3.4 Mechanical Tests
71
uses abrasive particles between the surface studied and the counterbody used in the
test, under either dry or wet conditions. Two-body sliding wear tests are of the most
common type for laboratory-scale wear test or relatively small objects. The relative
displacement of the test specimen and its counterbody is realized by a rotating motion
of either of the contacting bodies, and the test is standardized by fixing both the sliding
distance and the pressure (force) acting between the two contacting surfaces. These
methods can be further classified on the basis of the shape of the counterbody, such as
ball cratering, pin on wheel and thrust washer test. If the driver of the instrument that
provides the power for the relative motion of the two facing surfaces is equipped with
a suitable feedback system, the friction coefficient can be measured simultaneously
from the force required to maintain the desired relative motion. Several tribological
methods are defined by various national standards.
Wear resistance is strongly related to other properties of the specimen. As a thumb
rule, the wear resistance increases with hardness. The reduction of surface roughness
leads to improved wear resistance through a lower friction coefficient.
Adhesion tests include a number of methods of various complexity and are often
combined with other sorts of impact. In all cases, a load applied to the substrate–
coating system to provoke the detachment of a surface layer from the substrate.
The simplest qualitative adhesion test is the well-known scotch tape method. A
number of qualitative test methods include mechanical impact such as bending,
twisting, deep drawing, grinding, filing or local cupping. Other adhesion tests are
based on heat treatment exposing either the entire specimen or applying a local heat
shock. In these tests, the deformation difference of the coating and its substrate is
of importance, either due to the hardness/tensile strength difference of the mismatch
of the heat expansion coefficients. Hence, these tests are not of general validity but
refer to specific conditions that correspond to the load that the coating is expected to
withstand to. If the coating layer is flexible enough for rolling, a quantitative peel-off
test can be carried out by delaminating the entire coating layer by simultaneously
measuring the force required or the work performed during a complete delamination.
In both cases, the result is quantitatively elucidated and can be normalized (force
and work with respect to the width of the layer strip and to the entire surface area,
respectively).
3.5 Scanning Probe Methods
In scanning probe techniques, the surface of a solid specimen is scanned with a sharp
tip along a raster. The recorded parameter is a physical quantity that is converted
to a colour-scaled image which shows the local variation of the physical property
measured. The lateral dimension of the scanned area is at most a few micrometres.
The lateral resolution depends on the specific technique and is mainly determined
by the tip shape and size. The vertical resolution of all methods can be at the subnanometre scale, which is due to the precise motion control technique based on
piezoelectric drives.
71
uses abrasive particles between the surface studied and the counterbody used in the
test, under either dry or wet conditions. Two-body sliding wear tests are of the most
common type for laboratory-scale wear test or relatively small objects. The relative
displacement of the test specimen and its counterbody is realized by a rotating motion
of either of the contacting bodies, and the test is standardized by fixing both the sliding
distance and the pressure (force) acting between the two contacting surfaces. These
methods can be further classified on the basis of the shape of the counterbody, such as
ball cratering, pin on wheel and thrust washer test. If the driver of the instrument that
provides the power for the relative motion of the two facing surfaces is equipped with
a suitable feedback system, the friction coefficient can be measured simultaneously
from the force required to maintain the desired relative motion. Several tribological
methods are defined by various national standards.
Wear resistance is strongly related to other properties of the specimen. As a thumb
rule, the wear resistance increases with hardness. The reduction of surface roughness
leads to improved wear resistance through a lower friction coefficient.
Adhesion tests include a number of methods of various complexity and are often
combined with other sorts of impact. In all cases, a load applied to the substrate–
coating system to provoke the detachment of a surface layer from the substrate.
The simplest qualitative adhesion test is the well-known scotch tape method. A
number of qualitative test methods include mechanical impact such as bending,
twisting, deep drawing, grinding, filing or local cupping. Other adhesion tests are
based on heat treatment exposing either the entire specimen or applying a local heat
shock. In these tests, the deformation difference of the coating and its substrate is
of importance, either due to the hardness/tensile strength difference of the mismatch
of the heat expansion coefficients. Hence, these tests are not of general validity but
refer to specific conditions that correspond to the load that the coating is expected to
withstand to. If the coating layer is flexible enough for rolling, a quantitative peel-off
test can be carried out by delaminating the entire coating layer by simultaneously
measuring the force required or the work performed during a complete delamination.
In both cases, the result is quantitatively elucidated and can be normalized (force
and work with respect to the width of the layer strip and to the entire surface area,
respectively).
3.5 Scanning Probe Methods
In scanning probe techniques, the surface of a solid specimen is scanned with a sharp
tip along a raster. The recorded parameter is a physical quantity that is converted
to a colour-scaled image which shows the local variation of the physical property
measured. The lateral dimension of the scanned area is at most a few micrometres.
The lateral resolution depends on the specific technique and is mainly determined
by the tip shape and size. The vertical resolution of all methods can be at the subnanometre scale, which is due to the precise motion control technique based on
piezoelectric drives.