Formation of ultralow friction surface films on boron carbide

Article abstract of DOI:10.1063/1.115676

In this letter, we describe the formation and ultralow friction mechanisms of a surface film on boron carbide (B4C). This film results from sequential reactions between B4C and oxygen and between the resulting boron oxide (B2O3) and moisture; it can afford friction coefficients of 0.03 to 0.05 to sliding steel surfaces. At temperatures above 600 °C, B4C undergoes oxidation and forms a layer of boron oxide (B2O3) in the upper surface. During cooling to room temperature, the B2O3 reacts with moisture in the air to form a secondary film, boric acid (H3BO3). The sliding friction coefficient of 440C steel balls against this film is 0.04, compared to 0.7 against the bare B4C surfaces. Mechanistically, we propose that the ultralow friction behavior of the heat-treated B4C surface is due mainly to the layered-crystal structure of the H3BO3 film that forms on the sliding surface.

Full text of DOI:10.1063/1.115676

Formation of ultralow friction surface films on boron carbide
A. Erdemir, C. Bindal, and G. R. Fenske
Citation: Applied Physics Letters 68, 1637 (1996); doi: 10.1063/1.115676
View online: http://dx.doi.org/10.1063/1.115676
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Formation of ultralow friction surface films on boron carbide
A. Erdemir,a) C. Bindal,b) and G. R. Fenske
Argonne National Laboratory, Energy Technology Division, Argonne, Illinois 60439
͑Received 27 November 1995; accepted for publication 19 January 1996͒
In this letter, we describe the formation and ultralow friction mechanisms of a surface film on boron
carbide ͑B4C͒. This film results from sequential reactions between B4C and oxygen and between the
resulting boron oxide ͑B2O3) and moisture; it can afford friction coefficients of 0.03 to 0.05 to
sliding steel surfaces. At temperatures above 600 °C, B4C undergoes oxidation and forms a layer of
boron oxide ͑B2O3) in the upper surface. During cooling to room temperature, the B2O3 reacts with
moisture in the air to form a secondary film, boric acid ͑H3BO3). The sliding friction coefficient of
440C steel balls against this film is 0.04, compared to 0.7 against the bare B4C surfaces.
Mechanistically, we propose that the ultralow friction behavior of the heat-treated B4C surface is
due mainly to the layered-crystal structure of the H3BO3 film that forms on the sliding surface.
© 1996 American Institute of Physics. ͓S0003-6951͑96͒04012-3͔ © 1996 American Institute of
Physics. ͓S0003-6951͑96͒04012-3͔
Because of its exceptional hardness, outstanding elastic
modulus, and low specific gravity, B4C is used for a wide
range of engineering applications where high wear resistance
and light weight are desired. For example, it has been used as
wear parts in grinding wheels and in wheel-dressing sticks
for sharpening knives and other cutting edges.1 Fine powders
of B4C are used as super abrasives in polishing and grinding
of metals and ceramics.1 The low specific density and high
elastic modulus of B4C are exploited in the production of
B4C-whisker-reinforced composites.2,3 Lightweight armor
plates resistant to piercing by bullets are also made of
B4C.4 Despite its excellent wear resistance, B4C cannot pro-
vide low friction to sliding surfaces. In fact, because of its
very hard and abrasive nature, it causes high wear losses on
mating surfaces.5,6
In this letter, we describe a new and simple annealing
procedure that results in the formation of a super-slippery
boric acid film on hard B4C substrates. This film provides
0.03 to 0.05 friction coefficients to sliding steel surfaces. The
annealing procedure developed here is very simple in that it
involves only heating the B4C to high temperature ͑e.g.,
above 600 °C in open air͒ and holding it there for a short
duration. During annealing, B4C reacts with oxygen in the
surrounding air and forms a layer of B2O3 . During subse-
quent cooling to room temperature, the B 2O3 undergoes a
secondary reaction with moisture in the air and forms a thin
H3BO3 film that provides ultralow friction.
heat-treated and control samples in a ball-on-disk tribometer
under a load of 5 N, at room temperature ͑about 23 °C͒, and
in open air of 50Ϯ5% relative humidity. Rotational speed
was 5 r minϪ1 which translated into a sliding velocity of 5.2
mm sϪ1. The counterface material was made of 440C steel
balls, 9.5 mm in diameter, with a highly polished surface
finish of better than 0.01 ␮m CLA roughness. Laser-Raman
spectroscopy was used to characterize the structure and
chemical nature of the sliding surfaces. The Raman spectro-
scope used a HeNe laser at 632.8 nm with an output power
of 25 mW focused to a spot size of 2 to 3 ␮m.
Figure 1 shows the friction coefficients of 440C steel
balls during sliding against B4C before and after annealing.
As is clear, the friction coefficient of 440C steel against
B4C is initially low ͑about 0.3͒, but increases substantially as
sliding continues and reaches a value of 0.7 toward the end
of the test. This result verifies further that B4C is not a low-
friction material. The specific wear rate ͑measured by a
method desribed in Ref. 7͒ of the 440C steel ball slid against
The B4C material used in our experiments was hot-
pressed and obtained from a commercial source. The test
pieces were cut into squares having nominal dimensions of
35ϫ35ϫ6 mm. The surface finish of the test pieces was 0.1
␮m center-line-average ͑CLA͒. The annealing heat-treatment
for oxidation was done in a box furnace at 800 °C for one
hour.
Friction and wear tests were performed with both the
a͒
To whom all correspondence should be sent.
Electronic mail: a_erdemir@qmrgate.anl.gov
b͒
Permanent address: Sakarya University, Department of Metallurgical En-
gineering, Adapazari, Turkey.
FIG. 1. Variation of friction coefficients of 440C steel balls during sliding
against as-received and annealed B4C.
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FIG. 3. Schematic illustration of formation of B2O3 and H 3BO3 films on
B4C surfaces.
energies for oxidation. The principal reaction products are
CO2 and B2O 3 .9,10 B4C is thermodynamically stable up to
600 °C and does not oxidize, however at temperatures of
600 °C and higher, oxidation starts and proceeds at a fairly
slow pace.1,9,10
As illustrated schematically in Fig. 3 and described in
details in Refs. 11 and 12, boron oxide ͑layer 1͒ undergoes a
secondary reaction with moisture in air ͑because of a nega-
tive standard heat of reaction͒. The end product is a thin
boric acid film on the exposed surface ͑layer 2 in Fig. 3͒. As
revealed by Raman spectroscopy in Fig. 2, some traces of
graphite are also present in this film.
FIG. 2. Raman spectra of as-received and annealed B4C. Raman spectra of
H3BO3 ͑boric acid͒ and graphite standards are also included.
B4C was 2.9ϫ10Ϫ5 mm3NϪ1mϪ1, which can be considered
rather high.
As for the ultralow friction mechanism of boric acid
film, we propose the following explanation. Boric acid crys-
tallizes in a layered triclinic crystal structure.13 The atomic
layers are parallel to the basal plane and are made up of
boron, oxygen, and hydrogen atoms. These atoms are closely
packed and strongly bonded to each other by covalent, ionic,
and hydrogen bonds, whereas the atomic layers are widely
spaced and held together by weak forces, e.g., van der Waals
͑Fig. 4͒. The layered-crystal structure of boric acid resembles
those of MoS2 , graphite, and hexagonal boron nitride.
Therefore, we believe that the ultralow friction measured on
the heat-treated B4C surfaces is a direct consequence of the
formation of a slippery boric acid film. Mechanistically, we
propose that under shear forces, platelike crystallites of boric
acid align themselves parallel to the direction of relative mo-
tion; once so aligned, they can slide over one another with
relative ease to provide the low friction coefficient shown in
Fig. 1. A recent study by Bindal and Erdemir confirms that
low-friction boric acid films can also be formed on borided
steel surfaces.14
The friction coefficient of a 440C steel ball sliding
against the annealed B4C surface is initially 0.07, but as slid-
ing continues it decreases to 0.04 and remains constant for
the rest of the test. This demonstrates clearly that the simple
heat-treatment procedure developed here leads to the forma-
tion of a very slippery surface film on B4C. Furthermore, the
specific wear rate of 440C ball was 3ϫ10Ϫ7 mm3NϪ1mϪ1
.
This means that the lubricious film formed on the surface
reduced the wear rate of steel ball by nearly two orders of
magnitude.
Raman spectroscopy of the annealed B4C revealed two
strong Raman bands; one centered at approximately 498
cmϪ1 and the other at 879 cmϪ1 ͑see Fig. 2͒. These values
are very close to those ͑i.e., 500 and 881 cmϪ1) of the bulk
boric acid ͑H3BO3) reported in Ref. 8. For further confirma-
tion, we obtained reagent-grade H3BO3 powders from a com-
mercial vendor and ran Raman spectroscopy on them as
well. We included the Raman spectrum of this H3BO3 in Fig.
2 for comparison. As is clear, the Raman spectrum of the
reagent grade H3BO3 overlaps that formed on the surface of
B4C after annealing. The Raman spectrum of the as-received
B 4C is also included in Fig. 2, and as can be seen, it is very
different from those of the slippery surface film and H3BO3
standard.
In short, ultralow friction coefficients can be achieved on
B4C surfaces as a direct consequence of the sequential for-
mation on the B4C surface of, first, a boric oxide layer during
annealing, then a boric acid film during cooling. The boric
To verify the occurrence of graphitization on the an-
nealed surface of B4C, we included the standard Raman
spectrum of graphite as well. The Raman spectrum of an-
nealed B4C reveals two broad peaks centered at around 1350
and 1580 cmϪ1 which suggest that some graphitization has
occurred. The Raman spectrum of as-received B4C also ex-
hibited broad peaks corresponding to the principal Raman
bands of graphite. However, based on the spectra given in
Fig. 2, it is difficult to estimate the quantitative amounts of
graphite before and after annealing at 800 °C.
Based on the chemical analyses presented above, we be-
lieve that the ultralow friction coefficient of annealed B4C
surface is directly related to the formation of H3BO3 film on
the exposed surface, as depicted in Fig. 3. During exposure
to 800 °C, the boron and carbon atoms gain high activation
FIG. 4. Layered triclinic crystal structure of H3BO3 .
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1638 Appl. Phys. Lett., Vol. 68, No. 12, 18 March 1996 Erdemir, Bindal, and Fenske
137.195.150.201 On: Fri, 03 Oct 2014 04:52:31

acid film formed on the exposed surfaces has a layered crys-
tal structure; when interposed between two sliding surfaces,
it shears easily, thus providing low friction.
This work was supported by the U.S. Department of En-
ergy under Contract W-31-109-Eng-38. The authors thank
Dr. E. Savrun of Quest Integrated, Inc., Kent, WA, for fur-
nishing the B4C samples and Dr. C. Zuiker of Argonne Na-
tional Laboratory for performing the laser-Raman spectros-
copy.
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