The wear resistance of a material, in layman’s terms, refers to its ability to resist surface damage or material loss under repeated mechanical actions such as contact, friction, and sliding.
Alternatively, it can be defined as the capacity of a material to maintain structural integrity under dynamic frictional loading.
The wear resistance of rubber is closely related to the following properties:
- Coefficient of friction
- Yield strength and hardness
- Fatigue strength and crack propagation resistance
- Impact toughness
- Self-healing capability of surface structures
Among these, the coefficient of friction plays a significant role and is influenced by the following material parameters:
a. Surface Energy
Polymers with higher surface energy typically exhibit more polar surfaces, leading to stronger adsorption interactions with other surfaces and increased adhesive friction. For example, polymers containing carboxyl or hydroxyl groups (e.g., PVA) generally have higher coefficients of friction. In contrast, materials with extremely low surface energy, such as PTFE (polytetrafluoroethylene), exhibit minimal friction.
Surface energy reflects the ability of surface molecules to attract external substances, originating fundamentally from the asymmetric charge distribution in polar functional groups. This asymmetry induces dipole responses in surrounding materials, exacerbating adhesive friction.
| Rubber Type | Surface Energy Range (mJ/m²) | Chemical Structure Features | Typical Applications |
|---|---|---|---|
| Natural Rubber (NR) | 25–35 | Non-polar polyisoprene chains | Tire treads, shock absorbers |
| Styrene-Butadiene Rubber (SBR) | 28–36 | Styrene-butadiene copolymer (weakly polar) | Automotive tires, conveyor belts |
| Nitrile Rubber (NBR) | 35–45 | Strongly polar cyano groups (-CN) | Fuel hoses, seals |
| Chloroprene Rubber (CR) | 38–42 | Polar chlorine atoms | Oil-resistant seals, cable sheaths |
| Fluoroelastomer (FKM) | 45–55 | Highly polar fluorine atoms | Aerospace seals, chemical piping |
| Silicone Rubber (VMQ) | 20–25 | Polar Si-O backbone with methyl side chains | High-temperature seals, medical devices |
| EPDM Rubber | 30–34 | Non-polar saturated structure | Construction waterproofing, automotive seals |
b. Chain Segment Mobility
The flexibility (mobility) of polymer chain segments directly determines a material’s response to frictional stress. Higher chain mobility results in softer materials, which are prone to stick-slip transitions during friction. This unstable dynamic process causes fluctuations in friction force, noise, and localized wear.
Conversely, rigid chain structures (e.g., aromatic rings or highly symmetric linear segments) significantly suppress localized chain movement, reducing such frictional instabilities. Thus, the molecular chain architecture governs the microscopic deformation mechanisms and energy dissipation patterns under frictional loading.
c. Crystallinity
Higher crystallinity in materials leads to rigid, dense, and ordered surfaces, typically resulting in lower coefficients of friction. This is because crystalline regions feature highly aligned chain segments with strong intermolecular forces, low surface roughness, and reduced effective contact area, thereby minimizing adhesive friction. Highly crystalline polymers like PA (polyamide) and PEEK (polyether ether ketone) exhibit excellent friction performance under dry conditions.
In contrast, amorphous polymers (e.g., PSU, PC) have disordered chain arrangements, making surfaces more susceptible to local adhesion, deformation, creep, and adsorption of moisture/particles, which increase friction. A useful analogy is that crystalline regions resemble microscopic “brick walls,” while amorphous regions resemble “molecular tangles,” leading to distinct frictional behaviors.
| Rubber Type | Crystallinity at RT (%) | Strain-Induced Crystallization | Structural Impact on Crystallinity |
|---|---|---|---|
| Natural Rubber (NR) | 25–35 | Strong (strain > 300%) | Regular cis-polyisoprene chains |
| Chloroprene Rubber (CR) | 10–18 | Moderate | Chlorine atoms disrupt chain regularity |
| Nitrile Rubber (NBR) | <5 | None | Cyano side groups hinder chain alignment |
| Butyl Rubber (IIR) | 15–20 | Weak | Dense methyl groups restrict chain motion |
| Fluoroelastomer (FKM) | 5–15 | Crystallizes at high temps | Fluorine atoms form ordered crystalline regions |
| EPDM Rubber | <1 | None | Fully amorphous structure |
Strategies to Enhance Rubber Wear Resistance
1. Incorporating Self-Lubricating Fillers
- Graphite, MoS₂: Layered structures with weak van der Waals forces between layers, enabling easy shear-induced slippage.
- PTFE Micropowders: Extremely low surface energy minimizes molecular adsorption, while friction-induced migration forms a “solid lubricating film.”
These fillers preferentially form a lubricating layer at the contact interface, akin to a “low-friction coating,” preventing direct adhesive contact. Some lubricants also exhibit migration behavior, actively spreading over the friction interface to replenish lubrication—similar to an “automatic oiling” mechanism.
2. Adding Reinforcing Fillers
- Talc, Carbon Nanotubes, Nano-Oxides (SiO₂, Al₂O₃)
Key Functions:
- Increase Surface Hardness: Strengthen the polymer surface to resist scratching or plastic deformation by counterfaces, reducing micro-interlocking and friction.
- Suppress Chain Mobility (Especially with Nanofillers): At the molecular scale, nanofillers create physical “bridges” or “barriers” between polymer chains, restricting localized chain relaxation and motion. This stabilizes surface rigidity and mitigates friction fluctuations caused by thermally activated chain motion (“stick-slip” phenomena).
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