Availableonlineatwww.sciencedirect.com ScienceDirect CERAMICS INTERNATIONAL ELSEVIER Ceramics International 36(2010)299-306 Layered structures in ceramic nozzles for improved erosion wear resistance in industrial coal-water-slurry boilers Deng Jianxin,, Yun Dongling, Zhou Houmin, Tan Yuanqian School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, PR China red 25 March 2009: received in revised form 22 June 2009; accepted 3 August 2009 Available online 22 September 2009 Abstract The nozzle is the most critical part in the coal-water-slurry(Cws) boilers. Ceramics being highly wear resistant have great potential as CWs nozzle materials. In this paper, Al,O/(W,Ti)C Al2O3/TiC layered ceramics (LNI, LN2, and LN3)with different thickness ratios among constituent layers were developed to be used as nozzles in Cws boilers. CwS burning tests in a boiler with these nozzles were carried out. The erosion wear behavior of the layered nozzles was investigated and compared with an unstressed reference nozzle(N5). Results showed that the layered ceramic nozzles exhibited an apparent increase in erosion wear resistance over the unstressed reference one. The mechanisms responsible were found to be that layered structure in the Cws nozzles can improve the hardness and fracture toughness of the external layer, and reduce temperature gradients and the thermal stresses at the exit of the nozzle during Cws burning processes. It is suggested that layered structures in ceramic nozzles is an effective way to improve the erosion wear resistance over the stress-free ceramic nozzles in industrial CwS boilers. C 2009 Elsevier Ltd and Techna Group S.r.L. All rights reserved. Keywords: Nozzles; Ceramic materials; Residual stress; Layered materials; Slurry erosion 1. ntroduction and higher thermal stress were found to be the main reason that caused the failure of the nozzle exit The nozzle is the most critical part in a coal-water-slurry Layered structures constituted by alternate layers with (CwS) burning system. During CWS burning processes, the different compositions can be properly designed to induce a nozzle is eroded continuously by the abrasive action of CwS, surface compressive residual stress [3-6]. The basic idea is to the working environmental temperature of nozzle can reach up couple material layers with different thermal expansion to 1000C, and there is a very high temperature gradient inside coefficients( CTE) so that residual stresses arise during cooling nozzle [1, 2]. Therefore, the nozzles in the cws boiler must from the sintering temperature Residual stresses arise from a have high hardness, and good erosion and thermal shock mismatch between the CTE, sintering rates and elastic resistance. In the author's previous studies [1, 2), several constants of the constituent phases and neighbouring layers ceramic composites were produced by hot-pressing for use in and the residual stress field depends on the geometry of the CWS nozzles. Detailed observations and analyses of the nozzle layered structure and on the thickness ratio among layers [7-9 wear surface have revealed that the primary wear mechanisms The design of layered ceramics has been proved to be a viable of the CwS ceramic nozzle exhibited thermal shock damage strategy to obtain significant increases of the fracture with chipping at the nozzle exit. Greater temperature gradient resistance, wear resistance, and tribological properties [ 10-12 The idea of layered structures was first introduced to the design of sand-blasting ceramic nozzles so as to form compressive residual stresses at the nozzle entry(or exit) hor at: School of Mechanical Engineering, Shandong region in fabrication process, which may partially counteract University, Jil the tensile stresses resulting from external loadings [13-15] Tel:+8653188 Results showed that layered structures in ceramic nozzle can E-mailaddress:jxdeng@sdu.edu.cn(J.Deng) induce an excess residual stress in the nozzle during fabrication. x1 2-8842/$36.00 2009 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.1016 1-ceramint.2009.09003
Layered structures in ceramic nozzles for improved erosion wear resistance in industrial coal-water-slurry boilers Deng Jianxina, *, Yun Donglinga , Zhou Houminb , Tan Yuanqiangb a School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China b School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, PR China Received 25 March 2009; received in revised form 22 June 2009; accepted 3 August 2009 Available online 22 September 2009 Abstract The nozzle is the most critical part in the coal-water-slurry (CWS) boilers. Ceramics being highly wear resistant have great potential as CWS nozzle materials. In this paper, Al2O3/(W,Ti)C + Al2O3/TiC layered ceramics (LN1, LN2, and LN3) with different thickness ratios among constituent layers were developed to be used as nozzles in CWS boilers. CWS burning tests in a boiler with these nozzles were carried out. The erosion wear behavior of the layered nozzles was investigated and compared with an unstressed reference nozzle (N5). Results showed that the layered ceramic nozzles exhibited an apparent increase in erosion wear resistance over the unstressed reference one. The mechanisms responsible were found to be that layered structure in the CWS nozzles can improve the hardness and fracture toughness of the external layer, and reduce the temperature gradients and the thermal stresses at the exit of the nozzle during CWS burning processes. It is suggested that layered structures in ceramic nozzles is an effective way to improve the erosion wear resistance over the stress-free ceramic nozzles in industrial CWS boilers. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Nozzles; Ceramic materials; Residual stress; Layered materials; Slurry erosion 1. Introduction The nozzle is the most critical part in a coal-water-slurry (CWS) burning system. During CWS burning processes, the nozzle is eroded continuously by the abrasive action of CWS, the working environmental temperature of nozzle can reach up to 1000 8C, and there is a very high temperature gradient inside nozzle [1,2]. Therefore, the nozzles in the CWS boiler must have high hardness, and good erosion and thermal shock resistance. In the author’s previous studies [1,2], several ceramic composites were produced by hot-pressing for use in CWS nozzles. Detailed observations and analyses of the nozzle wear surface have revealed that the primary wear mechanisms of the CWS ceramic nozzle exhibited thermal shock damage with chipping at the nozzle exit. Greater temperature gradient and higher thermal stress were found to be the main reason that caused the failure of the nozzle exit. Layered structures constituted by alternate layers with different compositions can be properly designed to induce a surface compressive residual stress [3–6]. The basic idea is to couple material layers with different thermal expansion coefficients (CTE) so that residual stresses arise during cooling from the sintering temperature. Residual stresses arise from a mismatch between the CTE, sintering rates and elastic constants of the constituent phases and neighbouring layers, and the residual stress field depends on the geometry of the layered structure and on the thickness ratio among layers [7–9]. The design of layered ceramics has been proved to be a viable strategy to obtain significant increases of the fracture resistance, wear resistance, and tribological properties [10–12]. The idea of layered structures was first introduced to the design of sand-blasting ceramic nozzles so as to form compressive residual stresses at the nozzle entry (or exit) region in fabrication process, which may partially counteract the tensile stresses resulting from external loadings [13–15]. Results showed that layered structures in ceramic nozzle can induce an excess residual stress in the nozzle during fabrication, www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 36 (2010) 299–306 * Corresponding author at: School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China. Tel.: +86 531 88392047. E-mail address: jxdeng@sdu.edu.cn (J. Deng). 0272-8842/$36.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.09.003
J. Deng et al. /Ceramics International 36(2010)299-306 and this residual stress is compressive whatever the sintering a three-layer symmetrical structure. The composition at the temperature. This kind of compressive residual stress can result nozzle entry or exit is Al2O3 /45 vol %(W,Ti)C, while the in an improved erosion wear resistance of the layered ceramic cor position at the nozzle center area is Al2O3/ 55 voL %oTiC. nozzle compared with the homogeneous stress-free one in sand- Three layered nozzles with different thickness ratio p(p=A,/ blasting processes [13-15]. A2)among constituent layers were produced. The layered I2O,/(W,TiC and Al2O,/TiC ceramics are widely used in nozzles with the thickness ratio of 0. 2, 0.5, and I are named industrial applications such as cutting tools and dies [16-18], LNl, LN2, and LN3, respectively(see Fig. 1) they both have high hardness and wear resistance. These two The starting powders used to fabricate the layered nozzles materials have different thermal expansion coefficients; and are listed in Table I with their physical properties. Composite different shrinkage during sintering. The thermal expansion powders of different mixture ratios were prepared by wet ball coefficient(CTEof AlO/(W, Ti)Cis 7. 2x 10 K, and the milling in alcohol with cemented carbide balls for 80h. CTEof Al2O/TiCis 80x 10 K [16-18]. These differences Following drying, the composite powders with different are sufficient to induce residual stresses in the laminated mixture ratios were layered into a graphite mould. The sample structures, and compressive residual stresses are induced in the was then hot-pressed in flowing nitrogen for 15 min at 1700C layers with lower CTE. As for Al2O3/(W,Ti)C +Al2O/TiC temperature with 30 MPa pressure. For the purpose of layered ceramic material, the CTE of the surface layer(Al2O3/ comparison, a homogeneous stress-free ceramic nozzle was (W,Ti)c) is lower than that of the center layer(Al2O3/TiC), so also manufactured by hot-pressing. This stress-free nozzle compressive residual stresses will be formed in the surface layer made from Al 2O3 /45 vol. (W,Ti)c is named N5 the layered materials during fabrication. In the present study, Al2O3/(W,TI)C Al2O3/TiC layered ceramics with different 2.2. Hardness and fracture toughness measurements ar thickness ratios among constituent layers were produced to be surface layer of the layered nozzle materials used as the Cws nozzles. The mechanical properties at the surface layers of the layered materials were measured, and the fracture toughness measurement was performed using cro-structure was examined. The wear behaviors of the layered indentation method at the nozzle external layer(entry or exit) ceramic nozzles were investigated and compared to an unstressed in a hardness tester(MH6) using the formula proposed by Cook reference nozzle. The purpose was to characterize the erosion and Lawn [19]. Hardness measurements were performed by wear of the layered ceramic nozzle in industrial CWS boilers. placing Vickers indentations on external layer of the layered nozzle material. The indentation load was 200n and a 2. Materials and experimental procedures minimum of five indentations were tested. The vickers hardness(GPa)is given by 2.1. Preparation of the Al20y(w,Ti)C Al203/iC layered ceramic nozzles P Hu=1.8544 The dimension and compositional distribution of the layered ceramic nozzles with different thickness ratios among where P is the indentation load (N), 2a is the catercorner lengt constituent layers are shown in Fig. 1. These nozzles possess (um) due to indentation. 10 mm Nozzle exit O3/(W,Ti)c Al2OJTIC Al2O3/W,Ti)C Nozzle ent (a) Fig 1. Compositional distribution of the layered ceramic nozzles with different thickness ratio among constituent layers: (a) NI nozzle(p= A / A2=0.2),(b) nozzle(p=0.5), and (c)N3 nozzle (p=1)
and this residual stress is compressive whatever the sintering temperature. This kind of compressive residual stress can result in an improved erosion wear resistance of the layered ceramic nozzle compared with the homogeneous stress-free one in sandblasting processes [13–15]. Al2O3/(W,Ti)C and Al2O3/TiC ceramics are widely used in industrial applications such as cutting tools and dies [16–18], they both have high hardness and wear resistance. These two materials have different thermal expansion coefficients; and different shrinkage during sintering. The thermal expansion coefficient (CTE) of Al2O3/(W,Ti)C is 7.2 106 K1 , and the CTE of Al2O3/TiC is 8.0 106 K1 [16–18]. These differences are sufficient to induce residual stresses in the laminated structures, and compressive residual stresses are induced in the layers with lower CTE. As for Al2O3/(W,Ti)C + Al2O3/TiC layered ceramic material, the CTE of the surface layer (Al2O3/ (W,Ti)C) is lower than that of the center layer (Al2O3/TiC), so compressive residual stresses will be formed in the surface layer of the layered materials during fabrication. In the present study, Al2O3/(W,Ti)C + Al2O3/TiC layered ceramics with different thickness ratios among constituent layers were produced to be used as the CWS nozzles. The mechanical properties at the surface layers of the layered materials were measured, and the micro-structure was examined. Thewear behaviors of the layered ceramic nozzles were investigated and compared to an unstressed reference nozzle. The purpose was to characterize the erosion wear of the layered ceramic nozzle in industrial CWS boilers. 2. Materials and experimental procedures 2.1. Preparation of the Al2O3/(W,Ti)C + Al2O3/TiC layered ceramic nozzles The dimension and compositional distribution of the layered ceramic nozzles with different thickness ratios among constituent layers are shown in Fig. 1. These nozzles possess a three-layer symmetrical structure. The composition at the nozzle entry or exit is Al2O3/45 vol.%(W,Ti)C, while the composition at the nozzle center area is Al2O3/55 vol.%TiC. Three layered nozzles with different thickness ratio p ( p = A1/ A2) among constituent layers were produced. The layered nozzles with the thickness ratio of 0.2, 0.5, and 1 are named LN1, LN2, and LN3, respectively (see Fig. 1). The starting powders used to fabricate the layered nozzles are listed in Table 1 with their physical properties. Composite powders of different mixture ratios were prepared by wet ball milling in alcohol with cemented carbide balls for 80 h. Following drying, the composite powders with different mixture ratios were layered into a graphite mould. The sample was then hot-pressed in flowing nitrogen for 15 min at 1700 8C temperature with 30 MPa pressure. For the purpose of comparison, a homogeneous stress-free ceramic nozzle was also manufactured by hot-pressing. This stress-free nozzle made from Al2O3/45 vol.%(W,Ti)C is named N5. 2.2. Hardness and fracture toughness measurements at surface layer of the layered nozzle materials Fracture toughness measurement was performed using indentation method at the nozzle external layer (entry or exit) in a hardness tester (MH 6) using the formula proposed by Cook and Lawn [19]. Hardness measurements were performed by placing Vickers indentations on external layer of the layered nozzle material. The indentation load was 200 N and a minimum of five indentations were tested. The Vickers hardness (GPa) is given by: Hv ¼ 1:8544 P ð2aÞ 2 (1) where P is the indentation load (N), 2a is the catercorner length (mm) due to indentation. Fig. 1. Compositional distribution of the layered ceramic nozzles with different thickness ratio among constituent layers: (a) N1 nozzle ( p = A1/A2 = 0.2), (b) N2 nozzle ( p = 0.5), and (c) N3 nozzle ( p = 1). 300 J. Deng et al. / Ceramics International 36 (2010) 299–306
J. Deng et al./Ceramics international 36(2010)299-306 Physical properties of Al2O3, TiC and (W,Ti)C Starting Density Youngs Thermal conductivity ons Particle Purity (%) Manufacture powder (g/cm) modulus(GPa) coefficient(10-6K-)W/(m K) size(um) Al2O33.98 8.0 Beijing Antai Advanced Tech and Mater. Co. Ltd. 4.93500 7.4 24.3 0.20 Zhuzhou cemented carbide works W,T)C9.56480 214 0.25 Zhuzhou cemented carbide works pro rties of the coal-water-slurry. Consistency (% Quantity of heat(M/kg) Ash(%) Sulphur(%) Volatility (%) Adhesiveness(MPa s) Grit number(μm) 18.81-2048 2 40-80 2.3. Coal-water-slurry burning tests CWS nozzles were examined by scanning electron microscopy (SEMD) Coal-water-slurry burning tests were conducted with an industrial Cws boiler. The properties of the CwS used in this 3. Results and discussion study are listed in Table 2. The schematic diagram and the photo of the boiler are shown in Fig. 2. The Cws drawn by 3. 1. Surface layer properties and microstructural pump passed through pipeline, accelerated and mixed in the characterization of the layered nozzle materials pray-gun(Fig 3) by gas stream(commonly compressed air). The compressed air pressure was set at 0. 4 MPa, and CwS The results of hardness and fracture toughness of the layered pressure was set at 0.2 MPa. The nozzles have an internal nozzles with different thickness ratios are presented in Table 3 diameter 5.0 mm, external diameter 12.0 mm, and length It is indicated that the hardness and fracture toughness decrease 10.0 mm. Photo of a layered ceramic nozzle is shown in Fig 4. gradually from LNI nozzle to N5 nozzle. The external layer of The mass loss of the worn layered nozzles was measured the nozzle with low thickness ratio shows the highest hardness with an accurate electric balance(minimum 0.01 mg). The and fracture toughness. By comparison with the stress-free erosion rates(W) of the layered nozzles are defined as the nozzle (N5), the hardness at the external layer of the layered nozzle mass loss(m1) divided by the nozzle density(d)times nozzles is much more improved, and rose from 19.9 GPa for N5 the burn mass of Cws(m2) stress-free nozzle to 21.6 GPa for LNI layered nozzle, representing a maximum increase of 1.7 GPa. The fracture (2) toughness rose from 4.9 MPa m for N5 stress-free nozzle to 9.8 MPa m"for LNI layered nozzle, representing a maximum where W has the units of volume loss per unit mass of Cws increase of 4.9 MPa m".As reported in Ref. [20], layered structures in ceramic material can induce an excess compres Finite element method(FEM) was used as a means of sive residual stress at the nozzle external layer during numerically evaluating the temperature gradient and thermal fabrication. These compressive residual stresses are beneficial stresses inside the cws nozzle. The eroded wall surfaces of the for the increase of hardness and the fracture toughness at the O 50 mm ure gauge Air compressor CwS storage Pressure gauge Fig. 2. Schematic diagram and photo of the industry coal-water-slurry boiler
2.3. Coal-water-slurry burning tests Coal-water-slurry burning tests were conducted with an industrial CWS boiler. The properties of the CWS used in this study are listed in Table 2. The schematic diagram and the photo of the boiler are shown in Fig. 2. The CWS drawn by pump passed through pipeline, accelerated and mixed in the spray-gun (Fig. 3) by gas stream (commonly compressed air). The compressed air pressure was set at 0.4 MPa, and CWS pressure was set at 0.2 MPa. The nozzles have an internal diameter 5.0 mm, external diameter 12.0 mm, and length 10.0 mm. Photo of a layered ceramic nozzle is shown in Fig. 4. The mass loss of the worn layered nozzles was measured with an accurate electric balance (minimum 0.01 mg). The erosion rates (W) of the layered nozzles are defined as the nozzle mass loss (m1) divided by the nozzle density (d) times the burn mass of CWS (m2). W ¼ m1 d m2 (2) where W has the units of volume loss per unit mass of CWS (mm3 /kg). Finite element method (FEM) was used as a means of numerically evaluating the temperature gradient and thermal stresses inside the CWS nozzle. The eroded wall surfaces of the CWS nozzles were examined by scanning electron microscopy (SEM). 3. Results and discussion 3.1. Surface layer properties and microstructural characterization of the layered nozzle materials The results of hardness and fracture toughness of the layered nozzles with different thickness ratios are presented in Table 3. It is indicated that the hardness and fracture toughness decrease gradually from LN1 nozzle to N5 nozzle. The external layer of the nozzle with low thickness ratio shows the highest hardness and fracture toughness. By comparison with the stress-free nozzle (N5), the hardness at the external layer of the layered nozzles is much more improved, and rose from 19.9 GPa for N5 stress-free nozzle to 21.6 GPa for LN1 layered nozzle, representing a maximum increase of 1.7 GPa. The fracture toughness rose from 4.9 MPa m1/2 for N5 stress-free nozzle to 9.8 MPa m1/2 for LN1 layered nozzle, representing a maximum increase of 4.9 MPa m1/2. As reported in Ref. [20], layered structures in ceramic material can induce an excess compressive residual stress at the nozzle external layer during fabrication. These compressive residual stresses are beneficial for the increase of hardness and the fracture toughness at the Table 1 Physical properties of Al2O3, TiC and (W,Ti)C. Starting powder Density (g/cm3 ) Young’s modulus (GPa) Thermal expansion coefficient (106 K1 ) Thermal conductivity W/(m K) Poisson’s ratio Particle size (mm) Purity (%) Manufacture Al2O3 3.98 380 8.0 30.2 0.27 1–2 >99 Beijing Antai Advanced Tech. and Mater. Co., Ltd. TiC 4.93 500 7.4 24.3 0.20 1–2 >99 Zhuzhou cemented carbide works (W,Ti)C 9.56 480 8.5 21.4 0.25 1–2 >99 Zhuzhou cemented carbide works Table 2 Properties of the coal-water-slurry. Consistency (%) Quantity of heat (MJ/kg) Ash (%) Sulphur (%) Volatility (%) Adhesiveness (MPa s) Grit number (mm) 65 2 18.81–20.48 15 1000–2500 40–80 Fig. 2. Schematic diagram and photo of the industry coal-water-slurry boiler. J. Deng et al. / Ceramics International 36 (2010) 299–306 301
J. Deng et al. /Ceramics International 36(2010)299-306 Table 3 Hardness and fracture toughness at the extermal layer(entry or exit) of the layered ceramic nozzles with different thickness ratios among constituer lay Coal-water-slurry Code name Thickness ratio Hardness Fracture toughness P=AlA LNI layered nozzle 21.6±1.298±0.6 LN2 layered nozzle 0.5 21.1土1.184±0.5 Compressed air entry LN3 layered nozzle I 20.9±1.17.3±0.5 N5 stress-free nozzle 199±1.049±0.5 Fig. 3. Structure of the spray-gun. hole diameter variation of n5 stress-free nozzle is much higher than those of the LNl, LN2, and LN3 layered nozzles under the same test conditions and the hole diameter variation was also infuenced by the thickness ratio among constituent layers. The LNI nozzle with thickness ratio of 0.2 between adjacent layers had the smallest hole diameter variation among the layered The comparison of the erosion rates of the nozzles in Cws burning processes is shown in Fig. 7. It is obvious that the erosion rates of N5 stress-free nozzles are much higher than that of the layered nozzles, and the erosion rates of the nozzles from low to high is LNl, LN2, LN3, and N5 in turn. The LNi layere 10 mm nozzle with thickness ratio of 0.2 exhibited the highest erosion wear resistance g. 4. Photo of the layered ceramic nozzles. 3.3. Worn surface of the layered nozzles external layer, which is also reported by other researchers [4- The exit hole profiles of the CwS nozzles after 120 h operation are shown in Fig. 8. It was found that the exit of N5 The SEM micrographs section surface of the stress -free nozzles exhibits fracture marks. for determination LN3 layered nozzle material are in Fig. 5. A layered of erosion mechanisms, the worn CWS nozzles were sectioned structure can be clearly seen axially. Fig 9 shows the wall surface profiles of the worn CWs nozzles after 120 h operation. It is obvious that the hole 3. 2. Hole diameter variation and erosion rate of the diameter of N5 stress-free nozzles enlarges greatly compared avered ceramic nozzle with that of the layered nozzles Characteristic SEM pictures taken on the nozzle wall surface Fig 6 shows the comparison of hole diameter variation of of the LNI and N5 nozzles after 120 h operation are showed the CWS nozzles after 120 h operation. It is indicated that the Fig. 10. The wall surface of the LNI layered CwS nozzle is Center layer Exit laye Fig. 5. SEM micrographs of the cross-section surface of the LN3 layered nozzle material
external layer, which is also reported by other researchers [4– 6]. The SEM micrographs of the cross-section surface of the LN3 layered nozzle material are shown in Fig. 5. A layered structure can be clearly seen. 3.2. Hole diameter variation and erosion rate of the layered ceramic nozzles Fig. 6 shows the comparison of hole diameter variation of the CWS nozzles after 120 h operation. It is indicated that the hole diameter variation of N5 stress-free nozzle is much higher than those of the LN1, LN2, and LN3 layered nozzles under the same test conditions, and the hole diameter variation was also influenced by the thickness ratio among constituent layers. The LN1 nozzle with thickness ratio of 0.2 between adjacent layers had the smallest hole diameter variation among the layered nozzles. The comparison of the erosion rates of the nozzles in CWS burning processes is shown in Fig. 7. It is obvious that the erosion rates of N5 stress-free nozzles are much higher than that of the layered nozzles, and the erosion rates of the nozzles from low to high is LN1, LN2, LN3, and N5 in turn. The LN1 layered nozzle with thickness ratio of 0.2 exhibited the highest erosion wear resistance. 3.3. Worn surface of the layered nozzles The exit hole profiles of the CWS nozzles after 120 h operation are shown in Fig. 8. It was found that the exit of N5 stress-free nozzles exhibits fracture marks. For determination of erosion mechanisms, the worn CWS nozzles were sectioned axially. Fig. 9 shows the wall surface profiles of the worn CWS nozzles after 120 h operation. It is obvious that the hole diameter of N5 stress-free nozzles enlarges greatly compared with that of the layered nozzles. Characteristic SEM pictures taken on the nozzle wall surface of the LN1 and N5 nozzles after 120 h operation are showed in Fig. 10. The wall surface of the LN1 layered CWS nozzle is Fig. 4. Photo of the layered ceramic nozzles. Fig. 3. Structure of the spray-gun. Table 3 Hardness and fracture toughness at the external layer (entry or exit) of the layered ceramic nozzles with different thickness ratios among constituent layers. Code name Thickness ratio P=A1/A2 Hardness (GPa) Fracture toughness (MPa m1/2) LN1 layered nozzle 0.2 21.6 1.2 9.8 0.6 LN2 layered nozzle 0.5 21.1 1.1 8.4 0.5 LN3 layered nozzle 1 20.9 1.1 7.3 0.5 N5 stress-free nozzle 19.9 1.0 4.9 0.5 Fig. 5. SEM micrographs of the cross-section surface of the LN3 layered nozzle material. 302 J. Deng et al. / Ceramics International 36 (2010) 299–306
J. Deng et al./Ceramics international 36(2010)299-306 0.5 ON5 stress-free nozzle ON5 stress-free nozzle E5 ■LN3 layered nozzle ■LN3 layered nozze 口LN2 layered nozzle □LN2 nozzle 口 LNI layered nozzle 二 口LN 0.2 Nozzles Fye d Come ia t he bed rt aiai of e L:. LN, LN Flg ns ofte o le o e o NN N N uy ed ovales relatively smooth in contrast to that of the N5 stress-free nozzle. 3.4. Discussion From Fig. 10d, the micro-structure can be seen clearly. In this structure,the"white"phase with clear contrast is(W,TiC, and When the erosive particles hit the target at low angles such as the grey phase is Al2O3. No sign of plowing was found both at the nozzle wall surface in CwS burning processes, most of the the wall surface of lnl nozzle CwS particles traveled almost parallel to the nozzle wall, and Fig 8. Exit hole profiles of the worn CWS nozzles after 120 h operation: (a) LNI layered nozzle, (b)LN2 layered nozzle, (c)LN3 layered nozzle, and(d) N5 stress-
relatively smooth in contrast to that of the N5 stress-free nozzle. From Fig. 10d, the micro-structure can be seen clearly. In this structure, the ‘‘white’’ phase with clear contrast is (W,Ti)C, and the grey phase is Al2O3. No sign of plowing was found both at the wall surface of LN1 nozzle. 3.4. Discussion When the erosive particles hit the target at low angles such as the nozzle wall surface in CWS burning processes, most of the CWS particles traveled almost parallel to the nozzle wall, and Fig. 7. Comparison of the erosion rates of the LN1, LN2, LN3 layered nozzles and N5 stress-free nozzle. Fig. 6. Comparison of the hole diameter variation of the LN1, LN2, LN3 layered nozzles and N5 stress-free nozzle. Fig. 8. Exit hole profiles of the worn CWS nozzles after 120 h operation: (a) LN1 layered nozzle, (b) LN2 layered nozzle, (c) LN3 layered nozzle, and (d) N5 stressfree nozzle. J. Deng et al. / Ceramics International 36 (2010) 299–306 303
J. Deng et al. /Ceramics International 36(2010)299-306 Fig 9. Wall surface profiles of the worn CwS nozzles after 120 h operation: (a) N5 stress-free nozzle, ( b)INI layered nozzle, and (c) LN3 layered nozzle. (d) Fig. 10. SEM micrographs of the wall surface of the CwS nozzles after 120 h operation:(a)wall surface of N5 stress-free nozzle, (b)enlarged SEM micrograph corresponding to(a),(c)wall surface of LNI layered nozzle, and(d)enlarged SEM micrograph corresponding to(c) impact the wall surface of the nozzle at low angles. As resistance of the layered nozzles may be explained by their high erodent particles in CWS were much softer than the ceran surface hardness(see Table 3) nozzles, sliding abrasive particles in the CwS can act as a The damage of ceramics subjected to temperature gradient polishing medium on the nozzle wall surface. Therefore, the environments is another major limiting factor in relation to eroded wall surface of the nozzle is very smooth(see Fig. 10), service requirements and lifetime performance. Repetitive and the hardness of the CWs nozzles plays an important role thermal shock also results in thermal fatigue hich has a with respect to their erosive wear. The ceramic nozzles with significant effect on the life of the Cws nozzles. For high hardness had smaller erosion rates. The high erosion determination of the thermal stresses inside the nozzle when
impact the wall surface of the nozzle at low angles. As the erodent particles in CWS were much softer than the ceramic nozzles, sliding abrasive particles in the CWS can act as a polishing medium on the nozzle wall surface. Therefore, the eroded wall surface of the nozzle is very smooth (see Fig. 10), and the hardness of the CWS nozzles plays an important role with respect to their erosive wear. The ceramic nozzles with high hardness had smaller erosion rates. The high erosion resistance of the layered nozzles may be explained by their high surface hardness (see Table 3). The damage of ceramics subjected to temperature gradient environments is another major limiting factor in relation to service requirements and lifetime performance. Repetitive thermal shock also results in thermal fatigue, which has a significant effect on the life of the CWS nozzles. For determination of the thermal stresses inside the nozzle when Fig. 10. SEM micrographs of the wall surface of the CWS nozzles after 120 h operation: (a) wall surface of N5 stress-free nozzle, (b) enlarged SEM micrograph corresponding to (a), (c) wall surface of LN1 layered nozzle, and (d) enlarged SEM micrograph corresponding to (c). Fig. 9. Wall surface profiles of the worn CWS nozzles after 120 h operation: (a) N5 stress-free nozzle, (b) LN1 layered nozzle, and (c) LN3 layered nozzle. 304 J. Deng et al. / Ceramics International 36 (2010) 299–306
J. Deng et al./Ceramics international 36(2010)299-306 329.947 150.509 28.929 208368 Fig. 14. Thermal stresses in the LN3 layered nozzle (MPa). Fig. 11. Finite element method gridding model of the nozzle. 口 N5 stress-free nozzle ■LN3 layered nozzle 日 LNI layered nozzle 362449 8013 11838 5663 Fig. 15. Comparison of maximum thermal stress at the exit in different nozzles 2275 13751 (MPa) Fig. 12. Temperature gradients in the LN3 layered nozzle (C/m). The temperature gradient in LN3 layered nozzle is shown in Fig. 12. It was found that there is greater temperature gradient used in a Cws boiler, three-dimensional finite element method inside the CWS nozzles, and the highest temperature gradient is (FEM) was used. Owing to the symmetry, an axisymmetric 17576C/, and is located at the nozzle exit. Fig 13 shows the calculation was preferred and steady state boundary conditions comparison of maximum temperature gradient of LNl, LN2, were invoked. Details on the FEM model approach and the LN3, layered nozzles and N5 stress-free nozzle at the exit. It is boundary conditions employed are described in Refs. [1, 21]. indicated that the maximum temperature gradient of N5 stress- The materials constants are listed in Table 1. Fig. 1 l shows the free nozzle is much higher than that of the Lnl, lN2, and LN3 FEM gridding model of the nozzle. layered nozzles. Fig. 14 shows the thermal stress distribution in LN3 layered 18500 N5 stress-free nozzle nozzle. As can be seen. there are higher thermal stresses inside the ■LN3 layered nozzle CWS nozzles, and the highest thermal stress is located on the exit 日LN2 layered nozzle of the nozzle and the maximum value is 477. 525 MPa. Fig. 15 E 18000 DLNI layered nozzle shows the comparison of thermal stress of LNl, LN2, and LN3 <6 ered nozzles and N5 stress-free nozzle at the exit. Itis indicated As calculated above, there are higher temperature gradient 17500 (Figs. 12 and 13)at the exit of the nozzle, which lead to large hermal stresses(Figs. 14 and 15) during the Cws burning processes. These thermal stresses can lower the fracture strength of the ceramic material, which, in some cases, may be sufficient to cause considerable damage or even catastrophic Nozzles failure of cws ceramic nozzles Fg. S 13. Comparison of maximum temperature gradients at the exit in different There may be several factors that affect the erosive wear les (C/m) resistance of the cws nozzles One is the hardness of the cws
used in a CWS boiler, three-dimensional finite element method (FEM) was used. Owing to the symmetry, an axisymmetric calculation was preferred and steady state boundary conditions were invoked. Details on the FEM model approach and the boundary conditions employed are described in Refs. [1,21]. The materials constants are listed in Table 1. Fig. 11 shows the FEM gridding model of the nozzle. The temperature gradient in LN3 layered nozzle is shown in Fig. 12. It was found that there is greater temperature gradient inside the CWS nozzles, and the highest temperature gradient is 17576 8C/m, and is located at the nozzle exit. Fig. 13 shows the comparison of maximum temperature gradient of LN1, LN2, LN3, layered nozzles and N5 stress-free nozzle at the exit. It is indicated that the maximum temperature gradient of N5 stressfree nozzle is much higher than that of the LN1, LN2, and LN3 layered nozzles. Fig. 14 shows the thermal stress distribution in LN3 layered nozzle. As can be seen, there are higher thermal stresses inside the CWS nozzles, and the highest thermal stress is located on the exit of the nozzle, and the maximum value is 477.525 MPa. Fig. 15 shows the comparison of thermal stress of LN1, LN2, and LN3 layered nozzles and N5 stress-free nozzle at the exit. It is indicated that the layered nozzles possess lower maximum thermal stresses. As calculated above, there are higher temperature gradient (Figs. 12 and 13) at the exit of the nozzle, which lead to large thermal stresses (Figs. 14 and 15) during the CWS burning processes. These thermal stresses can lower the fracture strength of the ceramic material, which, in some cases, may be sufficient to cause considerable damage or even catastrophic failure of CWS ceramic nozzles. There may be several factors that affect the erosive wear resistance of the CWS nozzles. One is the hardness of the CWS Fig. 12. Temperature gradients in the LN3 layered nozzle (8C/m). Fig. 13. Comparison of maximum temperature gradients at the exit in different nozzles (8C/m). Fig. 15. Comparison of maximum thermal stress at the exit in different nozzles (MPa). Fig. 11. Finite element method gridding model of the nozzle. Fig. 14. Thermal stresses in the LN3 layered nozzle (MPa). J. Deng et al. / Ceramics International 36 (2010) 299–306 305
J. Deng et al. /Ceramics international 36(2010)299-306 nozzles. and the other is the thermal stress inside the cws [2] Z Ding. J. Deng, J. Li, Wear behavior of ceramic nozzles in coal water nozzles. Once the CwS ceramic nozzle possesses a layered slurry burning, Ceramics Intemational 34(2004)591-596 structure, the hardness and fracture toughness at the nozzle [3] R. Lakshminarayanan, D K Shetty, R.A. Cutler, Toughening of layered xternal layer are improved(see Table 3), and the temperature eramic composites with residual surface compression, Journal of Amer- ican Ceramic Society 79(1996)79-87 gradients and the thermal stresses at the nozzle exit can be greatly [4] P.Z. Cai, DJ. Green, G L Messing, Mechanical characterization of ALO3/ reduced due to the different thermal expansion coefficients of the ZrO2 hybrid laminates, Journal of the European Ceramic Society 5(1998) individual layers during Cws burning processes. Therefore, 025-2034. layered ceramic nozzles exhibited an apparent increase [5] G. de Portu, L. Micele, G. Pezzotti, Laminated ceramic structures from xide systems, Composites: Part B 37(2006)556-567 erosion wear resistance over the unstressed reference one, the [6] V.M. Sglavo, M. Bertoldi, Design and production of ceramic I mechanisms responsible were found to be that layered structure with high mechanical resistance and reliability, Acta Materialia s in the CWS nozzles can improve the hardness and fracture 4929-4937 toughness of the external layer, and reduce the temperature [7] S. Guicciardi, M. Nagliati, C. Melandri, G. Pezzotti, D. Sciti, Effects gradients and the thermal stresses at the exit of the nozzle residual stresses on the fracture properties of non-oxide laminated omposites, Journal of the European Ceramic Society 27(2007)351 4. Conclusions [8] H. Tomaszewski, H. Weglarz, A. Wailer, M. Boniecki, D. Kalin Multilayer ceramic composites with high failure resistance, Journal of Al2O/W,Ti)C Al2O3/TiC layered ceramics with different the European Ceramic Society 27(2007)1373-1377 thickness ratios among constituent layers were developed to be [9] V. Sergo, D.M. Lipkin, G. De Portu, D.R. Clarke, Edge stresses in alumina/zirconia laminates, Journal of American Ceramic Society 80 used as nozzles in CWs boilers. The performance and wear 997)1633-1638 characteristics of these nozzles were investigated. The [10]G De Portu, L. Micele, D. Prandstraller, G. Palombarini, G. Pezzotti, following conclusions were obtained: brasive wear in ceramic laminated composites, Wear 260(2006)1104- The layered ceramic nozzles (LNl, LN2, and exhibited an apparent increase in erosion wear resistand [11] A. Tarlazzi, E. Roncari, P. Pinasco. S. Guicciardi, C. Melandri, G. de the unstressed reference nozzle(n5)in industry CWs I Portu, Tribological behaviour of Al2O,/ZrOr-ZrO2 laminated composites, wear244(2000)29-40. The mechanisms responsible were found to be that layered [12] M Hadad, G. Bluga bler. E. Rosset. L. Rohr. J. Michler. Tribo- structure in the CwS nozzles can improve the hardness and logical behaviour of Si,N4 and Si3N4 -% tin based composites and fracture toughness of the external layer, and can reduce the ti-layer laminates, Wear 260(2006)634-64 temperature gradients and the thermal stresses at the exit of the 13] J. Deng, Wear behaviors of ceramic nozzles with laminated structure at their entry, Wear 266(2009)30-36. nozzle during CWs burning processes 14] J. Deng, L Liu, M. Ding, Effect of residual stresses on the erosion wear of It is suggested that layered structures in ceramic nozzles is laminated ceramic nozzles, Materials Characterization 59(2008)1-8. an effective way to improve the erosion wear resistance over the [15] J. Deng, L. Liu, M. Ding, Erosion wear behaviours of SiC/(w,)c stress-free ceramic nozzles in industry Cws boilers laminated ceramic nozzles in dry sand blasting processes, Materials Science Engineering A 444(2007)120-129. Acknowledgements 16]x. Al. Z. Li, J. Deng. Development and perspective of advanced ceramic ng tool materials, Key Engineering Materials 108(1995)53-66. 17]J.Barry, G. he machining of hardened steels. This work was supported by the"Taishan Scholar Program Part I. Alumina/TiC cutting tool wear, Wear 247(2001)139-15 Foundation of Shandong", the"Outstanding Young Scholar [181. Deng. X. Yang Wear mechanisms of Al/nic/Mo/Ni ceramic wire- Science Foundation of Shandong", and the"Key Science and 9]RF. Cook, B.R. Lawn, A modified indentation toughness technique. Technology Project of Shandong Province(2006GG2204017) Journal of American Ceramic Society 66(1983)200-201 [20] J. Deng, D. Yuan, Y. Tan, Design, fabrication and performance of Al2O3/ References Wo.7Tio.3)C+ Al2O,TiC multilayered ceramic nozzles, International Journal of Refractory Metals Hard Materials 27(2009)734-739 [I J. Deng, Z Ding, D. Yun, Erosion wear mechanisms of coal-water-slurry 21 Z. Ding, Development of assembled coal-water-slurry ceramic nozzles (CWS)ceramic nozzles in industry boilers, Materials Science Engineering and study on their failure mechanisms, Ph D. Dissertation, Shandong A417(2006)1-7
nozzles, and the other is the thermal stress inside the CWS nozzles. Once the CWS ceramic nozzle possesses a layered structure, the hardness and fracture toughness at the nozzle external layer are improved (see Table 3), and the temperature gradients and the thermal stresses at the nozzle exit can be greatly reduced due to the different thermal expansion coefficients of the individual layers during CWS burning processes. Therefore, the layered ceramic nozzles exhibited an apparent increase in erosion wear resistance over the unstressed reference one, the mechanisms responsible were found to be that layered structure in the CWS nozzles can improve the hardness and fracture toughness of the external layer, and reduce the temperature gradients and the thermal stresses at the exit of the nozzle. 4. Conclusions Al2O3/(W,Ti)C + Al2O3/TiC layered ceramics with different thickness ratios among constituent layers were developed to be used as nozzles in CWS boilers. The performance and wear characteristics of these nozzles were investigated. The following conclusions were obtained: The layered ceramic nozzles (LN1, LN2, and LN3) exhibited an apparent increase in erosion wear resistance over the unstressed reference nozzle (N5) in industry CWS boilers. The mechanisms responsible were found to be that layered structure in the CWS nozzles can improve the hardness and fracture toughness of the external layer, and can reduce the temperature gradients and the thermal stresses at the exit of the nozzle during CWS burning processes. It is suggested that layered structures in ceramic nozzles is an effective way to improve the erosion wear resistance over the stress-free ceramic nozzles in industry CWS boilers. Acknowledgements This work was supported by the ‘‘Taishan Scholar Program Foundation of Shandong’’, the ‘‘Outstanding Young Scholar Science Foundation of Shandong’’, and the ‘‘Key Science and Technology Project of Shandong Province (2006GG2204017)’’. References [1] J. Deng, Z. Ding, D. Yun, Erosion wear mechanisms of coal-water-slurry (CWS) ceramic nozzles in industry boilers, Materials Science Engineering A 417 (2006) 1–7. [2] Z. Ding, J. Deng, J. Li, Wear behavior of ceramic nozzles in coal water slurry burning, Ceramics International 34 (2004) 591–596. [3] R. Lakshminarayanan, D.K. Shetty, R.A. Cutler, Toughening of layered ceramic composites with residual surface compression, Journal of American Ceramic Society 79 (1996) 79–87. [4] P.Z. Cai, D.J. Green, G.L. Messing, Mechanical characterization of Al2O3/ ZrO2 hybrid laminates, Journal of the European Ceramic Society 5 (1998) 2025–2034. [5] G. de Portu, L. Micele, G. Pezzotti, Laminated ceramic structures from oxide systems, Composites: Part B 37 (2006) 556–567. [6] V.M. Sglavo, M. Bertoldi, Design and production of ceramic laminates with high mechanical resistance and reliability, Acta Materialia 54 (2006) 4929–4937. [7] S. Guicciardi, M. Nagliati, C. Melandri, G. Pezzotti, D. Sciti, Effects of residual stresses on the fracture properties of non-oxide laminated composites, Journal of the European Ceramic Society 27 (2007) 351– 356. [8] H. Tomaszewski, H. Weglarz, A. Wajler, M. Boniecki, D. Kalinski, Multilayer ceramic composites with high failure resistance, Journal of the European Ceramic Society 27 (2007) 1373–1377. [9] V. Sergo, D.M. Lipkin, G. De Portu, D.R. Clarke, Edge stresses in alumina/zirconia laminates, Journal of American Ceramic Society 80 (1997) 1633–1638. [10] G. De Portu, L. Micele, D. Prandstraller, G. Palombarini, G. Pezzotti, Abrasive wear in ceramic laminated composites, Wear 260 (2006) 1104– 1111. [11] A. Tarlazzi, E. Roncari, P. Pinasco, S. Guicciardi, C. Melandri, G. de Portu, Tribological behaviour of Al2O3/ZrO2–ZrO2 laminated composites, Wear 244 (2000) 29–40. [12] M. Hadad, G. Blugan, J. K¨ ubler, E. Rosset, L. Rohr, J. Michler, Tribological behaviour of Si3N4 and Si3N4 – % TiN based composites and multi-layer laminates, Wear 260 (2006) 634–641. [13] J. Deng, Wear behaviors of ceramic nozzles with laminated structure at their entry, Wear 266 (2009) 30–36. [14] J. Deng, L. Liu, M. Ding, Effect of residual stresses on the erosion wear of laminated ceramic nozzles, Materials Characterization 59 (2008) 1–8. [15] J. Deng, L. Liu, M. Ding, Erosion wear behaviours of SiC/(W,Ti)C laminated ceramic nozzles in dry sand blasting processes, Materials Science Engineering A 444 (2007) 120–129. [16] X. Ai, Z. Li, J. Deng, Development and perspective of advanced ceramic cutting tool materials, Key Engineering Materials 108 (1995) 53–66. [17] J. Barry, G. Byrne, Cutting tool wear in the machining of hardened steels. Part I. Alumina/TiC cutting tool wear, Wear 247 (2001) 139–151. [18] J. Deng, X. Yang, Wear mechanisms of Al2O3/TiC/Mo/Ni ceramic wiredrawing dies, Materials Science Engineering A 424 (2006) 347–354. [19] R.F. Cook, B.R. Lawn, A modified indentation toughness technique, Journal of American Ceramic Society 66 (1983) 200–201. [20] J. Deng, D. Yuan, Y. Tan, Design, fabrication and performance of Al2O3/ (W0.7Ti0.3)C + Al2O3/TiC multilayered ceramic nozzles, International Journal of Refractory Metals & Hard Materials 27 (2009) 734–739. [21] Z. Ding, Development of assembled coal-water-slurry ceramic nozzles and study on their failure mechanisms, Ph.D. Dissertation, Shandong University, 2004. 306 J. Deng et al. / Ceramics International 36 (2010) 299–306