THE EFFECT OF DEPOSITION PARAMETERS ON THE CHEMICAL COMPOSITION AND CORROSION RESISTANCE OF TIC X N Y COATINGS PRODUCED ON HIGH-SPEED STEEL SUBSTRATES

THE EFFECT OF DEPOSITION PARAMETERS ON THE CHEMICAL COMPOSITION AND CORROSION RESISTANCE OF TIC_XN_Y COATINGS PRODUCED ON HIGH-SPEED STEEL SUBSTRATES

L.F.Senna^1,2*, C.A.Achete²_, F.L.Freire Jr.³ and T.Hirsch4

¹ Universidade do Estado do Rio de Janeiro (UERJ), Instituto de Química,

Departamento de Química Analítica, Pavilhão Haroldo Lisboa, Sala 427,

Rua São Francisco Xavier 524, 20550 –013, Phone: [phone] ext. 49.

Fax: [phone], Rio de Janeiro - RJ, Brazil.

2 Programa de Engenharia Metalúrgica e de Materiais, COPPE/UFRJ,

Av. Brigadeiro Trompowski, s/n, Bl. F, Rio de Janeiro - RJ, Brazil.

³ Departamento de Física, Pontifície Universidade Católica,

Gávea, Rio de Janeiro - RJ, Brazil.

4 Stifftung Institut für Werkstofftechnik (IWT), Badgasteinstr. 3, Bremen, Germany.

(Received: April 26, 2001 ; Accepted: October 3, 2001)

Abstract - TiC_xN_y coatings deposited on high-speed steel substrates have been used to enhance the tribological properties of cutting tools (hardness, wear resistance, etc.) as well as their corrosion resistance in an aggressive environment. These layers are usually produced by plasma deposition techniques (PVD or CVD), and different coating properties can be obtained with each method. In this work, TiC_xN_y films were deposited on AISI M2 high-speed steel substrates by the reactive magnetron sputtering technique. A series of samples with a variety of reactive gas mixtures (nitrogen and methane), substrate biases, and deposition temperatures was produced. As a result, coatings with different chemical compositions were deposited for each group of deposition parameters. Gas mixture composition and substrate bias directly affected the chemical composition of the coating, while deposition temperature influenced the chemical composition of TiC_xN_y layers to a very low extent.

Keywords: Hard thin films; chemical composition; electrochemical behavior; deposition parameters

INTRODUCTION

Thin films are generally used to modify the surfaces of the substrate materials, enhancing their mechanical, electrical, electrochemical, optical or biological properties and extending their useful time. Thus, the use of thin films enables the application of less expensive materials as substrates, since substrate-thin film systems are able to guarantee the desired properties. For instance, hard titanium nitride and titanium carbide films (TiN and TiC) are widely used as tribological coatings on steel substrates. These coatings enhance substrate hardness and wear resistance, making it possible to use coated steels as cutting tools, drills, etc. [Stimmel, 1987; Lee & Bayer, 1995]. Excessive deterioration of the base material is then avoided by the hard coating.

Hard titanium carbonitride coatings (TiC_xN_y) have several TiN and TiC properties, most importantly the improvement of substrate hardness and wear resistance, which allows their use as high-speed steel and stainless steel coatings [Deng et al., 1994; Ghosh & Kohler, 1992]. When compared with TiC and TiN films, it was verified that the hardness of TiC_xN_y coatings increased with the amount of carbon in the film [Randhawa, 1987; Bergmann et al., 1990]. Thus, it is expected that the highest hardness value will be obtained when nitrogen is partially substituted by carbon, in a degree of stoichiometric film [Bergmann et al., 1990]. Consequently, the high hardness characterizing TiC_xN_y coatings could be due to the combined effect of the high hardness of titanium carbide and the high chemical stability of titanium nitride [Ghosh & Kohler, 1992]. Recently, the TiC_xN_y coatings have also been used as interlayers for DLC (diamond-like carbon) thin films on steel substrates. It seems that these interlayers increase the adherence of the DLC coatings to the steel, maintaining their high degree of hardness and enhancing their toughness [Schneider et al., 1995; Deng & Braun, 1996].

These hard films can be produced by both reactive PVD (physical vapor deposition) and CVD (chemical vapor deposition) techniques [Senna, 1998; George, 1992]. Carbon to nitrogen ratio in TiC_xN_y is directly related to the deposition method and the gas mixture flux. However, opinions differ as to whether there is an exact similarity between the carbon to nitrogen ratio in the film and the N₂ to CH₄ ratio in the gas flux [Randhawa, 1987; Damond et al., 1991]. Some authors claim that if the values were always the same, only stoichiometric films,Ti₁N_xC_(1-x), would be produced [Damond et al., 1991]. However, in order to deposit coatings with this composition, a high metal/nonmetal ratio would be needed in the gas phase. This is not easily obtained, since it depends on the deposition process used. Thus, nonstoichiometric coatings are also referred to in the literature [Damond et al., 1991; Eizenberg et al., 1995; Senna et al., 1997; Freire Jr. et al., 1998]. On the other hand, there is a general consensus that the variation in the CH₄/N₂ ratio modifies the colors of the films from reddish-yellow (CH₄ : N₂ = 1:5) to gray (CH₄ : N₂ = 1:1). Increasing the ratio to values higher than 1:1 (CH₄ : N₂ = 2:1, 3:1, ...), results in dark overstoichiometric films. The variation in coating composition is small and TiC nonstoichiometric films can also be produced [Damond et al., 1991; Senna, 1998]. Hydrogen is also detected in all these coatings, especially the overstoichiometric ones [Randhawa, 1987; Damond et al., 1991; Eizenberg, et al., 1995; Freire Jr. et al., 1998; Senna, 1998]. Deposition temperature is another parameter usually studied, although it seems to affect the stoichiometry of these coatings to a lesser extent [Senna, 1998; Achete et al., 1997; Freire Jr. et al., 1998]. However, the same result is never observed when voltage bias is applied to the substrate. Changes in the coating microstructure (pores, smaller grain sizes, higher lattice parameters, for instance) can also be observed [Achete et al., 1997; Senna, 1998].

Since these coatings are usually used to cover steel substrates, the electrochemical behavior of TiC_xN_y coatings has attracted interest recently [Senna et al., 1998; Senna et al., 1999; Precht, 1996]. It is known that point defects such as pinholes and small cracks, which are generally present on hard film surfaces, permit the attack on the steel substrate by an aggressive environment [Massiani & Medjahed, 1990; Senna et al., 1998; Jehn & Baumgärtner, 1992]. Film thickness also directly affects the corrosion resistance of the coating-substrate system [Jehn & Baumgärtner, 1992]. The electrochemical behavior of TiC_xN_y films has also shown a dependence on the residual stress as well as on the composition of the coatings [Senna, 1998; Achete et al., 1997].

In this work, TiC_xN_y films of an average thickness of 3.0mm were produced on high-speed steel substrates by the reactive magnetron sputtering technique. The target was pure titanium, while N₂ and CH₄ were used as Nitrogen and Carbon sources, respectively. The coatings were chemically and electrochemically analyzed as a function of the variation in deposition parameters. The influence of coating’s thickness on chemical composition was also evaluated.

EXPERIMENTAL METHODOLOGY

TiC_xN_y coatings were produced on AISI M2 high-speed steel substrates by using the reactive magnetron sputtering technique. Figure 1 shows a schematic view of the equipment used in this PVD process, while the chemical composition of the steel and the Vickers hardness are described in Table 1.

The high-speed steel substrates were mechanically polished and the final roughness, R_a, was less than 1.0 mm. The prepared samples were first degreased with a detergent solution of sodium lauryl sulfate and then ultrasonically cleaned with acetone for a period of 15 minutes. Finally, the steel substrates were dried with N₂ and loaded into a heater/substrate holder apparatus. The heating process involved a resistive heater, which was linked to a current source and a proportional temperature controller [Senna, 1998]. The target was pure titanium (99.95%). High purity argon (99.99%) was used as the sputtering gas, while a gas mixture of high purity methane and nitrogen (99.99%) was the reactive gas. The total pressure for all of the depositions was in the range of 0.08 to 0.093 Pa and the DC power was kept constant at 400W. For each reactive gas composition a set of samples was prepared by changing the substrate bias (0 to -150V) and deposition temperature (300 to 450⁰C). All deposition parameters were kept constant during the process.

Deposition time was used to control film thickness, and TiC_xN_y coatings of approximately 3.0 mm were then produced. The grazing incidence angle X-ray diffraction technique was used to determine the phase of the TiC_xN_y coating deposited [Senna, 1998; Hirsch & Mayr, 1988; Senna et al., 1997]. The Rutherford backscattering spectrometry (RBS) and glow discharge optical spectroscopy (GDOS) techniques were used to detect the chemical composition of the coatings. Details of the instrumentation as well as the processes themselves, as used in these two methods of analysis, are described elsewhere [Feldman & Mayer, 1989; Rose & Mayr, 1989; Freire Jr. et al., 1998]. The RBS technique is generally the method used to quantify the Ti:C:N atomic ratios [Feldman & Mayer, 1989; Rotberg et al., 1988], since direct measurements can be obtained and no pattern values are needed. However, its maximum depth resolution is 1.5 mm. On the other hand, the GDOS technique is a more sensitive method for detecting light elements. It also shows the transition between elements in the spectrum, permitting the observation of the coating-substrate interface. Nevertheless, it needs well-adjusted patterns to be used [Rose & Mayr, 1989]. Thus, the RBS data could be used here as a pattern for the GDOS analysis, while the depth data were obtained by the GDOS technique [Freire Jr. et al., 1998].

The electrochemical measurements were taken by the electrochemical impedance spectroscopy (EIS) technique, which uses electrical concepts to characterize electrochemical interfaces [Senna, 1998; Massiani & Medjahed, 1990; Mansfeld, 1990]. It is based on the excitation of the electrochemical process with sinusoidal signs in the frequency range of interest. Then the system response to electrochemical potential and current was analyzed. The impedance result for each frequency was used to plot the diagrams, which represent the electrochemical behavior of the material in an aggressive environment [Senna, 1998]. Details of the instrumentation as well as of the process itself are described elsewhere [Senna, 1998; Mansfeld, 1990]. All samples were electrically linked and then embedded in resin before being immersed in a solution of 0.5 M sodium sulfate (pH = 5.5), and a constant exposed area of approximately 1 cm² was maintained. Electrochemical behavior as a function of time was monitored by means of the EIS technique during at least 350 minutes. The experiments were always performed at the corrosion potential, and the frequency range used varied from 40 kHz to 4 mHz. The aim of these analyses was to verify qualitatively the performance of several TiC_xN_y coating compositions in an aggressive environment.

RESULTS AND DISCUSSION

Grazing incidence angle X-ray diffraction results showed that the d-TiC_xN_y phase was produced, no matter which deposition parameter was varied. Figure 2 shows a typical diffractogram obtained for these coatings. After confirming the deposited phase of the coating, the influence of the deposition parameters on the chemical composition and the electrochemical behavior of these coatings can now be described.

Chemical Composition of the Coatings

a) Effect of the Amount of Methane in the Gas Mixture

Figure 3 shows the chemical composition of the coatings as a function of the amount of methane in the gas mixture at a constant temperature (300, 350, 400 or 450⁰C), and at two substrate bias values (0 and -100 V). The data was achieved by RBS analysis. An increase in the amount of methane higher than 25% vol. tends to increase the carbon content in the coating, which agrees with data in the literature [Deng et al., 1994; Randhawa, 1987; Damond, et al. 1991; Freire Jr. et al., 1998]. A similar effect would be expected by decreasing the amount of nitrogen in the coating. However, this effect could only be verified for methane levels lower than 20% to 25% vol., depending on the substrate bias used. Higher amounts of methane caused an increase of nitrogen content in the TiC_xN_y coating (Figure 3b), which could be related to the production of overstoichiometric titanium carbonitrides. Overstoichiometric TiC_xN_y films, where C+N > 50%, have already been described in the literature, for both the PVD and the CVD deposition techniques [Damond et al., 1991; Eizenberg et al., 1995; Freire Jr. et al., 1998]. Moreover, understoichiometric (C+N < 50%), and stoichiometric (C+N = 50%) films could also be observed, when the samples had been produced with a small amount of methane in the gas mixture. In the present work, the overstoichiometric films could have been caused by poisoning of the titanium target. The gases (methane and nitrogen) always react with the target in a reactive sputter process. The target surface becomes poisoned by TiN and TiC and the sputtering rate decreases [Damond et al., 1991; Carlsson et al., 1993; Senna, 1998]. This effect possibly permitted a stoichiometric coating to be enriched with methane and nitrogen from the plasma. Thus, during the sputtering process, the grown film can be composed of layers originating in several deposition process regions, ranging from the metal operation mode (at the beginning of the deposition process) to the compound operation mode (at the end of the deposition process) [Abe & Yamashima, 1975]. In this way, films with larger amounts of carbon and nitrogen were produced. The amount of methane in the gas mixture required to start the overstoichiometric compound formation depends on the substrate bias voltage applied, as can be seen in Figure 3.

b) Effect of Deposition Temperature

The deposition temperature was varied from 300⁰C to 450⁰C. The influence of the deposition temperature on the chemical composition of the coatings in this range was very small, since the amounts of carbon, nitrogen and titanium in the films remained constant, as can be seen in Table 2. Similar results were observed for coatings produced with all of the gas mixture compositions studied. Table 2 also shows an interesting combined effect of deposition temperature and substrate bias on the chemical composition of the coatings. As the substrate bias increased, the amounts of carbon and nitrogen in the coating decreased, for the temperature range studied. Moreover, the TiC_xN_y coatings produced with a 0 V bias had an amount of titanium smaller than the sum of the amounts of carbon and nitrogen, even for the lowest methane content in the gas mixture (10% vol.). This result could be explained as a "focalization" plasma effect, coupled with the effect of deposition temperature on the substrate diffusion process. Gröning et al. (1993) observed that when a steel substrate was exposed to methane plasma, there was an increase in the ionic flux straight to the substrate as the RF substrate bias was increased. This caused the high rate of growth of an amorphous carbon-like diamond film. Nonetheless, at a negative substrate bias higher than -50 V, the sputtering effect of ions in the plasma became more effective, which in turn decreased the adsorption of CH radicals. It was also observed that the carbon growth rate on AISI 440C steel substrate exposed to Ar-CH₄ plasma decreased with the increase in deposition temperature up to 350^oC. However, this rate increased again in the temperature deposition range between 350^oC and 600^oC [Gröning et al., 1992; Gröning et al., 1993]. The coatings shown in Table 2 were produced with a gas mixture composed of methane, nitrogen and argon and in a temperature deposition range of between 300 and 450^oC with a 0 V substrate bias. Compared to the experiments performed by Gröning et al. (1992 and 1993), these are the best conditions to permit the direct addition of carbon to the substrate [Senna, 1998]. On the other hand, at -100 V, there was an improvement in the sputtering process and the coatings showed low carbon growth and adsorption rates.

c) Effect of Substrate Polarization

Substrate polarization directly affected the chemical composition of the TiC_xN_y coatings, as can be seen in Figure 4. The amounts of both nitrogen and carbon decreased as the negative substrate bias increased. This effect could be related to the re-sputtering process of carbon and nitrogen atoms, which could be intensified as the negative substrate bias increased. However, this phenomenon was observed at a negative substrate bias higher than -150 V for nitrogen [Rickerby & Burnett, 1988] and higher than -250 V for carbon [Gröning et al., 1993]. Nevertheless, the above mentioned effect could be related to a strong chemical bond between carbon and nitrogen, since both quantities increase with increasing amounts of methane in the gas mixture (Figure 3). Thus, the increase in negative substrate bias could apparently cause both carbon and nitrogen sputtering as a bonded CN compound, and not as carbon or nitrogen or a CH radical, due to a preferred chemical attack [Senna, 1998; Achete et al., 1997]. CN bonds are common in CVD TiC_xN_y coatings [Eizenberg et al., 1995; Dubois et al., 1992], which means that the coatings were not only composed of TiCN, but also of CN bonds. Residual stress analysis corroborated this proposal, since it shows that these coatings have a great distortion in the fcc lattice [Achete et al., 1997].

d) Effect of Film Thickness

TiC_xN_y coatings produced with a methane content in the gas mixture of higher than 30% had overstoichiometric chemical compositions (C+N>50%At. and Ti<50%At.). Figure 5 shows the GDOS depth profile analysis of these samples. It is possible to observe a change in the chemical composition of the coatings throughout their thickness and also that the titanium content increased from the film surface to the steel-film interface. The GDOS diagram shows that the coating is composed of TiC_xN_y inner layers, which were gradually enriched with carbon and depleted of titanium, due to target poisoning [Senna, 1998; Senna et al, 1997]. At the beginning of the process (interface), the target was clean and large numbers of titanium layers were deposited on the substrate. Then the reactive gases combined with the deposited titanium, and the coating was formed. However, these reactive gases could also react with the titanium target itself, forming TiC and TiN compounds on the target surface. This caused a decrease in the sputtering rate and also in the quantity of metal deposited on the substrate [Senna, 1998]. At the same time, carbon and nitrogen continued to react on the substrate surface, and were added to the growing film. The efficient adsorption of carbon [Gröning, et al., 1992] explains the high content found on the amorphous film surface [Senna, 1998; Achete et al., 1997]. Multilayer films (Ti/TiN/TiCN/TiC/Ti-DLC) have been reported earlier in the literature [Schneider et al., 1995; Deng et al., 1996]. These coatings present high hardness and mechanical strength [Senna, 1998; Achete et al., 1997; Senna et al., 1997; Cheng & Duh, 1991], since gradual interfaces usually increase adherence [Sproul, 1994; Schneider et al., 1995]. Thus, they can also be used to enhance the DLC coatings on steel substrates, releasing the hard coating stress [Schneider et al., 1995; Deng & Braun, 1996] and mechanically protecting the steel substrate.

Electrochemical Analysis

a) Effect of the Amount of Methane in the Reactive Gas Mixture

Figure 6.a shows the electrochemical behavior of the samples produced at 350^oC and at a -100V bias after being immersed in the aggressive medium for 15 minutes. Previous results for TiN coatings showed that these films had not been attacked by the electrolyte at the corrosion potential [Massiani & Medjahed, 1990; Senna et al., 1995]. Thus, the real axis should correspond to the performance of the coating as a protective barrier, thereby avoiding the electrolyte attack on the substrate. As a consequence, if pores and cracks were present on the coating surface, its ability to protect the substrate would decrease.

The electrochemical behaviors of the samples shown in Figure 6a are very similar, since all samples present a capacitive loop at the frequency range studied. However, in the high frequency region (Figure 6b), it is possible to note that the sample produced with a higher methane content in the gas mixture showed a small capacitive loop, even after being exposed to the aggressive environment for only 15 minutes. This behavior suggests that the coatings with large amounts of carbon have the lowest corrosion resistance. The increase in carbon content in the coating would probably imply an increase in fcc lattice distortion, creating several micro defects, favoring an electrolyte attack on the substrate [Achete et al., 1997; Senna, 1998]. As the experiment continued, the corrosion resistance of all coatings decreased, and after being exposed for 350 min, pitting and cracks were observed on the surfaces of all of the samples.

b) The Influence of Deposition Temperature

Deposition temperature seems to directly affect the electrochemical behavior of the TiC_xN_y coatings. Figure 7 shows that corrosion resistance was enhanced by the increase in deposition temperature. This effect could be associated with the release of residual stresses, which decreased the number of surface defects and consequently the contact between electrolyte and steel substrate [Senna, 1998]. We have already shown that there was a release of the third kind residual stress in TiC_xN_y coatings with an increase in deposition temperature [Achete et al., 1997]. Since this kind of residual stress is associated with point defects and voids [Noyan & Cohen, 1987], which permit the electrolyte to attack the substrate, the control of deposition temperature is an important parameter for producing a nonporous coating and enhancing the corrosion resistance of the coating-substrate.

c) The Effect of Substrate Bias

Substrate bias is another parameter that influences the electrochemical behavior of the coatings. Figure 8 shows that a coating produced with a 0V bias has higher values of corrosion resistance than another coating deposited with -100V, under the same experimental conditions (10% vol. CH₄+ 90% vol. N₂; 350⁰C). It is known that a 0V bias coating has a coarse microstructure with many pores. On the other hand, a biased coating shows a compact microstructure with point defects [Rickerby & Burnett, 1988]. However, it is also known that the residual stresses in hard coatings are enhanced by substrate bias, leading to the formation of microcracks and increasing the number of defects [Senna, 1998; Achete et al., 1997; Hirsch & Mayr, 1988; Rickerby & Burnett, 1988]. In an aggressive environment, these coatings would behave as large cathodes, while the exposed substrates would act as small anodes. This combined effect tends to intensify the corrosion of the substrate, decreasing the corrosion resistance.

CONCLUSIONS

Both carbon and nitrogen contents in TiC_xN_y films increased when the amount of methane in the reactive gas mixture was higher than 20 to 25% vol. This behavior could be due to a mechanism other than reactive sputtering when these elements were added to the films. It was also observed by chemical analysis that, for the same temperature deposition and substrate bias, the coating stoichiometry depended on the amount of methane in the reactive gas mixture and on the level of target poisoning. The overstoichiometric coatings, produced with a methane content higher than 30% vol., showed a variation in chemical composition throughout the coating thickness, which could be related to target poisoning, causing the deposition of multilayers.

Deposition temperature affects chemical composition to a very low extent. However, the combined effect of deposition temperature and substrate bias might favor the production of carbon- and nitrogen- rich layers, mainly at 0 V. It is suggested that a strong CN bond could have been formed and the PVD TiC_xN_y coatings could also contain CN compounds. Substrate bias alone affects the chemical composition of the coatings at any deposition temperature and reactive gas mixture composition, which agrees with data in the literature. A decrease in carbon and nitrogen in the coating was observed as the substrate bias increased. This could probably be due to a chemical attack on the CN bonds present in the film.

The electrochemical experiments showed that the TiC_xN_y coatings have defects and pores, permitting the electrolyte to attack the steel substrate. This resulted in cracks and pitting corrosion on the surfaces of the coatings. All of the deposition parameters affected the corrosion resistance of the films, and it seems that attention should be paid to carbon content in the film, deposition temperature and substrate bias in the production of a protective coating.

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Publication Dates

• Publication in this collection
  17 Apr 2002
• Date of issue
  Dec 2001

History

• Received
  26 Apr 2001
• Accepted
  03 Oct 2001