A Test of the Second‐Phase and Impurity‐Segregation Models for MgO‐Enhanced Densification of Sintered Alumina

Abstract

To determine if MgO added within thesolubility limit in A1203 is sufficient to suppress discontinuous grain growth, an undoped alumina pellet was sintered next to a preequilibrated, two-phase mixture of spinel (MgAI,O,) and alumina. The MgO-doped outer surface of the undoped pellet did sinter to full density; metallography confirmed that the sample was free of second phase. Grain boundaries were analyzed with a scanning Auger micloprobe, which showed that the grain boundaries of the inner porous region had approximately thesame Ca segregation as those in the dense outer shell region. Therefore, it was concluded that the beneficial effect of magnesium is due neither to second-phase pinning nor to Ca impurity segregation.

Solubility Limit of MgO in Al2O3 at 1600°C

“…Since Coble's work in the 1960s, the effect of doping alumina with MgO has been extensively studied 1–6 . MgO is one of the major dopants used to reduce grain‐boundary mobility during sintering 4,5 . In 1968, Roy and Coble 3 reported a solubility limit of 300 ppm at a temperature of 1630°C.…”

Alternative Explanation for the Role of Magnesia in the Sintering of Alumina Containing Small Amounts of a Liquid Phase

“…Furthermore, to determine the effect of the addition of an extremely small amount of MgO on the microstructural evolution of Al 2 O 3 , MgO was added through the vapor phase. The advantage of this experiment, which is similar to that of Bagley et al 26 and Johnson et al, 29 is that the possibility of nonuniform distribution of MgO as a result of mechanical mixing can be minimized. An Al 2 O 3 powder compact containing 0.2 mol% SiO 2 was sintered next to 70Al 2 O 3 ⅐30MgO (in mol%) in a closed crucible.…”

Function of magnesium aluminate hydrate and magnesium nitrate as MgO addition in crystal structure and grain size control of α-Al2O3 during sintering

“…Positive effect of MgO addition (Jorgensen and Westbrook 1964;Jorgensen 1965;Taylor et al 1974Taylor et al , 1976Johnson and Coble 1978) in alumina sintering is accompanied by its solid solution in alumina, and excess addition of MgO beyond its solid solubility limit exists as non-stoichiometric spinel in grain boundaries of sintered alumina. Li and Kingery (1984) reported segregation of magnesium aluminate spinel as second phase discrete particles with definite contact angle and no continuous second phase along grain boundaries of alumina, while investigating scanning transmission electron microscopy of MgO doped (0⋅2-0⋅5 wt% MgO) alumina sintered at 1575°C for 96 h in oxygen and quenched in liquid nitrogen.…”