Nature inspired, multi-functional, damage tolerant thermal spray coatings

Abstract

Design and fabrication of ceramic materials that simultaneously provide adequate strength and toughness is of importance of in engineering applications. Nature has demonstrated that it is feasible to achieve this duality in properties through a combination of materials and microstructural engineering. The nacreous layer within the abalone shell is perhaps the most notable demonstration of such combined properties through unique combination of ceramic and polymeric material in an ordered, layered assembly. Recent studies have shown that through appropriate control of thermal spray processes and post-spray polymer infiltration of the deposited ceramic, it is possible, to some extent, to harness nacre"s microstructural attributes resulting in the material's dramatic improvements in both strength and toughness. This paper seeks to build upon previous work of layered, natural inspired design seen in nacreous materials, with an emphasis on examining their abrasion and contact damages. Two distinct thermal spray ceramic templates, one nacreous analogue and one standard thermal spray template, have been reproduced here and evaluated with regards to their functional performance e.g. abrasive wear © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ 2 and particle impact. The results from this study confirm the mechanical property improvements after polymer infiltration, and further show that it is indeed feasible to achieve spray coatings improved surface properties from natural design principles and processing innovations.

Orientation‐dependent mechanical and thermal properties of plasma‐sprayed ceramics

“…The existing defects and cracks beneath the notch root in these defected structures act as stress concentration points and the intended “pre‐crack”. This results in stable crack propagation under loading with limited sensitivity to notch root radius variation, which has been confirmed in both internal testing, as well as literature results . Elaboration on the mechanical behavior and toughness testing of these complex structures has been well studied, with many additional literature references found in Dwivedi, et al The insert in Figure shows a tested sample, depicting the notch geometry and resulting crack, with similarity to existing publications …”

“…Typical thermal spray materials can experience strong anisotropy due to the splat based build‐up, with direct implications in many thermal spray applications (e.g., wear protection, fracture, and thermal conductivity, etc. ), and further understanding of these changes is important . As stated in the introduction, the layered assembly of droplets form discrete layers, and in many cases, maintain high aspect ratios (on the order of 20:1), introducing microstructural and property anisotropy.…”

“…This results in stable crack propagation under loading with limited sensitivity to notch root radius variation, which has been confirmed in both internal testing, as well as literature results. 23,26,27 Elaboration on the mechanical behavior and toughness testing of these complex structures has been well studied, with many additional literature references found in Dwivedi, et al 22 The insert in Figure 2 shows a tested sample, depicting the notch geometry and resulting crack, with similarity to existing publications. 20,21,28 Flexural strength samples were delaminated, sectioned, and prepared in similar fashion, however without notching.…”

“…In these cases, as well as others, assumptions about the continuity of mechanical properties are necessary in extrapolating values from the crack behavior in such loading systems . These can be supplemented with deliberate modification of processing conditions, heat treatments, and microstructure, which has revealed interesting attributes about the layered anisotropy, the mechanical strength, and the toughness response …”

“…11,21 These can be supplemented with deliberate modification of processing conditions, heat treatments, and microstructure, which has revealed interesting attributes about the layered anisotropy, the mechanical strength, and the toughness response. 11,22,23 These studies have elucidated the importance of the lamellar microstructure on the physical and mechanical properties of spray assembled ceramic coatings. But understanding fracture response, especially in the weak orientation (in-plane, along the lamellae, perpendicular to spray direction) has become critical in understanding failure in TBCs.…”