Not long ago, an engineer asked me:
They have a POM gear, and the initial validation process has proceeded smoothly.
Two rounds of wear testing were conducted, revealing minimal dimensional changes and excellent surface condition, confirming that the material exhibits no significant wear issues.
However, during recent whole-machine lifespan tests conducted after reaching approximately 200,000 cycles, several samples began exhibiting tooth breakage at remarkably consistent locations.
There is currently some debate within the community: some attribute it to processing stress, others to structural design flaws, while still others suspect variations in material batches.
He asked a very good question-
If the material really had issues, why did the previous wear test yield such excellent results?
This is actually a very common issue, and many teams fall into it.
What it truly points to is not a specific parameter anomaly, but rather a fundamental cognitive misconception.
Why does a material that exhibits excellent wear resistance actually become more prone to fatigue failure under real-world operating conditions?
01
Many people get the "type of problem" wrong right from the start.
When most people encounter the description "low wear + subsequent fracture," their first reaction is to identify the abnormality.
Is this batch of material unstable? Are there issues with the processing? Or are the testing conditions inconsistent?
But if you break down the problem, you'll actually realize that...
Wear and fatigue are fundamentally not the same category of issues.
Wear, in essence, is a surface phenomenon.
You can think of it as the material surface being continuously "cut" and "scratched".
What materials are required to withstand this process?
It's simple: hard, dense, and difficult to cut through.
Citation: Study and Characteristics of Abrasive Wear Mechanisms
As you can see, many wear-resistant materials share these characteristics:
High crystallinity
High modulus
Low surface energy (with lubricating components)
In other words, wear tests the following:
Can the surface withstand external damage?
But fatigue follows a completely different logic.
Fatigue occurs within the material and is a cumulative process.
Each gear engagement is essentially a tiny cycle of bending and stretching.
It's fine for a single viewing, but the problem is...
This action will be repeated hundreds of thousands or even millions of times
What happens inside the material during this process?
The molecular chain undergoes repeated stretching and rebounding.
Irreversible deformation occurs in localized regions.
Microscopic defects gradually enlarge into cracks.
At first, you won't notice any changes.
But when the crack grows to a certain critical size-
The macro manifestation is: a sudden rupture.
Therefore, the key issue is not "why it broke," but rather:
Using "wear testing" to assess a "fatigue-dominated" operating condition from the outset is fundamentally inappropriate.
02
Why does "improved wear resistance" actually lead to more frequent fatigue failures?
Once you correctly categorize the issues, you'll observe a phenomenon that defies intuition yet is highly prevalent among high-scoring cases:
Many methods for improving wear resistance essentially compromise the material's fatigue resistance.
This is no coincidence; it's determined by structure.
1. The higher the crystallinity, the less mobile the chain segments become.
Why do materials such as POM, PBT, and PET exhibit excellent wear resistance?
The molecular chains are arranged in a highly compact and well-defined structure.
It's extremely difficult for outsiders to penetrate that environment.
But that's where the problem lies.
When subjected to periodic stress, the material must perform one task:
Energy is released through the microscopic movement of molecular chains.
However, the issue with the high-crystallinity structure is:
The segment is bound.
Free volume is small.
There is almost no buffer space locally
The result is:
When external force is applied, there's no place for it to dissipate-thus it concentrates solely in specific segments of the chain-and ultimately causes the main chain to break.
2. The filler often exacerbates the problem.
Many formulation engineers intuitively believe that:
For wear resistance → Add hardening filler
For example, fiberglass, carbon fiber, and inorganic powders.
In the short term, this approach is sound:
The surface is harder
Enhanced scratch resistance
But what happens to these elements under fatigue conditions?
stress raiser
The reason is simple:
Resin is "deformable".
The filler is "basically non-deformable".
Under repeated loading, the interface between the two components experiences significant shear stress.
What happens over time?
Interface delamination
Form microholes
The crack begins to propagate from the edge of the filler.
Finally, what you observe at the fracture surface is not a uniform fracture.
Rather, it is a typical case of fatigue crack that develops internally.
To summarize this section, it can be summed up in one sentence:
You aim to make the material "more scratch-resistant" and more "rigid," yet fatigue resistance precisely requires it to be "dynamic and capable of energy dissipation."
03
How can this contradiction be resolved from an engineering perspective?
When manufacturing components like gears, bearings, or sliding joints, customer requirements are typically straightforward:
It must neither be damaged nor broken!
So here's the problem:
How can we strike a balance between "rigidity" and "resilience"?
Approach 1: Instead of solely increasing hardness, first focus on reducing friction.
If you focus all your efforts on "improving the material's resistance to cutting",
It is easy to steer the system toward a trajectory of "high rigidity → low fatigue."
Try a different approach:
Instead of resisting friction directly, reduce the friction.
How exactly should this be done?
Add a silicone-based lubricating phase
Add ultra-high molecular weight PE micro-powder
Developing a self-lubricating system
The characteristics of these items are:
✔ Does not significantly increase the modulus
✔ However, it can form a low shear layer on the surface
The result is:
A portion of the external force is "slipped off" rather than "cut into".
The damage caused by fatigue is much less significant.
Approach ②: Develop an "energy consumption structure" rather than relying on a single material
This is precisely where many modification plants demonstrate their true technical expertise.
The core isn't "更强," but rather:
Provide a space within the material for energy dissipation.
The typical approach is:
As an alloy system, for example POM/TPU
POM offers rigidity and wear resistance.
TPU provides elasticity and energy dissipation.
When the crack propagates:
Once the flexible phase is encountered...
Energy is absorbed.
The cracks are passivated or even terminated.
You may sacrifice a small amount of wear resistance.
However, this resulted in a significant increase in life expectancy due to fatigue.
Approach 3: If the budget allows, use materials with the correct structure directly.
Some materials inherently resolve this contradiction through their molecular structure.
for example PEEK.
Its defining feature is not "a single indicator being particularly extreme."
Rather, it possesses the following structural characteristics simultaneously:
Rigid units (aromatic rings, ketone groups)
Flexible bond (ether bond)
The result is:
Both strong and wear-resistant
also exhibits certain segmental mobility
So you'll find that:
Many high-end dynamic components (used in aviation and medical applications),
Finally, after circling back, we return to these materials.
It's not because it's expensive, but because:
The "structural defects" of inexpensive materials are difficult to fully compensate for through formulation alone.
The fundamental issue with this type of problem isn't "using the wrong materials," but rather:
The evaluation system was used incorrectly.
Wear is the energy interaction between surfaces.
Fatigue is an accumulation of internal energy.
If you focus solely on optimizing a single metric within TDS,
It's easy to reach perfection in one dimension while completely losing control in another.
Truly experienced engineers don't focus on "optimizing a single metric"; instead, they aim to:
Find a balance point between rigidity, toughness, friction, and energy dissipation that ensures long-term performance.
If you've ever encountered this:
The initial testing proceeded smoothly; however, issues arose regarding the product's lifespan once it was put into use.
Especially that one-
The situation where "it hardly needed any grinding, but still broke."
This is most likely not a coincidence, but rather a typical fatigue-induced failure.
Take a close look at the fracture under a microscope-the answer is often already there.