Improving tribological properties of sputtered boron carbide coatings by process modifications

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Abstract

Boron carbide coatings are well-known for extreme hardness and excellent wear resistance. In this paper a d.c. magnetron sputter process for the deposition of boron carbide coatings is described. It is shown that by adding small amounts of a hydrocarbon reactive gas (in this case acetylene) the coefficient of friction can be reduced from 0.8 down to 0.2. Results from a laboratory scale deposition device are successfully transferred to an industrial batch coater. The coating adhesion is well enhanced by a titanium interlayer. From the analysis of the chemical composition and from hardness values it is concluded that a structural modification is responsible for the improvement of sliding behaviour. It is suggested that the introduction of additional bondings reduces the brittleness of boron carbide. Furthermore, a comparison with metal-containing amorphous carbon coatings (Me-DLC) reveals several similarities.

Introduction

Boron carbide is well-known as bulk ceramic or powder material. Most of its applications are based on the combination of low specific density and extreme hardness [1], [2], [3]. Furthermore, boron carbide offers excellent chemical and thermal stability due to its mainly covalent bonding. The high hardness and abrasive wear resistance is used for example in sand-blasting nozzles [4]. Its high brittleness, however, prevents extended application as engineering ceramic because it causes severe wear on metallic counterparts. This property is used to advantage with boron carbide grinding powder.

Nevertheless, boron carbide's properties suggest applications in tribological systems under high load and/or elevated temperatures. One application are gear boxes where the sliding mechanism under high loads causes continuous wear, scuffing, micro pitting and pitting fatigue on the teeth which in turn leads to gear failure. By coating the gear boxes with boron carbide wear may be reduced and the lifetime of the gears is increased. Coating the gears reduces not only the gradual wear but also the emergency running properties (running without sufficient lubricant) are significantly improved. Other potential applications include parts for the new types of high-pressure fuel injection systems in diesel engines. Because of the extremely high pressure in these injection systems (up to 2000 bars), the sliding parts need to be coated with a low-friction and wear-resistant coating to prevent seizure, erosion and fatigue.

Several methods of coating preparation are reported: numerous publications refer to plasma-enhanced chemical vapour deposition (PECVD), using BCl3 [5], [6] or boranes [7], [8] as a source for boron. Physical vapour deposition (PVD) techniques, such as d.c. magnetron sputtering [9], [10], [11] or r.f. sputtering [12], [13], using boron or boron carbide targets are suitable for the preparation of boron carbide coatings, too.

In recent years sputtered boron carbide coatings based on patents of Eichen and Flasck [14], [15] are commercially produced and recommended for automotive gears, moulds, cutting and forming tools [16], [17], [18].

In this paper the development of a modified magnetron sputter process of boron carbide coatings is described. It will be shown that process modifications like reactive sputtering can alter tribological behaviour drastically.

Section snippets

Experimental details

Coating development by varying several process parameter was started in a smaller laboratory facility. Thereafter successful process parameters were transferred to an industrial batch coater from Hauzer Techno Coating Europe BV (The Netherlands).

Initial investigations

The experiments in the laboratory scale system were started investigating the parameters argon gas flow and target power. At 100 sccm argon and 4 kW optimum deposition parameters with regard to a high deposition rate was found. Keeping this adjustment constant, small amounts of acetylene gas (1–8 sccm) were added during the process. Fig. 2 demonstrates how the amounts of boron, carbon and hydrogen in the deposited coating change with this modification. The amount of boron decreases nearly

Summary and conclusions

Modified boron carbide coatings were prepared in a laboratory facility and afterwards successfully transferred to an industrial d.c. magnetron sputtering unit (HTC 1000). By adding small amounts of acetylene gas the hardness values of these boron carbide coatings were lowered but the coefficient of friction was effectively reduced to 0.2. There are several indications that adding acetylene to sputtered boron carbide results in structural changes by introducing additional bonds like B–H and C–H

Acknowledgements

The authors would like to thank J. Schröder, P. Willich, K. Schiffmann, U. Wischmann, C. Beckmann and M. Lutansieto from the Fraunhofer IST for preparation, analysis (SIMS, EPMA, AFM, SEM, nanoindentation) and tribological characterisation of the boron carbide coatings.

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