II. Experimental Investigations 51
5.2. Tribometry
5.2.1. Chemicals and Materials
The following chemicals have been employed in the experiments as received:
– “AnalaR NORMAPUR® Acetone” from BDH PROLABO® VWR International GmbH
– “ROTIPURAN® Ethanol” from Carl Roth GmbH & Co. KG – “2-Propanol puriss. p.a.” from Sigma Aldrich (#33539)
– “Aqua Ad Injectabilia” from DeltaSelect GmbH in 10 mL ampoules – “Krytox® GPL 104” PFPE oil from DuPont
– “UHU Metall Kontaktkleber” metal glue from UHU GmbH & Co.KG
– “Soluwash® S akachemie® Spezialreiniger” from PUFAS Werk KG / Decotric GmbH
The following materials have been used in the experiments:
– B.BRAUN Injekt® 5 mL syringe
– B.BRAUN size 2 hollow needle (0.80 mm×40 mm)
The investigations presented in this thesis are based on a DIN 623 type 6001 hybrid ball bearing supplied by our partner Cerobear GmbH, Herzogenrath, Germany. Hence 4.763 mm diameter silicon nitride balls were used in the coating process and in the tri-bometry experiments. The steel specimens for the tritri-bometry were also manufactured by Cerobear GmbH from the same X30CrMoN15 1 steel that is employed for the man-ufacturing of the ball bearings using the exact same machines and cubic boron nitride
Chapter 5. Experimental Details 5.2. Tribometry
lathe tools as for the hybrid ball bearing parts. Prior to lathing the steel was hardened by heat treatment. Three 7.8 mm deep M3 screw threads in 120° angle on a 16 mm diameter were tapped into the bottom side of the 25 mm diameter specimens.
The employed tribometer was a BASALT MUST, Tetra GmbH, Ilmenau, Germany. The two cantilever units used throughout all of the experiments had the order number 91007 at Tetra GmbH. The cantilevers had the springs constants in perpendicular direction of 22.2 N m−1 and 19.196 N m−1, respectively, while the spring constants in tangential direction have been 29.1 N m−1 and 22.459 N m−1, respectively.
5.2.2. Experimental Procedure
5.2.2.1. Preparation
Careful not to touch the sliding surfaces, the bearing steel specimens were handled with powder-free nitrile gloves. Furthermore, with tweezers that were cleaned each time si-multaneously with the balls, the silicon nitride balls were also handled with care. Before each of the experiments, ceramics balls and steel specimens were cleaned by brandishing in acetone, ethanol and 2-propanol consecutively. This procedure is in accordance with the general cleaning approach of the hybrid ball bearing manufacturer, Cerobear GmbH, Herzogenrath, Germany. Between the steps and afterwards the parts and tweezers were dried using pressurized air. After the parts have been dried, the silicon nitride balls were glued to the cantilever unit of the tribometer.
Figure 5.1 shows the tribometer with assembled rotation unit, specimen holder and spec-imen as well as with the mounted cantilever unit. The box in the lower part of the housing is the rotation unit, that rotates the bearing steel specimen, the smallest cylin-der visible on top of rotation unit and specimen holcylin-der. A silicon nitride ball is glued to the cantilever of the cantilever unit, while both black fiber optics sensor cables are visible in the upper part of the housing. A zoom on the cantilever and fiber optics unit is shown in figure 5.2.
The steel specimens were screwed on top of a specimen holder on the rotating unit (RM120) by three screws. Lubrication was applied using a syringe and a hollow needle.
The lubricants have been applied drop-wise on the sliding surface of the steel specimen.
For PFPE lubrication 0.2 mL Krytox® GPL 104 covered the entire specimen surface, while in case of lubrication with water 0.3 mL lubricant formed a droplet in the middle of the steel specimen.
rotation unit specimen holder specimen tangential force fiber optics
perpendicular force fiber optics
silicon nitride ball cantilever
Figure 5.1.: Photograph of the tribometer employed in the tribometry experiments, a BASALT MUST from Tetra GmbH, Ilmenau, Germany.
normal fiber
tangential fiber
tangential mirror
normal mirror
ceramics ball
Figure 5.2.: Fiber optics and cantilever unit of the tribometer employed for the work underlying the experimental part of this thesis. A silicon nitride ball is glued underneath the cantilever. Two perpendicular mirrors reflect the light emitted by the fiber optics sensors.
Chapter 5. Experimental Details 5.2. Tribometry
5.2.2.2. Procedure
Due to this droplet formation, for water lubrication only part of the specimen surface was covered. Hence for water lubrication, the experiments were carried out on the same radius of 2.632 mm at 20 RPM three times, after it had been clarified that no significant wear occurred. In case of PFPE lubrication or no lubrication at all (called NEAT) the whole specimen surface could be used. Hence three consecutive experiments employing the same ball and specimen were carried out on three different radii, 2.632 mm, 5.263 mm and 7.519 mm. A constant relative speed of the surfaces of 5.512·10−3m s−1 was maintained by rotation speeds of 20 RPM, 10 RPM and 7 RPM, respectively. This relative speed of the surfaces is about the same as the sliding speed due to slip at the ball-track-interface in a type 6001 ball bearing at 850 RPM. The target perpendicular force was 4 mN for all experiments, which results for these materials in a maximum Hertzian pressure of 874 bar. The experiments have been carried out at a temperature of 20 °C and a relative humidity of 50%. The duration of the experiments was 30 minutes. During this time about 100,000 data points each for tangential and perpendicular force were recorded. A cumulative average of the calculated friction coefficients was used as measurement result.
All possible different coating situations of ball and steel specimen have been considered:
Ball and specimen coated, both uncoated, ball uncoated and specimen coated and vice versa. In combination with three different lubrication states, this results in 12 sets of experiments. One set of experiments comprised at least fifteen experiments each: five different pairs of ball and specimen, for which at least the three usual measurements have been carried out. Longer-term measurements were performed at least once for each coating condition and lubrication state, to determine the wear characteristics of the systems. For water lubrication, longer-term measurements required compensating the evaporating lubricant because no cooling was applied. Hence 6 drops of water were added every half an hour. The shortest longer-term measurements were carried out for water lubrication, in which case the measurement duration was 6 hours, which is 12 times the usual measurement duration and revolutions. The longest measurement has been carried out for PFPE lubrication with 120 hours, while in case of no lubrication 66 hours was the longest measurement duration.