Indian Journal of Engineering & Materials Sciences

Vol. 15, June 2008, pp. 236-240

Comparison of properties of silver-tin oxide electrical contact materials through

different processing routes

Asmita Pandeya, P Verma

b & O P Pandey

c*

aDepartment of Computer Science, Baba Banda Singh Bahadur Engineering College,

Fatehgarh Sahib 140 407, India

bLovely Professional University, Jalandhar 144 402, India cSchool of Physics and Materials Science, Thapar University, Patiala 147 004, India

Received 23 August 2007; accepted 2 June 2008

In the present work silver-tin oxide (Ag-SnO2) electrical contact materials have been prepared by different processing

techniques, viz., powder metallurgy (P/M), internal oxidation (I/O) and internal oxidation of alloyed powders (IOAP). The

influence of processing routes on the physical properties such as electrical conductivity, density and microstructure of these

Ag-SnO2 composite materials are compared. The microstructural study reveals that internal oxidation (I/O) technique

provides most uniform structure.

Key words: Electrical contact materials, internal oxidation, silver tin oxide

Electrical contact materials are used in many

electromechanical devices as component which can

carry current intermittently through contact surfaces.

The basic properties required for these materials are

that they should possess high electrical and thermal

conductivity, high melting point and good oxidation

resistance. High melting point is required to avoid

any accidental overheating because of fusion of the

contact points whereas high thermal conductivity

helps to dissipate heat effectively. In order to keep the

contacts clean and free of insulating oxides, it is

essential that material must possess good oxidation

resistance. Metals like gold, silver, copper, palladium

and their alloys; composites of silver and copper with

various metal oxide combination are being used in

various low and high voltage switchgears1,2

.

Silver metal oxide composite electrical contact

materials find application in low voltage switchgear

devices such as contactors, relays, circuit breaker and

switches3-5

. Several composite systems like Ag-CdO,

Ag-ZnO, Ag-NiO, Ag-CuO, Ag-SnO2 and Ag-SnO2-

InO2 are the materials which are currently in use6-8

.

Efforts are being made to develop new alloy system

to achieve improved performance. Silver cadmium

oxide has been on the center stage for several decades

after having made its debut in 19509-11

. It has been

well established that Ag-CdO composites are suitable

for switching devices operating in the range of 15-

5000 A12, 13

. However, because of toxic nature of

cadmium and of increasingly stricter environmental

regulation imposed by RoHS (restriction of hazardous

substances, the European Union Directive

2002/95/EC) and WEEE (waste electronic and

electrical equipment in electrical and electronic

equipment set at July 2006 by European Union),

considerable efforts are underway to develop Cd free

silver metal oxide materials. In this regard, the oxides

of tin, zinc and indium are under investigation14-21

.

Among this Ag-SnO2 has emerged as a substitute of

Ag-CdO as it has superior corrosion resistance and

better anti welding properties as compared to Ag-

CdO. Ag-SnO2 derives its improved anti-corrosion

characteristics from thermally stable tin oxide which

prevents melting and scattering on the surface of the

contact by electric arc generated during contact

movement21-25

. Since Ag-SnO2 is a combination of

two dissimilar materials, namely, a metal and an

oxide, process routes other than the conventional

melting are followed to manufacture these materials.

Earlier reported work26

on erosion behaviour on

silver-metal oxide have shown that a continuous

increase in temperature from 45°C to 85°C occurs for ——————

*For correspondence (E mail: oppandey@tiet.ac.in)

PANDEY et al.: SILVER-TIN OXIDE ELECTRICAL CONTACT MATERIALS

237

a switching operation between 2-16 kilo cycles for

Ag-SnO2 alloys whereas very little fluctuation was

observed for Ag-CdO and Ag-ZnO alloys when tested

under similar conditions. The reason for it was non-

uniform dispersion of SnO2 phase in silver matrix.

Apart from this the eroded surface exhibited volcanic

craters, which is because of segregation of SnO2

phase along grain boundaries thus exposing the silver

matrix, which is soft, and get eroded26

. Currently two

methods are being commercially used to provide a

dispersion of tin oxide particles in silver. One method

which is used by German companies is the internal

oxidation of silver-tin alloys19,27

. At the same time

Japanese companies follow the powder metallurgy

process where mixing of tin oxide with silver powder

followed by pressing and sintering is done to achieve

fine dispersion of tin oxide in the matrix of silver28

.

The above methods have some limitations like

formation of needle like brittle phase and limitation

of oxidation27,28

. Considering the above fact the

present study is undertaken to develop Ag-SnO2

contact material with uniform distribution of second

phase SnO2 in the silver matrix.

Experimental Procedure

Ag-SnO2 composite contact materials are prepared

by powder metallurgy (P/M), internal oxidation (I/O)

and internal oxidation of alloyed powders (IOAP

techniques) by taking 5N purity elements and 4N

purity compounds. The composition of composite

made by above techniques is given in Table 1.

The P/M involves thorough mixing of fine powders

of silver and tin oxide in a ball mill. The mixed

powder is consolidated and pre-sintered to increase

density. However, in order to achieve high final

density, secondary processes, such as repressing,

sintering and extrusion are essential. The details of

these processes followed in the present investigation

are given in the flow chart as shown in Fig. 1. The

(I/O) process essentially consists of preparation of

silver-tin-indium alloy by induction melting under

vacuum. The alloy is rolled in the form of sheets of

suitable thickness and homogenized subsequently.

Pieces of suitable size and dimensions are punched

out from the homogenized sheet and internally

oxidized. Indium was added to silver-tin alloy to

facilitate internal oxidation. The flow chart of the

process adopted is shown in Fig. 2. In the IOAP

process, which is a hybrid of I/O and P/M processes

described earlier silver-tin-indium alloy was prepared

by induction melting. The melt was subsequently

atomized in an indigenously designed and fabricated

atomizer29

. The powders collected after atomization

Fig. 1 — Flow chart of powder metallurgy process (P/M)

Table 1— Important physical properties of Ag-SnO2 materials processed by different techniques

Process Composition Conductivity Hardness Density %Theoretical

(% IACS) (HV 1.0) g/cm2 density

Powder metallurgy Ag- 10 SnO2 64 89.2 9.6 97%

Internal oxidation Ag-7.6 SnO2 –4.8

In2O3

62 111.0 9.9 100%

Internal oxidation

of alloyed powder

Ag-7.6 SnO2 –4.8

In2O3

59 87.2 9.7 99%

INDIAN J. ENG. MATER. SCI., JUNE 2008

238

Fig. 2 — Flow chart of internal oxidation process (I/O)

are subsequently internally oxidized. The internally

oxidized powders are used for making the contact

points of different shapes and dimensions by P/M

techniques. The flow chart of the process is shown in

Fig. 3. The details of these processes are described

elsewhere26

.

Test pieces of 16 mm diameter were prepared from

the lot of samples prepared in each route. The

electrical conductivity of samples was measured with

eddy current conductivity meter. To minimize the

error in measurement, the samples were carefully

polished on 600 grit emery paper before

measurement. The hardness was measured with the

help of Vickers hardness tester (Buahler, USA). The

reported hardness values are an average of five

readings. Density was measured by dimensional

arrangement. In order to study the microstructure, the

samples were mechanically polished and observed

under scanning electron microscope (SEM). All the

microstructures were taken in back scattered electron

mode of the microscope.

Fig. 3 — Flow chart of internal oxidation of alloyed powder

process (I.O.A.P)

Results and Discussion

The physical properties of the contact materials

prepared by different process routes are compared in

Table 1. It can be noticed that P/M route gives higher

electrical conductivity despite of fact that 3%

porosity in the material exists. The pores are potential

scattering centers for the electrons and consequently

reduce the conductivity26

. However, P/M processed

materials without any pore are likely to exhibit higher

conductivity due to better connectivity. On the other

hand, despite of its better density, the I/O composites

show poor conductivity. This can be attributed to the

straining of silver lattice resulting from the formation

of bulky tin oxide and indium oxide particles. The

strain in lattice changes the mean free path of

electrons and hence the conductivity. The lowest

value of conductivity observed in I/O is the result of

pores as well as lattice strain in these materials. It is

evident from Table 1 that I/O has the best hardness.

High value hardness is attributed to the full density

and very uniform distribution of tin oxide and indium

PANDEY et al.: SILVER-TIN OXIDE ELECTRICAL CONTACT MATERIALS

239

Fig. 4 — Microstructure of sample prepared by powder

metallurgy (P/M) process

oxide in silver. P/M suffers from the disadvantage of

density and relatively poorer dispersion of oxide and

consequently the poor hardness. IOAP has the similar

features as that of P/M.

The I/O process suffers from the problem of high

inter-diffusion of tin in silver. It has been established

that a high content of tin (>4wt%) in silver is not

amenable to internal oxidation as it forms an

impervious layer of tin oxide, inhibiting further

diffusion of oxygen into the alloy29,30

. In order to

facilitate oxidation, several additives such as bismuth,

copper and indium have been tried with different

measures of success22,29

. Among these, on the indium

bearing the silver-tin alloy has been commercially

exploited to its full potential. Indium prevents the

formation of dense oxide bands of SnO2 and enables

oxygen to diffuse at faster rate into the silver-tin

alloys30, 31

.

The material resulting from such process leads to a

fine dispersion of In2O3 and SnO2 in the matrix of

silver. This dispersion of oxides in the matrix of

silver provides high hardness and reasonably good

electrical conductivity. The greatest disadvantage of

the process stems from the slow diffusion of oxygen

in the material resulting in higher process time. Use

of indium escalates the cost. Moreover, the internally

oxidized materials invariably have an oxide free zone

also called denuded zone in the center and non-

uniform microstructure across the depth. Because of

the small size of the powders, the time for internal

oxidation is reduced to a great extent16,30

. The

material processed by this route exhibit good

electrical conductivity and hardness. The material

also displays uniform microstructure and is devoid of

denuded zone.

Fig. 5 — Microstructure of sample prepared by internal oxidation

(I/O) process

Fig. 6 — Microstructure of sample prepared by internal oxidation

of alloyed process (IOAP)

The microstructure of the materials developed by

P/M, I/O and IOAP are given in Figs 4, 5 and 6

respectively. As is evident from the microstructure,

I/O gives the most uniform microstructure. However,

I/O has the problem of oxides precipitating along the

grain boundaries. This can be controlled to some

extent by reducing the solute percentage. I/O

sometimes also has features of divorced grain

boundary, i.e., region close to grain boundary are

devoid of oxides. The pores and state of dispersion of

oxides in P/M processed material are shown in Fig. 4.

Better connectivity of individual silver grains is

revealed in the microstructure.

Figure 6 shows the microstructure of the IOAP

processed material. It may be noted that a silver-rich

region envelops each grain. The silver-rich region

enveloping each grain is the result of diffusional

creep of silver following the formation of bulky SnO2

and In2O3 inside the lattice. This silver-rich region

facilitates sintering and gives rise to better

connectivity.

INDIAN J. ENG. MATER. SCI., JUNE 2008

240

Conclusions

From the above work it can be concluded that P/M

and IOAP processes are less time consuming but

require large infrastructure with more processes steps.

The materials processed by P/M route exhibit

homogenous microstructure, high electrical

conductivity and greater latitude in composition

selection and overall faster speed of process.

However, the materials processed by P/M route show

poor hardness and density. At the same time I/O

requires less process step with uniform microstructure

and full density. Thus, a judicious selection of the

process for the production of the tips requires a trade-

off between properties, investment and scale of

operation.

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