Other oxides

Magnesium aluminatehas been studied as a coating for electrical insulation.[101, 102]The addition of MgO into Al₂O₃increases electrical resistivity and gas perme-ability of the coatings compared to pure alumina coatings. An issue is the hydrophilic nature of the coatings which decreases the insulation manifold in higher humidities.

Another application is in lambda-sensors in combustion engines.[1]

Mullite and Zirconare compounds of Al₂O₃or ZrO₂and SiO₂, respectively. These coatings have low coefficients of thermal expansion making them useful in high temperature applications. [103]Indeed both can be used in heat-exchanger tubes to protect silicon-based ceramics against the alkali salts and water vapor.[103, 104]

Other application for mullite are cooling liners in combustion chambers of gas tur-bines[105], additives in TBCs[106]and components requiring electrical resistivity [107]. Zircon can be potentially used as a TBC or instead of PSZ especially when pro-ducing self-standing components, although it readily decomposes to its constituents, ZrO₂and glassy silica.[108]It is possible to recombine the coating back to zircon.

[109]

YAG (yttrium aluminum garnet) has the composition of Y₃Al₅O₁₂ can be used as a thin coating to measure surface temperatures by spraying undoped [110]or doped either with dysprosium [111] or europium[112] with solution precursor plasma spray (SPPS). The doping leads to photo-luminescent properties. Another promising future use of YAG is as a binder for silicon carbide (SiC) to create extremely hard SiC-containing coating. This is possible by surrounding the SiC particles by YAG to prevent oxidation of the SiC.[113]Additionally, YAG does not react with SiC.[114]

3 DAMAGE TOLERANCE IN THERMALLY SPRAYED CERAMIC COATINGS

Damage tolerance is a multi-dimensional system property of a coating. The term tolerance, indicates a type of reliability, which is indeed accurate for ceramic coatings where a single microscopic flaw can produce a catastrophic failure of the whole compo-nent. The flaw can be pre-existing or produced, for example, by an unexpected impact.

The property that is nowadays understood as damage tolerance is a combination of wear resistance, corrosion protection, fatigue resistance and adhesion to the substrate.

These properties are a product of the following processing effects: Residual stresses, feedstock alteration during spraying, microstructure of the coating and properties of the substrate. [115]The description of damage tolerance is illustrated in Figure 3.1. Since a majority of applications for thermal spray coatings have some form of impact, erosion or contact wear[116], it is of importance to assess these properties and how to improve them for ceramic coatings.

Figure 3.1 Performance and processing considerations in producing damage tolerant coatings. [115]

It is currently understood that the hardness of a brittle thermally sprayed coating is a poor indicator of its wear resistance, in experimental set-ups and in real life. Rather, fracture toughness has shown a stronger correlation to wear properties. [117, 118]

Additionally, based on the characteristics of ceramic coatings that they are intrinsi-cally more apt to resist chemical attacks than sharp mechanical impacts (as described in section 2.3.1), prioritizing wear resistance to corrosion resistance is agreeable.

The typical mechanism of wear for ceramics is brittle fracture with almost no prior plastic deformation. The phenomenon includes crack initiation and propagation perpendicular to the applied load. The crack propagation in crystalline solids, such as ceramics, occurs usually through the grains and along planes of high atomic density.

[2, 91]Once the crack reaches a critical size, determined by the fracture toughness of the coating, the component fails. [8, 9, 119]An illustration of this is shown in 3.2.

Damage tolerance in traditional ceramics is understood as a measure of load between the onset of first cracking and final failure[120], allowing time to replace a component before an abrupt process stoppage, for example. Increasing this means essentially giving more time to notice the impeding failure before a costly breakdown, while not compromising on the other required properties, such as corrosion resistance or low porosity. For thermally sprayed ceramic coatings, additional issues arise from

Figure 3.2 Illustrations of a) intergranular cracking and b) transgranular cracking. [7]

the defects in the coating structure (described in detail in 2.1.2). They have shown significantly lower fracture toughness values in the in-plane direction compared to out-of-plane direction.[121]This is a direct result of the anisotropy of the lamellar structure. Pores, pre-existing cracks from thermal mismatches, interlamellar inter-faces and un-/semi-melted particles all have to be considered when attempting to toughen coatings. [2]Some coating defects and properties can be detrimental to toughness, such as pores and poor cohesion between lamellae[122], while others

might be considered useful. These include poorly melted particles[13], pre-existing microcracks[9]and residual stresses[123]. In general, high density and homogeneity (or at least homogeneous distribution of dispersions) are desirable for coatings to optimize their structural integrity and thus mechanical properties.

The damage tolerance investigation in the scope of this thesis can be divided in two components as stated in 1.2: crack propagation resistance and impact resistance. The division is two dimensional — resulting from the discussion above, increasing damage tolerance means, in other words, to hinder crack propagation in the coating without compromising other properties. While these results are an indication of potential for the system, there is a need for practical validity of the results: when considering different applications and the environments that ceramic coatings must endure, often the situations where ceramic coatings fail mechanically are caused by either small repeated impacts or isolated higher energy impacts. Therefore it is essential to have sense of the applicability of the results to real-life conditions. In other words, crack propagation resistance is though of as a material/coating property, while impact resistance is a system property.

3.1 Improving the damage tolerance of thermally sprayed ceramic coatings

Significant improvement in the crack propagation resistance or fracture toughness and impact resistance of thermally sprayed coatings have been reported by optimizing the coating architecture through novel processing routes. These routes include the addition of a metallic constituent to the feedstock, use of nanostructured powder feedstock or the use of suspension feedstock.