Thermal Evolution and High- High-Temperature Performance of the Mixed

Amorphous/Crystalline Structure

This chapter involves evaluation of the thermal and mechanical behaviour of the composite coating of interest (pseudo-eutectic alumina- 8 wt% YSZ) in the presence of amorphous phase. It investigates the roles of the amorphous phase in as-deposited coating, as well as the properties of the coating after heat treatment.

6.1. Crystallization and Phase Transformation Temperatures versus Amorphous Content

One question that this research was to address is the role of the amorphous phase content on the crystallization and consequent transformation temperatures in the composite coating. Figure 6-1 represents the two main transformation temperatures versus crystallization peak area (representing amorphous content) extracted from DSC curves.

The crystallization temperature in this diagram varies in the range of 951-9560C while the crystallization peak area ranges from 2 to 152 units. As can be seen, the variation of crystallization temperature is not affected by the amorphous phase content. This observation contradicts the proposition by Kim et al. [24] about the probable importance of the amorphous content on crystallization temperature in this composite, causing discrepancy in different reports. In these reports, the composites were produced with different processes and impurity contents. Therefore, the sources for the differences in the

crystallization temperatures may be referred to the production processes and/or the

Figure 6-1 Transformation temperatures versus crystallization peak area, showing that neither crystallization temperature nor the ?- to a-alumina transformation temperatures are affected by the

amorphous content

In the same range of amorphous content, the transformation temperature of ?- to a-alumina, changes between 1258 and 13100C. Although this is not a negligible change, dependency between this transformation temperature and the amount of amorphous phase

is not apparent.

6.2. Crystalline Structure Changes after Heat

Treatments

The variation of the coatings' crystalline structure after 400°C/8 hr, 700°C/24 hr, 1000°C/10 hr, 1300°C/24 hr andl500°C/5 hr heat treatment were studied. Heat treatments at 400 0C for 8 hours and 700 0C for 24 hours were done to investigate if the

diffused peak in the DSC curve includes any residual stress energy relief. XRD pattern of the resulting coatings presented no peak shift; this suggests that the diffused peak in the DSC graph does not involve any considerable residual stress relief. However it was seen that in case of highly crystalline coating, after both heat treatments some peak sharpening happened due to grain growth, while the peaks in the high amorphous samples showed no

visible change.

Crystallization heat treatment at 10000C for 12 hours in the highly crystalline coatings caused almost no crystallographic changes. In amorphous containing coatings with 35%

and 53% amorphous content some reduction in amorphous humps could be considered, but the humps did not fully disappear after 10 hours and the calculated amorphous

content reduced to about 24% from 53% amorphous and to 21% from 35% amorphous content. This observation shows that the crystallization, as a diffusion-controlled process,

is time dependant. Later results from thermal cyclic tested samples in section 6.8.2 confirm this observation by showing that the crystallization has been completed in the samples after many heating cycles at the same temperature, when the time is long enough

for the completion of the crystallization process.

In the sample used for heat treatment at 13000C for 24 hours, the initial crystalline structure in high amorphous sample (with about 64% amorphous) in Figure 6-2 (a)

consists of a-alumina and cubic zirconia. The presence of some tetragonal pattern closely

similar to cubic phase cannot be denied. The highly crystalline structure in Figure 6-2(c) (with about 11% amorphous) is composed of both a- and ?-alumina and cubic zirconia in as-coated condition. The comparison of this pattern with that of the coating heat-treated at 13000C for 24, hours shown in Figure 6-2 (b) and (d), suggests that in both structures

a-alumina is the only alumina phase present in the coating. The difference is in the YSZ dominant phase, which in the case of the heat-treated highly amorphous structure in Figure 6-2(c) presents some tetragonal structure (revealed by peak splitting at angles between 34°-35° and 59°-60°). It suggests that the amorphous phase should have crystallized in the form of tetragonal mainly by releasing the dissolved alumina as the stabilizer of high-temperature cubic phase. In contrast, in the highly crystalline coating after the same heat treatment in Figure 6-2(d), the metastable cubic YSZ solid solution is

still the dominant phase.

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1000

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Figure 6-2 Comparison of crystalline structure of heat-treated coatings with fully crystalline and highly amorphous coating: a) coating with 64% amorphous; b) 64% amorphous coating heat-treated at 13000C for 24 hrs; c) highly crystalline (11% amorphous) coating not heat-treated; d) same as c

after heat treatment at 13000C for 24 hrs

Further heat treatment at 150O0C for 5 hours to investigate the possibility of monoclinic

zirconia phase formation showed that except for some grain growth, in any of the structures traces of this phase could not be found. Conversely, formation of monoclinic in 8YSZ (8 mol% equal to 13 wt% yttria stabilized zirconia) has been reported at 14000C [117]. This proposes that the very high temperature stability of the composite against martensitic transformation of tetragonal to monoclinic zirconia is due to the added stabilizing effect of alumina to that of yttria.

In addition to the coatings with high amorphous content, the composite in the crystalline as-coated sample shows the high stability of the cubic solid solution of YSZ even at temperatures as high as 15000C. These results support the role of alumina as a stabilizer

through extended solubility in the zirconia.

6.3. Microstructural Changes after Heat

Treatments

The microstructures of the coatings after heat treatments of 1000°C/10 hrs, 1300°C/24 hrs and 1500°C/5 hrs has been investigated. In Figure 6-3, the microstructures of the

as-deposited coatings with high amorphous content (that has appeared as extensive grey areas in Figure 6-3(a)) and coatings with low amorphous content (with distinctive black and white regions in Figure 6-3(b)) are shown. The crystallization heat treatment for 10

hours at 10000C did not end with any visible change in the coatings' microstructures.

However, after heat treatment at 13000C for 24 hours, the two coatings can be compared in Figure 6-3(c) and (d). It can be seen some spotty areas that are formed of precipitation

of alumina and zirconia as a result of crystallization of the amorphous phase. Therefore, there are clearly more spots (precipitates) formed in the case of high amorphous structure.

These precipitates are typically of minimum 50 nm size. Heat treatment at higher temperatures of 15000C for 5 hours, as visible in Figure 6-3(e) and (f), shows the growth of precipitates as well as commencement of some spheroidization of the splats in the

form of round corners and thickened splats.