Fracture toughness of thermal barrier coatings (TBCs) has gained significant interest in recent years as one of the dominant design parameters dictating selection of materials and assessing ...durability. Much progress has been made in characterizing and understanding fracture toughness of relevant TBC compositions in their bulk form, but it is also apparent that the toughness is significantly affected by process‐induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma‐sprayed TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different process conditions inducing different levels of porosity and defect densities. Fracture toughness was measured on free‐standing coatings in as‐processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative composition, Gd2Zr2O7 which, as anticipated, shows significantly lower fracture toughness compared to YSZ. The results not only point toward a need for process and microstructure optimization for enhancing TBC performance, but also a framework for establishing performance metrics for promising new TBC compositions.
Thermally sprayed ceramics, when infiltrated with polymer, exhibit synergistic increases in strength and toughness. The structure of such composites—a dense, brick–mortar arrangement—is strikingly ...similar to that of nacre, as are the mechanisms underlying the robust mechanical behavior. This industrial‐scale process thus presents an exciting tool for biomimetic exploration.
A combination of characterization techniques has been used to provide new understanding of the complex crystallization behavior of as-sprayed amorphous Yb2Si2O7-based air-plasma-sprayed environmental ...barrier coatings (EBCs). During crystallization heat-treatment, initially a mixture of metastable α-Yb2Si2O7 and X1-Yb2SiO5 phases form, along with stable β-Yb2Si2O7 and X2-Yb2SiO5 phases. Eventually the metastable phases transform to the stable β-Yb2Si2O7 (major) and X2-Yb2SiO5 (minor) phases. The significant volume expansion associated with these transformations partially contributes towards the anomalous expansion measured in these EBCs after crystallization, but it does not account for all the measured expansion. In this context, in similar EBCs, it is also observed that the porosity increases upon crystallization heat-treatment, primarily in the form of thin, interconnected pores, which also contributes to the measured anomalous expansion. Based on this understanding, guidelines are provided for ‘near-net-shape’ crystallization of phase-pure, dense β-Yb2Si2O7 EBCs that are free of vertical cracks.
Due to droplet‐based assembly, microstructure anisotropy is expected in atmospheric plasma‐sprayed coatings (APS), with lamellar separations and interfaces having critical effects on properties. ...Quantitative determination of these anisotropic properties is difficult due to geometric test constraints. This has been overcome in the literature through a variety of indirect, local, or modeled evaluation, however direct measurement on like‐dimensioned coatings is not available. In this work, 25‐mm thick ceramic coating variants, deposited at two different feed rates, were obtained from industry and macroscopic mechanical and thermal properties were evaluated in both in‐plane and out‐of‐plane orientations using identical specimen geometries. As expected, and confirming select past work, coating anisotropy has a direct influence on measured properties. The response of each property is microstructure‐dependent, highlighting the specific interaction: for instance, the fracture toughness is 120% higher in the through‐thickness orientation versus in‐plane after thermal aging, while the thermal conductivity was 24% lower in the through‐thickness. The former benefits from the lamellar interfaces that provide obstacles to crack propagation while the latter sees these interfaces as efficient phonon scatters. The results provide insights for design through robust property measurements and into operational mechanisms in this class of highly defected ceramics.
A modified three-point bending technique has been proposed to measure the interfacial toughness of thick plasma sprayed ceramic coatings. Four different ceramic materials (titania, yttria stabilized ...zirconia, alumina, and yttria) on steel substrate were investigated resulting in a range of toughness values. Plasma sprayed coatings were prepared on grit blasted substrates with two different roughnesses and subsequently a notch in the centre of the coated beam was fabricated using a SiC saw. A two-step three-point bend test was conducted: first, the notched sample was loaded with a small offset to create pre-crack. The pre-cracked specimens were further loaded with offset 0.5 to measure interfacial toughness. Crack propagation manifested as a pop-in in the load-deflection plot. Subsequently, finite element analysis was used to calculate interfacial toughness, Gc, corresponding to the pop-in load. The visualization of crack propagation after pop-in load suggested that the crack had propagated along the interface in all four coatings. The widely accepted tensile adhesion test (ASTM C633) was separately performed on the same four coatings to compare the results obtained from the offset bending test. A correlation between interfacial toughness measured using offset bending and adhesion strength measured using the tensile adhesion test has been proposed.
•A modified three-point bending technique has been established to measure the interfacial toughness of thick plasma sprayed ceramic coatings having wide range of interfacial toughness.•The interfacial toughness and adhesion strength of four ceramic coating on steel substrate having wide range of interfacial strength has been measured experimentally•A correlation between interfacial toughness measured using offset bending and adhesion strength measured using the tensile adhesion test has been proposed.
Tensile adhesion tests (TATs) are an economical and convenient tool to assess the bond strength of thermal spray coatings and provide insight into manufacturing reliability and potentially component ...performance in service. It is a common practice to employ the TAT as per the recommendations outlined in various standards like ASTM C633 or ISO 14196. The industry accepts the TAT results as a characterizing parameter of coating adhesion. Although widely used for decades, the fundamental aspects of this test concerning its fidelity and reliability, relevance to bonding mechanisms, linkage to performance in service, and their material/substrate parametric dependencies are significantly limited. This work critically examines the test attributes, specimen preparation, material responses, reliability, and processing linkages through a comprehensive assessment of literature data and industrial practices along with carefully designed in-house experiments (where necessary to sort through inconsistencies in the literature) on a range of coating-substrate systems and processes. This assessment points to several challenges in accepting available data, especially regarding a lack of definition of failure loci and widespread variability in test results. It is noted that variability can arise from testing practices (specimen geometry, edge effects, etc
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) and intrinsic material attributes primarily for brittle coatings (flaw variations at the interface and associated toughness issues). The integrated results and analysis presented in this paper suggest that the test is intrinsically variable and highly sensitive to testing nuances and flaws (especially in ceramics), leading to inconclusive outcomes. In non-brittle systems (metals and carbides), the test evaluates the strength of the interface, with possible linkages to operative mechanisms of adhesion. It is envisioned that these developments, along with more robust descriptions of processing-related attributes (residual stresses, modulus, etc.), will enable further refinement of the method along with innovations for more advanced measurements.
In article an attempt has been made to investigate the dielectric strengths of selected ceramic coatings namely alumina (Al2O3), yttria (Y2O3), YSZ (ZrO2–7% Y2O3) and ferroelectric BST ...(Ba0.68Sr0.32TiO3) deposited using a atmospheric plasma spray process. Samples of each material were deposited under three different spray distances. The hardness, moduli of elasticity, dielectric strengths of the deposits were evaluated. The microstructures and phases were analyzed using optical microscopy and X-ray diffraction analysis respectively. The results show the significant influence of spray distance on dielectric strengths of the coatings. The dielectric strengths as per hierarchy are 17.3±1.0kV/mm for yttria, 16.6±1.8kV/mm for alumina, 11.1±1.5kV/mm for YSZ and 7.8±0.8kV/mm for BST. The particles temperature and velocity during spraying were analyzed in order to find relation between process parameters and dielectric strengths. Correlation between crystallinity of deposited BST and dielectric strength was found in this study.
Thermal spray deposited WC-CoCr coatings are extensively used for surface protection of wear prone components in a variety of applications. Although the primary purpose of the coating is wear and ...corrosion protection, many of the coated components are structural systems (aero landing gear, hydraulic cylinders, drive shafts etc.) and as such experience cyclic loading during service and are potentially prone to fatigue failure. It is of interest to ensure that the coating and the application process does not deleteriously affect the fatigue strength of the parent structural metal. It has long been appreciated that the relative fatigue life of a thermal sprayed component can be affected by the residual stresses arising from coating deposition. The magnitude of these stresses can be managed by torch processing parameters and can also be influenced by deposition effects, particularly the deposition temperature. In this study, the effect of both torch operating parameters (particle states) and deposition conditions (notably substrate temperature) were investigated through rotating bending fatigue studies. The results indicate a strong influence of process parameters on relative fatigue life, including credit or debit to the substrate's fatigue life measured via rotating bend beam studies. Damage progression within the substrate was further explored by stripping the coating off part way through fatigue testing, revealing a delay in the onset of substrate damage with more fatigue resistant coatings but no benefit with coatings with inadequate properties. The results indicate that compressive residual stress and adequate load bearing capability of the coating (both controlled by torch and deposition parameters) delay onset of substrate damage, enabling fatigue credit of the coated component.
•Thermal spray coatings strongly influence fatigue life of a substrate.•Coatings may help prevent substrate damage from fatigue under proper conditions.•Coating processing and deposition parameters determine residual stress, strongly influencing fatigue life.•Compressive coating stress benefits fatigue life.
This paper reports the finding of the role of substrate surface chemistry on the interaction between the molten droplets (splats) and solid substrates in thermal spray coating. The substrate surfaces ...were modified by thermal treatments to grow specific types of oxide and hydroxide layers on the surface. It was found that water released from the dehydration of surface hydroxide triggered by the impact of the droplets provoked splat fragmentation and splashing, resulting in the poor splat–substrate bonding. Although this finding is used to address the question of the nature and the influence of surface adsorbates on the splat formation and morphology in thermal spray coating, it can be applied to other technologies and material processes involving the contact between the molten droplets and metal surfaces.
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