Friction Reduction and Wear Resistance Enhancement of SiC and Si3N4 Ceramics under Dry Conditions

In this study, an effort was made to control the friction and wear behavior of silicon carbide (SiC) and silicon nitride (Si₃N₄) ceramics using an ultrasonic nanocrystalline surface modification (UNSM) technique. The friction and wear behavior of the ceramic specimens was investigated using a ball-on-disk tribotester under dry conditions against two different Si₃N₄ and bearing steel (SUJ2) balls. The experimental test results revealed the possibility of controlling the friction and wear behavior of ceramics, where the friction coefficient and wear resistance of the specimens were improved by the UNSM technique. The hardness of the specimens also increased after UNSM treatment, but it decreased abruptly with increasing depth from the very top surface. Microscratch tests showed that the critical load of the specimens was improved by the UNSM technique. In addition, Raman spectra results revealed that no additional phase was detected after UNSM treatment, but the intensity decreased after UNSM treatment. Hence, the UNSM technique ensures stronger ceramics and enables better friction and wear behavior than available conventional sintered ceramics.     

Failure analysis for RF characteristics of GaAs MESFETs

The effect of thermal stress on the noise degradation of GaAs MESFETs was investigated. Minimum noise figure, associated gain, scattering parameters, and capacitance–voltage profiles were measured during the tests, and the electro-chemical properties after thermal stress were analyzed by means of Auger electron spectroscopy, X-ray diffractometery, cross-sectional transmission electron microscopy, and capacitance–voltage measurements. The RF failure mode consists of an increase of the minimum noise figure and a decrease of associated gain of the FETs. The extracted equivalent circuit elements from measured scattering parameters were used to evaluate the influence of each parameter on the noise degradation. The parametric estimation showed that the noise degradation was mainly attributed to the decrease of a.c. transconductance. From the C–V analysis, we found that the decrease of a.c. transconductance was caused by the decrease of effective carrier concentration. From the electro-chemical analysis, the decrease in effective carrier concentration was resulted from the gate-sinking by the thermally activated interdiffusion between the gate metal and the GaAs channel layer. Therefore, the thermally activated carrier compensation by Ga vacancies in the channel is proposed to be the main failure mechanism for noise degradation of GaAs MESFETs.     

LP-MOCVD grown (lnAs)m(GaAs)m short period superlattices on InP

(InAs)_m(GaAs)_m (1 ≤ m ≤ 12) short period superlattices (SPSs) have been grown on semi-insulated InP substrates with a 200 nm InP cap layer using low pressure metalorganic chemical vapor deposition (MOCVD). According to double crystal x-ray diffraction and transmission electron microscopy results, the critical layer thickness of (InAs)_m(GaAs)_m SPS was observed to be ～30Å (m = 5). For the SPS below the critical layer thickness, mirror-like surface morphology was found without defects, and strong intensity Fourier transformed photoluminescence (FT-PL) spectra were also obtained at room temperature. The SPS with m = 4 showed a drastic improvement in photoluminescence intensity of order of two compared to an InGaAs ternary layer. However, the SPS with a large value of m (m ≥ 6), rough surface was observed with defects, with broad and weak FT-PL spectra. The surface morphology of SPS was greatly affected by the substrate orientation. The SPS with m = 5 was grown on two degree tilted substrate from (100) direction and showed poor surface morphology as compared to the one grown on (100) exact substrate Moreover, the SPS grown on a (111)B substrate showed a rough triangular pattern with Nomarski optical microscopy. In-situ thermal annealed SPS with m = 4 showed a 18 meV increase in PL peak energy compared to the as-grown sample due to phase separation resulting from thermal interdiffusion.     

A new theory describing the hydrogen-assisted intergranular cracking of high-strength steels

A thermodynamic analysis of hydrogen-assisted intergranular brittle fracture of high-strength steels has been made. In this analysis the functional relationship between cohesive energy and hydrogen coverage is derived in the case of solute equilibrium constraint during the decohering process. This relationship is evaluated and discussed in the presence of a triaxial stress field. The variation of threshold-stress intensity, K _th, with hydrogen fugacity is calculated by a criterion for hydrogen-assisted intergranular fracture, and is also considered as it relates to the effects of several material parameters, such as trap-binding energy at a grain boundary, yield strength and work-hardening exponent. In particular the fracture mode transition by hydrogen-assisted cracking is discussed as it relates to the effects of hydrogen on the K _th necessary for the occurrence of the respective fracture modes.     

Densification of powder compacts by vibration

Various packing methods such as vibration, shaking, etc., in addition to normal gravitational settling, can be often used to density powder compacts. Many issues relevant to this matter are of great importance in advanced ceramic powder processing. In the present work, the relaxation of structure due to vibration is addressed by using a computer experimental model based on Monte Carlo method. Packing structures, diffraction patterns, radial distribution functions are used for the characterization of structures. Bulk properties such as packing fraction and average height of the deposit are examined. The results agree well with those observed in model experiments, even with more implication.     

Uniform and high coupling efficiency between InGaAsP-InP buried heterostructure optical amplifier and monolithically butt-coupled waveguide using reactive ion etching

We obtained uniform and high coupling efficiency for InGaAsP-InP buried heterostructure (BH) optical amplifiers integrated with butt-coupled waveguides using reactive ion etching (RIE) for mesa definition and low-pressure metalorganic vapor phase epitaxy (LPMOVPE) for waveguide layer regrowth. Measured average coupling efficiency was over 91% across a quarter of 2-in InP wafer. RIE etched vertical facet and a subsequent chemical etching using HBr-based solution for relief of RIE damage enabled us to reduce the coupling loss due to anomalous regrowth shape at the interface. RIE and selective regrowth processes are promising techniques for the fabrication of the photonic integrated circuit (PIC).