Materials Page 
Terminology
References 3 & 4

HfO2 (Hafnia)

Bulk Properties

Melting Point:  2812 ºC
Density: 9.7 gm/cc

Film Properties

Evap. Temp.: 2500 ºC
Film Growth:  crystalline with low packing density
Film Hardness:  Hard

Common uses:   Overcoat for IR mirrors.  Coating wires for emitters.   Used in UV laser antireflection coatings and high reflectors.  High damage threshold at 0.35mm (especially if deposited from metal form.)

Comments:  Very hard, adherent film.