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Plasma spray processing of Al2O3/AlN composite powders

L.H. Cao, K.A. Khor*, L. Fu, F. Boey

School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

Abstract

A novel method is proposed to prepare Al2O3/AlN composite powders. The composite powders were synthesized by direct nitridation of

Al2O3 powders in Ar/N2 plasma. The processing characteristics were studied. The results show that the particle size of the initial materials,

the nitrogen plasma gas ¯ow rate and the power of the plasma generator are important factors that in¯uence the phase composition of the

Al2O3/AlN composite powders. Post treatment of the Al2O3/AlN composites in nitrogen atmosphere was carried out. Microstructure

analysis showed that the composite powders are spheroids, with the small particle size AlN formed on the surface of the Al2O3. # 1999

Elsevier Science S.A. All rights reserved.

Keywords: Plasma spraying; Composite powder; Alumina and aluminium nitride; Phase composition; Post-treatment

1. Introduction

Plasma spraying is a well-established technique for pre-

paring a wide variety of coatings that has been used over the

past three decades [1±3]. Such coatings are increasingly

used in the automobile, aerospace, textile, biomedical,

electrical and optical industries to impart properties of wear

resistance, thermal barrier, corrosion resistance, biocompat-

ibility and electrical insulation [4±6]. Plasma spraying is

characterized by high temperatures (�10000 K), high spe-

ci®c energy densities and high cooling rates [7,8]. The

increasing applications of plasma spray have attracted con-

siderable attention over the last few years. Plasma spray

synthesis of particulate materials has been developed in

recent years. Early attempts were carried out in a DC plasma

reactor to form carbides from metal powders and gaseous

precursors [9]. Fine SiC powders were synthesized using

SiO2 particles and CH4 gas in a DC plasma jetreactor [10].

Arc plasma methods were also used for the direct production

of ultra®ne silicon powders and nitrides/carbides of silicon,

titanium and tungsten [11±13]. Nanocrystalline zirconia

powders were produced using zirconium butoxide solutions

by plasma spray pyrolysis [14]. The work to-date demon-

strated the feasibility of producing ®ne powders by reactive

plasma spray processing. Many investigations have con-

®rmed that the process depends mainly on the completeness

of the chemical reactions in the plasma environment. In

other words, plasma spray processing can provide a reason-

able method by which to prepare composite powders. Com-

posite materials have the propensity to improve the

mechanical, chemical and thermal behavior by combining

materials with distinctive or supplementary properties [15±

17]. This paper presents a study on the preparation of Al2O3/

AlN composite powders using plasma spraying technology.

Different particle sizes of alumina were used as the initial

materials to prepare the Al2O3/AlN composite powders. The

process characteristics and the properties of the composite

powders were also studied.

2. Experimental procedure

Fig. 1 presents a schematic illustration of the experimen-

tal apparatus. This system consists of a plasma gun (SG-100,

Praxair Technologies, USA), a powder feed hopper (Miller

Thermal), a reactor with cooling water and an exhaust gas

treatment system. Argon is used as the primary plasma

working gas and nitrogen is the secondary plasma gas.

The initial materials were injected into the plasma arc using

a vibration feeder by means of argon gas and were directly

nitrided. The reactants condensed rapidly on the substrate

and the wall of the reactor. After collection, the composite

powders were kept dry in a desiccator. The processing

parameters were studied in relation to the amount of AlN

formed.

Journal of Materials Processing Technology 89±90 (1999) 392±398

*Corresponding author. Tel.: +65-7995526; fax: +65-791-1859;

e-mail: [email protected]

0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 0 6 3 - 1

2.1. The starting materials

Commercial Al2O3 powders (Cerac, USA) were

used as the initial material. In order to obtain different

particle sizes, the Al2O3 powders were wet grounded

with acetone in a planetary ball mill at the rate of

200 rpm for different durations. The particle size and

its distribution of the powders were measured by laser

diffraction using a Fritch Particle Sizer Analysette 22.

Table 1 lists the particle size and the speci®c surface

area of the initial materials in relation to the milling

duration.

2.2. Post-treatment

In order to further examine the characteristics of Al2O3/

AlN composite powders, post-treatment was carried out in a

nitrogen atmosphere within the temperature range of 800±

12008C for 2 h.

2.3. Materials evaluation

The phase compositions of the Al2O3/AlN composite

powders were determined by X-ray diffraction using Cu

Ka radiation at 40 kV and 30 mA with a Philips MRD1880.

All of the peaks in the 2� range from 20 to 808 were used to

calculate the relative intensities. The particle size and its

distribution were measured by Laser-diffraction with a

Fritch Particle Sizer Analysette 22. A scanning electron

microscope (SEM) equipped with an energy-dispersive X-

ray analyzer (EDX, Link 5130) was used for the Al2O3/AlN

composite powders. A Perkin±Elmer FT2000 fourier trans-

formed infrared spectroscopy (FTIR) was used to observe

the Al±O and Al±N bonds in the powders produced.

3. Results and discussion

3.1. The influence of processing parameters

The experiments were carried out with different arc

currents and nitrogen ¯ow rates using Al2O3 feedstock that

was milled for 4 h, at a given feedrate of 1.0 rpm and 40 psi

of carrier gas. Fig. 2 shows the XRD pattern of the compo-

site powders produced at 900 A and 100 psi of nitrogen.

It can be seen that a-Al2O3 is the major phase. Cubic AlN

phase and the transient g-Al2O3 also appeared in the com-

posite powders. The relative amounts of the phases were

calculated from the X-ray diffraction intensity data. The

ratio R1 determined the AlN content in the composite

R1 �IAlN�311�

IaÿAl2O3

�113� � IAlN�311�� 100% (1)

where R2 is the content of g-Al2O3

R2 �I

gÿAl2O3

�440�IaÿAl2O3

�113� � IgÿAl2O3

�400�� 100% (2)

in which I (hkl) is the intensity of the peak diffraction for the

corresponding plane of the a-Al2O3, AlN and g-Al2O3

phases.

The in¯uence of the arc current and the nitrogen ¯ow rate

on R1 and R2 were investigated. The results are demonstrated

in Tables 2 and 3, respectively.

It can be seen from Tables 2 and 3 that the AlN content

(R1) and g- Al2O3 content (R2) increased gradually with

increasing arc current and nitrogen gas ¯ow rate, respec-

tively. Increasing arc current can lead to an increasing

Fig. 1. Schematic illustration of the experimental apparatus.

Table 1

The average particle size of different Al2O3 initial materials versus the

milling duration

Sample Time for

milled (h)

Particle

size (mm)

Spec. surf.

area (m2/cm3)

1 None 31.27 0.794

2 2 18.39 1.457

3 4 7.38 2.347

4 8 2.56 4.786

L.H. Cao et al. / Journal of Materials Processing Technology 89±90 (1999) 392±398 393

plasma ¯ame temperature, which enhances the nitriding

process by the thermal decomposition of more Al2O3.

The transformation of a-Al2O3 to transient g-Al2O3

occurred easily at high temperatures. Increasing nitrogen

gas ¯ow rate enhanced the nitriding reaction between Al2O3

and N2 plasma. Because the nitrogen plasma has a higher

enthalpy than argon plasma, it also improved the reactive

temperature.

3.2. Influence of different particle sizes of the initial

materials

The experiments were carried out using different particle

sizes of the Al2O3 as initial materials at arc current 900 A

and 100 psi of nitrogen. Figs. 3 and 4 show the AlN content

(R1) and g- Al2O3 content(R2) versus the particle size of

Al2O3 feedstock.

As seen from Figs. 3 and 4, both the AlN and g-Al2O3

contents increase with decreasing particle size of the Al2O3

feedstock. When the average particle size of the initial Al2O3

Fig. 2. XRD pattern of the composite powders produced at 900 A and 100 psi of N2.

Table 2

The influence of the arc current on the phase composition

Sample Current (A) Ar (psi) N2 (psi) Carrier gas (psi) Feedrate (rpm) R1 R2

1 700 40 100 30 1.0 8.56 9.30

2 800 40 100 30 1.0 9.19 12.86

3 900 40 100 30 1.0 11.52 15.67

Table 3

The influence of the N2 flow rate on the phase composition

Sample Current (A) Ar (psi) Carrier gas (psi) Feedrate (rpm) N2 (psi) R1 R2

1 900 40 30 1.0 60 9.97 13.16

2 900 40 30 1.0 80 10.56 14.68

3 900 40 30 1.0 100 11.52 15.67

Fig. 3. The AlN content (R1) versus the particle size of the Al2O3

feedstock.

394 L.H. Cao et al. / Journal of Materials Processing Technology 89±90 (1999) 392±398

materials decreased to 2.56 mm, the R1 increased nearly

three times compared to that of the average particle size

of 7.38 mm. The R2 also increased ®ve times correspond-

ingly (Fig. 4). The nitriding reaction between Al2O3 and

nitrogen plasma depends mainly on the state of the Al2O3

particles in the plasma. The smaller the particle size of

Al2O3, the easier is its melting or evaporating. Therefore,

these powders will exhibit higher reactivity with nitrogen

plasma and consequently more AlN can be formed.

3.3. Influence of post-treatment

Fig. 5 shows the XRD patterns for post-treatment of

Al2O3/AlN composite powders in nitrogen atmosphere at

the temperature range of 800±12008C for 2 h, at 2008Cintervals.

The diffraction patterns show that the cubic AlN and g-

Al2O3 gradually decrease after heat treatment. However, the

hexagonal AlN phase appeared and increased with an

increase of temperature. The results for phase composition

calculated by XRD are listed in Table 4, in which R3

represented the hexagonal AlN content in the composite

powders.

Table 4 indicates that R1 (the cubic AlN content) and R2

(the g-Al2O3 content) were about 31.26 and 35.76% at

8008C, respectively.

When the temperature reached 12008C, R1 decreased to

8.16% and g-Al2O3 disappeared completely. At the same

time R3 (the hexagonal AlN content) increased to 49.61%. It

is known that AlN has a hexagonal crystalline form with a

wurtzite-type structure, which is the most stable form.

However, it also has a oxygen-stabilized cubic structure

which is unstable at high temperature in nitrogen atmo-

sphere [18]. In the plasma spaying process, the AlN nuclei

®rstly formed on the surface of the Al2O3 particles and

produced the cubic structure, which contains nitrogen and

oxygen ions. With the AlN nuclei growth in a nitrogen

atmosphere, it gradually crystallized to form a single-phase

AlN hexagonal structure. Because of the high quenching rate

of the plasma ¯ame, there was not enough time for AlN

growth. This led to the oxygen-stablized cubic AlN forma-

tion. When it was heated in a nitrogen atmosphere for a

period of time, the oxygen ions in the cubic AlN phase were

gradually replaced by nitrogen ions and the cubic AlN

transformed to the hexagonal AlN. At the same time the

Fig. 4. The g-Al2O3 content (R2) versus the particle size of the Al2O3

feedstock.

Fig. 5. XRD patterns after post-treatment.

Table 4

Phase composition of powders after heat treatment

Sample Temperature

(8C)

R1 R2 R3

1 as-sprayed 43.21 63.17 0

2 800 31.26 35.76 15.13

3 1000 25.16 26.32 33.45

4 1200 8.16 0 49.61


transient g-Al2O3 not only were nitrided to form more AlN in

the nitrogen atmosphere because of its high reactivity, but

also it was converted into the stable a-Al2O3 phase at high

temperatures. The transformation mechanism will be stu-

died in detail in future work.

3.4. Morphology of Al2O3/AlN composite powders

SEM micrographs of Al2O3/AlN composite powders,

produced with Al2O3 feedstock milled for 2 and 8 h are

shown in Figs. 6 and 7, respectively. Fig. 8 shows the

morphology of Al2O3/AlN composite powders after post-

treatment. Fig. 6 indicates that most of the particles are

spherical, some are irregular, which resulted from the large

unmelted particles. Spheroidizing particles were obtained by

decreasing the particle size of the initial materials (Fig. 7).

Simultaneously, AlN with small particle sizes formed on

the surface of the Al2O3. This can be identi®ed by EDX, the

EDX spectrum being shown in Fig. 9.

Further examination of the Al±N bond was carried out by

FTIR. The results of FTIR spectroscopy for commercial

Al2O3 and AlN as well as Al2O3/AlN composite powders are

presented in Fig. 10. The infrared spectrum of the composite

powders showed two peaks related to the Al±N bond at

around 750 and 1334 cmÿ1, which con®rmed the presence

of AlN. This result agreed with the XRD and EDX results. A

strong Al±O spectrum was observed at 600 and 1636 cmÿ1.

The broad peak around 3400 cmÿ1 was the H±O bond due to

the composite powders absorbing moisture of the air. There-

fore, it is very important to keep the powders dry.

4. Conclusions

The Al2O3/AlN composite powders were prepared by the

plasma spraying of milled Al2O3 powders. The processing

characteristics were studied. The results showed that arc

current, N2 ¯owrate and the particle sizes of the initial

materials have a great in¯uence on the phase composition.

With increasing arc current, the contents of AlN and g-Al2O3

increased due to higher reaction temperature. Increasing the

N2 ¯owrate enhanced the nitridation of the Al2O3 feedstock

in the plasma. Decreasing the particle size of the initial

materials also resulted in increasing the AlN and g-Al2O3

contents. The smaller the particle size of the initial materials,

the higher the AlN and g-Al2O3 contents. Post-treatment of

Fig. 6. Microstructure of the composite powders.

Fig. 7. Microstructure of the composite powders.

Fig. 8. Microstructure of the composite powders after post-treatment.

Fig. 9. EDX spectrum of the composite powders.


Fig. 10. FTIR spectra of: (a) commercial Al2O3 and (b) AlN; as well as that of Al2O3/AlN composite powders.


Al2O3/AlN composite powders resulted in increasing the

AlN content and decreasing the g-Al2O3 content. At the

same time, the cubic AlN phase transformed to hexagonal

AlN after post treatment. The EDX and FTIR results further

con®rmed the AlN presence in the composite powders. SEM

results showed that the morphologies of the composite

powders are spheroids with ®ne AlN particles formed on

the surface of the Al2O3.

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