Biology 代写:Determine The Number Of Asperity Peaks

At present there are no generally accepted and experimentally confirmed, 2D or 3D, deterministic, asperity-deformation models to evaluate the real contact area in tribological applications. One of the key obstacles is that there are no clear and experimentally verified criteria about how to define and consequently determine the "actual" load-carrying asperity peaks. As a result, this work attempts to clarify how different, arbitrarily selected, asperity-peak identification criteria affect the calculated asperity-peak properties, i.e., the number, radii and heights. Such an analysis is still missing from the literature on 2D and 3D, asperity-peak analyses and is required for a better understanding of the physical meaning and engineering feasibility, and thus more realistic assumptions about these criteria.

Different criteria that take into account the number of required neighbouring points (i.e., 3, 5 and 7 points), the peak-threshold value (z-direction) and the effect of the data resolution in the x-direction were applied in this study. Five different real surface roughnesses in the broad engineering range from Ra = 0.003 µm to Ra = 0.70 µm were evaluated. The results show the huge influence of these pre-selected criteria for which no verified guidelines exist. Although contact-deformation conditions based on experimental evidence are still required, several obvious and relevant conclusions can be drawn: (i) the 3-point asperity-peak criteria are more trustworthy than the 5 or 7 point criteria; (ii) an x-direction data resolution Δx below 1 µm should be used to limit the important effect on the calculated number of asperity peaks; (iii) the peak threshold value (z-direction criteria) depends to a large extent on the surface roughness and lacks guidelines for use in its current form.

Keywords: surface topography, roughness, asperity peak, real contact area, identification criteria

1. Introduction

When trying to characterise the real contact area in tribological contacts, the topography, roughness, load and material properties are the main influencing parameters. However, these parameters are difficult to determine reliably due to them changing continuously at the micro-asperity level. It is probably for this reason that in the majority of tribological publications a nominal contact area is used to calculate the contact pressures and the temperatures. However, this simplification may greatly overestimate the size of the real contact area, and this underestimates the severity of the real contact conditions that occur between two rough surfaces [1]. More realistic, higher contact pressures and temperatures can cause different behaviour of the materials and lubricants in tribological contacts, which can have a significant influence on the tribological properties [1]. It is thus very important to estimate the real contact areas as accurately as possible in order to better understand the real contact conditions.

Surface topography is characterised by variations in the form, waviness and roughness [2, 3]. Because of the nature of surface topography, the real contact area in a tribological contact is a momentary sum of the "micro-contacts" Ai and is only a fraction of the nominal contact area [4-6], as shown in Figure 1.

Figure 1: Real contact area of flat/flat contact.

However, the definition of a micro-contact is very general in its nature, because there exist surface irregularities as well as surface roughnesses down to the atomic scale. Thus, even the micro-contacts Ai consist of other, smaller micro-contacts, such as Ai,i shown in Figure 1, if we scale down the lengths of interest. The level of what is considered as a relevant ''micro-asperity", or micro-contact, therefore depends on our ability to identify and quantify them, as well as our ability to determine their influence in bearing loads, heat transfer, etc. Accordingly, it is crucial to determine which asperity peaks do have an influence on the contact conditions and are able to resist external loads.

A determination of the load-carrying asperity peaks is always needed when using deterministic contact models for real contact-area calculations. However, to do this, in accordance with the above discussion, we first need to identify the asperity peaks. Methods for determining the asperity peaks are seldom described in the literature and are not well established, and this may also be one of the important obstacles to their use for the real contact area in tribological models.

At present there are no generally accepted and experimentally confirmed, 2D or 3D, deterministic, asperity-deformation models to evaluate the real contact area in tribological contacts. One of the key obstacles is that there are no general criteria about how to define and determine the "actual" load-carrying asperity peaks. As a result, this work attempts to clarify how different, arbitrarily selected, asperity-peak identification criteria affect the calculated asperity-peak properties, i.e., the number, radii and heights. Although there have been attempts in the past, there is no consensus on their relevance due to the lack of any experimental verification and complex mathematical analyses with many assumptions, limiting their use for a highly specialised audience rather than establishing a ready-to-use engineering basis.

For the purpose of this research, steel specimens with five distinctively different surface roughnesses were prepared and then measured using a stylus-tip profiler and the 2D data were analysed to calculate the number of asperity peaks, their radii and their heights. The effects of the different asperity-peak identification criteria (the number of neighbouring points that define an asperity peak - 3, 5 and 7 points) as well as the corrections in the z-direction and the resolution of the profile measurement in the x-direction were evaluated for a broad range of engineering surface roughnesses (Ra between 0.003 µm and 0.644 µm).

It is known from the literature that the asperity-peak properties, calculated or measured from 2D surface profiles, can be different compared to those that are calculated or measured from the 3D surface topography [7-11]. Although we realize that 2D surface profiles are less accurate than 3D surface topography, 2D profilometry is still widely used in both industry and academia, and so a 2D analysis remains important and relevant - especially for a comparison with scientific past results and industrial best practices and values (for tribological function), which are mostly 2D. Furthermore, a 2D analysis could provide us with an important basic understanding of asperity-peak identification criteria, which can easily be expanded to a 3D surface topography, and this is planned for future work. However, it should be noted that 3D surface analyses can - due to their complexity - also cause large errors if they are not properly used, as well as lack of standardised 3D parameters still limits their wider use in more profound and comprehensive analyses.

Biology 代写:Determine The Number Of Asperity Peaks