Wet Paint No Barrier to Timely, Accurate Measure of Coating Thickness

A newly demonstrated technology providing
the first-ever nondestructive means to
measure the thickness of specialty paint
coatings during their application will
generate substantial savings for the Air
Force. An AFRL materials engineering team
successfully demonstrated the integration
of a paint spray robot with a sensor—
specifically, a miniature Class I, Division
1–compliant time domain (TD) terahertz
(THz) sensor—for real-time cure monitoring
of coating materials as they are applied to
aircraft surfaces. By ensuring the application
of coatings to the correct thickness the first
time, the integrated spray booth/sensing
capability will eliminate costly sanding,
rework, and/or reapplication tasks, increasing
confidence in coating materials and helping
to ensure that production schedules—in this
case, for the F-35—are met.
Aircraft maintainers commonly apply
specialty coatings via robot or by hand.
Despite the critical relation of coating
thickness to material performance,
maintainers have historically relied on
destructive tests, costly and inaccurate
witness coupons, or otherwise limited and
unreliable measurement systems—all of
which require contact with a dry coating
surface. To address the issue, AFRL
partnered with Picometrix and Northrop
Grumman to conduct initial testing and
demonstration of a noncontact TD THz
instrument for in-process cure monitoring
of specialty material coatings developed
under an earlier, Small Business Innovation
Research (SBIR) Phase II effort. With the
aid of computer simulations, the AFRL/
industry team determined the optimal size
of the sensing package, as well as the best
location for incorporating the sensor into the
automated spray system.
Key to development success was the
capacity to mate the sensor to the spray
system without altering the production
spray path or qualified procedures. While
successful laboratory demonstrations of
the integrated system occurred earlier in
the program, the achievement of comparable
results in a production environment
was critical for program completion
and subsequent technology transition.
Ultimately, a production demonstration
confirmed that the measurement system
could be successfully mounted to the
robot and accurately measure wet coating
thickness during a spray event. Additional
data collection, validation tests, and system
deployment activities are slated to occur
under a SBIR Phase II extension effort
ongoing through Fiscal Year 2011.
Meanwhile, the TD THz instrument is
undergoing development for operation in the
production line/quality assurance process
at Northrop Grumman’s paint booth facility
in Palmdale, California. Recent testing
demonstrated the TD THz system installation
concept (i.e., placement of the control unit
in the control room, as well as routing of
the umbilical cord to the gantry and the
robot arm). Researchers achieved sensor
operation exhibiting no loss in fidelity or
electrical interference with the sensor
signal and thus enabling high-quality
measurements over the range of sensor-tilt
angles of interest.