The effect of current-carrying friction on the internal crystal phase of diamond coatings on WC-Co substrate

A great deal of attention has been paid to friction and wear caused by current-carrying friction. In this study, diamond coatings were deposited on the cemented carbides (WC-Co) substrate by hot filament chemical vapor deposition method. Although the diamond coating possessed a higher roughness than the WC-Co substrate, it was effective at reducing the friction coefficient and improving the effectiveness of WC-Co in the current-carrying friction process. In the presence of 1 A, the diamond coating exhibited a lower friction coefficient than the WC-Co matrix, and the diamond transformed into graphite as a result. The results show that diamond-coated surfaces have a better wear condition than WC-Co.


Introduction
The excellent performance of cemented carbides (WC-Co) made them widely used in aerospace, ocean-going vessels, electronics, and other industries [1].However, the poor thermal conductivity of WC-Co made it difficult in the application environment of current-carrying friction.Current-carrying friction was characterized by a stable current between the contact surfaces when the friction pair is moving relative to each other [2].There was an interaction between the conductive system and the frictional system [3].As the friction effect between contact surfaces declines, electrical conductivity will rapidly weaken.In the event of such a short and abrupt change, a large amount of friction heat and joule heat energy will be released between contact surfaces, damaging the working environment and contact state of the friction pair.For current-carrying friction, diamond coatings can achieve excellent results when deposited on the surface of WC-Co substrate due to its exceptional hardness, good friction performance, and high thermal conductivity [4,5].The purpose of this study was to design and prepare a diamond coating capable of improving the current-carrying friction effect between friction pairs.The intrinsic diamond is an insulator.By using various doping methods, researchers prepare highquality diamond coatings, aiming to utilize the diamond's excellent properties in practical electronic devices [6].A diamond has an excellent thermal conductivity (2000-2200 W/mꞏK), five times that of copper, which allows it to transfer Joule heat and friction heat rapidly during the friction fatigue process of current-carrying contacts [7,8].Furthermore, diamond has a low thermal expansion coefficient, which means that it will not deform to cause harmful stresses in the matrix [9,10].
In this paper, a hot filament chemical vapor deposition (HFCVD) method was used to deposit a diamond coating on the WC-Co substrate to study the current-carrying friction on diamond coating and WC-Co.The coating and substrate surfaces were examined using scanning electron microscopy (SEM) after friction and the depth of abrasion marks were measured.Diamond coating crystal composition was examined before and after friction testing using Raman spectroscopy.

Experiment
In this work, a flat sheet of WC-Co (YG6) with a geometric shape of 13×13×4 mm 3 was used as the substrate in the experiment.A pretreatment of the substrate was performed prior to deposition in order to increase the nucleation density of the diamond coating [11], as shown in Figure 1 (a).The Co element on the surface of the WC-Co substrate reacted with the carbon source gas when the diamond grew, forming graphite as a result.By using the acid-alkaline etching method, the Co element can be removed from the substrate.A conventional HFCVD technology was used to prepare the diamond coating in a bias-reinforced reactor.In this experiment, four pairs of stranded tantalum filaments were used, and the distance between the hot filaments and the substrate was fixed at approximately 10 mm.Diamond growth was suitable for an environment in which hot filaments were kept at 2000~2200°C and substrate surfaces were kept at 800-900°C, respectively.In this study, acetone and hydrogen were used as reaction gases, with concentrations set at 90 and 240 sccm, respectively.The diamond deposition process consists of two stages, nucleation and growth.In the nucleation and growth stage, the time was set at 0.5 and 4 h, and the pressure was 1.5 ~ 2.0 kPa and 4.0 ~ 5.0 kPa, respectively, which creates ideal conditions for chemical reactions.
To evaluate the tribological properties, a reciprocating sliding friction test was conducted using the HMS-200Z high-speed current-carrying friction and wear machine in Figure 1 (b).A closed loop is formed when the positive electrode connects to another pair of grindings and returns a current to the negative terminal.A 304 stainless steel was applied to the friction pair, along with WC-Co and diamond coating.During the experiment, the motor speed was 50 R/min, and the duration of the experiment was 30 minutes.The load of the friction test is 50 N, the disc radius is 185 mm, and the rated voltage is 12 V.Relative humidity of 40+2% HR and a temperature of 25+5°C were observed, respectively.Currents of 0, 1, and 3A were applied, and the sample and workbench were cleaned before each test.

Test Results and Discussions
The morphology of the diamond coating is shown in Figure 2(a), and the WC-Co was completely covered.Diamond coatings have a grain size of 3.5 um, which is favorable from a tribological standpoint [3].The surface roughness of diamond coating was much higher than that of the WC-Co substrate in Figure 1 (a).Figure 2 (b) shows the Raman spectrum of the diamond coating, and the characteristic peak of the diamond is located at 1338.2 cm -1 .Raman spectra were all flat, and no obvious graphite peak can be observed at 1550~1600 cm -1 .The friction coefficient of WC-Co and diamond coatings in the current-carrying friction experiment are shown in Figure 3. Since there is no current flowing at 0 A, and the surface roughness of WC-Co was much smaller than that of diamond coatings, the friction coefficient of WC-Co was smaller than that of the diamond coating.The friction coefficient of diamond coating and WC-Co increases significantly in 1 and 3 A compared to 0 A. When the current is switched on, the diamond coating has a lower friction coefficient than WC-Co.Diamond coatings exhibited a lower friction coefficient than WC-Co once the current was applied, which was due to the extremely high thermal conductivity and excellent electrical conductivity.According to the current-carrying friction experiment, diamond coatings had a better friction effect than WC-Co.The morphologies of the diamond coating and WC-Co substrate after the friction test are illustrated in Figure 4.Under the currents of 0, 1, and 3 A, the WC-Co and diamond coating exhibited different morphologies, indicating that friction was affected by the current flowing.Obvious wear marks on the surface of WC-Co were observed when there was a current between contact surfaces, which indicated that current-carrying friction had an important effect on the WC-Co.Based on the results of 0, 1, and 3 A current friction, the diamond coating did not exhibit any significant changes, which indicated that it has a significant impact on current-carrying friction.

Conclusion
The conclusions are obtained as below: (1) When evaluating the results when the friction coefficient changed abruptly after the current was turned on and when the friction coefficient was close to smooth, the diamond coating had a higher current-carrying friction effect than the WC-Co substrate.
(2) As a result of current-carrying friction, the diamond is converted to graphite in the coating.
(3) The reduction of the effect of this transformation will greatly improve the life of diamond coatings in current-carrying friction applications.
(a) Surface morphology of WC-Co after grinding (b) The current-carrying friction platform Figure 1.Surface morphology of WC-Co and the current-carrying friction platform.
Surface morphology of the diamond coating (b) Raman spectroscopy of the diamond coating Figure2.Surface morphology and Raman spectra of the diamond coating.

Figure 3 .
The friction coefficients of diamond coatings and WC-Co under different currents.

Figure 4 .Figure 5 .
Figure 5.The Raman curves of diamond coatings after friction experiments at different currents.