  High-harmonic generation is the cornerstone of nonlinear optics. It has been
demonstrated in a wide range of crystalline systems including dielectrics,
semiconductors, and semi-metals, as well as in gases, leaving metals out due to
their low damage threshold. Here, we report on the high-harmonic generation in
metallic titanium nitride (TiN) films. TiN is a refractory plasmonic metal,
known for its high melting temperature and laser damage threshold, with optical
properties similar to those of gold. We show that TiN can withstand laser
pulses with peak intensities as high as 13 TW/cm$^2$, one order of magnitude
higher than gold, enabling the emission of intraband harmonics up to photon
energies of 11 eV. These harmonics can pave the way for compact and efficient
plasmonic devices producing vacuum ultraviolet (VUV) frequency combs. Through
numerical calculations and experimental studies, we show that the intensity
scaling and angular anisotropy of the emitted VUV radiation stem from the
anisotropic conduction band structure of TiN, thus confirming its intraband
origin.
