Abrasive properties of modified chromia

Article abstract of DOI:10.1023/A:1004173632509

ZrO₂-, CeO₂-, Y₂O₃-, La ₂O₃-, and CaO-modified Cr₂O ₃abrasives were characterized by x-ray diffraction, electron microscopy, and sedimentation analysis. The results indicate the formation of solid solutions in narrow composition ranges. The materials offer a high abrasive ability and ensure residual roughness R_z= 0.04-0.09 μm in polishing precision parts of the ShKh-15 steel, with a product yield of up to 80-82%.

Full text of DOI:10.1023/A:1004173632509

Inorganic Materials, Vol. 37, No. 3, 2001, pp. 274–280. Translated from Neorganicheskie Materialy, Vol. 37, No. 3, 2001, pp. 342–348.
Original Russian Text Copyright © 2001 by Chekhomova.
Abrasive Properties of Modified Chromia
L. F. Chekhomova
Institute of Mechanical Engineering, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia
Received December 14, 1999; in final form, July 20, 2000
Abstract—ZrO2-, CeO2-, Y2O3-, La2O3-, and CaO-modified Cr2O3 abrasives were characterized by x-ray dif-
fraction, electron microscopy, and sedimentation analysis. The results indicate the formation of solid solutions
in narrow composition ranges. The materials offer a high abrasive ability and ensure residual roughness Rz =
0.04–0.09 µm in polishing precision parts of the ShKh-15 steel, with a product yield of up to 80–82%.
INTRODUCTION
ery’ rule for covalent crystals). At the same time, at
high temperatures, solid solutions incorporating cat-
ions markedly different in size may be formed in a nar-
row composition range. In this context, it is important
to locate the limits of solid solutions between Cr2O3
and oxides of larger sized cations.
Metal-oxide abrasives are used in final polishing
stages to produce precision metal parts. The perfor-
mance of abrasives depends on their chemical compo-
sition, particle-size distribution, and physicochemical
and mechanical properties [1].
Earlier work [9] showed that chromia may form
chromites and solid solutions with rare-earth and cal-
cium oxides. In particular, below 1600°C, the Cr2O3–
CaO system contains mixed oxides with the general
formula mMO · nCrO3 · pCr2O3 and the chromite
ë‡Cr2O4, which exists in two polymorphs: α (high-
In addition, it is generally believed that the abrasive
ability depends strongly on the chemical processes
involved. The highest abrasive ability is offered by
metal oxides with moderate hardness (Mohs’ hardness
in the range 6–7), including Cr2O3, which is tentatively
attributed to the chemical interaction between the abra-
sive and the surface of the material being polished [2–5].
temperature) and β (low-temperature). The β
α
phase transition occurs at 1570°C. The Cr2O3–ZrO2
system contains Cr2O3-based rhombohedral and ZrO2-
based monoclinic solid solutions. According to phase-
diagram data, the solid solution of ZrO2 in Cr2O3 exists
below 2000°C. In the Cr2O3–La2O3 system, La2O3 ·
Cr2O3 and La2O3 form a eutectic with a melting point of
1380°C. Below this temperature, La2O3 and LaCrO3
crystallize at Cr2O3 contents under 50 mol %. In the
range 1000–1650°C, LaCrO3 has a hexagonal struc-
ture [9].
The performance of the common “soft” abrasives
improves in the following order: Al2O3 < ZrO2 <
Fe2O3 < Cr2O3 < CeO2 ≤ ThO2 [1]. Rare-earth oxides
are of limited use because of the high cost but can be
added to other abrasives as modifiers. The abrasive
most widely used on a commercial scale is chromia, a
relatively inexpensive material. However, Cr2O3 is not
always capable of producing high-quality surfaces. Use
is also made of Al2O3–Cr2O3 solid solutions, but these
ensure product yields no higher than 50–55% [2, 3].
Thus, there is a need for improving the abrasive
properties of Cr2O3. Equally important is the ability to
reduce surface roughness by enhancing the reactivity of
the abrasive, which would raise the product yield.
Teruhisa et al. [10] and Peck et al. [11] reported the
formation of La1 – xCaxCrO3 – δ (x ≤ 0.3) perovskite solid
solutions at Ӎ1000°C and studied the mechanism of
Sr2+ substitution for Ca2+. The only compound in the
Cr2O3–Y2O3 system is the YCrO3 perovskite, which
forms eutectics with Y2O3 and Cr2O3. Zvereva et al.
[12] obtained YCaCrxAl1 – xO4 solid solutions.
One way to raise the abrasive ability of Cr2O3 is to
introduce modifiers. The most pronounced effect can
be attained by using reactive elements which form solid
solutions with Cr2O3, modifying its physicochemical
properties (surface energy, particle size, and reactivity).
Thus, according to phase-diagram data [9], Cr2O3 is
likely to form solid solutions with oxides of La, Y, Ce,
Zr, and Ca between 1600 and 2000°C.
According to isomorphism theory [6–8], the main
criterion for the formation of solid solutions is the cat-
ion–anion distance, determined by the sum of the ionic
radii: the difference in atomic radius between the host
cation and substituent should not exceed 15–20% CaO modifiers have on the abrasive properties of
(Goldschmidt’s rule for ionic crystals and Hume-Roth-
The purpose of this work was to study the effects
that small amounts of ZrO2, CeO2, La2O3, Y2O3, and
Cr2O3.
0020-1685/01/3703-0274$25.00 © 2001 MAIK “Nauka/Interperiodica”

ABRASIVE PROPERTIES OF MODIFIED CHROMIA
275
EXPERIMENTAL
intermediate compounds. The present results agree
with earlier data [13, 18–21], except for some distinc-
tions in the thermolysis products. The reaction
sequence can be represented by the following schemes:
To modify chromia with CaO, ZrO2, La2O3, Y2O3,
and CeO2 (≤0.18 mol %), mixtures of CrO3 with
Ca(OH)2, Zr(SO4)2, La2O3, Y2O3, and Ce(SO4)2 were
heat-treated first at 250°C for 1.5–2 h, then at 350–
400°C for 1.0–1.5 h, and finally at 1000–1200°C for
1.5–2 h. This schedule was chosen with consideration
for the mechanism of CrO3 thermolysis [13].
200–210°C
200–270°C
CrO3(l) + Ca(OH)2
CaCr3O10
280°C
+ Cr2(Cr10C31)3
CaCr2O7 + Cr3O8
(I)
400°C
465°C
X-ray diffraction (XRD) measurements were car-
ried out on a DRON-2 powder diffractometer (ëuKα
radiation) fitted with a UVD-2000 high-temperature
attachment. Interplanar spacings were calculated by the
centroid method. The lattice parameters of Cr2O3 and
solid solutions were determined using the 110, 300, and
1010 lines with an accuracy of ±0.03, ±0.004, and
±0.002 Å, respectively. The volume of the hexagonal
unit cell was calculated as V = (3/2)1/2(a2c) [14] with an
absolute uncertainty of ±0.3 Å3. The lattice parameters
were used to ascertain the formation of solid solutions.
The lattice parameters of α-Cr2O3 were determined to
be a0 = 4.955 Å and c0 = 13.600 Å, in agreement with
earlier data [5, 15]. Chemical and phase analyses were
carried out as described in [16] with a relative accuracy
of ±0.001 mol %. Particle-size analysis was performed
by gravity sedimentation using an SA-CP2 Shimadzu
analyzer (viscosity of the dispersion medium, 9.3 ×
10−4 Pa s; density of the liquid phase, 1.0 g/cm3) and by
electron microscopy (D-2 Carl Zeiss instrument).
+ Cr2O5
CaCrO4 + CrO2
CaCrO4
900–1200°C
+ CrO1.56
Cr2 – xCaxO3 – δ + CaCr2O4 + O2,
200–210°C
280–300°C
CrO3(l) + Zr(SO4)2
Cr2(Cr10O31)3
+ Zr2(OH)2SO4(CrO4)2
360–370°C
Zr2(OH)2SO4(CrO4)2
440°C
+ Cr2(Cr2O7)3
Zr2(OH)2SO4(CrO4)2
(II)
465°C
+ Cr2(CrO4)3
CrO1.56
1000–1100°C
+ Zr2(OH)2SO4(CrO4)2
Cr2 – xZrxO3 + δ
Abrasive ability was assessed by a standard proce-
dure using an S-15 polisher equipped with a sample
holding fixture made of Bitumen 5. Four polished disks
from the ShKh-15 quenched steel (HRC = 58–60) with
Rz = 0.3–0.4 µm over a base length of 0.25 mm were
fixed in the sample holder. Fine polishing was per-
formed for 30 min at a load of 17.5 MPa. The rotation
rates of the polishing wheel and the sample holder were
192 and 60 rpm, respectively. The polishing suspension
was prepared from the modified Cr2O3 powder under
investigation (25 g) and distilled water (175 ml). The
material removal rate was calculated from weight loss
data by the formula P = ∆M/(Sτ), where ∆M (mg) is the
mean weight loss due to polishing, S (cm2) is the sam-
ple surface area, and τ (min) is the polishing time. After
polishing, the residual roughness Rz was determined as
the average of ten measurements over a base length of
0.08 mm using an MII-4 optical microscope [17].
+ ZrO2 + SO2,
200–210°C
CrO3(l) + Zr(SO4)2
280–300°C
+ Ca(OH)2
Cr2(Cr10O31)3
+ Zr2(OH)2SO4(CrO4)2
350–370°C
+ CaCr2O7
Zr2(OH)2SO4(CrO4)2
440°C
+ Cr2(Cr2O7)3 + CaCr2O7
Cr2(CrO4)3 (III)
465°C
+ CaCrO4 + Zr2(OH)2SO4(CrO4)2
CrO1.56
RESULTS AND DISCUSSION
+ Zr2(OH)2SO4(CrO4)2
According to XRD data, heat treatment of the
CrO3 + Ca(OH)2, CrO3 + Zr(SO4)2, CrO3 + Ca(OH)2 +
Zr(SO4)2, CrO3 + La2O3, CrO3 + Y2O3, and CrO3 +
Ce(SO4)2 mixtures at the temperatures indicated above
leads to CrO3 decomposition into Cr2O3, possibly with
the formation of solid solutions, through a number of
1000–1100°C
+ CaCrO4
Cr2 – x – x Zrx Cax O3 – δ
1 2 1 2
+ ZrO2 + Zr(OH)2CrO4 + SO2,
INORGANIC MATERIALS Vol. 37
No. 3 2001

276
CHEKHOMOVA
200–250°C
350–370°C
465°C
a, Å
4.96
CrO3 + La2O3
Cr2(Cr10O31)3
Cr2(Cr2O7)3
CrO1.56
(a)
1
2
3
4
5
4.95
4.94
4.93
4.92
4.91
4.90
+ La2(Cr2O7)3
+ La2(CrO4)3
(IV)
600–660°C
+ La2(CrO4)3
LaCrO4 + Cr2O3
750°C
1000–1100°C
6
+ O2
LaCrO3 + Cr2O3
Cr2 – xLaxO3,
Ò, Å
15.0
(b)
200–210°C
200–250°C
456°C
CrO3 + Y2O3
Cr2(Cr10O31)3
14.6
14.2
13.8
13.4
+ Y2(Cr2O7)3
Y2(CrO4)3
YCrO4 + Cr2O3
YCrO3
560–600°C
+ CrO1.56
(V)
740°C
+ O2
900–1100°C
+ Cr2O3
YCrO3 + Cr2 – xYxO3,
V, Å3
305
(c)
200–210°C
280–300°C
300
295
290
285
CrO3(l) + Ce(SO4)2
Cr2(Cr10O31)3
350–370°C
+ Ce2(OH)2SO4(CrO4)2
Cr2(Cr2O7)3
Cr2(CrO4)3
450°C
+ Ce2(OH)2SO4(CrO4)2
+ Ce2(OH)2SO4(CrO4)2
(VI)
280
0
0.05
0.10
0.15
0.20
465°C
Molar percent
Ce2(OH)2SO4(CrO4)2
Fig. 1. Composition dependences of lattice parameters for
Cr O modified with (1) CaO, (2) ZrO , (3) CaO + ZrO
900–1100°C
2
3
2
2
+ CrO1.56
Cr2 – xCexO3 + δ
(2 : 1), (4) La O , (5) Y O , and (6) CeO .
2
3
2
3
2
+ CeO2 + CeOCrO4 + SO2.
P, mg/(min cm2)
0.7
The data displayed in Fig. 1 demonstrate that a
decreases with increasing modifier content in the
ranges 0–0.10 mol % CaO and 0–0.05 mol % ZrO2,
La2O3,Y2O3, and CeO2; c and V increase with modifier
content in the same composition ranges. The lattice
parameters of the La2O3-, Y2O3-, and CeO2-modified
materials decrease by 3–4% in the range 0–0.03 mol %.
The observed variations of lattice parameters sug-
gest that the Zr4+, La3+, Y3+, Ce4+, and Ca2+ ions substi-
tute for Cr3+. In the case of Zr4+ and Ca2+, the difference
in charge state and ionic radius between the substituent
and Cr3+ leads to the formation of a defective lattice and
an increase in unit-cell volume, which testifies to a
reduction in atomic density and an increase in Cr2O3
reactivity. The abrasive abilities of the CaO- and
1
2
3
4
5
6
0.6
0.5
0.4
0.3
0.2
0.1
0
0.05
0.10
0.15
0.20
Molar percent
Fig. 2. Composition dependences of abrasive ability for
modified Cr O : (1–6) same as in Fig. 1.
2
3
INORGANIC MATERIALS Vol. 37 No. 3
2001

ABRASIVE PROPERTIES OF MODIFIED CHROMIA
277
ZrO2-containing materials vary with modifier content
Rz, µm
in a similar way (Fig. 2).
0.20
1
2
3
4
5
6
The observed variations of the abrasive ability with
modifier content are not surprising. According to
Hauffe [22], substitutions at the Cr site and Cr3+ defi-
ciency reduce atomic density. The highest abrasive
ability was shown by Cr2O3 modified with
0.04−0.09 mol % CaO (3–6 times higher in comparison
with Cr2O3). Note that the increase in abrasive ability is
likely due to the formation of solid solutions, since,
with increasing calcium chromite content (0.12–
0.18 mol % CaO), the abrasive ability decreases to the
level typical of pure Cr2O3.
0.15
0.10
0.05
0
0.05
0.10
0.15
Molar percent
Fig. 3. Composition dependences of residual roughness for
modified Cr O : (1–6) same as in Fig. 1.
2
3
In the case of Cr2O3 modified with 0.02–0.035 mol %
CaO, the residual roughness was Rz = 0.10 µm (Fig. 3).
The largest Rz (0.17–0.19 µm) was observed at 0.05–
0.09 mol % CaO.
ë, %
60
(a)
1
2
3
4
5
6
7
50
40
30
20
10
0
Solid solutions can be formed by two mechanisms.
If the oxygen stoichiometry remains unchanged, some
of the Ca2+ ions are incorporated interstitially to give
the Cr2 – xCa1.5xO3 solid solution, in which one Cr ion is
replaced by 1.5 Ca ions. This mechanism is however
unlikely in view of the large ionic radius of Ca. In the
second mechanism, the cation stoichiometry remains
unchanged, and the solid solution contains oxygen
vacancies, as described by the formula ër2 – xCaxO3 – δ
with 0 ≤ δ ≤ 0.045.
,
According to sedimentation analysis and electron
microscopy data, the introduction of Ca2+ ions
increases the particle size of Cr2O3 from 0.5–1 to
3−5 µm (Figs. 4, 5). As the CaO content rises from 0.04
to 0.09 mol %, the particle size gradually increases to
d = 1–4 µm, the most probable size range being
1−2 µm. A small fraction of the particles fall in the
range d = 0.2–0.5 µm.
50
40
30
20
10
(b)
The introduction of Zr4+ ions increases the abrasive
ability to P = 0.24–0.27 mg/(min cm2) and reduces the
residual roughness to Rz = 0.07–0.09 µm (Figs. 2, 3).
The major size range in ZrO2-modified Cr2O3 is
1−2 µm; particles with d = 2–5 µm are also present. At
the same time, the fraction of fine particles with d in the
range 0.4–1 µm increases. The increase in the abrasive
ability is attributable to the formation of Cr2 – xZrxO3 + δ
solid solutions with 0 < x ≤ 0.04 and 0 ≤ δ ≤ 0.02.
0
1
2
3
4
5
6
7
d, µm
Fig. 4. Particle-size distributions of modified Cr O con-
2
3
taining (a) (1) 0, (2) 0.05, (3) 0.06, (4) 0.07, (5) 0.08,
(6) 0.09, and (7) 0.18 mol % CaO and (b) (1) 0.02–0.05 +
0.02, (2) 0.03 + 0.02, (3) 0.04 + 0.03, (4) 0.05 + 0.05,
(5) 0.03 + 0.05, (6) 0.08 + 0.04, and (7) 0.07 + 0.05 mol %
CaO + ZrO .
2
CaO + ZrO2 additions are, most likely, incorporated
substitutionally, yielding Cr2 – x – x Zr Ca O3 – δ oxy-
x1
1
2
x2
earlier report that Zr(OH)2CrO4 forms at 140–190°C
and is stable up to 1200°C [23]. The highest abrasive
ability, 0.28–0.34 mg/(min cm2), is observed in the
range 0.04–0.09 mol % CaO + 0.02–0.04 mol % ZrO2
(Fig. 2, curve 3), where the lattice parameters vary sub-
stantially. The residual roughness attained with these
compositions is Rz = 0.08–0.09 µm (Fig. 3, curve 3).
gen-deficient solid solutions with 0 < x1 ≤ 0.04, 0 < x2 ≤
0.09, and 0 < δ ≤ 0.025. The material also contains
pseudocubic ZrO2 and zirconium hydroxochromate,
Zr(OH)2CrO4, as evidenced by the XRD peaks with
d = 3.48 (100% intensity), 2.93 (60%), 2.84 (50%),
1.807 (39%), and 1.574 Å (30%), in agreement with an
INORGANIC MATERIALS Vol. 37
No. 3 2001

278
CHEKHOMOVA
1 µm
1 µm
(a)
(b)
1 µm
1 µm
(c)
(d)
Fig. 5. Electron micrographs of (a) Cr O , (b) Cr O + 0.05 mol % CaO; (c) Cr O + 0.02 mol % ZrO , and (d) Cr O +
2
3
2
3
2
3
2
2 3
0.04 mol % CaO + 0.02 mol % ZrO ; ×6000.
2
The introduction of Y3+ ions increases the abrasive
ability of Cr2O3 by a factor of 3 (Fig. 2, curve 5) and
reduces Rz to 0.04–0.07 µm, which can be accounted
for by the formation of Cr2 – xYxO3 solid solutions with
0 < x ≤ 0.026. The powders modified with 0.01–
0.03 mol % Y2O3 range in particle size from 0.05–0.5
to 5–6 µm (Fig. 6b).
Cr2O3 modified with Ca2+ and Zr4+ ions was found
to contain, along with particles 2–5 µm in size, fine par-
ticles with d from 0.04 to 0.50 µm, which are likely to
ensure a high quality of polished metal surfaces
(Figs. 3, 4b, 5d).
The composition dependence of the lattice parame-
ters for La2O3-modified Cr2O3 indicates the formation
of Cr2 – xLaxO3 solid solutions in the composition range
0 < x ≤ 0.018, where the abrasive ability increases two-
fold. The residual roughness is 0.10 µm (Fig. 3, curve 4).
Ce4+ ions raise the abrasive ability of Cr2O3 to 0.30–
0.32 mg/(min cm2), ensuring Rz = 0.07–0.09 µm. The
composition dependences of lattice parameters in the
The major size range is 1–2 µm. As the La2O3 content Cr2O3–CeO2 system indicate the formation of
Cr2 − xCexO3 + δ heterovalent substitutional solid solu-
tions with 0 < x ≤ 0.03 and 0 < δ ≤ 0.15. In addition, the
CeO2-modified materials contain CeOCrO4 [24]. The
increases from 0.02 to 0.05 mol %, the particle size
increases to 3–4 µm, as supported by sedimentation
analysis (Fig. 6a).
INORGANIC MATERIALS Vol. 37
No. 3 2001

ABRASIVE PROPERTIES OF MODIFIED CHROMIA
279
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CONCLUSION
The present results demonstrate that the abrasive
ability of Cr2O3 modified with rare-earth oxides and
CaO and the residual roughness correlate with the
observed variations in lattice parameters, indicating the
formation of solid solutions. The introduction of these
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=
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presence of both coarse and fine particles, the latter
exhibiting high reactivity in the polishing process. The
modified Cr2O3 abrasives ensure a product yield of 80–
82% in polishing precision metal parts.
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