北京化工大学：《材料导论》课程阅读材料（无极非金属）Densification_and_mechanical_properties_of_CaB6_with

CERAMICS INTERNATIONAL ELSEVIER amics Interational 31(2005)271-276 www.elscvicr.com/ocate/ceramint Densification and mechanical properties of CaB6 with nickel as a sintering aid Lixia Yang",Guanghui Min*,Huashun Yu",Jiande Han",Y.B.Paderno Abstract The 1.Introduction ssible Nonoxide c Because of their strong bonds between the boron atoms However,either large amounts of second phase,or extre racterize p ing temperatures are r All of espe these outstanding properties make it widely used in modem cially in large amounts,as sintering aids. engines and various other structures walent B-B honds inhibit diffusic the production of refractory boride p wders hy hot-r and fracture toughness of CaB have been measured and and vacuum sintering is very difficult Up tonow,calcium correlated with the variation in composition of the body. llin een made eith 2.Experimental procedure duction methods impose a high demand on equipment and moulds,which increas on costs distribution is illustrated in Fig.1.Up to 35 wt nickel was added as a sintering aid

Densification and mechanical properties of CaB6 with nickel as a sintering aid Lixia Yanga , Guanghui Mina, *, Huashun Yua , Jiande Hana , Y.B. Padernob a School of Materials Science and Engineering, South Campus of Shandong University, Jinan 250061, PR China b Institute for Problems of Materials Science, National Academy of Science of Ukraine, Kiev 03142, Ukraine Received 11 July 2003; received in revised form 25 February 2004; accepted 3 May 2004 Available online 3 August 2004 Abstract The densification behavior and mechanical properties of CaB6 with additions of nickel up to 35 wt.% were investigated. Sinterability was greatly improved by the addition of nickel. When 28 wt.% nickel was added, nearly full density was obtained. This improvement was attributed to the enhanced mobility of the melting of metals including the evaporated calcium or the formation of new phases in the Ca–B–Ni system at the grain boundaries. As a result of this improvement in the density, mechanical properties has been increased remarkably. Because of the elimination of pores, CaB6 with 28 wt.% nickel has a maximum value of flexural strength and fracture toughness. The enhancement of the fracture toughness was due to a mixed mode of intergranular and transgranular fracture with a distinct step-cleavage pattern. # 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Mechanical properties; Calcium boride; Densification 1. Introduction Because of their strong bonds between the boron atoms, borides are particularly promising. They are characterized by high melting-point, high hardness, high chemical stabi￾lity, high resistance to oxidation and wear and so on. All of these outstanding properties make it widely used in modern aircraft, parts of rocket engines and various other structures at high temperature [1]. Because their strong covalent B–B bonds inhibit diffusion, the production of refractory boride powders by hot-pressing and vacuum sintering is very difficult [2]. Up to now, calcium hexaboride polycrystalline has been made either under high pressure (3–5 GPa)[3] or at high temperature (2000–2200 8C) [4] in order to obtain high or full density bodies. These production methods impose a high demand on equipment and moulds, which increase production costs. For certain purposes, it is desirable to produce a material with a density that is as high as possible, and at temperature that is as low as possible. Nonoxide ceramics such as TiB2 have been found to be effective as sintering aid for CaB6. However, either large amounts of second phase, or extre￾mely high sintering temperatures are required for further densification and improvement of strength. Up to now, there have been a limited number of studies using metal, espe￾cially in large amounts, as sintering aids. In the present study, the effect of nickel addition on the densification behavior of CaB6 has been investigated. Mechanical properties, such as hardness, flexural strength and fracture toughness of CaB6 have been measured and correlated with the variation in composition of the body. 2. Experimental procedure The CaB6 powder prepared by carbonboride method was ball-milled for 16 h. The average size and specific surface area of the CaB6 powder were 14.73 mm and 0.68 m2 /g, respectively. The particle size distribution is illustrated in Fig. 1. Up to 35 wt.%, nickel was added as a sintering aid. www.elsevier.com/locate/ceramint Ceramics International 31 (2005) 271–276 * Corresponding author. Tel.: +86 531 839 5639; fax: +86 531 295 5999. E-mail address: ghmin@sdu.edu.cn (G. Min). 0272-8842/$30.00 # 2004 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2004.05.014

272 LYang et al/Ceramics Intemational 31(2005)271-276 164m article size(m) Fig.1.Particle size distribution of the used CaBs powder Powder mixtures were hot-pressed in a 40 mm inner graphite pressed at 16C/min and furnace cooling.respectively. All surfaces of sintered material were polished and the and) Eere e iE detect nickel in the sintered matrix. using 3 mm x 30 mm.Ben d out by a 6m spee beam method (SENB)was used for evaluation of fracture were 4 mmmm nd n noteh was curi shown in Fig.2(b).When 28 wt.%nickel was added.nearly full density was obtained,although the temperature is I00 3.Results and discussion above 1300C at hig The sintering behavior and microstructure of CaBs are [5].In this case.the elements in the second phases area were changed markedly by the addition of nicke.The detected using EDAX.and nickel.cobalt,calcium and iron I addit acdnihcticmisutatedby the SEM were found as shown n ts of thes hot-presed "r the impurity in the nickel powder.And accordingly.calcium seen in Fig.2(a).With the addition of 20 wt.%nickel the was formed due to the evaporation of CaB at high tem specimen shows a significantly improved densification.as perature.However.there was a great difference of calcium

Powder mixtures were hot-pressed in a 40 mm inner diameter circular graphite die for 1.5 h at 1750 8C under 32 MPa in a high vacuum. Nickel additions up to 20 wt.% was used. Considering the fast flow of the large amount of nickel, CaB6 powder with 28 and 35 wt.% nickel was hot￾pressed at 1650 8C under 32 MPa. The temperature was monitored by an optical pyrometer, which was calibrated using a thermocouple. The heating and cooling rates were 16 8C/min and furnace cooling, respectively. All surfaces of sintered material were polished and the microstructure of the sintered sample and fractured surfaces were analyzed by scanning electron microscopy (JXA-840). Energy dispersive analysis of X-ray (EDAX) was used to detect nickel in the sintered matrix. Hardness was measured on the polished surfaces using Rockwell apparatus. Four indentations were made for each preparation using 306 N loads. For bending strength testing, samples were cut and ground with dimensions of 4 mm
3 mm
30 mm. Bending strength was carried out by a three-point-bending with a crosshead speed of 0.5 mm/min and a span of 20 mm. The three-point single edge notched beam method (SENB) was used for evaluation of fracture toughness with 0.05 mm/min crosshead speed of and span of 20 mm. The dimensions of the fracture toughness samples were 4 mm
2 mm
30 mm, and a notch was cut with 0.2 mm in width and 1.5 mm in depth. 3. Results and discussion The sintering behavior and microstructure of CaB6 are changed markedly by the addition of nickel. The effect of nickel addition on the densification is illustrated by the SEM photographs in Fig. 2 Incomplete densification of the pure CaB6 specimen hot-pressed at 1750 8C for 1.5 h is clearly seen in Fig. 2(a). With the addition of 20 wt.% nickel the specimen shows a significantly improved densification, as shown in Fig. 2(b). When 28 wt.% nickel was added, nearly full density was obtained, although the temperature is 100 8C lower. It has been previously reported that above 1300 8C, atomic calcium enters the vapor–gas phase at high vacuum [5]. In this case, the elements in the second phases area were detected using EDAX, and nickel, cobalt, calcium and iron were found as is shown in Fig. 3 and the contents of these elements were illustrated in Table 1. Cobalt and iron were the impurity in the nickel powder. And accordingly, calcium was formed due to the evaporation of CaB6 at high tem￾perature. However, there was a great difference of calcium 272 L. Yang et al. / Ceramics International 31 (2005) 271–276 Fig. 1. Particle size distribution of the used CaB6 powder. Fig. 2. SEM photographs of CaB6 specimens hot-pressed at 1750 8C (a); CaB6 with 20 wt.% nickel hot-pressed at 1750 8C (b) and CaB6 with 28 wt.% nickel hot-pressed at 1650 8C (c)

L.Yang etal/Ceramies Intemational 31 (2005)271-276 273 8 ull Scale 298 cts Cursor 6 951 kev (25 cts ul Scsie 163 cts Cursor:3695 kev (43 cts Fig.3.EDAX profiles of second phase in CaBe with 35 wt.%nickel hot-pressed at 1650C though boron was not detected due to detection limitation the evaporation of calcium resulted in the production of free the rich-nickel areas.Previous studies showed that chemical boron.A possible reaction among calcium,boron and nickel reactions coul occur in the Ca-B- system 6.Ever nelting of the metals in the rain boundaries and the formation of new phases [6].In general.as we can see from Elemen Weight ( Atomic(%) the microstructure.under the current c nces (high of CaB resembled the morphology of CaB powder. was clear A enhanced up to 20 wt.%nickel.These increases were pre

content in the point 1 and 2, which further confirmed the evaporation of calcium from the boundary to the center of the rich-nickel areas. Previous studies showed that chemical reactions could occur in the Ca–B–Ni system [6]. Even though boron was not detected due to detection limitations, the evaporation of calcium resulted in the production of free boron. A possible reaction among calcium, boron and nickel at such high temperature was deemed to have occurred. The improved sinterability, therefore, was attributable to the melting of the metals in the grain boundaries and the formation of new phases [6]. In general, as we can see from the microstructure, under the current circumstances (high temperature and pressure), the metals melted and penetrated into the corner of CaB6 particles, which to some extent resembled the morphology of CaB6 powder. As a result of the improved density, the hardness of CaB6 was clearly enhanced. The effect of nickel addition on hardness of CaB6 is shown in Fig. 4. With the increasing amount of nickel, the hardness values were gradually enhanced up to 20 wt.% nickel. These increases were pre￾sumably due to the decrease in porosity. The hardness decrease of the specimen with 28 and 35 wt.% nickel was L. Yang et al. / Ceramics International 31 (2005) 271–276 273 Fig. 3. EDAX profiles of second phase in CaB6 with 35 wt.% nickel hot-pressed at 1650 8C. Table 1 Second phase element analysis results in CaB6 with 35 wt.% nickel hot￾pressed at 1650 8C Element Weight (%) Atomic (%) Point 1 Ca 1.27 1.85 Fe 2.16 2.26 Co 8.90 8.81 Ni 87.66 87.08 Point 2 Ca 10.99 15.32 Fe 1.53 1.53 Co 8.78 8.32 Ni 78.69 74.83

LYang et al/Ceramies Intemational 31 (2005)271-276 ea 90 99 0 Amount of nickel(w% Fig.4.Effect of nickel additions on the hardness of CaB 240 2 20 16似m 40 20 06105202505 Amount of nickel(w) Fig.5.Flexural strength of CaBs with different amounts of nickel. 5.0 35 25 050152025 30 Amount of nickel(w) 5u m Fig.6.Fracture toughness of CaBe with different amounts of nickel Fig.8.Practure surfaces of pure CaB specimens hot-pressed at 1750C

274 L. Yang et al. / Ceramics International 31 (2005) 271–276 Fig. 6. Fracture toughness of CaB6 with different amounts of nickel. Fig. 5. Flexural strength of CaB6 with different amounts of nickel. Fig. 4. Effect of nickel additions on the hardness of CaB6. Fig. 8. Fracture surfaces of pure CaB6 specimens hot-pressed at 1750 8C. Fig. 7. SEM photograph of CaB6 with 5 wt.% nickel specimens hot-pressed at 1750 8C (a) and nickel distribution (b)

LYang et al/Ceramies 31(2005)271-276 275 16μm deemed to be controlled by the rule of mixtures.When the can lowe 8μm strength and the fracture toughness of CaB as illustrated in Fig.5 and Fig.6.The specimen with 5 wt.%nickel has the e in thes materials her than th the Fig the nickel distribution photograph.it's clear that the small surfaces of CaBs with 10 and 28 wt.%nickel specimen shown in Fig.9.As a c When2 wtnckel was added.the highest strength was 28 wt nickel sintered specimens.the fracture routes a obtained,which was apparently due to the reduction in the more complex and the ratio of transgranular to intergranular nany pores type is enhanced.The existenc and barea i Different from the flexural strength,the fracture tough- ord tomake cear investigation of fracture mode.the nter was used tomake an indentation on the 0 greatly a the 06N Th number and size of the pores noted that the crack passed through the nickel area and the Fig.8 shows the fracture surfaces of CaB polycrystal path was not so straight.As the arrow indicated,the pore has cture mode of CaB een the intergranular and transgranular pore an

deemed to be controlled by the rule of mixtures. When the porosity has been controlled to a limited degree, the com￾paratively soft nickel can lower the hardness of CaB6. The addition of nickel also had great effect on the flexural strength and the fracture toughness of CaB6, as illustrated in Fig. 5 and Fig. 6. The specimen with 5 wt.% nickel has the lowest strength value in these materials, although its relative density is slightly higher than that of the pure CaB6. The SEM photograph of this specimen is shown in Fig. 7(a). On the nickel distribution photograph, it’s clear that the small amount of nickel was randomly filled in grain boundaries. So the inhomogeneity of nickel is partially responsible for the poor property. When 28 wt.% nickel was added, the highest strength was obtained, which was apparently due to the reduction in the number and size of many pores acting as fracture origin. However, the strength decreased slightly with further nickel addition. Different from the flexural strength, the fracture tough￾ness increased steadily with the addition of nickel up to 28%, which has reached a top value in Fig. 6. The predominant reason is that the addition of nickel greatly decreased the number and size of the pores. Fig. 8 shows the fracture surfaces of CaB6 polycrystal￾line. It is worth emphasizing that the fracture mode of CaB6 polycrystalline was a mix of intergranular and transgranular type. As shown in Fig. 8(b), the regular pore is considered to be due to the pull-out of a small particle. The fracture surfaces of CaB6 with 10 and 28 wt.% nickel specimen are shown in Fig. 9. As a consequence of nickel addition, the specimens were clearly densified. As for CaB6 with 28 wt.% nickel sintered specimens, the fracture routes are more complex and the ratio of transgranular to intergranular type is enhanced. The existence of terraces and steps makes the fracture area increase, which results in a high fracture energy and hence a high fracture toughness. In order to make clear investigation of fracture mode, the Vickers indenter was used to make an indentation on the surface of CaB6 with 35 wt.% nickel under the pressure of 306 N. The load direction was indicated in Fig. 10. It was noted that the crack passed through the nickel area and the path was not so straight. As the arrow indicated, the pore has been the fracture origin, the crack was propagated along the pore and formed the intergranular fracture failure. Com￾L. Yang et al. / Ceramics International 31 (2005) 271–276 275 Fig. 9. Fracture surfaces of hot-pressed specimens CaB6 with 10 wt.% nickel (a) and CaB6 with 28 wt.% nickel (b). Fig. 10. Crack propagation in CaB6 with 35 wt.% nickel (a) and fracture surfaces of hot-pressed specimens CaB6 with 35 wt.% nickel (b)

276 LYang et al/Ceramics Intematlonal 31(2005)271-276 bined with Fig.10(b),the main fracture mode was transgra- Acknowledgement e intens This significant contribution to the improvement of fracture toughness. References 4.Conclusions t nickel was aded.nearly full density at 1650C under 32 MPa.The improvement in sinterability 2]T.L Sere yakova.E.N.Martynenko,Sintering of highly dis Powder Metall.Met. m36(11-12)(1997 583 phases in the Ca-B Ni system at the new 3 on the Flexural strength and fracture toughness had their top [4T Yu.G.T values with 28 wt.%nickel.When the amount of nickel LF.Och as.TI.S theura strength the lowest partiall s of The enhancement of the fracture toushness was due to the [5]G.N.Ma nk on of nick 1993)828- proportior 6197736-35g9

bined with Fig. 10(b), the main fracture mode was transgra￾nular, and the particle was severely laniated. In some areas, there were serrated edges, which was the result of the intense integrity between two particles. The transgranular fracture surface is distinct with step-cleavage pattern, which is a significant contribution to the improvement of fracture toughness. 4. Conclusions The sinterability and mechanical properties of CaB6 with additions of nickel up to 35 wt.% were investigated. When 28 wt.% nickel was added, nearly full density was obtained at 1650 8C under 32 MPa.The improvement in sinterability was attributed to the enhanced mobility of the metals including the evaporated calcium or the formation of new phases in the Ca–B–Ni system at the grain boundaries. Flexural strength and fracture toughness had their top values with 28 wt.% nickel. When the amount of nickel was 5 wt.%, the flexural strength was the lowest partially because of the inhomogeneity of small amount of nickel. The enhancement of the fracture toughness was due to the mixture mode of intergranular and transgranular type with a distinct step-cleavage pattern. With the addition of nickel, the fracture routes become more complex and the proportion of transgranular fracture increased. Acknowledgement This research was jointly supported by Shandong Out￾standing Young Scientist Foundation (2000) and Shandong High Technology Foundation (GXB991). References [1] A.N. Paderno, Yu.B. Paderno, A.N. Martynenko, V.M. Volkogon, Effect of the production method on the structure formation and failure of the pseudoalloy CaB6–TiB2. 1. Sintering by hot pressing under high pressure, Soviet Powder Metall. Met. Ceram. 31 (10) (1992) 863–866. [2] T.I. Serebryakova, E.N. Martynenko, Sintering of highly dispersed titanium diboride and composite titanium diboride–calcium hexaboride powders, Powder Metall. Met. Ceram. 36 (11–12) (1997) 579–583. [3] V.N. Paderno, V.M. Volkogon, N.A. Martynenko, Effect of high pres￾sure on the microstructure of sintered polycrystalline calcium hexabor￾ide cakes, Soviet Powder Metall. Met. Ceram. 24 (7) (1985) 543–546. [4] T.I. Serebryakova, L.F. Ochkas, T.I. Shaposhnikova, Yu.G. Tkachenko, E.N. Martynenko, et al., Influence of addition of calcium hexaboride on the structure and properties of hot-pressed titanium boride ceramic, Powder Metall. Met. Ceram. 37 (9–10) (1998) 507–511. [5] G.N. Makarenko, S.P. Gordienko, V.B. Fedorus, et al., Interaction of boron carbide with calcium oxide, Soviet Powder Metall. Met. Ceram. 32 (1993) 828–831. [6] M.L. Fiedler, H.H. Stadelmaier, I.K. Simonsen, Nickel–boron side of the ternary system nickel–calcium–boron, Zeitschrift fuer Metallkunde 68 (1977) 356–358. 276 L. Yang et al. / Ceramics International 31 (2005) 271–276