Bimodal microstructure toughens plasma sprayed Al₂O₃-8YSZ-CNT coatings

Abstract

Current work pursues generating controlled bimodal microstructure by plasma spraying of micrometer-sized Al₂O₃ and nanostructured spray-dried agglomerate with reinforcement of 20 wt% of 8 mol % yttria stabilized zirconia (8YSZ) and 4 wt% carbon nanotube (CNT) as potential thermal barrier coating (TBC) on the Inconel 718 substrate. Composite coatings exhibit bimodal microstructure of: (i) fully melted and resolidified microstructured region (MR), and (ii) partially melted and solid state sintered nanostructured regions (NR). Reinforcement with 8YSZ has led to an increase in hardness from ∼12.8 GPa (for μ-Al₂O₃) to ∼13.9 GPa in MR of reinforced Al₂O₃-YSZ composite. Further, with the addition of CNT in Al₂O₃-8YSZ reinforced composite, hardness of MR has remained similar ∼13.9 GPa (8YSZ reinforced) and ∼13.5 GPa (8YSZ-CNT reinforced), which is attributed to acquiescent nature and non-metallurgical bonding of CNT with MR. Indentation fracture toughness increased from 3.4 MPam^0.5 (for μ-Al₂O₃) to a maximum of 5.4 MPam^0.5 (8YSZ- CNT reinforced) showing ∼57.7% improvement, which is due to crack termination at NR, retention of t-ZrO₂ (∼3.3 vol%) crack bridging, and CNT pull-out toughening mechanisms. Modified fractal models affirmed that the introduction of bimodal microstructure (NR) i.e., nanometer-sized- Al₂O₃, nanostructured 8YSZ and CNTs in the μ-Al₂O₃ (MR) contributes ∼44.6% and ∼72% towards fracture toughness enhancement for A8Y and A8YC coatings. An enhanced contribution of nanostructured phases in toughening microstructured Al₂O₃ matrix (in plasma sprayed A8YC coating) is established via modified fractal model affirming crack deflection and termination for potential TBC applications.

Introduction

Thermal barrier coatings (TBCs) are extensively used to offer thermal insulation to metallic components working in high temperature environment such as gas turbine blades, combustion zone of the engines. TBCs consist of several layers, every layer has its own function. The ceramic layer, known as top-coat (usually made of stabilized zirconia) serves as a thermal barrier owing to its lower thermal conductivity [1]. The metallic layer named as the bond coat (MCrAlY, M = Ni and Co) improves the oxidation and hot corrosion resistance of the metallic substrate. Thermal conductivity of atmospheric plasma sprayed yttria stabilized zirconia (YSZ) and Al₂O₃ are 2 and < 7 Wm^-1K^-1, respectively. Also, low coefficient of thermal expansion (YSZ: 5-10 × 10^-6 K^-1), and high thermal stability at elevated temperature [[2], [3], [4], [5], [6], [7], [8], [9]] of aluminum oxide (Al₂O₃) and YSZ makes these a suitable top coat material for TBC applications. On the other hand, Al₂O₃ possesses low fracture toughness, K_IC ∼3.5 ± 0.6 MPam^0.5, which limits its use in structural applications [10]. To overcome this problem, yttria doping (Y₂O₃, 8 mol%) in ZrO₂ (called as 8YSZ) can be used as reinforcement in Al₂O₃ to improve the fracture toughness with its t-ZrO₂ phase [11]. Additionally, 8YSZ also possesses low thermal conductivity (<2 Wm^-1K^-1) and high durability at high temperature (∼1473 K) in turbines making it an appropriate reinforcement for Al₂O₃ based TBCs. Xu et al. [12] synthesized a composite of Al₂O₃ fiber, glass beads (hollow), nano silica powder and sepiolite fiber with improved insulation properties (0.124 Wm^-1 K^-1 at temperature of 610 °C). However, use of this composite might be restricted to ∼ 1400 °C owing to its poor thermal stability and modest mechanical properties [12].

In the last decade, TBCs synthesized from nanoparticles have been broadly addressed and discussed due to its improved properties compared with their microstructured counterparts. In case of micrometer-sized TBCs, catastrophic failure due to sudden crack propagation is the major problem. Introduction of nanometer-sized regions in the final microstructure creates a large number of grain boundary area and, thus, ensues longer path for crack propagation, which results in the crack termination at the nanostructured regions and, thus, enhanced fracture toughness and also increase the thermal shock life. Lima et al. [13] have summarized that TBCs made from nanostructured YSZ has lower thermal diffusivity (heating and cooling steps, up to ∼1200 °C) and ∼4 times higher thermal shock resistance than microstructured feedstocks. Nanostructured TBCs show promising improvement in the properties, however, nanometer-sized particles cannot be directly thrown into plasma plume due to its poor flowability and low specific weight. To overcome this problem, nanoparticles are reconstituted into micrometer-sized sprayable granules via spray drying. Generally, these spray dried powders are further provided a thermal treatment in order to improve their sinterability [[14], [15], [16]]. Coatings made from such agglomerates lead to a bimodal microstructure, which consists of partially melted and solid-state sintered agglomerates particles (called as nanostructured regions) and fully melted and resolidified regions (called as microstructured regions) working as a binder [14,15]. Carpio et al. [17] synthesized YSZ coating via plasma spraying of thermally treated spray dried bimodal YSZ suspension (50:50 nano and sub-micrometer-sized particles) and reported increase in micro-hardness and fracture toughness of bimodal coating (micro-hardness- ∼7 GPa, fracture toughness ∼2 MPam^0.5) than that of microstructured coating (micro-hardness- ∼4.9 GPa, fracture toughness- ∼1.8 MPam^0.5).

Further, in order to toughen Al₂O₃ based composites for its continuous and prolonged use for the structural applications, several researchers incorporated high toughness reinforcements like CNTs, graphite, Kevlar fibers etc. [[18], [19], [20], [21], [22], [23], [24], [25]]. For instant, multi wall carbon nanotubes (MWCNTs) are attractive reinforcement due to higher Young's modulus (∼1 TPa), tensile strength (∼200 GPa) and exhibit extraordinary performance as compared to other reinforcements like graphite, Kevlar fibers, and other ductile materials (copper, stainless steel etc.) [[26], [27], [28], [29], [30], [31], [32]]. Balani et al. [3] synthesized Al₂O₃-4wt% CNT coating via plasma spraying and reported an increase in fracture toughness from 3.22 MPam^0.5(Al₂O₃ coating) to 4.60 MPa m^0.5(Al₂O₃-4 wt% CNT coating). Wei et al. [33] synthesized Al₂O₃-3 vol% CNT via hot pressing (1600 °C, at 20 MPa under nitrogen for 1 h) and reported fracture toughness value of 5.02 ± 0.2 MPam^0.5 which is 79% increase in toughness in comparison with monolithic Al₂O₃ (2.80 ± 0.3 MPam^0.5). An enhanced fracture toughness in Al₂O₃-CNT composites is attributed to bridging effect of CNTs during crack propagation [34]. Mazaheri et al. [35] synthesized yttria stabilized zirconia (YSZ) reinforced with 0.5–5 wt% CNT via spark plasma sintering (1350 °C, pressure of 50 MPa, soaking time of 2 min under vacuum) and reported an increase in Young's modulus (258 ± 22 GPa) and Vicker’s hardness (12.8 ± 0.18 GPa) for YSZ-5wt% CNT than that of pure YSZ (modulus: 198 ± 15 GPa, Vicker’s hardness: 12.1 ± 0.21 GPa). Apart from mechanical properties, when it comes to effect in thermal conductivity via addition of CNTs, in this aspect Bakshi et al. [24] fabricated Al₂O₃-4 wt% CNT coating and reported decrease in thermal conductivity even after adding the 4 wt% CNT in Al₂O₃ matrix from 5.4 Wm^-1K^-1 (Al₂O₃ coating) to 3 Wm^-1K^-1 (Al₂O₃-4 wt% CNT coating), which was ascribed to the inter-splat thermal resistance. Thermal conductivity of CNT is also depended on its orientation. The value of thermal conductivity along and across the CNT axis is ∼3000 Wm^-1K^-1 and ∼1.64 Wm^-1K^-1, respectively [24,36]. Ariharan et al. [37] theoretically calculated thermal conductivity via different models with the consideration of crystallite size, phase content, porosity and CNT dispersion in Al₂O₃-based composite coatings. The thermal conductivity of plasma sprayed Al₂O₃-8YSZ-4 wt% CNT coating is calculated to 71.76 Wm^-1K^-1 without considering CNT alignment, however, for same coating composition the out-of-plane thermal conductivity was calculated to be 1.55 Wm^-1K^-1 (incorporating the alignment factor of CNT).

The spray drying of nanostructured YSZ powder and its mixing with microstructured Al₂O₃ has resulted in the retainment of nanometer-sized powders and, thus, bimodal microstructure, which is the combination of partially melted and fully melted agglomerates particles. Researchers have reported better properties (fracture toughness, micro-hardness) for YSZ bimodal coating than conventional coating formed by totally melted particles [17]. Though several studies have reported the formation of bimodal structure using spray dried powders [15,17,38], however no study has shown the initial mixing of micrometer-sized powders with spray-dried nanostructured powders to engineer bimodal microstructure via plasma spraying. In the recent work by Huang et al. [39], the bimodal microstructure of the YSZ based thermal barrier coatings is produced by modified powder feeding method, where the nanometer and micrometer-sized 8YSZ powders are fed from the tail of plasma flame during plasma spraying. Modification in the powder feeding system as well as the post sintering process led to the formation of bimodal microstructure containing unmelted particles with porous structure. Presence of porosity in the final microstructure resulted in the poor mechanical properties as compared to the conventional coatings (macroscopic Young's modulus as low as 15 GPa even after sintering for 100 h). Additionally, feeding of nanometer-sized powder directly into flame may also result in the clogging of nozzles. Therefore, to overcome these limitations, present work focuses on intentionally preparing the bimodal microstructure by spray drying (nanometer-sized powders) to provide improved mechanical properties. For this purpose, micrometer-sized Al₂O₃ has been added to spray dried nano agglomerates before the plasma spraying which helps in retaining bimodality in the coating, thus enhancing fracture toughness of the coatings.

In the present work, spray dried nanometer-sized Al₂O₃ composite powder was blended with micrometer-sized Al₂O₃ powder to make it bimodal mixture before depositing the coatings via atmospheric plasma spraying (APS). APS coating was deposited on Inconel 718 alloy, which is a real-life turbine blade material. Al₂O₃ phase is used as a matrix, and 8YSZ, possessing low thermal conductivity and to further enhance its fracture toughness, is used as a reinforcement. In addition, CNT is used as a reinforcement along with 8YSZ for toughening the Al₂O₃ matrix. Microstructural and mechanical characterization (nanoindentation) were performed on the coatings. Also, coating thickness and porosity in coatings were analytically quantified via image analysis software. The conceptual contribution of bimodal microstructure and YSZ/CNT reinforcements on the indentation toughness of Al₂O₃ based coatings is correlated in the current work.