Elevated temperature wear behaviors of a Co–Mo–Si ternary metal silicide alloy
Introduction
In advanced industrial gas turbines and the power generation industries, there are many tribological components working under high temperature, aggressive service conditions, demanding novel high-performance wear resistant materials which also have outstanding corrosion and high-temperature oxidation properties. Transition metal silicide-based alloys, especially those ternary metal silicide alloys having the topologically closed-packed (TCP) hP12 MgZn2 type Laves phase crystal structure such as W2Ni3Si, Ti2Ni3Si, Mo2Ni3Si and Co3Mo2Si, etc. [1], [2], [3], [4], are demonstrated to have outstanding wear properties due to their inherent high hardness, strong anomalous hardness-temperature dependence, covalent-dominated strong atomic bonds and yield anomaly; they are emerging as a new group of advanced high-performance wear resistant candidate materials for many mechanical components working under aggressive tribological conditions. However, the existence of intrinsic brittleness from room to elevated temperatures and poor processing capability are currently the main obstacles restricting them from industrial applications as tribological components [3], [5]. In situ incorporation of a second ductile phase has been demonstrated to be one of the most effective methods to significantly enhance the toughness and ductility for many intermetallic alloys [6], [7], [8], [9], [10]. Cobalt-base solid solution (hereafter referred as Coss) is well known for its excellent combination of strength and ductility as well as high-temperature oxidation and corrosion resistance [11]. A dual-phase metal silicide alloy, with the hard ternary metal silicide Co3Mo2Si as the wear resistant reinforcing phase and the ductile Coss as the matrix, is expected to be a promising wear resistant material, possessing outstanding tribological properties. In our previous work [12], a group of Coss-toughened Co3Mo2Si ternary metal silicide alloys with a micro-structure consisting of Co3Mo2Si primary dendrites and the interdendrtic ductile Coss were designed. The effect of Co content on micro-structure, hardness and high-temperature sliding wear resistance of the alloys was systematically investigated at a fixed test condition. As a high-temperature wear resistant material, a basic understanding of its wear behavior, especially the load- and temperature-dependence of wear, are necessary from both academic and application points of view. In this paper, the effects of load and temperature on wear resistance and behaviors of the Coss/Co3Mo2Si dual-phase metal silicide alloy were investigated.
Section snippets
Experimental procedure
The chemical composition (wt.%) of the Coss/Co3Mo2Si metal silicide alloy studied is 51.67Co–40.44Mo–7.89Si. Short cylinder-shaped ingots of the Coss/Co3Mo2Si alloy with an average ingot diameter of approximately 18 mm and ingot height of approximately 14 mm were fabricated by melting the alloy powders in a newly patented laser melting furnace, using a high-power laser beam delivered from an 8 kW continuous-wave CO2 laser. Details of the laser melting procedures and process are reported elsewhere
Results
As shown in Fig. 2 the main phase constituents of the alloy are the ternary metal silicide Co3Mo2Si and the face-centered cubic β-Co solid solution (Coss). As shown in Fig. 3, the Coss/Co3Mo2Si metal silicide alloy has a dense and uniform dual-phase micro-structure consisting of the primary dendrites and the interdendritic irregular eutectics with small particles embedded in the matrix of another phase. EDS analysis indicates that chemical compositions (at.%) of the primary dendrite as well as
Discussions
The excellent wear resistance of the Coss/Co3Mo2Si metal silicide alloy under high-temperature sliding wear test conditions was attributed to its unique micro-structural characteristics and tribological behaviors of the phase constituents. First, the primary dendrites of ternary metal silicide Co3Mo2Si played the critical role in resisting adhesive wear because its covalent-dominated strong atomic bonds prevented the intermetallic phase from contact plastic deformation, adhesion and materials
Conclusions
The Coss-toughened Co3Mo2Si dual-phase metal silicide alloy exhibited excellent wear properties under high-temperature sliding wear test conditions, because of the high hardness and covalent-dominant strong atomic bonds of the ternary metal silicide Co3Mo2Si and the excellent ductility of the toughening Co-base solid solution. The alloy exhibited strong abnormal load- and temperature-dependence of wear. The wear mass loss of the alloy was extremely insensitive to the increasing applied test
Acknowledgements
The work was supported by National Natural Science Foundation of China (Grant No. 50331010), Trans-Century Outstanding Talents Program of Ministry of Education of China, National High-Tech Research and Development Program (Contract Nos. 2002AA305203, 2003AA305750), Beijing Municipal Natural Science Foundation (Grant No. 2022012), Basic Scientific Research Project of the Commission of Science, Technology and Industries for National Defense of China and Aeronautical Science Funds Office of AVIC
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