Towards engineering isotropic behaviour of mechanical properties in thermally sprayed ceramic coatings

Abstract

It is widely recognized by the scientific community that thermal spray coatings exhibit anisotropic behaviour of mechanical properties, e.g., the elastic modulus values of the coating in-plane (i.e., parallel to the substrate surface) or through-thickness (i.e., perpendicular to the substrate surface) will tend to be significantly different due to their anisotropic microstructures. This work shows that thermally sprayed ceramic coatings may exhibit isotropic mechanical behaviour similar to that of bulk materials even when exhibiting the typical anisotropic coating microstructure. Elastic modulus values on the in-plane and through-thickness directions were measured via Knoop indention and laser-ultrasonic techniques on a coating produced via flame spray (FS) using a nanostructured titania (TiO 2 ) powder. No significant differences were found between the coating directions. In addition, four major cracks with similar lengths were observed originating near or at the corners of Vickers indentation impressions on the coating cross-section (i.e., a typical characteristic of bulk ceramics), instead of two major cracks propagating parallel to the substrate surface, which is normally the case for these types of coatings. It was observed by scanning electron microscopy (SEM) that coatings tended to exhibit an isotropic behaviour when the average length of microcracks within the coating structure oriented perpendicular to the substrate surface was about twice that of the microcracks aligned parallel to the substrate surface. Modelling, based on scalar crack densities of horizontal and vertical cracks, was also used to estimate when thermal spray coatings tend to exhibit isotropic behaviour. Crown

Hardness of thermal sprayed coatings: Relevance of the scale of measurement

“…Indeed, the polishing could lead to a release of the residual stresses and/or to work hardening in a region very close to the polished zone. In addition, the anisotropy of thermal spray ceramic coatings namely interlamellar sliding may occur during indentation into the cross-section, which would explain why a lower hardness value is measured [18,19].…”

Review of Functionally Graded Thermal Sprayed Coatings

“…The fracture toughness for thermal spray coatings can be characterized by strain energy release rate (GI c ) or fracture toughness (KI c ). The following methods are mainly used for the coatings [1,204,[226][227][228]: (i) double-cantilever beam; (ii) double torsion; (iii) indentation test; (iv) bending test; (v) scratch test; and (vi) tensile test.…”

Identification of the Anisotropic Elastic Parameters of NiCrAlY Coating by Combining Nanoindentation and Finite Element Method

“…Due to the microstructure depending on the preparation technology, the anisotropy of the hard coating is often expressed as transversely isotropic, and the mechanical properties are the same in the plane parallel to the substrate (as shown in Figure 1); the mechanical properties in the direction vertical to the surface of the substrate are quite different from those in the isotropic plane [19][20][21][22].…”

“…Several recent studies [12][13][14][15] on the mechanical properties of hard coatings have also equated coatings with isotropic materials. However, the microstructure feature of the hard coating resulting from the special preparation process (such as the layered structure prepared by air plasma spraying [16] and the columnar structure prepared by physical vapor deposition [17]) makes the elastic modulus of the hard coating show a significant difference in horizontal and vertical directions in the coating surface [16][17][18][19][20][21][22]. Because the vibration response of the coating-substrate composite structure contains complex modes such as bending and torsion, the anisotropy of the elastic modulus will affect the vibration characteristics [23]; the isotropic elastic modulus is difficult to accurately predict the mechanical behavior of the coating-substrate composite structure in vibration and the damping effect.…”