J. Deng et al. /Ceramics International 36(2010)299-306 Table 3 Hardness and fracture toughness at the extermal layer(entry or exit) of the layered ceramic nozzles with different thickness ratios among constituer lay Coal-water-slurry Code name Thickness ratio Hardness Fracture toughness P=AlA LNI layered nozzle 21.6±1.298±0.6 LN2 layered nozzle 0.5 21.1土1.184±0.5 Compressed air entry LN3 layered nozzle I 20.9±1.17.3±0.5 N5 stress-free nozzle 199±1.049±0.5 Fig. 3. Structure of the spray-gun. hole diameter variation of n5 stress-free nozzle is much higher than those of the LNl, LN2, and LN3 layered nozzles under the same test conditions and the hole diameter variation was also infuenced by the thickness ratio among constituent layers. The LNI nozzle with thickness ratio of 0.2 between adjacent layers had the smallest hole diameter variation among the layered The comparison of the erosion rates of the nozzles in Cws burning processes is shown in Fig. 7. It is obvious that the erosion rates of N5 stress-free nozzles are much higher than that of the layered nozzles, and the erosion rates of the nozzles from low to high is LNl, LN2, LN3, and N5 in turn. The LNi layere 10 mm nozzle with thickness ratio of 0.2 exhibited the highest erosion wear resistance g. 4. Photo of the layered ceramic nozzles. 3.3. Worn surface of the layered nozzles external layer, which is also reported by other researchers [4- The exit hole profiles of the CwS nozzles after 120 h operation are shown in Fig. 8. It was found that the exit of N5 The SEM micrographs section surface of the stress -free nozzles exhibits fracture marks. for determination LN3 layered nozzle material are in Fig. 5. A layered of erosion mechanisms, the worn CWS nozzles were sectioned structure can be clearly seen axially. Fig 9 shows the wall surface profiles of the worn CWs nozzles after 120 h operation. It is obvious that the hole 3. 2. Hole diameter variation and erosion rate of the diameter of N5 stress-free nozzles enlarges greatly compared avered ceramic nozzle with that of the layered nozzles Characteristic SEM pictures taken on the nozzle wall surface Fig 6 shows the comparison of hole diameter variation of of the LNI and N5 nozzles after 120 h operation are showed the CWS nozzles after 120 h operation. It is indicated that the Fig. 10. The wall surface of the LNI layered CwS nozzle is Center layer Exit laye Fig. 5. SEM micrographs of the cross-section surface of the LN3 layered nozzle materialexternal layer, which is also reported by other researchers [4– 6]. The SEM micrographs of the cross-section surface of the LN3 layered nozzle material are shown in Fig. 5. A layered structure can be clearly seen. 3.2. Hole diameter variation and erosion rate of the layered ceramic nozzles Fig. 6 shows the comparison of hole diameter variation of the CWS nozzles after 120 h operation. It is indicated that the hole diameter variation of N5 stress-free nozzle is much higher than those of the LN1, LN2, and LN3 layered nozzles under the same test conditions, and the hole diameter variation was also influenced by the thickness ratio among constituent layers. The LN1 nozzle with thickness ratio of 0.2 between adjacent layers had the smallest hole diameter variation among the layered nozzles. The comparison of the erosion rates of the nozzles in CWS burning processes is shown in Fig. 7. It is obvious that the erosion rates of N5 stress-free nozzles are much higher than that of the layered nozzles, and the erosion rates of the nozzles from low to high is LN1, LN2, LN3, and N5 in turn. The LN1 layered nozzle with thickness ratio of 0.2 exhibited the highest erosion wear resistance. 3.3. Worn surface of the layered nozzles The exit hole profiles of the CWS nozzles after 120 h operation are shown in Fig. 8. It was found that the exit of N5 stress-free nozzles exhibits fracture marks. For determination of erosion mechanisms, the worn CWS nozzles were sectioned axially. Fig. 9 shows the wall surface profiles of the worn CWS nozzles after 120 h operation. It is obvious that the hole diameter of N5 stress-free nozzles enlarges greatly compared with that of the layered nozzles. Characteristic SEM pictures taken on the nozzle wall surface of the LN1 and N5 nozzles after 120 h operation are showed in Fig. 10. The wall surface of the LN1 layered CWS nozzle is Fig. 4. Photo of the layered ceramic nozzles. Fig. 3. Structure of the spray-gun. Table 3 Hardness and fracture toughness at the external layer (entry or exit) of the layered ceramic nozzles with different thickness ratios among constituent layers. Code name Thickness ratio P=A1/A2 Hardness (GPa) Fracture toughness (MPa m1/2) LN1 layered nozzle 0.2 21.6  1.2 9.8  0.6 LN2 layered nozzle 0.5 21.1  1.1 8.4  0.5 LN3 layered nozzle 1 20.9  1.1 7.3  0.5 N5 stress-free nozzle 19.9  1.0 4.9  0.5 Fig. 5. SEM micrographs of the cross-section surface of the LN3 layered nozzle material. 302 J. Deng et al. / Ceramics International 36 (2010) 299–306