Oxidation of Micro-Sized Spherical Aluminium Particles

Abstract

Micro-sized spherical Al particles have recently attracted interest for the development of a new concept for coatings based on their capability to form hollow alumina spheres and aluminized diffusion zones in the substrate. For understanding better their oxidation behaviour, spherical µm-Al particles with different sizes were oxidized in air on heating up to 1300°C and under isothermal conditions at 800°C and 850°C. The oxide formation was studiedin situby high temperature X-ray diffraction and the oxidised particles were analysed by scanning electron microscopy. On heating the µm-Al particles begin to form a g-Al2O3scale before reaching the melting point and the molten Al is kept within the g-Al2O3shell. On further heating q-Al2O3is detected, which forms simultaneously with the g-Al2O3. The g-Al2O3/ q-Al2O3scale is stable and protective under isothermal conditions up to 800°C within the investigated times. On further heating the g-Al2O3and q-Al2O3transform simultaneously to a-Al2O3in a temperature range of 850°C to 1100°C. Under isothermal conditions the g à a-Al2O3transformation is observed after 160 min at 850°C. During the g à a-Al2O3transformation shrinkage occurs that leads to formation of pores. A model is proposed describing the mechanism that leads to the formation of the observed whiskers morphologies during the g à a-Al2O3transformation.

Oxidation of Micro-Sized Aluminium Particles: Hollow Alumina Spheres

“…able to withstand the stresses due to the shrinkage during the transformation. This would be in agreement with Kolarik et al [25], who suggest h-Al 2 O 3 disappears at these temperatures using in situ XRD techniques. Another possibility is a mechanical damage during cooling: smaller particles will lead to thinner scales that are not able to withstand mechanical stresses due to temperature drop during cooling.…”

“…Molten aluminium is kept within an alumina shell at those temperatures. The formation of h-Al 2 O 3 both in nano-size [4,25,28] and micro-size powders [2,25] has been found, and it can appear in the studied powders. DTA (Fig.…”

Development of thermal barrier coating systems from Al microparticles. Part I: Influence of processing conditions on the mechanisms of formation

“…With the annealing step at 1100°C for 2 h, the top coatings got thinner (~10 µm and ~30 µm for Al_DS and Al_HS, respectivelysee Figures 9a and 9d) and micro-cracks appeared for the Al_DS sample (Figure 7d), although they were still observed to be adherent on the samples. The thinning and cracking of the coatings was attributed to the volume shrinkage of the thin alumina shells that occurs during the γ-Al2O3 to α-Al2O3 transition [9,21,26]. The same morphology was reported by Pedraza and Podor after exposure to 1150°C in 120 Pa He-4%H2…”

“…Alternatively, the presence of the NiAl3 structure for the Al_DS sample (Figure 8) and the bright particles observed within the top coatings (Figures 9b, 9c, 9e and 9f) indicate that some Al remained trapped in the particles and could thus not diffuse towards the substrate. The large mass gains observed upon heating between 700 and 1100°C in synthetic air and in Ar-10%H2O ( Figure 10) can therefore be attributed to the fast oxidation of the Al melt eventually released through the cracks of the shells [19,26]. This would explain the formation of quite thick oxide crusts [9,27] and the high level of sintering of the top coatings after the complete heat treatment in both synthetic air (Figures 7b and 7e) and Ar-10%H2O (Figures 7c and 7f).…”

Mechanical Properties and Reliability of Glass-Aluminium Composite as Insert Mold

“…Further heating caused the formation of θ-Al 2 O 3 scale, grew simultaneously with γ-Al 2 O 3 . The γ-Al 2 O 3 /θ-Al 2 O 3 scales remained stable and protective at isothermal conditions, up to a temperature reach 800°C [20]. The oxide layer decreased the properties of sinterability of aluminum powders.…”