February_AMP

ADVANCED MATERIALS & PROCESSES •

FEBRUARY 2014

Accepted Practice to Test Bond

Strength of Thermal Spray Coatings

sufficient adhesion bond be-

tween the coating and substrate

is critical for the functionality of

the coated part. Coating integrity and

durability directly depends on the cohe-

sion bond strength. ASTM C633 “Stan-

dard Test Method for Adhesion or

Cohesion Strength of Thermal Spray

Coatings” is the baseline and mandatory

process to follow for tensile testing of

bond strength of thermal spray coatings.

The test is required as a condition of ap-

proval for new coatings and their suppli-

ers, and is the core qualification test for

coatings for aviation, oil and gas, auto-

motive, power, marine, and many other

industries. This practice clarifies details

of ASTM C633 requirements and provides examples of

best practices confirmed by hundreds of tests performed

wordwide, adopted by numerous industrial standards, and

requested to comply by international technical standardiza-

tion and certification organizations such as ISO, AS, SAE,

and Nadcap.

Test scope

The ASTM C633 test applies tensile

stresses to coated systems consisting of a

coated sample glued to another cylindrical

sample as shown in Fig. 1. Alternatively, the

coating could be applied on a cylindrical

“button,” which is glued between two cylin-

drical samples as shown in Fig. 2. The load is

applied in a perpendicular direction to the in-

terface between the coating and substrate.

The amount of tensile load is gradually in-

creased from 0, which results in sample fail-

ure—the coating pulled out from the

substrate or fractured in two pieces inside the

coating. The coated surface is always a flat

circle with a diameter of 1 in. + 0/-0.005 in.,

and sample dimensions are standardized by

ASTM 633. Bond strength is calculated as the load at sam-

ple failure divided by the coating failure area.

Coating thickness

Coating thickness of 0.01 in. (0.25 mm) is recom-

mended if coating porosity is below 2%. If the coating has

high porosity, increase coating thickness to 0.015 in. (0.38

mm) and above, as recommended by ASTM C633 to pre-

vent glue penetration through the coating to the substrate,

but do not exceed the thickness required for the coated part

application. Thicker coatings have lower bond strength due

to residual stress accumulation that can lead to nonrepre-

sentative test results.

Coating thickness variation

The coating thickness should not vary across the surface

by more than 0.001 in. (0.025 mm). The coating surface

may be finished by grinding or machining when thickness

variation is excessive. Other treatment, such as grit blast-

ing, should not be used to level the coating thickness. Coat-

ing tapering increases sample misalignment, which results

in failure at a lower load.

Glue

Polyamide-epoxy FM 1000 Adhesive Film is recom-

mended (mandatory in many industrial specifications) as

a bonding glue for the coating tensile test. Advantages in-

clude allowing testing of high-porosity coatings (such as

abradable and ceramic coatings) without glue penetration

to the substrate. Disadvantages include losing its own

strength if stored at temperatures of 85°F (29°C) and

above, but can work for years if stored in a refrigerator.

The ASM Thermal Spray Society (TSS)

Accepted Practices Committee on

Metallography released a new document on

the accepted practice to test the bond

strength of thermal spray coatings.

Bond test step-by-step

• Inspect coating quality on bond caps. (Coating chipping, cracks,

delamination, separation, or overspray are not allowed.)

• Prepare mating caps; grit blast their flat surface. (Do not grit blast

coating surface).

• Apply a layer of glue on coating surface.

• Place coated samples with applied glue layer into V-groove of

curing fixture (see example of V-groove gravity fixture in Fig. 3).

• Add mating cap in the fixture with the grit-blasted surface facing the

glue layer on coated sample.

• Apply compression pressure to assembled samples. Note: Fig. 3

doesn’t show the additional weight on the top of assembled sample

to keep them in compression during glue curing (see below under

requirements).

• Place fixture with assembled and compressed samples in preheated

oven.

• Keep samples under pressure in heated oven until glue cures; cool

samples and release pressure.

• Carefully remove excess glue from each assembly with grinding.

Recommended abrasive size for grinding paper or wheel is not

coarser than 120 mesh; grinding direction is parallel to glued

surface; interface damage/removal of sample material(s) not allowed.

• Place assembled samples in grips of tensile machine, gradually apply

tensile load, and record failure load.

• Measure diameter (D) of sample failed face, and calculate the

failure area as follows: 3.14 × D

/4.

• Bond strength = load of failure/failure area.

To obtain representative results, compare properties of coatings de-

posited and tested at different facilities; test conditions and processes

require consistency and standardization.

Mating

cap

Adhesive

Coating

Bond cap

Fig. 1 —

Samples assembled

for bond test and applied tensile

load.