Evaluation of Nano-Scale ZnO Coatings Prepared via the Sol–Gel Method on Cast Iron Piston Rings: Tribological Performance
DOI:
https://doi.org/10.51747/energy.v16i2.p501-518Keywords:
ZnO coating, grey cast iron, piston ring, sol–gel method, dip-coating, tribological performance, wear, pin-on-diskAbstract
This study evaluated the structural, thermal, and tribological performance of nano-scale ZnO coatings synthesized via a sol–gel method and deposited on FC-25 grey cast iron substrates using dip-coating followed by annealing at 400 °C. Three precursor molarities (0.1 M, 0.2 M, and 0.4 M) and two dipping cycles (1× and 2× dip) were investigated. Structural characterization confirmed the formation of crystalline hexagonal wurtzite ZnO after annealing. Thermogravimetric analysis indicated a 4.48% mass loss in non-furnaced samples due to organic precursor decomposition, whereas annealed samples exhibited a stable relative mass change (+4.68%). Scanning electron microscopy revealed post-annealing microcracks resulting from volumetric shrinkage and thermal expansion mismatch. Tribological testing under ASTM G99 conditions demonstrated that the coated specimens did not outperform the untreated cast iron substrate (friction force: 0.89±0.22" N" ; CoF: 0.18±0.05; wear depth: 6.6±2.1 μ"m" ). Among the coated conditions, performance was strongly governed by process parameters: double-dipping at low molarity (2× Dip, 0.1 M) produced the highest wear and friction (friction force: 4.91±1.55" N" ; CoF: 1.00±0.32; wear depth: 131.4±69.3 μ"m" ), whereas increasing precursor molarity (2× Dip, 0.4 M) substantially mitigated wear degradation (friction force: 1.63±0.57" N" ; CoF: 0.33±0.10; wear depth: 41.5±14.8 μ"m" ). Two-way ANOVA confirmed that precursor molarity exerts a primary statistically significant effect (p<0.05) alongside a significant interaction with dipping cycle (p<0.05). These results demonstrate that coating structural integrity and stress management play a more critical role than bulk crystallinity in dictating tribological performance.
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