5th International Biennial Conference on Ultrafine Grained and Nanostructured Materials, UFGNSM15

An Investigation on the fabrication of Al2O3-Co-TiO2 Nano-cermets

via metal coated ceramic particles

A. Esmaeilzaeia*, S. A. Sajjadia, S. Mollazadeh Beidokhtia

aDepartment of Material Engineering, faculty of Engineering, Ferdowsi university, P.O. BOX 9177948974, Mashhad, Iran.

Abstract

Ceramic-metal composites (Cermets), which are a well-known category of advance materials, attract so much interest during

decades due to their excellent abrasion and corrosion resistance. Powder metallurgy is a favorite route for fabrication of such

materials; however this method is accompanied with the non-uniform distribution of reinforcements in the matrix, high sintering

temperature and advanced equipment required. In order to reduce such limitations, in the present study Al2O3-Co-TiO2 nano-

cermets were fabricated by using metal coated Al2O3 powders. In this regard electroless cobalt deposition technique was used for

metal coating of micronized Al2O3 particles. Subsequently, different compositions of coated particles were used to fabricate

variety of nano-cermets. Also In this investigation, spark plasma sintering (SPS) has been used for obtaining fully dense samples.

The morphology and chemical composition of the Co-coated powders were characterized by scanning electron microscopy

(SEM), energy dispersive spectroscopy (EDS). The effects of TiO2 nano particle percentages were investigated on hardness of

fabricated nano-cermets. Besides, the obtained results indicated that mechanical properties of composite samples fabricated by

optimized percentage of nanoparticles showed some improvement and also uniform shells were coated on Al2O3 particle.

Keywords: nano-cermet; electroless;hardness; spark plasma sintering

1. Introduction

Excellent abrasion resistance, high elastic modulus and good chemical properties make Alumina ceramics widely

applicable in various industries such as oil, gas, automotive, aerospace, Wang et al (2003), sajjadi et al (2011).

Besides, good biocompatibility of alumina, make it a highly regarded and used candidate biomaterial in dentistry

and hip joint prosthesis in the past few decades, Byong- Taek, Lee et al (2008). However, the widespread

application of Al2O3 ceramics has been limited due to their low fracture toughness, Scheppokat et al (2005). The

addition of soft metal powders in the ceramic matrix is an effective way which can increase the fracture toughness

of cermet made by powder metallurgy method, Li et al (2001). Poor wettability of ceramic particles by surronding

metals, inhomogeneous distribution of particles in the matrix and high-temperature sintering are some restrictions of

* Corresponding author. Tel.: +985138763305; fax: +985138763305.

E-mail address: A.Esmaeilzaei70@yahoo.com

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powder metallurgy process. Spark plasma sintering (SPS) method developed quickly to produce a wide range of

advanced materials as nano crystals, nano cermet, etc, Byong- Taek, Lee et al (2008). In this method, the powder is

placed into a cylindrical mold made of graphite or insulator materials and compressed by two punch from top and

bottom. Applying electric current through powder mass provides the driving force for sintering and makes the

production process run over in a little time. Faster heating rate, lower sintering temperature, less time for

maintenance at temperature, no need to compress the powder before sintering and improving the properties of

sintered materials are some of the characteristics which distingueshes this sintering method in comparison with other

methods. Accordingly, to reduce restrictions listed in producing of Al2O3 - Co - TiO2 nano cermet of, Al2O3 micro

powder were coated with a thin layer of Co by electroless method, in this study. After wards, TiO2 nanoparticles

with different percentages was added to nano cermet to improve the strength of the nano cermet. Finally, nano

cermet Al2O3 - Co - TiO2 coated with electroless cobalt and sintering was done by SPS method.

2. Experimental

Raw materials used in the present study consisted of pure Al2O3 powder (purity 99, average particle size 5

micrometers) and TiO2 (purity 99, average grain size 20 nm). Surface powder activation and electroless deposition

in water bath, are the major steps which were organized to coat alumina particle in this study. The details of

mentioned steps described eleswhere but in briefly.

Table 1 shows steps applied to coat particles, chemical compounds used and the powder ratio in the electroless

bath. The powder activated in the first step is dissolved in deionized water on magnetic stirrer, Beygi et al (2012).

After which, the solution temperature reaches to 95ºC, cobalt chloride, sodium hypophosphite, and sodium citrate

are added to the solution and PH is set at 9.5 by NaOH. At the beginning of the reaction, a lot of hydrogen gas

comes out of solution and the solution gets black because of cobalt coatings on powders. At the end of coating

process, the color of solution changes to dark purple shade because of cobalt ions reduction. The coated powder was

washed with deionized water several times, separated by centrifuge and finally was dried in an oven at 84 ° C for 2

hours, Zhang et al (2003). After that, alumina powders coated with different percentages of Tio2 (20-25-30 wt%) are

mixed together by low-energy mixer and cast into cylindrical mold made of graphite with an inner diameter of 13

mm. The sintering of the obtained powder was done in vacuum using SPS and applying direct pulse electrical

current with duration of 5 min during simultaneous application of uniaxial pressure of 30 MPa at a temperature of

1300 ºC.

Table 1. Formulae of chemical deposition solution

Step Composition Concentration

1-pretreatment washing Aceton 200 ml Coarsening HNO3 100ml/l

Sensitization SnCl2 15g/l

HCl 60ml/l Activation PdCl2 0.5g/l

HCl 10ml/l

2-electroless deposition CoCl2.6 H2O 30g/l

Na3C6H5O7.2 H2O 100g/l

NaH2PO2. H2O 60g/l

NaOH 50g/l

3. Results and discussion

SEM images of Micro-alumina powder are given in Figure 1 before and after the coating process. As it can be

seen a thin coating of cobalt with a thickness of 40 nm has been created on the surface of the particles. Related EDS

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spectrum is also given in fig. 1(b). Al and Co peaks were detected as the main element in the EDS spectrum. The

appearances of Au peaks were due to using an Au grid for the preparation of the SEM sample.

Fig. 1. SEM picture of a) initial alumina powders ̦ b) Al2O3/Co nanocermets powder̦ c) EDS profile

Figure 2 shows the microstructure of reinforced nano cermet with different percentages of TiO2 nano-particles

which are coated with cobalt. Microstructure of nano cermet represented that TiO2 particles are evenly distributed in

the treated matrix. Heterogeneous distribution of particles are observed in cases which have higher percentage of

TiO2 phase.

b

c

a

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Fig. 2. SEM picture of Al2O3-Co-TiO2 nanocermets with a) 20%wt TiO2, b) 25%wt TiO2, c) 30%wt TiO2

Table 2 shows the results of rockwell hardness values for nano cermet samples which composed of different

percentages of TiO2.

Table 2. Rockwell hardness of specimens

Samples

Average hardness 20%wt TiO2 25%Wt TiO2 30%Wt TiO2

50.1 56.7 51.6

By comparing hardness values of different nano cermet samples, it can be observed that samples hardness first

increases and then decreases as TiO2 wt% increases. The main reason of hardness increase in the samples reinforced

with nanoparticles, could be due to the high hardness of reinforcing particles, prevented grain growth of aluminum

oxide and movement dislocations movement prohibition which caused by the increase in the fraction of mentioned

particles. The reason of observed hardness decline in nano-cermet samples at high percentages of TiO2 is because of

TiO2 nanoparticles agglomeration.

4. Conclusions

1. electroless Sedimentation is an effective way for precipitation of uniform Co coating on Al2O3 particles.

2. Spark plasma sintering could be considered as an appropriate method for the production of nano Al2O3 - Co -

TiO2 cermet. The created spark between alumina powder and coated titania could decrease the sintering temperature.

3. Coating of Al2O3 particles by Co could increase wetting ability of ceramic reinforcing particles and improve

their distribution in metal matrix.

a b

c

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