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International Journal of Machine Tools & Manufacture 47 (2007) 521–528

www.elsevier.com/locate/ijmactool

Application of ultra-thin fluorine-content lubricating films to reducetool/workpiece adhesive interaction during thread-cutting operations

S.C. Veldhuisa,�, G.K. Dosbaevaa, G. Bengab

aDepartment of Mechanical Engineering, McMaster University, Hamilton, Ont., CanadabFaculty of Engineering and Management for Technical Systems, University of Craiova, Drobeta Turnu Severin, Mehedinti, Romania

Received 13 March 2006; received in revised form 26 May 2006; accepted 6 June 2006

Available online 24 July 2006

Abstract

This paper considers the application of lubricious fluorine-organic-based surface-active substances such as perfluorpolyether (PFPE)

films for thread-cutting operations. The coefficient of friction of samples prepared both with and without the lubricating film was studied

over a range of temperatures from 20 to 550 1C, to test the potential for this film to reduce tool/workpiece adhesive interaction. An 18%

reduction in the coefficient of friction was observed at temperatures as high as 400–450 1C for this film. This temperature is within the

typical range of thread-tapping operations. To explore this potential application further, the PFPE films were applied on the surface of

thread-cutting taps made of high-speed steel. Cutting tool life was also investigated for regular, spiral point, and spiral tap designs, both

with and without the PFPE films. The surface morphology of the worn-cutting tools was studied using an SEM and the shape as well as

the micro-structure of the chips was also investigated in detail. Overall it was shown that the application of the PFPE film improved the

cutting tool life by a factor of two, decreased the average cutting torque by 8% and produced tightly curled chips.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Thread cutting; Tapping; Tool wear; Tool/workpiece adhesive interaction; Perfluorpolyether (PFPE) films

1. Introduction

Internal thread cutting is a challenging operation inmodern machining. Typically it is performed late in themanufacturing cycle so there is already a considerableinvestment in the part and breakage of the tap often resultsin scrapping the part or extra rework activity. Thus,breakage of a thread-cutting tool can significantly impactthe productivity of the process. A leading cause of toolbreakage in this application is the transfer of the workpiecematerial onto the tool. This adhesive interaction affects thesurface of the tool, increases the cutting forces and makes itmore difficult for the chips to be cleared from the cuttingzone. Once the intensity of adhesive interaction reaches acritical level, chips fail to clear and thus fill the flutes of the

e front matter r 2006 Elsevier Ltd. All rights reserved.

achtools.2006.06.003

ing author. Tel: +1905 525 9140x27044;

9742.

ess: veldhu@mcmaster.ca (S.C. Veldhuis).

tool causing the torque to increase, with the probability ofbreakage increasing substantially [1]. Two ways to addressthis problem are through improved designs of tools, whichfacilitate the clearing of chips from the cutting zone andthrough surface engineering of the tool to reduce theintensity of the tool/workpiece adhesive interaction. Theapproach investigated in this work is to further improve ona good tool design by reducing the intensity of the adhesiveinteraction between the workpiece and the tool through theuse of an ultra-thin lubricating layer on the tool.Fluorine-based materials like perfluorpolyether (PFPE)

or Z-DOL are widely used as lubricants in harshenvironments. Example applications include seizure pre-vention at joints on astronaut’s pressurized suits, bearingson spacecraft antenna arrays [2], and lubrication betweenthe read head and magnetic disk surfaces in hard drives[3,4]. It has been shown that friction during slider-disccontact can be reduced considerably with the presence of aPFPE lubricant [5,6]. Due to their low surface energy and

ARTICLE IN PRESSS.C. Veldhuis et al. / International Journal of Machine Tools & Manufacture 47 (2007) 521–528522

good oxidation stability, PFPE films are often employed aslubricants in micro-electromechanical devices and for indexmatching of fluids in optoelectronic devices [7].

A synthetic fluid based on PFPE called VT-Nano-plus,which is commonly used for surface treatment, wasconsidered in this work. This material represents the latestgeneration of this family of lubricating agents as manu-factured by VT Verschleiteiltechnik GmbH, Langenhagen.The process of film deposition consists of applying thefluorine-surface-active substances (SAS) molecules on thesurface of a frictional body such as a cutting tool. The thinfilm consists of a close-packed molecular monolayer (nano-layer) that provides an even coating to a rough tool surface[8]. These films are multi-component systems, whichcontain fluorine-organic (fluorine-SAS) and controllingagents in various solvents. These components give thesurface unique anti-friction and anti-adhesive properties.

Usually PFPE, which is a 0.5% solution of perfluorinepolyester acid (Rf-CH2OH) in Freon 113, can be depositedby dipping the part into the solution. As a result of thisprocess, thin (40–80 A thick) nano-scaled films of orientedfluorine-SAS molecules are formed. These molecules arefixed on the surface by chemosorption forces with the filmspenetrating into the micro-pores and micro-cracks typicallyfound at the surface layer of the cutting tools [3].

PFPE has the following chemical structure: HO–CH2–C-F2O–(C2F4O)6–(CF2O)20–CF2–CH2–OH [1,2]. The physi-cochemical properties of PFPE are shown in Table 1. Thethin film consists of a close-packed molecular monolayerthat provides an even coating to a rough tool surface.These organic-based SAS molecules are widely used inpractice for specific industrial applications and have thefollowing features [6–12]:

�

Ta

Phy

Pro

Mo

De

Th

Lo

Ma

able to reduce the surface energy of the materialapproximately 1000 times, this facilitates the spreadingand retention of additional liquid-based lubricants onthe surface;

� reduces the probability of dry friction conditions

occurring through the prevention of the displacementof liquid lubricants in heavily loaded tribo-systems suchas cutting tools;

� prevents electrochemical corrosion such as hydrogen

embrittlement, due to the high penetrability of thefluorine-SAS molecules, which fills up and degases all ofthe pores and cracks; this further protects the surfacesfrom exposure to aggressive substances;

ble 1

sicochemical properties of perfluorpolyether (PFPE) [7]

perty Value

lecular mass 2194 g/mol

nsity 1560 kg/m3

ickness range of the film 40–80 A

ad-caring capacity range 3–5GPa

ximum service temperature 450 1C

�

enhances tribological compatibility due to the reductionof the coefficient of friction and the lowering/preventionof adhesive interaction during friction; � high temperature stability (up to 400–450 1C, with short-

term temperature flashes allowable up to 700 1C);

� high load bearing capacity (the approved unit pressure is

3000MN/m2);

� shifts the friction to a milder condition due to a

reduction of the adhesive interaction at the frictionalbody interface.

In the literature most of the attention is paid to the PFPEgrades that are used for general tribological applications,for example, to prevent adhesion in mechanical systems.This is a major cause of the failure of integratedaccelerometers used in automotive air bag triggeringmechanisms [13]. So far, less attention has been paid tothe wear behavior of heavy loaded tribo-systems usingthese lubricious films. However, some types of PFPE havebeen proposed for use in heavy loaded tribo-systems suchas cutting and stamping tools [12].The goal of this paper is to study the impact of PFPE

film application on the wear performance of thread-cuttingtools, working under conditions involving intensive adhe-sive wear. In these applications, adhesive interactionbetween the tool and workpiece material is the processthat controls the wear rate intensity and ultimately the toollife. A typical cutting tool exposed to these conditions,which does not exceed the PFPE films working tempera-ture, is the thread-cutting tool (taps). The effectiveness ofthe PFPE film in this application was studied for thisapplication using three different tap designs and its effecton tool wear and machining torque was investigated.

2. Experimental studies

The coefficient of friction was determined with the aid ofa specially designed apparatus, which is described in [6]. Inthis test, a rotating sample of the coated substrate wasplaced between two polished specimens made of P20 moldsteel HRC 30–35. To simulate tool friction conditionsduring cutting, the specimens were heated using a resistiveheating method varying the temperature range from 150 to550 1C. A standard force of 2400N was applied to generatethe plastic strain in the contact zone. To evaluate the anti-friction properties of the layer, the adhesion component ofthe coefficient of friction was used, because adhesion ofworkpiece material is typical for cutting operations [14].This component is mainly responsible for the catastrophicwear stage of HSS tools when adhesive wear dominates. Itwas determined as the ratio of the shear strength inducedby the adhesion bonds between the tool and the workpieceto the normal contact stress developed on the contactsurface at the test temperatures (t/prn). No less than threetests were performed for each kind of sample. The scatterof the friction parameter measurement was approximately5% in this case.

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The process of PFPE film deposition includes thefollowing steps: (a) surface preparation (cleaning inacetone), (b) application of the fluorine-SAS film, followedby (c) drying. In this study, two variations of the processwere compared. During the first ‘‘hot PFPE and cool tap’’process, the room temperature taps were placed in a boilingfluorine-SAS-containing solution and kept at a tempera-ture of 50 1C for 1 h. The process was run in a sealedcontainer. During the second ‘‘cool PFPE and hot tap’’process, the taps were heated in a furnace held at atemperature of 60 1C for 0.5 h. After that the heated tapswere dipped in a room temperature fluorine-SAS-contain-ing solution and kept there for 20min at room tempera-ture. After the deposition of the PFPE film was complete, adrying process was performed in a lab furnace at atemperature 100 1C for 1 h. The drying process is animportant step in strengthening the chemosorption bondbetween the fluorine-SAS film and the treated surfaces.

Table 2

Tap tool life depending on the PFPE film deposition technique used

Deposition technique Tool life (avg.)

Cool PEPE and hot tap 27 holes

Hot PEPE and cool tap 19 holes

Fig. 1. Images of different types of taps tested:

To check the nominal quality of the deposited film thewetting angle y was observed. The wetting angle isdetermined by measuring the contact angle that a liquidoil lubricant makes when in contact with the coatedsurface. The wetting angle value characterizes the ability ofa liquid to form a boundary surface with a solid. Thewetting angle y should be equal to or more than 721 for thiscoating and an oil drop, having a diameter of 2–4mm,should stay still on the surface. In all cases, theseconditions were met.To compare the efficiency of the method of the film

application outlined above, cutting tests were performedwith the coolant and the resultant tool life of the taps werecompared. Not less than five cutting tests were performedfor each type of tap and technique. The scatter in the toollife measurements was approximately 10%. The cuttingconditions used were as follows: tool—HSS 3/800 � 16 taps(approx. 9.5mm +); workpiece material—P20 mold steel,HRC 30—35; speed—8m/min; (260RPM); feed rate—1.587mm/rev (feed rate is based on the pitch of thethreads); length of the thread—10mm; through holes. Thetool wear criterion was 0.3mm flank wear. The tool lifedata as a function of the deposition technique is presentedin Table 2 and shows that the ‘‘cool PFPE and hot tap’’technique for depositing the PFPE films provided thelonger tool life and was thus used for the detailed tool lifestudies. Three types of HSS taps with different designs

(a) regular; (b) spiral point; and (c) spiral.

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00 200 300 400

Fla

nk w

ear,

mm

without PFPE

with PFPE

00 10 30 40 50 60

Fla

nk w

ear,

mm

00 5 10 15 20 25 30 35

Fla

nk w

ear,

mm

0.6

0.5

0.4

0.3

0.2

0.1

20Number of holes

0.35

0.3

0.25

0.2

0.15

0.1

0.05

Number of holes

0.25

0.2

0.15

0.1

0.05

Number of holes100

without PFPE

with PFPE

without PFPE

with PFPE

(a)

(b)

(c)

S.C. Veldhuis et al. / International Journal of Machine Tools & Manufacture 47 (2007) 521–528524

(regular, spiral point and spiral) were tested using thesame cutting conditions. Images of the taps are presentedin Fig. 1.

3. Results and discussion

The coefficient of friction data vs. temperature of thecoated sample in contact with the P20mold steel withhardness HRC 30–35 is presented in Fig. 2. The applicationof the PFPE films in this case resulted in a coefficient offriction improvement in the range of temperatures up to500 1C. This upper level of temperature range is below thattypically found in tapping operations. Thus, this coatingmaterial was considered a reasonable candidate forreducing the intensity of adhesive interaction, which takesplace in thread-cutting applications and was applieddirectly to the taps and tested.

Fig. 3 presents the tool life data for the taps both withand without the PFPE films. The data presented shows thatthe design of the taps affects tool life in this applicationboth with and without the PFPE films. Based on previouswork [1], this difference is largely attributed to the chip-clearing properties of the tool. The highest tool life in thisstudy was found for the spiral taps (Fig. 3(c)) and thelowest was with the spiral pointed taps (Fig. 3(b)).

There are two interesting features on the wear curves inFig. 3. The regular and spiral pointed taps without PFPEhave a short running-in stage and a narrow stable stage ofwear. This implies that deep surface damage occurs duringthe initial stage due to intensive tool/workpiece adhesiveinteraction, which impacts the ability of the tool towithstand cutting loads over time. In contrast, the tapswith PFPE have a more gradual wear rate within therunning-in stage, which is associated with a lower level ofinitial surface damage and thus a much wider stable stage.It is interesting to note that as soon as the taps without thePFPE film transform to the catastrophic wear stage, after

00 100 200 300 400 500 600

Co

effi

cien

t o

f fr

icti

on

HSS HSS+PFPE

0.2

0.15

0.1

0.05

Temperature °C

Fig. 2. Coefficient of friction vs. temperatures for the vapor oxidized HSS

steels samples with and without PFPE films.

Fig. 3. Flank wear of the taps with and without PFPE films vs. number of

holes: (a) regular, (b) spiral point, and (c) spiral.

machining 20 holes for the regular tap design (Fig. 3(a)),the wear rate for the taps with PFPE were found tostabilize after the same length of cutting time. This isattributed to the reduced level of surface damage resultingfrom the lower level of adhesive interaction at the tool/workpiece interface. A similar phenomenon is observed forthe spiral pointed taps (Fig. 3(b)).In the case of an improved tap design, two methods of

friction control [12] are realized, i.e. wear intensity duringthe running-in stage is lowered and the stable wear stage iswidened. This is associated with lower intensity of tool/workpiece adhesive interaction as shown in Fig. 3(c).Fig. 4 shows a comparison of the cutting torque data for

the spiral taps for both the cases i.e. with and without thePFPE film. The torque values were measured using a

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0

5

10

15

20

25

30

0 100 150 200 250 300 400

Tor

que

(Nm

)

Without PFPEWith PFPE

50

Hole Number

350

Fig. 4. Comparison of cutting torques for tools with and without PFPE films.

Fig. 5. SEM images of the ‘regular’ taps’ worn surface with and without PFPE films: (a) without PFPE films after 23 holes, overview; (b) with PFPE films

after 51 holes, overview; (c) detailed view of the image shown in ‘a’; and (d) detailed view of the image shown in ‘b’.

S.C. Veldhuis et al. / International Journal of Machine Tools & Manufacture 47 (2007) 521–528 525

Kistler table dynamometer model 9255B setup to measureFx, Fy, Fz, and Mz. The reported values are based on anaverage of 1 s of data taken at a sampling rate of 1000samples/s. The average was calculated during the sameperiod of tapping, with the taps engaged in the hole. Thepresence of the PFPE film shows a moderate reduction intorque of approximately 8%, which can be attributed tothe reduction in friction. The reduction in torque becomesmore dominant, earlier on the surface of the tool withoutthe PFPE film, as the tool wear progresses and tool/workpiece material adhesive interaction intensifies.

SEM data on the surface morphology of the taps bothwith and without the PFPE films is shown in Figs. 5 and 6.Fig. 5 presents the surface morphology of the worn tapswith a ‘regular’ design and Fig. 6 presents similar data forthe worn spiral taps, which provided the best tool life ascompared to the other types of taps tested in this study. Itwas found that taps without the PFPE films exhibitintensive adhesion of the workpiece material on the tool.This is typical for a tapping operation with this material asthese service conditions can lead to a wear mode involvingintensive surface damage to the cutting tools through

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Fig. 6. SEM images of the worn surface of the spiral tap with and without PFPE films: (a) without PFPE films after 194 holes, overview; (b) with PFPE

films after 380 holes, overview; (c) detailed view of the image shown in ‘a’; and (d) detailed view of the image shown in ‘b’.

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intensive adhesion of workpiece material on the tool. Thisadhesive wear mode ultimately dominates the workingsurfaces of the cutting tool at low to moderate speeds [8].In this application, the use of PFPE films significantlyreduces the intensity of tool/workpiece adhesive interac-tion, which in turn reduces the damage done to the tool,facilitates chip clearing, and overall results in a better toollife.

The use of PFPE films also changes the shape andsurface morphology of the chips generated during atapping operation as shown in Fig. 7. Relatively thickchips are formed when using the taps without the PFPEfilms (Fig. 7(b)), they also showed less curling (Fig. 7(a andc)) and had a rougher undersurface (Fig. 7(e)). In contrast,thinner chips (Fig. 7(d)) were obtained from the tapswith PFPE films. Chips also curled in a tight spiral(Fig. 7(b and d)) and had a smoother undersurface(Fig. 7(f)). These factors also facilitate the evacuation ofthe chips from the hole during tapping.

Cross-sections of the chips show two areas [14]: a contactflow zone and a zone of extended deformation (Fig. 8).There is a very wide flow zone for chips produced usingtaps without PFPE (Fig. 8(a)). In contrast, the flow zonetogether with the zone of extended deformation is muchnarrower for the taps with PFPE (Fig. 8(b)). This is

interpreted as an improvement in the flow of the workpiecematerial over the tool during cutting due to the improvedlubricity of the taps with the PFPE films.

4. Conclusion

It was found that the application of PFPE films reducedthe coefficient of friction by 18% over uncoated surfaces attemperatures up to 450–500 1C. This result indicated thatthis coating is a good candidate for low-temperaturemachining operations like thread tapping where high-speedsteel-cutting tools are widely used. The ‘‘cool PFPE and hottap’’ application process of the lubricious PFPE nano-filmswas found to provide the best adhesion in this application.When applied to a tapping operation, the PFPE films werefound to reduce the tool/workpiece adhesive interactionand thus reduce torque by 8% as well as improve the overallwear behavior of the cutting tools by a factor of two.Having superior tribological properties, lubricious PFPEfilms also beneficially change the type of chips to be morecurled and improve the workpiece metal flow along thecutting tool surface. The PFPE film application is arelatively simple and cheap process to implement and canbe done with minimal capital investment.

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Fig. 8. SEM images of the chip cross-sections: (a) without PFPE and (b) with PFPE films. Zones 2—contact flow zone and 3—zone of extended

deformation.

Fig. 7. Types of chips after tapping P20mold steel: (a, b) chip shape; (c, d) side view; and (e, f) undersurface. (a, c, e) without PFPE; and (b, d, f) with

PFPE.

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Acknowledgements

The authors are grateful to NSERC for their NATOScience Fellowship which supported Dr. G. Benga duringhis stay at McMaster University and to the Centre forMaterials and Manufacturing for their research support.

References

[1] T. Cao, J.W. Sutherland, Investigation of thread tapping load

characteristics through mechanistic modeling and experimentation,

International Journal of Machine Tools and Manufacture 42 (2002)

1527–1538.

[2] T.W. Del Pesco, Perfluoropolyethers, in: R.L. Shubkin (Ed.),

Synthetic Lubricants and High-Performance Functional Liquids,

Marcel Dekker, New York, 1993.

[3] B. Bhushan, Magnetic media tribology: state of the art and future

challenges, Wear 136 (1990) 169–197.

[4] T. Kato, M. Kawaguchi, M. Sajjad, J. Choi, Friction and durability

characteristics of ultrathin perfluoropolyether lubricant film com-

posed of bonded and mobile molecular layers on diamond-like

carbon surfaces, Wear 257 (2004) 909–915.

[5] T. Kita, The influence of lubrication on head and disk wear,

Tribology Transactions 35 (1992) 551–555.

[6] H.J. Lee, R. Zubeck, D. Hollars, J.K. Lee, A. Chao, M. Smallen,

Enhanced tribological performance of rigid disk by using chemically

bonded lubricant, Journal of Vacuum Science and Technology A 11

(1993) 711–714.

[7] H. Liu, B. Bhushan, Nanotribological characterization of molecu-

larly thick lubricant films for applications to MEMS/NEMS by

AFM, Ultramicroscopy 97 (2003) 321–340.

[8] I.S. Napreev, Control over tribological characteristics of bearing

units by the methods of epilam formation, Ph.D. Thesis, Bella

Russian State Technology University, 1999.

[9] T.E. Karis, B. Marchon, D.A. Hopper, R.L. Siemens, Perfluoropo-

lyether characterization by nuclear magnetic resonance spectroscopy

and gel permeation chromatography, Journal of Fluorine Chemistry

118 (1–2) (2002) 81–94.

[10] R.J. Greve, S.C. Langford, J.T. Dickinson, Oxidation and reduction

reactions responsible for galvanic corrosion of ferrous and reactive

metals in the presence of a perfluoropolyether lubricant: Fomblin

Z-DOL, Wear 249 (2001) 727–732.

[11] J.T. Dickinson, S.C. Langford, W. Faultersack, H. Yoshiaki,

Application of transient current measurements: evidence for galvanic

corrosive wear of aluminum by a polyperfluoroether lubricant, Wear

215 (1998) 211–222.

[12] G.S. Fox-Rabinovich, A.I. Kovalev, D.L. Wainstein, L. Sh. Shuster,

G.K. Dosbaeva, Improvement of ‘duplex’ PVD coatings for HSS

cutting tools by PFPE (perfluorpolyether ‘Z-DOL’), Surface and

Coatings Technology 160 (1) (2002) 99–107.

[13] R.E. Sulouff, MEMS opportunities in accelerometers and gyros

and the microtribology problems limiting commercialization, in: B.

Bhushan (Ed.), Tribology Issues and Opportunities in MEMS,

Kluwer Academic, Dordrecht, The Netherlands, 1998.

[14] E.M. Trent, P.K. Wright, Metal Cutting, fouth ed, Butterworth-

Heinemann, Woburn, MA, 2000.