Full length articleGrowth kinetics and isothermal oxidation behavior of a Si pack cementation-coated Mo-Si-B alloy
Introduction
Refractory metal systems with high melting points have received attention for their potential to increase temperatures in high-temperature applications such as turbine blades or nozzles [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. It has been reported that the turbine blades made from Ni based alloys reached temperature limits, and attempts have been made in order to increase turbine temperatures for achieving high fuel efficiency [3]. Refractory metal systems, especially the Mo-rich Mo-Si-B alloy system, have melting points higher than 2000 °C and show excellent mechanical properties above 1500 °C [10,11]. Mo-Si-B systems are categorized as two-phase alloys (Mo (s.s.) + Mo5SiB2 (T2)) and three-phase alloys (Mo (s.s.) + Mo3Si (A15) + Mo5SiB2 (T2)) depending on the fraction of Si and B. Mo-Si-B alloys with a precipitated T2 phase have excellent mechanical properties at high temperatures. In particular, three-phase Mo-Si-B alloys have compression strengths above 750 MPa and high creep resistances at 1500 °C [4,12,13]. The reason these alloys exhibit excellent mechanical properties at high temperatures is mainly due to the presence of the Mo5SiB2 (T2) phase. Additionally, the T2 phase is in thermodynamic equilibrium with the other phases in the Mo-Si-B ternary system [14]. Among the alloys, the aforementioned three phase alloys with a composition of Mo-3Si-1B alloy has been specially focused, since the composition exhibits a balance of oxidation resistance and mechanical properties [14].
Mo-Si-B alloys prevent their oxidation by forming a borosilicate scale on their surface at high temperatures under ambient atmosphere [4,12,13]. Furthermore, the addition of Al, Ce, La, Y or Fe to a Mo-Si-B alloy has been shown to result in oxide formation, i.e., Mo0-xAyOz (A = Al, Ce, La, Y and Fe), and each alloy improves oxidation resistance at certain temperature conditions [10,[14], [15], [16], [17], [18], [19]]. However, the increase in oxidation resistance from changing the alloy composition is often limited. Therefore, surface coatings, such as thermal plasma spray, sputtering or pack cementation coatings, on Mo-Si-B alloys have been investigated to improve their oxidation resistance [3,5,14,[20], [21], [22], [23], [24], [25], [26], [27], [28]]. Since pack cementation coatings can produce uniform coatings on complex components and excellent adhesion of the coating layer to substrates, pack cementation coatings have been widely studied for Mo-Si-B alloys [3,5,13,[23], [24], [25], [26], [27], [28]].
Pack cementation coatings with Si- and Al-based coating sources have been reported on Mo-Si-B alloys. For Si pack cementation coatings, a chemical reaction, such as xSi (s) + Al2O3 (s) + xNaF (s) → SiFx (g) + NaF (s) + Al2O3 (s), may occur during the coating heat treatment. SiFx (g) is the activating gas containing Si and simultaneously includes SiF, SiF2, SiF3 and SiF4. SiF2 (g) has the highest pressure in the pack and is the main component of Si pack cementation coatings [28]. When a high-temperature SiFx activating gas atmosphere is formed during heat treatment, Si coats a Mo-Si-B alloy through solid-state diffusion. Similarly, Al coatings using NH4Cl activators produce an AlxCly activating gas during the coating process, and Al diffuses towards the substrate [5,25]. Thus, Si or Al pack cementation coatings result in the formation of an outer MoSi2 or Mo3Al8 layer on a substrate. During the oxidation process of coated Mo-Si-B alloys, the SiO2 or Al2O3 ceramic protective layers have high oxidation resistances at temperatures above 1300 °C [3,5,[23], [24], [25], [26]]. Co-deposition (Si/Al, Si/B, Si/Y, Si/Y2O3) and/or two-step (Si and B) pack cementation coating processes improve oxidation resistance through formation of a complex coating-layer phase and oxidation behavior [3,14,[26], [27], [28], [29], [30], [31], [32], [33]]. In the coating process, a correct analysis of the thickness requires estimating the coating kinetics with respect to time, temperature, coating source and activator.
Regarding the previous models used for estimating coating thicknesses, Xiang et al. [35] and Majumdar et al. [28,36] reported analytical estimation methods for calculating the thicknesses of coating layers. However, the two models do not have consistent growth relationships regarding the coating-layer kinetics. Furthermore, there is no governing equation that can be applied to Mo-Si-B alloy. In this study, a new governing equation that can predict thicknesses of coating layer was suggested based upon experimental evaluation. To correctly identify the coating-layer kinetics, these two models were investigated with respect to Si pack cementation coating conditions. The variable factors examined to determine the coating layer thickness were the amount of Si and NaF in the pack, the heat treatment temperature and the time. Then, the experimental results were applied to the controlling equation that originated from the two models, and the revised controlling equation was compared with the experimental values to investigate the validity of the proposed equations. Finally, the microstructures of the coated alloys were discussed based on the structural analysis performed during isothermal oxidation exposure.
Section snippets
Experimental procedure
Mo-Si-B ingots with the composition of Mo-3Si-1B (wt%) were prepared by vacuum arc melting under an inert Ar gas atmosphere. The specimen was remelted five times to ensure homogeneity. The ingot was sliced into 10 mm × 10 mm × 4 mm pieces and sanded with SiC paper. The standard powder mixture composed of 65 wt% Al2O3 (6 μm), 30 wt% Si (25 μm) and 5 wt% NaF (35 μm) was loaded in an alumina crucible with the substrate, and the crucible was covered with an alumina lid, which was tightly bonded
Pack cementation coatings and growth models
A three-phase Mo-Si-B alloy with a composition of Mo-3Si-1B was fabricated, and the SEM backscattered electron (BSE) image is shown in Fig. 1. The SEM BSE image shows that the alloy contained three phases, Mo(s.s.) + Mo3Si + Mo5SiB2, corresponding to the previous results [3,4,[12], [13], [14]].
Typical cross-sectional SEM BSE images of Mo-3Si-1B coated by pack cementation at 1000 °C for 12 h and 1100 °C for 48 h are shown in Fig. 2. Fig. 2(a) shows an SEM BSE image of the specimen coated at
Summary
This study investigated the growth rates of Si pack cementation coating layers under various coating conditions on Mo-Si-B alloys. The pack cementation coatings contained an outer MoSi2 layer and a MoB layer between the MoSi2 and the substrate (Mo-3Si-1B), which can increase oxidation resistance during high-temperature exposure in ambient atmosphere. Previous theories on the growth rate of the diffusion coating layer were limited to the activator amount and the coating-layer growth relationship
Acknowledgements
Financial support from ADD (Agency for Defense Development) in Republic of Korea is gratefully acknowledged. Additionally, JSP appreciates financial support from the Basic Science Research Program through the National Research Foundation of Republic of Korea (NRF) funded by the Ministry of Education, Science and Technology (contract No. 2016R1D1A1A0991905).
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