Cryogenic Processing of Biomaterials for Improved Surface Integrity

7/28/2019 Cryogenic Processing of Biomaterials for Improved Surface Integrity

1/6

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


2/6

S. Yang et al.176

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


3/6

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


4/6

S. Yang et al.178

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).


5/6

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

179


6/6

S. Yang et al.180

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.

6 REFERENCES

[1] Wang, Z.B. et al., Effect of surface nanocrystallization

on friction and wear properties in low carbon steel,

2003, Mater. Sci. Eng., 352:144-149.

[2] Karimpoor, A.A., Erb U., Aust, K.T. and Palumbo, G.,

High strength nanocrystalline cobalt with high tensile

ductility, 2003, Scripta Material, 49: 651656.

[3] Shi, Y.N. and Han, Z., Tribological behaviors of

nanostructured surface layer processed by means of

surface mechanical attrition treatment, 2008, Key Eng.Mater., 384: 321-334.

[4] Swaminathan, S., Shankar, M.R., Lee, S., Hwang, J.,

King, A.H., Kezar, R.F., Rao, B.C., Brown, T.L.,

Chandrasekar, S., Compton, W.D., and Trumble, K.P.,

2005, Large strain deformation and ultra-fine grained

materials by machining, Mater. Sci. Eng. A, 410411:

358363.

[5] Altenberger I., Scholtes B., Martin U. and Oettel H.,

Cyclic deformation and near surface microstructures of

shot peened or deep rolled austenitic stainless steel

AISI 304, 1999, Mater. Sci. Eng. A, 264: 116.

[6] Nalla, R.K., Altenberger, I., Noster, U., Liu, G.Y.,

Scholtes, B., and Ritchie, R.O., On the influence ofmechanical surface treatments-deep rolling and laser

shock peening on the fatigue behavior of Ti-6Al-4V at

ambient and elevated temperatures, 2003, Mater. Sci.

Eng. A, 355: 216-230.

[7] Nikitin, I., Altenberger, I., Maier, H.J., Scholtes, B.,

2005, Mechanical and thermal stability of mechanically

induced near-surface nanostructures, Mater. Sci. Eng.

A 403: 318327

[8] Yang, L. and Zhang, E., 2009, Biocorrosion behavior of

magnesium alloy in different simulated fluids for

biomedical application, Mater. Sci. Eng. C, 29: 1691

1696.

[9] Prevey, P. and Cammett, J., 2001, Low cost corrosion

damage mitigation and improved fatigue performance of

low plasticity burnished 7075-T6, J. Mater. Eng.

Perform. 10: 54855.

[10] Prevey, P., Telesman, J., Gabb, T. and Kantzos, P.,

2000, FOD resistance and fatigue crack arrest in low

plasticity burnished IN718, In: Proceedings of the 5th

Nat. Turbine Eng. HCF Conf..

[11] Prevey, P., Hornbach, D., Maston, P., Jacobs, T. and

Commett, J., 2001, Low cost surface enhancement

method for improved fatigue life of superalloys at

engine temperature, NAS3-99116 SBIR, NASA Final

Report.

[12] Iglesias, P., Bermudez, M.D., Moscoso, W., Rao, B.C.,

Shankar, M.R. and Chandrasekar, S., Friction and wear

of nanostructured metals created by large strain

extrusion machining, 2007, Wear, 263: 636642.

[13] Ramesh, A., Melkote, S.N., Allard, L.F., Riester, L.,

Watkins, T.R., 2005, Analysis of white layers formed in

hard turning of AISI 52100 steel, Mater. Sci. Eng. A,

390: 8897.

[14] Pu, Z., Puleo, D.A., Dillon, Jr., O.W. and Jawahir, I.S.,controlling the biodegradation rate of magnesium-based

implants through surface nanocrystallization induced by

cryogenic machining, 2011, TMS, in press.

[15] Li, W.L., Tao, N.R. and Lu, K., Fabrication of a gradient

nano-micro-structured surface layer on bulk copper by

means of a surface mechanical grinding treatment,

2008, Scripta Material, 59: 546549.

[16] Ni, H., Elmadagli M. and Alpas, A.T., Mechanical

properties and microstructures of 1100 aluminum

subjected to dry machining, 2004, Mater. Sci. Eng. A,

385: 267278.

[17] Disegi, J.A., Kennedy, R.L. and Pilliar, R., Cobalt-based

alloys for biomedical applications, 1999, Technology &

Engineering, Book Chapter.

[18] Hsu, H.C. and Lian, S.S., Wear properties of Co-Cr-Mo-

N plasma-melted surgical implant alloys, 2003, J.

Mater. Process. Technol., 138: 231-235.

[19] Nasab, M.B. and Hassan M.R., Metallic biomaterials of

knee and hip - A review, 2010, Trends Biomater. Artif.

Organs, Vol. 24(1): 69-82.

[20] Yamanaka, K., Mori, M., Kurosu, S, Matsumoto, H., and

Chiba, A., Ultrafine grain refinement of biomedical Co-

29Cr-6Mo alloy during conventional hot-compression

deformation, 2009, Metallurgical and Mater. Trans. A,

40: 1980-1994.[21] http://www.cartech.com/news.aspx?id=578, Feb. 2008.

[22] Wu, X., Tao, N., Hong, Y., Liu, G., Xu, B. and Lu, J.,

Strain-induced grain refinement of cobalt during surface

mechanical attrition treatment, 2005, J. Acta Mater, 53:

681-691.

[23] Meyers, M. and Chawla, K., Mechanical behavior of

materials, 2009, 2nd

edition, Cambridge Univ. Press.

[24] Hallab, N.J. and Jacobs, J.J., 2003, Orthopedic Implant

Fretting Corrosion, Corrosion Reviews, Vol. 2 J, Nos. 2-

3: 184-213.

[25] Pu, Z., Puleo, D.A., Dillon, Jr., O.W. and Jawahir, I.S.,

2010, Microstructural changes of AZ31 magnesium

alloys induced by cryogenic machining and its influence

on corrosion resistance in simulated body fluid for

biomedical applications, MSEC2010, in press.
http://www.cartech.com/news.aspx?id=578http://www.cartech.com/news.aspx?id=578

Cryogenic Processing of Biomaterials for Improved Surface Integrity

Documents

Transcript of Cryogenic Processing of Biomaterials for Improved Surface Integrity