DE LA PATERNIDAD Y FILIACION CAPITULO I

6.1.2.1 Oxide application

Both naturally “generated” and artificially “applied” iron oxides were used over the course of this testing. Methods were created that could be used on as many of the test rigs as possible to allow continuity between methods. The difference in specimens meant that the same method could not be used between test rigs, but the methods were kept as similar as possible.

Naturally “generated” oxides could be formed on steel test specimens. This was often done by simply cleaning and then wetting the steel surface, allowing oxide to form as it dried. A humidity chamber was used if a thicker oxide layer was required. When using the SUROS rig, the heat and pressure of the contact could produce an oxide layer without needing any water addition.

These generated layers produced a natural oxide layer which better replicates what would occur on the railhead, but the oxide formed varied in thickness and colour so tests were often hard to repeat and small differences in conditions could have a large impact on the final oxide layer.

To create a more repeatable oxide layer, iron oxide and water pastes were used. Iron oxide pastes were formed from iron oxide particles and water and applied to the contact, allowing conditions to be kept more constant than those in the generated

6.1 Introduction

oxides due to not having a condition dependent pre-oxidation step. The pastes were made by mixing a known mass of iron oxide powder, bought from a chemical supplier, with a known mass of water in a beaker to form a suspension of particles in a liquid. Hematite and magnetite powders were used because the literature review found them to be two common oxides found on the railhead and were readily available as powders.

The pastes are made as a weight percentage (wt %), for example with 10 g of 60 % hematite paste being made by mixing 6 g of hematite powder and 4 g water. Hematite (Fe2O3) and magnetite (Fe3O4) powders were used, each with a large size

distribution of particles which were thought to better resemble what would be seen in a real wheel-rail contact, rather than very small particle size distributions. SEM images of the hematite and magnetite powders that were used are shown in Figures 6.1 and 6.2.

The pastes could be applied in a specific volume using a syringe. The oxides, es- pecially the higher viscosities, would spread out very little when dropped onto a surface so it was difficult to add the same thickness of oxide paste for the UMT and pendulum rigs in particular. However, the amount of excess oxide pushed out during each test implies that the contact was saturated with oxide paste and the excess pushed out, which may mean that over a certain threshold, the thickness does not effect the results.

Figure 6.2: SEM image of magnetite powder

The two types of iron oxide particles look very different, the red hematite having a much more rounded structure than the black and more angular magnetite, Mag- netite seems to be present as separate particles whilst the hematite is present as an agglomerate, it was expected that the pastes would have different properties in the contact. Particle size distributions were not available for these powders but these powders were chosen because of the large particle size distribution as would likely be found in the real contact, compared to those that could be bought with a known distribution, which generally had a limited range. Hematite paste was more stable and “stickier”, meaning it remained as a suspension and stuck to any substrate it was left on. Magnetite in comparison, separated into a layer of powder below a layer of water very quickly and ran off any substrate unless it was level, rather than sticking to it. The difference between the two suspensions is seen in Figure 6.3. Each rig has been tested with both naturally “generated” and artificially “applied” oxide pastes to allow a range of conditions to be assessed.

6.1 Introduction

Figure 6.3: 50 wt % hematite (left) and magnetite (right), 2 minutes after mixing

6.1.2.2 Water application

Water is known to play a large role in low adhesion and both the volume and application method will likely affect test results, as discussed in the literature review. Due to the variation in test rigs and methods, the water application method could not be kept identical between test methods but the application was kept as consistent as possible.

Water was applied using 4 main methods. A syringe was used for the majority of this work, able to simply apply an accurate volume of water. When a continuous water application was needed the syringe could be used with a syringe plunger, which pushed the syringe at a user defined rate so the correct water flow rate could be achieved.

A micro-pipette was used to accurately dispense water when smaller water volumes were needed. An issue with both of these methods was that water could only be applied to a small area and surface tension often meant that large droplets were formed, rather than a thin layer over a larger area.

To overcome this a “spray” bottle was used. This sprayed water over a large area when the trigger was pulled and ensured better coverage than a syringe. The amount sprayed for each trigger pull was not completely consistent but was measured to be approximately 7 ml.

An aerosol water spray was used in later tests, this was a bottle containing water and nitrogen that could finely mist the surface that it was sprayed on, producing very small droplets of water. Once again the spray was not accurate at dispensing specific amounts of water, but the water dispensed per second of spraying was measured to be approximately 0.7 ml/sec.