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Abstract: The visualization of the detailed microstructure of plasma-sprayed ceramic coating was realized through energy dispersed X-rays (EDX) line analysis of the distribution of an infiltrated element as a tracer. A method was proposed based on an idealized lamellar structure model that the coating consisted of a stacking of lamellae with an identical thickness and the EDX line distribution of a tracer in the coating to quantitatively characterize the microstructure of plasma-sprayed coatings. The characterization was performed using structural parameters including lamellar bonding ratio, mean lamellae thickness and interlamellar gap size distribution. The experiment was carried out using plasma-sprayed alumina coating with the infiltration of Cr2O3 as the tracer through chromic acid. The measurement yielded a mean bonding ratio of 32% between the lamellar interfaces, a mean lamellae thickness of 1.2μm and a bimodal distribution for interlamellar gaps. The mean bonding ratio was defined as the ratio of the total bonded area to the total apparent interface area. These results agreed well with previously reported results obtained by copper plating technique. It was revealed that the proposed method could be applied to quantitatively characterize the lamellar structure of plasma-sprayed coatings. [Copyright &y& Elsevier]

The fracture toughness of plasma-sprayed Al2O3 coatings in terms of critical strain energy release rate G Ic was investigated using a tapered double cantilever beam (TDCB) approach. This approach makes the fracture toughness be measured only using the critical fracture load disregarding crack length during test. The Al2O3 coatings were deposited under different spray distances and plasma powers to clarify the effect of spray parameters on the G Ic of the coatings. The fracture surfaces were examined using scanning electron microscope. On the basis of an idealized layer microstructure model for thermal sprayed coatings, the theoretical relationship between the cohesive fracture toughness and microstructure is proposed. The correlation between the calculated fracture toughness and observed value is examined. It was found that the fracture toughness of plasma sprayed Al2O3 coatings is not significantly influenced by spray distance up to 110 mm, and further increase in spray distance to 130 mm resulted in large decrease in the fracture toughness of the coatings. The G Ic value predicted based on the proposed model using lamellar interface mean bonding ratio and the effective surface energy of bulk ceramics agreed well with the observed G Ic data. Such agreement evidently shows that the fracture toughness of thermally sprayed ceramic coatings at the direction along coating surface is determined by lamellar interface bonding. [ABSTRACT FROM AUTHOR]

The measurement of fracture toughness of plasma-sprayed ceramic coatings is usually performed using a standard double cantilever beam approach. The design of specimen geometry using a tapered double cantilever beam (TDCB) approach makes the compliance of specimens be linearly proportional to crack length. Fracture toughness in terms of critical strain energy release rate can be obtained by measuring fracture load only. Three varieties of the TDCB specimens are used to examine the effect of the geometry and size of specimens on the results. These are simple TDCB, modified and long TDCBs. Plasma-sprayed Al2O3 coatings are used in test. It was found that linearly proportional relations are experimentally obtained between the compliance of the specimen and crack length for all three varieties of the TDCB samples. Fracture toughness of plasma-sprayed Al2O3 coating is independent on the geometry of the test piece used in the study. The TDCB approach is a simple and reliable approach to measure fracture toughness of thermal spray coatings without the measurement of crack length. [Copyright &y& Elsevier]