J. Deng et al. /Ceramics International 36(2010)299-306 and this residual stress is compressive whatever the sintering a three-layer symmetrical structure. The composition at the temperature. This kind of compressive residual stress can result nozzle entry or exit is Al2O3 /45 vol %(W,Ti)C, while the in an improved erosion wear resistance of the layered ceramic cor position at the nozzle center area is Al2O3/ 55 voL %oTiC. nozzle compared with the homogeneous stress-free one in sand- Three layered nozzles with different thickness ratio p(p=A,/ blasting processes [13-15]. A2)among constituent layers were produced. The layered I2O,/(W,TiC and Al2O,/TiC ceramics are widely used in nozzles with the thickness ratio of 0. 2, 0.5, and I are named industrial applications such as cutting tools and dies [16-18], LNl, LN2, and LN3, respectively(see Fig. 1) they both have high hardness and wear resistance. These two The starting powders used to fabricate the layered nozzles materials have different thermal expansion coefficients; and are listed in Table I with their physical properties. Composite different shrinkage during sintering. The thermal expansion powders of different mixture ratios were prepared by wet ball coefficient(CTEof AlO/(W, Ti)Cis 7. 2x 10 K, and the milling in alcohol with cemented carbide balls for 80h. CTEof Al2O/TiCis 80x 10 K [16-18]. These differences Following drying, the composite powders with different are sufficient to induce residual stresses in the laminated mixture ratios were layered into a graphite mould. The sample structures, and compressive residual stresses are induced in the was then hot-pressed in flowing nitrogen for 15 min at 1700C layers with lower CTE. As for Al2O3/(W,Ti)C +Al2O/TiC temperature with 30 MPa pressure. For the purpose of layered ceramic material, the CTE of the surface layer(Al2O3/ comparison, a homogeneous stress-free ceramic nozzle was (W,Ti)c) is lower than that of the center layer(Al2O3/TiC), so also manufactured by hot-pressing. This stress-free nozzle compressive residual stresses will be formed in the surface layer made from Al 2O3 /45 vol. (W,Ti)c is named N5 the layered materials during fabrication. In the present study, Al2O3/(W,TI)C Al2O3/TiC layered ceramics with different 2.2. Hardness and fracture toughness measurements ar thickness ratios among constituent layers were produced to be surface layer of the layered nozzle materials used as the Cws nozzles. The mechanical properties at the surface layers of the layered materials were measured, and the fracture toughness measurement was performed using cro-structure was examined. The wear behaviors of the layered indentation method at the nozzle external layer(entry or exit) ceramic nozzles were investigated and compared to an unstressed in a hardness tester(MH6) using the formula proposed by Cook reference nozzle. The purpose was to characterize the erosion and Lawn [19]. Hardness measurements were performed by wear of the layered ceramic nozzle in industrial CWS boilers. placing Vickers indentations on external layer of the layered nozzle material. The indentation load was 200n and a 2. Materials and experimental procedures minimum of five indentations were tested. The vickers hardness(GPa)is given by 2.1. Preparation of the Al20y(w,Ti)C Al203/iC layered ceramic nozzles P Hu=1.8544 The dimension and compositional distribution of the layered ceramic nozzles with different thickness ratios among where P is the indentation load (N), 2a is the catercorner lengt constituent layers are shown in Fig. 1. These nozzles possess (um) due to indentation. 10 mm Nozzle exit O3/(W,Ti)c Al2OJTIC Al2O3/W,Ti)C Nozzle ent (a) Fig 1. Compositional distribution of the layered ceramic nozzles with different thickness ratio among constituent layers: (a) NI nozzle(p= A / A2=0.2),(b) nozzle(p=0.5), and (c)N3 nozzle (p=1)and this residual stress is compressive whatever the sintering temperature. This kind of compressive residual stress can result in an improved erosion wear resistance of the layered ceramic nozzle compared with the homogeneous stress-free one in sand￾blasting processes [13–15]. Al2O3/(W,Ti)C and Al2O3/TiC ceramics are widely used in industrial applications such as cutting tools and dies [16–18], they both have high hardness and wear resistance. These two materials have different thermal expansion coefficients; and different shrinkage during sintering. The thermal expansion coefficient (CTE) of Al2O3/(W,Ti)C is 7.2 106 K1 , and the CTE of Al2O3/TiC is 8.0 106 K1 [16–18]. These differences are sufficient to induce residual stresses in the laminated structures, and compressive residual stresses are induced in the layers with lower CTE. As for Al2O3/(W,Ti)C + Al2O3/TiC layered ceramic material, the CTE of the surface layer (Al2O3/ (W,Ti)C) is lower than that of the center layer (Al2O3/TiC), so compressive residual stresses will be formed in the surface layer of the layered materials during fabrication. In the present study, Al2O3/(W,Ti)C + Al2O3/TiC layered ceramics with different thickness ratios among constituent layers were produced to be used as the CWS nozzles. The mechanical properties at the surface layers of the layered materials were measured, and the micro-structure was examined. Thewear behaviors of the layered ceramic nozzles were investigated and compared to an unstressed reference nozzle. The purpose was to characterize the erosion wear of the layered ceramic nozzle in industrial CWS boilers. 2. Materials and experimental procedures 2.1. Preparation of the Al2O3/(W,Ti)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 or exit is Al2O3/45 vol.%(W,Ti)C, while the composition at the nozzle center area is Al2O3/55 vol.%TiC. Three 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, and 1 are named LN1, LN2, and LN3, 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. Following drying, the composite powders with different mixture ratios were layered into a graphite mould. The sample was then hot-pressed in flowing nitrogen for 15 min at 1700 8C temperature with 30 MPa pressure. For the purpose of comparison, a homogeneous stress-free ceramic nozzle was also manufactured by hot-pressing. This stress-free nozzle made from Al2O3/45 vol.%(W,Ti)C is named N5. 2.2. Hardness and fracture toughness measurements at surface layer of the layered nozzle materials Fracture toughness measurement was performed using indentation method at the nozzle external layer (entry or exit) in a hardness tester (MH 6) using the formula proposed by Cook and Lawn [19]. Hardness measurements were performed by placing Vickers indentations on external 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 (1) where P is the indentation load (N), 2a is the catercorner length (mm) due to indentation. Fig. 1. Compositional distribution of the layered ceramic nozzles with different thickness ratio among constituent layers: (a) N1 nozzle ( p = A1/A2 = 0.2), (b) N2 nozzle ( p = 0.5), and (c) N3 nozzle ( p = 1). 300 J. Deng et al. / Ceramics International 36 (2010) 299–306