Microstructure and Oxidation Behavior of Plasma Sprayed Mo-Si-B Intermetallic Coatings

In this study, low pressure plasma spraying (LPPS) demonstrated as a viable technique to producing fine-grained samples from complex Mo-Si-B intermetallics. Microstructure and oxidation behavior of the coatings were studied. Introduction Due to its excellent oxidation resistance, high melting temperature, and relatively easy processibility one candidate material that could be used at significantly higher temperatures is MoSi2. However, MoSi2–based intermetallics exhibit poor low temperature mechanical strength and low creep resistance at high temperatures [1,2]. Another Mo–based silicide material for high temperature applications is Mo5Si3, having superior creep resistance compared to MoSi2 [3] but its oxidation resistance is poor [4,5]. Research at Ames Laboratory has focused on the mechanical and physical properties of phase assemblages within the Mo–Si–B ternary phase diagram [6,7]. In–situ multiphase composites of Mo5Si3Bx (T1)–MoSi2–MoB and T1–Mo3Si–Mo5SiB2 (T2) offer an interesting alternative to MoSi2–based intermetallics and these composites offer a combination of higher creep strength [3] and oxidation resistance comparable to MoSi2 [8,9]. The metal phase containing assemblages, in particular Mo–T2–Mo3Si and Mo–T2, have higher room temperature fracture toughness values but they also suffer from decreased oxidative stability. The current thought for using these materials in high temperature applications is to coat a refractory substrate of the Mo rich alloy with a T1–based alloy [10]. The refractory substrate would provide enhanced elevated temperature strength and fracture toughness while the T1–based coating would protect the substrate material from the aggressive combustion environment. In this regard, understanding the oxidation of T1–based coatings is crucial. Experimental Procedure Spray deposited samples were prepared on stainless steel substrate in a sealed chamber filled with argon at a reduced pressure, using the powder procured from a commercial supplier. Bulk coating was then removed from the substrate to prepare specimens. The specific details of spray process are published elsewhere [11]. Elemental analyses were performed using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Powder had the following elemental composition; 81.5 wt % Mo, 16.2 wt % Si, 1.6 wt % B and 0.7 wt % O. Initial annealing experiments based on previous work showed that the near equilibrium phase assemblage could be achieved by annealing for 2 hours at 1800°C in argon [12]. Phase array of bulk coating was 83 vol % T1 – (matrix phase), 15.6 vol % MoB – (secondary phase) and 1.4 vol % MoSi2 – (minor phase) used in this study. Coupons (~1 cm 2 ) were suspended from a sapphire wire in a vertical tube thermogravimetic Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 509-512 doi:10.4028/www.scientific.net/KEM.264-268.509 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:13:35) analyzer (TGA) for isothermal oxidation testing. Samples were characterized prior to and following oxidation. Phases were identified by measuring the Mo to Si ratio with EDS. Uniformly light gray regions are T1, the dark grays are MoSi2 and slightly recessed regions are MoB. Oxide scale composition and microstructure were analyzed using XRD and scanning electron microscopy coupled with energy dispersive analysis (SEM/EDS). Auger electron spectroscopy (AES) was used to qualitatively determine the composition of the oxide scale, including its boron content. Figure 1. SEM micrograph of as–sprayed coating, spray direction is from the top to bottom in the micrograph (etched with Murakami’s reagent). Results and Discussion The sprayed material showed a layered–like structure perpendicular to the spraying direction (Fig. 1). Individual splats were elongated (up to 10 μm by 1 to 3 μm thick) yet irregular in shape. Both inter– and intra-splat fracturing were observed. The liquid immersion bulk density for the coating showed that the samples had a density of approximately 90 % of theoretical. XRD of the sample indicated substantial chemical inhomogeneity. The crystalline phases were predominately T1, MoB and a small amount of MoSi2 within the interior of the splats and not at the splat boundaries. The phase proportions and the grain sizes within individual and between neighboring splats varied considerably. Some splats were almost entirely T1, while others had high proportions of MoB and MoSi2. Plots of oxidation induced mass change versus time for the Mo–Si–B coating at 800 – 1400°C are shown in Fig. 2. The samples show the characteristic oxidation behavior for Mo–Si–B materials that has been previously discussed [13]. Figure 2. Mass change of LPPS-deposited bulk Mo–Si–B intermetallic coating as a function of time on oxidation in air at 800 – 1400 °C. For all temperatures, there is an initial mass gain due to formation of MoO3 and SiO2 on the silicide surface. As the sample is heated past 750 °C, a rapid mass loss is observed due to evaporation of MoO3, which leaves a porous silica (or borosilicate) layer behind. Unless flow can occur to seal the pores and provide a passivating layer, transport through the scale occurs rapidly. Rate constants for oxidation of the Mo–Si–B bulk coating are given in Table 1. These rate kinetics are reasonable compared to that previously seen for T1–MoSi2–MoB materials over the temperature range of 800 – 1450 °C [8,9]. Fig. 3 shows the scale cross-section formed on the alloys after oxidation. At 800 °C the scale is glassy in appearance, containing cracks and pores. EDS of the 510 Euro Ceramics VIII