D Jianxin et al. Materials Science and Engineering A 444 (2007)120-129 Nozzle Shallow impact or abrasive zone Fig. 1. Schematic diagram of the interaction between the erodent particle and the nozzle in sand blasting processe A novel technique, by which a compressive residual stress was investigated in comparison with the homologous stress-free n be generated into the surface of a material, is the produc- nozzles. The purpose is to evaluate whether laminated ceramic tion of laminated structures designed to combine the advanta- nozzles have superior erosion wear resistance to that of the geous characteristics of the different materials involved, thereby homologous stress-free nozzles. improving the overall mechanical behaviour of the materials. It has been shown that laminated hybrid structures constituted by 2. Materials and experimental procedures alternate layers of different materials can be properly designed in order to induce a surface compressive residual stress leading to 2.1. Preparation of sic/(w, Ti)C laminated ceramic nozzle an improved surface mechanical properties and wear resistance materials [19-22]. Residual stresses arise from a mismatch between the coefficients of thermal expansion(CTE), sintering rates and elas- The starting materials were(W,Ti C solid-solution powders ic constants of the constituent phases and neighbouring layers. with average grain size of approximately 0.8 um, purity 99.9%0 Compressive residual stresses are induced in layers with lower and Sic powders with average grain size of 1 um, purity 99.8%. CTE, while tensile stresses arise in those with higher CtE. The Six different volume fractions of (W,Ti)C(55, 57, 59, 61, 63, residual stress field also depends on the geometry of the layered 65 vol %)were selected in designing the SiC/(W,Ti)C laminated structure and on the thickness ratio among layers [23-26] nozzle material with a six-layer structure The effectiveness of laminated hybrid structures in improving The compositional distribution of the laminated ceramic noz the sliding wear resistance of alumina has been already reported zle materials is shown in Fig. 2. It is indicated that the composi by Toschi [22]. In the present study, Sic/(W,Ti)C laminated tional distribution of the laminated nozzle materials changes in ceramic nozzles were produced by hot pressing. The residual nozzle axial direction(see Fig. 2(a)and( b)). As the heat conduc thermal stress of the laminated nozzle in the fabricating process tivity of SiC is higher than that of (w,Ti)C solid-solution, while was calculated by means of the finite element method(FEM). its thermal expansion coefficient is lower than that of (,Ti)C, The erosion wear behaviour of the laminated ceramic nozzle the layer with the highest volume fraction of SiC was put in the Nozzle exit Nozzle exit Nozzle exit C/65Vol%(W,Tn)c iC/57Vol%(W,TO)C SiC/55Vol%(W, TO)C Nozzle entry Nozzle entry Nozzle entry Fig. 2. Compositional distribution of (a) the ceramic nozzle laminated only in entry area. (b) the ceramic nozzle laminated both in entry and exit area, and (c)the stress-free nozzles (a) GN-2 laminated nozzle, (b) GN-3 laminated nozzle, (c)CN-2 stress-free nozzleD. Jianxin et al. / Materials Science and Engineering A 444 (2007) 120–129 121 Fig. 1. Schematic diagram of the interaction between the erodent particle and the nozzle in sand blasting processes. A novel technique, by which a compressive residual stress can be generated into the surface of a material, is the produc￾tion of laminated structures designed to combine the advanta￾geous characteristics of the different materials involved, thereby improving the overall mechanical behaviour of the materials. It has been shown that laminated hybrid structures constituted by alternate layers of different materials can be properly designed in order to induce a surface compressive residual stress leading to an improved surface mechanical properties and wear resistance [19–22]. Residual stresses arise from a mismatch between the coefficients of thermal expansion (CTE), sintering rates and elas￾tic constants of the constituent phases and neighbouring layers. Compressive residual stresses are induced in layers with lower CTE, while tensile stresses arise in those with higher CTE. The residual stress field also depends on the geometry of the layered structure and on the thickness ratio among layers [23–26]. The effectiveness of laminated hybrid structures in improving the sliding wear resistance of alumina has been already reported by Toschi [22]. In the present study, SiC/(W,Ti)C laminated ceramic nozzles were produced by hot pressing. The residual thermal stress of the laminated nozzle in the fabricating process was calculated by means of the finite element method (FEM). The erosion wear behaviour of the laminated ceramic nozzle was investigated in comparison with the homologous stress-free nozzles. The purpose is to evaluate whether laminated ceramic nozzles have superior erosion wear resistance to that of the homologous stress-free nozzles. 2. Materials and experimental procedures 2.1. Preparation of SiC/(W,Ti)C laminated ceramic nozzle materials The starting materials were (W,Ti)C solid-solution powders with average grain size of approximately 0.8 m, purity 99.9%, and SiC powders with average grain size of 1 m, purity 99.8%. Six different volume fractions of (W,Ti)C (55, 57, 59, 61, 63, 65 vol.%) were selected in designing the SiC/(W,Ti)C laminated nozzle material with a six-layer structure. The compositional distribution of the laminated ceramic noz￾zle materials is shown in Fig. 2. It is indicated that the composi￾tional distribution of the laminated nozzle materials changes in nozzle axial direction (see Fig. 2(a) and (b)). As the heat conduc￾tivity of SiC is higher than that of (W,Ti)C solid-solution, while its thermal expansion coefficient is lower than that of (W,Ti)C, the layer with the highest volume fraction of SiC was put in the Fig. 2. Compositional distribution of (a) the ceramic nozzle laminated only in entry area, (b) the ceramic nozzle laminated both in entry and exit area, and (c) the stress-free nozzles. (a) GN-2 laminated nozzle, (b) GN-3 laminated nozzle, (c) CN-2 stress-free nozzle.