:PROPERTIES:
:ID:       6e35d9c4-787d-4334-9c68-457868bcc91b
:END:
#+TITLE: Tribolology
#+AUTHOR: Vishakh Pradeep Kumar
#+EMAIL: v.kumar@hw.ac.uk


# https://youtube.com/playlist?list=PL9nARBd2NBsYWarjXrv4ZxTUl0Qx0izkF

Thermal spraying is an established industrial method for the surfacing and resurfacing of engineered components. Metals, alloys, metal oxides, metal/ceramic blends, carbides, wires, rods and various composite materials can be deposited on a variety of substrate materials to form unique coating microstructures or near-net-shape components. Thermal spray coatings provide a functional surface to protect or modify the behavior of a substrate material and/or component. A substantial number of the world’s industries utilize thermal spray for many critical applications. 4 Key application functions include restoration and repair; corrosion protection; various forms of wear such as abrasion, erosion and scuff; heat insulation or conduction; oxidation and hot corrosion; electrical conductors or insulators; near-net-shape manufacturing; seals, engineered emissivity; abradable coatings; decorative purposes; and more.

Thermal spraying surfaces and resurfaces engineered components to protect or modify the behaviour of a substrate component. Unique coating microstructures made from blending metals, alloys, oxides, ceramics, carbides, and composites are used to resist wear & corrosion, manage thermal efficiency, and enhance electrical properties, among others.

Thermal spray coatings require a heat/energy source, consumable materials, and gases to propel the consumable as fine molten droplets against the part surface. The droplets strike the part, solidify and adhere, with each layer forming a lamellar "pancake-like" splat structure. The metallurgical properties of the coating are dependent on the thermal and kinetic energy used.


* Introduction

SEM and x-ray microanalysis capture magnified images and chemical data from metallographic samples. They work in a high vacuum, using a powerful electron beam to analyze backscattered and secondary electrons for insight.
A conductive specimen is crucial for directing most electrons to the ground. Image formation relies on collecting scattered signals from the beam-sample interaction.

Backscattered and secondary electrons, the primary signals for imaging, arise within the interaction volume. Backscattered electrons are reflected back after elastic interactions between the electron beam and the sample, while secondary electrons originate from the atoms of the sample. Backscattered electrons come from deeper regions of the sample and increase with higher atomic number while secondary electrons originate from surface regions and are less affected by atomic number.


* Sample Preparation for Scanning Electron Microscopy
SEM delves into surfaces, yet interior details are often more interesting; one must uncover them gently while preserving the sample's essence & structures. Mechanical prep boils down to gradual material removal with abrasives in finer steps until the desired outcome.

For metals & alloys, samples are sectioned by saws and prepared by grinding & polishing. Serrated saw blades cut hard - they take a chunk, but mar the surface. Grinding and polishing smooth the surface step by step to reveal the true undeformed microstructure; etching may follow, depending on imaging needs. For best analysis, a pristine, mirror-like flat surface is imperative.


** Sample Labeling

Properly marking the SEM specimen is vital. Use an alphanumeric code, unique and constant from sample selection to final data. Direct labeling is rare, but an indelible marker under a metal specimen suffices. If immediate labeling isn't possible, store in a labeled container. Preserve identity, especially with multiple samples on one stage - it is remarkably hard to distinguish between polished samples.

Quality tools aid small specimen labeling and prep. A sturdy 100-mm glass lens with 5x magnification, fixed to adjustable support, near flexible lights, is handy; one can be found with soldering hands. A smaller 10x lens for closer inspection helps.

** Abrasive Sectioning

Cutting aims for minimal damage to microstructure; the right blade is crucial to avoid burning and heat generation. Abrasive discs with silicon carbide or alumina abrasives in a resin or rubber bond are used for abrasive cutting. Be sure to orient the specimen correctly! Blades should enter coating and exit base to keep the coating compressed; cracks should not be formed (by tension) when one wishes to study failure mechanisms. Lubricate for safe & damage-free cutting; in addition to removing swarf, the right cutting fluid protects from rust.

Blades break, samples crack or burn in abrasive cutting. Here's how to identify causes and rectify them:
- Chipped or broken blades
  Sample may have moved during cut, secure sample properly.
  The cutting force might be too high, reduce cutting force weight.
- Bluish burnt color on specimen
  Incorrect cutting fluid, blade, or excessive force.
  Consult applications guideline for proper blade & cutting fluid.

** Precision Sectioning

Thin saws, rim-pressed with diamond abrasive, equipped with micrometers ensure exact alignment and positioning. Finer abrasives produce less damage but take longer; fine grit diamond blades have particle sizes of 10-20 microns, like 600 grit sandpaper, while medium grit wafering blades have particle sizes of 60-70 microns, akin to 220 grit. Regular dressing keeps these blades cutting their best; metal smeared over the abrasive's cutting edge is removed by a ceramic abrasive in a soft matrix.

** Sample Mounting

Mounting secures samples for grinding in controlled orientation while protecting sample edges. The sample is mounted by casting in a hardenable plastic or be compressing it into a hot plastic.

*** Mounting for small sample (eg. powder)
Most specimens examined in the SEM are much smaller than the SEM chamber and can usually fit on a Cambridge specimen stub, which is 12 mm in diameter and 3 mm thick.
The general approach is to make the specimen as small as possible without compromising the appearance of the features of interest and the ability of the microscope to image and analyze these features.

** Grinding & Polishing
Grinding is performed by abrasive particles bonded in resin or metal matrix - silicon carbide for softer materials, diamond for harder ones. Choose abrasives harder than the material but choose methods carefully, as aggressiveness may erode vital features (eg. alumina coatings). Polishing employs loose abrasives for gentler, superior finish although with slower removal.


* COMMENT Operational parameters used to chemically analyze samples in the scanning electron microscope
Depending on the accelerating voltage of the primary beam,
the BSE and XRP signals are generated deep (μ ms) below the
surface of the sample, in contrast to the shallow (nms) genera-
tion depth of the SE signals.

The BSE topographic images are generally inferior to the high
quality images provided by the SE.
3. The ideal specimen surface for the most accurate and precise
analytical information is flat and highly polished in order
to reduce surface roughness to no more than 100 nm. This
requirement is particularly important for x-ray microanalysis.
Resolution of these problems is dependent on sample preparation.
4. The deeper generation of the BSE and x-ray photons in the
sample may create operational problems with the analysis of
thin films on substrates of particles, rough surfaces, and beam
sensitive specimens.

Table 9.1. Comparison of the different signals and the range of
operational voltage and current commonly used in scanning elec-
tron microscopy and x-ray microanalysis. Kv = voltage, A = beam
current, SE = secondary electrons, BSE = backscattered electrons,
EBSD = electron backscattered diffraction, EDS = energy dispersive
x-ray photon spectroscopy, WDS = wavelength dispersive x-ray pho-
ton spectroscopy, CL = cathodoluminescence
| Signal | Operational voltage Kv | Operational current A |
| SE     | 10eV to 30Kv           | 10pA to 200nA         |
| BSE    | 500eV to 30Kv          | 100pA to 200nA        |
| EBSD   | 10Kv to 30Kv           | 1–10nA                |
| EDS    | 1Kv to 20Kv            | 250pA to 200nA        |
| WDS    | 1Kv to 20kv            | 10nA to 200nA         |
| CL     | 2Kv to 20Kv            | 300pA to 1nA          |

The two main variable parameters of the incoming primary
beam are the acceleration voltage and beam current. In simple
terms,
the higher the voltage the faster the electrons move and the further they penetrate into the specimen.
The higher the beam current, the greater the number of electrons.
High accelerating voltage is associated with increased resolution, and high beam current is associated with an increase in the signal emitted from the specimen.

The actual voltage and beam current used to examine and analyze a given specimen is very variable and strongly influenced by the following sample parameters:

1. The magnification and spatial resolution needed to obtain the
appropriate information from the specimen
2. The type of signals needed from the sample
3. The voltage and current needed to get this information
4. The density and conductivity of the sample
5. The sensitivity of the sample to radiation damage



Backscattered Electron Imaging: Useful for Distinguishing
Differences Between Broad Groups of Elements
Unlike the SE signal, the BSE coefficient (η ) increases nearly
monotonically with the atomic number of the specimen. For
example, ten times more of the incident electron beam is back-
scattered by gold than by carbon. The differences in the BSE
coefficient form the basis of the qualitative analytical procedure.
A high-atomic-number inclusion in low-atomic-number material
gives a strong BSE signal, which can be used to give sufficient
differential contrast in an image, but can only locate the inclusion.

There are four conditions that should be met in order to make
full and proper use of this analytical technique:
1. The best signals come from flat, highly polished, conductive
samples with no preparation induced surface deformations.
2. There should be maximal specific localization of high-atomic-
number elements within the sample.
3. There should be minimal nonspecific distribution of high-
atomic-number elements within the sample.
4. The BSE signal must provide an adequate structural images.



* COMMENT Sample Cleaning

Once cleaned, the specimen either goes directly into the
specimen chamber of a scrupulously clean SEM. This is not to
suggest that this is the only time cleaning occurs; it is a continu-
ous and repetitive process during sample preparation to remove
every thing that is not an original and integral part of the sample.
If a particular preparative procedure contaminates the specimen,
clean it off before going on to the next task.

It is best to assume that all specimens are to some extent
contaminated (and so too are our fingers). As a golden rule,
always wear disposable plastic gloves and use clean metal tools,
such as forceps or tweezers, when handling specimens, placing
prepared specimens onto the microscope stage, and removing
them after examination.

There are two general types of cleaning, non-contact cleaning
in which there is no physical contact between the cleaning agent
and the specimen, and contact cleaning in which there is physi-
cal and chemical contact between the cleaning agent and the
sample. Ideally we would like only to use non-contact processes
to ensure that the sample surface is undamaged. For example,
loosely adherent dust can be removed from a dry specimen by a
puff of clean air or, better still, a low pressure jet of an inert gas
such as nitrogen. Most cleaning is achieved using solids, fluids,
chemicals, or high energy beams that are applied to the sample
surface by varying degrees of physical contact. These processes
should not leave any traces of the cleaning materials as this may
compromise any subsequent chemical analysis.

As a general rule the cleaning process should start by using
the mildest cleaning agent and the least physical contact. There
is only one rule; the cleaning process must not damage or modify
the specimen.


The cleaning process must remove mineral and organic oils, grease, and paint together with any traces of inorganic and organic chemicals on the surface.
One of the principal contaminants on metal samples is carbon and organic carbon compounds derived either from faulty cleaning or the microscope itself.

Mechanical means such as high pressure particle abrasion
such as sandblasting and lapping procedures that will certainly
clean the sample but at the expense of surface erosion and
damage. This approach should not be used as a final stage
for topographical studies, but is a good preliminary cleaning
process for preparing specimens for subsequent surface analy-
sis.
2. Chemical processes such as powerful detergents, organic
solvents, reactive acids, and alkalis. This approach can be used
to clean metal surfaces for imaging in the SEM

For topographic imaging, the surface must be degreased by
washing in high purity solvents such as acetone, toluene, or
alcohol, using an ultrasonic cleaner. It is important to use several
changes of the degreasing agent.


Table 10.1. A simple procedure for cleaning a metal surface
1. Remove adherent dry material with a tooth brush.
2. Remove oil and grease with a suitable solvent and a soft rag.
3. Treat metal surface with a suitable acid.
4. Dip into a boiling solution of ammonia and detergent solution
and scrub well.
5. Rinse the metal surface ten times first in tap water and then distilled
water.
6. Air dry the metal at ambient temperature.




A more vigorous cleaning of non-porous specimens is achieved
with an ultrasonic cleaner, in which the high frequency vibrations
are transferred to the cleaning fluid producing a turbulent pen-
etrating action. These cleaners are used in conjunction with water
or solvents to remove contaminating material from crevices and
small holes. Prolonged exposure to an ultrasonic cleaner may
damage some softer specimens. A final rinse with methanol helps
to remove any remaining surface films.


Plasma cleaning is a very effective way of removing organic
contamination before the sample goes into the microscope



This phenomenon was referred to as charg-
ing and is a common feature of most secondary electron images of
non-conducting specimens. This is because the secondary electrons
are emitted with such low energies, 5–50 eV, that local potentials
due to charging can have a large effect on the collection of the SE
signal detector, which typically has a potential of +300 V.





Modifying the Microscope by Optimizing the Operating
Conditions to Reduce Charging

However, specimen charging is a unique phenome-
non of the SEM and it would be erroneous not to briefly consider
the changes that may be made to the operation of the microscope
in order to reduce charging. Some of these changes have already
been hinted at in the Introduction.
1. Lowering both the voltage and current of the incident beam
diminishes charging. This is the first and easiest change to
make.
2. Changing the scan speed from slow to TV rate diminishes the
time the sample is exposed to the incoming beam.
3. Use the higher energy backscattered electron to image the
specimen rather than the much lower energy secondary elec-
trons, which are more readily influenced by both positive and
negative charging.
4. Charging may be minimized for a given material by operat-
ing the microscope at a low voltage, with the E1 selected to
correspond to the E2 upper crossover point as shown in Table
11.2 . In the modern SEM, the accelerating voltage easily may
be changed by small increments of 100 eV or less


The E2 values may be determined using the scan square test
described by Joy and Joy (1996). The uncoated sample is placed
in the SEM and imaged at ×100 at 3–5 Kev and a TV scan rate.
The magnification is very quickly increased to ×1,000, maintained
there for 5 s, and then immediately returned to ×100. The small
scan area at the center of the screen is then examined.

1. If the scan area is brighter than the background, then the sample
is charging negatively and the beam energy is greater than E2
(or less than E1
).
2. If the scan area is darker than the background, then the sample
is charging positively and the beam energy is less than E2 (or
greater than E1
).
Set the SEM at its lowest operating voltage and repeat the scan
square test. If the sample is charging positively, then E1 < E2
.
Carefully increase the voltage and image the sample at the point
where charging is minimized. If the sample is charging negatively,
then E1 > E2 and it is not possible to lower the voltage any further,
the sample should be tilted 45° and the scan square repeated.
1. When carrying out energy dispersive x-ray microanalysis,
check that the maximum x-ray photon energy emitted from
the specimen is equal to the energy of the incident electron
beam producing the x-rays. This is referred to as the Duana-
Hunt Limit.
2. Pre-bombard the sample with an argon beam that traps posi-
tive ions in porous surfaces, which suppresses the build-up of
a negative charge.



* COMMENT Sample Artifacts and Damage
The sole reason for using a scanning electron microscope is to obtain
accurate, precise and reproducible information about their structure
and chemical identity. We seek information either to confirm and
extend our existing knowledge about an object or investigate a new
and unknown object. The information we obtain is either in the
form of a picture (image) or as files of numerical data. We need to
be able to validate this information because the processes of obtain-
ing images and data using the SEM are usually very invasive and
totally alien to the environment in which we and our specimens
exist. We must be satisfied that the procedures used to obtain infor-
mation do not damage the object or introduce artifacts.
Damage is an unexpected and irreversible change in the object and
can occur before and during microscopy. In many cases, damage is
very obvious in an image and some examples are shown later in
this chapter. However, sometimes the damage is less immediately
obvious.
Artifacts are perceived structural distortions or misrepresentative
chemical changes to the original object that arise as a consequence of
the techniques used in preparing objects for subsequent microscopy
and analysis. Artifacts are frequently not immediately obvious.

Specific Discipline Journals Containing
Specimen Preparation Methods

Acta Materials www.elsevier.com/locate/actamat
Corrosion Science www.elsevier.com/locate/corsci
Intermetallicswww.elsevier.com/locate/intermet
Journal of Electronic Materials www.springer.com
Journal of Materials Research www.mrs.com
Journal of Materials Science www.springer.com
Materials Science and Engineering www.elsevier.com/locate/msea
Metallurgy and Materials www.asm.com
Metallurgical Transactions www.tms.com
Powder Metallurgy www.maney.co.uk
