Wear266(2009)30-36 Contents lists available at Science Direct WEAR Wear ELSEVIER journalhomepagewww.elsevier.com/locate/wear Wear behaviors of ceramic nozzles with laminated structure at their entry Deng jianxin a, b, a School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, PR China b School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China ARTICLE INFO ABSTRACT Laminated ceramic nozzles(GNl, GN2, and GN3)with different architecture and composition at the con- eceived in revised form 1 May 2008 stituent layers were produced by hot pressing. The mechanical properties of the constituent layers of the laminated materials were measured. the microstructure was examined. The wear behaviors of these lam- Available online 15 July 2008 nated nozzles were investigated and compared with an unstressed reference nozzle( Cn). results showed that the surface layer(nozzle entry )possessed higher hardness and fracture toughness than those of other Keywords layers of the laminated nozzles, and is greatly improved over the stress-free nozzle. The laminated noz. oston wear zles had superior erosion wear resistance to that of the stress-free one, especially at the nozzle entry. The mechanism responsible was explained as the formation of compressive residual stresses at the entry of the laminated nozzles during fabricating proces erosion wear resistance of the laminated nozzles constituent layers, but also by the difference in with 2% difference of (W,Tic adjacent layers exhibited higher erosion wear resistance over the GNI and gN3 nozzles. Surface mechan- ical properties improvement at the entry area was found to be the main reason for the increase of erosion wear resistance of the gn2 nozzle o 2008 Elsevier B V. All rights reserved 1. Introduction is to couple material layers with different thermal expansion coef- ficients(CTE)so that residual stresses arise during cooling from the Nozzle is the most critical part in the abrasive air-jet equipment. sintering temperature. It has been shown that laminated structure ramics being with high wear resistance have great potentials as constituted by alternate layers of different materials can induce a the nozzle materials. There are many factors that influence the surface compressive stress leading to improved mechanical proper nozzle wear such as: flow rate and impact angle, erodent ties. The design of ceramic laminates has been proved to be a viable abrasive properties, nozzle material and its geometry [1-4. As the strategy to obtain significant increases of the fracture resistance of erosive particles hit the nozzle at high impact angles (nearly 90) ceramic materials [12]. The effectiveness of laminated structures at the entry in abrasive air-jet machining, the nozzle entry suffers in improving the tribological properties has been also reported form severe abrasive impact, and generates large tensile stress [5].[13-16. direction of the nozzle decreases fromentry to center, and increases tures was firstly introduced to the design of cer aminated struc- Several studies [5-8 have shown that the stress along the axial In the authors previous studies, the idea of rom center to exit. The highest tensile stresses are located at the and then, a Sic/w Ti)c ceramic nozzle with six-layer structures nozzle entry. Therefore, the erosion wear of the nozzle entry is was produced by hot pressing [ 18, 19, the(W,Ti)C volume fraction always serious contrast with that of the center and exit [ 5-8. in the constituent layers of this laminated nozzle is 55, 57, 59, 61 A novel technique, by which a compressive residual stress can 63, 65 voL%, respectively, the difference of(W, Ti)C content between generated into the surface of a material is the production of adjacent layers is 2%. Results showed that laminated structures in laminated structures designed to combine the advantageous char- this nozzle can induce ss residual stress in the nozzle during cteristics of the different materials involved, thereby improving fabricating process, and this residual stress at the nozzle entry zone the overall mechanical behavior of the system 9-11.The basic idea is compressive whatever the sintering temperature. This kind of oppressive residual stress can result in an improved erosion wear resistance of the laminated nozzle compared with the homologous stress-free nozzle 18, 19]. A model for the design of symmetrical correspondence address. School of mechanical Engineering. Shandong Univer- multilayered ceramic nozzle was also presented (20, 21). and this ax:+865312955999 nozzle has a symmetrical structure at its entry and exit. Among all E-mailaddress:jxdeng@sdu.edu.cn. these works, effect of the composition and mechanical properties 648/s-see front matter o 2008 Elsevier B.V. All rights reserved
Wear 266 (2009) 30–36 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Wear behaviors of ceramic nozzles with laminated structure at their entry Deng Jianxina,b,∗ a School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, PR China b School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China article info Article history: Received 27 June 2007 Received in revised form 1 May 2008 Accepted 27 May 2008 Available online 15 July 2008 Keywords: Erosion wear Nozzles Laminated structures Ceramic materials abstract Laminated ceramic nozzles (GN1, GN2, and GN3) with different architecture and composition at the constituent layers were produced by hot pressing. The mechanical properties of the constituent layers of the laminated materials were measured, the microstructure was examined. The wear behaviors of these laminated nozzles were investigated and compared with an unstressed reference nozzle (CN). Results showed that the surface layer (nozzle entry) possessed higher hardness and fracture toughness than those of other layers of the laminated nozzles, and is greatly improved over the stress-free nozzle. The laminated nozzles had superior erosion wear resistance to that of the stress-free one, especially at the nozzle entry. The mechanism responsible was explained as the formation of compressive residual stresses at the entry of the laminated nozzles during fabricating processes. The erosion wear resistance of the laminated nozzles was influenced not only by the volume fraction of the constituent layers, but also by the difference in composition between adjacent layers. The GN2 nozzle with 2% difference of (W,Ti)C content between adjacent layers exhibited higher erosion wear resistance over the GN1 and GN3 nozzles. Surface mechanical properties improvement at the entry area was found to be the main reason for the increase of erosion wear resistance of the GN2 nozzle. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Nozzle is the most critical part in the abrasive air-jet equipment. Ceramics being with high wear resistance have great potentials as the nozzle materials. There are many factors that influence the nozzle wear such as: mass flow rate and impact angle, erodent abrasive properties, nozzle material and its geometry [1–4]. As the erosive particles hit the nozzle at high impact angles (nearly 90◦) at the entry in abrasive air-jet machining, the nozzle entry suffers form severe abrasive impact, and generates large tensile stress [5]. Several studies [5–8] have shown that the stress along the axial direction of the nozzle decreases from entry to center, and increases from center to exit. The highest tensile stresses are located at the nozzle entry. Therefore, the erosion wear of the nozzle entry is always serious contrast with that of the center and exit areas [5–8]. 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 behavior of the system [9–11]. The basic idea ∗ Correspondence address. School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China. Tel.: +86 531 88392047; fax: +86 531 2955999. E-mail address: jxdeng@sdu.edu.cn. is to couple material layers with different thermal expansion coef- ficients (CTE) so that residual stresses arise during cooling from the sintering temperature. It has been shown that laminated structures constituted by alternate layers of different materials can induce a surface compressive stress leading to improved mechanical properties. The design of ceramic laminates has been proved to be a viable strategy to obtain significant increases of the fracture resistance of ceramic materials [12]. The effectiveness of laminated structures in improving the tribological properties has been also reported [13–16]. In the author’s previous studies, the idea of laminated structures was firstly introduced to the design of ceramic nozzles [17], and then, a SiC/(W,Ti)C ceramic nozzle with six-layer structures was produced by hot pressing [18,19], the (W,Ti)C volume fraction in the constituent layers of this laminated nozzle is 55, 57, 59, 61, 63, 65 vol.%, respectively, the difference of (W,Ti)C content between adjacent layers is 2%. Results showed that laminated structures in this nozzle can induce an excess residual stress in the nozzle during fabricating process, and this residual stress at the nozzle entry zone is compressive whatever the sintering temperature. This kind of compressive residual stress can result in an improved erosion wear resistance of the laminated nozzle compared with the homologous stress-free nozzle [18,19]. A model for the design of symmetrical multilayered ceramic nozzle was also presented [20,21], and this nozzle has a symmetrical structure at its entry and exit. Among all these works, effect of the composition and mechanical properties 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.05.012
J. Deng/wear266(2009)30-36 Table 1 Particle size, purity and manufacturer of the starting powders Powders Average particle size (um) Purity (6) Beijing Antai Advanced Tech and Materials Co, Ltd. w,Ti)c >999 Zhuzhou cemented carbide works Exit cps mm The sixth layer(nozzle exit) The fifth layer Φ20mm Entry Entry Fig. 1. Compositional distribution of (a)the laminated nozzle and (b)the homologous stress-free nozzle. of the constituent layers on the wear behaviors of the laminated ual stress [9-12]. Compressive residual stresses are induced nozzles was not considered in layers with lower coefficients of thermal expansion(CTE). t In the present study, laminated ceramic nozzles with different while tensile stresses arise in those with higher CTE.There- hitecture and composition at the constituent layers were pro- fore, to form compressive residual stresses at the nozzle entr duced by hot pressing. The mechanical properties of the constituent the Cte of the nozzle entry( the first layer) should be the low layers of the laminated materials were measured, the microstruc- est among all the layers of the material. As the cte of Sic is ture was examined. Particular attention was paid to the effect of lower than that of (W,ti)c, the layer with the highest volume the composition and mechanical properties of the constituent lay fraction of Sic was put in the nozzle entry with the composi ers on wear behaviors of ceramic nozzles with laminated structure tional distribution changing from the entry layer to the exit layer at their entry. with the highest volume fraction of (W,Ti)c as can be seen in 2. Materials and experimental procedures Six SiC/(W,Ti)C composite powders with different mixture ratios were prepared respectively by wet ball milling in alcohol with 2.1. Preparation of the laminated ceramic nozzle cemented carbide balls for 150 h Following drying, the mixtures composite powders with different mixture ratios were laminated The starting powders used to fabricate the laminated ceramic into the mould. The sample was then hot-pressed in flowing nozzle are listed in Table 1 with their particle size, purity and nitrogen for 40 min at 1900C temperature and 30 MPa pres manufacturer. Silicon carbide( SiC)was used as the baseline mate- sure. As can be seen in Table 2, the laminated nozzles with 1.2 rial. Additions of(W, Ti)C solid solution particles were added to Sic and 5% difference of (w tic content between adjacent layers are matrix. Six different volume fractions of (W, Ti)c were selected in named GNl, GN2, and GN3, respectively, all constituent layers in designing the laminated nozzles with a five-layer structure at their GNl, GN2, and GN3 laminated nozzles were kept 1 mm in thick- entry. The compositional distribution of the laminated ceramic noz ness zle is shown in Fig. 1(a). It is indicated that the compositional For the purpose of son, a homologous stress-free distribution changes in axial direction at the entry of the nozzle. ceramic nozzle was also were manufactured by hot pressing as can The range of (W, Ti)C additions to Sic was from 55 to 80 vol% as can be seen 1(b). This stress-free nozzle with no compositional change is named CN, its compositions and mechanical properties It has been shown that laminated structure constituted by are listed in Table 3. All of the four nozzles have the same compo- alternate layers of different materials can induce a surface resid- sitions at their entry section (the first layer or surface layer) Difference of (w, Ti)c content W,Ti)c content(vol%) between adjacent layers(%) Layer 1(nozzle entry) Layer 2 Layer 3 Layer 4 Layer Layer 6(nozzle exit)
J. Deng / Wear 266 (2009) 30–36 31 Table 1 Particle size, purity and manufacturer of the starting powders Powders Average particle size (m) Purity (%) Manufacture SiC 2–3 >98.9 Beijing Antai Advanced Tech. and Materials Co., Ltd. (W,Ti)C 1–2 >99.9 Zhuzhou cemented carbide works Fig. 1. Compositional distribution of (a) the laminated nozzle and (b) the homologous stress-free nozzle. of the constituent layers on the wear behaviors of the laminated nozzles was not considered. In the present study, laminated ceramic nozzles with different architecture and composition at the constituent layers were produced by hot pressing. The mechanical properties of the constituent layers of the laminated materials were measured, the microstructure was examined. Particular attention was paid to the effect of the composition and mechanical properties of the constituent layers on wear behaviors of ceramic nozzles with laminated structure at their entry. 2. Materials and experimental procedures 2.1. Preparation of the laminated ceramic nozzle The starting powders used to fabricate the laminated ceramic nozzle are listed in Table 1 with their particle size, purity and manufacturer. Silicon carbide (SiC) was used as the baseline material. Additions of (W,Ti)C solid solution particles were added to SiC matrix. Six different volume fractions of (W,Ti)C were selected in designing the laminated nozzles with a five-layer structure at their entry. The compositional distribution of the laminated ceramic nozzle is shown in Fig. 1(a). It is indicated that the compositional distribution changes in axial direction at the entry of the nozzle. The range of (W,Ti)C additions to SiC was from 55 to 80 vol.% as can be seen in Table 2. It has been shown that laminated structure constituted by alternate layers of different materials can induce a surface residual stress [9–12]. Compressive residual stresses are induced in layers with lower coefficients of thermal expansion (CTE), while tensile stresses arise in those with higher CTE. Therefore, to form compressive residual stresses at the nozzle entry, the CTE of the nozzle entry (the first layer) should be the lowest among all the layers of the material. As the CTE of SiC is lower than that of (W,Ti)C, the layer with the highest volume fraction of SiC was put in the nozzle entry with the compositional distribution changing from the entry layer to the exit layer with the highest volume fraction of (W,Ti)C as can be seen in Table 2. Six SiC/(W,Ti)C composite powders with different mixture ratios were prepared respectively by wet ball milling in alcohol with cemented carbide balls for 150 h. Following drying, the mixtures composite powders with different mixture ratios were laminated into the mould. The sample was then hot-pressed in flowing nitrogen for 40 min at 1900 ◦C temperature and 30 MPa pressure. As can be seen in Table 2, the laminated nozzles with 1, 2, and 5% difference of (W,Ti)C content between adjacent layers are named GN1, GN2, and GN3, respectively, all constituent layers in GN1, GN2, and GN3 laminated nozzles were kept 1 mm in thickness. For the purpose of comparison, a homologous stress-free ceramic nozzle was also were manufactured by hot pressing as can be seen in Fig. 1(b). This stress-free nozzle with no compositional change is named CN, its compositions and mechanical properties are listed in Table 3. All of the four nozzles have the same compositions at their entry section (the first layer or surface layer). Table 2 Compositions of the laminated ceramic nozzles Code name Difference of (W,Ti)C content between adjacent layers (%) (W,Ti)C content (vol.%) Layer 1 (nozzle entry) Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 (nozzle exit) GN1 1 55 56 57 58 59 60 GN2 2 55 57 59 61 63 65 GN3 5 55 60 65 70 75 80
J. Deng/Wear266(2009)30-36 Table 3 Table 4 Mechanical properties of CN homologous stress-free ceramic nozzle Abrasive air-jet machining conditions Compositions(vol.s SiC55%(W, TIc Abrasive air-jet equipment GS-6 type abrasive air-jet machine tool lexural strength(MPa) Nozzle material GN1, GN2, and gN3 laminated nozzles ardness(GPa) CN stress-free nozzle Fracture toughness(MPa m/2 492±03 Dimension of the nozzles damm(internal diameter)x 30mm Filter Pressure gauge Control valve Compressed air pressure Cumulative mass weight Balance(minimum 0.01 mg) ● density(d) times the mass of the erodent abrasive particles (m2). Air supply tu W (1) g gun Flow meter where the W has the units of volume loss per unit mass(mm /g) For observation of the microdamage and determination of ero- sion mechanisms, the worn nozzles were sectioned axially. the eroded bore surfaces of the nozzles were examined by scanning electron microscopy (SEM). Sand hopper le supply tube 3. Results and discussion 3.1. Microstructural characterization and properties of the Fig. 2. Schematic diagram of the abrasive air-jet machine tool. laminated nozzles The SEM micrograph of the cross-section surface of the GN2 2. 2. Abrasive air-jet machining laminated nozzle material is shown in Fig 4. A five-layer laminated structure can be clearly seen. SEM micrographs of the polished lay Fig. 2 shows the schematic diagram of the abrasive air-jet ers of the GN2 laminated nozzle material are shown in Fig.5. The machine tool(GS-6 type, made in China), which consists of an air black areas were identified by EDX analysis as SiC, and the white compressor,a blasting gun, a control valve a particle supply tube, phases with clear contrast were(W, Ti)C. It can be seen that the a filter, a desiccator, an adjusting press valve, an abrasive hopper W, Ti)c particles are quite uniformly distributed throughout the and a nozzle. The air and grit flow adjusting was controlled by the microstructure, porosity is virtually absent. valves and regulators. The gas flow rate is controlled by the com- Hardness measurements were performed by placing Vickers pressed air. The compressed air pressure was set at 0. 4 MPa, and the indentations(MH 6)on each layer of the cross-sectional surface The erodent abrasives were of silicon carbide(SiC) powders with The results gathered on 10 specimens were presented in Table 5 50-100 um grit size. The SEM micrograph of the Sic powders The surface layer(the first layer or the nozzle entry) with high Sic shown in Fig 3. All the test conditions are listed in Table 4. volume fraction shows higher hardness than that of other layers of The mass loss of the worn ceramic nozzle was measured with the laminated nozzles. The GN2 laminated nozzle( with 2% differ- a balance(minimum 0.01 mg). The erosion rates(w) of the noz- ence of (w, nc content between adjacent layers) shows the highest zles are defined as the nozzle mass loss(m1)divided by the nozzle surface hardness. By comparison with the CN stress-free ceramic nozzle(see Table 3), the hardness at the entry area of the lami nated nozzles is much more improved, and rose from 25.5 GPa for CN nozzle to 26.9 GPa for GN2 laminated nozzle, representing a maximum increase of 1. 4 gpa iC/61 Vole The fourth layer SiC/59Vol%(W,IOC The third layer SiC/55Vol %(W,T I mm 70 um Fig 4. SEM micrograph of the cross-section surface of the GN2 laminated nozzle 3. SEN
32 J. Deng / Wear 266 (2009) 30–36 Table 3 Mechanical properties of CN homologous stress-free ceramic nozzle Compositions (vol.%) SiC/55%(W,Ti)C Flexural strength (MPa) 548.6 ± 43 Hardness (GPa) 25.5 ± 0.6 Fracture toughness (MPa m1/2) 4.92 ± 0.3 Fig. 2. Schematic diagram of the abrasive air-jet machine tool. 2.2. Abrasive air-jet machining Fig. 2 shows the schematic diagram of the abrasive air-jet machine tool (GS-6 type, made in China), which consists of an air compressor, a blasting gun, a control valve, a particle supply tube, a filter, a desiccator, an adjusting press valve, 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. The compressed air pressure was set at 0.4 MPa, and the abrasive particle velocity through the nozzle is adjusted to 60 m/s. The erodent abrasives were of silicon carbide (SiC) powders with 50–100 m grit size. The SEM micrograph of the SiC powders is shown in Fig. 3. All the test conditions are listed in Table 4. The mass loss of the worn ceramic nozzle was measured with a balance (minimum 0.01 mg). The erosion rates (W) of the nozzles are defined as the nozzle mass loss (m1) divided by the nozzle Fig. 3. SEM micrograph of the SiC abrasives used for abrasive air-jet machining. Table 4 Abrasive air-jet machining conditions Abrasive air-jet equipment GS-6 type abrasive air-jet machine tool Nozzle material GN1, GN2, and GN3 laminated nozzles CN stress-free nozzle Dimension of the nozzles ˚ 8 mm (internal diameter) × 30 mm (length) Erodent abrasives 50–100 m SiC powders Compressed air pressure 0.4 MPa Cumulative mass weight Balance (minimum 0.01 mg) density (d) times the mass of the erodent abrasive particles (m2). W = m1 (dm2) (1) where the W has the units of volume loss per unit mass (mm3/g). For observation of the microdamage and determination of erosion mechanisms, the worn nozzles were sectioned axially. The eroded bore surfaces of the nozzles were examined by scanning electron microscopy (SEM). 3. Results and discussion 3.1. Microstructural characterization and properties of the laminated nozzles The SEM micrograph of the cross-section surface of the GN2 laminated nozzle material is shown in Fig. 4. A five-layer laminated structure can be clearly seen. SEM micrographs of the polished layers of the GN2 laminated nozzle material are shown in Fig. 5. 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 (W,Ti)C particles are quite uniformly distributed throughout the microstructure, porosity is virtually absent. Hardness measurements were performed by placing Vickers indentations (MH 6) on each layer of the cross-sectional surface of the laminated nozzle material. The indentation load was 20 N. The results gathered on 10 specimens were presented in Table 5. The surface layer (the first layer or the nozzle entry) with high SiC volume fraction shows higher hardness than that of other layers of the laminated nozzles. The GN2 laminated nozzle (with 2% difference of (W,Ti)C content between adjacent layers) shows the highest surface hardness. By comparison with the CN stress-free ceramic nozzle (see Table 3), the hardness at the entry area of the laminated nozzles is much more improved, and rose from 25.5 GPa for CN nozzle to 26.9 GPa for GN2 laminated nozzle, representing a maximum increase of 1.4 GPa. Fig. 4. SEM micrograph of the cross-section surface of the GN2 laminated nozzle material.
J. Deng/wear266(2009)30-36 乳 Fig. 5. SEM micrographs of the polished surface of the GN2 laminated nozzle:(a)the first layer(nozzle entry) (b)the third layer, and (c) the sixth layer (nozzle exit). Table 5 hardness of different layers of the laminated ceramic nozzles Code name Hardness(GPa) Layer 1(nozzle entry) Layer 2 Layer 3 Layer 4 Layer 6(nozzle ex 26.6±0 263± 26.0±0.5 25.7±06 248±0 24.1士0.5 69±0.7 6.5±0.5 260±0.6 8±0.6 44.7±0.5 42士0 GN3 6.6±0.6 260士0.7 5.8±0.5 25.5±0.5 246±0.5 24.1±0.5 of the laminated ceramic nozzles Layer 1(nozzle entry) Layer 2 Layer 3 Layer 5 Layer 6(nozzle exit) ±0.3 5.3±0.3 4.1±03 4.0±0.2 58±0.2 45士02 5.6±03 5.0±0.2 3.8±0.2 3.5±0.2 Fracture toughness measurement was performed using inden- Fig 8 shows the increase of exit bore diameter with the eros tation method in a hardness tester on every layer of the time for the laminated and the stress-free nozzles. The increase of cross-sectional surface of the laminated nozzle using the formula the exit bore diameter follows an almost linear rate law, there is no oposed by Cook and Lawn 22]. Number of tests used to measure much difference in the increase of exit bore diameter between the the fracture toughness is 10, and the results are listed in Table 6. laminated and the stress-free nozzles. The surface layer(nozzle entry) shows higher fracture toughness Fig 9 shows the comparison of the erosion rates for GNl, GN2, han that of other layers of the laminated nozzles, and is greatly GN3, and CN nozzles in abrasive air-jet machining. It is obvious aproved compared with that of the CN stress-free nozzle(see that the erosion rate of the CN stress-free nozzle is the high- Table 3). The GN2 laminated nozzle shows the highest surface frac- est, and the gn2 laminated nozzle the smallest. Therefore, it is ture toughness. apparently that the laminated nozzles exhibit higher erosion wear 3. 2. Wear behaviors of the laminated nozzles -o- CN stress-free nozzle The wear behaviors of the laminated ceramic nozzles(gn1, TH GNI laminated nozzle GN2, and gn3)in abrasive air-jet machining were investigated comparison with the homologous stress-free nozzle(CN). Fig. 6 99-·aN2 laminated nozzle shows the cumulative mass loss of the laminated and the stress- 一口GN3 Laminated nozzle free nozzles. It is noted that the cumulative mass loss continuously creased with the operation time. Compared with the laminated ozzie the CN stress-free nozzle showed higher cumulative mass loss under the same test conditions the gn2 laminated nozzle (with 2% difference of (W, Ti)c content between adjacent layers) showed the lowest cumulative mass among all the nozzles tested. The results of the increase of entry bore diameter with the ero sion time for the laminated and the stress-free nozzles are shown in Fig. 7. The entry bore diameter enlarges greatly with the opera- tion time for CN stress-free nozzle; while the entry bore diameter increases slowly with the operation time for laminated nozzles Erosion time(min) The GN2 laminated nozzle showed the lowest entry bore diameter Fig. 6. Cumulative mass loss of CN, GNI, GN2, and GN3 nozzles with the erosion increase among all the nozzles tested
J. Deng / Wear 266 (2009) 30–36 33 Fig. 5. SEM micrographs of the polished surface of the GN2 laminated nozzle: (a) the first layer (nozzle entry), (b) the third layer, and (c) the sixth layer (nozzle exit). Table 5 Hardness of different layers of the laminated ceramic nozzles Code name Hardness (GPa) Layer 1 (nozzle entry) Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 (nozzle exit) GN1 26.6 ± 0.7 26.3 ± 0.6 26.0 ± 0.5 25.7 ± 0.6 24.8 ± 0.5 24.1 ± 0.5 GN2 26.9 ± 0.7 26.5 ± 0.5 26.0 ± 0.6 25.8 ± 0.6 24.7 ± 0.5 24.2 ± 0.4 GN3 26.6 ± 0.6 26.0 ± 0.7 25.8 ± 0.5 25.5 ± 0.5 24.6 ± 0.5 24.1 ± 0.5 Table 6 Fracture toughness of different layers of the laminated ceramic nozzles Code name Fracture toughness (MPa m1/2) Layer 1 (nozzle entry) Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 (nozzle exit) GN1 5.7 ± 0.4 5.5 ± 0.3 5.3 ± 0.3 4.4 ± 0.2 4.1 ± 0.3 4.0 ± 0.2 GN2 5.8 ± 0.2 5.7 ± 0.3 5.5 ± 0.2 5.0 ± 0.2 4.5 ± 0.2 4.3 ± 0.1 GN3 5.6 ± 0.3 5.3 ± 0.4 5.0 ± 0.2 4.1 ± 0.3 3.8 ± 0.2 3.5 ± 0.2 Fracture toughness measurement was performed using indentation method in a hardness tester on every layer of the cross-sectional surface of the laminated nozzle using the formula proposed by Cook and Lawn [22]. Number of tests used to measure the fracture toughness is 10, and the results are listed in Table 6. The surface layer (nozzle entry) shows higher fracture toughness than that of other layers of the laminated nozzles, and is greatly improved compared with that of the CN stress-free nozzle (see Table 3). The GN2 laminated nozzle shows the highest surface fracture toughness. 3.2. Wear behaviors of the laminated nozzles The wear behaviors of the laminated ceramic nozzles (GN1, GN2, and GN3) in abrasive air-jet machining were investigated in comparison with the homologous stress-free nozzle (CN). Fig. 6 shows the cumulative mass loss of the laminated and the stressfree nozzles. It is noted that the cumulative mass loss continuously increased with the operation time. Compared with the laminated nozzle, the CN stress-free nozzle showed higher cumulative mass loss under the same test conditions. The GN2 laminated nozzle (with 2% difference of (W,Ti)C content between adjacent layers) showed the lowest cumulative mass among all the nozzles tested. The results of the increase of entry bore diameter with the erosion time for the laminated and the stress-free nozzles are shown in Fig. 7. The entry bore diameter enlarges greatly with the operation time for CN stress-free nozzle; while the entry bore diameter increases slowly with the operation time for laminated nozzles. The GN2 laminated nozzle showed the lowest entry bore diameter increase among all the nozzles tested. Fig. 8 shows the increase of exit bore diameter with the erosion time for the laminated and the stress-free nozzles. The increase of the exit bore diameter follows an almost linear rate law, there is no much difference in the increase of exit bore diameter between the laminated and the stress-free nozzles. Fig. 9 shows the comparison of the erosion rates for GN1, GN2, GN3, and CN nozzles in abrasive air-jet machining. It is obvious that the erosion rate of the CN stress-free nozzle is the highest, and the GN2 laminated nozzle the smallest. Therefore, it is apparently that the laminated nozzles exhibit higher erosion wear Fig. 6. Cumulative mass loss of CN, GN1, GN2, and GN3 nozzles with the erosion time
J. Deng/Wear266(2009)30-36 CN stress-free nozzle a CN stress-free nozzle GNI laminated nozzle -o GN2 laminated nozzle ■GN3 laminated nozzle -O GN3 laminated nozzle Entry bore diamete 口GN2 laminated nozzle 1202403 Erosion time(m Fig 9. Comparison of the erosion rates of CN, GNl, GN2, and GN3 nozzles. Fig. 7. Increase of entry bore diameter of CN, GNI, GN2, and GN3 nozzles with the The worn ceramic nozzles were cut after operation in longitudi nal directions for failure analysis, and the worn inner bore surfaces were analysed by SEM. Fig. 11 shows the sEm micrographs of the 2< Exit bore diameter nner bore profile of the whole nozzle after abrasive air-jet machin- ing for 540 min. The inner bore diameter of the worn CN nozzle H GN3 laminated nozzle along the nozzle longitudinal directions is larger than that of the -o--CN stress-free nozzle worn GN2 laminated nozzle, especially at the nozzle entry ●GN2 laminated nozzle t GNI laminated nozzle 3.3. Discussion Ceramic nozzle failure by erosion wear is generally caused by fracture owing to the large tensile stress at the nozzle entry 5-8 The erosion wear at the entry is always serious contrast with that of the center and exit area for the stress-free nozzles nozzle failure by erosion wear is usually accompanied by chipping in the nozzle entry. Because the nozzle entrance suffers severe abrasive impact. 0 and generates large tensile stress, which may cause the subsurface lateral cracks and facilitates removal of the material chips. Thus, the erosion wear of the nozzle depends on the stress distribution at the entry. Once the maximum tensile stress exceeds the ultimate Fig.8. Increase of exit bore diameter of CN, GNl GN2, and GN3 nozzles with the strength of the ceramic nozzle material, fracture will occur 5-8- As can be seen in Tables 2 and 3 and Figs. 8-11, all of the four nozzles have the same compositions at their entry section(surface resistance over the stress-free one under the same test condi- layer), but they showed different erosion wear resistance, especially at the nozzle entry area. The Cn stress-free nozzles revealed lower The entry bore profiles of worn GN2 and CN nozzles after abra- wear resistance than that of the laminated ones(GNl, GN2, and sive air-jet machining for 540 min are shown in Fig. 10. The entry GN3), while the GN2 nozzles presented the highest wear resistance bore of CN stress-free nozzle was severely worn; while the entry among the laminated nozzles. There may be two reasons, one is the ore of GN2 laminated nozzle had worn slightly compared with formation of residual stresses at the entry area of the laminated hat of the stress-free one nozzles, and the other is the difference in mechanical properties Fig. 10. Entry profile of the nozzles, (a)before operation. (b) GN2 laminated nozzle after 540 min operation, and (c)oN stress-free nozzle after 540 min operation
34 J. Deng / Wear 266 (2009) 30–36 Fig. 7. Increase of entry bore diameter of CN, GN1, GN2, and GN3 nozzles with the erosion time. Fig. 8. Increase of exit bore diameter of CN, GN1, GN2, and GN3 nozzles with the erosion time. resistance over the stress-free one under the same test conditions. The entry bore profiles of worn GN2 and CN nozzles after abrasive air-jet machining for 540 min are shown in Fig. 10. The entry bore of CN stress-free nozzle was severely worn; while the entry bore of GN2 laminated nozzle had worn slightly compared with that of the stress-free one. Fig. 9. Comparison of the erosion rates of CN, GN1, GN2, and GN3 nozzles. The worn ceramic nozzles were cut after operation in longitudinal directions for failure analysis, and the worn inner bore surfaces were analysed by SEM. Fig. 11 shows the SEM micrographs of the inner bore profile of the whole nozzle after abrasive air-jet machining for 540 min. The inner bore diameter of the worn CN nozzle along the nozzle longitudinal directions is larger than that of the worn GN2 laminated nozzle, especially at the nozzle entry. 3.3. Discussion Ceramic nozzle failure by erosion wear is generally caused by fracture owing to the large tensile stress at the nozzle entry [5–8]. The erosion wear at the entry is always serious contrast with that of the center and exit area for the stress-free nozzles. Nozzle failure by erosion wear is usually accompanied by chipping in the nozzle entry. Because the nozzle entrance suffers severe abrasive impact, and generates large tensile stress, which may cause the subsurface lateral cracks and facilitates removal of the material chips. Thus, the erosion wear of the nozzle depends on the stress distribution at the entry. Once the maximum tensile stress exceeds the ultimate strength of the ceramic nozzle material, fracture will occur [5–8]. As can be seen in Tables 2 and 3 and Figs. 8–11, all of the four nozzles have the same compositions at their entry section (surface layer), but they showed different erosion wear resistance, especially at the nozzle entry area. The CN stress-free nozzles revealed lower wear resistance than that of the laminated ones (GN1, GN2, and GN3), while the GN2 nozzles presented the highest wear resistance among the laminated nozzles. There may be two reasons, one is the formation of residual stresses at the entry area of the laminated nozzles, and the other is the difference in mechanical properties Fig. 10. Entry profile of the nozzles, (a) before operation, (b) GN2 laminated nozzle after 540 min operation, and (c) CN stress-free nozzle after 540 min operation
J. Deng/wear266(2009)30-36 Ent CN nozzle GN2 laminated nozzle Fig. I1. Worn inner-hole profiles of (a)CN stress-free nozzle and(b)GN2 laminated nozzle after 540 min operation. owing to the variety of volume fraction of the constituent layers the general form as expressed below: nd the (w,ti c content between adjacent layers in the laminated nozzle The higher erosion wear resistance especially at the entry of the V=AE HKC laminated nozzle compared with the stress-free one can be anal- ysed in terms of the formation of compressive residual stresses at where V is the volume of material removed, E is the elastic modu- the entry. As can be seen in Refs. [18, 19]. there is an excess compres- lus, Hy is the hardness, Kic is the fracture toughness, A is a constant, sive residual stress formed at the entry of the laminated nozzle and the exponents n, p, and q all depend on the model of mate- during cooling from the sintering temperature, which may par- rial removal. In the case of a brittle material response, the lateral ly counteract the tensile stresses at the nozzle entry resulting crack length that contributes to material removal is a function of fromexternal loadings during abrasive air-jet machining. This effect the applied load. In erosion, the applied load changes during the ly lead to the increase in resistance to fracture and thus increase particle impact event. However, the maximum load will occur at the nozzle entry. It has been reported that the presence of com- function of the hardness and kinetic energy the particle loses a y the erosion wear resistance of the laminated nozzle, especially at maximum penetration of the particle into the target, which is ressive residual stresses can increase the toughness of the surface ing impact. The form of Eq (2 )deduced by Buijs for the case of layer[10, 14]. Consequently, the propagation of a crack generated erosion is given by (251 during the er iderably hindered This phe- size, become responsible for the material removal in form of wear V dE5/4 omenon reduces the number of cracks which, reaching a critical debris. Fig 12 shows the SEM micrographs of the entry bore surface of the Cn and gn2 laminated nozzle after abrasive air-jet machining for 540 min The CN nozzle at the entry failed in a highly brittle man- where d is the density of the target material. In this equation the ner, and exhibited a brittle fracture induced removal process. There wear volume is much stronger dependence on hardness and frac- re a lot of obvious micro-cracks and small pits located on the bore ture toughness, the higher the hardness and fracture toughness, the surface indicating that brittle fracture took place( Fig. 12(a)). The lower the wear volume. probability of finding such features on the eroded surface was sig- As can be seen in Tables 3. 5 and 6. the hardness and fracture GN2 nozzle showed a relative smooth surface by contrast with that nozzle are improved with respect to the gNl and GN3 nozzle of the stress-free one, no obvious micro-cracks can be observed. From this point of view, surface hardness and fracture toughness The erosion wear resistance of ceramics has been reported to improvement at the entry of the gN2 nozzle may be one of the rea- increase with both increasing hardness and fracture toughness sons for the increase of erosion wear resistance within laminated 3. 24]. Buijs [25 has noted that erosion wear can be modeled in nozzles Fig. 12. SEM micrographs of the entry bore surface of (a)the CN stress-free nozzle and (b) the GN2 laminated nozzle after 540 min operation
J. Deng / Wear 266 (2009) 30–36 35 Fig. 11. Worn inner-hole profiles of (a) CN stress-free nozzle and (b) GN2 laminated nozzle after 540 min operation. owing to the variety of volume fraction of the constituent layers and the (W,Ti)C content between adjacent layers in the laminated nozzles. The higher erosion wear resistance especially at the entry of the laminated nozzle compared with the stress-free one can be analysed in terms of the formation of compressive residual stresses at the entry. As can be seen in Refs.[18,19], there is an excess compressive residual stress formed at the entry of the laminated nozzle during cooling from the sintering temperature, which may partially counteract the tensile stresses at the nozzle entry resulting from external loadings during abrasive air-jetmachining. This effect may lead to the increase in resistance to fracture, and thus increase the erosion wear resistance of the laminated nozzle, especially at the nozzle entry. It has been reported that the presence of compressive residual stresses can increase the toughness of the surface layer [10,14]. Consequently, the propagation of a crack generated during the erosion process can be considerably hindered. This phenomenon reduces the number of cracks which, reaching a critical size, become responsible for the material removal in form of wear debris. Fig. 12 shows the SEM micrographs of the entry bore surface of the CN and GN2 laminated nozzle after abrasive air-jet machining for 540 min. The CN nozzle at the entry failed in a highly brittlemanner, and exhibited a brittle fracture induced removal process. There are a lot of obvious micro-cracks and small pits located on the bore surface indicating that brittle fracture took place (Fig. 12(a)). The probability of finding such features on the eroded surface was significantly greater. While the appearance of the eroded areas of the GN2 nozzle showed a relative smooth surface by contrast with that of the stress-free one, no obvious micro-cracks can be observed. The erosion wear resistance of ceramics has been reported to increase with both increasing hardness and fracture toughness [23,24]. Buijs [25] has noted that erosion wear can be modeled in the general form as expressed below: V = AEnHp VKq IC (2) where V is the volume of material removed, E is the elastic modulus, Hv is the hardness, KIC is the fracture toughness, A is a constant, and the exponents n, p, and q all depend on the model of material removal. In the case of a brittle material response, the lateral crack length that contributes to material removal is a function of the applied load. In erosion, the applied load changes during the particle impact event. However, the maximum load will occur at maximum penetration of the particle into the target, which is a function of the hardness and kinetic energy the particle loses during impact. The form of Eq. (2) deduced by Buijs for the case of erosion is given by [25]: V = dE5/4 (H17/12 V KIC) (3) where d is the density of the target material. In this equation the wear volume is much stronger dependence on hardness and fracture toughness, the higher the hardness and fracture toughness, the lower the wear volume. As can be seen in Tables 3, 5 and 6, the hardness and fracture toughness at the surface layer (nozzle entry) of the GN2 laminated nozzle are improved with respect to the GN1 and GN3 nozzle. From this point of view, surface hardness and fracture toughness improvement at the entry of the GN2 nozzle may be one of the reasons for the increase of erosion wear resistance within laminated nozzles. Fig. 12. SEM micrographs of the entry bore surface of (a) the CN stress-free nozzle and (b) the GN2 laminated nozzle after 540 min operation
J. Deng/Wear266(2009)30-36 4. Conclusions [4] S Srinivasan, R.O. Scattergood. Effect of erodent hardness erials.Wear128(1988)139-145. Ceramic nozzles with laminated structure at their entry and with different composition at the constituent layers were produced [|JDer A408(2005)227-233 by hot pressing. Particular attention was paid on wear behaviors of these nozzles. the following conclusions were obtained [7J. Deng. Y Feng, Z Ding Wear behaviors of the ceramic nozzles in sand blasting treatments, J. Eur. Ceram Soc. 23(2003)323-329 [8J. Deng. X Zhang P Niu, Wear of ceramic nozzles by dry sa 1. The surface layer(nozzle entry of the laminated nozzles pos- sessed higher hardness and fracture toughness compared with 9vM. sglavo, M, Bertold. design and production Iminates ciardi, M. Nagliati, C. Melandri, G. Pezzotti, D. Sciti, Effects of residual zle showed the highest surface hardness and fracture toughness xide laminated composites. ]. Et 2. The laminated nozzles(GNl, GN2, and GN3)had superiorerosion [111 Gde Portu.L G. Pezzotti Laminated ceramic structures from oxide wear resistance to that of the stress-free nozzle(cn), especially at the nozzle entry. The mechanism responsible was explained [12T. Henryk, W Anna, B. Marek, K. Dariusz, Multilayer ceramic as the formation of compressive residual stress in entry region composites with high failure resistance, J. Eur. Ceram. Soc. 27(2007) in fabricating process of the laminated nozzle [13] M. Hadad, G. Blugan, ]. Kubler, E. Rosset, L Rohr, J. Michler, Tribological The erosion wear resistance of the laminated nozzles was influ- mposites and multi-layer lami- enced not only by the volume fraction of the constituent layers, [141 E Toschi, C. Melandri, P Pinasco, E. Roncari, S. Guicciardi, G. De Portu, Influ- layers. The GN2 nozzle w vear in ceramic lamina edi comps Guicciardi, C. Melandri, G de portu Tribo- tance over the GNI and GN3 ones. Surface mechanical properties [16] A Tarlazzi, E Roncari, P Pinasco, S( improvement at the entry area was found to be the main reason logical behaviour of AlzO3/ZrOz-zroz laminated composites, Wear 244(2000) for the increase of erosion wear resistance of the gn2 laminated [171 J. Deng, L Liu, J. Li, M. Ding. X Yang, Development of gradient ceramic nozzle erials for sand blasting surface treatments, Int. J. Refract. Met. Hard Mater. 5(2007)130-137 Acknowledgements on the erosion wear of lami- nated ceramic nozzles. Mater. Charact. 59 [193. Deng. F Wu, . Zh This work was supported by"the National Natural Science Foun dation of China(50675120)" and"the Key Science and Technology [201 J. Deng. L Liu, M Ding, Erosion wear behaviours of SicK(W,Ti)c laminated Project of Shandong Province (2006GG2204017) nozzles in dry sand blasting processes, Mater. nozzles, Int References [22]R F Cook, B.R. Lawn, A modified indentation toughness technique. .Am. Ceram ow angles of incidence, Wear 48(1978)181-187. carbide coated tungsten carbide, Wear 233-235(1999)134-15 [31 P.H. Shipway, L.M. Hutchings, The influence of particle properties on the erosive [25] M. Buijs, Erosion of glass as modeled by indentation theory. J. Am. Ceram Soc. wear of sintered boron carbide, Wear 149(1991)85-98. 77(1994)1678-1682
36 J. Deng / Wear 266 (2009) 30–36 4. Conclusions Ceramic nozzles with laminated structure at their entry and with different composition at the constituent layers were produced by hot pressing. Particular attention was paid on wear behaviors of these nozzles. The following conclusions were obtained: 1. The surface layer (nozzle entry) of the laminated nozzles possessed higher hardness and fracture toughness compared with those of other layers, and is greatly improved over the stress-free nozzle. Among the three laminated nozzles tested, the GN2 nozzle showed the highest surface hardness and fracture toughness. 2. The laminated nozzles (GN1, GN2, and GN3) had superior erosion wear resistance to that of the stress-free nozzle (CN), especially at the nozzle entry. The mechanism responsible was explained as the formation of compressive residual stress in entry region in fabricating process of the laminated nozzles. 3. The erosion wear resistance of the laminated nozzles was influenced not only by the volume fraction of the constituent layers, but also by the difference in composition between adjacent layers. The GN2 nozzle with 2% difference in (W,Ti)C content between adjacent layers exhibited higher erosion wear resistance over the GN1 and GN3 ones. Surface mechanical properties improvement at the entry area was found to be the main reason for the increase of erosion wear resistance of the GN2 laminated nozzle. Acknowledgements This work was supported by “the National Natural Science Foundation of China (50675120)”, and “the Key Science and Technology Project of Shandong Province (2006GG2204017)”. References [1] Y.I. Oka, H. Ohnogi, The impact angle dependence of erosion damage caused by solid particle impact, Wear 203/204 (1997) 573–579. [2] I. Finnie, D.H. Mcfadden, On the velocity dependence of the erosion of ductile metals by solid particles at low angles of incidence, Wear 48 (1978) 181–187. 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