• Journal of the Korean Vacuum Society
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• v.13 no.4
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• pp.175-181
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• 2004

We investigated the stability of the DLC film coated on 304 stainless steel substrate by Radio frequency assisted chemical vapor deposition method. Fracture and spallation behaviour of the coating was observed during micro-tensile test of the fil $m_strate composite. As the tensile deformation progressed, the cracks of the film were observed in the perpendicular direction to the tensile axis. Further deformation resulted in the plastic deformation with $45^{\circ}$ slip bands on the substrate surface. Spallation of the film occurred with the plastic deformation, which was initiated at the cracks of the film and was aligned along the slip directions. We found that both the cracking and the spallation behaviors are strongly dependent on the pre-treatment condition, such as Ar plasma pre-treatment. The spallation of the film was considerably suppressed in an optimized condition of the substrate cleaning by Ar glow discharge. We observed the improved stability with increasing duration of Ar plasma pre-treatment.nt.

• The Journal of Advanced Prosthodontics
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• v.6 no.3
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• pp.165-170
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• 2014

PURPOSE. This study was aimed to evaluate effect of the desensitizing pretreatments on the micro-tensile bond strengths (${\mu}TBS$) to eroded dentin and sound dentin. MATERIALS AND METHODS. Forty-two extracted molars were prepared to form a flat dentin surface, and then they were divided into two groups. Group I was stored in distilled water while group II was subjected to a pH cycling. Each group was then subdivided into three subgroups according to desensitizing pretreatment used: a) pretreatment with desensitizer (Gluma); b) pretreatment with $CO_2$ Laser (Ultra Dream Pluse); c) without any pretreatment. All prepared surfaces were bonded with Single Bond 2 and built up with resin composite (Filtek Z250). The micro-tensile bond test was performed. Fracture modes were evaluated by stereomicroscopy. Pretreated surfaces and bonded interfaces were characterized by scanning electron microscope (SEM). The data obtained was analyzed by two-way ANOVA (${\alpha}$=0.05). RESULTS. For both sound and eroded dentin, samples treated with desensitizer showed the greatest ${\mu}TBS$, followed by samples without any treatment. And samples treated with $CO_2$ laser showed the lowest ${\mu}TBS$. SEM study indicated that teeth with eroded dentin appeared prone to debonding, as demonstrated by existence of large gaps between adhesive layers and dentin. CONCLUSION. Pretreatment with Gluma increased the ${\mu}TBS$ of Single Bond 2 for eroded and sound teeth. $CO_2$ laser irradiation weakened bond performance for sound teeth but had no effect on eroded teeth.

• Journal of the Korea Institute of Military Science and Technology
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• v.13 no.6
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• pp.1121-1126
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• 2010

Borosilicate glass (81% $SiO_2$-2% $Al_2O_3$-13% $B_2O_3$-4% $Na_2O_3$) was prepared, and the glass was ion exchanged in $KNO_3$ powder containing different temperature and time. The $K^+-Na^+$ ion exchange takes place at the glass surface and creates compressed stress, which raise the mechanical strength of the glass. The depth profile of $Na^+$ and $K^+$ was observed by electron probe micro analyzer. With the increasing heat-treatment time from 0min to 20min, the depth profile was increased from 17.1um to 29.4um, but mechanical properties were decreased. It was also found out that excessive heat treatment brings stress relaxation. The Vickers hardness, Fracture Toughness and bending strength of ion exchanged samples at $570^{\circ}C$ for 10min were $821.8H_v$, $1.3404MPa{\cdot}m^{1/2}$, and 953MPa, which is about 120%, 180%, and 450% higher than parent borosilicate glass, respectively. Transmittance was analyzed by UV-VIS-NIR spectrophotometer. Transmittance of ion exchanged borosilicate glass was decreased slightly at visible-range. It can be expected that transparent bulletproof materials in more light-weight and thinner by ion exchanged borosilicate glass.

• Bulletin of the Society of Naval Architects of Korea
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• v.9 no.2
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• pp.43-48
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• 1972

Inertia friction welding, a relatively recent innovation in the art of joining materials, is a forge-welding process that releases kinetic energy stored in the flywheel as frictional heat when two parts are rubbed together under the right conditions. In a comparatively short time, the process has become a reliable method for joining ferrous, and dissimilar metals. The process is based on thrusting one part, attached to a flywheel and rotating at a relatively high speed, against a stationary part. The contacting surfaces, heated to plastic temperatures, are forged together to produce a reliable, high-strength weld. Welds are made with little or no workpiece preparation and without filler metal or fluxes. However, In order to obtain a good weld, the determination of the optimum weld parameters is an important problem. Especially, because the amount of the flywheel mass will be determined according to the initial rotating velocity values at the constant thrust load, the initial rotating velocity is an important factor to affect a weld character of the inertia-welded IN713C-SAE8630, which is used for the wheel-shafts of turbine rotors or turbochargers, exhausting valves, etc. In this paper, the effects of initial rotational velocity on a weld character of inertia-welded IN713C-SAE8630 was studied through considerations of weld parameters determination, micro-structural observations and tensile tests. The results are as the following: 1) As initial rotating velocity was reduced to 267 FPM, cracks and carbide stringers were completely eliminated in the micro-structure of welded zone. 2) As initial rotating velocity was reduced and flywheel mass was increased correspondingly, the maximum welding temperatures were decreased and the plastic working in the weld zone was increased. 3) As initial rotating velocity was progressively decreased and carbides were decreased, the tensile strengths were increased. 4) And also the fracture location moved out of the weld zone and the tensile tests produced, the failures only in the cast superalloy IN713C which do not extend into the weld area. 5) The proper initial rotating velocity could be determined as about 250 thru 350 FPM for the better weld character.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.30 no.1
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• pp.7-12
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• 2017

3YSZ + (x) $Al_2O_3$ composites (x = 20, 40, 60, 80 wt%) were fabricated and the influences of particle sizes of $Al_2O_3$ on their microstructures and mechanical properties were investigated with XRD, SEM, vickers hardness and fracture toughness. $Al_2O_3$-3YSZ composites containing $Al_2O_3$ powder of a $0.3{\mu}m$ and an $1.0{\mu}m$, which are here in after named as $Al_2O_3$($0.3{\mu}m$)-3YSZ and $Al_2O_3$($1.0{\mu}m$)-3YSZ, respectively, were made by mixing raw materials, uni-axial pressing and sintering at $1,400^{\circ}C$, $1,500^{\circ}C$, and $1,600^{\circ}C$. $Al_2O_3$($0.3{\mu}m$)-3YSZ composites show the higher density and the better mechanical properties than $Al_2O_3$($1.0{\mu}m$)-3YSZ composites. The Vickers hardness of the $Al_2O_3$($0.3{\mu}m$)-3YSZ composites show a peak value of 1,997 Hv at the content of 60 wt% $Al_2O_3$, which is a slightly higher value in comparison with 1,938 Hv of the $Al_2O_3$($1.0{\mu}m$)-3YSZ composite. However, the fracture toughness of $Al_2O_3$-3YSZ composites monotonically increases with decreasing the content of $Al_2O_3$ without any peak values. $Al_2O_3$($0.3{\mu}m$)-3YSZ and $Al_2O_3$($1.0{\mu}m$)-3YSZ composites sintered at $1,600^{\circ}C$ have a maximum value of a $6.9MPa{\cdot}m^{1/2}$ and a $6.2MPa{\cdot}m^{1/2}$, respectively at the composition of containing 20 wt% $Al_2O_3$. It should be noticed that the mechanical properties and the sintering density of the $Al_2O_3$-3YSZ composites can be enhanced by using more fine $Al_2O_3$ powder due to their denser microstructure and smaller grain size.

• Journal of the Korean Ceramic Society
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• v.40 no.9
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• pp.867-873
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• 2003

$Si_{3}N_{4}$-BN based machinable ceramics were fabricated by hot-pressing at $1800^{\circ}C$ for 2 h under a pressure of 25 MPa. The microstructure, mechanical properties, and machinability were investigated. With increasing h-BN content from 5 vol% to 30 vol%, three point flexural strength decreased from 1000 MPa of monolithic S $i_3$ $N_4$ to 720～400 MPa. The fracture toughness, $K_{IC}$ , was decreased from 7.6 MPaㆍ$m^{1/2}$ of monolithic S $i_3$ $N_4$ to 6.5～4.1 MPaㆍ$m^{1/2}$. The grain size and aspect ratio of $\beta$-S $i_3$ $N_4$ slightly decreased with increasing h-BN content. S $i_3$ $N_4$ monolith could not be machined due to brittle fracture, but S $i_3$ $N_4$-BN based machinable ceramics could be machined without fracture, showing excellent machinability. With increasing h-BN content, the thurst force during cutting and micro-drilling process was decreased.

• Journal of Welding and Joining
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• v.26 no.3
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• pp.67-73
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• 2008

Fatigue life evaluation of welded structures in a range of high cycles is one of the most difficult problems since extremely small plastic deformation and damage occur during the loading cycles. Moreover, it is very difficult to identify the strong non-linearities of welding, inducing residual stress. In this paper, numerical fatigue test method for welded structures was developed using continuum damage mechanics with inherent strain. Recently, continuum damage mechanics, which can simulate both crack initiation at the micro-scale level and crack propagation at the meso-scale level, has been adopted in the fracture related problem. In order to consider the residual stresses in the welded strictures, damage calculation in conjunction with welding, inducing inherent strain, was proposed. The numerical results obtained from the damage calculation were compared to experimental results.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.7 s.250
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• pp.857-864
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• 2006

Various compressive specimens were fabricated using autoclave do-gassing moulding to find out the compressive characteristics of the carbon/epoxy plain weave fabric composites with respect to the bias and shear angles. The stacking angles of the bias specimens are $[0]_{10T,}\;[3]_{10T,}\;[6]_{10T,}\;[9]_{10T,}\;[12]_{10T,}\;[15]_{10T,}\;[30]_{10T,}\;[45]_{10T}$ and those of the sheared specimens are $[{\pm}37]_{10T,}\;[{\pm}32]_{10T,}\;[{\pm}28]_{10T,}\;[{\pm}22]_{10T,}$ respectively. In order to verify the effect of micro-tow structures on compressive strength and modulus of the composites, compressive test specimens of uni-directional carbon/epoxy composites with the same materials and the same stacking conditions were fabricated. The modulus and strength of both types of composite specimens were compared with the prediction results based on the CLPT and a proposed strength formula. The tow deformation and fracture modes were investigated by microscopic observation.

• Transactions of Materials Processing
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• v.20 no.3
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• pp.193-205
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• 2011

Because of its rustproof property, stainless steel is widely used in kitchen appliances, building materials, electronics, chemical plants and automobile exhausts. In addition, the utilization of stainless steel for fuel cell application is growing. As the demand for this material increases, it is necessary to study the basic properties of stainless steel such as corrosion resistance, heat transfer, formability, cutting or shearing ability and weldability. In this article, the mechanical properties, formability and press forming performance of stainless steel are reviewed. Since temperature and strain rate affect the press forming performance of STS304(austenitic) stainless steel, the influence of these parameters on the plastic behavior should be investigated. Moreover, measures for the prevention of ridging of STS430(ferritic) and delayed fracture of STS430, which respectively appear during and after press forming, should be considered. Recently, stainless steel sheets with a thickness lower than 0.2 mm have been widely used in applications for mobile phone, digital camera and fuel cell separator. Therefore, there is a growing interest of studying the grain size effect and plasticity at the crystal scale in order to understand the anisotropic behavior and micro forming ability of thin sheets. This review paper was written with the objective of helping engineers and researchers to understand the forming characteristics of stainless steel and to establish standards in plastic forming techniques.

• Smart Structures and Systems
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• v.3 no.2
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• pp.133-146
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• 2007

This paper addresses a new type of broad and stage-based hybrid carbon fiber reinforced polymer (HCFRP) sensor that is suitable for the sensing of infrastructures. The HCFRP sensors, a type of composite sensor, are fabricated with three types of carbon tows of different strength and moduli. For all of the specimens, the active materials are carbon tows by virtue of their electrical conductivity and piezoresistivity. The measurement principles are based on the micro- and macro-fractures of different types of carbon tows. A series of experiments are carried out to investigate the sensing performances of the HCFRP sensors. The main variables include the stack order and volume fractions of different types of carbon tows. It is shown that the change in electrical resistance is in direct proportion to the strain/load in low strain ranges. However, the fractional change in electrical resistance (${\Delta}R/R_0$) is smaller than 2% prior to the macrofractures of carbon tows. In order to improve the resistance changes, measures are taken that can enhance the values of ${\Delta}R/R_0$ by more than 2 times during low strain ranges. In high strain ranges, the electrical resistance changes markedly with strain/load in a step-wise manner due to the gradual ruptures of different types of carbon tows at different strain amplitudes. The values of ${\Delta}R/R_0$ due to the fracture of high modulus carbon tows are larger than 36%. Thus, it is demonstrated that the HCFRP sensors have a broad and stage-based sensing capability.