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

https://doi.org/10.1016/j.jeurceramsoc.2010.08.021Get rights and content

Abstract

It has previously been suggested that Al2O3/SiC nanocomposites develop higher surface residual stresses than Al2O3 on grinding and polishing. In this work, high spatial resolution measurements of residual stresses in ground surfaces of alumina and nanocomposites were made by Cr3+ fluorescence microspectroscopy. The residual stresses from grinding were highly inhomogeneous in alumina and 2 vol.% SiC nanocomposites, with stresses ranging from ∼ −2 GPa within the plastically deformed surface layers to ∼ +0.8 GPa in the material beneath them. Out of plane tensile stresses were also present. The stresses were much more uniform in 5 and 10 vol% SiC nanocomposites; no significant tensile stresses were present and the compressive stresses in the surface were ∼ −2.7 GPa. The depth and extent of plastic deformation were similar in all the materials (depth ∼ 0.7–0.85 μm); the greater uniformity and compressive stress in the nanocomposites with 5 and 10 vol% SiC was primarily a consequence of the lack of surface fracture and pullout during grinding. The results help to explain the improved strength and resistance to severe wear of the 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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