Copyright Ⓒ 2013 JSPE㪬㪉㪋Wear monitoring of amorphous carbon films by embedding
luminescent wear-sensing layer
Tokyo Institute of Technology { Atsawin SALEE, Atsushi HIRATA
This research aims to present the concept of wear monitoring of amorphous carbon (a-C) films throughluminescence from a wear sensing layer embedded between a-C film and substrate. Fabrication andcharacterisation of the coatings are also presented. The wear sensing layer consists of luminescent ZnS:Cu
powder dispersed in epoxy resin (EP/ZnS:Cu) coating. The a-C films are deposited onto EP/ZnS:Cu coatings
by sublimation of fullerenes (C60) in electron beam excited plasma (EBEP) method and pulsed vacuum arc
deposition (VAD) method. The results show that the EP/ZnS:Cu coatings exhibit the green luminescencewhen irradiated with the UV light. After deposition of a-C films, the luminescence from the EP/ZnS:Cu
coating is undetected. A ball-on-disk friction test is carried out not only to examine the coefficient of friction,
but also generate the wear track on the a-C films. It is found that after the friction test, the a-C films
deposited by EBEP method are completely worn out and the luminescence from the EP/ZnS:Cu can be redetected. In contrast, the a-C film deposited by VAD is durable and is not worn out, therefore theluminescence from the EP/ZnS:Cu cannot be re-detected.

1. Introduction
Amorphous carbon (a-C) films have been attractive fora several decades in the mechanical field due to their
exceptional properties such as high hardness, wear
resistance and low friction properties 1,2). With proper
design, they can offer a long lifetime as a tribologicalcoating. However, these properties depend on many factors,
for example, sliding speed, temperature, load andenvironment 3), in which wear monitoring is difficult while
in service. This occasionally leads to the pre-mature failureof the coating.
Luminescent spectroscopy has been proposed andapplied to monitor the health of thermal barrier coatings4,5). The coatings are multilayer consisting of a top coating
and a luminescent layer embedded between the top coating
and substrate. The monitoring mechanism is that when
the top layer is worn out, the luminescence can be detected.
The present research aims to extend such concept formonitoring the wear life of amorphous carbon (a-C) films.
Deposition of a-C films onto the wear sensing layers, which
consist of luminescent ZnS:Cu powder dispersed in epoxy
resin is presented. The coating performance are examinedand discussed.
2. Experimental
The wear sensing layer, which consists of luminescent
ZnS:Cu powder dispersed in epoxy resin (EP/ZnS:Cu), wasfabricated initially. As-purchased ZnS:Cu powder of 5 wt.%
to the base resin was mixed with an epoxy resin. The
mixture was dropped onto silicon wafers and covered with
polystyrene films. The mixture coatings were finally cured
at 40qC for 12 h.
The cured EP/ZnS:Cu coatings were subsequently
coated with amorphous carbon (a-C) films by two methods,
namely sublimation of fullerenes (C 60) in electron beam
excited plasma (EBEP) method and pulsed vacuum arc
deposition (VAD) method. The detail of deposition of a-C
films by sublimation of C60 in EBEP method was given inelsewhere 6). In this method, a-C films were deposited at-200 V and 0 V bias voltages for 60 min. For the a-C coatedby the pulsed VAD method, the EP/ZnS:Cu coatings wereplaced 12 cm apart from the arc plasma gun in which thegraphite rod was used as the carbon source. The arc
plasma voltage was 100 V, capacitance for arc discharge
was 720 PF, discharge frequency was 1.0 Hz and thenumber of discharge was 6000 counts, respectively.
Photoluminescence (PL) properties of the EP/ZnS:Cu
before and after being coated with a-C films wereinvestigated by irradiating with the UV light at 365 nmand collecting the PL spectra from the coating by thespectrometer. The coatings were tested in the ball-on-disk
friction test. The coatings were rubbed with stainless steel(SUS440C) balls 4.76 mm in diameter at the normal forceof 0.3 N and rotational speed of 50 rpm. The sliding
distance of the test was 50 m. After the friction test, PLspectra were collected again at the wear tracks of thecoatings in order to investigate the sensing capability.
3. Results and discussion
Photoluminescence (PL) spectra of the as-prepared
EP/ZnS:Cu coating is shown in Fig.1(a). The spectrum
shows the peak center at 525 nm with a small shoulder at
450 nm, corresponding to the characteristic spectrum ofZnS:Cu powder 7). The PL spectra of EP/ZnS:Cu after being
coated with a-C films by various deposition conditions asshown in Fig. 1(b)-(d) are not detected. This suggests thata-C films can protect EP/ZnS:Cu from the UV excitation.
Friction behaviour of a-C coated EP/ZnS:Cu coatings
obtained from various conditions are illustrated in Fig. 2.
The a-C coated EP/ZnS:Cu coatings deposited by EBEP
show the coefficient of friction (COF) of 0.2 at the starting
period and the COF suddenly increases to the higher value
in the short sliding distance. Finally both coatings reach
the COF values of 0.7 as shown in Fig. 2(a) and (b). On theother hand, a-C coated EP/ZnS:Cu coatings deposited byVAD at 6000 counts shows the lower COF value of 0.36 asshown in Fig. 2(c).

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

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Copyright Ⓒ 2013 JSPE㪬㪉㪋Fig. 3 shows the photoluminescence (PL) spectra
plotted together with the fitted spectra collected from thewear track of the coatings after the friction test. It showsthat PL spectra with the peak position of 525 nm can bedetected from the wear tracks of the a-C coated EP/ZnS:Cucoatings deposited by EBEP at both -200 V and 0 V asshown in Fig. 3(a) and (b). This corresponds to the PL
spectra of the EP/ZnS:Cu coating. Moreover, this also
indicates that a-C films has been worn out resulting in theexposure of EP/ZnS:Cu to the UV excitation. Therefore, theluminescence can be redetected. However, the PL
spectrum from the a-C coated EP/ZnS:Cu coating deposited
by VAD at 6000 counts is undetected, which indicatingthat the coatings are not removed and still durable.
Fig. 1 Photoluminescence spectra of EP/Zns:Cu coatings(a) before and (b) after being deposited with a-C films byEBEP at -200 V, (c) EBEP at 0 V and (d) VAD at 6000
countsFig. 2 Friction behaviour of a-C coated EP/ZnS:Cu coatings
deposited by (a) EBEP at -200 V, (b) EBEP at 0 V and (c)VAD at 6000 counts

Fig. 3 PL spectra collected at the wear track of a-C coated
EP/ZnS:Cu coatings deposited by (a) EBEP at -200 V, (b)EBEP at 0 V and (c) VAD at 6000 counts

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4. Conclusion
The fabrication and demonstration of amorphous
carbon films with wear sensing layer is presented. The
wear sensing layer consists of EP/ZnS:Cu coating, whichshows strong and detectable green luminescence. After theEP/ZnS:Cu coatings were coated with amorphous carbon(a-C) films by sublimation of fullerenes in electron beam
excited plasma (EBEP) and pulsed vacuum arc deposition(VAD), the luminescence from the EP/ZnS:Cu was not bedetected. This suggests that the a-C films can protect thesensing layer from the excitation. The a-C deposited on
EP/ZnS:Cu by two methods show the different friction
behaviour. The a-C films deposited by EBEP method showa higher value of COF than those deposited by VAD
method. Moreover, the luminescence from the wear tracks
could be detected for the coatings deposited by EBEP
method, while it could not be detected for the coatings
deposited by VAD. This is caused by the removal of a-Cfilms deposited by EBEP methods after the friction test
and durability of the a-C film deposited by VAD method.
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ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.