Abstract
TiBx thin films with a B content of 1.43 ≤ x ≤ 2.70 were synthesized using high-power impulse magnetron sputtering (HiPIMS) and direct-current magnetron sputtering (DCMS). HiPIMS allows compositions ranging from understoichiometric to overstoichiometric dense TiBx thin films with a B/Ti ratio between 1.43 and 2.06, while DCMS yields overstoichiometric TiBx films with a B/Ti ratio ranging from 2.20 to 2.70. Excess B in overstoichiometric TiBx thin films from DCMS results in a hardness up to 37.7 ± 0.8 GPa, attributed to the formation of an amorphous B-rich tissue phase interlacing stoichiometric TiB2 columnar structures. We furthermore show that understoichiometric TiB1.43 thin films synthesized by HiPIMS, where the deficiency of B is found to be accommodated by Ti-rich planar defects, exhibit a superior hardness of 43.9 ± 0.9 GPa. The apparent fracture toughness and thermal conductivity of understoichiometric TiB1.43 HiPIMS films are 4.2 ± 0.1 MPa√m and 2.46 ± 0.22 W/(m·K), respectively, as compared to corresponding values for overstoichiometric TiB2.70 DCMS film samples of 3.1 ± 0.1 MPa√m and 4.52 ± 0.45 W/(m·K). This work increases the fundamental understanding of understoichiometric TiBx thin films and their materials properties, and shows that understoichiometric films have properties matching or going beyond those with excess B.
Highlights
Titanium diboride, TiB2, is one of the more thoroughly investigated transition-metal diborides to date
The elemental composition of the films determined by ToF-ERDA is given in Table 1; the composition is used for sample notation
The composition of the thin films deposited with direct-current magnetron sputtering (DCMS) was TiB2.70 and TiB2.20 for deposition pressures of 5 mTorr and 20 mTorr, respectively. This overstoichiometry is consistent with pre vious reports on TiBx thin films from DCMS [17,18]
Summary
TiB2, is one of the more thoroughly investigated transition-metal diborides to date This hard ceramic has attractive properties motivating this interest [1,2,3], including high thermal and electrical conductivity [4], good thermal and chemical stability [5] as well as good oxidation and mechanical erosion resistance [6,7,8]. In contrast to TiBx thin films grown with DCMS, reports indicate that TiBx films grown with HiPIMS realize an understoichiometric composition. The mechanism for this behavior has not yet been addressed, with limited investigation of the resulting mechanical properties. We here investigate the effect of gas pressure on HiPIMS and DCMS ion flux, atomic structure, microstruc ture, and expand on the properties of over- and understoichiometric TiBx films grown with HiPIMS and compare with corresponding DCMS grown films
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