Biomedical Engineering Reference
In-Depth Information
Adhesion testing is essential to ensure the coating will adhere properly to the substrate
to which it is applied. Many techniques have been used for adhesion measurement of films
and coatings as demonstrated by Mittal (1995). The most popular test methods for measur-
ing the bond strength of film on substrate systems include pull-off, cross-cut, indentation
scratching at increasing loads, and pin-on-disk. Microtensile testing is also an excellent
method to determine the adhesion integrity of nanocoatings.
Pull-off testing is perhaps the most common method for determining the bond strength
between a thin film and a thick substrate. Many types of configurations have been pro-
posed (Vallin et al. 2005) using universal testing machines or dedicated tensile testers. The
main requirements are that the alignment of grips and sample fixture is precise to ensure
uniform loading and that the adhesive used to bond the sample to the fixture is sufficiently
strong (stronger than the bond strength of the film-substrate) and does not penetrate the
interface of interest. The bond strength is determined from the critical load required to
separate the film-substrate contact area. The main advantage of pull-off testing is its appli-
cability and versatility to a wide range of coating/substrate systems. It can be used for both
soft flexible coating as well as hard brittle ones. The weakness in typical pull testing is the
wide variation in data. One must perform multiple tests on a given sample and statistical
analysis to obtain reliable quantitative data. In biomaterials, pull-out is a more common
mode of failure of a bone implant than bone fracture. Pull-out may not necessarily occur at
the interface between bone and coating, but instead between the metal and the hydroxy-
apatite coating.
Pin-on-disk tribometers are used to determine the wear resistance and friction coef-
ficient of surfaces. The tester consists of a pin (normally a ruby, tungsten carbide, or metal
sphere) under a static load in contact with a rotating sample. Wear coefficients for the pin
and disk material are calculated from the volume of material lost during the test. The
method facilitates the study of friction and wear behavior of many material combinations
with or without lubricant. Pin-on-disk test can be run for an extended period of time either
in air or a simulated body environment to provide information on the degradation of the
coating with time (sliding distance).
Similar devices that are used in biomaterials research are pin-on-plate type wear test-
ing machines, such as the Ortho-Pod instrument (Advanced Mechanical Testing, Inc.,
Watertown, MA) (Figure 2.12d). This has six stations for the pins with independent servo-
controlled variables that correspond to rotary motion of both the plate and the pins and
the normal load applied to each of the six pins. The pin rotation feature gives this machine
the ability to generate pin/plate sliding motions that are typically not available on stan-
dard pin-on-disk machines. The Ortho-pod is an excellent tool for testing implant materi-
als whose wear characteristics depend upon various sliding directions (crossings) as well
as load, in either dry or a fully lubricated environment (serum or other fluids).
Scratch testers (Figure 2.12a) use a small diamond stylus or hard metal (such as tungsten
carbide) normally with a 200-µm radius that is drawn across the coated surface at a con-
stant velocity at increasing load. The load on the diamond causes stresses to be increased
at the interface between the coating and the substrate, which can result in flaking or chip-
ping of the coating. The load at which the coating first delaminates is called the critical
load and is a measure of coating adhesion. Coating failure can be determined from load
and the distance at which delamination begins using microscopy, or from the change in
friction or using an acoustic emission sensor. In addition to the critical load, the applied
normal force, the tangential (friction) force, and the penetration depth are obtained.
Examining scratch test results is difficult due to the complex stress states involved and
the broad array of damage processes that can occur depending on film thickness and the
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