Grey iron casting stands as a cornerstone material in mechanical engineering, prized for its excellent castability, good machinability, inherent damping capacity, and notably, its self-lubricating properties imparted by the graphite flakes within its microstructure. This combination of favorable characteristics and cost-effectiveness has cemented its role as the material of choice for numerous sliding and wear components, particularly within the critical moving assemblies of refrigeration and air conditioning compressors. The very graphite that provides lubrication, however, also imposes a limit on strength and wear resistance. Historically, the operational demands on these components were moderate, and the intrinsic properties of grey iron casting were sufficient. Yet, the global shift towards environmentally friendly refrigerants with higher compression ratios—such as R32, R410A, and CO₂—has driven compressor designs towards higher speeds and increased specific loads. In this intensified operational landscape, the native tribological performance of standard grey iron casting often falls short, leading to accelerated wear, increased leakage paths, elevated motor power losses, and ultimately, a constraint on compressor efficiency and lifespan.
For decades, the standard industrial solution to enhance the friction and wear performance of grey iron casting components has been phosphating—a chemical conversion coating process. This treatment forms a porous, crystalline layer of manganese or zinc-manganese phosphate on the surface. This layer’s primary function is to act as an oil reservoir, improving run-in behavior, providing emergency lubrication, and reducing the risk of adhesive wear (scuffing). While effective for mild conditions, phosphate layers possess inherent limitations. Their structure, composed of brittle crystalline particles with weak cohesive strength, offers poor abrasion resistance. Under the severe loading of modern compressors, these layers can fracture and delaminate, losing their protective function and potentially generating abrasive debris, thus failing to ensure long-term operational stability. This persistent challenge has spurred the search for more robust and durable surface engineering solutions for grey iron casting.
An advanced alternative emerging from the field of thermochemical surface engineering is low-temperature ion sulfurization. This process involves the diffusion of active sulfur atoms into the substrate material at temperatures typically between 150°C and 300°C, forming a thin, integral layer of iron sulfides (primarily FeS) on the surface. The key to its effectiveness lies in the crystal structure of FeS, which is hexagonal close-packed (HCP). This layered structure possesses low shear strength parallel to the basal planes, granting it solid lubricant properties. When incorporated into the surface of a grey iron casting, this layer can significantly reduce friction and wear. While extensive research has documented the benefits of sulfurized layers on steels, alloyed irons, and powder metallurgy parts, a comprehensive investigation focusing on their application and performance on common grey iron casting, particularly in direct comparison to the ubiquitous phosphate coating, has been less prevalent. This article presents a detailed comparative study, employing a suite of advanced characterization techniques and tribological tests, to quantitatively evaluate the microstructure, mechanical properties, and, most importantly, the friction and wear performance of sulfurized layers versus traditional phosphate coatings on grey iron casting.
1. Experimental Materials and Methodology
1.1 Substrate Material and Coating Preparation
The substrate used throughout this investigation was as-cast grey iron casting, grade HT250, supplied by a domestic foundry. Its microstructure consisted of type A graphite flakes with a length grade of 4, within a matrix of 75-85% pearlite. The chemical composition, determined by optical emission spectrometry, 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 prepare the solid lubricant layers:
- Phosphating (Phosphate Layer): A standard immersion manganese phosphating process was employed. Samples were first cleaned and then subjected to a 60-second surface activation treatment at room temperature using a commercial conditioner. The phosphating was carried out in a PF-M1AM manganese-based solution at 85°C for 160 seconds, followed by rinsing and drying.
- Low-Temperature Ion Sulfurization (Sulfurized Layer): The treatment was conducted in a LGM-500 pulsed plasma ion sulfurization furnace. The process parameters were optimized as follows: pulsed voltage of 1000 V, chamber pressure of 100 Pa, substrate temperature of 150°C, and a treatment duration of 9 hours.
1.2 Characterization Techniques
A multi-faceted analytical approach was adopted to thoroughly characterize the produced layers:
- Microstructure and Composition: Surface and cross-sectional morphology were examined using a FEI Quanta 250 FEG Scanning Electron Microscope (SEM). Elemental distribution and approximate layer thickness were assessed via Energy Dispersive X-ray Spectroscopy (EDS) attached to the SEM. Phase identification was performed using a Rigaku Miniflex 600 X-ray Diffractometer (XRD) with Cu Kα radiation (40 kV, 35 mA).
- Mechanical Properties: Surface hardness was measured using a Fischerscope HM2000 nanoindentation system with a Berkovich indenter. A load of 15 mN was applied with a 5-second hold time to minimize creep effects, ensuring indents were placed on the metal matrix, avoiding graphite flakes. Residual stress at the surface was determined using a Rigaku Automate II micro-area X-ray stress analyzer (Cu Kα, 40 kV, 40 mA) employing the sin²ψ method.
- Tribological Testing: Two types of tests were conducted under lubricated conditions using Polyvinyl Ether (PVE) oil, common in compressor applications.
- Friction Evaluation (Pin-on-Disc): Tests were performed on an MMW-1 tester. The upper specimen (pin) was untreated grey iron casting. The lower specimens (discs) were: (a) untreated substrate (S), (b) phosphated (S+P), and (c) sulfurized (S+S). Conditions: Load = 300 N, Speed = 1200 rpm, Duration = 30 min.
- Wear Resistance Evaluation (Block-on-Ring): Tests were conducted on a Falex 001-001-331 tester. The ring (counterface) was untreated grey iron casting. The block specimens were the same three types: S, S+P, S+S. Conditions: Load = 360 N, Speed = 1000 rpm, Duration = 60 min. Wear volume on the blocks was quantified using a 3D white light interferometer (ContourGT-K).
2. Results and Discussion: Layer Characterization
2.1 Surface Morphology and Phase Constitution
The SEM micrographs reveal fundamentally different surface topographies. The phosphate layer on the grey iron casting (Figure 2a) exhibits a characteristic coarse, crystalline structure. The plate-like crystals, ranging from 5.0 to 8.5 μm in size, are densely packed but create a network of micro-porosity due to interstitial spaces. This porosity is crucial for its oil-retention capability. In contrast, the surface of the sulfurized grey iron casting (Figure 2b) is covered with a much finer, uniformly distributed鳞片状 (scale-like) morphology. These individual scales are significantly smaller (2.0-4.0 μm) and their overlapping arrangement also generates a micro-porous structure, though likely with different pore size distribution and connectivity compared to the phosphate layer.
The EDS and XRD analyses provide definitive evidence of the formed compounds. For the phosphate layer, a strong phosphorus signal is detected alongside manganese and iron. The XRD pattern confirms the presence of manganese phosphate hydrates: Mn₃(PO₄)₂·3H₂O and (Mn,Fe)₃(PO₄)₂·4H₂O. These are typical products of manganese phosphating on ferrous substrates. For the sulfurized grey iron casting, the EDS spectrum shows a pronounced sulfur peak. The XRD pattern is dominated by strong peaks for troilite (FeS, hexagonal P6₃/mmc), with minor traces of pyrite (FeS₂, cubic Pa-3). The formation of FeS is the primary goal of the sulfurization process, as its HCP structure is key to the tribological enhancement.
2.2 Layer Thickness and Surface Hardness
Cross-sectional EDS line scans across the interface provide an estimate of layer thickness. For the phosphated grey iron casting, the phosphorus intensity increases within approximately 6 μm of the surface. For the sulfurized grey iron casting, the sulfur intensity rises within about 4 μm. Therefore, the phosphate layer is ~6 μm thick, while the sulfurized layer is ~4 μm thick. It is critical to note the nature of the interface: the phosphate layer appears as a distinct, deposited layer, whereas the sulfurized layer shows a more diffuse interface, suggesting a graded composition as sulfur atoms diffuse into the substrate, promoting better adhesion.
Nanoindentation hardness results are summarized in Table 2. The untreated grey iron casting substrate has a hardness of approximately 213 HV. Phosphating reduces the surface hardness to about 159 HV, a decrease of ~25%. This softening is expected due to the relatively soft and brittle phosphate crystals. Sulfurization causes a much smaller reduction in hardness to about 197 HV (~8% decrease). The higher hardness of the sulfurized layer on grey iron casting compared to the phosphate layer suggests intrinsically better resistance to plastic deformation and penetration under contact stress, which is generally correlated with improved wear resistance, all else being equal. The hardness, $H$, can be derived from the nanoindentation load ($P_{max}$) and the projected contact area ($A_c$) using the standard Oliver-Pharr method:
$$H = \frac{P_{max}}{A_c}$$
| Sample | Description | Surface Hardness (HV0.0015) | Surface Residual Stress (MPa) |
|---|---|---|---|
| S | Untreated Substrate | 213.2 ± 15 | -320 ± 25 |
| S+P | Phosphated | 158.9 ± 20 | -120 ± 30 |
| S+S | Sulfurized | 196.6 ± 12 | -280 ± 20 |
2.3 Surface Residual Stress State
Residual stress measurements, also in Table 2, reveal another significant difference. The untreated grey iron casting surface exhibits a compressive residual stress of about -320 MPa, typical from casting and machining processes. The phosphated surface shows a markedly lower compressive stress of approximately -120 MPa. This reduction is attributed to two factors: first, the phosphate layer itself, being a deposited coating, likely possesses little intrinsic stress at the measurement scale; second, the X-rays penetrate the coating and sample the underlying substrate, but the porous phosphate layer may dampen the diffraction signal and/or the phosphating process itself may slightly relax the very near-surface stresses of the substrate. In contrast, the sulfurized grey iron casting retains a high level of compressive residual stress (-280 MPa), very close to the original substrate. The low-temperature (150°C) sulfurization process does not provide sufficient thermal energy to relieve the casting stresses. High compressive surface stress is generally beneficial for fatigue and wear resistance, as it hinders crack initiation and propagation. The residual stress, $\sigma_\phi$, is calculated from the X-ray diffraction data using the fundamental equation:
$$\sigma_\phi = \frac{E}{(1+\nu)} \cdot \frac{\partial (2\theta)}{\partial (\sin^2 \psi)} \cdot \frac{\pi}{360} \cdot \cot \theta_0$$
where $E$ is Young’s modulus, $\nu$ is Poisson’s ratio, $2\theta$ is the diffraction angle, $\psi$ is the tilt angle, and $\theta_0$ is the Bragg angle for the stress-free condition.
3. Results and Discussion: Tribological Performance
3.1 Friction Behavior (Pin-on-Disc Test)
The evolution of the coefficient of friction (COF) with time is shown in Figure 3a, and the average values are compared in Figure 3b and Table 3. The friction pair consisting of two untreated grey iron casting surfaces (S vs. S) exhibits the highest and most unstable COF, averaging 0.142. Significant fluctuations indicate periods of severe interaction, likely involving adhesive junctions and their subsequent shearing. When a phosphated surface (S+P) slides against untreated grey iron casting, the average COF drops by about 39% to 0.086. The curve is more stable, indicating the beneficial effect of the phosphate layer in preventing metal-to-metal contact and providing a smoother run-in. Most impressively, the sulfurized grey iron casting (S+S) against untreated grey iron casting achieves the lowest and steadiest friction, with an average COF of 0.066. This represents a reduction of approximately 53% compared to the untreated pair and a 23% reduction compared to the phosphated pair.
SEM examination of the wear scars on the treated surfaces after testing provides insight into the wear mechanisms. The untreated grey iron casting disc shows deep abrasive grooves, fatigue cracks, and large spallation pits, indicative of severe abrasive and fatigue wear. The phosphated surface shows shallower grooves but numerous small pits where phosphate crystals have been fractured and removed, exposing the substrate. The sulfurized surface on the grey iron casting displays only very fine, shallow scratches with no evidence of pitting or cracking, characteristic of mild abrasive wear.
3.2 Wear Resistance (Block-on-Ring Test)
Under the more severe, high-pressure line contact of the block-on-ring test, the differences in durability become even more pronounced. The 3D wear scar morphologies and the quantified wear volumes are summarized in Table 3. The phosphated grey iron casting block suffered the highest wear volume, approximately 37.5% greater than that of the untreated substrate. This confirms the poor load-bearing capacity and cohesive strength of the phosphate layer; under high load, it fractures and is quickly removed, offering little protection. In stark contrast, the sulfurized grey iron casting block exhibited the lowest wear volume, about 6% lower than the untreated substrate and, critically, 31.6% lower than the phosphated sample. This demonstrates that the sulfurized layer is not only a friction modifier but also a wear-resistant layer. It remains intact and functional under conditions that destroy the phosphate coating.
The dominant wear mechanism can be related to the Archard wear equation in a simplified form:
$$V = k \frac{N \cdot s}{H}$$
where $V$ is wear volume, $k$ is a wear coefficient, $N$ is the normal load, $s$ is the sliding distance, and $H$ is hardness. While all parameters except $k$ and $H$ were constant, the sulfurized grey iron casting’s combination of higher hardness ($H_{S+S} > H_{S+P}$) and a lower intrinsic wear coefficient ($k_{S+S} << k_{S+P}$ due to its solid lubricant nature) leads to significantly reduced wear volume.
| Sample (Block/Disc) | Avg. Coefficient of Friction (Pin-on-Disc) | Wear Scar Width (Block-on-Ring) [mm] | Wear Volume (Block-on-Ring) [mm³] | Relative Wear vs. Substrate |
|---|---|---|---|---|
| Untreated Substrate (S) | 0.142 | 0.98 | 0.0285 | 0% (Baseline) |
| Phosphated (S+P) | 0.086 | 1.09 | 0.0392 | +37.5% |
| Sulfurized (S+S) | 0.066 | 0.96 | 0.0268 | -6.0% |
3.3 Analysis of Friction Reduction Mechanism
The superior performance of the sulfurized layer on grey iron casting can be attributed to a synergistic combination of effects, fundamentally different from the primarily mechanical oil-reservoir action of phosphating.
1. Crystal Structure and Shear Properties: The key phase, FeS (troilite), has a hexagonal close-packed structure. The bonding within the basal planes is strong, but the shear strength between these planes is very low. This anisotropy allows easy relative sliding under shear stress, effectively turning the surface layer into a deposited solid lubricant. The critical shear stress $\tau_c$ for slip in an HCP crystal is highly orientation-dependent:
$$\tau_c = \frac{\sigma \cdot \cos \phi \cdot \cos \lambda}{m}$$
where for basal slip, the Schmid factor $m = \cos \phi \cdot \cos \lambda$ is maximized when the basal plane is oriented at 45° to the shear direction. The FeS layer, with its fine, multi-oriented scales, provides numerous easy-slip orientations. In contrast, the manganese phosphate hydrates have complex monoclinic crystal structures (e.g., P2₁/c for Mn₃(PO₄)₂·3H₂O) with no such readily activated low-shear slip systems, making them more brittle under shear.
2. Dynamic Tribochemical Reaction and Transfer Film Formation: A pivotal finding from EDS analysis of the untreated counterface pin after testing against the sulfurized grey iron casting disc was the presence of sulfur on its wear scar. This was not observed after testing against the phosphate layer or the untreated substrate. This indicates a dynamic process during sliding: frictional heating and mechanical activation can cause the release of active sulfur from the FeS layer. This sulfur then reacts with the fresh iron surface of the counterface to form a secondary, in-situ FeS transfer film. This process can be conceptualized as a reversible tribochemical reaction:
$$\text{FeS (on disc)} \xrightarrow[\text{Shear, Heat}]{\text{}} \text{Fe}^{2+} + \text{S}^{2-} \xrightarrow[\text{on counterface}]{\text{Reaction}} \text{FeS (transfer film)}$$
This creates a self-replenishing lubricating interface between two FeS-covered surfaces, dramatically reducing adhesion and friction. The phosphate layer lacks this ability; once its crystals are removed, they are lost from the contact zone.
3. Enhanced Mechanical Integrity: As shown by the hardness and residual stress measurements, the sulfurized layer on grey iron casting is harder and possesses higher compressive stress than the phosphate layer. Furthermore, it is not a mere deposit but a diffusion-based layer with a graded interface, ensuring excellent adhesion to the substrate. This allows it to withstand higher contact pressures without spallation, maintaining its lubricating function over extended periods and under severe loads where phosphate layers fail.
4. Practical Application and Performance Validation
The theoretical and laboratory advantages of sulfurizing grey iron casting components translate directly into tangible performance gains in real-world applications. To validate this, a comparative test was conducted on commercial 1.0 HP rotary compressors. One compressor was assembled with standard untreated grey iron casting pistons, while an identical compressor was assembled with pistons that had undergone the low-temperature ion sulfurization process. Both compressors were tested under identical conditions according to the national standard GB/T 15765-2004.
The key performance metric is the Coefficient of Performance (COP), defined as the ratio of cooling capacity ($Q_{evap}$) to input power ($W_{in}$):
$$\text{COP} = \frac{Q_{evap}}{W_{in}}$$
The test results, summarized in Table 4, show a clear improvement. While the compressor with sulfurized pistons showed a marginally lower cooling capacity (attributable to potentially tighter clearances or other minor assembly variations), the reduction in input power was significantly greater. This reduction in power consumption is a direct consequence of the lower friction losses in the piston-cylinder assembly. The COP of the compressor with sulfurized grey iron casting pistons was measured at 4.48, compared to 4.32 for the standard compressor. This represents a 3.7% increase in COP, a substantial improvement in efficiency for a mature technology like small-displacement refrigeration compressors. This gain directly translates to lower energy consumption for the end-user and reduced operational costs.
| Compressor Type | Cooling Capacity [kW] | Input Power [kW] | Coefficient of Performance (COP) | COP Improvement |
|---|---|---|---|---|
| Standard (Untreated Pistons) | 2.60 | 0.602 | 4.32 | Baseline |
| With Sulfurized Pistons | 2.58 | 0.576 | 4.48 | +3.7% |
5. Conclusion
This comprehensive investigation conclusively demonstrates that low-temperature ion sulfurization is a markedly superior surface engineering technology compared to traditional phosphating for enhancing the tribological performance of grey iron casting, especially in demanding applications like compressor components.
- Structural and Mechanical Superiority: The sulfurized layer on grey iron casting, primarily composed of hexagonal FeS, is a diffusion-based coating with excellent adhesion, higher surface hardness (~197 HV vs. ~159 HV), and greater beneficial compressive residual stress (-280 MPa vs. -120 MPa) than a manganese phosphate layer. It is an integral part of the surface, not a brittle, deposited coating.
- Exceptional Friction Reduction: The easy-slip HCP structure of FeS provides intrinsic solid lubrication. Under sliding conditions against grey iron casting, the sulfurized layer reduced the average coefficient of friction by 53% compared to the untreated substrate and by 23% compared to the phosphated surface.
- Superior Wear Resistance: Contrary to the phosphate layer which increased wear volume by 37.5%, the sulfurized layer on grey iron casting actually reduced wear by 6% compared to the untreated substrate under high load. This translates to a 31.6% lower wear rate than the phosphate coating, highlighting its durability and load-bearing capability.
- Dynamic Lubrication Mechanism: The sulfurization process offers a unique, active lubricating mechanism. The formation of a secondary FeS transfer film on the counterface via a dynamic tribochemical process ensures sustained low friction and protection, a feature completely absent in passive phosphate layers.
- Proven Application Benefit: Implementing this technology on grey iron casting pistons in a commercial rotary compressor resulted in a measurable 3.7% increase in system COP, directly attributable to reduced mechanical friction losses. This validates the technology’s potential for significant energy savings and performance enhancement in real-world machinery.
In summary, for grey iron casting components operating under the high loads and demanding conditions of modern efficient compressors, low-temperature ion sulfurization presents a robust, high-performance alternative to conventional phosphating. It moves beyond mere run-in protection to provide a durable, actively lubricating surface that enhances efficiency, reliability, and service life. The adoption of this advanced surface treatment for grey iron casting parts represents a strategic step towards meeting the escalating performance demands of next-generation, energy-efficient mechanical systems.