J. Am. Ceram. Soc., 83, 2000, 1219-25

POLISHING BEHAVIOR AND SURFACE QUALITY OF ALUMINA AND ALUMINA / SILICON CARBIDE NANOCOMPOSITES$

Hudai Kara, Steve G. Roberts

Department of Materials, University of Oxford,

Parks Road, Oxford, U.K., OX1 3PH

ABSTRACT

The response of alumina and alumina/SiC nanocomposites to lapping and polishing after initial grinding was investigated in terms of changes in surface quality with time for various grit sizes. The surface quality was quantified by surface roughness (Ra) and by the relative areas of smooth polished surfaces as opposed to rough “as-ground” areas. Polishing behavior of the materials is discussed in terms of SiC content and grain size. It was concluded that nanocomposites are more resistant to surface damage than alumina, and this behavior does not depend on the amount of SiC in the range 1-5% by volume. SiC addition as low as 1% is enough to produce a noticeable improvement in surface quality during lapping and polishing.

$ Supported by EPSRC under Brite Euram III Project: BRE3 CT96 0212, and by the Turkish Government

Preprint - differs in formatting from final published version.

I. Introduction

After Niihara’s original paper1 on alumina/SiC nanocomposites, a significant amount of work2-6 has been carried out to explain the so-called ‘nanocomposite effect’, where the addition of nanometer sized SiC particles into the alumina matrix gives a pronounced improvement in fracture strength and toughness. Such improvements have been attributed to various mechanisms, including microstructural refinement1, grain boundary strengthening7, crack deflection8 and bridging9, changes in surface flaw population3,6 and most recently surface residual stresses10,11. However, none of the proposals have given a fully satisfactory explanation for the observed changes in mechanical properties. There is a well-defined change in fracture mode from intergranular in alumina to transgranular in the nanocomposite by the addition of nano-sized SiC particles into alumina1. However, it is still unclear whether the particles on the grain boundaries or those inside the grains are responsible for this change. However, it is generally considered that strengthening of grain boundaries occurs in SiC-containing material. This is also found to result in a reduction of grain pullout during grinding and polishing12, so that the nanocomposite materials show better polishing characteristics than those of aluminas, when the same polishing treatment is applied. Winn and Todd13 tried to quantify this improved ‘polishability’. They assessed the extent of grain pullout after polishing ground alumina and alumina/5% SiC materials with different abrasive grit sizes. They measured the area of highly reflective ‘smooth’ regions under an optical microscope, as opposed to darker rough regions due either to grain fracture and pullout or remaining from the original grinding. They found that the nanocomposite material with as low as 1.8 vol. % SiC content performed in a similar manner in the grinding wear test to the 11.0 vol. % SiC content material: all SiC-containing materials had much better quality surfaces than alumina material that had undergone similar treatment. However, Chou et al.14 found that alumina and alumina – 5%SiC nanocomposites have the same surface roughness after coarse grinding (128µm grit). This paper aims to investigate the surface quality and polishing behavior of different alumina materials and nanocomposites with 1-5% SiC using different polishing grit sizes and polishing times. The quality of the ground and polished surfaces is quantified and the polishing behavior is discussed in terms of polishing grit size and time, SiC content and grain size.

II. Experimental Procedure

Material Preparation Alumina and Al2O3 / SiC nanocomposites were produced by a conventional powder mixing method. The full details of the method can be found elsewhere.4 Here a short description is given. A reinforcement of 1-5 % vol. of α-SiC powders (UF 45 - Lonza, Germany) with an average particle size of 200 nm was dispersed ultrasonically in water for 20 minutes prior to mixing with α-Al2O3 matrix (AES11C - Sumitomo, Japan) with a 400 nm average particle size. The mixture was then attrition milled (Szegvari-01 HD) in distilled water for 2

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hours at 500 rpm. The resulting slurry was vacuum dried for 24 hours followed by 4 hours freeze-drying. After freeze-drying all powders were sieved through a 150 µm mesh. The same treatment was also applied to alumina powders with no SiC content. Powders were compacted either by uniaxial pressing at 40 MPa pressure or a combination of uniaxial and cold isostatic pressing (CIP) at 40 and 200 MPa respectively. Three different sintering conditions; 1550 and 1600 oC for 2 hrs and 1600 oC for 6 hrs were chosen for alumina. The nanocomposites compacted by both uniaxial pressing and CIP were sintered at 1600 oC for 2 hrs; uniaxially pressed compacts were sintered at 1700 oC for 2 hrs. These conditions were chosen to achieve full densities and a range of grain sizes. To prevent SiC particles from oxidation during sintering the nanocomposite compacts were embedded in a bed of SiC grits and a flowing nitrogen gas atmosphere was used. Alumina compacts were sintered in air in a bed of alumina grits. After sintering, density measurements were made by Archimedes’ method using water as an immersion medium. The relative density values were calculated using the theoretical densities of α-Al2O3 (3.96 gcm-3) and α-SiC (3.05 gcm-3). Elastic properties (Young’s modulus, E and Poisson’s ratio, ν) were measured by a resonance method using an MK5 “Grindosonic” machine (J.W. Lemmens, Belgium). The linear intercept method was used to evaluate grain sizes from SEM micrographs, counting at least 400 intercepts for each micrograph. The grain size distributions of materials used in the polishing test were assessed from the number of intercepts for a certain intercept length and are shown in Figure 1. Properties of the materials manufactured are shown in Tables 1 and 2. (2) Grinding and polishing

Two sets of polishing experiments were performed: 1) Specimens of alumina and nanocomposites were separately mounted on two steel

holders 6 cm in diameter and ground using a flat bed grinder (Jones & Shipman 1400L, U.K.) with the following conditions: 25 cm diameter resin-bonded 150-grit diamond wheel; grinding wheel speed 1240 rpm; table velocity 0.8 m/s; and depth of cut 0.125 mm/pass. After grinding, polishing was performed with a Kemet 3 disc polishing machine (Engis Ltd., U.K.) with a surface pressure of 1.47x106 Nm-2. Three different grades of diamond slurry, namely 25, 8 and 3 µm grit size, were used successively for a total of 45 minutes at each grit size. Fresh grit and lubricant were supplied throughout at a fixed rate for all experiments.

2) The same procedure was followed as in the first set, except that the steel holders, in which alumina and nanocomposites were mounted, were 3.5 cm in diameter. Otherwise, the polishing was performed with a standard Kent polishing machine using the same conditions as in (1).

For each grade of grit, polishing was initially performed for 15 minutes. For the first set of specimens, the polishing damage was measured by an image analyzer (Kontron Elektronik, Germany) as the percentage of smooth, reflective surfaces (transgranular fracture) to rough, non-reflective surfaces (grain pull out and intergranular fracture). At least 10 different areas of polished surfaces were selected randomly under the optical microscope and mean

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values and standard deviation calculated. For the second set of specimens, the polishing damage was measured by a Surface Profilometer (Rank Taylor Hobson Ltd., England). The profilometer has a stylus tip detecting the surface topography of the samples; the data collected are converted to surface roughness, Ra (µm), the arithmetic mean of the departure of the profile from the mean line. The surface roughness values of ground surfaces were measured when the stylus tip was normal to the surface. Measurements were then taken both perpendicular and parallel to the groove direction; for as-ground surfaces, the Ra value parallel to the grinding direction was significantly lower than that of the value perpendicular to the grinding direction, while for all polished specimens, there were no significant differences in Ra values in the two directions (see section III).

III. Results and Discussions Firstly, the microstructures of typical alumina and nanocomposite materials and the effect of SiC particles on the microstructure are discussed. Secondly, results of progressive polishing of selected alumina and nanocomposite materials will be presented and their polishing behavior with polishing time and polishing grits will also be discussed. Finally, the effect of SiC content on surface quality in the absence of the effect of grain size and the effect of grain size on surface quality for similar materials will be shown.

(1) Microstructures Grain size distributions of selected alumina and nanocomposite materials are shown in Fig.1. At low sintering temperature (1600 oC) Al2O3-5% SiC composites showed narrower grain size distributions with a smaller mean grain size compared to alumina (Fig. 1.a and b). When the sintering temperature is increased to 1700 oC, 1% SiC nanocomposite (Fig. 1.d) shows a wide grain size distribution indicating that the SiC content is not enough to prevent grain growth at this temperature. However, grain growth was suppressed when the SiC content is increased further up to 5% (Fig. 1.c). Figure 2 shows typical microstructures of a pure alumina and a 5% vol. SiC nanocomposite both sintered at 1600 oC for 2 hours. Alumina shows a wide grain size distribution (fig. 2.a) with elongated grains whereas the nanocomposite has a relatively homogeneous grain size distribution (fig. 2.b) with equiaxed grains, indicating grain boundary pinning by SiC particles. The SiC particles retard densification of the composite leaving a number of pores in the microstructure (3.48 % for the material in fig. 2.b). Porosity decreases with increasing sintering temperature (see Table 2). Detailed TEM investigation of the microstructures of these and related alumina-SiC nanocomposites is under way and will be reported elsewhere.

(2) Progressive Polishing Ground surfaces of typical alumina and nanocomposite specimens are shown in Fig.3. Whatever the grain size and SiC content, all materials responded similarly to grinding (except for the finer grain alumina, which had Ra values half those of the other materials). This is consistent with the findings of Chou et al.14

Figure 4 shows optical micrographs of polished surfaces for alumina sintered at 1600 oC and Al2O3/5 % vol. SiC nanocomposite sintered at 1700 oC after polishing successively with grits of each size for 45 minutes. Material removal from the surface in the nanocomposite is mainly due to transgranular fracture within grains. In contrast, even at 3

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µm final finish, alumina shows a large number of dark areas where material removal is by grain detachment due to intergranular fracture. The area percentage of reflective surfaces and Ra values for typical alumina materials are plotted against polishing time for various diamond grit sizes in Figure 5 (Note that the surface roughness scale is inverted, so that higher quality surfaces have the same visual trend on the figures). Ground surfaces appeared dark under optical microscopy so it was not possible to quantify the percentage of polished areas; the initial polished area was thus taken as being zero. Ra values of ground surfaces were different in the direction parallel to and vertical to the ground direction; the two values are shown separately on Figure 5. Alumina materials show a strong grain size effect on polishing behavior. The polishing response of the finest grain size alumina for each polishing grit size is better than that of the coarser grain sizes (figure 5.a). The polishing response of the 3 and 5.3 µm alumina samples is similar to each other (figure 5.b and c). Also, alumina with the smallest grain size (2.01 µm) showed high resistance to grinding, with a surface roughness of 0.5 µm (compared to Ra = 0.69 µm and 0.74 µm, respectively, for the alumina materials of 3.08 µm and the 5.31 µm grain size). This resulted in a quicker response to polishing with a good final polished surface. For the grit sizes of 25 and 8 µm there is an optimum time to achieve the best result as assessed by optical microscopy for all three aluminas and this time is roughly the same for each material (30 min. for 25 µm, 45 min. for 8 µm). It is difficult to draw similar conclusions from the surface roughness results. In contrast, for grit size of 3 µm the polished area increases with polishing time. This possibly indicates that the polishing mechanism is changing from predominantly grain pullout to trans-granular fracture when the grit size is comparable to the grain size of the alumina materials. Figure 6 shows the detailed polishing behavior of typical nanocomposites with polishing time for different successive grit sizes. All the ground nanocomposite materials show a similar initial surface roughness. Surface roughness values are around 1.0 µm when the stylus tip is perpendicular to the groove direction. This decreases when the stylus tip is parallel to the groove direction to 0.53 µm for the 1.41 µm grain size nanocomposite and 0.77 µm for the other two nanocomposites. The response to polishing of nanocomposite materials, like the pure aluminas, varies with grain size. For samples with 5% SiC content (figure 6.a and b), the composite with the finer grain size showed, at given grit sizes, slightly better polished surfaces measured as the rate of reduction of surface roughness compared to coarser-grain material. In Figure 6, the 5% SiC nanocomposite (b) shows better surface quality than the 1% SiC composite (c) with decreasing polishing grit size. The difference in polishing behavior might be because of the differences in either grain size or SiC content. This is discussed later in this paper. Comparing results from nanocomposites with aluminas, the nanocomposite showed an immediate and strong increase in surface quality on polishing with 25 µm grits, whereas the aluminas showed a weaker response. This possibly implies that material removal processes during polishing are different for alumina and nanocomposites. The final surfaces of all nanocomposites, after 45 min. of polishing with 3 µm grit, are very similar either by “fraction of polished area” or by Ra measurements. All nanocomposites

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showed a highly polished surface regardless of SiC content or grain size after only 15 minutes at the final polishing step (3 µm). Aluminas, on the other hand, showed a slow increase in surface quality with time and even after 45 minutes of polishing, only 80 % of the surfaces were polished to a shiny finish. Compared to alumina, nanocomposites display a better surface finish after each successive polishing step. Sternitzke et al.6 found similar behavior. They measured the surface quality of ground and polished alumina and Al2O3 / 5 % SiC nanocomposite by line focus acoustic microscopy (LFAM) and found that Rayleigh wave velocities for both materials increase with decreasing polishing grit sizes, indicating better surface finish. However, in contrast to our results, the amount of increase in surface quality for nanocomposites was higher than for alumina, while the surface quality for alumina with a good polish (polishing grade < 3 µm) was found to be similar to those for the nanocomposites. The difference in results for fine polishing might be due to the different defects detected by different methods15. The acoustic microscopy technique is not principally sensitive to the surface roughness, but mostly to surface-breaking cracks introduced during machining which might not be detected by the image analyzer or the surface profilometer used here.

(3) Effect of SiC content and grain size

3.1. Effect of Grain size Grain sizes of manufactured aluminas vary between 1 and 6 µm depending on the sintering temperature. Nanocomposites have grain sizes between 1 and 8 µm depending not only on sintering temperatures but also on SiC volume content (See Table 2). The effect of grain size on surface roughness at the end of each polishing stage for alumina and nanocomposites is shown in Figure 7. At a given grain size, all nanocomposites (irrespective of SiC content) have better polished surfaces than aluminas with the exception of the finest grain alumina, which shows similar surface finish to high volume SiC nanocomposites sintered at 1600 oC. Marshall et al.16 also found that for aluminas of comparable purity, those with finer grain sizes have higher grinding resistance. In alumina (Fig. 7.a) surface roughness at a given stage of polishing increases with increasing grain size. This grain size dependence of surface roughness is also apparent in nanocomposites for 25 and 8 µm polishing grit sizes, although this dependence is not as strong as in alumina. However, this sensitivity to grain size is not apparent for nanocomposites when 3 µm polishing grit is used. This can more easily be seen in Figure 8 where the surface roughness is plotted against grain size for a polishing grit size of 3 µm for alumina and nanocomposites. It can be concluded that the material removal processes are different for the two types of material. Material removal in alumina is still predominantly by grain pull-out or grain dislodgement due to grain boundary fracture although the polishing grit size is comparable to grain sizes of aluminas. In nanocomposites, it is difficult to dislodge or remove individual grains with 3 µm grits. Grain boundaries are strengthened by SiC particles so that the polishing might be mainly due to local plastic deformations and/or fine-scale removal processes. 3.2. Effect of SiC content To compare the effect of SiC content on polishing, in the absence of any effects of grain size, we show data from cold isostatically pressed alumina sintered at 1600 oC and nanocomposites with 2 and 5% SiC content sintered at 1600 oC and 1700 oC respectively.

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These materials have comparable grain sizes of 3.08 µm, 3.20 µm and 3.15 µm respectively (See Tables 1 and 2). The surface roughness data at the end of each polishing stage for these materials are shown in Figure 9. For a given polishing stage, nanocomposites have better surface finish than alumina; the differences in quality of the surfaces is more noticeable after the 3 µm grit polishing stage. There is not much improvement in surface quality when the SiC vol. content is increased above 2%. This result is in good agreement with the findings of Winn and Todd13, who also found that the addition of as little as 1% SiC was sufficient to prevent grain boundary cracking usually associated with alumina materials and to promote the transgranular fracture characteristic of nanocomposites.

IV. Summary 1. We have investigated the response of alumina and alumina/SiC nanocomposites (made

by similar sintering routes) to progressive polishing with 25 µm, 8 µm and 3 µm diamond grits. Polishing quality was measured by the fraction of optically reflective surface, and by profilometry.

2. Alumina and alumina-SiC nanocomposites have different polishing responses. 3. Grain size has a strong effect on the polishing of alumina whereas this effect in

nanocomposites is less strong and decreases with polishing grit size. 4. For a given grain size, nanocomposites with SiC content between 1% and 5% have a

better surface finish than alumina at all stages of a progressive polishing treatment. 5. With the schedules used here, the nanocomposites could be readily polished to a good

surface finish (near “100% reflective”) regardless of grain size or SiC content (above 1% by volume), largely due to the suppression of grain pull-out. The aluminas used could only be polished to, at best, an “80% reflective” surface; this was largely because of grain pull-out.

6. The performance of the 1% SiC nanocomposite is similar to that of 2-5% SiC nanocomposites.

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REFERENCES 1K. Niihara, "New design concept of structural ceramics, ceramic nanocomposites" J.

Ceram. Soc. Jpn., 99, 974-982 (1991). 2J. Zhao, L.C. Stearns, M.P. Harmer, H.M. Chan, G.A. Miller and R.F. Cook, "Mechanical

behavior of alumina-silicon carbide nanocomposites" J. Am. Ceram. Soc., 76 [2] 503-510 (1993).

3C. C. Anya and S. G. Roberts, "Indentation fracture toughness and surface flaw analysis of sintered alumina/SiC nanocomposites" J. Euro. Ceram. Soc., 16, 1107-1114 (1996).

4R.W. Davidge, R.J. Brook, F. Cambier, M. Poorteman, A. Leriche, D. O'Sullivan and T. Kennedy, "Fabrication, properties and modelling of engineering ceramics reinforced with nanoparticles of silicon carbide" Brit. Ceram. Trans., 96 [3] 121-127 (1997).

5S.J. Jiao, M.L. Jenkins and R.W. Davidge, "Interfacial fracture energy - mechanical behavior relationship in Al2O3 / SiC and Al2O3 /TiN nanocomposites" Acta Mater., 45 [1] 149-156 (1997).

6M. Sternitzke, E. Dupas, P. Twigg and B. Derby, "Surface mechanical properties of alumina matrix nanocomposites" Acta Mater., 45 [10] 3963 - 3973 (1997).

7I. Levin, W.D. Kaplan, D.G. Brandon and A.A. Layyous, "Effect of SiC submicrometer particle size and content on fracture toughness of alumina-SiC Nanocomposites" J. Am. Ceram. Soc., 78 [1] 254 - 256 (1995).

8Y. Xu, A. Zangvil and A. Kerber, "SiC nanoparticle - reinforced Al2O3 matrix composites: role of intra- and intergranular particles" J. Euro. Ceram. Soc., 17, 921-928 (1997).

9T. Ohji, Y-K. Jeong, Y-H. Choa and K. Niihara, "Strengthening and toughening mechanisms of ceramic nanocomposites" J. Am. Ceram. Soc., 81 [6] 1453 -1460 (1998).

10J. Luo and R. Stevens, "The role of residual stress on the mechanical properties of Al2O3 - 5 vol % SiC nanocomposites" J. Euro. Ceram. Soc., 17, 1565-1572 (1997).

11H.Z. Wu, C.W. Lawrence, S.G. Roberts and B. Derby, "The strength of Al2O3 / SiC nanocomposites after grinding and annealing" Acta Mater., 46 [11] 3839-3848 (1998).

12A.J. Winn and R.I. Todd, "Microstructural requirements for alumina - SiC nanocomposites" pp 153-164 in “Engineering with Ceramics” (edited by W.E. Lee & B. Derby), British Ceramic Proceedings 59, Institute of Materials, London, 1999.

13A.J. Winn and R.I. Todd, "Microstructural requirements for alumina - SiC nanocomposites", J. Euro. Ceram. Soc., in press

14I.A. Chou, H.M. Chan and M.P. Harmer, "Maching-induced surface residual stress behavior in Al2O3-SiC nanocomposites", J. Am. Ceram. Soc., 79, 2403-409 (1996).

15P.D. Warren, C. Pecorari, O.V. Kolosov, S.G. Roberts and G.A.D. Brigss, "Characterisation of surface damage via surface acoustic waves", Nanotechnology, 7, 295-301 (1996)

16D.B. Marshal, B.R. Lawn, and R.F. Cook, "Microstructural effects on grinding of alumina and glass-ceramics," J. Am. Ceram. Soc., 70 [6] C139-140 (1987)

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FIGURE CAPTIONS

Figure 1 Grain size distribution of: (a) alumina, 1600 ºC; (b) Al2O3-5% SiC, 1600 ºC; (c) Al2O3- 5% SiC, 1700 ºC; (d) Al2O3- 1% SiC, 1700 ºC.

Figure 2 Typical microstructures of: (a) alumina; (b) Al2O3/5 % SiC nanocomposite, both sintered at 1600 oC for 2 hours. Specimens were thermally etched.

Figure 3 SEM micrographs of ground surfaces of: (a) alumina; (b) 5 % SiC nanocomposite (both sintered at 1600ºC). Grinding direction from right to left.

Figure 4 Optical micrographs of: (left) alumina (1600 ºC); (right) Al2O3/5 % SiC nanocomposite (1700 ºC) after polishing successively with grit of the sizes shown for 45 minutes at each grit size.

Figure 5 Variation of surface quality of aluminas with polishing time with different grit sizes: Sintering conditions: (a) 1550 ºC, 2 hrs; (b) 1600 ºC, 2 hrs; (c) 1600 ºC, 6 hrs. Open symbols: % polished area, filled symbols: surface roughness, Ra (µm). / 150 µm grinding ( parallel, ⊥ perpendicular to groove direction); / 25 µm grit; / 8 µm grit; ∆/▲ 3 µm grit.

Figure 6 Variation of surface quality of nanocomposites with polishing time with different grit sizes. (a) 5 % SiC, 1600 ºC; (b) 5 % SiC, 1700 ºC; (c) 1 % SiC, 1700 ºC. Open symbols: % polished area, filled symbols: surface roughness, Ra (µm). / 150 µm grinding ( parallel, ⊥ perpendicular to groove direction); Ò/ 25 µm grit; / 8 µm grit; ∆/▲ 3 µm grit.

Figure 7 Effect of grain size on surface roughness after polishing successively for 45minutes for each of the grit sizes shown: (a) alumina (compaction method and sintering temperature are given in brackets); (b) nanocomposites (sintering temperature and SiC content are given in brackets). Sintering time is 2 hrs unless stated.

Figure 8 Surface roughness after final polishing with 3 µm grit as a function of grain size for alumina and nanocomposites.

Figure 9 Effect of SiC content on the surface roughness (at the end of each polishing stage with the grit size shown) of alumina and nanocomposites of similar grain sizes

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Table 1: Properties of alumina produced at different sintering conditions.

Sintering Temperature,

ºC

Density g/cm3

Density % theoretical

Mean Grain Size, µm

Young’s Modulus GPa

Poisson’s Ratio ν

1550 3.970 ± 0.01 100.0 ± 0.25 2.01 ± 0.28 384 ± 5.0 0.24 1600 3.957 ± 0.02 99.93 ± 0.5 3.08 ± 0.54 380 ± 6.2 0.25

1600 § 3.920 ±0.02 99.00 ± 0.5 4.43 ± 0.22 373 ± 5.5 0.25 1600 + 3.970 ± 0.01 100.0 ± 0.25 5.31 ± 0.19 322 ± 22 0.25

§ uniaxial pressing; all others CIP. + 6 hr sintering time; all others 2hr.

Table 2: Properties of Al2O3/SiC nanocomposites produced at different sintering conditions. Composite Density

g/cm3 Density

% theoretical Mean Grain

Size, µm Young’s Modulus

GPa Poisson’s Ratio

ν 1 % SiC 3.968 ± 0.004 100 ± 0.09 6.45 ± 0.73 383 ± 3.3 0.25 2 % SiC 3.887 ± 0.064 98.61 ± 1.63 3.20 ± 0.49 375 ± 2.8 0.24 3 % SiC 3.859 ± 0.05 98.14 ± 1.27 2.03 ± 0.18 382 ± 3.3 0.25 4 % SiC 3.812 ± 0.04 97.16 ± 1.01 1.74 ± 0.11 355 ± 3.0 0.24

1600

o C (C

IP),

2hr

5 % SiC 3.778 ± 0.031 96.52 ± 0.79 1.41 ± 0.12 345 ± 0.2 0.25

1 % SiC 3.928 ± 0.06 99.81 ± 1.43 7.16 ± 0.14 408 ± 3.5 0.25 2 % SiC 3.918 ± 0.04 99.50 ± 1.09 5.90 ± 0.53 387 ± 2.1 0.25 3 % SiC 3.932 ± 0.02 99.47 ± 0.4 4.34 ± 0.14 385 ± 1.0 0.25 4 % SiC 3.909 ± 0.01 99.11 ± 0.16 3.72 ± 0.10 379 ± 1.6 0.25 17

00 o C

(U

P)2h

r

5 % SiC 3.866 ± 0.01 98.60 ± 0.37 3.15 ± 0.16 363 ± 8.3 0.25

0102030405060

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22Grain Size, d (µm)

Fig.1: Grain size distribution of: (a) alumina, 1600 ºC; (b) Al2O3-5% SiC, 1600 ºC; (c) Al2O3- 5% SiC, 1700 ºC; (d) Al2O3- 1% SiC, 1700 ºC.

Fig 1

10

µm

10 µm

(a)

3.8 µm

(b)

Fig 2: Typical microstructures of: (a) alumina; (b) Al2O3/5 % SiC nanocomposite, both sintered at 1600 oC for 2 hours. Specimens were thermally etched.

Fig 2

30 µm

(a)

30 µm

(b)

Fig. 3: SEM micrographs of ground surfaces of: (a) alumina; (b) 5 % SiCnanocomposite (both sintered at 1600ºC). Grinding direction from right to left.

Fig 3

100 µm 100 µm

25 µm grit

100 µm 100 µm

8 µm grit

100 µm 100 µm

Alumina Al2O3 / 5% SiC3 µm grit

Fig. 4: Optical micrographs of: (left) alumina (1600 ºC); (right) Al2O3/5 % SiC nanocomposite (1700 ºC) after polishing successively with grit of the sizes shown for 45 minutes at each grit size.

Fig 4

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Fig. 5: Variation of surface quality of aluminas with polishing time with different grit sizes: Sintering conditions: (a) 1550 oC, 2 hrs; (b) 1600 oC, 2 hrs; (c) 1600 oC, 6 hrs. Open symbols: % polished area, filled symbols: surface roughness, Ra (µm). / 150 µm grinding ( parallel, ⊥ perpendicular to groove direction); / 25 µm grit; / 8 µm grit; ∆/ 3 µm grit.

0

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Are

a (%

) Surface Roughness,

Ra (µm

)

(c)

d = 7.16 µm

⊥

0

0.2

Polishing Time (min.)

Fig. 6: Variation of surface quality of nanocomposites with polishing time with different grit sizes. (a) 5 % SiC, 1600 oC; (b) 5 % SiC, 1700 oC; (c) 1 % SiC, 1700 oC. Open symbols: % polished area, filled symbols: surface roughness, Ra (µm). / 150 µm grinding ( parallel, ⊥ perpendicular to groove direction); Ò/ 25 µm grit; / 8 µm grit; ∆/ 3 µm grit.

Fig 6

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 7 8

(CIP

, 155

0) (CIP

, 160

0)

(UP,

160

0)

(CIP

, 160

0, 6

hrs

)

150 µm grinding

25 µm grit

8 µm grit

3 µm grit

Surf

ace

Rou

ghne

ss, R

a (µ

m)

Grain size, d (µm)

(a)

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 7 8

(160

0,4%

) (1

600,

3%

)

(170

0, 5

%)

(160

0, 2

%)

(170

0, 4

%)

(170

0, 3

%)

(170

0, 2

%)

(160

0, 1

%)

(170

0, 1

%)

(160

0,5%

)

150 µm grinding

25 µm grit

8 µm grit

3 µm grit

Surf

ace

Rou

ghne

ss, R

a (µ

m)

Grain size, d (µm)

(b)

Fig. 7: Effect of grain size on surface roughness after polishing succesively for 45minutes for each of the grit sizes shown: (a) alumina (compaction method and sintering temperature are given in brackets); (b) nanocomposites (sintering temperature and SiC content are given in brackets). Sintering time is 2 hrs unless stated.

Fig 7

0

0.1

0.2

0.3

0 2 4 6 8

Alumina

Nanocomposites

Surf

ace

Rou

ghne

ss, R

a (µ

m)

Grain size, d (µm)

Fig. 8: Surface roughness after final polishing with 3 µm grit as a function of grain size for alumina and nanocomposites.

Fig 8

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5

vol.% SiC

Surf

ace

Rou

ghne

ss, R

a (µ

m)

d =3.08 µm

d =3.15 µmd =

3.20 µm

150 µm grinding

25 µm grit

8 µm grit

3 µm grit

Fig. 9: Effect of SiC content on the surface roughness (at the end of each polishing stage with the grit size shown) of alumina and nanocomposites of similar grain sizes.

Fig 9