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Ni nanocomposites produced by heterogeneous precipitation method with varying nickel contents
Int. Journal of Refractory Metals and Hard Materials 57 (2016) 139–144
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Pressureless sintering of Al2O3/Ni nanocomposites produced by heterogeneous precipitation method with varying nickel contents Betül Kafkaslıoğlu ⁎, Yahya Kemal Tür Department of Materials Science and Engineering, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 26 January 2016 Received in revised form 7 March 2016 Accepted 15 March 2016 Available online 16 March 2016 Keywords: Pressureless sintering Nanocomposites Hardness Al2O3
a b s t r a c t Al2O3/Ni nanocomposites with varying Ni contents (1, 3, 5 vol.%) were produced by pressureless sintering of Al2O3 and nano Ni powder mixture, prepared by a heterogeneous precipitation method. Better densification was achieved by increasing the sintering temperature and decreasing the Ni content. The highest density was obtained from the samples with 1 vol.% Ni content sintered at 1550 °C. X-ray diffraction analysis was used in order to verify the Ni particle formation in the sintered specimens. SEM analyses were conducted on the polished and thermally etched specimens and Al2O3 grain and Ni metal particle sizes were measured. It was observed that Ni particles were dispersed homogeneously in the alumina matrix and they were commonly located at the grain boundaries or at the triple junctions. Vickers hardness of the specimens was measured to investigate the hardness-material relationship which was strongly dependent on microstructure which is affected by the sintering temperature and Ni content. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Alumina (Al2O3) possesses desired properties such as high hardness, corrosion resistance with chemical stability. Also, it is a very costeffective material in the family of advanced ceramics. These properties make it an ideal material to perform in a variety of applications including alumina-based wearing parts, biomaterials and armor. It would not be wrong to consider alumina as the “workhorse material” of the structural ceramics [1]. Nevertheless, the applications of alumina are limited by the material's low tensile strength, fracture toughness and thermal shock resistance [2]. To improve these, Al2O3 matrix composites have been widely explored since 1990s [3–6]. Ceramic nanocomposites which contain sub-micron secondary phase particles dispersed in micron grain sized Al2O3 matrix are known to enhance mechanical properties [5–7]. Considering the nanocomposite concept, Al2O3/metal nanocomposites have an important place: with Al2O3/metal nanocomposites, not only the mechanical properties were improved, but also other attractive qualities such as magnetic, electric or optical properties can be enhanced [6,8,9]. However, these properties are especially related to the microstructure and homogeneity of the microstructure, therefore, it is critical to choose the most appropriate fabrication method. Compared to other methods such as mechanical mixing of Al2O3 with metallic powders or the sol-gel method, coating processes (like electroless plating, co-precipitation and heterogeneous precipitation) have important advantages in achieving homogeneity, so they are being investigated comprehensively recently [10–12]. ⁎ Corresponding author. E-mail address: [email protected] (B. Kafkaslıoğlu).
http://dx.doi.org/10.1016/j.ijrmhm.2016.03.009 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
The microstructure control of metal reinforced ceramic nanocomposites is harder than ceramic particle reinforced ceramic composites because the metal phases generally used in those composites have low melting temperature, and metals are in the molten state during the sintering stage. The diffusion rate is higher in liquid phase than solid state [6]. Moreover, wetting of the molten metal phase on the ceramic is usually insufficient [11,13]. The coarsening of the molten metal particles is thus more pronounced than hard ceramic particles in a ceramic matrix [4,6]. Therefore, most of the metal reinforced nanocomposites are prepared by using hot pressing techniques because hot pressing lowers the sintering temperature and allows the production of dense specimens in a shorter time. On the other hand, high cost of hot pressing imposes limits on its applications [14]. In recent years, spark plasma sintering has been used as an another technique in nanocomposite processing, due to its advantages of achieving fine microstructures of the ceramic phase, preserving the nano size of the metal phase and attaining small flaw size [15,16]. However, this technique has also some disadvantages such as high cost, heating source effects, and undesired phases which can formed by reactive sintering [15,17]. Pressureless sintering techniques are also widely used to prepare metal reinforced nanocomposites [4,15,18,19]. It is a low cost method and also easier to apply to mass production. However, pressureless sintering is usually accompanied by grain growth. Rodriguez-Suarez and his co-workers produced nano nickel reinforced alumina composites by pressureless sintering to obtain the material with high toughness and hardness to use in wear resistant components [15]. In the beginning of the 2000s, Guo Jun Li and his co-workers used heterogeneous precipitation method to prepare Al2O3/Ni nanocomposites [10,20–22]. The Ni volume content was varied from 4 to 10 vol.% in
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the composite and hot pressing at various temperatures (1300–1500 °C with an applied pressure of 20 MPa) was used to densify the green bodies. They attained the highest relative density (98.6%) at 1450 °C for 4 vol.% Ni [10]. It was shown that at low metal contents nano metal particles enhances the hardness of the ceramic material and hardness values higher than predicted by the rule of mixtures were achieved [23,24]. In the current work, Al2O3 powders were coated with nanosized nickel particles by using the heterogeneous precipitation method. Pressureless sintering was used and the composites were prepared by changing the nickel volume content as 1, 3 and 5 vol.%. The main goal of this work is to densify Al2O3/Ni composites with homogeneous metal particle dispersion without using hot pressing. The effects of low nickel content and pressureless sintering temperatures on density, microstructure and hardness were investigated. 2. Experimental procedure Al2O3-Ni composite powder mixtures with different volume ratios of Ni (1, 3, 5 vol.%) were prepared by heterogeneous precipitation. The following commercially available raw materials have been used: Al2O3 powder with average diameter of 0.60 μm (Almatis Calcined Alumina, Germany), nickel nitrate hexahydrate (Ni(NO3)2·6H2O) (BDH Chemicals Ltd., Poole, England, 98% purity), ammonium bicarbonate (NH4HCO3) (Sigma Aldrich, 99% purity). Al2O3, Ni(NO3)2·6H2O and 0.5 wt.% polyacrylic acid (Darvan 821A from MSE Tech Co. Ltd., Turkey) were used as dispersants and were first mixed in distilled water and ball milled for 24 h with yttria-stabilized zirconia balls (ball: Al2O3 powder ratio of 10:1 by weight). The amount of Ni(NO3)2·6H2O was adjusted according to the predetermined volume ratio of Ni in the final composite. Then NH4HCO3 aqueous solution of 1.0 M was added dropwise into the as-prepared slurry under vigorous stirring. Precipitation reaction of Ni precursor is given by the following equation [25]: 3Ni2þ þ 6HCO− 3 þ H2 O→NiCO3 2NiðOHÞ2 2H2 O↓ þ 5CO2 ↑:
ð1Þ
Complete precipitation was accomplished by using an excess amount of NH4HCO3 and by keeping the pH value of the solution between 8 and 9 during precipitation. The slurry was filtered and thoroughly washed with distilled water and ethanol. The precipitants were dried in air for 24 h, calcined in air at 550 °C for 2 h and reduced in a 90% Ar + 10% H2 atmosphere at 700 °C for 4 h. Calcining and reducing cycles were applied at a 5 °C/min heating rate and then the samples
Fig. 1. XRD patterns of (a) monolithic alumina powder in as-received form, and of the composite with 5 vol.% Ni content after each stage of processing: (b) precipitation, (c) calcination at 550 °C/2 h, (d) reduction at 700 °C/4 h, and (e) sintering at 1550 °C/2 h.
Fig. 2. XRD patterns of the monolithic alumina after: (a) ball milling with zirconia balls, (b) after eliminating the dispersant at 550 °C/2 h, and (c) sintering at 1450 °C/2 h.
were cooled down to room temperature in the furnace. Previous works were considered to determine the parameters (temperature, duration and atmosphere) of calcination and reduction heat treatments [15,21, 26]. After reduction and sieving to 90 μm, nano Ni-coated Al2O3 composite powder mixtures were pressed as disc form in a 15.9 mm diameter steel mold under a uniaxial pressure of 70 MPa, then discs were cold-isostatically pressed (CIP) under 90 MPa to increase the green strength and density. Monolithic alumina specimens were prepared under the same conditions. The composite specimens were pressureless sintered at three different temperatures (1450, 1500, and 1550 °C) in a vertical tube furnace under a 90% Ar + 10% H2 atmosphere. Sintering cycles were applied as: 5 °C/min heating rate, held 2 h at sintering temperature and then cooled down to room temperature in furnace. The monolithic alumina specimens were sintered in air. The microstructure of the sintered specimens was studied by Scanning Electron Microscopy (SEM) (Philips XL 30 SFEG). After grinding and polishing down to 0.5 μm, the composite specimens were thermally etched at 1400 °C/0.5 h under a 90% Ar + 10% H2 atmosphere and the monolithic alumina specimens were also etched at 1400 °C/2 h in air to analyze the microstructure in detail. The alumina matrix grain size was measured by using SEM micrographs with the linear intercept method where more than 100 intercepts are counted and averaged for each composition. Ni particle sizes were also measured using SEM micrographs. Phase analyses were performed by X-ray diffractometry (XRD) (Bruker® D8 Advance) at a scanning rate of 4°/min with a range from 20° to 70°. The bulk densities of the sintered samples were
Fig. 3. Relative densities of the monolithic alumina and Al2O3/Ni composites as a function of sintering temperature.
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Fig. 4. SEM-BSE mode micrographs of Al2O3/Ni composites sintered at 1450 °C: (a) 1 vol.% Ni, (b) 3 vol.% Ni, and (c) 5 vol.% Ni. Alumina matrix appears darker.
Fig. 5. SEM-SE mode micrographs of Al2O3/Ni composites: (a) 1 vol.% Ni, (b) 3 vol.% Ni, and (c) 5 vol.% Ni composites sintered at 1450 °C, and (d) 1 vol.% Ni, (e) 3 vol.% Ni and (f) 5 vol.% Ni composites sintered at 1500 °C. All specimens were thermally etched at 1400 °C/0.5 h under 90% Ar + 10% H2 atmosphere. Alumina matrix appears darker.
measured by the Archimedes' water replacement method. The hardness was measured by an Instron® Wolpert Testor 2100 equipped with a diamond pyramid Vickers indenter. Five measurements were made from three specimens (15 measurements) for each composition and sintering temperature. Loading time and load was selected as 10 s and 5 kg, respectively. The equation used to evaluate the hardness is [27]: H¼2
F 2
d
ð2Þ
where H = hardness (GPa), F = applied load (N), and d = average of diagonal length (mm).
reported by Guo Jun Li and co-workers [10,20–22]. Finally, by sintering in reducing atmosphere, Ni was kept in metal form and Al2O3/Ni composites were produced (Fig. 1.e). The XRD patterns of the composites with 1 and 3 vol.% Ni content were similar to the 5 vol.% Ni content composite, although, with increased metal volume content, Ni peaks become more evident.
Table 1 Relative density, Al2O3 matrix grain size, Ni particle size and hardness of the specimens with respect to Ni content and pressureless sintering temperature. Ni content (%)
Pressureless Relative density (%) sintering temperature (°C)
Al2O3 grain size (μm)
Ni grain size (μm)
0 1 3 5 0 1 3 5 0 1 3 5
1450 1450 1450 1450 1500 1500 1500 1500 1550 1550 1550 1550
0.93 ± 0.08 0.93 ± 0.08 0.93 ± 0.06 0.89 ± 0.07 1.52 ± 0.15 1.32 ± 0.21 1.40 ± 0.23 1.40 ± 0.13 1.81 ± 0.22 2.09 ± 0.19 2.08 ± 0.19 2.09 ± 0.28
15.40 ± 1.42 0.22 ± 0.08 15.43 ± 0.72 0.36 ± 0.13 14.42 ± 0.43 0.42 ± 0.12 12.54 ± 0.64 21.02 ± 1.10 0.26 ± 0.10 21.10 ± 0.70 0.39 ± 0.11 20.03 ± 0.48 0.46 ± 0.16 19.62 ± 0.70 21.96 ± 1.38 0.28 ± 0.07 22.53 ± 0.59 0.43 ± 0.14 22.49 ± 1.32 0.55 ± 0.19 21.95 ± 0.88
3. Results and discussion Fig. 1 is the XRD patterns of Al2O3 powder, the Al2O3-Ni powder mixtures and sintered Al2O3/Ni composite for 5 vol.% Ni content sintered at 1550 °C. The XRD results show that the precipitated Ni precursor NiCO3·2Ni(OH)2·2H2O was amorphous, because no new peaks were present (only Al2O3 peaks were present) (Fig. 1.b). After calcination at 550 °C/2 h in air, the amorphous phase crystallized to nano-sized discrete NiO particles (Fig. 1.c), and after the heat treatment at 700 °C/ 4 h in 90% Ar + 10% H2 atmosphere, NiO particles were all reduced to nano-sized Ni particles (Fig. 1.d). By using the heterogeneous precipitation method, the possibility of acquiring nano-sized Ni coated Al2O3 was
90.9 ± 0.36 90.9 ± 0.25 87.9 ± 0.21 84.9 ± 0.12 98.5 ± 0.12 97.4 ± 0.66 96.2 ± 0.29 94.7 ± 0.34 99.6 ± 0.25 98.2 ± 0.51 97.4 ± 0.50 97.1 ± 0.25
Hardness (GPa)
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Fig. 6. SEM-SE mode micrographs of Al2O3/Ni composites after pressureless sintering at 1550 °C and thermal etching: (a) 1 vol.% Ni, (b) 3 vol.% Ni, and (c) 5 vol.% Ni. Alumina matrix appears darker.
Fig. 2 indicates the XRD patterns of the monolithic alumina ball milled with zirconia balls for 24 h (Fig. 2.a) together with the patterns of alumina after eliminating the dispersant at 550 °C/2 h (Fig. 2.b) and after sintering at 1450 °C/2 h in air (Fig. 2.c). After sintering, apart from alumina peaks, new peaks belonging to zirconia appeared as an impurity; they were attributed to wear of zirconia balls used in ball milling while preparing the monolithic specimens. These peaks were not recognizable in ball milled (Fig. 2.a) and calcined alumina powder (Fig. 2.b) due to poor crystallinity of the ZrO2 particles, probably the impact energy of the milling media resulted in the broadening of these peaks. After sintering at 1450 °C/2 h, the grain growth of zirconia caused more pronounced peaks. Fig. 3 shows the variation of the relative densities with pressureless sintering temperature for the monolithic alumina and all composites with different nickel contents. When the sintering temperature was increased, the relative density was enhanced for all specimens. Conversely, at a given sintering temperature, the relative density of specimens decreased with increasing nickel content. The highest densification was obtained at 1550 °C for all specimens and when Ni content was 1 vol.%, the highest relative density of 98.2% was attained. When the Ni content reached to 5 vol.%, the relative density decreased to 97.1%. The melting temperature of Ni is 1453 °C [14] and it is liquid in the range of the sintering temperatures. The wetting ability of liquid Ni on Al2O3 is unsatisfactory with a wetting angle of 128° [28]. The nonwetting behavior of Ni hinders the densification of the microstructure, especially at higher Ni contents because of the larger interface of Al2O3/Ni surfaces. Li and co-workers [10] used hot pressing and also observed that relative density decreased with increasing Ni content. Interpolating their results, for 5 vol.% Ni content they would achieve 98% relative density by hot pressing at 1450 °C, which is 100 °C lower than pressureless sintering results presented in Fig. 3. Fig. 4 presents SEM-BSE (backscattered electron mode) images of the polished surfaces of the composites with increasing Ni contents sintered at 1450 °C. Nickel has a higher atomic number than Al2O3 so Ni particles appear white and Al2O3 grains appear darker in these images. The porosity increased with increasing Ni content which is in agreement with the decreasing relative density of these composites.
Also, with increasing Ni content, the average Ni particle size and the average distance between Ni particles decreased considerably, although, all composites had homogeneous metal dispersion. Fig. 5 presents the SEM-SE (secondary electron mode) images of the polished and thermal etched surfaces of the composites sintered at 1450 °C and 1500 °C. The density enhancement from 1450 °C to 1500 °C is clearly indicated, with less porosity at 1500 °C. Homogeneous matrix grain sizes could be obtained with no exaggerated Al2O3 grains. This observation is also supported by the relatively small distribution of the alumina grain sizes given in Table 1. Ni particles were located at triple junctions or at the grain boundaries which signifies that rapid alumina grain growth did not occur and Ni particles had enough time for diffusion and were not trapped in the alumina matrix [14]. Table 1 gives the average Ni particle size as a function of Ni content and the sintering temperature. From Table 1 and Fig. 5, with increasing sintering temperature, Ni particle size increased. For example, for 1 vol.% Ni content, the Ni particle size increased from 0.22 ± 0.08 μm to 0.28 ± 0.07 μm with increased sintering temperature. However, Ni content had a more pronounced effect on Ni particle size. For example, at 1450 °C, the Ni particle size increased from 0.22 ± 0.08 μm to 0.42 ± 0.12 μm with increasing Ni content. The average Ni particle size reaches to 0.55 ± 0.19 μm for 5 vol.% Ni sintered at 1550 °C. Fig. 6 shows SEM-SE micrographs of thermally etched surfaces of Al2O3/Ni composites after pressureless sintering at 1550 °C. In these micrographs the scale length is given as 5 μm for a better representation of the surface and average Al2O3 grain size because alumina matrix grain size increased at 1550 °C. Although the highest densification was obtained at 1550 °C for all specimens, the relative densification was 97 to 98%, and there were still pores, especially gathered around Ni particles. As stated above, poor wetting ability of Ni obstructs the densification near these particles and increases the porosity. The preferred processing method in this study, pressureless sintering, did not provide enough driving force for full densification. SEM-SE micrographs of thermally etched surfaces of monolithic alumina specimens for all sintering temperatures are shown in Fig. 7. In addition to the increase in the relative density with the increasing sintering temperature, the alumina grain size also increased. X-ray
Fig. 7. SEM-SE mode micrographs of monolithic Al2O3 specimens after sintering at: (a) 1450 °C, (b) 1500 °C, and (c) 1550 °C. The arrows show the zirconia particles.
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Fig. 8. Hardness measurements as a function of vol.% Ni content and sintering temperature.
analysis showed zirconia particle impurities were present and these zirconia particles can be seen in the micrographs. The weight loss of zirconia balls indicates that the zirconia phase was about 0.5 vol.% of the monolithic alumina and the particles were located randomly in the microstructure. The average size of Al2O3 grains measured by the linear intercept method for the monolithic alumina and for all composites with different nickel contents are given in Table 1. With the increasing sintering temperature, monolithic Al2O3 grain size increased for all Ni contents. The grain size increase increment was more pronounced from 1500 °C to 1550 °C. At 1450 °C and 1500 °C, the presence of Ni inhibited the grain growth of Al2O3. When the sintering temperature was increased to 1550 °C, the liquefaction of Ni occurred and liquid phase sintering became active. Thus, the inhibiting role of Ni on the grain growth of Al2O3 was replaced by a driving role for grain growth. Table 1 shows that at 1550 °C, the Al2O3 grain size was considerably greater for Ni containing composites compared to 0 vol.% Ni content Al2O3. Table 1also shows that the Ni content did not have much effect on the grain growth of Al2O3. It is thought that the low metal content (1, 3, 5 vol.%) and achievement of homogeneous metal particle dispersion caused uniform and similar matrix grain sizes for all composites. The average hardness values of three specimens are also given in Table 1 and shown as a function of Ni content in Fig. 8. With increasing sintering temperature, the hardness of the specimens increased and the increment was greatest from 1450 °C to 1500 °C. These results agree with the density variation in the same temperature range. Therefore,
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the hardness increased with the enhancement of density for all specimens. The highest hardness values were obtained after sintering at 1550 °C. A decrease in the hardness of the composite with increasing Ni volume content was expected [23]. In metal particle reinforced ceramic materials, when the metal phase content increases, there is a decrease in the hardness of the composite, due to the softer character of the metal phase in comparison to the hard ceramic matrix phase. Among the specimens, the composite with 1 vol.% Ni content had the highest hardness value, as 22.53 GPa at 1550 °C. At the same sintering temperature, the hardness was 21.95 GPa for the composite with 5 vol.% Ni content. This value is high contrary to the expectations and the small difference between the hardness values is attributed to the decrease in relative density. Similarly, the observed decrease in hardness with increasing Ni content for specimens sintered at 1450 °C and 1500 °C can be attributed to the decrease in relative density; i.e., increase in porosity. Fig. 9 shows the correlation between hardness and porosity. The hardness decreased exponentially with increasing porosity. Using a best fit curve, this correlation was expressed in exponential form as: H ¼ H0 e−4:2P
ð3Þ
where H = hardness and P = porosity. For P = 0 (i.e., 100% relative density) the predicted hardness, H0, would be 23.6 GPa. Since hardness of Ni is lower than alumina [23], the rule of mixtures would predict that increasing Ni content reduces the hardness. However, from Fig. 9 it is seen that at lower porosities (at higher relative densities) 3 and 5 vol.% data points are on or above the best fit curve signifying their hardness are higher than predicted from their porosity. Ni particle does not reduce the hardness of the composite material so strongly, and relative density is the main parameter in controlling the hardness. In nano metal particles, the dislocation motion is restricted and hardness of the metal particle is much higher than that of bulk metal due to the Hall-Petch effect [29]. In pressureless sintered Al2O3/Ni composites, average metal particle size is above 100 nm which is considered the critical particle size for the hardening effect [30]. There is also a contribution to hardness from the Ni particles because a fraction of the particles maintain sizes near 100 nm. SEM-SE mode micrograph (Fig. 10.a) shows Ni particles with particle sizes about 50 nm precipitated on alumina powders. When these powders are pressureless sintered, the Ni particles have a tendency to grow as Table 1 shows. However, Ni particle sizes have a relatively large error and as seen in SEM-BSE mode micrograph (Fig. 10.b), some of the Ni particles remain relatively small. The hardness increase through these smaller Ni particles compensates for the softening effect of the larger particles; therefore, hardness of the Al2O3/Ni composite is maintained. If this effect was not present, the hardness values of 3 and 5 vol.% Ni composites would be below the best fit curve due to softer character of the Ni particles. It is seen that in Al2O3/Ni composites produced by heterogeneous precipitation method, high hardness of the alumina can be retained if high densification is succeeded with pressureless sintering. 4. Conclusions
Fig. 9. Hardness measurements as a function of porosity.
Al2O3/Ni nanocomposite powders with different Ni contents (1, 3, 5 vol.%) were prepared successfully by using the heterogeneous precipitation method. Pressureless sintering was used to densify the specimens at 1450–1550 °C for 2 h under 90% Ar + 10% H2 atmosphere. The highest relative density was obtained at 1550 °C for the composite with 1 vol.% Ni as 98.2%. Homogeneous metal particle dispersion was achieved for all composites. However, with increasing Ni contents, decreased densification was observed and attributed to poor wetting of alumina by liquid Ni. Since the highest densities were achieved at 1550 °C, the highest hardness values were obtained from specimens sintered at this temperature. Increased Ni content resulted in a slight decrease in hardness for all sintering temperatures and the decrease
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Fig. 10. (a) SEM-SE mode micrograph of 3 vol.% Ni-Al2O3 powder mixture after 700/4 h under 90% Ar + 10% H2 atmosphere, and (b) SEM-BSE mode micrograph of Al2O3/Ni composite with 3 vol.% Ni sintered at 1550 °C.
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Pressureless sintering of Al2O3/Ni nanocomposites produced by heterogeneous precipitation method with varying nickel contents Betül Kafkaslıoğlu ⁎, Yahya Kemal Tür Department of Materials Science and Engineering, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey
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Article history: Received 26 January 2016 Received in revised form 7 March 2016 Accepted 15 March 2016 Available online 16 March 2016 Keywords: Pressureless sintering Nanocomposites Hardness Al2O3
a b s t r a c t Al2O3/Ni nanocomposites with varying Ni contents (1, 3, 5 vol.%) were produced by pressureless sintering of Al2O3 and nano Ni powder mixture, prepared by a heterogeneous precipitation method. Better densification was achieved by increasing the sintering temperature and decreasing the Ni content. The highest density was obtained from the samples with 1 vol.% Ni content sintered at 1550 °C. X-ray diffraction analysis was used in order to verify the Ni particle formation in the sintered specimens. SEM analyses were conducted on the polished and thermally etched specimens and Al2O3 grain and Ni metal particle sizes were measured. It was observed that Ni particles were dispersed homogeneously in the alumina matrix and they were commonly located at the grain boundaries or at the triple junctions. Vickers hardness of the specimens was measured to investigate the hardness-material relationship which was strongly dependent on microstructure which is affected by the sintering temperature and Ni content. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Alumina (Al2O3) possesses desired properties such as high hardness, corrosion resistance with chemical stability. Also, it is a very costeffective material in the family of advanced ceramics. These properties make it an ideal material to perform in a variety of applications including alumina-based wearing parts, biomaterials and armor. It would not be wrong to consider alumina as the “workhorse material” of the structural ceramics [1]. Nevertheless, the applications of alumina are limited by the material's low tensile strength, fracture toughness and thermal shock resistance [2]. To improve these, Al2O3 matrix composites have been widely explored since 1990s [3–6]. Ceramic nanocomposites which contain sub-micron secondary phase particles dispersed in micron grain sized Al2O3 matrix are known to enhance mechanical properties [5–7]. Considering the nanocomposite concept, Al2O3/metal nanocomposites have an important place: with Al2O3/metal nanocomposites, not only the mechanical properties were improved, but also other attractive qualities such as magnetic, electric or optical properties can be enhanced [6,8,9]. However, these properties are especially related to the microstructure and homogeneity of the microstructure, therefore, it is critical to choose the most appropriate fabrication method. Compared to other methods such as mechanical mixing of Al2O3 with metallic powders or the sol-gel method, coating processes (like electroless plating, co-precipitation and heterogeneous precipitation) have important advantages in achieving homogeneity, so they are being investigated comprehensively recently [10–12]. ⁎ Corresponding author. E-mail address: [email protected] (B. Kafkaslıoğlu).
http://dx.doi.org/10.1016/j.ijrmhm.2016.03.009 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
The microstructure control of metal reinforced ceramic nanocomposites is harder than ceramic particle reinforced ceramic composites because the metal phases generally used in those composites have low melting temperature, and metals are in the molten state during the sintering stage. The diffusion rate is higher in liquid phase than solid state [6]. Moreover, wetting of the molten metal phase on the ceramic is usually insufficient [11,13]. The coarsening of the molten metal particles is thus more pronounced than hard ceramic particles in a ceramic matrix [4,6]. Therefore, most of the metal reinforced nanocomposites are prepared by using hot pressing techniques because hot pressing lowers the sintering temperature and allows the production of dense specimens in a shorter time. On the other hand, high cost of hot pressing imposes limits on its applications [14]. In recent years, spark plasma sintering has been used as an another technique in nanocomposite processing, due to its advantages of achieving fine microstructures of the ceramic phase, preserving the nano size of the metal phase and attaining small flaw size [15,16]. However, this technique has also some disadvantages such as high cost, heating source effects, and undesired phases which can formed by reactive sintering [15,17]. Pressureless sintering techniques are also widely used to prepare metal reinforced nanocomposites [4,15,18,19]. It is a low cost method and also easier to apply to mass production. However, pressureless sintering is usually accompanied by grain growth. Rodriguez-Suarez and his co-workers produced nano nickel reinforced alumina composites by pressureless sintering to obtain the material with high toughness and hardness to use in wear resistant components [15]. In the beginning of the 2000s, Guo Jun Li and his co-workers used heterogeneous precipitation method to prepare Al2O3/Ni nanocomposites [10,20–22]. The Ni volume content was varied from 4 to 10 vol.% in
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the composite and hot pressing at various temperatures (1300–1500 °C with an applied pressure of 20 MPa) was used to densify the green bodies. They attained the highest relative density (98.6%) at 1450 °C for 4 vol.% Ni [10]. It was shown that at low metal contents nano metal particles enhances the hardness of the ceramic material and hardness values higher than predicted by the rule of mixtures were achieved [23,24]. In the current work, Al2O3 powders were coated with nanosized nickel particles by using the heterogeneous precipitation method. Pressureless sintering was used and the composites were prepared by changing the nickel volume content as 1, 3 and 5 vol.%. The main goal of this work is to densify Al2O3/Ni composites with homogeneous metal particle dispersion without using hot pressing. The effects of low nickel content and pressureless sintering temperatures on density, microstructure and hardness were investigated. 2. Experimental procedure Al2O3-Ni composite powder mixtures with different volume ratios of Ni (1, 3, 5 vol.%) were prepared by heterogeneous precipitation. The following commercially available raw materials have been used: Al2O3 powder with average diameter of 0.60 μm (Almatis Calcined Alumina, Germany), nickel nitrate hexahydrate (Ni(NO3)2·6H2O) (BDH Chemicals Ltd., Poole, England, 98% purity), ammonium bicarbonate (NH4HCO3) (Sigma Aldrich, 99% purity). Al2O3, Ni(NO3)2·6H2O and 0.5 wt.% polyacrylic acid (Darvan 821A from MSE Tech Co. Ltd., Turkey) were used as dispersants and were first mixed in distilled water and ball milled for 24 h with yttria-stabilized zirconia balls (ball: Al2O3 powder ratio of 10:1 by weight). The amount of Ni(NO3)2·6H2O was adjusted according to the predetermined volume ratio of Ni in the final composite. Then NH4HCO3 aqueous solution of 1.0 M was added dropwise into the as-prepared slurry under vigorous stirring. Precipitation reaction of Ni precursor is given by the following equation [25]: 3Ni2þ þ 6HCO− 3 þ H2 O→NiCO3 2NiðOHÞ2 2H2 O↓ þ 5CO2 ↑:
ð1Þ
Complete precipitation was accomplished by using an excess amount of NH4HCO3 and by keeping the pH value of the solution between 8 and 9 during precipitation. The slurry was filtered and thoroughly washed with distilled water and ethanol. The precipitants were dried in air for 24 h, calcined in air at 550 °C for 2 h and reduced in a 90% Ar + 10% H2 atmosphere at 700 °C for 4 h. Calcining and reducing cycles were applied at a 5 °C/min heating rate and then the samples
Fig. 1. XRD patterns of (a) monolithic alumina powder in as-received form, and of the composite with 5 vol.% Ni content after each stage of processing: (b) precipitation, (c) calcination at 550 °C/2 h, (d) reduction at 700 °C/4 h, and (e) sintering at 1550 °C/2 h.
Fig. 2. XRD patterns of the monolithic alumina after: (a) ball milling with zirconia balls, (b) after eliminating the dispersant at 550 °C/2 h, and (c) sintering at 1450 °C/2 h.
were cooled down to room temperature in the furnace. Previous works were considered to determine the parameters (temperature, duration and atmosphere) of calcination and reduction heat treatments [15,21, 26]. After reduction and sieving to 90 μm, nano Ni-coated Al2O3 composite powder mixtures were pressed as disc form in a 15.9 mm diameter steel mold under a uniaxial pressure of 70 MPa, then discs were cold-isostatically pressed (CIP) under 90 MPa to increase the green strength and density. Monolithic alumina specimens were prepared under the same conditions. The composite specimens were pressureless sintered at three different temperatures (1450, 1500, and 1550 °C) in a vertical tube furnace under a 90% Ar + 10% H2 atmosphere. Sintering cycles were applied as: 5 °C/min heating rate, held 2 h at sintering temperature and then cooled down to room temperature in furnace. The monolithic alumina specimens were sintered in air. The microstructure of the sintered specimens was studied by Scanning Electron Microscopy (SEM) (Philips XL 30 SFEG). After grinding and polishing down to 0.5 μm, the composite specimens were thermally etched at 1400 °C/0.5 h under a 90% Ar + 10% H2 atmosphere and the monolithic alumina specimens were also etched at 1400 °C/2 h in air to analyze the microstructure in detail. The alumina matrix grain size was measured by using SEM micrographs with the linear intercept method where more than 100 intercepts are counted and averaged for each composition. Ni particle sizes were also measured using SEM micrographs. Phase analyses were performed by X-ray diffractometry (XRD) (Bruker® D8 Advance) at a scanning rate of 4°/min with a range from 20° to 70°. The bulk densities of the sintered samples were
Fig. 3. Relative densities of the monolithic alumina and Al2O3/Ni composites as a function of sintering temperature.
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Fig. 4. SEM-BSE mode micrographs of Al2O3/Ni composites sintered at 1450 °C: (a) 1 vol.% Ni, (b) 3 vol.% Ni, and (c) 5 vol.% Ni. Alumina matrix appears darker.
Fig. 5. SEM-SE mode micrographs of Al2O3/Ni composites: (a) 1 vol.% Ni, (b) 3 vol.% Ni, and (c) 5 vol.% Ni composites sintered at 1450 °C, and (d) 1 vol.% Ni, (e) 3 vol.% Ni and (f) 5 vol.% Ni composites sintered at 1500 °C. All specimens were thermally etched at 1400 °C/0.5 h under 90% Ar + 10% H2 atmosphere. Alumina matrix appears darker.
measured by the Archimedes' water replacement method. The hardness was measured by an Instron® Wolpert Testor 2100 equipped with a diamond pyramid Vickers indenter. Five measurements were made from three specimens (15 measurements) for each composition and sintering temperature. Loading time and load was selected as 10 s and 5 kg, respectively. The equation used to evaluate the hardness is [27]: H¼2
F 2
d
ð2Þ
where H = hardness (GPa), F = applied load (N), and d = average of diagonal length (mm).
reported by Guo Jun Li and co-workers [10,20–22]. Finally, by sintering in reducing atmosphere, Ni was kept in metal form and Al2O3/Ni composites were produced (Fig. 1.e). The XRD patterns of the composites with 1 and 3 vol.% Ni content were similar to the 5 vol.% Ni content composite, although, with increased metal volume content, Ni peaks become more evident.
Table 1 Relative density, Al2O3 matrix grain size, Ni particle size and hardness of the specimens with respect to Ni content and pressureless sintering temperature. Ni content (%)
Pressureless Relative density (%) sintering temperature (°C)
Al2O3 grain size (μm)
Ni grain size (μm)
0 1 3 5 0 1 3 5 0 1 3 5
1450 1450 1450 1450 1500 1500 1500 1500 1550 1550 1550 1550
0.93 ± 0.08 0.93 ± 0.08 0.93 ± 0.06 0.89 ± 0.07 1.52 ± 0.15 1.32 ± 0.21 1.40 ± 0.23 1.40 ± 0.13 1.81 ± 0.22 2.09 ± 0.19 2.08 ± 0.19 2.09 ± 0.28
15.40 ± 1.42 0.22 ± 0.08 15.43 ± 0.72 0.36 ± 0.13 14.42 ± 0.43 0.42 ± 0.12 12.54 ± 0.64 21.02 ± 1.10 0.26 ± 0.10 21.10 ± 0.70 0.39 ± 0.11 20.03 ± 0.48 0.46 ± 0.16 19.62 ± 0.70 21.96 ± 1.38 0.28 ± 0.07 22.53 ± 0.59 0.43 ± 0.14 22.49 ± 1.32 0.55 ± 0.19 21.95 ± 0.88
3. Results and discussion Fig. 1 is the XRD patterns of Al2O3 powder, the Al2O3-Ni powder mixtures and sintered Al2O3/Ni composite for 5 vol.% Ni content sintered at 1550 °C. The XRD results show that the precipitated Ni precursor NiCO3·2Ni(OH)2·2H2O was amorphous, because no new peaks were present (only Al2O3 peaks were present) (Fig. 1.b). After calcination at 550 °C/2 h in air, the amorphous phase crystallized to nano-sized discrete NiO particles (Fig. 1.c), and after the heat treatment at 700 °C/ 4 h in 90% Ar + 10% H2 atmosphere, NiO particles were all reduced to nano-sized Ni particles (Fig. 1.d). By using the heterogeneous precipitation method, the possibility of acquiring nano-sized Ni coated Al2O3 was
90.9 ± 0.36 90.9 ± 0.25 87.9 ± 0.21 84.9 ± 0.12 98.5 ± 0.12 97.4 ± 0.66 96.2 ± 0.29 94.7 ± 0.34 99.6 ± 0.25 98.2 ± 0.51 97.4 ± 0.50 97.1 ± 0.25
Hardness (GPa)
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Fig. 6. SEM-SE mode micrographs of Al2O3/Ni composites after pressureless sintering at 1550 °C and thermal etching: (a) 1 vol.% Ni, (b) 3 vol.% Ni, and (c) 5 vol.% Ni. Alumina matrix appears darker.
Fig. 2 indicates the XRD patterns of the monolithic alumina ball milled with zirconia balls for 24 h (Fig. 2.a) together with the patterns of alumina after eliminating the dispersant at 550 °C/2 h (Fig. 2.b) and after sintering at 1450 °C/2 h in air (Fig. 2.c). After sintering, apart from alumina peaks, new peaks belonging to zirconia appeared as an impurity; they were attributed to wear of zirconia balls used in ball milling while preparing the monolithic specimens. These peaks were not recognizable in ball milled (Fig. 2.a) and calcined alumina powder (Fig. 2.b) due to poor crystallinity of the ZrO2 particles, probably the impact energy of the milling media resulted in the broadening of these peaks. After sintering at 1450 °C/2 h, the grain growth of zirconia caused more pronounced peaks. Fig. 3 shows the variation of the relative densities with pressureless sintering temperature for the monolithic alumina and all composites with different nickel contents. When the sintering temperature was increased, the relative density was enhanced for all specimens. Conversely, at a given sintering temperature, the relative density of specimens decreased with increasing nickel content. The highest densification was obtained at 1550 °C for all specimens and when Ni content was 1 vol.%, the highest relative density of 98.2% was attained. When the Ni content reached to 5 vol.%, the relative density decreased to 97.1%. The melting temperature of Ni is 1453 °C [14] and it is liquid in the range of the sintering temperatures. The wetting ability of liquid Ni on Al2O3 is unsatisfactory with a wetting angle of 128° [28]. The nonwetting behavior of Ni hinders the densification of the microstructure, especially at higher Ni contents because of the larger interface of Al2O3/Ni surfaces. Li and co-workers [10] used hot pressing and also observed that relative density decreased with increasing Ni content. Interpolating their results, for 5 vol.% Ni content they would achieve 98% relative density by hot pressing at 1450 °C, which is 100 °C lower than pressureless sintering results presented in Fig. 3. Fig. 4 presents SEM-BSE (backscattered electron mode) images of the polished surfaces of the composites with increasing Ni contents sintered at 1450 °C. Nickel has a higher atomic number than Al2O3 so Ni particles appear white and Al2O3 grains appear darker in these images. The porosity increased with increasing Ni content which is in agreement with the decreasing relative density of these composites.
Also, with increasing Ni content, the average Ni particle size and the average distance between Ni particles decreased considerably, although, all composites had homogeneous metal dispersion. Fig. 5 presents the SEM-SE (secondary electron mode) images of the polished and thermal etched surfaces of the composites sintered at 1450 °C and 1500 °C. The density enhancement from 1450 °C to 1500 °C is clearly indicated, with less porosity at 1500 °C. Homogeneous matrix grain sizes could be obtained with no exaggerated Al2O3 grains. This observation is also supported by the relatively small distribution of the alumina grain sizes given in Table 1. Ni particles were located at triple junctions or at the grain boundaries which signifies that rapid alumina grain growth did not occur and Ni particles had enough time for diffusion and were not trapped in the alumina matrix [14]. Table 1 gives the average Ni particle size as a function of Ni content and the sintering temperature. From Table 1 and Fig. 5, with increasing sintering temperature, Ni particle size increased. For example, for 1 vol.% Ni content, the Ni particle size increased from 0.22 ± 0.08 μm to 0.28 ± 0.07 μm with increased sintering temperature. However, Ni content had a more pronounced effect on Ni particle size. For example, at 1450 °C, the Ni particle size increased from 0.22 ± 0.08 μm to 0.42 ± 0.12 μm with increasing Ni content. The average Ni particle size reaches to 0.55 ± 0.19 μm for 5 vol.% Ni sintered at 1550 °C. Fig. 6 shows SEM-SE micrographs of thermally etched surfaces of Al2O3/Ni composites after pressureless sintering at 1550 °C. In these micrographs the scale length is given as 5 μm for a better representation of the surface and average Al2O3 grain size because alumina matrix grain size increased at 1550 °C. Although the highest densification was obtained at 1550 °C for all specimens, the relative densification was 97 to 98%, and there were still pores, especially gathered around Ni particles. As stated above, poor wetting ability of Ni obstructs the densification near these particles and increases the porosity. The preferred processing method in this study, pressureless sintering, did not provide enough driving force for full densification. SEM-SE micrographs of thermally etched surfaces of monolithic alumina specimens for all sintering temperatures are shown in Fig. 7. In addition to the increase in the relative density with the increasing sintering temperature, the alumina grain size also increased. X-ray
Fig. 7. SEM-SE mode micrographs of monolithic Al2O3 specimens after sintering at: (a) 1450 °C, (b) 1500 °C, and (c) 1550 °C. The arrows show the zirconia particles.
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Fig. 8. Hardness measurements as a function of vol.% Ni content and sintering temperature.
analysis showed zirconia particle impurities were present and these zirconia particles can be seen in the micrographs. The weight loss of zirconia balls indicates that the zirconia phase was about 0.5 vol.% of the monolithic alumina and the particles were located randomly in the microstructure. The average size of Al2O3 grains measured by the linear intercept method for the monolithic alumina and for all composites with different nickel contents are given in Table 1. With the increasing sintering temperature, monolithic Al2O3 grain size increased for all Ni contents. The grain size increase increment was more pronounced from 1500 °C to 1550 °C. At 1450 °C and 1500 °C, the presence of Ni inhibited the grain growth of Al2O3. When the sintering temperature was increased to 1550 °C, the liquefaction of Ni occurred and liquid phase sintering became active. Thus, the inhibiting role of Ni on the grain growth of Al2O3 was replaced by a driving role for grain growth. Table 1 shows that at 1550 °C, the Al2O3 grain size was considerably greater for Ni containing composites compared to 0 vol.% Ni content Al2O3. Table 1also shows that the Ni content did not have much effect on the grain growth of Al2O3. It is thought that the low metal content (1, 3, 5 vol.%) and achievement of homogeneous metal particle dispersion caused uniform and similar matrix grain sizes for all composites. The average hardness values of three specimens are also given in Table 1 and shown as a function of Ni content in Fig. 8. With increasing sintering temperature, the hardness of the specimens increased and the increment was greatest from 1450 °C to 1500 °C. These results agree with the density variation in the same temperature range. Therefore,
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the hardness increased with the enhancement of density for all specimens. The highest hardness values were obtained after sintering at 1550 °C. A decrease in the hardness of the composite with increasing Ni volume content was expected [23]. In metal particle reinforced ceramic materials, when the metal phase content increases, there is a decrease in the hardness of the composite, due to the softer character of the metal phase in comparison to the hard ceramic matrix phase. Among the specimens, the composite with 1 vol.% Ni content had the highest hardness value, as 22.53 GPa at 1550 °C. At the same sintering temperature, the hardness was 21.95 GPa for the composite with 5 vol.% Ni content. This value is high contrary to the expectations and the small difference between the hardness values is attributed to the decrease in relative density. Similarly, the observed decrease in hardness with increasing Ni content for specimens sintered at 1450 °C and 1500 °C can be attributed to the decrease in relative density; i.e., increase in porosity. Fig. 9 shows the correlation between hardness and porosity. The hardness decreased exponentially with increasing porosity. Using a best fit curve, this correlation was expressed in exponential form as: H ¼ H0 e−4:2P
ð3Þ
where H = hardness and P = porosity. For P = 0 (i.e., 100% relative density) the predicted hardness, H0, would be 23.6 GPa. Since hardness of Ni is lower than alumina [23], the rule of mixtures would predict that increasing Ni content reduces the hardness. However, from Fig. 9 it is seen that at lower porosities (at higher relative densities) 3 and 5 vol.% data points are on or above the best fit curve signifying their hardness are higher than predicted from their porosity. Ni particle does not reduce the hardness of the composite material so strongly, and relative density is the main parameter in controlling the hardness. In nano metal particles, the dislocation motion is restricted and hardness of the metal particle is much higher than that of bulk metal due to the Hall-Petch effect [29]. In pressureless sintered Al2O3/Ni composites, average metal particle size is above 100 nm which is considered the critical particle size for the hardening effect [30]. There is also a contribution to hardness from the Ni particles because a fraction of the particles maintain sizes near 100 nm. SEM-SE mode micrograph (Fig. 10.a) shows Ni particles with particle sizes about 50 nm precipitated on alumina powders. When these powders are pressureless sintered, the Ni particles have a tendency to grow as Table 1 shows. However, Ni particle sizes have a relatively large error and as seen in SEM-BSE mode micrograph (Fig. 10.b), some of the Ni particles remain relatively small. The hardness increase through these smaller Ni particles compensates for the softening effect of the larger particles; therefore, hardness of the Al2O3/Ni composite is maintained. If this effect was not present, the hardness values of 3 and 5 vol.% Ni composites would be below the best fit curve due to softer character of the Ni particles. It is seen that in Al2O3/Ni composites produced by heterogeneous precipitation method, high hardness of the alumina can be retained if high densification is succeeded with pressureless sintering. 4. Conclusions
Fig. 9. Hardness measurements as a function of porosity.
Al2O3/Ni nanocomposite powders with different Ni contents (1, 3, 5 vol.%) were prepared successfully by using the heterogeneous precipitation method. Pressureless sintering was used to densify the specimens at 1450–1550 °C for 2 h under 90% Ar + 10% H2 atmosphere. The highest relative density was obtained at 1550 °C for the composite with 1 vol.% Ni as 98.2%. Homogeneous metal particle dispersion was achieved for all composites. However, with increasing Ni contents, decreased densification was observed and attributed to poor wetting of alumina by liquid Ni. Since the highest densities were achieved at 1550 °C, the highest hardness values were obtained from specimens sintered at this temperature. Increased Ni content resulted in a slight decrease in hardness for all sintering temperatures and the decrease
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Fig. 10. (a) SEM-SE mode micrograph of 3 vol.% Ni-Al2O3 powder mixture after 700/4 h under 90% Ar + 10% H2 atmosphere, and (b) SEM-BSE mode micrograph of Al2O3/Ni composite with 3 vol.% Ni sintered at 1550 °C.
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