Availableonlineatwww.sciencedirect.com ScienceDirect CERAMICS INTERNATIONAL ELSEVIER Ceramics International 36(2010)299-306 Layered structures in ceramic nozzles for improved erosion wear resistance in industrial coal-water-slurry boilers Deng Jianxin,, Yun Dongling, Zhou Houmin, Tan Yuanqian School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, PR China red 25 March 2009: received in revised form 22 June 2009; accepted 3 August 2009 Available online 22 September 2009 Abstract The nozzle is the most critical part in the coal-water-slurry(Cws) boilers. Ceramics being highly wear resistant have great potential as CWs nozzle materials. In this paper, Al,O/(W,Ti)C Al2O3/TiC layered ceramics (LNI, LN2, and LN3)with different thickness ratios among constituent layers were developed to be used as nozzles in Cws boilers. CwS burning tests in a boiler with these nozzles were carried out. The erosion wear behavior of the layered nozzles was investigated and compared with an unstressed reference nozzle(N5). Results showed that the layered ceramic nozzles exhibited an apparent increase in erosion wear resistance over the unstressed reference one. The mechanisms responsible were found to be that layered structure in the Cws nozzles can improve the hardness and fracture toughness of the external layer, and reduce temperature gradients and the thermal stresses at the exit of the nozzle during Cws burning processes. It is suggested that layered structures in ceramic nozzles is an effective way to improve the erosion wear resistance over the stress-free ceramic nozzles in industrial CwS boilers. C 2009 Elsevier Ltd and Techna Group S.r.L. All rights reserved. Keywords: Nozzles; Ceramic materials; Residual stress; Layered materials; Slurry erosion 1. ntroduction and higher thermal stress were found to be the main reason that caused the failure of the nozzle exit The nozzle is the most critical part in a coal-water-slurry Layered structures constituted by alternate layers with (CwS) burning system. During CWS burning processes, the different compositions can be properly designed to induce a nozzle is eroded continuously by the abrasive action of CwS, surface compressive residual stress [3-6]. The basic idea is to the working environmental temperature of nozzle can reach up couple material layers with different thermal expansion to 1000C, and there is a very high temperature gradient inside coefficients( CTE) so that residual stresses arise during cooling nozzle [1, 2]. Therefore, the nozzles in the cws boiler must from the sintering temperature Residual stresses arise from a have high hardness, and good erosion and thermal shock mismatch between the CTE, sintering rates and elastic resistance. In the author's previous studies [1, 2), several constants of the constituent phases and neighbouring layers ceramic composites were produced by hot-pressing for use in and the residual stress field depends on the geometry of the CWS nozzles. Detailed observations and analyses of the nozzle layered structure and on the thickness ratio among layers [7-9 wear surface have revealed that the primary wear mechanisms The design of layered ceramics has been proved to be a viable of the CwS ceramic nozzle exhibited thermal shock damage strategy to obtain significant increases of the fracture with chipping at the nozzle exit. Greater temperature gradient resistance, wear resistance, and tribological properties [ 10-12 The idea of layered structures was first introduced to the design of sand-blasting ceramic nozzles so as to form compressive residual stresses at the nozzle entry(or exit) hor at: School of Mechanical Engineering, Shandong region in fabrication process, which may partially counteract University, Jil the tensile stresses resulting from external loadings [13-15] Tel:+8653188 Results showed that layered structures in ceramic nozzle can E-mailaddress:jxdeng@sdu.edu.cn(J.Deng) induce an excess residual stress in the nozzle during fabrication. x1 2-8842/$36.00 2009 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.1016 1-ceramint.2009.09003Layered structures in ceramic nozzles for improved erosion wear resistance in industrial coal-water-slurry boilers Deng Jianxina, *, Yun Donglinga , Zhou Houminb , Tan Yuanqiangb a School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China b School of Mechanical Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, PR China Received 25 March 2009; received in revised form 22 June 2009; accepted 3 August 2009 Available online 22 September 2009 Abstract The nozzle is the most critical part in the coal-water-slurry (CWS) boilers. Ceramics being highly wear resistant have great potential as CWS nozzle materials. In this paper, Al2O3/(W,Ti)C + Al2O3/TiC layered ceramics (LN1, LN2, and LN3) with different thickness ratios among constituent layers were developed to be used as nozzles in CWS boilers. CWS burning tests in a boiler with these nozzles were carried out. The erosion wear behavior of the layered nozzles was investigated and compared with an unstressed reference nozzle (N5). Results showed that the layered ceramic nozzles exhibited an apparent increase in erosion wear resistance over the unstressed reference one. The mechanisms responsible were found to be that layered structure in the CWS nozzles can improve the hardness and fracture toughness of the external layer, and reduce the temperature gradients and the thermal stresses at the exit of the nozzle during CWS burning processes. It is suggested that layered structures in ceramic nozzles is an effective way to improve the erosion wear resistance over the stress-free ceramic nozzles in industrial CWS boilers. # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Nozzles; Ceramic materials; Residual stress; Layered materials; Slurry erosion 1. Introduction The nozzle is the most critical part in a coal-water-slurry (CWS) burning system. During CWS burning processes, the nozzle is eroded continuously by the abrasive action of CWS, the working environmental temperature of nozzle can reach up to 1000 8C, and there is a very high temperature gradient inside nozzle [1,2]. Therefore, the nozzles in the CWS boiler must have high hardness, and good erosion and thermal shock resistance. In the author’s previous studies [1,2], several ceramic composites were produced by hot-pressing for use in CWS nozzles. Detailed observations and analyses of the nozzle wear surface have revealed that the primary wear mechanisms of the CWS ceramic nozzle exhibited thermal shock damage with chipping at the nozzle exit. Greater temperature gradient and higher thermal stress were found to be the main reason that caused the failure of the nozzle exit. Layered structures constituted by alternate layers with different compositions can be properly designed to induce a surface compressive residual stress [3–6]. The basic idea is to couple material layers with different thermal expansion coefficients (CTE) so that residual stresses arise during cooling from the sintering temperature. Residual stresses arise from a mismatch between the CTE, sintering rates and elastic constants of the constituent phases and neighbouring layers, and the residual stress field depends on the geometry of the layered structure and on the thickness ratio among layers [7–9]. The design of layered ceramics has been proved to be a viable strategy to obtain significant increases of the fracture resistance, wear resistance, and tribological properties [10–12]. The idea of layered structures was first introduced to the design of sand-blasting ceramic nozzles so as to form compressive residual stresses at the nozzle entry (or exit) region in fabrication process, which may partially counteract the tensile stresses resulting from external loadings [13–15]. Results showed that layered structures in ceramic nozzle can induce an excess residual stress in the nozzle during fabrication, www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 36 (2010) 299–306 * Corresponding author at: School of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China. Tel.: +86 531 88392047. E-mail address: jxdeng@sdu.edu.cn (J. Deng). 0272-8842/$36.00 # 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2009.09.003