Understanding integrated effects of humidity and interfacial transfer film formation on tribological behaviors of sintered polycrystalline diamond

Abstract

Polycrystalline diamond (PCD) used in down-hole tool applications is ideally suit for harsh environments such as drilling at circulation break and poor lubrication, in which the water molecules of physical and chemical adsorption can severely affect the tribochemistry effect across cutting interface. Here, tribological behaviors of PCD are studied in a controlled humid atmosphere (5%-50% RH). The friction coefficient is ~ 0.04 under 5% RH condition, which is significantly increased to ~ 0.11 under 50% condition.The run-in period and wear rate of PCD decrease while increasing RH levels during the tribotest. Such an ultra-low friction coefficient regime is explained coincide with the formation of efficient carbonaceous transfer films in the run-in periods through ex situ, micro-laser Raman spectroscopy and Atomic Force Microscope (AFM) measurements.Abstract graph: This paper highlights an intimate relationship among RH levels, dangling bonds passivation, run-in periods, transfer films formation and tribological behaviors.3 bits due to its extremely high wear resistance, high hardness and excellent toughness.Among these applications, fluids such as water, drilling mud and gas, are always required to lubricate the surfaces and interfaces of PCD in down-hole applications and its counter frictional pair. 1-4 Previous works have focused on that diamond rubbing against diamond in the presence of oil and water based drilling fluids had a coefficient of friction between 0.05 and 0.08. 5 Sexton et al. 6 investigated the bearing wear rates in laboratory testing and results showed that PCD thrust bearings provide a long-lasting bearing solution for down-hole drilling tools. Knuteson et al. 7 reported that the friction coefficient values of PCD thrusting bearing decreased in the lubrication regime moving from boundary to mixed-mode and may even become hydrodynamic. Mehan et al. 8 measured the wear rates of PCD and pointed out that wear mechanism of PCD was the result of diamond particles falling off in pin-on-disk tribometer experiments. However, the above works only concentrated on the mechanical aspects of wear performance and have not addressed the role of service environment factors on the friction coefficient, wear and ultimate failure wear of PCD. Considering the operating conditions such as high speeds, high loads, run-in period, frequent starting and stopping, cyclic fluctuating loads and extreme conditions such as drilling at circulation break and poor lubrication, the PCD contact surfaces cannot be ensured keeping in liquid lubrication condition all the time. We have confirmed from tribological behaviors of PCD under different relative humidity (RH) that friction coefficient, wear and adhesion behaviors are significantly affected by the adsorbed water molecules which can have distinct impacts on physical and chemical properties of solid surfaces. Understanding the effects of the relative humidity on sliding interface would be 5 formation of a transfer layer, possibly graphitic in nature. 19 Transfer films are known t...

Effect of Substrate Bias and Temperature on Friction and Wear Properties for ta-C Coating Prepared under Different Substrate Bias Voltages with Filtered Cathodic Vacuum Arc Deposition

“…The 100 V ta-C coating presents a much higher wear rate with a longer running-in cycles despite of having higher hardness of ta-C coating. This severe wear with longer runningin cycles agrees well with previous reports [5,18,19]. During the running-in cycles, the abrasive particles generated from the surface of 100 V ta-C coating with higher hardness results in severe wear with disturbing to form a transfer layer on the wear scar.…”

Effect of sliding mating materials on vacuum tribological behaviors of sintered polycrystalline diamond

“…9(b)) and Al 2 O 3 ( Fig. 9(d)) presents characteristic peaks proving the existence of SiC [26], Si 3 N 4 [27] and Al 2 O 3 [25]. These confirm that no other materials are observed on the wear scars of SiC, Si 3 N 4 and Al 2 O 3 balls.…”

Controllable wear behaviors of silicon nitride sliding against sintered polycrystalline diamond via altering humidity

“…However, the specific friction and wear mechanism of ceramics sliding with PDC under humidity conditions is complex. In previous studies, under 5%‐50% RH, we have found that the formation of transfer films at the sliding interfaces significantly affected the tribological behavior of Si 3 N 4 and PDC . Nevertheless, when RH level reached and exceeded 55%, a totally different wear mechanism of Si 3 N 4 arose.…”

“…In previous studies, under 5%-50% RH, we have found that the formation of transfer films at the sliding interfaces significantly affected the tribological behavior of Si 3 N 4 and PDC. 14 Nevertheless, when RH level reached and exceeded 55%, a totally different wear mechanism of Si 3 N 4 arose. Consequently, the effects of RH levels on the wear mechanism should be systematically investigated via exploring interfacial tribochemical reactions.…”