736 D. Jianxin et aL/Int. Journal of Refractory Metals 8 Hard Materials 27(2009)734-739 Physical properties of AlzO3, TiC and(wozTio3)c arting Density Youngs modulus Thermal expansion coefficient Thermal Poissons Particle size Purity Manufacture (g/cm)(GPa) (10-6K-1) inductivity rato Al,O3 3.98 Beijing Antai Advanced Tech. an Zhuzhou cemented ca WozTioa)c 9.5 214 Zhuzhou cemented ca works Fracture toughness measurement was performed using inden- tation method at the nozzle surface layer(nozzle entry )in a hard- Abrasive air-jet equipment GS-6 type abrasive air-jet machine tool ness tester(MH 6) using the formula proposed by Cook and Lawn Nozzle material Ni, N2, N3 and N4 layered nozzles [19]. Hardness measurements were performed by placing Vickers indentations on surface layer of the layered nozzle material. The 0-250 um Al2O3 powders indentation load was 200n and a minimum of five indentations were tested. The Vickers hardness(GPa)is given by Cumulative mass weigh Balance(minimum 0.01 mg H ar residual stresses at the nozzle entry or exit area are all compres- where P is the indentation load (N), and 2a is the catercorner length sive whatever the thickness ratios. The Ni layered nozzle with (um)due to indentation. thickness ratio of 0.2 among constituent layers had the highest The results of hardness and fracture toughness of the layered compressive stress(-315 MPa)at the nozzle entry; while the n4 nozzle with different thickness ratios among constituent layers nozzle with thickness ratio of two had the smallest compressive are presented in Table 3. It is indicated that the hardness and frac stress(-24 4 MPa)at the nozzle entry Therefore, layered struc- ture toughness decrease gradually from NI nozzle to N5 nozzle. tures in these ceramic nozzles can form an excess compressive The external layer(nozzle entry or exit) of the nozzle with low residual stresses at the nozzle entry(or exit)area during fabricat- thickness ratio shows the highest hardness and fracture toughness ng process. The smaller the thickness ratio, the higher the com- By comparison with the stress-free nozzle(N5). the hardness at the pressive stress at the nozzle entry or exit area. external layer of the layered nozzles is much more improved, and From Fig. 3, we can also see that the radial residual stress or rose from 19. 9 GPa for N5 stress-free nozzle to 21.6 GPa for NI lay- showed no much change along the nozzle axial direction for ered nozzle, representing a maximum increase of 1.7 GPa. While Al2O3/Wo.7Tio3)C+Al203TiC layered material The reason is that the fracture toughness rose from 4.9 MPa m/2 for N5 stress-free multilayered material (the compositions are only changed in noz- imum increase of 4.9 MPa m e Vered nozzle, representing a max- the compositions in nozzle radial direction are the same for this nozzle to 9.8 MPa m for NI la zle axial direction, see Fig. 1) so there is no difference in thermal As can be that layered structures in Al2O3/(Wo.Tio3)C+ expansion coefficient in the nozzle radial direction, which leads Al203/TiC material can form an excess compressive residual stress to almost identical radial residual stress o, along the nozzle axial at the nozzle entry (or exit area during fabricating process. The direction smaller the thickness ratio, the higher the compressive stress at he nozzle entry or exit area. These compressive residual stresses ructural characterization and surface layer properties of are benefit for the increase of hardness and the fracture toughne AlyO3 lo3)C+ Al,O TiC layered nozzle materials at the surface layer of the layered structures, which is also reported by other researchers [1-4 The high hardness and fracture tough- The SEM micrographs of the cross-section surface of the N4 lay- ness of the layered nozzles with low thickness ratios among con- ered nozzle material are shown in Fig 4. a layered structure can be stituent layers may attribute to their high compressive stresses clearly seen. at the external layer (entry or exit ) b -320-261-203-14485-263190149 Fig.2. Distribution of (a)axial residual stress dz and (b)radial residual stress dr in N2 layered nozzle during fabricating process from sintering temperature 1700Ctorr residual stresses at the nozzle entry or exit area are all compres￾sive whatever the thickness ratios. The N1 layered nozzle with thickness ratio of 0.2 among constituent layers had the highest compressive stress (31.5 MPa) at the nozzle entry; while the N4 nozzle with thickness ratio of two had the smallest compressive stress (24.4 MPa) at the nozzle entry. Therefore, layered struc￾tures in these ceramic nozzles can form an excess compressive residual stresses at the nozzle entry (or exit) area during fabricat￾ing process. The smaller the thickness ratio, the higher the com￾pressive stress at the nozzle entry or exit area. From Fig. 3, we can also see that the radial residual stress rr showed no much change along the nozzle axial direction for Al2O3/(W0.7Ti0.3)C + Al2O3/TiC layered material. The reason is that the compositions in nozzle radial direction are the same for this multilayered material (the compositions are only changed in noz￾zle axial direction, see Fig. 1), so there is no difference in thermal expansion coefficient in the nozzle radial direction, which leads to almost identical radial residual stress rr along the nozzle axial direction. 3.2. Microstructural characterization and surface layer properties of Al2O3/(W0.7Ti0.3)C + Al2O3/TiC layered nozzle materials The SEM micrographs of the cross-section surface of the N4 lay￾ered nozzle material are shown in Fig. 4. A layered structure can be clearly seen. Fracture toughness measurement was performed using inden￾tation method at the nozzle surface layer (nozzle entry) in a hard￾ness tester (MH 6) using the formula proposed by Cook and Lawn [19]. Hardness measurements were performed by placing Vickers indentations on surface layer of the layered nozzle material. The indentation load was 200 N and a minimum of five indentations were tested. The Vickers hardness (GPa) is given by: Hv ¼ 1:8544 P ð2aÞ 2 ð2Þ where P is the indentation load (N), and 2a is the catercorner length (lm) due to indentation. The results of hardness and fracture toughness of the layered nozzle with different thickness ratios among constituent layers are presented in Table 3. It is indicated that the hardness and frac￾ture toughness decrease gradually from N1 nozzle to N5 nozzle. The external layer (nozzle entry or exit) of the nozzle with low thickness ratio shows the highest hardness and fracture toughness. By comparison with the stress-free nozzle (N5), the hardness at the external layer of the layered nozzles is much more improved, and rose from 19.9 GPa for N5 stress-free nozzle to 21.6 GPa for N1 lay￾ered nozzle, representing a maximum increase of 1.7 GPa. While the fracture toughness rose from 4.9 MPa m1/2 for N5 stress-free nozzle to 9.8 MPa m1/2 for N1 layered nozzle, representing a max￾imum increase of 4.9 MPa m1/2. As can be seen that layered structures in Al2O3/(W0.7Ti0.3)C + Al2O3/TiC material can form an excess compressive residual stress at the nozzle entry (or exit) area during fabricating process. The smaller the thickness ratio, the higher the compressive stress at the nozzle entry or exit area. These compressive residual stresses are benefit for the increase of hardness and the fracture toughness at the surface layer of the layered structures, which is also reported by other researchers [1–4]. The high hardness and fracture tough￾ness of the layered nozzles with low thickness ratios among con￾stituent layers may attribute to their high compressive stresses at the external layer (entry or exit). Table 1 Physical properties of Al2O3, TiC and (W0.7Ti0.3)C. Starting powder Density (g/cm3 ) Young’s modulus (GPa) Thermal expansion coefficient (106 K1 ) Thermal conductivity (W/m K) Poisson’s ratio Particle size (lm) Purity (%) Manufacture Al2O3 3.98 380 8.0 30.2 0.27 1–2 >99 Beijing Antai Advanced Tech. and Mater. Co., Ltd. TiC 4.93 500 7.4 24.3 0.20 1–2 >99 Zhuzhou cemented carbide works (W0.7Ti0.3)C 9.56 480 8.5 21.4 0.25 1–2 >99 Zhuzhou cemented carbide works Table 2 Abrasive air-jet machining conditions. Abrasive air-jet equipment GS-6 type abrasive air-jet machine tool Nozzle material N1, N2, N3 and N4 layered nozzles N5 stress-free nozzle Dimension of the nozzles U 8 mm (internal diameter) 30 mm (length) Erodent abrasives 150–250 lm Al2O3 powders Compressed air pressure 0.4 MPa Cumulative mass weigh Balance (minimum 0.01 mg) -320 -261 -203 -144 -85 -26 31 90 149 (MPa) (MPa) -290 -226 -162 -98 -34 29 92 156 220 a b Fig. 2. Distribution of (a) axial residual stress rz and (b) radial residual stress rr in N2 layered nozzle during fabricating process from sintering temperature 1700 C to room temperature 20 C. 736 D. Jianxin et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 734–739