In modern machinery, particularly within compressors operating under increasingly demanding conditions with high-pressure refrigerants, the tribological performance of moving components is paramount. Gray iron castings have been widely favored for such applications due to their good castability, damping capacity, and inherent self-lubricating properties from the graphite phase. However, their inherent wear resistance often falls short under high-speed, heavy-load operations, leading to increased energy consumption and potential failure. Traditional surface modifications, such as phosphating, create a porous layer that aids in oil retention but often lacks the necessary durability and load-bearing capacity. This investigation explores an alternative surface engineering approach: low-temperature ion sulfurization. This study systematically compares the microstructure, mechanical properties, and, most importantly, the friction and wear performance of sulfurized layers against conventional phosphate coatings on gray iron castings.
The core objective is to evaluate whether the solid lubricating layer formed by sulfur infiltration offers superior anti-friction and anti-wear characteristics compared to the chemically converted phosphate layer. The performance of these coatings is critical for enhancing the efficiency and longevity of components made from gray iron castings. We will delve into the phase composition, hardness, residual stress state, and the fundamental wear mechanisms active in both surface treatments.
1. Experimental Materials and Methodology
The substrate material used in this study was a standard as-cast grade of gray iron, conforming to HT250 specifications. The chemical composition of these gray iron castings is detailed in Table 1. The microstructure typically consists of a pearlitic matrix with type A graphite flakes, providing a consistent baseline for surface treatment evaluation.
| C | Si | Mn | P | S | Sn | Cu | Fe |
|---|---|---|---|---|---|---|---|
| 3.57 | 2.70 | 0.81 | 0.020 | 0.083 | 0.12 | 0.06 | Bal. |
Two distinct surface treatment processes were applied to the gray iron castings:
1. Phosphating: A standard immersion manganese phosphating process was employed. The castings were first treated with a surface conditioner at room temperature for 60 seconds. Subsequently, they were immersed in a commercial manganese-based phosphating solution (PF-M1AM) at 85°C for 160 seconds to form the phosphate layer.
2. Low-Temperature Ion Sulfurization: This treatment was conducted in a dedicated LDM-500 furnace. The process parameters included a pulsed voltage of 1000 V, a vacuum of 100 Pa, a temperature of 150°C, and a treatment duration of 9 hours to form the sulfurized layer.
The characterization of the treated gray iron castings involved several advanced techniques:
– Microstructural and Compositional Analysis: Surface and cross-sectional morphology were examined using Scanning Electron Microscopy (SEM, FEI Quanta250 FEG). Elemental distribution and coating thickness were analyzed via Energy Dispersive X-ray Spectroscopy (EDS). Phase identification was performed using X-ray Diffraction (XRD, Rigaku Minin Flex600) with Cu-Kα radiation.
– Mechanical Property Evaluation: Surface hardness was measured using a Fischerscope HM2000 nanoindenter with a load of 15 mN. Residual stress on the surface was determined using a Rigaku Automate II micro-area X-ray stress analyzer, employing the sin²ψ method.
– Tribological Testing:
a) Friction Coefficient: Evaluated using a pin-on-disk tribometer (MMW-1). The upper specimen (pin) was untreated gray iron, sliding against lower specimens (disks) of either untreated, phosphated, or sulfurized gray iron castings under lubrication with PVE oil (Load: 300 N, Speed: 1200 rpm, Time: 30 min).
b) Wear Resistance: Assessed using a ring-on-block tribometer (Falex 001-001-331). A ring of untreated gray iron rotated against blocks of the three different surface conditions under lubrication with PVE oil (Load: 360 N, Speed: 1000 rpm, Time: 60 min). Wear volume was subsequently quantified using a 3D white light interferometer.
2. Results: Microstructure, Composition, and Properties
2.1 Surface Morphology and Phase Constitution
The SEM analysis revealed distinct surface topographies for the two coatings on the gray iron castings. The phosphate layer exhibited a microstructure composed of densely packed, blocky crystalline grains approximately 5.0–8.5 μm in size. The packing of these crystals created a porous, somewhat loose structure. In contrast, the surface of the sulfurized layer consisted of uniformly distributed鳞片状 (scale-like) structures, significantly smaller at 2.0–4.0 μm, which also formed a micro-porous network but with a finer texture.
EDS and XRD analyses confirmed the expected chemical and phase differences. The phosphate layer showed a significant increase in phosphorus content and was identified as a mixture of manganese phosphate hydrates: Mn₃(PO₄)₂·3H₂O and (Mn,Fe)₃(PO₄)₂·4H₂O. For the sulfurized layer, a substantial sulfur peak was detected. The XRD pattern confirmed the successful formation of iron sulfides, primarily the covalently bonded compound FeS (with a hexagonal close-packed crystal structure) along with minor FeS₂. This phase is crucial as FeS is known for its solid lubricating properties due to its low shear strength on the basal planes.
2.2 Coating Thickness, Hardness, and Residual Stress
Cross-sectional EDS line scans were used to estimate coating thickness. The phosphorus signal increased within approximately 6 μm of the surface for the phosphate layer, while the sulfur signal increased within about 4 μm for the sulfurized layer. Although thinner, the sulfurized layer exhibited a characteristic interdiffusion zone with a jagged interface, suggesting stronger metallurgical bonding compared to the primarily deposited phosphate layer.
Nanoindentation hardness tests revealed significant differences. The untreated gray iron castings substrate had a hardness of approximately 213 HV. The phosphating process reduced the surface hardness to about 159 HV, a decrease of ~25%. The sulfurization process resulted in a much smaller reduction to about 197 HV, a decrease of only ~8%. This indicates that the sulfurized layer better preserves the substrate’s load-bearing capacity.
The residual stress measurements provided further insight. The untreated substrate exhibited the highest surface compressive stress. Phosphating, being a purely chemical conversion process, did not introduce significant mechanical strain, and the measured stress was lower, partly due to the attenuation of X-rays by the coating. Sulfurization, performed at a low temperature (150°C), did not relieve the substrate’s residual stress significantly, leaving the surface with a compressive stress state closer to that of the base material, which is beneficial for fatigue and wear resistance. The data for hardness, residual stress, and thickness are summarized in Table 2.
| Sample | Coating Thickness (μm) | Surface Hardness (HV0.0015) | Residual Stress (MPa, approx.) | Primary Phases Identified |
|---|---|---|---|---|
| Untreated Substrate (S) | – | 213.2 | -300 | α-Fe, Graphite, Fe3C |
| Phosphate Layer (S+P) | ~6 | 158.9 | -200 | Mn3(PO4)2·3H2O, (Mn,Fe)3(PO4)2·4H2O |
| Sulfurized Layer (S+S) | ~4 | 196.6 | -250 | FeS, FeS2 |
3. Results: Tribological Performance Evaluation
3.1 Friction Coefficient Behavior
The pin-on-disk tests under lubricated conditions yielded clear distinctions in frictional performance. The friction curve for the untreated gray iron castings pair (substrate vs. substrate) was unsteady, with significant fluctuations, indicating unstable wear and possible adhesive interactions. Both surface treatments stabilized the friction process. The average friction coefficient was substantially reduced.
The quantitative analysis showed that the phosphate coating reduced the average friction coefficient by approximately 39.3% compared to the untreated pair. Remarkably, the sulfurized layer achieved an even greater reduction of about 53.6%. Consequently, under identical test conditions, the sulfurized layer on gray iron castings provided a further 23.5% reduction in the coefficient of friction compared to the phosphate layer. This can be expressed by the relation for relative improvement:
$$\text{Reduction vs. Phosphate} = \frac{\mu_{P} – \mu_{S}}{\mu_{P}} \approx 23.5\%$$
where $\mu_{P}$ and $\mu_{S}$ are the average friction coefficients for the phosphate and sulfurized layers, respectively.
3.2 Wear Resistance and Wear Scar Analysis
The ring-on-block tests, designed to be more severe, provided critical data on wear volume. The results, measured via 3D interferometry, are presented in Table 3. Contrary to merely reducing friction, the phosphate layer exhibited a significantly higher wear volume than the untreated substrate—an increase of about 37.5%. This indicates that under the applied load of 360 N, the phosphate coating was rapidly worn away due to its brittle nature and weak cohesion. In contrast, the sulfurized layer demonstrated genuine wear resistance, showing a slight decrease (≈6.0%) in wear volume compared to the untreated substrate. Most importantly, the wear volume for the sulfurized layer was about 31.6% lower than that of the phosphate layer.
SEM examination of the wear scars confirmed the mechanisms. The untreated gray iron castings surface showed deep grooves, fatigue cracks, and large spallation pits, indicative of severe abrasive and fatigue wear. The worn phosphate surface revealed many small pits from the fracture and detachment of phosphate crystals, confirming its poor durability. The wear scar on the sulfurized layer was remarkably mild, showing only shallow scratches characteristic of mild abrasive wear, with no evidence of coating delamination or severe fatigue.
| Specimen Surface Condition | Average Wear Scar Width (mm) | Wear Volume (mm³) | Wear Volume vs. Substrate | Wear Volume vs. Phosphate Layer |
|---|---|---|---|---|
| Untreated Substrate | 0.98 | 0.0285 | 0% (Baseline) | – |
| Phosphate Layer | 1.09 | 0.0392 | +37.5% | 0% (Baseline) |
| Sulfurized Layer | 0.96 | 0.0268 | -6.0% | -31.6% |
4. Discussion: Mechanisms of Performance Enhancement
The superior tribological performance of the sulfurized layer on gray iron castings can be attributed to a synergistic combination of factors, fundamentally different from the action of the phosphate layer.
1. Nature of the Coating: The phosphate layer is a deposited crystalline coating with mechanical adhesion to the substrate. Its blocky, porous structure aids in oil retention but offers limited mechanical strength. The sulfurized layer, however, is formed by the inward diffusion of active sulfur atoms and their chemical reaction with the iron matrix to form FeS. This creates a graded, metallurgically bonded layer with superior adhesion and load-bearing capacity, as evidenced by its higher hardness and intact wear scar.
2. Solid Lubrication Mechanism: This is the most critical distinction. The primary phase in the sulfurized layer is FeS, which possesses a hexagonal close-packed (HCP) crystal structure (space group P6₃/mmc). In HCP structures like FeS, the basal planes have low shear strength. During sliding, easy shear occurs along these planes, effectively reducing friction. The friction coefficient $\mu$ is directly related to the shear strength $\tau$ of the interfacial material: $$\mu \propto \frac{\tau}{H}$$ where H is the hardness. The low $\tau$ of FeS dominates despite its relatively higher H compared to the phosphate coating. The phosphate crystals (monoclinic structure) lack such easy-slip planes.
3. Dynamic Transfer and Replenishment: A unique phenomenon was observed via EDS analysis of the counterface (the untreated pin that slid against the sulfurized disk). After testing against the sulfurized layer, the pin’s surface contained traces of sulfur, which were absent after tests against the substrate or phosphate layer. This suggests that during frictional heating and contact, active sulfur from the FeS layer can transfer to the counterface, potentially forming a thin, lubricious tribofilm. This represents a dynamic process of sulfur transfer and re-formation, extending the lubricating effect beyond the original coated surface and providing a “self-replenishing” characteristic to the friction pair. No such transfer mechanism exists for the phosphate coating, which simply wears away.
4. Synergy with Lubrication: Both coatings possess micro-porosity for oil retention, aiding in hydrodynamic lubrication. However, the sulfurized layer’s durability ensures this micro-reservoir structure persists longer under load, maintaining a more consistent oil film supplementary to the solid lubrication from FeS.
5. Practical Application and Performance Impact
The practical value of this surface treatment for gray iron castings was evaluated in a functional test. Pistons for a 1.0 HP rotary compressor, made from gray iron castings, were treated using the low-temperature ion sulfurization process. These were assembled into compressors, and their performance was compared against standard compressors with untreated pistons according to GB/T 15765-2004. The key performance metric is the Coefficient of Performance (COP), defined as: $$COP = \frac{Cooling Capacity}{Power Input}$$
The test results demonstrated a clear efficiency gain. The compressor with sulfurized pistons showed a COP of 4.48, compared to 4.32 for the standard compressor. This represents a 3.7% improvement in COP. For small-displacement compressors where efficiency margins are critical, this is a significant enhancement directly attributable to the reduced mechanical losses (friction and wear) within the compression mechanism. This application confirms that the benefits observed in laboratory tribological tests translate into measurable energy savings for components made from gray iron castings.
6. Conclusion
This comprehensive investigation into surface treatments for gray iron castings leads to the following conclusions:
- Both phosphate and sulfurized layers provide porous surfaces that enhance oil retention on gray iron castings. However, their fundamental natures differ: phosphate is a deposited coating, while the sulfurized layer is a diffused, chemically bonded compound layer.
- The sulfurized layer, primarily composed of the HCP-phase FeS, provides superior solid lubrication via easy shear on basal planes. This mechanism, combined with a dynamic sulfur transfer process to the counterface, results in a significantly lower and more stable coefficient of friction—23.5% lower than the phosphate layer under tested conditions.
- The sulfurized layer exhibits markedly better wear resistance due to its higher hardness, stronger metallurgical bond to the gray iron castings substrate, and maintained residual compressive stress. Its wear volume was 31.6% lower than that of the phosphate layer under a heavy load, where the phosphate coating failed rapidly.
- The practical application of sulfurization on compressor pistons made from gray iron castings yielded a 3.7% improvement in system COP, validating the laboratory findings and demonstrating the technology’s potential for enhancing energy efficiency in mechanical systems.
Therefore, for gray iron castings operating in demanding tribological environments, low-temperature ion sulfurization presents a technologically superior alternative to conventional phosphating, offering enhanced durability, lower friction, and tangible gains in operational efficiency.