M.H. Bocanegra-Bermal B. Matovic/ Materials Science and Engineering A 500(2009) 2 Fig 1. Typical microstructure of a liquid phase sintered SiaNa ceramics schematic(a)and SEM micrograph(b)-(1)Si3N4 matrix grains: (2)crystalline secondary phase: and ()amorphous residue at triple junctions and grain boundaries [From Ref [3511- After pressureless sintering of silicon nitride ceramics, the grain growth resulting in the highest aspect ratio grains compared formed intergranular phase could be crystallized by slow cool with other rare earth oxides[174. However, Luo et al. [175 used ing or alternately by heat treatment. The devitrification increases high oxynitride glass with y+ and La+ in order to control the high temperature strength of sintered body substantially, for microstructure development and B-Si3N4 grain growth forming example, the strength at 1000C can be increased about 50% glassy phase with high softening temperature and high viscosity ver that of as-sintered one[12]. Various authors have reported at grain boundaries during sintering increasing the elevated tem- different techniques in order to improve the high temperature perature properties of Si3N4 ceramics. strength: i)decreasing of impurities such as Ca and Fe contained he commercializing of silicon nitride ceramics for structural in silicon nitride powders[165). ii)devitrification of intergranu- applications as bearings, requires an extensive effort to obtain an lar glassy phase [11, 155, 166]. iii)addition of sintering aid which advanced silicon nitride bearing and it is very important to correlate does not leave any residual glassy phase [167]. iv) adoption of the physical/mechanical properties to the cture which dvanced techniques such as Hot isostatic Pressing(HIP), to con- develops during synthesis and processing [16, 142, 176-179 Liu and solidate silicon nitride with or without a small amount of additives Nemat-Nasser[16 obtained Si3 Na ceramics exceeding 99% theoret 13,44,168-170 ical density with pressureless sintering and 6 wt% Y2 03, 6wt% AIN Rice[171 ]reported the preparation of Si3 N4 ceramic composites and 1 wt% TiO as sintering aids where the sintering parameters reinforced with whiskers, for example with Sic whiskers, because (temperature and time)were optimized in an effort to control, i ) the lese prevent viscous sliding between grains at high temperatures percentage of a-phase to B-phase transformation, ii) the stabiliza- and improve the strength at high temperatures practically. Mit- tion of the a-Si3N4 structure and iii) the acicular B-grain growth. omo et al. [172 reported to dissolve the ingredients of the glass Takatori et al. [12 examined the devitrification of the intergran- phase, which act as the liquid phase in the early and middle stages ular glassy phase in silicon nitride ceramics sintered with 5 wt% of sintering, into a-and B-Si3 Na grains at the final stage of sintering. Y203 and 5 wt% MgAl2O4. Mg-spinel in one of the most effective It is important to point out the study of lanthanum oxide additives for the densification of silicon nitride at relatively low (La2O3)doped B-Si3Ng sintered sample carried out by Shibata et sintering temperature al. [173 where they observed rare earth segregation in silicon herefore, the simultaneous addition of Y203 and Mg-spinel nitride ceramics at subnanometre dimensions, taking into account is effective to produce a high strength silicon nitride alloy. the that Laz O3 additions are known to strongly promote anisotropic temperature dependence of the bending strength for as-sintered and heat-treated bodies where each specimen had about the same able 1 strength of 700 MPa at room temperature was revealed by Takatori Oxide additives used for the densification of Si3 N4 [from Ref [3511. et al. [12]. Similarly, the strength of the as-sintered sample dropped Additive(M,Oy) Temperature of liquid formation( to 450 MPa at 1000.C, which was improved 50% with the heat treatment at approximately 1250 and 1350 C On the other hand Silicate(MrOy-SiO2) Oxynitride(MxOy-SiOz-Si3 N4) 480122 Summary of sintering studies for Si3 Na at atmospheric pressure with magnesia and CeO2 yttria additives and their compounds [from Ref [351. Additive Sintered density 1435 435121l mperature(c) Al203 1595 1470121 00-1700 1650-1900 Y,03+2 wtX Al,03 10 wt% MgO Al203 600-1750 10wt%Y203+3 wt% Al 1600-1750 3.5-20wt%Y2O3+20wt 1750-1825 Pr,03 10 mol% Y203+20 mol% SiO2 1750 4-17wt%Y2O3+2-4wt%A2O3 500-1750M.H. Bocanegra-Bernal, B. Matovic / Materials Science and Engineering A 500 (2009) 130–149 139 Fig. 1. Typical microstructure of a liquid phase sintered Si3N4 ceramics [schematic (a) and SEM micrograph (b)]. (1) Si3N4 matrix grains; (2) crystalline secondary phase; and (3) amorphous residue at triple junctions and grain boundaries [From Ref. [35]]. After pressureless sintering of silicon nitride ceramics, the formed intergranular phase could be crystallized by slow cool￾ing or alternately by heat treatment. The devitrification increases high temperature strength of sintered body substantially, for example, the strength at 1000 ◦C can be increased about 50% over that of as-sintered one [12]. Various authors have reported different techniques in order to improve the high temperature strength; i) decreasing of impurities such as Ca and Fe contained in silicon nitride powders [165], ii) devitrification of intergranu￾lar glassy phase [11,155,166], iii) addition of sintering aid which does not leave any residual glassy phase [167], iv) adoption of advanced techniques such as Hot Isostatic Pressing (HIP), to con￾solidate silicon nitride with or without a small amount of additives [13,44,168–170]. Rice [171] reported the preparation of Si3N4 ceramic composites reinforced with whiskers, for example with SiC whiskers, because these prevent viscous sliding between grains at high temperatures and improve the strength at high temperatures practically. Mit￾omo et al. [172] reported to dissolve the ingredients of the glass phase, which act as the liquid phase in the early and middle stages of sintering, into -and -Si3N4 grains at the final stage of sintering. It is important to point out the study of lanthanum oxide (La2O3) doped -Si3N4 sintered sample carried out by Shibata et al. [173] where they observed rare earth segregation in silicon nitride ceramics at subnanometre dimensions, taking into account that La2O3 additions are known to strongly promote anisotropic Table 1 Oxide additives used for the densification of Si3N4 [from Ref. [35]]. Additive (MxOy) Temperature of liquid formation (◦C) Silicate (MxOy–SiO2) Oxynitride (MxOy–SiO2–Si3N4) Li2O 1030 1030 [121] MgO 1543 1390 [121] Y2O3 1650 1480 [122] CeO2 1560 1460 [121] ZrO2 1640 1590 [121] CaO 1435 1435 [121] Al2O3 1595 1470 [122] Additive (MxOy) Melting temperature (◦C) Sc2O3 2300 Ce2O3 2776 La2O3 2315 BeO 2530 HfO2 2758 SrO 2430 Nd2O3 2272 Pr2O3 2200 Sm2O3 2300 Ac2O3 1596 grain growth resulting in the highest aspect ratio grains compared with other rare earth oxides [174]. However, Luo et al. [175] used high oxynitride glass with Y3+ and La3+ in order to control the microstructure development and -Si3N4 grain growth forming glassy phase with high softening temperature and high viscosity at grain boundaries during sintering increasing the elevated tem￾perature properties of Si3N4 ceramics. The commercializing of silicon nitride ceramics for structural applications as bearings, requires an extensive effort to obtain an advanced silicon nitride bearing and it is very important to correlate the physical/mechanical properties to the microstructure which develops during synthesis and processing [16,142,176–179]. Liu and Nemat-Nasser [16] obtained Si3N4 ceramics exceeding 99% theoret￾ical density with pressureless sintering and 6 wt% Y2O3, 6 wt% AlN and 1 wt% TiO2 as sintering aids where the sintering parameters (temperature and time) were optimized in an effort to control, i) the percentage of -phase to -phase transformation, ii) the stabiliza￾tion of the -Si3N4 structure and iii) the acicular -grain growth. Takatori et al. [12] examined the devitrification of the intergran￾ular glassy phase in silicon nitride ceramics sintered with 5 wt% Y2O3 and 5 wt% MgAl2O4. Mg-spinel in one of the most effective additives for the densification of silicon nitride at relatively low sintering temperature. Therefore, the simultaneous addition of Y2O3 and Mg-spinel is effective to produce a high strength silicon nitride alloy. The temperature dependence of the bending strength for as-sintered and heat-treated bodies where each specimen had about the same strength of 700 MPa at room temperature was revealed by Takatori et al. [12]. Similarly, the strength of the as-sintered sample dropped to 450 MPa at 1000 ◦C, which was improved ≈50% with the heat treatment at approximately 1250 and 1350 ◦C. On the other hand, Table 2 Summary of sintering studies for Si3N4 at atmospheric pressure with magnesia and yttria additives and their compounds [from Ref. [35]]. Additive Sintering temperature (◦C) Sintered density (%Th. D.) 5 mol% MgO 1500–1700 86 10 mol% spinel (MgO.Al2O3) 1650–1900 96 5 wt% MgO + 0.15 wt% CaO + 0.8 wt% FeO + 4 wt% Y2O3 + 2 wt% Al2O3 1750 95 10 wt% MgO.Al2O3 1600–1750 97 5 wt% MgO + BeO + CeO2 1800 97 4 mol% Y2O3 + 2 mol% Al2O3 1725 Not mentioned 10 wt% Y2O3 + 3 wt% Al2O3 1600–1750 98 3.5–20 wt% Y2O3 + 20 wt% Al2O3 1750–1825 100 10 mol% Y2O3 + 20 mol% SiO2 1750 90 4–17 wt% Y2O3 + 2–4 wt% Al2O3 1500–1750 95