Coating Abrasion Resistance Testing by the Falling Sand Method (ASTM D968): Principles, Procedures,

Coating abrasion resistance testing by the falling sand method, standardized in ASTM D968, is a widely used laboratory procedure that measures the volume of falling abrasive required to wear through an organic coating to its substrate. The result, expressed in liters of abrasive per mil (25.4 µm) of dry film thickness, provides a quantitative index of a coating's ability to withstand erosion-type wear. This article explains the test principle, sample requirements, apparatus, step-by-step procedure, calculation of results, typical applications, and answers to frequently asked questions for manufacturers, coating formulators, and quality engineers.

Technical Overview

The falling sand method (also called the falling abrasive or sand-drop test) is a classical laboratory technique for evaluating the abrasion resistance of organic coatings such as paints, varnishes, lacquers, enamels, and powder coatings. In this test, a dry, graded abrasive—natural silica sand (Method A) or silicon carbide (Method B)—flows under gravity from a hopper through a vertical guide tube and impinges on a coated panel mounted at a 45-degree angle. The abrasive stream is allowed to fall until the coating is worn through and the substrate becomes visible at the point of impact. The quantity of abrasive consumed, converted to volume and normalized to film thickness, is reported as the abrasion resistance value in liters per mil (L/mil) of coating. A higher value indicates greater resistance to abrasive wear.

The principal standard governing this method is ASTM D968 (Standard Test Methods for Abrasion Resistance of Organic Coatings by Falling Abrasive). Related and complementary abrasion tests include the Taber rotary abrasion method (ASTM D4060, ISO 7784-2), wet scrub resistance for architectural coatings (ASTM D2486, ISO 11998), and gravel impact chipping (ASTM D3170). Falling sand procedures of similar design are also referenced in various national and industry product specifications. Because of its simple, low-cost apparatus and straightforward operation, the falling sand method remains a popular quality-control and formulation-screening tool, particularly for ranking coatings of the same generic type against one another or against a specified acceptance limit.

• Wear mechanism simulated: erosion by free-falling hard particles striking the surface at 45 degrees
• Primary standard: ASTM D968 (Method A—silica sand; Method B—silicon carbide)
• Reported result: liters of abrasive per mil (25.4 µm) of dry film thickness
• Typical use: comparative ranking, specification compliance, formulation development, and failure analysis

Testing Samples

Standard Flat Panels

Most falling sand tests are performed on flat, rigid panels of approximately 100 mm × 100 mm (4 in. × 4 in.). Common substrates include cold-rolled steel, tinplate, galvanized steel, aluminum, and glass. The coating is applied by the specified method—typically spray application, drawdown, or coil coating—to achieve a uniform dry film thickness representative of the intended product, and the panels are fully cured according to the coating system's documented schedule before testing.

Coated Components and Product Samples

Actual production parts can also be evaluated provided they contain a sufficiently flat region that can be clamped in the specimen holder at the required angle and position. Sections cut from extrusions, coil-coated sheet, appliance panels, furniture boards, or plastic trim parts are regularly tested in accredited third-party laboratories using this approach. Strongly curved or complex three-dimensional surfaces are generally unsuitable because the abrasive stream must strike a planar area at the standard geometry.

Sample Preparation and Conditioning

• Minimum recommended quantity: at least three replicate panels per coating to allow averaging and assessment of scatter
• Dry film thickness measured at several points on each panel (for example, per ASTM D7091 for magnetic substrates or equivalent non-destructive methods for non-magnetic substrates)
• Conditioning at standard laboratory atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity, for a defined period prior to testing
• Panels must be clean, free of defects such as bubbles, craters, or unmelted particles at the test location, since flaws accelerate premature wear-through

Testing Items

A complete falling sand abrasion evaluation typically includes the following items, which together determine the reported abrasion resistance value:

• Falling sand abrasion resistance, expressed as liters of abrasive per mil (L/mil or L/25.4 µm) of coating thickness
• Dry film thickness determination at the wear location and over the whole panel
• Verification of the wear-through endpoint, i.e., continuous exposure of the substrate at the impact spot
• Abrasive flow rate calibration and, where volume is derived from mass, measurement of the abrasive's bulk density
• Documentation of abrasive type, grading, and condition, together with ambient laboratory conditions during testing

Testing Methods

Apparatus and Abrasive Media

The test apparatus consists of an abrasive hopper or reservoir with a control gate, a straight vertical guide tube with an internal diameter of 19 mm (0.75 in.) and a length of approximately 914 mm (36 in.), and a specimen holder that clamps the coated panel so that the abrasive stream strikes the surface at a 45-degree angle, with the impact point located approximately 25 mm (1 in.) below the lower end of the tube. A balance readable to 0.1 g, a timer, calibrated sieves for verifying abrasive grading, and a magnifier for inspecting the wear spot complete the setup. Because silica dust presents an inhalation hazard, the apparatus should be operated within an enclosure with appropriate dust extraction, and operators must use suitable respiratory protection.

For Method A, the abrasive is a natural silica sand of specified grading, commonly retained between 600 µm and 850 µm sieves, with rounded to sub-rounded particles. For Method B, a sharper and more aggressive silicon carbide abrasive of specified fineness is used. The two abrasives give different numerical results and are not interchangeable; the method used must always be reported. Abrasive should be dry, clean, and consistent between runs, since moisture uptake, contamination, or reuse of spent abrasive can alter the flow rate and cutting action.

Test Procedure

• Condition the panels and measure the dry film thickness at multiple points; record the thickness in the area to be abraded.
• Verify the apparatus: confirm the guide tube is vertical and undamaged, the specimen holder geometry is correct, and the flow rate meets the standardized target by collecting the abrasive over a timed interval (flow is typically on the order of 0.6–0.8 kg per minute for graded sand).
• Clamp the specimen at 45 degrees with the test area centered under the tube outlet, maintaining the specified distance between the tube end and the panel surface.
• Charge the hopper and open the gate so a continuous, centered stream of abrasive falls onto the coating; keep the hopper filled to maintain a constant head and steady flow throughout the run.
• Periodically interrupt the flow, brush loose abrasive from the panel, and inspect the impact spot under a magnifier with strong illumination.
• Stop the test at the endpoint, defined as a continuous wear-through of the coating to the substrate over a spot approximately 4 mm in diameter at the impact area.
• Weigh the total abrasive consumed, or collect and measure its volume directly in a graduated cylinder, and record the value for the panel.
• Repeat the determination on the remaining replicate panels and calculate the mean abrasion resistance.

Calculation and Expression of Results

The abrasion resistance value is obtained by dividing the volume of abrasive used by the dry film thickness expressed in mils. In practical terms, if the abrasive volume is measured by mass, it is first converted to liters using the measured bulk density of the abrasive; the liters per mil value is then calculated as the abrasive volume in liters multiplied by 25.4 and divided by the film thickness in micrometers. Results are reported as the average of the replicate determinations, together with the individual values, the abrasive type and grading, the flow rate, the film thickness, and the test standard applied. When silicon carbide is used (Method B), substantially lower volumes are expected than with sand, so the values must never be compared across methods.

Factors Influencing Results

• Abrasive mineralogy, particle shape, grading, moisture content, and whether fresh or reused material is employed
• Consistency of the flow rate and hopper head, and the verticality and bore condition of the guide tube
• Accuracy and uniformity of the dry film thickness, which is one of the largest sources of error since results are normalized to thickness
• State of cure: under-cured or soft films wear through rapidly and give misleadingly low values
• Substrate hardness and operator judgment in identifying the wear-through endpoint
• Conditioning temperature and humidity before and during the test

Comparison with Other Abrasion Test Methods

It is important to recognize that the falling sand method simulates erosion by loose particles, whereas the widely used Taber rotary abrasion test (ASTM D4060, ISO 7784-2) produces sliding wear under a loaded grinding wheel and reports mass loss per 1000 cycles. Wet scrub tests (ASTM D2486, ISO 11998) evaluate architectural wall paints under wet cleaning conditions, and gravel impact devices (ASTM D3170) assess chip resistance of automotive coatings. Because each method imposes a distinct wear mechanism, numerical results are not transferable between methods, and the test specified in the product standard or purchase specification should always govern. In practice, the falling sand method is favored for its low equipment cost, minimal maintenance, and suitability for hard, glossy, high-build industrial coatings, while Taber testing is often preferred where sliding abrasion is the service-relevant mechanism.

Applications

Falling sand abrasion testing is applied across many industries wherever coated surfaces are exposed to dust, grit, repeated cleaning, or handling wear:

• Coil coatings for architectural metal panels, roofing, and cladding exposed to wind-blown dust
• Appliance and HVAC finishes, including liquid and powder coatings on steel and aluminum
• Wood coatings for furniture, cabinetry, joinery, and flooring lacquers
• Architectural coatings for window frames, trim, and exterior maintenance paints
• Automotive refinish and component coatings, plastics coatings, and hardware finishes
• Industrial maintenance coatings for structures and equipment operating in dusty or abrasive environments
• Research and development: optimization of binder type, pigment volume concentration, hard fillers, crosslink density, and clearcoat technology
• Quality control, supplier qualification, incoming inspection, and comparative failure analysis of field-returned samples

Frequently Asked Questions

What does the liters-per-mil result mean, and what is a good value?

The result expresses how many liters of abrasive are needed to wear through one mil (25.4 µm) of coating under the standardized conditions. Higher values indicate better abrasion resistance. Absolute numbers vary widely with coating chemistry—weak decorative films may register only a few liters per mil, while high-performance industrial or coil coatings can exceed one hundred liters per mil—so values should always be judged against the specification or against a reference coating of the same generic type tested in parallel.

Are sand (Method A) and silicon carbide (Method B) results interchangeable?

No. Silicon carbide is harder and sharper than silica sand and cuts through films considerably faster, producing lower volumes per mil. The two methods yield systematically different numbers, and the report must state which abrasive was used. Cross-comparison of sand and silicon carbide results is not valid.

How many samples are required for a reliable determination?

A minimum of three replicate coated panels is recommended so that an average and a measure of scatter can be reported. Because the endpoint depends partly on operator observation, replication is essential for defensible results, especially in disputed or certification contexts.

Can finished products be tested instead of laboratory panels?

Yes, provided the part has a flat region large enough to be clamped at the required 45-degree orientation with the impact point correctly positioned under the guide tube. Severely curved or textured surfaces are not suitable for the standard geometry.

How long does the test take?

The abrasion run itself may last from a few minutes for soft, thin films to an hour or more for hard, thick, high-performance coatings, since abrasive must often be replenished several times to reach wear-through. Including conditioning, film thickness measurement, and replicate determinations, the complete laboratory evaluation is typically completed within a few working days.

Is the falling sand result a direct prediction of service life?

No single laboratory abrasion test predicts field life exactly. The falling sand method is best regarded as a comparative tool for ranking coatings, verifying specification compliance, and supporting formulation decisions. Correlation with actual service performance should be established for each product family and exposure environment.