Page 246 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surfuce treatments qf,fibers and effects on composite properties 227

at the fiber-Sn02 interface is purely mechanical, whereas that between SnO2 and
glass is a combination of chemical and mechanical bonds. Fig. 5.39 shows a
characteristic planar brittle fracture and pull-out fibers in uncoated and Sn02 coated
PRD- 166 fiber-glass matrix composites, respectively. The major toughening
mechanisms in the coated fiber composite are mainly crack bridging and crack
deflection (Chawla, 1993). The beneficial effects of Sn02 coating on A1203 fiber has
also been demonstrated in flexure and compression tests (Siadati et al., 1991).
A rnicromechanics analysis of the residual thermal stresses present in glass matrix
composites with and without Sn02 coating has been studied by Chawla (1993), and
a summary is given in Fig. 5.40. Both the radial and axial stresses in the fiber are
greater for the coated fibers than the uncoated fibers, whereas these stresses remain
almost constant in the matrix. From the composite toughness viewpoint, the
presence of the high tensile radial stress at the fiber-coating and coating-matrix
interfaces is deemed particularly desirable. It is also interesting to note that there is a
large axial stress discontinuity at the interface region when the coating layer is
present.

I

Radial distance (p)

400 (bl
-
.........................
200
ID
a PRD-166 Fiber Matrix
E
3 0- ................................
2
c
.- Coating
v1
- -200-
m
X
a -
1 I I I
5
Radial distance @rn)
Fig. 5.40. Distributions of thermal residual stresses in the (a) radial and (b) axial directions of SnOz
coated PRD-166 Al2O3 fiber reinforced glass matrix composites: (. ... .) uncoated fiber; (-) coated fiber.
After Chawla (1993), Fig. 9.29, p. 335. Reproduced by permission of Chapman & Hall.