Fracture Origin and Strength in Advanced Pressureless‐Sintered Alumina

Abstract

Advanced raw materials and shaping approaches enable the production of pressureless-sintered alumina parts where, in bending, the average maximum stress at the fracture origin is as high as 800 MPa. In individual specimens that fracture at lower stresses (450-600 MPa), failure often originates at volume flaws, as known for hot-pressed alumina with a similar strength. Also, transgranular and intergranular fracture modes along the crack path are the same as those observed in hot-pressed alumina. If the size and the frequency of volume flaws are reduced, fracture initiates at smaller defects in the ground surfaces and bodies with a bending strength of >800 MPa are produced without hot pressing. The grain-size dependence of grinding-induced surface damage contributes to a grain-size effect for the strength.

Subcritical crack growth in Al2O3 with submicron grain size

“…7 The results obtained here for the subcritical crack growth resistance, however, are rather surprising:…”

“…These cracks are ''short'' compared with the crack length in precracked bending bars but large compared with the typical flaw sizes in high-strength ceramics. 7 In the fine-grained ceramics investigated here, R-curve effects are improbable, but, nevertheless, no final information on the equivalence of the present results with the subcritical growth of shorter flaws is available, and any interpretation should be done with care.…”

“…Additional tests in 4-point-bending with standard 20/40 mm span An analysis of flaws in gelcast specimens revealed that fracture of low-strength samples (local stress at origin < 600 MPa) was initiated by pores (diameter > 50 mm) in the volume of the microstructures, whereas the fracture of stronger bars started from small (probably grinding induced) crack near the surfaces. 7 To investigate the effect of different flaw distributions, additional samples were prepared from the same powder by uniaxial pressing (200 MPa). Due to the less homogeneous compaction, these samples required a high sintering temperature of 1450 C for a relative density > 99% (gelcasting: 1260-1280 C!).…”

Failure Investigation of Direct Write Pen Tips

“…2, the pen tips were most likely composed of sintered alumina ceramic, a claim which was reinforced by the microstructure both of the fracture surface and the undamaged surfaces of the pen tip. The fracture surfaces matched examples of brittle fracture of sintered alumina found in literature, namely, Krell [2]. Close examination of the fracture surfaces revealed a mix of intergranular and transgranular cleavage.…”

The application of automated image analysis to dense heterogeneities in partially sintered alumina

“…If the largest heterogeneities in the population affect the strength of alumina ceramics then the ranking from the extreme value analysis should agree with previous reports of the effect of forming method on strength. 21 …”