Friction Behavior of Ceramic Materials for the Development of Bridge-Bearing Friction Materials

Featured Application

Ceramic bridge bearings are designed for long-term durability, extreme low- and high-temperature environments, and applications where the use of metallic components is restricted.

Abstract

This study evaluates the potential of ceramic materials as friction components in bridge bearings, focusing on durability and frictional behavior under high-load conditions. Bridge bearings traditionally use materials such as PTFE and UHMWPE, which suffer from wear, oxidation, and deformation over time, leading to costly maintenance and frequent replacements. To address these limitations, zirconia-based ceramics were selected for their high hardness, wear resistance, and low friction coefficient. Frictional tests on ceramic samples, including surface roughness adjustments and stress conditions, indicated a stable frictional performance with minimal wear over extended cycles. The results suggest that ceramic materials can maintain consistent frictional properties without lubricant use, potentially reducing bearing maintenance costs and extending their service life. These findings suggest that ceramics could serve as a promising alternative to conventional friction materials in bridge bearings by offering enhanced durability, reduced maintenance requirements, and improved operational reliability.

1. Introduction

Bridge bearings play a critical role as ancillary components that significantly contribute to bridge durability. This is achieved by distributing superstructure loads to the substructure through frictional behavior, while accommodating thermal expansion, contraction displacements, and deflections. To ensure efficient load accommodation, transmission, and movement, the performance of the bridge bearings is crucial. However, the friction material, which is a key component of a bearing, is a decisive factor [1]. The materials used in bridge bearings must support the vertical loads of the superstructure, accommodate displacements due to horizontal movements, and transfer them to the substructure. These materials are positioned between the bridge deck and bearing element (Figure 1).

Traditional friction materials used in bridge bearings, such as polytetrafluoroethylene (PTFE), ultrahigh-molecular-weight polyethylene (UHMWPE), and engineering plastics (EP), are fluoropolymer-based materials known for their excellent compressive-stress performance and superior coefficient of friction (COF) when combined with lubricants [2,3,4,5]. However, owing to recurring frictional behavior during service, issues such as oxidation and loss of lubricants, wear, and deformation of materials (Figure 2) can occur, leading to performance degradation and necessitating the periodic maintenance of bridge bearings. In this context, partial repair or replacement of the material within the bridge bearing is not feasible, resulting in the replacement of the entire bearing unit, even though its average service life is 20–25 years. In addition, reinstalling bearings after material failure leads to shorter damage cycles, causing excessive financial losses.

Currently, most friction materials used in bearing supports are EP, ultrahigh-molecular-weight polyethylene (UHMWPE), and polytetrafluoroethylene (PTFE) [3,6,7,8]. These materials consist of inorganic compounds, with fluoropolymers as their main components [9]. Ref. [10] applied PTFE to standard bearing supports in a 40 m simple span and a two-span continuous bridge. However, PTFE is prone to tearing or deformation over prolonged use, leading to the widespread adoption of UHMWPE and EP, owing to their superior properties and higher durability. Long-term friction tests revealed that the sliding distance of EP was two to five times longer than that of PTFE [11] and its COF was three times higher than that of PTFE. Lubricants have been used to ensure smooth structural behavior [12,13,14,15,16]. However, the friction performance of these materials diminishes over time owing to lubricant depletion or deformation of the material during its extended service life.

Ref. [17] developed a friction material by substituting magnesium oxide with polyvinylidene fluoride (PVDF) and demonstrated through performance analysis that it exhibited improved wear resistance and a COF similar to that of conventional PTFE. Ref. [18] analyzed the COF and long-term friction behavior of bridge bearings using EP, which can enhance the durability of PTFE, whereas [19] conducted tests and analyses of the friction characteristics of modified PTFE.

However, the commonly used friction materials in bridge bearings are insufficient to accommodate the wear and damage caused by cumulative travel distances and the dynamic behavior of friction materials resulting from increased vehicle speeds and traffic volumes [9,11]. Consequently, the demand for new materials with high compressive strength, durability, and safety is growing [9,20,21,22,23,24]. Ceramic materials have the potential to address and overcome the issues associated with these materials.

Ceramic materials possess excellent durability, wear resistance, chemical resistance, and heat resistance while exhibiting high hardness, high strength, and low coefficients of friction [25,26,27]. In addition, ceramics do not deform even after prolonged use, allowing for semi-permanent use, which can significantly reduce the maintenance costs. These properties present a potential solution to the issues of high deformation and corrosion associated with existing friction materials and suggest that ceramics could serve as an alternative to friction materials in bridge bearings, ensuring their efficiency. Although studies on the application of ceramics as friction materials for bridge bearings remain limited, their potential advantages, including high wear resistance, low coefficient of friction, and excellent thermal stability, suggest significant opportunities for future development. As part of this study, we explored these properties through friction tests and discussed how ceramic-based materials could be effectively applied in structural components such as bridge bearings. Recent advancements in material science have led to the development of reinforced and hybrid ceramics by incorporating metals or graphene, thereby enhancing both mechanical strength and flexibility [28,29,30]. These hybrid materials effectively address the inherent brittleness of conventional ceramics while maintaining superior hardness and thermal stability. In particular, ceramic-based composites have emerged as promising candidates for structural applications, including seismic supports. These supports enable controlled sliding and energy dissipation during seismic events, ensuring improved resilience and structural integrity in earthquake-prone environments. This unique combination of properties underscores the potential of ceramics in advanced structural applications where reliability under dynamic loading is critical.

This study investigates the potential of ceramic materials as alternative friction components for bridge bearings by evaluating their performance characteristics, including low coefficient of friction (COF) and durability. To ensure their suitability for structural applications, comprehensive compression and friction tests were conducted on large-scale bridge bearings incorporating ceramic materials. The experimental results provide critical insights into the feasibility of using ceramics in bridge bearing systems.

2. Properties of Ceramics

Zirconia-based (ZrO₂) ceramics supplied by DKFC (Dongpu Korea Fine Ceramics, Icheon, Republic of Korea) were selected as bridge-bearing friction materials considering their economic feasibility, performance, and applicability. The key properties of the materials previously used for bridge bearings and the characteristics of the zirconia-based ceramics used in this study are presented in

Table 1. Comparison of properties by material.

Properties	Ceramic (ZrO2)	Stainless Steel	PTFE	EP	UHMWPE
Young’s modulus (GPa)	220	210	0.41	1.3	31
Flexural strength (MPa)	1000	515	500	70~80	600
Hardness (HV)	1550	500	-	-	-

. Their compositions are listed in

Table 2. Components of ceramic materials.

Component	Result (%)
ZrO2	94.66
Y2O3	5.34
Al2O3	0.23
Fe2O3	0.001
TiO3	0.001
SiO3	0.004
CaO	0.001
Na2O	0.003

. Zirconia ceramics exhibit significantly higher strengths than conventional materials, offering excellent wear resistance owing to their high hardness. However, their high strength results in brittleness, which makes them susceptible to vibration.

Despite their brittleness, when they are applied as a friction material in bridge bearings subjected to continuous wheel loads, the vibrations and energy transmitted from the bridge superstructure dissipate as they reach the bearing. Moreover, the frictional behavior under high loads contributes to damping, allowing the ceramic material to overcome its inherent brittleness. This makes zirconia ceramics suitable candidates for bridge-bearing applications, where durability and wear resistance are critical.

Ceramics, formed by sintering at high temperatures, exhibit excellent chemical and heat resistance, enabling them to withstand harsh environments and maintain durability under repeated friction. Their application in bridge bearings reduces reliance on lubricants, minimizing wear and deformation. This improvement extends the replacement cycle and lowers maintenance costs, making ceramics a promising alternative for enhancing the longevity and reliability of bridge bearings.

3. Friction Test Results and Analysis of Ceramic Materials

3.1. Friction Test Plan for Ceramic Materials

To utilize ceramics as friction materials, an analysis of their friction characteristics is essential, leading to an evaluation of the frictional behavior of the ceramic materials themselves. The friction test was conducted according to the standards for friction materials outlined in the AASHTO LRFD Bridge Construction Specifications (BCS) [31], considering the specifications of the testing equipment and fixed jig standards. The friction material (upper plate) was manufactured with a diameter of 76 mm and a thickness of 5.5 mm, whereas the counter material (lower plate) was produced with dimensions of 130 mm × 140 mm and a thickness of 5.5 mm. The fixed jig was designed to securely hold both the friction material and the counter material in place, ensuring precise alignment, minimizing unintended movement during testing, and enabling accurate and repeatable friction measurements [32,33,34].

The testing equipment used was a sliding pad performance evaluation device with a vertical load capacity of 1200 kN, a horizontal load capacity of 250 kN, and a stroke of ±100 mm (Figure 3). Friction tests were performed under bridge-loading conditions by applying a vertical contact pressure of either 15 or 60 MPa. The horizontal movement was controlled at a displacement of ±12.7 mm from the origin at a speed of 1.058 mm/s. Finally, the COF was determined using the ratio of the vertical force to the horizontal force. To perform and evaluate tribological experiments, a ceramic-friction material with a diameter of 76 mm and a thickness of 5 mm was prepared, along with a rectangular ceramic floor plate designed to allow horizontal movement.

Factors influencing the frictional behavior of bridge bearings include the inherent COF of the material and the surface roughness coefficient, both of which are critical variables [6]. Given the goal of avoiding lubricant use, surface roughness, which significantly affects frictional behavior, can be optimized through surface polishing to achieve lower roughness coefficients, thereby facilitating the expression of lower friction coefficients. To understand the frictional behavior of the ceramic materials, two surface roughness variables were established based on the polishing processes: 0.8 and 0.027. A surface roughness of 0.8 represents the average value typically achieved through ceramic manufacturing, whereas a surface roughness of 0.027 represents the minimum achievable value through polishing without causing performance degradation.

3.2. Frictional Behavior of Ceramic Materials

The surfaces of the ceramic materials were visually inspected before and after the friction tests (Figure 4). Despite repeated horizontal movements under the applied contact pressures of 15 and 60 MPa, the surface of the ceramic-friction material remained exceptionally smooth with no observed defects or damaged areas. The superior hardness of ceramic materials contributes to their high wear resistance, demonstrating that, even under strong contact conditions owing to high vertical loads, ceramics maintain significant resistance to wear from horizontal frictional movement. This indicates that the ceramic materials exhibit excellent durability and stability against abrasive forces, even under rigorous loading conditions.

The friction test results for the ceramic materials with different roughness coefficients and loads are shown in Figure 5. For ceramic-friction materials with a roughness coefficient of 0.8, the results showed a consistent trend without a significant variation in the horizontal load, even as the number of friction cycles increased under contact pressures of 15 and 60 MPa. This indicates the stable frictional behavior of the ceramic material during the repeated friction tests. The average COF for the ceramic material with a roughness coefficient of 0.8 was determined to be 0.16 under 15 MPa and 0.13 under 60 MPa.

For ceramic materials with a roughness coefficient of 0.027, the horizontal load progressively increased with the number of friction cycles, as observed in the horizontal load–horizontal displacement curves. Initially, the COF was approximately 0.1. However, it gradually increased with the number of cycles, reaching an average value of 0.16. The increase in the horizontal load corresponded to an increase in the COF, suggesting a diminishing effect of the surface roughness on the frictional performance over time.

During the initial stages, the friction coefficient of the ceramic material with a roughness coefficient of 0.027 was lower than that of the ceramic material with a roughness coefficient of 0.8. However, as the number of friction cycles increased, the difference decreased and eventually reversed. Although a roughness coefficient of 0.027 theoretically results in a lower COF owing to a smoother surface, the combined effect of the high hardness of the ceramics and strong surface contact under high-load conditions appears to offset the influence of surface roughness at the microscopic level [35]. This effect amplifies the inherent COF of the ceramic material, which is the dominant factor influencing frictional behavior. In conclusion, achieving a low COF through surface polishing to reduce the surface roughness does not yield significant results under high-load conditions for ceramic materials. The intrinsic properties of ceramics, particularly their high hardness and strong contact conditions, overshadow the effects of surface roughness, making surface polishing less impactful in these scenarios.

3.3. Long-Term Durability Evaluation of Ceramic Materials

To simulate the actual frictional behavior, a repeated friction test was conducted on the ceramic-friction material under a vertical load of 15 MPa for 100 cycles. The 15 MPa stress level corresponds to the vertical stresses typically encountered in bearings used in standard beam bridges. Figure 6 shows the horizontal load–displacement curves and capacity of the horizontal friction load over time. The friction load exhibited a highly consistent trend within the load–displacement curves, with stable frictional behavior observed throughout 100 cycles.

The COF evaluation yielded an average value of 0.17, with a coefficient of variation of approximately 0.031. A small variation of approximately 3%, as well as a consistent friction load and stable COF at 0.17, indicate that ceramic-friction materials have the potential to provide stable frictional behavior when applied in bridge bearings. Additionally, visual inspection of the ceramic material after the test revealed no visible damage or defects, confirming its high durability.

These results demonstrate that ceramic materials can maintain stable frictional performance under repeated loading conditions, suggesting their suitability for use in bridge bearings, where long-term durability and consistent behavior are crucial.

4. Test Results and Analysis of Bridge Bearings with Ceramic Applications

4.1. Test Plan for Bridge Bearings

A test plan was developed to analyze the application of ceramic-friction materials in full-scale bridge bearings. The target bridge-bearing capacity in this study was set to 3000 kN. Tests were conducted based on the bridge-bearing test standards outlined in the AASHTO LRFD BCS. A bridge-bearing test evaluation device with a vertical load capacity of 30,000 kN, horizontal load capacity of 5000 kN, and stroke of ±1000 mm was used to perform the compression and friction tests (Figure 7). This setup allowed a comprehensive evaluation of the performance of ceramic materials under realistic bridge-loading conditions, providing insights into the feasibility of using ceramics as materials in high-capacity bridge bearings.

For a comparative evaluation with conventional friction materials, tests were conducted on bridge bearings using ceramic and PTFE friction materials. The PTFE material used had a thickness of 4.5 mm, diameter of 350 mm, and dimple pattern with lubricant application conditions, simulating the environment of actual bridge bearings. For the ceramic material, a roughness coefficient of 0.8 was set, as indicated by previous findings, indicated that achieving a lower surface roughness through polishing was not significantly beneficial. The ceramic material was designed to have dimensions of 50 mm width and 8 mm thickness, with the lower plate featuring side sections of 250 mm height and a central section of 350 mm, whereas the upper plate consistently measured 350 mm. This configuration was used because of the limitations of the ceramic manufacturing equipment, which constrained the achievable length and thickness. In addition, the use of ceramics in the full-plate form proved less economical than the use of PTFE. Therefore, the optimal configuration was determined to be the partial application of ceramics, considering both the manufacturing constraints and cost efficiency. As shown in Figure 8, a partial plate layout was designed to enhance the economic feasibility by selectively, rather than fully, applying ceramic materials. The detailed size and application positions were determined based on the loading conditions of the bridge bearing and typical movement ranges, with an average travel of ±10 mm in the transverse direction and ±50 mm in the longitudinal direction. Consequently, as shown in Figure 8, ceramic materials were arranged in a grid pattern on the upper and lower sections of the bearing to optimize the performance and cost.

For the compression test, a load equivalent to 1.5 times the capacity of the bridge bearing was applied at a constant rate, maintained at the maximum load for 5 min, and then removed. A visual inspection was conducted after the test. To assess potential failure behaviors such as damage and defects due to excessive loading on the bridge bearing, an additional test was performed under extreme conditions with a load set to 3.0 times the bearing capacity.

The friction test was conducted by applying a vertical load equivalent to the full capacity of the bridge bearing (1.0 times its capacity), controlling the displacement at a rate of 1.058 mm/s with a movement range of ±25.4 mm from the origin. The COF was determined as the ratio of vertical to horizontal forces.

4.2. Compression Test Results of Bridge Bearings by Friction Material

The results of the compression tests for bridge bearings with ceramic and PTFE friction materials are shown in Figure 9. Both the ceramic and PTFE materials demonstrated sufficient compressive stress under a load of 4500 kN, which is 1.5 times the capacity of the 3000 kN bridge bearing, showing ideal compressive behavior during both the loading and unloading phases. Additionally, when subjected to a load of 9000 kN, representing 3.0 times the bearing capacity to simulate extreme conditions, both ceramic and PTFE materials exhibited compressive stress with adequate load-bearing capability, indicating that both materials are capable of maintaining sufficient performance under standard and extreme loading conditions, suggesting their suitability for use in bridge-bearing applications.

Ceramics exhibit sufficient compressive durability under both normal and extreme compressive loads, which can be attributed to their superior strength and hardness. Bridge bearings are structural components subjected to significant continuous compressive forces owing to the self-weight of the superstructure and live loads. Although the magnitude of the compressive force varies momentarily owing to the external dynamic loads, these variations are relatively small compared with the continuous compressive load, with a minimal fluctuation range. In addition, even if impacts and vibrations occur, the energy dissipates as it travels to the location of the ceramic-friction material, effectively damping the forces. Although ceramics are brittle materials, they do not require special consideration for impacts and vibrations under typical bridge conditions.

Figure 10 presents the friction test results of the 3000 kN bridge bearings with ceramic and PTFE friction materials. For a bridge bearing with a ceramic material, the friction test results showed that the horizontal load gradually increased with the number of friction cycles and eventually converged to a consistent horizontal load level. Similarly, the COF increased with the number of friction cycles, but gradually stabilized, converging toward an average COF of 0.16. This indicates that, although the frictional behavior of the ceramic material initially increased, the rate of increase diminished over time, resulting in a consistent frictional performance. This stability suggests that ceramic materials do not exhibit defects or failures under repeated friction, confirming their potential as durable materials for bridge bearings.

In contrast, the friction test results of the bridge bearing with PTFE exhibited stable horizontal loads, regardless of the number of friction cycles, with an average COF of 0.0027. The low COF of PTFE is influenced by the inherent properties of the material, which are significantly enhanced by the application of lubricants, resulting in a markedly improved frictional performance. These findings underscore the potential of ceramic-friction materials to provide durable and stable frictional behavior under repeated loading, and highlight the efficiency of PTFE in achieving extremely low friction through the use of lubricants.

After completing all the compression and friction tests, the bridge bearings with the ceramic and PTFE friction materials were disassembled for visual inspection, and the results are shown in Figure 11. The ceramic material exhibited no damage or defects on its edges or surface, and was confirmed to remain securely in place within the bridge bearing without any displacement. This indicates that ceramic materials maintain excellent wear resistance and durability, even under high compressive forces and repeated frictional behavior.

In contrast, although no damage or defects were observed in the PTFE friction material, most of the applied lubricant migrated toward the edge of the friction material. In addition, some of the lubricant flowed outside the bridge bearing through the edges of the material, resulting in lubricant loss. The loss of lubricant can lead to increased wear of the PTFE material, degradation of the bridge-bearing performance, and reduction in replacement intervals, ultimately resulting in increased maintenance costs.

These findings highlight the potential durability advantages of ceramic-friction materials in bridge bearings, while also drawing attention to the challenges associated with lubricant loss in PTFE bearings, which could impact long-term performance and cost efficiency.

5. Conclusions

This study evaluated the performance and behavior of ceramic-friction materials as an alternative to conventional friction materials in bridge bearings using compression and friction tests. The key findings are summarized as follows:

• During the friction tests of ceramic materials under a contact pressure of 15 MPa, the friction behavior differed between roughness coefficients of 0.8 and 0.027; however, the average COF was 0.16 in both cases, indicating a minimal influence of surface roughness. This suggests that the high strength and hardness of ceramic materials, combined with strong surface contact under high-load conditions, minimize the microscale effects of surface roughness and amplify the inherent COF of the ceramics.
• Under both normal and extreme compressive loads corresponding to a 3000 kN bridge-bearing capacity, the ceramic-friction material exhibited ideal compressive behavior during both the loading and unloading phases, demonstrating excellent compressive-stress performance.
• During the friction tests of bridge bearings with ceramic-friction materials, the COF gradually increased with the number of friction cycles; however, the rate of increase diminished over time, eventually converging toward an average value of 0.16. This indicates that ceramic materials can maintain a stable frictional performance with minimal fluctuation.
• Ceramic materials demonstrated excellent wear resistance and durability without defects or damage, even under high compressive loads and repeated horizontal friction. This consistent friction performance suggests that ceramics are advantageous for use as bridge-bearing friction materials and offer benefits in terms of durability, maintenance, and installation.
• Although the COF required for fluoropolymer-based friction materials in bridge bearings is less than 0.05, the ceramic-friction materials do not satisfy this threshold. However, the unique physical and chemical properties of ceramics, along with their superior mechanical characteristics and COF of approximately 0.16, suggest their potential applications in energy dissipation mechanisms, extremely low- and high-temperature environments, and situations where the use of metallic components is restricted.

These findings highlight the potential of ceramic materials to serve as reliable friction components in specialized structural applications, despite their higher coefficients of friction compared with conventional fluoropolymer materials.