Tribological properties of laser-textured and ta-C coated surfaces with burnished WS2 at elevated temperatures
Graphical abstract
Introduction
Mechanical components such as engine valves must typically withstand severe tribological contacts at elevated temperatures. To maintain the dimension tolerances, there is a demand for wear resistant and low friction coatings for such valves.
Diamond-like carbon (DLC) films have been of interest due to their unique mechanical, chemical and tribological properties. Hydrogen-free tetrahedral amorphous carbon (ta-C) films have high hardness and low coefficient of friction (COF) in humid conditions [1], which make them good candidates for many tribological applications. The ta-C coatings with high hardness inherently have a high internal compressive stress. Sufficient adhesion of the coating is achieved only by proper substrate surface pretreatment and by applying intermediate coatings. However, their friction properties deteriorate at higher temperatures as low friction values of ta-C films are believed to be dependent on the formation of a thin water layer or water molecules [2], [3], which evaporate from the surfaces at elevated temperatures.
Transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) are known for their solid state lubricating properties [4], [5]. Contrary to ta-C film, these TMDs provide extremely low friction coefficients in vacuum and dry atmospheres [6]. WS2 has also been reported to provide good lubrication at 300 °C in air [7]. Due to their low hardness and load-bearing capacity [8], TMD films alone are inadequate to operate in high-load contacts, as they will wear off rapidly. To overcome this problem, attempts of utilizing surface texturing with solid lubricants have been made [9], [10], [11], [12]. Surface texturing has been widely used with fluid lubrication [13], [14], [15], [16] but it has also been shown to increase significantly the wear life of a burnished MoS2 layer when compared to non-textured surfaces [9]. Various texturing techniques such as machining, reactive ion etching, ion beam and laser have been studied [17]. The fundamental idea of these techniques is the controlled preparation of small, micron-scale dimples to act as hydrodynamic bearings, lubricant reservoirs or as a sink for capturing the wear debris. Laser has been of interest since it provides excellent control over shape and size of the dimples.
The concept of enhancing the tribological properties of DLC by surface texturing is not entirely new, for efforts have already been made in other studies [18], [19]. But to the best knowledge of the authors no solid lubricants or elevated temperatures have been applied in these kinds of reports. In addition, they have concentrated on hydrogenated DLC while studies of ta-C have been lacking. Dumitru et al. did the texturing of a-C:H films by using two different techniques [18]: (1) by texturing steel substrates and subsequently depositing DLC on them (indirect process), and (2) by texturing DLC coatings (direct process). They found out that it was difficult to laser texture DLC coated surfaces for the moment and only the indirectly processed samples were good enough for tribological tests.
The aim of this work was to study the effect of indirect laser surface texturing (LST) and WS2 addition on the tribological properties of ta-C films at elevated temperature (250 °C). The fundamental idea was that as a hard material ta-C would provide wear resistance, load bearing capacity, and low friction in humid conditions, i.e. at room temperature, whereas WS2 would help maintain low friction at elevated temperatures. As shown later, LST would increase the wear life of WS2.
Section snippets
Sample preparation
Polished stainless steel (AISI 316) discs with diameter of 40 mm and surface roughness less than 0.01 µm and hardness of 276±11 HV (average of 5 measurements using 1 kg) were used as substrates. Four different sample variants were fabricated for tribological tests:
- (1)
ta-C only,
- (2)
ta-C with WS2 addition,
- (3)
LST substrate with WS2 addition, and
- (4)
ta-C coated LST substrate with WS2 addition.
In LST process, dimples with diameter of 50 µm, 5–10 µm depth, and a spatial period of 50 µm were laser processed on the whole
Steel surface modifications by LST, ta-C and WS2
LST surfaces had evenly distributed dimples, whose cross-sectional shapes can be seen in 2D profile measurement in Fig. 1. Due to the laser process used, dimples had a rough bottom surface and small, less than 100 nm high bulges. These characteristics were also apparent in SEM (Fig. 2a). Dimples were not fully identical as the depth and shape slightly varied. After the post-polishing process, dimple bulges were removed (Fig. 2b). The coating process resulted in a uniform ta-C coating on the
Discussion
Typically, laser texturing causes relatively high bulges around the dimples, which has been an issue in some studies [10], [22]. In our experiments, a short wavelength picosecond laser was used, which enabled sensitive approach on laser texturing with small dimple bulges. This was essential for maintaining the overall fine surface roughness of steel substrates. Laser textured dimples had a rough, cauliflower-like bottom surface due to the used laser process. Adhesion has been noted to be an
Conclusions
The present study demonstrated possibilities for enhancing the tribological properties of ta-C at elevated temperature (250 °C) by laser surface texturing and WS2 addition. At 250 °C, ta-C film broke down rapidly but with burnished WS2 the wear life was increased remarkably. In addition, low COF values (0.01–0.02) were achieved. Laser surface texturing was shown to be beneficial as it more than doubled the wear life of WS2/ta-C surfaces. Wear analyses showed that LST dimples not only functioned
Acknowledgments
The authors want to thank The MATERA Plus program for funding the research, Raimo Penttilä for laser texturing, Ander Amasorrain and Simo Varjus for technical support, and Erkin Cura for Raman microscopy measurements.
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