Effect of reflected Ar neutrals on tantalum diboride coatings prepared by direct current magnetron sputtering

ABSTRACT:
The magnetron sputtered tantalum diboride (TaB_y) coatings from stoichiometric TaB₂ target are often reported to be deposited in broad B/Ta interval with diverse structure and mechanical properties. In this article, the effect of Ar neutrals reflected from TaB₂ target on the B/Ta ratio is examined. Two targets with different thickness are used to influence the current-voltage characteristic of the discharge and the energy of reflected Ar neutrals. In addition, external magnetic field from Helmholtz coils is applied to influence the plasma density in the substrate region. It is demonstrated that the reflected Ar neutrals have a significant effect on B/Ta ratio reduction from 1.9 to 1.4. While decreasing the B/Ta ratio, preferred TaB_y crystal orientation changes from (0001) to (101̅1). Intense Ar bombardment results in loss of crystallinity exemplified by diffraction maxima broadening. The variation of B/Ta ratio is accompanied by change of hardness and Young’s modulus in range from 48 GPa to 32 GPa and from 532 GPa to 390 GPa, respectively. The coatings with B/Ta ratio < 1.6 show material pile-up around cube-corner indents, an indication for improved ductility.