Damage Tolerance of Thermally Sprayed Oxide Coatings

Thermally sprayed ceramic coatings are utilized in various applications in industries, such as paper- and process, aerospace and energy production. The requirements for the coatings vary from wear resistance and chemical stability to functional properties, such as low surface energy or thermal conductivity. Oxide coatings, such as yttriastabilized zirconium oxide, aluminum oxide, titanium oxide and chromium oxide
are commonly deposited by thermal spray processes using an atmospheric plasma, a high-velocity oxy-fuel or a flame spray torch.

The biggest drawback of the oxide coatings is their susceptibility to catastrophic failure from sudden, unexpected impacts, consequently leading to the functional failure of the component. The possibility of such impacts is omnipresent in most applications where ceramic coatings are used, which makes the topic attractive to a wide range of industries. This property of the coatings — named damage tolerance for the purposes of this thesis — additionally limits the number of possible applications. Therefore, any improvement in damage tolerance could open doors to various new technologies. Multiple workarounds have been attempted in improving the damage tolerance of ceramic coatings, such as metallic additions, oxide mixtures and nanostructured coatings, but so far increases in performance have been modest or have deteriorated other beneficial functions of the coating.

Furthermore, there lies a challenge in accurate and repeatable measurement of the damage tolerance. Current methodology includes testing in laboratory scale, giving information on the nature of the material and coating, and application-based testing, where the obtained information is not widely applicable in other conditions.

In this study, the primary focus was to evaluate different methods of measuring the damage tolerance of thermally sprayed ceramic coatings. Damage tolerance was divided in two distinguishable properties: crack propagation resistance and resistance to low-energy impacts. The former is akin to fracture toughness, but aims to give a more transferable result. Measurement methods of crack propagation resistance evaluated include four-point bending with acoustic emission instrumentation and high-energy impacts from spherical projectiles with crack path tracing. These methods provided insights into the effect of microstructure on the toughness of the coating. Interlamellar cohesion was shown to be the weakest link of toughness in that the weak interfaces provide the path of least resistance for crack propagation. Additionally, denser HVOF coatings proved more brittle than their plasma-sprayed counterparts as they did not have stress-relieving zones from pre-cracked areas.

The low-energy impact approach is slightly more application-oriented, aiming to emulate impact damage conditions in real-life environments. The methods used to measure it are micro-impact fatigue, where a small indenter is repeatedly impacted on the surface with high frequency, and cavitation erosion, where a vast number of impacts from collapsing bubbles create a statistical approach to measuring coating cohesion in the micrometer scale. The results of these tests correlated well with the concept of damage-tolerance as they measured the properties of the coating in a more general level. Since these methods rely on small impacts, hardness of the coating was a determining factor of damage-tolerance until the energy of the impact rose past a coating-specific threshold. Above this value, the coatings either failed catastrophically, or showed a more gradual failure propagation. The latter of these behaviors is highly preferred, as it gives time to react before the component fails in real conditions.

The secondary focus was to create ceramic coatings with increased damage tolerance through novel spray processes, as measured by the screened testing methods. The spray methods were suspension HVOF-spraying and solution-precursor -powder hybrid HVOF spraying. The suspension sprayed Cr2O3-coatings provided improvements in damage tolerance with similar or improved levels of wear resistance and hardness. The hybrid-spraying of Al2O3 powder and a zirconium acetate based precursor proved to still require further optimization of the spray process, as unmolten agglomerates of precursor-derived nanoparticles rather weakened the coating, instead of improving the cohesion. Nonetheless, promising potential is foreseen for the hybrid-spraying in the future due to its ability to tailor the coating composition rather seamlessly.