Metallographic Preparation of Orthopedic Medical Devices
D. J. Medlin Zimmer Incorporated, Warsaw, Indiana, USA
G. M. Lucas and G. F. Vander Voort Buehler Ltd, Lake Bluff, Illinois, USA
Abstract
Metallographic sample preparation methods for porous coated
implant devices can be difficult due to inadequate fill of the
mounting materials into the porous metallic structures.
Inadequate fill of the mounting material during sample
preparation leads to problems such as edge rounding, uneven
etching, and metal smearing during polishing. These
problems make proper microstructural identification and
analysis difficult and/or inaccuate.
Two porous coated implant components were
metallographically prepared by five different sample
preparation methods to determine which method would
develop the best metallographic specimens. Edge retention
was best when the specimens were electroless nickel-plated
and mounted in Epomet®-F thermosetting resin. This
mounting material had the best fill in the porous coating areas
and resulted in superior microstructural clarity. Three other
preparation methods, including vacuum impregating with two
epoxy resins and mounting in phenolic resin, resulted in
adequate metallographic images. The Sample-Kwick® cast
acrylic resin resulted in more edge rounding and uneven
etching than the other mounting materials when evaulating
porous coated metallographic specimens.
Introduction
Metals have a diverse application in the medical field as
implantable, internal (in-vivo), structural, load-bearing
replacement components and surgical instruments. A few
examples of metallic components include hip and knee
replacements, fracture fixation plates, screws, cables, surgical
blades and tools, etc. The field of metallography plays a
significant role in the quality control of metals used to
manufacture medical implants. Metallographic techniques are
used to examine raw materials (metals) prior to fabrication of
the devices and systematic examinations during and after
specific processing steps to insure the final product will be
safe and effective when used in patients.
Some implant designs have porous metallic coatings on the
surface to improve the adhesion at the bone/metal interface by
bone in-growth (or on-growth) of bone tissues into the
metallic coatings. Traditional metallographic techniques can
be insufficient in properly preparing porous metallographic
specimens and revealing microstructures due to problems such
as: edge rounding, incomplete fill of mounting material,
porous metal smearing, bimetallic polishing and etching
problems, color metallographic etching issues, etc.
The purpose of this investigation is to find more efficient and
thorough methods to prepare porous coated metallic
specimens by resolving typical porous metal preparation
issues and allowing improved and more complete
examinations of the microstructures.
Specimens Evaluated
Two porous coated metallic samples were evaluated in this
evaluation. Sample 1 was an acetabular cup made from Ti-
6Al-4V-ELI alloy (ASTM F-136) with a commercially pure
(CP) titanium (ASTM F-67) fiber metal wire coating (mesh)
on the surface1. Sample 2 was a femoral hip stem made from
a Co-Cr-Mo alloy forging (ASTM F-799) with Co-Cr-Mo
beads (ASTM F-75) sintered to the surface1.
Specimen Mounting Procedure
The scope of this study was to find an improved method of
metallogrphically preparing porous coated specimens by
obtaining more complete impregnation and fill of the
metallographic mounting materials into the voids of porous
metal coatings. Five different combinations of mounting
compounds, specimen coating materials, and mechanical
impregnation procedures were evaluated in an effort to reduce
edge rounding and incomplete mounting material fill. Table 1
list the five different mounting procedure combinations.
Table 1: The five different mounting procedures used in this
analysis.
Test
Number
Mounting Combination
1
Phenocure™ thermosetting phenolic resin
2
Electroless Ni-plating and Epomet®-F
thermosetting resin
3
Vacuum impregnation with low-viscosity Epo-
Thin® epoxy resin
4
Vacuum impregnation with Epo-Heat™
epoxy resin
5
Sample-Kwick® cast acrylic resin
Titanium Alloy Preparation Procedure
Sample 1 was a Ti-6Al-4V acetabular cup with a
commercially pure (CP) titanium wire mesh diffusion bonded
to the surface. After mounting the specimens, refer to Table 1,
the specimens were ground with a 320-grit abrasive silicon-
carbide Carbimet® paper. A force of 18 N (4 lbs) at 250 rpm
was used with the specimen holder and platen rotating in
opposite directions (contra rotation). This process was water
cooled and was ground until the specimen was planar. The
polishing process initiated with a 9-μm Metadi Supreme®
polycrystalline diamond suspension on a Ultra-Pol™ silk
cloth. A force of 18 N (4 lbs) at 200 rpm was used with a
contra rotation between the specimen holder and the platen for
approximately 4 minutes. Next, the specimens were polished
with 3-μm Metadi Supreme® polycrystalline diamond
suspension on a Texmet® 1000 pad with 18N (4 lbs) of force
at 200 rpm. Contra rotation was used for approximately 4
minutes. The final polishing procedure was Mastermet-2®
slurry on a Chemomet® pad with 31N (7 lbs) at 150 rpm. The
Mastermet-2® slurry is a 0.02-μm colloidal silica attack polish
made by mixing 1 part H2O2 (30% concentration) to 6 parts
Mastermet-2. The specimen holder and the platen were
rotated in the same direction (comp rotation) for 7 about
minutes.
Cobalt Alloy Preparation Procedure
Sample 2 was a Co/Cr/Mo femoral hip stem with Co/Cr/Mo
beads sintered to the surface. After mounting the specimens,
see Table 1, the specimens were ground planar with a 125-μm
diamond Apex™ DGD disk with 18N (4 lbs) of force at 250
rpm. The specimens polished with contra rotation between the
specimen holder and the platen and ground until the specimen
was planar. The specimens were then ground with 320-grit
silicon-carbide Carbimet paper with 18N (4 lbs) of force at
250 rpm and contra rotation for approximately 2 minutes.
Polishing was done with 9-μm Metadi Supreme®
polycrystalline diamond suspension on a Ultra-Pol™ silk
cloth. A force of 18 N (4 lbs) at 200 rpm was used with a
contra rotation between the specimen holder and the platen for
approximately 4 minutes. Next, the specimens were polished
with 3-μm Metadi Supreme® polycrystalline diamond
suspension on a Texmet® 1000 pad with 18N (4 lbs) of force
at 200 rpm. Contra rotation was used for approximately 3
minutes. The next polishing procedure was a two part 4
minute cycle. First, a Mastermet-2® slurry on a Chemomet®
pad with 36N (8 lbs) at 150 rpm was used and then at mid-
cycle (after about 2 minutes) a Mastermet alumina suspension
was used. The specimen holder and the platen were rotated in
the same direction (comp rotation). The final procedure was a
1 hour vibratory polish using Masterprep™ alumina
suspension on a Microcloth® pad.
Etching Procedure
The titanium alloys were etched with Kroll’s Reagent and
modified Weck’s Reagent, as shown in Table-2. The Weck’s
Reagent was used for color metallographic imaging. The Co-
Cr-Mo alloys were etched with an HCl and H2O2 (3%
concentration) mixed in a 5 to 1 ratio, also shown in Table 2.
Table 2. The etchants used for the titanium and Co/Cr/Mo
alloys. Weck’s Reagent is a color etchant2-4
.
Etchant
Name
Procedure Composition
Kroll’s
Reagent
(titanium)
Immerse 5-
30 seconds
10 ml HF
5 ml HNO3
85 ml H2O
Weck’s
Reagent
(titanium)
Immerse
for 15-30
seconds
5 g ammonium bifluoride
4 ml HCl
100 ml H2O
Colbalt
Etchant
Immerse 2-
4 minutes
100 ml HCl
20 ml H2O2 (3% conc.)
Metallographic Results – Acetabular Cup
Sample 1 (Ti-6Al-4V acetabular cup with CP titanium wire
mesh) was initially mounted in a Phenocure™ thermosetting
phenolic resin and polished with the titanium alloy
preparation method. Figure 1a shows the microstructure
etched with Kroll’s Reagent and Figure 1b shows the color
tinted microstructure etched with Weck’s Reagent. These
acetabualr cup components were diffusion bonded to
metallurgically attach the CP-titanium wire mesh to the Ti-
6Al-4V substrate. The metallurgical bond between the CP-
titanium wires and between the wires and the Ti-6Al-4V
substrates can be seen. Minimal edge rounding and excess
edge etching is apparent in these images.
Figure 1:Titanium fiber metal diffusion bonded to Ti-6Al-4V
substrate and mounted in phenolic mounting material. Figure
1a (top) is etched with Kroll’s Reagent and Figure 1b (above)
is color etched with Weck’s Reagent.
Additional specimens from Sample 1 were electroless nickel-
plated and then mounted in an Epomet®-F thermosetting
resin. Coating the specimen with a layer of nickel helps
maintain the integrity of the specimen edges during polishing
and keeps the entire surface of the metal within the same focal
plane when examining the specimen in a metallograph. Figure
2a shows the microstructure etched with Kroll’s Reagent and
Figure 2b shows the microstructure colored etched with
Weck’s Reagent. Both micrographs reveal the nickel-plating
layer around the surface of the wires and substrate and show
very little evidence of edge rounding or uneven etching at the
interface between the metal and mounting material. Figure 3
shows the microstructure at a higher magnification and the
nickel-plating layer completely coats the exposed metal
surfaces. The metallic bond between the round wires and the
substrtate are very clear at this magnification.
Figure 2:Titanium fiber metal diffusion bonded to Ti-6Al-4V
substrate and electroless nickel plated and then mounted in
and Epomet®-F thermosetting resin.. Figure 2a (top) is etched
with Kroll’s Reagent and Figure 2b (above) is color etched
with Weck’s Reagent.
Another set of specimens from sample 1 were vacuum
impregnated with low-viscosity Epo-Thin® epoxy resin.
After etching, this mounting method and mounting material
exhibited some evidence of edge rounding and uneven etching
at the interface between the Epo-Thin epoxy and the metal.
The uneven etching is due to the retention of acids during
etching usually due to interface cracks forming between the
mounting material and the metal. The entrapped acid slowly
leaks out of the interface crack and etches the immediate area
more than the rest of the metal surface. Figure 4a shows the
microstructure etched with Kroll’s Reagent and Figure 4b
shows the microstructure after color etching with Weck’s
Reagent. The arrows indiates the areas with edge rounding
and uneven etching. Figure 5 shows the uneven etching at the
interface between the metal and the mounting material.
200 µm a
200 µm 200 µm
a
200 µm
200 µm b 200 µm 200 µm b
Figure 3:Titanium fiber metal diffusion bonded to Ti-6Al-4V
substrate and electroless nickel plated and then mounted in
and Epomet®-F thermosetting resin. This specimen was
etched with Kroll’s Reagent.
The fourth mounting material used on specimens from Sample
1 were prepared by vacuum impregnating with Epo-Heat™
epoxy resin. This mounting material showed similar edge
retention and uneven etching when compared to the Epo-Thin
resin. Figures 6a, 6b and 7 show the microstructures.
The last mounting material evaluated on specimens from
Sample 1 was the Sample-Kwick® cast acrylic resin. Figures
8a and 8b show substantial amounts of edge rounding and
uneven etching at the interfaces between the mounting
material and the metal. These edge retention problems would
make interface microstructural analysis more difficult when
compared with the other mounting materials.
Metallography Results – Femoral Hip Stem
The same five specimen preparation methods shown in Table
1 were used for Sample 2, the bead coated femoral hip stem
made from a Co/Cr/Mo alloy. In general, metallographically
preparing and etching the Co/Cr/Mo alloy is more difficult
than the titanium based alloys. Etching of the Co/Cr/Mo
alloys must be performed within a few minutes of final
polishing to obtain optimum results. Waiting several hours
between the final polishing procedure and etching may make
proper etching difficult.
The results from Sample 2 were similar to the results from
Sample 1. Electroless nickel-plating and mounting with
Epomet® thermosetting resin produced the best
metallographic images with minimal amounts of edge
rounding and very uniform etching. The Phenocure™
thermosetting phenolic resin and the two vacuum impregnated
epoxy resin preparation methods revealed adequate results
with minimal edge retention issues. Figure 9 shows the bead
coated layer mounted with the phenolic resin and Figure 10
Figure 4:Titanium fiber metal diffusion bonded to Ti-6Al-4V
substrate vacuum impregnated with low-viscosity Epo-Thin®
epoxy resin. Figure 4a (top) is etched with Kroll’s Reagent
and Figure 4b (above) is etched with Weck’s Reagent.
shows the excellent results using the nickel-plated Epomet®-F
resin. Comparing these two micrographs with the results in
Figure 11, it is apparent that the Sample-Kwick acrylic resin
does not retain the sample edges like the previous two
preparation methods. The Co/Cr/Mo beads in Figure 11 are
not as clear and defined as they are in Figures 9 and 10 and
this could result in misleading or inaccurate metallographic
interpretation and analysis.
The two vacuum impregnation sample preparation methods
resulted in satisfactory results with minimal edge retention
problems.
.
50 µm 200 µm
a
200 µm b 200 µm
Figure 5:Titanium fiber metal cup sample vacuum
impregnated with low-viscosity Epo-Thin® epoxy resin.
.
Figure 6:Titanium fiber metal cup sample vacuum
impregnating with Epo-Heat™ epoxy resin. Etched with
Kroll’s Reagent (top) and Weck’s Reagent (above).
Figure 7:Titanium fiber metal cup sample vacuum
impregnating with Epo-Heat™ epoxy resin and etched with
Kroll’s Reagent.
Figure 8:Titanium fiber metal cup sample was mounted in
Sample-Kwick® cast acrylic resin and etched in Kroll’s
Reagent. Figure 8a (top) and Figure 8b (above).
50 µm
200 µm a
200 µm
200 µm b
50 µm
200 µm
a 200 µm
b 50 µm
Figure 9. Bead coated Co/Cr/Mo hip stem in Phenolic
mounting material etched in HCl-H2O2.
Figure 10. Bead coated Co/Cr/Mo hip stem electroless nickel-
plated, mounted in Epomet®-F thermosetting resin, and
etched in HCl-H2O2.
Summary
Metallographic specimen preparation procedures have been
developed to adequately prepare porous implant devices with
minimal problems such as edge rounding, uneven etching,
incomplete fill of the mounting material, metal smearing, and
color etching problems. Edge retention and uniform etching is
best with the electroless nickel-plating and Epomet®-F
thermosetting resin preparation procedure. Adequate results
were obtained with the Phenocure™ thermosetting phenolic
resin and the two vacuum impregnated epoxy resins. The
poorest edge retention resulted from the Sample-Kwick®
acrylic resin for this type of application.
Figure 11. Bead coated Co/Cr/Mo hip stem mounted in
Sample-Kwick acrylic resin and etched in HCl-H2O2.
References
1. American Society for Testing Materials, Annual Book of
Standards, Medical Devices and Services, Volume 13.01,
2004.
2. G. Vander Voort, Metallography: Principles and Practice,
ASM International, 1984.
3. D.J. Medlin and R. Compton, Metallography of
Biomedical Orthopedic Alloys, ASM Handbook, Volume
9, Metallography and Microstructures, 10th
Edition, 2004.
4. L.E. Samuels, Metallographic Polishing by Mechanical
Methods, Third Edition, ASM International, 1985.
200 µm
200 µm
200 µm