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Title: Interactions between glass-ceramic coatings and metals
Author: Hong, Feng
ISNI:       0000 0001 3581 1653
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1991
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A study of glass-ceramic coatings on metal substrates was carried out in order to understand how the coatings and substrates interact and how these interactions affect coating microstructures and properties. There are two systems involved. One is a lithium-silicate glass-ceramic coating on Ni/Cr/Co alloy and the other is a lithium-aluminosilicate glass-ceramic coatings (with some other optional compositions) on titanium metal. Simple techniques such as screen printing or the "droplet” method were used to coat the substrates with a layer of fine glass powder, and then a firing procedure followed to create a vitreous enamel layer on the metals. Heat treatment was usually applied to convert the vitreous coating into a highly crystallized, glass- ceramic coating since this generally results in superior properties. Because both the Ni/Cr/Co alloy and titanium metal are active metals, complicated interactions were observed at the coating/metal interface. In the Ni/Cr/Co case. Cr from the substrate tends to diffuse into the coating very rapidly at the firing temperature. The rapid diffusion of mainly Cr++ ions and subsequent change into Cr+++ ions results in saturation of Cr+++ ions in the glass coating. The second oxidation step proceeds more rapidly at the coating surface and LiCr(Si03)2 crystals start to precipitate on the surface of the coating. Mismatch of TEC, thermol expansion coefficient, between LiCr(Si03)2 and the coating causes severe disruption. In addition, some components in the coating, for instance, P2O5, react with Cr to form Cr^Pî. As a result, in the reaction zone, consumption of the intended nucleating agent P2O5 leads to a poorly crystallized structure affecting coating properties. In the titanium system, though long range diffusion across the interface was not observed, the major problem is the reactivity of titanium with coating components including Si( > 2. Direct reaction between titanium and silica gives TisSia as an interfacial layer and the gaseous O2 produced may disrupt the molten glass structure during firing. Other interactions such as Ti/P2Os can also proceed to give other damaging effects. In general, the coating on titanium after firing is usually very porous if these interactions are not prevented. Preoxidation of Ni/Cr/Co alloy created an adherent Cr203 layer on the alloy surface, and this layer is very stable in the coating glass due to its low diffusivity and solubility. The existence of this layer prevented direct contact of alloy and coating at the firing temperature, minimizing interfacial reactions and leading to the desired coating structure. From a chemical point of view, preoxidation of titanium metal can create a barrier of Ti02 between the coating and metal to hinder the formation of Ti5Si3. However, the poor bonding strength of this layer meant that it was ineffective in forming a transition layer between the metal and coating. Furthermore, the Ti02 can be readily dissolved by the coating glass during firing. Addition of an adherence oxide, CoO, was successful in 1) producing Ti02 in situ at the interfacial area and Co/Ti dendrites, both of which are necessary in maintaining chemical as well as mechanical bonding across the interface 2) diverting and hence minimizing the damaging direct reaction between Ti and SiC > 2 because the reaction between Ti and CoO always takes place prior to the reaction between Ti and Si02 thermodynamically. In the systems of glass-ceramic coatings for reactive metal substrates, various complications may occur. In this study, interactions of lithium-silicate/Ni/Cr/Co alloy and a wide range of glass-ceramic coatings/titanium have been studied. Detailed observation has been given together with explanation. Further work has also been suggested so that better understanding and application may generate from what has been observed in this project.
Supervisor: Not available Sponsor: British Council ; Jiao yu bu, China
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; TA Engineering (General). Civil engineering (General)