D Jianxin et al/Int. Journal of Refractory Metals E Hard Materials 27(2009)734-739 2. Materials and experimental procedure where Vi is the nozzle volume loss, w has the units of volume loss per unit mass(mm/g). 2.1. Preparation of the Al2O/Wo.7Ti03)C+ Al2Oa/TiC layered ceramic The finite element method (FEM) sed numerically evaluating the residual stress and its distribution within the layered nozzle in the fabricating processes The dimension and compositional distribution of the layered For observation of the micro-damage and determination of ero- ceramic nozzles with different thickness ratios among constituent sion mechanisms, the worn nozzles were sectioned axially. The layers are shown in Fig. 1. These nozzles possess a three-layer eroded bore surface of the nozzles was examined by scanning elec symmetrical structure. The composition at the nozzle entry and tron mi exit is Al203 45 voL%(Wo. 7Tio.3) C, while the composition at the nozzle center area is Al203 55 vol% TiC. Four layered nozzles with 3 Results and discussion different thickness ratio p(p=Al Az) among constituent layers were produced. The layered nozzles with the thickness ratio of 3. 1. Residual stress in the Al203/(Wa.7Tio3)C+Al203/Tic layered nozzle 0. 2, 0.5, 1 and 2 are named Nl, N2, N3 and N4, respectively(see The residual stress of the layered ceramic nozzles during fabri- The starting powders used to fabricate the layered nozzles are cating process was calculated by means of the finite element meth- listed in Table 1 with their physical properties. Composite powders od by assuming that the compact is cooled from sintering of different mixture ratios were prepared by wet ball milling temperature 1700C to room temperature 20C. The physical alcohol with cemented carbide balls for 80 h, respectively. Follow- properties of Al2O3, (Wo.7Tio.)C and TiC are listed in Table 1 ing drying, the composite powders with different mixture ratios Owing to the symmetry, an axisymmetric calculation was pre were layered into the graphite mould in turn. The sample was then ferred. Presume that it was steady state boundary condition. The hot-pressed in flowing nitrogen for 15 min at 1700C temperature results of the distribution of axial (oz)and radial(o)residual stres with 30 MPa pressure. For the purpose of comparison, a homolo- ses in the N2 layered nozzle in fabricating process from sintering gous stress-free ceramic nozzle was also manufactured by hot- temperature 1700C to room temperature 20C is shown in pressing. This stress-free nozzle made from A203 /45 voL% Fig. 2. It is indicated that an excess residual stress is formed both (Wo7Tio3)C is named N5 Layered structures constituted by alternate layers with different 2. 2. Abrasive air-jet machining compositions can be properly designed to induce a surface com- pressive residual stress. These residual stresses mainly arise from ressure was set at 0.4 MPa. The erodent abrasives used were of among layers [5-8]. Compressive residual stresses are induced in A203 powders with 150-250 um grain size. The mass loss of the layers with lower CTE, while tensile stresses arise in those with yorn ceramic nozzle was measured with a balance(minimum higher CTE. As for Al2O3/(Wo.7Tio3)C+Al2O3/TiC layered material 0.01 mg). Nozzles with internal diameter 8 mm and length the CTE of Al203/Wo.7Tio3)C is 7.2 x 10-K- and the CTE of 30 mm made from layered structure(Nl, N2, N3 and N4)and Al 03/TiC is 8.0 x 10-6K-I. That is to say, the thermal expansion stress-free structure(N5)were manufactured by hot-pressing. All coefficient of the entry section(surface layer)of the layered nozzle the test conditions are listed in table 2 The erosion rates(W) of the nozzles are defined as the nozzle stresses will be formed in the nozzle entry region during fabricat mass loss(mn)divided by the nozzle density (di) times the mass ing process of the erodent abrasive particles(m2) Fig 3 shows the residual stresses (oz and or)inside the N1, N2. (1) N and N4 layered nozzles at the inner-hole surtace along the noz. (20mn Al O,(W,Ti)c AhOLiC b) Fig. 1. Dimension and compositional distribution of the layered ceramic nozzles with different thickness ratios among constituent layers, (a)NI nozzle (p=0.2).(b)N2 ozzie(p=0.5. (c)N3 nozzle(p=1)and(d)N4 nozzle(p=2).2. Materials and experimental procedures 2.1. Preparation of the Al2O3/(W0.7Ti0.3)C + Al2O3/TiC layered ceramic nozzles The dimension and compositional distribution of the layered ceramic nozzles with different thickness ratios among constituent layers are shown in Fig. 1. These nozzles possess a three-layer symmetrical structure. The composition at the nozzle entry and exit is Al2O3/45 vol.% (W0.7Ti0.3)C, while the composition at the nozzle center area is Al2O3/55 vol.% TiC. Four layered nozzles with different thickness ratio p (p = A1/A2) among constituent layers were produced. The layered nozzles with the thickness ratio of 0.2, 0.5, 1 and 2 are named N1, N2, N3 and N4, respectively (see Fig. 1). The starting powders used to fabricate the layered nozzles are listed in Table 1 with their physical properties. Composite powders of different mixture ratios were prepared by wet ball milling in alcohol with cemented carbide balls for 80 h, respectively. Follow￾ing drying, the composite powders with different mixture ratios were layered into the graphite mould in turn. The sample was then hot-pressed in flowing nitrogen for 15 min at 1700 C temperature with 30 MPa pressure. For the purpose of comparison, a homolo￾gous stress-free ceramic nozzle was also manufactured by hot￾pressing. This stress-free nozzle made from Al2O3/45 vol.% (W0.7Ti0.3)C is named N5. 2.2. Abrasive air-jet machining The erosion wear tests were conducted with a GS-6 type abrasive air-jet machining machine tool [14]. The compressed air pressure was set at 0.4 MPa. The erodent abrasives used were of Al2O3 powders with 150–250 lm grain size. The mass loss of the worn ceramic nozzle was measured with a balance (minimum 0.01 mg). Nozzles with internal diameter 8 mm and length 30 mm made from layered structure (N1, N2, N3 and N4) and stress-free structure (N5) were manufactured by hot-pressing. All the test conditions are listed in Table 2. The erosion rates (W) of the nozzles are defined as the nozzle mass loss (m1) divided by the nozzle density (d1) times the mass of the erodent abrasive particles (m2): W ¼ m1=ðd1 m2Þ ¼ V1=m2 ð1Þ where V1 is the nozzle volume loss, 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 layered nozzle in the fabricating processes. For observation of the micro-damage and determination of ero￾sion mechanisms, the worn nozzles were sectioned axially. The eroded bore surface of the nozzles was examined by scanning elec￾tron microscopy (SEM). 3. Results and discussion 3.1. Residual stress in the Al2O3/(W0.7Ti0.3)C + Al2O3/TiC layered nozzle The residual stress of the layered ceramic nozzles during fabri￾cating process was calculated by means of the finite element meth￾od by assuming that the compact is cooled from sintering temperature 1700 C to room temperature 20 C. The physical properties of Al2O3, (W0.7Ti0.3)C and TiC are listed in Table 1. Owing to the symmetry, an axisymmetric calculation was pre￾ferred. Presume that it was steady state boundary condition. The results of the distribution of axial (rz) and radial (rr) residual stres￾ses in the N2 layered nozzle in fabricating process from sintering temperature 1700 C to room temperature 20 C is shown in Fig. 2. It is indicated that an excess residual stress is formed both at the entry and the exit of the layered nozzle. Layered structures constituted by alternate layers with different compositions can be properly designed to induce a surface com￾pressive residual stress. These residual stresses mainly arise from a mismatch between the coefficient of thermal expansion (CTE) of the constituent layers, and are influenced by the thickness ratio among layers [5–8]. Compressive residual stresses are induced in layers with lower CTE, while tensile stresses arise in those with higher CTE. As for Al2O3/(W0.7Ti0.3)C + Al2O3/TiC layered material, the CTE of Al2O3/(W0.7Ti0.3)C is 7.2 106 K1 and the CTE of Al2O3/TiC is 8.0 106 K1 . That is to say, the thermal expansion coefficient of the entry section (surface layer) of the layered nozzle is lower than that of the center layer, so that compressive residual stresses will be formed in the nozzle entry region during fabricat￾ing process. Fig. 3 shows the residual stresses (rz and rr) inside the N1, N2, N3 and N4 layered nozzles at the inner-hole surface along the noz￾zle axial direction in fabricating process. It is indicated that rz and (a) (b) (c) (d) A1 A2 A1 30 mm Al2O3/(W,Ti)C Al2O3/TiC Al2O3/(W,Ti)C Entry Exit Φ20mm Φ8mm Fig. 1. Dimension and compositional distribution of the layered ceramic nozzles with different thickness ratios among constituent layers, (a) N1 nozzle (p = 0.2), (b) N2 nozzle (p = 0.5), (c) N3 nozzle (p = 1) and (d) N4 nozzle (p = 2). D. Jianxin et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 734–739 735