The diamond stylus
typically has a Rockwell C geometry with an angle of 120 degrees and a
spherical tip radius of 200 microns. Source: CSM Instruments
ASTM standard for scratch adhesion testing, ASTM C1624, covers the
determination of the adhesion strength and failure modes of hard, thin ceramic
coatings on metal or ceramic substrates. Scratch testers can be used for
industrial quality control as well as scientific research. They should allow
both lightly skilled operators and more experienced personnel to use the
instrument to characterize the mechanical properties of Physical Vapor
Deposition (PVD) or Chemical Vapor Deposition (CVD) coatings.
In scratch testing, a diamond stylus of defined
geometry is drawn across the surface of a coated sample at a constant speed
with a defined normal force over a defined distance. The normal force can be
constant, progressively increasing or incrementally increasing. The diamond
stylus typically has a Rockwell C geometry with an angle of 120 degrees and a
spherical tip radius of 200 microns. Different tip radii can be used to change
the contact pressure.
During the test the tangential force, the penetration
depth and the acoustic emission signals are recorded as secondary test data
along with the normal force. After the completion of the scratch test, the
scratch track is microscopically analyzed for specific, well-defined damage
such as cracking, deformation, buckling, spallation or delamination of the
coating. The load, which corresponds to the failure event, is denoted as the
critical load, Lc, and is related to the practical adhesion strength and the
damage resistance of the given coating or substrate system.
The critical load depends on the test parameters such
as stylus parameters and geometry, loading rate and scratch speed, as well as
on the properties of the coated sample such as coating thickness, surface
roughness and microstructure, damage mechanism, hardness, modulus of elasticity
and fracture strength.
In constant load (CL) scratch testing, the normal
force is maintained at a constant level while scratching the sample. By
increasing the load for each subsequent scratch, a scratch map can be generated
to determine the critical load corresponding to a specified damage. In progressive
load (PL) scratch testing, the stylus is drawn along the sample while the
normal force is linearly increased to a maximum pre-determined value. The
critical load is recorded as the normal force at which the damage is first
observed. Incremental load (IL) scratch testing consists of incrementally
increasing constant load scratch segments and is useful if space is limited on
The tangential force (Ft) is the force that opposes
the relative motion between the stylus and the sample. During the scratch test
the tangential force, or frictional force, may change in amplitude as different
types of damages occur with the increasing load. The Acoustic Emission (AE)
sensor can detect any high frequency elastic waves produced in the coating or substrate
system by brittle damage events like cracking, delamination, chipping and
This shows a selection of typical progressive
load scratch tests on a range of different coating-substrate combinations in
common use. Each scratch path is labeled with the coating, its exact thickness
and the substrate material on which it is deposited. It is obvious how
different the failure modes are for these seven examples. Source: CSM Instruments
Typical scratch test instruments are designed
for ease of use in industrial quality control and production lines.
Comprehensive scratch test software enables the operator to predefine the
measurement protocol. The protocol is exported via a USB memory stick that is
inserted in the front panel of the instrument and the test can be performed
immediately. After the test is completed, the scratch track can be investigated
with the optical microscope.
With optional tangential force and acoustic emission
sensors, the scratch tester also is a complete coating characterization station
for research facilities. Scratch test protocols can be sent to different
production plants to maintain an internal test standard within the company.
A typical scratch test example is shown here, along with the sample
information and the data set.
Another scratch test feature is its active
feedback of the applied load. This active feedback control allows the
measurement of flat and curved surfaces. The indentation mode allows operators
to perform conventional Rockwell and Vickers indentation tests to determine the
hardness of the sample. The available pre-scan and post-scan options enable
profiling of the surface before and after the scratch.
With the pre-scan profiling, the surface roughness
and topography are taken into account in order to get the true penetration
depth (PD) during scratching. The post-scan profiling is used to
obtain the residual depth (RD) data that is important for certain
materials experiencing visco-elastic relaxation.
Three different scratch test modes are shown
here: constant load, progressive load and incremental load. Source: CSM Instruments
Test in action
A DLC coating on steel substrate is a typical
scratch test example. (See pg. 17.) The thickness of the coating is 5 microns.
A progressive load scratch test with a maximum load of 35 Newton was performed
on the sample. The scratch length was 1 millimeter and the loading rate was 35
N/min. The first critical load (Lc1) is 12 Newton and corresponds to the point
where the first damage is observed. This first damage has the shape of an
interfacial shell-shaped spallation. Note that Lc1 corresponds to the first
small jump on the acoustic emission signal as well as on the friction force
The second critical load (Lc2) is 18 Newton and it is
the point where the damage becomes continuous and complete delamination of the
coating starts. After this point, all of the acoustic emission, friction force
and penetration depth signals become noisier. These critical points are shown
on the scratch track with arrows.
With growing interest in scratch testing for
industrial quality control and a new ASTM standard (see sidebar), scratch
testers are proving to be a valuable tool for coating characterization. Such
technology will enable coating companies to have better control over their
deposition methods and build up a complete database of coating failure modes.
In addition, scratch test information can be used to justify the adhesion of a
coating to a prospective customer and offers a simple and quantitative method.
Sidebar: ASTM Standard
Due to the acceptance of the scratch test as the primary quantitative
analysis method in coating production and a high demand for scratch test
equipment, a new ASTM standard has been developed. The new ASTM standard for
scratch testing is designated as ASTM C1624, “Standard Test Method for Adhesion
Strength and Mechanical Failure Modes of Ceramic Coatings by Quantitative
Single Point Scratch Testing.” This standard explains the principles of the
scratch test in detail along with the limitations of the test, applicability to
different coatings, terminology, test methodology, specimen requirements,
apparatus requirements, calibration, test procedure, calculations, and
repeatability and reproducibility requirements.
A comprehensive bibliography and a scratch atlas
describing and illustrating the common damage types are included in the
standard. This standard provides a complete and accurate document for helping
coating manufacturers develop an in-house quality control scratch test, which
can be used regularly to screen coated components and evaluate their adhesion.