Dry Friction Surface Roughness Influence Test: Evaluating the Effect of Surface Texture on Unlubrica

The dry friction surface roughness influence test is a tribological evaluation that quantifies how surface texture parameters such as Ra, Rq, and Rz affect the coefficient of friction and wear behavior of engineering materials under unlubricated sliding contact. Performed with standardized tribometers such as pin-on-disc, ball-on-flat, and block-on-ring systems in accordance with ASTM and ISO standards, this test delivers essential design data for bearings, brakes, seals, fasteners, and other components operating in dry contact. This article explains the test principles, specimen preparation, measured parameters, standard methods, industrial applications, and frequently asked questions.

Technical Overview

Dry friction, also referred to as unlubricated or solid friction, occurs when two solid surfaces slide against each other without an intentional lubricant film separating them. Under these conditions, friction and wear are governed by the interaction of surface asperities, the microscopic peaks and valleys that constitute surface texture. Because dry contact is concentrated at a small number of asperity junctions rather than distributed over the nominal contact area, even modest changes in surface roughness can produce significant changes in frictional force, heat generation, and material removal.

According to the classical Bowden and Tabor model, the dry friction force consists of an adhesion component, arising from the shear strength of welded asperity junctions, and a plowing or deformation component, arising when harder asperities cut or plastically displace the counterface. The adhesion component tends to dominate on very smooth surfaces, where the real contact area is large, while the plowing and abrasion components dominate on rough surfaces, where sharp asperities mechanically excavate the softer material. As a result, the relationship between initial roughness and the coefficient of friction (COF) is frequently non-monotonic and often exhibits a minimum at an intermediate, application-specific roughness level.

The dry friction surface roughness influence test is designed to characterize this relationship systematically. Specimens are prepared with a controlled series of surface finishes, subjected to dry sliding against a defined counterface under fixed load and speed, and evaluated for friction coefficient, wear volume, and roughness evolution. The resulting data support surface finishing decisions, running-in strategies, material pairing selection, and durability predictions for components that must operate without liquid lubrication.

Key Physical Mechanisms Investigated

• Real contact area formation at asperity summits and its dependence on roughness amplitude parameters such as Ra and Rq.
• Plowing and micro-cutting wear caused by sharp asperities on ground or turned surfaces.
• Adhesive friction and galling risk on polished surfaces with high real contact area.
• Running-in behavior, in which tribo-surfaces evolve toward an equilibrium roughness regardless of the initial finish.
• Third-body effects, including wear debris trapping in roughness valleys and the formation of transfer films.
• Frictional heating and its feedback on material hardness, oxidation, and asperity flattening.

Testing Samples

Metallic Material Couples

Metal-on-metal pairs are the most common subjects of dry friction roughness studies because they exhibit strong sensitivity to surface finish. Typical combinations include hardened bearing steel against bearing steel, cast iron against steel, aluminum alloys against steel, copper alloys against steel, and stainless steel against stainless steel. Disc specimens are commonly 30 to 60 mm in diameter and 5 to 10 mm thick, while stationary partners take the form of pins with 4 to 10 mm diameter flat or spherical ends, or balls of 3 to 10 mm diameter.

Polymer and Composite Materials

Engineering polymers such as PTFE, POM, PA66, PEEK, and UHMW-PE are frequently tested against polished or ground steel counterfaces to simulate dry-running bushings and sliding guides. Composite materials, including bronze-filled or carbon-fiber-reinforced polymers, sintered bronze, and metal-backed polymer liners, are also evaluated. Polymer tests are especially sensitive to counterface roughness because the polymer preferentially smooths or transfers onto the steel surface, and the counterface finish strongly controls whether a beneficial transfer film forms.

Ceramics, Coatings, and Hard Layers

Advanced ceramics such as alumina and silicon nitride, thermal spray coatings, PVD/CVD hard coatings, and diamond-like carbon (DLC) layers are tested to quantify how deposition texture and post-deposition polishing influence dry friction. For these systems, roughness control is often the decisive factor in achieving low-friction transfer-free sliding.

Surface Preparation for Roughness Series

• Grinding with progressively finer abrasive grits to produce target Ra levels, for example 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 micrometers.
• Polishing with diamond or alumina suspensions for mirror finishes below Ra 0.05 micrometers.
• Turning or milling with controlled feed to generate deterministic textures with known Ra and Rz values.
• Degassing, ultrasonic cleaning in acetone or ethanol, and drying to remove residual fluids before testing.
• Verification of every specimen with a stylus or optical profilometer in accordance with ISO 21920 (formerly ISO 4287) before the tribological test begins.

Testing Items

A comprehensive dry friction roughness influence study reports the following measured and derived quantities for each roughness level in the test series:

• Static coefficient of friction, determined at the onset of sliding.
• Kinetic (dynamic) coefficient of friction as a continuous function of sliding time or sliding distance.
• Running-in duration and the transition behavior from initial to steady-state friction.
• Steady-state average COF and friction force or friction torque traces.
• Wear volume and mass loss of both the specimen and the counterface.
• Specific wear rate (wear coefficient), expressed in mm³/(N·m), calculated using the Archard relationship.
• Initial and post-test roughness parameters, including Ra, Rq, Rz, Rsk (skewness), Rku (kurtosis), Rmr (material ratio), and areal parameters Sa and Sq where 3D profilometry is available.
• Contact surface temperature rise measured by embedded thermocouples or infrared sensing.
• Wear mechanism classification based on optical microscopy, scanning electron microscopy, and energy-dispersive spectroscopy of wear tracks and wear debris.
• Critical roughness thresholds for transitions between mild wear, abrasive wear, adhesive wear, and severe galling or scuffing.

Testing Methods

Applicable Standards

• ASTM G99, Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus, the primary method for rotary dry sliding tests.
• ASTM G133, Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear, suitable for fretting and reciprocating dry contact.
• ASTM G115, Standard Guide for Measuring and Reporting Friction Coefficients, which governs friction data quality and reporting.
• ASTM G77, Standard Test Method for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear Test.
• ISO 20808, Fine ceramics determination of friction and wear characteristics of monolithic ceramics by ball-on-disc method.
• ISO 6601, Plastics friction and wear by sliding against a steel ball on a flat specimen.
• GB/T 3960, Chinese national standard for determining friction and wear properties of plastics in sliding contact.
• ASTM D1894 and ISO 8295 for coefficient of friction of plastic films and sheeting, applicable to flexible dry-contact materials.
• ISO 21920 (successor to ISO 4287 and ISO 4288) for surface roughness measurement, profiling, and parameter definitions.

Typical Test Configurations

Rotary pin-on-disc and ball-on-disc rigs are the most widely used configurations. A stationary pin or ball is pressed against a rotating disc under deadweight or pneumatic normal loading while a force transducer records the tangential friction force. Reciprocating ball-on-flat rigs are preferred for short stroke amplitudes, oscillatory motion, and the study of early running-in. Block-on-ring rigs apply line contact and are used for ranking materials under higher contact stress, while inclined-plane or horizontal friction sled testers are used for static COF determination on flat couples.

Representative Test Conditions

• Normal load: typically 1 to 50 N for point contact, up to several hundred newtons for conformal block-on-ring contact.
• Sliding speed: commonly 0.01 to 1.0 m/s, selected to represent the intended service condition and avoid uncontrolled flash temperature.
• Sliding distance: 100 to 5,000 m, or fixed durations from 10 minutes to several hours, sufficient to reach steady state.
• Environment: laboratory air at 23 ± 5 °C and 50 ± 10 % relative humidity unless a dry atmosphere or elevated temperature is specified; no lubricant is applied at any stage.
• Replicates: a minimum of three valid runs per roughness level, with fresh or re-conditioned counterfaces for each run.

Test Procedure Summary

• Prepare the roughness series specimens, measure and record initial roughness on the actual wear track location, and document the texture direction relative to sliding.
• Clean all specimens ultrasonically, dry them, and weigh them on an analytical balance with 0.1 mg resolution.
• Mount the specimens in the tribometer, verify alignment, and confirm zero-load friction of the fixture is negligible.
• Apply the normal load and set the rotational speed or stroke parameters, then acquire friction force, time, and temperature data continuously.
• Terminate the run at the defined sliding distance, allow cooldown, and re-weigh and re-profile the worn surfaces.
• Compute average steady-state COF, wear volume by mass difference or profilometric wear scar cross-section, and specific wear rate, then plot friction and wear against initial Ra, Rq, and Rz to identify the roughness optimum and mechanism transitions.
• Examine wear tracks by microscopy to confirm the dominant mechanism for each roughness level and document any transfer film or debris bed.

Data Interpretation

Results are typically presented as COF versus sliding time curves for each roughness level, COF and wear rate versus initial Ra on scatter plots, and before/after roughness comparisons. A classic outcome is a U-shaped COF curve: very smooth surfaces show elevated adhesion-driven friction and galling tendency, intermediate finishes give the lowest steady-state friction after a short running-in, and rough finishes sustain high abrasive friction and plowing wear. Deviations from this pattern indicate material-specific effects such as polymer transfer film formation, oxide film lubricity in steels, or graphitic layer formation in cast irons.

Applications

• Automotive engineering: brake pad and disc pairing optimization, clutch facing development, and dry sliding components where finish specifications directly control friction stability and judder.
• Dry-running and self-lubricating bearings: selecting counterface finish ranges that maximize the life of PTFE, POM, and composite bushings in machinery that cannot accept lubricants.
• Seals and gaskets: defining shaft and rod surface finishes that balance seal wear against leakage control in reciprocating and rotary seals.
• Fasteners and bolted joints: measuring friction coefficients on fastener bearing faces and threads as a function of coating roughness for accurate torque-tension prediction.
• Manufacturing processes: die surface finishing in metal forming to reduce galling, and cutting tool surface conditioning to manage chip friction and built-up edge.
• Railway systems: wheel-rail interface studies, where surface conditioning of rails under dry contact governs traction, wear, and rolling contact fatigue.
• Precision instruments and electronics: connector contact finishes, textile machinery guides, and printer or scanner feed rollers requiring repeatable dry friction.
• Quality control and supplier qualification: incoming inspection of surface finishes against friction-related specifications, verified by accredited third-party laboratories when independent certification is required.

FAQ

Why does surface roughness change the dry friction coefficient?

Roughness controls the real contact area and the deformation mode at asperity junctions. Smoother surfaces increase the real contact area and raise the adhesion component of friction, while rougher surfaces increase plowing and micro-abrasion. Because the total friction is the sum of these components, the COF typically passes through a minimum at an intermediate roughness.

Is a smoother surface always better for dry sliding?

No. Very smooth, polished surfaces under dry conditions can promote adhesive wear, scuffing, and even cold welding between similar metals, particularly steels. Many dry tribosystems perform best with a moderately finished surface that traps beneficial debris or transfer film while limiting asperity interlocking.

Which roughness parameters should be reported besides Ra?

Ra alone is insufficient to describe tribological texture. Rz captures peak-to-valley height, Rq reflects RMS amplitude and is sensitive to outliers, Rsk indicates whether the surface is plateaued or spiky, Rku describes sharpness of the asperity height distribution, and Rmr expresses the load-bearing material ratio. For advanced studies, areal parameters Sa and Sq from 3D profilometry are recommended.

What is running-in and why does it matter in this test?

Running-in is the initial period in which asperity peaks are flattened or sheared and the surface converges toward an equilibrium roughness. During running-in, friction is unstable and wear rate is at its highest. Roughness influence tests must therefore distinguish initial transient friction from steady-state friction, and the test duration must be long enough to reach steady state for the comparison to be valid.

How are test load and speed chosen?

Load and speed should reproduce the contact pressure and sliding velocity of the intended application, often guided by ASTM G115 reporting requirements. Excessive speed under dry contact generates flash temperatures that alter the wear mechanism, so a preliminary scoping run is advisable when no service data exist.

How reproducible are the results?

With disciplined control of roughness, cleaning, humidity, and counterface condition, inter-laboratory COF scatter of roughly 10 to 25 percent is typical for dry sliding tests. Friction is a system property, not a material constant, so results are always reported together with the complete contact conditions, and replicate testing at every roughness level is mandatory.

Can this test be combined with lubricated or high-temperature conditions?

Yes. Tribometers used for dry roughness studies are frequently equipped with lubricant reservoirs, environmental chambers, or heating stages. Comparing dry baseline data from this test with lubricated or elevated-temperature results helps isolate the specific contribution of surface texture across operating regimes.