High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites
Introduction
Al2O3/SiC nanocomposites combine polycrystalline alumina and small amounts of sub-micron SiC particles [1], [2], [3]. The typical microstructure of Al2O3/SiC nanocomposites is composed of a polycrystalline matrix with an average size of 1–5 μm and SiC particles with size ranging from 100 to 200 nm. The addition of a small amount of sub-micron sized SiC to the alumina matrix can significantly improve the surface finish after machining, the resistance to severe wear, and the strength [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The nanocomposites have better surface finish and wear resistance both because the mean size of the individual pieces of material removed by brittle fracture at the surface is reduced and because the initiation of fracture is itself suppressed by the SiC additions [12], [13]. The strengthening mechanism of nanocomposites, however, is still controversial and a number of possible mechanisms have been proposed. 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 Al2O3 and Al2O3/SiC nanocomposites are measured and compared, in order to investigate the validity of the proposed residual stress strengthening mechanism.
Previously, grinding induced surface residual stresses in Al2O3 and Al2O3/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. Furthermore, the mean stress deduced depends on estimating a thickness for the compressive surface layer and often there is little information about what value this should take.
To probe the local stress variation in the ground surfaces more directly, a higher spatial resolution technique is required. In this work, confocal Cr3+ fluorescence microscopy was used, with lateral and axial (depth) resolutions of ∼1.5 μm and ∼3 μm, respectively [21], [22]. Previous work on alumina based materials using Cr3+ fluorescence microscopy investigated only residual stresses induced by indentation or scratching [23], [24]; in addition, it used weakly confocal microscopes with depth resolution of ∼10 μm. From both TEM observations [18] and results in our previous work [25], it is known that grinding stresses are expected to be found at depths of ∼1 μm for monolithic alumina. Considering the translucency of alumina materials, therefore, the conclusion in Ref. [24] that the residual stresses around indentations and scratches in alumina were lower than in alumina/SiC nanocomposites may be an artefact of lower transparency in the nanocomposites, which would confine the sampled volume more closely to the stressed region.
The confocal microscope used in this work alleviates this problem but does not entirely remove it because the axial resolution is still not sufficient to make simple point measurements of surface stress. The experimentally measured stress is actually the convolution of the real stress with the axial probe response function (PRF) [26] which describes the relative collection efficiency as a function of depth and depends on the instrument and the translucency of the material. In our previous work, on ground surfaces of alumina, residual stress distributions were estimated by modelling the plastic displacement of material resulting from grinding as an array of continuously distributed edge dislocations [21], [25], and established the PRF of our instrument when used with Al2O3 and Al2O3/SiC [21], [22]. The convolution of the fluorescence response predicted by the model with the PRF allowed the local residual stress variation for polycrystalline alumina after grinding and polishing to be estimated by adjusting the physical parameters in the model to fit the experimental results. In the current work, the same method will be used to compare the local stress distributions in surface ground monolithic Al2O3 and Al2O3/SiC nanocomposites.
Section snippets
Materials and specimen preparation
The starting powders were AKP50 alumina (200 nm, Sumitomo, Japan, 99.995% purity) and UF45 SiC (260 nm, Lonza, Germany, contains 0.2% free Si, 0.6% free C and 3.5% oxygen) respectively. 0.25 wt% MgO was added to all materials to prevent abnormal grain growth. Mechanical mixing by attrition milling (Szegvari HD, USA) using yttria stabilized zirconia milling media was performed at a speed of 300 rpm for 2 h. The ratio of water to powder was 4:1 by volume and 2.1 wt.% of Dispex A40 (Allied Colloids, UK)
Microstructure observation after grinding
SEM micrographs for the ground surfaces of monolithic alumina and Al2O3/x vol.% SiC (x = 2, 5, 10) nanocomposites are shown in Fig. 2. From the micrographs, it is observed that:
(1) 5 and 10 vol.% SiC nanocomposites have much better surface finish after grinding compared to those of monolithic alumina and 2 vol.% SiC nanocomposite. This can be attributed to the decrease of pullout size and also the suppression of pullout formation [9], [12]. The pullout size decreases as the amount of SiC increases (
Surface residual stresses and their distribution
By substituting D′, d, s and p listed in Table 2 into the stress model (Eqs. (A2a), (A2b)), the stress distributions in both Al2O3 and Al2O3/SiC nanocomposites for the nominal plane strain stress state can be extracted from the model and are plotted in Fig. 9. Tensile σxx and σzz (the coordinates for the stress model are defined in Fig. A1) were present for some distance below the “ground” regions of the surface in alumina (in agreement with our previous work [25]) and the 2 vol.% SiC
Summary
High spatial resolution measurements of surface residual stresses in ground surfaces of Al2O3 and Al2O3/x vol.% SiC nanocomposites (x = 2, 5, 10) were made by Cr3+ fluorescence microspectroscopy. In order to allow correctly for the translucency of the materials in interpreting the results, the probe response function was measured for the different materials and convoluted with the predictions of a model for the grinding stresses. The surfaces of the Al2O3 and 2 vol.% SiC nanocomposite exhibited
Acknowledgements
S. Guo would like to thank the K C Wong Education Foundation and the Overseas Research Students Awards Scheme, and A. Limpichaipanit would like to thank the Thai government, for financial support of their D.Phil study in the University of Oxford.
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2018, Ceramics InternationalCitation Excerpt :After increasing the SiC content from 0 to 20 vol%, the pull-out of grains are only observed in a certain region with poor distribution of SiC. The SiC present at grain boundaries causes the change of fracture mode [32]. The interfacial friction between different phases is caused by residual stress resulting from the mismatch of thermal expansion coefficient and Young’s modulus between Al2O3-GdAlO3 matrix and SiC particles.
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2016, Acta MaterialiaCitation Excerpt :Pezzotti and colleagues also applied the fluorescence shift methodology to measure stress distributions in Al2O3–ZrO2 composites [34] (similar to [35]), but more importantly made the first direct determination of stresses in bridging ligaments behind crack tips in large-grained polycrystalline Al2O3 [36] and in a series of Al2O3 materials with microstructures tailored with Al2O3 platelets [37,38] and metal particles [37,39–41] to maximize bridging. More recently, Todd and colleagues have used the methodology to measure stresses in polycrystalline Al2O3 and Al2O3–SiC composites [42], including the effects of surface grinding [43,44], and proximity to indentations [45,46] and high strain rate impacts [47] in polycrystalline Al2O3, Al2O3–SiC, and Al2O3–ZrO2. Despite the above advances and demonstrated applications, very few works have used fluorescence shift measurements to generate images (two-dimensional, 2-D, maps) of stress heterogeneity in Al2O3 systems.
- 1
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong.
- 2
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.