Materials Science and Engineering, A 140 ( 1991 ) 549-553 549

Properties and performance of plasma-assisted physically vapor- deposited TiC coatings

K. Upadhya GE Medical A~vsterns, P.O. Box 414, M/C 7B-36, Milwaukee, W153201 (U.S.A.)

Abstract

Hard carbide coatings such as TiC, HfC, TaC and Cr7C3 have found numerous industrial applications. For example, TaC and SiC have been investigated as emissive coatings for X-ray tubes, TiC for ball- bearing and (Hf, Ta)C for space applications. In particular, chemically vapor-deposited TiC coatings have been reported to be extremely useful, because of their good wear resistance and very low co- efficient of friction. Also, owing to their high hardness and fine single-phase microstructure, TiC coat- ings can be polished to yield an extremely smooth surface. In this paper, the morphology and tribological properties of TiC deposited by the plasma-assisted physical vapor deposition technique have been reported. Also, the major functions of the plasma in the film deposition have been discussed.

1. Introduction

Because of the recent availability of plasma- generating devices and their reliable perform- ance, the application of these devices in coating deposition technologies is rapidly increasing. Plasma-assisted deposition techniques have wit- nessed a surge in the research activities for fabri- cation of thin films of superconducting materials, diamond or diamond-like carbon, cubic boron nitride, r-sic, and also borides, oxides, nitrides and carbides for wear-resistance, corrosion- resistance and protective hard coatings.

Hard coatings can be produced by a number of different techniques which can be broadly divided into two categories; (i) film deposition; (ii) surface modification. There are several film deposition techniques including plasma spraying, electroplating, fused salt electrolysis, chemical vapor deposition (CVD), physical vapor deposi- tion (PVD), plasma-enhanced CVD (PECVD) and plasma-enhanced PVD (PEPVD). Surface modification techniques include spark-hardening, laser-induced surface-hardening, ion implanta- tion and plasma flame-hardening processes.

However, in this paper, our discussion will be limited to only thin film deposition techniques. These techniques essentially are variants of CVD and PVD processes which rely on vapor trans-

port of materials to construct new surfaces. The mean free energy E of a gas atom with respect to temperature is given by

where k is the Boltzmann constant and T(K) is the temperature.

Thus vapor transport of materials for con- structing new surfaces by simply increasing the temperature is not promising, as it will raise the substrate temperature correspondingly. There- fore plasma-assisted ionization of the vapor atoms of the material that is to be used for coat- ings will provide an alternative suitable method for coatings. The ionized vapor species have greater reactivity and also they can be accelerated in the electric field; the combination of these two properties will provide a coating with better physical and mechanical properties.

Therefore the main theme for developing a powerful efficient coating process is clear: ionize the coating species and transport them to the surface to be modified in an electric field. When ionization of species is used for surface modifica- tion, then two different methods can be adopted for coating the substrate material. In the first option, a small ion flux with a high mean energy per ion is used while, in the second option, a high ion flux with sufficient mean energy per ion to

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TABLE 1

Main characteristics of the coating process

CVD Electro-deposition Thermal spraying

Mechanism of species production

Deposition rate

Species deposited

Throwing power for complex-shaped objects into small blind holes

Metal deposition

Alloy deposition

Refractory compound deposition

Energy of species deposited

Bombardment of substrate and/or deposit by inert gas ions

Growth interface (by external means)

Chemical reaction Deposition from solution From flames or plasma

Moderate Low to high Very high (200-2500 A rain- J)

Atoms Ions Droplets

Good Good No Limited Limited Very limited

Yes Yes, limited Yes

Yes Quite limited Yes

Yes Limited Yes

Can be high with Carl be high Can be high PACVD

Possible No Yes

Yes (by rubbing) Yes No

ensure good adhesion of the coating to the sub- strate is used. The first option is called "ion implantation", while the other is termed "plasma- enhanced deposition" processes, i.e. PEPVD, PECVD, ion beam mixing etc. It is worth noting that in both instances there is an upper limit to the energy flux which can be deposited on the surface to be modified if the substrate properties are to be kept unaltered. Table 1 lists the main characteristics of established and relatively newer coating deposition processes and Table 2 presents the major differences between the three main physical vapor deposition processes. Each of these above-listed techniques has specific applications to which it is best suited.

2. Experimental procedures and materials

The coating process involves placing clean coupons with 5-10 ~in surface finish in the fixtures which become the cathode of a high voltage circuit in the reaction chamber. The chamber is evacuated and filled with argon. A plasma is generated with argon as the carrier gas and the coupons are sputter cleaned for 20 min. An electron beam from a hollow-cathode discharge, which is similar in design and con- struction to that described by Morley [1} and

Bunshah [2], melts the titanium and Ti ÷ ions are transported to the surface of rear coupons where it reacts with carbon atoms from the decomposi- tion of C2H2 and forms the TiC film at the coupon's surface. The coated coupon is vacuum cooled and the surface is cleaned and polished. The optimum TiC coating thickness on the rear coupon has been established to be 5-7 am. All the deposited TiC had approximately stoichio- metric composition.

A schematic diagram of the apparatus system is shown in Fig. 1. The TiC deposition rate is

Vacuum , W o r k piece

, , , , , I / . IReact ive ---J I \ ~ , ' , ' " , 3 I ~ - -

_-t-

-~" Plasma _

.e..J Fig. 1. Schematic diagram of the apparatus.

TABLE 2

Major differences between evaporation, ion plating and sputtering

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Evaporation Ion plating Sputtering

Mechanism of species Thermal energy Thermal energy Momentum transfer production

Deposition rate Can be very high Can be very high Low except for pure metals (up to 75 ~m min-J) (up to 25 # min 1) (Cu, 0.2 ~m min 1)

Species deposited Atoms and ions Atoms and ions Atoms and ions

Throwing power for complex-shaped objects Poor; line-of-sight coverage Good but non-uniform Good, but non-uniform into small blind holes except for gas scattering thickness distribution thickness distribution

Poor Poor Poor

Metal deposition Yes Yes Yes

Alloy deposition Yes Yes Yes

Refractory compound Yes Yes Yes deposition

Energy of deposited Low Can be high Can be high species

Bombardment of substrate Not normally Yes Yes or no, depending on and/or deposit by inert geometry gas ions

Growth interface Not normally Yes Yes perturbation

Substrate heating (by Yes, normally Yes or no Not generally external means)

TABLE 3

Experimental parameters

Electron beam power 39 V, 380 A Focusing coil power 0.9 V, 90 A Plating bias 100 V, 6 A Sputtering power 500 V, 1.0 A C2H 2 flow rate 180 cm 3 min Plating temperature 450 °C Plating pressure (6-7) x 10 - s Torr TiC deposition rate 4-5/~m h-

approximately 4-5 ~m h-1. The main coating parameters are given in Table 3.

3. Results and discussion

According to the Hiigg rule [3, 4], the structure of transition metal carbides, nitrides, borides and hydrides is determined by the ratio R of the atomic radius of the interstitial element to that of the transition metal:

v~ R =

v~

where Vi is the radius of interstitial elements and V M is the radius of the transition metals.

If R<0.59, then simple NaCI or simple hexagonal structures will be formed. On the contrary, if R>0.59, the transition metals and interstitial atoms form complicated structures sometimes containing 100 atoms per unit cell. In the absence of any impurity, TiC and in fact all group IV (TIC, ZrC and HfC) and group V (VC, NbC and TaC) metal carbides possess B1 NaCi structures, while group VI (Cr-C, Mo-C and W-C) carbides have more complex structures. A representative micrograph of TiC deposited by PEPVD is shown in Fig. 2.

TiC films grown by PVD methods have been extensively studied particularly during the last 10 years. Bunshah and coworkers [5-8] have reported the structure and properties of the TiC films grown both by activated reactive evapora- tion and by direct evaporation of TiC billets. Also, several ion-plating and ion-sputtering tech- niques have been used to deposit TiC films [9-19].

From all these investigations, it is clear that the structure of carbide films is very complicated,

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TABLE 4 Physical and mechanical properties of TiC

Property Data

Compressive strength Tensile strength Young's modulus Poisson's ratio Microhardness Thermal expansion coefficient

1376-2958 MPa 241-275 MPa 447 x 10 3 MPa 0.19 37000 N mm -2 8xl0-6K-I

Fig. 2. Scanning electron micrograph of TiC deposited by PEPVD.

o

÷ Ti / 80 PC ("graph~e")

I

70

~. 60

5o

°ii 10 -5 10 10 "3 10 -2

CzH z PARTIAL PRESSURE {TORR)

Fig. 3. Composition of TiC as a function ofpc2H2.

especially for carbon-to-metal ratios exceeding unity• According to the phase diagram for Ti-C [4] a two-phase structure of TiC plus graphite should exist for these compositions• However, owing to the non-equilibrium structure of TiC and also because of the chemisorption of hydro- carbons, generally low density metal carbides (TIC, ZrC and HfC) films with free carbon located in grain boundaries may occur even for carbon-to-metal ratios of less than unity.

For the plasma-assisted reactive process (similar to the present investigation), usually the amount of carbon in the TiC films increases with

increasing flow rate or partial pressure of the reactive gas--in the present study C2H 2. A typical example is shown in Fig. 3. It can be clearly seen from this figure that, at a critical partial pressure of C2H2, decomposition of the hydrocarbons takes place in the plasma and carbon is deposited directly from the plasma. This establishes a narrow range close to the stoichiometric com- position where the TiC films rapidly change to a two-phase structure. For TiC films deposited by PEPVD, some of the excess carbon can also be incorporated interstitially as has been observed by several investigators [9, 10]. This was also observed in the present investigation, causing large lattice distortions of TiC and thereby increasing the hardness values far above the stoichiometric composition of TiC. The hardness for TiC deposited in the present study by PEPVD was found to be 37000 N mm -2.

For TiC films close to the stoichiometric com- position, the grain sizes are relatively small (less than 100 /am) for a deposition temperature below 600 °C. In the present investigation, the TiC grain size was in the range 0.1-0.2/am for a deposition temperature of 400°C. However, Bunshah and coworkers [5] observed a grain size of up to 4/am at a deposition temperature of 600 °C. These investigators also found a large dislocation density which caused a high intrinsic stress in the TiC films which was usually com- pressive in nature and had values in the range 107-109 Pa [11, 14, 16]. Table 4 lists the physical and mechanical properties of TiC films.

4. Applications

Because of its extreme hardness (37000 N mm-2), good wear resistance, low coefficient of friction against several metals and alloys, and fine single-phase microstructure, TiC coatings have emerged as a strong candidate for solid lubricants for high vacuum, high temperature applications.

Also, TiC, ZrC and HfC are currently being investigated as coating materials for SP-100 reactor bearing components. In X-ray tube bear- ing components [20] and in spacecraft bearings because of the space vacuum environment and contamination problems and extremely hostile environments, TiC- together with ZrC- and HfC- coated bearing components have provided many tribological solutions. In the literature, several applications of TiC-coated bearing components for space applications have been reported [21-24].

5. Concluding remarks

The need for hard coatings (group IV-VI carbides, nitrides and borides) for space applica- tions and hostile environments is rapidly increas- ing. As a result, the last decade has witnessed an amazing growth in the methods to produce a unique microstructure and therefore the physical and mechanical properties of these coating materials. These techniques have made increasing utilization of plasma, laser-, ion- and electron- beam-enhanced physical and chemical deposition processes. These newer techniques can be tailored to produce a unique microstructure for these coating materials which determines the physical properties of the deposited films. For example, in the present study, the plasma- enhanced physical deposition process was employed to deposit TiC at a very low substrate temperature of 350-450°C. This produced a non-equilibrium, highly stressed and extremely hard TiC film for the following reasons.

(i) Because of a high defect concentration and smaller grain size, the TiC coating was much harder.

(ii) Owing to the high growth rate of the film coupled with continuous energetic bombardment of the growing film, extended solubilities and non-equilibrium phase compositions were obtained.

(iii) The incorporation of Ar + during the growth of the TiC film generated high compres- sive stress levels in the deposited coating. Also,

553

substitutional and interstitial impurities may have contributed to the hardness of the TiC film.

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