JOURNAL OF RARE EARTHS Vol 25, Suppl., Jun, 2007, P. 309 Study on Toughness Mechanism of Ceramic Cutting Tools Qiu like(邱立科), Li Xikun(李喜坤)2, Qiu guanming(邱关明), Ma weimin(马伟民), Sun yanbin(孙彦彬)3, Yu huadong(于化东)3 (I. Jilin Ruid Electrical Technology Development Co, Ltd, Jilin 132013, China; 2. Materials Science Engi- neering College, Shenyang University of Science and Technology, Shenyang 110168, China; 3. School of Material and Chemical Engineering, Changchun University of Science and Technology, Changchun 130022, China 4. Department of Materials Science and Engineering, Shenyang University, Shenyang 110044, China; 5. Department of Materials Science and Engineering, Jilin University, Changchun 130026, China Abstract: Ceramic cutting tools can be used in metal cutting, and can keep their hardness, strength, abrasion resistance ng performance life under high cutting speed. With these special mechanical properties, ceramic cutting tools and hard alloy cutting tools with ceramic coat will certainly substitute high speed steel cutting tools in many fields. But the fa tal weakness of brittleness of ceramic materials makes their reliability lower. The mechanical properties of ceramic cutting tools, such as toughness, strength, abrasion resistance and fatigue resistance, can be improved by rare earth additives. In this paper, a various kinds of methods, such as bio-inspired ion, nano technique, granule dispersion, grain fiber, stress inducing phase change, microcracks, residual stress ase change, synergistic toughening and strengthening were studied in improving the toughness of ceramic cutting Key words: ceramics; ceramic cutting tools; toughness; strengthening and toughening CLC number: 0614 33 Document code: A Article ID: 1002-0721(2007)-0309-08 Metal cutting is a complicated process of physical dding additives to form the second chemistry; it requires cutting tool materials of high temperature chemical reaction or phase hardness, high abrasion resistance, high strength, under controlled condition and reaction process. The high fracture toughness, high thermal hardness, good evenly distributed crystal whiskers, grains of big as- thermal-shock resistance, corrosion resistance and pect ratio or crystal chips were produced to reinforce wettability resistance. Various kinds of difficult cutting the main phase matrix. This method can avoid incom materials,developed with the development of science patibility between two phases and uneven distribution and technology, make it necessary to develop new cut ting tool material higher then introducing the second phase directly, and The application of ceramic tools is limited be it does not undermine the strength while enhancing ause of their big fracture, low toughness. There are toughness of ceramics as other toughening methods two ways to improve the strength and toughness: (1) To avoid or reduce the original cracks and defects of 1. 1 New approach of bio-inspired ceramics during preparation process; (2) Toughening preparation and Strengthening through the importation of the sec Recently, the toughening mechanisms of nacre ond phase. The methods and mechanisms of Toughen- shell, such as crack deflection, crystal fiber pulling ing and Strengthening of ceramic cutting tools were out and bridge of organic matter, were studied by studied in this pape some scientists of China; it was noticed that the high 1 Preparation of Ceramic Cutting toughness of nacre shell was closely related to the in- Tools by Bio-Inspired Toughen terface of aragonite with soft-hard layered structure and organic matter. The composite of Al203/Aramid fiber ing and Strengthening Methoe enhanced epoxy resin folded layers was designed; its Ceramic composite materials were prepared by work to rupture was raised by two orders of magnitude Received date: 2006-10-16: revised date: 2007-03-15 Foundation item: Project supported by the Scientific and Technological Project of Shenyang City (1053090-2-05) Biography: Qiu Like(1968-), Male, Bachelor, Senior engineer Correspondingauthor(E-mailsunyanbin1965@163.com)
JOURNALOFRAREEARTHS Vol.25, Suppl., Jun. 2007, p.309 Study on Toughness Mechanism of Ceramic Cutting Tools Qiu Like (*a#)’, Li Xikun (+*-)$)’, Qiu Guanming (5FlfxYI)3, Ma Weimin (J!J~+R)~, Sun Yanbin (818&@)5*, Yu Huadong (T4k$)3 ( 1 . Jilin Ruid Electrical Technology Development Co . , Ltd , Jilin 132013, China ; 2. Materials Science & Engineering College , Shenyang University of Science and Technology, Shenyang 110168, China ; 3. School of Material and Chemical Engineering, Changchun University of Science and Technology, Changchun 130022, China ; 4. Department .f Materials Science and Engineering, Shenyang University, Shenyang IIW , China ; 5. Department of Materials Science and Engineering , Jilin University , Changchun 130026, China ) Abstract : Ceramic cutting tools can be used in metal cutting, and can keep their hardness, strength, abrasion resistance and long performance life under high cutting speed. With these special mechanical properties, ceramic cutting tools and hard alloy cutting tools with ceramic coat will certainly substitute high speed steel cutting tools in many fields. But the fatal weakness of brittleness of ceramic materials makes their reliability lower. The mechanical properties of ceramic cutting tools, such as toughness, strength, abrasion resistance and fatigue resistance, can be improved by rare earth additives. In this paper, a various kinds of methods, such as bio-inspired preparation, nano technique, granule dispersion, grain fiber, stress inducing phase change, microcracks, residual stress, and phase change, synergistic toughening and strengthening, were studied in improving the toughness of ceramic cutting materials. Key words : ceramics ; ceramic cutting tools ; toughness ; strengthening and toughening CLC number: 0614.33 Document code: A Article ID: 1002 - 0721(2007) - 0309 - 08 Metal cutting is a complicated process of physical chemistry; it requires cutting tool materials of high hardness, high abrasion resistance, high strength, high fracture toughness, high thermal hardness, good thermal-shock resistance, corrosion resistance and wettability resistance . Various kinds of difficult cutting materials, developed with the development of science and technology, make it necessary to develop new cutting tool materials. The application of ceramic tools is limited because of their big fracture, low toughness. There are two ways to improve the strength and toughness: ( 1) To avoid or reduce the original cracks and defects of ceramics during preparation process ; ( 2) Toughening and Strengthening through the importation of the second phase. The methods and mechanisms of Toughening and Strengthening of ceramic cutting tools were studied in this paper. 1 Preparation of Ceramic Cutting Tools by Bio-Inspired Toughening and Strengthening Methoe Ceramic composite materials were prepared by adding additives to form the second phase via high temperature chemical reaction or phase change process under controlled condition and reaction process. The evenly distributed crystal whiskers, grains of big dspect ratio or crystal chips were produced to reinforce the main phase matrix. This method can avoid incompatibility between two phases and uneven distribution, the strength and toughness of the composites were higher then introducing the second phase directly, and it does not undermine the strength while enhancing toughness of ceramics as other toughening methods. 1.1 New approach of bio-inspired preparation Recently, the toughening mechanisms of nacre shell, such as crack deflection, crystal fiber pulling out and bridge of organic matter, were studied by some scientists of China; it was noticed that the high toughness of nacre shell was closely related to the interface of aragonite with soft-hard layered structure and organic matter. The composite of A1203/Aramid fiber enhanced epoxy resin folded layers was designed; its work to rupture was raised by two orders of magnitude Received date: 2006 - 10 - 16; revised date: 2OO7 - 03 - 15 Foundation item: Project supported by the Scientific and Technological Project of Shenyang City (1053090-2-05) Biography : Qiu Like ( 1968 - ) , Male, Bachelor, Senior engineer * Corresponding author (E-mail: sunyanbinl965 @ 163. corn)
OURNAL OF RARE EARTHS, Vol 25, Suppl., Jun. 2007 comparing with its counterparts energy, and inhibits cracks from expanding to improve veral bio-inspired materials were prepared fracture toughness. When the glass phase decreases bio-inspired preparation. The bio-inspired SiC whiskers it evenly distributes between brains to form a very thin were composed of dumbbell crystal whiskers and minili- film to lead to passivation of cracks and close of gas form beads, the reaction is Sio(g)+3C0(g)-SiC holes to improve fracture toughness. When the glass 2002(g). The root style bio-inspired carbon fiber was phase increases, the growth of B-Si3N4 columnar crys prepared by vapor deposition on ceramic base. The tals is prome tals is promoted, the aspect ratio is enlarged so as to tree style ZnO crystal whiskers, the bio-i make crack deflect to improve fracture toughness. Or carbon fiber and Self-healing anti-oxide ceramic/car. the other hand, if the glass phase is treated as ba bon composite were also developed Si3 N4 columnar crystals acts as toughening phase 1. 2 Self toughness- enhancing ceramic the pulling-out effect will occur, which will also im prove the fracture toughne cutting tools It is known to all that the bigger the porosity, the The strengthening ceramics of even-distributed lower the density, the smaller the virtual area of ce crystal whiskers, rod-shaped grains or chips were pre- ramics the lower the elastic modulus and fracture ener pared via high temperature chemical reaction or phase gy. If the size of gas holes is proper and gas holes are transformation to form big aspect ratio through in situ of round or oval shape and evenly distribute in the formation.This method was widely used in ceramic base, atoms around the air hole arrange abnormally to composite cutting materials, such as Si, N4, Sialon, bring higher distortion energy. The air holes play a Al-Zr-C, Ti-B-C, SiC, Al2O3, ZrB2/ZrCo. 6/Zr etc, In nning role to the expansion of cracks to create crack SiN, ceramic material with B-Si, Na columnar crystals branches so as to improve fracture toughness as the main crystal phase, the cracks will deflect on 2 Toughening of Nano-Ceramic the surface of B-Si3 N4 columnar crystals. The bigger Cutting tools the deflection angle, the better the toughening effect In nano-ceramics, the grains, the boundary and If the growing direction of B-Si3N, columnar crystals is their combination are of nano-size. The strength random. the cracks will deflect in 3d directions, thus toughness are increased greatly because of the refine the fracture toughness of ceramics was improved. The ment of grains and the significant increase in number Si,Na columnar crystals must have sufficient size to of grain boundaries play a similar toughening role as crystal whiskers. But E=B20n8D,d KT because the expansion coefficients of different direc where E is the degree of creep deformation, d is the tions are different in polycrystalline system average grain size. If the size of ceramic grains phase ceramics, there will be a big residual stress dur dropped from the usual um level to nm level, in fo ing the cooling process in the sintered ceramics. It was mula(1)E is inversely proportional to d, so the de- proved that the grain size of Si3N4 crystals should be formation rate will be increased to 109 times. That is controlled under 5 um with big aspect ratio to avoid to say the nano-size grains and boundaries have great concentration of residual stress. In hot-pressing sin impact on the mechanical properties of ceramics tered Si,N, ceramics, the main phase is B-Si3N4 long columnar crystals. When the B-Si]N, columnar crystal 2.1 Toughening of nano ceramic base grows to certain size, the Pulling out toughening Karch etc, in the middle of the 20th century model will occur when the ceramics was broken reports firstly that nano(8 nm) TiO2 particles plate The additives in ceramics formed glass phase in specimen was heated to 1800 C in a customized the grain boundary. The composition of additives has mold, and then bended by angle of 180, which has influence on the composition and content of the grain no expansion of the pre-existing cracks, this demon boundary phase, and the sintering temperature is als strated its high toughness and super plasticity affected by additives. Stress from the linear expansion The quick microwave sintering method is adopt coefficient difference of the grain boundary phase and ed. Y-ZrO2 nano powder with the particle size the main phase leads to crack formation in the grain 20 nm was sintered with the heating and cooling speed boundary. If the main crystal phase is weakly com of at 500C.min, and the temperature was kept in bined with the grain boundary phase, cracks will dif 1200C for 2 min. The sintered density can reach fuse along the grain boundary. Crack bifurcatic more than 98% of the theoretical value, and the aver- brought by crack diffusion process absorbs the fracture age grain size of the sintered samples is around 120
310 JOURNAL OF RARE EARTHS, Vol. 25, Suppl. , Jun . 2007 comparing with its counterparts. Several bio-inspired materials were prepared via bio-inspired preparation. The bio-inspired Sic whiskers were composed of dumbbell crystal whiskers and miniliform beads, the reaction is : SiO( g) + 3CO( g)+SiC + 2C02(g). The root style bio-inspired carbon fiber was prepared by vapor deposition on ceramic base. The tree style ZnO crystal whiskers, the bio-inspired helix carbon fiber and Self-healing anti-oxide ceramidcarbon composite were also developed. 1.2 Self toughness-enhancing ceramic cutting tools The strengthening ceramics of even-distributed crystal whiskers, rod-shaped grains or chips were prepared via high temperature chemical reaction or phase transformation to form big aspect ratio through in situ formation. This method was widely used in ceramic composite cutting materials, such as Si3N4, Sialon , Al-Zr-C , Ti-B-C, Sic, A1203, ZrB2/ZrCo,6/Zr etc. In Si3N4 ceramic material with P-Si3N4 columnar crystals as the main crystal phase, the cracks will deflect on the surface of P-Si3N4 columnar crystals. The bigger the deflection angle, the better the toughening effect. If the growing direction of /3-Si3N4 columnar crystals is random, the cracks will deflect in 3D directions, thus the fracture toughness of ceramics was improved. The Si3N4 columnar crystals must have sufficient size to play a similar toughening role as crystal whiskers. But because the expansion coefficients of different directions are different in polycrystalline system, multiphase ceramics, there will be a big residual stress during the cooling process in the sintered ceramics. It was proved that the grain size of Si3N4 crystals should be controlled under 5 pm with big aspect ratio to avoid concentration of residual stress. In hot-pressing sintered Si3N4 ceramics, the main phase is P-Si3N4 long columnar crystals. When the P-Si3N4 columnar crystal grows to certain size, the “ Pulling out toughening ” model will occur when the ceramics was broken. The additives in ceramics formed glass phase in the grain boundary. The composition of additives has influence on the composition and content of the grain boundary phase, and the sintering temperature is also affected by additives. Stress from the linear expansion coefficient difference of the grain boundary phase and the main phase leads to crack formation in the grain boundary. If the main crystal phase is weakly combined with the grain boundary phase, cracks will diffuse along the grain boundary. Crack bifurcation brought by crack diffusion process absorbs the fracture energy, and inhibits cracks from expanding to improve fracture toughness. When the glass phase decreases, it evenly distributes between brains to form a very thin film to lead to passivation of cracks and close of gas holes to improve fracture toughness. When the glass phase increases, the growth of P-Si3N4 columnar crystals is promoted, the aspect ratio is enlarged so as to make crack deflect to improve fracture toughness. On the other hand, if the glass phase is treated as base, P-Si3N4 columnar crystals acts as toughening phase, the pulling-out effect will occur, which will also improve the fracture toughness. It is known to all that the bigger the porosity, the lower the density, the smaller the virtual area of ceramics the lower the elastic modulus and fracture energy. If the size of gas holes is proper and gas holes are of round or oval shape and evenly distribute in the base, atoms around the air hole arrange abnormally to bring higher distortion energy. The air holes play a pinning role to the expansion of cracks to create crack branches so as to improve fracture toughness. 2 Toughening of Nano-Ceramic Cutting Tools In nano-ceramics , the grains, the boundary and their combination are of nano-size . The strength, toughness are increased greatly because of the refinement of grains and the significant increase in number of grain boundaries”’ . where E is the degree of creep deformation, d is the average grain size. If the size of ceramic grains dropped from the usual pm level to nm level, in formula (1) € is inversely proportional to d3, so the deformation rate will be increased to 109 times. That is to say the nano-size grains and boundaries have great impact on the mechanical properties of ceramics[*’ . E = B201XDb/d3KT (1) 2.1 Toughening of nano ceramic base Karch etc. , in the middle of the 20th century, reports firstly that nano (8 nm) TiO, particles plate specimen was heated to 1800 “c in a customized mold, and then bended by angle of 180°, which has no expansion of the pre-existing cracks, this demonstrated its high toughness and super plasticity. The quick microwave sintering method is adopted. Y-ZrO, nano powder with the particle size of 20 nm was sintered with the heating and cooling speed of at 500 “c amin-’, and the temperature was kept in 1200 “c for 2 min. The sintered density can reach more than 98 % of the theoretical value, and the average grain size of the sintered samples is around 120
Qiu L K et al. Study on Toughness Mechanism of Ceramic Cutting Tools 311 nm. Adding some additives can also promote nano- particles, the strength and toughness of the tool have with an average particle size of 20 nm, an appropriate ening effect of nanoparticles mainly comes from ih &s ceramic sintering, for example, in the alumina powder been improved dramatically. Strengthening and to amount of the average particl e of 0. 5 uum Y-PSZ finement of structure and the inhibition of grain growth powder was added and sintered. It was found that Y- during sintering process PSZ powder could promote the densification of sinter- ing, lower sintering temperature, and inhibit grains of Table 1 Properties of ceramic base and nano ceramic AlO, from growing. If sintered at 1600 C, 99 of rticle composite cutting tools theoretical density can be achieved Ceramic base"nano ceramic particle cor The result of Gleiter's study shows that if poly- crystalline ceramics are formed by the grain size of nm Fracture toughness Strength Temperature Krc/(MPa·m2) g /MPa℃ several nanometers, it could turn into ductility at low SiCn/Al2O3<3003.5-4.8 1520800-1200 temperature. 100% of plastic deformation occurred SiN4/A12O3<3003.54.7 350→850800→·1300 and TiO2 nano ceramic materials was found to have ex SiCn2/Mg0<3001.2-4.5 340700600→·1400 cellent toughness at room temperature. There were no SiCn /Si3 N4 <300 4.5-7.5 850→·15501200·1400 new cracks if this material was bended by angle of 180 C. Many experts believe that if the technique of 2.3 Nano particles toughening hibiting the growth of grains during the single-phase nano-ceramic sintering process to control nano-ceram composite cutting tool grain size in less than 50 nm, it will have advantages Ceramic Nano composite tools classified as three of high hardness, high toughness, lower temperat categories:(1)second phase of nanoparticles disper super plasticity, easy processing which the traditional sed in matrix grains; (2)second phase of nanoparti ceramics is incomparable. a study shows that after cles dispersed between matrix grains;(3)matrix and tensile cycle test in ambient temperature, the super the second phase are all of nanocrystals. In that,(I nation occurred in nano-3Y-TZP(100(2)will not only improve the mechanical properties m)samples fracture, the deformation is 380%. Sli room temperature, but also can improve the high-tem- lines were observed that often appears in metal frac- perature mechanical properties;(3)can lead to new Although there are still many critical technolo- functions of machinability and superplasticity. When solve in ceramic nano-technology, but its ex- density of single component nano-ceramics is over mechanical properties at high temperature, 95%, the grain size of the ceramics is more than 100 nm which makes it difficult to prepare nano ceram application in cutting tools, bearings, engine parts for cutting tool of un-growing grains and dense bulk. It was automobiles and many other aspects. It also competes found that the nano-particles dispersed in the heteroge other materials and plays an irreplaceable role in ultra- neity matrix components, because nano particles are dis- high temperature and strong corrosion environment persed by matrix composition and the differences of sin 2.2 Toughening of ceramic nano-particles ering activity, the shortcomings of abnormal growth of nanoparticles in the densification process can be over- In the crack deflection toughening mechanis come. This ceramic nano composite(CNC)is easier for the second phase particles with high surface energy is preparing pure nano-solid-tool, also more practical needed, and these particles can afford high stress. It It is the strengthening of the main grain boundary is noticed that the smaller the particle size and the and differentiation of grains that result in toughening more nearly spherical the particles, the more possible reinforcement effect of nano particles. Nano phase in the crack extension bypasses the particles. However, grain boundary can combine with the base to when the expansion coefficient a, of the second phase in grain boundary of high strength, and play a is smaller than the expansion coefficient a m of matrix, effect to cracks along grains; nano-crysta the cracks can extend directly and easily through the toughening effect is to create a trans stress field, coupled with the hypo-interface cracks trix. If the second phase is of nano-particles, crack to enhance the grain boundary, but of the deflection will take place in the particles, the ceramics is toughened. Table I shows properties of toughening 1)Nanoparticles toughening oxide-based ceram nano ceramic composite cutting tools and that of the ic composite cutting tools, such as SiCn /Al203, Nano corresponding matrix. It can be seen that adding nano- SiC particles refine the base of Al2O, to improve the
Qiu L K et a1 . Study on Toughness Mechanism of Ceramic Cutting Tools 311 nmL2’. Adding some additives can also promote nanoceramic sintering , for example, in the alumina powder with an average particle size of 20 nm, an appropriate amount of the average particle size of 0.5 pm Y-PSZ powder was added and sintered. It was found that YPSZ powder could promote the densification of sintering, lower sintering temperature, and inhibit grains of A1203 from growing. If sintered at 1600 “c, 99% of theoretical density can be achievedL3] . The result of Gleiter’s study shows that if polycrystalline ceramics are formed by the grain size of several nanometers, it could turn into ductility at lowtemperature. 100% of plastic deformation occurred, and Ti02 nano ceramic materials was found to have excellent toughness at room temperature. There were no new cracks if this material was bended by angle of 180 “c. Many experts believe that if the technique of inhibiting the growth of grains during the single-phase nano-ceramic sintering process to control nano-ceramic grain size in less than 50 nm, it will have advantages of high hardness, high toughness, lower temperature super plasticity, easy processing which the traditional ceramics is incomparable. A study shows that after tensile cycle test in ambient temperature, the super plastic deformation occurred in nano-3Y-TZP ( 100 nm) samples fracture, the deformation is 380% . Sliplines were observed that often appears in metal fracturef4]. Although there are still many critical technologies to solve in ceramic nano-technology, but its excellent mechanical properties at high temperature, bending strength, fracture toughness make it a wide application in cutting tools, bearings, engine parts for automobiles and many other aspects. It also competes other materials and plays an irreplaceable role in ultrahigh temperature and strong corrosion environment. 2.2 Toughening of ceramic nano-particles In the crack deflection toughening mechanisms, the second phase particles with high surface energy is needed, and these particles can afford high stress. It is noticed that the smaller the particle size and the more nearly spherical the particles, the more possible the crack extension bypasses the particles. However, when the expansion coefficient ap of the second phase is smaller than the expansion coefficient a,,, of matrix, the cracks can extend directly and easily through the particles because the residual tensile stress ot in matrix. If the second phase is of nano-particles, crack deflection will take place in the particles, the ceramics is toughened. Table 1 shows properties of toughening nano ceramic composite cutting tools and that of the corresponding matrix. It can be seen that adding nanoparticles, the strength and toughness of the tqol have been improved dramatically. Strengthening and toughening effect of nanoparticles mainly comes from the refinement of structure and the inhibition of grain growth during sintering process. Table 1 Properties of ceramic base and nano ceramic particle composite cutting tools ~ ~ Ceramic base+nano ceramic particle composite size/ cutting tool nm Fracture toughness Strength Temperature/ Composite Krr/(MPa.m’”) u4MPa T SiCn,/&O, < 300 3.5-4.8 35-1520 800-1200 Si3NdA1203 < 300 3.5-4.7 35W8.50 80+1300 SiCn,/MgO < 300 1.2-4.5 34-700 60+1400 SiCnJSisNd < 300 4.5+7.5 85-1550 12-1400 2.3 Nan0 particles toughening ceramic composite cutting tool Ceramic Nano composite tools classified as three categories : ( 1 ) second phase of nanoparticles dispersed in matrix grains ; (2) second phase of nanoparticles dispersed between matrix grains; (3) matrix and the second phase are all of nanocrystals . In that, ( 1 ) , (2) will not only improve the mechanical properties at room temperature, but also can improve the high-temperature mechanical properties; ( 3) can lead to new functions of machinability and superplasticity . When density of single component nano-ceramics is over 95%, the grain size of the ceramics is more than 100 nm which makes it difficult to prepare nano ceramic cutting tool of un-growing grains an8 dense bulk. It was found that the nano-particles dispelsed in the heterogeneity matrix components, because nano particles are dispersed by matrix composition and the differences of sintering activity, the shortcomings of abnormal growth of nanoparticles in the densification process can be overcome. This ceramic nano composite (CNC) is easier for preparing pure nano-solid-tool , also more practical. It is the strengthening of the main grain boundary and differentiation of grains that result in toughening reinforcement effect of nano particles. Nano phase in grain boundary can combine with the base to form main grain boundary of high strength, and play a pinning effect to cracks along grains; nano-crystal phase toughening effect is to create a transgranular stress field, coupled with the hypo-interface and microcracks to enhance the grain boundary, but the grains of the base tend to fracture. ( 1 ) Nanoparticles toughening oxide-based ceramic composite cutting tools, such as SiCn,/A1,03, Nano Sic particles refine the base of A1203 to improve the
312 JOURNAL OF RARE EARTHS, Vol 25, Suppl., Jun, 2007 trength markedly, but fracture toughness increased less. Nano SiC particles increase the sintering temper- ature of Al2O3, And alumina grain size decreases obvi- usly, the grains distribute evenly, and it is not ob served that the size of single crystal of Al2O, increased with temperature. Most SiCn, nano grains are inside of Al,O3 grains to cause stress concentration in grains and lead to dislocation cells, it may transmitted to the grain boundaries to produce stress on the grain bound aries to reinforce the boundaries, In SiCn, /AlO3,the Sic volume fraction/% presence of nano SiCn, lead to the formation of sub rain boundaries, which can greatly improve the Fig. 2 Influence of Vp on fracture roughness KIc of SiC/Si,Na h. Although crack deflection and microcracks can also play the role of toughening, but it is noticed that the stricted in all directions and high aspect ratio grains of smaller the SiC particles the better of toughening B-Si3N4 decreased. When Vp=30%, the matrix grains (2) Nanoparticles toughening non- oxide cer are equiaxed, and with the increase of Vp, the aver- matrix composite cutting tools, such as SiCnp/Si3N4 age grain size of SiN4 decreases from 0. 7 to 0. 3 um ig.I shows the affection of grain size to fracture (V,=30%),which undermines the toughening ef- oughness. When the grain size is of um level, the fect. SiCmp particles mainly distribute in the grain grains of bigger size(32 um) have better effect of boundaries of Si, Na, a small number of SiCnp particles oughening, the toughening effect comes from the in- distribute in Si3N4 grains. The grain size of P-SiCnp in teraction of SiC micron particles, cracks and grains of the grain boundaries is 50-100 nm, the size of those the base; when the grain size is of nm level (< 300 in Si, N, grains is less than 30 nm. There were a lot of m), the fracture toughness is improved, it is because dislocations inside Si, N4 grains of the hexagonal crys the growth of p-Si3N4 grains promoted by nano Sic par- tal it is because of SiCap grains lead to the deforma tion of Si,N. grains during their growth. Since the lin- Nano SiC particles act as nucleating agent. When ear expansion coefficient of SiC is bigger than that of volume fraction(Vp)of the second phase SiCnp is pure Si,N4, the axial tensile stress and horizontal com lower than 20%, with the increase of Vp, the growth pressive stress occurred when the Si, N4 matrix coolin of slender Si, N4 grains is promoted, the bridging effect when the residual stress is large enough, the hyr d crack deflection are increased and the fracture grain boundaries will produce inside the Si3 N4 grair toughness of composite ceramics is enhanced. When The formation of dislocations and hypo-grain bound V, is more than a certain amount (23 %, KIc=6.7), aries are the main mechanism of SiCn enhancing frac Nano Sic grains hindered the growth of slender p SinN, ture toughness of SigN grains, fracture toughness decreases with the e increase Nano-particle reinforced ceramic matrix compos of V. A maximum of Kic exists to certain Ve ite materials are mainly used in superhard, high Fig 2 shows the influence of Vp on Kic of SiC/ strength materials, SiCn /Si N, material has been used SiaN4, where the grain size of SiC is less than 100 to produce ceramic cutting tools, ball bearings, mode- nm.When Vp =10%, KIc =8. 27, the fracture ls and piston pump; In the field of high-temperature toughness increased 23% over that of the Si,N4 ma- nano-particle reinforced ceramic matrix composite ma trix. With the increase of nano-SiC particles, KIc terials can be used as rotor, stator, non-water-cooled creased slowly, it is because Si, N grain growth is re- engine piston ceramic roof and the snail-shaped tube of gas turbine in ceramic engines. These materials can λ6.5(≤300nm) 65(<300nm) e engine spark plugs and pisto er,combustion chamber and parts of piston-turbine 3(9um) for air engi 4.12(5μ nm level An evel 3 Toughening Reinforcement by Particle Dispersion Fig. I Influence of SiCnp grain size on Kic in NA com- The dispersed second phase particles are intro duced to inhibit slip and climb of dislocations to pre
312 JOURNAL OF RARE EARTHS, Vol. 25, Suppl. , Jun . 2007 strength markedly, but fracture toughness increased less. Nan0 Sic particles increase the sintering temperature of A1203. And alumina grain size decreases obviously, the grains distribute evenly, and it is not observed that the size of single crystal of A1203 increased with temperature. Most SiCn, nano grains are inside of grains to cause stress concentration in grains and lead to dislocation cells, it may transmitted to the grain boundaries to produce stress on the grain boundaries to reinforce the boundaries. In SiCnp/A1203, the presence of nano SiCn, lead to the formation of subgrain boundaries, which can greatly improve the strength. Although crack deflection and microcracks can also play the role of toughening, but it is noticed that the smaller the Sic particles the better of toughening. (2) Nanoparticles toughening non-oxide ceramic matrix composite cutting tools, such as SiC,,/Si3N4, Fig. 1 shows the affection of grain size to fracture toughness. When the grain size is of pm level, the grains of bigger size (32 pm) have better effect of toughening, the toughening effect comes from the interaction of Sic micron particles, cracks and grains of the base; when the grain size is of nm level ( < 300 nm) , the fracture toughness is improved, it is because the growth of P-Si3N4 grains promoted by nano Sic particles. Nan0 Sic particles act as nucleating agent. When the volume fraction ( V,) of the second phase Sic,, is lower than 20%, with the increase of V,, the growth of slender Si3N4 grains is promoted, the bridging effect and crack deflection are increased and the fracture toughness of composite ceramics is enhanced. When V, is more than a certain amount (23%, K,, = 6.7) , Nan0 Sic grains hindered the growth of slender p Si3N4 grains, fracture toughness decreases with the increase of V,. A maximum of Klc exists to certain V,. Fig. 2 shows the influence of V, on KIc of Sic/ Si3N4, where the grain size of Sic is less than 100 nm. When V, = lo%, KIc = 8. 27, the fracture toughness increased 23% over that of the Si3N4 matrix. With the increase of nano-Sic particles, KIc increased slowly, it is because Si3N4 grain growth is re- 4 6.5 (<300 nm) 6.5 (<300 nm) Fig. 1 Influence of Sic,, grain size on Kc in SiCp/Si3N4 composite cutting tool 51 0 5 10 15 20 25 30 Sicp volume fraction/% Fig. 2 Influence of Vp on fracture roughness Klc of SiC/Si3N4 stricted in all directions and high aspect ratio grains of /3-Si3N4 decreased. When V, = 30% , the matrix grains are equiaxed, and with the increase of V,, the average grain size of Si3N4 decreases from 0.7 to 0.3 pm ( V, = 30%), which undermines the toughening effect. Sic,, particles mainly distribute in the grain boundaries of Si3N4, a small number of Sic,, particles distribute in Si3N4 grains. The grain size of /3-Sic,, in the grain boundaries is 50 - 100 nm, the size of those in Si3N4 grains is less than 30 nm. There were a lot of dislocations inside Si3N4 grains of the hexagonal crystal; it is because of Sic,, grains lead to the deformation of Si3N4 grains during their growth. Since the linear expansion coefficient of Sic is bigger than that of pure Si3N4, the axial tensile stress and horizontal compressive stress occurred when the Si3N4 matrix cooling, when the residual stress is large enough, the hypograin boundaries will produce inside the Si3N4 grains. The formation of dislocations and hypo-grain boundaries are the main mechanism of Sic,, enhancing fracture toughness of Si3N4. Nano-particle reinforced ceramic matrix composite materials are mainly used in superhard, highstrength materials, SiC,,/Si3N4 material has been used to produce ceramic cutting tools, ball bearings, models and piston pump; In the field of high-temperature, nano-particle reinforced ceramic matrix composite materials can be used as rotor, stator, non-water-cooled engine piston ceramic roof and the snail-shaped tube of gas turbine in ceramic engines. These materials can also be used as the engine spark plugs and piston cover, combustion chamber and parts of piston- turbine for air engine. 3 Toughening Reinforcement by Particle Dispersion The dispersed second phase particles are introduced to inhibit slip and climb of dislocations to pre-
Qiu L K et aI. Study on Toughness Mechanism of Ceramic Cutting Too vent the cracks from expanding to achieve the purpose lid more attention. The key probl of toughening reinforcement. It is an important method ceramic particle reinforced composites is particle d and the basis to reinforce and toughen composite c persion, methods of ultrasonic dispersion and ramics. There are various kinds of strengthening and surfactants are commonly used to prevent toughening mecha that field, microcracks, crack deflection, bending crack, n the matrix crack bifurcation, crack bridging, crack pinning etc The above mechanisms can 4 Crystal Fiber Toughening follows:(1)Stress-induced The pulling out or bridging effect of crystal whis- ing;(2)Non-planar fracture toughness, including two kers or fibers during fracture process makes the dissi forms: crack Bifurcation and deflection; (3)Strength- pation energy increase when the Ceramic matrix con- ening and toughening through particles directly taining discontinuous toughening phase of a certain cluding crack bridging, Crack bending and crack pin- pect ratio of crystal fiber or crystal whiskers. Thereby ning;(4)Residual stress toughening composite ceramIcs In In practical applications, because of particle type creased. Under external stress, the ceramic matrix and the different crystal fractured firstly, and then crystal fibers or whiskers properties between particles and the matrix, when dif- cracked continuously. A bridge connecting the surface ferent types of particles dispersed in different matrix, of two cracks formed at the end of cracks to play a role the dominant mechanisms of strengthening and tough- of inhibiting crack expansion; In addition, the consec ening are also different. For ductile phase enhanced utive pulling-out of crystal fibers or whiskers from the matrix composite system(high elastic modulus, high matrix under certain load will also consume energy strength), by adding the second phase particles, a thus bridging and crystal fiber pulling-out can improve certain ductile deformation or creep deformation by the toughness of the matrix slip along the grain boundary will occur under external There are two kinds of crystal fibers used to rein force to delay stress concentration to achieve strength- force composites, long crystal fibers or whiskers. For ning and toughening effect. For rigid particles disper lor sed composite ceramics, because of the differences of their anisotropy of crystal fiber orientation arrange elastic modulus and thermal expansion coefficient be- ment, some properties of crystal fiber vertical arrange tween the second phase particles and the matrix ment is significantly higher than that of hor zontal ar grains, residual stress field formed around particles rangement, the direction of crystal fibers are often and the matrix grains during the cooling process, the ranged in accordance with the practical requirements residual stress field interacted with crack tip stress Crystal whiskers toughening composite materials devel create crack deflection, bifurcation, bridging and pin- op very fast in resent years because their preparation ning effect to toughen the matrix. Among them, the process is simple and part advantages of long crystal latter is used more widely than the former. In addit- fiber composite materials are reserved. The studyl 12 ion, researchers also notice that 7, the particle dis- shows that the ceramic whiskers will enhance tough persion toughening only comes from those disperse ness of 30%-100%. Moreover, crystal whisker articles larger in size. When the dispersed particle toughening also can be used in the combination ma- size is close to the particle size of the matrix, only a chine and automatic processing technology small deflection can be created by the residual stres 5 Stress-Induced Phase Transforma field, the toughening effect is not clear. No toughen ing effect for the particle size less than that of the base tion Toughening particl In ceramics containing metastable ph Toughening reinforcement by particle dispersion ZrO2, when cracks expand into the region with t- technique has characteristics of simple process, easier phase, the t-m phase transformation will occur be- to control of the particle size and distribution, stable cause of the stress field of crack tip, to form a phase performance, and toughening is not affected by tem- change process zone[ 2). In the process zone, new perature, which can be used as a high temperature crack surface occurs due to crack expansion, this pro toughening mechanism. It has been widely used in cess will absorb some energy; on the other hand, the practice. At present, these particle dispersed compos- expansion effect caused by phase transformation will ite ceramics, such as ZrO2/A120,8-10 ZO2 /Si, N4, also consume energy; meanwhile the crack expansion SiC,/AL,O O3,( ZNOz+ SiC, )/A12O3, are is hindered because phase change leads to expansion
Qiu L K et a1 . Study on Toughness Mechanism of Ceramic Cutting Tools 313 vent the cracks from expanding to achieve the purpose of toughening reinforcement. It is an important method and the basis to reinforce and toughen composite ceramics. There are various kinds of strengthening and toughening mechanisms, such as the residual stress field, microcracks crack deflection, bending crack, crack bifurcation, crack bridging, crack pinning etc . The above mechanisms can be summarized as follows[51 : ( 1 ) Stress-induced microcracks toughening; (2) Non-planar fracture toughness, including two forms : crack Bifurcation and deflection; (3) Strengthening and toughening through particles directly, including crack bridging, Crack bending and crack pinning; (4) Residual stress toughening. In practical applications, because of particle type and the different crystal shape and size, the different properties between particles and the matrix, when different types of particles dispersed in different matrix, the dominant mechanisms of strengthening and toughening are also different. For ductile phase enhanced matrix composite system (high elastic modulus, high strength), by adding the second phase particles, a certain ductile deformation or creep deformation by slip along the grain boundary will occur under external force to delay stress concentration to achieve strengthening and toughening effect. For rigid particles dispersed composite ceramics, because of the differences of elastic modulus and thermal expansion coefficient between the second phase particles and the matrix grains, residual stress field formed around particles and the matrix grains during the cooling process, the residual stress field interacted with crack tip stress to create crack deflection, bifurcation, bridging and pinning effect to toughen the matrix. Among them, the latter is used more widely than the former. In addition, researchers also notice that"], the particle dispersion toughening only comes from those dispersed particles larger in size. When the dispersed particle size is close to the particle size of the matrix, only a small deflection can be created by the residual stress field, the toughening effect is not clear. No toughening effect for the particle size less than that of the base particles. Toughening reinforcement by particle dispersion technique has characteristics of simple process, easier to control of the particle size and distribution, stable performance, and toughening is not affected by temperature, which can be used as a high temperature toughening mechanism. It has been widely used in practice. At present, these particle dispersed composite ceramics, such as Zr02/A1203[8-101 9 ZrO 2p 1s' 13N4, SiCP/Al2O3, TiCP/Al2O3, ( ZrO,, + Sic, )/A1203, are paid more attention. The key problem to preparation of ceramic particle reinforced composites is particle dispersion, methods of ultrasonic dispersion and adding surfactants are commonly used to prevent particles from agglomerating so that particles dispersed evenly in the matrix. 4 Crystal Fiber Toughening The pulling out or bridging effect of crystal whiskers or fibers during fracture process makes the dissipation energy increase when the Ceramic matrix containing discontinuous toughening phase of a certain aspect ratio of crystal fiber or crystal whiskers. Thereby the fracture toughness of composite ceramics increased. Under external stress, the ceramic matrix fractured firstly, and then crystal fibers or whiskers cracked continuously. A bridge connecting the surface of two cracks formed at the end of cracks to play a role of inhibiting crack expansion ; In addition, the consecutive pulling-out of crystal fibers or whiskers from the matrix under certain load will also consume energy, thus bridging and crystal fiber pulling-out can improve the toughness of the matrix. There are two kinds of crystal fibers used to reinforce composites, long crystal fibers or whiskers, For long crystal fiber-reinforced composites, because of their anisotropy of crystal fiber orientation arrangement, some properties of crystal fiber vertical arrangement is significantly higher than that of horizontal arrangement, the direction of crystal fibers are often arranged in accordance with the practical requirements. Crystal whiskers toughening composite materials develop very fast in resent years because their preparation process is simple and part advantages of long crystal fiber composite materials are reserved. The study"" shows that the ceramic whiskers will enhance toughness of 30% - 100%. Moreover, crystal whisker toughening also can be used in the combination machine and automatic processing technology. 5 Stress-Induced Phase Transformation Toughening In ceramics containing metastable phase of tZr02, when cracks expand into the region with tphase, the t-m phase transformation will occur because of the stress field of crack tip, to form a phase change process zone"". In the process zone, new crack surface occurs due to crack expansion, this process will absorb some energy ; on the other hand, the expansion effect caused by phase transformation will also consume energy ; meanwhile the crack expansion is hindered because phase change leads to expansion
JOURNAL OF RARE EARTHS, Vol 25, Suppl., Jun. 2007 of grains in volume to create pressure on cracks 13) ng cooling but no microcrack produced, instead This shows that stress-induced phase transformation there is a residual stress around the m phase grains af- leads to consumption of external stress, and reduces ter t-m phase change 14. When the main crack ex the stress strength factor of the crack tip. This makes pands into the residual stress area, the release of re- the cracks, those may continue to expand, stop ex sidual stress will consume energy, and close or hinder panding, thereby increasing the fracture toughness of expansion of cracks: on the other hand, the material the materials. After phase transformation, the strength structure will be uneven because of internal stress,it of extermal stress must be increased to assure the ex- will influence the form of crack expansion and lead to pansion of cracks. With the continuous increase of distortion deflection. This is the mechanism of crack such stress strength, the cracks will continue to ex- ection tougher pand. It is noteworthy that the role of the phase transi In ZrO2 ceramics, because of the co-existence of t tion makes the crack propagation resistance become and m phase, toughening comes from the comprehe greater and greater, more and more difficult to ex ive interaction of above three mechanisms Because of the mechanical properties of m phase is lower than that 6 Microcrack Toughening of t phase, m phase will undermine other properties when it roughening ceramics to some extent. There For those grains of size d>dm(dm is the critical fore, a proper content of m phase must be controlled size of grains of m phase), there will be a t-m phase in order to acquire PSZ with better comprehensive me transformation during the cooling process to lead a bi chanical properties duce microcrack toughening. Th cracks of very small size may reduce the local elas 8 Phase Transformation Toughening modulus of materials to reduce the stress strength fac- According to the micro structure style of ceram- or of crack tip. When the main crack expands into s, phase transformation toughening ceramics is act crack area, the main crack will bifurcate to ally a kind of self-toughening ceramics, its toughen form many sub-cracks to produce new surface to mechanism is to produce toughening phase inside ing sume strain energy greatly, so as to reduce crack cro structure by controlling of sintering process, and panding force to improve the toughness of material the most widely used ceramics is ZrO, phase change This is the mechanism of microcrack toughening 12-14. toughening ceramics. There are three kinds of crystal Fig 3 shows the SEM pattern of crack expansion form for ZrO2 phase: the cubic phase at temperature polishing surface of ZY A20(3Y-PSZ/Al203)compos- above 2370C(p= 6. 09 g cm 3), the monoclinic ite, it can be seen that the main crack bifurcates in phase at temperature below 1170 C(p=5.83 microcrack area and then stops expanding g."), the tetragonal phase at temperature between 7 Residual Stress Toughening 110~2370℃(p=5.83g·cm-3). When ZrO2 tetragonal phase at When the grain size d <d< dm(d. is the criti- temperature to the monoclinic phase at low tempera- cal grain size of t-m phase transformation), there still ture, there is a volume expansion of 3 %-5% and is a t-m phase transformation in ceramic materials dur- shear strain of 16%. The tetragonal phase can be kept as metastable form to low temperature through certain method such as adding stabilizer and heat treat ment). The t-m phase change will occur under certain crack tip stress during heat treatment; both the ne produced crack surface and volume expansion during phase change will absorb energy. And the particles of phase transformation will produce compression stress to strength factor so as to improve the fracture toughness of ceramics In addition, because of phase change, residual stress field produces around ZrO2 particles of mono Fig 3 SEM pattern of indent crack expansion on polishing sur- clinic phase; cracks will deflect, bend or bifurcate to e of ZYA20 play a toughening effect when expanding into stress field. At present, phase change toughening mec
314 JOURNAL OF RARE EARTHS, Vol. 25, Suppl, , Jun . 2007 of grains in volume to create pressure on cracks“31. This shows that stress-induced phase transformation leads to consumption of external stress, and reduces the stress strength factor of the crack tip. This makes the cracks, those may continue to expand, stop expanding, thereby increasing the fracture toughness of the materials. After phase transformation, the strength of external stress must be increased to assure the expansion of cracks. With the continuous increase of such stress strength, the cracks will continue to expand. It is noteworthy that the role of the phase transition makes the crack propagation resistance become greater and greater, more and more difficult to expand. 6 Microcrack Toughening For those grains of size d > d,( d, is the critical size of grains of m phase), there will be a t-m phase transformation during the cooling process to lead a big volume effect to induce microcrack toughening. The cracks of very small size may reduce the local elastic modulus of materials to reduce the stress strength factor of crack tip. When the main crack expands into the microcrack area, the main crack will bifurcate to form many sub-cracks to produce new surface to consume strain energy greatly, so as to reduce crack expanding force to improve the toughness of materials . This is the mechanism of microcrack toughening“*- 14] . Fig.3 shows the SEM pattern of crack expansion on polishing surface of ZYA20 ( 3Y -PSZ/AI2O3 ) composite, it can be seen that the main crack bifurcates in microcrack area and then stops expanding. 7 Residual Stress Toughening When the grain size d, < d < d,( d, is the critical grain size of t-m phase transformation), there still is a t-m phase transfornation in ceramic materials durFig. 3 SEM pattern of indent crack expansion on polishing surface of ZYA20 ing cooling, but no microcrack produced, instead there is a residual stress around the m phase grains after t-m’phase change[l4]. When the main crack expands into the residual stress area, the release of residual stress will consume energy, and close or hinder expansion of cracks; on the other hand, the material structure will be uneven because of internal stress, it will influence the form of crack expansion and lead to distortion deflection. This is the mechanism of crack deflection toughening. In Zr02 ceramics, because of the co-existence of t and m phase, toughening comes from the comprehensive interaction of above three mechanisms. Because of the mechanical properties of m phase is lower than that of t phase, m phase will undermine other properties when it roughening ceramics to some extent. Therefore, a proper content of m phase must be controlled in order to acquire PSZ with better comprehensive mechanical properties. 8 Phase Pansformation Toughening According to the micro structure style of ceramics, phase transformation toughening ceramics is actually a kind of self-toughening ceramics, its toughening mechanism is to produce toughening phase inside micro structure by controlling of sintering process, and the most widely used ceramics is ZrO, phase change toughening ceramics. There are three kinds of crystal form for Zr02 phase: the cubic phase at temperature above 2370 “c ( p = 6.09 g * , the monoclinic phase at temperature below 1170 “c ( p = 5. 83 g* cm-’) , the tetragonal phase at temperature between 1170-2370 “c (p =5.83 gmcm-’). When Zr02ceramics transforms from the tetragonal phase at high temperature to the monoclinic phase at low temperature, there is a volume expansion of 3% - 5% and shear strain of 16%. The tetragonal phase can be kept as metastable form to low temperature through certain method (such as adding stabilizer and heat treatment). The t-m phase change will occur under certain crack tip stress during heat treatment; both the new produced crack surface and volume expansion during phase change will absorb energy. And. the particles of phase transformation will produce compression stress to counteract tensile stress of crack tip, and reduce stress strength factor so as to improve the fracture toughness of ceramics. In addition, because of phase change, residual stress field produces around Zr02 particles of monoclinic phase; cracks will deflect, bend or bifurcate to play a toughening effect when expanding into stress field. At present, phase change toughening mecha-
Qiu L K et al. Study on Toughness Mechanism of Ceramic Cutting Tools 315 ism is mainly used in two kinds of ceramic materials, and many works and analysis have been carried ot one is ZrO2 phase change toughening ceramics, such toughening strengthening theory and material prepara as ZTA, ZTM(ZrO2 toughening mullite)and Si,N. tion udies of synergistic toug ma another one is ZrO, matrix ceramics, such as 3Y-TZP, inly focused on phase change toughening, whisker Mg-PSZ and Ce-PSZ etc strengthening and particle dispersion, particle and particle dispersion etc. on the basis of particle disper 9 Synergistic Toughening sion toughening and strengthening. Metral et al. [isJ It is paid more attention in recent years that high studied the temary system of TiB2-TiC-SiC composite performance composite ceramics are designed by using ceramics, the best strength and toughness acquired the synergistic effect of interaction of multi-toughening with different composition. It can be seen that syner echanisms. Studies 7 show that, the different tough- gistic toughening with two or more mechanisms is ening mechanism can interact with each other, the great latent research value. So one can expect more synergistic effect will not always happen through any deep researches will be carried on the relation of com arbitrary toughening mechanism fold. The synergistic ponent technique and structure performance of multi toughening effect is not the simple fold of some tough component ceramIcs ening mechanisms. In recent years, new principles Table 2 shows the ordinary method and toughen are provided on synergistic toughening mechanism ing mechanism of ceramic cutting tools Table 2 Ordinary method and toughening mechanism of ceramic cutting tools i9 Toughening Toughening method ffect/(MPam 2)Toughening mechanism (1)The pulling out of crystal whiskers or fibers to consume crack energy: (2)Crack deflection on the surface of crystal whiskers of fibers to consume crack es (3)The pinning effect of crystal whiskers or fibers on cracks inhibits the expansion of cracks 5-15 1)Phase change toughening, in that, t-ZnO changes to m-ZrO2 to consume crack expanding energy: (2)Microcrack toughening, during phase transformation of t-ZrO2 to m-Z 0,, the phase change or the expansion volume produces microcracks to inhibit the ex of the main cracks (3)Surface induced strengthening and toughening, during the process of surface machining, the volume expan sion produced by phase change to form stress on the surface to inhibit the formation and expansion of cracks (4)Crack bending, the second phase and the residual stress field in the base make cracks expand and deflect and crack deflection increases the path of crack expansion to reduce expanding force; (5)Microcrack bifurcation toughening (W, Ti)C (1)Elastic modulus of the composite is 1.2 times of that of pure Al2O, ceramic (2)The second phase of(W, Ti )C has pinning effect on cracks: (3)The consecutive and interlude one another framework structure improves toughne 4,2~4.5 The dispersed hard phase of TiC plays a pinning effect on cracks to deflect and prevent them from expanding Metal-TiC 57-7.2 (1)Using metal as even dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin racks and improves fracture toughness (2)The pinning effect of TiC hard phase to cracks leads to crack deflection, bypass to hinder crack expansion by consuming expanding force energy SiC whisker 5-7 Crystal fiber toughening or whisker bridge toughening, in that, both of the pulling out of whiskers and crack de- flection by whiskers inhibits the random crack expansion 9-10 Using metal as evenly dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and Ceramics-ce. 5=10. 8 (1)The interaction of crack tip and dispersed phase leads to crack tip pinning, bending, deflection and distor- (2)The microcracks from stress induced phase change during crack expansion and the pulling out of crystal whis- kers, release the main crack tip stress or produce compressive stress to inhibit main crack expansion and to im Sub-micron Effective It has fine, dense and evenly distributed structure Metal-TiN 5.7-7.2 Using metal as even dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks es fracture toughn
Qiu L K et a1 . Study on Toughness Mechanism of Ceramic Cutting Tools 315 nism is mainly used in two kinds of ceramic materials, one is ZrOz phase change toughening ceramics, such as ZTA, ZTM ( ZrOz toughening mullite) and Si3N4 ; another one is ZrOz matrix ceramics, such as 3Y-TZP, Ma-PSZ and Ce-PSZ etc. and many works and analysis have been carried out on toughening strengthening theory and material preparation. Up to now, studies of synergistic toughening mainly focused on phase change toughening, whisker strengthening and particle dispersion , particle and 9 Synergistic Toughening It is paid more attention in recent years that high performance composite ceramics are designed by using the synergistic effect of interaction of multi-toughening mechanisms. Studies"'] show that, the different toughening mechanism can interact with each other, the synergistic effect will not always happen through any arbitrary toughening mechanism fold. The synergistic toughening effect is not the simple fold of some toughening mechanisms. In recent years, new principles are provided on synergistic toughening mechanism, particle dispersion etc. on the basis of particle dispersion toughening and strengthening. Metral et a1 . ['*I , studied the ternary system of TiB,-Tic-Sic composite ceramics, the best strength and toughness acquired with different composition. It can be seen that synergistic toughening with two or more mechanisms is of great latent research value. So one can expect more deep researches will be carried on the relation of component technique and structure performance of multicomponent ceramics . Table 2 shows the ordinary method and toughening mechanism of ceramic cutting tools. Table 2 Ordinary method and toughening mechanism of ceramic cutting tools"" Toughening Toughening method effect/(MPa*m") Toughening mechanism Sic 8- 10 ( 1 ) The pulling out of crystal whiskers or fibers to consume crack energy; (2) Crack deflection on the surface of crystal whiskers of fibers to consume crack expanding energy; (3) The pinning effect of crystal whiskers or fibers on cracks inhibits the expansion of cracks. Zr02 6.5 - 15 ( 1 )Phase change toughening, in that, t-ZrOz changes to m-ZrO2 to consume crack expanding energy; (2) Microcrack toughening, during phase transformation of t-ZrOz to m-ZrO2, the phase change or the expansion of volume produces microcracks to inhibit the expansion of the main cracks; (3) Surface induced strengthening and toughening, during the process of surface machining, the volume expansion produced by phase change to form stress on the surface to inhibit the formation and expansion of cracks; (4) Crack bending, the second phase and the residual stress field in the base make cracks expand and deflect, and crack deflection increases the path of crack expansion to reduce expanding force; (5) Microcrack bifurcation toughening. (1) Elastic modulus of the composite is 1.2 times of that of pure A1203 ceramics; (2) The second phase of (W, Ti )C has pinning effect on cracks; (3) The consecutive and interlude one another framework structure improves toughness; (4) It has fine, dense and evenly distributed structure. (W, Ti)C >5.8 TIC 4.2-4.5 Metal-Tic 5.7 - 7.2 Sic whisker 5 - 7 Metal 9- 10 Ceramics-ce- 5 - 10.8 ramics composite The dispersed hard phase of Tic plays a pinning effect on cracks to deflect and prevent them from expanding. (1) Using metal as even dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and improves fracture toughness; (2) The pinning effect of TIC hard phase to cracks leads to crack deflection, bypass to hinder crack expansion by consuming expanding force energy. Crystal fiber toughening or whisker bridge toughening, in that, both of the pulling out of whiskers and crack deflection by whiskers inhibits the random crack expansion. Using metal as evenly dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and improves fracture toughness. (1) The interaction of crack tip and dispersed phase leads to crack tip pinning, bending, deflection and distortion ; (2) The microcracks from stress induced phase change during crack expansion and the pulling out of crystal whiskers, release the main crack tip stress or produce compressive stress to inhibit main crack expansion and to improve fracture toughness. Sub-micron Effective It has fine, dense and evenly distributed structure A1203 Metal-TiN 5.7-7.2 Using metal as even dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and improves fracture toughness;
JOURNAL OF RARE EARTHS, Vol 25, Suppl., Jun. 2007 Toughening Toughening effect/(MPa. 2) Toughening mechanism TI(NC) 4.5~5 The dispersed hard phase of TiC. plays a pinning effect on cracks to deflect and prevents them from expanding Metal-Ti The dispersed hard phase of TiC plays a pinning effect on cracks to deflect and prevents them from expandi Using metal as evenly dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and improves fracture toughness Metal- 6-6.5 (1) Elastic modulus of the composite is 1. 2 times of that of pure AlO, Ceramics; (2)the second phase of (W, Ti)C has pinning effect on cracks (3)the consecutive and interlude one another framework structure improv (4)It has fine, dense and evenly distributed structure (5)Using metal as evenly dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and improves fracture toughness; Micro-structure Effective The grain form, size and boundary characters can be changed to improve fracture toughness control 2O, purifica- Effective mproving structure, increasing toughness 5.3 (1)The expansion coefficients of the base and Si,N2 dispersed particles are quite different, there resid ual stresses around the particles dispersed in Si]Na boundaries which enhance the destroy energy of the boundar 2)Particles dispersed in the boundary and inside grains of Si,N, prevent microcracks or dislocation of the base from expanding: 3)The combining state of ceramic grain boundary is improved by introducing si, N, articles 10 Conclusion [6Kan Yanmei,Jin Ceramic cutting tool is an important application nisin In tes and their effecting factors Journal of Ceramics, 1998, 19(4 area in modern structural ceramics. It has advantages [7] Yasuliro Goto, et al. Mechanical properties of unidirec of high hardness, high abrasion resistance, and excel tionally oriented SiCW whisker-reinforced Si3N4 fabricated by lent mechanical performance at high temperature. But extrusion and hot-pressing [J].J.Am.Ceran 1993,76(6):1420 ceramic cutting tool is certainly not for all purpose [8] Ge Qilu, Gao Zengseng, et al. Microstructure Now studies on strengthening and toughening of ceram- chanical properties of hot-pressed Al,0,-ZH02(6%Y ic cutting tools mainly focused on mechanisms of part ramic composite [J]. Bulletin of the Chinese Ceram cle dispersion toughening, grain fiber or grain whisker ( in Chin.),1995,(2) toughening and synergistic toughening. These mecha hot-pressed Al2O3-ZrO2 ics [J].Bulletin nisms all are confined in certain area. It can be ex- nese Ceramic societ Chin.),1993,(4):15 pected that with the use of various new ceramic cutting [10] Lange FF, Margaret M. Hirlinger, Hindrance of Grain ols, the development of high efficient machine anc Growth in Al, 0, by ZO, Inaclusions [J].J.Am.Cerami high speed cutting technology will be promoted, and Soc.,1984,67(3):64 further more the application of ceramic cutting tools [ll] Guo Jingkun. The progress and application prospect of will be promoted ramic Society(in Chin. ) 1995,(4) ramics(I)[J]. ceramic Er References seeing,197,3l(1):40 [13]Mou Jun, Li Jian, Guo Shaoyi, et al. Transformation and [1] Qiu Guan ghening in zirconia toughened ceramics [J].Materials nry Industry Press(in Chin ),1993. 79 Science ande 1994,12(3):6 [2] Zhu Xiaoping, ZhouZhou, Chen Shilu, Technological [14] Yang Zhengfang, Xu Haiyang, Tan Jiaqi, et al.Mechani alleges on flight mechanic with applications of intelligent cal properties of mullite-based composites [J]. Journal of structures [J]. Flight dynami 5(3):13 the Chinese Ceramic Society(in Chin. ) 1990, 5(4): 467. [3] Zheng Xiuhua, et al. Efects of Y-PSZ on Densificatic [15] Xu Lihua, Ding Zishang, Huang Yong. Study progress and nal of Materials Research 10(3)8 3 /0. of several dispersions ()-multipl ceramIcs composite [4] Guo Jingkun, u Yueping. State-of-Art of [J]. Bulletin of the Chinese Ceramic Society(in Chin s[J]. Journal of the Chinese Ceramic Society(in 1996,(6):42 Chin.),1992,20(3):286 [5]Zhao Hong, Jin Zonghe. Analysis of residual stress and [17] Ma Laipeng, Yin Yansheng. Al20-based cerrmic cutting ghening mechanism for particulate composites [J].Jour- ools and toughening mechanisms [J]. Jiangsu Ceramics(in nal of the Chinese Ceramic Society(in Chin. ) 1996, 24(5 Chin.),2004,37(2);:9
316 JOURNAL OF RARE EARTHS, Vol. 25, Suppl. , Jun . 2007 Toughening Toughening method effect/(MPa.m”2) Toughening mechanism Ti(NC) 4.5-5 Metal-Ti 6-6.5 (N,C) The dispersed hard phase of Tic plays a pinning effect on cracks to deflect and prevents them from expanding. The dispersed hard phase of Tic plays a pinning effect on cracks to deflect and prevents them from expanding; Using metal as evenly dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and improves fracture toughness. (1) Elastic modulus of the composite is 1.2 times of that of pure A1203 Ceramics; (2) the second phase of (W, Ti)C has pinning effect on cracks; (3) the consecutive and interlude one another framework structure improves toughness; (4) It has fine, dense and evenly distributed structure. (5) Using metal as evenly dispersed phase, the crack tip stress is reduced by plastic deformation of metal to pin cracks and improves fracture toughness; Metal- 6 - 6.5 (Ti,W) The grain form, size and boundary characters can be changed to improve fracture toughness. Improving structure, increasing toughness Micro-structure Effective control A1203 purifica- Effective tion Si,N4 >5.3 (1 ) The expansion coefficients of the base and Si3N4 dispersed particles are quite different, there are many residual stresses around the particles dispersed in Si3N4 boundaries which enhance the destroy energy of the boundary of A1203 ceramics; (2) Particles dispersed in the boundary and inside grains of Si3N4 prevent microcracks or dislocation of the base from expanding; .. ;e T cr ’i i SiqNep~dicles. .. 10 Conclusion Ceramic cutting tool is an important application area in modern structural ceramics. It has advantages of high hardness, high abrasion resistance, and excellent mechanical performance at high temperature. But ceramic cutting tool is certainly not for all purpose. Now studies on strengthening and toughening of ceramic cutting tools mainly focused on mechanisms of particle dispersion toughening, grain fiber or grain whisker toughening and synergistic toughening. These mechanisms all are confined in certain area. It can be expected that with the use of various new ceramic cutting tools, the development of high efficient machine and high speed cutting technology will be promoted, and further more the application of ceramic cutting tools will be promoted. References : [ 11 Qiu Guanming. Newtype Ceramics [MI. Beijing: Weaponry Industry Press (in Chin. ) , 1993. 79. [2] Zhu Xiaoping, Zhou Zhou, Chen Shilu. Technological challenges on flight mechanic with applications of intelligent structures [J]. Flight Dynamics, 1997, l5(3): 13. 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