High Density Of Microcracks Research Articles

Oxide layer development under thermal cycling and its role on damage evolution and spallation in TBC system

The nature and cause of failure of thermal barrier coatings (TBCs) consisting of physical vapor deposited (PVD) yttria stabilized zirconia (YSZ, 8 wt.% Y2O3) and a diffusion aluminide bond coat (Pt-Al) were investigated after oxidative thermal cycling and isothermal heat treatment at 1177 °C in air. Experiments were conducted for 45 and 10-minute hold times and for isothermal condition for disk specimens with and without TBC. It is found that microcracks starts in the oxide scales at the bond coat grain boundary protrusions. Total number of thermal cycles affect the density of microcracks within the TGO layer. Evidence is presented that higher density of microcracks in the 10-min hold-time experiments tend to separate the TBC from the TGO layer via extensive coating “micro-decohesion” and promotes 'complete' TBC separation as opposed to traditional 'partial' spallation of TBC from the substrate as in the 45-min hold-time and isothermal experiments.

MoSi2–ZrO2 composites—fabrication, microstructures and properties

Abstract In this study, dense monolithic MoSi2 and MoSi2–ZrO2 composites were fabricated by hot-pressing from mechanically alloyed (MA) and commercial MoSi2 powders, unstabilized zirconia (UZ) and yttria partially stabilized zirconia (YPSZ) powders. The microstructures and properties of these materials were characterized. Hot-pressed monolithic materials and composites were consolidated to 96.1–99.5% density. Zirconia phases identified in the composites MoSi2–UZ and MoSi2–YPSZ were monoclinic and tetragonal, respectively. The MA–MoSi2 powder prepared materials have a higher hardness due to the refinement of the matrix. The toughness of the composites UZ and YPSZ were 2.4 times and 20% higher than that of MoSi2. The predominating fracture modes have been identified. High density of microcracks and dislocations were observed in UZ composites but not in YPSZ composites. Microcracking is suggested to be the main toughening mechanism for the former; while for the latter, the improvement in toughness could be attributed, at least partly, to crack deflection.

GaAs / AlGaAs pin MQW structures grown on patterned Si substrates

We report structural and optical studies of a pin MQW structure grown on a patterned Si substrate. We find that a high density of microcracks in the <110> directions are formed on the plain area of the substrate but that these are almost totally absent on the patterned areas. Photoluminescence measurements show a shift of QW emission to higher energies as island size is reduced with little change in intensity. These results show that good quality MQW device structures with reduced strain and freedom from microcracks can be grown on patterned Si.

CoSi2/Si(111) interface: Determination of the interfacial metal coordination number.

The morphology and the surface and interface structure of flat ${\mathrm{CoSi}}_{2}$ films (thickness \ensuremath{\sim}2 Si-Co-Si triple layers) epitaxially grown on Si(111) have been investigated using medium-energy ion scattering with ultrahigh energy resolution. The data show that the silicide is attached to the substrate through Co-Si bonds rather than through Si-Si bonds. At the interface, the silicide is terminated by a full Si-Co-Si triple layer. The interface metal atoms are thus eightfold coordinated. The silicide-substrate distance is expanded by 0.16\ifmmode\pm\else\textpm\fi{}0.08 \AA{}. The first Si substrate layer is relaxed outward by 0.08\ifmmode\pm\else\textpm\fi{}0.08 \AA{}. The length of the Co-Si interface bond is thus dilated by 0.08 \AA{}. At the silicide surface, the first Co-Si interplanar distance is found to be relaxed inward by 0.14\ifmmode\pm\else\textpm\fi{}0.04 \AA{}. Our measurements suggest that the silicide lattice strain is almost fully relaxed. We propose that such relaxation occurs through a high density of microcracks in the film. The average spacing between cracks is then smaller than roughly 50 \AA{}.

The effective elastic properties of a linear elastic solid with microcracks

The effective elastic properties of elastic solid with small density and high density of microcracks is discussed. In order to analyse the relation between effective elastic properties and strength, the effective elastic properties of elastic solids with high density microcracks is obtained by a simple model (beam model for 2-D case and plate model for 3-D case). Statistical analysis of effective elastic properties and strength show that the failure criteria appears to be sensitive to the distribution of size, orientation and distance between microcracks, where the effective elastic properties are quite insensitive to such morphological detail.

ChemInform Abstract: THERMAL‐SHOCK STRUCTURES IN A PARTIALLY STABILIZED ZRO2 IN THE SYSTEM CAO‐ZRO2

AbstractDie Veränderungen der Mikrostrukttlr an 2 mit je 3.4 Gewf CaO stabilisierten ZiOz‐Proben mit unterschiedlichen Gehalt an monokliner Phase bei Wärmeschoekeinwirkung wurden untersucht, Bei Proben mit 40 Gew.‐′‐52‐ an monokliner Phase tritt starke Mikroriffsbilclung auf.