Protective trivalent Cr-based electrochemical coatings for gun barrels

Highlights: • Electro-deposition of Cr coatings from trivalent Cr bath. • Reinforcement of Cr with YSZ, CNT and YSZ+CNT. • Enhanced hardness and elastic modulus from nano and micro-indentation in Cr-YSZ-CNT. • High wear resistance (wear rate ∼3.7 × 10⁻⁵ mm³/Nm) from fretting in Cr-YSZ-CNT. • High compressive stress (−918 MPa) exist in Cr-YSZ-CNT coating which restricts wear. Conventional Cr as a protective coating on gun barrels is electro-deposited from toxic hexavalent chromium electrolyte, which can be susceptible to cracking and tends to spall off. Herein, environment friendly trivalent Cr bath was utilized to electrodeposit chromium based composite coatings on armour steel with the incorporation of 3 mol% yttria stabilised zirconia (3YSZ) and carbon nano tubes (CNT). Higher dislocation density (17 × 10¹⁵ m⁻²), yield strength (4.6 GPa), calculated using Taylor's model, and reduced plasticity index (0.29) resulted in an enhanced hardness (∼25 GPa) and elastic modulus (∼206 GPa) for the synergistically incorporated YSZ and CNT in Cr matrix when compared to the Cr, Cr-YSZ and Cr-CNT coatings (using micro and nano-indentation techniques). Moreover, the fretting wear testing also elicited Cr-YSZ-CNT as most adroit coating having enhanced wear resistance (wear rate ∼3.7 × 10⁻⁵ mm³/Nm) due to the efficient presence of compressive stress (−918 MPa) and fine crystallite size (11 nm). The reduction in Hertzian contact diameter, from 32 μm for Cr to 27 μm for Cr-YSZ-CNT coating, results in a decreased tribological interaction between the counter body and coating material during the wear test. The mechanism involved in Cr-YSZ-CNT coating, grafted significantly with the mechanical and tribological performance, was attributed to toughening by YSZ nanoparticles that strengthen Cr matrix and impart higher resistance to crack-propagation during fretting wear. While the lubrication and bridging phenomenon of CNTs can potentially seal-up the initiation and propagation of cracks and pores within the matrix, thereby reducing the damage due to the wear. Thus, the dexterous mechanical and tribological performance of Cr-YSZ-CNT coating over Cr, Cr-YSZ and Cr-CNT coatings, makes it suitable composite material for gun barrel protection.