5.3 KiB
| Rockwell | ASTM E18 |
| Vickers | ASTM E384 |
| Adhesion Strength Tensile Test | ASTM C633 |
| Area Percent Porosity | ASTM E2109 |
| Coating Thickness | ASTM B487 |
| Metallographic Preparation | ASTM E3, E1920 |
| Grain Size (Comparison Method Only) | ASTM E112, E930 |
| Microetching | ASTM E407 |
| Alpha Case | MCL III-273 |
| IGA/IGO / Casting Mold Reaction/Alloy Depletion | MCL III-251 |
| Delta Ferrite | MCL III 237.06 |
| Heat Treatment Solution/Incipient Melting Measurement | MCL III-221 |
| Wrought Titanium Microstructure | MCL III-273 |
| Micro Porosity by Image Analysis and Point Count | MCL III-270, ASTM E562 |
| Oxidation Test | HRC LM-100 |
Specimen Pairs & Wear
The first issue to address in designing a test is which way round, in terms of relative hardness, to have the specimen pair. Traditionally, many wear tests have involved running a soft pin or ball on a hard disc or plate. Under these conditions, the wear occurs on the softer material, sometimes accompanied by the generation of a transfer film on the harder material.
Measurement of material lost from the softer pin or ball is relatively easy. It should however be remembered that if material has been transferred to the disc or plate, its mass may increase.
If the specimen pairs are reversed, with a harder pin or ball running on a softer disc or plate, we generate a different mechanism, depending on the relative hardness, the contact pressure and contact shape. What happens to the disc or plate specimen depends on the nature of the material. With metallic specimens, plastic deformation of the surface and work hardening may take place, thus changing the nature of the material. With coated surfaces, repeated passes by a hardened pin or ball may give rise to adhesion-de-lamination and subsequent failure of the coating. If we define wear exclusively as the removal of material, it will be apparent that if the scar generated on the disc or plate specimen involves plastic deformation (material is redistributed but not removed), then it cannot be considered in the true sense as a “wear” scar. With this contact configuration, the processes involved may be more analogous to forming or machining processes. In the case of forming, we would anticipate plastic deformation, and in the case of machining, removal of material by cutting or ploughing action.
In real machines, we frequently find contacting materials of similar hardness, with the result that wear is shared between the two contacting surfaces. The only solution here is to measure the wear on both surfaces, not forgetting that, if the materials are different, the wear rate will still be dependent on which material is used for the pin or ball and which is used for the disc or plate. This is because the energy inputs are different for the two specimens.
Overlap Parameter
Now let’s move on to the overlap parameter. If we have a 10 mm diameter pin running on a 100 mm circumference disc track, then in one revolution, a point on the pin experiences 100 mm of sliding. However, a similar point on the disc sees only a single pass of the pin, hence a sliding distance of just 10 mm. Double the track circumference and the point on the pin sees 200 mm sliding per revolution whereas the point on the disc still only sees 10 mm. Hence, in this example, changing the track diameter has a direct impact on how the sliding distance, hence the wear, is shared between the two surfaces. It also means that running repeat tests at different track diameters, at the same surface speed on the same disc, will generate different wear rates. By contrast, with the thrust washer arrangement, the sliding distance for a point on either sample has to be the same. This probably makes it a better arrangement for testing many materials, unless, of course, we wish deliberately to confine the majority of wear to one surface. The "overlap parameter" (Czichos) is defined as the ratio of sliding distance for "body" divided by sliding distance for "counter body". For the thrust washer this is 1, for fretting tests it is close to 1, but for pin on disc tests it is variable, but is typically less than 0.05. The overlap parameter also applies for reciprocating tests, but here there is not the temptation to use the equivalent of different pin on disc track diameters, as one would sensibly keep the stroke the same and index the specimen plate sideways to run a fresh wear track.
Specimen Orientation
Slide 22Specimen Orientation Let’s think a bit about specimen orientation. If we run a pin on disc machine with the pin loaded onto the disc from above, any wear debris generated will tend to accumulate on the surface. This will give different behaviour from exactly the same configuration turned upside down. In this case, the debris will fall off the disc surface, giving different friction and wear behaviour.