Interface coatings processing

Surface tension and contact angle phenomena play a major role in many practical things in life. Whether a liquid will spread on a surface or will break up into small droplets depends on the above properties of interfaces and determines well-known operations such as detergency and coating processes and others that are, perhaps, not so well known, for example, preparation of thin films for resist lithography in microelectronic applications. The challenge for the colloid scientist is to relate the macroscopic effects to the interfacial properties of the materials involved and to learn how to manipulate the latter to achieve the desired effects. Vignette VI provides an example. 

A remaining question is the morphology of the interface between the silica top-layer and the intermediate Y-AI2O3 support layer. In the dip-coating process the silica layer may either form an almost flat layer on top of the polished support surface or partly infiltrate the pores in the y-layer. With conventional techniques (SEM-EDS, or TEM) the microscopic geometry of this interface cannot be elucidated. With RBS analysis a more clear indication can be obtained on the depth distribution of the silicon atoms with respect to the y-alumina surface. 

Materials flexibility has thus been achieved— albeit at the cost of introducing an internal interface within the photoreceptor structure—an interface which must be carefully controlled in the coating process to prevent undesirable intermixing of layers, delamination, and the formation of barriers impeding charge carrier injection. 

These film thickness equations can be affected by the formation of a skin layer during the coating process. Skin layer formation is caused by the difference in the properties of the spinning material near the air interface and the properties of the bulk fluid. In particular, if the skin layer forms early in the spin coating process, little evaporation occurs and the value of n is close to 1. If the skin layer forms late in the spin coating process, most of the evaporation would already have taken place, and the value of n is close to 1/2. A detailed and mathematically involved treatment can be found in contributions by Emslie, Bonner and Peck (1958) and more recently Sahu, Parija and Panigrahi (2009). 

Wider use of fiber-reinforced ceramic matrix composites for high temperature structural applications is hindered by several factors including (1) absence of a low cost, thermally stable fiber, (2) decrease in toughness caused by oxidation of the commonly used carbon and boron nitride fiber-matrix interface coatings, and (3) composite fabrication (consolidation) processes that are expensive or degrade the fiber. This chapter addresses how these shortcomings may be overcome by CVD and chemical vapor infiltration (CVI). Much of this chapter is based on recent experimental research at Georgia Tech. 

---