N. Chaula et al Roughness FIG1. Qualitative effect of roughness on the sliding behavior at the fiber/ matrix interface in a composite several surface studies have been con- by Nippon Carbon) contains substantially ducted in the areas of polymers, semicon- less free oxygen, so it is very attractive as a ductors, and biology [9-12]. The AFM has a reinforcement for ceramic matrices at high distinct advantage over the scanning tun-temperatures heling microscope because the technique require a conductive surface Chawla ct al. were the first to usc an AFM EXPERIMENTAL PROCEDURE as a tool for measuring surface roughness of ceramic fibers [13]. They examined three In the AFM, a very sharp gold-coated Si3n4 different continuous AlO fibers and quan- tip a few microns in diameter is attached to titatively described the extent of roughness a cantilever probe and placed a few ang- and its effect on propertie ay from the specimen surface (Fig. 2). In this study, the AFM(Digital In- In this study we have used the afm to struments na scope Ill, Santa barbara examine and quantify the surface rough- CA)was used in contact mode, where the ness of two polymer-derived ceramic fi- tip was in actual contact with the specimen bers, NicalonTM and HI-NicalonTM(Nippon surface. IL should be noted that the sharper Carbon, Tokyo, Japan). The composition of in AFM tip, the higher is the resolution in the two fibers is given in Table 1. Nicalon is tracing contours of a given surface. The in- used extensively in ceramic composites be- teratomic repulsion that exists between the low cost, and surface and the tip causes a deflection of simall diameter [14, 15]. Unfortunately, it the cantilever. A laser is used to measure contains a substantial amount of free oxy- the deflection, and the signal is processed gen so at high temperatures the oxygen to obtain images as well as profiles of sur- combines with free silicon to form a glassy face roughness phase, SiO2, that forms a strong bond be- To prepare the specimen for the AFM, tween the fiber and matrix, and is therefore the fibers were placed in an acetone bath in detrimental to operties of the com- an ultrasonic cleaner to remove the protec- posite [16 HI-Nicalon(recently developed tive sizing on the surface. Next, a thin layer Table 1 Properties of Nicalon and HI-Nicalon Fibers nicalon HI-Nicalon Composition(wt %) 58%Si31%C,11%O63.7%Si,35.8%C,0.5%O 12-18 Elastic modulus(GPa) Coefficient of thermal expansion(10-6K-) 3.9200 N. Chawla et al. > \ matrix FIG. 1. Qualitative effect of roughness on the sliding behavior at the fiber/matrix interface in a composite. several surface studies have been con￾ducted in the areas of polymers, semicon￾ductors, and biology [9-121. The AFM has a distinct advantage over the scanning tun￾neling microscope because the technique does not require a conductive surface. Chawla et al. were the first to use an AFM as a tool for measuring surface roughness of ceramic fibers [13]. They examined three different continuous A1203 fibers and quan￾titatively described the extent of roughness and its effect on properties of the compos￾ite. In this study, we have used the AFM to examine and quantify the surface rough￾ness of two polymer-derived ceramic fi￾bers, NicalonT”r and HI-NicalonT” (Nippon Carbon, Tokyo, Japan). The composition of the two fibers is given in Table 1. Nicalon is used extensively in ceramic composites be￾cause of its high strength, low cost, and small diameter [14, 151. Unfortunately, it contains a substantial amount of free oxy￾gen so at high temperatures the oxygen combines with free silicon to form a glassy phase, SiOt that forms a strong bond be￾tween the fiber and matrix, and is therefore detrimental to the properties of the com￾posite [16]. HI-Nicalon (recently developed by Nippon Carbon) contains substantially less free oxygen, so it is very attractive as a reinforcement for ceramic matrices at high temperatures. EXPERIMENTAL PROCEDURE In the AFM, a very sharp gold-coated Si3N4 tip a few microns in diameter is attached to a cantilever probe and placed a few ang￾stroms away from the specimen surface (Fig. 2). In this study, the AFM (Digital In￾struments Nanoscope III, Santa Barbara, CA) was used in contact mode, where the tip was in actual contact with the specimen surface. It should be noted that the sharper in AFM tip, the higher is the resolution in tracing contours of a given surface. The in￾teratomic repulsion that exists between the surface and the tip causes a deflection of the cantilever. A laser is used to measure the deflection, and the signal is processed to obtain images as well as profiles of sur￾face roughness. To prepare the specimen for the AFM, the fibers were placed in an acetone bath in an ultrasonic cleaner to remove the protec￾tive sizing on the surface. Next, a thin layer Table 1 Properties of Nicalon and HI-Nicalon FiberP Composition (wt.%) Fiber diameter (pm) Elastic modulus (GPa) Tensile strength (GPa) Coefficient of thermal expansion (10e6 K-l) Nlcalon HI-Nlcalotl 58% Si I 31% C 11% 0 63.7% Si, 35.8% C, 0.5% 0 12-18 ’ 12-18 193 269 2.96 2.80 3.9 - “(Manufacturer’s reported data, Dow Corning Corp., 1995)