.05) 본 연구결과에 따르면, ProFile과 WaveOne은 Mtwo에 비하여 만곡 근관에서의 형태 유지능이 통계적으로 유의하게 높게 나타났으므로, ProFile과 WaveOne이 만곡근관에 대한 형태 유지능이 우수하다고 할 수 있다. 만곡근관에서의 형태 유지능에 있어서 continuous rotary system과 single file system은 만곡근관에서의 형태 유지능에 차이가 없다.