Figure 2. Brittle fracture surface of NiAl
imaged with contact mode AFM. 5.7µm scan.

Figure 3. Microstructure of the superalloy
CMSX-6. The cuboidal precipitates (green)correspond to the γ' phase. 4µm scan.

near-atomically-flat terraces. Here, theenrichment of chromium on the steppedsurface prohibits oxidation in small
bands on the upper side of the steps.

necessary for most microstructuralinvestigations, can be developed bymany chemical preparation methods aswell. In comparison to these smoothsurfaces, fractured specimens often
show very high roughness levels. In
Figure 2 an example of a brittle fracturesurface is shown, where fracture facets
with distinct orientations are resolved
with the AFM.

Surface Preparation
and Roughness
Since AFM allows imaging ofconductive and nonconductive surfaces
directly in ambient air, surface
preparation is generally an easy task.
To image very small particles, however,etching and/or polishing is sometimes
required to prepare a surface which isflat enough to resolve the particles (themean remaining roughness of thesurface must be kept lower than theparticle size). The roughness itself isdetermined easily from AFM images.
Some typical roughness values after
different preparation steps are listed
in Table 1.
Electropolishing is often the best choicefor preparation of metallic specimens.
Topographic contrasts, which areSpecimen
Fracture Surfaces
Steel
NiAl Single Crystal
Superalloy
Superalloy
Microalloyed Steel

Quantitative Evaluationof Microstructures
For exact quantitative measurements,
the etching or preparation depth of thesurface should be minimized – i.e.
substantially less than the mean particlesize. The superior vertical resolutionof the AFM is optimal for thesemeasurements. Figure 3 shows a 3D
topographic image from a chemicallyetched specimen of the superalloy
CMSX-6. The lower (green) parts of theimage corresponds to the γ' phase,
which is responsible for the excellent

Preparation Method
None
Mechanical polishing(1µm diamond paste)
Mechanical polishing(0.25µm)
Chemical etching
Electropolishing
Electropolishing

Roughness (Ra)
100nm - 5µm
2 - 5nm
1nm
3 - 4nm
0.8nm
0.6nm

Table 1. Typical roughness values for different metallic surfaces as measured with AFM.

high temperature properties of thisalloy. Volume fraction and precipitation
size obviously are important parameters
for optimizing the high temperature
performance of this alloy. A quantitativeevaluation of such parameters isillustrated in Figure 4. The low etching
depth (14nm) of the γ' phase isillustrated through the linescan. The
area fraction of the precipitates wasdetermined from a histogram of theimage data, where two maxima
reveal the microstructure of the γ' andmatrix phases.
In other superalloys, spherical particles(instead of cuboidal) are precipitatedthrough the heat treatment. Waspaloy,for example, shows a bimodal
distribution of precipitates. Here, thesmall precipitate size prevents reliable
quantitative evaluation of the γ' volume
fraction with most microscopic
techniques, and quantitative evaluationof TEM images is difficult, since
determination of the transmitted film
thickness is a time-consuming task.
Again, AFM overcomes thesedifficulties. Figure 5 shows an example,
where the bimodal size distribution wasdetermined from a single AFM image
after an electropolishing procedure.
Radii of 30nm and 200nm were
found for the small and large
precipitates, respectively.

Nanomechanical Studies
The AFM can also be used to indent
the specimen surface plastically.
Indentation techniques, as for example
Vickers hardness testing, are well-known
methods in the field of materials sciencefor fast estimations of yield stress andtensile strength with hardness values
calculated from optical inspection of
the indentation size. With AFM,indentation techniques can now be
performed at much smaller size andforce scales.

For “nanoindentation,” diamond tips
are usually used on metal samples.
The size of the indents are determineddirectly after indenting by scanning
the affected surface with the same
AFM tip that produced the indent(s).
Nanoindenting is performed withmaximum loads in the range of only afew hundred micro-Newtons, which
also makes this technique very attractive
for studying thin films. Figure 6 shows
how the different phases of thesuperalloy CMSX-6 were indented witha diamond tip, and the different sizesof the indents reveal the lower hardnessof the matrix phase. Indentation sizes
can be reduced to ten nanometersor less, which makes possible highly
localized studies of the mechanicalproperties of the microstructure(Figure 7).

In-Situ Loading andInvestigation of Cracks
Another application of AFM is imaging
of defects like cracks, dislocations andpores. The high resolution capabilityof AFM permits study of the processesof brittle and ductile crack growth indetail. To study such crack growth, asmall specimen was mounted inside
the AFM with a small force loading
device which was built for the Digital
Instruments MultiMode AFM. Preloading
was done outside the AFM with amicrometer screw, and further loading
inside the AFM occurred by applying
an electric voltage to a small
piezostack. Figure 8 shows two images
from a loaded mode “I“ crack in asingle-crystalline specimen of NiAl. Thecrack grows after loading (from the left
to the right image) in a quasi-stablebrittle manner in steps of nearly 1µm
length. In both images, the high-stressed
red region around the crack tip wascaused by small elastic displacements.
The high stress intensity of the crack
produces a small elastic depression
zone (red) with a depth of only someten nanometers. These elastic
displacements around the brittle crack
tip are best visible with the high verticalresolution of the AFM.
For larger specimens, Digital Instruments
Dimension™ Series AFMs allow
investigations with commonly used
testing machines, where stresses andstrains are applied and measured on
tensile or even bend specimens. Figure
9 shows an example of a cracked
specimen of NiAl with a fracture
toughness (KIC) of 4 MPam1/2 loaded
under force control. The AFM image
shows that even this material – which isextremely brittle at room temperature –shows small amounts of ductile
deformation behavior, i.e. dislocation
emission at the crack tip. Two glideplanes, which include an angle of 45°
with the crack, are activated throughdislocation emission. The number anddistribution of dislocations emitted fromthe crack can be counted by studying
the kink motion of parallel polishing
traces remaining on the surface after
mechanical polishing. As a dislocation
crosses a polishing trace, it will bekinked in a step corresponding to theBurgers vector length of the dislocation.
From these plastic deformations andalso from the elastic depression zonearound the crack tip, the fracture
toughness can be calculated. Thiswork helps to develop a better
understanding of fracture mechanism
and the brittle-to-ductile-transition inmetals and intermetallic compounds.

Figure 4. AFM image (top left) can beused to measure sections through themicrostructure of the superalloy CMSX-6,12µm scan. The height difference (14nm)
between the two phases in CMSX-6 isobtained from a line section (left, bottom).
The area fraction of the γ' phase is
obtained from the histogram (above).

Figure 5. Waspaloy’s bimodal precipitation
distribution is responsible for its high temperature
properties. The volume fraction of the small γ'
precipitation and the particle size distribution canbe determined from AFM images with highaccuracy. 2.2µm scan.

Figure 6. The hardness of both phases inCMSX-6 was compared through small
nanoindentations obtained using a diamondprobe tip. The smaller size of the triangularindents in the γ' phase (yellow) reveal it’s greater
hardness. 2µm scan.