Dopant activation process in Mg-implanted GaN studied by monoenergetic positron beam

A process for activating Mg and its relationship with vacancy-type defects in Mg-implanted GaN were studied by positron annihilation spectroscopy. Mg+ ions were implanted with an energy of 10 keV, and the Mg concentration in the subsurface region (≤ 50 nm) was on the order of 1019 cm−3. After the Mg-implantation, N+ ions were implanted to provide a 300-nm-deep box profile with a N concentration of 6 × 1018 cm−3. From capacitance–voltage measurements, the sequential implantation of N was found to enhance the activation of Mg. For N-implanted GaN before annealing, the major defect species were determined to Ga-vacancy related defects such as divacancy. After annealing below 1000 °C, the clustering of vacancies was observed. Above 1200 °C annealing, however, the size of the vacancies started to decrease, which was due to recombinations of vacancy clusters and excess N atoms in the damaged region. The suppression of vacancy clustering by sequential N-implantation in Mg-implanted GaN was attributed to the origin of the enhancement of the Mg activation.

Gradient of Residual Stress and Lattice Parameter in Mechanically Polished Tungsten Measured Using Classical X-rays and Synchrotron Radiation

In this work, the stress gradient in mechanically polished tungsten sample was studied using X-ray diffraction methods. To determine in-depth stress evolution in the very shallow subsurface region (up to 10 μm), special methods based on reflection geometry were applied. The subsurface stresses (depth up to 1 μm) were measured using the multiple-reflection grazing incidence X-ray diffraction method with classical characteristic X-rays, while the deeper volumes (depth up to 10 μm) were investigated using energy-dispersive diffraction with white high energy synchrotron beam. Both complementary methods allowed for determining in-depth stress profile and the evolution of stress-free lattice parameter. It was confirmed that the crystals of tungsten are elastically isotropic, which simplifies the stress analysis and makes tungsten a suitable material for testing stress measurement methods. Furthermore, it was found that an important compressive stress of about − 1000 MPa was generated on the surface of the mechanically polished sample, and this stress decreases to zero value at the depth of about 9 μm. On the other hand, the strain-free lattice parameter does not change significantly in the examined subsurface region.

Application of a rolling bearing life model with surface and subsurface survival to hybrid bearing cases

A previously published rolling bearing life model that separates the surface and subsurface survival is briefly summarised. The model is applied to the case of hybrid bearings and discussed with regard to a selected set of application examples. Ball hybrid bearings under equal load condition show 12% higher Hertzian stress than all-steel bearings. However, field applications, typically under light load, poor lubrication and contamination, show that hybrid bearings have longer fatigue life than all-steel bearings. Traditional all-steel life models fail to predict this type of behaviour. In this paper, it is shown that hybrid bearing unique fatigue performance can be described using the idea of separation of surface and subsurface survival. The model applies the classical rolling contact fatigue in the subsurface region of the rolling contact while a newly developed tribologically dependent surface degradation models is used for the ceramic-steel raceway interface. It is found that the particular fatigue resistance of the ceramic-steel interface of the hybrid bearing raceway can, in most cases, compensate for the additional stress present in the subsurface region of the contact.

Cyclic carburization-oxidation behavior of Hastelloy-X at 1000°C

Cyclic corrosion, consisting of carburization in Ar+4% CH4+3% H2 for 1 h and oxidation in air for 1 h, of Ni-base in a wrought alloy, Hastelloy-X, was investigated at 1000°C to understand the significant metal loss occurring in fuel injection nozzles of gas turbine combustor operated using natural gas. The mass gain of cyclic corrosion was relatively low up to about eight cycles of corrosion by formation of a Cr2O3 scale in each oxidation stage, but it increased rapidly due to formation of a Fe- and Ni-rich oxide scale as the number of corrosion cycles increases. The Cr content in the subsurface region rapidly decreased under the cyclic corrosion condition compared with a continuous single oxidation or carburization, which resulted in the formation of non-protective Fe- and Ni-rich oxide scale in a small number of corrosion cycles. The formation of a Cr3C2 scale from the protective Cr2O3 scale in the carburization stage was detrimental for the oxidation resistance of Hastelloy-X and was considered to have a similar effect to spallation of a Cr2O3 scale; therefore, it accelerated Cr consumption from the alloy subsurface region.