Variations in nanostructure and composition for controlling the interfacial properties of metal matrix composites and ceramic matrix composites

MATERIALS S~IEllE & ENGINEERING ELSEVIER

Materials Science and Engineering A204 (1995) 135-139

A

Variations in nanostructure and composition for controlling the interfacial properties of metal matrix composites and ceramic matrix composites J.T. McGinn a, B. Singh a, T. Mukherji b ~David Sarnoff Research Center, CN 5300, Princeton, N J, USA bAllied Signal Engines, 550 Main Street, Strat[~rd, CT 06497, USA

Abstract

Interface properties are critical to the strength and toughness of metal matrix composites and ceramic matrix composites. These interfaces provide both diffusion barriers and load transfer functions. The nanostructure across the interface was varied to fulfill conflicting diffusion and load transfer demands. The deposition conditions developed allow nanostructure control of TiN fiber coatings. The TiN coating varied from a dense, diffusion-limiting layer to a columnar bond debond layer. TiN was deposited in a reel-to-reel, cylindrical magnetron coater. The fiber bias and gas flow rates were the dominant deposition parameters controlling the nanostructure. Nanostructure changes were examined after consolidation in a Ti matrix and additional heat treatments at 1000 °C.

Keywords: Interfacial properties; Titanium; Heat treatments

1. Introduction

High temperature performance demands placed on advanced metal matrix composites (MMCs) and ceramic matrix composites (CMCs) have centered significant attention on the fiber interface. It is generally accepted that interface control is critical to mechanical success under extreme conditions. Three generic attributes demanded of a fiber-matrix interface are (1) preventing deleterious fiber-matrix interactions, (2) controlling fiber-matrix debonding characteristics, and (3) diminishing the negative effects of mismatch between the fiber and matrix coefficients of thermal expansion (CTE). Limiting mass transport across the interface prevents deleterious fiber-matrix interactions by slowing interdiffusion and undesirable phase formation. Often associated with embrittlement and fiber depletion, such phases form during high temperature consolidation or extended periods under lower temperature operating conditions. Proper bonding insures crack deflection along the fiber-matrix interface rather than shearing the fiber in the crack plane. This is particularly true of CMCs 0921-5093/95/$09.50 ~ 1995 - Elsevier Science S.A. All rights reserved S S D I 0921-5093(95)09949-2

where strong fiber matrix bonding results in a single fracture plane through fiber and matrix. Fracture surface deflection increases the crack propagation energy. Additional energy required to pull debonded fibers from matrix cavities further toughens the material. The effects of CTE mismatch arise owing to abrupt changes in material properties at the fiber matrix interface. Finite element modeling has shown hoop stresses are displaced from the interface by coatings with properties that vary gradually between the fiber and the matrix [1]. These demands are in conflict. Limiting fiber-matrix interactions requires a high temperature barrier with low diffusivity. Suitable materials typically contain few internal diffusion paths (e.g. voids, grain boundaries and dislocations), have close packed structures, and often display covalent bonding. Conversely, crack deflecting layers benefit from high densities of stress-risers and crack-initators. CTE mismatch layers containing materials foreign to the interface increase the chemical complexity. Multilayer coatings represent a solution to those conflicting demands. Typical multilayer coatings containing distinct chemical layers add to interface corn-

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plexity and increased manufacturing cost. An alternative, that has received little attention, is the use of nanostructure to alter the properties of successive coating layers. In this approach, deposition would vary, forming a dense, low-defect density diffusion barrier and a porous, crack-deflecting layer in a single coating. Columnar grain removal from diffusion barriers is important. These grains are commonly formed by many deposition processes. The associated grain boundaries offer rapid diffusion paths. Loosely packed grains and voids offer the possiblity of modifying the bond strength across the crack-deflecting layer. Additional compatibility can be introduced if coating layers incorporate transition regions with material properties varying gradually from fiber-like to matrix-like. This requires coating-material properties that vary continuously with composition over an extended phase field. Titanium nitride possesses such an extended phase field with a Ti-to-N ratio varying from 28 to 55 at.%. Our goal was to develop a single material coating with nanostructure variation suitable to accomplish the three functions of a fiber matrix interface. This work demonstrates the ability to change the nanostructure by depositing symmetric coatings with layers varying from columnar to equiaxed and back to columnar. This symmetry insures that a layer's nanostructure is not predetermined by the structure of the preceding layer. A cylindrical magnetron, shown schematically in Fig. 1, was used to deposit all coatings. The cylindrical mangetron was developed at Sarnoff specifically to coat fibers. The procedure to control the nanostructure and vary the coating composition has not previously been demonstrated on fiber-like substrates.

~

~

Pump Flange

System Baseplate

Diognosfic langes

Fig. 2. Top view of the reel-to-reel system developed for coating long fiber lengths.

2. Experimental details

2.1. Experimental system Although variations in nanostructure and composition have applications to both MMCs and CMCs, only MMC matrix materials was explored. The composite studied for this work was a Ti-1100 matrix with embedded SCS-6 fibers from Textron Specialty Materials. Rhodes characterized the interface formed in this system, and strong fiber-matrix interactions were found [2]. Titanium nitride was chosen as the coating. Titanium nitride has strong covalent bonds and a resultant low self diffusion coefficient. It is a highly effective diffusion barrier for integrated circuits (ICs) [3]. Our choice of this system was guided by previous expericence and the large literature base relating deposition conditions and nanostructure.

2.2. Coating deposition, cylindrical magnetron

FeedthrougbTube

Wate,~od R~ pewe,)

Fig. 1. Cylindrical. PECVD reactor module used for d e p o s m o n ot multi-layer coatings onto moving SCS fibers. Three of these source modules were installed in the base of the vacuum chamber.

Coatings were applied to SCS fibers using a specially designed cylindrical magnetron sputtering source shown in Fig. 1. The system has three modules that can deposite multiple layers or undertaken fiber pre-cleaning. Fig. 1 shows a cross-section of a sputtering module and Fig. 2 is a top view of the deposition system. The cylindrical magnetron provides uniform, highrate coating capability for SCS-6 fibers. The fiber is centered in each module by the fiber transport system. The sputtering target is cylindrical and arranged as shown in Fig. 1. Plasma is generated using r.f. (13.56 MHz) excitation and a capacitively coupled " L " type impedance matching network. Commerically available Plasma Therm, Inc., r.f. power supplies and matching networks were also used.

J.T. McG#m el al.

Materials Science and Engineering .4204 (1995) 135 I39

The source configuration in Fig. 1 embodies some unique construction features enabling the cathode or "target" to be water cooled and rapidly changed. The sputtering module's diameter is 2.0 inches and the active region's length is approximately 3.0 inches. The device employs an auxiliary anode and a proprietary fiber biasing capability. Fig. 2 shows the top view of the coating system. Computerized gas flow meters and pressure controllers are used to introduce and control the process gas mixture, flow rate, and pressure in real time. The specially designed, in situ reel-to-reel fiber transport system is computer controlled. The control algorithm enables selection of fiber speed, duration of specific fiber sections in the coating zone, number of repeated back and forth steps, etc, A stepper motor is used for the transport movement. The combination of the stepper motor and the computerized control system enables selected fiber sections to be positioned accurately in the coating zone. To ensure reliable operation of the transport, the tension and synchronization of the take-up and source spools had to be carefully controlled. Thus the fiber transport system required considerable development and optimization to make it suitable for monofilament SCS fibers. 2.3. Parameter space explored

Graded titanium nitride coatings were achieved by varying the N, flow during deposition. A single fiber pass through the coater was made at a fixed N2 flow rate. The N, flow rate was then altered and a subsequent pass made. This process continued until the desired structure was achieved. A pure titanium layer was deposited in initial and final passes to promote adhesion. Argon was used as a carrier gas. The coatings were deposited using multiple sources. The parameters used for the experiments are shown in Table 1. Since the SCS fiber are sufficiently conductive, a d.c. bias can be employed. The d.c. bias effects the composition of the TiN films. This requires calibrating the deposition conditions for each bias used. Calibration runs were analyzed for composition and nanostructure Table 1 Parameter space investigated Range High density Columnar explored coating coating Bias (V) Power (W) Pressure (gm Hg) Flow ( A r + N2) (standard c m ' min ~) N 2 flow (standard cm ~ rain

~)

0 400 100 700 5 20 100

80 400 20 100

0.5 5

2

40 400 2{) 100 0.5 2

137

Table 2 Gas flow rates and resultant Ti:N ration for deposited film Ar flow (standard cm ~ m i n ~)

N 2 flow (standard cm 3 rain

99.5 99.0 98.0

0.5 1.0 2.0

I)

Ti-to-N ratio 0.90 0.57 0.52

Cylindrical magnetron processing parameters, shown in Table 1, are similar to traditionally planar magnetron values. 2.4. Consolidation

A standard consolidation procedure was used to embed both coated and uncoated SCS-6 fibers in a Ti-1100 matrix. Sheets of Ti-ll00 was etched in 0.5% HNO3 1.0% HF, 98% H 2 0 , rinsed in double distilled H 2 0 with a final rinse in ethyl alcohol. A foil fiber foil consolidation scheme was used with a consolidation frame providing laterial confinement. Frame and sample were evacuated to 5 x 10 (' Torr and heated to 1000 °C. The system was brought to atmospheric pressure under argon flow. The consolidation force and temperature were raised slowly to 7000 psi and 1000 °C respectively. Consolidations continued for 2 h. Subsequently, the consolidation force was removed slowly and the sample cooled under argon. Strips were cut from the consolidated coupon for separate anneals of 3 and 9 h at 1000 °C. 3. Results

The effects of various magnetron and process variables on thin film nanostructure and compositon were studied. The selected range for each parameter was based on previous experience. The parameter's ranges and the optimum value for a titanium nitride film with a compact nanostructure and a golden-yellow color, typical of a stoichiometric TiN, are given in Table 1. Table 2 gives the titanium-to-nitrogen ratios for coatings deposited at - 8 0 V bias as determined by electron microprobe analysis. Fiber bias was the dominant parameter controlling the coating's nanostructure. Colunmar or densely packed nanostructures could be deposited over a wide range of nitrogen flow conditions by fixing the fiber bias. The most equiaxed coatings were deposited with a bias of - 80 V. With increasingly positive bias, intermediate and eventually columnar nanostructures resulted. The equiaxed and columnar structures are shown in Fig. 3(a) and (b). Biases more negative than - 8 0 V caused a decrease in deposition rate. The coating adhesion deteriorated with increasing negative bias. Coatings deposited

138

J.T. MeGinn et al. ,' Materials Science and Engineering A204 (1995) 135 139

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lOgm

(a)

(b)

Fig. 3. With increasingly negative bias, the deposited titanium nitride's nanostructure changes from columnar to equiaxed: (a} bias -40 V, columnar nanostructure with grain boundaries passing through the layer and acting as rapid diffusion path; (b) - 8 0 V bias, equiaxed grain boundary structure. with a - 8 0 V bias lacked sufficient a d h e s i o n to survive c o n s o l i d a t i o n . F i b e r - c o a t i n g a d h e s i o n was i m p r o v e d by d e p o s i t i n g the first nitride layer with a - 4 0 V bias. To retain c o a t i n g s y m m e t r y , the final nitride layer was also d e p o s i t e d at - 4 0 V bias. All internal layers were d e p o s i t e d at - 8 0 V bias. T i t a n i u m - t o - n i t r o g e n ratios are shown in T a b l e 2. All c o a t i n g s were d e p o s i t e d with a - 8 0 V bias. C o m p o s i tions n e a r s t o i c h i o m e t r i c T i N are d e p o s i t e d at the lowest nigtrogen flow rate. A t the highest nitrogen flow rate, the c o m p o s i t i o n a p p r o a c h e s stoichiometric Ti2N. Crystalline phases were identified by X - r a y diffraction. G r a p h i t e , r - S i C , a n d fl-SiC, f r o m the SCS-6 fiber, were always detected. C o a t i n g phases associated with a - 8 0 V bias a n d v a r i o u s flow rates are listed in T a b l e 3. Oxides, when observed, were associated with residual gaseous oxygen in the s p u t t e r i n g c h a m b e r . T i N was f o u n d for all gas flow c o n d i t i o n s while Ti2N was observed sporadically. The final c o a t i n g c o n t a i n e d eight layers differing in

t i t a n i u m - t o - n i t r o g e n ratio a n d bias. T o i m p r o v e the a d h e s i o n further, an initial layer o f t i t a n i u m was deposited. C o a t i n g s y m m e t r y was m a i n t a i n e d with the final four layers d e p o s i t e d in reverse o r d e r to the first four. D e p o s i t i o n c o n d i t i o n s for each layer are given in T a b l e 4 a n d the resultant structure is shown in Fig. 4. A s - c o n s o l i d a t e d samples d e m o n s t r a t e d g o o d protection o f the fiber m a t r i x interface. Etching o f polished sections revealed a c o m p l e x structure a r o u n d c o a t e d fibers after c o n s o l i d a t i o n . Fig. 5 shows a cross-section after a 3 min etch in I% H N O 3 , 2% H F , 97'70 H 2 0 . Q u a l i t a t i v e A u g e r analysis i n d i c a t e d interdiffusion across the t i t a n i u m g r a p h i t e and T i N m a t r i x interfaces. TiC h a d f o r m e d at the t i t a n i u m - g r a p h i t e interface. The trend seen in Fig. 5 was t o o thin for A u g e r analysis b u t from its etching characteristics it is surmised to be u n r e a c t e d titanium. Table 4 Deposition parameters for symmetric coating layers Layer

Table 3 Crystalline phases identified by X-ray diffraction, all layers deposited under 80 V bias Flow rate (standard cm3 min ~) 0.5 N

2,

99.5 Ar

TiN

Ti2N

x

X

1.0 N 2, 99.0 Ar

X

2.0 N2, 98 Ar

X

TiO,

x

Bias (V)

1

0

2 3 4 5 6 7 8

-40 80 -80 80 80 -40 0

N, flow (standard cm3 min i)

AF flow

0

100

0.5 1 2 2 1 0.05 0

99.5 99 98 98 99 99.6 100

(standard cm3 min " J)

J. 7". MeGinn el al./ Materials" Science and Engineering A204 (1995) 135 139

iiiii

139

! !~ il iii

!i

lOgm Fig. 4. Symmetric coating showing variation in the nanostructure and composition of titanium nitride layers. The first and last layers of the deposition are pure titanium.

4. Discussion and conclusions

Simultaneous nanostructure and composition control of a single-phase system deposited onto cylindrical fiber surfaces has been demonstrated. This capability offers the possibility of a single material system meeting multiple coating demands. While nitrogen diffusion into the matrix clearly indicates that titanium nitride is unsuitable as a coating in the SCS-6/Ti-ll00 system, the principles demonstrated can serve as a useful step in designing a system that possesses the required coating attributes. Fiber bias is the principle parameter for controlling the deposition nanostructure. Coatings change from columnar to equiaxed as the fiber bias is changed from 0 V to - 80 V. It is believed the bias supplies additional energy to arriving ions and promotes local rearrangment of the ions into the deposited film. Extensive work on ion-assisted deposition of very high performance coatings has shown optimum properties for films bombarded with reative species having energies of a few tens of electronvolts [4,5]. It appears the key role of ion b o m b a r d m e n t during growth is imparting adatom mobility to the condensing species. This enhances epitaxial growth, increases film density by eliminating columnar growth, reduces film stress, and improves many other desirable film properties [5]. Typically, it is desirable that impinging ions have energies approximately equal to the growth material's chemical bond strength. Ener-

--.-,-

20pm

Fig. 5. SEM image of coated fiber after consolidation. gies in excess of few hundred electronvolts, however, can cause structural and crystalline imperfections in the deposited layers. An optimum bias or ion energy bombardment level must be determined empirically. Varying the coating composition from fiber-like to matrix-like allows tailoring mechanical properties across the fiber-matrix interface. Matching CTE and elastic properties across the coating are attractive possibilities. Clearly titanium nitride is not suitable for the SCS-6/Ti-ll00 system owing to the rapid diffusion of nitrogen in Ti-1 100. The work does demonstrate simultaneous nanostructure and composition modification for fiber coatings using a cylindrical magnetron and standard sputtering practices.

References

[1] M.E. Labib, H. Merrick, L.S. Hu, C.-Y. gai and B. Singh, Graded composition fiber coatings for intermetallic matrix cornposits, NASA Conll Publ. Proe. HITEMP Review 1991, Advanced High Temperature Engineering Materials Technology Program, Westlake Holiday Inn, Cleveland, OH, October 1991. [2] C.G. Rhodes, Study of titanium-matrix composites, Teehnical Report SC528& iFR, 1982 (Rockwell-International Science Center, Thousand Oaks, CA). [3] N. Kumar, K. Pourrezaei, J.T. McGinn, B. Lee and E.C. Douglas, TEM study of brown golden titanium nitride thin films in VLSI diffusion barriers, J. Vac. Sci. Teehnol. A, 6 (1988) 1602. [4] P.J. Martin, H.A. Macleod, R.P. Nenerfield, E.G. Packey and W.G. Sainty, Assisted deposition of optical films, App. Opt., 22 (1983) 178. [5] R. Messier and Y.E. Yehoda, Investigation of film microstructures in thin flms, J. App. Phys., 58 (1985) 3739.