Low-temperature deposition of titanium nitride

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COATIN6S H6JIAIOLOT Surfaceand Coatings Technology76-77 (1995) 815-820

Low-temperature deposition of titanium nitride Anthony J. Perry, James R. Treglio, Albert F. Tian ISM Technologies Inc., 9965 Carroll Canyon Road, San Diego, CA 92131, USA

Abstract

A new technique for depositing hard, dense, well-adhering TiN with excellent adhesion is described. Taking advantage of the high degree of ionization of the material emitted from a cathodic arc source, a short, very high voltage pulse (the order of 5-20 kV for 1-3 gs at a frequency of some 1-2 kHz) is applied to the substrate in addition to the usual d.c. bias during reactive deposition. The high bias accelerates the ions located within the sheath during the short pulse and their momentum modifies the properties of the coatings as indicated above. The technology, termed Hyper-Ion, is readily retrofitted to existing physical vapor deposition systems equipped with cathodic arc sources. The present work presents results on titanium nitride deposited onto temperaturesensitive materials including low alloy steel, and aluminum 6061-T6 at a substrate temperature of 150 °C. The coating has a type-T microstructure and the substrates do not lose mechanical strength. Keywords: Low temperature deposition; Titanium nitride; Hyper-ion; Cathodic arc; Short high voitage pulse

1. Introduction

In current industrial physical vapor deposition (PVD) technology, coatings of TiN and similar hard coatings are deposited on substrates such as cemented carbide and high-speed steel tools. It is usual to select the deposition temperature to be as high as possible to ensure a dense microstructure and good adhesion. As high speed steels are tempered at temperatures of the order of 550-600 °C, PVD coating is normally done at about 450 °C with a process floor-to-floor time of the order of 3 h. The same coating deposition conditions are often used for cemented carbide where the temperature is low enough that there is no carbon loss from the substrate to the coating such as occurs in chemical vapor deposition (CVD) at temperatures of the order of 950 °C, and which leads to loss of transverse rupture strength. As the group IVB metals are very reactive and known "getters", TiN and ZrN coatings can be formed readily at much lower temperatures than these, low enough indeed that they can be used for decorative purposes on substrates such as chrome-plated brass and die-cast zinc parts• The coatings deposited at the low deposition temperatures required by such materials (chrome plating on die-cast zinc will 'bubble' due to zinc vapor generation below 100 °C) do not display the excellent wear resistance of the tool coatings because the microstructure changes from the dense columnar-type T (using the ~,34[,"/ £Q"/'3/Q~/~NQ ~N ~ 1QC~ " ~ l ~ v i ~ r £ei~nea ,£ A All r ~ h t ~ reserved

Thornton microstructure designators F1]) to the opentype I with its intergranular voids. Although it was long since established that the application of a negative electrical bias to the substrate reduces the transition temperature between the zones, bias alone cannot reduce it far enough to achieve zone T microstructures in the coatings at temperatures in the range 160-180 °C. As a result of this lack of wear resistance, constructional parts made from aluminum alloys, some of which soften at 160 °C (and some at lower temperatures still), cannot be coated using current technology. Also, tools made from low alloy steels, which are tempered in this range of 160-180 °C, cannot be used without a post-deposition heat treatment, which carries an economic sacrifice and the risk of tool distortion as associated with the volume changes which accompany the heat treatment of steel. At the present time the lowest practical deposition temperature for wear-resistant TiN coatings is of the order of 190-250 °C, depending on the application and coating center, and more specifically, the point loading on the part in service, i.e. the property requirements in specific applications. An additional energy input in the form Of ion bombardment during deposition (termed generically ionbeam-assisted deposition (IBAD) [-2]) has been used to improve the microstructure, reduce the residual stress, and enhance the wear-resistant properties of coatings [-3]. This follows from the effect of binary collision cascades [-4,5] produced by the ions which cause a

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A J Perry et at/Swface and Coat#zgs Technology 76-77 (1995) 815-820

reordering of the atoms being deposited, Ieading to denser microstructure and reduced intrinsic stress. A disadvantage of the present IBAD methods is the complexity of industrial deposition equipment [6] because (i) the ion beam should be near-congruent with the depositing material to maximize its effect and to minimize ion beam sputtering, and (ii) the need for additional vacuum pumping capacity if a gas ion-beam source is used. As a result of these considerations, there is a clear need for a simpler approach which is applicationoriented and suitable for complex parts. A direct approach to solving these limitations is provided by the Hyper-Ion technology discussed here, where the energy input is provided by a high-voltage pulsed bias applied to the substrate in addition to the usual d.c. bias; no ion source is needed and the additional pumping requirement is eliminated because the ion bombardment is provided by the depositing ions themselves. A significant aspect of this development is that it can be readily fitted to much existing industrial PVD deposition equipment.

and are comparable with those used in industrial ion implantation (which is generally done in the range 30-80 kV) because the ions must penetrate to a depth of several atomic layers. The frequency of the H-I pulse is set sufficiently high so that there is no electrical breakdown to allow arcing to the plasma or vacuum enclosure, i.e. the order of kHz with a pulse length in the microsecond range, giving a duty cycle typically of the order of 0.1%. The Hyper-Ion pulse acts only on this fraction of the incoming ions, thus the conditions can be defined and set for optimum performance with respect to microstructure and residual stress. Further, as any sputter loss from the coating can only occur during the high voltage pulse, this is negligible. Finally, the effect of the presence of nitrogen in the chamber needs be taken into account in defining the deposition conditions because the ions emitted by the arc cathode can be neutralized by collision en route and thus no longer be influenced by the H-I field. This effect is known in standard PVD technology where the presence of a reactive gas reduces the residual stress in the coating to an extent depending on the operating pressure in the chamber [ 11].

2. Hyper-Ion technology

3. Experimental ISM Technologies has developed a technology, termed Hyper-Ion (H-I), for the non-reactive deposition of coatings such as TiB2 and TiN starting from these as the cathodes for arc evaporation [7,8], and applying short, very high-voltage pulses to the substrate during the deposition process, which has also been studied by others [9,10]. In the present work we report the extension of our work to the reactive deposition of coatings such as TiN from titanium cathodes through a lowpressure nitrogen atmosphere. The material evaporated by a cathodic arc source is very highly ionized, and so these ions can be influenced by an electric field. Thus, applying a pulsed high-voltage bias to the substrate being coated has the effect of accelerating the ions within the cathode sheath to the energy levels required for implantation, leading to collision cascades and the densification and stress relief of the coating. Clearly, these ions are part of the depositing material, and comprise those ions found within the sheath over the time interval that the pulse is applied to the substrate. In between these pulses, the reactive deposition of the coating proceeds as usual. In should be added that the H-I technology can be applied to any system where sufficient ionization of the coating vapor or working gas is present, e.g. either using cathodic arc evaporation where the depositing material is highly ionized, or unbalanced magnetron deposition where the argon supporting the magnetron discharge is ionized. The acceleration voltages required by the Hyper-Ion process are generally in the range ( - ) 5-20 kV. These are well above those used in current PVD technology

The substrates studied in the work were primarily 440C steel and 6061-T6 aluminum (i.e. tempered or aged at 175 °C), both chosen because of the sensitivity of their mechanical properties to thermal exposure. In addition, stainless steel and titanium-6Al-4V samples were coated. As titanium alloys can be coated at the more usual PVD deposition temperatures, it needs to be added that the interest here is related to the properties of the native oxide. At ambient temperatures this is amorphous, changing to the crystalline anatase phase and then the harder rutile phase as the temperature is raised. As the amorphous form is most easily removed by ion bombardment, the use of H-I technology in this case has practical and economic advantages also. Coatings were deposited from an unfiltered titanium cathodic arc source 100 mm in diameter operated in a pulsed mode to give a time-averaged current set in the range 45-70 A. This is a low current in terms of standard industrial tool-coating PVD practice which normally operates at the higher currents needed to cause cathode spot splitting which reduces the macro-particle count. The pressure of nitrogen was set in the range 14-20 mTorr. The substrates were mounted on a hot plate as in our previous work, with a source-to-substrate holder distance of 400 ram, preheated to 120 °C, and followed by a sputter cleaning step. If, during the deposition the temperature exceeded 150 °C, deposition was stopped until the temperature fell to about 130 °C. As noted below, the H-I was effective in removing any native oxide on the substrate surface, so it is considered

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that any impurities absorbed on the coating surface during such a cool-down would be rapidly removed during subsequent deposition under the action of the H-I high voltage. The substrate bias applied was set at a standard - 7 5 V d.c., which is consistent with industrial practice and is well beyond the lower (negative) bias range where a high residual stress is observed [12,13]. The HyperIon bias was set at typically - 5 kV in the runs reported here because of limitations in the power supply used initially; runs in the bias range 10-20 kV have also been carried out. The H-I pulse length is typically 1-3 las at a frequency of 1-2 kHz.

4. Results 4.1. Substrate properties

The tensile and four-point bend test properties of both the 440C steel and the 6061-T6 aluminum were measured before and after coating with approximately 2 pm of TiN using the H-I process. In the case of these

samples, the bias applied to the substrates during deposition was - 7 5 V d.c. and the H-I parameters were set at - 5 kV, 3 ps and 1 kHz (i.e. a duty cycle of 0.3%). The tensile properties of the coated (average of five samples) and uncoated (average of two samples) materials were tested using flat tensile samples with gauge length and width of 1.7 inch and 0.5 inch, respectively (43.2 mm and 12.7 mm, respectively); the loading rate was 60 ksi rain -1 (42.3 kg min-1). The results are listed in Table 1 from which it is seen that there is no significant change in properties of the steel after the coating process, but some softening of the aluminum. Similarly, fourpoint bend tests were performed on flat samples of length and width 2 inch and 0.25 inch, respectively (50.8 mm and 6.4 mm, respectively), with loading and support point spacings of 0.875 inch and 1.75 inch respectively (22.2 mm and 43.2 mm, respectively). Two replicates were tested in each case, for coated and uncoated material. Again, no change due to the coating process was found in the steel but some softening was found in the 6061-T6 aluminum samples, as before. Typical four-point bend test curves measured on this material (with a crosshead rate of 0.2 inch min -1, i.e.

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A.Z Perry et aL/Szoface and Coatings Technology 76--77 (1995) 815-820

5 mm min -1) are shown in Fig. 1, where the change in the substrate mechanical properties is apparent. We are advised that this change is not significant.

4.2. Coating properties Typical scanning electron micrographs are shown in Figs. 2(a) and 2(b) as taken from coatings made with the same deposition geometry and under identical deposition conditions (except for the application of the H-I bias). These show about 3-4 gm of TiN deposited onto 6061-T6 aluminum at a substrate temperature of 150 °C, without and with H-I, respectively. The change in microstructure from type I to type T is evident from the more dense microstructure and, more particularly, from the change in surface morphology. These are, excluding the macro-particles coming from the unfiltered source, very reminiscent of the microstructure and surface of a coating made at 450 °C by industrial low-voltage, highcurrent ion plating 1-14]. It is clear that the present coatings need to be studied by transmission electron microscopy to allow the role of the H-I to be understood in more detail. A fractograph of a (microstructurally and crystallographically) amorphous TiB2 coating on a titanium 6A1-4 V substrate made under the same substrate temperature and H-I conditions is shown in Fig. 2(c) for comparison purposes. The composition of selected typical coatings was confirmed as being stoichiometric using Auger electron spectroscopy (AES), when made with and without H-I. The microhardness of a coating made with H-I was measured as 2100 _+200 Hvo.ozs. The residual stress was measured by laser reflection from coated thin stainless steel shim-stock, as in previous work ['8]. The results without H-I were typically a compressive stress of the order of 5 GPa. The use of H-I reduced this to the range 0.9-2.9 GPa, depending on the profile of the pulse; this is to be studied in more detail.

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4.3. Substrate-coating intelface The scratch adhesion testing was carried out on a number of coated samples which had been coated with and without the use of H-I. These were stainless steel substrates coated with TiN at 150 °C; this substrate was chosen to allow comparison with existing data [14] measured on samples made with industrial-standard low-voltage, high-current ion plating with a substrate temperature of about 450 °C. The results were measured with a 200 gm diameter diamond stylus at the recommended [151 scratch speed of 100 mm rain -1 and rate of load increase of 100 N min -1. The results are shown in Fig. 3 where the open and filled symbols are coatings made with and without H-I, respectively, and the fine curve is taken from existing data on industrial-standard

(c) Fig. 2. Scanning electron fractographs: (a) and (b) coatings of TiN deposited onto 6061-T6 aluminum at about 150 °C without (3.4 Nn) and with Hyper-Ion (4.3 pan), as discussed in the text, and (c) 3.5 gm thick coatings of TiBz deposited onto titanium-6Al-4V at about 150 °C with Hyper-Ion.

A.J. Perry et aL/Surface and Coathzgs Technology 76-77 (1995) 815-820

819

Table 1 Tensile properties of 440C stainless steel and 6061-T6 aluminum before (two samples) and after coating with TiN (five samples) using the HyperIon process. The data are reported in the units used by the testing laboratory; for conversion purposes 1 ksi=7.05 GPa Material

Fracture stress (ksi)

0.2% yield stress (ksi)

U.T.S. (ksi)

Elongation (%)

440C 440C + TiN 6061-T6 6061-T6 + TiN

119.50, 119.79 117.33 + 5.49 75.49, 72.01 71.23 + 17.76

48.13, 47.93 47.87 + 1.18 36.81, 36.56 39.50 _+8.00

99.80, 100.02 99.57 + 0.91 47.56, 47.56 48.30 +_1.01

17.56, 16.90 16.87 _+ 1.34 19.76, 19.76 17.35 + 2.68

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are shown in Fig. 4 and indicate that the H-I has removed the native oxide layer from the stainless steel leading to the observed enhanced adhesion. There is no indication of ion-beam-induced intermixing of the coating and substrate 1-16].

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TiN made at 450 °C. The increase in critical ioad on using H-I is apparent. The reason for the increase in the adhesion was sought by carrying out a profile study using AES. The results

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The present work demonstrates that the Hyper-Ion process produces dense, low residual stress TiN with a type-T microstructure, and excellent adhesion. The steel substrates were coated without loss of mechanical properties. In the case of the aluminum alloy, the deposition temperature still needs to be lowered further, which is not a process limitation. The results seem to indicate that the increase in adhesion is associated with the removal of the native oxide film by ion bombardment due to the H-I pulse. It is considered that any impurity pick-up by the coated part in the chamber during a cooling step (in an industrial deposition chamber, the substrates can be rotated away from the deposition zone) will be removed when the substrates return to the coating zone. These changes in properties of the coating are generally found when it is bombarded by energetic ions during deposition, i.e. as per IBAD techniques. The H-I differs from the more usual forms of IBAD in that the ions in the former, those subjected to the H-I field and which carry high energy to penetrate into the growing film, are of the same material as the coating itself, originating from the cathode. H-I has the advantage geometrically that any substrate to which a bias can be applied can be readily coated with this process. As is well known, during deposition of TiN from a cathodic arc source a bias voltage in the range ( - ) 30-100V has the effect of producing a more dense microstructure. A bias set at ( - ) 800-1500 V is used typically to sputter clean. Pulsing the bias in the high range of ( - ) 5000-20000 V, causes the ions located within the cathode sheath during the pulse to become implanted in the growing coating and this appears to produce atomic re-ordering and a reduction in the residual stress. Our work confirms that the mode of energy input during coating deposition is significant, e.g. the time average of the H-I pulse is of the order of ( - )

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A.J. Perry et al./Smface and Coatings Technology 76-77 (1995) 815-820

10-20 V, where such a change in the applied d.c. bias voltage causes no change in the microstructure, as found during the present studies and in our earlier studies of TiB 2 coatings [7].

6. Conclusions It has been shown that TiN coatings can be deposited at 150 °C with dense type-T microstructure, low residual stress and excellent hardness and adhesion by applying a very high-voltage pulsed bias, in the range ( - ) 5-20 kV, over the usual d.c. bias during deposition. This technology, termed Hyper-Ion, can be retrofitted to current industrial PVD systems fitted with cathodic arc sources. It can be readily scaled up to treat large numbers of parts made from temperature-sensitive materials such as aluminum alloys or low alloy steels for application as constructional or wear-resistant parts.

Acknowledgments The authors wish to thank the following for their experimental support: Dr. Cheryl R. B1anchard of SwRI, Dr. Keith O. Legg of BIRL and Mr. Don Wall of GA, which was very much appreciated. This work was carried out under D o E Contract DE-FG03-93ERS1577.

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