Preparation of rare earth oxide doped alumina ceramics, their hardness and fracture toughness determinations

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 13, October 2006, pp Preparation of rare earth oxide doped alumina ceramics, their hardness and fracture toughness determinations Kuntal Maiti & Anjan Sil* Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee , India Received 8 September 2005; accepted 3 July 2006 Alumina based ceramics doped with rare earth oxide (Y 2 O 3 ) of 500 ppm, 1000 ppm and 2000 ppm were prepared by following ceramic powder processing and solid state sintering at 1500 C for different soaking time periods of 3 h, 6 h and 9 h. Microstructural investigations on all the samples were carried out. Hardness measurements were conducted using Vickers Indentation technique. Fracture toughness of the samples was estimated by inducing radial cracks by various indentation loads on the samples. The measured hardness values were found to lie between 139 MPa and 1444 MPa, while the fracture toughness values lie in the range MPa m 1/2. IPC Code: C04B35/40 Ceramics in general have intrinsic characteristics, such as high melting point, high hardness and good chemical inertness. These above properties make ceramics promising candidates for high temperature structural and wear-resistant components. The ceramics are widely used in a variety of high temperature engine parts, cutting tools, dies for drawing or extrusion, seal rings and bearing parts. However, the ceramics, even if fully densified, in general show variability in their mechanical strength and poor fracture toughness and due to these the ceramics are limited to be used in structural or wear applications. Considerable improvement in mechanical properties, i.e., high hardness, excellent chemical and mechanical stability in wide range of temperatures and high specific stiffness were found to achieve in different ceramic composites such as Al 2 O 3 /TiC, Al 2 O 3 /TiB 2, and Al 2 O 3 /SiC as reported in the literature. These materials are being used in various engineering applications and offer advantages with respect to friction and wear behaviour 1-3. In recent years, the improvement of mechanical properties of alumina-based ceramics by doping with rare earth elements has been realized by researchers 4. The addition of a small amount of metal oxide, sensitively change the mechanical properties 5. However, investigations 6,7 have, so far, been limited to the high temperature properties modification such as a significant improvement in high temperature *For correspondence ( asil1fmt@iitr.ernet.in, anj_sil@yahoo.co.uk) creep resistance, mechanical strength and fracture toughness. At this point it may be mentioned that the rare earths are standard dopants used for luminescent and magnetic materials. But, in recent years, the application of rare earths has been extended to engineering ceramics also. In the present work, Y 2 O 3 doped Al 2 O 3 ceramics were prepared and their hardness, fracture toughness values were determined at room temperature and microstructural characteristics were studied. The doping of impurities into structural ceramics has become one of the effective methods of tailoring various aspects such as grain size and shape, grain boundary structures and strengthening. Though there have been many reports on the effect of rare earth dopants to structural ceramics for enhancement of high temperature mechanical properties, practically no studies has yet been reported regarding the effect of rare earths doping on room temperature mechanical properties of structural ceramics especially alumina 4. However, the room temperature mechanical properties of ZrO 2 added Al 2 O 3 system were studied and reported 8. Indentation hardness testing is a convenient means of investigating the mechanical properties of a small volume of materials. The conventional procedure of hardness testing consists of applying a fixed load on diamond indenter and measuring the impression with the help of optical microscope. The dimensions of the resultant indentation on the test material were measured. Though variety of indenter geometrics in the hardness testing are used, the Vickers indentation

2 444 INDIAN J. ENG. MATER. SCI., OCTOBER 2006 method is most wide spreading 9. The Vickers diamond pyramid hardness number, H v, is defined as H v = P/A = α P/d 2 (1) where P is the applied load, A the pyramidal contact area of the indentation, d the average length of the diagonals of the resultant impression and α = for Vickers indenter. It has been established that an apparent hardness of a given ceramic is a function of applied test load and this phenomenon is sometimes referred as indentation size effect (ISE). Several relationships between the applied load P, and the resulting indentation size d were suggested. The simplest and commonly used relationship is power law proposed by Meyer 10 P = Ad n, where A and n are constants that can be derived directly from the curve fitting of the experimental data. The most popular approaches found in the literature to explain the source of the ISE, are energy balance consideration and the proportional specimen resistance (PSR) model. In this model, the applied test load, P, and the resultant indentation size, d, are predicted to follow the relationship 10 P = a 1 d +a 2 d 2, (2) where a 1 and a 2 can be related to the load-independent hardness. The Vickers indentation technique has been considered as an attractive method and an easy way (low cost of conducting experiments) to estimate the fracture toughness of brittle materials by measuring the length of radial cracks appearing at the indent corners. Such radial cracks are thought to emanate from the indent as a result of residual tensile stresses that develop during loading. Measured crack lengths are correlated to K IC through the following relationship 11 K IC = χ (E/H v ) 1/2 P/a 3/2 (3) where E is Young s modulus, H v the Vickers hardness, a the radial crack length measured from the centre of the indent and χ an empirical calibration constant which has been taken as ± Experimental Procedure The powders used were high purity Al 2 O 3 (Qualigens Fine Chemicals, 99%, particle size range μm), Y 2 O 3 (HIMEDIA Laboratory Pvt Ltd, 99.99%, particle size range μm). Samples were prepared with three different dopant concentrations of 500 ppm, 1000 ppm and 2000 ppm. The powders of Al 2 O 3 and Y 2 O 3 were mixed in liquid phase (acetone medium) using pestle and mortar for 2-3 h in order to develop homogeneous mixtures. The mixtures were dried at room temperature. The dried mixtures were pressed in the form of cylindrical pellets (diameter 16.4 mm, thickness 2.8 mm) uniaxially by hydraulic press at a load level of about 12.5 ton. The pellets (2-3 samples for each set) were sintered at 1500 C for three different soaking time periods of 3 h, 6 h and 9 h. Volume and linear shrinkages due to sintering of the samples were estimated by measuring their dimensions (diameter and thickness). Each sample was polished in one face with diamond paste to mirror finish, then indented under various loads of 30, 40, 50, 60 and 100 kg at a constant indenter dwell time of 30 s. All indentation tests were carried out under ambient laboratory conditions. After indentation, the length of each diagonal of the square shapes Vickers indentation was measured by optical microscope (Axiovert 200 MAT, ZEISS) with a magnification of 50X. The hardness values were calculated from the Eq. (1) by substituting the P and d values of the corresponding loads. Three indentations on average were taken in each case and the hardness data were found fairly well to be consistent. The fracture toughness (K IC ) values were estimated by determining the crack length (a) and substituting these data into the Eq. (3). The samples for those the indentation surface cracks were not observed, were further polished to check the presence of subsurface cracks, if any. In order to know the grain structure developed in the samples due to sintering, microstructures were observed by SEM. The polished samples were gold coated for the SEM studies. The grain size of the sintered samples was determined by mean linear intercept method on the micrographs of the samples. The average grain size ( G ) was calculated by using the following formula G = 1.56 L, where L is the average grain-boundary intercept length of a series of random lines drawn on the micrograph. Results and Discussion Volume shrinkage and linear shrinkage of the samples The shrinkage data are given in Table 1. It can be seen from the Table 1 that the respective volume and

3 MAITI & SIL: RARE EARTH OXIDE DOPED ALUMINA CERAMICS 445 Table 1 Volume shrinkage and linear shrinkage due to sintering (at 1500 C) of the Y 2 O 3 doped Al 2 O 3 samples studied Yttrium oxide (ppm) Soaking time (h) Volume shrinkage (fractional) Linear shrinkage (fractional) the linear shrinkages vary from as low as 11.0%, 7.0% to as high as 27.0%, 9.7%. There is an average increase in shrinkages with increase in soaking time period and this effect commonly occurs in normal sintering operation. Hardness measurements and indentation size effect (ISE) The Vickers hardness number, H v, for the samples were determined from the Eq. (1) and are plotted in Figs 1a, 1b and 1c which show H v versus applied test load, P. Figures 1a, 1b and 1c show H v versus applied test load for the samples sintered for 3 h, 6 h and 9 h respectively. In almost all the samples ISE was observed. Though the hardness data points in the figures are shown joined point to point, it is evident that the best fit lines of the data points can be shown to have a positive slope, i.e., the hardness increases with increase in applied test load. The sample doped with 1000 ppm Y 2 O 3 shows higher hardness values for all the test loads used. However, the hardness values for 500 ppm and 2000 ppm Y 2 O 3 doped samples lie in the lower range relative to that of 1000 ppm doped samples. A relatively wide range of applied test load was used to study the load dependence of measured hardness of the samples prepared. It can be seen that the hardness values of 6 h sintered 1000 ppm doped samples in relation to 500 ppm and 2000 ppm doped samples are almost double for all the corresponding test loads. This higher hardness results may be attributed to the combined effect of equiaxed smaller grain size, relatively high density of the material leading to high specimen resistance against deformation due to indentation. Figures 2a, 2b and 2c show P/d versus d curves for the 500 ppm, 1000 ppm and 2000 ppm Y 2 O 3 doped Fig. 1 Vickers hardness as a function of the applied test load for various samples sintered for (a) 3 h, (b) 6 h and (c) 9 h Al 2 O 3 samples. The best fit straight line plots of the scattered data points for respective soaking time periods were drawn with the help of Microsoft Excel. It is evident from the figures that the lines show apparent linearity. This is based on proportional specimen resistance (PSR) model by Li and Bradt 10 to explain the observed ISE in ceramics in relatively wider range of applied test load. The occurrence of apparent linearity between P/d versus d plots indicates that the PSR model could be applied to describe the observed ISE in a relatively wider range of applied test load. However, the experimental result shows that in some situations the linear relationship between P/d and d can be observed over a narrow range of applied test loads. The sintering process in general and the variables of sintering in particular influence the properties that result in the materials. The plots were

4 446 INDIAN J. ENG. MATER. SCI., OCTOBER 2006 Fig. 2 P/d plotted against d for samples with Y 2 O 3 doping concentration of (a) 500 ppm, (b) 1000 ppm and (c) 2000 ppm also presented for measured hardness versus soaking time. This is just to demonstrate the effect of soaking time, which is the only sintering variable in the present study. It can be seen from the Figs 3a, 3b, 3c, 3d, and 3e that maximum hardness values were obtained for 1000 ppm Y 2 O 3 doped samples sintered for 6 h. While the hardness values of 2000 ppm Y 2 O 3 doped samples sintered for 6 h are also different from 3 h and 9 h sintered samples. However, the hardness difference is not as significant as the case of 1000 ppm Y 2 O 3 doped samples. From the above observations it may be concluded that the 6 h is the optimum soaking period for giving rise to materials with high hardness. It can be inferred that the combination of 1000 ppm Y 2 O 3 doping along with a soaking period of 6 h is the suitable for giving rise to high hardness samples in the present study. Fig. 3 Hardness plotted against soaking time period for an applied load of (a) 30 kg; (b) 40 kg, (c) 50 kg, (d) 60 kg and (e) 100 kg

5 MAITI & SIL: RARE EARTH OXIDE DOPED ALUMINA CERAMICS 447 The fracture toughness values of the samples were presented in Table 2. The toughness values were calculated from the Eq. (3). It is to mention that the cracks were not found to be developed in every sample at all the applied loads. This may be due to the variation in physical characteristic, i.e., bulk density, surface density, pressure contours, strength, hardness from one sample to another, even when they are of same type. Moreover, the ceramic being generally brittle, ideally a large number of samples are to be made of same material for characterization and a statistical approach is to be applied to design the ceramic component. The cracks found in the samples due to indentation under the applied loads are described in Table 2. The radial crack lengths were estimated employing axio vision software in the optical microscopic observations. Fig. 4 presents a typical micrograph showing the indent with radial Table 2 Fracture toughness of yttrium oxide doped samples sintered at 1500 C Yttrium oxide (ppm) Soaking time (h) Applied load for indentation (kg) Radial crack length, a (µm) Fracture toughness (MPa m 1/2 ) Fig. 4 Optical micrograph showing a typical indent with subsurface crack profile in a 2000 ppm Y 2 O 3 doped sample sintered at 1500 C for a soaking time period of 3 h cracks developed in a sample doped with 2000 ppm Y 2 O 3 and sintered for the soaking time period of 3 h. The a values in the table are the average values of crack lengths. In the present study the toughness is found to vary from as low as 1.65 MPa m 1/2 to as high as 5.76 MPa m 1/2, the higher toughness values are obtained for 500 ppm Y 2 O 3 doped samples. While 40 kg load was adequate to generate radial cracks in 500 ppm Y 2 O 3 doped samples, cracks could not be observed with 50 and 60 kg loads. The non-occurance of the cracks perhaps due to the absence of residual stresses required to produce crack. However, one can refine the observations and establish reproducibility by checking more number of samples. In the case of 1000 ppm Y 2 O 3 doped sample, crack formation for 6 h sintering, the required starting load level was again 40 kg. In 2000 ppm Y 2 O 3 doped samples the cracks were found at all loads ranging from 30 kg to 60 kg. Therefore, it may be inferred that while hardness is more for 1000 ppm Y 2 O 3 doped samples sintered for 6 h, high fracture toughness could be obtained for the 500 ppm Y 2 O 3 doped samples. Microstructural observations SEM micrographs for 500 ppm Y 2 O 3 doped samples are shown in Figs 5a, 5b, and 5c. All the micrographs were taken under same magnification that is 20KX. It can be seen from micrographs that the microstructure becomes coarser with increase in soaking time and the average grain sizes are estimated to be 1.48, 1.87 and 2.41 μm for the soaking time periods of 3 h, 6 h and 9 h respectively. The grains are found to be equiaxed in shape and the degree of this shaping is more for 9 h sintered samples. The grain size increase is not very significant with the increase in soaking time and this clearly indicates that the doping retards/restricts grain growth during sintering. For 1000 ppm Y 2 O 3 doped samples the micrograph in Figs 6a, 6b and 6c show that average grain size increases with increase in soaking time and the grain sizes are 1.57, 2.07 and 2.37 µm for the soaking time periods of 3 h, 6 h and 9 h respectively. For 2000 ppm Y 2 O 3 doped samples the average grain sizes (estimated from the micrograph in Figs 7a, 7b and 7c) are found to be 1.60, 1.93 and 1.45 μm for soaking time of 3 h, 6 h and 9 h respectively. Figure 8 shows SEM micrographs of pure alumina sample sintered at 1500 C for soaking time period of 6 h. Though it is difficult to observe the grain structure from the micrograph, however, on careful observation one can see the presence of bigger grains.

6 448 INDIAN J. ENG. MATER. SCI., OCTOBER 2006 Fig. 5 SEM micrographs of 500 ppm Y2O3 doped samples sintered at 1500 C for (a) 3 h, (b) 6 h and (c) 9 h Fig. 6 SEM micrograph of 1000 ppm Y2O3 doped samples sintered at 1500 C for (a) 3 h, (b) 6 h and (c) 9 h

7 MAITI & SIL: RARE EARTH OXIDE DOPED ALUMINA CERAMICS 449 Fig. 7 SEM micrograph of 2000 ppm Y 2 O 3 doped samples sintered at 1500 C for (a) 3 h, (b) 6 h and (c) 9 h Fig. 8 SEM micrograph of Alumina sample sintered at 1500 C for 6 h Conclusions The following conclusions can be drawn from this study: (i) (ii) (iii) (iv) (v) (vi) Depending on the doping level and the soaking time period, the measured hardness values are found to lie in the range MPa. The dependence of hardness on applied test loads is clear for all the samples. The hardness values at all the loads are higher for 1000 ppm doped samples and a maximum hardness of 1444 MPa was obtained for the 6 h sintered samples. Indentation size effect was observed in all the samples. P/d versus d plots are apparently linear indicating the data points for the ceramics under study could also be described by the PSR model. The micrographs show the equiaxed grain structure of alumina ceramics due to yttrium oxide doping. (vii) The evidence of grain growth restriction was clear from the micrographs. (viii) The fracture toughness values of the ceramics as determined by the method of crack opening profiles are found to lie in the range MPa m 1/2. (ix) (x) The highest value of fracture toughness was found for 500 ppm doped sample sintered for 3 h. Radial crack length is found to be minimum in this case. One can further refine the results by taking number of indentations for every load and taking the statistical average of the deduced hardness data or toughness data.

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