2016 STLE Annual Meeting & Exhibition
May 15-19, 2016
Bally’s Las Vegas Hotel and Casino
Las Vegas, Nevada, USA
Friction and wear performance of Titanium alloy against tungsten carbide lubricated
with phosphate ester
Ye Yang
Chenhui Zhang
Yan Wang
Yuanjing Dai
Jianbin Luo
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
1. Introduction
Due to the relatively low density, outstanding strength-to-density ratio and excellent corrosion resistance,
titanium alloys have been widely utilized in aeroengine and airframe manufacturing fields [1,2]. However,
titanium alloys are recognized as difficult-to-cut materials due to their low thermal conductivity, high
chemical activity and small elastic modulus [3,4]. Cutting fluid is considered as an accessory in a
machining process in order to increase the productivity. It lubricates the contact areas between rake face
and chips, flank face and machined surface as well as reduces adhesion and abrasion thus producing a
good surface finish. As to the mechanism of this action, the chemical compositions of the cutting fluid
are of vital importance and the interactions with the metal surface have been investigated [5]. Researches
try to find effective additives which have good tribological properties with steel or aluminum alloys, for
example, borate ester compounds, phosphorus-containing molecules and so on[6,7]. However, the
chemical properties of metal surfaces differ significantly. As a consequence, the type and intensity of
interaction of cutting fluid additives with titanium alloys are very different with steel and aluminum
alloys because of the bad tribological properties of titanium alloys [8]. Up to now, few reaches have been
done to investigate the effective cutting fluid additives with titanium alloys considering the lubricating
and anti-adhesion properties. This paper aims to provide an effective water soluble additive for titanium
alloys and investigate the mechanism of the additives acting with the metal surface. It can be further used
for the development of titanium alloys cutting fluid.
2. Experimental details
Nonylphenol polyoxyethylene ether phosphate ester (PPE) aqueous solution PPE was bought from
Haian Company, China, which was synthesized by nonylphenol polyoxyethylene ether (NP-10) and
phosphorus pentoxide in the presence of a catalyst. The triethanolamine borate used in this study was
purchased from Haian Company. It was diluted to the concentration of 1 wt% by deionized water. As a
comparison, one kind of commercial emulsion with concentration of 5% which is widely used for
machining titanium alloys in aviation industry was also tested lubricating properties.
The frictional tests were carried out utilizing a ball-on-disc apparatus SRV4 (Optimal) under different
lubricating conditions at atmosphere during the tests. The discs were TC4 (Ti6Al4V) alloy with hardness
of HRC35. The tungsten carbide YG8 (WC-Co) ball with diameter of 10mm and hardness of 89HRA
was chosen as the counter specimen. The samples were ultrasonically cleaned by acetone and ethanol
and then ultrapure water successively each for 10 min before tests. A normal load of 100N was applied
and the initial maximum Hertz contact pressure was calculated to be 1267MPa. The upper ball slid
reciprocally against the stationary disc with amplitude of 1 mm and frequency of 20 Hz for 5 min. A
Quanta200 scanning electron microscopy (SEM) combined with energy dispersion spectrometry (EDS)
was used for the surface analysis of the investigated materials. The chemical compositions of the worn
surfaces were characterized by a PHI Quantera SXM X-ray photoelectron spectrometer (XPS).
3. Results and Discussion
3.1 Tribological properties of various aqueous solutions
Figure 1 showed the friction coefficients during the sliding process with various lubricants. As for
deionize water and borate solution, the friction coefficient increased from 0.3 to 0.45 with increasing
sliding time. The fluctuation of the friction coefficient was fairly large during the tests and cacophony
was heard caused by rubbing. It meant that deionized water failed to lubricate titanium alloy and tungsten
carbide. Compared to them, emulsion showed better lubricating properties. After a short period of
running-in, the friction coefficient kept stable with a value of 0.18. During the tests, it always happened
that the friction coefficient jumped to a high value (0.35 in fig.1 for example) and then went back to the
original value and kept constant till next jump. This might be caused by material transfer and stick-slip
friction. At the beginning of the test of PPE solution, the friction coefficient wasn’t stable for running-in
but it dropped to 0.14 quickly and kept stable to the end of the test. From this figure we can conclude
that for tungsten carbide and titanium alloy tribopair, PPE can provide effective lubrication, even better
than the emulsion used in our tests while triethanolamine borate has no effect on this system under the
experimental conditions.
Material:Ti6Al4V
Ball:φ10mm WC-Co
Fn:100N
f=20Hz
L=2mm
Fig.1 The friction coefficient of water, borate solution, emulsion and PPE solutions
The wear tracks on the surfaces of TC4 and the surface of balls were observed by SEM as shown in
Figure 2. The SEM pictures showed that there were many delaminated scars and irregular abrasive
particles on the tracks lubricated by water and triethanolamine borate solutions. Furthermore micro
patches were detected on surface of the balls (Fig. 2a, b). To ascertain the ingredients of the material on
the balls, EDS analysis were made. The results indicated that titanium alloys existed on the surfaces of
the balls which were transferred from the discs and adhered on the ball surface (Fig. 2e). It was deduced
that seriously adhesive wear occurred for the chemical activity of titanium and the poor lubricating
condition. The rubbing surfaces lubricated by emulsion and PPE solutions were much smoother, only
some micro-grooves exhibited (Fig. 2c, d). What’s more, micro particles but not patches existed on the
surface of balls. The elements were so alike that it was just listed once in Fig. 2e. This meant that adhesive
wear was strongly decreased and a well finish surface was obtained. It was easy to understand that oilcontained emulsion can provide effective lubrication. To our surprise, the PPE solution exhibited even
better lubrication and anti-wear ability to titanium alloy.
(a)
disc
50 μm
(b)
20 μm
(c)
disc
50 μm
20 μm
50 μm
disc
ball
ball
(d)
20 μm
ball
50 μm
disc
20 μm
ball
(e)
1.
4
KCnt
1.
1
Ti
0.
9
0.
6
0.
3
0.
0
O
Ti
C
A
l
V
1.00 2.00
3.00 4.00Energy
5.00
6.00
7.00
8.00
9.00
10.0
0
(keV)
Fig.2 SEM micrographs of worn surface on titanium alloy and carbide ball lubricated by
(a) water (b) triethanolamine borate (c) emulsion (d) PPE and (e) EDS analysis of the balls
3.2 Chemical characterization
XPS is a practical method to clarity the chemical states of elements within adsorption film on surface
of tribopair. Figure3 (a) showed typical XPS survey scans inside and outside the wear track. Phosphoric
peak at 133eV appeared at the position inside the wear track, while no phosphorus was detected outside
the track. It demonstrated that phosphoric compounds remained after frictional test when lubricated with
PPE solution. The peak of Ti2p inside the wear track was apparently lower than the substrate which
further demonstrated that some film existed on the wear track.
To further investigate the mechanism, detailed high resolution XPS scans of oxygen were recorded.
The high resolution O1s spectrum outside the wear track was shown in Figure3 (b). According to the
possible existent compounds in this work, the spectrum was resolved by Gaussian fitting. Figure3 (c)
showed the O1s spectrum inside the wear track which was fitted with three peaks. Because of the high
chemical activity of titanium, it adsorbed oxygen and water molecules easily thus an oxidation layer
formed on the surface of titanium alloy though the titanium alloy used in our work were freshly polished.
So a high peak at 530.3eV (assigned to TiO2 ) and 531.9eV (assigned to Ti-OH) can be found in Fig.3
(b).The O1s peaks inside the wear track were quite different. As shown in Fig.3 (c), a peak at 531.6
(assigned to P-O-Ti) and 532.8 (assigned to P-OH) were detected. It can be deduced that PPE molecules
were adhered onto Ti6Al4V surface through covalently bond P-O-Ti.
The test results showed that the PPE can adsorb on the surfaces of titanium alloy. The viscosity of
0.1wt% PPE solution was about 0.96mPa.s which was almost the same as the viscosity of water at 22℃.
Even the concentration increased to 1 wt%, the viscosity of solution was 1.3 m Pa.s. It was not high
enough to form hydrodynamic film under this situation. So the adsorption film played the most important
role to reduce the friction coefficient and significantly improve the anti-wear property of water by
separating the contact surfaces to some extent.
x 10
4
O1s
6
O1s
5
C/S
1400
inside the wear track
Ti2p
4
1200
P2p
outside the wear track
3
inside the wear track
1000
800
2
outside the wear track
600
1
0
1200
P2p
1600
C1s
C/S
7
400
140
1000
800
600
400
Binding energy (eV)
200
135
130
Binding energy (eV)
0
125
(a)
P-O-Ti
TiO2
P-OH
TiO2
Ti-OH
(b) outside the wear track
(c) inside the wear track
Fig.3 (a) XPS survey scans of the titanium alloy surface after tribological test; O1s components (b) ouside and (c)
inside the wear track
4. Conclusions
The friction and wear performance of titanium alloys against tungsten carbide balls under four kinds
of aqueous solutions were investigated. The results showed that triethanolamine borate had no effect on
lubricating titanium alloy. The friction coefficient was high and the worn surface was as rough as that
lubricated with water. While PPE solutions reduced the friction coefficient and wear volume thus a well
finish surface being obtained. It exhibited even better lubricating and anti-wear abilities than the
commercial emulsion used for cutting titanium alloys. The adhesive wear was weaken at the same time.
The XPS analysis demonstrated that adsorption film formed by P-O-Ti covalent bond on titanium
alloy surface avoided the directly contacting with the counter surface.
This paper found a new aqueous system to lubricate titanium alloys/tungsten carbide effectively which
gave a direction on choosing water-based additives for machining titanium alloys cutting fluid.
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Key words Nonylphenol polyoxyethylene ether phosphate ester(PPE), titanium alloy, water-based
lubrication, anti-wear properties