Impurity‐Dependent Morphology and Grain Growth in Liquid‐Phase‐Sintered Alumina

Abstract

The alumina grains in liquid-phase-sintered (LPS) materials prepared from different commercial sources have a predominantly platelet morphology. Generally, the MgO:(CaO ؉ BaO ؉ Na 2 O ؉ K 2 O) ratio in the chemical composition controls the morphology in LPS alumina that is 91-94 wt% pure. Within a given range of SiO 2 content (i.e., 4.3-5.2 wt% in the chemical composition), a low MgO:(CaO ؉ BaO ؉ Na 2 O ؉ K 2 O) ratio (i.e., <1.0) in the LPS compositions favors the formation of elongated grains, whereas ratios of >1.0 result in equiaxed grains. SiO 2 contents outside the 4.3-5.2 wt% range favor the formation of elongated grains. A tendency to form platelike grains is observed for LPS alumina with a purity of 91-94 wt% when both the MgO:(CaO ؉ BaO ؉ Na 2 O ؉ K 2 O) ratio and the SiO 2 content are relatively low. The sintered density generally increases as the SiO 2 content in the chemical composition decreases.

Microstructural development of SrAl12O19 in alumina–strontia composites

“…Since the pores are trapped within or between the large grains, further densification is hindered and hence the decrease in density. 22 Similar densification is reported for calcia by Goswami et al 14 and Vishista and Gnanam. 23 When the additive concentration is within the solubility limit, the homogeneous mixing accelerates the densification by diffusion phenomena.…”

“…The morphology of sintered alumina varies significantly with type of impurities such as magnesia, calcia, silica, soda and baria. 14 The addition of strontium oxide to alumina in the ratio of 1:6 yields strontium hexa-aluminate, a compound of formula SrAl 12 O 19 (SA 6 ). A characteristic feature of SA 6 is its strongly anisotropic growth, which results in elongated grains and platelets.…”

Effect of LZSA Glass-Ceramic Addition on Pressureless Sintered Alumina. Part II: Mechanical Behavior

“…Composite A F 21 achieved a high relative density at a lower temperature (1470 °C); and because of this, alumina grains did not experience as much growth. In fact, sintering at lower temperatures in the presence of the liquid phase may cause a suppression of grain growth 38 . Although composites A F 7 and A C 7 show the presence of elongated grains, one can also see coarse, equiaxed grains.…”

“…Peak displacement occurred The LZSA addition also changed the morphology of the alumina, as shown in Figure 11 for the etched specimens. The morphology is influenced by the chemical composition of the intergranular phase of materials obtained by LPS and can significantly change the mechanical properties of alumina [36][37][38] . As shown in the SEM observations, the composites have elongated grains.…”

“…As shown in the SEM observations, the composites have elongated grains. Elongated grains can act as reinforcement material in the microstructure and indeed contribute to the increased fracture toughness 36,38 . Composite A F 21 achieved a high relative density at a lower temperature (1470 °C); and because of this, alumina grains did not experience as much growth.…”