Effect of La2O3 content on wear resistance of alumina ceramics

Effect of La2O3 content on wear resistance of alumina ceramics

JOURNAL OF RARE EARTHS, Vol. 34, No. 3, Mar. 2016, P. 288 Effect of La2O3 content on wear resistance of alumina ceramics WU Tingting (吴婷婷)1, ZHOU Jia...

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JOURNAL OF RARE EARTHS, Vol. 34, No. 3, Mar. 2016, P. 288

Effect of La2O3 content on wear resistance of alumina ceramics WU Tingting (吴婷婷)1, ZHOU Jian (周 建)1, WU Bolin (吴伯麟)2,*, LI Wenjie (李文杰)2 (1. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China; 2. College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China) Received 18 June 2015; revised 4 January 2016

Abstract: In order to improve the wear resistance, a kind of alumina ceramic with good wear resistance was created in an Al2O3-CaCO3-SiO2-MgO-La2O3 (ACSML) system. The effects of La2O3 content on sintering temperature, bulk density, and wear rate were investigated. The wear rate of sample was as low as 0.0393‰. The wear resistance of the sample containing La2O3 has improved 43% than that of the sample without La2O3. Appropriate La2O3 doping could inhibit grain growth, enhance density, and purify grain boundary. La2O3 could diffuse into Al2O3 to form a solid solution and react with Al2O3 to form high-aluminum low-lanthanum complex oxides. The combination among Al2O3, the solid solution layer, and the layer of high-aluminum low-lanthanum complex oxides combined closely, which could improve grain boundary cohesion. Besides, the homogeneous distributions of elements made uniform structure. Finally, the wear resistance of alumina ceramic was improved. Keywords: alumina ceramic; abrasive media; lanthanum oxide; wear resistance; rare earths

Ball milling is a critical unit process in many industries. The quality of abrasive media directly affects the quality of products. Therefore, it is very important to select the right material for milling. Alumina ceramics are attractive for many industrial applications due to their excellent properties, such as high wear resistance, and the relatively low cost of manufacture[1]. So they are used as one kind of abrasive media material. The demand for high purity alumina ceramic is increasing with the rapid development of industrial technology. The impurity content is the key issue in preparing high purity alumina ceramics. However, abrasive media is one of the main pollution sources. So it is very important to prepare high-alumina abrasive media with good wear resistance. In the current market, the content of alumina is generally less than 95% in high-alumina abrasive media. There are hardly abrasive media containing more than 95% alumina with good wear resistance. In general, processing and manufacturing of pure alumina products are a difficult and expensive task. Therefore, additional compounds are added to alumina to minimize the product processing and manufacturing costs[2]. The application of rare earth elements in ceramic industry has made a satisfactory progress in recent years[3–8]. However, few investigations on the effect of rear earth on the wear resistance have been reported.

In the present study, we focused on the effect of La2O3 content on the wear resistance of alumina ceramic in an ACSML system. Through the design of formula and control of the preparation process, an alumina ceramic with good wear resistance was gained. The sintering temperature, wear rate, phases, and microstructure were investigated. Furthermore, two extra experiments, solid solution of La3+ in CaAl12O19 and a high temperature reaction model of La2O3 with single-crystal alumina were done so as to provide strong evidences for the wear resistance mechanism.

1 Materials and methods Samples were prepared using a commercial, monosized α-Al2O3 powder (>99.8% purity and a mean particle size of 0.65 μm). The powder was mixed with magnesia, calcium carbonate, silicon dioxide and lanthanum oxide. The sum of Al2O3 and La2O3 was 98 wt.%. When the contents of lanthanum oxide were 0, 0.2 wt.%, 0.8 wt.%, 1.6 wt.% and 2.4 wt.%, the samples were referred to as L0, L2, L8, L16, and L24. The powders were mixed and ball milled for 24 h in a water medium. Then, the slurry was dried in an oven. The dried powders were subsequently crushed to remove soft agglomerates that resulted from the drying procedure. Next, the powders were pressed at 100 MPa by a cold isostatic pressing (CIP). Samples were shaped into sphere of 30 mm in

Foundation item: Project supported by the National Natural Science Foundation of China (51172049), State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (WUT, China) (2015-KF-4), and Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials (13AA-1) * Corresponding author: WU Bolin (E-mail: [email protected]; Tel.: +86-773-5897060) DOI: 10.1016/S1002-0721(16)60027-3

WU Tingting et al., Effect of La2O3 content on wear resistance of alumina ceramics

diameter and sintered in a box-type electrical resistance furnace at 5 ºC/min up to a right temperature for 1 h in air. After those processes, bulk density and wear rate were tested. Bulk density was measured to an accuracy of 0.01 g/cm3 by the Archimedes method with deionized water as the immersion medium. The wear rate testing process was as follows for each sample: weighed sample (M1) and measured diameter (Dx). Then put the sample into a polyurethane pot (inner diameter: 200 mm; length: 220 mm) and mill for 24 h in water medium. Dry the sample and weigh it again (M2). The wear rate is calculated by the following equation: W=KD (M1–M2)/M1, where W is wear rate (‰), K a constant (4.17×10–4 mm–1), D the mean diameter (mm) of samples, M1 the mass before wear (g), and M2 the mass after wear (g). Solid solution experiment: La0.3Ca0.7Al12O19.15 and CaAl12O19 were synthesized for study. The sintering temperature is 1600 ºC for 3 h in air. High temperature reaction model experiment: put La2O3 powder to coat on single-crystal alumina. And then, they were sintered at 1600 ºC for 3 h. X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to analyze the samples. The XRD tests were carried out in an X’Pert PRO multi-purpose X-ray diffractometer (PANalytical B.V., Almelo, Netherlands). It was used to analyze the phase composition. Samples were polished and coated with gold, and analyzed by a field emission scanning electron microscope (FESEM; S-4800, Hitachi, Japan) with energy dispersive spectroscopy (EDS).

2 Results 2.1 Effect of La2O3 content on sintering temperature The sintering temperature of samples increase with the increasing of La2O3 content (Fig. 1). Water absorption of sample was measured as a function of sintering temperature. The sintering temperature was determined by water absorption being 0%.

Fig. 1 Sintering temperature curve of samples

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The sintering temperature of sample L0 without La2O3 was 1500 ºC. When 0.2 wt.% La2O3 was added to the raw materials (Sample L2), the sintering temperature was 1520 ºC. When the La2O3 content was added from 0.2 wt.% to 2.4 wt.%, the sintering temperatures increased from 1520 to 1575 ºC. Adding La2O3 to alumina can raise sintering temperature. The melting point of La2O3 (2217 ºC) is higher than that of Al2O3 (2000–2050 ºC). Add high melting substances to the raw materials and there is no eutectic point in the ingredient range, which will lead to the increase of sintering temperature. Additionally, the large rare-earth ions can block the migration of particles, thus affecting the sintering of ceramic. 2.2

Effect of La2O3 content on bulk density and wear rate

The wear rates and bulk densities of La2O3-free and La2O3-added samples were measured. The results are presented in Fig. 2. It can be seen that the wear rates of samples vary parabolically with the increase of La2O3 content. The wear rate of sample L0 without La2O3 is 0.070‰. When the La2O3 content is 0.2 wt.%, the wear rate declines to 0.058‰. The wear rates of samples still decrease with the increasing of La2O3 content. When the La2O3 content is 1.6 wt.%, the wear rate is as low as 0.039‰ and the bulk density is 3.90 g/cm3. The wear rate of sample L16 is the lowest, and the bulk density is the maximum. It is reported that the addition of La2O3 is helpful to promote densification[9–12]. However, when the La2O3 content is 2.4 wt.%, the bulk density and wear rate are worse than sample L16. The chart shows an upward trend in wear rate and a downward trend in bulk density when the La2O3 content exceeds 1.6 wt.%. But the densities of La2O3-doped samples are higher than that of La2O3-free sample; and the wear rates of La2O3-doped samples are lower than that of La2O3-free sample. Under the equivalent conditions, the wear rate of a product with good resistance on the market is 0.172‰. The wear resistance of sample L16 is enhanced about 77% over the product. The experimental results indicate that La2O3 additive

Fig. 2 Wear rate and bulk density curves of samples

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can improve the wear resistance and bulk density of alumina ceramic. The optimum range is 0.8 wt.%–1.6 wt.%. Moreover, wear resistance has a correlation with bulk density. 2.3 Effect of La2O3 content on phase composition Fig. 3 shows the XRD results of samples with different La2O3 contents. The diffraction peaks of Al2O3, CaAl12O19, and Al2.4Mg0.4O4 are identified in five samples. However, a new phase (LaAl11O18) was formed with the increasing of La2O3 content (Sample L8, L16 and L24). And, the diffraction intensity of LaAl11O18 increases as the content of La2O3 increases. It is reported that the bulk solubility limit of lanthanum in Al2O3 is calculated to be ~80 mg/L[13]. Nevertheless, LaAl11O18 was undetectable in sample L2 because the content was too low to be detected by XRD. Besides, the diffraction intensity of CaAl12O19 increases as the content of La2O3 increases. La2O3 can improve the sintering of CaAl12O19[14]. This revealed that doping La2O3 promoted the crystallization of CaAl12O19 to reduce glass phases. To further analyze the effect of La2O3 on CaAl12O19, a solid solution experiment of La3+ in CaAl12O19 was done (Fig. 4). In Fig. 4, the diffraction peaks of

Fig. 3 X-ray diffraction patterns of samples

Fig. 4 X-ray diffraction patterns of La0.3Ca0.7Al12O19.15 and CaAl12O19

La0.3Ca0.7Al12O19.15 match well with CaAl12O19 and there is no impurity in it. It is indicated that La2O3 and CaAl12O19 can form a solid solution. A slow scan was taken by measuring 2θ from 33.5º to 37º, at a step size of 0.017º and a dwell time of 122 s per step. The result shows that the diffraction peaks of La0.3Ca0.7Al12O19.15 shift slightly to the low-angle region. The ionic radius of La3+ is larger than that of Ca2+. When La3+ displaces Ca2+, crystalline interplanar spacing (d) will increase. According to the Bragg equation (2dsinθ=nλ), θ will decrease as d increases, so the diffraction peaks of La0.3Ca0.7Al12O19.15 shift slightly to the low-angle region. This experiment confirmed that La3+ can replace Ca2+ in CaAl12O19 to form a solid solution. 2.4 Effect of La2O3 content on microstructure Samples L0, L2, L8, L16, and L24 were cut and polished. The structures of cross-section and the ratio between the mass are shown in Figs. 5 and 6. In sample L0, the mean grain size is about 2–3 μm and a small amount of round pores exist. There are molten substances on the grain surface and in between grains. Sintering aids (CaCO3, MgO, and SiO2) react with the compounds that contained only a trace of K+, and Na+ in Al2O3 powder to form liquid phase[15]. The liquid phase becomes glass phase after being cooled. The molten substances may be the glass phase. Compared with L0, the grain sizes of L2 and L8 do not grow excessively at elevated temperatures. The Al2O3 grains present idiomorphic structure. It is indicated that rare earth ions on the surface of Al2O3 are beneficial to the completely developed crystal grain and inhabit grain growth. Many researchers[11,16] have reported that La3+ acts as a grain growth inhibitor in the sintering. There are some plate-like crystals in samples. This kind of crystals leads to the structure of stagger growth between grains, which makes intergranular bond more closely. The molten substances are still seen in sample L2, but the content is less than sample L0. However, there are no molten substances in samples L8, L16, and L24. In samples L16 and L24, a part of grains grow slightly, and small grains increase. Fig. 1 shows that the sintering temperatures of samples increase with the increase of La2O3 content. Rising temperature leads to grain growth. However, the inhibition of La2O3 on grain growth causes that the grains did not grow excessively and the number of small grain increased. Compactness of sample L24 is worse than that of sample L16, consistent with the results of bulk density. The difference in microstructure causes that the wear resistance of sample L16 is better than that of sample L24. Sample L16 has the best wear resistance. Elemental distribution of sample L16 was analyzed. The result is shown in Fig. 7. In sample L16, the elemental distribution of La is relatively homogeneous. This result implies that

WU Tingting et al., Effect of La2O3 content on wear resistance of alumina ceramics

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Fig. 5 Microstructures of samples L0 (a), L2 (b), L8 (c), L16 (d) and L24 (e)

Fig. 6 EDS of samples L0, L2, L8, L16, and L24

La2O3 disperses well in alumina ceramic. It is speculated that part of La2O3 dissolved in Al2O3 to form a solid solution, and part of La2O3 reacted with Al2O3 to form aluminum-lanthanum complex oxides. The complex oxides dispersed homogeneously in grain boundaries, which helped to sinter and strengthen structure of ceramic. Similarly, the elemental distributions of Ca, Mg, and Si are homogeneous, too. It is indicated that the structure of ceramic system is uniform. Uniform structure is important for improving wear resistance of ceramic. 2.5 Results of the high temperature reaction model of La2O3 with Al2O3 Elemental distribution of La is homogeneous in the alumina ceramic (Fig. 7). It is speculated that part of La2O3 dissolved in Al2O3 to form a solid solution. In order to figure out the effect of La2O3 on alumina ceramic, the experiment of the high temperature reaction model was studied. Fig. 8 shows the border of single-crystal

alumina and the single-crystal alumina coating with La2O3. As seen in the figure, there are four layers of structure. The first layer A is single-crystal alumina. The second layer B presents a hexagonal structure and a quadrilateral structure containing a 120-degree angle. The EDS spectra of B show that the atomic percent ratio of Al to O is approximately 2:3, and it also contains trace amounts of La (0.22 at.%). Therefore, it can be judged that the second layer B is La3+ being dissolved in Al2O3 to form a solid solution. The second layer B is tightly bound to the first layer A. Grains in the third layer C are small. Parts of crystals exist as euhedral-granular texture. The atomic percent ratio of La, Al, and O is about 1:8:12, which is similar to LaAl11O12. And the third layer C is tightly bound to the second layer B. In the fourth layer D, the atomic percent ratio of Al to La is nearly 1:1. It is observed that the growth of the crystalline grain was poor. The edges of the grains are round. Some of aluminum-lanthanum complex oxides present spherical

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Fig. 7 Elemental distribution maps of Sample L16

Fig. 8 FESEM micrograph (a) and EDS spectra (b) of the high temperature reaction model

WU Tingting et al., Effect of La2O3 content on wear resistance of alumina ceramics

shape and scatter on the third layer C. It is speculated that compounds in layer D have low surface wettability with the compounds in layer C. However, high surface wettability is prerequisite to good performance of interfacial bonding[17]. The experiment has confirmed that La3+ can diffuse into Al2O3 to form a solid solution. The layer of solid solution is tightly bound to the basal layer of Al2O3. Al3+ also can diffuse to La2O3. During diffusion, Al2O3 together with La2O3 generates some aluminum-lanthanum complex oxides having different ratios of Al to La. The complex oxides near Al2O3 contain very little La2O3 and grain size is smaller (low-La2O3 layer). This layer is tightly bound to the layer of solid solution. However, La2O3 content is higher in the complex oxides away from Al2O3 (high-La2O3 layer). The high-La2O3 layer has low surface wettability with its next layer, which leads to poor adhesion. In addition, the crystal growth in the high-La2O3 layer needs a higher temperature. Those are the reasons that adding too many La2O3 can cause the sintering temperature increment and poor wear resistance of alumina ceramic.

3 Discussion In this study, we discovered that the La2O3 addition can improve the wear resistance of alumina ceramic. Adding La2O3 can increase the sintering temperature. Rising temperature accelerates pores to be discharged and mass transfer process, which promote grains rearrangement to increase bulk density and wear resistance. However, rising temperature can also cause grain growth. Oversize grains lead to performance degradation[1]. When the positive effect coming from rising temperature does not offset the negative impact such as grain growth, the performance of material will be reduced (Fig. 2). Adding appropriate La2O3 can restrain the grain growth of alumina effectively, which causes alumina ceramic with fine crystal particles and dense structure (Fig. 5). Deng[18] thought that La2O3 basically consisted in the grain boundaries of alumina ceramics to refine the grain. Yang et al. had reported that La2O3 preferably exists at alumina grain boundaries because of the discrepancy of ionic radius between La3+ and A13+ (rLa3+=0.103 nm, rAl3+=0.053 nm)[12]. The XRD result indicated that there was LaAl11O18 in La2O3-doped samples (Fig. 3). Qian et al.[10] also reported that LaAl11O18 was formed by the reaction of Al2O3 and La2O3 during the sintering, which was located among the grain boundaries and restrained the grain growth in the ceramic. The experiment of the high temperature reaction model revealed that La2O3 can diffuse into Al2O3 to form a solid solution and react with Al2O3 to form high-aluminum low-lanthanum complex oxides when add a small amount of La2O3. The combina-

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tion among Al2O3, the solid solution layer, and the low-La2O3 layer combine closely (Fig. 8). These are a part of reasons that La2O3 can improve wear resistance of alumina ceramic. The elemental distributions are homogeneous in ceramic, which makes homogeneous structure. Homogeneous structure helps to improve wear resistance of ceramic, too. Besides, the content of the liquid phase decreases with the increasing of La2O3 content (Fig. 5). Fig. 3 shows that the contents of CaAl12O19 increase as the content of La2O3 increases. The experiment of solid solution confirmed that La3+ can replace Ca2+ in CaAl12O19 to form a solid solution (Fig. 4). This means that adding La2O3 has a certain purifying effect on grain boundary, which leads to the liquid phase reduction and improvement of the strength of grain boundaries[19]. In ceramics, if stresses created by the boundary glass are tensile, the ceramics will be more susceptible to wear. Whereas if these stresses are compressive, the abrasive wear resistance of the material will be enhanced[20]. The thermal expansion coefficient of the glass phase in alkali-silicate system[21] is usually higher than that of alumina (9.1×10–6/ºC)[22]. Thus, the interface between glass and Al2O3 grains can form tensile stresses, which will result in a weaker boundary. The reduction of glass phase helps to improve wear resistance in alumina ceramic.

4 Conclusions (1) Adding La2O3 to Al2O3 ceramic could improve wear resistance of material. The wear rate was as low as 0.0393‰. The optimum quantity was about 1.6 wt.%. (2) Wear resistance had a close relationship with density. The density and wear resistance of alumina ceramic were improved by a proper addition of La2O3. But excessive addition would cause grain growth, degrading performance and density. (3) The wear resistance mechanism: adding appropriate La2O3 could inhibit grain growth to refine grain and purify the grain boundary by reducing the glass phase, improving grain boundary cohesion. The homogeneous distributions of elements made uniform structure. Finally, the wear resistance of alumina ceramic was improved.

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