Nathalie Renevier

Low-temperature plasma-assisted nitriding

Plasma-assisted nitriding is an attractive surface treatment for metallurgical surface modification to improve wear, hardness and fatigue resistance of ferrous and non-ferrous materials. For this purpose, ion nitriding by a d.c. glow discharge is generally efficient for numerous materials. However, for some metals and alloys, the processing temperature, dominated by the discharge parameters, is too high and cannot be controlled independently from the plasma reactivity. This paper reviews the following solutions for low-temperature plasma-assisted nitriding: pulsed d.c. discharge, thermionically assisted d.c. triode arrangements, plasma implantation, electron cyclotron resonance systems and thermionic arc discharges. We focus on metallurgical results obtained by these techniques on austenitic stainless steel and aluminium.

Coating characteristics and tribological properties of sputter-deposited MoS2/metal composite coatings deposited by closed field unbalanced magnetron sputter ion plating☆

As previously reported (V.C. Fox, N.M. Renevier, D.G., Teer, J. Hampshire, V. Rigato, Proceedings of the PSE Conference in Garmisch Partenkirchen, 14–18 September 1998, Germany, Surf. Coating Technol., in press) the properties of MoS2 coatings deposited by closed field unbalanced magnetron sputter ion plating (CFUBMSIP) can be improved by the co-deposition of a small amount of titanium. These MoS2/Ti composite non-multilayer coatings known as MoST™, are harder (10–15 GPa) and more adherent (critical load scratch above 120 N), much more wear resistant (10−16–10−17 m3/mN), and higher load bearing (3.5 GPa) than pure MoS2 coatings. They have similar low friction to pure MoS2 coating (μ of 0.02 at 40% humidity) but are much less sensitive to atmospheric water vapour during tribological testing at high humidity. MoS2/Ti composite coatings have given excellent industrial results for a wide range of cutting and forming applications. Other metal additions such as Cr, W, Mo and Zr have been studied with varying compositions and substrates. Laboratory test results using micro hardness and nanoindentation testing, scratch adhesion testing, pin on disc and reciprocating friction and wear tests are presented. Structural analysis was carried out using X-ray diffraction, optical microscopy, scanning electron microscopy and transmission electron microscopy of cross-sections through the coating. This paper presents further analysis on the MoS2/Ti composite coatings and with other metal additions such as chromium, tungsten or zirconium.

Advantages of using self-lubricating, hard, wear-resistant MoS2-based coatings

The paper reports a summary of MoS2 development through years. The properties of MoS2 coatings can be improved by the co-deposition of small amounts of titanium. These MoS2/Ti composite coatings known as MoST produced by closed-field unbalanced magnetron sputtering, are harder (1000–2000 compared to 400 HV for MoS2), much more wear-resistant (by a factor of 100) and also less sensitive to atmospheric water vapour (improvement by a factor of 2800 compared to MoS2) during tribological testing. They retain a low specific wear rate of 4×10−17 m3/N m, a low friction coefficient of 0.02–0.1, and a high load-bearing capacity up to 5 GPa. Alternative MoS2/material composite coatings have been developed with similar properties. These coatings have given excellent industrial results for a wide range of cutting and forming applications. Recent industrial performance data related to the characteristics of these MoST auto-lubricating coatings, which are utilised today in large-scale production, are presented.

Performance of MoS2/metal composite coatings used for dry machining and other industrial applications

As previously reported (Fox et al., Proc. PSE Conf., Garmisch Partenkirchen, 14–18 September, 1998), the properties of MoS2 coatings can be improved by the co-deposition of a small amount of titanium. These MoS2/Ti coatings, known as MoST™ produced by closed field unbalanced magnetron sputtering, are harder, much more wear resistant and less sensitive to atmospheric water vapour. These coatings have given excellent industrial results for a wide range of cutting and forming applications. Two forms of these MoS2/titanium composite coatings have been developed: MoS2/titanium composite (low titanium, 10 at%) and MoS2/titanium composite (high titanium, 20 at%). The MoS2/titanium composite (low titanium) exhibits a coating hardness of 500 HV, a coefficient of friction of 0.02 during 100 N applied load reciprocating wear testing, and a low wear rate, while the MoS2/titanium composite (high titanium) exhibits a coating hardness similar to that of TiN, a coefficient of friction of 0.04 during 100 N applied load reciprocating wear testing, and an extremely low wear rate. The choice of coatings is dependent upon the application. Recent industrial performance data related to the characteristics of these MoS2/titanium composite (high titanium) self-lubricant coatings, which are utilised now in large-scale production, are presented.

Tribological properties and wear mechanism of sputtered C/Cr coating

Carbon/chromium (C/Cr) coatings were co-deposited using unbalanced magnetron sputtering deposition technique. The tribological properties including wear mechanism and microstructures of the coatings were investigated using pin-on-disc tribometer, X-ray diffraction (XRD) and transmission electron microscope (TEM). The coatings were characterised by a high hardness (∼2200 HV), a low coefficient of friction (<0.1) and a very low wear rate under very high loads. The pin-on-disc tests of C/Cr coated tool steel against WC ball (5 mm of diameter) revealed low friction coefficient 0.06–0.1 depending on load and low wear rate ∼10−17 m3/Nm with little dependence on load and sliding speed for the test using load up to 80 N and siding speed up to 400 mm/s for 1 h. Critical sliding speed was found above which, the wear rate was higher. The pin-on-disc tests of C/Cr coated austenitic 316 stainless steel against WC ball under a load of 10 N at siding speed of 400 mm/s for 1 h revealed a low friction coefficient ∼0.1 and a very low wear beyond the limitation of the as used wear measurement technique could detect. Microstructure of the coated stainless steel was investigated. XRD profiles of the coating exhibited typical patterns for amorphous-like material, with no crystal-like peaks identified. Cross-sectional TEM analysis of the rubbed tracks of the coated stainless steel revealed some deformation in the substrate near the interface. However, no sign of interfacial spallation was observed, indicative of excellent adhesion of the coating to the substrate. The selected area electron diffraction patterns taken from the rubbed and unrubbed regions of the coating were interpreted and discussed. The tribological performance of this novel C/Cr coating is believed to be closely related to its graphite nature. The high resolution TEM (HRTEM) cross-section analysis of the coating on stainless steel substrate revealed that the outer-most surface of the rubbed track was reoriented due to the rubbing in a 1-h pin-on-disc test under a load of 10 N. Finally, the relationship between deposition parameters and tribological performance of the coating is also discussed in this paper.

Low temperature nitriding of AISI 316L stainless steel and titanium in a low pressure arc discharge

AISI 316L stainless steel (SS) and titanium nitriding were studied in a low pressure arc-assisted nitriding process where the substrate temperature and the plasma parameters are uncoupled. Lower nitriding temperature limits were explored for constant plasma parameters in Ar–N2 gas mixtures and substrates at floating potential. Nitrogen superficial concentration, layer thicknesses and X-ray diffraction analyses were performed on SS specimens nitrided at two temperatures (580 and 680 K) for different times and titanium nitriding was studied in the temperature range 750–1025 K. At low temperature, the nitriding performances are limited by a plasma–surface phenomenon that probably involves recombination of nitrogen atoms.

Hard lubricating coatings for cutting and forming tools and mechanical components

Two new coatings based on graphite and MoS2 have been developed. They combine low friction with high hardness, high load capacity and exceptionally low wear. Both coatings act as solid lubricants, providing protection for both the coated surface and any opposing uncoated surface. The coatings are finding application in improving the general performance of cutting and forming tools and also make possible high-speed machining. The graphite-based coatings have exceptional wear properties under water or oil and results from wear tests under a wide range of conditions are given. A number of practical applications are given, including the protection of artificial hip joints. The advantages offered by the use of such coatings for many mechanical components are demonstrated.

Performance and limitations of MoS2/Ti composite coated inserts

Abstract Self-lubricating low friction MoS2/Ti composite coatings were deposited onto hard coated carbide inserts using a hybrid process and were tested for dry high-speed milling and turning of steel. Dry machining is an important objective in industry to reduce environmental and production costs. It was shown that dry machining using MoS2/Ti composite coatings is possible in some cases. Following an already good knowledge in low speed dry operations including drilling, tapping and threading, the coatings were tested under dry high-speed machining operations (milling and turning) where the temperatures involved are higher. Cutting tool parameters and tool grade and geometry were found to have an influence on the performance of the tools. Temperature and oxidation were investigated separately and correlated to the mechanical, chemical, oxidation and structural behaviour of the tools during machining tests.

An improved diamond-like carbon coating with exceptional wear properties

Parameters for the deposition of diamond-like carbon thin films using a hybrid magnetron sputter/plasma-enhanced chemical vapour deposition (PECVD) system have been optimised in order to improve the properties for high load-bearing applications. An extensive tribological testing program has been carried out. The main results from this testing program are described. A typical unlubricated pin-on-disc test in air using a 5-mm-diameter WC–Co pin under a load of 100 N gave a specific wear rate of 9×10−18 m3 N−1 m−1 and a coefficient of friction of 0.02. Extremely low counter-surface wear was also observed. Dry cold forming simulation tests, carried out using coated WC–Co balls against tin, indicate much lower friction than for uncoated carbide tools under the same conditions. Due to these properties, the coatings are considered to be suitable for a wide range of industrial applications. Some current applications are described.

The structure of tribologically improved MoS2–metal composite coatings and their industrial applications

Abstract The properties of MoS2 coatings deposited by closed field unbalanced magnetron sputter ion plating have been improved by the co-deposition of small amounts of metal [D.G. Teer et al., Surf. Coat. Technol. 94–95 (1997) 572]. These initial MoS2–metal composite (MoST) coatings were hard, adherent (critical load above 120 N) low friction (μ=0.02 at 40% humidity), wear resistant and less sensitive to water vapour than pure MoS2 coatings. The MoST coating has now been further developed and improved to give a coating with higher wear resistance than that originally developed and has been tested in a variety of industrial applications, showing excellent results for a wide range of cutting and forming applications. Industrial testing of coated tools has been performed and the results are presented. Laboratory test results using microhardness testing, scratch adhesion testing, pin-on-disc and reciprocating friction and wear tests are presented. The structure of the coating has been extensively studied by a variety of techniques, including optical microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy. The MoST coating is deposited by rotating the substrates between four targets: three MoS2 and one titanium. Because of the way in which the coating is deposited, it was initially assumed that the coating deposited could be a multilayer coating consisting of alternating layers of MoS2 and titanium. TEM and XRD analysis has been unable to detect the presence of any multilayers within the coating. Further analysis has been carried out to determine the detailed structure of the coating and the location of the titanium. TEM analysis also revealed that the MoST coating was quasi-amorphous and selected-area diffraction was unable to detect any crystalline structure. Further analysis on the amorphous nature of this coating and its stoichiometry is presented.