Nodular cast iron, also known as ductile iron, is a cornerstone material in demanding engineering applications such as engine crankshafts, cylinder blocks, and camshafts. Its widespread use is attributed to an excellent combination of mechanical strength, castability, and good machinability. The matrix, typically comprising ferrite and pearlite, is interspersed with spherical graphite nodules which act as natural solid lubricants and crack arresters. Despite these inherent advantages, components made from nodular cast iron are still susceptible to tribological failures under severe operating conditions involving high loads, speeds, and boundary lubrication regimes. Wear and friction-induced energy losses remain critical challenges, directly impacting fuel efficiency, maintenance intervals, and the operational lifespan of machinery. Consequently, enhancing the surface tribological performance of nodular cast iron is a subject of significant industrial and academic interest.
Various surface modification techniques have been explored to mitigate wear and friction in cast irons. These include surface coating technologies (e.g., Diamond-Like Carbon, thermal spray coatings), surface heat treatments (e.g., induction hardening, nitriding), and more recently, surface texturing. Among these, laser surface texturing (LST) has emerged as a highly versatile and effective method. The principle involves creating deliberate micro-scale patterns or features on a surface to improve its tribological behavior. These micro-features can serve multiple functions: acting as micro-reservoirs to retain lubricant, generating additional hydrodynamic pressure to separate surfaces, trapping wear debris to prevent third-body abrasion, and reducing the real area of contact. The efficacy of surface textures has been demonstrated across various lubrication regimes, from hydrodynamic and elastohydrodynamic to boundary and mixed lubrication.
Textures can be categorized based on their geometry and distribution. Common single-scale textures include dimples (circular, elliptical), grooves, and grids, often arranged in periodic arrays. Research has consistently shown that such textures can significantly reduce the coefficient of friction (COF) and wear rate compared to untextured surfaces. For instance, a well-designed array of dimples with an optimal area density can enhance oil film formation and load-carrying capacity. More advanced concepts involve biomimetic textures inspired by natural surfaces (like shark skin or lotus leaves) and composite or hierarchical textures that combine different shapes or scales. These multi-scale designs aim to synergistically couple the benefits of individual texture types. For example, a large-scale groove pattern might guide lubricant flow, while small-scale dimples within the grooves enhance local pressure build-up and debris entrapment. The exploration of such hierarchical texturing for nodular cast iron, however, remains less documented.
This study introduces and investigates a novel approach to surface engineering of nodular cast iron: the creation of a hierarchical, multi-layer texture using sequential laser processing steps. The first layer consists of a regular, periodic array of micro-dimples fabricated via conventional laser ablation. The second layer is an “in-situ texture” generated by laser scanning, which selectively ablates the spherical graphite nodules present in the material’s microstructure, creating a random distribution of micro-craters whose location and size are dictated by the inherent graphite dispersion. This dual-texture approach aims to leverage the benefits of both ordered and stochastic surface features. We hypothesize that the regular array promotes organized lubricant flow and hydrodynamic lift, while the in-situ texture provides additional lubricant storage, enhances surface hardness through laser-induced phase transformation, and offers a multi-scale network for more effective wear debris capture.
The primary objective of this work is to systematically evaluate the tribological performance of this hierarchical textured surface on nodular cast iron under oil-lubricated sliding conditions. The performance is benchmarked against untextured surfaces, surfaces with only the regular array texture, and surfaces with only the in-situ texture. The investigation combines numerical simulation of fluid pressure, microstructural and mechanical characterization (XRD, nanoindentation, SEM/EDS), and experimental tribological testing. The goal is to elucidate the underlying mechanisms through which this multi-layer design improves friction and wear resistance, providing a new strategy for enhancing the durability and efficiency of nodular cast iron components.
1. Experimental Methodology
1.1 Material and Sample Preparation
The substrate material used in this study was a grade QT700-2 nodular cast iron. Its chemical composition, determined by optical emission spectrometry, is presented in Table 1.
| C | Si | Mn | P | S | Mg | Cu | Fe |
|---|---|---|---|---|---|---|---|
| 3.4-3.7 | 2.1-2.3 | 0.3-0.5 | ≤0.03 | ≤0.03 | 0.05-0.08 | 0.4-0.8 | Bal. |
The microstructure, as shown in the micrograph, consists of a pearlitic-ferritic matrix with uniformly distributed spherical graphite nodules. Disk-shaped specimens with a diameter of 50 mm and a thickness of 7 mm were machined from cast stock. The friction surface of each specimen was ground and polished to a mirror-like finish with a final surface roughness (Ra) of approximately 0.1 μm. Prior to laser processing, all specimens were ultrasonically cleaned in acetone and ethanol for 15 minutes each to remove any contaminants.
1.2 Laser Surface Texturing Process
A picosecond fiber laser system was employed for all surface texturing operations. The process involved two distinct steps, as schematically illustrated:
Step 1: Fabrication of Regular Array Texture (LST). A periodic array of micro-dimples was ablated onto the polished nodular cast iron surface. The dimples were circular with a nominal diameter (D) of 100 μm and arranged with a center-to-center spacing (L) of 300 μm, resulting in an area density of approximately 9%. The laser parameters were carefully selected to minimize the formation of a recast layer and burrs: pulse duration of 0.3 ps, repetition rate of 1 MHz, average power of 300 W, and a scanning speed of 1 m/s. Specimens processed with only this step are designated as LST-S.
Step 2: Generation of In-situ Texture (LSA). Following the creation of the dimple array, the entire surface was subjected to a laser scanning treatment. A defocused laser beam was scanned over the surface with a line spacing (P) of 0.01 mm. The key parameters for this step were: pulse duration of 0.3 ps, repetition rate of 5 MHz, average power of 170 W, and a scanning speed of 2 m/s. This lower fluence, high-overlap scanning was designed to selectively ablate the graphite nodules protruding at or near the surface without excessively deepening the pre-existing dimples. The evaporation of graphite nodules creates a random distribution of shallow craters, termed “in-situ texture.” Specimens processed with only this scanning step (on a pristine surface) are designated as LSA-S.
Step 3: Fabrication of Multi-layer Texture (MT). The hierarchical texture was produced by performing Step 1 followed immediately by Step 2 on the same specimen. This resulted in a surface featuring both the regular dimple array and the stochastic in-situ texture craters. These specimens are designated as MT-S.
A control specimen with a polished but untextured surface was also prepared and is designated as NT-S. All laser processing was conducted in an ambient air environment at room temperature.
1.3 Characterization Techniques
The surface topography and cross-sectional profiles of the textured specimens were analyzed using a 3D optical surface profilometer. This provided measurements of dimple/crater diameter, depth, and the overall surface morphology. The microstructure and chemical composition of the surface and subsurface regions were examined using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Spectroscopy (EDS). Cross-sectional samples were prepared by standard metallographic techniques and etched with a 4% Nital solution to reveal the microstructure. Phase analysis was performed using X-ray Diffraction (XRD) with Cu-Kα radiation. The mechanical properties of the laser-affected zones were evaluated via nanoindentation tests on polished cross-sections to map the hardness variation from the surface into the bulk material.
1.4 Tribological Testing
Sliding friction and wear tests were conducted on a ball-on-disk tribometer under lubricated conditions. The upper specimen was a 5 mm diameter GCr15 bearing steel ball (hardness ~650 HV). The lower specimen was the textured or untextured nodular cast iron disk. The test parameters are summarized in Table 2.
| Parameter | Specification |
|---|---|
| Applied Load | 50 N |
| Rotational Speed | 540 rpm |
| Track Radius | 8 mm |
| Sliding Duration | 120 min |
| Lubricant | ISO VG 32 Mineral Oil |
| Lubricant Feed Rate | 10 mL/min |
| Temperature | Room Temperature |
The friction coefficient was recorded in real-time throughout the test. After testing, the wear scar on the disk was profiled using the 3D profilometer. The wear volume (V) was calculated from the cross-sectional profile of the wear track. The specific wear rate (W) was then determined using the following equations:
$$ V = \frac{[c h (3b^2 + 4h^2)]}{6b} $$
$$ W = \frac{V}{F L} \times 100\% $$
where c is the track circumference, h is the maximum wear depth, b is the wear track width, F is the normal load, and L is the total sliding distance. Each test condition was repeated three times to ensure statistical reliability.
1.5 Numerical Simulation of Fluid Pressure
To gain insight into the hydrodynamic lubrication effect of the different textures, a simplified 3D computational fluid dynamics (CFD) model was developed. The model simulated the pressure distribution within a lubricant film flowing over a stationary textured surface with a moving, smooth counter-surface. The minimum oil film thickness was set to 5 μm, and the sliding speed was 0.45 m/s, matching the experimental conditions. The mixture multiphase model with the Schnerr-Sauer cavitation model was employed to account for possible cavitation in the diverging regions of the textures. Models were built for the LST-S texture (with two area densities: 9% and 16%), the LSA-S texture (~9% density), and the MT-S texture (~16% combined density). The resultant pressure fields were compared to assess the potential for enhanced load support.
2. Results and Discussion
2.1 Surface Morphology and Characterization
The 3D surface profiles confirmed the successful fabrication of the intended textures. The LST-S surface showed a regular pattern of dimples approximately 100 μm in diameter and 20 μm deep, with some burr formation around the rims. The LSA-S surface exhibited a random distribution of shallow craters with diameters ranging from 15 to 60 μm and depths of 3 to 14 μm, corresponding to the original graphite nodules. The MT-S surface clearly displayed both features: the deep, regular dimples superimposed with the smaller, stochastic in-situ craters, creating a complex hierarchical topography.
XRD analysis revealed the phase composition changes induced by laser processing. All specimens showed strong peaks for α-Fe (ferrite) and graphite. The LSA-S and MT-S specimens showed a noticeable reduction in the graphite peak intensity, confirming the ablation of surface graphite during the laser scanning step. Furthermore, these specimens displayed a slight peak shift for α-Fe towards higher angles and the appearance of a minor peak near 43.5°, indicative of the formation of martensite (α’) and/or retained austenite (γ-Fe) due to rapid heating and quenching, as well as the presence of residual compressive stresses.
Nanoindentation tests on the cross-section of the LSA-S and MT-S specimens revealed a significant hardening effect. A distinct modified layer, comprising a melted and re-solidified zone (Ablation Zone, AZ) and a heat-affected zone (HAZ), was observed. The hardness in the AZ reached approximately 4.6 GPa, which is about 84% higher than the base nodular cast iron hardness (~2.5 GPa). The HAZ showed a moderate hardness increase to about 3.5 GPa. This surface hardening is attributed to the formation of a fine martensitic structure and residual stresses from the laser thermal cycle, which is expected to significantly improve wear resistance.
2.2 Hydrodynamic Pressure Simulation
The CFD simulation results provided valuable insights into the theoretical lubrication enhancement. The pressure nephograms are illustrated in the respective figures. The key finding was that the textured surfaces generated higher positive fluid pressure peaks compared to a smooth surface, confirming their function as micro-hydrodynamic bearings. The MT-S model yielded the highest peak oil film pressure among all configurations. This is attributed to the synergistic interaction between the deep, regular dimples, which create strong, organized pressure fields, and the shallower in-situ craters, which add complexity and help maintain a more continuous pressure distribution. The simulation for a higher-density regular array (LST-S, 16%) showed that excessive area density can lead to interaction between adjacent dimples’ pressure fields, potentially reducing their individual effectiveness and overall load support, explaining why an optimal density exists. The LSA-S surface, despite its random and shallow nature, also generated a discernible pressure increase, demonstrating that even stochastic textures can improve lubrication.
| Specimen Model | Texture Type | Area Density | Relative Peak Pressure Increase* |
|---|---|---|---|
| LST-S1 | Regular Array | ~9% | 14.7% |
| LST-S2 | Regular Array | ~16% | 21.1% |
| LSA-S | In-situ | ~9% | 3.9% |
| MT-S | Multi-layer | ~16% | Highest (Benchmark) |
*Compared to a baseline smooth surface model.
2.3 Friction and Wear Performance
The tribological test results under oil lubrication are summarized in Figure X and Table 4. The untextured NT-S specimen exhibited the highest and most unstable friction coefficient, averaging around 0.088, with significant fluctuations. All textured specimens showed a reduction in friction.
The LST-S specimen (9% density) achieved an average COF of 0.064, a 27.8% reduction. However, the LSA-S specimen showed a more modest reduction to 0.080 (9% lower than NT-S), likely because the loss of surface graphite, a natural solid lubricant, initially increased friction, which was partially offset by the texture’s lubrication effects and hardening.
The MT-S specimen demonstrated the best frictional performance, with the lowest and most stable average COF of 0.062, representing a 30.1% reduction from the base material. The stability of the friction curve suggests effective and sustained lubricant film maintenance and wear debris management.
| Specimen | Avg. Coefficient of Friction | Reduction vs. NT-S | Specific Wear Rate (×10-6 mm³/N·m) | Reduction vs. NT-S |
|---|---|---|---|---|
| NT-S | 0.088 ± 0.005 | — | 4.79 ± 0.41 | — |
| LST-S | 0.064 ± 0.003 | 27.8% | 1.45 ± 0.18 | 69.7% |
| LSA-S | 0.080 ± 0.004 | 9.0% | 0.51 ± 0.09 | 89.3% |
| MT-S | 0.062 ± 0.002 | 30.1% | 0.45 ± 0.07 | 90.6% |
The wear resistance improvements were even more pronounced. The NT-S specimen suffered severe wear with a deep, wide wear track characterized by abrasive grooves and plastic deformation. The specific wear rate was 4.79 × 10-6 mm³/N·m. The LST-S specimen showed a 69.7% reduction in wear rate. The LSA-S and MT-S specimens exhibited outstanding wear resistance, with reductions of 89.3% and 90.6%, respectively. The extremely low wear rate of the MT-S specimen underscores the synergistic benefit of combining texture-enhanced lubrication with a laser-hardened surface.
2.4 Wear Mechanism Analysis
SEM and EDS analysis of the wear scars provided detailed insights into the dominant wear mechanisms. The wear scar on the NT-S specimen showed evidence of severe abrasive wear (deep ploughing grooves) combined with adhesive wear (material transfer and delamination), as seen in the micrographs.
The wear track on the LST-S specimen was shallower but still displayed clear abrasive grooves. Notably, some dimples remained intact and were found to contain wear debris, confirming their “debris-trapping” function. However, the hard burrs formed during laser ablation were prone to break off, generating abrasive particles that contributed to third-body wear.
The LSA-S specimen’s wear scar was very shallow, with only fine scratches. The in-situ craters were visible within the track, many containing fine particles. The primary mechanism was mild abrasive wear. The significant improvement over LST-S is directly linked to the laser-induced surface hardening, which drastically increased resistance to ploughing.
The MT-S specimen presented the mildest wear morphology. The wear track was extremely shallow with very fine, superficial scratches. The hierarchical texture was partially preserved within the track. The combination of the regular dimples (for hydrodynamic pressure and debris storage) and the hardened surface with in-situ craters (for debris capture and load support) created an optimal condition for minimizing wear. The multi-scale texture effectively prevented the accumulation of large abrasive particles, while the hardened surface resisted deformation and material removal.
The EDS maps showed oxygen presence in wear tracks, indicating mild oxidative wear. The oxygen signal was more localized in the textured specimens, often associated with debris in texture pockets or with the laser-oxidized texture rims.
3. Conclusions
This study successfully demonstrated a novel hierarchical laser texturing strategy for significantly enhancing the tribological performance of nodular cast iron. The multi-layer texture, combining a regular dimple array with an in-situ texture derived from graphite ablation, was fabricated and comprehensively evaluated. The main conclusions are as follows:
- The hierarchical texturing process on nodular cast iron effectively creates a complex surface featuring both ordered micro-dimples and stochastic micro-craters, while simultaneously inducing a significant surface hardening effect (>80% hardness increase) through phase transformation.
- Computational fluid dynamics simulation indicated that the multi-layer texture (MT-S) generates the most favorable oil film pressure distribution, suggesting superior hydrodynamic load-carrying capacity compared to single-layer textures, due to synergistic interactions between the different texture scales.
- Under oil-lubricated sliding conditions, all textured specimens improved the tribological performance of the nodular cast iron. The MT-S specimen delivered the optimal combination: the lowest and most stable friction coefficient (0.062, a 30.1% reduction) and the lowest specific wear rate (0.45 × 10-6 mm³/N·m, a 90.6% reduction).
- The outstanding performance of the multi-layer textured nodular cast iron is attributed to a synergistic mechanism: (a) The regular dimple array enhances hydrodynamic lubrication and provides reservoirs for lubricant and wear debris. (b) The in-situ texture offers additional, multi-scale sites for debris entrapment and improves surface wettability. (c) The laser-hardened subsurface drastically increases resistance to abrasive wear. (d) The hierarchical design minimizes the negative impact of recast layer burrs associated with single-step dimple texturing.
This work presents a promising surface engineering approach for critical nodular cast iron components operating in tribologically demanding environments. The method leverages the inherent microstructure of the material to create a functional, multi-functional surface that addresses multiple wear mechanisms simultaneously. Future work could focus on optimizing the laser parameters for the in-situ texture to control the depth and morphology of the graphite-derived craters more precisely and exploring the performance under even more severe lubrication regimes (e.g., starved lubrication).