HIGH-RESOLUTION PARALLEL-BEAM POWDER DIFFRACTION MEASUREMENT OF SUB-SURFACE DAMAGE IN ALUMINA-SILICON CARBIDE NANOCOMPOSITE

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1 169 HIGH-RESOLUTION PARALLEL-BEAM POWDER DIFFRACTION MEASUREMENT OF SUB-SURFACE DAMAGE IN ALUMINA-SILICON CARBIDE NANOCOMPOSITE B K Tanner, H Z Wu + and S G Roberts * Department of Physics, University of Durham, South Road, Durham, DH1 3LE, U.K. + School of Engineering, Coventry University, Priory Street, Coventry CV15FB, U.K * Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, U.K. ABSTRACT The sub-surface damage in ground and subsequently annealed alumina/silicon carbide nanocomposite has been studied by analysis of the x-ray diffraction line width as a function of incidence angle. Bragg peak broadening fell as the depth of penetration of the x-ray beam increased. Despite substantial anisotropy in the uniform compressional strain in the surface of the ground material, no significant azimuthal anisotropy was found in the peak widths. The surface extrapolated broadening decreased by an order of magnitude on annealing but the rate of fall of broadening with depth did not change. No such changes were observed on annealing polished material. Williamson-Hall analysis of the annealed ground material showed an order of magnitude decrease in the diffracting domain size at the surface compared with that of the bulk material, where the diffracting domain size was of the order of the grain size. INTRODUCTION In 1991 Niihara and Nakahira [1] reported that significant mechanical property improvements are found for polycrystalline alumina that contain a dispersion of 5 1% sub-micron silicon carbide particles. Al 2 O 3 /SiC nanocomposites also show excellent surface mechanical properties compared to polycrystalline alumina of similar grain size. Winn and Todd [2] and Kara and Roberts [3] found that the Al 2 O 3 /SiC nanocomposite materials polished more readily and, under identical polishing conditions, produced a surface with much less pull-out than polycrystalline alumina. There has been considerable interest in determining the strain below the surface associated with the polishing process and in pioneering laboratory experiments [4], Odén and Ericsson used parallel beam, x-ray powder diffraction to attempt to determine the near-surface strains in Al 2 O 3 and Al 2 O 3 /SiC composite. As they used a laboratory diffractometer with slit-collimated beams and a graphite-monochromated detector to measure the diffraction peak positions and widths as a function of incidence angle of the beam, they were unable to detect significant broadening of the diffraction peaks in the polished samples. They concluded that deformation was confined to a region within 5nm of surfaces polished with 3μm diamond paste, a conclusion subsequently confirmed and quantified by synchrotron radiation experiments [5]. Odén and Ericsson found that the strain due to grinding extended to between 1. 5 and 2μm below the surface in both pure Al 2 O 3 and the Al 2 O 3 /SiC composite. We have undertaken high-resolution, grazing incidence parallel-beam x-ray powder diffraction experiments at beamline ID31 of the ESRF, Grenoble [6] and have shown that the variation with angle of the full width half height maximum (FWHM) of the Bragg peaks can be successfully modelled by a FWHM distribution that falls exponentially with depth. Correlation was found between the depth to which the FWHM fell to 1/e of the

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3 17 surface value and transmission electron microscopy observation of the extent below the surface of dislocations associated with the grinding damage. The FWHM broadening extrapolated to the surface was found to increase linearly with increasing diameter of diamond partic les used to polish ground nanocomposite. In this paper we analyse the x-ray data from ground Al 2 O 3 /SiC nanocomposite and also ground material that had been annealed in flowing argon at 125 C for 2 hours. EXPERIMENTAL DETAILS AND ANALYSIS TECHNIQUE Samples of alumina-silicon carbide na nocomposites were fabricated by hot pressing in a graphite die at 165 to 168 C for 1 hr under a pressure of 2-25 MPa under flowing argon. The nanocomposite consisted of an alumina matrix material containing 5% by volume submicron SiC. Alumina powder with submicron particle size (AKP53 from Sumitomo, Japan) and of reported chemical purity of 99.99% α-al 2 O 3 was used as the matrix. The SiC particles were a commercial α-sic powder (UF 45 from Lonza, Germany), with a mean particle size of ~2 nm. The hot pressed discs were ground to remove the top surface on both sides with an epoxy resin bonded diamond wheel (grit size 15 μm) to achieve a specimen thickness of about 3 mm. A wheel speed of 125 rpm, table translation speed of.8 ms -1 and feed depth of 12.5 μm per pass was used in this process. Specimens with polished surfaces were produced by lapping successively with 25, 8, 3 and 1 μm diamond slurry on a Kent III polishing machine with Kemet plates rotating at about 6 rpm and with an external load of 15 N. High resolution powder diffraction measurements were performed on beamline ID31 at the ESRF. The two circle diffractometer has nine Ge 111 analyser crystals displaced by 2º with respect to one another and this permits rapid parallel data collection at very high resolution [7]. For each incidence angle, the sample was kept stationary and the diffraction pattern was recorded by scanning the detectors. The divergence in the scattering plane of the 8keV incident beam, sagitally focused by a 111 Si monochromator, was 8 arc seconds. For this energy, the Ge analyser reflection full width at half height maximum (FWHM) was 17 arc seconds giving extremely high angular resolution and strain sensitivity. The overall instrument resolution was approximately 5 x 1-3. We selected 8keV x-ray energy as a compromise between scattering strength, undulator spectral output and photoelectric absorption. Fig 1 shows the depth at which the intensity is attenuated by a factor 1/e as a function of incidence angle for 8keV x-rays incident at angle φ with respect to the surface of alumina. By varying φ between.25 and 2, we were able to vary the probe depth from 25nm to 25μm. As the scattering angles for all Bragg reflections studied were large, the attenuation of the diffracted beam was neglected in comparison to the incident beam attenuation. Peak centroids and FWHM were determined by fitting pseudo-voigt functions to the data. RESULTS From precise measurement of the Bragg peak positions as a function of φ we were recently able to show directly that there exists a uniform compressive strain in the near surface region of the nanocomposite [8]. The strain disappeared on annealing and was not found in comparable samples of the single phase alumina. In the nanocomposite, the near-surface strain was strongly

4 171 anisotropic (Fig 2), the direction of the principal strain component being normal to the grinding directions and damage lines observed on the surface of the material. Absorption depth (μm) Peak shift Δ(2θ) [degrees] Ground and polished Ground parallel to beam Ground perp. to beam Refraction correction Incidence angle (º) Incidence angle [degrees] Fig. 1. Absorption depth (depth at which the intensity drops by 1/e) versus incidence angle for alumina Fig. 2. Displacement of the 22.6 Bragg peak as a function of incidence angle for the ground nanocomposite with the incident beam parallel and perpendicular to the grinding direction As reported previously, there is a dramatic rise in the Bragg peak width as the incidence angle decreases. For the ground nanocomposite, this can be up to 1.5 (Fig 3) for very small angles that approach the critical angle for total external reflection. In fig.3, there are two independent measurements of the peak broadening for the grinding direction aligned perpendicular to the incidence plane and one for the grinding direction in the incidence plane. There is no anisotropy in the maximum peak broadening observed at the lowest angles from which data could be collected. When the data are fitted to the equation derived on the assumption of an exponential fall of peak width with depth [5], δ ( φ) δ o = δ ( κ sin φ / μ ), Equation [1] there is an indication that the rate of fall is greatest when the grinding direction is in the incidence plane. (In equation 1, δ(φ) is the Bragg peak width at incidence angle φ, δ and δ are fitting parameters, μ is the linear absorption coefficient and 1/κ is the depth at which the broadening falls to 1/e.) However, the data scatter for the two equivalent measurements is greater than that between them and with the grinding direction in the incidence plane. Despite the uniform compressive strain being extremely strongly anisotropic, we find no substantial anisotropy in the peak broadening. Annealing of the ground nanocomposite results in a dramatic change in the peak width extrapolated to the surface (fig. 4). The broadening at the surface drops by almost an order of magnitude on annealing and transmission electron microscopy reveals that there are almost no

5 172 dislocations or microcracks present in the near-surface region after annealing. The rate of fall of the broadening, on the other hand does not change substantially indicating that the relaxation process proceeds locally and proportionately to the broadening Bragg peak FWHM Δ(2θ)( ) Inciden ce p lane parallel to grinding direction Inciden ce p lane perpe ndicular to grinding direction Inciden ce p lane perpe ndicular to grinding direction FWHM (2θ) degrees ground annealed ground Incidence angle ( ) Incidence angle (degrees) Fig Bragg peak width versus incidence Fig. 4. Bragg peak width as a function of angle for nanocomposite oriented with grinding incidence angle for ground and annealed direction parallel (triangles) and perpendicular nanocomposite. (squares and circles) to the incidence plane The substantial changes on annealing are not seen below the nanocomposite surfaces that have been polished. For all polished surfaces that we have examined, the rate of decrease in damage is very much faster than for the ground material and the peak widths extrapolated to the surface are much smaller. In the nanocomposite, the rate of fall is extremely fast, broadening being seen only at the very lowest angles of incidence. Annealing has no effect on the sub-surface damage (fig 5). Due to the very high brilliance of the ID31 undulator, we have been able to collect data out to scattering angles of over 12 before the low peak intensity and poor signal to noise ratio makes it impossible to fit profiles to peaks when the incidence angle is small. This has enabled us to perform analysis of the line profiles as a function of scattering vector to separate the effects of size and strain broadening on the Bragg peak width Δ(2θ). In the Williamson-Hall equation, Δ(2θ) cos θ = 2Δε sinθ +(λ /L), Equation 2 that we have used for the analysis, θ is the Bragg angle of the reflection and λ is the x-ray wavelength. Δε is the strain dispersion, often referred to as the microstrain and L is the dimension of the diffracting domain parallel to the diffraction vector. It is important to note that this is not necessarily the grain size and, particularly in annealed material, may represent the size of dislocation cells. Although we see from fig. 3 that there is little anisotropy parallel and perpendicular to the grinding direction, different reflections probe the domain size in different directions with respect to the surface and we may expect significant scatter in the Williamson- Hall plot, particularly at very low angles of the incident beam with respect to the surface.

6 173 Equation 1 has been used to extrapolate, to zero and to infinite depth, the broadening for various reflections. Williamson-Hall plots for the broadening extrapolated deep into the material and at the surface are shown, in fig. 6, for the annealed nanocomposite material. The first observation is that the plot corresponding to scattering from deep in the material shows little deviation from a straight line. The diffracting domain size, from the intercept, is found to be 5 (+15/-4) μm, very comparable to the grain size. The strain dispersion, determined from the slope, is 7.8 (±.8) x1-4. Such a non-zero value might well be expected from a two phase material. The diffracting domain size at the surface, determined from the extrapolations using equation 1, is seen to be very substantially reduced compared with that deep inside the material. While the data for the low order reflections is too unreliable to include, the intercept is quite well conditioned and gives a diffracting domain size of.2 (+.2/-.1) μm. The increased strain dispersion of 22 (±4) x 1-4 is interesting, as it is not consistent with the lack of variation in values from specific incidence angles. There may well, however, be a sharp gradient in strain dispersion just at the surface..3.5 FWHM Δ(2θ) degrees.2.1 Polished (1μm diamond) Po lish ed and an nealed Δ(2θ) cos(θ) Surface extrapolation Infinite depth extrapolation Incidence angle (degrees) sin θ Fig. 5. Bragg peak width as a function of incidence angle for polished and annealed material. Fig. 6. Williamson-Hall plots for the peak broadening extrapolated to deep within the material and at the surface DISCUSSION The increase in Bragg peak broadening as a function of depth below ground surfaces of Al 2 O 3 /SiC nanocomposite is consistent with similar broadening seen in single phase alumina. The damage introduced in the grinding process is large and extends to a depth of more than a micrometre. Lapping and polishing must therefore remove more than several micrometres of material if the grinding damage is not to be left below the surface. This is somewhat surprising from a brittle material. In the example of polished material shown here, the ground surface initially was lapped using 25 μm diamond slurry to remove at least 15 μm of material. After this, 8 μm diamond slurry was used to remove at least a further 1 μm and then 3 μm diamond slurry was used to remove at least 5 μm. Finally 1 μm diamond slurry was used to remove an additional 5 μm. Each step of the polishing sequence was carefully selected to eliminate all

7 174 surface damage introduced by the previous step. The result, as evident from fig 5, is that the subsurface damage has been removed from all but an extremely thin region close to the surface. CONCLUSIONS Grazing incidence, parallel beam, high resolution powder diffraction has been able to show quantitatively how the damage profile below ground surfaces extends into brittle ceramic materials. Use of an undulator on a third generation synchrotron radiation source provides sufficient intensity to study reflections with high scattering vectors and a large component of diffraction vector parallel to the surface. Sufficient reflections can be found which give satisfactory statistics at very low angles of incidence for Williamson-Hall analysis to be performed on the peak widths extrapolated to the surface and deep inside the material. The most dramatic change between the depth and surface of annealed nanocomposites is in the diffracting domain size, which becomes much smaller than the grain size at the surface. ACKNOWLEDGEMENTS Thanks are given to Andy Fitch and the staff of beamline ID31 for their excellent technical support. REFERENCES [1] K. Niihara. and A. Nakahira, J Ceram. Soc. Japan, 99 (1991) 974 [2] A. J. Winn and R. I. Todd, Brit. Ceram. Trans. 98 (1999) 219 [3] H. Kara and S. G. Roberts J. Am. Ceram. Soc. 83 (2) 1219 [4] M. Odén, and T. J. Ericsson, Am. Ceram. Soc. 79 (1996) 2154 [5] B. K. Tanner, T. P. A. Hase and H. Z. Wu, Phil. Mag. Letts. 81 (21) 351 [6] B. K.Tanner, H. Z. Wu, S. G. Roberts and T. P. A. Hase, Phil. Mag. 84 (24) 1219 [7] A. N. Fitch, Materials Science Forum 228 (1996) 219 [8] B. K. Tanner, H. Z. Wu and S. G. Roberts, Appl. Phys. Lett. 86 (25) 6199