MATERIALS HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 444 (2007)120-129 www.elsevier.com/locate/msea Erosion wear behaviours of Sic/(w,Ti)c laminated ceramic nozzles in dry sand blasting processes Deng jianxin. Liu Lili. dir Department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China Received 13 April 2006: accepted 17 August 2006 In sand blasting processes, the nozzle entry region suffers form severe abrasive impact, which may cause large tensile stress and lead to an increased erosion wear at the nozzle entry area. In this paper, SiC/W, Ti)c laminated ceramic nozzles were produced by hot pressing. The purpose to reduce the tensile stress at the entry region of the nozzle. Due to the different thermal expansion coefficients and shrinkage of the Sic and W,Ti)C solid-solution, the entry region of the Sic/(w,Ti)C laminated ceramic nozzles in the fabricating process exhibit a compressive residual stress. The value of this residual stress was calculated by means of the finite element method. The erosion wear behaviour of the laminated ceramic nozzle and of a stress-free nozzle, with the same composition, was assessed by dry sand blasting Results showed that the laminated ceramic nozzles have superior erosion wear resistance to that of the homologous stress-free nozzles. The mechanism responsible was explained as the formation of compressive residual stresses in nozzle entry region in fabricating process of the laminated ceramic nozzles, which may partially counteract the tensile stresses resulting from extermal loadings, and leads to an improvement of erosion wear resistance. It is indicated that laminated structures in ceramic nozzles is an effective way to improve the erosion wear resistance of the stress-free nozzles C 2006 Elsevier B. V. All rights reserved. Keywords: Nozzles: Ceramic materials: Laminated materials: SiC 1. Introduction The nozzle is the most critical part in the sand blasting treatment equipment. There are many factors that influence the Sand blasting treatment is an abrasive machining process and nozzle wear such as: the mass flow rate and impact angle [5-7] is widely used for surface strengthening[l], surface modification the erodent abrasive properties [8-10, the nozzle material and 21, surface clearing and rust removal [3, 4], etc. It is suitable for its geometry [11-16], and the temperatures [17, 18]. Ce the treatment of hard and brittle materials, ductile metals, alloys, being highly wear resistance, have great potential as and nonmetallic materials, and can provide perfect surface treat- blasting nozzle materials ment to all kinds workpieces from hull, steel structure, container Several studies [11-15] have shown that the entry area of to watchcase, button and inject needle In the sand blasting pro- a ceramic nozzle exhibited a brittle fracture induced removal cess, a very high velocity jet of fine abrasive particles and carrier process, while the centre area showed plowing type of material gas coming out from a nozzle impinges on the target surface and removal mode. As the erosive particles hit the nozzle at high erodes it. The fine particles are accelerated by the gas stream, angles(nearly 90%) at the nozzle entry section in sand blasting commonly compressed air at a few times atmospheric pressure. (see Fig. 1), the nozzle entry region suffers form severe abrasive The particles are directed towards the surfaces to be treated. impact, which may cause large tensile stresses. The stress alon As the particles impact the surface, they cause a small fracture, the axial direction of the nozzle decreases from entry to centre, and the gas stream carries both the abrasive particles and the and increases from centre to exit. The highest tensile stresses fractured particles away are located at the entry region of the nozzle. While the wear of the nozzle centre area changes from impact to slidin the tensile stresses caused by the abrasive impact in this area are much smaller than those at the entry section. Thus, the erosion Corresponding author. Tel: +86 531 88392047 wear of the nozzle entry region is always serious in contrast with E-mail address sdu.edu. cn(D. Jianxin) that of the centre area [11-15] )6 Elsevier B v. All rights reserved
Materials Science and Engineering A 444 (2007) 120–129 Erosion wear behaviours of SiC/(W,Ti)C laminated ceramic nozzles in dry sand blasting processes Deng Jianxin ∗, Liu Lili, Ding Mingwei Department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China Received 13 April 2006; accepted 17 August 2006 Abstract In sand blasting processes, the nozzle entry region suffers form severe abrasive impact, which may cause large tensile stress and lead to an increased erosion wear at the nozzle entry area. In this paper, SiC/(W,Ti)C laminated ceramic nozzles were produced by hot pressing. The purpose is to reduce the tensile stress at the entry region of the nozzle. Due to the different thermal expansion coefficients and shrinkage of the SiC and (W,Ti)C solid-solution, the entry region of the SiC/(W,Ti)C laminated ceramic nozzles in the fabricating process exhibit a compressive residual stress. The value of this residual stress was calculated by means of the finite element method. The erosion wear behaviour of the laminated ceramic nozzle and of a stress-free nozzle, with the same composition, was assessed by dry sand blasting. Results showed that the laminated ceramic nozzles have superior erosion wear resistance to that of the homologous stress-free nozzles. The mechanism responsible was explained as the formation of compressive residual stresses in nozzle entry region in fabricating process of the laminated ceramic nozzles, which may partially counteract the tensile stresses resulting from external loadings, and leads to an improvement of erosion wear resistance. It is indicated that laminated structures in ceramic nozzles is an effective way to improve the erosion wear resistance of the stress-free nozzles. © 2006 Elsevier B.V. All rights reserved. Keywords: Nozzles; 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. It is suitable for the treatment of hard and brittle materials, ductile metals, alloys, and nonmetallic materials, and can provide perfect surface treatment to all kinds workpieces from hull, steel structure, container to watchcase, button and inject needle. 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 fine particles are accelerated by the gas stream, commonly compressed air at a few times atmospheric pressure. The particles are directed towards the surfaces to be treated. As the particles impact the surface, they cause a small fracture, and the gas stream carries both the abrasive particles and the fractured particles away. ∗ Corresponding author. Tel.: +86 531 88392047. E-mail address: jxdeng@sdu.edu.cn (D. Jianxin). The nozzle is the most critical part in the sand blasting treatment 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 centre area showed plowing type of material removal mode. As the erosive particles hit the nozzle at high angles (nearly 90◦) at the nozzle entry section in sand blasting (see Fig. 1), the nozzle entry region suffers form severe abrasive impact, which may cause large tensile stresses. The stress along the axial direction of the nozzle decreases from entry to centre, and increases from centre to exit. The highest tensile stresses are located at the entry region of the nozzle. While the wear of the nozzle centre area changes from impact to sliding erosion, the tensile stresses caused by the abrasive impact in this area are much smaller than those at the entry section. Thus, the erosion wear of the nozzle entry region is always serious in contrast with that of the centre area [11–15]. 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.08.090
D Jianxin et al. Materials Science and Engineering A 444 (2007)120-129 Nozzle Shallow impact or abrasive zone Fig. 1. Schematic diagram of the interaction between the erodent particle and the nozzle in sand blasting processe A novel technique, by which a compressive residual stress was investigated in comparison with the homologous stress-free n be generated into the surface of a material, is the produc- nozzles. The purpose is to evaluate whether laminated ceramic tion of laminated structures designed to combine the advanta- nozzles have superior erosion wear resistance to that of the geous characteristics of the different materials involved, thereby homologous stress-free nozzles. improving the overall mechanical behaviour of the materials. It has been shown that laminated hybrid structures constituted by 2. Materials and experimental procedures alternate layers of different materials can be properly designed in order to induce a surface compressive residual stress leading to 2.1. Preparation of sic/(w, Ti)C laminated ceramic nozzle an improved surface mechanical properties and wear resistance materials [19-22]. Residual stresses arise from a mismatch between the coefficients of thermal expansion(CTE), sintering rates and elas- The starting materials were(W,Ti C solid-solution powders ic constants of the constituent phases and neighbouring layers. with average grain size of approximately 0.8 um, purity 99.9%0 Compressive residual stresses are induced in layers with lower and Sic powders with average grain size of 1 um, purity 99.8%. CTE, while tensile stresses arise in those with higher CtE. The Six different volume fractions of (W,Ti)C(55, 57, 59, 61, 63, residual stress field also depends on the geometry of the layered 65 vol %)were selected in designing the SiC/(W,Ti)C laminated structure and on the thickness ratio among layers [23-26] nozzle material with a six-layer structure The effectiveness of laminated hybrid structures in improving The compositional distribution of the laminated ceramic noz the sliding wear resistance of alumina has been already reported zle materials is shown in Fig. 2. It is indicated that the composi by Toschi [22]. In the present study, Sic/(W,Ti)C laminated tional distribution of the laminated nozzle materials changes in ceramic nozzles were produced by hot pressing. The residual nozzle axial direction(see Fig. 2(a)and( b)). As the heat conduc thermal stress of the laminated nozzle in the fabricating process tivity of SiC is higher than that of (w,Ti)C solid-solution, while was calculated by means of the finite element method(FEM). its thermal expansion coefficient is lower than that of (,Ti)C, The erosion wear behaviour of the laminated ceramic nozzle the layer with the highest volume fraction of SiC was put in the Nozzle exit Nozzle exit Nozzle exit C/65Vol%(W,Tn)c iC/57Vol%(W,TO)C SiC/55Vol%(W, TO)C Nozzle entry Nozzle entry Nozzle entry Fig. 2. Compositional distribution of (a) the ceramic nozzle laminated only in entry area. (b) the ceramic nozzle laminated both in entry and exit area, and (c)the stress-free nozzles (a) GN-2 laminated nozzle, (b) GN-3 laminated nozzle, (c)CN-2 stress-free nozzle
D. Jianxin et al. / Materials Science and Engineering A 444 (2007) 120–129 121 Fig. 1. Schematic diagram of the interaction between the erodent particle and the nozzle in sand blasting processes. A novel technique, by which a compressive residual stress can be generated into the surface of a material, is the production of laminated structures designed to combine the advantageous characteristics of the different materials involved, thereby improving the overall mechanical behaviour of the materials. It has been shown that laminated hybrid structures constituted by alternate layers of different materials can be properly designed in order 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. Compressive residual stresses are induced in layers with lower CTE, while tensile stresses arise in those with higher CTE. The residual stress field also depends on the geometry of the layered structure and on the thickness ratio among layers [23–26]. The effectiveness of laminated hybrid structures in improving the sliding wear resistance of alumina has been already reported by Toschi [22]. In the present study, SiC/(W,Ti)C laminated ceramic nozzles were produced by hot pressing. The residual thermal stress of the laminated nozzle in the fabricating process was calculated by means of the finite element method (FEM). The erosion wear behaviour of the laminated ceramic nozzle was investigated in comparison with the homologous stress-free nozzles. The purpose is to evaluate whether laminated ceramic nozzles have superior erosion wear resistance to that of the homologous stress-free nozzles. 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 m, purity 99.9%, and SiC powders with average grain size of 1 m, 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 laminated ceramic nozzle materials is shown in Fig. 2. It is indicated that the compositional distribution of the laminated nozzle materials changes in nozzle axial direction (see Fig. 2(a) and (b)). 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 Fig. 2. Compositional distribution of (a) the ceramic nozzle laminated only in entry area, (b) the ceramic nozzle laminated both in entry and exit area, and (c) the stress-free nozzles. (a) GN-2 laminated nozzle, (b) GN-3 laminated nozzle, (c) CN-2 stress-free nozzle.
D Jianxin et al./ Materials Science and Engineering A 444(2007)120-129 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). 851115K沁总;:mm nozzle entry with the compositional distribution changing from the entry layer to the exit layer with the lowest volume frac Fig. 5. SEM micrograph of the SiC abrasives used for dry sand blasting ion of SiC(see Fig. 2(a)). While in Fig. 2(b), the compositional istribution of the laminated ceramic nozzle is symmetrical, the trolled by the valves and regulators. The abrasive air jet is formed layer with the highest volume fraction of Sic was put both in in the blasting gun using a suction-type process as schematically the entry layer and in the exit layer. The homologous stress-free illustrated in Fig. 4. The gas flow rate is controlled by the com- nozzle with no compositional change is shown in Fig. 2(c).The pressed air, and the abrasive particle velocity through the nozzle ceramic nozzle laminated only in entry area is named GN-2, the is adjusted to 60m/s ceramic nozzle laminated both in entry and exit area is named The erodent abrasives used in this study were of silicon GN-3. while the stress-free nozzle is named CN-2. carbide(Sic) powders with 50-150 um grain size. The SEM Six Sic/(w,Ti)c composite powders of different mixture micrograph of the SiC powders used for the dry sand blasting is ratios were prepared by wet ball milling in alcohol with shown in Fig. 5. As these abrasives are more durable and create cemented carbide balls for 80h. Following drying, the mixtures less dust than sand, and typically are reclaimed and reused. composite powders with different mixture ratios were laminate Nozzles with internal diameter mm and length 30 mm made into the mould in turn. The sample was then hot-pressed in flow- from SiC/(W, Ti)C laminated structure(GN-2 and GN-3)and ing nitrogen for 40 min at 1900C temperature with 30MPa stress-free structure(CN-2)were manufactured by hot-pressing as can be seen in Fig. 6. The mass loss of the worn nozzles was measured with an accurate electronic balance(minimum 2.2. Sand blasting tests 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(mD) .. Fig. 3 shows the schematic diagram of the abrasive air-jet divided by the nozzle density (d) times the mass of the erodent chine tool(GS-6 type), which consists of an air compressor, abrasive particles(m2) a blasting gun, a control valve, a particle supply tube, a filter, a desiccator, an adjusting press valve. a dust catcher, an abra- W= ml (1) sive hopper, and a nozzle. The dust catcher was used to prevent fugitive dust emissions. The air and grit flow adjusting was con- where the Whas the units of volume loss per unit mass(mm/g) Abrasive flow orifice Fig. 4. Schematic diagram of blasting gun structure(1, gun support; 2, air flow nozzle; 3, adjusting gasket; 4, ceramic nozzle; 5, plastic jacket for the nozzle)
122 D. Jianxin et al. / Materials Science and Engineering A 444 (2007) 120–129 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). nozzle entry with the compositional distribution changing from the entry layer to the exit layer with the lowest volume fraction of SiC (see Fig. 2(a)). While in Fig. 2(b), the compositional distribution of the laminated ceramic nozzle is symmetrical, the layer with the highest volume fraction of SiC was put both in the entry layer and in the exit layer. The homologous stress-free nozzle with no compositional change is shown in Fig. 2(c). The ceramic nozzle laminated only in entry area is named GN-2, the ceramic nozzle laminated both in entry and exit area is named GN-3, 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 h. Following drying, the mixtures composite powders with different mixture ratios were laminated into the mould in turn. The sample was then hot-pressed in flowing nitrogen for 40 min at 1900 ◦C temperature with 30 MPa pressure. 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 dust catcher was used to prevent fugitive dust emissions. The air and grit flow adjusting was conFig. 5. SEM micrograph of the SiC abrasives used for dry sand blasting. trolled by the valves and regulators. The abrasive air jet is formed in the blasting gun using a suction-type process as schematically illustrated in Fig. 4. 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 m grain size. The SEM micrograph of the SiC powders used for the dry sand blasting is shown in Fig. 5. As these abrasives are more durable and create less dust than sand, and typically are reclaimed and reused. Nozzles with internal diameter 8 mm and length 30 mm made from SiC/(W,Ti)C laminated structure (GN-2 and GN-3) and stress-free structure (CN-2) were manufactured by hot-pressing as can be seen in Fig. 6. 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 = m1 (d × m2) (1) where the W has the units of volume loss per unit mass (mm3/g). Fig. 4. Schematic diagram of blasting gun structure (1, gun support; 2, air flow nozzle; 3, adjusting gasket; 4, ceramic nozzle; 5, plastic jacket for the nozzle).
D Jianxin et al. Materials Science and Engineering A 444 (2007)120-129 Hardness of different layers of the SiC/(,TiC laminated nozzle(GN-2) Layer (W,TiC content (vol %) Vickers hardness, Hy(GPa) 26.52 3 4 5 24.67 6 where P is the indentation load (N), 2a is the catercorner length (um) due to indentation. Hardness of each layer of SiC/(W,Ti)C laminated nozzle(GN-2)material is presented in Table 2 ig. 7 illustrates -ray diffraction analysis Sic/W,Ti)C laminated ceramic nozzle(GN-2) material after Fig. 6. Photo of the SiC/w,Ti)C laminated ceramic nozzles. sintering at 1900 C for 40 min. It can be seen that both (w,Tic and Sic existed in the sintered specimens. SEM micrograph of each polished layer of Sic/(W,Ti)C laminated ceramic noz- The finite element method(FEM) was used as a means of zle(Gn-2)material are shown in Fig 8. The black areas were numerically evaluating the residual thermal stress and its dis- identified by EDX analysis as SiC, and the white phases with tribution of the laminated ceramic nozzle in the fabricating clear contrast were(W,Ti)C. It can be seen that the Sic particles processes are quite uniformly distributed throughout the microstructure. For observation of the micro-damage and determination of porosity is virtually absent. erosion mechanisms, the worn nozzles were sectioned axially The eroded bore surfaces of the nozzles were examined by scan- ning electron microscopy. 3.2. Residual thermal stress analysis of sic/(W,Ti)C laminated nozzle material 3. Results and discussion The residual thermal stress of the laminated ceramic noz- zle in the fabricating process was calculated by means of the 3.1. Microstructural characterization and properties of finite element method by assuming that the compact is cooled Sic(W, Ti)C laminated nozzle materials from sintering temperature 1900C to room temperature 20oC Thermo-mechanical properties of (W,Ti)C and Sic are as fol- Hardness measurements were performed by placing Vick- lows: rs indentations on every layer of the cross-sectional surface of SiC/W,Ti)C laminated nozzle (GN-2)material. The indentation (W, Ti)C: E=480 GPa, v=0.25, a=85x10 K load was 200N and a minimum of three indentations were tested for each layer. The Vickers hardness( GPa)of each layer is given k=214W/mK) P Hy=1.8544 (2a)2 4000 Table 1 3000 Dry sand blasting test conditions Sand blasting equipment GS-6 type sand blasting machine tool Sic/(W,Ti)C ceramic nozzle laminated only in Nozzle material entry area( GN-2 SiC/W, Ti)C ceramic nozzle laminated both in entry and exit area(GN-3) SiC/(W,Ti)C stress-free nozzle(CN-2) Dimension of nozzle omm(internal diameter)x 30 mm(length) 50-150um SiC powders 0.4 MPa Cumulative mass weigh Accurate electronic balance(minimum 0.1 mg) Fig. 7. X-ray diffraction analysis of the SiC/(W,Ti)c laminated ceramic nozzle material(GN-2)after sintering at 1900C for 40 min
D. Jianxin et al. / Materials Science and Engineering A 444 (2007) 120–129 123 Fig. 6. Photo of the SiC/(W,Ti)C laminated ceramic nozzles. The finite element method (FEM) was used as a means of numerically evaluating the residual thermal stress and its distribution of the laminated ceramic 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. Microstructural characterization and properties of SiC/(W,Ti)C laminated nozzle materials Hardness measurements were performed by placing Vickers indentations on every layer of the cross-sectional surface of SiC/(W,Ti)C laminated nozzle (GN-2) material. The indentation load was 200 N and a minimum of three indentations were tested for each layer. The Vickers hardness (GPa) of each layer is given by: Hv = 1.8544 P (2a) 2 (2) Table 1 Dry sand blasting test conditions Sand blasting equipment GS-6 type sand blasting machine tool Nozzle material SiC/(W,Ti)C ceramic nozzle laminated only in entry area (GN-2) SiC/(W,Ti)C ceramic nozzle laminated both in entry and exit area (GN-3) SiC/(W,Ti)C stress-free nozzle (CN-2) Dimension of nozzle Ø 8 mm (internal diameter) × 30 mm (length) Erodent abrasives 50–150m SiC powders Compressed air pressure 0.4 MPa Cumulative mass weigh Accurate electronic balance (minimum 0.1 mg) Table 2 Hardness of different layers of the SiC/(W,Ti)C laminated nozzle (GN-2) materials Layer (W,Ti)C content (vol.%) Vickers hardness, Hv (GPa) 1 55 26.89 2 57 26.52 3 59 25.93 4 61 25.70 5 63 24.67 6 65 24.15 where P is the indentation load (N), 2a is the catercorner length (m) due to indentation. Hardness of each layer of SiC/(W,Ti)C laminated nozzle (GN-2) material is presented in Table 2. Fig. 7 illustrates the X-ray diffraction analysis of the SiC/(W,Ti)C laminated ceramic nozzle (GN-2) 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. SEM micrographs of each polished layer of SiC/(W,Ti)C laminated ceramic nozzle (GN-2) material are shown in Fig. 8. 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. 3.2. Residual thermal stress analysis of SiC/(W,Ti)C laminated nozzle material The residual thermal stress of the laminated ceramic nozzle in the fabricating process was calculated by means of the finite element method by assuming that the compact is cooled from sintering temperature 1900 ◦C to room temperature 20 ◦C. Thermo-mechanical properties of (W,Ti)C and SiC are as follows: (W, Ti)C : E = 480 GPa, ν = 0.25, α = 8.5 × 10−6 K−1, k = 21.4 W/(m K). Fig. 7. X-ray diffraction analysis of the SiC/(W,Ti)C laminated ceramic nozzle material (GN-2) after sintering at 1900 ◦C for 40 min.
D Jianxin et al./ Materials Science and Engineering A 444(2007)120-129 05e1115gy氵i:u a) ()L6561169:636n ?虑 Fig 8. SEM micrographs of each polished layer of SiC/(W,Ti)C laminated ceramic nozzle material (GN-2):(a)the first layer(entry zone). (b)the second layer, (c) the third layer, (d)the fourth layer, (e)the fifth second layer, and(f)the sixth layer(exit zone) SiC:E=450GPa,u=0.16.,a=46×10-6K-1 and the maximum value is.003 MPa,-130949 MPa, and k=33.5W/(mK) -265.368 MPa, respectively. Therefore, laminated structures in ceramic nozzles can form an excess compressive residual stresses in the nozzle entry (or exit)region during fabricating Owing to the symmetry, an axisymmetric calculation was process preferred Presume that it was steady state boundary conditions The FEM gridding model of the laminated nozzle is shown in Fig 9. The results of the distribution of axial (oz), radial(or), and circumferential(oe)residual thermal stresses in the GN-2 laminated nozzle in cooling process from sintering temperature to room temperature are showed in Fig. 10. As can be seen, an excess residual thermal stress is formed in the nozzle entry region for the GN-2 laminated nozzle. It is indicated that axial(oz), radial(o), and circumferential(oe) residual thermal stresses at the nozzle entry zone are compressive, and the maximum value is -71.018 MPa,-121578 MPa, and -276 204 MPa, respec- tively Fig. 11 shows the distribution of axial (oz), radial (or). and circumferential(oe)residual thermal stresses in the GN-3 laminated nozzle in cooling process from sintering tempera ture to room temperature. It is obvious that an excess resid ual thermal stress is formed both in nozzle entry and exit region for the GN-3 laminated nozzle, and the axial(oz), radial (or), and circumferential(oe) residual thermal stresses both at the nozzle entry zone and at the exit zone are compressive Fig. 9. FEM gridding model of the laminated nozzle
124 D. Jianxin et al. / Materials Science and Engineering A 444 (2007) 120–129 Fig. 8. SEM micrographs of each polished layer of SiC/(W,Ti)C laminated ceramic nozzle material (GN-2): (a) the first layer (entry zone), (b) the second layer, (c) the third layer, (d) the fourth layer, (e) the fifth second layer, and (f) the sixth layer (exit zone). SiC : E = 450 GPa, ν = 0.16, α = 4.6 × 10−6 K−1, k = 33.5 W/(m K). Owing to the symmetry, an axisymmetric calculation was preferred. Presume that it was steady state boundary conditions. The FEM gridding model of the laminated nozzle is shown in Fig. 9. The results of the distribution of axial (σz), radial (σr), and circumferential (σ) residual thermal stresses in the GN-2 laminated nozzle in cooling process from sintering temperature to room temperature are showed in Fig. 10. As can be seen, an excess residual thermal stress is formed in the nozzle entry region for the GN-2 laminated nozzle. It is indicated that axial (σz), radial (σr), and circumferential (σ) residual thermal stresses at the nozzle entry zone are compressive, and the maximum value is −71.018 MPa, −121.578 MPa, and −276.204 MPa, respectively. Fig. 11 shows the distribution of axial (σz), radial (σr), and circumferential (σ) residual thermal stresses in the GN-3 laminated nozzle in cooling process from sintering temperature to room temperature. It is obvious that an excess residual thermal stress is formed both in nozzle entry and exit region for the GN-3 laminated nozzle, and the axial (σz), radial (σr), and circumferential (σ) residual thermal stresses both at the nozzle entry zone and at the exit zone are compressive, and the maximum value is −94.003 MPa, −130.949 MPa, and −265.368 MPa, respectively. Therefore, laminated structures in ceramic nozzles can form an excess compressive residual stresses in the nozzle entry (or exit) region during fabricating process. Fig. 9. FEM gridding model of the laminated nozzle.