Copper (Cu) is widely used as electrical contact materials because of its relatively low electrical wear and low cost compared to metallic materials such as silver (Ag), platinum (Pt), palladium (Pd) and cadmium (Cd). However, the electrical and mechanical properties of copper are not good compared to other metal contacts.

The working conditions of contact materials require thermal, electrical, mechanical and chemical properties to be excellent. It is because when capacitive and inductive loads are switched on and off, the temperature of contact generated at the contact surface is high, with a large number of switches and high contact pressure, and they work under the conditions of electrical wear and mechanical wear caused by arc discharges. The electrical and mechanical properties are not negligible in contact materials.

Several methods have been applied to improve the properties of contact materials. Recently, worldwide research on carbon nanotubes (CNT) has been active and the range of their application has been extended. CNTs have been widely used as reinforcement materials for metal matrix composites because of their good mechanical, physicochemical and electrical properties.

The tensile strength of CNT is 50-200GPa and its modulus of elasticity is about 1TPa. When carbon nanotubes are added to copper matrix, the tensile and flexural strength of carbon nanotube reinforced copper matrix composites increases by over 20% compared to pure copper. In addition, carbon nanotubes can be either conductors or semiconductors depending on what angle the hexagonal ring takes on the tube wall. In the case of conductors, the electrical transport capacity of carbon nanotubes is 1×10⁹A/cm², which is 1 000 times higher than that of copper wires.

This indicates that carbon nanotubes can be incorporated into metal matrix to enhance the mechanical and electrical properties of metal matrix composites. The addition of carbon nanotubes, excellent carbon nanomaterial, is the best way to realize the refinement of the matrix microstructure. Carbon nanotubes are placed at the boundary of the matrix microstructure and hinder grain growth, and thus grain refinement has a positive effect on both strength and toughness.

Nevertheless, the content of carbon nanotubes should not be too high. Because of the large surface area of carbon nanotubes, carbon nanotubes with relatively large surface activation energy are easily aggregated and negatively affect the microstructure and properties of metal matrix.

Jon Song Won, a researcher at the Faculty of Material Science and Technology, investigated the effect of carbon nanotubes on the properties and texture of contact carbon nanotube reinforced copper matrix composites prepared by direct addition of carbon nanotubes by blocking them in molten state.

When the CNT content was 2.0vol.%, the grain size of the CNT-reinforced Cu matrix composites was 21-23μm, and grain refinement was observed in their microstructure. However, no change in size, distribution and shape of inclusions was observed. The electrical and mechanical properties of carbon nanotube reinforced copper matrix composites are as follows: Tensile strength is 315MPa, elongation is 12%, hardness is HB 80 and electrical resistivity is 0.017 3μΩ·m.

Gears, major components for power and motion, are widely used in industries like automotive, aerospace, ship, power, mining, etc.

Gears are subject to external forces during motion and power transfer for their working characteristics, which causes bending stresses at their teeth roots and contact stresses on their contact parts. The teeth wear by sliding friction at the contact parts. In addition, improper gear meshing causes impact. Common failures in gears include wear and scratches, pitting corrosion, abrasive wear and teeth breakage by flexural fatigue and impact fatigue.

In order to improve the mechanical properties of gear steel, alloying elements such as niobium, titanium, boron, rare earths and vanadium are microalloyed.

Jong Chung Bing, a researcher at the Faculty of Material Science and Technology, has investigated the effect of the complex microalloying of boron and RE elements on the mechanical properties of low C-Mn steel in order to achieve high performance of gear steel without the need for import-dependent chromium.

As a result, he has found that when the chemical composition of steel is C 0.23-0.27%, Mn 1.1-1.3%, Si 0.2-0.45%, P and S less than 0.03%, B 0.001-0.004% and RE 0.025-0.05%, the mechanical properties of this steel quenched and tempered at low temperature are tensile strength 1 100-1 300MPa, elongation 9-15% and impact value 90-130J/cm², which means it meets the technical requirements of steel for gears.

In system reliability analyses, computation of rare (failure) event probability is a main task. In most cases, the numerical model of a rare event is nonlinear and the resulting failure domain is often multimodal.

One strategy for estimating failure probability is the importance sampling (IS) method. The efficiency of IS depends on the choice of IS density.

With the focus on the sequential importance sampling, Kim Sang Rim, a researcher at the Faculty of Applied Mathematics, has investigated the improvement of its initial density.

First, he reviewed the sequential importance sampling (SIS). Then, he newly found the approximate optimal IS density using the scaling density. On this basis, he proposed a technique for estimating target failure probability by IS.

The numerical simulation showed that the proposed method seems to be appropriate to the problems for estimating probability lower than 10^-6.