Year

1992

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

A thermochemical method has been developed in recent years to coat ceramics on metal substrates. With this method, ceramic coatings can be applied to metal substrates at temperatures lower than 540°C, and the coatings obtained are referred to by the proprietary name of C-ramic coatings. Although the C-ramic coatings are believed to have great prospects of application in various service environments, relatively little work has been conducted on the fundamental study and systematic evaluation of the coatings and the real potential of the coatings is still in doubt. The present work was carried out to investigate the characteristics of some C-ramic coating systems and their behaviour under thermal cycling, with particular emphasis on their thermal shock behaviour during cooling. Eight types of C-ramic coating systems, involving four types of coatings and three types of substrates, were investigated in the present research. The four types of coatings are two C Z coatings (C1Z1 and C2Z2) with a mixture of chromic acid and phosphoric acid as the binder/densifier (Z type), one A X coating with chromic acid as the binder/densifier (X type), and one C i X coating with both Z and X type binder/densifiers. The three types of substrates were stainless steel (AISI316), carbon steel (0.9%C) and copper. Material characterization was carried out using scanning electron microscopy and Xray diffractometry in order to have a better understanding of the fundamental features of the coating microstructure. Investigation of the influences of thermal shock and coating types with different composition on coating microstructure was also conducted. In addition, thermomechanical and thermogravimetric analyses were used to study the thermal stability of the coating materials. Thermal cycle tests with heating in a furnace and cooling in both water and air were conducted to observe the performance of the coating systems under various thermal cycle conditions. From this investigation, the general behaviour of C-ramic coating systems under thermal shock, the effects of coating types and substrate types on thermal shock resistance of C-ramic coating systems, and the possible causes of coating failure under thermal shock were revealed and analysed. Performances of the coating systems were evaluated mainly in terms of the critical peak temperature reached on heating and the type of failure. It was found that all coating systems fail by spalling during cooling when the peak temperature of the thermal cycles reached a critical level. Coating spalling occurred in four modes, viz. decohesion at the coating/substrate interface, decohesion along substrate peaks, decohesion within the coating, and decohesion within the substrate. Both critical peak temperatures and spalling m o d e depended on the type of coating system. Mechanisms for different spalling modes were also analysed and presented in the thesis. Coating spalling occurs mainly due to temperature induced stresses which in turn, magnitude, depend on the transient temperature gradient in the system and the mismatch in thermal expansion coefficient between the coating and substrate. To gain a better understanding of the spalling mechanism for general coating systems, a theoretical analysis was also conducted to study the effect of temperature gradient on coating spalling tendency during rapid cooling in terms of unconstrained strain mismatch (USM). Based on this analysis, together with the experimental investigation, it is suggested that coating spalling is probably mainly related to inelastic deformation occurring at high temperature attributed to viscous flow of amorphous phases. A s a result of inelastic deformation, the stress free point of a system m a y shift from the fabrication temperature to a higher value and thus a more severe mismatch in coating and substrate deformation tendency occurs during the cooling process and consequently causes coating spalling

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