Columnar microstructure, growth

In Chapter 7 various growth models were described layer growth (Section 7.9), nucleation-coalescence growth (Section 7.10), development of texture (Section 7.11), columnar microstructure (Section 7.12), and other structural forms (Section 7.13). In this section we discuss the effects of additives on these growth mechanisms. 

Both columnar and equiaxed microstructures are obtained from polycrystal growth. Columnar micro structure is composed of columnar grains of preferred orientation that grow from the first nucleated location on a substrate surface. The microstructure is attributed to high supersaturation and low temperature and hence more limited diffusion. A columnar microstructure is suitable for high-temperature structural applications, such as thermal barrier coatings. Figure 6.6 shows the typical columnar CVD SiC microstructure in C/SiC composites. 

As follows from electron diffraction, films of crystalline materials formed on amorphous substrates under normal conditions are generally polycrystalline in their structure. During further thickness growth, a columnar microstructure can often be observed and thicker film may develop a texture. 

D. Moldovan, D. Wolf, S. R. PhiUpot, and A. J. Haslam. Role of grain rotation during grain growth in a columnar microstructure by mesoscale simulation. Acta Materialia, 50 3397-3414, 2002... 

Researchers who have focused more on understanding cause-effect relationships in solution processing have given attention to film drying and pyrolysis behavior, densification processes, and nucleation and growth into the desired crystalline state. Both thermodynamic and kinetic factors associated with the phase transformation from the amorphous state to the crystalline state have been considered.11 119 Control of these factors can lead to improvements in the ability to influence the microstructure. It is noted that in the previous sentence, influence has been carefully chosen, since the ability to manipulate the factors that govern the nature of the phase transformation to the extent that full control of the microstructure is possible remains to be demonstrated. However, trends in characteristics such as film orientation and columnar versus uniaxial grains have certainly already been achieved.120... 

The microstructure of the tube taken in a direction normal to the growth direction is shown in Figure 6.24b. The multi-layer columnar grain structure in this figure is the result of cyclically interrupted deposition because of P-CVD. The deposition rate is calculated to be 30 pm-h 1. This is well below the deposition rate of 125 pm-h 1 normally achieved with the same parameters of temperature and... 

Microstructural analysis of suspension plasma sprayed Ti02 coatings by Raman spectroscopy and transmission electron microscopy correlated well with results obtained by XRD. The anatase content in the coatings varied between 10 and 22 mass%, dependent on spraying parameters. Raman spectroscopy enabled the imaging of the spatial 2D-distribution of the rutile and anatase (Podlesak et al., 2008). Transmission electron microscope enabled to visualise the contact areas between the lamellae inside sprayed coatings as well as the columnar growth of crystal inside the lamellae. 

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