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powders and Ni3Al (produced by self-propagating high tempera-

ture synthesis in our laboratory) powders with mean particle size

of 3070 mm were used as the starting materials. The nominal

composition in mass of the composite is: Ni3Al, BaF2/CaF2(520%), Ag (015%) and Mo (515%). The milling operation was

carried out in a Fritsch Pulverisette 5 planetary high-energy ball

milling system in argon at room temperature. A 250-ml tungsten

carbide vial and tungsten carbide balls were used in milling. Three

kinds of powders with an average particle size of 20, 10 and 5 mm(denoted as AC, BM and CF) were obtained by adjusting mill

variables, which is listed inTable 1. The as-milled powders were

put into an hBN-coated graphite die, and then heated at a rate of

10 1C/min in a hot-press-sintering furnace at a dynamic vacuum of

about 102 Pa. The powders were pressed at 900 1C for 15 min

under 35 MPa, and then heated to 980 1C and held for 20 min.

The size and the morphology of the three kinds of powder

particles were examined using a scanning electron microscopy

(SEM, JSM-5600LV). The densities of the hot-pressed samples

were measured using Archimedes method. The details on the

mechanical properties and tribological tests were described else-

where[14]. The tribological tests were conducted on an HT-1000

ball-on-disk high temperature tribometer. Before test, the sur-faces of the disk were cleaned with acetone and then dried in hot

air. The counterpart ball was the commercial Si3N4 ceramic ball

with a diameter of 6 mm (about HV 15 GPa). The selected test

temperatures were 20, 200, 400, 600, 800 and 1000 1C. The sliding

speed was 0.188 m/s, the applied load was 10 N and the testing

time was 20 min.

Microstructures of the sintered composites, phase composi-

tions and morphologies of the worn surfaces at different wear

condition were examined using scanning electron microscopy

(SEM, JSM-5600LV), energy dispersive spectroscopy (EDS, Kevex,

USA) and X-ray Diffraction (XRD, Philips X Pert-MRD X-ray

diffractometer, 40 kV, 30 mA, Cu Ka radiation). Samples for SEM

observations were polished using colloidal alumina (0.05 mm) and

chemical etch with aqua regia solution.

Table 1

Milling variables of the three kinds of powders produced by high-energy ball

milling.

Sample Milling time

(h)

Ball-to-powder ratio (in

weight)

Milling speed

(rpm)

AC 8 2.5:1 300

BM 8 10:1 300

CF 16 10:1 300

Fig. 1. SEM morphology of the milled powders and XRD results of the milled powders: AC, BM and CF.

S. Zhu et al. / Tribology International 44 (2011) 18001809 1801

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3. Results

3.1. Microstructural and mechanical properties

Three kinds of powder mixtures (AC, BM and CF) are used in

our experiments. It can be found fromFig. 1that most of the AC

particles (mainly Ni3Al) are coarse particles with an average

particle size of about 20 mm, the mean particle size of BM is about

10 mm and that of CF is about 5 mm. Meanwhile, it can be alsoobserved that particle shape of AC is flaky and irregular, whereas

CF turns round after the milling process. XRD results of the milled

powders indicate that the peaks of Ni3Al and Mo become broader

and the intensity get weaker with reducing particle size, which

can be attributed to the refinement of the grain size and the

increase of atomic level strain. In addition, few peaks of Ag can be

found and peaks of fluorides disappear. One reason for this is that

fluorides grains easily crack during mechanical alloying and

undergo deformation and/or fracture processes. The other reason

is that the decomposition of fluorides could happen. The amor-

phization and internal stress of these phases during mechanical

milling can make their peaks to become weaker, broader or

disappear [1113].The microstructures of AC, BM and CF composites are shown in

Figs. 24. EDS analysis indicates that the gray area is the

continuous bulk Ni3Al phase with uniformly dispersed Mo ele-

ment; the white phase is Ag-rich phase and the deep gray area is

Fig. 2. Microstructure and elemental distribution of AC, 1000.

S. Zhu et al. / Tribology International 44 (2011) 180018091802

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fluoride-rich phase. It can be observed that the isolated Ag phase

and fluorides phase are surrounded by the continuous bulk Ni3Al

phase. The AC Ni3Al phase forms coarser continuous phase

compared to BM and CF, while CF provides finer microstructure

than AC. In other words, the second-phase of CF distribute more

uniformly in Ni3Al matrix phase.

Table 2 shows densities and mechanical properties of the

sintered Ni3Al matrix composites. AC has the lowest density,

and its microhardness (3.60 GPa) is significantly lower than

microhardness of BM (4.40 GPa) and CF (4.90 GPa). However, AC

possesses the highest yield stress (1220 MPa) while CF has the

lowest (790 MPa). In addition, AC (1390 MPa) and BM (1400 MPa)

have high compressive strength. It suggests that with decreasing

particle size, the density and microhardness increase but the yield

stress and compressive strength decrease.

The variation of the above mechanical properties is related to

the microstructure of the composites. It can be found fromFigs.

24that element Mo as the hard phase uniformly disperses into

Ni3Al phase, while lubricants as the soft phase locate at the grain

boundary. It is well known from the sintering theory that small

particles possess larger driving force of sintering. The finer the

particles, the easier the solid solution and dispersion process, and

also the denser and harder the sintered materials. Furthermore,

the finer the particles, the more volume percent of the low load

bearing lubricant phase and the less the continuous load bearing

phase is, correspondingly, the lower the strength is.

Fig. 3. Microstructure and elemental distribution of BM, 1000.


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In the case of AC, the lubricants are enclosed by the coarse Ni3Al

bulk phase. As for CF, with the finer microstructure, the lubricants

have the larger contact area with the Ni3Al bulk phase. This lead to

weakness in boundary strength of CF compared to that of AC.

Furthermore, it should be noted that the volume fraction of soft

lubricants is larger than that of Mo. In the compressive test, yield

stress determines the deformation resistance of material. There-

fore, AC with the coarse Ni3Al bulk phase has a better yield stress

and compressive resistance than CF with the fine one.

For BM and CF, since the lubricants (Ag and fluorides) and

reinforcement phase (Mo) can be distributed uniformly in the

Ni3Al matrix, the hardness has small error. However, for the

hardness of AC, indentation contacts with an inhomogeneous

microstructure. Namely, there is more hard phase like Mo or soft

phase like Ag at the contact region. Therefore, a large test error is

obtained.

Fig. 4. Microstructure and elemental distribution of CF, 1000.

Table 2

Densities and mechanical properties of the sintered Ni3Al matrix composites.

Samples r (g/cm3) Hardness (GPa) Yield stress (MPa) Compressive

strength (MPa)

AC 7.020 3.7070.50 122075 139075

BM 7.060 4.4070.15 113575 140075

CF 7.070 4.9070.10 79075 114075


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3.2. Tribological properties

Fig. 5 shows the variations in friction coefficients of the

sintered Ni3Al matrix composites with different particle size after

tests. It can be found that the friction coefficient of the three

composites is in the window from 0.3 to 0.4 at a wide tempera-

ture range from room temperature to 1000 1C. Additionally, the

friction coefficient of the coarse particle AC is the lowest, ranged

from 0.3 to 0.35. And the friction coefficient of BM is slightlylower than that of CF.

Variations in wear rates of the sintered Ni3Al matrix composites

with different particle size after tests are presented inFig. 6. From

room temperature to 1000 1C, it can be noted that the wear rate

increases with decreasing particle size. AC has the lowest wear

rate. At room temperature, the low wear rate is about 4

106 mm3 N1 m1. With increasing temperature, the wear rate

tends to increase and rises to the peak point at 600 1C, AC to

4.11104 mm3 N1 m1, BM to 4.47104 mm3 N1 m1 and

CF to 4.92104 mm3 N1 m1. And then the wear rate remarkably

decreases to the low point about 1104 mm3 N1 m1 at

800 1C. As the temperature further rises to 1000 1C, the wear rate

of AC increases to 3.02104 mm3 N1 m1, BM to 4.93

104 mm3 N1 m1 and CF to 8.02104 mm3 N1 m1. Still, AC

is the lowest.

XRD results in Fig. 7 show that the peaks of the sintered AC

sample are stronger than those of the milled powders, suggesting

that the soluble Ag and embedded fluorides precipitate from

Ni3Al matrix. At 600 1C, the peaks of a new phase BaMoO4occur inXRD results. With increasing temperature, the peaks of BaF2 and

CaF2 disappear, but the peaks of BaMoO4 and CaMoO4 get

stronger. The presence of BaMoO4 and CaMoO4 on the worn

surface is attributed to the complex reaction, including high

temperature and tribo-chemical reaction.

At room temperature, some plastic deformed debris, which is the

Ag-rich phase by EDS analysis, is attached on the worn surface

together with some fine grooves, as shown inFig. 8. It suggests that

the discontinuously lubricating Ag film develops on the worn surface.

In addition, the patch of Ag-rich phase is present on the AC worn

surface, whereas the stripe of Ag-rich phase takes place on the BM

and CF worn surface. The wear mechanism is mainly microploughing.

The worn surfaces appear to be alike when the test tempera-

tures are 200 and 400 1C. The typical morphologies of the worn

surfaces of the three samples tested at 200 1C are shown inFig. 9.

Significant differences in the wear behavior are found. For AC, it

can be observed fromFig. 9(a) that cracks and flake of debris take

place at some regions, which are the Ni3Al-rich and Mo-rich phase

by EDS analysis, as well as the discontinuous grooves. The

discontinuous grooves and delaminated debris reveal that there

exists interaction between this area and the coupled Si3N4 ball

during sliding. Therefore, it is proposed that the area containing

Ni3Al and Mo should be the wear resistant area. The wear resistant

area is also found on the worn surface of BM. However, for the fine

particle CF, it is clear that some fine grooves are present on the

worn surface, the wear resistant area like those of AC and BM

cannot be found. Additionally, the discontinuously deformed Ag

film on the worn surface at room temperature does not occur. It

indicates that the wear mechanism is transformed from delamina-

tion to microploughing with decreasing particle size.

At 600 1C, it is similar to the characteristic of worn surface at

200 1C that some delaminated layers occur on the worn surface ofFig. 5. Variation of friction coefficients of the sintered Ni3Al matrix compositeswith different particle size.

Fig. 6. Variation of wear rates of the sintered Ni3Al matrix composites with

different particle size.

Fig. 7. XRD patterns of the sintered sample (a); and worn surfaces of AC after

sliding for 20 min at an applied 10 N loads and different temperatures: 600 1C (b);

8001

C (c); 10001

C (d).


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AC, while fine grooves appear on the worn surfaces of BM and CF,

as shown inFig. 10. Furthermore, few wear debris remains on the

worn surface. The wear mechanisms are delamination for AC and

microploughing for BM and CF.

As the temperature rises to 800 1C, the deformed surface and

grooves are present on the worn surface in all the three samples

(shown in Fig. 11). Compared to BM and CF, AC provides a

relatively smooth surface and glaze film, which is composed of

Fig. 8. Worn surfaces at room temperature: AC (a); BM (b); CF (c). Fig. 9. Worn surfaces at 200 1C: AC (a); BM (b); CF (c).


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NiO, BaMoO4and CaMoO4by XRD analysis. The wear mechanism

is dominant surface deformation.

When the temperature increases up to 1000 1C, the coarse

grooves break down the smooth surface of AC (see Fig. 12a),

whereas severe plow grooves and the delaminated pits are found

on the worn surfaces of BM and CF (see Fig. 12b, c). XRD results

show that large numbers of oxides, which consist of BaMoO4,

CaMoO4 and NiO, develop on the worn surface. Moreover, the

Fig. 10. Worn surfaces at 600 1C: AC (a); BM (b); CF (c). Fig. 11. Worn surfaces at 800 1C: AC (a); BM (b); CF (c).


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worn track of AC is found to be much smoother compared to

those of BM and CF. The wear mechanism is mainly microplough-

ing and oxidative wear.

4. Discussion

In general, the tribological property of material is closely

related to its microstructure and mechanical properties. Although

a higher hardness of the fine particle size composite always

implies a higher wear resistance, this rule seems not to be

completely suitable for the high temperature self-lubricating

composite.

From the tribology principle point of view, the ideal composi-tion of a high temperature self-lubricating composite should

be composed of high temperature oxidation resistant and

high strength matrix, favorable solid lubricant, as well as wear

resistant phase. Solid lubricant plays a dominant role in friction

behavior, and wear behavior is determined considerably by the

wear resistant phase. In the case of Ni3AlBaF2CaF2AgMo

composite, Ni3Al acts as a high strength matrix, Ag and fluorides

act as solid lubricants and Mo acts as reinforcement. Therefore,

Ag and fluorides in the composite provide a lubricity, while

Ni3Al and Mo are the wear resistant phase and the load-carried

phase.

The three composites show good lubricating property at a

wide temperature range from room temperature to 1000 1C. It

could be attributed to Ni3

Al matrix with favorable high tempera-

ture combined properties and the coaction of Ag (as a lubricant at

relatively low temperatures below 450 1C), fluorides (act above

400 1C) and molybdates formed by the complex reaction (as high

temperature solid lubricants) [1517]. In addition, AC exhibits

better frictional performance compared to BM and CF. The reason

is briefly explained as following.

During mechanical milling, with increasing milling time and

enhanced milling intensity, decomposition of fluorides and solid

solubility of Ag in BM and CF decrease the lubricity (seen in

Fig. 1). Therefore, AC provides more effective lubricity.

At low temperatures, the soft and coarse Ag particles on the

sliding surface of AC were scratched and squeezed gradually by

the Si3N4ball, and then spread on the sliding surface and formed a

Ag-rich film on the worn circle. Therefore, the Ag-rich film of AC

expressed more effective lubricating ability than those of BM and

CF. At high temperatures (above 600 1C), the higher load-carried

capacity and the lubricity of fluorides and molybdates formed

during the sliding process allow AC to provide lower friction

coefficient.

With respect to wear behavior, the following explanations are

given to clarify obtained results. The coarse bulk phase can

provide better deformation resistance and higher load bearing

capacity than the finer microstructure and the presence of the

lubricating film on the worn surface also plays an important role

in wear of the three composites. At low temperatures, the low

wear rate is obtained, which is due to the discontinued Ag film on

the worn tracks, as shown in Fig. 8. Although strength of the

monolithic Ni3Al increases with temperature, the presence of the

soft lubricants make the strength to decrease a lot. Therefore, itresults in decrease in the combined strength of the composite.

With the rise of temperature, the absence of the lubricating film

leads to a higher wear rate (seen inFigs. 9 and 10) as a result of

decrease in strength. However, at 800 1C, the formation of the

glaze film effectively reduces wear rate, as shown in Fig. 11. As

the temperature rises to 1000 1C, the decrease in strength

degrades wear resistance (seeFig. 12).

At low temperatures, the presence of the more effective Ag-

rich film on the worn track allows AC to obtain the lower wear

rate. In the temperature range from 200 to 600 1C, there is the

absence of the lubricating film on the worn surface. The Si 3N4ceramic ball directly contact with the asperities on the worn

surface. Meanwhile, the area containing Ni3Al and Mo can be the

wear resistant area. The coarse microstructure possesses the high

Fig. 12. Worn surfaces at 1000 1C: AC (a); BM (b); CF (c).


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deformation resistance and protects the worn surface from being

gouged out during the sliding process (Figs. 10 and 11). Above

800 1C, the higher strength of the coarse Ni3Al and Mo phases

can ensure the formation of the smooth surface film (seen in

Figs. 10 and 11), which is accounted for the improved anti-wear

performance of AC.

5. Conclusions

(1) The Ni3Al matrix high temperature self-lubricating compo-

sites with different particle size (about 20, 10 and 5 mm) were

fabricated by the powder metallurgy technique.

(2) At a wide temperature range from room temperature to

1000 1C, the three composites provide good lubricating prop-

erties, which can be attributed to the coaction of Ag, fluorides

and molybdates formed by the complex reaction.

(3) The coarse particle AC exhibits excellent frictional property

compared to BM and CF because AC provides more effective

lubricity and higher load-carried capacity.

(4) In the case of the coarse particle AC, the low wear rate is

obtained. The reason is that the coarse bulk phase can provide

better deformation resistance and higher load bearing capa-

city than the fine microstructure.

Acknowledgments

The authors are grateful to the National Natural Science Founda-

tion of China (51075383), the Innovation Group Foundation from

NSFC (50721062), and the National 973 Project (2007CB607601) for

financial support.

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