When designing friction units or analyzing mechanical connections, there is often a need to know exactly how two aluminum surfaces will behave when interacting. Coefficient of friction between aluminum and aluminum - the value is not constant, but variable, depending on many operating conditions. Unlike steel or cast iron, light alloys are prone to adhesion, which can lead to scoring and jamming if not properly prepared.
In dry conditions, without the use of lubricants, this indicator can reach critical values, making the movement of parts impossible. However, introducing a lubricant or changing the surface roughness radically changes the picture. Engineers need to consider that bare metal behaves differently than oxidized or coated metal.
This article is intended to systematize scattered data and provide up-to-date information for calculations. We will consider the influence of various factors on the force of resistance to movement. This knowledge is critical for preventing accidents in structures where aluminum is the main structural material.
The physical nature of friction of aluminum alloys
The mechanism of interaction between two aluminum surfaces is based on the adhesion-deformation theory. Since aluminum is a soft metal, the actual contact area during compression quickly increases, approaching the nominal one. Adhesive interaction between metal atoms becomes the dominant resistance factor.
A special feature is instant oxidation in air. A thin film of aluminum oxide (Al2O3) typically reduces friction, protecting the bare metal from seizing. If this film breaks down under high pressure, “seizing” occurs—cold welding at the contact points. That's why sliding friction coefficient may jump sharply during node operation.
⚠️ Attention: Under high loads without lubrication, growths may form on the surface (hardening), which will lead to scuffing and irreversible damage to parts.
It is important to distinguish between static and dynamic friction. To break apart parts at rest, much more force is required than to maintain their movement. This is due to the fact that in a state of rest, microroughnesses have time to penetrate each other deeper.
Why does aluminum stick?
Aluminum has high chemical activity. At the micro level, clean surfaces tend to form strong interatomic bonds. If you remove the oxide film in a vacuum, the two aluminum parts will be welded instantly. In air, this process is inhibited by oxidation, but during friction the film breaks, and local welding still occurs.
Factors affecting the coefficient of friction
The value of the coefficient is influenced by a set of parameters, ignoring which will make any theoretical calculations useless. The condition of the surface plays a primary role. Roughness, the presence of microcracks and contamination change the interaction pattern.
The temperature regime also makes its own adjustments. When heated, the mechanical properties of the alloy change; it becomes softer, which increases the actual contact area. Temperature dependence especially relevant for piston groups and brake systems, where heating is inevitable.
There is a direct dependence on the pressure in the contact zone. At low loads, friction can be high due to the mechanical engagement of the asperities. At medium levels it decreases. At very high levels, it grows again due to setting.
- 🔹 Surface roughness: Grinding or polishing can either reduce or increase friction depending on the presence of lubrication.
- 🔹 Sliding speed: affects heat generation and behavior of the lubricating layer.
- 🔹 Presence of oxide film: natural protection against welding, but easily destroyed.
To reduce friction in sliding units without liquid lubrication, anodizing is often used. The hard anodic coating significantly increases wear resistance and reduces the coefficient of friction compared to bare metal.
Table values of friction coefficient
For engineering calculations, it is customary to use averaged data obtained experimentally. Below is a table showing the range of values depending on the conditions.
| Terms of interaction | Friction coefficient (min) | Friction coefficient (max) |
|---|---|---|
| Dry surfaces (clean) | 1.05 | 1.35 |
| Dry surfaces (oxidized) | 0.30 | 0.50 |
| Oil lubrication | 0.10 | 0.15 |
| Graphite lubrication | 0.15 | 0.25 |
As can be seen from the data, the presence of a lubricant reduces the resistance to movement significantly. However, it is worth remembering that with boundary friction, when the lubricating layer breaks, the values may tend to dry friction values.
The spread of values in the first row of the table is due to the purity of the experiment. In real conditions, it is difficult to achieve perfectly clean surfaces, but it is also impossible to completely eliminate oxidation. Therefore, for reliability calculations, the maximum value is usually taken.
For dry contact without special treatment, always use a factor of 1.35 to ensure a safety margin.
Effect of roughness and surface treatment
Paradoxically, perfect polishing does not always lead to reduced friction. Surfaces that are too smooth (cleanliness class above 9-10) may “stick” more strongly upon contact due to an increase in the contact area and molecular forces. Optimal roughness allows you to retain microscopic amounts of lubricant or wear products, working as an anti-friction additive.
Mechanical processing, such as milling or turning, creates a directional relief (mark). If the direction of movement coincides with the direction of the marks, friction will be lower. When moving sideways, the resistance will increase.
Heat treatment and hardening change the hardness of the surface layer. A harder alloy deforms less under load, which reduces the contact area and, accordingly, the friction force. However, fragility may increase.
The role of lubricants and coatings
Using lubricants is the most effective way to control friction. Oils and greases create a separating layer that prevents direct metal contact. For aluminum, it is important to select lubricants that do not cause corrosion and do not contain aggressive additives.
Solid lubricants such as molybdenum disulfide or graphite work even under extreme conditions. They fill the depressions of the microrelief and form a sliding film. Molybdenum disulfide (MoS2) Particularly effective at high pressures.
⚠️ Caution: Some synthetic lubricants may contain components that are corrosive to aluminum alloys. Always check the compatibility of the lubricant with a specific alloy (for example, D16T or AMg6).
Coatings such as Teflon or nickel-fluoroplastic compositions radically change the properties of the surface. They create an inert layer with a low coefficient of friction, which is independent of the oxide film of the base metal.
☑️ Selecting a lubricant for aluminum
Practical application in engineering
In the automotive and aerospace industries, the issue of friction is addressed in a comprehensive manner. Engine pistons, caliper guides, suspension components - wherever aluminum rubs against aluminum or steel, special solutions are used.
Combined friction pairs are often used, where one part is made of a material with a low coefficient of friction, and the other is made of a durable alloy. For example, installing bronze or brass bushings in aluminum housings.
When designing, it is necessary to provide clearances that take into account thermal expansion. Aluminum has a high coefficient of linear expansion. When heated, the gap may disappear, and dry friction will begin with all the ensuing consequences.
The force for shearing parts in an assembly is calculated using the formula where the friction force is equal to the product of the coefficient and the normal reaction force. An error in choosing a coefficient can lead to either jamming or, conversely, spontaneous disconnection of the unit.
How to calculate the shear force for an aluminum bushing?
To calculate, you need to multiply the mass of the part (in kg) by the acceleration of gravity (9.81 m/s²) and by the selected friction coefficient. For example, for a part weighing 10 kg and a coefficient of 0.3, the force will be: 10 9.81 0.3 ≈ 29.4 N. This is the minimum force required to start movement.
Can graphite grease be used on aluminum?
Yes, you can, but with caution. Pure graphite in the presence of moisture can create a galvanic couple with aluminum, accelerating corrosion. It is better to use modern compositions based on lithium or synthetic oils with the addition of molybdenum disulfide.
Why does anodized aluminum rub better?
Aluminum oxide obtained by anodizing has high hardness and a porous structure. The pores can retain lubricant, and the oxide surface itself is chemically inert and less prone to adhesion than pure metal.