Surface Roughness Characterization of Nicalon TM and HI-Nicalon tM Ceramic Fibers by Atomic Force Microscopy N. Chawla, " J. w. Holmes, and J. F. mansfield "Ceramic Composites Research Laboratory, and 'North Campus Electron Microbeam Analysis Laboratory, university of Michigan, Ann Arbor, MI 48109 The behavior of ceramic composites is governed by the nature of the fiber/matrix interface Fiber surface roughness is a key parameter in the behavior at the fiber/matrix interface (e. g,debonding, interfacial sliding) and the overall behavior of a composite. Using an atomic force microscope(AFM), quantitative surface roughness values of ceramic fibers can be obtained, with an uncertainty of Inm. The AFM technique was used to obtain surface roughness profiles and analysis on Si-C-O and Si-C fibers(Nicalon, and a new, virtually ox ygen-free Si-C fiber, HI-Nicalon). The latter fiber had a slightly higher roughness ampli tude, which may be caused by differences in processing. Although the differences in rough- ness between the fibers were small, the calculated radial strain and radial normal stress in composites reinforced with HI-Nicalon were higher than in those reinforced with Nicalon This result indicates that small changes in the roughness of a fiber can significantly affect the debonding and sliding properties between the fiber and matrix INTRODUCTION residual stresses from processing will in- duce radial stresses at the interface(which The nature of bonding between the fiber for most composites, are compressive)and and matrix in continuous fiber-reinforced axial stresses between the matrix and fibers ceramic matrix composites determines the [4, 5]. Fiber surface roughness can also con- properties of the composite [1-3]. If a fiber tribute a mechanical component to the ra is strongly bonded to the matrix, the energy dial clamping stresses between the fibers at a crack tip in the vicinity of an interface and matrix, because the matrix becomes may be high enough to fracture the fiber In mechanically keyed to the fiber [6-8]. A this case, the crack propagates straight roughness-induced strain arises because of through the interface, into the fiber; the this mechanical keying. This strain can be composite usually fails in a catastrophic estimated by the roughness amplitude be- manner. If the fiber/matrix interface is ta ween the fiber and matrix. Figure 1 shows lored(e. g, by an appropriate fiber coating), schematically the qualitative effect of fiber such that the degree of bonding between roughness on the sliding behavior at the fi the fiber and matrix is weak, a propagating ber/matrix interface of a composite rack will be deflected at the interface, An adequate technique for the measure- causing it to lose energy. In this manner, ment of surface roughness in ceramic fibers debonding and sliding of the fiber with re- is not available. Conventional profilometry spect to the matrix acts as an energy-ab- techniques are accurate only to lmm, while sorbing mechanism. the roughness in ceramic fibers may be as Several important factors control the fi- low as Inm. With the commercial availabil ber/matrix behavior. For example, thermal ity of the atomic force microscope(AFM), 1044-5803/95/59.50 655 Avenue of the Amencas New York, NY 10010 ssDI10445803(9501036ELSEVIER Surface Roughness Characterization of NicalonTM and HLNicalon TM Ceramic Fibers by Atomic Force Microscopy N. Chawl.a,* J. W. Holmes,” and J. E Mansfield+ *Ceramic Composites Research Laboratory, arm! +North Campus Electron Microbeam Analysis Laboratoy, University of Michigan, Ann Arbor, Ml 48109 The behavior of ceramic composites is governed by the nature of the fiber/matrix interface. Fiber surface roughness is a key parameter in the behavior at the fiber/matrix interface (e.g., debonding, interfacial sliding) and the overall behavior of a composite. Using an atomic force microscope (AFM), quantitative surface roughness values of ceramic fibers can be obtained, with an uncertainty of lnm. The AFM technique was used to obtain surface roughness profiles and analysis on Si-C-O and Si-C fibers (Nicalon, and a new, virtually ox￾ygen-free Si-C fiber, HI-Nicalon). The latter fiber had a slightly higher roughness ampli￾tude, which may be caused by differences in processing. Although the differences in rough￾ness between the fibers were small, the calculated radial strain and radial normal stress in composites reinforced with HI-Nicalon were higher than in those reinforced with Nicalon. This result indicates that small changes in the roughness of a fiber can significantly affect the debond ing and sliding properties between the fiber and matrix. INTRODUCTION The nature of bonding between the fiber and matrix in continuous fiber-reinforced ceramic matrix composites determines the properties of the composite [l-3]. If a fiber is strongly bonded to the matrix, the energy at a crack tip in the vicinity of an interface may be high enough to fracture the fiber. In this case, the crack propagates straight through the interface, into the fiber; the composite usually fails in a catastrophic manner. If the fiber/matrix interface is tai￾lored (e.g., by an appropriate fiber coating), such that the degree of bonding between the fiber and matrix is weak, a propagating crack will be deflected at the interface, causing it to lose energy. In this manner, debonding and sliding of the fiber with re￾spect to the matrix acts as an energy-ab￾sorbing mechanism. Several I.mportant factors control the fi￾ber/matrix behavior. For example, thermal residual stresses from processing will in￾duce radial stresses at the interface (which, for most composites, are compressive) and axial stresses between the matrix and fibers [4,5]. Fiber surface roughness can also con￾tribute a mechanical component to the ra￾dial clamping stresses between the fibers and matrix, because the matrix becomes mechanically keyed to the fiber [6-B]. A roughness-induced strain arises because of this mechanical keying. This strain can be estimated by the roughness amplitude be￾tween the fiber and matrix. Figure 1 shows schematically the qualitative effect of fiber roughness on the sliding behavior at the fi￾ber/matrix interface of a composite. An adequate technique for the measure￾ment of surface roughness in ceramic fibers is not available. Conventional profilometry techniques are accurate only to lmm, while the roughness in ceramic fibers may be as low as lnm. With the commercial availabil￾ity of the atomic force microscope (AFM), 199 MATERIALS CHARACTERIZATION 35199-206 (1995) Q Elsev~er Science Inc., (1995) 655 Avenue of the Americas New York, NY 10010 10445803/95/$9.50 SSDI 10445803(95)001034