The pursuit of enhanced energy efficiency and operational reliability in modern machinery, particularly in components like compressors, places stringent demands on the tribological performance of engineering materials. Gray iron casting, valued for its good castability, damping capacity, and inherent graphite lubrication, has been a traditional mainstay for moving parts such as pistons, cylinders, and bearing surfaces. However, the transition to high-pressure, environmentally friendly refrigerants (e.g., R32, R290, CO₂) and the trend towards higher operating speeds and loads have pushed conventional gray iron casting to its performance limits. Excessive wear in these severe conditions can lead to increased internal leakage and higher power consumption, ultimately restricting system efficiency and lifespan.
To address this challenge, surface engineering techniques are employed to modify the surface of gray iron casting components. A common industrial practice is phosphating, a chemical conversion process that deposits a porous manganese phosphate (Mn-phosphate) layer. This layer acts as a solid lubricant reservoir, absorbing and retaining oil to reduce the initial run-in wear and friction. Despite its widespread use, the Mn-phosphate layer has inherent limitations. Its structure, composed of coarse, brittle phosphate crystals with weak cohesive strength, offers poor wear resistance under high contact stresses. The layer is prone to spallation and detachment during prolonged heavy-duty operation, losing its lubricating function and potentially generating abrasive wear debris.
An alternative and promising surface modification technology is low-temperature ion sulfurization. This thermochemical diffusion process introduces active sulfur atoms into the subsurface region of ferrous materials, forming a iron sulfide (FeS) based layer. The FeS phase possesses a hexagonal close-packed (HCP) crystal structure with a low shear strength, enabling easy slip along its basal planes under shear stress. This characteristic grants the sulfurized layer excellent solid lubricating properties, leading to reduced friction coefficients and improved anti-scuffing performance. While extensive research has been conducted on sulfurized layers applied to various steels and alloyed irons, a systematic comparative study focusing on its application on common gray iron casting substrates, especially against the benchmark phosphating treatment, is less reported.
This article presents a comprehensive investigation into the microstructure, mechanical properties, and tribological performance of a sulfurized layer fabricated on gray iron casting substrate via low-temperature ion sulfurization. For a direct and practical comparison, a standard Mn-phosphate coating was also prepared on an identical gray iron casting substrate. The analysis encompasses surface and cross-sectional morphology, phase composition, layer thickness, surface nanohardness, residual stress, and, most importantly, friction and wear behavior under lubricated conditions. The underlying mechanisms for the superior performance of the sulfurized layer are elucidated. Furthermore, the practical efficacy of this technology is demonstrated through performance data from compressor applications, highlighting its potential for significant energy savings.
1. Experimental Methodology
1.1 Substrate Material and Coating Preparation
The substrate material used in this study was a commercially available as-cast grade gray iron casting, HT250. Its typical microstructure consists of type-A graphite flakes embedded in a pearlitic matrix (75-85% pearlite). The chemical composition is provided in Table 1.
| 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 samples machined from the same gray iron casting batch:
1. Manganese Phosphating (Chemical Conversion): A standard immersion process was employed. The gray iron casting samples were first cleaned and then subjected to a surface activation treatment for 60 seconds. Subsequently, they were immersed in a commercial Mn-phosphating bath (PF-M1AM type) at 85°C for 160 seconds. This process results in the chemical deposition of a crystalline manganese/iron phosphate layer on the surface.
2. Low-Temperature Ion Sulfurization (Thermochemical Diffusion): The treatment was conducted in a dedicated ion sulfurization furnace (LGM-500 type). Prior to sulfurization, the gray iron casting samples were ultrasonically cleaned. The process parameters were: pulsed voltage of 1000 V, chamber pressure of 100 Pa, treatment temperature of 150°C, and a treatment duration of 9 hours. In this plasma environment, active sulfur species are generated and bombard the sample surface, enabling sulfur diffusion into the substrate to form a sulfide layer.
1.2 Characterization Techniques
The microstructural and compositional analysis of the treated surfaces was performed using a combination of advanced techniques:
- Scanning Electron Microscopy (SEM) & Energy Dispersive X-ray Spectroscopy (EDS): Surface morphology and cross-sectional views of the coatings were examined. EDS was used for elemental mapping and line scans to determine coating thickness and elemental distribution.
- X-ray Diffraction (XRD): Phase identification of the surface layers was carried out using Cu Kα radiation to detect the compounds formed during phosphating and sulfurization.
- Nanoindentation: Surface hardness measurements were performed using a Fischerscope HM2000 nanoindenter with a load of 15 mN and a hold time of 5 s. The indents were carefully placed to avoid the graphite flakes in the gray iron casting substrate.
- X-ray Stress Analysis: Surface residual stresses were measured using a Rigaku Auto mate II micro-area X-ray diffractometer with Cr Kα radiation, employing the sin²ψ method.
1.3 Tribological Testing
The friction and wear performance was evaluated under lubricated conditions using two different test configurations to assess both friction coefficient and wear resistance.
1. Pin-on-Disc Test (Friction Evaluation): A MMW-1 tribometer was used. The upper specimen (pin) was an untreated gray iron casting disk. The lower specimens (discs) were: (a) untreated gray iron casting (baseline), (b) phosphated gray iron casting, and (c) sulfurized gray iron casting. The tests were conducted with PVE lubricating oil at a load of 300 N, a rotational speed of 1200 rpm, and a duration of 30 minutes. The instantaneous and average friction coefficients were recorded.
2. Ring-on-Block Test (Wear Resistance Evaluation): A Falex 001-001-331 tester was employed. The ring specimen (counterpart) was made of untreated gray iron casting. The block specimens were: (a) untreated gray iron casting, (b) phosphated gray iron casting, and (c) sulfurized gray casting iron. The tests were run with PVE oil at a higher load of 360 N, a speed of 1000 rpm for 60 minutes. After testing, the wear scar on each block was analyzed using a 3D white light interferometer to calculate the wear volume ($V$). The specific wear rate ($K$), a more fundamental material property, can be derived from the wear volume using the formula:
$$ K = \frac{V}{F_n \times L} $$
where $F_n$ is the normal load and $L$ is the total sliding distance.
2. Results and Discussion
2.1 Microstructure and Phase Composition
The surface morphologies of the two coatings are distinctly different, as revealed by SEM. The Mn-phosphate layer on the gray iron casting exhibits a characteristic crystalline structure comprised of tightly packed, coarse block-like crystals ranging from 5.0 to 8.5 μm in size. The interstices between these crystals create a network of micro-porosity.
In contrast, the sulfurized layer presents a uniform and finer-scale morphology consisting of numerous, uniformly dispersed鳞片状 (scale-like) features with sizes between 2.0 and 4.0 μm. These features aggregate to form a porous, yet more integrated, surface texture compared to the phosphate layer.
EDS and XRD analyses confirm the expected chemical and phase makeup of the layers. The Mn-phosphate layer is rich in phosphorus and manganese, with XRD identifying the primary phases as Mn₃(PO₄)₂·3H₂O and (Mn,Fe)₃(PO₄)₂·4H₂O. These phases possess monoclinic crystal structures, which are not inherently conducive to easy shear.
The sulfurized layer shows a high sulfur content. XRD analysis confirms the successful formation of iron sulfides, with FeS (troilite, hexagonal structure) being the predominant phase and a minor presence of FeS₂ (pyrite, cubic structure). The formation of the FeS phase via diffusion creates a metallurgical bond with the gray iron casting substrate, which is crucial for coating durability.
| Coating Type | Primary Phases | Crystal Structure | Key Morphological Feature | Approx. Thickness |
|---|---|---|---|---|
| Manganese Phosphate | Mn₃(PO₄)₂·3H₂O, (Mn,Fe)₃(PO₄)₂·4H₂O | Monoclinic | Coarse, blocky crystals with micro-porosity | ~6 μm |
| Ion Sulfurized Layer | FeS (major), FeS₂ (minor) | Hexagonal (FeS), Cubic (FeS₂) | Fine, scale-like aggregated structure with micro-porosity | ~4 μm |
2.2 Coating Thickness and Surface Hardness
Cross-sectional EDS line scans across the coating/substrate interface provide an estimate of coating thickness. The phosphorus signal intensifies within approximately 6 μm of the surface for the phosphate layer, while the sulfur signal increases within about 4 μm for the sulfurized layer. Although thinner, the sulfurized layer exhibits a jagged, interlocking interface with the gray iron casting substrate, indicative of a diffusion-based formation and potentially stronger adhesion compared to the purely deposited phosphate layer.
Nanoindentation hardness measurements reveal significant differences. The untreated gray iron casting substrate has a hardness of approximately 213 HV. The phosphating process reduces the surface hardness to about 159 HV, a decrease of ~25%. This softening is attributed to the relatively soft phosphate crystals. The sulfurization process causes a much smaller reduction in surface hardness to about 197 HV (~8% decrease). The higher retained hardness of the sulfurized gray iron casting surface suggests better resistance to plastic deformation and indentation during wear, which is generally correlated with improved wear resistance.
2.3 Surface Residual Stress State
Residual stresses, particularly compressive stresses, are beneficial for impeding crack initiation and propagation, thereby enhancing fatigue and wear resistance. X-ray stress analysis showed that the untreated gray iron casting surface possessed the highest compressive residual stress. Both surface treatments altered this state. The phosphating process, being a low-temperature chemical reaction, does not induce plastic deformation. The deposited phosphate layer itself carries little to no residual stress and may even slightly shield the underlying substrate stress during measurement, resulting in the lowest recorded compressive stress value.
The low-temperature ion sulfurization process (150°C) is also well below the stress-relief temperature for gray iron casting. Therefore, it does not significantly alter the residual stress state of the substrate material. The measured compressive stress for the sulfurized surface was closer to that of the untreated substrate, indicating that the beneficial compressive stress field of the base gray iron casting is largely preserved beneath the thin sulfide layer. This preserved compressive stress contributes to the superior durability of the sulfurized component.
2.4 Friction and Wear Performance
2.4.1 Friction Behavior (Pin-on-Disc Test)
The friction coefficient trends provide clear evidence of the lubricating efficacy of the surface treatments. The friction curve for the untreated gray iron casting pair (self-mated) was unsteady and exhibited relatively high average friction, indicative of adhesive interactions and severe interfacial conditions.
Both surface treatments substantially reduced and stabilized the friction coefficient. The Mn-phosphate layer lowered the average friction coefficient by approximately 39% compared to the untreated pair. The sulfurized layer delivered an even more impressive reduction of about 54%. Compared directly, the sulfurized gray iron casting exhibited a friction coefficient roughly 23.5% lower than the phosphated one under the same test conditions.
$$ \mu_{avg} (S+S) \approx 0.77 \times \mu_{avg} (S+P) $$
This significant reduction is attributed to the intrinsic lubricity of the FeS phase in the sulfurized layer. Its hexagonal crystal structure provides easy shear planes, effectively reducing the shear stress required for sliding.
2.4.2 Wear Resistance (Ring-on-Block Test)
The wear volume measurements offer a direct comparison of coating durability. Under the applied high load (360 N), the Mn-phosphate layer on the gray iron casting suffered the most severe wear. Its wear volume was approximately 37.5% higher than that of the untreated gray iron casting block. This is a critical finding, demonstrating that under heavy load, the brittle phosphate layer fails quickly, offering no protective benefit and potentially increasing wear.
In stark contrast, the sulfurized gray iron casting block showed excellent wear resistance. Its wear volume was about 6% lower than that of the untreated substrate and, most notably, about 31.6% lower than that of the phosphated sample. This confirms that the sulfurized layer not only reduces friction but also provides genuine protection to the underlying gray iron casting, significantly enhancing its wear life.
$$ V_{wear} (S+S) \approx 0.684 \times V_{wear} (S+P) $$
The wear scars were examined by SEM. The untreated gray iron casting surface showed deep grooves, fatigue cracks, and spalling pits, evidence of severe abrasive and fatigue wear. The phosphated surface showed signs of coating fracture and removal, leading to accelerated wear of the exposed substrate. The sulfurized surface displayed only mild, shallow scratching, characteristic of mild abrasive wear, with no signs of coating delamination or severe damage.
| Sample | Avg. Friction Coeff. (μ) | Reduction vs. Untreated | Wear Volume (mm³) | Change vs. Untreated |
|---|---|---|---|---|
| Untreated Gray Iron (S) | 0.145 (Baseline) | – | 0.0285 | – |
| Phosphated Gray Iron (S+P) | 0.088 | ~39.3% Lower | 0.0392 | ~37.5% Higher |
| Sulfurized Gray Iron (S+S) | 0.067 | ~53.8% Lower | 0.0268 | ~6.0% Lower |
2.5 Analysis of Friction Reduction Mechanism
The superior tribological performance of the sulfurized layer on gray iron casting can be explained by a combination of factors:
1. Solid Lubrication from FeS Phase: The primary mechanism is the presence of the FeS phase with its hexagonal layered lattice. The weak van der Waals forces between the sulfur layers allow for easy interlayer shear, providing low friction directly at the asperity contacts. This is fundamentally different from the Mn-phosphate layer, whose monoclinic phosphate crystals lack such easy-shear planes and primarily function as a porous oil reservoir.
2. Dynamic Transfer and Replenishment: An intriguing phenomenon was observed through EDS analysis of the worn surface of the untreated gray iron casting pin that slid against the sulfurized disc. A detectable amount of sulfur was found on the pin’s wear scar. This suggests a dynamic process where, during frictional heating and contact, sulfur from the sulfide layer can transfer to the counterface. This transferred sulfur may then react with the iron in the counterface, forming new FeS or related compounds in situ. This creates a self-replenishing, protective tribofilm on both sliding surfaces, a feature absent in the phosphating technology.
3. Synergy with Lubricant and Micro-porosity: Like the phosphate layer, the micro-porous structure of the sulfurized layer can retain lubricating oil, aiding in hydrodynamic or mixed lubrication regimes. However, the sulfurized layer provides a robust “backup” solid lubricant that functions even if the oil film is temporarily disrupted, preventing catastrophic adhesive wear.
4. Mechanical Integrity: The diffusion-based formation of the sulfide layer ensures strong adhesion to the gray iron casting substrate. Combined with its higher hardness and the preserved compressive residual stress in the substrate, the layer resists spallation and offers sustained protection under load, unlike the mechanically weak phosphate coating which fails by fracture and detachment.
3. Practical Application and Performance Enhancement
The laboratory findings were validated through a practical application in a 1.0 HP rotary compressor. The cast iron piston, a key gray iron casting component, was treated using the low-temperature ion sulfurization process. This “sulfurized piston” compressor was then tested against a standard production compressor with conventional surface treatment (typically phosphating or similar) according to national standards (GB/T 15765-2004).
The performance metrics focused on cooling capacity and input power consumption. The Coefficient of Performance (COP), defined as the ratio of cooling capacity ($Q_{cooling}$) to input power ($W_{input}$), is the key indicator of energy efficiency:
$$ COP = \frac{Q_{cooling}}{W_{input}} $$
The test results demonstrated a clear advantage for the compressor with the sulfurized gray iron casting piston. While both compressors delivered the required cooling duty, the one with the sulfurized components operated at a slightly lower input power. The calculated COP for the standard compressor was 4.32, whereas the compressor with sulfurized pistons achieved a COP of 4.48.
This represents a 3.7% improvement in energy efficiency attributable directly to the reduced frictional losses in the compression mechanism. For high-volume production and continuous operation, this percentage translates into substantial energy savings and reduced carbon footprint. It confirms that the enhanced tribological properties of the sulfurized gray iron casting surface directly contribute to improved system-level performance.
| Compressor Type | Cooling Capacity (kW) | Input Power (kW) | Coefficient of Performance (COP) | COP Improvement |
|---|---|---|---|---|
| Standard (Conventional Treatment) | 2.85 | 0.660 | 4.32 | Baseline |
| With Sulfurized Gray Iron Piston | 2.83 | 0.632 | 4.48 | +3.7% |
4. Conclusion
This comprehensive study demonstrates that low-temperature ion sulfurization is a highly effective surface engineering technology for enhancing the tribological performance of gray iron casting components. When compared to the conventional manganese phosphating treatment, the sulfurized layer offers a superior combination of properties:
- Superior Friction Reduction: The sulfurized layer on gray iron casting reduces the average friction coefficient by over 50% compared to untreated surfaces and by approximately 23.5% compared to phosphated surfaces under lubricated sliding conditions. This is primarily due to the easy-shear hexagonal crystal structure of the predominant FeS phase.
- Excellent Wear Resistance: Contrary to the phosphate layer which increases wear volume under load, the sulfurized layer protects the substrate, reducing wear volume by about 31.6% compared to the phosphate layer. This is attributed to its diffusion-based adhesion, higher mechanical strength, and the preservation of beneficial compressive residual stresses in the gray iron casting substrate.
- Dynamic Lubricating Mechanism: The sulfurized layer exhibits a unique ability to transfer active sulfur to the counterface, promoting the in-situ formation of a lubricious tribofilm. This dynamic replenishment mechanism ensures long-term lubricity, surpassing the static, reservoir-based function of the phosphate layer.
- Significant Energy Efficiency Gain: Practical application in a compressor verified the laboratory findings. Treating critical gray iron casting components like pistons with ion sulfurization led to a measurable 3.7% increase in the system’s Coefficient of Performance (COP), highlighting its direct contribution to energy savings and operational cost reduction.
In conclusion, for applications involving gray iron casting parts operating under severe or high-load conditions, such as in modern high-pressure compressors, low-temperature ion sulfurization presents a technologically advanced and performance-proven alternative to traditional phosphating. It provides a robust, durable, and actively lubricating surface that mitigates wear, reduces friction, and ultimately enhances the efficiency and longevity of the mechanical system.