Development Of Growth Stresses Research Articles

Temperature Dependence of Oxidation Behaviour of a Ferritic–Martensitic Steel in Supercritical Water at 600–700 °C

Oxidation tests on a ferritic–martensitic steel were carried out in deaerated supercritical water at 600–700 °C under 25 MPa pressure. The oxidation kinetics followed near-parabolic rate law at 600 °C and obeyed near-cubic rate law at 650–700 °C. The deviations from parabolic behaviour may have been related to the development of growth stresses within the oxide. The oxidation rate did not always increase significantly with increasing temperature. The oxidation rates at 650 and 700 °C were approximately equal. The reason for this may be attributed to the formation of large pores at the interface between oxide scale and substrate at 700 °C. The influence of temperature on the microstructure of oxide scale and oxidation kinetics is discussed.

The evolution of growth stresses in chemical vapor deposited tungsten films studied by in situ wafer curvature measurements

An in situ study of the evolution of the biaxial state of intrinsic stress during nucleation and growth of polycrystalline tungsten chemical vapor deposition films deposited by the hydrogen reduction of tungsten hexafluoride is presented. The evolution of biaxial stress was determined from in situ wafer curvature measurements. It is shown that the intrinsic stress is a growth stress, i.e., a stress developing in close vicinity to the advancing surface of the film due to metastable film growth processes. The stress developing depends strongly on the thickness of the film. High tensile stress (≊4 GPa) is observed during the initial stage of growth, compressive stress (up to −1 GPa) is observed in an intermediate thickness regime after film closure and tensile stress (0.1–1 GPa) is observed in the thick film regime. The associated stress gradients in the film are preserved during and after growth. The development of growth stress is determined by deposition temperature and growth rate. The tensile stress in the thick film regime is larger at a higher growth rate or a lower deposition temperature, while the compressive stress in the intermediate thickness regime showed the opposite dependency. Film properties as the evolution of grain size, impurity content, and resistivity are found not to vary significantly with the growth conditions. Therefore, the development of growth stress is ascribed to kinetical processes. The development of tensile stress in the thick film regime is described with a (kinetic) grain boundary formation and relaxation model. The compressive stress in the intermediate thickness regime is tentatively ascribed to compressive coherency strains induced by interfacial tensions of the grains in the stage of island growth.

Growth Stress in CVD-WGex Films Deposited by Reduction of WF6 by GeH4

ABSTRACTGrowth stresses in thin metal films on a substrate are important for integrated circuit device fabrication and reliability. The development of growth stress in blanket CVD-WGex films deposited by GeH4 reduction of WF6 is studied by in situ wafer curvature measurements. Depending on process conditions, the growth stress varies from 600 MPa tensile in β-WGex films deposited at low temperatures (T < 400 °C) to 400 MPa compressive in α-WGex films deposited at high temperatures (T > 400 °C). At very low temperatures amorphous WGex films grow without significant stress. Development of tensile growth stress is favoured by low temperatures and high growth rates, whereas development of compressive growth stress is favoured by high temperatures and low growth rates. Although the material properties of CVD-WGex films depend on process conditions, the development of growth stress in these films shows a dependence on deposition temperature and growth rate which is similar to pure α-W films deposited by H2 reduction of WF6. The results will be discussed in view of a model which has recently been put forward to explain development of growth stress in polycrystalline metal films on a substrate.

Evolution of Intrinsic Stress During Nucleation and Growth of Polycrystalline Tungsten Films by Chemical Vapor Deposition.

AbstractWe present in situ measurements of the intrinsic (growth) stress during nucleation and growth of thin tungsten films by chemical vapor deposition. It is shown that interfering stress sources, as recrystallization or plasic flow, do not contribute to the intrinsic stress in these films. This makes W-CVD a model system for the experimental study of the relation between the evolution of microstructure and the development ofgrowth stress.

The role of oxide grain boundaries in the development of growth stresses during oxidation

A model is described which identifies the oxide growth stresses observed in many systems with the diffusion of anions along oxide grain boundaries and the subsequent formation of an oxide intrusions at the oxide/metal interface. Detailed calculations are made of the elastic stress system within the metal and of the average stress developed in the oxide. For a Pilling-Bedworth ratio greater than unity, the oxide intrusion acts as a linear centre of dilatation generating short-range tensile stresses within the metal and equivalent compressive stresses within the oxide. It is shown that the average value of the oxide stress increases with both the oxide grain size and with the lateral dimensions of the oxide intrusion.

The nature of reaction wood. VI. The reaction anatomy of seedlings of woody perennials.

The reaction anatomy of the stems of seedlings of A 1 species of hroad-leaved trees was examined. The reaction was induced by bending the stems at right angles or growing the plants horizontally. As a result of greater cambial activity on the upper side such stems developed a structural asymmetry characterized by enhanced xylem and phloem development. The structural features of the lower side were similar to those of stems grown vertically. The xylem on the upper side contained fewer vessels, more fibres, and fewer medullary rays than that on the lower side of the same stem. "Gelatinous" layers developed within the reaction fibres as well as in fibres of the phloem. Vessels, medullary ray cells, and parenchyma cells were often compressed and sometimes obliterated. Reaction fibres showed reduced wall pitting and tended to retain their contents until they lay relatively deep within the stem. There were no differences in fibre length between upper and lower sides of the stem, but comparisons were probably confounded with an age difference. Cortical cells and, in species where they occurred, canals were compressed on that side of the stem. Such cortical cells were not only smaller in cross section than those on the lower side, but were also shorter in length. This was also a feature of medullary ray cells. There was a greater accumulation of tannins in phloem and cortex on the upper side of the stem. The structure of reaction fibres was examined in ultraviolet, polarizing, and electron microscopes. In the first of these, the "gelatinous" layer was seen to consist of a series of concentric bands convoluted towards the cell lumen, the bands containing radial striations. The polarizing microscope showed that the innermost part of the wall possessed some birefringence. Both convolutions and birefringence tended to disappear as the cells matured; staining with Congo red became less intense. The heterogeneity of the "gelatinous" layer was confirmed by electron microscopic examination. The results are discussed in connection with the gravimorphic responses of displaced stems, the altered patterns of lignification, and the development of growth stresses in such stems.