Metal Ceramic LT-1, from Pyrosales is a composite material made up of chromium and aluminium oxide. This combination of metal and ceramic exhibits excellent resistance to oxidation above 1200°C and also makes it resistant to wetting by many metals and alloys.
UCAR Metal Ceramic LT-1 is a combination of a metal matrix, chromium, and a pure ceramic phase, aluminium oxide (alumina). It is composed 65% by volume of metallic phase and 35% by volume of ceramic phase. The material is slip cast, sintered, and then oxidised. The exact nature of the bond between the phases is not known but a physical-chemical bond may be formed through the sharing of oxygen by the chromium and the alumina. There is no evidence of wetting or solution.
Slip casting is a process whereby finely divided solid constituents are put into liquid vehicle to form a colloidal suspension called a “slip”. This slip or suspension is poured into a porous plaster mould. The mould absorbs the liquid leaving the solids in the shape of the mould cavity. Axial ID holes are obtained by drain casting which involves draining off the excess “slip” after the wall has built up to the desired thickness.
By the nature of its constituents, UCAR Metal Ceramic LT-1 exhibits properties that are not found in either a metal or pure ceramic alone.
Metal Ceramic LT-1 has excellent oxidation resistance and also resists wetting by many metals and alloys, as well as basic furnace slays. The chromium-metal phase takes on a very tightly bonded layer of chromium oxide which, together with the naturally inert nature of the alumina, provides this material with its remarkable resistance to oxidising atmospheres over 1200°C, good corrosion resistance, and the ability to resist wetting by molten metals.
High thermal conductivity and the resultant excellent sensitivity to temperature changes accounts in part for its demand in the high-temperature pyrometry field as a thermocouple protection tube.
Metal Ceramics LT-1 have good strength at temperatures where many high-temperature metals melt. Above approximately 1540°C, it begins to soften and becomes plastic. Metal Ceramic LT-1 thermocouple protection tubes have, however, been used successfully for dip immersion at a temperature of 1650°C. In use or service, care must be taken to avoid conditions of extreme thermal shock, extreme thermal gradients, mechanical shock, and impact.
Although Metal Ceramic LT-1 is less resistant to shock and impact than the metallic alloys. Therefore, a standard thermocouple protection tube should be preheated to about 480°C before immersion in molten metal at 1100°C or higher. If practical, the following preheat procedure can also be used: hold the tube immediately above the molten metal for one minute before immersing. In tests conducted, this procedure proved to be adequate to prevent thermal shock failure.
UCAR metal-ceramic Metal Ceramic LT-1 exhibits good resistance to wear under conditions of sliding friction and also resistance to abrasion at high temperatures. The hardness of this material (Rockwell C 37) is more indicative of the crushing strength of the material than its true hardness because the individual particles have a greater hardness than the combined body.
UCAR Metal Ceramic LT-1 is less porous than most compacts. There is no significant passage of gases through the body at high temperature, except under high vacuum. For the usual industrial application, it is sufficiently impermeable. For example, S02 and S03 gases have not penetrated LT-l thermowells over a three year period to affect thermocouple wires.