Macroscopic and Microscopic Residual Stresses in Ceramics Due to Contact Loading
The paper deals with microplastic deformation and macroscopic residual stresses in silicon nitride ceramics due to contact loading and their effect on the near-surface strength. Depth-resolving X-ray residual stress measurements on shot peened samples detect high microplastic deformation and macroscopic compressive residual stresses up to 1.3 GPa at the surface decreasing to zero within 10 pm. These high compressive stresses enhance the load bearing capacity in the ball-on plate test up to 50 % compared to polished samples.
Characterisation and assessment of machined ceramic surfaces
L'article compile les caracteristiques essentielles des zones superficielles de ceramiques usinees, des schemas etablis et des methodes modernes d'investigation, et donne quelques exemples pour une evaluation pertinente pour l'emploi des surfaces modifiees par un usinage. Une attention particuliere a ete apportee aux methodes de diffraction des rayons X pour la caracterisation et aux methodes de la mecanique de la rupture pour une evaluation quantitative des surfaces usinees. Les exemples typiques concernent l'usinage par abrasion du carbure de silicium, du nitrure de silicium et de l'alumine et le comportement en service d'echantillons usines sollicites en fatigue par flexion et traction.
Phase formation during mechanical alloying in the Ti–Si system
Abstract Mechanical alloying of Ti–Si powder mixture or TiH 1.924 –Si powder mixture was performed by high-energy ball milling, in which the Ti to Si mole ratio was 1:2. The phase formations in both mixtures during milling were investigated by X-ray diffraction. Mechanical alloying a mixture of elemental powders led to the formation of TiSi 2 equilibrium compound, while when starting from Si + TiH 1.924 , a metastable phase was formed. The metastable phase completely transforms to TiSi 2 when the mechanically alloyed powder is annealed at temperatures above 800 °C. Two different pathways of phase transitions during mechanical alloying are attributed to the existence of hydrogen atoms, i.e., when starting from Si + TiH 1.924 compared to Si + Ti, the kinetics for the formation of the TiSi 2 intermetallic compound slows down. The kinetics related to the different phase transitions depending on the starting powder mixture are discussed.