Hot-Pressed Si3N4
Hot-Pressed Si3N4
复制标题
热压氮化硅
DOI：
10.1111/j.1151-2916.1973.tb12447.x
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发表时间：
1973
影响因子：
3.9
通讯作者：
G. Gazza
中科院分区：
文献类型：
--
作者：
G. Gazza
由于Si3Na在高温燃气轮机部件中的适用性，其热压制技术受到了广泛的关注。据报道，通常需要高达5 wt%的MgO作为n-Si3Na粉末的添加剂，通过热压生产出完全致密的高强度体。然而，高温强度和抗蠕变性能明显受到晶界相硅酸镁（顽火辉石）形成的限制。通过从高纯度的Si， Na粉末（特别是Ca， Na和K）开始并通过减少所需的MgO量来增强高温性能。另一种方法是使用一种添加剂，这种添加剂不仅可以促进致密化和高强度，而且还可以产生比硅酸镁更耐火的晶界相。如果边界玻璃相可以很容易地结晶，那么与粘性蠕变有关的高温问题也可能得到缓解。在考虑耐火边界方法时，应优化而不是最小化所需添加剂的量。一种添加剂被发现可以有效地生产高密度高强度的Si， Na样品，并具有满足所需添加剂标准的高潜力，即YIO，。*添加1.0 ~ 3.3 wC% Y， O至高相硅粉；允许粉末在1750℃下热压至满密度。在氮气气氛下，用6000到7000 psi的单轴压力在石墨模具中热压。石墨箔，一面涂上BN浆料，用作模具，柱塞和粉末之间的间隔材料。有效热压温度为1750℃~ 1800℃，保温时间为2 h。通过液体浸泡测定的热压试样的密度从3.22 g/cm3到3.26 g/cm3不等，随添加剂用量的增加而增加（从1.0 wt%到3.3 wt%）。形成的晶界相，虽然尚未定量鉴定，显然是硅酸钇。Toropov和BondaZ报道的硅酸钇的组成是YzO，。SiO(单斜),2 y。01.3 Si02（六边形）和Y203。2 sioz(单斜)。这些硅酸盐的密度分别为4.49、4.39和4.06 g/cm3，熔点分别为1980、1950和1780℃。从热压盘上加工弯曲试样，以确定室温和1315℃下的断裂模量（MOR），采用四点弯曲，压板率为0.002 in。/min，应变速率为z6。4 x10 - /英寸。高温下最小。大多数MOR测试是在试样的拉伸面与热压方向垂直的平面上进行的。最后的表面光洁度是用平行于试样长度的220粒金刚石磨削得到的。在室温下进行的25次测试中，平均MOR为118,000 psi，数据范围为100,000至140,000 psi。MOR值的变化可能归因于添加剂的不完全分布和试样内晶粒尺寸的变化，这些变化导致了热各向异性引起的高边界应力。在1315℃时，用4点弯曲法测定了添加Y. 0的Si， Na试样在Ar气氛中的高温more值。MOR值范围为58000 ~ 69000 psi。由于在热压Si， N，的边界处明显产生了新的玻璃相，因此确定热冲击的影响是合适的。几个0.100英寸的标本。在管式炉（空气气氛）中加热到935℃，并在室温下淬入水中。到目前为止，随后的MOR测试显示强度没有下降。
OT-PRESSING of Si3Na has received considerable attention H because of its applicability in components used in hightemperature gas turbines. It has been reported’that up to 5 wt% MgO is generally required as an additive to n-Si3Na powder to produce fully dense high-strength bodies by hotpressing. However, high-temperature strength and creep re-sistance are apparently limited by the formation of magnesium silicate (enstatite) as a grain-boundary phase. High-temperature properties are enhanced by starting with a higher-purity Si, Na powder (particularly with respect to Ca, Na, and K) and by minimizing the amount OF MgO required. Another approach is to use an additive which would not only promote densification and high strength but would also produce a grain-boundary phase more refractory than magnesium silicate. If the boundary glass phase could be readily crystallized, high-temperature problems associated with viscous creep might also be alleviated. In considering the refractory boundary approach, the amount of additive required should be optimized rather than minimized. An additive found to be effective in producing dense high-strength specimens of Si, Na with high potential for satisfying the desirable additive criteria mentioned is YIO,.* Additions of from 1.0 to 3.3 wC% Y, O, to a high-a-phase SilN, powder; permitted hot-pressing of the pwder to full density at 1750 C. Specimens 1.25 in. in diameter were hot-pressed in graphite dies using 6000 to 7000 psi uniaxial pressure in an N2 atmosphere. Graphite foil, coated on one side with a BN slurry, was used as spacer material between the die, plungers, and powder. Effective hot-pressing temperatures were from 1750’to 1800 C with a 2-h hold time. Density of the hot-pressed specimens, determined by liquid immersion, varied from 3.22 to 3.26 g/cm3, increasing with the aniount of additive used (from 1.0 to 3.3 wt%). The grainboundary phase formed, although not as yet quantitatively identified, is apparently yttrium silicate. The compositions of yttrium silicate reported by Toropov and BondaZ are YzO,. SiO,(monoclinic), 2Y. 01.3 Si02 (hexagonal), and Y203. 2SiOz (monoclinic). These silicates have densities of 4.49, 4.39, and 4.06 g/cm3 and melting points of 1980, 1950”, and 1780” C, re-spectively.Bend-test specimens were machined from the hot-pressed disks to determine modulus of rupture (MOR) at room temperature and at 1315 C. Four-point bending was used with a platen rate of 0.002 in./min at room temperature and a strain rate of z6. 4X10-’in./in. min at high temperatures. Most MOR testing was performed with the tensile surface of the specimen representing a plane perpendicular to the hotpressing direction. The final surface finish was obtained using 220-grit diamond grinding parallel to the specimen length. An average MOR of 118,000 psi was calculated from 25 tests at room temperature, with a data range of 100,000 to 140,000 psi. Variations in MOR values may be attributed to probable incomplete distribution of additive and variations in grain size within a specimen resulting in high boundary stresses caused by thermal anisotropy. High-temperature MOR values were determined at 1315OC for Si, Na specimens with Y. 0, additive by 4-point bending in an Ar atm0sphere. t MOR values ranged from 58,000 to 69,000 psi. Since a new glass phase is apparently produced at the boundaries in hot-pressed Si, N,, it was appropriate to determine the effects of thermal shock. Several specimens 0.100 in. square were heated in a tube furnace (air atmosphere) to 935 C and quenched into water at room temperature. Thus far, subsequent MOR testing has shown no degradation of strength …