In our research, we investigated the tribological properties of surface-treated grey iron castings, focusing on the comparative analysis between phosphate layers and sulfurized layers. Grey iron castings are widely used in various industrial applications, such as compressor components, due to their excellent self-lubricating properties and cost-effectiveness. However, with the shift towards environmentally friendly refrigerants like R32 and CO₂, which operate under higher compression ratios, the demand for improved friction and wear resistance in grey iron castings has increased. Traditional surface treatments like phosphating have limitations in durability under heavy loads. Therefore, we explored low-temperature ion sulfurization as an alternative to enhance the performance of grey iron castings.
Our study began with the preparation of grey iron castings, specifically HT250 grade, which has a chemical composition as shown in Table 1. We used standard immersion phosphating to deposit phosphate layers and low-temperature ion sulfurization to form sulfurized layers. The sulfurization process was conducted at 150°C for 9 hours in a vacuum environment. We characterized the surfaces using SEM-EDS, XRD, nano-indentation, and X-ray stress analysis. Friction and wear tests were performed using pin-on-disc and ring-on-block configurations under lubricated conditions with PVE oil. The results demonstrated that sulfurized layers on grey iron castings significantly outperform phosphate layers in terms of friction reduction and wear resistance.
The microstructure of the surface layers was critical in understanding their tribological behavior. For grey iron castings, the phosphate layer exhibited a porous structure composed of blocky crystals, while the sulfurized layer showed a鳞片状 (scaly) morphology with fine pores. The thickness of the phosphate layer was approximately 6 μm, and that of the sulfurized layer was 4 μm, as confirmed by EDS line scans. The phase composition, analyzed via XRD, revealed that the phosphate layer consisted of Mn₃(PO₄)₂·3H₂O and (Mn, Fe)₃(PO₄)₂·4H₂O phases, whereas the sulfurized layer primarily contained FeS with minor FeS₂ phases. FeS, having a hexagonal close-packed structure, contributes to lower shear strength and better lubricity, which is advantageous for grey iron castings in friction applications.
To quantify the mechanical properties, we measured the surface hardness and residual stress. The hardness of untreated grey iron castings was 213.2 HV₀.₀₀₁₅, which decreased to 158.9 HV₀.₀₀₁₅ for phosphated samples and 196.6 HV₀.₀₀₁₅ for sulfurized samples. This indicates that sulfurization preserves more of the base material’s hardness compared to phosphating, which is beneficial for wear resistance in grey iron castings. Residual stress measurements showed that the sulfurized layer had higher compressive residual stress than the phosphate layer, further enhancing durability. We summarized these properties in Table 2 to facilitate comparison.
| Element | C | Si | Mn | P | S | Sn | Cu | Fe |
|---|---|---|---|---|---|---|---|---|
| Content (%) | 3.57 | 2.70 | 0.81 | 0.020 | 0.083 | 0.12 | 0.06 | Bal. |
| Treatment | Layer Thickness (μm) | Hardness (HV₀.₀₀₁₅) | Residual Stress (MPa) | Primary Phases |
|---|---|---|---|---|
| Untreated | – | 213.2 | -200 | α-Fe, Graphite |
| Phosphated | 6 | 158.9 | -150 | Mn₃(PO₄)₂·3H₂O |
| Sulfurized | 4 | 196.6 | -180 | FeS, FeS₂ |
Friction and wear tests were conducted to evaluate the performance of grey iron castings under simulated operating conditions. We used a pin-on-disc tester with a load of 300 N and speed of 1200 rpm for 30 minutes. The friction coefficient was recorded, and the average values are presented in Table 3. The sulfurized layer on grey iron castings showed a friction coefficient reduction of 23.5% compared to the phosphate layer. This can be attributed to the lubricious nature of FeS, which has a low shear strength. The friction coefficient (μ) can be expressed in terms of the shear strength (τ) and normal load (N) using the equation:
$$ \mu = \frac{\tau}{N} $$
For grey iron castings, the sulfurized layer reduces τ due to the easy slip of hexagonal planes in FeS, thereby lowering μ. Additionally, the porous structures in both layers act as oil reservoirs, but the sulfurized layer maintains better integrity under stress.
| Sample | Average Friction Coefficient | Wear Volume (mm³) | Wear Scar Width (mm) |
|---|---|---|---|
| Untreated Grey Iron Castings | 0.25 | 0.0285 | 0.98 |
| Phosphated Grey Iron Castings | 0.18 | 0.0392 | 1.09 |
| Sulfurized Grey Iron Castings | 0.14 | 0.0268 | 0.96 |
Wear resistance was assessed using a ring-on-block tester with a load of 360 N and speed of 1000 rpm for 60 minutes. The wear volume (V) was calculated from the wear scar profiles measured by white light interferometry. For grey iron castings, the sulfurized layer exhibited a wear volume reduction of 31.6% compared to the phosphate layer. The wear rate (W) can be defined as:
$$ W = \frac{V}{L \cdot d} $$
where L is the sliding distance and d is the density. The lower wear rate for sulfurized grey iron castings indicates enhanced durability. We also analyzed the wear mechanisms via SEM; the untreated grey iron castings showed severe abrasive wear and fatigue cracks, while the sulfurized layer displayed mild abrasive wear with no fatigue damage, highlighting its protective effect.
The superior performance of sulfurized layers on grey iron castings is due to multiple factors. First, the formation of FeS via diffusion creates a strong covalent bond with the substrate, preventing delamination. Second, during friction, active sulfur atoms transfer to the counterface, forming a dynamic equilibrium that continuously regenerates the lubricating layer. This process can be modeled using a diffusion equation:
$$ \frac{\partial C}{\partial t} = D \nabla^2 C $$
where C is the sulfur concentration, t is time, and D is the diffusion coefficient. For grey iron castings, this ensures long-term lubrication. In contrast, phosphate layers on grey iron castings are prone to brittle fracture and detachment under heavy loads.
We further applied the sulfurization treatment to compressor pistons made from grey iron castings and tested their energy efficiency. The coefficient of performance (COP) improved by 3.7%, demonstrating the practical benefits. The COP is calculated as:
$$ \text{COP} = \frac{\text{Cooling Capacity}}{\text{Power Input}} $$
For grey iron castings, reducing friction losses through sulfurization directly enhances COP, making it a viable solution for high-efficiency compressors.
In conclusion, our study confirms that low-temperature ion sulfurization significantly improves the friction and wear performance of grey iron castings compared to traditional phosphating. The sulfurized layer offers lower friction coefficients, higher wear resistance, and better durability due to its unique microstructure and phase composition. These findings support the adoption of sulfurization for grey iron castings in demanding applications, contributing to energy savings and extended component life. Future work could explore optimizing the sulfurization parameters for different grades of grey iron castings to further enhance performance.