Structure and Properties of 

Aluminum Nitride and AlON 

Ceramics 

by James W. McCauley 

ARL-TR-2740 May 2002 

Approved for public release; distribution is unlimited. 

20020625 071 
. 

The findings in this report are not to be construed as an 

official Department of the Army position unless so 

designated by other authorized documents. 

Citation of manufacturer’s or trade names does not 

constitute an official endorsement or approval of the use 

thereof. 

Destroy this report when it is no longer needed. Do not 

return it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 210053069 

ARL-TR-2740 May 2002 

Structure and Properties of 

Aluminum Nitride and AlON 

Ceramics 

James W. McCauley 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distribution is unlimited. 
Abstract 

A brief review of aluminum nitride and AlON ceramics is presented in the 

context of the pseudobinary aluminum nitride-aluminum oxide phase 

equilibrium system. AlON is the name applied to the nitrogen stabilized cubic 

spine1 in this system, with a composition centered at 35.7 mole-percent AlN. 

First, the phase equilibria and crystal chemistry of these phases are presented, 

focusing on a constant anion oxynitride spine1 model and the various phases, 

including polytypoids, in this system. Then, a brief s ummary is given on the 

processing and microstructures of AIN and AlON, with comparisons to other 

microstructures in this system. Finally, selected properties of AlN and AlON are 

discussed and tabulated, with special reference to alumina. A comprehensive 

reference list is given. 

. 

ii 
Acknowledgments 

Thanks to Dr. Lynn Neergaard of the Advanced Materials and Processes 

Technology Information Analysis Center, Huntsville, AL, for help in the 

literature survey. 

. . . 

Ill 
iv 
Contents 

Acknowledgments . . . 

111 

List of Figures vii 

List of Tables 

1. 

2. 

3. 

4. 

5. 

Introduction 1 

Phase Equilibria and Crystal Chemistry 1 

2.1 AlON-Al203 Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

2.2 AION Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

2.3 AlN Polytypoids (Corbin 1989; Tabary and Servant 1999a; 

Tabary et al. 2000) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Processing and Microstructures of AlN and AlON 5 

Properties of AIN and AlON 7 

References 9 

Distribution List 17 

Report Documentation Page 19 
vi 

. 
List of Figures 

Figure 1. Proposed experimental phase equilibrium diagram for the 

pseudobinary AhOsAlN composition join at one atmosphere of 

flowing nitrogen (McCauley and Corbm 1983). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Figure 2. Crystal chemistry relationships in the A1203-AlN system 

(McCauley and Corbin 1983). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Figure 3. Representative microstructures of AlN, AlON/polytypoid 

mixture, AlON, and Al203 _....................................................................................... 6 

List of Tables 

Table 1. Stoichiometries calculated from the constant anion model 

(McCauley 1978). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .._.................................... 4 

Table 2. Representative properties of polycrystalhne AlN, AlON, and 

a-AkO3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Vii 
. . . 

Vlll 
1. Introduction 

The AIN-Al203 system contains three important materials: AlN, AlON, and 

Ab.03, which have a wide range of properties and uses. Excellent reviews for 

AlN can be found in Sarnsonov (1969), Dugger (1975), Sheppard (1990), and 

Prohaska and Miller (1990), in Corbin (1989) for AlON, and in Munro (1997) for 

polycrystalline a-alumina. Some discussion and references are provided in this 

review for alumina for ease of comparison to the other two materials. 

2. Phase Equilibria and Crystal Chemistry 

Substitution of nitrogen for oxygen in Al203 or, conversely, substitution of 

oxygen into AIN stabilizes new phases with significantly different crystal 

structures and symmetry (space group): a-Ah03 = R %, AION = Fd3m, and 

AIN = P6arnc-rhombohedral, cubic, and hexagonal, respectively. 

AlON, having the cubic spine1 structure, can be thought of as a nitrogen 

stabilized cubic aluminum oxide. It has many properties comparable to o-A203, 

but because of its cubic crystal structure, fully dense, polycrystalline bodies can 

be completely transparent if processed properly (McCauley and Corbin 1979). 

Other properties, like dielectric loss tangent, can be extremely low because of the 

lack of thermal expansion-induced residual strain at grain boundaries. AlN is an 

intriguing material because of its theoretical thermal conductivity of about 

320 W/n-X at room temperature-extremely high for a dielectric material and 

comparable or higher than many metals. 

Figure 1 illustrates the proposed phase equilibrium diagram for the 

pseudobinary A1203-m composition join determined experimentally by 

McCauley et al. (1988). In a recent series of papers (Willems et al. 1992a, 1992b, 

1993; Dumitrescu and Sundman 1995; Qiu and Metselaar 1997), the 

thermodynamics of AlON have been worked out and the phase relations 

calculated, confinning the main aspects of the experimental diagram. Tabary 

and Servant (1998) have also reassessed the phase relationships in this system. 

Fukuyama et al. (1999), in using plasma arc melting to produce AlON powder, 

seem to have confirmed the vapor-liquid relationship in the extreme temperature 

region. Besides the three primary phases, there are also many intermediate 

phases, listed in the following subsections. 
Aluminum OqnlMde Phaw 

; AIN lm 

a- :i$g 

ill BB 

21R 83 

A16@.N4 EH 

WWkJ ALDN f-/l El 

m _ AI@%& cb’ Splncl 16.7 

A1203 Corundum 0 

tam 

LIQUID 

AM 2lR 21R +V 

". UH*L 

l2H + AWN 

2lR + ALON 

27R l ALON 

Figure 1. Proposed experimental phase equilibrium diagram for the pseudobinary 

Alr03-AIN composition join at one atmosphere of flowing nitrogen 

(McCauley and Corbin 1983). 

2.1 AION-ALOJ Region 

l 13 Al~Os:l ALN = Q - Al203 (Michell972). 

l 9 Al$&:lAlN = S - A1203 (Long and Foster 1961; Lefebvre et al. 1972; 

Tabary and Servant 1999b). 

l 10 Al203:2AlN = < - A1203 or $’ - Al203 (Long and Foster 1961; Michel 

1972); Al~030N2 = AlON’ or +’ (McCauley 1978; Tabary 

et al. 1999). 

2.2 AlON Region 

. 9A1203:5AlN = A1~027Ns = AlON (McCauley 1978; Tabary and Servant 

1999a). 

2 
2.3 AlN Polytypoids (Corbin 1989; Tabary and Servant 1999a; Tabary et al. 

2000) 

l lAlz03:4AlN = 12H. 

l lA1203:5AlN = 21R. 

l lAl~O3:7AlN = 27R &rid-man et al. 1985). 

l lAl~03:14AlN = 32H (Krishnan et al. 1985). 

Other polytypoids have also been identified: SH, 15R, 16H, 33R, 24H, and 39R. 

However, the phase equilibria have not yet been clarified. Tabary and Servant 

(1999a) and Tabary et al. (2000) have carried out a fairly detailed microstructural 

characterization of the polytypoids in this system. 

Figure 2 (McCauley and Corbin 1983) summarizes selected phase equilibria, 

bonding and atomic structure changes in this system. The bonding changes from 

primarily ionic in a-Al203 to covalent in AIN. The electronegativities of Al = 1.5, 

0 = 3.5, and N = 3.0 result in predicted ionic character of o-Al203 at 63%, AlN at 

43%, and AlON at about 56% (Pauling 1963). With a magnesium 

electronegativity of 1.2, MgAh.04 has a predicted ionic character of about 68%. 

Structurally, the substitution of oxygen into AlN or nitrogen into M203 

destabilizes the parent structures with the resulting formation of modulated 

structures. Conceptually, nitrogen substitution into a203 causes a local charge 

imbalance on the substituted nitrogen. This can be reduced by a shift in anion 

coordination around Al from six to four, driving the a-A.hOs-based phase toward 

a spine1 (MgAhO&type structure, where the cations are distributed between 

octahedral and tetrahedral coordination. This has been confirmed by crystal 

structure analysis of AlON, first determined by Goursat et al. (1981) and more 

recently refined by Tabary and Servant (1999a), confirming the predictive model 

of McCauley (1978). Basically, in the AlON spine1 unit cell, there are 8 Al cations 

in tetrahedral sites and 15 Al and 1 vacancy in the 16 octahedral sites. The model 

proposed by McCauley (1978), which assumes a constant number (32) of anions 

in a spine1 unit cell, has successfully been used to describe and account for the 

unconventional (seemingly nonstoichiometric) composition of both the AlON (N 

= 5) and the Q’ (N = 2) phases. The model assumes the following formula: 

Al(~+,&ation Vacancy(h)/3 O(32-x)Nx. 

Table 1 lists the various compositions predicted by this model from N = 0 to 11. 

The “normal” spine1 composition (N = 8) does not seem to be a stable phase, 

whereas AlON (N = 5) and @’ (N = 2) are seemingly stable stoichiometric phases 

verified by many investigators. Yamaguchi and Yanagida (1959) were the first 

investigators to propose that nitrogen additions to A1203 would stabilize a 

spinel-type material. 

3 
T’iPE SF 

P~UILIBRIA SOLID'VAFOR 

CAl,Oii ,,d /I::CRWSING VI 

CLOSE io 

\DECREASING IV 

114 IV 

COORDINATION 314 VI 

I 

PACKli,!C OF HCP (All CCP IOINI 

RA~Y\'IORK ATOM 

HCP IO) I 

lv~lcx~ ,v 

COORDINATIOi! , 

I v I L' IV IV ’ 

SiRUCTURiS 

PHASES 

(RATIO - 

A&:AIN) 

AllJ STRUClURES CONSTANT ANlON 

WJRTZITE F.;ODULATED E!:lltl YIWCL 

Al-OI:d OCTAHEDRA PWEL. uoov:A'cl 

SPiNELhlODEl C0R"p~D"hl 

I 

_-.r 

I 

mole % AIN 

Figure 2. Crystal chemistry relationships in the Ah@-AlN system (McCauley and 

Corbin 1983). 

Table 1. Stoichiometies calculated from the constant anion model (McCauley 1978). 

N 0 Al 

lla 21 25.00 

Mole-Percent 

Interstitials Vacancies ALN 

1.00 - 61.1 

10 

9 

8” 

7 

6 

5” 

22 

23 

24 

25 

26 

27 

J 

24.67 0.67 - 57.7 

24.33 0.33 - 54.0 

24.00 0 0 50.0 Normal 

23.67 - I 0.33 I 45.7 ~ I 

23.33 - 0.67 40.9 

23.00 - 1.00 35.7 AlON (y) 

4 28 22.67 - 1.33 30.0 

3 29 22.33 - 1.67 23.7 

, I 2= 1 30 1 22.00 1 - ( 2.00 16.7AlON’ (9’) 

1 31 1 21.67 1 - / 2.33 8.8 

0 32 1 21.33 1 - 1 2.67 0 

a Stoichiometric compounds - integral numbers of atoms. 

As indicated in Figure 2, AlN has the hexagonal, wurtzite structure, where Al is 

tetrahedrally coordinated with nitrogen. The substitution of oxygen for nitrogen 

in the wurtzite AlN structure results in structural irregularities (Dorignac et al. 

1994) and the formation of many polytype-like (polytypoid) phases. This is 

apparently different than Sic, which has a vast array of polytypes. Recently, 

however, Ueno et al. (1992) and Xia et al. (1993) have confirmed that at pressures 

4 
of about 16-20 GPa, AlN transforms into an octahedrally coordinated rock 

Salt SkLdure (@h). 

3. Processing and Microstructures of AlN and AlON 

AlN powder can be produced by nitridation of Al metal powder or also by the 

reduction of alumina with admixed fine carbon particles in the presence of 

nitrogen or ammonia (Huseby 1983). AION can be produced by the simple 

reaction of Al203 with AlN in nitrogen or by the carbothermal reduction of Al203 

+ carbon mixtures (Yawes et al. 1997a, 1997b; Ish-Shalom 1982). 

Even though AlN is hexagonal, it can be sintered into translucent, almost 

transparent, polycrystalline ceramics (Kuramoto and Taniguchi 1984; Kuramoto 

et al. 1989). Coble (1962) demonstrated similar results for c%d203, called Lucalox 

by the General Electric Company, Schenectady, NY. Both of these materials can 

never be truly transparent in polycrystalline bodies because of their anisotropic 

optical properties or birefringence, unless they have grain sizes significantly 

smaller than the wavelength of light. On the other hand, AlON, being optically 

isotropic, can be produced in completely transparent ceramic bodies (McCauley 

and Corbin 1979). Raytheon Corporation, Lexington, MA, manufactures AlON 

in sizes up to about 12 x 12 x 0.25 in for special high-performance applications, 

including transparent armor and radar windows and domes. 

Figure 3 illustrates typical microstructures of AlN (Kuramoto 1984), AlON, and 

Lucalox. Of course, there are many variations of these microstructures, including 

grain size, shape, porosity, etc. The microstructure of AlON is hard to resolve 

optically because the lack of residual stress at the grain boundaries prevents the 

polishing treatment from preferentially eroding the boundaries for better 

contrast. (See McCauley et al. [19SS] for a comprehensive review of 

microstructures in the A1203-m system, focusing on the AlON’, AlON, and 

polytypoid regions.) 

Both pressureless sintering and hot pressing have been used to produce 

pore-free, fully dense AlN and AlON ceramics. Small amounts of additives 

(MgO in Al203 and CaO and Rare Earth compounds in AlN) are typically 

utilized to achieve fully dense, pore-free material. Various chemical vapor 

deposition techniques (e.g., Irene et al. [1975] and Silvestri et al. [1975]) can be 

used to produce Al,O,N, films. The effect of oxygen on the sintering and hot 

pressing of AlN has been studied extensively (Sakai and Iwata 1977; Sakai et al. 

1978). Besides McCauley and Corbin (1979), other investigators have studied 

the pressureless sintering of AlON (Chen et al. 1991; Kollenberg and 

Rymon-Lipinska 1992). Several investigators have also studied the hot pressing 
(4 m (b) 21R +AlON+12H 

(c) AlON (d) A1203 

Figure 3. Representative microstructures of AlN, AlON/polytypoid mixture, AlON, and 

A1203. 

of AlzOsAlN powder mixtures and determined the resulting properties; 

however, the formation of the various polytypoids makes the interpretation 

of their results difficult (Turpm-Launay et al. 1983; Shimpo et al. 1992; Sakai 

1978a, 197810, 1981a, 1981b). Figure 3(b) is an example of a mixed phase 

microstructure. 

Sakai (1981a, 1981b) and Sakai et al. (1978) has reported on the room and high 

temperature fleXUral strength of db&Al203 material in the 0-70 mole-percent 

Al203 region. The final products contained variable amounts of AlN poytypoids. 

Shimpo et al. (1992) investigated the properties of pure AlON as compared to 

AlON-BN composite materials. In a more comprehensive study, Turpin-Launay 

et al. (1983) and Launay et al. (1984) investigated the properties of materials in 

the Alb&M203 system by both reactive hot pressing and reaction sintering; they 

have referred to these materials as ALUMINALON. Significant variations in 

hardness, flexure strength, and fracture toughness were observed. Kim et al. 

(1998) have also studied composites in this system. Djenkal et al. (2000) 

investigated the Sic-reinforcement of a203 ~10N composites. The friction, 

wear resistance, and other mechanical properties of the ALUMINALON family 
of materials were systematically studied by Trabelsi et al. (1987) and Berriche 

et al. (2000). 

4. Properties of AlN and AlON 

AlN is a high band gap (about 6.2 eV) dielectric material with a very high 

thermal conductivity. The electronic band structure of A1203, AlON, and AlN is 

reviewed by French (1990). Slack (1973) estimated the thermal conductivity of a 

single crystal to be about 320 W/mK at 300 K. Since that time, there has been a 

great deal of effort in attempting to produce ceramics (Buhr et al. 1991; Iwamoto 

et al. 1992) that approach this value for a variety of high-performance substrate 

and packaging applications in microelectronics and high power devices (Ichinose 

1988; Sheppard 1990). It also has excellent molten metal corrosion resistance and 

stability at high temperatures. Extensive high-temperature creep and 

compressive strength measurements have been carried out on AlN (e.g., Masson 

et al. [1994]). It is also very interesting to note that Heard and Cline (1980) have 

reported a brittle-ductile transformation in AlN at 0.55 GPa confining pressure; 

Al203 does not seem to exhibit this transformation up to a confining pressure of 

1.25 GPa. AlON has not yet been studied in this way. 

AlON has not been studied as extensively as AlN or a-Al203. The elastic 

properties have been worked out by Graham et al. (1988); the thermomechanical 

properties by Quinn et al. (1984) and Swab et al. (1999); dielectric properties by 

Westphal (1979) and Corbin and McCauley (1981); and the optical properties by 

Tropf and Thomas (1991). Kim and Richards (1985) investigated the electrical 

conductivity of AlON. Neutron irradiation studies are detailed by Jeanne et al. 

(1987). Goursat et al. (1976, 1981) have carried out extensive studies on the 

oxidation of AlON-based material at high temperature. The use of AlON-based 

materials has also been evaluated as refractory ceramics (Lepkova et al. 1996; Li 

et al. 2000). Table 2 summarizes a variety of representative properties for AlN, 

AlON, and a-AhO3. 

7 
Table 2. Representative properties of polycrystalline AlN, AlON, and C(-a203.a 

I Property AIONb &Ah03 

Hardness (GPa) 

3.26 3.711 3.98 

1 12 13.8 1.5 I I 

1 Young’s Modulus (GPa) I 308 1 3Oi-320 1 416 -- 

rength &Pa)’ 

I -- -~- I --_ 

I 400 I 228-307 I 380 I 

i 

Flexure St 

Compressive Strength (GPa) 

I --- 

1 1.540 1 NA 1 1.340 

Fracture Toughn ess (MPa ml/r) 1 3.0-4.0 ( 2.4-2.9 1 3.5 

Bulk Modulus (GPa) I 200 1 206-214 t 257 

Shear Modulus (GPa) 1 127-130 1 12S128 1 150-169 1 

I Poisson’s Ratio 1 0.24 1 0.25 1 0.23 1 

I Thermal Conductivitv fW/m “C) 

1 Coefficient of Expansion (/“C) 5.6 

1982. 

b35.7 mole-percent AIN material. 

‘Sapphire dielectric constants: parallel to c axis = 9.39; perpendicular to c = 11.58 (Harris 1999). 

8 
5. References 

Berriche, Y., J. Vallayer, R. Trabelsi, and D. Treheux. “Severe Wear Mechanisms 

in Al203-AION Ceramic Composites.” Journal of the European Ceramic Society, 

vol. 20, pp. 1311-1318,200O. 

Bti, H., G. Muller, and H. Wiggers. “Phase Composition, Oxygen Content, and 

Thermal Conductivity of AIN (Y2O3) Ceramics.” Journal of the American 

Ceramic Society, vol. 74, pp. 718-723,199l. 

Cazamias, J. U., I?. S. Fiske, and S. J. Bless. “Shock Properties of AlON.” 

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New York: Elsevier Science, pp. X31-188, edited by K. I?. Staudhamer, 

L. E. MLIIT, and M. A. Meyers, 2000. 

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Transformation of Pressureless Reactive Sintered AlON Ceramics.” Materials 

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Coble, R. L. U.S. Patent 3,026,210. 1962. 

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Ceramic Society, vol. 5, pp. 143-154,1989. 

Corbin, N. D., and J. W. McCauley. “Nitrogen-Stabilized Aluminum Oxide 

Spine1 (AlON).” Proceedings of SITE, vol. 297, pp. 19-23,198l. 

Djenkal, D., D. Goeuriot, and F. Thevenot. “Sic-Reinforcement of an a203 

AlON Composite.” Journal of the European Ceramic Society, vol. 20, 

pp. 2585-2590,200O. 

Dorignac, D., A. Mazel, Y. Kihn, J. Sevely, B. Aspar, B. Armas, and 

C. Combescure. “Transmission Electron Microscopy Studies of AlN.” JournuZ 

of the European Ceramic Society, vol. 14, pp. 345-353,1994. 

Dugger, C. 0. “The Single Crystal Synthesis and Some Properties of Ahm+num 

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Dumitrescu, L., and B. Sundman. “A Thermodynamic Reassessment of the 

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French, R. H. “Electronic Band Structure of Al203 With Comparison to AlON 

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9 
Fukuyama, H., W. Nakao, M. Susa, and K. Nagata. “New Synthetic Method of 

Forming Aluminum Oxynitride by Plasma Arc Melting.” Journal of the 

American Ceramic Society, vol. 82, pp. 1381-1387,1999. 

Goursat, P., P. Goeuriot, and M. Billy. “Contribution a L’etude du Systeme 

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Goursat, P., M. BiIIy, I?. Goeuriot, J. C. Labbe, J. M. ViIIeche-Noux, G. RouIt, and 

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Vapor Deposited Films of Al,O,N, on Silicon.” Journal of EZectronic Materials, 

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Ish-Shalom, M. “Formation of Ahuninum Oxynitride by Carbothermal 

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10 
Iwamoto, Y., A. Kuibira, I. Sugiura, and J. Tsubaki. “Effects of Powder Properties 

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Kim, Y. W., B. H. Park, H. C. Park, Y. B. Lee, K. D. Oh, and F. L. Riley. 

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KoIIenberg, W., E. Rymon-Lipinska. “Sintem von Ahuninum-Oxinitrad 

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Krishnan, K. M., R. S. Rai, G. Thomas, N. D. Corbin, and J. W. McCauley. 

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System.” Materials Research Society Symposium Proceedings, vol. 60, 

pp. 211-218,1985. 

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Japan, 1984. 

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1. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Structure and Properties of Aluminum Nitride and AlON Ceramics 18VVS4 

5. AUTHOR(S) 

tames W. McCauley 

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13. ABSTRACT(Maximum 200 words) 

4 brief review of aluminum nitride and AlON ceramics is presented in the context of the pseudobinary aluminum 

ritride-aluminum oxide phase equilibrium system. AlON is the name applied to the nitrogen stabilized cubic spine1 in 

his system, with a composition centered at 35.7 mole-percent AlN. First, the phase equilibria and crystal chemistry of 

hese phases are presented, focusin, 0 on a constant anion oxynitride spine1 model and the various phases, including 

)olytypoids, in this system. Then, a brief summary is given on the processing and microstructures of AlN and AlON, 

with comparisons to other microstructures in this system. Finally, selected properties of AlN and AlON are discussed 

urd tabulated, with special reference to alumina. A comprehensive reference list is given. 

14. SUBJECTTERMS 15. NUMBER OF PAGES 

:eramics, aluminum nitride, AlON, properties, processing 23 

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