As an essential part of the project high temperature mechanical test equipment was developed to carry out tensile creep, bending creep, tension/compression cyclic fatigue, and measurement of crack growth rates in doubled torsion and compact tension. The tensile creep and fatigue rigs utilised long rod specimens (150mm) which protruded from the furnace ends allowing cold gripping. For both types of test bending on the specimens was maintained at less than 5%. One of the aims of the project was to obtain mechanical property data from a variety of test methods and specimen geometries in an attempt to determine real material specific properties.
Three materials have been produced. The first was a mullite material, the second was a zircon material and the third was developed from the zircon material to have a lower volume fraction glassy phase and to consist of the crystalline phases zircon-mullite-zirconia. The mullite material had 2 vol % glass, the zircon material 9 vol % and the zircon-mullite-zirconia had a value nearer to that of mullite.
For the mullite and the zircon-mullite-zirconia materials creep rates measured in tension and bending were comparable, with bending being 2-4 times faster. For the zircon material creep measured in tension was two orders of magnitude faster than that measured in bending. This is probably due to the fact that the presence of such a large volume fraction of glassy/liquid phase resulted in creep in tension being much faster than in compression. with bending creep being controlled by the slower creep rate on the compression face. For all materials, particularly at the highest test temperatures, the most damaging loading condition was static tension. Cyclic loading prolonged specimen lives, whether tension/compression or simply partial unloading tension/tension. Furthermore, measured cyclic crack growth rates were always slower than crack growth rates in statically loaded specimens. This behaviour is the result of crack healing due to the presence of the glassy phase.
The mullite and zircon-mullite-zirconia materials have potential for further development. For the current materials in the temperature range 1200(C - 1400(C, the minimum creep rates for mullite are 2 orders of magnitude slower, the times to fracture are 3 orders of magnitude longer and the cyclic behaviours at 1200(C (zircon-mullite-zirconia) and 1400(C are comparable. The mullite material is the more expensive as a result of the high sintering temperature. However, the zircon-mullite-zirconia material has by far the greater potential for improvement of its high temperature properties. Both should be considered as possible materials for use as energy efficient gas burning radiant heater tubes or radiating plates.
This proposal concerns the development of relatively inexpensive, non toxic, environmentally friendly, Zircon and Mullite-Zirconia high grade refractories/Technical ceramics for use at temperatures up to 1600 C.
These materials show great potential for use in materials show great potential for use in melting or heat treatment furnacezs for glasses, ceramics and metallic alloys, as both the lining refractory and particularly as energy efficient gas burming radiant heater tubes.
Mullite-Zirconia and Zircon ceramics of different compositions (Mullite-ZrO2, Mullite-ZrO2AL2O3,SiO3Zr,SiO4Zr-Mullite,SiO4Zr-ZrO2) will be prepared using freely available Zircon sand and waste fine calcined bauxite.
The micro structures of the ceramics and their long term stability, are to be controlled by the use of novel preparation methods and subsequent thermal treatments to yield good fatigue, creep and thermal shock resistant materials at elevated temperatures.
Measurements will be made of, tenile and flexural creep, tension/compression and torsion fatigue, thermal shock resistance and thermal fatigue. Crack growth velocities will be measured directly under both static and cyclic loading. Studies will be carried out at temperature up to 1600 C using novel radiant heating techniques developed and patented by one of the applicaThe crack growth and creep/fatigue behaviour will be modelled utilising concepts of fracture mechanics and plastic flow developed by the applicants.
These models will be used to evaluate the role played by the major phases and the glassy phase or recrystallised glass phase, in determining mechanical properties.
It is by these means that the particular systems with potential for commercial development as energy saving, environmentally safe, high temperature heating systems will be identified.
Funding SchemeCSC - Cost-sharing contracts
33428 Llanera Asturias