Variation of the temperature during the burning of straw pellets is shown in Fig. 3. The temperature of the first thermocouple (below the samples) is quite lower, that is due to the constant air flow from the bottom of the cylinder, and the maximum temperature is considerably lower (450–480 °C), because the burning of straw pellets is happening on top of the samples. The rapid growth of temperature can be observed at 17 min., that is when the straw pellets start to burn, the peak temperature is reached after 25 min. and is between 520–600 °C. Meanwhile, the temperature on the surface was about 100 °C higher than on the bottom when the straw pellets were burning. Once the burning process is over the temperature starts to decrease and stabilizes at 430–470 °C range. The temperature obtained on the top surface of the coatings was about ~ 50 oC higher after burning process was finished.
Surface morphology of uncoated steel before the heat treatment and after 20 cycles is shown in Fig. 4. The surface of untreated sample (Fig. 4a) is relatively clean, although some particles can be observed. After 20 heat treatment cycles the chance of the surface is evident. A lot more particles can be observed on the surface, that remained even after ultrasonic cleaning, these are mainly leftover products of burned straw pellets. In addition, the surface seems to be etched, this is the result of ongoing chemical reactions and oxidation during the burning of straw pellets. The SEM images clearly indicated that the surface of P265GH steel was changes and damaged.
Surface of sample with the Al2O3 coating is shown in Fig. 5. The surface of untreated sample (Fig. 5a) consists of splats and somewhat unmolten particles, also no defects, such as cracks or delamination, can be seen. Surface of the sample after 20 cycles (Fig. 5b) is similar, but the quantity of small particles is considerably higher. Much like in steel sample, a lot of those small particles are leftover products of burned straw pellets, that were not cleaned completely. Just as in steel sample, the surface is etched and cavities do form, but they are considerably smaller. This is due to the fact, that Al2O3 coating is a lot more resistant to chemical reactions, happening during the burning process of straw pellets, that steel.
The surface of Al2O3-13%wt.TiO2 sample is shown in Fig. 6. As in previous sample, untreated coating (Fig. 6a) consists of splats and somewhat unmolten particles, also no cracks or delamination zones can be seen. But unlike in Al2O3 sample the surface after 20 cycles is relatively the same, roughly the same number of small particles and no formed cavities are observed (Fig. 6b). The addition of titania increased coatings resistance to chemical and thermal impact.
Elemental composition was determined using energy dispersive X-ray spectroscopy. Samples were tested before treatment, and after 5 and 20 cycles. Each sample was measured in 4 different spots, then mean values were calculated (deviation 1%). The first sample (uncoated steel) consisted mainly of iron (93 at.%), also a small amount of oxygen (4 at.%) and other materials, that came from elemental composition of steel, were found. Even after 5 cycles the amount of oxygen dramatically increased to 46%, while iron lowered to 50%, and further increase of cycles had no effect to the amount of oxygen in the sample. This is due to absorbed oxygen during the combustion process, during the burning of straw pellets. Additionally, low traces of silicon, potassium, calcium and sulfur are found, that are leftover products of combustion reactions and were not completely removed with ultrasonic cleaning.
Al2O3 sample before treatment consisted of aluminum (31 at.%), oxygen (58 at.%), nickel (6 at.%) and chromium (2 at.%). Also, low amounts of other elements can also be found (carbon, iron, silicon etc.) and are attributed to impurities within the sample. Chromium and nickel originate from NiCr underlayer, that was applied before the coating in order to increase the adhesion of the coating. After 20 cycles aluminum decreased by 4% (to 27 at.%) and oxygen by 1% (to 57 at.%), nickel increased by 2% (to 8 at.%) and chromium remained the same (2 at.%). This happened due to the increase of other elements (iron, carbon, silicon etc.) during burning of straw pellets, and because the combustion products could not be completely removed.
Al2O3-13wt.% TiO2 sample consisted of aluminum (22 at.%), oxygen (57 at.%), titanium (4 at.%), nickel (9 at.%) and chromium (3 at.%). Much like in the Al2O3 sample, nickel and chromium are attributed to NiCr underlayer and small traces of other elements (carbon, iron, silicon etc.) are found due to impurities of the sample. However, after 20 cycles the amount of aluminum, titanium and chromium remained the same (22 at.%, 4 at.% and 3 at.% respectively), while oxygen increased by 2% (to 59 at.%) and nickel decreased by 1% (to 8 at.%). Also, just as in Al2O3 coating, number of other elements coming from combustion products increased. The data indicates that addition of titania did increase stability in elemental composition, since even after 20 heat treatment cycles only amount of oxygen altered by more than 1%.
Phase composition of the P265GH steel sample is shown in Fig. 7. Before the treatment there is only 2 peaks of Fe at 2θ = 44.7° and 65.1°, but even after 5 cycles several peaks of Fe3O4 appear and become the dominant phase. The further increase of heating cycles continues to increase the intensity of Fe3O4 peaks. These results support the previous statement, that steel sample was heavily damaged during the treatment cycles and protection is necessary under these working conditions.
There is noticeably less phase composition change in Al2O3 sample. Firstly, the dominant phase remains α-Al2O3 with peaks when 2θ = 25.6°, 35.7°, 43.5°, 57.7° and 63.1°. Secondly, another phase of alumina also apparent, that is γ-Al2O3 at 2θ = 37.4°, 38.9°, 45.8° and 67.1°. The ratio of α-Al2O3 and γ-Al2O3 most intense peaks before treatment is 1.16 but increases to 2.06 after 20 cycles. This is since α-Al2O3 peak intensity increased, while γ-Al2O3 remained similar. This happens because the temperature in not high enough for phase transition reactions from α-Al2O3 to γ-Al2O3 to occur. The temperature is only enough to initiate transition of amorphous Al2O3 to α-Al2O3. Similar results were found by Dhakar et al  where Al2O3 coating were heat treated at 900 °C. Thirdly, besides alumina there are two more peaks that are attributed to iron and nickel at 2θ = 44.3°and 51.6°, their intensities remained the same before and after 20 heat cycles.
Addition of titania in the final sample increased stability of phase composition of alumina coating, therefore even less changes can be observed (Fig. 9). There is a less peaks of α-Al2O3: when 2θ = 35.7°, 43.5° and 63.1°. Also, there are a few γ-Al2O3 peaks at 2θ = 37.4°, 45.8° and 67.1°. The ratio of α-Al2O3 to γ-Al2O3 the most intense peaks of the coating was 1.46. Meanwhile after 20 cycles of treatment the α-Al2O3/γ-Al2O3 ratio increased very slightly up to 1.51, which confirms, that the resistance to heat treatment increased with the addition of titania. Just as in α-Al2O3 sample there are two peaks of at 2θ = 44.3°and 51.6° attributed to the bonding NiCr layer. These results agree with the work of other authors , where no significant changes were observed for γ-Al2O3 phase during the annealing of up to 700 °C of plasma sprayed NiCrAl/Al2O3-13wt.%TiO2 coatings. The authors obtained only minor changes in the peak’s intensities of alpha phase.