Bilayer systems of tantalum or zirconium nitrides and molybdenum for optimized diamond deposition
Introduction
Diamond films grown by Chemical Vapor Deposition (CVD) are widely used as surface overlay coating onto WC–Co cutting tools to improve their performances (endurance, surface hardness, thermal conductivity, friction, wear protection…). It is well known that the Co content has to be lower than 6 wt.% and the fraction of other carbides should be as small as possible to achieve satisfactory adhesion levels of CVD diamond films. The reduction of the Co content from the substrate causes toughness deterioration and reduces the tool lifetime. Whereas the reduction of the cemented carbide grain size (< 1 μm) combined with an increase of the Co content (> 10 wt.%) leads to mechanical properties close to those obtained with conventional grain size (> 10 μm) and 3 wt.% Co content [1]. However the increase of the cemented carbide Co content is detrimental for the diamond adhesion. During deposition of sp3 diamond film by CVD process onto sintered WC–Co substrates, formation of sp² graphitic species can occur because of the Co binder catalytic properties related to their valence electrons [2], [3], [4]. To improve the diamond adhesion with the substrate and the disagreements encountered, many pretreatment methods have been applied such as selective etching of Co [5], [6], [7], [8], [9], forming stable Co compounds [10], [11], [12], [13], giving a suitable diffusion barrier layer on the substrates [14], [15], [16], [17], [18], [19], [20] or a tungsten-carbide gradient coating [21] etc. The latter three strategies aim at blocking the way through which elemental Co could reach the diamond WC–Co substrate interface. According to the phase diagram of Co–C system in the temperature range of 975–1275 K (typical diamond CVD temperature), carbon is soluble in Co up to 0.2–0.3 wt.%. During the initial stage of the CVD process, the WC–Co substrate is exposed to a hydrocarbon rich atmosphere and carbon species can diffuse into the bulk of the binder phase until the carbon solubility is exceeded. Once the carbon concentration at the substrate surface is sufficiently large to promote solid carbon condensation, the preferential formation of graphite layer is promoted by the presence of the binder. Moreover, as a further consequence of carbon solubility in the binder phase, diffusion of carbon from the deposited diamond into the metal binder can also occur. To suppress the interaction between Co and the deposited diamond film, an interlayer with low diffusion coefficients for C and Co has to be used. Moreover, an interlayer material with an intermediate thermal expansion coefficient between WC–Co and diamond could relieve the residual thermal stresses. Improvement in adhesion using various interlayers, ranging from amorphous carbon to metallic materials (Cu, Mo, Nb, Pt, Ti, Ta, W and Ag) and ceramics (SiC, Si3N4, TiC, TiN and WC) has already been reported [22].
In a previous paper [17], we have shown that refractory nitrides like ZrN, NbN and TaN can act as efficient Co diffusion barriers for CVD diamond deposition on WC–Co substrates containing 12 wt.% of cobalt. In this study, we propose to focus on bilayer systems composed of a Co diffusion barrier (TaN or ZrN) and a thin metallic layer to improve the diamond nucleation. Amorphous Si coating can act as a promoter for diamond nucleation thanks to its good carbide former properties [23]. Unfortunately, Si is not compatible with most of transition or refractory nitrides because the lowest chemical potential values of formed silicides lead to inescapable destruction of the barrier. In this study, molybdenum is used as another candidate to improve the nuclei density of diamond during CVD processing.
Section snippets
Deposition of nitrides layers
The substrates were WC–Co 12 wt.% plates 16 × 16 mm² and 5 mm thick. Prior to deposition, WC–Co subtrates have been blasted with alumina micro particles under the same conditions (air flow absolute pressure : 2 × 105 Pa; distance 10 cm), then cleaned in acetone for 1 h in an ultrasonic bath and finished under propan-2-ol vapor for 1 min. The roughness Ra measured (White light confocal profilometer VEECO) was lower than 500 nm.
Nitride layers were deposited by reactive magnetron sputtering using Zr, Ta and
Bilayer morphology before diamond growth
For Zr and Ta, binary phase diagrams show mainly the existence of two compounds M2N and MN with M═Zr or Ta. Due to its lower enthalpy of formation M2N compounds should be preferentially synthesized under low temperature and pressure conditions. Thanks to the reactive magnetron sputtering process, the formation of the MN phase is possible when the partial pressure of nitrogen is increased. X-ray diffraction spectra (Fig. 1) show the presence of the hexagonal TaN (JCPDS 39-1484) and cubic ZrN
Discussion
The results clearly demonstrate that as intermediate layers of Mo, TaN or ZrN on WC–Co substrates show different effects on subsequent diamond growth. Based on the comparison with results obtained without the Mo layer used on the top of TaN or ZrN intermediate layers, the diamond nucleation has been significantly promoted. For TaN and ZrN, the increase is time 20. The consequences on the adhesion of the diamond film have to be quantified. We have already showed [17] that an intermediate layer
Conclusion
The efficiency of the different bilayer systems (TaN–Mo and ZrN–Mo selected using thermochemical computation) with the aim to increase the diamond film growth has been investigated. TaN and ZrN thin film pre-coated on WC–Co substrates as Co diffusion barrier leads to a lower diamond nucleation density compared to the same system with a Mo top layer. After diamond deposition, a massive carburization of molybdenum and tantalum nitride is observable whereas zirconium nitride is not. Nevertheless,
Acknowledgments
The author would like to thank “Le Conseil Regional d'Aquitaine” for its financial support, Dr. Michel Lahaye (CeCaMa, Université de Bordeaux1) for AES characterization and Dr. Christine Labrugère for XPS quantifications.
References (32)
- et al.
Wear
(1999) - et al.
Surf. Coat. Technol.
(2001) - et al.
Carbon
(2004) - et al.
Diamond Relat. Mater.
(2006) - et al.
Int. J. Refract. Met. H
(2002) - et al.
Int. J. Refract. Met. H
(2007) - et al.
Appl. Surf. Sci.
(2008) - et al.
Diamond Relat. Mater.
(2001) - et al.
Diamond Relat. Mater.
(2002) - et al.
Surf. Coat. Technol.
(2000)
Diamond Relat. Mater.
Surf. Coat. Technol.
Diamond Relat. Mater.
Diamond Relat. Mater.
Surf. Coat. Technol.
Int. J. Refract. Met. H
Cited by (11)
CVD diamond coating on WC-Co substrate with Al-based interlayer
2016, Surface and Coatings TechnologyA multilayer innovative solution to improve the adhesion of nanocrystalline diamond coatings
2015, Applied Surface ScienceCitation Excerpt :Multilayer system: multilayer coating composed of 9 periods of a TaN(50 nm) and ZrN(30 nm) bilayer system, is covered with a TaN(0.5 μm)–Mo(0.5 μm) TFG bilayer system. Because, micro-grained TaN layer is completely carburized during the diamond deposition process and ZrN(1 μm)–Mo(0.5 μm) bilayer system is more efficient than TaN(1 μm)–Mo(0.5 μm) to prevent Co diffusion from the substrate when a bias is used during the process [7], the use of a TaN–ZrN multilayer system was thought: (1) to prevent efficiently the diffusion of cobalt thanks to the changing of the ZrN and TaN layers and the multiplication of the interfaces, and (2) to regulate the diffusion of carbon in addition to the regulatory effect of our conventional top TaN–Mo bilayer system dedicated to control both carbon diffusion and diamond nucleation close to the interface with diamond. TaN and ZrN layer thicknesses in the multilayer system were selected to keep constant the final grain boundary diffusion path length compared to the previous system.
Machining of hypereutectic aluminum silicon alloys
2014, Procedia CIRPMechanisms of time-modulated polarized nano-crystalline diamond growth
2013, Surface and Coatings TechnologyCitation Excerpt :This layer is then coated with a thin molybdenum [18] layer dedicated to enhance the diamond nucleation density (called here the “diamond nucleation layer”). Some authors have also proposed to use titanium [4,9–11], but thermochemical computation has shown that the bilayer system TaN–Ti is thermodynamically unstable for diamond CVD range of process temperature [19]. Hence, this work focuses on the TaN–Mo and ZrN–Mo bilayer systems used to enhance the time-modulated polarized diamond growth with a control of cobalt and carbon diffusion phenomena during the diamond formation at the interface and a control of the NCD cluster morphology.