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Solid particle erosion of alumina ceramics at elevated temperature
Materials Chemistry and Physics 139 (2013) 765e769
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Solid particle erosion of alumina ceramics at elevated temperature Xiaojun Wang a, b, Minghao Fang a, *, Lai-Chang Zhang c, Hao Ding a, Yan-Gai Liu a, Zhaohui Huang a, Shaoping Huang d, Jingzhou Yang b a
School of Materials Science and Technology, China University of Geosciences (Beijing), No. 29 Xueyuan Road, Haidian District, Beijing 100083, China School of Mechanical and Chemical Engineering, University of Western Australia, Perth, WA 6009, Australia c School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, WA 6027, Australia d Luoyang Precondar Instruments for Testing Refractoriness Co., Ltd, Luoyang 471039, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< The erosion rate of alumina ceramics increases slowly from room temperature to 800 C, then sharply rising above 800 C. < The maximum erosion rate took place at 75 and 60 for 1200 C and 1400 C. < There is no obviously evidence of plastic deformation at 1200 C and 1400 C. < The harder SiC particles lead to more severe erosion damage of alumina ceramics, compared with corundum grits.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2012 Received in revised form 18 October 2012 Accepted 12 February 2013
A big gap exists for the understanding of the influence of temperature on erosion behavior at elevated temperature between experimental observations (up to 1100 C) and high temperature industrial application (up to 1600 C). This work the first time investigated the effect of higher temperature on the erosion resistance and mechanism of alumina ceramics. The solid particle erosion behavior of high purity alumina ceramics has been studied at elevated temperatures up to 1400 C and different impingement angles (30 , 45 , 60 , 75 , and 90 ), using corundum and SiC particles as erodent. Erosion rate of alumina ceramics slowly increases from 0.32 mm3 g1 to 0.44 mm3 g1 below 800 C, and then significantly increases up to 1.30 mm3 g1 at 1400 C. With increasing the impingement angle, the erosion rate increases slightly and reaches maximum value at 90 from room temperature to 800 C. However, the maximum value of erosion rate occurs at 75 and 60 for 1200 C and 1400 C, respectively. The brittle erosion mechanisms still dominate the material removal at elevated temperature. The material removal is mainly resulted from transgranular cleavage and grain removal. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Ceramics Erosion Microstructure Crack
1. Introduction High temperature solid particle erosion, one of the main failure modes in materials, refers to the material loss caused by solid * Corresponding author. Tel./fax: þ86 10 8232 2186. E-mail address: [email protected] (M. Fang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.02.029
particles impinging on a solid surface at elevated temperatures. This type of material damage has become major issue in many industrial systems, including metallurgical industry, cement industry, thermal power, garbage incineration, fluidized bed combustion systems and turbine engine, as well as engine parts, such as nozzles, shroud rings, and combustor components [1e3]. Due to the erosion by coal powders and limestone grits, the circulating
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fluidized bed (CFB) furnace usually has to be shut down for maintenance. High temperature erosion dramatically reduces the life and efficiency of such a device. Therefore, it is crucial to fully understand the process and mechanisms of erosion at elevated temperature, which will help to develop new materials with improved erosion performance at elevated temperature. It has been demonstrated that the erosion experimental factors (impact velocity, impingement angle, erodent size, erosion temperature, etc.) significantly affect the erosion behaviors of materials [4,5]. The main erosion mechanisms have been studied using micro-cutting model and elasticeplastic fracture mechanics. In the case of ductile material, it involves series of plastic deformation in which the material loss caused by the displacing or cutting action of eroding particle. In contrast, in a brittle material, the model of material loss is the formation and intersection of cracks that cause grain ejection from the surface of the tested material [1,6e12]. Alumina (Al2O3) ceramics, one of the most important engineering oxide ceramics, have the widest range of applications, due to low cost, high hardness and high erosion resistance. The effects of different parameters on the erosion behavior of alumina ceramics have been previously studied [1,2,12e15]. It was reported that the erosion rate of sintered alumina decreases from 23 C to 1000 C, owing to the change from brittle erosion at 23 C to ductile erosion at 1000 C [16]. Zhou and Bahadur [1] have also found that elevated erosion temperatures resulted in a large increase in erosion rate. Note that, the erosion temperature investigated so far in alumina ceramics has been limited to up to 1100 C. However, the working temperature of many oxidation furnaces is usually above 1600 C in the high-temperature chemical industry. Therefore, it is apparent that a big gap exists for the understanding of the influence of temperature on erosion behavior and mechanisms at elevated temperature between industrial application and experimental observations. More studies are needed to further understand the role of temperature in erosion at elevated temperatures. In this work, we detailed investigated the influence of temperature up to 1400 C (the first time with erosion temperature higher than 1100 C) and other factors on the erosion behavior of alumina ceramics at elevated temperatures. Our goals were to expose the erosion morphologies and to reveal the major erosion mechanisms at elevated temperatures. 2. Experimental Alumina ceramics was chosen as the target material of solid particle erosion experiments. Alumina powder (Aluminum Corporation of China Limited) with grain size of 3e5 mm and purity of 99.99% was shaped in a cylindrical die with 50 mm in diameter under uniaxial pressure of 50 MPa. The preformed green samples were then isostatic cold pressed under 200 MPa. Samples were firstly heated to 1670 C with a heating rate of 10 C min1, cooling to a lower temperature of 1620 C with a cooling rate of 35 C min1 and held for 10 h. The relative density of sintered samples was 95% measured by Archimedes method. The erosion tests were carried out using an experimentally designed solid particle erosion machine (up to 1400 C) in accordance with ASTM G76-04 [17], as shown in Fig. 1. The erosion particles are accelerated by high velocity air stream down to the surface of targets. There is a gap between two tubes (3 and 5) which will dramatically reduce the change in the furnace caused by cold air flow especially at high temperature. This is because a balance of air pressure will be formed when the pressure inside of furnace is equal to the sealed chamber. So it is needed to switch the compressed air on about 30 s to stabilize the temperature before the erosion particles insert into the erodent feeder. It is also essential to pre-heat the targets to minimize the effect of thermal shock on the target materials
Fig. 1. The schematic diagram of high-temperature erosion testing machine (1 e gas inlet; 2 e erodent feeder; 3 e accelerating tube; 4 e sealed cabin; 5 e alumina nozzle; 6 e furnace chamber; 7 e sample).
especially for high temperature. As described in our previous work [3], erosion particles are accelerated in a high velocity air stream down along an accelerating tube (3: 10 mm in diameter) and alumina nozzle (5: 20 mm in diameter) to impact on the targets at different temperatures. The feeding rate of erosion particles was 60 g min1. Each test was set to about 5 min, during which the target surface would be impacted by about 300 g of erosion particles. The impingement angles of erosion particle stream on the targets were 30 , 45 , 60 , 75 and 90 . The impact velocity was 50 m s1 measured by the rotating double-disc method [18]. Two different particles (corundum and SiC) with a size range of 375e550 mm were used as erosion particles. The erosion performance of samples was assessed using volume erosion rate which was defined as the volume loss of specimen material divided by the total mass of erosion particles (mm3/g), i.e. E ¼ (M1 M2)/mr, where M1 M2 is the average mass loss of samples after erosion, m is the mass of erosion particles, and r is the density of samples. The morphologies of erosion surfaces were characterized by optical microscopy and scanning electron microscopy (SEM) (JEOL JSM 6460LV). The bending strength and fracture were tested using conventional three-point bending and single-edge notched beam methods, respectively. 3. Results and discussion 3.1. Erosion behaviors Fig. 2 shows the erosion rate of alumina ceramics under corundum grits and SiC particles at different temperatures and impingement angle of 90 . As seen from Fig. 2, the erosion rate of samples eroded by corundum grits increases slowly from room
Fig. 2. Variation of erosion rate as a function of temperature at impingement angle of 90 .
X. Wang et al. / Materials Chemistry and Physics 139 (2013) 765e769
Fig. 3. Variation of erosion rate as a function of the impingement angle with different temperatures.
temperature to 800 C. The effect of temperature on erosion is apparent above 800 C and the erosion rate increases rapidly with increasing temperatures. The maximum erosion rate is 1.30 mm3 g1 at 1400 C. The trend of erosion rate eroded by SiC particles is similar with that eroded by corundum. However, the harder SiC particles lead to more severe erosion damage of alumina ceramics (2.08 mm3 g1 at 1400 C), compared with corundum grits. It has been reported that the yield strength of sintered alumina decreases with increasing temperature. The drop in hardness at high temperature occurs even more rapidly. The impact strength of sintered alumina also slightly reduces with increasing temperature but significantly drops above 800 C [19]. The decreases in the strength and hardness of alumina ceramics with increasing temperature lead to more damage of samples, especially for the temperature above 800 C. Fig. 3 shows the variation of volume erosion rate as a function of the impingement angle from room temperature to 1400 C. As seen
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in the figure, the erosion rate is strongly dependent on the impingement angle. The erosion rate slightly increases with the increasing impingement angle at room temperature and 800 C. However, the erosion rate increases significantly with the increasing impingement angle at 1200 C and 1400 C. The erosion rate reaches its highest value at 75 for 1200 C, which is more than twice of that at 30 . The maximum erosion rate occurs at 60 for 1400 C. Many literature on erosion have reported that materials are broadly classified as ductile or brittle, based on the dependence of their erosion rate. Ductile materials, such as pure metals, have a maximum erosion rate at low angles of incidence (typically 15e 30 ), while the maximum impact erosion rate usually occurs at 90 for brittle materials [20]. Due to the typical brittle characteristics of ceramic materials, maximum erosion rate would be expected to occur at normal impact (i.e. 90 ). However, some researchers [1,3,12,21e23] have found that maximum erosion would occur at lower impact angles with increasing temperature. In our present work, the brittle alumina ceramics have the maximum erosion rate at an impingement angle of 90 when eroded at room temperature and 800 C, which is well consistent with those results in the literature. However, with further increasing the erosion temperature, the maximum value no longer occurs at 90 , i.e. the erosion rate reaches its highest value at impingement angles of 75 and 60 for 1200 C and 1400 C, respectively. 3.2. Erosion mechanisms Fig. 4 shows the surface morphologies of samples eroded at different temperatures with corundum particles and impingement angle of 90 . After erosion tests, different surface morphologies are exposed due to variant erosion temperature. As shown in Fig. 4(a), the surface of sample eroded only presents some small scattered spots due to some original defects on the erosion sample surface. When the erosion temperature is above 600 C (Fig. 4(b)e(f)), the surface morphology of eroded samples shows obvious erosion pits
Fig. 4. Surface morphologies of samples eroded at different temperatures with corundum grits and impingement angle of 90 : (a) RT, (b) 600 C, (c) 800 C, (d) 1000 C, (e) 1200 C, and (f) 1400 C.
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Fig. 5. SEM micrographs of alumina surfaces eroded at room temperature with corundum grits at impingement angle of 90 .
with the diameter of 30 mm. As seen in the Fig. 4(e) and (f), another steeper pit appears at the center of former pit with elevated temperatures of 1200 C and 1400 C. Fig. 5 presents the SEM microstructure of a typical impact pit with corundum grits at room temperature and impingement angle of 90 . The erosion scar shows the typical brittle failure pattern of indentation friction with the grains exposed on the erosion surface. Some work [8,9] has indicated radial and lateral cracks occur when the brittle materials are eroded by hard particles. In general, the radial crack decreases the strength of the target, while the lateral crack leads to the material removal. The erosion pit is typical in high-energy impact on brittle materials and has been described from quasi-static indentation studies [24e27]. The impact pits formed by continuous expansion of the radialelateral
crack systems on and around the site. It can be clearly seen that plastic indentation region formed due to the impact of erosion particles on grain surfaces, and then radialelateral crack systems generated by repetitive impact. Bruce [28] has also indicated that impact by small hard projectiles at high temperature can cause cracks to form near the impact site. The impact induces a residual tensile stress which provides the primary driving force for the crack configuration at final stage of material loss. Many of these cracks will gradually coalesce and lead to failure on and around the impact site [29]. Fig. 6 shows the overview and enlarged morphologies of eroded surface with the temperatures of 1200 C and 1400 C. As seen from the microstructural features of alumina ceramics after elevated temperature erosion, there is no obviously evidence of plastic deformation at the elevated temperatures of 1200 C and 1400 C. It can be clearly seen the typical brittle mechanism of transgranular cleavage and grain removal (Fig. 6(b)). Fig. 6(d) shows an erosion pit formed by the loss of a couple of grains due to intergranular cracking or chipping. The arrow indicates the cracks located a few microns below the erosion surface and generated by the accumulated impact stress and energy of the erodent particles, which will have a significant contribution to the dislocation of grains. Fig. 7 shows the transgranular cleavage at room temperature and 1200 C with corundum grits and impingement angle of 90 . It can be found that the brittle mechanism is still the dominant model of material removal at the elevated temperature for the high purity alumina ceramics. However, compared with the sharp edges at room temperature, the soften phenomenon can be clearly seen from the transgranular cleavage surface at 1200 C. This is owing to that the yield strength and hardness of sintered alumina decrease dramatically at the elevated temperature [19]. Nevertheless, there is still no sign of plastic deformation at elevated temperature of 1200 C, which is significantly different from the predicted plastic deformation at high temperature.
Fig. 6. Erosion morphologies of alumina ceramics eroded at impingement angle of 90 with temperature of 1200 C: (a) overview and (b) magnified, and 1400 C: (c) overview and (d) magnified.
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(2) The erosion rate increases slightly with increasing impingement angle and reaches peak at 90 for room temperature and 800 C. However, the maximum erosion rate took place at 75 and 60 for 1200 C and 1400 C, respectively. (3) There is no obviously evidence of plastic deformation at 1200 C and 1400 C. The material removal is mainly resulted from transgranular cleavage and grain removal. The brittle erosion mechanisms still dominate the material removal at elevated temperature. (4) The harder SiC particles lead to more severe erosion damage of alumina ceramics, compared with corundum grits. Acknowledgments The authors are grateful to National Natural Science Foundation of China (Grant nos. 50802091 and 50972134) and Fundamental Research Funds of the Central Universities for financial support (2011PY0168). Xiaojun Wang is supported by China Scholarship Council. Jingzhou Yang would like to thank the ECM Small Development Grant, University of Western Australia. References
Fig. 7. SEM micrographs of transgranular cleavage at: (a) room temperature and (b) 1200 C.
4. Conclusions The aim of the investigation is to analyze the erosion behaviors of alumina ceramics with different temperatures from room temperature to 1400 C and variant impact angles from 30 to 90 . The following observations and conclusions are made from the investigations: (1) The erosion rate of alumina ceramics is highly dependent on the erosion temperature, increasing slowly from room temperature to 800 C, then sharply rising above 800 C.
[1] J. Zhou, S. Bahadur, Wear 181e183 (1995) 178. [2] L. Curkovi c, I. Kumi c, K. Grilec, Ceram. Int. 37 (2011) 29. [3] J.-Z. Yang, M.-H. Fang, Z.-H. Huang, X.-Z. Hu, Y.-G. Liu, H.-R. Sun, J.-T. Huang, X.-C. Li, J. Eur. Ceram. Soc. 32 (2012) 283. [4] A. Patnaik, T.G. Mamatha, S. Biswas, P. Kumar, Mater. Des. 36 (2012) 511e521. [5] T.G. Mamatha, A. Patnaik, S. Biswas, P. Kumar, J. Eng. Tribol. 225 (2011) 1106e1120. [6] M.G. Gee, A. Gant, B. Roebuck, Wear 263 (2007) 137e148. [7] K.C. Goretta, A.J. Cunninghama, N. Chen, D. Singh, J.L. Routbort, R.G. Rateick Jr., Wear 262 (2007) 1056e1060. [8] A.G. Evans, J.C. Chesnutt, H. Nadler, Acta Metall. 24 (1976) 867. [9] A.G. Evans, T.R. Wilshaw, Acta Metall. 24 (1976) 939. [10] A.G. Evans, T.R. Wilshaw, J. Mater. Sci. 12 (1977) 97. [11] I. Finnie, Wear 19 (1972) 81. [12] Z. Jianren, S. Bahadur, Wear 162e164 (1993) 285. [13] J. Zhou, S. Bahadur, Wear 150 (1991) 343. [14] M. Wakuda, Y. Yamauchi, S. Kanzaki, J. Mater. Process. Technol. 132 (2003) 177. [15] K.R. Gopi, R. Nagarajan, S.S. Rao, S. Mandal, Wear 264 (2008) 211. [16] J.E. Ritter, L. Rosenfeld, K. Jakus, Wear 111 (1986) 335. [17] ASTMG76-04, Standard Test Method for Conducting Erosion Tests by Solid Particle Impingement Using Gas Jets. [18] A.W. Ruff, L.K. Ives, Wear 35 (1975) 195. [19] R. Morrell, Handbook of Properties of Technical & Engineering Ceramics, Her Majesty’s Stationary Office, London, 1985. [20] ASM International, Friction, Lubrication, and Wear Technology, tenth ed., ASM Handbooks, 1992. [21] G. Sundararajan, M. Roy, Tribol. Int. 30 (1997) 339. [22] I. Hussainova, J. Pirso, M. Antonov, K. Juhani, Wear 267 (2009) 1894. [23] K. Shimizu, Y. Xinba, S. Araya, Wear 271 (2011) 1357. [24] D.B. Marshall, J. Am. Ceram. Soc. 67 (1984) 57. [25] K. Bose, R.J.K. Wood, Wear 258 (2005) 366. [26] D.B. Marshall, B.R. Lawn, A.G. Evans, J. Am. Ceram. Soc. 65 (1982) 561. [27] Y. Ballout, J.A. Mathis, J.E. Talia, Wear 196 (1996) 263. [28] R.W. Bruce, Tribol. Trans. 41 (1998) 399. [29] A.G. Evans, D.R. Mumm, J.W. Hutchinson, G.H. Meier, F.S. Pettit, Prog. Mater. Sci. 46 (2001) 505.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Solid particle erosion of alumina ceramics at elevated temperature Xiaojun Wang a, b, Minghao Fang a, *, Lai-Chang Zhang c, Hao Ding a, Yan-Gai Liu a, Zhaohui Huang a, Shaoping Huang d, Jingzhou Yang b a
School of Materials Science and Technology, China University of Geosciences (Beijing), No. 29 Xueyuan Road, Haidian District, Beijing 100083, China School of Mechanical and Chemical Engineering, University of Western Australia, Perth, WA 6009, Australia c School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, WA 6027, Australia d Luoyang Precondar Instruments for Testing Refractoriness Co., Ltd, Luoyang 471039, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< The erosion rate of alumina ceramics increases slowly from room temperature to 800 C, then sharply rising above 800 C. < The maximum erosion rate took place at 75 and 60 for 1200 C and 1400 C. < There is no obviously evidence of plastic deformation at 1200 C and 1400 C. < The harder SiC particles lead to more severe erosion damage of alumina ceramics, compared with corundum grits.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2012 Received in revised form 18 October 2012 Accepted 12 February 2013
A big gap exists for the understanding of the influence of temperature on erosion behavior at elevated temperature between experimental observations (up to 1100 C) and high temperature industrial application (up to 1600 C). This work the first time investigated the effect of higher temperature on the erosion resistance and mechanism of alumina ceramics. The solid particle erosion behavior of high purity alumina ceramics has been studied at elevated temperatures up to 1400 C and different impingement angles (30 , 45 , 60 , 75 , and 90 ), using corundum and SiC particles as erodent. Erosion rate of alumina ceramics slowly increases from 0.32 mm3 g1 to 0.44 mm3 g1 below 800 C, and then significantly increases up to 1.30 mm3 g1 at 1400 C. With increasing the impingement angle, the erosion rate increases slightly and reaches maximum value at 90 from room temperature to 800 C. However, the maximum value of erosion rate occurs at 75 and 60 for 1200 C and 1400 C, respectively. The brittle erosion mechanisms still dominate the material removal at elevated temperature. The material removal is mainly resulted from transgranular cleavage and grain removal. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Ceramics Erosion Microstructure Crack
1. Introduction High temperature solid particle erosion, one of the main failure modes in materials, refers to the material loss caused by solid * Corresponding author. Tel./fax: þ86 10 8232 2186. E-mail address: [email protected] (M. Fang). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.02.029
particles impinging on a solid surface at elevated temperatures. This type of material damage has become major issue in many industrial systems, including metallurgical industry, cement industry, thermal power, garbage incineration, fluidized bed combustion systems and turbine engine, as well as engine parts, such as nozzles, shroud rings, and combustor components [1e3]. Due to the erosion by coal powders and limestone grits, the circulating
766
X. Wang et al. / Materials Chemistry and Physics 139 (2013) 765e769
fluidized bed (CFB) furnace usually has to be shut down for maintenance. High temperature erosion dramatically reduces the life and efficiency of such a device. Therefore, it is crucial to fully understand the process and mechanisms of erosion at elevated temperature, which will help to develop new materials with improved erosion performance at elevated temperature. It has been demonstrated that the erosion experimental factors (impact velocity, impingement angle, erodent size, erosion temperature, etc.) significantly affect the erosion behaviors of materials [4,5]. The main erosion mechanisms have been studied using micro-cutting model and elasticeplastic fracture mechanics. In the case of ductile material, it involves series of plastic deformation in which the material loss caused by the displacing or cutting action of eroding particle. In contrast, in a brittle material, the model of material loss is the formation and intersection of cracks that cause grain ejection from the surface of the tested material [1,6e12]. Alumina (Al2O3) ceramics, one of the most important engineering oxide ceramics, have the widest range of applications, due to low cost, high hardness and high erosion resistance. The effects of different parameters on the erosion behavior of alumina ceramics have been previously studied [1,2,12e15]. It was reported that the erosion rate of sintered alumina decreases from 23 C to 1000 C, owing to the change from brittle erosion at 23 C to ductile erosion at 1000 C [16]. Zhou and Bahadur [1] have also found that elevated erosion temperatures resulted in a large increase in erosion rate. Note that, the erosion temperature investigated so far in alumina ceramics has been limited to up to 1100 C. However, the working temperature of many oxidation furnaces is usually above 1600 C in the high-temperature chemical industry. Therefore, it is apparent that a big gap exists for the understanding of the influence of temperature on erosion behavior and mechanisms at elevated temperature between industrial application and experimental observations. More studies are needed to further understand the role of temperature in erosion at elevated temperatures. In this work, we detailed investigated the influence of temperature up to 1400 C (the first time with erosion temperature higher than 1100 C) and other factors on the erosion behavior of alumina ceramics at elevated temperatures. Our goals were to expose the erosion morphologies and to reveal the major erosion mechanisms at elevated temperatures. 2. Experimental Alumina ceramics was chosen as the target material of solid particle erosion experiments. Alumina powder (Aluminum Corporation of China Limited) with grain size of 3e5 mm and purity of 99.99% was shaped in a cylindrical die with 50 mm in diameter under uniaxial pressure of 50 MPa. The preformed green samples were then isostatic cold pressed under 200 MPa. Samples were firstly heated to 1670 C with a heating rate of 10 C min1, cooling to a lower temperature of 1620 C with a cooling rate of 35 C min1 and held for 10 h. The relative density of sintered samples was 95% measured by Archimedes method. The erosion tests were carried out using an experimentally designed solid particle erosion machine (up to 1400 C) in accordance with ASTM G76-04 [17], as shown in Fig. 1. The erosion particles are accelerated by high velocity air stream down to the surface of targets. There is a gap between two tubes (3 and 5) which will dramatically reduce the change in the furnace caused by cold air flow especially at high temperature. This is because a balance of air pressure will be formed when the pressure inside of furnace is equal to the sealed chamber. So it is needed to switch the compressed air on about 30 s to stabilize the temperature before the erosion particles insert into the erodent feeder. It is also essential to pre-heat the targets to minimize the effect of thermal shock on the target materials
Fig. 1. The schematic diagram of high-temperature erosion testing machine (1 e gas inlet; 2 e erodent feeder; 3 e accelerating tube; 4 e sealed cabin; 5 e alumina nozzle; 6 e furnace chamber; 7 e sample).
especially for high temperature. As described in our previous work [3], erosion particles are accelerated in a high velocity air stream down along an accelerating tube (3: 10 mm in diameter) and alumina nozzle (5: 20 mm in diameter) to impact on the targets at different temperatures. The feeding rate of erosion particles was 60 g min1. Each test was set to about 5 min, during which the target surface would be impacted by about 300 g of erosion particles. The impingement angles of erosion particle stream on the targets were 30 , 45 , 60 , 75 and 90 . The impact velocity was 50 m s1 measured by the rotating double-disc method [18]. Two different particles (corundum and SiC) with a size range of 375e550 mm were used as erosion particles. The erosion performance of samples was assessed using volume erosion rate which was defined as the volume loss of specimen material divided by the total mass of erosion particles (mm3/g), i.e. E ¼ (M1 M2)/mr, where M1 M2 is the average mass loss of samples after erosion, m is the mass of erosion particles, and r is the density of samples. The morphologies of erosion surfaces were characterized by optical microscopy and scanning electron microscopy (SEM) (JEOL JSM 6460LV). The bending strength and fracture were tested using conventional three-point bending and single-edge notched beam methods, respectively. 3. Results and discussion 3.1. Erosion behaviors Fig. 2 shows the erosion rate of alumina ceramics under corundum grits and SiC particles at different temperatures and impingement angle of 90 . As seen from Fig. 2, the erosion rate of samples eroded by corundum grits increases slowly from room
Fig. 2. Variation of erosion rate as a function of temperature at impingement angle of 90 .
X. Wang et al. / Materials Chemistry and Physics 139 (2013) 765e769
Fig. 3. Variation of erosion rate as a function of the impingement angle with different temperatures.
temperature to 800 C. The effect of temperature on erosion is apparent above 800 C and the erosion rate increases rapidly with increasing temperatures. The maximum erosion rate is 1.30 mm3 g1 at 1400 C. The trend of erosion rate eroded by SiC particles is similar with that eroded by corundum. However, the harder SiC particles lead to more severe erosion damage of alumina ceramics (2.08 mm3 g1 at 1400 C), compared with corundum grits. It has been reported that the yield strength of sintered alumina decreases with increasing temperature. The drop in hardness at high temperature occurs even more rapidly. The impact strength of sintered alumina also slightly reduces with increasing temperature but significantly drops above 800 C [19]. The decreases in the strength and hardness of alumina ceramics with increasing temperature lead to more damage of samples, especially for the temperature above 800 C. Fig. 3 shows the variation of volume erosion rate as a function of the impingement angle from room temperature to 1400 C. As seen
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in the figure, the erosion rate is strongly dependent on the impingement angle. The erosion rate slightly increases with the increasing impingement angle at room temperature and 800 C. However, the erosion rate increases significantly with the increasing impingement angle at 1200 C and 1400 C. The erosion rate reaches its highest value at 75 for 1200 C, which is more than twice of that at 30 . The maximum erosion rate occurs at 60 for 1400 C. Many literature on erosion have reported that materials are broadly classified as ductile or brittle, based on the dependence of their erosion rate. Ductile materials, such as pure metals, have a maximum erosion rate at low angles of incidence (typically 15e 30 ), while the maximum impact erosion rate usually occurs at 90 for brittle materials [20]. Due to the typical brittle characteristics of ceramic materials, maximum erosion rate would be expected to occur at normal impact (i.e. 90 ). However, some researchers [1,3,12,21e23] have found that maximum erosion would occur at lower impact angles with increasing temperature. In our present work, the brittle alumina ceramics have the maximum erosion rate at an impingement angle of 90 when eroded at room temperature and 800 C, which is well consistent with those results in the literature. However, with further increasing the erosion temperature, the maximum value no longer occurs at 90 , i.e. the erosion rate reaches its highest value at impingement angles of 75 and 60 for 1200 C and 1400 C, respectively. 3.2. Erosion mechanisms Fig. 4 shows the surface morphologies of samples eroded at different temperatures with corundum particles and impingement angle of 90 . After erosion tests, different surface morphologies are exposed due to variant erosion temperature. As shown in Fig. 4(a), the surface of sample eroded only presents some small scattered spots due to some original defects on the erosion sample surface. When the erosion temperature is above 600 C (Fig. 4(b)e(f)), the surface morphology of eroded samples shows obvious erosion pits
Fig. 4. Surface morphologies of samples eroded at different temperatures with corundum grits and impingement angle of 90 : (a) RT, (b) 600 C, (c) 800 C, (d) 1000 C, (e) 1200 C, and (f) 1400 C.
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Fig. 5. SEM micrographs of alumina surfaces eroded at room temperature with corundum grits at impingement angle of 90 .
with the diameter of 30 mm. As seen in the Fig. 4(e) and (f), another steeper pit appears at the center of former pit with elevated temperatures of 1200 C and 1400 C. Fig. 5 presents the SEM microstructure of a typical impact pit with corundum grits at room temperature and impingement angle of 90 . The erosion scar shows the typical brittle failure pattern of indentation friction with the grains exposed on the erosion surface. Some work [8,9] has indicated radial and lateral cracks occur when the brittle materials are eroded by hard particles. In general, the radial crack decreases the strength of the target, while the lateral crack leads to the material removal. The erosion pit is typical in high-energy impact on brittle materials and has been described from quasi-static indentation studies [24e27]. The impact pits formed by continuous expansion of the radialelateral
crack systems on and around the site. It can be clearly seen that plastic indentation region formed due to the impact of erosion particles on grain surfaces, and then radialelateral crack systems generated by repetitive impact. Bruce [28] has also indicated that impact by small hard projectiles at high temperature can cause cracks to form near the impact site. The impact induces a residual tensile stress which provides the primary driving force for the crack configuration at final stage of material loss. Many of these cracks will gradually coalesce and lead to failure on and around the impact site [29]. Fig. 6 shows the overview and enlarged morphologies of eroded surface with the temperatures of 1200 C and 1400 C. As seen from the microstructural features of alumina ceramics after elevated temperature erosion, there is no obviously evidence of plastic deformation at the elevated temperatures of 1200 C and 1400 C. It can be clearly seen the typical brittle mechanism of transgranular cleavage and grain removal (Fig. 6(b)). Fig. 6(d) shows an erosion pit formed by the loss of a couple of grains due to intergranular cracking or chipping. The arrow indicates the cracks located a few microns below the erosion surface and generated by the accumulated impact stress and energy of the erodent particles, which will have a significant contribution to the dislocation of grains. Fig. 7 shows the transgranular cleavage at room temperature and 1200 C with corundum grits and impingement angle of 90 . It can be found that the brittle mechanism is still the dominant model of material removal at the elevated temperature for the high purity alumina ceramics. However, compared with the sharp edges at room temperature, the soften phenomenon can be clearly seen from the transgranular cleavage surface at 1200 C. This is owing to that the yield strength and hardness of sintered alumina decrease dramatically at the elevated temperature [19]. Nevertheless, there is still no sign of plastic deformation at elevated temperature of 1200 C, which is significantly different from the predicted plastic deformation at high temperature.
Fig. 6. Erosion morphologies of alumina ceramics eroded at impingement angle of 90 with temperature of 1200 C: (a) overview and (b) magnified, and 1400 C: (c) overview and (d) magnified.
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(2) The erosion rate increases slightly with increasing impingement angle and reaches peak at 90 for room temperature and 800 C. However, the maximum erosion rate took place at 75 and 60 for 1200 C and 1400 C, respectively. (3) There is no obviously evidence of plastic deformation at 1200 C and 1400 C. The material removal is mainly resulted from transgranular cleavage and grain removal. The brittle erosion mechanisms still dominate the material removal at elevated temperature. (4) The harder SiC particles lead to more severe erosion damage of alumina ceramics, compared with corundum grits. Acknowledgments The authors are grateful to National Natural Science Foundation of China (Grant nos. 50802091 and 50972134) and Fundamental Research Funds of the Central Universities for financial support (2011PY0168). Xiaojun Wang is supported by China Scholarship Council. Jingzhou Yang would like to thank the ECM Small Development Grant, University of Western Australia. References
Fig. 7. SEM micrographs of transgranular cleavage at: (a) room temperature and (b) 1200 C.
4. Conclusions The aim of the investigation is to analyze the erosion behaviors of alumina ceramics with different temperatures from room temperature to 1400 C and variant impact angles from 30 to 90 . The following observations and conclusions are made from the investigations: (1) The erosion rate of alumina ceramics is highly dependent on the erosion temperature, increasing slowly from room temperature to 800 C, then sharply rising above 800 C.
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