J. Deng et al./Ceramics international 36(2010)299-306 Physical properties of Al2O3, TiC and (W,Ti)C Starting Density Youngs Thermal conductivity ons Particle Purity (%) Manufacture powder (g/cm) modulus(GPa) coefficient(10-6K-)W/(m K) size(um) Al2O33.98 8.0 Beijing Antai Advanced Tech and Mater. Co. Ltd. 4.93500 7.4 24.3 0.20 Zhuzhou cemented carbide works W,T)C9.56480 214 0.25 Zhuzhou cemented carbide works pro rties of the coal-water-slurry. Consistency (% Quantity of heat(M/kg) Ash(%) Sulphur(%) Volatility (%) Adhesiveness(MPa s) Grit number(μm) 18.81-2048 2 40-80 2.3. Coal-water-slurry burning tests CWS nozzles were examined by scanning electron microscopy (SEMD) Coal-water-slurry burning tests were conducted with an industrial Cws boiler. The properties of the CwS used in this 3. Results and discussion study are listed in Table 2. The schematic diagram and the photo of the boiler are shown in Fig. 2. The Cws drawn by 3. 1. Surface layer properties and microstructural pump passed through pipeline, accelerated and mixed in the characterization of the layered nozzle materials pray-gun(Fig 3) by gas stream(commonly compressed air). The compressed air pressure was set at 0. 4 MPa, and CwS The results of hardness and fracture toughness of the layered pressure was set at 0.2 MPa. The nozzles have an internal nozzles with different thickness ratios are presented in Table 3 diameter 5.0 mm, external diameter 12.0 mm, and length It is indicated that the hardness and fracture toughness decrease 10.0 mm. Photo of a layered ceramic nozzle is shown in Fig 4. gradually from LNI nozzle to N5 nozzle. The external layer of The mass loss of the worn layered nozzles was measured the nozzle with low thickness ratio shows the highest hardness with an accurate electric balance(minimum 0.01 mg). The and fracture toughness. By comparison with the stress-free erosion rates(W) of the layered nozzles are defined as the nozzle (N5), the hardness at the external layer of the layered nozzle mass loss(m1) divided by the nozzle density(d)times nozzles is much more improved, and rose from 19.9 GPa for N5 the burn mass of Cws(m2) stress-free nozzle to 21.6 GPa for LNI layered nozzle, representing a maximum increase of 1.7 GPa. The fracture (2) toughness rose from 4.9 MPa m for N5 stress-free nozzle to 9.8 MPa m"for LNI layered nozzle, representing a maximum where W has the units of volume loss per unit mass of Cws increase of 4.9 MPa m".As reported in Ref. [20], layered structures in ceramic material can induce an excess compres Finite element method(FEM) was used as a means of sive residual stress at the nozzle external layer during numerically evaluating the temperature gradient and thermal fabrication. These compressive residual stresses are beneficial stresses inside the cws nozzle. The eroded wall surfaces of the for the increase of hardness and the fracture toughness at the O 50 mm ure gauge Air compressor CwS storage Pressure gauge Fig. 2. Schematic diagram and photo of the industry coal-water-slurry boiler.2.3. Coal-water-slurry burning tests Coal-water-slurry burning tests were conducted with an industrial CWS boiler. The properties of the CWS used in this study are listed in Table 2. The schematic diagram and the photo of the boiler are shown in Fig. 2. The CWS drawn by pump passed through pipeline, accelerated and mixed in the spray-gun (Fig. 3) by gas stream (commonly compressed air). The compressed air pressure was set at 0.4 MPa, and CWS pressure was set at 0.2 MPa. The nozzles have an internal diameter 5.0 mm, external diameter 12.0 mm, and length 10.0 mm. Photo of a layered ceramic nozzle is shown in Fig. 4. The mass loss of the worn layered nozzles was measured with an accurate electric balance (minimum 0.01 mg). The erosion rates (W) of the layered nozzles are defined as the nozzle mass loss (m1) divided by the nozzle density (d) times the burn mass of CWS (m2). W ¼ m1 d  m2 (2) where W has the units of volume loss per unit mass of CWS (mm3 /kg). Finite element method (FEM) was used as a means of numerically evaluating the temperature gradient and thermal stresses inside the CWS nozzle. The eroded wall surfaces of the CWS nozzles were examined by scanning electron microscopy (SEM). 3. Results and discussion 3.1. Surface layer properties and microstructural characterization of the layered nozzle materials The results of hardness and fracture toughness of the layered nozzles with different thickness ratios are presented in Table 3. It is indicated that the hardness and fracture toughness decrease gradually from LN1 nozzle to N5 nozzle. The external layer 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 LN1 layered nozzle, representing a maximum increase of 1.7 GPa. The fracture toughness rose from 4.9 MPa m1/2 for N5 stress-free nozzle to 9.8 MPa m1/2 for LN1 layered nozzle, representing a maximum increase of 4.9 MPa m1/2. As reported in Ref. [20], layered structures in ceramic material can induce an excess compres￾sive residual stress at the nozzle external layer during fabrication. These compressive residual stresses are beneficial for the increase of hardness and the fracture toughness at the Table 1 Physical properties of Al2O3, TiC and (W,Ti)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 (mm) 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 (W,Ti)C 9.56 480 8.5 21.4 0.25 1–2 >99 Zhuzhou cemented carbide works Table 2 Properties of the coal-water-slurry. Consistency (%) Quantity of heat (MJ/kg) Ash (%) Sulphur (%) Volatility (%) Adhesiveness (MPa s) Grit number (mm) 65  2 18.81–20.48 <12 <0.8 >15 1000–2500 40–80 Fig. 2. Schematic diagram and photo of the industry coal-water-slurry boiler. J. Deng et al. / Ceramics International 36 (2010) 299–306 301