Direct evidence for compressive elastic strain at ground surfaces of nanocomposite ceramics

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

Cr3+ microspectroscopy measurements and modelling of local variations in surface grinding stresses in polycrystalline alumina

“…Both ground and polished surfaces also contain very strong in-plane compressive stresses (σ t ∼ −1.5 GPa) in the plastically deformed layer (Table 3). It is difficult to compare these directly with other works in which stresses have been measured by the curvature method, 8 X-ray diffraction [5][6][7] or Hertzian indentation 9,10 because of the different grinding and polishing treatments used but the maximum compressive residual stresses obtained in this work are at the high end of the ranges reported using other methods (ranging from ∼−50 MPa to −1.5 GPa). One reason for the high values is that the residual stresses obtained in this work are not volume averaged and reflect the local stress at different points in the stressed surface layer.…”

“…There are two important conclusions to draw from this. First, although more severe grinding leads to a greater mean compressive surface stress, which has previously been seen as a strengthening mechanism, 6,8,34 its potential benefits are severely compromised not only by the cracks nucleated by grinding, but also by the appearance of local tensile stresses in some regions which act to help such cracks to grow. Secondly, once pullouts start to form by surface fracture, it becomes easier to form further pullouts owing to the increasingly tensile stress normal to the surface and just below the plastically deformed layer.…”

High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites

“…Previously, grinding induced surface residual stresses in Al 2 O 3 and Al 2 O 3 /SiC materials have been measured by X-ray diffraction [14,16,17], curvature measurement [15] and Hertzian indentation [18,19]. The disadvantage of these techniques is that they all have poor spatial resolution compared with the scale of the microstructure [20] and as a result the measured stress is volume averaged rather than reflecting the local stress at the surface and its spatial distribution.…”

“…One obvious explanation is simply the improved surface finish and reduction in cracking during specimen preparation mentioned above. Another related suggestion is that the compressive surface residual stress after machining is increased [9,14,15]. In this work, the grinding induced surface residual stresses in Al 2 O 3 and Al 2 O 3 /SiC nanocomposites are measured and compared, in order to investigate the validity of the proposed residual stress strengthening mechanism.…”

Confocal fluorescence microscopy in alumina-based ceramics: Where does the signal come from?

“…Characterisation of residual stresses caused by thermal expansion mismatch, 1-4 indentation 5 or machining induced residual stress [6][7][8] is an important issue in ceramics. It is crucial to a better understanding of procedures such as the indentation fracture toughness test 9,10 and to understanding the effect of the residual stresses on crack initiation and propagation and hence on mechanical performance.…”