PSI - Issue 42

Mushfiq Hasan et al. / Procedia Structural Integrity 42 (2022) 1169–1176 Mushfiq Hasan et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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3.4. Effect of Surface Treatment Two different types of surface treatment were experimentally tested to evaluate their performance against wear and micropitting. This surface treatment technology is commercially known as ‘Triboconditioning’ and is provided by Applied Nano Surfaces (Tribonex), Sweden . ‘Triboconditioning’ is a mechanochemical surface finish process which consists of extreme pressure mechanical burnishing with tribochemical deposition of an antiwear film or solid lubricant (19). Treatment was applied on a steel sample having a surface roughness of 0.5 µm. The main difference between the two treatments is based on tooling material, burnishing time and pressure. The effect of surface treatment on surface parameters is listed in Table 2. Surface treatment 2 has a major effect compared to surface treatment 1 as roughness is reduced by more than 50%. So, the irregularities due to peaks and valleys should be more stable than the untreated steel sample. Moreover, RMS slope values became smaller, referring to the reduction in asperity sharpness. A steep RMS slope can increase the contact pressure as well as fatigue. Therefore, a decrease in slope value might help to reduce pitting risk. Furthermore, lower skewness and peak height can also be beneficial for mitigating fatigue initiation.

Table 2: Tribological parameters before and after surface treatment Parameters Symbol

Treatment 1

Treatment 2

Before (µm)

After (µm)

Before(µm)

After(µm)

Sq Sa

RMS Roughness

0.611 0.477 -0.533 2.085 3.283 0.059

0.577 0.439 -0.837 1.385 2.688 0.048

0.545 0.434 -0.180 1.965 2.238 0.053

0.230 0.182 -0.444 0.637 1.168 0.013

Arithmetic Roughness

Ssk

Skewness

Sp Sv

Peak Height Valley Depth RMS Slope

Sdq

Fig.7 represents the outcome of surface treatments 1 and 2 over 16 hours period, where the tests were stopped at four different time intervals for observation. Surface treatment 1 has a similar damage progression pattern compared to untreated steel contact but at a lower magnitude. Initially, the micropitted area grew with running time. Later pits containing area decreased slowly with increasing wear. Despite having a drop in wear volume, micropitting performance was not improved. On the contrary, surface treatment 2 was better to restrict pitting and wear damage. A very negligible amount of micropitting can be traced after 16 hours of the test. The maximum recorded wear volume was 0.01mm3, around 90% lower than the untreated steel-steel contact that was discussed earlier and depicted in Fig.6. Several reasons can be outlined behind this performance. Firstly, improvement in surface irregularities makes the surface smoother and reduces the chance of surface-initiated fatigue. Moreover, the thin deposited surface layer by mechanochemical process might be helpful to enhance wear behaviour by reducing friction. Lastly, the burnishing process under high pressure can increase the compressive stress that might mitigate the tensile stress that is responsible for pitting cracks.

(a) (b) Fig.7: Wear volume and micropitted area over time intervals (a) Surface treatment 1 (b) Surface Treatment 2