Cryogenic Processing of Biomaterials for Improved Surface Integrity
Transcript of Cryogenic Processing of Biomaterials for Improved Surface Integrity
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G. Seliger et al. (eds.),Advances in Sustainable Manufacturing: Proceedings of the 8th Global Conference 175on Sustainable Manufacturing, DOI 10.1007/978-3-642-20183-7_26, Springer-Verlag Berlin Heidelberg 2011
Cryogenic Processing of Biomaterials for Improved Surface Integrity andProduct Sustainability
S. Yang1, Z. Pu
1, D.A. Puleo
2, O.W. Dillon, Jr.
1, I.S. Jawahir
1
1Department of Mechanical Engineering, University of Kentucky, Lexington, USA
2
Center for Biomedical Engineering, University of Kentucky, Lexington, USA
Abstract
Improved functional performance and longer service life of biomedical products offer great sustainability
benefits. Surface integrity, which can be modified by severe plastic deformation (SPD) processes, affects the
functional performance of materials. Two SPD processes burnishing and machining were studied under
cryogenic conditions. Cryogenic burnishing of a Co-Cr-Mo biomedical alloy using a novel burnishing tool led to
significant grain refinement and 80% greater surface microhardness relative to the bulk. Cryogenic burnishing
ofAZ31 Mgalloy led to a more than 2 mm thick SPD surface layer with remarkably refined microstructure
(grains
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treatment at cryogenic temperatures, Li et al. [15]
synthesized a gradient nano-micro-structure in the surface
layer of bulk pure copper. The average grain sizes vary from
about 22 nm in the topmost surface to sub-micrometers at
200 m deep. Ni et al. [16] also reported that by using
cooling fluid during machining, grain size in the secondary
deformation zone was reduced from 1.2 m to 360 nm.
In the current study, two SPD processes burnishing and
machining were studied at cryogenic conditions, whereliquid nitrogen was applied so as to reduce the temperature
rise created during and following processing. The effects of
cooling conditions on microstructure and microhardness
changes in two materials are reported herein.
2 EXPERIMENTAL
2.1 Work materials
Co-Cr-Mo alloy
Cobalt-chromium-molybdenum (Co-Cr-Mo) alloy has good
mechanical properties, wear resistance, and corrosion
resistance, which combine to make it biocompatible [17, 18].
It has been extensively used in joint implants such as
artificial hips and knees [19, 20]. BioDur Carpenter CCM
alloy, which is a high nitrogen, low carbon wrought version of
ASTM F75 Cast Alloy with an average initial hardness of 43
HRC is used as the work material in the burnishing
experiments reported here. A Co-Cr-Mo alloy bar (50.8 mm
diameter) is used to prepare disk samples which have a
diameter of 50.8 mm and a thickness of 3 mm.
AZ31 Mg alloy
Magnesium alloys are emerging as a new class of
biodegradable implant materials for internal bone fixation.
They provide good temporary fixation and do not need to be
surgically removed after healing occurs, providing relief to
the patients and reducing the healthcare costs [14].The
particular material studied was the commercial AZ31 B-O
magnesium alloy. It was obtained in the form of a 3 mm thick
sheet. Disc specimens (3 mm in thickness and 130 mm in
diameter) were made from the sheet.
2.2 Processing
Burnishing
Burnishing experiments were conducted on a Mazak CNC
lathe along with ICEFLYTM
cryogenic equipment. Liquid
nitrogen as a cryogenic coolant was used. It has the
advantages of better surface finish for workpieces,
environmentally safer and healthier for the worker. A
specially designed and fabricated burnishing tool, with a
fixed burnishing roller which was used, is shown in Figure
1(a). The spherical cavity in the tool holder permits the use
of rollers with different diameters. Figure 1(b) and (c) show
the experiment setup used for cryogenic burnishing. The
roller is spring loaded for approximate adjustment of the
burnishing force. The forces developed during the
processing were measured by a KISTLER 3-Component
Tool Dynamometer. A M2/M7 high-speed tool steel roller
with a diameter of 14.3 mm was chosen as the burnishing
tool for the current experiments. The hardness and surface
roughness of these rollers were measured and found to be
63 HRC and 0.01 m (Ra), respectively. The roller head is
fixed in order to induce enough shear stress and strain to the
surface region to cause grain refinement via SPD and
possibly dynamic recrystallization (DRX).
The processing conditions used for the burnishing
experiments on Co-Cr-Mo and AZ31 Mgalloysare shown in
Table 1. Co-Cr-Mo discs are burnished with and without
liquid nitrogen for better study of the effects of cryogenic
cooling.
Table 1: Burnishing conditions forCo-Cr-Mo and AZ31 Mg
alloys
Material Burnishing
time
Burnishing
speed
Feed
rate
Radial
Force
Co-Cr-
Mo
20 s 100 m/min 0.05
mm/rev
230 N
AZ31 Mg 60 s 100 m/min 0.05
mm/rev
240 N
The application of liquid nitrogen is expected to effectively
suppress grain growth after SPD processing and the
occurrence of DRX within the surface region. Due to large
strains, high strain-rates and lower temperatures, under the
burnishing conditions used, the cryogenic SPD processintroduces significant grain refinement to the material surface
layer.
Machining
A Mazak CNC lathe equipped with an Air Products liquid
nitrogen delivery system is also used to conduct orthogonal
turning of the AZ31 Mgdiscs. As shown in Figure 2, liquid
nitrogen was sprayed on the machined surface from theclearance side of the cutting tool for what is being called
cryogenic machining. The cutting tools used were
Kennametal uncoated carbide C5/C6 inserts with 68 m
edge radius.AZ31 Mgdisc was cryogenically machined with
100 m/min cutting speed and 0.01 mm/rev feed rate.
2.3 Material characterization
Co-Cr-Mo alloy
Metallurgical Co-Cr-Mo specimens were cut from the
processed discs. After hot mounting, grinding, and polishing,
the specimens were chemically etched (120 ml 37%
hydrochloric acid + 12 g cupric chloride dehydrate,
(a)
Nozzle for liquidnitrogen
Fixed roller
Work material
(c)(b)
Cavity fordifferentdiametertools
Figure 1: (a) Burnishing tool illustration; (b) application ofliquid nitrogen during cryogenic burnishing; (c) experiment
setup for cryogenic burnishing
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Cryogenic Processing of Biomaterials for Improved Surface Integrity and Product Sustainability 177
crystalline + 10 ml R.O. Water) [21] for 15 s at 120 c to
reveal their microstructures.
AZ31 Mg alloy
MetallurgicalAZ31 Mgsamples were cut from the machined
discs. After cold mounting, grinding and polishing, acetic
picric solution was used as the etchant to reveal the grain
structure as shown below.
Characterization methods
The materials microhardness and microstructure in the
surface region were measured before and after the
processing. Microindentation tests were undertaken for Co-
Cr-Mo specimens by using a Vickers indenter on a CSM
Micro-Combi Tester with 300 mN applied load. The hardness
ofAZ31 Mgalloy was measured by a CLARK Digital Micro
hardness Tester (CM-700 AT) with 50 mN load and 15 s
dwell time. Metallurgical analysis was conducted by using
optical and scanning electron microscope (SEM). Chemical
compositions were determined from energy dispersive
spectroscopy (EDS).
3 RESULTS AND DISCUSSION
3.1 Co-Cr-Mo alloy
SEM investigations were conducted to study the burnished
workpieces deformed microstructures. A comparison
between SEM photomicrographs of the initial surface and
burnished surface using different burnishing conditions is
shown in Figure 3. The burnishing direction is parallel to the
plane of the figures, and the grains shown are exactly at the
edge of the burnished surfaces. Comparisons between the
initial microstructure (Figure 3(a)) and the burnished ones
(Figure 3(b) and (c)) show that the grains were elongated
and more condensed in a thin layer near the surface of the
workpiece, grain structures within this layer are not
discernable. This layer of indiscernible grain structure wasalso reported in other materials after burnishing [6] [15],
which is defined as the SPD layer. Nano-grains were not
observed. When Wu et al. [22] conducted SMAT process on
cobalt, a microstructural evolution in the deformed surface
layer was observed, which contained recrystallized nano-
grains, subgrain subdivisions, elongated subgrains, grains
with heavily twins, and equiaxed bulk grains with stacking
faults sequentially from the depth of 15 m down to 180 m.
This is in line with the present investigation. However,
recrystallized nano-grains have not been induced by the
currently used burnishing conditions.
From Figure 3(b) and (c); it is clearly visible that the depth of
the process-influenced layer from the cryogenic burnishing
conditions is much larger than that from using dry conditions.This suggests that cryogenic cooling has substantial
influence on the surface layer developed during SPD
processing. During burnishing, the top surface layer is
subjected to the most severe SPD state; the layers beneath
the surface are subjected to less severe deformation
conditions. For dry burnishing, the effects of plastic
deformation on the subsurface layer are compromised by the
large amount of heat generated during processing. This layer
is hardened by plastic deformation and softened by heat
simultaneously; the mechanical and thermal effects oppose
each other and finally lead to minor or no influence on
microstructure changes. On the other hand, liquid nitrogen
application effectively suppresses the heating effect and
increases the process influenced depth to a larger extent. As
shown in Figure 3(a), a large amount of twinning is present
within the initial grain interiors, which may be attributed to
pre-existing residual stresses prior to burnishing. This has a
substantial influence on the subsequent effects of
burnishing.
(a) (b) (c)
Figure 3: SEM micrographs ofCo-Cr-Mo discs before (a) and after burnishing: (b) dry, (c) cryogenic
In order to characterize the hardness variation in the surface
region of the processed Co-Cr-Mo workpieces,
microhardness measurements were made. The minimum
measurement depth from the surface is 5 m in order to
avoid the edge softening effect. For each depth, at least
three measurements were taken. The microhardness profiles
shown in Figure 4 were averaged values from these
measurements.
Nozzle
for
liquid
nitrogen
Cutting tool
Workmaterial
Figure 2: Machining setup with the liquid nitrogendelivery system
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Figure 4:Subsurface microhardness profiles for cryogenicand dry burnished Co-Cr-Mo discs
The measured microhardness of the virgin disk is 430 HV on
average. Comparing to the initial workpiece hardness, an
increase of up to 80% was achieved after cryogenic
burnishing. With the same burnishing conditions, the
application of liquid nitrogen led to higher hardness value
and larger process-influenced depth comparing to dry
burnishing. The general trend of the curves shows a gradual
decrease in microhardness with distance below the surface
until the bulk value is reached.
Based on the well-known Hall-Petch relationship [23]:
1 2
0
/
y kDV V
(1)
between yield stress (y) and grain size (D) as well as the
close interrelations among hardness, yield stress, and
residual stresses, high hardness values often indicate fine
grain size and large residual stresses. In other word, grain
size and residual stresses collectively contribute to the
hardness value. In our current work, microstructure changes
due to different cooling conditions were only observed within20 m depth from the surface. In contrast, microhardness
differences were measured to the depth of 250 m. It is
reasonable to state that the variations in microhardness were
due to the different residual stresses being generated during
processing. The residual stresses of the processed samples
will be measured using X-ray diffraction techniques to
validate this hypothesis. These results will be published later.
Figure 5 shows the results of the EDS analysis.
Measurements were taken on the processed surface and
bulk of specimens. The higher oxygen amount on the
surface is due the formation of chromium oxide during
processing, which is believed to protect the surface from
electrochemical degradation and improve corrosion
resistance [24].
(a) (b)
Figure 5: EDS results ofCo-Cr-Mo discs from cryogenic burnishing: (a) bulk, (b) surface
3.2 AZ31 Mgalloy
The initial microstructure of the AZ31 Mg disc is shown in
Figure 6. There is no twinning in the bulk material since the
as-received material is annealed. However, twinning is
visible near the surface of the disc. This is due to the sample
preparation in the machine shop where a turning operation is
used as the final step in making the disc.
Figure 6: Initial microstructure ofAZ31 Mgdiscsbefore
processing
The microstructure obtained from cryogenic machining is
shown in Figure 7. Significant changes in microstructure
near the processed surface were found; a SPD surface layer
of about 8 m in which grain boundaries were no longer
visible (at this magnification) was created by machining.
Figure 7: Microstructure ofAZ31 Mgdiscs after cryogenic
machining
Figure 8(a) shows the microstructure after cryogenic
burnishing, in which a 2 mm process-influenced layer was
formed, which is similar to the machined surface. The grain
structure in the burnished surface region (Figure 8(b)) was
no longer discernable compared to the clearly defined
microstructure prior to processing (Figure 5). The transition
between the initial microstructure and the process-influenced
microstructure can clearly be seen in Figure 8(c).
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Cryogenic Processing of Biomaterials for Improved Surface Integrity and Product Sustainability
(a) (b) (c)
Figure 8: Microstructure ofAZ31 Mgdiscs after cryogenic burnishing: (a) cryogenic burnishing; (b) surface layer in (a); (c)
transition layer in (a)
SEM investigations were undertaken for better study of the
severe plastic deformed layer from burnishing. As shown in
Figure 9, highly uniform grains were present in the burnished
surface layers; the grain sizes are generally less than 500
nm, although grains less than 300 nm can be found close to
the surface. Comparing to the 10 m grains before
burnishing, the grain size on the burnished surface was more
than 20 times smaller.
Figure 9: SEM micrograph ofAZ31Mgdisc surface region
from cryogenic burnishing
Figure 10 shows the microhardness profiles ofAZ31 Mg
discs before and after cryogenic processing. The initial
hardness of the material was measured to be about 50 HV.
The hardness values near the surface on these three profiles
(Figure 10) were increased to different extents. The increase
of hardness on the edge of the initial disc is due to the discpreparation in the machine shop. Comparing to the bulk
material, the measured hardness at about 15 m from the
surfaces was increased about 60% during cryogenic
machining and 95% by cryogenic burnishing, which indicate
significant grain refinement based on the classical Hall-Petch
equation (1).
4 SUMMARY
Two manufacturing processes machining and burnishing -
are shown to be viable SPD routes for introducing ultrafine or
nano-sized grains into the surface region of Co-Cr-Mo and
AZ31 Mg alloys. Cryogenic burnishing of Co-Cr-Mo alloy
experiments resulted in significant grain refinement in the
surface region through burnishing-induced SPD.
Microhardness in the SPD layer was increased up to 80%
relative to the bulk value.
Figure 10: Subsurface microhardness profiles ofAZ31 Mg
discs before and after cryogenic processing
AZ31 Mg alloy was subjected to both cryogenic machining
and burnishing processes. The cryogenic burnishing process
led to a more than 2 mm thick surface layer with remarkably
refined microstructures formed on the burnished surface. A
95% increase in hardness was obtained on the burnished
surface, where grains less than 300 nm were observed
under scanning electron microscopy. A SPD layer was
shown to form on the surface of AZ31 Mg disc after
cryogenic machining. The hardness of this layer was about
60% larger than the bulk material. It has been reported that
the corrosion resistance of the AZ31 Mg alloy in simulated
body fluid was enhanced due to the formation of this SPD
layer [14][25].
The present results demonstrate that both cryogenic
processes significantly modify the surface properties ofCo-
Cr-Mo and AZ31 Mg alloys and, therefore, may enhance
their performances for improved sustainability.
Systematic studies will be done to further investigate the
influence of various processing conditions on microstructural
changes ofCo-Cr-Mo andAZ31 Mgalloys. Pin-on-disc wear
testes will be conducted for studying the relationship
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between microstructure and wear resistance of Co-Cr-Mo
alloy.
5 ACKNOWLEDGMENTS
We sincerely thank to Air Products and Chemicals for
supplying the equipment for liquid nitrogen application and
Dr. Fuqian Yang (Material Science Dept., Univ. of Kentucky)
for providing the equipment for microhardness
measurements. Additional thanks to our technicians Bill
Young and Richard Anderson for their valuable help onconducting the experimental work.
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