Wang Yun Google is you friend. Properties are identical to zirconia with which it is intimately found. Wiki gives you:
https://en.wikipedia.org/wiki/Hafnium(IV)_oxide
"Hafnium(IV) oxide is quite inert. It reacts with strong acids such as concentrated sulfuric acid and with strong bases. It dissolves slowly in hydrofluoric acid to give fluorohafnate anions. At elevated temperatures, it reacts with chlorine in the presence of graphite or carbon tetrachloride to give hafnium tetrachloride"