Residual stress distributions around indentations and scratches in polycrystalline Al2O3 and Al2O3/SiC nanocomposites measured using fluorescence probes
Introduction
Alumina/silicon carbide nanocomposites (alumina polycrystals containing small fractions of sub-micron-sized SiC particles) can show large increments of strength compared to polycrystalline alumina samples with equivalent grain size; any increase in strength is not accompanied by a significant increase in material toughness [1], [2], [3], [4]. A number of workers have proposed that the surface properties of alumina/SiC nanocomposites – in particular the near-surface in-plane compressive stresses introduced by surface finishing operations – are the key to their enhanced strength.
Recent work has found intriguing differences between the surface behaviour of alumina polycrystals and alumina/SiC nanocomposites. Sternitzke et al. [5] first reported a quantitative difference in the sub-surface damage between alumina and alumina/SiC nanocomposites after grinding and polishing. Other studies also reported enhanced strength in these nanocomposites after surface grinding [1] and a much superior polished surface with suppressed grain pull-out [6], [7]. Wu et al. showed [8] that grinding the surface of alumina/SiC nanocomposites with a coarse diamond grit wheel resulted in a four-point bend strength of ∼400 MPa, whereas similarly ground alumina specimens had a strength of ∼300 MPa. The enhanced strength of the nanocomposites was shown to be associated with a large compressive surface stress induced by surface grinding, while such a magnitude of residual stress was not present in the alumina after identical surface treatment. A more detailed study of the surface stress state, and the sub-surface dislocation structure after deformation, found that the nanocomposite material showed a significantly greater dislocation density beneath ground and polished surfaces, compared to identically treated polycrystalline alumina [9], [10], [11].
In order to investigate further the influence of surface grinding on the mechanical behaviour of alumina and alumina/SiC nanocomposites, we have undertaken a study of the deformation around single-point indentations and simple scratch grooves. Such surface deformation can be taken as a model for the processes occurring around individual grit particles during grinding. The mechanisms of deformation around indentations in ceramics and brittle materials have been the subject of considerable research in the past. It is now generally accepted that even in highly brittle materials there is a region of plastic deformation immediately below the indentation [12]; this is shown schematically in Fig. 1. During indentation an elastic stress field is generated, of radial compression and tangential tension, extending into the material outside the plastic zone, the magnitude of which decays with distance, r, from the centre of the plastic zone as 1/r2 [13], [14]. The tensile components of this stress field can lead to the nucleation of radial and median cracks on loading. The magnitude of the residual elastic stress field after unloading varies as 1/r3 from the contact point [14], [15] (though it may be influenced by the presence of the crack systems illustrated in Fig. 1). The residual stress field close to the surface has a tension component normal to the surface, which may lead to the nucleation of lateral cracks. For a sliding indentation, there is as yet no model that can completely describe the elastic/plastic stress fields under the indenter. An extension of Yoffe’s model of static indentation to sliding contacts has been reported [16], though further validation is needed.
Conventional methods of residual stress measurement, e.g. X-ray diffraction or surface curvature changes, measure the residual stress averaged over a considerable region of material. In order to obtain a high-resolution local picture of the residual stress state on the surface close to scratches and indentations, we have used the Cr3+ fluorescence spectra obtained from natural concentrations of Cr impurities within alumina crystals. Grabner [17] first described the relationship found between the measured line shift in Cr3+ fluorescence and the local stress state. Ma and Clarke developed this analysis for a more general case to relate the mean line shift to the local stress tensor [18]. They also presented an analysis of the broadening of the fluorescence peak to determine the mean stress distribution in an illuminated area. Thus, by using an optical microscope of sufficient resolution fitted with an appropriate spectrometer, it is possible to determine the surface stress within alumina specimens with a lateral spatial resolution of about 1 μm and over a sampled depth slightly greater than this [19].
Section snippets
Experimental procedure
The materials used in this study consist of a polycrystalline alumina (AKP53, Sumitomo Chemical Co., Tokyo, Japan) of mean grain size about 3 μm and an alumina/SiC nanocomposite with a matrix of the AKP53 alumina containing 1, 5 and 10 vol.% α-SiC particles (UF 45, Lonza, Waldshut, Germany), with a mean particle diameter of 90 nm. Processing conditions were chosen to ensure that the nanocomposite and polycrystalline alumina had equivalent mean grain size. The processing routes used have been
Analysis of fluorescence line shift and broadening
The Cr3+ fluorescence measurements show significant levels of residual stress close to the scratches and indentations (see Section 4). These residual stresses are caused by the superposition of stress fields from a number of independent mechanisms (where indicates a stress tensor)
- (i)
is caused by the influence of the anisotropic thermal expansion of the hexagonal α-alumina phase on the polycrystalline aggregate.
- (ii)
is the stress from the mismatch in
Results and discussion
Typical examples of the resolved shift and broadening (full width at half maximum (FWHM)) for R1 and R2 fluorescence lines across the indentations and scratches in alumina and the nanocomposites are shown in Fig. 3, Fig. 4. A load of 3 N was used in both cases. The origin on the horizontal axis represents the centre of the indentation or scratch, and the negative and positive numbers indicate respectively the distance (in μm) away from the central position on left- and right-hand side relative
Conclusions
Cr3+ fluorescence spectra can be used to analyse residual stress distributions in polycrystalline alumina and alumina/SiC nanocomposites. Changes in local stress on a scale smaller than sampling volume of the optical probe result in a broadening of the fluorescence line along with a shift in the line related to the mean hydrostatic stress of the sampled volume. The maximum compressive surface stress measured using fluorescence data increases with indentation load on polycrystalline alumina
Acknowledgements
We gratefully acknowledge the support of the EPSRC structural materials programme under grant GR/L95908. We would also like to acknowledge the assistance of Ms. R. Sinclair, Dr. S. Eichhorn and Dr. M. Montes in the Raman group of the School of Materials, University of Manchester.
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