Page 226 - Engineered Interfaces in Fiber Reinforced Composites

mechanical performance of the coated fibers and the composites fabricated
therefrom. In particular, large tensile circumferential and axial stresses may be
generated in the coating layer during cooling if the fiber has a very low CTE
compared to the coating material, such as in yttria coated-Sic monofilament (see
Section 5.5.5). The coating layer undergoes severe cracking or spallation due to the
large difference in the residual stresses, as shown in the SEM photo micrograph in
Fig. 5.25.
The thickness of coating applied onto the fiber is a predominant parameter which
limits the performance of the coating. The coating thickness should be chosen to
protect the fiber from environmental attack, minimize the residual thermal stresses,
and induce a non-brittle failure mode. Coating thicknesses in the range of 0.1-
1.0 pm are commonly applied for MMCs and CMCs. Furthermore, a uniform,
reliable and reproducible application of a coating material without generating pores
is a very difficult task. Numerous areas of poor bonding and porosity can lead to a
general failure of the coating itself.

5.5.2.2. Coating techniques
The first major obstacle encountered in developing a successful coating seems to
be the method of application. The coating must wet the fiber properly, be uniform in
thickness and, most preferably, be free of porosity extending to the surface of the
substrate. The conventional methods of applying coatings to bulk substrates are not
suitable nor adequate for fibers of small diameter. It is important to note that the
selection criteria for one coating technique over another are most often different.

Fig. 5.25. A scanning electron microphotograph of fractured yttria coating on a Sic monofilament after
cooling through 500°K. After Kiescheke et al. (1991a). Reproduced by permission of Elsevier Science Ltd.