Elsevier

Wear

Volume 261, Issues 5–6, 20 September 2006, Pages 540-548
Wear

Characterization of chemical vapor deposited HfN multilayer coatings on cemented carbide cutting tools

https://doi.org/10.1016/j.wear.2006.01.005Get rights and content

Abstract

The present study has been carried out in order to evaluate the tribological behavior of HfN-based multilayer-coated tools used for milling. The initial results of the wear behavior of multilayer HfN coatings, during the laboratory friction and wear tests conducted at room temperature against WC as tribological pair, are reported. As part of the characterization of the mechanical properties of the coatings, Vickers microindentation tests were also carried out employing loads in the range of 0.01–10 N, in order to evaluate the composite hardness of the coating-substrate systems. Information regarding the absolute hardness of the HfN and TiCN monolayer films has been obtained employing a model recently developed by one of the authors. This information is then used in order to determine the properties of the HfN/TiCN multilayer coating. It has been found that the wear volume of the single layer HfN coatings is approximately 15 times greater than the wear volume corresponding to the HfN/TiCN for the same amount of cycles.

Introduction

During the high-speed milling of steel, the repeated high-velocity impacts generate thermo-mechanical shocks at the sharp cutting edge of the tool. These shocks propagate through the surface of the tool into the bulk. A protective coating of a hard material, such as TiN, TiCN, TiAlN, Al2O3, or their combinations, provide the necessary wear and abrasion resistance, as well as heat resistance to the tool substrate [1], [2]. In conventional, uninterrupted cutting, these enhanced properties are often adequate to prevent premature wear, softening or deformation of the tool. But when high-energy impact forces are added to the cutting environment, it is often found that the tool fractures during the cut. The fracture process may start as microscopic chips forming at the cutting edge and spreading along the tool, or by the formation of transverse cracks perpendicular to the cutting edge. Under repeated impacts, these cracks may be joined by lateral subsurface cracks, leading to fracture of the cutting edge. Coatings often serve to minimize the thermal gradients in the cutting tool, by deflecting the heat generated at the tool-chip contact surface away from the tool itself. The interrupted cut imposes a condition of rapid heating and cooling of the cutting edge as it moves in and out of the cut at every pass, in addition to the mechanical forces imposed on it due to the cutting action.

Typical hard coatings on cutting tool are deposited by chemical vapor deposition (CVD). The CVD method operates at a temperature of about 1000 °C, and as a result, imposes a slightly tensile residual stress in the coating when the tool is cooled to room temperature, primarily as a result of a slight mismatch in the coefficient of thermal expansion between the coating and the substrate. Even though that the magnitude of this stress, as measured by the X-ray techniques, is relatively small, it nonetheless can aid in the formation or growth of micro-cracks in the coating during impacts experienced in milling. Hard coatings deposited by the physical vapor deposition (PVD) methods, on the other hand, tend to develop a residual compressive stress, caused mainly due to the non-equilibrium plasma deposition environment. The compressive stress can delay the onset of thermal cracking if the coating can withstand the heat and remain adherent at the cutting edge of the tool. It is clear that if the onset of thermal cracking is delayed, tool life can be significantly enhanced during the milling operation. While the conventional CVD (or PVD) hard coatings provide a certain level of protection against wear and chemical reactivity with the work-piece material, there is a need for an improved coating design to enhance the thermo-mechanical damage resistance of the cutting tool.

Tests were conducted at the Machinability Laboratory of Stellram to provide some insight into the relative behavior of various hard coatings and tool substrate materials during dry milling of AISI 4140 steel. In these tests, the standard Stellram CVD-coated grades were compared with experimental substrate/coating combinations and a competitive coated grade. The nominal compositions of the substrates and coatings are shown in Table 1. The machining conditions were as follows: speed = 290 m/min, feed = 206 mm, depth of cut = 2.5 mm and length of cut = 305 mm. The results are summarized in the graphs shown in Fig. 1 and these show that:

  • 1.

    A coating of HfN as the first layer in a multilayer coating provided the best protection against thermal cracking during dry milling.

  • 2.

    The wear resistance of HfN-coated tools was not as good as that offered by the conventional CVD-coated grades.

  • 3.

    In order to achieve an optimum combination of wear resistance and thermal cracking resistance, it is necessary to improve the wear resistance of HfN-coated tools.

Therefore, a multilayer combination containing HfN and a harder coating such as TiCN or HfCN may provide added wear resistance to the tools.

Based on the results of this preliminary study, it was decided to test the wear behavior of HfN coating and to compare it to the wear behavior of HfN/TiCN multilayer coatings, deposited on two different tool substrates. Since the early 1980s, there are different research reports in the literature on the properties of HfN coatings obtained by both PVD [3], [4], [5], [6] on different substrates and by HT-CVD [7], [8] on cemented carbides. Most of these studies were dedicated to the microstructural characterization and/or to hardness measurements. For example, Vickers hardness values varying between 32 and 49 GPa were determined for HfN coatings when they were prepared via high-rate reactive sputtering with dopants [3].

Oakes [7] has shown, when evaluating the use of the cemented carbide cutting tools coated with HfN, Al2O3, TiC and TiN during the continuous and interrupted turning of alloy steel, that HfN was the most effective coating in prolonging the tool life. A more recent comparative study between the mechanical and tribological properties of TiN and HfN carried out by Berg et al. [6] indicated a better wear performance of nearly five times for TiN as compared to HfN, when the experiments were performed using a plate-on-cylinder tribometer, and attributed this result mainly to a lesser adhesion of the latter coatings.

However, the wear resistance of the HfN/TiCN multilayer coatings has not been reported yet nor the mechanical properties of the whole system. Therefore, the goal of this work was to determine if the improved wear resistance of such multilayer coating system can be demonstrated using standard tribological test methods for hard coatings.

Section snippets

Coating deposition

The coatings used in the present study were prepared by the conventional chemical vapor deposition technique. A Bernex-type CVD reactor, fitted with a HfCl4 generator, was used for this purpose. Hafnium chloride was generated in situ by passing a flow of Cl2 over Hf chips and introduced in the deposition chamber with other process gases. Samples of different cemented carbide substrates were loaded into the deposition chamber, and coatings of HfN and HfN/TiCN multilayers were deposited. The

Compositional analysis

The AES results of different coatings are shown in Fig. 2. For both systems, oxygen is detected throughout the profiles and there is generally more oxide detected in the Hf layers than in the TiCN layers, as a consequence of the higher affinity of Hf towards oxygen. Also, since the ion gun was off during the time that the crater profile data was accumulated, some of this oxide could be due to a reaction with trace impurities in the AES vacuum system. It is interesting to notice from the AES

Conclusions

The role of multilayers HfN/TiCN coatings in improving the sliding wear resistance of the HfN coated cemented carbides against WC has been reported in the present investigation. It has been found that the wear volume of the single layer HfN coatings is approximately 15 times greater than the wear volume corresponding to the HfN/TiCN for the same amount of cycles.

Both coatings exhibited an excellent adhesion to the substrate, and a value of the critical load of 144 N was found for the

Acknowledgements

The authors wish to acknowledge the financial support from the National Council for Science, Technology and Innovation-FONACIT-Venezuela through the project UCV-F-2001000600 and from the Irma and Raymond Giffels Chair Endowment Research Fund at the University of Arkansas.

References (19)

  • D.T. Quinto

    Int. J. Refract. Met. Hard Mater.

    (1996)
  • S. PalDey et al.

    Mater. Sci. Eng.

    (2003)
  • A.J. Perry et al.

    Thin Solid Films

    (1984)
  • W.D. Sproul

    Surf. Coat. Technol.

    (1988)
  • G. Berg et al.

    Surf. Coat. Technol.

    (1995)
  • J.J. Oakes

    Thin Solid Films

    (1983)
  • J.E. Sundgren et al.

    Surf. Sci.

    (1983)
  • J.R. Tuck et al.

    Surf. Coat. Technol.

    (2001)
  • M.H. Staia et al.

    Surf. Coat. Technol.

    (1996)
There are more references available in the full text version of this article.

Cited by (0)

View full text