Molybdenum can be used to over 2900F but only in a vacuum where oxygen isn’t present, otherwise it will oxidize quickly. For the temperatures you are dealing with in an air-rich environment, I would suggest Inconel 600 which is usable to 2000F in air or a ceramic like alumina which is usable to 3000F.

Q: Why does Zirconia cost more than Alumina?

A:  1) Raw material cost: Aluminia is far more abundant, whereas Yttrium oxide which is a stabilizer for Zirconia ceramic, is a rare earth element.

2) Shaping costs: Due to its higher density it takes approximately 10x longer to grind down and shape Zirconia into parts.

3) Higher sintering temperatures:  The thermal shock resistance for Zirconia is poor so it requires a more costly sintering process.

Q: I’m using a high temperature vacuum furnace at 2500F. What type of material do you suggest using and is there any advantage of using a metal over a ceramic?

A:   Under vacuum conditions with little to no oxygen present, there are some metallic options for fasteners which include molybdenum and tantalum. These metals are considered referactory metals and could be used up 2900F and 3600F for molybdenum and tantalum fasteners respectively. In addition to metals, alumina and zirconia ceramics could be used as well.

The advantages of each come from what is most important and the conditions of your application. For example, if quenching is involved, you might not want to use ceramic fasteners due to the thermal shock which will cause failure. However if having low thermal conductivity through the fastener is important, then ceramics are a good choice. If the fasteners are to hold higher loads, the molybdenum and tantalum metals might be more robust.