DEVELOPMENT OF A VACUUM TRIBOMETRIC STAND FOR HIGH-TEMPERATURE TESTING AND RESEARCH OF SOLID-LUBRICATING MoS2 COATINGS
The development results of a vacuum tribometric stand for high-temperature tribological tests of materials and coatings under high vacuum conditions at temperatures up to 300 °C are presented. The stand implements the standard method of tribological tests for reciprocating sliding of the ball along the plane (ASTM G133-05). The molybdenum disulfide solid-lubricant coatings deposited by magnetron sputtering of a MoS2 target at a various modes resource tests results are presented.
Solutions of the problems associated with the development of solid-lubricant materials and coatings for extreme operating conditions (under vacuum and high temperature conditions) are connected with working out of technological processes during which it is necessary to carry out tribological tests of samples in the corresponding conditions. For friction units of vacuum equipment, functioning under conditions of vacuum, high temperatures and radiation, the use of traditional liquid lubricants and greases is unacceptable; these conditions require solid lubricants and coatings, namely, the coatings based on molybdenum disulphide (MoS2) are most common. In this connection, the LLC "Electrovacuum Technologies" specialists developed and manufactured a specialised high-vacuum tribometric stand for high-temperature tests, as well as developed and studied thin-film solid-lubricant MoS2-coatings formed by magnetron deposition.
The stand design is based on a scheme that provides a standardised method of testing for reciprocating ball sliding on a plane (ASTM G133-05).
Tribometric stand (Fig.1) is assembled on carrier frame (4) and includes a cylindrical vacuum chamber (1) with a testing unit of tribometric system for testing samples located inside. The flanges of the chamber are connected to the nozzles and angular valve-shutter (7) of vacuum system, as well as a vacuum inlet incorporating a mechanism of reciprocating motion transfer (2) to the test block slide with a drive based on stepper motor and gearbox (3) with crank mechanism. The chamber contains two doors: on the front side of the test bench (with a viewing window) and on the back side – to provide access to the test unit when mounting the test specimens, as well as to perform calibration of the measuring system. The vacuum system of the test stand is based on the spiral pump (5) NVSp-35 (SC Vakuummash) and turbomolecular pump (6) VGTN-150 (VMC). Electro-pneumatic valves (7, 8) are used for switching of the vacuum system lines. Vacuum gauges with manometric transducers (10–12) of domestic production are used to control pressure in the vacuum system.
To transfer the reciprocating motion to the object stage inside the vacuum chamber, a bellows-based vacuum motion input was developed (Fig.2). Bellows (1) is connected to the vacuum chamber by a flange (2). The reciprocating movement is provided by an actuator based on a stepper motor with a reduction gear and a crank mechanism which rod is connected to the movable flange (3), which moves along guides (4) fixed in clamps (6) on slide bearings (5).
Tribometric system of the stand (Fig.3) is located on a pedestal mounted on the bottom flange in the centre of the vacuum chamber. The object stage (1) with the specimen (2) mounted on it is installed on the movable platform (3) which is supported on the body with bushings (13) sliding along cylindrical guides (12). The measuring system includes a measuring bar (4). A rod (6) is attached to the end of the measuring bar (5) and a counterbody is inserted into its hole, which is brought into contact with the sample to be tested. A weight (9) is attached to the rod to create a normal force and the necessary contact stresses in the ball/plane contact. The balance of the unloaded beam is ensured by counterweights (7) attached to the opposite side of the rod. The friction force arising in the contact when moving the stage with the sample relative to the stationary counterbody is perpendicular to the beam and causes deformation of strain plates (8) attached to the beam with high-temperature strain gauges welded to them so as to make a bridge circuit. The measuring strain gauge is connected to the ZET 058 measuring controller (ETMS LCC). The object stage is heated by cartridge heaters placed in openings (10) of the stage. Thermal isolation between stage (1) and movable platform (3) is ensured by ceramic bars (11). The rod (15) placed in the support (14) is attached to the platform (3) to transfer the reciprocating motion from the vacuum input ensuring motion. The construction of the stand includes thermal decouplers and vibration isolation elements that reduce the magnitude of external vibrations passing to the table and the measuring columns.
For the application of MoS2 solid-lubricant coatings, a vacuum process unit was used which provided evacuation of the working chamber to a residual pressure of 10–3 Pa. Coatings were applied by magnetron sputtering of a dia 75 mm and 99.72% purity MoS2 target under DC mode. Argon of 99.9995% purity was used as the working gas. Bronze plates for tribological tests and silicon witness wafers with surface roughness of about 3 nm were used as substrates to measure thickness of obtained coatings by step profile using TR220 roughness profilometer-meter (TIME GROUP INC., PRC). A mask was applied to the surface of silicon wafers to prepare the step, and the mask was removed after coating.
Prior to application of the MoS2 coating, the substrate surfaces were pretreated by liquid cleaning in alkaline solutions in an ultrasonic bath with rinsing and drying. Immediately prior to coating, sample surfaces were treated with an ion beam using an autonomous ion source. Coating samples were obtained using heating (250 °C) and without heating (substrate temperature was about 50 °C). The distance between target and substrate was 95 mm. The coating deposition modes for the prepared samples are shown in Table 1.
Tribological tests were carried out by heating the coated bronze samples to 250 °C. The temperature value was in the range of 247–255 °C throughout the test. A temperature sensor (Pt100) was attached directly to the test surface. The pressure in the chamber was maintained below 10–2 Pa. The reciprocating movement of the stage was carried out at a frequency of 0.5 Hz, the average linear displacement rate of the sample relative to the counterbody was 5 mm/s. The counterbody material was 316L steel rods with a hemisphere dia. 3 mm at the end. The counter-body load provided a maximum Hertz-stress value at the contact of about 1.04 GPa (calculated value; calculations were made according to the formulas given in ). The calculations used the elastic moduli of the materials for a temperature of 250 °C.
RESULTS AND DISCUSSION
The coated specimens were tested under above modes until complete wear which was determined by increasing the friction coefficient to values characteristic of uncoated bronze. After testing, bronze was visible on the friction paths. Each specimen was tested over three working days (6–8 hours per day – test time), with stops, with the specimens remaining in the chamber, under vacuum conditions, until the next test. Figure 4 shows summary plots of the change in friction coefficient of the MoS2-coatings on specimens No.1 and No.2 over the full test cycle.
The results show the higher tribological performance of MoS2-coated sample No.2 without substrate heating. During seven hours the coefficient of friction for sample No.2 did not exceed the value of 0.08, and, then, for another 10 hours its value was about 0.1 time to complete wear – more than 20 hours. For specimen No.1 friction coefficient was less stable, increasing to 0.1 during 3 hours, rising from 0.1 to 0.15 at 6 to 12 hours. The coating life on sample No.1 was less than 13 hours.
Thus, the research results indicate performance of the solutions adopted in the design of the test stand. The vacuum system of the stand provides vacuum in the chamber up to 10–5 Pa. The existing system of heating samples provides their heating up to 300 °C under vacuum conditions, and this limitation is related to the design of the used cartridge heaters of small diameter (6 mm). By replacing the heaters for higher temperature ones, it is possible to heat samples up to 450 °C.
The results of the tribological tests of the coated samples show the higher performance and service life of coating No. 2, applied without substrate heating.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.