Surface roughness and effects

Surface roughness,often simply called roughness,is a component of surface finish (surface texture).It is quantified by the deviation of the normal vector direction of the real surface from its ideal form.If these deviations are large, the surface is rough;if they are small, the surface is smooth.In surface metrology,roughness is generally considered to be the high-frequency,short-wavelength component of the surface being measured.In practice,however,it is often necessary to know the amplitude and frequency to ensure that the surface is fit for purpose.Roughness plays an important role in determining how real objects interact with their environment.In tribology,rough surfaces typically wear faster and have a higher coefficient of friction than smooth surfaces.Roughness is often a good predictor of mechanical component performance,as surface irregularities can form nucleation sites for cracks or corrosion. Roughness,on the other hand,can promote adhesion.In general,cross-scale descriptors such as surface fractals are not scale-specific descriptors,but provide more meaningful predictions of surface-mechanical interactions,including contact stiffness and static friction.While high roughness values are generally undesirable,they are difficult and expensive to control during fabrication.For example,controlling the surface roughness of fused deposition modeling (FDM) manufactured parts is difficult and expensive. Reducing the roughness of a surface usually increases its manufacturing cost.This often results in a trade-off between the manufacturing cost of the component and its application performance.Roughness can be measured by manual comparison with a "surface roughness comparator" (a sample of known surface roughness),but more commonly surface profile measurements are made using a profilometer.These can be contact (usually a diamond stylus) or optical (for example:white light interferometer or laser scanning confocal microscope).Often,however,roughness needs to be controlled.For example,glossy surfaces are too shiny for the eyes and too slippery for fingers (touchpads are a good example),so roughness needs to be controlled.This is the case where both amplitude and frequency are very important.

Fractal theory 

Mathematician Benoit Mandelbrot noted the link between surface roughness and fractal dimension.The description povided by fractals at the level of microscopic roughness can allow control over material properties and the type of chip formation that occurs.But fractals cannot provide a full-scale representation of a typical machined surface affected by tool feed traces; it ignores the geometry of the cutting edge.(J. Paulo Davim, 2010, op.cit.).Fractal descriptors of surfaces play an important role in correlating physical surface properties with surface structure.Linking physical,electrical,and mechanical behavior to traditional surface descriptors of roughness or slope has been challenging in multiple domains.Certain interfacial phenomena,including contact mechanics,friction,and electrical contact resistance,can better explain surface structure by employing surface fractal measurements along with roughness or surface shape measurements.

Soil-surface roughness

Soil surface roughness (SSR) refers to the vertical variation of the microscopic and macroscopic relief of the soil surface and its random distribution.There are four distinct categories of SSR,each representing a characteristic vertical length scale; the first category includes microtopographic variations from individual soil particles to aggregates on the order of 0.053-2.0 mm; the second category consists of 2 to 100 mm; the third category of soil surface roughness is the systematic elevation difference due to tillage,called directional roughness (OR), and ranges from 100 to 300 mm; the fourth category includes Planar curvature or macroscopic terrain features.The first two classes account for the so-called microroughness,which has been shown to be largely influenced by the event and seasonal timescales of rainfall and tillage,respectively.Microroughness is most commonly quantified by stochastic roughness, which is essentially the standard deviation of bed elevation data around the mean elevation,after correcting for slope using a best-fit plane and removing tillage effects in individual height readings.Depending on the initial microroughness conditions and soil properties, the rainfall effect can result in attenuation or increase in microroughness.On rough soil surfaces, the action of rainsplash separation tends to smooth the edges of the soil surface roughness,leading to an overall decrease in RR.However,a recent study examining the response of smooth soil surfaces to rainfall revealed that RR can be significantly increased for low initial microroughness length scales of 0–5 mm.It is also shown that the increase or decrease is consistent between the various SSR indices.

Practical effects

Surface structure plays a key role in controlling contact mechanics,that is, the mechanical behavior that two solid objects exhibit at an interface when they approach each other and transition from non-contact to full contact.In particular, the normal contact stiffness is mainly controlled by the roughness structure (roughness,surface slope,and fractality) and material properties.As far as engineered surfaces are concerned roughness is considered detrimental to part performance.Therefore,most manufactured prints have an upper limit on roughness,but no lower limit.One exception is in cylinder bores where oil remains in the surface profile and minimum roughness is required.