Characterization and investigation of the tribological properties of sol-gel zirconia thin films

Article abstract of DOI:10.1111/j.1151-2916.2002.tb00462.x

Sol-gel zirconia thin films were prepared by dip coating in an ethanol solution of zirconium oxychloride. The zirconia films consisted of a completely tetragonal phase and exhibited nanoscale uniformity. They displayed excellent antiwear and friction-reduction performance in sliding against steel. The friction coefficient (0.13-0.15) and the wear life over 5000 sliding cycles were recorded for the films at a sliding speed of 90 mm/min and a load of 0.5 N. The film was characterized by slight scuffing and abrasion at low loads and sliding speeds.

Full text of DOI:10.1111/j.1151-2916.2002.tb00462.x

J. Am. Ceram. Soc., 85 [9] 2367–69 (2002)
journal
Characterization and Investigation of the Tribological Properties of
Sol–Gel Zirconia Thin Films
Yunxia Chen and Weimin Liu*
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, People’s Republic of China
Sol–gel zirconia thin films were prepared by dip coating in an
ethanol solution of zirconium oxychloride. The zirconia films
consisted of a completely tetragonal phase and exhibited
nanoscale uniformity. They displayed excellent antiwear and
friction-reduction performance in sliding against steel. The
friction coefficient (0.13–0.15) and the wear life over 5000
sliding cycles were recorded for the films at a sliding speed of
90 mm/min and a load of 0.5 N. The film was characterized by
slight scuffing and abrasion at low loads and sliding speeds.
II. Experimental Procedure
Single-crystal silicon (Si(100)) and glass sheets were used as
the substrates. The substrates were immersed in piranha solution
(3:7 volume fraction of 30% hydrogen peroxide (H2O2) and 98%
sulfuric acid (H2SO4)), heated to a temperature of 70°C for 15 min,
and rinsed with distilled water and ethanol. The precursor sol was
a mixture of 0.02 mol of ZrOCl2⅐8H2O and 100 mL of anhydrous
ethanol, and the sol was aged at 30°C for 72 h. The sol–gel films
were deposited onto the substrates by dip coating in air at a relative
humidity (RH) of 45%–55% and speed of 42.4 cm/min. The films
were dried at 50°C for 15 min, then sintered to 550°C at a rate of
10°C/min and held there for 30 min. The above-described proce-
dures were repeated for the preparation of multilayer films. The
thickness of the monolayer ZrO2 film was ϳ50 nm, as measured
by ellipsometry (Model L116-E, Gaertner Scientific Corp., Chi-
cago, IL).
The thermal behavior of the dried gel was investigated via
thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) in flowing nitrogen at a scanning rate of 10°C/min
(Model 7 Series system, Perkin–Elmer, Norwalk, CT). The surface
morphology of the films was observed via atomic force micros-
copy (AFM) (Model SPM-9500, Shimadzu Corp., Japan). The
chemical states in the films were identified via multifunctional
X-ray photoelectron spectroscopy (XPS) (Model PHI-5702 Phys-
ical Electronics Division, Eden Prairie, MN), using a pass energy
I. Introduction
URING the last decade, rapidly increasing interest has been
Ddirected toward thin-film materials, particularly wear-resistant
coatings.1 Of these materials, zirconia (ZrO2) thin films have been
focused on as a potential wear-resistant coating in harsh environ-
ments, because of its promising physical and chemical properties,
such as high mechanical strength, high-temperature resistance,
corrosion resistance, low friction coefficient, and long wear life.2–5
ZrO2 thin films have been prepared using numerous coating
techniques, including physical vapor deposition,6 sputtering,7
spraying,8 plasma-assisted deposition,3,5 and sol–gel deposi-
tion.2,4,9–12 Of these methods, sol–gel deposition has attracted
much interest, because (i) it involves a simple process, (ii)
excellent control of the stoichiometry can be maintained, (iii)
homogeneous films with a large area can be formed at relatively
low cost, and (iv) it has flexible deposition parameters.11 However,
not much work has been conducted in regard to the tribological
properties of sol–gel thin films,13,14 especially those of sol–gel
ZrO2 thin films.
Organo-zirconium compounds, such as zirconium alkoxides,
zirconium alkanoates, and zirconium acetylacetonate, have been
widely used as the starting materials for the deposition of sol–gel
ZrO2 films.4,9–11 These organo-zirconium compounds generally
are expensive and sensitive to moisture.2 Thus, inorganic zirco-
nium salts have been recommended as a popular source of
zirconium for the preparation of a very stable precursor solution in
an ambient atmosphere.
of 29.4 eV and an excitation source of MgK␣ radiation (h
ϭ
␯
1253.6 eV). The binding energy of contaminated carbon (C 1s,
284.6 eV) was used as the reference. The microstructure of the
film and the morphologies of the worn surfaces were analyzed via
X-ray diffractometry (XRD) (Model D/max-RB, Rigaku, Tokyo,
Japan), using CuK␣ radiation, and scanning electron microscopy
(SEM) (Model EPMA-810Q, JEOL, Tokyo, Japan).
Friction and wear tests were conducted on a dynamic-static
tribometer (Model DF-PM, Kyowa Kagaku Corp., Ltd., Tokyo,
Japan) in a one-way reciprocating configuration and at a sliding
distance of 10 mm. A steel ball 3 mm in diameter (SAE52100
steel, HRc 59–61, with the following mass composition: carbon,
0.95%–1.05%; chromium, 1.30%–1.65%; silicon, 0.15%–0.35%;
manganese, 0.20%–0.40%; phosphorus, Ͻ0.027%; sulfur,
Ͻ0.020%; nickel, Ͻ0.30%; copper, Ͻ0.25%; and the balance was
iron) was used as the counterpart. All the tests were conducted at
room temperature and RH ϭ 50%.
In the present paper, the preparation of ZrO2 thin films, using
zirconium oxychloride octahydrate (ZrOCl2⅐8H2O) as the starting
material and anhydrous ethanol as the solvent, is reported. The
morphology, microstructure, and tribological properties of the
films have been investigated.
III. Results and Discussion
(1) Characterization of Sol–Gel Zirconia Films
R. O. Scattergood—contributing editor
Figure 1 shows the TGA and DSC curves for ZrO2 gel powders.
The weight loss of 40% from 25°C to 500°C is attributed to
evaporation of the residual organic solvents and decomposition of
the organo-zirconium compounds that had formed via hydrolysis
and condensation during the preparation of the precursor sol. No
Manuscript No. 187814. Received June 18, 2001; approved May 31, 2002.
*Member, American Ceramic Society.
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Communications of the American Ceramic Society
Vol. 85, No. 9
Fig. 1. TGA and DSC curves for ZrO2 gel powders.
Fig. 3. XRD pattern of ZrO2 films deposited on glass substrate.
thermal weight loss was observed at Ͼ500°C. The weak exother-
mic peak at ϳ480°C in the DSC curve corresponds to crystalliza-
tion of the gel powders.
thick sustain a metastable tetragonal structure at low temperature
because of the crystallite size effect.15
AFM observation of the film morphology indicated that the film
was compact and uniform, with a root-mean-square roughness of
Ͻ0.3 nm. The film was crack-free and consisted of nanoscale
crystallites. Thus, ZrOCl2⅐8H2O was determined to be an excellent
zirconium source in the sol–gel preparation of the ZrO2 films.
The XPS spectra of Zr 3d and O 1s in ZrO2 films that had been
sintered at 550°C are given in Fig. 2. The binding energy of Zr 3d
at 182.22 eV is identified as the crystallized ZrO2 phase.12 The O
1s spectrum consists of two branches: the stronger peak at 530.25
eV corresponds to ZrO2, and the smaller shoulder at 530.25 eV is
attributed to SiOx.9 The SiOx interlayer behaves as a buffer;
therefore, the adhesion of the ZrO2 films on a silicon wafer would
be greatly improved.
Figure 3 shows the XRD pattern of the multilayer ZrO2 film.
The ZrO2 films that were deposited on the glass substrates have a
well-developed crystalline structure after sintering at 550°C. The
peaks at 2␪ ϭ 30.25°, 35.11°, 50.65°, 60.17°, and 63.10° are
respectively assigned to the (111), (200), (220), (131), and (222)
lattice planes of the tetragonal ZrO2 phase. This finding conforms
to that which has been reported previously: ZrO2 films ϳ450 nm
(2) Friction and Wear Behavior
Figure 4 shows the tribological properties of the monolayer
ZrO2 films on silicon substrates sliding against the steel. ZrO2
films display excellent antiwear and friction-reduction perfor-
mance under a low load of 0.5 N (see Fig. 4(a)). Up to 5000 sliding
cycles, the friction coefficient stabilizes at ϳ0.14, and no wear
track is visible on the surface of the film in this case. An increase
in load causes a considerable increase in the friction coefficient at
extended sliding cycles. In particular, the friction coefficient
increases sharply after ϳ800 sliding cycles as the load increases to
3 N. Moreover, as shown in Fig. 4(b), the friction coefficient
increases as the sliding speed increases at a fixed load of 3 N. In
other words, the film is more liable to fail at larger sliding
Fig. 4. Friction coefficients, as a function of the number of sliding cycles,
at (a) a sliding speed of 90 mm/min and various loads and (b) an applied
load of 3.0 N and various sliding speeds.
Fig. 2. XPS spectra of (a) Zr 3d and (b) O 1s in zirconia films.

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Communications of the American Ceramic Society
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Fig. 5. SEM micrographs of the worn surfaces of ZrO2 film and the counterpart steel ball at various testing conditions ((a) ZrO2 film and (b) steel ball at sliding
speed 90 mm/min and load 1.0 N for 5000 sliding cycles; (c) ZrO2 film and (d) steel ball at sliding speed 120 mm/min and load 3.0 N for 700 sliding cycles).
velocities and loads. Thus, it can be concluded that sol–gel ZrO2
film on a silicon substrate is unsuitable for high-speed and
high-load applications.
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IV. Conclusions
Nanoscale sol–gel ZrO2 films on Si(100) and glass substrates have
been prepared by dip coating in an ethanol solution of zirconium
oxychloride (ZrOCl2). ZrOCl2 is a suitable zirconium source in the
sol–gel preparation of ZrO2 films. The ZrO2 films consist of a
completely tetragonal phase and exhibit nanoscale uniformity. They
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sliding against steel. The friction coefficient stabilizes at ϳ0.13–0.15
at a sliding speed of 90 mm/min and loads of 0.5 N, and the wear life
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scuffing and abrasion under a low load and sliding speed, whereas
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