In my research, I focused on developing a novel approach to strengthen ductile iron crankshafts by embedding continuous carbon fibers using a sand casting foundry process. The automotive industry heavily relies on ductile iron crankshafts, which often fail under high tensile stress. Traditional strengthening methods, such as alloying and heat treatment, are costly and have reached their performance limits. Carbon fibers offer high strength, high modulus, low density, and moderate cost, making them attractive reinforcements. However, poor interfacial bonding between carbon fibers and iron matrices remains a critical challenge. My work addressed this by applying nickel electroplating to the carbon fibers, designing a manual sand mold casting technique, and analyzing the interfacial microstructure and mechanical properties.
1. Introduction and Background
Ductile iron, also known as nodular cast iron, is characterized by graphite nodules within the ferrous matrix. It combines high strength with adequate ductility and toughness, making it a preferred material for automotive crankshafts. The crankshaft is subjected to cyclic bending, torsion, and high inertial forces. Fatigue fractures commonly initiate at fillet regions where stress concentration occurs. Conventional improvements include adding alloying elements such as copper and molybdenum, or performing heat treatments like normalizing and quenching-tempering. These methods increase costs and have limited potential for further enhancement. Carbon fiber reinforced metal matrix composites have been explored for aluminum and magnesium matrices, but their application in iron-based systems is rare due to severe interfacial reactions and poor wetting.
Carbon fibers, especially PAN-based T700 grade, exhibit tensile strengths exceeding 4.9 GPa and modulus around 230 GPa. Their density is only 1.8 g/cm³, offering excellent specific properties. At temperatures above 700°C in oxidizing atmospheres, carbon fibers react with oxygen, forming CO and CO₂, which degrades their mechanical performance. In liquid iron, the fiber-matrix interface suffers from lack of wetting and formation of brittle carbides such as Fe₃C, which is detrimental. Therefore, surface modification of carbon fibers is essential. I chose electroplating nickel onto the fiber surface to act as a barrier and wetting agent. Nickel is compatible with iron, being fully soluble in austenite, and it can form a diffusion gradient that promotes metallurgical bonding.
The objectives of my study were: (1) to establish a reliable surface treatment route for carbon fibers, including de-sizing, roughening, and nickel plating; (2) to develop a manual sand casting method that places continuous carbon fibers at precise locations within a Y-shaped test block; (3) to compare tensile properties of composites containing nickel-plated and unplated carbon fibers; (4) to analyze the oxidation behavior of carbon fibers at high temperatures; and (5) to investigate the interfacial microstructure and elemental distribution using SEM and EDS.
2. Surface Metallization of Carbon Fibers
2.1 Selection of Surface Modification Method
Several surface treatment methods exist, including acid etching, electrochemical oxidation, plasma treatment, and dip coating. Each method aims to increase surface roughness, introduce functional groups, or apply a metallic coating. For iron matrices, a metallic coating that is compatible with both carbon and iron is required. Nickel is an ideal candidate because it wets carbon fibers moderately after initial deposition, and it forms a solid solution with iron at high temperatures. Electroplating offers advantages of uniform thickness control, low cost, and relatively simple equipment compared to electroless plating. I selected electroplating for this study.
2.2 De-sizing and Roughening
Commercial carbon fibers are coated with an organic sizing agent to protect them during handling and weaving. This sizing must be removed to expose the active fiber surface. I used a heat treatment furnace at 400°C for 5 minutes, which effectively volatilized the organic layer without causing significant oxidation. SEM images showed that the desized fibers had a cleaner surface with fine grooves, which enhanced adhesion of subsequent coatings.
To further increase the surface area and mechanical interlocking, I roughened the fibers chemically using a mixture of sulfuric acid and nitric acid (3:7 by volume). The treatment was conducted at 30–40°C for 80 minutes. During the roughening, small pits and micro-crevices formed on the fiber surface. The optimal time was determined experimentally; 80 minutes created moderate roughness, while 100 minutes caused excessive surface damage and flaking.
2.3 Electrodeposition of Nickel
The electroplating bath composition and parameters are summarized in Table 1. I used a two-electrode system with the carbon fibers as the cathode and a nickel plate as the anode. The bath was agitated gently to ensure uniform ion transport. The pH was maintained at 3–4.5, temperature at 25°C, and current density at 0.5 A/dm². The plating time varied from 2 to 10 minutes to study thickness evolution.
| Component | Concentration | Function |
|---|---|---|
| NiSO₄ | 225 g/L | Main nickel source |
| NiCl₂ | 65 g/L | Anode activator, conductivity |
| NaCl | 55 g/L | Supporting electrolyte |
| H₃BO₃ | 40 g/L | Buffer |
| C₁₂H₂₅SO₄Na | 0.05 g/L | Surfactant |
| pH | 3–4.5 | — |
| Temperature | 25°C | — |
| Current density | 0.5 A/dm² | — |
| Time | 5–10 min | — |
Figure 1 shows the growth sequence: nickel grains initially nucleated on active sites, then coalesced into islands, and eventually formed a continuous film. The optimal coating thickness was identified as 0.3 μm. When the thickness exceeded 0.5 μm, dendritic structures and rough deposits appeared, reducing coating quality. For my composites, I used a plating time of 8 minutes to achieve a uniform, dense nickel layer of about 0.3 μm.
3. Sand Casting Foundry Process with Embedded Carbon Fibers
3.1 Mold Design and Fiber Placement
The sand casting foundry process was chosen due to its flexibility and low cost for prototype production. I used a three-dimensional woven carbon fiber bundle (continuous, 6K tow) and embedded it directly into the sand mold. The mold cavity was shaped as a Y-block according to standard tensile test specimen requirements. The fiber bundle was fixed at both ends of the mold box before ramming the sand. This pre-embedding method ensured that the fibers were taut and precisely located along the central axis of the Y-block. When molten iron was poured, the lightweight fibers would otherwise float or shift, causing misalignment. By fixing both ends and keeping the fiber taut during ramming, I minimized this problem.
Two approaches were tested. The first involved burying the fiber bundle during mold preparation, then pulling it tight after the mold was compacted. The second involved placing fibers after the mold was finished, fixing them with steel pins. The first method proved superior because the steel pins softened near the molten iron and allowed fiber movement. Therefore, I adopted the pre-embedding method for all subsequent castings.
3.2 Gating and Pouring
Before pouring, I placed the mold on a flat surface and made a pouring cup with a 60° funnel shape. Vent holes were created using a vent rod to allow gases to escape. The Y-block orientation was such that the carbon fiber region was at the bottom, away from the ingate, to ensure that inclusions and slag floating upward did not contaminate the critical fiber region. Approximately 10 minutes before pouring, I purged the mold cavity with argon gas to reduce the oxygen content and protect the carbon fibers from oxidation. Although a complete inert atmosphere was not achieved, this reduced the extent of oxidation.
The ductile iron melt was prepared with the composition given in Table 2. The pouring temperature was approximately 1350°C. After pouring, the mold was covered with an insulating agent to slow cooling and promote better melt infiltration around the individual carbon filaments. The Y-block was allowed to cool for about 4 hours before shakeout.
| Element | Base iron | After spheroidization |
|---|---|---|
| C | 3.6–4.0 | 3.5–3.8 |
| Si | — | 1.8–2.0 |
| Mn | 0.3–0.5 | 0.3–0.5 |
| S | 0.008–0.025 | 0.008–0.025 |
| P | ≤0.05 | ≤0.05 |
| Mg | — | 0.035–0.055 |
| RE | — | 0.006–0.03 |
| Cr | ≤0.08 | ≤0.08 |
| Cu | — | 0.2–0.3 |
| Sn | — | 0.015–0.025 |
3.3 Specimen Preparation
The Y-block was machined into standard round tensile specimens with a diameter of 10 mm and a gauge length of 50 mm. Before machining, I marked the carbon fiber location using X-ray or visual inspection of the block surface. This step was critical to avoid cutting the fibers incorrectly. Eight specimens were produced in the same batch: specimens 1# to 3# contained unplated carbon fibers, and specimens 4# to 8# contained nickel-plated carbon fibers.
4. Tensile Testing and High-Temperature Oxidation Analysis
4.1 Tensile Test Results
I performed tensile tests using an MTS Exceed E45 universal testing machine with a crosshead speed of 12.70 mm/min. The results are summarized in Table 3. The average ultimate tensile strength for unplated fiber composites was 437 MPa, while nickel-plated fiber composites reached 535 MPa, an improvement of 98 MPa. Visual inspection of the fractured surfaces revealed distinct behaviors: in unplated fiber specimens, fibers were pulled out cleanly from the matrix, indicating poor adhesion and the presence of voids. In contrast, nickel-plated fibers fractured together with the matrix, showing effective load transfer.
| Specimen | Fiber type | Tensile strength (MPa) |
|---|---|---|
| 1# | Unplated | 430 |
| 2# | Unplated | 441 |
| 3# | Unplated | 440 |
| Average unplated | — | 437 |
| 4# | Nickel-plated | 530 |
| 5# | Nickel-plated | 538 |
| 6# | Nickel-plated | 532 |
| 7# | Nickel-plated | 536 |
| 8# | Nickel-plated | 539 |
| Average plated | — | 535 |
4.2 Oxidation of Carbon Fibers
After high-temperature exposure during casting, both types of fibers suffered oxidation. I analyzed the fibers by SEM-EDS. Results are presented in Table 4. Unplated fibers exhibited 68.65% C, 21.10% O, and 6.43% Fe, suggesting the formation of iron oxides or carbides. Nickel-plated fibers contained 78.65% C, 7.77% O, and 2.21% Ni, with negligible Fe. The nickel layer acted as a barrier, limiting direct contact between carbon and iron, and reduced the formation of brittle Fe₃C.
| Element | Unplated fiber | Nickel-plated fiber |
|---|---|---|
| C | 68.65 | 78.65 |
| O | 21.10 | 7.77 |
| Si | 0.31 | 0.00 |
| Cr | 3.52 | 0.35 |
| Fe | 6.43 | 0.00 |
| Ni | — | 2.21 |
4.3 Oxidation Reaction Mechanism
Carbon fibers react with oxygen at temperatures above 700°C according to:
$$ \text{C} + \text{O}_2 \rightarrow \text{CO}_2 $$
$$ 2\text{C} + \text{O}_2 \rightarrow 2\text{CO} $$
$$ 2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2 $$
$$ \text{C} + \text{CO}_2 \rightarrow 2\text{CO} $$
These reactions consume carbon atoms from the fiber surface. The CO₂ generated can further react with carbon, creating a self-sustaining cycle that rapidly degrades the fiber. Using a gas-solid reaction model, I derived the oxidation rate in terms of fiber radius change:
$$ \frac{dr}{dt} = -\frac{k c_0}{\rho} $$
where r is the fiber radius, k is the rate constant, c₀ is the oxygen concentration, and ρ is the fiber density. The time for complete consumption is:
$$ t_{\text{ox}} = \frac{\rho r_0}{k c_0} $$
This shows that higher oxygen concentrations and smaller fiber diameters accelerate oxidation. Therefore, to protect carbon fibers during casting, I recommend (1) pouring under vacuum or in an inert gas environment, (2) applying a protective coating that blocks oxygen diffusion, or (3) increasing fiber diameter, although that is not practical. In my sand casting foundry experiments, argon purging partially protected the fibers, but not completely. Further optimization would involve sealed molds and continuous inert gas flow.
5. Interfacial Behavior and Microstructural Analysis
5.1 Interfacial Bonding Mechanisms
The interface between carbon fibers and the iron matrix is crucial for load transfer. I identified four basic bonding types: mechanical, physical, diffusion, and chemical bonding. In practice, multiple mechanisms coexist. For unplated carbon fibers in iron, the lack of wetting leads to mechanical interlocking only, with poor adhesion and void formation. The nickel coating changes this by introducing a metallic layer that can dissolve into the iron matrix, creating a diffusion bond and possibly a chemical reaction zone.
5.2 Optical Microscopy Observations
I prepared cross-sectional specimens from both composite types. Polishing was performed with grinding papers up to 2000 grit, followed by diamond suspension. Optical micrographs at magnifications of 10×, 50×, and 100× revealed clear differences. In the nickel-plated fiber composites, the iron matrix closely followed the contours of the carbon fibers, with no visible gaps. In contrast, unplated fiber composites showed large gaps and many individual fibers were detached, creating porosity.
5.3 SEM Morphology of Fracture Surfaces
Using a QUANT FEG 250 field emission scanning electron microscope, I examined the fracture surfaces after tensile testing. Figure 2 (not shown here) demonstrated that unplated carbon fibers remained clustered and were not wetted by iron. Large voids were present at the fiber bundle locations. Nickel-plated fibers, however, were individually surrounded by iron, and the fiber-matrix interface was continuous. The improved wetting is attributed to the nickel layer, which reduces the contact angle between carbon and iron. This allowed molten iron to penetrate between individual filaments, leading to better mechanical and chemical bonding.
5.4 EDS Elemental Mapping and Line Scanning
I performed EDS mapping on cross-sections of nickel-plated fiber composites. The distribution of C, Ni, and Fe is shown in Table 5 (representative results). The nickel signal was concentrated around the fiber boundaries and diffused outward into the iron matrix. This indicates that during solidification, nickel atoms migrated from the coating into the adjacent iron, forming a solid solution zone. The presence of a nickel-rich zone near the fiber and a gradual decrease in nickel content with distance conforms to a concentration gradient driven by diffusion.
| Position | C (wt%) | Ni (wt%) | Fe (wt%) | Interpretation |
|---|---|---|---|---|
| Point a (matrix) | low | low | high | Bulk iron matrix |
| Point b (interface) | moderate | high | moderate | Fe-Ni diffusion zone |
| Point c (fiber center) | high | low | low | Carbon fiber interior |
The EDS line scan indicated that the width of the nickel diffusion zone was approximately 2–5 μm. This zone corresponds to a Fe-Ni solid solution, which has higher strength and toughness than pure iron. Moreover, nickel may react with carbon to form Ni₃C, but this compound is less stable than Fe₃C and may decompose during solidification. Overall, the nickel layer facilitated a metallurgical Fe-Ni-C interface, significantly enhancing interfacial adhesion.
6. Discussion
6.1 Effect of Ni Coating on Mechanical Properties
The tensile strength improvement of 98 MPa corresponds to about 22% increase relative to unplated fiber composites. This improvement is linked to the elimination of voids and the enhanced interfacial shear strength. In unplated composites, the carbon fibers acted as stress concentrators because they were not bonded to the matrix; when the specimen was loaded, the fibers could not carry any significant load and instead caused premature crack initiation. With nickel plating, the fibers were firmly bonded, allowing load transfer from the matrix to the high-strength carbon filaments. This mechanism is consistent with the rule of mixtures for continuous fiber composites:
$$ \sigma_c = \sigma_f V_f + \sigma_m (1 – V_f) $$
where σ_c is the composite strength, σ_f is the fiber strength, σ_m is the matrix strength, and V_f is the fiber volume fraction. In my specimens, the actual fiber volume fraction was low (about 1–2%), so the theoretical contribution is limited. However, the nickel coating also improved the cast matrix quality by reducing porosity, which explains the observed strength increase.
6.2 Oxidation Control in Sand Casting Foundry
Despite the benefits of nickel plating, carbon fibers still suffered some oxidation during casting because the argon purge was not a complete vacuum or inert atmosphere. The oxidation reactions were more severe near the outer regions of the fiber bundle. To fully avoid oxidation, I suggest implementing a sealed sand mold with continuous argon flow during the entire pouring and solidification process. Alternatively, a vacuum-assisted sand casting foundry setup could be used, although that is more costly. Another approach is to apply a protective ceramic coating on the carbon fibers before nickel plating, creating a dual-layer system that blocks oxygen diffusion.
6.3 Interfacial Strengthening Mechanisms
The nickel layer enhances the interface through several mechanisms: (1) improved wetting reduces the critical pressure required for molten iron to infiltrate the fiber bundle; (2) nickel diffuses into iron, forming a solid solution that increases the cohesive strength of the interface; (3) the nickel layer acts as a compliant medium that accommodates thermal expansion mismatches between carbon (1–2×10⁻⁶/°C) and iron (10–12×10⁻⁶/°C), reducing internal stresses; (4) the nickel layer limits harmful chemical reactions between carbon and iron, preventing the formation of brittle Fe₃C at the interface. Together, these effects produced the observed denser, more defect-free microstructure.
7. Conclusions
From my investigation, I draw the following conclusions:
1. Carbon fibers (T700) can be successfully electroless-nickel plated after de-sizing at 400°C for 5 min, acid roughening in H₂SO₄/HNO₃ (3:7) at 30–40°C for 80 min, followed by electroplating at pH 3–4.5, 25°C, and 0.5 A/dm² for 8 min. This produced a uniform, dense nickel layer of 0.3 μm thickness.
2. A sand casting foundry process was developed to place continuous carbon fibers into a Y-block mold. Pre-embedding the fiber bundle during sand ramming and keeping it taut was essential to maintain fiber positioning. Argon purging before pouring partially protected the fibers from oxidation.
3. Tensile tests showed that nickel-plated fiber composites had an average strength of 535 MPa, compared to 437 MPa for unplated fiber composites—a 98 MPa improvement. The nickel layer improved fiber-matrix bonding and eliminated voids that otherwise caused premature failure.
4. High-temperature oxidation models indicated that carbon fibers react with oxygen and CO₂ at casting temperatures, leading to significant mass loss. To minimize oxidation, complete inert gas shielding or vacuum casting is necessary during the sand casting foundry process.
5. SEM and EDS analyses revealed that the nickel coating created a Fe-Ni diffusion zone at the interface. Nickel atoms diffused into the iron matrix, forming a concentration gradient. This promoted a metallurgical Fe-Ni-C bond, enhancing interfacial adhesion and enabling effective load transfer from the ductile iron matrix to the carbon fibers.
8. Future Work
To further improve the performance of carbon fiber reinforced ductile iron crankshafts, I propose the following directions for future research: optimizing the fiber volume fraction and weave architecture to maximize strengthening without compromising castability; developing a preform concept where a controlled amount of carbon fiber is incorporated into a disposable insert that melts during casting; evaluating the fatigue behavior of the composite crankshafts under cyclic loading; and refining the surface treatment to create a multi-layer coating that enhances oxidation resistance while maintaining interfacial bond strength. Moreover, scaling up the sand casting foundry procedure to industrial production requires automated fiber placement and mold cavity evacuation systems to ensure reproducibility.
In conclusion, this research demonstrates that continuous carbon fibers, modified with a nickel coating and embedded via a sand casting foundry route, can effectively enhance the tensile strength of ductile iron. The interfacial analysis confirmed that nickel acts as a beneficial transition layer, forming a strong Fe-Ni-C bond. With further optimization, this method may lead to lightweight, high-strength crankshafts for next-generation automotive engines.