Engineered carbons like graphite, carbon fibers, and carbon-carbon composites are used in aerospace and other industries. Oxidation and evaporation are common problems for carbon materials, which limit their applications at high temperatures. The development of methods for protecting oxidation of carbon materials has been given considerable attention lately [1].
A wide variety of coatings are used as conventional techniques for the preservation of carbon products against oxidation, such as SiC [2], TiC [3], TiN [4], Si3N4[5], B4C [6], SiO2[7], ZrSiO4[8], ZrO2[9], HfC-ZrC-SiC [10], Al2O3[11], mullite[12], hexagonal-BN [13], LaB6-MoSi2-ZrB2[14], MoSi2-SiC[15], ZrB2–ZrC–SiC [16] and ZrB2[17].
The very common techniques of applying these ceramic coatings are the chemical vapor deposition (CVD) and pre-ceramic polymer pyrolysis[18], which are complex and expensive. Among the high-temperature ceramics, SiC has gained a lot of attention and is being extensively recruited for protecting graphite and C/C composites from oxidation because of its strong compatibility with the carbon substratum and the creation of a SiO2 glass film, enjoying low oxygen permeability at high temperatures [19, 20].
At present, SiC coatings’ preparation for protecting carbonaceous substrates have multiple methods, including pack cementation [21–23], laser-induced chemical decomposition (LICD) [24], and chemical vapor deposition (CVD) [25]. Several researchers have used pack cementation of SiC to reduce the thermal expansion variations between carbon-based substrates and coating[26]. Nevertheless, in the coating preparation process, the unavoidable mismatching of the thermal expansion coefficients between them is linked to crack development [27]. These micro-cracks are pathways for oxygen to reach the carbonaceous layers such as SiC. Therefore, the oxidation resistance of the SiC coatings should be improved. Al2O3 has been widely used to provide oxidation shielding of C/C composites coated with SiC, thanks to its excellent corrosion resistance, good thermal stability, and low oxygen diffusion rate [28]. To coat alumina layers on different materials, abundant techniques such as atomic layer deposition [29], plasma spray [30], laser-cladding [31], and sol-gel [32] have been utilized, which are expensive and need special equipment. Thus, in the present work, the electrophoretic deposition technique was applied to obtain crack-free and uniform layers of α-Al2O3 nanoparticles on the carbon layer. The EPD can be either used alone or in combination with other methods such as sol-gel for protective coatings of carbon parts [33].
In addition to coat complex pieces due to throwing power ability of EPD, this method has other advantages such as composite coatings of oxide-carbide ceramics with additives which are commonly used for their better performance. Moreover, EPD method is simple, low-cost, and low-temperature [34].
In order to further improve the efficiency of the coating and make it denser, microwave sintering was used, which is a suitable technique to densify ceramic materials. Despite SPS and HP techniques, microwave processing is cheap and enables to sinter complex shape samples. In addition, the sintering in a microwave is fast and requires lower working temperatures, as compared to traditional sintering techniques [35].