Magnetron sputtering of boron carbide coating on aluminum substrate—Experimental considerations for enhancing adhesion

Boron carbide is a ceramic material whose high cross-section for neutron absorption renders it practical as a coating for neutron-based detection applications, while aluminum is typically used as a substrate material since it is transparent to neutrons. However, adhesion between these two distinct materials is challenging because of the difference in their thermal expansion coefficients as well as the formation of unstable aluminum-carbon bonds that may cause adhesive failure. Toward improving the efficient attachment of boron carbide and aluminum, we deposited micron-sized (∼ 1–3.5 µm) boron carbide coatings with an adhesive intermediate titanium layer on an aluminum substrate using pulsed-DC magnetron sputtering. Qualitative testing of adhesion strength showed that coatings deposited with a bias voltage higher than ∼ −150 V presented high residual stress of > −1 GPa and poor adherence to the substrate. We determined that the optimal bias voltage applied to the substrate during deposition lies between ∼ −25±5 V (a floating potential value) and −60 V. Together with the support of a titanium layer adjusted to reduce the discrepancies in the thermal expansion coefficients, boron carbide coatings have proven continuous and strongly adherent for more than 27 months.