To counteract the severe fretting wear and progressive oxidation experienced by pure-silver electrodes embedded in smart bolts that monitor axial clamping loads in gas-turbine discs, wind-turbine gearboxes and high-speed rail bogies, the work aims to evaluate the technical feasibility of replacing monolithic Ag with Cr-alloyed Ag coatings whose chemistry, microstructure and thickness are deliberately tailored by radio-frequency (RF) magnetron co-sputtering. Three composite Ag-Cr targets (nominal atom ratios of Ag3Cr7, Ag5Cr5 and Ag7Cr3) were sputtered in 1 Pa high-purity Ar at two contrasting RF powers of 250 W and 500 W onto mirror-polished 304 stainless-steel plates (for mechanical and tribological tests), AlN flats (for four-probe electrical characterization) and Si(100) chips (for cross-sectional scanning-electron microscopy and energy-dispersive X-ray spectroscopy). The as-deposited films were systematically characterized and then subjected to a 3 h vacuum anneal at 700 ℃ to accelerate thermally activated microstructural evolution and to rank their likely durability under realistic service temperatures. Film thickness, measured by SEM on fracture cross sections, increased almost linearly with both Ag content and RF power, reaching a maximum of (892±12) nm for the Ag-rich Ag7Cr3 coating grown at 500 W. EDS confirmed that the Ag:Cr ratio in every film closely mirrored the target composition, validating the reliability of the co-sputtering protocol. Micro-indentation (0.49 N load, 15 s dwell) revealed that Cr acted as the dominant solid-solution strengthener: Vickers hardness rose monotonically from 405HV (Ag7Cr3, 250 W) to 686HV (Ag3Cr7, 250 W) and from 504HV (Ag7Cr3, 500 W) to 782HV (Ag3Cr7, 500 W). Conversely, the steady-state coefficient of friction (COF) obtained from 180 s reciprocating dry-sliding tests (6.35 mm Si3N4 ball, 0.2 N normal load, 5 mm stroke, 1.25 mm/s) increased with Cr content because the harder surface promoted greater ploughing resistance. Raising RF power from 250 W to 500 W produced denser, better-oriented columnar grains and higher intrinsic compressive stress, so hardness improved further while COF dropped by up to 50% (from 0.331 to 0.173 for Ag7Cr3) because the harder film reduced real contact area and subsurface plasticity. Scratch adhesion tests showed, however, that excessive Cr or higher power was detrimental to coating-substrate cohesion: the critical load for coating detachment fell from 13.1 N (Ag3Cr7, 250 W) to 3.2 N (Ag3Cr7, 500 W), emphasizing the need for a balanced composition. Four-probe resistance measurements performed on AlN indicated that electrical resistivity scaled directly with Cr content (because Cr scatters conduction electrons) but decreased with film thickness (because the current path widened). Consequently, the 500 W Ag5Cr5 coating exhibited the lowest as-deposited sheet resistance (≈4 Ω along the short edge) among all six variants. After 3 h vacuum annealing at 700 ℃, every coating softened as Ag-rich phases coarsened, residual stress relaxed and grain boundaries annihilated. The hardness dropped by 2.6%-16.2%, yet the COF fell even further (0.114-0.149) because the softened surface accommodated interfacial shear more readily, evidencing excellent high-temperature lubrication stability. Notably, the Ag5Cr5 coating deposited at 500 W experienced the smallest hardness loss (3%), retained the lowest post-anneal COF (0.121) and maintained the smallest electrical resistance, thereby offering the best compromise among mechanical durability, tribological reliability and electrical conductivity. These findings demonstrate that modest Cr alloying (≈50at.%) coupled with a moderate increase in sputtering power (500 W) is an effective and manufacturable strategy for extending the service life of Ag-based electrodes in high-temperature sliding contacts, and they provide a practical materials route for realizing robust smart bolt sensors capable of stable operation above 700 ℃.
Key words
AgCr coating electrode /
sputtering power /
annealing /
hardness /
friction coefficient /
resistance
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Funding
Open Fund of Tianjin Key Laboratory of Fastening and Joining Technology (TKLF2024-02-A-01)
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