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(9501036
ELSEVIER 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 oxygen-free Si-C fiber, HI-Nicalon). The latter fiber had a slightly higher roughness amplitude, which may be caused by differences in processing. Although the differences in roughness 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 tailored (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 respect to the matrix acts as an energy-absorbing mechanism. Several I.mportant factors control the fiber/matrix behavior. For example, thermal residual stresses from processing will induce 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 contribute a mechanical component to the radial 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 between the fiber and matrix. Figure 1 shows schematically the qualitative effect of fiber roughness on the sliding behavior at the fiber/matrix interface of a composite. An adequate technique for the measurement 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 availability 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
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-) 3.9
200 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 conducted in the areas of polymers, semiconductors, and biology [9-121. The AFM has a distinct advantage over the scanning tunneling 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 quantitatively described the extent of roughness and its effect on properties of the composite. In this study, we have used the AFM to examine and quantify the surface roughness of two polymer-derived ceramic fibers, 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 because of its high strength, low cost, and small diameter [14, 151. Unfortunately, it contains a substantial amount of free oxygen so at high temperatures the oxygen combines with free silicon to form a glassy phase, SiOt that forms a strong bond between the fiber and matrix, and is therefore detrimental to the properties of the composite [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 angstroms away from the specimen surface (Fig. 2). In this study, the AFM (Digital Instruments 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 interatomic 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 surface roughness. To prepare the specimen for the AFM, the fibers were placed in an acetone bath in an ultrasonic cleaner to remove the protective 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)
Itomic Force Microscopy of Ceramic Fibers Laee Mirror specimen Cantilever probo FIG. 2. Schematic of the atomic force microscope (AFM). of hot wax was placed on a steel magnetic where L is the length of the roughness pro- disk. Using a light microscope, dry single file and f(x)is the roughness curve. Addi- fibers were placed on the wax and allowed tional roughness parameters that were cal- to cool down Care was taken to ensure that culated are rmax, the difference between no wax was present on the surface of the fi- highest and lowest point in a given image, ber and the: fiber was completely flat(bow- and Rrms, the root-mean squared rough ng or movement of th ring AFM scanning would yield erroneous results (Fig. 3). The magnetic disk was positioned in the afm and the cantilever tip was low red close to the fiber when the cantilever descended until the cantilever touched he fiber surface. several locations were canned at the top of the fiber surface, and he scan direction was varied to ensure that a given image was not an artifact of the AFM tip After capturing the images, polynomial fits were applied to"flatten"the curved surface obtained from the afm. three-d mensional and section profiles of the fiber surface were obtained and a variety of quantitative roughness values were calcu- lated, such which is defined as: 10 um f(x)dx FIG3. Scanning electron micro h of nicalon fi bers on the magnetic disk before AFM imaging
Atomic Force Microscopy of Cemmic Fibers 201 Diode V d FIG. 2. Schematic of the atomic force microscope (AFM). of hot wax was placed on a steel magnetic disk. Using a light microscope, dry single fibers were placed on the wax and allowed to cool down. Care was taken to ensure that no wax was present on the surface of the fiber and the fiber was completely flat (bowing or movement of the fiber during AFM scanning would yield erroneous results) (Fig. 3). The magnetic disk was positioned in the AFM and the cantilever tip was lowered close to the fiber. When the cantilever was close enough, the controlling computer descended until the cantilever touched the fiber surface. Several locations were scanned at the top of the fiber surface, and the scan direction was varied to ensure that a given image was not an artifact of the AFM tip. After capturing the images, polynomial fits were applied to “flatten” the curved surface obtained from the AFM. Three-dimensional and section profiles of the fiber surface were obtained and a variety of quantitative roughness values were calculated, such as the mean roughness, R,, which is defined as: L R, = ;jf(x)dx (1) 0 where L is the length of the roughness profile and f(x) is the roughness curve. Additional roughness parameters that were calculated are R,,,, the difference between highest and lowest point in a given image, and R,,,, the root-mean squared roughness. FIG. 3. Scanning electron micrograph of Nicalon fibers on the magnetic disk before AFM imagmg
N. Chala et al 175 875nm FIG4. Height mode image of the surface of Nicalon fiber RESULTS AND DISCUSSION from the areas scanned in both images (Ta- ble 2). HI-Nicalon has Figures 4 and 5 show height mode images roughness than Nicalon, but the roughness of Nicalon and HI-Nicalon fibers, respec- amplitude of both fibers is quite low, on the tively. Topographic views of the profiles order of 45nm. Taking into consideration are shown in Figs. 6 and 7. Notice the nod- such factors as the radius of the tip, type of ular surface profile in both fibers. It is spec- scanner, size of features being imaged, and ulated that these nodules form during pro- number of pixels in the image, the uncer cessing Melt-spinning processes are used tainty in the roughness measurements was to manufacturc Nicalon and HI-Nicalon fi- calculated as +0.67nm From Figs 6 and 7 bers(Fig 8). Polycarbosilane is spun and a section analyses were also conducted to green fiber, ready for sintering, is obtained. provide a graphic profile of the roughness The difference in processing between fibers( Fig 9). A line was drawn on the image and lies in the next steps. In Nicalon, the green the corresponding roughness profiles were fiber is cured by oxidation and pyrolyze plotted. Notice that both fibers have similar HI-Nicalon, on the other hand, is cured by profiles. Notice, however, that the rough an electron beam and then pyrolyzed this ness is more uniform in Nicalon than in procedure lowers the oxygen content HI-Nicalon. This may be critical during fi- verged roughness profiles were taken ber sliding, as a large amplitude between a FIG. 5. Height mode image of the surface of HI-Nicalon fiber
202 N. Chawla et al. 87.5 nm FIG. 4. Height mode image of the surface of Nicalon fiber. RESULTS AND DISCUSSION Figures 4 and 5 show height mode images of Nicalon and HI-Nicalon fibers, respectively. Topographic views of the profiles are shown in Figs. 6 and 7. Notice the nodular surface profile in both fibers. It is speculated that these nodules form during processing. Melt-spinning processes are used to manufacture Nicalon and HI-Nicalon fibers (Fig. 8). Polycarbosilane is spun and a green fiber, ready for sintering, is obtained. The difference in processing between fibers lies in the next steps. In Nicalon, the green fiber is cured by oxidation and pyrolyzed. HI-Nicalon, on the other hand, is cured by an electron beam and then pyrolyzed; this procedure lowers the oxygen content. Averaged roughness profiles were taken from the areas scanned in both images (Table 2). HI-Nicalon has a somewhat higher roughness than Nicalon, but the roughness amplitude of both fibers is quite low, on the order of 4-5nm. Taking into consideration such factors as the radius of the tip, type of scanner, size of features being imaged, and number of pixels in the image, the uncertainty in the roughness measurements was calculated as ?0.67nm. From Figs. 6 and 7, section analyses were also conducted to provide a graphic profile of the roughness (Fig. 9). A line was drawn on the image and the corresponding roughness profiles were plotted. Notice that both fibers have similar profiles. Notice, however, that the roughness is more uniform in Nicalon than in HI-Nicalon. This may be critical during fiber sliding, as a large amplitude between a 100 50 nm 0 FIG. 5. Height mode image of the surface of HI-Nicalon fiber
Itomic Force Microscopy of Ceramic Fibers 203 nm)100 幅m FIG. 6. Topographic view of the surface of Nicalon fiber given peak and valley may serve as an ob- tion of the roughness-induced radial stress stacle to sliding and the residual normal stress that arises It should be noted that with conventional from the mismatch in coefficient of therma i-expansion(CTE), Aa, between the fiber and croscopy, coating of the specimen would matrix [17, 18]. The roughness-induced ra- obliterate any desired features. Using low dial stress in their model, rough is given b olta without coating the specimen, but height EmEr nformation would not be possible without rough E(1+vm)+E( some kind of stereo microscopy which may not be nearly as accurate as the aFm The surface roughness information ob- where A is the characteristic roughness am- ained from the AFM can be used to deter- plitude, r is the radius of the fiber, and Em, nine the roughness-induced contribu- Et, Vm, and vf are the elastic moduli and tion to the radial stress on the fiber rough, Poisson ratios of fiber and matrix, respec from the matrix Kerans and Parthasarathy tively. This term is added to the residual showed that the radial stress is a combina- normal stress contribution from CtE mis- 200 ium) FIG. 7. Topographuc view of the surface of HI-Nicalon fiber
Atomic Force Microscopy of Ceramic Fibers 203 [nm) 100 FIG. 6. Topographic view of the surface of Nicalon fiber. given peak and valley may serve as an obstacle to sliding. It should be noted that with conventional techniques such as scanning electron microscopy, coating of the specimen would obliterate any desired features. Using lowvoltage microscopy, one can view images without coating the specimen, but height information would not be possible without some kind of stereo microscopy which may not be nearly as accurate as the AFM. tion of the roughness-induced radial stress and the residual normal stress that arises from the mismatch in coefficient of thermal expansion (CTE), ho, between the fiber and matrix [17, 181. The roughness-induced radial stress in their model, Trough, is given by: KtlE f u,ough = E,(l + vm) + E,(l - uf) (2) The surface roughness information ob- where A is the characteristic roughness amtained from the AFM can be used to deter- plitude, r is the radius of the fiber, and E,, mine the roughness-induced contribu- Er, v,, and vf are the elastic moduli and tion to the radial stress on the fiber (Trough, Poisson ratios of fiber and matrix, respecfrom the matrix. Kerans and Parthasarathy tively. This term is added to the residual showed that the radial stress is a combina- normal stress contribution from CTE mis- (nm) 100 (WI 3 ! 3 f w) 4 : ‘4 5 FIG. 7. Topographic view of the surface of HI-Nicalon fiber
Polycarbosilane matrix/coating interface are ignored, and the stresses in Eqs. (2) and (3)can be con- sider at the fiber/ Spinning interface, so all matrix terms(subscript m) correspond to fiber-coating terms. The roughness-induced strain, -A/r, was cal- culated by using the maximum roughness Rmav as a conservative estimate of the cha dation curing Electron beam acteristic roughness amplitude, A. The thermally induced strain component, △o△T, was assumed to be the same in both Pyrolysis fibers and was calculated by taking am acarbon=28.3X 10-K (pyrolitic carbon, perpendicular to the graphitic layers [19D), S-C fiber I C1- nIcalon= CHI-Nicalon=3.9 x 10-6K-I and for conventional chemical vapor depo- HI-NICALON sition of the coating, AT= 1400 K [20].Due t-spinning process used to manufacture to the large cte difference between coating and HI-Nicalon fibers, which tay influence and fiber, the thermally induced strains are roughness of fiber larger than the roughness-induced strain but due to higher roughness in HI-Nicalon match. The total residual normal stress, on, the net strain in the composite is higher is then given by [17, 18] with HI-Nicalon fibers( table 3) The radial clamping stresses were also cal -emEr △a△T+ culated using Eqs. (2)and(3), using ENicalon Er(l+ vm)Em(I-v) 193 GPa, EHI-Nisalon 269 GPa, nIcalon (3) VHI-Nicalon0. 12, Em=Carbon=12 GI a (per pendicular to graphitic layers),and carbon here△a d△ T is the 0. 4 ( perpendicular to grap hitec layers) ture gradient due to processing [19, 21]. Notice that, due solely to moduli In a model ceramic composite, Nicalon differences in the fiber, the thermally in- HI-Nicalon fibers would be coated with a duced stresses are greater with HI-Nicalon material such as pyrolitic carbon to pro- fibers(Table 3). Coupled with the higher mote a weak bond at the fiber/matrix inter- roughness-induced stresses in the later fi- face and interfacial sliding. It has been ber, the net radial clamping stress will be shown that debonding and crack deflection about 20 MPa higher in a composite with take place at the fiber/coating interface [2, HI-Nicalon fibers. With both fibers, how- 3]. Furthermore, in this model the pyrolitic ever, it is important to note that the net ra carbon coating is assumed to be compliant dial stress is quite high and compressive, so enough to relieve thermal-induced and that a substantial clamping stress is applied roughness-induced stresses at the coating/ to the fiber. Future studies will include the matrix interface. Thus, the stresses at the use of the aFM to examine wear damage to Table 2 Roughness Values of Nicalon and HI-Nicalon Fibers(uncertainty in roughness values is±0.67nm) HL-icaion Mean roughness(r,),nm 329 Root-mean square roughness(r ms),nm Max height(rmax),nm 43.14
204 1 Green,fiber 1 t Electron beam curing & pzq FSI_C NICALON Hi-NICALON FIG. 8. Melt-spinning process used to manufacture Nicalon and HI-Nicalon fibers, which may influence surface roughness of fibers. match. The total residual normal stress, (TN, is then given by [17,X3]: -wf UN = E,( 1 + v,)E,( 1 - Y~)(‘~‘~ + :) (3) where ACX = (Y, - tyf and AT is the temperature gradient due to processing. In a model ceramic composite, Nicalon or HI-Nicalon fibers would be coated with a material such as pyrolitic carbon to promote a weak bond at the fiber/matrix interface and interfacial sliding. It has been shown that debonding and crack deflection take place at the fiber/coating interface [2, 31. Furthermore, in this model the pyrolitic carbon coating is assumed to be compliant enough to relieve thermal-induced and roughness-induced stresses at the coating/ matrix interface. Thus, the stresses at the N. Chawla et al. matrix/coating interface are ignored, and the stresses in Eqs. (2) and (3) can be considered as the stresses at the fiber/coating interface, so all matrix terms (subscript m) correspond to fiber-coating terms. The roughness-induced strain, -A/r, was calculated by using the maximum roughness, R may, as a conservative estimate of the characteristic roughness amplitude, A. The thermally induced strain component, AoAT, was assumed to be the same in both fibers and was calculated by taking OL, = Ol,,,bOn = 28.3 X 10m6 Km1 (pyrolitic carbon, perpendicular to the graphitic layers [19]), ol = ~Nmlon = UHI-Nsalon = 3.9 x 1O-6 K-l, and for conventional chemical vapor deposition of the coating, AT = 1400 K [20]. Due to the large CTE difference between coating and fiber, the thermally induced strains are larger than the roughness-induced strains, but due to higher roughness in HI-Nicalon, the net strain in the composite is higher with HI-Nicalon fibers (Table 3). The radial clamping stresses were also calculated using Eqs. (2) and (3), using ENicaion = 193 GPa, EH~_.P.J,~~~~~ = 269 GPa, ~~~~~~~~ = ~~~~~~~~~~~ = 0.12, E, = Ecarbon = 12 GPa (perpendicular to graphitic layers), and &a&,,, = 0.4 (perpendicular to graphitic layers) [19, 211. Notice that, due solely to moduli differences in the fiber, the thermally induced stresses are greater with HI-Nicalon fibers (Table 3). Coupled with the higher roughness-induced stresses in the later fiber, the net radial clamping stress will be about 20 MPa higher in a composite with HI-Nicalon fibers. With both fibers, however, it is important to note that the net radial stress is quite high and compressive, so that a substantial clamping stress is applied to the fiber. Future studies will include the use of the AFM to examine wear damage to Table 2 Roughness Values of Nicalon and HI-Nicalon Fibers (uncertainty in roughness values is 2 0.67nm) Nicalon HI-Nzcalon Mean roughness (RJ, nm 3.29 4.28 Root-mean square roughness (R,), nm 4.08 5.42 Max height (R,,,), run 30.84 43.14
Atomic Force Microscopy of Ceramic Fibers 5 ength (um) (a) FIG9. Section analyses of surface roughness in(a)HI-Nicalon fiber and(b)Nicalon fiber the fibers during cyclic fatigue of ceramic calon and HI-Nicalon ceramic fibers using matrix composites. atomic force microsco The AFM is a simple and effective tool CONCLUSIONS for measuring surface roughness, if used carefully and consistently, with an un- everal conclusions can be drawn from the certainty in measurements of lnm quantification of surface roughness of Ni- Roughness profiles and analysis show Table 3 Roughness and Thermally Induced Strains and Stresses in Composites with Nicalon and HI-Nicalon Fibers nicalon HI-NIcalon Roughness-induced strain, rmah 00071 Thermally induced strain,△a△T Net radial strain 0.0412 luce stress, Rough (MPa) 42.4 Thermally induced stress, Othermal(MPa) Net radial normal stress, ON(MPa) -344
Atomic Force Microscopy of Ceramic Fibers 205 0 1 2 3 4 5 Length (urn) -20 -30 -40 t---,....,... 4w 0 1 2 3 4 Length Cm) 6 FIG. 9. Section analyses of surface roughness in (a) HI-Nicalon fiber and (b) Nicalon fiber. the fibers during cyclic fatigue of ceramic calon and HI-Nicalon ceramic fibers using matrix composites. atomic force microscopy: l The AFM is a simple and effective tool CONCLUSIONS for measuring surface roughness, if used carefully and consistently, with an unSeveral conclusions can be drawn from the certainty in measurements of lnm. quantification of surface roughness of Ni- l Roughness profiles and analysis show Table 3 Roughness and Thermally Induced Strains and Stresses in Composites with Nicalon and HLNicalon Fibers Nicalotz Hl-Nlcalon Roughness-induced strain, R,,,Jr -0.0051 -0.0071 Thermally induced strain, AaAT -0.0341 -0.0341 Net radial strain -0.0392 -0.0412 Roughness-induced stress, u‘rough (MPa) -42.4 -59.9 Thermally induced stress, vthermal (MPa) -281 -284 Net radial normal stress, UN (MPa) -323 -344
40:1243(1992) rous ughness amplitude than Nicalon fi- 9. G Binning, C F Quate, and Ch. Gerber, Atomic bers. This difference may be attributed to force microscope, Phys Reo. Lett. 56: 930(1986) differences in processing 10. T. R. Albrecth, M.M. dovek, C Higher roughness in HI-Nicalon induces Grutter, C. F. Quate, S w.J. Kuan, C. w. Frank, and R. F. W. Pease, Imaging and modification of greater roughness-induced strains and stresses which contribute to even higher atomic force microscopy, /. Appl. Phys. 64: 1178 clamping stresses than are observed with (1988) Nicalon fibers 11. S. Alexander, L. Hellemans, O Marti, ]. Schneir, V. Elings, P.K. Hansma, M. he authors thank R A Lowden of oak ridge National Laboratory and M. N. Gross of lemented using an optica 12. H. B. Butt, K H Downing, and P K. Hansma, Im- Nicalon fibers, respectively. N. C. thunks Prof. aging the membrane protein bacteriorhodop K. K. Chawla of New Mexico Tech and J with the atomic force microscope, J, Byophys Soc Honeyman of digital Instruments for useful 58:1473(1990) discussions.This work was supported by Dr. A. 13. K. K. Chawla, Z. R. Xu, A. Hlinak, and y- w Pechenik at the Air Force Ofice of Scientific Research (#F49620-95-1-0206)and the na Alumina-Type Fibers by Atomic Force Microscopy, Proc Conf on Advances in Ceramic Matrix Com tional science foundation osites, Indianapolis, IN(A 14. S. Yajima, K. Okamura, J. Hayashi, and F. E Wawner, Synthesis of continuous SiC fiber with References high tensile strength, J. Am. Ceram. Soc. 59: 324 (1976 K.KChawla, Ceramic MatrIx Compostes, Chap- 15..Ishika Silicon carbide continuous fiber man& Hall, London, p. 291(1993) (Nicalon), in Silicon Carbide Ceramics--2, S. Somiya 2. R. A. Lowden and D. P Stinton Interface modifi and Y. Inomata, eds. Elsevier, New York, p. 81 (1991) cation in Nicalon/SiC composites, Ceram. Eng SCl. 16. E. Bischoff, M. Ruhle, O. Sbaizero, and A.G. Pro.9:705(1988) 3. R. Venkatesh and K. K. Chawla, Effect of fiber Evans, Microstructural studies of the interfac one of a SiC- fiber-reinforced lithium aluminum roughness on the pullout of alumina/gl Posites, Mater. Sci. Lett. 11: 650(190 silicate glass-ceramic, A. Ceram. Sac. 72: 741 4.YMikata and M. Taya, Stress field in a coated 17.R.I. Kerans and T AParthasarathy, Theoretical continuous fiber composite subjected to thermo- mechanical loadings, / Cump Mater. 19: 554(1985) nalysis of the fiber pullout and pushout tests, J m. Ceram.Soc.74:1585(1991) 5. Z.R.Xu, K.K. Chawla, A. Neuman, A. Wolf- 18. T. A. Parthasarathy, P D Jero, and R J. Kerans, enden, G. M. Liggett, and N. Chawla, Stiffness los and density decrease due to thermal cycling in an erties from a fiber alumina fiber/magnesium alloy composite, Mat push-out test, Scripta Met. Mater. 25: 2457(1991) Sci Eng in press(1995) 19.Y.S. Toloukian and C. Y. Ho, eds, Thermophysical 6. R. W. Goettler and K.T. Faber. Interfacial shear Properties of Matter, Vol 13, Plenum, New York, P. tresses in SiC and alumina fiber reinforced 79(1977 glasses, Comp. Sci. Tech 37: 129 (1989 20. S. Shanmugham, D. P. Stinton, F. Rebillat, A Bleier T. M. Bes 7. P. D Jero and R J Kerans, The contribution of in terfacial roughness to sliding friction of ceramic fi Liaw, Oxidation-resistant interfacial coatings fo hers in a glass matrix, Scripta Met. Mater. 24: 231 ontinuous fiber ceramic composites, Ceram. Eng cl. Proc. in press(1995) 8. PD.Warren,T]. Mackin, and A G. Evans, De. 21. Engineered Materials Handbook, Vol. 4, ASM Inter- national, Materials Park, OH (1991 push-through test for the measurement of inter- face properties in composites, Acta. Metall. Mater. Received June 1995; accepted July 1995
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