Surface modification to improve friction and galling

Journal of Materials Processing Technology 174 (2006) 334–341
Surface modification to improve friction and galling
properties of forming tools
B. Podgornika,∗, S. Hogmarkb
a Centre for Tribology and Technical Diagnostics, University of Ljubljana, Bogisiceva 8, SI-1000 Ljubljana, Slovenia
b The Tribomaterials Group, ˚Angström Laboratory, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden
Received 23 September 2003; accepted 20 January 2006
Abstract
The aim of the present work was to investigate and compare different surface modification techniques in terms of galling properties when applied
on forming tools. The surface modification techniques included were different grades of polishing, plasma nitriding and DLC coating, all applied
to cold work tool steel, respectively. Tribological evaluation against austenitic stainless steel was carried out in a load-scanning test rig. A special
test configuration makes it possible to gradually increase the normal load during testing, with the load being in the range between 100 and 1300 N
(1–3.5 GPa).
It is concluded that the galling tendency can be greatly reduced by proper polishing of the tool surface. However, selection of a carbon-based
multilayer WC/C coating leads to substantially reduced probability of work material transfer even at a relatively high surface roughness, and also
under starved lubrication. If polished after treatment, a plasma nitrided surface will provide improved friction and wear properties of the tool steel
and to some extend also reduce the galling tendency in sliding against austenitic stainless steel. A combination of nitriding, surface polishing and
DLC coating is suggested as the optimum surface treatment of tools aimed at forming austenitic stainless steel.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Forming tools; Surface engineering; Friction; Galling
1. Introduction
Manufacturing of parts is confronted with ever-increasing
demands on higher productivity, which put increased require-
ments on wear and fatigue resistance of the tools. In the case
of cutting tools, deposition of hard PVD and CVD coatings
(i.e. TiN, TiAlN, CrN, . . .) has proven to provide consider-
able enhancement in wear resistance and therefore tool life in
numerous cases [1–3]. However, in competitive environment
only increased production at maintained product quality can be
successful because this directly influencing the profitability.
The surface finish is a critical quality parameter in the
case of sheet metal parts. A smooth surface often results in
marketing advantages, as well as improved performance. The
major obstacles to generation of superior smooth surfaces
are smudge, orange peel and galling [4], with galling also
being the dominant cause for tool failure in sheet forming
∗ Corresponding author. Tel.: +386 1 4771463; fax: +386 1 4771469.
E-mail address: bojan.podgornik@ctd.uni-lj.si (B. Podgornik).
processes for austenitic stainless steel, aluminium-, nickel- and
titanium-based alloys. Typically, galling develops gradually
during forming operations as an accumulation of sheet material
pick-ups on the tool surface. This causes aesthetic problems for
the work-peace and generates unstable friction conditions for
the continuing forming process [5].
The main focus on improving the tribological properties in
sheet forming has been on modifying lubricants for better reten-
tionandpermeabilityatthetool/sheetcontactarea.Nevertheless,
galling can successfully be hindered also by modification of the
tool surface [6,7]. One way of modifying the surface, already
effectively used in the forming industry to improve wear resis-
tance of the tools, is thermo-chemical surface treatment, i.e.
hardening, nitriding, carbonitriding, etc. [7–9]. In the last couple
of years hard coatings have started to compete successfully with
the traditional thermo-chemical treatments. However, although
hard and corrosion-resistant coatings are frequently used to pro-
tect and enhance the lifetime of cutting tools, the majority of
the industrial forming tools is still uncoated. Besides a complex
shape that often makes the forming tools difficult to coat, com-
mercial hard ceramic coatings used in cutting tool applications
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2006.01.016

B. Podgornik, S. Hogmark / Journal of Materials Processing Technology 174 (2006) 334–341 335
often give a relatively high friction coefficient and a high ten-
dency to galling when in contact with soft metals [3,10,11]. With
the introduction of carbon-based coatings such as diamond and
diamond like carbon (DLC), which show high wear resistance
and excellent frictional properties [12–14], regular use of coated
forming tools can be expected for the near future.
Another way to improve the galling properties of forming
tools is to optimise the surface topography. Investigations on
the influence of surface roughness showed that it has a great
influence on the tribological behaviour of contacting surfaces
[15–17], thus playing an important role in the process of improv-
ing the galling properties of forming tools.
The aim of the present work was to investigate and com-
pare different surface modification techniques aimed to reduce
the galling tendency of forming tools. A tribological evaluation
of plasma nitrided, DLC coated, and polished cold work tool
steel was carried out in a load-scanning test rig, using austenitic
stainless steel as the counter material.
2. Experimental
The material used in this investigation was a powder metallurgy cold work
tool steel, VANADIS 6, from Uddeholm Tooling AB, Sweden. It has a nominal
chemical composition (wt.%) of 2.1 C, 1.0 Si, 0.4 Mn, 6.8 Cr, 1.5 Mo, 5.4 V.
The cylindrical test rods (Ø 10 mm, 100 mm long) were hardened and tempered
Fig. 1. Representative friction coefficient vs. normal load curves for hardened
and plasma nitrided tool steel, recorder during dry, single stroke sliding against
austenitic stainless steel. All test rods had a surface roughness of Ra = 0.25 ␮m.
to 850 HV, and ground to an average Ra value of ≈0.25 ␮m. Three groups of
test specimens were prepared in terms of surface modification.
The first group contained two plasma nitrided steels, obtained with different
gas mixtures, see Table 1. These treatments resulted in nitrided layers with and
without a compound layer, respectively. After nitriding all samples were re-
Fig. 2. Typical appearance of the contact surface of hardened steel specimens
from the dry, single stroke sliding test against austenitic stainless steel. (a) At
about 200 N load where the material transfer begins (light contrast). (b) At about
250 N load where a layer of transferred material has formed. The arrows indicate
the direction of sliding.
Table 1
Details of substrate treatment processes and obtained surface hardness values
Treatment Atmosphere Temperature (◦C) Time (h) Case depth (␮m) Compound layer (␮m) Surface hardness (HV0.1)
A Plasma nitriding 75% H2–25% N2 500 6 60 2-Fe4N (␥ ) 1300
B Plasma nitriding 95% H2–5% N2 500 9 55 0 1130
C Hardening Air quenched 1060/525 0.5/2 × 2 Through – 850
Table 2
Deposition parameters and resulting coating properties
Coating Process Temp. (◦C) Substrate bias (V) Interlayer (␮m) Hardness (GPa) Young’s modulus (GPa) Residual stress (GPa)
TiN Reactive e-beam 320–420 −110 Ti-0.1 30 ± 2 500 ± 50 −3.8 ± 0.4
WC/C Reactive sputtering 230 NA Cr-0.1 12 ± 1 130 ± 7 −0.3 ± 0.1
NA, not available.

336 B. Podgornik, S. Hogmark / Journal of Materials Processing Technology 174 (2006) 334–341
ground in order to re-establish the original surface roughness of Ra ≈ 0.25 ␮m.
The second set of specimens was hard coated. Two commercial PVD coatings
were included in this investigation; TiN as the most commonly used coating
in cutting tool applications and WC doped hydrogenated diamond like carbon
with a multilayer structure of WC and C (WC/C), representing the class of hard
low-friction coatings, see Table 2. The third group of specimens was prepared
using different grades of grinding and polishing, which gave average surface
roughness values Ra of 0.05, 0.1, 0.15, 0.25 and 0.4 ␮m, respectively.
Thetribologicalevaluationincludedcomparisonbetweensurfaceengineered
steel samples and hardened and ground reference ones. It was performed in a
new type of load-scanning test rig [18,19]. An austenitic stainless steel (AISI
304; 350 HV) was used as counter material. The test configuration, involves two
crossed cylinders, which are forced to slide reciprocally against each other under
a constant speed. The normal load increases gradually during forward strokes
and decreases correspondingly during reversed strokes. Thus, each point along
the contact path of both specimens will experience a unique load and display a
unique tribological history after test completion.
In the first phase of this investigation, the test equipment was set to a single,
forward stroke mode, and operated under dry conditions with the sliding speed
fixed to 0.01 m/s. Some additional tests were performed dry or under starved
lubricated conditions, with the test rig set to a multicycle mode. An approxi-
mately 10 ␮m thick film of pure poly-alpha-olefin (PAO) oil (v40 = 46.6 mm2/s)
was applied on the work material sample before each starved lubricated test. The
sliding speed was set to 0.1 m/s, and the highest number of test cycles was 50. In
both types of test, the normal load was gradually increased from 100 to 1300 N
(3.5 GPa maximum Hertzian contact pressure), reached at the end of each stroke.
Results of this test were displayed as friction versus load, and by imagining
the worn surfaces by optical (OM) or scanning electron microscopy (SEM).
Monitoring of the friction coefficient as a function of load and time has been
Fig. 3. Critical loads for the beginning of transfer (Lc1) and formation of a
layer of transfered stainless steel (Lc2) on surface treated tool steel samples, as
estimated from the dry, single stroke test.
Fig. 4. Representative friction coefficient vs. normal load curves for coated tool
steel, recorder during dry, single stroke sliding against austenitic stainless steel.
utilised for the lubricated multicycle tests to prepare friction maps, which show
any transition point in the tribological behaviour. From the OM micrographs,
critical loads for the initiation of galling (Lc1) and for development of a whole
layer of transferred material (Lc2) could be established.
Fig. 5. Critical loads for the beginning of transfer (Lc1) and formation of a layer
of transfered stainless steel (Lc2) on uncoated and coated tool steel samples, as
estimated from the dry, single stroke test.
Fig. 6. Occasional spalling of WC/C coated steel after repeated dry sliding
against austenitic stainless steel. (a) After 30, and (b) 50 loading cycles at 550 N
load. Note that the material transfer is associated to the spalling.

3. Results and discussion
3.1. Influence of plasma nitriding
In the single stroke test, hardened forming tool steel initially
displayed a friction coefficient between 0.3 and 0.4. An increase
in friction, indicating transfer of work material to the tool steel
surface, was observed between 200 and 250 N load, as shown in
Fig. 1. This transfer is confirmed by the OM pictures in Fig. 2
that shows beginning of work material transfer at about 200 N
(Fig. 2a) and building-up of a relatively thick layer of transferred
material above 250 N load (Fig. 2b).
Plasma nitriding followed by polishing to the original sur-
face roughness, reduced the initial friction coefficient against
austenitic stainless steel to about 0.25 and improved the galling
properties, with the critical load for the beginning of material
transfer being in the range between 250 and 350 N load, see
Figs. 1 and 3. However, depending on the nitriding conditions,
nitriding increases the surface roughness [20,21] and if not re-
ground, nitrided surfaces will lead to an almost instantaneous
stainless steel transfer, as indicated in Fig. 3.
A comparison between nitrided forming tool steel with and
without a thin ␥ compound layer showed somewhat improved
friction and galling properties when using a combination of
diffusion and compound layer (treatment A in Table 1), see
Figs. 1 and 3. However, previous investigations [21–23] have
shown that a compound layer may lead to an increased wear
rate. Due to its brittleness and possible porosity, the compound
layer may start to break up under high loads, leading to for-
mation of hard abrasive particles that may accelerate the wear
rate.
3.2. Influence of TiN and WC/C coating
As compared to uncoated hardened forming tool steel, depo-
sition of a TiN coating, up till now one of the most successfully
used hard wear resistant coatings in cutting tool applications,
gave similar friction but almost instantaneous transfer of stain-
less steel to the tool steel surface, see Figs. 4 and 5. Although
hard ceramic coatings show high wear resistance, their high
tendency to pick up work material when in contact with soft
metals [23,24] make them highly unsuitable for forming tools.
Fig. 7. Representative surface roughness profiles and average roughness parameters for the tool steel samples after: (a and b) grinding and (c–e) polishing.

Fig. 8. Representative friction coefficient vs. normal load curves for the ground
and polished tool steel, recorder during dry, sinle stroeke sliding against
austenitic stainless steel.
Fig. 9. Critical loads for the beginning of transferred stainless steel (Lc1) and
formation of a layer of transfered material (Lc2) to ground and polished tool
steel samples.
Fig. 10. Proportion of the tool steel contact area covered by transferred stainless
steel for the tool steels of different surface roughness.
On the other hand, the low-friction coating WC/C reduced
the friction against austenitic stainless steel dramatically, to a
level of about 0.15 in the whole load range investigated, see
Fig. 4. Furthermore, the WC/C coating gave highly improved
galling properties, with transfer of stainless steel being pre-
vented almost up to the maximum load of 1300 N, as indicated in
Fig. 5.
These results clearly indicate, that carbon based low-friction
coatings with excellent frictional properties [23,25] also have a
high potential for improving the galling properties of forming
tools. However, some coating spallation was observed for the
WC/C coating in a dry multicycle test. This produced sharp
edges, see Fig. 6a, which increased the amount of work material
transfer, as shown in Fig. 6b.
3.3. Influence of surface topography
The investigation on galling properties of the VANADIS 6
tool steel also included five different grades of grinding and
polishing, resulting in an average surface roughness Ra of 0.4,
0.25, 0.15, 0.1 and 0.05 ␮m, respectively, see Fig. 7.
Fig. 11. Representative SEM micrographs of polished tool steel (Ra = 0.10 ␮m)
tested in dry, single stroke sliding. (a) At a position corresponding to a normal
load of 800 N. (b) At 1000 N load. The arrows indicate the direction of sliding.

Polishing of the surface reduced the friction and improved the
galling properties of the tool steel when sliding against austenitic
stainless steel, see Figs. 8 and 9. The two roughest surfaces
(Ra = 0.4 and 0.25 ␮m, respectively) gave the highest initial fric-
tion coefficient, about 0.4, and an almost instantaneous transfer
of stainless steel. Reducing the roughness leads to a substantial
reduction in friction and a corresponding significant increase in
critical load for the beginning of stainless steel transfer. Note
that the finest polishing (Ra = 0.05 ␮m) gave a low and very uni-
form friction coefficient of about 0.15, and an almost complete
protection against work material transfer in the entire tested load
interval, see Fig. 9. Besides an increased critical load, polishing
of the surface also reduces the amount of stainless steel being
transferred at loads higher than the critical, as shown in Fig. 10.
SEM observations of the contact surfaces showed that the
transfer of work material to the tool steel surface preferen-
tially starts at the edge of grinding grooves and roughness
asperities, see Fig. 11a. At further increasing load, the trans-
formation gradually creates a layer of transferred material, see
Fig. 11b. Obviously, any irregularity in the tool surface repre-
sents a potential source for initiation of work material transfer.
Therefore, by removing groves and roughness asperities, pol-
ishing of the surface eliminates potential sources for material
transfer, which leads to improved galling properties of forming
tools.
3.4. Friction and galling at starved lubrication
Friction maps for surface engineered tool steel loaded against
austenitic stainless steel under starved lubrication conditions are
shown in Fig. 12. For the hardened tool steel (Ra = 0.25) an
abrupt increase in friction was detected already during the sec-
ond stroke, see Fig. 12a. The test had to be stopped after the third
stroke due to extensive transfer of stainless steel. These results
indicate that, as the reciprocal sliding proceeds, the initial regime
of boundary lubrication moves towards a mixture of boundary
Fig. 12. Representative friction maps for surface engineered tool steel, sliding against PAO lubricated austenitic stainless steel. (a) Hardened reference steel
(Ra = 0.25 ␮m) (b), hardened and polished steel (Ra = 0.1 ␮m), (c) TiN coated and (d) WC/C coated.

lubrication and dry sliding. Similar results, with the initial fric-
tion in the range of 0.15 and 0.20 and transfer of work material
starting already during the second stroke, were observed for the
plasma nitrided samples.
Polishingofthesurfaceimprovedthefrictionpropertiesofthe
toolsteelunderboundarylubrication,reducingtheinitialfriction
to about 0.1 and increasing the number of strokes needed for the
lubrication conditions to deteriorate, see Fig. 12b. Five to 10
strokes were needed to substantially increase the friction for the
steel polished to Ra = 0.10 ␮m. However, the increase in friction,
indicating a less effective lubrication, was load dependant, with
extensive adhesion of work material being restricted to loads
above 500 N.
In the case of TiN coated steel a rapid increase in friction,
correspondingtoarapidtransferfromboundarylubricatedtodry
sliding, started at approximately 1000 N load already during the
first stroke, see Fig. 12c. On the other hand, the WC/C coated
steel displayed a uniform frictional behaviour with a friction
coefficient of ∼0.1 during the whole 50-cycle test, see Fig. 12d.
In addition, deposition of the WC/C coating gave a complete
protection against transfer of austenitic stainless steel during
the whole test. Similar conditions of low friction and no galling
have only been obtained through the use of fully formulated
forming oil [26].
4. Conclusions
• The major observation in this investigation is that both friction
and tendency to galling reduces with the surface roughness.
Polishing of the surface removes irregularities and asperities
from the contact surface, thus eliminating potential sources
for material transfer.
• Carbon based low friction coatings of the DLC type give
a certain tolerance to surface roughness and can maintain
low friction and galling resistance at high loads in dry
sliding.
• Provided that sufficient coating to substrate cohesion, adhe-
sion and load-carrying capacity of the substrate is met, DLC
coatings also have the potential to give excellent protection
against work material pick-up and transfer layer formation in
lubricated contacts.
• However, fine polishing seems to be competitive alternative
to DLC coating.
• Plasma nitriding, a commonly used surface treatment to
improve wear resistance and durability of forming tools, can
give up to 40% improvement in galling properties, but only if
the surface is polished after the treatment.
• Hard TiN coating shows high friction and high affinity to pick-
up work material when in contact with austenitic stainless
steel, and does not improve the galling properties of forming
tools.
• An optimum solution to providing superior friction and
galling properties of forming tools should therefore include
the following steps. Plasma nitriding to improve the load-
carrying capacity of forming tool steel. Post-polishing of the
contact surface to eliminate any surface asperities. Deposition
of a hard low-friction coating.
Acknowledgements
Uddeholm Tooling AB, The Swedish Research Council and
Carl Trygger Foundation are greatly acknowledged for financial
support. The supply of test materials and coatings from Udde-
holm Tooling and Balzers Sandvik Coating AB, respectively, is
much appreciated.
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