Correlation between nitriding process and
microstructure of reaction bonded aluminium
nitride ceramics
E. Salahi and J. G. Heinrich
in production methods, higher purity, and better knowledge
of physical and chemical properties initiated intensive
Aluminium nitride samples were produced by a reaction
research for new applications to utilise the very special
bonding process using AlN and Al powders with ake-
properties of these materials.
like and equiaxed morphologies as starting materials.
Various methods of producing AlN powder are
Changing the particle sizes and morphologies of the
known. 4–15 Nearly 130 years ago, the initial preparation of
aluminium starting powders led to changes in degree
AlN via the reaction of liquid Al with N
2
gas was reported. 16
of reaction and microstructure of the resulting reaction
Slack and McNelly17 used aluminium uoride and ammonia
bonded aluminium nitride ceramics. At 25 wt-%Al, the
gas to produce AlN. Total nitriding of Al powders was
degree of reaction showed a maximum, decreasing at
achieved in the work of Boitier et al.18 by the addition of
higher aluminium concentrations. T he degree of
CaCO
3
. Another method of manufacturing AlN is carbo-
reaction was increased by increasing nitrogen gas
thermal reduction of aluminium oxide, 19–21 which involves
owrate or nitriding temperature. It was also found to
reacting nely mixed Al
2
O
3
and carbon powders in N
2
be increased by reducing the average particle size of
containing gas at temperatures in the range 1373–2073 K.
the Al starting powder, the green bulk density, or the
The aerosol synthesis route for producing AlN oVers an
sample thickness, and also by choosing akelike morph-
alternative to conventional powder preparation methods. 22
ology. SEM and optical micrographs of reaction bonded
High purity powders can be obtained by using molecular
aluminium nitride samples revealed the pore structure
precursors as starting reactants. One group has described
and morphology of primary and secondary aluminium
aerosol reactors for AlN synthesis using AlCl
3
and NH
3
as
nitride. BCT /604
gas phase reactants. 23 Other workers have described solu-
tion phase methods for producing AlN, using metal organic
Dr Salahi (kesalahi@yahoo.com) is in the Ceramics
compounds instead of AlCl
3
as reactant: examples include
Division, Materials and Energy Research Centre, PO
aluminium alkyls,24,25 polyiminoalanes, 26,27 and methyl-
Box 14155–4777, T ehran, Iran and Professor Heinrich
silylamino aluminium compounds. 28
is at the Non-Metallic Institute, TU Clausthal,
In the present study, correlations between the nitriding
Zehntnerstrasse 2A, D–38678 Clausthal-Zellerfeld,
process and the microstructure of reaction bonded alumin-
Germany. Manuscript received 17 March 2003;
ium nitride (RBAN) ceramics are investigated.
accepted 19 June 2003.
© 2003 IoM Communications L td. Published by Maney for
MATERIALS AND METHODS
the Institute of Materials, Minerals and Mining.
Two diVerent kinds of aluminium powders were used,
one with akelike morphology (Eckart Art 44/102) and
the other with equiaxed morphology (Schlenk Rogal
Aluminium VP 0/40). The former had an average particle
size of 10 mm and in the latter case three diVerent average INTRODUCTION
particle sizes of 20, 30, and 40 mm were chosen. Chemical
Reaction forming of ceramics exhibits various advantages
analysis of the aluminium starting powders (Philips PU9400
when compared with conventional processing. The low
atomic absorption spectrometer) is shown in Table 1. A
processing temperature and glass phase free grain bound-
commercial aluminium nitride (H. C. Starck grade C) with
aries are the most attractive attributes for many technical
an average particle size of 1 mm was also used (3·6 m2 gÕ 1
and high performance applications. 1,2 The low to zero
specic surface area, according to manufacturer’s specica-
shrinkage capability in particular makes most reaction
tion). Oxygen and carbon impurities were specied as 0·9%
forming techniques suitable for fabrication of composites.
and less than 0·1% respectively. The Fe, Ca, W, and Mo
Direct nitridation of aluminium powder is an example of
contents were less than 50 ppm, Si less than 80 ppm, and
the reaction forming technique. A mixture of Al and AlN
other metallic impurities less than 200 ppm.
powder compacts is heated in a nitrogen atmosphere such
The aluminium nitride starting materials with diVerent
that Al nitridises to new AlN crystals, which bond together
contents of aluminium powders were mixed in ethyl alcohol
the original (primary) AlN particles. The main features of
this method are reduced shrinkage and acceptable strength,
as a result of the grain growth of the new AlN which
Table 1 Aluminium starting powder characteristics
develops during the reaction bonding process. The nitrid-
ation of Al particles thus represents the key to a technology
Elemental content, ppm
leading to novel AlN based ceramics, which are important
advanced ceramics and are becoming increasingly signi-
Al powder morphology Cd Cr Cu Ni Mn Zn Mg
cant as high thermal conductivity materials. The rst papers
on AlN were published in the late 1850s, 3 with rapidly
Flakelike 5 5 100 50 200 100 40
increasing activity in subsequent years because of new and
Equiaxed 5 50 100 50 100 100 40
higher requirements on refractory materials. Developments
British Ceramic Transactions 2003 Vol. 102 No. 4 161 DOI 10.1179 /096797803225005007