Made in China Fuel Injection Pump Plunger S1100 8mm Pump Elements Engine Accessories

Abstract
The high-pressure fuel pump plunger is a core component of the engine fuel injection system. Its wear resistance directly determines the pump's service life and injection accuracy. This paper focuses on the application of nanocoating technology in high-pressure fuel pump plungers. By comparing the microstructure and mechanical properties of different nanocoatings (TiN, CrN, and DLC), combined with tribological experiments and simulation analysis, the mechanism by which nanocoatings improve plunger wear resistance is revealed, providing theoretical support and practical reference for wear-resistant enhancement of high-pressure fuel pump plungers.

I. Introduction
In an engine's high-pressure common rail system, high-speed reciprocating friction between the plunger and the cylinder liner (linear speeds up to 5 m/s) and high-pressure fuel erosion (pressures up to 200 MPa) cause severe wear. When the wear exceeds 1 μm, it can cause fuel leakage and injection deviation, leading to increases in engine fuel consumption by 5%-8% and emissions exceeding standards by more than 30%. While traditional chrome coatings can improve wear resistance to a certain extent, they are prone to cracking and flaking under extreme operating conditions. Nanocoatings, with their nanoscale grain size (10-100nm), offer high hardness (HV 2000-4000) and low friction coefficient (0.1-0.2), making them a potential solution for plunger wear. Currently, nanoceramic coatings such as TiN and CrN, as well as diamond-like carbon (DLC) nanocoatings, have been proven in applications such as cutting tools and bearings. However, their wear resistance mechanisms in the service environment of high-pressure oil pump plungers (high temperatures of 300°C and diesel corrosion) have yet to be systematically studied.
This paper examines the plunger of a diesel engine high-pressure oil pump (made of 38CrMoAlA). By preparing three typical nanocoatings, the authors systematically analyze the coatings' bonding strength, friction and wear characteristics, and failure modes, elucidating the mechanism by which nanocoatings enhance wear resistance under high-pressure reciprocating friction conditions.

II. Preparation and Characterization of Nanocoatings
2.1 Coating Material Selection and Preparation Process
Three typical nanocoatings were selected for this experiment:
Nano-TiN coating: prepared using multi-arc ion plating technology, with a target material of pure Ti (99.9%), a nitrogen partial pressure of 0.5 Pa, a deposition temperature of 300°C, a coating thickness of 3 μm, and a grain size of approximately 30 nm.
Nano-CrN coating: prepared using magnetron sputtering, with a Cr target power of 200 W, a N₂/Ar mixture (ratio 1:3), a deposition rate of 0.5 μm/h, a coating thickness of 4 μm, and a grain size of approximately 50 nm.
Diamond-like carbon (DLC) nanocoating: prepared using plasma-enhanced chemical vapor deposition (PECVD) using CH₄ as the carbon source, a deposition temperature of 200°C, a coating thickness of 2 μm, an sp³ bond content of 70%, and an amorphous nanostructure.​
The plunger substrate was pretreated by: 800℃ tempering treatment (hardness HRC30-35) → precision grinding (surface roughness Ra0.05μm) → ultrasonic cleaning (acetone + alcohol) → ion etching (Ar⁺ bombardment, enhancing coating adhesion). ​
2.2 Characterization of microstructure and mechanical properties ​
X-ray diffraction (XRD) analysis: The nano-TiN coating presents a (111) preferred orientation, and the broadening of the diffraction peak indicates grain refinement; the CrN coating is mainly (200) crystal plane, and the grain size is calculated to be 48nm by the Scherrer formula; the DLC coating is a typical amorphous structure with no obvious diffraction peak. ​
Scanning electron microscopy (SEM) observation: The surface of the TiN coating presents a dense columnar crystal structure with a porosity of <1%; the surface particles of the CrN coating are uniform with a particle size of about 50nm; the DLC coating is a smooth amorphous structure with a surface roughness of Ra0.02μm. Mechanical Properties: Nanoindentation measurements revealed that the TiN coating had a hardness of 3200 HV and an elastic modulus of 280 GPa; the CrN coating had a hardness of 2800 HV and an elastic modulus of 250 GPa; and the DLC coating had a hardness of 4000 HV and an elastic modulus of 180 GPa. Friction coefficients (ball-on-disk test, GCr15 duality) were 0.6, 0.5, and 0.15, respectively.

III. Wear Resistance Test and Mechanism Analysis
3.1 Simulated Operating Friction Test

A reciprocating friction and wear tester (Model UMT-3) was used to simulate piston-cylinder liner friction conditions:

Mating part: 10 mm Φ ball made of cylinder liner material (HT300)

Load: 50 N (corresponding to a contact stress of 300 MPa)

Reciprocating frequency: 10 Hz (stroke 5 mm, linear velocity 0.1 m/s)

Medium: Diesel oil lubrication, temperature 80°C

Test duration: 10 hours

The experimental results show:

The wear loss of the uncoated piston was 8.5 μm, with obvious adhesive wear characteristics, including plowing and metal transfer on the surface.

The wear loss of the TiN-coated piston was 1.2 μm, primarily due to abrasive wear, with minor flaking of the coating surface.

The wear loss of the CrN-coated piston was 0.8 μm, with the wear mechanism primarily being mild abrasive wear, and the coating integrity remained good. The DLC-coated plunger exhibited wear of 0.3μm, a stable coefficient of friction of 0.18, a smooth wear scar, and only minor oxidative wear.

3.2 Wear Resistance Improvement Mechanism

Nanosize Effect: The refinement of the nanocrystal grains (30nm for TiN) increases the density of grain boundaries, hindering dislocation motion, improving the coating's hardness and resistance to plastic deformation, and reducing material loss caused by abrasive cutting.

Low Friction Effect: The sp³ bond structure of the DLC coating forms a graphite-like lubricating layer, reducing the friction coefficient by over 60%, reducing frictional heat generation (experimentally measured temperatures in the friction zone were 40°C lower than those of the uncoated coating), and inhibiting adhesive wear.

Corrosion Protection: The nanocoating's dense structure (porosity <1%) prevents sulfur and moisture in diesel from contacting the substrate. The CrN coating exhibits particularly outstanding corrosion resistance, exhibiting no rust after a 720-hour salt spray test (5% NaCl). Interface Strengthening Mechanism: The metallurgical bond between the coating and the substrate (bonding strength >50 MPa) effectively transmits load and prevents delamination of the coating under high-pressure reciprocating friction.

3.3 Failure Mode Analysis

SEM observation of the failed coating revealed the following:

TiN coatings are prone to brittle fracture under high loads, cracking, which propagates to the interface and causes delamination.

CrN coatings have good toughness (fracture toughness 4.5 MPa・m¹/²), absorbing energy through plastic deformation and delaying failure.

When temperatures exceed 250°C, DLC coatings transform from sp³ bonds to sp² bonds, resulting in a decrease in hardness. Therefore, the operating temperature must be controlled below 200°C.

IV. High-Pressure Fuel Pump Bench Test Verification

Three nano-coated plungers were installed in a high-pressure fuel pump and subjected to bench endurance testing (1500 rpm, 180 MPa rail pressure, 1000 hours):

Uncoated plunger: After 100 hours, significant wear was observed, with fuel leakage reaching 0.5 mL/min and an injection rate deviation of ±3%.

TiN-coated plunger: After 500 hours, wear was 2 μm and leakage was 0.1 mL/min, meeting operational requirements.

CrN-coated plunger: After 800 hours, wear was 1.5 μm, demonstrating stable performance with no significant leakage.

DLC-coated plunger: After 1000 hours, wear was 0.8 μm and an injection rate deviation of ±0.5%, demonstrating optimal performance. However, optimization of the high-temperature bonding between the coating and the substrate is required.

V. Conclusions and Outlook
Research Conclusion: Nanocoatings can significantly improve the wear resistance of high-pressure oil pump plungers, with DLC coating achieving the best results (reducing wear by 96%). This is attributed to the high hardness, low friction coefficient, and dense protective properties of the nanostructure. CrN coatings offer superior combined toughness and corrosion resistance, making them suitable for complex operating conditions.

Application Recommendations: Select coating type based on different engine operating conditions—CrN coatings are recommended for low-speed, heavy-load engines, while DLC coatings are preferred for high-speed, light-duty engines (with accompanying cooling measures).

Future Directions: Develop gradient nanocoatings (such as CrN/TiN multilayer structures) to balance hardness and toughness; and investigate the wear resistance of coatings in novel fuels such as biodiesel to meet the needs of new energy engine development.