Hrant Khatchadourian
MANE-6960
Friction & Wear of Materials
10-04-12
1. Question:
Write a brief essay (≈100 words) describing how and explaining why the surface and near
surface regions of a solid material are different from the bulk of the same material.
Answer:
When examining the near surface regions of a solid material compared to the bulk of the same
material there are a few layers (micro layers) that exist. These near surface regions that are of interest are
usually located within the first 100 micrometers from the surface. When considering the atomic structure
of the material, it can be visualized that the atoms in the bulk of the material are all surrounded by other
neighboring atoms while the atoms and molecules near the surface of the material only have neighbors on
the inner side of that material. For this mentioned reason, as well as other factors such as machining,
temperature and oxide formation, the near surface micro layers of a material are in discontinuity with the
bulk.
The main (thickest) layer that exists just above the bulk material is the Working layer. This layer,
approximately 1-100 micrometers in thickness, is plastically deformed (relatively speaking) and is
mainly contributed to the type of manufacturing process used to produce the material or piece.
Closer to the surface just above the working layer is the Beilby layer. The Beilby layer,
approximately 1-100 nanometers in thickness, is an amorphous or microcrystalline structure that
is severely distorted compared to the bulk. This layer is created due to the surface finishing
process employed onto the material which is a result of melting and surface flow during
machining. Directly above the Beilby layer is the Oxide layer which can be anywhere between
10-100 nanometers in thickness. The oxide layer forms through a chemical reaction between the
surface of the material and its surrounding environment. The thickness or amount of oxidation
depends on the available oxygen in the surrounding environment and other surface oxidation
mechanisms. The final 1-2 nanometers of material that sit on top of the oxide layer are a layer of
absorbed gases and water vapor. This layer forms from water vapor or hydrocarbons from the
environment that have condensed and physically or chemically been absorbed onto the surface.
2. Question:
Do some research on profilometers used to characterize the roughness characteristics of
solid surfaces and write a short essay describing some of the key features of currently available
models.
Answer:
A Profilometer is a device that measures a surface’s profile in order to quantify its roughness.
The first types of profilometers worked similar to how a phonograph works, where it uses a fine point
stylus to measure the samples vertical displacement with respect to position. With technological
advantages optical or non-contact profilometers are also being used today.
Contact profilometers can measure surface variations as low as 10 nanometers (nm) using a
diamond tipped stylus that generates an analog signal which is converted to a digital signal in order to be
processed and analyzed. Different horizontal scan speeds and sample rates can be used to acquire
different resolutions. The obvious disadvantage is that this type of profilometer comes in direct contact
with the samples surface. One advantage of using stylus type profilometers is for use in dirty
environments where non-contact types may inadvertently measure surface contaminants. Another
advantage is that the stylus tip radius can be as small as 20 nm which gives it significantly better profiling
than white-light optical measurements. For engineers or anyone wanting to publish any measurements,
contact profilometers are a better choice since most of the world's surface finish standards are written for
contact profilometers.
Optical non-contact and Fiber-based optical devices are the other types of profilometers that can
measure a surface’s profile in detail. These types of devices use lasers or other light interference
detection methods so that the device does not come in contact with the samples surface. Although
vertical resolution is in the nanometer region, horizontal resolutions are in the range of micrometer to
some devices as low as 150 nm. One prominent advantage of non-contact profilometers is that they have
better reliability due to less damage caused by surface wear (friction and surface wear). The fiber-based
profilometers are a good choice for use in tight quarters, harsh environments and in industrial processes
due to their small profiles and ease of implementation.
There are some key features that current profilometers come with thanks to improved technology.
For instance, the Zygo NewView 7300 non-contact profilometer is sold with a Dell laptop and LCD
monitor that has Zygo Metropro software running behind Microsoft Windows XP. It also features a
motorized 3-position auto or manual zoom system that has a vertical scan range up to 150 micrometers
(µm) with a data scan rate of ≤135 µm/sec. Another top model to consider is the KLA- Tencor HRP-350
contact profilometer which uses a 20 nm diamond stylus which allows for measurements as low as 45 nm.
This model also allows for a high resolution scan mode to enable accurate control of nano-scale features
and also a long-scan mode to operate at high throughput. The HRP-350 is also up to five times faster than
the previous HRP-340 surface measurement system. There are many different types and models of
profilometers that can be taken into consideration, so the choice really comes down to the user and
application of the device.
3. Question:
The attached Excel worksheet contains raw measured asperity elevation data as a
function of distance along the surface for a certain material surface (values in micrometers).
a) Compute the mean asperity height, adopt the computed value as the reference asperity height and
create a new column of transformed data showing the difference between the local asperity
height and the reference value.
b) Use the data to compute the values of Ra, and Rq.
c) Use the data to construct a histogram. Check that the data is approximately normally distributed
and find a closed form expression for a good fit Gaussian. Compute then the mean μ, variance σ,
Skewness Sk and Kurtosis K. Comment on your results.
d) Compute values of the autocorrelation function and discuss.
e) Compute the bearing ratio tp for various preselected asperity heights h* and also the bearing area
curve for the surface. Comment on your results.
Answer:
Please refer to HW1 – prb.3&4.xls for more detail.
a) The mean asperity height is 4.0146µm. See xls spreadsheet for data showing the difference
between the local asperity height and the reference value.
b) Ra, = 0.0695 and Rq = 0.0854
c) See the Excel spreadsheet for data and histogram. Below is the equation of the closed form
expression for a good fit Gaussian. From analyzing the histogram and calculated values of
mean, variance, skewness and kurtosis (given below) it can be seen that the histogram does seem
similar to a good fit gaussin. There is slight skewness of the curve to the left which is
represented by the negative Skewness value of -0.33997. Also, the curve is slightly flater on top
near the mean value which is represented by the negative Kurtosis value of -0.34885.
Mean μ = -2.3340E-16, Variance σ = 0.0854, Skewness Sk = -0.33997
and Kurtosis K = -0.34885.
d) See the Excel spreadsheet for values and a plot of Autocorrelation function, which provides
useful information on the spatial distribution of roughness. This is useful for examining surface
finishes.
e) See the Excel spreadsheet for tp Bearing Ration values and plot.
4. Question:
The following table summarizes experimental data on the asperity heights of a shot peened
aluminum surface in terms of the cumulative distribution function.
z(μm)
F(z)(−)
1
0.005
2
0.05
3
0.22
4
0.5
5
0.8
6
0.95
7
0.99
8
0.9999
Use the data to obtain the various statistical functions useful in the description of the surface.
Answer:
Please refer to HW1 – prb.3&4.xls for more detail.