The Influence Of Substrate On Partially Rapidly Solidified Alumina-3 wt % titania Nanocrystalline Coatings Deposited by
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0890-Y02-08.1
The Influence Of Substrate On Partially Rapidly Solidified Alumina-3 wt % titania Nanocrystalline Coatings Deposited by Plasma Spray Technique. R.Venkataraman, B.Parmanick, B.Ravikumar, L.C.Pathak, D.K.Das1 and B.Venkataraman1 Materials Science & Technology Division, National Metallurgical Laboratory, Jamshedpur, India-831 007. 1 Surface engineering group, Defence Metallurgical Research laboratory, Hyderabad, India-560 058. ABSTRACT In this paper an attempt was made to impose different degrees of rapid solidification by spraying on diverse substrates of varying thermal properties. Substrates such as Copper, Aluminum, Stainless steel, Low alloy steel substrates were used to alter the imposed cooling rate and thereby the amount of residual α phase. A start powder of 3 wt % Alumina-titania powder was used for spraying to a thickness of 250 µm on the different substrates specified. In all cases the rapidly solidified phases show nanocrystalline sizes with the most rapidly solidified metastable γ phase showing finer grain size of less than 25 nm. The surface roughness of the substrate and the coating were characterized by Atomic force microscopy. In contrary to the Alumina-13 wt % titania, coupons of Alumina-3 wt % titania had shown poor indentation fracture toughness with increased amount of residual α phase. Coupons of stainless steel and low alloy steel had shown the lowest fracture toughness when tested by Vickers type indentation at loads of 3 N and 5 N. In contrast to these results the interfacial toughness when measured by Rockwell indentation technique at loads of 150 N was found to be dependent on the elastic modulus of the substrate more than the coating hardness. The interfacial toughness was found to be lower for softer material such as aluminum and copper than stainless steel and low alloy steel. INTRODUCTION Rapid solidification of pure alumina of equilibrium α phase with a rhomohedral structure always results in metastable γ, a FCC type distorted spinel structures [1]. It is not uncommon to observe to an extent of 10% residual α phase in the pure alumina as sprayed coatings. The residual α phase in these coatings always has been attributed to unmelted core of the start powder or trapped start powder. In contrast with the pure alumina, the scenario for rapid solidification of Alumina-13 wt %Titania powder in the as sprayed condition is entirely different and preferential stability of α phase to an extent of 40-45 % has been observed. The preferentially stabilized α phase when delineated to mesoscale dimensions were found to exist as splats or in rounded morphology [2]. Similar to pure alumina spraying partially melted or unmelted particles appearing in round morphology to an extent of 10-15 % is as expected were found to be prevalent. But, the remaining residual α phase appearing as splats were found to be crucial for toughening the matrix as observed in a separate micro indentation type of studies [3].
0890-Y02-08.2
The equilibrium phase diagram shows little solubility of Titania to any o
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