Microstructure and mechanical properties of TiCAl2O3 coatings

Thin Solid Films. 118 (1984) 293-299 METALLURGICAL

AND PROTECTIVE

MICROSTRUCTURE COATINGS * R. C. BUDHANI,

293

COATINGS

AND MECHANICAL

H. MEMARIAN,

H. J. DOERR,

PROPERTIES

C. V. DESHPANDEY

School @‘Engineering and Applied Science, University q/California, (Received April 9.1984;

accepted

OF Tic-Al,O,

AND R. F. BUNSHAH

Los Angeles, CA 90024 (U.S.A.)

April 11,1984)

The microstructure, phase distribution and hardness of Tic-Al,O, two-phase coatings prepared by high rate physical vapor deposition have been studied as functions of deposition temperature and feed composition. Structural analysis using X-ray diffraction shows that the coatings consist of mA1,03 and TiC (cubic) phases. Transmission and scanning electron microscopy have been used to study the distribution of the two phases in the coatings. The growth morphology is fine grained at low temperatures and becomes a dense columnar type at high temperatures of deposition. The microhardness shows a corresponding increase with deposition temperature. Details of the relationship between the microstructure, composition and hardness of the coatings are reported.

1.

INTRODUCTION

Deposits of refractory metal oxides, carbides and nitrides are of considerable importance for wear and corrosion resistance applications’. These deposits are readily prepared by techniques such as activated reactive evaporation (ARE) and reactive sputtering of the respective metals or by chemical vapor deposition (CVD). On direct thermal evaporation or sputtering of the compounds, however, the material is not transformed to the vapor state as compound molecules but as fragments of various compositions thereof. The fragments are adsorbed onto the surface, diffuse on the surface and ultimately react to reconstitute the compound. For example, aluminum oxide dissociates on evaporation into aluminum and oxygen-rich fragments which then react on the substrate to form aluminum oxide. The stoichiometry of the deposit, therefore, depends on factors such as the deposition rate, surface mobility of the fragments, mean residence time of the fragments on the substrate, reaction rate for reconstitution and gaseous impurities present in the environment. The stoichiometry variations of the deposit affect its mechanical and physical properties. This problem becomes more acute when a composite coating of two compounds is being deposited. * Paper presented April 9-13, 1984. 0040-6090/84/$3.00

at the International

Conference

on Metallurgical

Coatings,

San Diego, CA, U.S.A.,

0 Elsevier Sequoia/Printed

in The Netherlands

294

R. C. BUDHANI

et al.

In this paper we deal with structural and mechanical aspects of (TiC),(Al,O,)ioO_, two-phase coatings prepared by thermal evaporation of composite billets. The hardness, microstructures, crystal structure and surface topography of the coatings have been studied as functions of composition and substrate temperature. 2.

EXPERIMENTAL

PROCEDURE

(TiC),(Al,O,),,,_, billets with x = 80,70,50 and 30 were made by thoroughly mixing high purity (more than 99.9%) powders and subsequent compaction in a hydraulic press. Films of 4-5 urn thickness were then deposited in vacuum onto stainless steel and molybdenum substrates by electron beam evaporation of the composite billets. The deposition was carried out at six temperatures, varying from room temperature to 600°C in steps of 100°C. A typical rate of deposition for a source-to-substrate distance of 10 cm was 2000 A s- ‘. For transmission electron microscopy (TEM) and diffraction studies, films about 1000 8, thick were deposited onto freshly cleaved NaCl crystals. TEM studies were carried out with a JEOL (100 CX) scanning transmission electron microscope operated at 100 keV. The surface topography and fracture cross section of the coatings were examined in the scanning electron microscope. Knoop microhardness measurements on the films were performed at 50 gf load. 3.

RESULTS

AND DISCUSSION

Figure 1 shows the X-ray diffraction patterns taken over a 28 range from 30” to 75” for a coating containing 80 at.%TiC. A comparison of the d spacings correspond-

74

70

66

Fig. 1. X-ray diffraction

62

58

54

50

pattern of (TiC),,(Ai,O,),,

46

42

38

34

films deposited at 400°C

30

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ing to the peaks in Fig. 1 with standard diffraction data on carbides and oxides of titanium and aluminum reveals that the coatings comprise cubic TiC and a-Al,O,. The formation of only these two compounds can be understood if we consider the possible decomposition modes of Al,O, and TiC during evaporation and subsequent recombination on the substrate. Al,O, can dissociate into aluminum, oxygenrich fragments and oxygen. Similarly, TIC can dissociate into atomic titanium and carbon. A heterogeneous reaction between the products of dissociation at the substrate during the condensation process may lead to the formation of Al,O,, Tic, AI,C, and various suboxides of titanium. The free energies of formation of AlzOJ, Tic, TiO and Al,C, at 600°C are -225 kcal mol-‘, - 180 kcal mol- ‘, -42 kcal mall ’ and - 15 kcal mol- ’ respectively. From a thermodynamic point of view, therefore, the probabilities of compound formation should be in the order Al,O,, Tic, TiOz, AI,C,. The formation of AlzO, and TiC in our films as indicated by X-ray diffraction is consistent with the thermodynamic arguments. The formation of TiC instead of TiO, is presumably due to the higher affinity of aluminum for oxygen which leads to the consumption of all the available oxygen. If other compounds are present, they amount to less than approximately 2%. Figure 2(a) and Fig. 2(b) are scanning electron micrographs of the surface topography and fracture section of two coatings with 80 at.% TiC deposited at 600 “C and room temperature respectively. The surface structure of both the films reveals randomly spaced growth flaws. X-ray chemical mappings for titanium and aluminum, shown in Fig. 2(c) and Fig. 2(d) respectively, do not show any elemental contrast at the positions corresponding to the flaws. This observation suggests that these defect structures contain TIC as well as Al,O, phases. A comparison of Figs. 2(a) and 2(b) also reveals that the flaw density is insensitive to the deposition temperature. The fracture cross section of the film deposited at higher temperatures shows a dense columnar microstructure growing perpendicular to the substrate plane. The fracture section of the films deposited at lower temperatures reveals a featureless structure. The fracture section also shows the growth morphology of the defects. Microscopy studies on several fracture sections showed that these defects originate at various locations along the thickness. This suggests that these structures nucleate at various stages of film growth. Similar defect structures have also been observed in crystalline and amorphous films prepared by evaporation2 and CVD processes3. The “cauliflower’‘-type growth morphology of these features indicates that small particles of the evaporant materials were spattered from the source and acted as nucleation centers for the abnormal growth. To explore the validity of this hypothesis, films approximately 1000 A thick of (TiC)s,(A1203)2, were deposited onto NaCl crystals at 400°C and subsequently examined in the transmission microscope. Figure 3 shows a transmission electron micrograph and corresponding electron diffraction pattern of the films. The micrograph shows small high density regions, almost like particles, uniformly dispersed on a relatively smooth background. These high density regions can subsequently act as nuclei for the abnormal growth. Figures 4(a)-4(d) are a series of scanning electron micrographs of the coatings deposited at 600°C from billets with decreasing Tic content. It can be seen from the micrographs that the defect density in the films decreases with decreasing Tic

Tic-Al,O,

297

COATINGS

Fig. 3. Transmission electron micrograph (TiC),,(AI,O,),, deposited at 400 “C.

and corresponding

electron

diffraction

pattern

of

content. The observation suggests that the defect growth in the coatings originates from TiC particles. Figure 5 shows the variation in the Knoop hardness of the films as a function of deposition temperature and evaporant concentration. The microhardness of (TiC),,(Al,O,),, coatings increases with increasing deposition temperature. The variation in the microhardness with composition for a fixed deposition temperature (600 “C) shows an increasing trend with increasing TiC concentration. The increase in the hardness of the films with deposition temperature can be understood in terms of the microstructure. As can be seen from the fracture cross sections (Fig. 2(a)), the microstructure of films deposited at high temperatures shows a highly dense columnar structure. These growth features are similar to the zone 2 structures proposed by Movchan and Demchishin4 for single-phase materials and subsequently extended by Thornton’. The films deposited at low temperatures, in contrast, exhibit a zone 1 structure. In most of the single-phase ceramic coatings a sudden increase in the hardness is observed on transition from zone 1 to zone 2

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BUDHANI

et al.

structures. However, the hardness data in Fig. 5 show a linear dependence on the deposition temperature. The linear variation is presumably because the transition from zone 1 to zone 2 is not abrupt in two-phase ceramic deposits. The increase in

(4 Fig. 4. Scanning electron micrographs of the surface topography TiC concentrations of (a) lOOO,/,,(b) 70%,(c) 50% and(d) 30%. TIC

IB Y z

4000

-

3000

-

2000

-

1000

-

80

70

I

I

of the coating

deposited

at 600 “C with

(%) ENTENT 50

40

I

I

I

30

1

2 P 2 : 4 Y

/ 200

L 300 DEPOSITION

1

I

I

400

TEMPERATURE

I

600

500 (‘Cl

coatings as a function of deposition temperature. Fig. 5. Knoop microhardness of (TiC),0(A1,03)20 plot also shows the concentration dependence of the microhardness for films deposited at 600°C.

The

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299

the Knoop hardness of the films with increasing TiC concentration can be explained if we consider the hardness of TiC and Al,O, films prepared under similar conditions. Movchan and Demchishin4 have measured the hardness of evaporated Al,O, coatings. Their values for a substrate temperature of about 600 “C is about 100 kgfmm-‘. Measurements by Raghuram and Bunshah on TiC coatings deposited at 600 “C by the ARE process show a hardness of about 4000 kgf mm-*. The concentration dependence of the hardness as shown in Fig. 5 thus follows the law of mixtures. 4. CONCLUSlONS (1) Two-phase coatings consisting of cubic TiC and a-Al,O, have been successfully deposited by electron beam evaporation of composite billets. (2) The coatings show evenly spaced growth flaws with a characteristic “cauliflower’‘-type growth morphology. The size and distribution of the flaws decreases with decreasing TIC content. (3) The hardness of the coatings increases with increasing temperature of deposition. The hardness increase at higher temperatures of deposition is because of a dense columnar microstructure. The compositional dependence of the hardness follows the law of mixtures. ACKNOWLEDGMENTS

This work was supported by Sumitomo Electric Industries Ltd., Japan. The interest and encouragement of Dr. A. Doi is sincerely appreciated. REFERENCES 1 R. F. Bunshah (ed.), Deposition Technologies for Films and Coatings, Noyes Publications, Park Ridge, NJ, 1982. 2 D. H. Boone, T. E. Strongman and L. M. Wilson, J. Vat. Sci. Technol., II (1974) 611. 3 J. C. Knights, J. Non-Crysf. Solids, 35-36 (1980)159. 4 B. A. Movchan and A. V. Demchishin, Fiz. Met. Metalloved., 28 (1969) 653. 5 J. A. Thornton, Annu. Rev. Mater. Sci., 7 (1977) 239. 6 A. C. Raghuram and R. F. Bunshah, J. Vat. Sci. Technol., 9 (1972) 1389.