Reducing the friction coefficient of the film will help increase its wear resistance, which can extend the service life of the workpiece. From the above results and, we can see the arc source current has a great influence on the surface quality of the film, and therefore has an important influence on the friction coefficient.
Fig. 1 shows the tribological characteristics curve of the film and the relationship between the arc source current and the friction coefficient. As shown in the figure, the friction coefficient of the film gradually increases as the arc source current increases. At high currents, the arc source target has a higher temperature, resulting in more macroscopic particles and clusters, and these particles or clusters carry higher energy and they are not easily blocked by other particles during the migration from the arc source target to the substrate (sample). The temperature is high when they reaching the surface of the substrate (sample), so the diffusion on the surface is fast, and the deposition rate of the film is fast too. The presence of these large particles leads to the rough surface of the final coated film surface as the particles are large in size and quantity. Therefore, under the same conditions, the friction coefficient of the high-current-plated TiN film is larger than that of the small-current-plated film.
a) I=40A; b) I=70A; c) I=80A; d) I=90A; e) polished stainless steel substrate; f) relationship between arc source current and film friction coefficient
Fig. 1 Curve of frictional characteristics of films coated with different arc source currents and polished high-speed steels
From the tribological characteristic curve of the film, it can be seen that when the arc source current is 40 A, the friction force of the TiN film is low, but when the arc source current increases, the surface quality of the film decreases due to the increase of droplets, and friction force increased. At the beginning, due to vibration, some curves fluctuate, but it gradually stabilizes with time passing.
Comparing the friction curves of films coated with 40 A of arc source current and the friction curves of polished high-speed steel, it can be seen that the friction force of the film is lower than that of polished high-speed steel, the droplets of the surface of the TiN thin film that deposited under low current are small in size and quantity, the roughness is low. With the increase of the current, the friction of the coated TiN film is correspondingly increased, which means that the surface quality of the thin film coated with a large current declines. In addition, these curves can also reflect the difference in the curve transition from the initial unstable zone to the stable zone. When I=40 A, the transition zone is very short. The transition curve of polished high-speed steel is flat and smooth. As the current increasing, the curve in the transition region also gradually becomes sharp, which, as already discussed above, is the effect of droplet particles on the surface of the film.
There are certain rules for the changes in the friction characteristic curve. At the beginning, the surface of the film is subjected to a sudden force, and the droplet particles are affected by the impact. Since these droplet particles only adhere to the surface of the film, they are easily knocked off, and the friction is relatively small. Afterwards, the friction force interacts with the large droplet particles buried deep inside the film, these droplet particles are not easily knocked off, so the friction suddenly rises and then stabilizes. The droplets buried deep in the film are related to the arc source current. If the arc source current is small, the droplets are small in size and quantity, so the droplets buried in the film are small too, and the film has small friction force. As the arc source current increases, the droplets also increase and become larger, so the friction force increases.