Surface engineering, coatings and tribology testing

Dry sand rubber wheel test (ASTM G65)

The ASTM G65 test specification simulates moderate abrasion. Silica sand (~200 µm diameter) is used as the test abrasive, and the counterface is a compliant rubber rimmed wheel, so wear tends to be relatively mild. Sample geometry is 76.2 x 25.4 x 12.7 mm, although smaller sizes (length >40 mm, width >20 mm) can be accommodated. 

NPL can also modify ASTM G65 to enable testing with other abrasives and under wet or dry conditions. 

High stress abrasion resistance test

ASTM B611 simulates very severe abrasion and was developed for hard metals or cemented carbides. Alumina grit is pressed against a test sample by a rotating steel wheel under a load of 20 kg. The sample geometry is 40 x 20 x 5 mm.  

Micro-scale abrasion test

The micro-scale abrasion test simulates abrasion by fine particles under light load conditions. Silicon carbide abrasive (4 µm) is dripped between the sample and a rotating steel ball (normally 25 or 25.4 mm) under an applied load of 0.2 N. The volume of wear that occurs is calculated from optical measurements of the size of the crater produced by the abrasion on the surface of the sample.  

Micro-scale abrasion can be used to evaluate the wear resistance of coatings just a few micrometres thick, by measuring the size of the scar produced in the coating and the substrate after perforation has occurred. A flat sample is required, with a thickness of 5 mm and less than about 50 mm across. Tests are easier to carry out and measure with a polished sample.

Macro-scale scratch tests

Scratch testing involves pulling an indenter – typically a diamond tip – across the surface of a sample and investigating the material’s response. Originally developed to assess coating adhesion – where the coating was rapidly removed by spallation at a certain critical load – today, the scratch test is used to look at other aspects of a material, such as surface damage during testing. Typical ramping loads are in the range of 2-200 N, and the resulting damage is examined by optical microscopy.

The friction force and the acoustic emission generated as the indenter moves over the surface can also be recorded to determine the critical load of failure. A flat sample is required, with a thickness of 5 mm and less than about 50 mm across. Tests are easier to carry out and measure with a polished sample. 

Microtribology

Microtribology experiments are used to examine and model the response of materials to asperity size contacts, or to simulate single abrasion events with conditions relevant to particle abrasion. Computer-controlled sequences of experiments are carried out under different test loads or ramping load conditions to study the variation in damage with changes in the applied conditions. 

NPL has two microtribometers that simulate scratch damage and single abrasion events to coatings and surfaces using an indenter with a ramping load of <250 mN. This includes: 

• a laboratory benchtop system, for experiments under ambient atmosphere conditions; and  
• high resolution SEM in situ, for image or video sequencing of damage build-up during testing, which can be correlated with the microstructure of the material under examination.  

The probes can be diamond indenters with radii ranging from 1-200 µm, or made from steel or another appropriate material. A flat sample is required, with a thickness of 5 mm and less than about 50 mm across. Tests are easier to carry out and measure with a polished sample. 

Sliding wear tests involve either pressing a pin with a rounded end or a ball against a rotating disc (pin-on-disc testing) or moving it back and forth against a flat plate in a controlled manner (reciprocating wear). A large radius of curvature can be used, providing a wide area of contact with good alignment between the samples. Flat-on-flat contact geometry can also be used, but there are often issues in getting good alignment. Factors such as the type of motion, contact geometry, applied load and the environment surrounding the contact points, including lubrication, can all be controlled.  

Pin-on-disc (ASTM G99)

NPL has a pin-on-disc system for testing at loads up to 450 N and speeds up to 7.5 m/s for a 100 mm wear track diameter.  

The test system measures friction and wear displacement (the total movement of the pin towards the disc, giving a measure of wear to both pin and disc) continuously. It also incorporates additional instrumentation, including: 

• a line-scan camera that can deliver real-time optical images and videos of the wear surface with a 1 µm resolution; 

• a non-contact optical chromatic aberration probe that enables the wear to the pin and flat to be measured separately; and 

• a self-zeroing friction measurement system that eliminates drift in the measurement of friction. This is particularly relevant for accurate measurement of modern low friction coatings. 

Plint TE77 reciprocating testing (ASTM G133)

NPL’s fully equipped Plint Tribology TE77 reciprocating tribometer can carry out tests at frequencies of up to 50 Hz, stroke lengths of 25 mm and temperatures of up to 600 °C. Housed in an extracted enclosure, the system enables measurement of both friction and wear. Wear scars can be examined by profilometry, and optical and scanning electron microscopy. 

Erosion testing is the analysis of damage that occurs when a small object – such as a grain of sand, a particle of metal or oxide, or a droplet of water – strikes a surface and removes material. Testing involves either accelerating the particle into the surface, or moving the surface onto the particle, depending on the design of the apparatus. Typically, the mass change of the sample is measured, however, tests are often tailored to the specific requirements of the customer and may include other analytical techniques. Sample preparation and polishing services are also available. 

Gas borne particulate erosion (ASTM G76)

Low velocity, room temperature solid particle erosion is carried out with a gas borne particulate erosion test system that broadly follows ASTM G76. Typically – but not exclusively – alumina or garnet with a diameter of ~200 µm is used as the erodent, at particle velocities up to about 75 m/s. Mass loss is measured over time and used to calculate the erosion rate. The mechanisms of wear are evaluated post testing by optical, scanning electron and 3D optical microscopy. Flat samples smaller than about 50 x 50 mm are required. 

Solid particle erosion tests (ASTM G76-18 and ASTM G211-14(2020))

NPL’s solid particle erosion test system enables high velocity, high temperature particulate erosion testing at temperatures up to 600 °C and velocities up to 300 m/s. It incorporates in situ measurement of mass change and laser triangulation of wear volumes, avoiding the need for periodic sample cooling during testing. It therefore measures in situ, real-time mass and wear volume without cooling interruptions, and performs post-testing evaluation of the mechanisms of wear using optical, scanning electron and 3D microscopy. 

Water droplet erosion test (ASTM G73-10, ISO/TS 19392-2)

The water droplet erosion test is used to study the damage caused by water droplets on fast moving surfaces at speeds of up to 300 m/s for steam turbine and aero engine applications, and 100-150 m/s for wind turbines. Samples in a rotor arm pass through a jet of water droplets, adjusting the speed of the rotor to control the linear speed of the samples. Mass loss is measured throughout testing, using optical and scanning electron microscopy to study the wear scar and the evolution of damage.