A dry ice-assisted ethanol–ester oil hybrid lubri-cooling strategy for enhanced surface integrity of hastelloy C-276

Machining of Hastelloy C-276 is challenging because of its high strength, pronounced work-hardening tendency, low thermal conductivity, and associated thermal and tribological loads. This study develops and experimentally evaluates a dry-ice-assisted ethanol–ester oil hybrid lubri-cooling strategy within a minimum quantity lubrication (MQL) framework, integrating cryogenic cooling, evaporative heat dissipation, and biodegradable ester-oil lubrication. Ethanol–ester oil mixtures with volumetric ratios of 1:0.25, 1:0.5, and 1:1 were characterized in terms of density, thermal conductivity, specific heat capacity, and viscosity under ambient and dry-ice-assisted conditions. The 1:0.5 formulation was selected for milling experiments based on its balance between fluid transport and lubrication characteristics. Dry-ice incorporation increased the measured effective thermal conductivity by approximately 11–17%, while reducing the apparent viscosity by 73–95% and measured specific heat capacity by 40–50%. These cryogenic measurements represent effective responses of the transient liquid–CO 2 gas system and may be affected by sublimation, gas fraction, and bubble formation. Milling tests demonstrated that the hybrid strategy increased surface hardness to approximately 525 HV, compared with 435 HV under dry machining, while enhanced subsurface hardness extended to approximately 50 μm. The hybrid condition also produced a more uniform surface morphology and reduced average surface roughness from 1.32 ± 0.04 to 0.61 ± 0.02 μm, corresponding to an approximately 54% reduction. Overall, the results demonstrate that the proposed hybrid strategy can provide effective thermal control and lubrication while substantially improving the surface integrity of Hastelloy C-276.