WEAR ELSEVIER wear225-229(1999)868-873 Tribological behavior of alumina reinforced with unidirectionally oriented sic whiskers D-S. Lim a, *D-S Park b B-D. Han T-SKan Department of Materials Engineering, Korea Unicersity Dong, Sung- Buk-Gu Seoul, South Korea Ceramic Materials Group. K/MM, 66 San Won City, Kyong-Nam, South Korea Abstract Silicon carbide whisker reinforced alumina ceramic composites were subjected to tribological tests by using ball-on-reciprocating fla geometry at 403 K in order to study wear progress under mild wear regime. Alumina composites were prepared by a modified tape casting followed by lamination, binder removal and hot pressing in order to align the whiskers in tape casting direction. Wear coefficients on three directions were measured; parallel and normal to the tape casting direction on the tape casting surface and normal to lamination direction on surface normal to the tape casting direction. For comparison, samples were prepared by hot pressing the powder mixture. The direction.The results of this study also indicated that matrix grain size and roughness due to whisker orientation were responsible . o highest wear rate was obtained in the direction parallel to the tape casting direction and the lowest in the direction normal to lamina variations of wear rate and friction coefficient. o 1999 Published by Elsevier Science S.A. All rights reserved Keywords: Alumina, SiC whiskers, Unidirectionally oriented; Tape casting 1. Introduction tance increased. DellaCorte [5]reported that wear behavior of the pin was different from that of the disk as the Silicon carbide whiskers have been used for reinforcing temperature increased. In the case of pin wear, the behav- brittle alumina ceramics [1]. The reinforcements were ef- ior was similar to what Yust reported, but disk wear fective in toughening the ceramic composites by crack decreased as the temperature increased. DellaCorte exam- deflection [2] and crack bridging [1]. In other words, when ined the wear scar area by using TEM and SEM. His the crack met the whisker in the composite, it propagated analysis indicated that the predominant failure mode at along the boundary between alumina matrix and the rein- room temperature was crack initiation and growth fol- forcement instead of cutting through it. Due to the differ- lowed by delamination and removal of the fractured parti ence in thermal expansion coefficients between alumina cles. So, brittle fracture of both the matrix and the whiskers and SiC, the whisker is under compressive stress and the was primary wear mode of the composite. Potential appli matrix under tensile stress. Improved fracture toughness of cations of the whisker reinforced ceramic composites are alumina-SiC whisker composites made the composites not limited to the cutting tools. So, a silicon nitride ball attractive for cutting tool applications where both wear was chosen as a counterpart material to examine how the resistance and fracture toughness were needed [3]. They orientation and content of the whiskers influence the abra- exhibited an excellent performance in machining Inconel sive wear behavior of alumina based ceramics 718, a nickel based superalloy. Also, tribological character- istics of alumina-SiC whisker composites was investigated by researchers [4, 5]. Yust [4] reported that wear of the 2. Experimental details composite went through a transition from mild wear to severe wear as the temperature increased or sliding dis Table 1 shows compositions of samples used for this study For sample A, the two ceramic powders were mixed by planetary ball milling for 8 h. Ethanol, alumina balls of 5 mm in diameter(SSA999, Nikkato, Tokyo, Japan)and a Corresponding author. Fax: +82-29295344: e-mail plastic jar were used for mixing. The mixed powder was dslimakuccnx. korea. ac kr hot pressed at 1823 K for I h 30 MP d. san 0043-1648/99/s- see front matter o 1999 Published by Elsevier Science S.A. All rights reserved P:S0043-1648(99)00068-X
Wear 225–229 1999 868–873 Ž . Tribological behavior of alumina reinforced with unidirectionally oriented SiC whiskers D.-S. Lim a,), D.-S. Park b , B.-D. Han b , T.-S. Kan a a Department of Materials Engineering, Korea UniÕersity, 1 Anam-Dong, Sung-Buk-Gu, Seoul, South Korea b Ceramic Materials Group, KIMM, 66 Sang-Nam-Dong, Chang-Won City, Kyong-Nam, South Korea Abstract Silicon carbide whisker reinforced alumina ceramic composites were subjected to tribological tests by using ball-on-reciprocating flat geometry at 403 K in order to study wear progress under mild wear regime. Alumina composites were prepared by a modified tape casting followed by lamination, binder removal and hot pressing in order to align the whiskers in tape casting direction. Wear coefficients on three directions were measured; parallel and normal to the tape casting direction on the tape casting surface and normal to lamination direction on surface normal to the tape casting direction. For comparison, samples were prepared by hot pressing the powder mixture. The highest wear rate was obtained in the direction parallel to the tape casting direction and the lowest in the direction normal to lamination direction. The results of this study also indicated that matrix grain size and roughness due to whisker orientation were responsible for variations of wear rate and friction coefficient. q 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Alumina; SiC whiskers; Unidirectionally oriented; Tape casting 1. Introduction Silicon carbide whiskers have been used for reinforcing brittle alumina ceramics 1 . The reinforcements were ef- w x fective in toughening the ceramic composites by crack deflection 2 and crack bridging 1 . In other words, when wx wx the crack met the whisker in the composite, it propagated along the boundary between alumina matrix and the reinforcement instead of cutting through it. Due to the difference in thermal expansion coefficients between alumina and SiC, the whisker is under compressive stress and the matrix under tensile stress. Improved fracture toughness of alumina–SiC whisker composites made the composites attractive for cutting tool applications where both wear resistance and fracture toughness were needed 3 . They w x exhibited an excellent performance in machining Inconel 718, a nickel based superalloy. Also, tribological characteristics of alumina–SiC whisker composites was investigated by researchers 4,5 . Yust 4 reported that wear of the w x wx composite went through a transition from mild wear to severe wear as the temperature increased or sliding dis- ) Corresponding author. Fax: q 82-29295344; e-mail: dslim@kuccnx.korea.ac.kr tance increased. DellaCorte 5 reported that wear behavior w x of the pin was different from that of the disk as the temperature increased. In the case of pin wear, the behavior was similar to what Yust reported, but disk wear decreased as the temperature increased. DellaCorte examined the wear scar area by using TEM and SEM. His analysis indicated that the predominant failure mode at room temperature was crack initiation and growth followed by delamination and removal of the fractured particles. So, brittle fracture of both the matrix and the whiskers was primary wear mode of the composite. Potential applications of the whisker reinforced ceramic composites are not limited to the cutting tools. So, a silicon nitride ball was chosen as a counterpart material to examine how the orientation and content of the whiskers influence the abrasive wear behavior of alumina based ceramics. 2. Experimental details Table 1 shows compositions of samples used for this study. For sample A, the two ceramic powders were mixed by planetary ball milling for 8 h. Ethanol, alumina balls of 5 mm in diameter SSA999, Nikkato, Tokyo, Japan and a Ž . plastic jar were used for mixing. The mixed powder was hot pressed at 1823 K for 1 h under 30 MPa. Samples W10 0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0043- 1648 99 00068-X Ž
surements were expressed in the results. Wear scar was Starting ceramic powder compositions of samples(vol % examined by scanning electron microscope equipped with T20 an energy dispersive X-ray analyzer 995 79.5 AKP-30, Sumitomo Chemical, Osaka, Japan EP, Junsei Chemical, Tokyo, Japan The density of sample was higher than 99% of theoreti SCW#1, Tateho Chemical Industries, Hyogo, Japan. cal density. Fig. I shows friction coefficient of samples The friction coefficient of sample A against the silicon nitride ball is 0.7 which was lower than that of a sample and w20 were prepared according to the same procedure with 10 vol. SiC whiskers. Friction coefficients of sam- for sample A except for the fact that planetary ball ples with 20 vol. whiskers were lower than the corre- milling without whisker was stopped after 7.5 h and then sponding values of samples with 10 vol. whiskers. The esumed for 0.5 h after adding the whiskers. Hot pressing measured friction coefficient was sensitive to the contact was performed at 2123 K under 30 MPa for I h after that. surface roughness. Fig. 2 shows worn surface of sample A Diameter and length of the whiskers were 1-1. 4 and and W10 Worn area of samples were smooth and showed 10-20 um, respectively according to information supplied plastic deformation. This is thought to be due to the fact from the manufacturer. For samples T10 and T20, a modi- that the sample was at a very early stage of wear and the fied tape casting method was employed for aligning the mild wear regime prevailed. Sample A has a smoother whiskers. Tape casting slurry was prepared by using plane- wear track than the other, which was consistent with tary ball milling. Metyl-Isobutyl-Ketone, KD-1(ICI Chem- measured friction coefficients. Some grain boundaries of ical, Barcelona, Spain), polyvinyl-butyral (Aldrich Chemi- the samples showed up as a result of the wear test, which cal, New York, USA)and di-butyl phathalate(Aldrich contributed to increasing roughness of the contact surface Chemical) were used as solvent, dispersant, binder andand variations in the friction coefficient. Grain size of plasticizer, respectively. Care was exercised to minimize sample A was smaller than that of sample w10 due to damage of the whiskers during mixing, mixing without the lower hot pressing temperature. When the sample con- whiskers was carried out for 7.5 h and then for 0.5 h after tained Sic whiskers harder than the silicon nitride ball. the whisker addition. The slurry was poured into a reservoir of whiskers scratched the surface of the ball and ridges tape casting equipment that was modified for better align- formed starting from the whisker on the surface of the ment of the whiskers. Details of the modification was composite sample as shown in Fig. 2(b) Reported knoop described elsewhere [6]. Tape cast products were cut and hardness values of single crystal SiC for(0001) plane in laminated at 353 K under 50 MPa for 0.5 h. Binder [1010] and [1120] direction were 28.6 and 28.98 GPa, burn-out was performed at 823 K for 10 h in open air, and respectively [8]. The values for (1010) plane in [0001] and then the sample was hot pressed at 2123 K under 30 MPa [1120] direction(parallel to c-axis and normal to c-axis on Tribological behavior was studied by using the same ball-on-reciprocating flat geometry that was previously reported [7]. Silicon nitride ball (NDB100, Norton, North surface normal to hot pressing direction(conventional H P. boro, MA, USA)of 6.35 mm in diameter was used against normal to tape casting direction(tape casting flat alumina-SiC whisker composite sample surfaces which surface normal to lamination direction(tape casting were polished with I um diamond slurry. Normal load was 40N and average speed was 10 mm/s. Reciprocating stroke was 5.64 mm and duration was I h. Wear tests were performed at 403 K in order to eliminate moisture ad sorbed on the surface. In order to examine the effect of whisker orientation on wear rate. tests were carried out on two surfaces(tape surface and surface normal to tape casting direction) for samples W10 and W20, and in two o=9= directions on the tape surface for samples(parallel and normal to tape casting direction) T10 and Wear volume of the flat sample was obtained by wear track length times cross-sectional worn area that was measured from the center portion of the groove by a surface pro- Whisker content(vol % ometer after cleaning in a ultrasonic bath. Each wear test Fig. 1. Friction coefficients between the alumina-Sic whisker composite was repeated three times and average values of the mea- and the silicon nitride ball
D.-S. Lim et al.rWear 225–229 1999 868–873 ( ) 869 Table 1 Starting ceramic powder compositions of samples vol.% Ž . Sample A W10 W20 T10 T20 a Al O 99.5 89.5 79.5 89.5 79.5 2 3 MgO 0.5 0.5 0.5 0.5 0.5 b c SiC whisker – 10 20 10 20 a AKP-30, Sumitomo Chemical, Osaka, Japan. b EP, Junsei Chemical, Tokyo, Japan. c SCWa1, Tateho Chemical Industries, Hyogo, Japan. and W20 were prepared according to the same procedure as for sample A except for the fact that planetary ball milling without whisker was stopped after 7.5 h and then resumed for 0.5 h after adding the whiskers. Hot pressing was performed at 2123 K under 30 MPa for 1 h after that. Diameter and length of the whiskers were 1–1.4 and 10–20 mm, respectively according to information supplied from the manufacturer. For samples T10 and T20, a modified tape casting method was employed for aligning the whiskers. Tape casting slurry was prepared by using planetary ball milling. Metyl-Isobutyl-Ketone, KD-1 ICI Chem- Ž ical, Barcelona, Spain , polyvinyl-butyral Aldrich Chemi- . Ž cal, New York, USA and di-butyl phathalate Aldrich . Ž Chemical were used as solvent, dispersant, binder and . plasticizer, respectively. Care was exercised to minimize damage of the whiskers during mixing; mixing without the whiskers was carried out for 7.5 h and then for 0.5 h after whisker addition. The slurry was poured into a reservoir of tape casting equipment that was modified for better alignment of the whiskers. Details of the modification was described elsewhere 6 . Tape cast products were cut and w x laminated at 353 K under 50 MPa for 0.5 h. Binder burn-out was performed at 823 K for 10 h in open air, and then the sample was hot pressed at 2123 K under 30 MPa for 1 h. Tribological behavior was studied by using the same ball-on-reciprocating flat geometry that was previously reported 7 . Silicon nitride ball NDB100, Norton, North- w x Ž boro, MA, USA of 6.35 mm in diameter was used against . flat alumina–SiC whisker composite sample surfaces which were polished with 1 mm diamond slurry. Normal load was 40 N and average speed was 10 mmrs. Reciprocating stroke was 5.64 mm and duration was 1 h. Wear tests were performed at 403 K in order to eliminate moisture adsorbed on the surface. In order to examine the effect of whisker orientation on wear rate, tests were carried out on two surfaces tape surface and surface normal to tape Ž casting direction for samples W10 and W20, and in two . directions on the tape surface for samples parallel and Ž normal to tape casting direction T10 and T20. Wear . volume of the flat sample was obtained by wear track length times cross-sectional worn area that was measured from the center portion of the groove by a surface profilometer after cleaning in a ultrasonic bath. Each wear test was repeated three times and average values of the measurements were expressed in the results. Wear scar was examined by scanning electron microscope equipped with an energy dispersive X-ray analyzer. 3. Results and discussions The density of sample was higher than 99% of theoretical density. Fig. 1 shows friction coefficient of samples. The friction coefficient of sample A against the silicon nitride ball is 0.7 which was lower than that of a sample with 10 vol.% SiC whiskers. Friction coefficients of samples with 20 vol.% whiskers were lower than the corresponding values of samples with 10 vol.% whiskers. The measured friction coefficient was sensitive to the contact surface roughness. Fig. 2 shows worn surface of sample A and W10. Worn area of samples were smooth and showed plastic deformation. This is thought to be due to the fact that the sample was at a very early stage of wear and the mild wear regime prevailed. Sample A has a smoother wear track than the other, which was consistent with measured friction coefficients. Some grain boundaries of the samples showed up as a result of the wear test, which contributed to increasing roughness of the contact surface and variations in the friction coefficient. Grain size of sample A was smaller than that of sample W10 due to lower hot pressing temperature. When the sample contained SiC whiskers harder than the silicon nitride ball, the whiskers scratched the surface of the ball and ridges formed starting from the whisker on the surface of the composite sample as shown in Fig. 2 b . Reported knoop Ž . hardness values of single crystal SiC for 0001 plane in Ž . wx wx 1010 and 1120 direction were 28.6 and 28.98 GPa, respectively 8 . The values for 1010 plane in 0001 and w x Ž . w x w x 1120 direction parallel to Ž c-axis and normal to c-axis on Fig. 1. Friction coefficients between the alumina–SiC whisker composite and the silicon nitride ball
D.-S. Lim et al./Wea225-2291999868-873 al to hot pressing direction(conventional H P) o. parallel to tape casting direction(tape casting -+-normal to tape casting direction(tape casting --V--surface normal to lamination direction( tape casting) m260226k0-sx 989 Whisker content(vol% Fig. 3. Wear of the alumina-SiC whisker composite samples and the balls EUl 28kU prismatic plane, respectively) were 20.9 and 27.03 GPa. It also interesting to note that the width of the ridge KIMM 的e2kU5x4,8B日 determined by projection length of the whisker normal to the direction of motion. It implies that whiskers were held strongly by the matrix. It was understood that SiC whiskers did not provide the weak grain boundaries for improving the fracture toughness. This is quite different from some other ceramic composite materials where cracks propa gated through weak grain boundary and the fracture tough ness increased [9]. Ceramic composites with weak grain boundary were reported not to exhibit good wear resis tance in spite of the high fracture toughness [10]. In fact unlike the composites with weak boundary, a significant portion of the reinforcements(which were whiskers in this were present within the matrix grains(alumina s). So, SiC whiskers in the sample were thought to protect the surface and formed ridges which, however KIMM 包白28kU5μmx4;88 ere thought to increase the surface roughness and friction coefficient Fig. 4. SEM micrographs of etched surface of w10 and W20 sample with Fig. 3 shows wear of the samples and the balls (wear w20 can be seen. The samples were thermally etched for 0.5 h at 1723K scar diameter). Sample W10 exhibited higher wear rate
870 D.-S. Lim et al.rWear 225–229 1999 868–873 ( ) Fig. 2. SEM micrographs of wear track on sample A a and sample W10 Ž . Ž . b . prismatic plane, respectively were 20.9 and 27.03 GPa. It . is also interesting to note that the width of the ridge was determined by projection length of the whisker normal to the direction of motion. It implies that whiskers were held strongly by the matrix. It was understood that SiC whiskers did not provide the weak grain boundaries for improving the fracture toughness. This is quite different from some other ceramic composite materials where cracks propagated through weak grain boundary and the fracture toughness increased 9 . Ceramic composites with weak grain w x boundary were reported not to exhibit good wear resistance in spite of the high fracture toughness 10 . In fact, w x unlike the composites with weak boundary, a significant portion of the reinforcements which were whiskers in this Ž case were present within the matrix grains alumina . Ž grains . So, SiC whiskers in the sample were thought to . protect the surface and formed ridges which, however, were thought to increase the surface roughness and friction coefficient. Fig. 3 shows wear of the samples and the balls wear Ž scar diameter . Sample W10 exhibited higher wear rate . Fig. 3. Wear of the alumina–SiC whisker composite samples and the balls. Fig. 4. SEM micrographs of etched surface of W10 and W20 sample with aligned whiskers; smaller grain size of sample W20 than that of sample W20 can be seen. The samples were thermally etched for 0.5 h at 1723 K in Ar environment
D.-S. Lim et al./Wea225-2291999868-873 than sample A. Cho et al. [11] reported that alumina with tained in the direction normal to tape casting direction on larger grains was vulnerable to intergranular damage due tape surface and highest in the direction normal to the to the residual stress and accumulated deformation. The lamination direction as shown in Fig. 1. Miyoshi and higher wear rate of sample w10 than that of sample A was Buckley [12] reported that the friction coefficient of single thought to be due to larger matrix grain size. Wear scar crystal SiC against the spherical diamond indenter was diameters on the ball were measured. The diameter on the lowest on the basal plane and highest in the direction ball worn against sample a was similar to that against parallel to the c-axis. They also reported that the wear sample W10. When whisker content increased from 10 groove width was smallest on the basal plane and largest vol. to 20 vol % the friction coefficient decreased from in the direction parallel to the c-axis. Their friction coeffi 0.91 to 0.67. Less wear of sample W20 than that of sample cient measurements showed a different tendency from the 10 shown in Fig 3 was explained by smaller matrix results of this study shown in Fig. I due to the fact that not grain growth, retarded by the whiskers(Fig. 4). However, only SiC whiskers but also the alumina matrix was present ear of sample w20 was still larger than sample a due to in the current samples. The friction coefficient parallel to rger matrix grains. Grain size of the ceramic exerted a tape casting direction was similar to that normal to thelam- strong influence on the wear behavior. ination direction. This was due to the fact that the projec- Fig 5 shows optical micrographs of the wear track of tion lengths of the whiskers normal to the direction of amples with 10 vol. whiskers. Whiskers were randomly motion that was parallel to the tape casting direction and oriented in sample w10. The whiskers of sample T10 were normal to lamination direction were similar to each other aligned in the tape casting direction. Sliding on sample Friction coefficients in the two directions were also close 10 was carried out in three directions with respect to the to the friction coefficient of W10. The friction coefficient whisker orientation, but the whiskers were well aligned as was lower in the direction normal to the tape casting shown in Fig. 5(b)-(d). Sample T10 showed different direction on the tape surface. As already mentioned, the friction coefficients depending on the orientation and sur- friction coefficient depended on the contact surface condi- face of the test. The lowest friction coefficient was ob- tions. Sliding in the two directions (i.e, parallel to the tape Sliding direction (b) (d),餐 5. Optical micrographs of wear track on the samples; (a)sample W10, (b) sample T1O-sliding parallel to the whisker length direction, (c) ple T10-sliding normal to the whisker length direction, and (d)sample T10-sliding normal to lamination direction; bright contrast represents the
D.-S. Lim et al.rWear 225–229 1999 868–873 ( ) 871 than sample A. Cho et al. 11 reported that alumina with w x larger grains was vulnerable to intergranular damage due to the residual stress and accumulated deformation. The higher wear rate of sample W10 than that of sample A was thought to be due to larger matrix grain size. Wear scar diameters on the ball were measured. The diameter on the ball worn against sample A was similar to that against sample W10. When whisker content increased from 10 vol.% to 20 vol.%, the friction coefficient decreased from 0.91 to 0.67. Less wear of sample W20 than that of sample W10 shown in Fig. 3 was explained by smaller matrix grain growth, retarded by the whiskers Fig. 4 . However, Ž . wear of sample W20 was still larger than sample A due to larger matrix grains. Grain size of the ceramic exerted a strong influence on the wear behavior. Fig. 5 shows optical micrographs of the wear track of samples with 10 vol.% whiskers. Whiskers were randomly oriented in sample W10. The whiskers of sample T10 were aligned in the tape casting direction. Sliding on sample T10 was carried out in three directions with respect to the whisker orientation, but the whiskers were well aligned as shown in Fig. 5 b – d . Sample T10 showed different Ž. Ž. friction coefficients depending on the orientation and surface of the test. The lowest friction coefficient was obtained in the direction normal to tape casting direction on tape surface and highest in the direction normal to the lamination direction as shown in Fig. 1. Miyoshi and Buckley 12 reported that the friction coefficient of single w x crystal SiC against the spherical diamond indenter was lowest on the basal plane and highest in the direction parallel to the c-axis. They also reported that the wear groove width was smallest on the basal plane and largest in the direction parallel to the c-axis. Their friction coefficient measurements showed a different tendency from the results of this study shown in Fig. 1 due to the fact that not only SiC whiskers but also the alumina matrix was present in the current samples. The friction coefficient parallel to tape casting direction was similar to that normal to thelamination direction. This was due to the fact that the projection lengths of the whiskers normal to the direction of motion that was parallel to the tape casting direction and normal to lamination direction were similar to each other. Friction coefficients in the two directions were also close to the friction coefficient of W10. The friction coefficient was lower in the direction normal to the tape casting direction on the tape surface. As already mentioned, the friction coefficient depended on the contact surface conditions. Sliding in the two directions i.e., parallel to the tape Ž Fig. 5. Optical micrographs of wear track on the composite samples; a sample W10, b sample T10-sliding parallel to the whisker length direction, c Ž. Ž. Ž. sample T10-sliding normal to the whisker length direction, and d sample T10-sliding normal to lamination direction; bright contrast represents the Ž . whiskers
D.-S. Lim et al./Wea225-2291999868-873 Sliding direction Fig. 6. SEM micrographs of wear track on the composite samples; (a) sample TIO-sliding parallel to whisker length direction, (b) sample T10-sliding normal to whisker length direction, (c)sample TIO-sliding normal to lamination direction, and(d) higher magnification of the track shown in(c) casting direction and normal to the lamination direction) sliding direction in the surface and formed the layer that produced fine grooves and ridges, the widths of which smeared on the whiskers and protected the surface as were similar to the diameters of the whiskers as shown in shown in Fig. 7. Some of the debris was hard enoug ig. 6(a) and(c). Whiskers were often discerned from the surface as indicated by arrowhead in Fig. 6(a). Some of the whiskers shown in Fig. 6(a) were off alignment. Fig. 6(b) shows that whiskers were normal to the sliding direction Even though some of the whiskers were broken and off the surface as indicated by the arrowhead, the friction coeffi- cient was lower than the values obtained from sliding in the other two directions. Fine ridges started from the whiskers, these appeared as gray spots and areas. Fine grooves and ridges increased actual contact surface area and the friction force. Examination of the wear track revealed that part of the surface of altered alumina grain due to the plastic deformation and part of it was covered with wear debris layer as indicated by the arrowhead in Fig 6(d). Formation of the debris layer was clearly shown in closer examination (Fig. 7). In the area exhibiting plastic deformation, the Sic whiskers stood up above the surface and started the ridges. Wear debris were from the Fig. 7. SEM micrograph showing formation of debris layer behind the ball as it went over the whiskers lying normal to the whisker on the surface of wear track
872 D.-S. Lim et al.rWear 225–229 1999 868–873 ( ) Fig. 6. SEM micrographs of wear track on the composite samples; a sample T10-sliding parallel to whisker length direction, b sample T10-sliding Ž. Ž. normal to whisker length direction, c sample T10-sliding normal to lamination direction, and d higher magnification of the track shown in c . Ž. Ž. Ž. casting direction and normal to the lamination direction. produced fine grooves and ridges; the widths of which were similar to the diameters of the whiskers as shown in Fig. 6 a and c . Whiskers were often discerned from the Ž. Ž. surface as indicated by arrowhead in Fig. 6 a . Some of the Ž . whiskers shown in Fig. 6 a were off alignment. Fig. 6 b Ž. Ž. shows that whiskers were normal to the sliding direction. Even though some of the whiskers were broken and off the surface as indicated by the arrowhead, the friction coefficient was lower than the values obtained from sliding in the other two directions. Fine ridges started from the whiskers; these appeared as gray spots and areas. Fine grooves and ridges increased actual contact surface area and the friction force. Examination of the wear track revealed that part of the surface of altered alumina grain due to the plastic deformation and part of it was covered with wear debris layer as indicated by the arrowhead in Fig. 6 d . Formation of the debris layer was clearly shown Ž . in closer examination Fig. 7 . In the area exhibiting Ž . plastic deformation, the SiC whiskers stood up above the surface and started the ridges. Wear debris were from the ball as it went over the whiskers lying normal to the sliding direction in the surface and formed the layer that smeared on the whiskers and protected the surface as shown in Fig. 7. Some of the debris was hard enough to Fig. 7. SEM micrograph showing formation of debris layer behind the whisker on the surface of wear track
D.-S. Lim et al./Wea225-2291999868-873 873 scratch the whisker as well as the alumina matrix by three smaller than that parallel to the direction in spite of the body wear. No visible inter-and intra-granular cracks were fact that the friction coefficients showed the opposite present. High compressive residual stress prevented the whiskers from being pulled out during the tests. Even though it should depend on the crystallographic orienta 4. Conclusions ion, elastic modulus of Sic was comparable to that of alumina. So, the grain boundary was thought to experience The monolithic alumina showed smallest wear because small deformation resulting from the difference in elastic of the smallest matrix grain size. Depending on sliding deformations of the matrix and the whisker under the orientation, friction and wear of the samples varied as contact stress. When whiskers were lying normal to the follows The friction coefficient was higher on the surface sliding direction in sample T10, they eliminate fine as- where more whiskers exposed their basal plane. Sliding perites on the contact surface of the ball. The longer the normal to the whisker length direction caused less wear projection length of the whiskers normal to the sliding than that parallel to it. As the whisker content increased direction, the larger the area covered with the debris layer from 10 vol. to 20 vol% both friction coefficient and as shown in Fig. 6(b). So, the contact surface became wear decreased due to the smaller matrix n size ther and the friction coefficient lower as shown in Even though whiskers were randomly oriented in the contact surface of sample w10 and were aligned Acknowledgements parallel to the sliding direction, the friction coefficients were quite similar to each other. It implied that the fine The authors(Korea University) would like to acknowl- lying normal to the sliding direction to be removed, and Korea c.hancial support for this study provided by the asperites on the ball took many passes over the whisker the friction stayed at a high value. When the whisker through the Ceramic Processing Research Center(CPRc) content increased from 10 vol% to 20 vol % the friction at Han Yang University coefficients decreased Wear also depended on sliding direction with respect to orientation of the whiskers as shown in Fig 3. Hardness of references single crystal Sic depends on the crystallographic orienta tion and plane. The basal plane exhibited the highest hardness and higher hardness was reported in the direction Ceran.Soc.74(1991)255-2 normal to the c-axis than parallel to it on prismatic plane 2] K.T. Farber, A.G. Evans, Crack deflection processes: 1. Theory, [12]. Based on the above information, sliding in the direc- cta Metal.31(1983)565-576 tion normal to the lamination direction should have the F. Shuster. CW whiskers of highest hardness because it involved more with alumina-SiC whisker cutting tools, Ceram. Eng. Sci. Proc. 9 contacts with basal plane of the SiC. So, the smallest wear [4] C.S. Yust, Tribological behavior of whisker-reinforced ceramic com in the direction normal to lamination direction was due to posite materials, in: S. Jahanmir(Ed. ) Friction and Wear of Ceram- highest hardness of the sic whiskers that slowed down the of shown in Fig. 6, the whiskers stood [5]C. DellaCorte, Tribological characteristics of silicon carbide p higher than the surface. The harder the whiskers were whisker-reinforced alumina at elevated temperatures, Friction and ar of Ceramics, Marcel-Dekker, New York, 1994, pp. 225-59 the longer they last and the slower the wear progressed [6] D.S. Park, C.-W. Kim, C. Park, Self-reinforced silicon nitride But the long lasting sharp whiskers increased the friction ning unidirectionally oriented silicon nitride whisker coefficients due to rougher contact surface. Sliding in tape seeds, to be published in Ceram. Eng. Sci. Proc casting direction involved contacts with whiskers length [7] D-S. Park, B.-D. Han, D-S. Lim, L-w. Yeo, A study on wear and erosion of sialon-Si3N4 whisker ceramic composites, Wear 203-204 direction where the hardness was lowest. The exposed (1997)284-290 whiskers were worn out faster than those of other orienta [8]M. Moser, Microstructure of Ceramics-Structure and Properties of tions, and the biggest wear was measured. The wear scar nding Tools, Akademiai Kiado, Budapest, 1980, p. 136 diameter of the ball was the largest and smallest in the [9 M. Yasuoka, K. Hirao, S. Kanzaki, Effects of TIN direction parallel to the whisker in the article size on mechanical pre direction on the tape surface, respectively. Sliding in the s,J. Ceran.Soc.Jpn.100(1992)617-620 [10l S.J. Cho, C.-D. Um, S.-S. Kim, Wear and wear transition in silicon direction normal to the lamination direction resulted in the carbide ceramics during sliding, J. Am. Ceram. Soc. 79(1996) smallest wear on the composite sample, but it caused more 1247-1251 vear of the ball than sliding normal to the whisker length [11] S.J. Cho, B. Hockey, B.R. Lawn, S.J. Bennison, Grain-size and direction. As the whisker content increased from 10 vol% R-curve effects in the abrasive wear of alumina, J. Am. Ceram Soc. 72(1989)1249-1252 to 20 vol % wear of the sample and the ball decreased. [12] T K. Miyoshi, D.H. Buckely, Ceramic wear in indentation and However. wear normal to whisker length direction was still sliding contact, ASLE Trans. 28(1985)296-302
D.-S. Lim et al.rWear 225–229 1999 868–873 ( ) 873 scratch the whisker as well as the alumina matrix by three body wear. No visible inter- and intra-granular cracks were present. High compressive residual stress prevented the whiskers from being pulled out during the tests. Even though it should depend on the crystallographic orientation, elastic modulus of SiC was comparable to that of alumina. So, the grain boundary was thought to experience small deformation resulting from the difference in elastic deformations of the matrix and the whisker under the contact stress. When whiskers were lying normal to the sliding direction in sample T10, they eliminate fine asperites on the contact surface of the ball. The longer the projection length of the whiskers normal to the sliding direction, the larger the area covered with the debris layer as shown in Fig. 6 b . So, the contact surface became Ž . smoother and the friction coefficient lower as shown in Fig. 1. Even though whiskers were randomly oriented in the contact surface of sample W10 and were aligned parallel to the sliding direction, the friction coefficients were quite similar to each other. It implied that the fine asperites on the ball took many passes over the whisker lying normal to the sliding direction to be removed, and the friction stayed at a high value. When the whisker content increased from 10 vol.% to 20 vol.%, the friction coefficients decreased. Wear also depended on sliding direction with respect to orientation of the whiskers as shown in Fig. 3. Hardness of single crystal SiC depends on the crystallographic orientation and plane. The basal plane exhibited the highest hardness and higher hardness was reported in the direction normal to the c-axis than parallel to it on prismatic plane w x 12 . Based on the above information, sliding in the direction normal to the lamination direction should have the whiskers of highest hardness because it involved more contacts with basal plane of the SiC. So, the smallest wear in the direction normal to lamination direction was due to highest hardness of the SiC whiskers that slowed down the progress of wear. As shown in Fig. 6, the whiskers stood up higher than the surface. The harder the whiskers were, the longer they last and the slower the wear progressed. But the long lasting sharp whiskers increased the friction coefficients due to rougher contact surface. Sliding in tape casting direction involved contacts with whiskers length direction where the hardness was lowest. The exposed whiskers were worn out faster than those of other orientations, and the biggest wear was measured. The wear scar diameter of the ball was the largest and smallest in the direction parallel to and normal to the whisker in the direction on the tape surface, respectively. Sliding in the direction normal to the lamination direction resulted in the smallest wear on the composite sample, but it caused more wear of the ball than sliding normal to the whisker length direction. As the whisker content increased from 10 vol.% to 20 vol.%, wear of the sample and the ball decreased. However, wear normal to whisker length direction was still smaller than that parallel to the direction in spite of the fact that the friction coefficients showed the opposite. 4. Conclusions The monolithic alumina showed smallest wear because of the smallest matrix grain size. Depending on sliding orientation, friction and wear of the samples varied as follows. The friction coefficient was higher on the surface where more whiskers exposed their basal plane. Sliding normal to the whisker length direction caused less wear than that parallel to it. As the whisker content increased from 10 vol.% to 20 vol.%, both friction coefficient and wear decreased due to the smaller matrix grain size. Acknowledgements The authors Korea University would like to acknowl- Ž . edge the financial support for this study provided by the Korea Science and Engineering Foundation KOSEF Ž . through the Ceramic Processing Research Center CPRC Ž . at Han Yang University. References w x 1 P.F. Becher, Microstructural design of toughened ceramics, J. Am. Ceram. Soc. 74 1991 255–269. Ž . w x 2 K.T. Farber, A.G. Evans, Crack deflection processes: I. Theory, Acta Metal. 31 1983 565–576. Ž . w x 3 E.R. Billman, P.K. Mehrotra, A.F. Shuster, C.W. Beegly, Machining with alumina–SiC whisker cutting tools, Ceram. Eng. Sci. Proc. 9 Ž . 1988 543–552. w x 4 C.S. Yust, Tribological behavior of whisker-reinforced ceramic composite materials, in: S. Jahanmir Ed. , Friction and Wear of Ceram- Ž . ics, Marcel-Dekker, New York, 1994, pp. 199–223. w x 5 C. DellaCorte, Tribological characteristics of silicon carbide whisker-reinforced alumina at elevated temperatures, Friction and Wear of Ceramics, Marcel-Dekker, New York, 1994, pp. 225–59. w x 6 D.-S. Park, C.-W. Kim, C. Park, Self-reinforced silicon nitride composite containing unidirectionally oriented silicon nitride whisker seeds, to be published in Ceram. Eng. Sci. Proc. w x 7 D.-S. Park, B.-D. Han, D.-S. Lim, I.-W. Yeo, A study on wear and erosion of sialon–Si N whisker ceramic composites, Wear 203–204 3 4 Ž . 1997 284–290. w x 8 M. Moser, Microstructure of Ceramics—Structure and Properties of Grinding Tools, Akademiai Kiado, Budapest, 1980, p. 136. w x 9 T. Nagaoka, M. Yasuoka, K. Hirao, S. Kanzaki, Effects of TiN particle size on mechanical properties of Si N rTiN particulate 3 4 composites, J. Ceram. Soc. Jpn. 100 1992 617–620. Ž . w x 10 S.-J. Cho, C.-D. Um, S.-S. Kim, Wear and wear transition in silicon carbide ceramics during sliding, J. Am. Ceram. Soc. 79 1996 Ž . 1247–1251. w x 11 S.-J. Cho, B. Hockey, B.R. Lawn, S.J. Bennison, Grain-size and R-curve effects in the abrasive wear of alumina, J. Am. Ceram. Soc. 72 1989 1249–1252. Ž . w x 12 T.K. Miyoshi, D.H. Buckely, Ceramic wear in indentation and sliding contact, ASLE Trans. 28 1985 296–302. Ž
