[ ME 539 ]

Surface Science & Engineering

Assoc. Prof. Dr. A. Tolga BOZDANA

Mechanical Engineering Department

© 2020

[ Chapter 1 ]

Surface Texture (Topography)

#1

Surface Texture (Topography): refers to total profile (i.e. form, waviness, roughness).

Surface Finish (Quality): refers to only roughness profile (ignoring form and waviness).

Form Error: non-cyclic long-period (macro-scale) deviations.

Waviness: wide-spaced (meso-scale) irregularities.

Roughness: close-spaced (micro-scale) irregularities.

Flaw: surface defects (i.e. scratches, cracks, holes, inclusions, etc.)

Lay: directionality of surface pattern.

Definition & Terminology

#2

Surface texture is significant in:

Friction of contact (mating) surfaces

Bearing and lubrication capabilities

Surface protection (i.e. coating, painting, plating, etc.)

Resistance to wear and corrosion

Tolerancing and fitting

Noise reduction

and so on…

Importance of Surface Texture

#3

Surface texture is measured using profilometer (roughness tester), consisting of a stylus (tracing probe) with perfectly sharp tip made of hard material.

Stylus is set in such a way that its tip must have contact with surface at all times. Then, it is moved horizontally to follow contours on the surface.

The path achieved by stylus is always smoother than the actual path.

Surface Texture Measurement

#4

Profilometer with skidded gage: In skidded gages, the sensitive diamond-tipped stylus is contained within a probe, which has a skid that rests on the workpiece. It uses the part itself as the reference surface, thus only roughness is measured.

Skidless gage profilometer: Skidless gages use an internal surface as reference. This enables measurement of not only roughness, but also waviness and form.

Profilometer (Roughness Tester)

Profilometer with

skidded gage

Skidless gage

profilometer

#5

The parameters related with measurement of surface texture are given below.

Only roughness measurements will be explained in this chapter. For others, refer to related standards given at the last slide.

Surface Texture Parameters

Parameter

Group Explanation

Parameters

Primary Waviness Roughness

Amplitude

Parameters

Vertical characteristics of

surface deviations

Pa, Pq, Pv, Pp, Pt, Psk, Pku, Pz

Wa, Wq, Wv, Wp, Wt, Wsk, Wku, Wz

Ra, Rq, Rv, Rp, Rt, Rsk, Rku, Rz, R3z

Spacing

Parameters

Horizontal characteristics of

surface deviations Psm Wsm Rsm, RHSC, RPc

Hybrid

Parameters

Combination of vertical and

horizontal characteristics of

surface deviations

PΔq, Pλq WΔq, Wλq RΔq, Rλq, Rmr,

Rpk, Rk, Rvk, Mr1

and Mr2

#6

The profile shown below is a typical 2D roughness profile:

Assesment (evaluation) length (L): This is the length of profile to be assessed. For this length, at least five consecutive lengths are taken as standard.

Sampling length (l): The mean line is determined, and the profile is divided into equal sampling lengths (from l1 to l5).

Cut-off length (λC): It is a filter to remove or reduce unwanted data to look at wavelengths in assesment region. Cut-off length is the same as sampling length.

Roughness Profile

l1 l2 l5 l4 l3

L

x

z

#7

Roughness Average (Ra): Universally recognized and commonly used parameter, which is the arithmetic mean of departures from the mean line. It is also known as Center Line Average (CLA) or Arithmetic Average (AA).

Root Mean Square (RMS) Roughness (Rq): It is RMS average of profile ordinates.

Ra is stable and repeatable parameter, which makes it good for random type surfaces. However, it cannot provide distinction between peaks and valleys.

Rq is more sensitive to peaks and valleys because the amplitudes are squared.

Roughness Parameters – Ra and Rq

xdxzL

RL

a 0

)(1

xdxzL

RL

q 0

2 )(1

Ra Rq

#8

As mentioned before, it is not possible to make a distinction between peaks and valleys based on Ra.

The profiles below have the same Ra value although they seem to be different. So, assessment of these profiles using Ra will cause inaccurate conclusions.

For this purpose, there is need for more specific and sensitive parameters so that more reliable assessment can be made.

Misinterpretation of Roughness

#9

Mean Roughness Height/Depth (Rz): The mean of heights/depths at each sample length.

Maximum Roughness (Rzmax): The largest of five heights/depths at each sample length.

Rz is more sensitive than Ra to changes on the surface since the maximum profile heights are examined rather than average of peak and valleys. Rzmax is useful when a single defect is not permissible (e.g. a seal with a scratch).

Rz and Rzmax are used together to monitor the variations of surface finish in production. Similar values of them indicate a consistent surface finish while a significant difference between them indicates a surface defect on consistent surface.

Rz to Ra conversion: Based on BS 1134/1-1972, 4 < Rz / Ra < 7 (depending on profile)

Roughness Parameters – Rz and Rzmax

Rz1 Rz2 Rz3= Rzmax Rz4 Rz5

n

i

zz iR

nR

1

1

maxmax izz RR

#10

Maximum Height (Rp): The maximum height (peak) within each sampling length.

Maximum Depth (Rv): The maximum depth (valley) within each sampling length.

Mean Levelling (Rpm & Rvm ): The mean of five consecutive peaks/valleys.

Peak-to-Valley Roughness (Rt): The largest peak-to-valley in the entire profile.

Rv is a good parameter where stress is a major factor. Rp is used to assess coating quality.

Rpm is for bearing and sliding surfaces and surface substrates prior to coating.

A low value of Rpm and a large value of Rz indicates a plateau surface.

The ratio of Rpm / Rz quantifies the asymmetry of profile.

Roughness Parameters – Rp , Rv , Rt

Rt

Rp5

Rv5

maxip pR R

maxiv vR R

t p vR R R

#11

Changing cut-off value (i.e. changing amount of "averaging" and "smoothing") can have big effect on measurement of roughness and waviness.

Choosing smaller cut-off length will result in smaller roughness value (though the real surface could be very rough). The profiles (shown below) are for the same surface with different cut-offs. Profile on the left gives twice of Ra value of profile on the right.

There are recommended values for choosing appropriate cut-off (given in table below), which were defined by ISO 4288-1996.

Selection of Cut-off Length

RECOMMENDED CUT-OFF LENGTHS (ISO 4288-1996)

Roughness Values Cut-off Profile Length

Rz (μm) Ra (μm) λC (mm) L (mm)

(0.025) to 0.1 (0.006) to 0.02 0.08 0.4

>0.1 to 0.5 >0.02 to 0.1 0.25 1.25

>0.5 to 10 >0.1 to 2 0.8 4

>10 to 50 >2 to 10 2.5 12.5

>50 to 200 >10 to 80 8 40

#12

Surface finish is strictly dependent on manufacturing process to be applied.

Some processes give rough surfaces (e.g. sand casting, sawing, etc.) while some can provide relatively smoother surfaces (i.e. finishing processes like reaming, lapping, burnishing, so on).

Therefore, it is crucial to have process planning for desired surface quality.

Surface Finish in Manufacturing

#13 ISO Standards on Surface Texture

ISO 1302 - 2001 Indication of Surface Texture

ISO 3274 - 1996 Nominal Characteristics of Contact (Stylus) Instruments

ISO 4287 - 1997 Terms, Definition and Surface Texture Parameters

ISO 4288 - 1996 Rules and Procedures for Assessment of Surface Texture

ISO 5436-1 - 2000 Calibration, Measurement Standards

ISO 5436-2 - 2000 Calibration, Soft Gages

ISO 8785 - 1999 Surface Imperfections - Terms, Definitions and Parameters

ISO 11562 - 1996 Metrological Characteristics of Phase Correct Filters

ISO 12085 - 1996 Motif Parameters

ISO 12179 - 2000 Calibration of Contact (Stylus) Instruments

ISO 13565 - 1996 Characterization of Surfaces Having Stratified Functional Properties

More information on surface texture measurement: www.taylor-hobson.com