Some thermal spray coatings are being used on structure steels in industry to prevent surface degradation by corrosion, erosion or combinations of erosion and corrosion. The ability of the coatings to protect base materials against erosion-corrosion is related to their composition and processing, and especially to their morphology, i. e. the form and structure of the coatings. Generally, the hardness of the coatings does not directly relate to their erosion resistance [1-3]. However, some data in the literatures [4-6] reported that the relative hardness values of the erodent particles and the target materials might play a significant role in the erosion behavior of the brittle materials. Decreasing the ratio of the target material hardness to the hardness of impacting particles resulted in a dramatic increase in the erosion rates, mainly when the ratio was in the range of unity [5-7]. In previous work [8] nine thermal spray coatings on mild steel, including hard tungsten carbide-cobalt coatings and soft, more ductile metal coatings were eroded at elevated temperature using fluidized bed combustor bed ash and quartz particles. It was found that when the coatings were eroded by bed ash particles, the ductile metal coatings had the same or lower erosion rate as compared to some harder coatings. When quartz particles were used as the erodent, the ratio of the target coating hardness to the hardness of the erodent particles appeared to play a significant role in erosion behavior of the coatings. There are two ranges; above a ratio of 0.56 it shows a slight reduction in erosion wastage with an increase in the ratio, below a ratio of 0.33 showing a dramatic increase in erosion wastage with a decrease in the ratio. The former range corresponds to the hard material coatings and the latter range covers the softer, ductile metal coatings.
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