Spray Coating Wear Resistance Testing: Methods, Standards, and Evaluation

Spray coating wear resistance testing evaluates how effectively thermally sprayed coatings, such as plasma, arc, flame, and HVOF coatings, resist abrasive, erosive, and sliding wear under demanding service conditions. Using internationally recognized methods such as ASTM G65, ASTM G99, ASTM G76, and ASTM D4060, accredited third-party laboratories quantify wear volume, wear rate, and friction behavior to support coating qualification, process optimization, and quality control. This article explains the sample requirements, testing items, standard test methods, applications, and frequently asked questions related to wear resistance testing of spray coatings.

Technical Overview

Spray coating refers to a family of surface engineering processes in which molten or semi-molten particles of metal, ceramic, cermet, or polymer materials are propelled onto a prepared substrate to form a protective layer. Common processes include flame spraying, wire arc spraying, atmospheric plasma spraying (APS), and high-velocity oxy-fuel (HVOF) spraying. Typical wear-resistant coating systems include WC-Co, WC-CoCr, Cr2O3, Al2O3-TiO2, Mo-based alloys, and Ni-based self-fluxing alloys.

Wear resistance is one of the most critical performance attributes of spray coatings because coated components are routinely exposed to abrasive particles, erosive media, sliding contact, fretting, and slurry impingement. Wear testing is therefore essential for coating qualification, supplier comparison, process parameter optimization, and failure analysis. The principal wear mechanisms evaluated in the laboratory include:

• Three-body and two-body abrasion caused by hard particles sliding or rolling between surfaces
• Solid particle erosion from high-velocity impact of airborne or gas-borne particles
• Adhesive sliding wear at tribological contact interfaces
• Slurry abrasion in wet, particle-laden fluids
• Cavitation erosion from collapsing vapor bubbles in liquids

Relevant standards are published by ASTM International, ISO, and the Chinese national standards body (GB series). Because wear behavior is highly system-dependent, test results are meaningful only when the test method, counterbody, abrasive medium, load, velocity, and environment are clearly documented and controlled.

Testing Samples

Flat Coupons and Disc Specimens

Flat plates (commonly 50 mm x 50 mm x 5 mm) and cylindrical discs (commonly 25 mm to 30 mm in diameter, 5 mm to 8 mm thick) are the standard sample formats for rubber-wheel abrasion, pin-on-disk, and Taber testing. The coating must be applied at a controlled thickness, typically 0.2 mm to 0.5 mm or greater for hard cermet coatings, and the test surface is usually ground and polished to a roughness of Ra 0.8 µm or better to eliminate surface-finish effects from the wear measurement.

Cylindrical Specimens

Rod or tube specimens coated on the outer diameter are used for block-on-ring, reciprocating, and service-simulative tribotests. These samples are particularly relevant for hydraulic piston rods, pump shafts, and printing rolls, where the counterface geometry differs from that of a flat disc.

Actual Components and Parts

Finished or near-finished parts such as valve balls, gate seats, impellers, and landing-gear cylinders can often be tested directly when the fixture permits. Testing real components captures the influence of edge quality, coating build-up distribution, and final surface finishing on wear performance.

Reference and Comparison Specimens

Baseline samples, such as uncoated substrate material, electroplated hard chrome, or a competitor coating, are frequently tested in the same run under identical conditions. Because wear data from different laboratories and rigs are not directly comparable, same-run reference testing provides the most reliable basis for ranking coating systems.

For all sample types, preparation before testing includes ultrasonic cleaning in acetone or alcohol, drying, identification marking on the non-test surface, and weighing on an analytical balance with a resolution of 0.1 mg. Coating thickness and surface roughness should be measured and recorded before each wear test.

Testing Items

Wear resistance testing of spray coatings typically covers the following measured and derived quantities:

• Mass loss and volume loss of the coating after a defined test duration or number of cycles
• Specific wear rate, k, expressed in mm3/(N·m), normalized by applied load and sliding distance
• Wear depth and wear scar profile, measured by stylus or optical profilometry
• Wear scar width and cross-sectional area for pin-on-disk and erosion tests
• Friction coefficient as a function of sliding distance or time
• Taber wear index (mass loss per specified number of cycles) for rotary abrasion
• Erosion rate expressed as volume loss per unit mass of impinging particles (mm3/g)
• Cavitation erosion rate as cumulative mass or volume loss versus exposure time
• Surface roughness change before and after testing
• Post-test examination of the wear track by optical microscopy, SEM, and EDS to identify wear mechanisms such as micro-cutting, plowing, coating pull-out, or delamination
• Complementary characterization including coating microhardness, porosity, and adhesion strength, which are commonly correlated with wear performance

Testing Methods

Dry Sand/Rubber Wheel Abrasion (ASTM G65)

In the dry sand rubber-wheel test, the coated specimen is pressed against a rotating rubber-rimmed wheel while AFS 50/70 quartz sand is fed into the contact zone, producing three-body low-stress abrasion. Typical parameters include a wheel speed of approximately 200 rpm, applied loads of 45 N to 130 N depending on the procedure selected, a sand feed rate of about 300 g/min to 400 g/min, and test durations of up to 6000 revolutions. Wear is reported as volume loss in mm3, calculated from mass loss and coating density. This method is widely used to qualify HVOF WC-Co and WC-CoCr coatings as replacements for hard chrome plating.

Sliding Wear - Pin-on-Disk Testing (ASTM G99, GB/T 12444, ISO 20808)

A stationary ball or pin, typically alumina, hardened steel, or WC, slides against the rotating coated disc under a defined normal load, commonly 1 N to 20 N, at sliding speeds of 0.1 m/s to 1 m/s over total distances of several hundred meters to several kilometers. Volume loss is determined from mass loss or from wear-scar profilometry, and the specific wear rate is calculated for both the coating and the counterbody. The continuous friction coefficient trace reveals running-in behavior and the onset of coating instability. Reciprocating ball-on-flat testing (ASTM G133) is used where oscillating contact better represents service conditions.

Rotary Taber Abrasion (ASTM D4060, ASTM D1044)

The Taber abrader subjects the coated surface to rotating abrasive wheels under a defined dead-weight load, commonly 500 g to 1000 g per wheel, with CS-10 (resilient) or CS-17 (aggressive) wheels and optional resurfacing discs. Results are expressed as mass loss or as a wear index per 1000 cycles. This method is particularly suited to thinner sprayed coatings and organic or metal-polymer overlays where material removal rates are moderate.

Solid Particle Erosion (ASTM G76)

A pressurized gas stream carrying angular alumina particles, typically 50 µm in size, is directed at the coating surface at controlled velocities, commonly 70 m/s to 170 m/s, and at impact angles of 30° and 90°. Erosion resistance is reported as erosion rate in volume loss per gram of erodent. This test is essential for coatings on gas turbine components, boiler tubes, and fan blades, where high-velocity particulate impact dominates material degradation.

Micro-Scale Abrasion / Ball Cratering (ISO 26424)

A rotating steel ball rolls against the coating in the presence of a fine diamond or silicon carbide slurry, generating a shallow spherical crater. Measuring the crater diameter allows calculation of the specific wear rate and, when the coating is perforated, simultaneous determination of coating thickness. The method requires only small flat areas and is well suited to hard, thin sprayed coatings.

Cavitation Erosion (ASTM G32)

The specimen face is immersed in water and exposed to cavitation generated by an ultrasonic horn, typically oscillating at 20 kHz with a peak-to-peak amplitude of about 50 µm at a controlled temperature near 25 °C. Cumulative mass loss is plotted against exposure time, and the maximum erosion rate is derived from the steady-state region. This method is applied to coatings for ship propellers, pump impellers, and water-turbine runners.

Good laboratory practice requires testing in a controlled environment of 23 °C ± 5 °C, cleaning and reweighing specimens immediately after testing using the identical cleaning procedure applied before testing, and testing a minimum of two or three replicate specimens per condition to account for scatter. The final report should document the coating system and process, substrate and surface preparation, coating thickness and hardness, complete test parameters, raw mass-loss data, calculated wear rates, friction behavior, and photographic or SEM evidence of the wear mechanisms observed.

Applications

Wear resistance testing of spray coatings supports design, qualification, and quality control across many industries:

• Aerospace: HVOF WC-CoCr coatings on landing gear, actuator rods, and mid-span shrouds of turbine blades, replacing electroplated hard chrome
• Oil and gas: plasma and HVOF coatings on valve gates, balls, and seats; drill tool joints; and rod couplings exposed to abrasive slurries and sand-laden fluids
• Power generation: erosion-protective coatings on boiler tubes, fan blades, steam turbine components, and water-turbine runners
• Automotive and heavy equipment: arc-sprayed and plasma coatings on cylinder liners, piston rings, synchronizer rings, and earthmoving wear parts
• Paper, printing, and textile machinery: ceramic and cermet coatings on dryer rolls, calender rolls, and thread guides subjected to sliding and fibrous abrasion
• Hydraulics and fluid handling: hardened coatings on piston rods, plungers, and pump shafts, qualified through pin-on-disk and rubber-wheel abrasion testing
• Mining and mineral processing: slurry abrasion qualification of coatings for chutes, screens, mixer components, and slurry pump wet ends

FAQ

Which standard is most commonly used for the wear resistance of thermal spray coatings?

ASTM G65 dry sand/rubber wheel abrasion is the most widely specified method for qualification of wear-resistant sprayed coatings such as HVOF WC-CoCr. For sliding wear, ASTM G99 pin-on-disk is the most common complement. The appropriate choice depends on the dominant wear mechanism expected in service.

Can wear resistance be judged from hardness alone?

No. Hardness correlates with abrasion resistance only within similar material classes. Coating porosity, microstructure, residual stress, adhesion, and the toughness of the carbide phase strongly influence wear behavior, so standardized tribological testing is necessary for reliable performance assessment.

How is the specific wear rate calculated?

The specific wear rate is obtained by dividing the measured wear volume by the product of the normal load and the sliding distance, expressed in mm3/(N·m). This normalization allows meaningful comparison between coatings tested at different loads or test durations, provided the wear mechanism remains unchanged.

What coating thickness and surface finish are required for testing?

Coating thickness should be sufficient that the wear scar remains entirely within the coating layer during the test, typically at least 0.2 mm for abrasive tests on hard coatings. The test surface is normally ground to Ra 0.8 µm or finer so that measured material loss reflects intrinsic wear resistance rather than initial surface texture.

Can coated parts be tested without cutting samples from them?

Yes, many wear rigs can accommodate actual components directly or through custom fixtures, and portable micro-abrasion testers allow on-site cratering tests. When direct testing is impractical, witness coupons coated in the same production run are used as the basis for acceptance.

How many samples should be tested?

A minimum of two to three replicate specimens per condition is recommended because wear data are inherently scattered. Testing a reference material, such as uncoated substrate or hard chrome, in the same run further improves the reliability of comparative conclusions.