MATERIALS CHARACTERIZATION 59(2008)1-8 SUMATERI ELSEVIER Effect of residual stresses on the erosion wear of laminated ceramic nozzles Jianxin Deng, Lili Liu, Mingwei Ding Department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Prouince, PR China ARTICLE DATA ABSTRACT Article history C/W, Tic laminated ceramic nozzles were produced by hot pressing. The purpose is to Received 11 April 2006 induce a compressive residual stress at the entry area of the laminated ceramic nozzles in Received in revised form fabricating process, and leading an improved erosion wear resistance of the laminated 21 September 2006 ceramic nozzles. The residual stresses arise from a mismatch between the coefficients of Accepted 17 October 2006 thermal expansion of the constituent phases and neighbouring layers. The value of this residual stress was calculated by means of the finite element method. Effect of this kind of Keyword residual stress on the erosion wear of the laminated ceramic nozzles was investigated. nozzles 9 2006 Elsevier In Residual stress Ceramic materials Laminated materials Introduction erosive particles travel nearly parallel to the inner surface of the nozzle, and the impact force can be divided into two constitu- Sand blasting treatment is an abrasive machining process and ents. One is parallel (Fp) to the surface of the material and the is widely used for surface strengthening 1], surface modifica- other is vertical(Fv). So the nozzle entry region suffers form tion [2], surface clearing and rust removal 3, 4, etc In the sand severe abrasive impact, which may cause large tensile stresses blasting process, a very high velocity jet of fine abrasive The highest tensile stresses are located at the entry area of the particles and carrier gas coming out from a nozzle impinges on nozzle. Thus, the erosion wear of the nozzle entry area is alway the target surface and erodes it. The nozzle is the most critical serious in contrast with that of the center area [11, 15] part in the sand blasting equipment. There are many factors Laminated hybrid structures constituted by altemate that influence the nozzle wear such as: the mass flow rate and layers with different compositions can be properly designed impact angle [5-7 the erodent abrasive properties[8-10, the to induce a surface compressive residual stress leading to an nozzle material and its geometry [11-16, and the tempera- improved surface mechanical properties and wear resistance tures (17, 18. Ceramics, being highly wear resistance, have [19-22 Residual stresses arise from a mismatch between the great potential as the sand blasting nozzle materials. coefficients of thermal expansion(CTE), sintering rates and Several studies (11, 15] have shown that the entry area of a elastic constants of the constituent phases and neighbouring ceramic nozzle exhibited a brittle fracture induced removal layers, and the residual stress field depends on the geometry process, while the center area showed plowing type of material of the layered structure and on the thickness ratio among removal mode. Fig. 1 shows the schematic diagram of the layers [23-26]. Toschiet al [22 reported that laminated hybrid interaction between the erodent particle and the nozzle in structures can improve the sliding wear resistance of alumina. sand-blasting surface treatments In section I of Fig. 1, the Portu et al. [27 showed that laminated structures with erosive particles hit the nozzle vertically; while in section II, the compressive residual stresses within the surface regions was Corresponding author E-mail address: jxdengesduedu cn ( Deng) 1044-5803/$- see fror er C 2006 Elsevier Inc. All rights reserved. doi: 10.1016/imatcha
Effect of residual stresses on the erosion wear of laminated ceramic nozzles Jianxin Deng⁎, Lili Liu, Mingwei Ding Department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China ARTICLE DATA ABSTRACT Article history: Received 11 April 2006 Received in revised form 21 September 2006 Accepted 17 October 2006 SiC/(W,Ti)C laminated ceramic nozzles were produced by hot pressing. The purpose is to induce a compressive residual stress at the entry area of the laminated ceramic nozzles in fabricating process, and leading an improved erosion wear resistance of the laminated ceramic nozzles. The residual stresses arise from a mismatch between the coefficients of thermal expansion of the constituent phases and neighbouring layers. The value of this residual stress was calculated by means of the finite element method. Effect of this kind of residual stress on the erosion wear of the laminated ceramic nozzles was investigated. © 2006 Elsevier Inc. All rights reserved. Keywords: Nozzles Residual stress Ceramic materials Laminated materials SiC 1. Introduction Sand blasting treatment is an abrasive machining process and is widely used for surface strengthening [1], surface modification [2], surface clearing and rust removal [3,4], etc. In the sand blasting process, a very high velocity jet of fine abrasive particles and carrier gas coming out from a nozzle impinges on the target surface and erodes it. The nozzle is the most critical part in the sand blasting equipment. There are many factors that influence the nozzle wear such as: the mass flow rate and impact angle [5–7], the erodent abrasive properties [8–10], the nozzle material and its geometry [11–16], and the temperatures [17,18]. Ceramics, being highly wear resistance, have great potential as the sand blasting nozzle materials. Several studies [11,15] have shown that the entry area of a ceramic nozzle exhibited a brittle fracture induced removal process, while the center area showed plowing type of material removal mode. Fig. 1 shows the schematic diagram of the interaction between the erodent particle and the nozzle in sand-blasting surface treatments. In section I of Fig. 1, the erosive particles hit the nozzle vertically; while in section II, the erosive particles travel nearly parallel to the inner surface of the nozzle, and the impact force can be divided into two constituents. One is parallel (Fp) to the surface of the material and the other is vertical (Fv). So the nozzle entry region suffers form severe abrasive impact, which may cause large tensile stresses. The highest tensile stresses are located at the entry area of the nozzle. Thus, the erosion wear of the nozzle entry area is always serious in contrast with that of the center area [11,15]. Laminated hybrid structures constituted by alternate layers with different compositions can be properly designed to induce a surface compressive residual stress leading to an improved surface mechanical properties and wear resistance [19–22]. Residual stresses arise from a mismatch between the coefficients of thermal expansion (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 [23–26]. Toschi et al. [22] reported that laminated hybrid structures can improve the sliding wear resistance of alumina. Portu et al. [27] showed that laminated structures with compressive residual stresses within the surface regions was MATERIALS CHARACTERIZATION 59 (2008) 1 – 8 ⁎ Corresponding author. E-mail address: jxdeng@sdu.edu.cn (J. Deng). 1044-5803/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.10.009
2 MATERIALS CHARACTERIZATION 59(2008)1-8 High velocity air and abrasive particles Jet spreading- Nozzle FY Fig. 3-Schematic diagram of the sand blasting machine tool (1. air compressor, 2. control valve, 3. filter, 4. desiccator, 5. press adjusting valve, 6. dust catcher, 7. blasting gun,8. abrasive hopper, 9. ceramic nozzle Fig. 1-Schematic diagram of the interaction between the erodent particle and the nozzle in sand blasting processes. a suitable way to obtain composite materials with superior and Sic powders with average grain size of 1 um, purity 99.8% tribological propertie Six different volume fractions of (W,Ti)c(55, 57, 59, 61, 63, In the present study, Sic/(w, Ti)C ceramic nozzles with 65 vol % were selected in designing the Sic/(W,Tic laminated laminated structures were produced by hot pressing in order nozzle material with a six-layer structure. The compositional to induce a compressive residual stress at the nozzle entry distribution of the ceramic nozzle materials is shown in Fig. 2 regions. The residual stress of the laminated nozzle during the It is indicated that the compositional distribution of the sintering process was calculated by means of the finite element laminated nozzle materials changes in the nozzle axial method(FEM). Effect of this kind of residual stress on the direction(Fig. 2(a)). The homologous stress-free nozzle with erosion wear of the laminated ceramic nozzle was investigated. no compositional change is shown in Fig. 2(b). The laminated ceramic nozzle is named GN-2. while the stress-free nozzle is Materials and Experimental Procedures Six Sic/(W, Tic composite powders of different mixture 2.1. Preparation of sic/(W, Ti)C Laminated Ceramic Nozzle ratos were prepared by wet ball milling in alcohol with cemented carbide balls for 80 hours respectively. Following drying, the mixtures composite powders with different The starting materials were (W, Ti)c solid-solution powders turn. as the heat conductivity of sic is higher than that of(W with average grain size of approximately 0.8 um, purity 99.9%o, T)C solid-solution, while its thermal expansion coefficient is lower than that of (w, Ti)C, the layer with the highest volume Nozzle exit Nozzle exit fraction of Sic was put in the entry layer. The sample was then hot-pressed in flowing nitrogen for 40 min at 1800C-1900C temperature with 30 MPa pressure ozzie entry Nozzle entry Fig. 2-Compositional distribution of (a)the GN-2 laminated nozzle, and(b)the CN-2 stress-free 4-Photo of W,TiC laminated ceramic nozzles
a suitable way to obtain composite materials with superior tribological properties. In the present study, SiC/(W,Ti)C ceramic nozzles with laminated structures were produced by hot pressing in order to induce a compressive residual stress at the nozzle entry regions. The residual stress of the laminated nozzle during the sintering process was calculated by means of the finite element method (FEM). Effect of this kind of residual stress on the erosion wear of the laminated ceramic nozzle was investigated. 2. Materials and Experimental Procedures 2.1. Preparation of SiC/(W,Ti)C Laminated Ceramic Nozzle Materials The starting materials were (W,Ti)C solid-solution powders with average grain size of approximately 0.8 μm, purity 99.9%, and SiC powders with average grain size of 1 μm, purity 99.8%. Six different volume fractions of (W,Ti)C (55, 57, 59, 61, 63, 65 vol.%) were selected in designing the SiC/(W,Ti)C laminated nozzle material with a six-layer structure. The compositional distribution of the ceramic nozzle materials is shown in Fig. 2. It is indicated that the compositional distribution of the laminated nozzle materials changes in the nozzle axial direction (Fig. 2(a)). The homologous stress-free nozzle with no compositional change is shown in Fig. 2(b). The laminated ceramic nozzle is named GN-2, while the stress-free nozzle is named CN-2. Six SiC/(W,Ti)C composite powders of different mixture ratios were prepared by wet ball milling in alcohol with cemented carbide balls for 80 hours respectively. Following drying, the mixtures composite powders with different mixture ratios were laminated into the graphite mould in turn. As the heat conductivity of SiC is higher than that of (W, Ti)C solid-solution, while its thermal expansion coefficient is lower than that of (W,Ti)C, the layer with the highest volume fraction of SiC was put in the entry layer. The sample was then hot-pressed in flowing nitrogen for 40 min at 1800 °C–1900 °C temperature with 30 MPa pressure. Fig. 1 – Schematic diagram of the interaction between the erodent particle and the nozzle in sand blasting processes. Fig. 2 –Compositional distribution of (a) the GN-2 laminated ceramic nozzle, and (b) the CN-2 stress-free ceramic nozzle. Fig. 3 – Schematic diagram of the sand blasting machine tool (1. air compressor, 2. control valve, 3. filter, 4. desiccator, 5. press adjusting valve, 6. dust catcher, 7. blasting gun, 8. abrasive hopper, 9. ceramic nozzle). Fig. 4 – Photo of the SiC/(W,Ti)C laminated ceramic nozzles. 2 MATERIALS CHARACTERIZATION 59 (2008) 1 – 8
MATERIALS CHARACTERIZATION 59(2008)1-8 Table 1-Sand blasting test condition The mass loss of the won nozzles was measured with an Sand blasting GS-6 type sand blasting machine tool accurate electronic balance (minimum 0. 1 mg). All the test conditions are listed in Table 1. The erosion rates(W) of the Nozzle material GN-2 laminated nozzle CN-2 stress-free nozzle nozzles are defined as the nozzle mass loss( s(mi)divided by (ength) nozzle density (a) times the mass of the erodent abrasive Erodent abrasives 50-150 um Sic powders particles(m小 W=m/(d- m2) Cumulative mass Accurate electronic balance(minimum 0.1 mg) Where the W has the units of volume loss per unit mass (mm/g The finite element method(FEM was used as a means of 2.2. Sand Blasting Tests numerically evaluating the residual stress and its distribution within the laminated nozzle in the fabricating processes Fig 3 shows the schematic diagram of the abrasive air-jet For observation of the micro-damage and determination of machine tool(GS-6 type), which consists of an air compressor, erosion mechanisms, the worm nozzles were sectioned axially a blasting gun, a control valve a particle supply tube, a filter, a The eroded bore surfaces of the nozzles were examined by desiccator, an adjusting press valve, a dust catcher, an scanning electron microscopy. abrasive hopper, and a nozzle. The air and grit flow adjusting was controlled by the valves and regulators. The gas flow rate is controlled by the compressed air, and the abrasive particle 3 Results and discussion velocity through the nozzle is adjusted to 60 m/s. The erodent abrasives used in this study were of silicon carbide(Sic) 3. 1. Residual Stress Analysis of Sic/W, Tic Laminated powders with 50-150 um grain size. Nozzles with intemal Nozzle Material diameter 8 mm and length 30 mm made from Sic/(W,Tic laminated structure(GN-2) and stress-free structure(CN-2) The residual stress of the laminated ceramic nozzle in the were manufactured by hot-pressing as can be seen in Fig 4. fabricating process was calculated by means of the finite B 55.478 87.101 13.597 167,0 5L⊥2
2.2. Sand Blasting Tests Fig. 3 shows the schematic diagram of the abrasive air-jet machine tool (GS-6 type), which consists of an air compressor, a blasting gun, a control valve, a particle supply tube, a filter, a desiccator, an adjusting press valve, a dust catcher, an abrasive hopper, and a nozzle. The air and grit flow adjusting was controlled by the valves and regulators. The gas flow rate is controlled by the compressed air, and the abrasive particle velocity through the nozzle is adjusted to 60 m/s. The erodent abrasives used in this study were of silicon carbide (SiC) powders with 50–150 μm grain size. Nozzles with internal diameter 8 mm and length 30 mm made from SiC/(W,Ti)C laminated structure (GN-2) and stress-free structure (CN-2) were manufactured by hot-pressing as can be seen in Fig. 4. The mass loss of the worn nozzles was measured with an accurate electronic balance (minimum 0.1 mg). All the test conditions are listed in Table 1. The erosion rates (W) of the nozzles are defined as the nozzle mass loss (m1) divided by the nozzle density (d) times the mass of the erodent abrasive particles (m2): W ¼ m1=ðdd m2Þ ð1Þ Where the W has the units of volume loss per unit mass (mm3 /g). The finite element method (FEM) was used as a means of numerically evaluating the residual stress and its distribution within the laminated nozzle in the fabricating processes. For observation of the micro-damage and determination of erosion mechanisms, the worn nozzles were sectioned axially. The eroded bore surfaces of the nozzles were examined by scanning electron microscopy. 3. Results and Discussion 3.1. Residual Stress Analysis of SiC/(W,Ti)C Laminated Nozzle Material The residual stress of the laminated ceramic nozzle in the fabricating process was calculated by means of the finite Table 1 – Sand blasting test conditions Sand blasting equipment GS-6 type sand blasting machine tool Nozzle material GN-2 laminated nozzle CN-2 stress-free nozzle Dimension of nozzle Φ 8 mm (internal diameter) × 30 mm (length) Erodent abrasives 50–150 μm SiC powders Compressed air pressure 0.4 MPa Cumulative mass weight Accurate electronic balance (minimum 0.1 mg) Fig. 5 – Distribution of axial (σz), radial (σr), and circumferential (σθ) residual stresses in GN-2 laminated nozzle in fabricating process from sintering temperature 1900 °C to room temperature 20 °C. (a) 1800 °C (b) 1850 °C (c) 1900 °C (d) 1950 °C. MATERIALS CHARACTERIZATION 59 (2008) 1 – 8 3
4 MATERIALS CHARACTERIZATION 59(2008)1-8 Ae 99 费品 8oooo088888888 o Radial stress o Radial stress O Axial stress o Circumferential stress Nozzle axial direction(mm) Nozzle axial direction(mm) c D 没 o Radial stress o Radial stress 200 O Axial stress ● Axial stress O Circumferential stress Circumferential stress Nozzle axial direction(mm) Nozzle axial direction (mm) 6-The residual stress at the inner-hole surface of the GN-2 laminated nozzle along the nozzle axial direction in fabricating cess with different sintering temperature. element method by assuming that the compact is cooled from sintering temperature. Therefore, laminated structures in sintering temperature 1800C-1900C to room temperature ceramic nozzle can form an excess compressive resid 20'C Thermo-mechanical properties of (W, Ti)C and Sic are as stresses at the nozzle entry area during fabricating process follow (W, Ti)C: Youngs modulus E=480 GPa, Poisson's ratio 3. 2. Microstructural Characterization and Properties of v=0.25,thermal expansion coefficient a=8.5x10-K, ther- Sic/(W, Ti C Laminated Nozzle Materials mal conductivity k=21.4 W/(m K SiC: Young s modulus E=450 GPa, Poissons ratio v=. 16, Hardness measurements were performed by placing Vickers thermal expansion coefficient a=4.6x10-K, thermal con dentations on each layer of the cross-sectional surface of ductivity k=33.5 W/(m K) GN-2 laminated nozzle material. The indentation load was Owing to the symmetry, an axisymmetric calculation was 200 N and a minimum of three indentations were tested for preferred Presume that it was steady state boundary condi- tions. The results of the distribution of axial (oz), radial (o, and circumferential (oe) residual stresses in the GN-2 lami- nated nozzle in fabricating process from sintering tempera- 2-Hardness of different layers of GN-2 laminated ture 1900C to room temperature 20C are showed in Fig. 5.As le material can be seen, an excess residual stress is formed in the GN-2 Nozzle Layer (W,Ti)C content Vickers hardnes laminated nozzle (vo%) HV(GPa) Fig 6 shows the residual stresses(oz, Or, and ae) at the GN-2 laminated 1 inner-hole surface of GN-2 laminated nozzle along the nozzle nozzle axial direction in fabricating process with different sintering temperature Itin indicated that oz, ar, and de residual stresses 57916 at the nozzle entry area are all compressive whatever the sintering temperature, and increase with the increasing of the
element method by assuming that the compact is cooled from sintering temperature 1800 °C–1900 °C to room temperature 20 °C. Thermo-mechanical properties of (W,Ti)C and SiC are as follows: (W,Ti)C: Young's modulus E= 480 GPa, Poisson's ratio ν= 0.25, thermal expansion coefficient α= 8.5 × 10−6 K−1 , thermal conductivity k= 21.4 W/(m K). SiC: Young's modulus E= 450 GPa, Poisson's ratio ν= 0.16, thermal expansion coefficient α= 4.6 × 10−6 K−1 , thermal conductivity k= 33.5 W/(m K). Owing to the symmetry, an axisymmetric calculation was preferred. Presume that it was steady state boundary conditions. The results of the distribution of axial (σz), radial (σr), and circumferential (σθ) residual stresses in the GN-2 laminated nozzle in fabricating process from sintering temperature 1900 °C to room temperature 20 °C are showed in Fig. 5. As can be seen, an excess residual stress is formed in the GN-2 laminated nozzle. Fig. 6 shows the residual stresses (σz, σr, and σθ) at the inner-hole surface of GN-2 laminated nozzle along the nozzle axial direction in fabricating process with different sintering temperature. It in indicated that σz, σr, and σθ residual stresses at the nozzle entry area are all compressive whatever the sintering temperature, and increase with the increasing of the sintering temperature. Therefore, laminated structures in ceramic nozzle can form an excess compressive residual stresses at the nozzle entry area during fabricating process. 3.2. Microstructural Characterization and Properties of SiC/(W,Ti)C Laminated Nozzle Materials Hardness measurements were performed by placing Vickers indentations on each layer of the cross-sectional surface of GN-2 laminated nozzle material. The indentation load was 200 N and a minimum of three indentations were tested for Fig. 6 –The residual stress at the inner-hole surface of the GN-2 laminated nozzle along the nozzle axial direction in fabricating process with different sintering temperature. Table 2 – Hardness of different layers of GN-2 laminated nozzle material Nozzle Layer (W,Ti)C content (vol.%) Vickers hardness Hv (GPa) GN-2 laminated nozzle 1 55 26.89 2 57 26.52 3 59 25.93 4 61 25.70 5 63 24.67 6 65 24.15 4 MATERIALS CHARACTERIZATION 59 (2008) 1 – 8
MATERIALS CHARACTERIZATION 59(2008)1-8 5000 o(, Tc Fig 12 shows the comparison of the erosion rates of GN-2 6000 and CN-2 nozzles in sand blasting processes. It is obvious that the erosion rate of the CN-2 stress-free nozzles is higher than that of the GN-2 laminated nozzles. Therefore, it is apparently ≥3000 that the GN-2 laminated nozzles exhibited higher erosion wear resistance over the cn-2 stress-free nozzle under the 2000 same test conditions 2xtheta(degrees Fig. 7-X-ray diffraction analysis of the GN-2 laminated ceramic nozzle material after sintering at 1900C for 40 mir (A)The first layer(entry zone).(B)The third layer(C) The sixth layer. each layer. The Vickers hardness(GPa) of each layer is given H=18544P Where P is the indentation load(N), 2a is the catercorner length (um)due to indentation Hardness of different layers of GN-2 laminated nozzle materials is presented in Table 2. Fig. 7 illustrates the X-ray diffraction analysis of the GN-2 laminated ceramic nozzle material after sintering at 1900C for 40 min. It can be seen that both (W,TiC and Sic existed the sintered specimens. Fig 8 shows the SEM micrographs of the fracture surface of the gn-2 laminated material. The seM micrographs of the polished surface of the laminated material are shown in Fig 9. The black areas were identified by EDX analysis as SiC, and the white phases with clear contrast were ,TiC. It can be seen that the Sic particles are quite uniformly distributed throughout the microstructure, porosity 9591!2K氵i’距 virtually absent. 3.3. Erosion Behavior of the Sic/(W, Tic Laminated Nozzle The erosion behavior of gn-2 laminated ceramic nozzle in sand blasting processes was investigated in comparison with the CN-2 stress-free ceramic nozzle. Fig. 10 shows the cumulative mass loss of gn-2 and cN-2 nozzles in sand blasting processes. It is indicated that the cumulative mass loss continuously increased with the operation time. Com pared with GN-2 laminated nozzle, the CN-2 stress-free nozzle showed higher cumulative mass loss under the same test The won ceramic nozzles were cut after operation in longitudinal directions for failure analysis. Fig. 11 shows the photos of the inner-hole profile of the whole ceramic nozzle after 540 min operation. It is showed that inner-hole diameter 85881128Kv氵’是弱u of the worn CN-2 nozzle along the nozzle longitudinal directions is larger than that of the won GN-2 laminated Fig. 8-SEM micrographs of the fracture surfaces of the GN-2 nozzles under the same test conditions, especially at the laminated ceramic nozzle material. () The first layer(B)The nozzle entry region. third layer. (C)The sixth layer
each layer. The Vickers hardness (GPa) of each layer is given by: Hv ¼ 1:8544 P ð2aÞ 2 ð2Þ Where P is the indentation load (N), 2a is the catercorner length (μm) due to indentation. Hardness of different layers of GN-2 laminated nozzle materials is presented in Table 2. Fig. 7 illustrates the X-ray diffraction analysis of the GN-2 laminated ceramic nozzle material after sintering at 1900 °C for 40 min. It can be seen that both (W,Ti)C and SiC existed in the sintered specimens. Fig. 8 shows the SEM micrographs of the fracture surface of the GN-2 laminated material. The SEM micrographs of the polished surface of the laminated material are shown in Fig. 9. The black areas were identified by EDX analysis as SiC, and the white phases with clear contrast were (W,Ti)C. It can be seen that the SiC particles are quite uniformly distributed throughout the microstructure, porosity is virtually absent. 3.3. Erosion Behavior of the SiC/(W,Ti)C Laminated Nozzle The erosion behavior of GN-2 laminated ceramic nozzle in sand blasting processes was investigated in comparison with the CN-2 stress-free ceramic nozzle. Fig. 10 shows the cumulative mass loss of GN-2 and CN-2 nozzles in sand blasting processes. It is indicated that the cumulative mass loss continuously increased with the operation time. Compared with GN-2 laminated nozzle, the CN-2 stress-free nozzle showed higher cumulative mass loss under the same test conditions. The worn ceramic nozzles were cut after operation in longitudinal directions for failure analysis. Fig. 11 shows the photos of the inner-hole profile of the whole ceramic nozzle after 540 min operation. It is showed that inner-hole diameter of the worn CN-2 nozzle along the nozzle longitudinal directions is larger than that of the worn GN-2 laminated nozzles under the same test conditions, especially at the nozzle entry region. Fig. 12 shows the comparison of the erosion rates of GN-2 and CN-2 nozzles in sand blasting processes. It is obvious that the erosion rate of the CN-2 stress-free nozzles is higher than that of the GN-2 laminated nozzles. Therefore, it is apparently that the GN-2 laminated nozzles exhibited higher erosion wear resistance over the CN-2 stress-free nozzle under the same test conditions. Fig. 7 – X-ray diffraction analysis of the GN-2 laminated ceramic nozzle material after sintering at 1900 °C for 40 min. (A) The first layer (entry zone). (B) The third layer. (C) The sixth layer. Fig. 8 – SEM micrographs of the fracture surfaces of the GN-2 laminated ceramic nozzle material. (A) The first layer. (B) The third layer. (C) The sixth layer. MATERIALS CHARACTERIZATION 59 (2008) 1 – 8 5