Microstructure and adherence of porcelain enamel to low carbon steel
Introduction
Ceramic coatings have been an important research area in materials technology over the years and their applications cover from civilian industries to space technology. Coatings are applied to substrates for different purposes, e.g. improving corrosion resistance, wear resistance, barrier property, aesthetics, and so forth. Regardless of its end use, two important criteria must be satisfied for a coating to function properly during service; that is, good coating properties, e.g. physical, chemical and mechanical properties, and good adherence between coating and substrate. In general, coating properties are closely related to a material’s bulk properties, which can be controlled by tailoring the chemical composition, microstructure, and residual stresses of the coating1, 2, 3, 4. For example, the coefficient of linear thermal expansion of a porcelain enamel can be estimated using the rule of mixture from its major constituent oxides[1]. On the other hand, the adherence of coating to substrate is a somewhat complex issue and depends upon various factors such as pretreatments of a substrate before enamelling, which may involve surface modifications, interlayers, or interfacial reactions at the coating/substrate interface. Pickling and transition-metal (nickel or cobalt) dip are the common practices used by the enamel industry for improving the adherence of porcelain enamel to low carbon steels5, 6, 7.
Various techniques have been proposed and utilised to evaluate the adherence of a ceramic coating to a substrate, e.g. indentation method[8], scratch test[9], bending test[10], and conductivity measurement[11]. Among them, the indentation method and the scratch test are popular in the field of thin-film technology, and are often used by production lines as a quality control tool. In contrast, the bending test and the conductivity measurement are widely accepted in thick-film technology, for instance, by enamel industry. Recently a periodic cracking method (see Section 3.2) for measuring the ultimate shear strength of a metal/ceramic interface was proposed by Agrawal and Raj[12]. This method has been applied to ceramic/metal systems such as SiO2/Cu[12]and NiO/Pt[13]with planar specimen geometry for measurement of the interfacial shear strength, and it was shown experimentally that the method is best fit for systems that involved a brittle coating on a ductile substrate.
In this paper, two techniques, conductivity measurement and periodic cracking method, were used to evaluate the adherence of porcelain enamel to low carbon steel, by which the relationship between these two test methods was investigated. The microstructure of the enamel/steel interfaces with different transition-metal treatments was studied by cross-sectional scanning electron microscopy (SEM).
Section snippets
Experimental Details
A SPP (special purpose for porcelain) grade low carbon steel, provided by the China Steel Co., Taiwan, was used as the substrate for this study. The nominal concentrations of C, Mn, P, S, Al, and Ti in the steel are 0.003, 0.16, 0.009, 0.005, 0.045, and 0.074%, respectively. Two different sizes of specimens were prepared: one has the dimensions of 100×100×0.8 mm used for conductivity measurement, and the other has an apparent size of 120×25×0.8 mm, by which a dumbbell-shaped specimen is made for
Results and discussion
To provide a basic knowledge of the testing methods used to evaluate the adherence of porcelain enamels to low carbon steels, a brief description of the principle of the techniques along with the test results is given below.
Conclusions
Two techniques, conductivity measurement and periodic cracking method, were used to evaluate the adherence of porcelain enamel to low carbon steel with different pretreatments. It was found from the test results that the adherence of porcelain enamel to steel treated with cobalt is higher than those treated with nickel and without transition metal. The difference in the adherence can be explained from an examination of the microstructure of enamel/steel interfaces by scanning electron
Acknowledgements
The authors would like to thank Professor R. Raj of Cornell University for valuable and stimulating discussion on the periodic cracking method. The assistance of Mr S. C. Lin at China Steel Co., Taiwan, with the conductivity measurement is gratefully appreciated. Financial support of this research by the National Science Council of Taiwan, is acknowledged.
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