Tribological Property of Al3BC3 Ceramic: A Lightweight Material
by
, , ,
Jinjun Lu
1,2,*
,
Rong Qu
1,
Fuyan Liu
2,3,
Tao Wang
2,4,
Qinglun Che
2,5,
Yanan Qiao
1 and
Ruiqing Yao
1,2,* 1
Key Laboratory of Synthetic and Natural Functional Molecule of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710127, China
2
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
3
Changzhou Institute of Technology, Changzhou 213032, China
4
Beijing Institute of Aeronautical Materials, Beijing 100095, China
5
School of Mechanical and Automotive Engineering, Qingdao University of Technology, Qingdao 266520, China
*
Authors to whom correspondence should be addressed.
Lubricants 2023, 11(11), 492; https://doi.org/10.3390/lubricants11110492
Submission received: 20 October 2023
/
Revised: 12 November 2023
/
Accepted: 13 November 2023
/
Published: 14 November 2023
(This article belongs to the Special Issue Functional Lubricating Materials)
Abstract
:Lightweight materials with a density less than 3 g/cm3 as potential tribo-materials for tribological applications (e.g., space tribology) are always desired. Al3BC3 ceramic, a kind of ternary material, is one of the lightweight materials. In this study, dense Al3BC3 ceramic is prepared via a reactive hot-pressing process in a vacuum furnace. Its tribological properties are investigated in two unlubricated conditions (one is at elevated temperature up to 700 °C in air, and another is in a vacuum chamber of back pressures from 105 Pa to 10−2 Pa at room temperature) and lubricated conditions (i.e., water and ethanol as low-viscosity fluids). At 400 °C and lower temperatures in air, as well as in vacuum, the tribological property of Al3BC3 ceramic is poor due to the fracture of grains and formation of a mechanically mixed layer. The beneficial influence of adsorbed gas species on reducing friction is very limited. Due to the formation of lubricious tribo-oxide at 600 °C and 700 °C, the friction coefficient is reduced from ca. 0.9 at room temperature and 400 °C to ca. 0.4. In the presence of low-viscosity fluids, a high friction coefficient and wear but a polished surface are observed in water, while a low friction coefficient and wear occur in ethanol. A lubricious carbide-derived carbon (CDC) coating on top of Al3BC3 ceramic through high-temperature chlorination can be fabricated and the wear resistance of CDC can be improved by adjusting the chlorination parameters. The above results suggest that Al3BC3 ceramic is a potential lubricating material for some tribological applications.
1. Introduction
There is ongoing demand for light-weight lubricating materials for many tribological applications (e.g., gyro for space tribology). Because of its low density (2.66 g/cm3) and high elastic modulus (137 GPa), Al3BC3 ceramic is an attractive material for light-weight structural components [1,2,3]. From the viewpoint of tribology, lamellar Al3BC3 ceramic is also considered as an attractive candidate for tribological components under both unlubricated and lubricated conditions. For example, based on the ratio of hardness to Young’s modulus (Hv/E = 0.068), Al3BC3 ceramic is predicted to have excellent wear resistance at room temperature [1]. In addition, the stiffness of Al3BC3 ceramic at 1600 °C is 79%, as high as that at room temperature, which is much higher than that of Ti3SiC2 [1]. This suggests that Al3BC3 ceramic is a candidate for the production of high-temperature wear-resistant materials [1,2,3,4].
Up until now, there have been no published reports on the fundamental tribological property of Al3BC3 ceramic under both unlubricated conditions (e.g., air bearings for gyro) and lubricated conditions (e.g., bearings and surface finishing). As a result, the application of Al3BC3 ceramic is greatly hindered due to the lack of basic data of the tribological property. Currently, a tentative exploration of the tribological properties of Al3BC3 seems to be the first step.
The importance of the tribological properties in air at elevated temperatures and in vacuum is highly valued due to the physical, chemical, and mechanical properties of Al3BC3 ceramic. In addition, technical solutions to obtain low friction of the Al3BC3 ceramic by liquid lubrication or solid lubrication are no doubt necessary in some cases. It is well known that water and ethanol are environmentally friendly and efficient as a lubricant and a polishing agent as well. A carbide-derived carbon coating on top of Al3BC3 ceramic is also expected to be an effective method of self-lubrication. In this connection, the fundamental tribological property, as well as the liquid lubrication and self-lubrication of Al3BC3 ceramic are presented in this study.
2. Materials and Methods
2.1. Preparation of Al3BC3 Ceramic and Carbide-Derived Carbon Coating
Lamellar Al3BC3 ceramic is prepared by reactive hot-pressing at 1800 °C and 15 MPa starting from powders of Al, B4C, and graphite in a vacuum furnace. The powders of Al, B4C, and graphite (−200 mesh) with a mole proportion of 3.2:0.25:2.30 are used to prepare Al3BC3 ceramic with low impurities. Al3BC3 has a lamellar structure (see Figure 1). The Al3BC3 ceramic for the tribological tests is dense with a porosity less than 2%. Some of the physical and mechanical properties of Al3BC3 ceramic include a density of 2.56 g/cm3, three-point bending strength of 175 MPa, microhardness (10 gf) of 12.1 GPa, and indentation fracture toughness of 2.1 MPa⋅m1/2.
Carbide-derived carbon (CDC) coating on top of Al3BC3 ceramic is prepared via a high-temperature chlorination process [5]. By varying the concentration of Cl2 in a Cl2 + Ar mixture and the duration of chlorination, two CDC coatings are prepared. Schedule A is prepared in 6.67% Cl2 + Ar at 800 °C for 2 h and schedule B is a two-step process of 6.67% Cl2 + Ar at 800 °C for 20 min and 3.33% Cl2 + Ar at 800 °C for 1 hr. Dechlorination is not conducted and residual adsorbed Cl2 is not removed. The thickness of the CDC coating is 20 to 30 μm. The advantage of CDC coating is its low internal stress [6]. The microstructure of the CDC coating on top of the Al3BC3 ceramic is different from that of the CDC coating on SiC and Ti3SiC2 (see Figure 2). A possible chemical reaction for high-temperature chlorination is as follows:
Al3BC3(s) + Cl2(g)→AlCl3(g) + BCl4(g) + C(s),
2.2. Tribological Tests
Two kinds of tribo-meters are employed to conduct tribological tests. A CSM THT tribo-meter with a ball-on-disk configuration is used for the evaluation of tribological properties (1) at high temperature, (2) in liquid lubrication, and (3) in the presence of a CDC coating. A home-made vacuum friction tester with a ball-on-disk configuration is used for tribological tests in vacuum. A G3 grade Si3N4 ball with a diameter of 3.175 mm is used for both tribo-meters. Fluctuation of the friction coefficient data of a given tribo-couple is frequently observed in this study, and it depends on the materials and assembly of the tribo-couple and sensitivity of the sensor to detect a friction force.
2.2.1. CSM THT Tribo-Meter with a Ball-on-Disk Configuration
A silicon nitride ball (3.175 mm in diameter) sliding on a bare Al3BC3 ceramic disk or CDC on Al3BC3 ceramic disk (25 mm in diameter and 3 mm in thickness) is used. For liquid lubrication, the disk is immersed in either flooded distilled water or flooded ethanol. Test conditions are 0.1 m/s for sliding speed, 5 N for normal load, temperature from 25 °C (for liquid lubrication and CDC coating) to 400 °C, 600 °C, and 700 °C. The wear volume is determined by a surface stylus profiler.
2.2.2. A Home-Made Tribo-Meter with a Ball-on-Disk Configuration
A silicon nitride ball (3.175 mm in diameter) sliding on a bare Al3BC3 ceramic disk is used. The test begins at a back pressure of 105 Pa for 10 min. After evacuation to a back pressure of 104 Pa, the test starts by using the same Si3N4 ball on the same wear track. The same process is repeated for back pressures of 103 Pa, 102 Pa, 101 Pa, 100 Pa, 10−1 Pa, and 10−2 Pa. The test conditions are 10 rpm rotation speed at a diameter of 12 mm, 2 N for normal load, and room temperature. The wear volume is determined by a surface stylus profiler.
3. Results
3.1. Tribological Property at Elevated Temperatures and in Vacuum
At room temperature, a stage of low friction coefficient (SLFC, ca. 0.2 in vacuum test, Figure 3a and ca. 0.4 in tests of high-temperature series, Figure 3b) before a stage of high friction coefficient (SHFC) is observed. In Figure 3a, such a SLFC occurs at back pressures of 105 Pa and 104 Pa, while no such SHFC occurs at back pressures of 103 Pa and lower. This implies that the SLFC is associated with an adsorption and desorption mechanism of gas species on the worn surface. In the vacuum test, a tribological test is begun at a back pressure of 105 Pa. After a ten-minute sliding test, the chamber is evacuated to a back pressure of 104 Pa and the re-adsorption of gas species on the sliding surface occurs. This enables a SLFC stage at 104 Pa. At back pressures of 103 Pa and lower, the time for the re-adsorption of gas species might be too long to fulfill multi-layer adsorption. Therefore, SHFC occurs at the very beginning of the sliding at back pressures of 103 Pa and lower.
The SLFC is not observed due to the desorption of gas species at 400 °C together with a lack of lubricious tribo-oxide, Figure 3b and Figure 4. The low friction coefficients at 600 °C and 700 °C are mainly attributed to tribo-oxides (B2O3). On one hand, the ionic potential of B2O3 is as high as 12, suggesting that B2O3 is a good high-temperature solid lubricant [7]. On the other hand, B2O3 is a glaze substance at 600 °C and 700 °C.
In summary, the tribological property of Al3BC3 ceramic in sliding against a Si3N4 ball is not plausible in vacuum at room temperature or in air at temperatures of 400 °C and lower. At temperatures of 600 °C and 700 °C, the tribological property of Al3BC3 ceramic is much better. Notably, the friction coefficients of Al3BC3 ceramic at 600 °C and 700 °C are comparable to that of a high temperature self-lubricating composite PM304 at 630 °C (Figure 3b, data from the authors’ laboratory). The frictional traces at elevated temperatures in air can be found in Figure 4.
3.2. Lubrication in Water and Ethanol
It is clear that the high friction (Figure 5a) and severe wear (Figure 5b) of Al3BC3 ceramic and its counterpart material under an unlubricated condition can be reduced by using either flooded water or flooded ethanol as a lubricant. Specifically, Figure 5a suggests that under unlubricated conditions, a SLFC stage occurs (the same reason as the above-mentioned) and the friction coefficient increases gradually to a high friction coefficient of 0.6. It is obvious that the fracture of grains and formation of a discontinuous mechanically mixed layer (Figure 5c) are the two main characteristics of the worn surface of Al3BC3 ceramic under unlubricated sliding. The fracture of grains and formation of such a tribo-layer on Al3BC3 ceramic are commonly found in ceramics, including MAX phase materials [8,9].
Although no such SLFC and friction coefficient are stably as high as ca. 0.5 in distilled water, the worn surface is smooth as a result of a polishing effect. Water, at the least, can be a good polishing agent for the planarization of Al3BC3 ceramic. Additionally, it is reasonable that a well-polished surface enables the application of self-lubricating film (e.g., diamond-like carbon film and graphene film) in good quality.
Ethanol, as a good lubricant, significantly reduces the friction coefficient, which is observed for many ceramics [9]. Some reports emphasize the role of tribo-chemical products, while some focus on the role of the physical adsorption of ethanol on the sliding surface in the lubricating behavior of ethanol. It is an interesting but also challenging topic.
It is well known that water and ethanol have almost identical viscosities at room temperature. Therefore, the role of viscosity for the different tribological properties in water and in ethanol is insignificant. The water dissolution of friction-induced fresh asperities might be a good reason for the polished worn surface. A low and stable friction coefficient suggests that the robust physical adsorption of ethanol on the sliding surface rather than tribochemistry is the main reason. In case of friction dominated by tribochemistry, the friction coefficient will decrease with sliding duration.
3.3. Self-Lubrication by CDC Coating
As seen in Figure 6a, CDC coatings significantly reduce the friction coefficient to 0.3 or even lower. The problem of the CDC coating is its relatively high wear rate (see Figure 6a). Similar results are found for CDC coating on top of Ti3SiC2. The wear resistance of CDC can be improved by adjusting the chlorination parameters (see Figure 6a). As seen in Figure 6b, the two CDC coatings are composed of poly-crystalline graphite, similar to Ti3SiC2 [5]. In addition, schedule A produces a ‘thicker’ CDC coating while schedule B produces a ‘thinner’ CDC coating. That might be the reason for the better wear resistance by schedule B.
The CDC coating is a highly porous coating of low density. The porosity of CDC is higher than 50% [5,6]. Therefore, it will not increase the density of the Al3BC3 ceramic. In addition, the internal stress of the CDC coating is much lower than diamond-like carbon film, which represents an advantage over the diamond-like carbon film. This means that the thickness of the CDC coating can be as high as several millimeters without risk of peeling [6].
4. Discussion
Al3BC3 ceramic behaves like brittle ceramics (i.e., fracture of grains rather than plastic deformation) under tribological loading. Al3BC3 ceramic is not a plausible lubricating material in vacuum at room temperature as well as at temperatures lower than 600 °C. Tribo-oxidation (glazed B2O3) in oxidizing atmosphere renders Al3BC3 ceramic with reduced friction and wear at 600 °C and 700 °C. Liquid lubrication by low-viscosity fluids and solid lubrication by carbon coating suggest that Al3BC3 ceramic can be used as a tribological component, either by liquid lubrication or by surface modification. The surface chemistry of Al3BC3 ceramic in various atmospheres and liquids is the key to understanding its tribological behavior. There are still many topics that deserve further investigation. For example, Al3BC3 ceramic with a low-friction coating (e.g., diamond-like carbon film) can be a candidate material for fabricating gyro for space tribology.
Author Contributions
Investigation, F.L., T.W. and Q.C.; data curation, F.L., T.W., Q.C. and Y.Q.; methodology, R.Q.; investigation, R.Q., F.L., T.W. and Q.C.; writing—original draft preparation, J.L.; writing—review and editing, J.L. and R.Y.; supervision, J.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.
Funding
This work is supported by Natural Science Foundation of China (51775434), Hundred Talent of Shaanxi Province.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
SEM micrograph of the edge of Al3BC3 grains.
Figure 2.
(a) Microstructure of CDC coating under low magnification; (b) the white particles are AlCl3 (EDS spectrum in (b)).
Figure 2.
(a) Microstructure of CDC coating under low magnification; (b) the white particles are AlCl3 (EDS spectrum in (b)).
Figure 3.
(a) Frictional trace of Al3BC3 ceramic in vacuum at room temperature; (b) average friction coefficient and wear rate at elevated temperatures in air. The black dash line is the average friction coefficient of PM304.
Figure 3.
(a) Frictional trace of Al3BC3 ceramic in vacuum at room temperature; (b) average friction coefficient and wear rate at elevated temperatures in air. The black dash line is the average friction coefficient of PM304.
Figure 4.
Frictional traces of Al3BC3 ceramic disk in sliding against a Si3N4 ball at elevated temperatures.
Figure 4.
Frictional traces of Al3BC3 ceramic disk in sliding against a Si3N4 ball at elevated temperatures.
Figure 5.
(a) Frictional traces of Al3BC3 ceramic under unlubricated and lubricated conditions; (b) wear rates of Al3BC3 ceramic and Si3N4 ceramic under unlubricated and lubricated conditions; (c) SEM micrograph of the worn surfaces of Al3BC3 ceramic under unlubricated conditions; (d) SEM micrograph of the worn surfaces of Al3BC3 ceramic in water.
Figure 5.
(a) Frictional traces of Al3BC3 ceramic under unlubricated and lubricated conditions; (b) wear rates of Al3BC3 ceramic and Si3N4 ceramic under unlubricated and lubricated conditions; (c) SEM micrograph of the worn surfaces of Al3BC3 ceramic under unlubricated conditions; (d) SEM micrograph of the worn surfaces of Al3BC3 ceramic in water.
Figure 6.
(a) Friction coefficient and wear rate of CDC coatings prepared using different chlorination schedules; (b) XRD patterns of Al3BC3 ceramic, CDC coating by schedule A, and CDC coating by schedule B. < indicates the (002) peak of graphite. Small amount of graphite in Al3BC3 ceramic.
Figure 6.
(a) Friction coefficient and wear rate of CDC coatings prepared using different chlorination schedules; (b) XRD patterns of Al3BC3 ceramic, CDC coating by schedule A, and CDC coating by schedule B. < indicates the (002) peak of graphite. Small amount of graphite in Al3BC3 ceramic.
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MDPI and ACS Style
Lu, J.; Qu, R.; Liu, F.; Wang, T.; Che, Q.; Qiao, Y.; Yao, R. Tribological Property of Al3BC3 Ceramic: A Lightweight Material. Lubricants 2023, 11, 492. https://doi.org/10.3390/lubricants11110492
AMA Style
Lu J, Qu R, Liu F, Wang T, Che Q, Qiao Y, Yao R. Tribological Property of Al3BC3 Ceramic: A Lightweight Material. Lubricants. 2023; 11(11):492. https://doi.org/10.3390/lubricants11110492
Chicago/Turabian StyleLu, Jinjun, Rong Qu, Fuyan Liu, Tao Wang, Qinglun Che, Yanan Qiao, and Ruiqing Yao. 2023. "Tribological Property of Al3BC3 Ceramic: A Lightweight Material" Lubricants 11, no. 11: 492. https://doi.org/10.3390/lubricants11110492
APA StyleLu, J., Qu, R., Liu, F., Wang, T., Che, Q., Qiao, Y., & Yao, R. (2023). Tribological Property of Al3BC3 Ceramic: A Lightweight Material. Lubricants, 11(11), 492. https://doi.org/10.3390/lubricants11110492
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