RESUMEN
Formation dynamics of laser-induced periodic surface structures (LIPSSs) on the SiC substrates were described with varying pulse numbers and pulse duration. As the number of laser pulses increases, two significant transformations become evident in the progression of structural formations. First from surface roughening with nanoparticles to LIPSS with the period that is slightly shorter than the laser wavelength. Second it turns to LIPSS with a period less than half the laser wavelength. It is found that maintaining the crystallinity is the key to changing the structures. In the cases of longer pulse width than sub-nanoseconds, no LIPSS formations are observed or LSFL does not change to HSFL because the irradiated area is poly-crystallized.
RESUMEN
We have previously shown that sharpening the cutting edge of a cemented carbide tool by chemical-mechanical polishing (CMP) improved the cutting speed by approximately 150% and reduced the wear on the flank surface by approximately 50% compared to a commercial tool when cutting a heat-resistant alloy. In addition, the cutting edges of carbide tools treated by laser doping (LD) using boron nitride as doping material achieved approximately 100 times longer cutting distance in glass machining than the edges of carbide tools treated with CMP grinding wheels. In this study, LD was conducted on a tool base material (WC-Co) to investigate and understand the crystal structure changes of the base material upon treatment. Electron backscatter diffraction and X-ray diffraction results show that the effect of LD was observed in the region 50 nm below the surface. LD improved the strength by approximately 11.7% without destroying the surface crystal structure. Thus, doping can be performed on tool tips while maintaining the WC structure to improve the performance of WC-Co cutting tools.
RESUMEN
Laser-induced periodic surface structure (LIPSS), which has a period smaller than the laser wavelength, is expected to become a potential technique for fine surface processing. We report the microscopic and macroscopic observations of the crystallinity of LIPSSs, where the characteristics such as defects generation and residual strain were analyzed, respectively. The LIPSSs were formed on a Si substrate using two different femtosecond pulses from Ti:Sapphire laser with near-infrared wavelength (0.8 µm) and free-electron laser (FEL) with mid-infrared wavelength (11.4 µm). The photon energies of the former and latter lasers used here are higher and lower than the Si bandgap energies, respectively. These LIPSSs exhibit different crystalline states, where LIPSS induced by Ti:Sapphire laser show residual strain while having a stable crystallinity; in contrast, FEL-LIPSS generates defects without residual strain. This multiple analysis (microscopic and macroscopic observations) provides such previously-unknown structural characteristics with high spatial resolution. To obtain LIPSS with suitable properties and characteristics based on each application it is paramount to identify the laser sources that can achieve such properties. Therefore, identifying the structural information of the LIPSS generated by each specific laser is of great importance.
RESUMEN
The effect of additional doping of charge impurities was investigated in a ferromagnetic semiconductor Zn1-xCrxTe. It was found that the doping of iodine, which is expected to act as an n-type dopant in ZnTe, brought about a drastic enhancement of the ferromagnetism in Zn1-xCrxTe, while the grown films remained electrically insulating. In particular, at a fixed Cr composition of x=0.05, the ferromagnetic transition temperature TC increased up to 300 K at maximum due to the iodine doping from TC=30 K of the undoped counterpart, while the ferromagnetism disappeared due to the doping of nitrogen as a p-type dopant. The observed systematic correlation of ferromagnetism with the doping of charge impurities of both the p and n type, suggesting a key role of the position of Fermi level within the impurity d state, is discussed on the basis of the double-exchange interaction as a mechanism of ferromagnetism in this material.
