Reinforcing Cold Sprayed Al Coatings with Boron Nitride Nanotubes and Micro-Boron Carbide and their Effect on Surface Mechanical and Dry Sliding Behavior

This investigation explores the reinforcement effects of both boron nitride nanotubes (BNNTs) and micro-boron carbide (μB4C) on the tribological and mechanical properties of aluminum matrix composite (MMC) cold-sprayed coatings. The synthesis process involved high energy ball milling (HEBM) and cold spraying with helium to create four distinct Al-MMC coatings on a magnesium (AZ31) substrate. These coatings consisted of pure aluminum, a composition containing 4 vol% B4C, a composition with 4 vol% BNNTs, and a composition with 2 vol% B4C and 2 vol% BNNTs. Successful dispersion of nanoparticles within the aluminum matrix was achieved. The hardness of the coatings exhibited significant improvements compared to the pure aluminum coating. Specifically, the Al-BNNT coating showed a hardness increase of 14.1%, the Al-B4C-BNNT coating displayed a hardness increase of 20.8%, and the Al-B4C coating demonstrated the highest increase at 33.3% over the pure aluminum coating. Furthermore, the Al-B4C coating exhibited remarkable reductions in wear volume loss and wear track depth, amounting to eight and two orders of magnitude, respectively. Adhesion testing revealed that the Al-B4C-BNNT coating failed cohesively, while the pure aluminum coating failed adhesively at approximately the same force. The Al-B4C coating experienced a combination of the two failure modes at a 31.2% increase in force compared to the pure aluminum coating. Tensile testing stress vs. strain curves indicated that the load was partially supported by the cold spray coating until the coating ruptured.