Types
of Thickness Standard
There
are two basic types of coating thickness standard, foils and pre-coated
metal. The following sections describe the two types in more detail
and discuss their relative merits and disadvantages so that you can
make a choice appropriate to the condition under which you will be working.
Foil
Thickness Standards
The lowest
cost and most convenient thickness standards are made from plastic foil
material and have been supplied over the years as measured and unmeasured
foils or shims. The current range of thickness extends from 12.5 µm
(0.5 mil/thou) to 20 mm (787.4 mil/thou 0.787 inch).
The unmeasured
foils are 25 x 50 mm strips of non-compressible plastic in the range
25 to 500 µm.
Measured
foils use the same non-compressible plastic that is designed for use
as gasket material but these are measured using a linear displacement
transducer (LVDT) system so that the measurement is independent of the
coating thickness gauges for which they provide the thickness standard.
In this lies one of the major advantages of the foil thickness standards.
The foils
can easily be measured from either side and, therefore, the transfer
standards used to calibrate the LVDT can themselves be calibrated and
certified to National Standards. Stainless steel slip gauges are used
and these are sent annually to a UKAS (Formally NAMAS) Measurement Laboratory
for independent certification.
The LVDT
system is also independently calibrated and certified by a UKAS Laboratory
on an annual basis. The slip gauges are used regularly through the working
day to ensure that the system stays within calibration and the system
is located in a designated area with restricted access to further maintain
its integrity.
A further
service is offered where a set of foils is supplied with a calibration
certificate. Up to 8 foils can be listed on an individual certificate
where they are identified with a serial number that traces the operator
who performed the measurement and the date it was carried out.
The certificate
also gives details of the certificates that apply to the measuring system
and the slip gauges used to calibrate it. The foil measurement and the
associated calibration techniques are recorded within the ISO 9002 Quality
Management procedures and therefore fall within the scope of Elcometer's
ISO 9002 accreditation.
The
Correct Use of Foils.
The point
at which thickness measurements are taken on a foil is at the intersection
of two imaginary lines drawn from corner to corner across the foil.
In practise, three measurements are taken at this point and these measurements
must be within 1 µm of each other for the foil to be acceptable as a
calibration standard.
The final
reading is recorded and printed on the label in both metric and imperial
(English) units. When using the foil it should be placed flat on the
surface and the coating thickness gauge probe placed at the point on
the foil where the two imaginary lines intersect.
Care must
be taken to ensure that no dirt or dust is trapped under the foil between
the foil and the substrate, as this will affect the reading taken. Foils
must be considered as consumable items and replaced regularly.
The probe
is brought into contact each time the foil is used and wear will occur
with use particularly in the case of the thinner values, 12.5, 25 and
50 µm (0.5, 1.0 and 2.0 mil/thou) and where blast cleaned, roughened
surfaces are used.
The thinner
foils can be used on curved surfaces, both concave and convex. Foils
of 250 µm (10 mil/thou) and above are rigid and are not capable of adopting
the shape of a curved surface.
Typical
Foil Thickness Standard Pre-Coated Thickness Standards.
The original
coated thickness standards available from National Standards bodies
were the, so-called, NBS standards. These originated with the National
Bureau of Standards in the USA. This body has now changed its name to
NIST. (The National Institute of Standards and Testing.)
Although
NIST has been operating for some years these coated standards are often
referred to as NBS Standards. The standards were made using steel plates
plated with copper to build the thickness required and finished with
a layer of chrome plating to provide a hard wearing surface. As these
films are adhered to the substrate, a non-destructive coating thickness
method is used to measure the final coating thickness.
The technique
chosen is beta-backscatter where the coating is illuminated with beta
radiation from a calibrated source and the absorbed energy is a measure
of the coating thickness. The back-scattered energy is detected using
a Geiger counter and the system is calibrated using coatings of known
thickness.
The fundamental
accuracy of the count is ± 5% so the accuracy of measurement on NIST
standards is no better than this limitation. When these standards were
used exclusively for the mechanical pull-off gauges, originally designed
to a ± 10% accuracy specification, ± 5% accuracy was sufficient.
These
days even pull-off type gauges have ± 5% accuracy specifications so
the NIST standards are no longer sufficient for checking these gauges.
Electronic gauges are capable of much better accuracy and NIST standards
are not relevant to these gauges.
n recent
years, pre-coated standards, using hard wearing epoxy coatings on both
steel and aluminium plates, have become available. With these standards
the substrate is carefully prepared and the individual plates are serialised
and characterised for thickness using a linear displacement transducer
system. The coating is then applied, lapped and polished back to the
required thickness before the total thickness of plate and coating is
re-measured.
The coating
thickness is the difference between the total thickness and the thickness
of the plate alone. The accuracy of this process is typically ± 2% but
with care and over restricted thickness ranges a ± 1% accuracy, traceable
to National Standards using the LVDT slip gauges previously described,
can be obtained.
Sets of
pre-coated epoxy coating thickness standards are supplied in binders
with a zero plate and two or three thickness values together with a
calibration certificate. The certificate can be renewed by re-measuring
the total thickness with reference to the original substrate data traceable
through the serial number of the set.
Advantages
and Disadvantages of Foils and Pre-Coated Standards.
As with
many engineering solutions there are advantages and disadvantages with
both types of the thickness standards in current use. These are listed
in the following table.
|
Advantages
and Disadvantages of Coating Thickness Standard Types
|
|
Type
of Standard
|
Advantages
|
Disadvantages
|
|
Foil
|
-
Low
Cost
-
Can
be used on sample of substrate to be tested
-
Thin
values can be used on complex shapes
-
Can
easily be re-measured
-
Traceability
is easy to establish and prove
|
-
Easily
damaged
-
Must
be kept free of dust and contamination on BOTH sides
-
Can
introduce an air gap at the substrate causing small errors
-
Can
cause pull-off magnet probes to be force off prematurely
-
Are
often supplied unmeasured
|
|
Pre-Coated
|
-
Hard
wearing
-
Can
be re-calibrated by the manufacturer
-
Do
not cause premature lift-off of permanent magnet probes
-
Easy
to use off site
-
Always
supplied with measurements
-
Can
be used for gauge verification
|
-
High
Cost
-
Substrate
not always representative of substrate to be tested
-
Not
easily re-measured
-
Can
be of insufficient area for certain gauge types
-
Some
types have been shown to be inaccurate, alloying of the coating
to the substrate can occur.
-
Not
suitable for curved or roughened surfaces
|
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