Mechanical and tribological properties of cBN films on silicon and tungsten carbide substrates

Thin Solid Films 398 – 399 (2001) 142–149

Mechanical and tribological properties of cBN films on silicon and tungsten carbide substrates M. Keuneckea,*, K. Yamamotob, K. Bewiloguaa a

¨ Schicht- und Oberflachentechnik, ¨ Fraunhofer Institut fur Bienroder Weg 54E, 38108 Braunschweig, Germany b Materials Research Laboratory, Kobe Steel Ltd., 1-5-5 Takatsuka-dai, Nishi-ku, Hyogo, 651-2271 Japan

Abstract Cubic boron nitride (cBN) films with a thickness of more than 2 mm on silicon and up to 0.8 mm on tungsten carbide substrates were prepared by reactive rf sputtering, in an AryN2 discharge, using an electrically conducting boron carbide (B4C) target. The increase in cBN film thickness was reached with a process including: a boron carbide interlayer with a variable thickness; and a subsequent gradient layer consisting of B, C and N. First, the boron carbide layer was deposited in a pure Ar discharge followed by a graded interlayer, which was conducted by continuously replacing Ar by N2 gas. Structure and composition of the films were investigated by: IR spectroscopy; TEM and SEM; as well as by SIMS. Tribological and mechanical properties of relatively thick (;1 mm) and long time stable films were investigated in detail. Quantities like: friction coefficients against different materials; abrasive wear rates; as well as hardness; Young’s modulus; or surface tension data of cBN films were determined. The hardness measured with a nano-indentation technique was approximately 65 GPa and corresponds to the known cBN bulk value. Abrasive wear rates were clearly lower than for hard coating materials like TiN. Furthermore, results concerning process transfer to technically relevant substrates like tungsten carbide (WC-Co) will be presented. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Sputter deposition; Cubic boron nitride; cBN; Superhard; Hardness

1. Introduction The outstanding mechanical and physical properties of cubic boron nitride (cBN), like high hardness, chemical inertness and high wear resistance, make this synthetic material a current and interesting topic of research. Following the first synthesis of cBN by Wentorf w1x in a high pressure and high temperature process approximately 30 years ago, the focus changed from the bulk material to the synthesis of cBN coatings, especially for applications as a hard and wear resistant tool coatings. Various PVD w2–6x processes succeeded in cBN film deposition. CVD methods are in use as well. Commonly the portion of the cubic phase in CVD prepared films seems to be lower compared to PVD processes w7x. In * Corresponding author. Tel.: q49-531-2155-652; fax: q49-5312155-900. E-mail address: [email protected] (M. Keunecke).

many CVD processes, a supporting plasma was applied (PACVD) for cBN synthesis w8x. Unfortunately, nearly all cBN films possess some drawbacks which limit the spectrum of application. Most publications only report on cBN films in the thickness range of a few hundred nanometers. In spite of the above mentioned successes, the deposition of thick cubic boron nitride (cBN) films ()1 mm), and the development of a cBN coating on substrate materials other than silicon, is still a scientific challenge. This is mainly caused by enormous residual intrinsic compressive stress (up to 20 GPa), poor adhesion and a lack of long-term stability under ambient conditions. Only few recent publications report on thicker cBN films. But these cBN films with a thickness )1 mm could be synthesized only at very high substrate temperatures of )10008C w9,10x. Another approach to achieve thicker films is stress reduction, either with elevated temperatures with the objective to anneal the

0040-6090/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 3 9 3 - 1

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film w11x, or with an intermittent deposition technique in combination with a high energy ion irradiation w12x. Our approach to thicker cBN films yields an adhesion improvement of the cBN layer without an essential reduction of stress w13x. After intensive research work, we succeeded in depositing cBN films with a high quality and thickness of more than 2 mm on silicon substrates. The progress was based on a layer system consisting of a boron carbide base layer, a B–C–N gradient layer and finally the cBN layer. A pre-condition of this layer system is the application of a boron carbide (B4C) target for sputtering. Among other properties, this material can be deposited using a dc sputtering process. Further description of the process is published elsewhere w13x. The transfer of these results to other substrate materials is one of the big challenges leading to an application of cBN-based coatings on tools. In this paper, the structural and compositional features, and the mechanical and tribological properties of thick cBN films will be reported. A method to deposit a superhard coating on tool materials with a significant cBN portion will also be reported. 2. Experimental details The preparation of the cBN layer and the B–C–N gradient layer was performed in an rf (13.56 MHz) diode sputtering apparatus using a boron carbide (B4C; purity 99.5%) target and an AryN2 atmosphere with N2 as reactive gas. The substrate was driven in an rf mode too, for an additional biasing. To enhance the incoming ion density at the substrate position, an auxiliary magnetic field was realized by a coil surrounding the substrate table. A detailed setup of the sputtering ¨ system is described by Schutze et al. w4x. The standard substrate material was a polished (100) oriented silicon wafer. The deposition of the layer system was carried out in the diode sputtering system. For the boron carbide base layer as well as for the following layers, the pressure was in the range of 0.2– 0.3 Pa. The substrate temperature, measured with an isolated thermocouple, increased from approximately 3008C for boron carbide deposition to approximately 6508C during cBN deposition. The substrate bias changed from floating potential respectively medium negative bias (approx. 100 V) for B4C deposition to 200 V negative rf self-bias for cBN nucleation. After nucleation the substrate bias could be reduced, because the ion energy threshold for cBN growth is lower than for cBN nucleation w14x. The deposition rate strongly depended on the applied substrate bias and consequently ranged from 450 nmyh for B4C to 200 nmyh for cBN. The gas flow changed from pure argon (50 sccm) for boron carbide deposition over a gradual exchange from argon to nitrogen for the B–C–N layer, and at approxi-

143

mately 10% N2, the threshold for cBN nucleation was reached w15x. After cBN nucleation the atmosphere was changed to pure nitrogen (50 sccm) for cBN growth. The total flow was kept constant at 50 sccm during the entire deposition run. More detailed information on the deposition process is described by Yamamoto et al. w13,16x. Due to the excellent properties of cBN, one of the most interesting fields of research is focused on an application of cBN films for cutting operations of steel. Therefore, we carried out experiments with tungsten carbide and also high-speed steel as substrate material. To achieve a well adhering layer system on tungsten carbide substrates, a pre-coating is advisable. It turned out that the deposition of a base metal layer, in our case, for instance titanium, was useful w17x. The deposition of the titanium base adhesion layer was conducted in a dc magnetron sputtering laboratory facility (Balzers 450 PM) with a titanium target in an argon atmosphere. The steps of this deposition were first evacuated down to -10y3 Pa, then argon ion etching with a negative substrate rf bias of 1000 V and finally sputter deposition of the titanium adhesion layer. The gas flow was 75 sccm, corresponding to a pressure of approximately 0.6 Pa. The target power for the titanium deposition was 1.2. kW. Following the transfer into the diode sputtering facility, the deposition process was continued as described above. In order to achieve comparable results, different substrate materials, i.e. silicon as well as tungsten carbide and high-speed steel, were coated in one process. After the deposition all samples were kept at ambient conditions. In order to determine the mechanical, tribological, structural and compositional features, several investigations were carried out. The microstructure and appearance of the deposited films were specified with cross-section SEM images. The chemical composition was characterized by electron probe microanalysis (EPMA) and secondary ion mass spectroscopy (SIMS). The quantitative analysis of light elements using EPMA and SIMS has been described in detail by Willich and Bethke w18x, and Willich and Wischmann w19x. Depth profiling was carried out by SIMS (Cameca ims 5f) with a Csq ion beam of 5.5 or 4 keV. Infra-red spectroscopy was used for the primary phase identification. Comparable IR-spectra have already been published in w13x. TEM investigations were carried out to determine the microstructure of the deposited films as well as to deliver another proof for the formation of the cBN phase. Besides the structural and compositional features, the mechanical and tribological properties were of particular interest. On this account, hardness and elastic modulus measurements were carried out. Two different hardness measurement methods were applied. One was the depth

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The surface tension of the coatings was calculated by measuring the contact angles between sessile drops of known liquids and the coating surface. The calculation of the surface tension, as well as the method of measuring the contact angle, was described by Grischke et al. w23x. 3. Results and discussion 3.1. Structure and composition

Fig. 1. SEM cross-section image of a cBN layer system.

sensing indentation method on a nanometer scale, called nano-indentation using a diamond Berkovich indentor with up to a 25 mN load. The microhardness and Young’s modulus were calculated according to the model by Oliver and Pharr w20x. The second method used was based on German and international standards for hardness measurements w21x. These measurements were carried out with a commercial Fischerscope䉸 hardness measuring device that determines a derived Vickers hardness from load displacement curves. Another characterization test concerned the determination of the abrasive wear rate. A commercial ballcratering device (CSEM) with a glycerinyalumina suspension (particle diameter approx. 1 mm) was used to grind a calotte. With the number of turns of the ball and the depth of the calotte, a measure for the abrasive wear rate was calculated. Another important property of coatings is its friction coefficient m. The coefficient of friction m was investigated using a pin-on-disc tester with a ball (diameter 4.76 mm) made of different materials as counterpart, a load of 1 N and a speed of 0.04 m sy1. The tangential force of the pin was measured, and with the perpendicular force caused by the load, the friction coefficient could easily be derived. The temperature and humidity were kept constant during the measurement of tribological properties at 218C and 45–50% relative humidity. A very important issue for applications, is sufficient adhesion of the newly developed film on the substrate material. The adhesion was characterized using a scratch test w22x, where the normal load on a diamond tip moving over the coated surface is continuously increased. The critical load Lc corresponds to that normal load where the first failures in the film will be observed.

Fig. 1 shows a cross-sectional SEM image of a cBN film with a thickness of approximately 2 mm. The cBN layer, the gradient layer, the boron carbide (B4C) base layer and the silicon substrate can clearly be distinguished from each other. A grain structure is neither recognizable for the cBN layer, nor for the B4C layer. This suggests that the structure of the layers is either nano-crystalline or amorphous. Evidence for this assumption was obtained by TEM investigations. In Fig. 2, a structural overview of the complete layer system is shown. The single layers beginning with the boron carbide up to the cBN-layer are recognizable (from the right to the left side in Fig. 2). The images in Fig. 3a–d are focussed on special regions of the layer system in Fig. 2 (marked with letters a–d).

Fig. 2. TEM image of whole layer system; cBN thickness approx. 1.2 mm.

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Fig. 3. (a) High resolution image of boron carbide layer. (b) Darkfield image of gradient layer region; (0002) hBN reflection. (c) Darkfield image of nucleation region; (111) cBN reflection. (d) Darkfield image of cBN growth region; (111) cBN reflection.

The high resolution image of the boron carbide (Fig. 3a) revealed the amorphous structure of this layer. The other micrographs show dark field TEM images (Fig. 3b–d). Fig. 3b is focussed on the gradient layer region. The brighter parts correspond to the (0002) reflection of hexagonal boron nitride (hBN). A columnar hBN grain structure is clearly recognizable. This confirms the assumption that the gradient layer mostly consists of hBN comparable to the structure known for thin cBN films. The bright parts in Fig. 3c were taken with the (111) reflection of cBN. The image shows the nucleation region and the first growing region of cBN. The nucleation zone is relatively rough; this means that the nucleation of cBN takes place over an extended region. The grain size of cBN could be estimated to be approximately 10–20 nm. As well as Fig. 3c, Fig. 3d is a dark field image and was taken with the (111) reflection of cBN.

The image was taken from the cBN growth region. It confirms the existence of cBN in thicker films. To our knowledge, this is the first TEM preparation of a thick ()1mm) cBN sample. Another interesting point is that the cBN grain size did not change significantly during the growth of thicker films. From the TEM image in Fig. 3d of the cBN region, a comparable but slightly larger grain size, as for the nucleation region, was detected. However, the grain size of the cBN region is not significantly different from thin cBN films. Apart from this, the TEM investigations, particularly the dark field images which were taken with the cBN (111) reflection, gave additional evidence for the synthesis of cubic boron nitride. Further results of TEM investigations will be published later w24x. Commonly, the cubic state of boron nitride films is proved by IR spectra w25x. The results of IR spectros-

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Fig. 4. SIMS depth profile of a cBN layer system on a cutting insert of WC-Co. I: cemented carbide cutting insert; II: titanium interlayer; III: boron carbide layer; IV: gradient layer and boron nitride layer.

copy as well as results of XRD investigations were presented by Yamamoto et al. w16x. It shall be noted that the nanocrystalline structure of the cBN phase could be revealed both from the XRD diagrams and the TEM images. Beside the deposition and characterization of thick cBN films on silicon substrates, the main intention of our experiments is the application of cBN as a coating for cutting inserts. As mentioned above, we succeeded in depositing adherent cBN films on tungsten carbide substrates with an adjusted layer system, including an additional titanium adhesion layer. In Fig. 4, a SIMS depth profile of a cBN layer system on a cutting insert is displayed. In the SIMS depth profile in Fig. 4, the different layers could be distinguished very well. The composition of the boron carbide layer is: boron 70 at.% and carbon 18 at.%. The concentrations of the other elements like nitrogen, oxygen, hydrogen, or argon are lower than 5 at.%. The ratio of boron to carbon is nearly the ratio for stoichiometric boron carbide (4:1). Also the B–C–N gradient layer is easy to detect. This layer corresponds with the incremental change from argon plasma to nitrogen plasma in the reactor. The nitrogen content of the film rose from approximately 4 at.% in the boron carbide layer to approximately 43 at.% in the boron nitride layer. Simultaneously, the boron content decreased from approximately 70 at.% to approximately 45 at.%, while the carbon content decreased from approximately 19 at.% to approximately 5 at.%. The combined concentrations of oxygen, argon and hydrogen are less than 6 at.%. The ratio of boron to nitrogen is close to unity, which is a requirement for cBN synthesis. To adjust our layer system for a different substrate material and to minimize the substrate influence, the

boron carbide layer thickness was enlarged. For good adhesion, we developed a system consisting of a titanium interlayer of approximately 0.5 mm, and a relatively thick boron carbide layer (1–1.7 mm). The maximum cBN thickness reached on a cutting insert so far is approximately 0.8 mm, based on such a layer system. Following the base layers of Ti and B4C, the normal sequence of our cBN layer system was deposited. A conventional B–C–N gradient layer of approximately 150 nm thickness is added and then the cBN nucleation and growth follows. The cBN film thickness of this sample, investigated with SIMS, was approximately 0.5 mm. 3.2. Mechanical and tribological properties of cBN films To confirm the cBN phase in coatings deposited on tungsten carbide substrates, as well as to determine the properties of the films and to compare them with those deposited onto silicon, we carried out many mechanical and tribological experiments. The results of the mechanical and tribological experiments are summarized in Table 1. As one of the most important properties, we measured the hardness and the Young’s modulus of the films with two techniques, as described above. The nano-indentation method provided microhardness values between 55 and 65 GPa on silicon substrates and between 53 and 60 GPa on tungsten carbide substrates. The nano-indentation measurements were carried out according to the Bueckle rule w26x, with high accuracy. A rough estimation of the Young’s modulus, using a Hertzian contact model and assuming a poisson ratio of 0.3, provided values from 500 to 550 GPa for both

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substrates. For comparison, typical values of TiN, DLC and diamond films were listed additionally in Table 1. The second method provided a derived Vickers hardness. However, as shown in Table 1, we received 5100 HV for the cBN layer system on tungsten carbide and 5800 HV for cBN layer system on silicon. Comparing these values with TiN or DLC films, the cBN layer system is nearly two times harder. The smaller Vickers hardness values for the tungsten carbide were due to the smaller cBN thickness on these substrates. In Fig. 5, a load displacement curve of an indentation experiment with the Fischerscope䉸 hardness measuring device is displayed. The derived elastic recovery amount is approximately 80% for a cBN layer system on tungsten carbide (cBN thickness approx. 0.8 mm). The indentation depth was approximately 320 nm and the maximum load was 100 mN. The problem that occurs for such an indentation depth is that the evaluation did not follow the Bueckle rule (maximum indentation depth is 1y10 of film thickness) w26x. This means we determined only a composite average of Vickers hardness values of the single layers of the whole system. Of course, the conversion to Vickers hardness included a variance. However, these results showed the outstanding hardness and elasticity of the cBN layer system. Certainly we have to relate the obtained values to the values of other coatings as they are presented in Table 1. However, the measured hardness and elastic recovery values agreed very well with the previous results of Mirkarimi et al. w27x. The measured value of Young’s modulus was significantly smaller than that of bulk cBN, reported to be approximately 800 GPa w7x. This behavior was also observed by Mirkarimi et al. as depressed elastic modulus w27x. This means that the Young’s modulus measurement of thin films is affected by an underlying soft substrate such as silicon or tungsten carbide. The measurements of Young’s modulus are much more affected by the substrate than the hardness measurements, because the elastic volume is

147

Fig. 5. Load displacement curve of indentation experiment with Fischerscope䉸 device.

much bigger than the plastically deformed volume w28x. However, it might be possible that Young’s modulus values were affected by other factors, such as crystallinity of cBN films, or texture of the films. Results of the abrasive wear rate measurements are also shown in Table 1. The abrasive wear rate of cBN films is comparable to DLC coatings and 10 times lower than that of TiN coatings. These measurements indicate the high potential of cBN films as a wear protection coating. Also listed in Table 1 are friction coefficient values of different films against a ball bearing steel (100 Cr6). The friction coefficient of the cBN films, with ball bearing steel as a counterpart with a value of 0.4, is in a medium range. Compared to a DLC coating, the friction coefficient of cBN is high, but small in relation to a TiN coating. In Table 2, the friction coefficients of cBN films with different materials as counterpart are summarized. An interesting result is the very low friction coefficient (0.1) of the cBN coating against a DLC coated steel ball. In contrast to this behavior, the values of the friction coefficient with other materials as counterpart were in a medium range. A first attempt to explain this interesting behavior could be due to the

Table 1 Properties of cBN films in relation to other wear protection and hard coatings

Microhardness by nanoindentationyGPa Vickers hardness by Fischerscope䉸yHV0.06 Young’s ModulusyGPa Abrasive wear ratey10y15 m3 my1 Ny1 Friction coefficient against steel Surface tensionymNym Critical load in scratch testyN a

cBN on silicon

cBN on tools

TiN

DLC

Diamond

60–65

55–60

25–30

25–33

80–100a

5800

5100

2600

2800

–

500–550 ;0.4

500–550 ;0.6

300 5–7

250–300 ;0.6

1050a –

0.4

0.4

0.7

0.2

G0.2

;40 20

– 25

– 30–50

;41 29

)DLC –

Hardness and elastic modulus of diamond described in w30x.

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Table 2 Friction coefficient of cBN with different materials as counterpart Counterpart of cBN film

Steel 100Cr6

Alumina (Al2O3)

Hard metal (WC-Co)

DLC-film

Friction coefficient m of cBN film against«

mf0.4

mf0.5

mf0.3

mf0.1

comparable properties of the two counterparts, like high hardness and very low abrasive wear. Another approach for explanation of this behavior could be given by a material transfer of the softer DLC ball to the harder cBN film. In this case, the measured friction coefficient corresponds to that of DLC against DLC counterpart w29x, and for this tribological system, low friction coefficient are well known. However, the tribological behavior strongly depends on the specific tribological system, and general statements are complicated. Another important property of coatings is the adhesion strength as determined by the scratch test results. Some scratch values are listed in Table 1. Up to now the obtained scratch values for cBN films of 20 N and 25 N for silicon and tungsten carbide substrates, respectively, could be sufficient for some applications, but they have to be increased for others. Another point which must be taken into account is the different thickness of the investigated films. Even though the overall thickness of the layer system reached approximately 3 mm, the single cBN film thickness was lower (approx. 2 mm for Si and 0.8 mm for WC) compared to other coatings displayed in Table 1 (common thickness of thin film coatings is approx. 3 mm). An impression of the potential of cBN coatings is given in Fig. 6. In the image, the failure region of a cBN film on a silicon substrate during a scratch test is displayed. The diamond tip was moved from the right side over the film. No scratch tracks were visible until film failure at 20-N critical load. But a closer inspection of Fig. 6 reveals that not the film but the silicon substrate failed. Due to the observation that no scratch marks could be detected until substrate failure, this suggests that the real critical load of the layer system is larger. Additionally, in Table 1, the surface tension of deposited cBN films was determined to be approximately 40 mNym, which was comparable to that of DLC films w23x.

imately 4=10y16 m3 yNm indicate the outstanding properties of cBN films and are comparable to bulk values. The cubic phase was confirmed by TEM investigations, which also confirmed the nanocrystalline structure of the films. Furthermore, the composition of each layer was identified by SIMS analysis. It was demonstrated that with an adjusted and optimized interlayer and interface system, the creation of thick cBN films is feasible on different substrate materials in spite of the enormous intrinsic stress w16x. The possibility for the synthesis of cBN films on substrate materials other than silicon, for example, tungsten carbide substrates, was demonstrated using an adjusted interlayer system including a relatively thick boron carbide layer. This boron carbide layer, in conjunction with the whole interlayer system, enabled a wide independence from substrate properties. This supports the assertion that a transfer to additional substrate materials should be feasible. However, the development status up to now is not sufficient for applications. The cBN thickness has to be increased and the adhesion must be improved, for example to critical loads )40 N, but the results are very promising for future development. Particularly, the possibility of depositing a superhard coating with more or less good adhesion to a tool substrate provides encouragement to carry out cutting performance experiments in the near future. Acknowledgements The authors are grateful to Mr T. Staedler and Mrs M. Lutansieto for indentation measurements, Dr P. Wil-

4. Summary and conclusions The presented results confirm the high potential of thick cBN layer systems for applications, especially as wear protection coating in steel cutting operations. The determined hardness values up to 65 GPa micro-hardness, the elasticity and the abrasive wear rate of approx-

Fig. 6. Scratch test tracks of cBN film on silicon substrate.

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