Surface engineering and coatings

An extensive range of expertise and facilities

NPL has the capability for a wide range of tribological testing, coupled with facilities and expertise for the evaluation of tribologically based failures. 

Tribology involves the contact between materials moving against one another under load, is complex and there are many different modes of wear that are possible. Different tests need to be applied to simulate these different modes of wear. Often more than one mode of wear occurs at the same time making interpretation of the original problem a non-trivial issue in itself.   In many cases forensic examination of the worn surfaces by experts is a key technique to determine the mechanisms of wear that have occurred so that any testing that is carried out will yield relevant results. 

The usual measurement will be of mass loss of the sample, however tests are often tailored to the specific requirements of the customer and may include: varying test length and frequency or speed; further analysis such as 3D microscopy, image subtraction, scanning electron microscopy and failure examination. Sample preparation and polishing are also available 

Abrasion Testing

• ASTM G65

Abrasion testing according to the ASTM G65 dry sand rubber wheel test specification.  Test abrasive is ~ 200 µm diameter silica sand. The counterface is a compliant rubber rimmed wheel, so wear tends to be relatively mild. The test conditions laid down in ASTM G65 are:

Sample geometry is 76.2 x 25.4 x 12.7 mm, although smaller sizes can be accommodated as long as the length of the sample is greater than 40 mm and width greater than 20 mm.

This test simulates moderate abrasion. An NPL modification to this test can be used to enable tests to be carried out with other abrasives and under wet or dry conditions.

• ASTM B611

Abrasion testing according to the ASTM B611 test specification which simulates very severe abrasion.. This was developed for hardmetals or cemented carbides and uses alumina grit which 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

The test is suitable to measure the response of materials to abrasion by fine particles under light load conditions.  In this test 4 µm SiC abrasive 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 that is produced by the abrasion on the surface of the sample. The test can be used to measure the wear resistance of thin (few micrometre) coatings by measuring the size of the scar produced in the coating and in the substrate once perforation of the coating has taken place. 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.

Scratch Testing

• Macro scale scratch tests

In the scratch test an indenter, typically a tip, is pulled across the surface of a test sample and the response of the material is investigated. Scratch testing was originally developed to assess the adhesion of coatings so that the diamond rapidly removed the coating by spallation at a certain critical load, but as time has progressed the adhesion of coatings has improved such that the scratch test is now used to look at other aspects such as the surface damage that is caused. Ramping loads in the range 2-200 N are typically used, and the scratches made are examined with an optical microscope to look at the damage that is caused. The friction force that is generated and the acoustic emission generated as the indenter moves over the surface can also be recorded. When a coating is tested, and if there is a critical load at which spallation occurs, the friction force and acoustic emission can show discontinuities which can be related to 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

NPL has two microtribology test systems which can be used to carry out experiments where a probe is moved over a test samples under smaller loads (up to 250 mN) than in the scratch test. The probes can be diamond indenters with radii ranging from 1 µm to 200 µm, or can be made from steel or any other appropriate material.

Microtribology experiments are carried out to examine and model the response of materials to asperity size contacts or to simulate single abrasion events with conditions relevant to abrasion by particles. Using to computer control sequences of experiments can be carried out at a range of different test loads or under ramping load conditions to look at the variation in damage with changes in applied conditions.

One of the NPL microtribometers is used for experiments on the laboratory bench under ambient atmosphere conditions. The other system is incorporated in situ in a high resolution SEM to enable sequences of images or videos of how damage builds up during a test, and to relate this damage to the microstructure of the material under test. 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  

• Pin-on-disc

Pin-on disc testing is where a pin (or ball) is pressed against a rotating disc. The contact geometry can be ball on flat or a pin with a rounded end on flat. The radius of curvature can be made very large so that a large area of contact is obtained, but with good alignment between the samples. Flat on flat contact geometry can be used, but there are often issues in getting good alignment. Factors such as the type of motion, the contact geometry, the applied load and the environment surrounding the contact points including lubrication can all be controlled. 

Two systems are available: 

1. tests under loads up to 100 N and speeds up to 0.2 ms-1. 
2. tests at load up to 250 N and speeds up to 2 ms-1. 

In both test systems friction and the wear displacement (total movement of pin towards disc giving a measure of wear to both pin and disc) are both measured continuously. The second test system is fitted with additional instruments including a linescan camera which can give optical images and videos with a 1 µm resolution of the wear surface in real time throughout a test, a non-contact optical chromatic aberration probe that enables separate measurements of the wear to the pin and flat to be obtained as the test proceeds, and a self-zeroing friction measurement system that eliminates drift in the measurement of friction which is particularly relevant for the accurate measurement of friction for modern low friction coatings.

• Plint TE77 Reciprocating Testing

Reciprocating wear is where a pin (or ball) is pressed against a flat plate that is moved back and forth in a reciprocating way. The contact geometry can be ball on flat or a pin with a rounded end on flat. The radius of curvature can be made very large so that a large area of contact is obtained, but with good alignment between the samples. Flat on flat contact geometry can be used, but there are often issues in getting good alignment. Factors such as the type of motion, the contact geometry, the applied load and the environment surrounding the contact points including lubrication can all be controlled. 

NPL’s fully equipped Plint Tribology TE77 reciprocating test system  can carry out reciprocating tests at frequencies up to 50 Hz, strokes of 25 mm and the flat test sample can be heated to 600 ^oC. The system at NPL is enclosed in a stainless steel environmental enclosure to facilitate tests in controlled atmospheres. Friction and wear displacement can both be measured. Wear scars can be examined by profilometry, and by optical and scanning electron microscopy.

• High temperature reciprocating wear

Reciprocating wear is where a pin (or ball) is pressed against a flat plate that is moved back and forth in a reciprocating way. The contact geometry can be ball on flat or a pin with a rounded end on flat. The radius of curvature can be made very large so that a large area of contact is obtained, but with good alignment between the samples. Flat on flat contact geometry can be used, but there are often issues in getting good alignment. Factors such as the type of motion, the contact geometry, the applied load and the environment surrounding the contact points can all be controlled. 

Tests can normally be carried out at temperatures up to 800 oC with nimonic sample holders, but with ceramic holders and SiC ceramic elements tests can be carried out up to 1300 oC. Loads up to 800 N can be applied. This test system has a relatively slow movement giving a maximum speed of about 20 mm per minute.|

• Erosion testing

 Erosion testing involved the analysis of damage occurring when a small object strikes a surface and removes material. The object may be a grain of sand, particle of metal or oxide, or a droplet of water. Testing either involves accelerating the particle into the surface, or moving the surface into the particle, depending on the design of the apparatus. 

• Low velocity, room temperature solid particle erosion follows ASTMG76.
• Higher velocity, high temperature testing uses the SPE apparatus
• Water droplet erosion tests utilise the WDE rig

The usual measurement will be of mass change of the sample, however tests are often tailored to the specific requirements of the customer and other options may be included. Sample preparation and polishing are also available 

• ASTM G76 Gas Borne Particulate Erosion

Particulate erosion is carried out with a gas borne particulate erosion test system which broadly follows ASTM G76. Silica sand with a diameter ~ 200 µm is typically used, but other erodants can also be used. Particle velocities up to about 75 ms^-1 are used. The mass of the sample is measured periodically through the test so that results for mass loss through erosion against erodent mass can be drawn up and erosion rate calculated. Wear scars can be examined by optical and scanning electron microscopy after the test to get information on the mechanisms of wear that are occurring. 3D optical microscopy can also be carried out to give a more direct measure of wear volume. Flat samples smaller than about 50 mm x 50 mm are required.

• Solid Particle Erosion (SPE apparatus) tests  

High temperature and high velocity particulate erosion testing is carried out in NPL’s new Solid Particle Erosion Test system.  This enables tests to be carried out at temperatures up to 600 °C and velocities up to 300 ms-1. This system also incorporates in situ mass change measurements and in situ laser triangulation measurement of wear volumes so that samples do not need to be cooled down periodically for measurements of sample mass during a test. Wear scars can also be examined by optical and scanning electron microscopy after the test to get information on the mechanisms of wear that are occurring and 3D optical microscopy can be carried out to give further  measures of wear volume.

• Water droplet erosion 

The water droplet erosion test system can study the damage caused by water droplets on fast moving surfaces. Samples are held in a rotor arm so that they pass through a jet of water droplets. The linear speed of the samples as they hit the water droplets can be controlled by adjusting the speed of the rotor. Tests are normally carried out at speeds of 300 ms-1 for steam turbine and aero engine applications, and 80 ms-1 for wind turbine leading engine applications. The samples are weighed to determine the loss of mass at intervals throughout a test.  The wear scar on the sample can be examined through the test by optical and scanning electron microscopy to examine the evolution of damage to the sample.