18-02-2013, 09:19 AM
Coatings
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Introduction:
Coatings have been used extensively in the past, either resulting in the novel applications or enhancing the performance of the already existing component. Coating technology has been universally applied to many engineering materials such as metals, ceramics and polymers successfully. Depending on the requirements of the particular application, the purpose of coating is to enhance the engineering performance of the original underlying substrate and confer additional tailored properties to the material. Although coatings were initially developed for decoration and environmental protection, recently they have been successful in imparting electrical, wear and corrosion resistance to many metals [1]. For example aluminum coated steels are used successfully in corrosive and oxidizing atmosphere environments in which the temperature ranges from that of outdoor exposure to 11500C. Aluminum coatings protect steel from attack by forming a very resistant barrier between the corrosive atmosphere and steel. The aluminum oxide that forms at the aluminum surface is highly resistant to a wide range of environments [2].
Applications of Aluminizing [3]:
• Aluminized coatings for power generation applications
• Automotive poppet valves
• Turbine blades and vanes
Laboratory work
Procedure:
• First of all aluminized steel sample was taken
• Then cutting and mounting of the sample was done
• After mounting polishing operation was performed
• Then etching with 2% Nital was done
• Finally microstructure was observed on the optical microscope
Discussion:
The measured coating thickness of our aluminized sample was 62.5 which lie in the range of hot dip aluminized process of aluminizing of steel (10-80) in the literature [4] which shows that our sample was hot dip aluminized.
Microstructure of hot dip aluminized carbon steel consists of three distinct regions, the outer pure aluminum layer, Intermetallic layer, The substrate layer and also the typical feature of the aluminized steel in pure aluminum (in the temperature range of 700-750ºC) was the un-even interface between the intermetallic compound layer and the substrate steel[5].
The micrograph of our coated sample revealed two distinct regions, a surface layer (aluminum) and the substrate (steel), but it could not be identified difference of intermetallic layer and the pure aluminum layer. As the interface between the substrate and intermetallic compound of aluminum and iron has a characteristics of tongue like morphology[6], so on the basis of this we can say the part of the coating just adjacent the substrate having dendritic shape could be of intermetallic. The standard interface between pure aluminum and intermetallic compound used to be smooth [6] but in our case it could not be revealed properly smooth which might be due to the conditions that prevailed during the experiment or the chemical composition of the substrate.
In hot dip aluminized steels the depth of intermetallic layer depends upon the dipping time and dipping temperature, greater the value of these parameters greater will be the thickness of intermetallic layer [7]. So in our case the reason for small intermetallic layer might be small dipping time or temperature or both.