Splat Formation and Nanohardness Investigation of Atmospheric Plasma Sprayed Alumina

Atmospheric plasma spraying (APS) is a well established process for preparing ceramic coatings. In this process, powder particles injected into the plasma jet are heated and accelerated simultaneously in the plasma stream and are deposited in molten or semi-molten droplets conditions onto a prepared substrate. During droplet impingement, these molten droplets rapidly solidify to form single splats. The coating is built up by piling up the layers of splats. Among plasma-sprayed ceramic and cermet coatings, alumina (Al2O3) is the most widely established coating material because of their dielectric and wear resistance properties. Much work has been done to characterize the complex structures which form plasma sprayed ceramic deposits. The majority of alumina related research has been carried out on deposits close to the interface with the substrate, and macro-features of the microstructure, e.g. splat shape, distribution alignment, and inter-splat interfaces. To enhance the quality and performance of APS coatings, a degree of understanding of coating structure and mechanical properties at the microscale and the nanoscale is required. However, this information is generally not available. Mechanical properties of ceramic coatings are usually difficult to measure, due to their comparatively small size in thickness and high brittleness. Depth-sensing nanoindentation is a very useful tool for investigating mechanical properties of plasma sprayed ceramic coatings [1]. The nanoindentation results of brittle materials can be made with higher scatter and reduced reproducibility, which is caused by the stochastic indentation response. After a detailed literature search, the author found that although a lot of effects have been made to study the plasma sprayed coatings using nanoindentation technique [2,3], studies on the nanoscale mechanical properties of alumina splats are very rare in the open literature. Therefore, the object of the present investigation is to carry out a detailed investigation on as-sprayed alumina splats using nanoindentation. Two distinct forms of alumina splats, the disc shaped and the distorted splashing shaped splats were prepared on smooth silicon wafers at different spray distances from the plasma gun. Also thick APS alumina coatings layer were prepared on a grit-blasted mild steel substrate to study their nanohardness and elastic modulus. For the alumina coating (Fig.1) on mild steel sample, indentation tests were performed at different locations on the sample surface separated by a spacing of at least 50 μm. The APS alumina coating exhibits bimodal distribution of nanohardness values. The average nanohardness value of one apex is 10.8 GPa, and the second one is 15.5 GPa. As shown in Fig. 2 (top), the APS alumina coating mainly consists of α Al2O3 and γ Al2O3 phases. The starting powder in an APS process is usually α Al2O3. Previous studies [4,5] reported that the transformation of α-alumina to γ-alumina could take place only when the powder particle is completely molten. It is well-known that the alpha phase is harder than gamma phase. Thus, the bimodal character in hardness testing on the APS alumina coating is due to the presence of two different alumina phases, α and γ, in the top coating. The average Young’s modulus calculated from the nanoindentation tests also has bimodal character, one is 195.6 GPa, and the other one is 253.2 GPa. For the disc shaped splat sample, a total of three kinds of indentations were made on a single splat. Two indentations (A and B) were made in the center region of the splat and the last indentation (C) was made on the rim of the splat, shown in Fig. 3. For the splashing shaped splat sample, only one indentation test was performed on each suitable location due to the small size of the splashing Fig.1 SEM micrograph of APS alumina coating Fig.2 XRD patterns of alumina starting powders and coating