Ti-C and CFs Work Together to Enhance the Comprehensive Tribological Properties of PTFE-Based Composites for the Manufacture of Wave Glider Power Shafts

Abstract

Wave gliders’ power system shafts face complex conditions. To enhance their operational stability, it is crucial to study PTFE, a polymer material that could replace traditional metals. This study added carbon fiber (CF), titanium carbide (Ti-C), and both to a PTFE matrix. The impact of seawater immersion on water absorption and the mechanical properties was examined, as well as friction and wear characteristics under constant amplitude cyclic (CAC) loading and seawater lubrication. The results indicated that while Ti-C boosts PTFE matrix hardness, its poor binding with the PTFE matrix leads to high water absorption in Ti-C/PTFE (PTFE-3), causing a significant decrease in the mechanical properties post-immersion and poor friction and wear performance. In contrast, CFs and the PTFE matrix have good interfacial bonding and greatly improve the resistance of the PTFE matrix to cyclic loading and seawater immersion. Therefore, CF/PTFE (PTFE-2) shows good mechanical and tribological properties. Moreover, incorporating a certain amount of CFs into Ti-C enhances its adhesion to the PTFE matrix, reducing the occurrence three-body wear and allowing Ti-C to fully utilize its high hardness. Thus, the combination of Ti-C and CFs markedly improves PTFE’s mechanical and tribological properties under cyclic loading and in seawater.

1. Introduction

In the context of the burgeoning marine and defense industries, there is an escalating demand for enhanced stability, technological, and safety in wave glider operations [1,2,3]. The wave glider spring hydrofoil system, a pivotal component of the wave glider system, is tasked with the conversion of wave energy into propulsion force, thereby facilitating the autonomous navigation of the wave glider. However, the sliding friction pairs, comprising the shaft and sleeve in the spring hydrofoil system of the wave glider, encounter intricate operational conditions in marine environments [4,5]. The simplified three-dimensional structure is shown in Figure 1a. The shaft is subjected to a multitude of forces, including the pull and thrust exerted by the underwater tractor, as well as the gravitational and buoyant forces that the wave glider itself must overcome, rendering it susceptible to wear [6,7]. Furthermore, given that the wave glider operates in seawater, the shaft must bear the load generated by the spring stretching and stretching, which are classified as Constant Amplitude Cycle (CAC) loads [8,9]. These loads induce torsion and bending vibrations in the shaft, which can disrupt the normal functioning of the wave glider and pose significant safety hazards to its structure. Taking Figure 1b, c as an example, the wave glider’s load during operation causes serious wear on the working face of the shaft, and even a risk of fracture. Consequently, investigating the impacts of CAC loads on the wave glider shaft’s new wear- and corrosion-resistant materials that were developed is crucial for improving operational efficiency and ensuring the safe navigation of the wave glider.

In the realm of marine technology, the traditional reliance on oil lubrication for the shafts of wave gliders has been increasingly scrutinized due to the marine environmental repercussions of energy depletion and the risks of oil spills. Consequently, there is a growing imperative to explore alternative, environmentally benign lubricants. The researchers [10,11,12] involved found that the utilization of natural seawater as a lubricating medium itself is a viable and sustainable alternative to conventional oil lubrication, offering significant potential for energy conservation and the optimization of seawater resource utilization. However, the inherent challenges posed by seawater’s low viscosity and aggressively corrosive nature cannot be overlooked, particularly in relation to the metal and alloy components that are susceptible to accelerated electrochemical corrosion and wear under seawater-lubricated conditions [13,14]. This reality underscores an urgent demand for the development and deployment of high-performance polymer materials that can effectively replace traditional alloy materials in marine applications, thereby addressing the forthcoming challenges in the domain of marine engineering and environmental sustainability.

In recent years, many authors [15,16,17] conducted a plethora of research that has demonstrated that tribological pairs comprising metals and polymers exhibit markedly enhanced tribological characteristics within the marine environment. Polymers possess a multitude of advantages over metallic counterparts, such as a reduced weight, enhanced corrosion resistance, chemical inertness, and inherent self-lubricating attributes, rendering them preferred material for applications in the marine sector. Among various polymer materials, PTFE has emerged as a material of choice across various industrial domains, which is attributable to its exceptional resistance to corrosion, absence of stickiness, and robust resistance to oxidation. Moreover, related authors [18,19,20] found that the exceedingly low coefficient of friction of PTFE positions it as one of the most prospective materials for friction-reducing components. However, the inherent limitations of pure PTFE, such as its low tensile strength, inadequate wear resistance, and susceptibility to creep, necessitate the exploration of enhancement strategies. Numerous researchers [21,22] have identified that the incorporation of a specific proportion of inorganic fillers, including graphite (GR) and carbon fiber (CF), etc., into the PTFE matrix can significantly ameliorate its tribological properties and broaden its practical applications. This approach not only addresses the material’s shortcomings but also paves the way for the development of advanced polymer composites that are better suited for the demanding conditions encountered in marine applications.

In the realm of material science, integrating Ti-C into composite materials has been a subject of extensive research. For instance, de Viteri et al. [23] have developed a Ti-C coating that exhibits superior tribological behavior and antibacterial properties. Comparative studies have demonstrated that Ti-C-coated materials outperform their uncoated counterparts, with the coating providing an exceptional protective effect that effectively mitigates frictional corrosion and fretting wear. In another study, Wang et al. [24] investigated the impacts of Ti-C coatings with varying carbon (C) contents on the sliding friction and wear properties of Ti-C and Si-C balls. They discovered that the optimal tribological properties were achieved when C was deposited with a current of 3 A, highlighting the critical role of the carbon content in determining the performance of Ti-C coatings. Sadeghi et al. [25] fabricated Ti-C/Cu composites using Cu, Ti, and C powders. They found that utilizing graphene with a high carbon content as a carbon source led to the formation of finer Ti-C particles. This refinement significantly enhanced the tribological properties of the composites, reducing the friction coefficient and wear loss by 16% and 6%, respectively. Similarly, Cheng et al. [26] explored the effects of different C contents on the mechanical and tribological properties of Ti-C coatings. Their findings revealed that as the C content increased from 0% to 9.3%, there were notable reductions in the friction coefficient and wear rate. These studies highlight the great potential of Ti-C to improve the properties of composite materials and confirm the strong connection between Ti-C and high-carbon carbon fibers, especially in marine applications requiring high wear resistance and corrosion resistance. Strategic manipulation of the carbon content in Ti-C coatings becomes critical in customizing material properties to suit specific tribological applications.

While previous studies have delineated the potential of Ti-C as a reinforcing filler, they have yet to extensively investigate its practical applications post-reinforcement or its tribological behavior within composite materials. Most of these studies have concentrated on tests under relatively static and low loads, with scant reporting on experiments conducted under complex conditions. Especially for the application in wave glider drive shafts, it is imperative to consider the impact of CAC loading during operation. Addressing these gaps, this paper examines the mechanical and tribological behaviors of PTFE composites reinforced with Ti-C and CF fillers in a seawater environment and under CAC loadings. Four types of PTFE and its composites were prepared via hot pressing, and their water absorption was evaluated in a seawater environment. Furthermore, a Shore hardness tester and universal tensile testing machine were employed to assess the mechanical properties of the four PTFE materials before and after seawater immersion. The tribological properties were tested by a test bench designed and built by ourselves. The three-dimensional wear patterns, surface roughness values, and microscopic wear patterns of four PTFE and its composites after abrasion were characterized by laser confocal microscopy (LSCM) and scanning electron microscopy (SEM). The results of this study can provide valuable insights for the selection of new shaft-surface materials for marine equipment such as wave gliders, which need to withstand complex conditions.

2. Materials and Methods

2.1. Preparation of the Materials and Friction Pairs

The external component of the friction pairs, constituted from solution-treated 316L stainless steel, was sourced from Shanghai Baosteel Group (Shanghai, China).

Table 1. Mechanical properties of 316L stainless steel.

Properties	Values
Density (g/cm3)	8.03
Elasticity modulus (GPa)	206
Brinell hardness (HB)	230
Elongation (%)	30
Heat conductivity coefficient (W/(m·k))	16.3
Thermal expansivity (10−6·°C)	16.0
Tensile strength (MPa)	620
Yield strength (MPa)	310

shows its mechanical characteristics, while its chemical composition and elemental content are detailed in

Table 2. Chemical composition of 316L stainless steel.

Composition	C	Si	Mn	P	S	Gr	Ni	Mo
Content (wt.%)	≤0.030	≤1.00	≤2.00	≤0.035	≤0.030	16.00 ~18.00	10.00 ~14.00	2.00 ~3.00

. The specimen was subsequently machined into a bearing configuration with dimensions of 40 mm long, an outer diameter of 40 mm, and an inner diameter of 30 mm. The specimen’s upper surface features a threaded hole and two optical holes, measuring 8 mm in diameter with a depth of 5 mm and 6 mm in diameter with a depth of 4 mm, respectively. The threaded hole is designated for load attachment, whereas the optical holes are engineered to facilitate the inflow of the lubricating medium.

The internal components of the friction pairs were fabricated from four distinct PTFE matrix composites: PTFE-1, PTFE-2, PTFE-3, and PTFE-4. These composites were prepared via hot pressing technology and consisted of a pure PTFE matrix (PTFE-1), 30% CF-reinforced PTFE (PTFE-2), 30% Ti-C-reinforced PTFE (PTFE-3), and a composite filled with 20% CFs and 10% Ti-C in the PTFE matrix (PTFE-4). The PTFE powder was FR104-4, with an average particle size of 23 microns and density of 2.1 g/cm³. The carbon fibers (CFs) had an average particle size of 12 microns and density of 1.84 g/cm³. The titanium carbide (Ti-C) had an average particle size of 25 microns and density of 4.93 g/cm³. The Ti-C particles were surface treated. The surface oxide layer was first removed by pickling, and then a layer of alloy was deposited on the surface by electroless plating to improve its conductivity and oxidation resistance. All powders and reinforced pellets were procured from Shandong Dongyue Group (Zibo, China). The processed dimensions of these composites were 40 mm long, a 30 mm outer diameter, and a 20 mm inner diameter. A keyway was incorporated in the inner hole of the sample, with dimensions of 40 mm in length and 4 mm in height.

Figure 2 presents the contact diagram of the shaft–surface sliding friction pairs engineered to emulate the shaft–surface matching structure of a wave glider drive shaft and sleeve. The external component of the friction pairs was fabricated from 316L stainless steel, while PTFE and its composite materials constituted the internal component. To ensure a uniform surface finish, the mating shaft surface of the materials was meticulously polished to achieve a surface roughness within the range of 0.3 to 0.5 μm. Furthermore, the depicted key in Figure 2 serves the purpose of affixing the internal sample of the friction pairs to the rotating shaft, thereby transferring rotational motion and facilitating the relative sliding between the friction pairs components.

2.2. Preparation of Natural Seawater

The lubricating medium employed in this experiment is natural seawater sourced from Qingdao, China. Before the experimental procedures, the seawater was allowed to stand for approximately 48 h to facilitate the sedimentation of suspended particles. Subsequently, the seawater was filtered using filter paper to remove any residual impurities, ensuring the purity required for the test. The post-experimental analysis revealed a salinity of 7.23 and a PH value 7.681 for the seawater [27]. A comprehensive list of the chemical constituents and their respective concentrations in the seawater is presented in

Table 3. Chemical composition of and contents in seawater.

Compound	NaCl	MgCl2	Na2SO4	NaHCO3	KBr	H3BO3	SrCl2	NaF	CaCl2
Concentration /(g/L)	24.95	5.77	4.52	0.221	0.196	0.044	0.031	0.005	1.34

.

2.3. Test Equipment and Principle

The equipment used for tribology samples is a friction and wear test bench specially designed and built by ourselves to study shaft–surface sliding pair, as shown in Figure 3. It includes a driving module, working loading module, lubrication module, and data acquisition module. The test bench can be divided into two parts: the upper part by starting the stepper motor to drive the crank rotation and stimulate the loading spring. The hooks at the ends of the loading spring are attached to the sleeve and the external specimen of the friction sub-sample by two bolts with rings. After being excited, the loading spring applies CAC loading to the external specimen of the friction pairs to simulate the cyclic stress borne by the wave glider drive shaft during operation [28]. In the lower part, the variable frequency motor drives the internal sample of the friction pairs to rotate through the rotating shaft to form dynamic sliding pairs with the external sample. The peristaltic pump completes the lubrication system, which can continuously provide the lubricating medium to the sample. The experimental data are collected by a dynamic torque sensor, which can monitor the friction pairs’ relative sliding speed and torque in real time and output it to the computer.

Table 4. Performance parameters of loading springs.

Parameters	Loading Spring
Length	130 mm
Wire diameter	1.5 mm
Spring diameter	15 mm
Spring coil number	64
Stiffness	0.64 N/mm
Vibration period	0.75 s

delineates the parameters of the selected loading spring. Figure 4a illustrates the loading mechanism of the crank-driven loading spring system. During the crank’s cyclic rotation, the loading spring undergoes continuous expansion and contraction, imparting a periodic cyclic loading to the frictional sub-sample. The relationship between the loading exerted by the loading spring and the crank’s rotation angle is described as follows:

$$N=k\delta$$

(1)

$$\delta =\left(l-x\right)$$

(2)

In the formulas, N represents the loading applied by the loading spring to the friction pairs, k represents the stiffness of the loading spring, l represents the tensile length of the loading spring, and x represents the extension of the loading spring. This relationship is precisely determined by tracking the crank rotations over one week to quantify the load imposed on the friction pairs by the loading spring.

Subsequently, MATLAB 2024 software was utilized to visualize the findings. Figure 4b presents the cyclic loading characteristics of loading spring in four cycles after excitation.

2.4. Experimental Methods

2.4.1. Water Absorption Tests

To investigate the hydrophilic characteristics of PTFE and its composites under the loading of CAC in a marine environment, a series of water absorption tests were conducted on four distinct PTFE samples after seawater immersion. The methodology is delineated as follows: Initially, the four samples underwent drying at a controlled temperature of 60 °C to ensure uniformity in the moisture content. Subsequently, an electronic balance (ME104E, purchased from METTLER, Zurich, Switzerland, with an accuracy of 0.1 mg) was employed to measure the mass of each sample, denoted as m1; each sample was subjected to triplicate weighing to ascertain the mean mass, thereby establishing a baseline for the original quality of the samples. The experimental conditions were as follows: the initial temperature for seawater immersion was set at 16 °C, with a test duration of 24 h. At intervals of every 4 h, the samples was retrieved, and any changes in mass were documented as m².

2.4.2. Mechanical Property Tests

In the realm of materials science, the determination of hardness is a critical parameter for evaluating the mechanical properties of various materials. This study adhered to the ASTM D2240 [29] standard test method, employing a Shore hardness tester to ascertain the hardness variations of four PTFE samples before and after 24 h of immersion in seawater. The experimental conditions were meticulously controlled, with the relative sliding speed of the friction pairs set at 0.3 m/s. To mitigate the risk of severe plastic deformation or fracture in the loading spring during the cyclic loading process, it was replaced every 2 h, totaling 12 springs throughout the test. Given the hardness characteristics of the PTFE materials, a D-type indenter was selected for the Shore hardness test, with the hardness values being expressed in terms of Shore D hardness units. The protocol for the test was stringent. Prior to commencement, the surface of each sample was carefully wiped with a clean cloth to remove any excess water, impurities, pollutants, and oxidation layers that could affect the accuracy of the test results. The samples were then securely placed on a test stand, and the D-indenter was pressed into the material’s surface with a force of 1 kg and held for 10 s, allowing for the precise measurement of the indentation depth. Indentations were taken at three distinct locations on each sample to ensure statistical reliability, and the average hardness values were calculated and recorded.

Using a universal tensile testing machine, the tensile properties of the four types of PTFE and its composites were evaluated before and after 24 h of immersion in seawater. The ASTM D638-14 standard [30] test method was used to prepare the specimens, which involved shaping the sample into a dumbbell configuration. The post-processing dimensions were 25 mm long at both ends, 20 mm wide, and an intermediate section of 40 mm long and 12 mm wide. The test was conducted at a rate of 10 mm/min, and the reported values represent the means of triplicate measurements to ensure data reliability and precision.

By the ASTM D790 standard [31] test method, a comprehensive evaluation of the flexural strength of four types of PTFE and its composites was conducted before and after a 24 h of immersion in seawater. Following the 24 h soaking period, the specimens were horizontally sectioned along their central axis, and a segment constituting half of each specimen was extracted for further flexural performance analysis. Subsequently, the samples were fashioned into a rod-like configuration, with the processed dimensions measuring 40 mm in length, 15 mm in width, and 5 mm in thickness. The flexural test was conducted at a controlled loading rate of 10 mm/min. The reported values represent the means of three independent replicate tests, ensuring the reliability and reproducibility of the obtained results.

The specimen was prepared into cylindrical tube shape according to ASTM D695-15 standard [32] for the compression performance test. The sample size after preparation was 40 mm × 30 mm × 40 mm in outer diameter, inner diameter and length, respectively. The loading rate was 10 mm/min, and the average value of the three measurements was recorded.

2.4.3. Friction and Wear Tests

Figure 5 shows the tribological performance test details. The experiments were conducted at an ambient temperature of 26 °C, encompassing two distinct lubrication environments, dry friction and seawater lubrication, with a comparative analysis being executed. To ensure a consistent and ample supply of seawater across the sliding surface of the specimens, a peristaltic pump was employed for continuous circulation of the lubricant, thereby obviating any interruption in the operational continuity throughout the experimental duration. In light of the necessity to prevent significant plastic deformation of the loading spring under cyclic loading, the test duration was prudently set to 100 min, with a sliding velocity of 0.3 m/s. Each tribological test was conducted in triplicate to ascertain the reliability and reproducibility of the results, and the mean values were subsequently computed to present the findings.

Before and after tribological testing, the mass of the specimens was ascertained utilizing an electronic balance with a precision of 0.1 mg, with each sample being weighed thrice to ensure accuracy and consistency. The pre-and post-test weight data were meticulously documented to quantify the extent of wear. Subsequently, a laser confocal microscope (LSCM, LEXT OLS5000, Olympus, Japan) and a scanning electron microscope (SEM, Apreo S, Waltham, MA, USA) were deployed to analyze the three-dimensional wear profile, surface roughness, and the microwear morphology of the specimens post-wear. Each characterization test was repeated a minimum of two times to ensure the reliability of the results.

3. Results

3.1. Analysis of the Water Absorption Results

In researching and developing fiber-reinforced composites, water absorption is one of the key factors to measure the properties of the materials [33]. In particular, the effect of seawater immersion on the material’s properties will weaken the interface flexural strength between the fiber and the matrix, including synergistic effects with other aging factors, and eventually lead to the aging failure of the composites, affecting its service life [34]. In previous studies, researchers often only paid attention to the mechanical properties of materials before and after water absorption but ignored the study of the water absorption characteristics. Therefore, the water absorption properties of PTFE and its composites after 24 h of seawater immersion were recorded in detail in this study.

As depicted in Figure 6a, the water absorption of the four PTFE composites escalated with the increment of the test duration.

Table 5. Changes in the water absorption characteristics of PTFE and its composites before and after 24 h of immersion in seawater.

Time (h)	Water Absorption ( $\mathbf{\times }{\mathbf{10}}^{\mathbf{-}\mathbf{6}}$ g/cm2)
	PTFE-1	PTFE-2	PTFE-3	PTFE-4
4	3.64	27.61	97.33	17.95
8	4.79	49.26	186.39	28.64
12	5.28	61.32	271.41	35.87
16	6.97	77.97	345.65	44.15
20	8.42	89.43	448.17	59.26
24	9.12	112.54	541.72	67.31
Rise (%)	150.55	303.22	456.58	274.99

shows the specific values. Notably, throughout the entire testing phase, PTFE-1 exhibited lower water absorption than the other three fiber-reinforced composites, with a slower rate of increase. This suggests that incorporating fiber particles intensifies seawater penetration into the material to a certain extent, thereby augmenting its water absorption. This observation corroborates previous findings [35]. Notably, the water absorption characteristics of PTFE-3 composite surpassed those of PTFE-2 and PTFE-4 composites, potentially due to more binding gaps between pure Ti-C particles and the PTFE matrix, which facilitated seawater infiltration. Seawater infiltration also amplified the adhesion gap between the fiber and the matrix, culminating in heightened water absorption in the PTFE-3 composite. Furthermore, the water absorption properties of the PTFE-2 and PTFE-4 composites demonstrated a steady improvement over time. The overall water absorption of the PTFE-4 composite was significantly lower than that of the PTFE-2 composite throughout the seawater immersion process. This could be attributed to CFs enhancing the adhesion between Ti-C and PTFE matrix, rendering the surface smoother and reducing porosity, thus impeding seawater infiltration into the material’s matrix. Moreover, the incorporation of part of CFs in Ti-C changed the original carbon content of the material to a certain extent, which may make it reach the best ratio and help to form an oxide film on the surface of the material, thus effectively inhibiting seawater penetration.

Figure 6b illustrates the percentage increase in the weight of PTFE and its composites following immersion in seawater (due to the inherently low water absorption rate of PTFE-1, it did not exhibit a significant weight increase and is thus not highlighted in the figure), with the specific values detailed in

Table 6. Weight and percentage changes of the four kinds of PTFE and its composites during immersion in seawater.

Time (h)	PTFE-1		PTFE-2		PTFE-3		PTFE-4
	Weight (g)	Augment (%)	Weight (g)	Augment (%)	Weight (g)	Augment (%)	Weight (g)	Augment (%)
0	31.27	0	32.28	0	33.89	0	32.81	0
4	31.281	0.035	32.37	0.28	34.22	0.97	32.87	0.18
8	31.285	0.045	32.44	0.49	34.52	1.86	32.91	0.31
12	31.287	0.054	32.48	0.62	34.81	2.71	32.93	0.37
16	31.292	0.071	32.53	0.77	35.06	3.45	32.95	0.43
20	31.296	0.083	32.57	0.89	35.41	4.49	33.01	0.61
24	31.299	0.093	32.64	1.16	35.73	5.43	33.03	0.67

. The data reveal that the weights of all four materials increased with increasing water absorption time. Notably, the PTFE-3 composite experienced the most pronounced weight increase, characterized by a rapid growth rate. This observation is indicative of the uneven binding gaps between the Ti-C particles and the PTFE matrix, which facilitates enhanced water absorption. In contrast, the PTFE-2 and PTFE-4 composites demonstrated more gradual increases in water absorption weight. The PTFE-4 composite demonstrated the least weight change throughout the test, exhibiting a decrease of approximately 87.66% after 24 h of seawater immersion compared to the PTFE-3 composite. This outcome further validates the effectiveness of the doping of CFs in enhancing the bonding gap between Ti-C and the PTFE matrix. This suggests that the synergistic incorporation of Ti-C and CFs can significantly enhance the interfacial bonding and partially mitigate the effects of seawater immersion. It has been demonstrated through prior research that dendritic PTFE nanoribbons function as an interface between the CFs and PTFE matrix, binding them together in a manner reminiscent of Boston ivy. This binding process has been shown to effectively enhance the bonding properties between the reinforcing filler and the PTFE matrix.

3.2. Analysis of Mechanical Property Test Results

Figure 7A illustrates the Shore hardness values of four types of PTFE and its composites. The results showed that adding fiber particles significantly increased the hardness of the PTFE matrix. Especially for the PTFE-3 composite, compared with PTFE-1, the hardness increased by about 53.21% (Figure 7a), which was attributed to the very high hardness value of Ti-C, and the addition of Ti-C could significantly improve the hardness of PTFE composites. The hardness of PTFE-2 reinforced by CFs was only increased by about 19.38% compared with PTFE-1. It shows that the hardness properties of CFs are not ideal compared with Ti-C.

Figure 7B presents the tensile strength measurements of the four PTFE composites and their respective matrices. The results demonstrated that the incorporation of fibrous particles significantly bolstered the tensile properties of the PTFE matrix. Notably, the PTFE-2 composite exhibited the most pronounced enhancement in tensile strength, with an increase of approximately 46.55% (Figure 7b), ascribed to the superior mechanical attributes of CFs. Furthermore, the tensile properties of the PTFE matrix reinforced solely by Ti-C, renowned for its high hardness, did not match those enhanced by CFs. Consequently, the tensile properties of the PTFE-4 composite, which was co-reinforced by both Ti-C and CFs, were marginally lower than those of the PTFE-2 composite after the CF enhancement.

Figure 7C illustrates the flexural strength values of the four types of PTFE and its composites. In the same way, the incorporation of fibrous reinforcements notably enhanced the flexural properties of the PTFE matrix. Relative to the tensile properties, the enhancement in bending strength was more pronounced, with increases of 85.23%, 36.26%, and 57.28% observed, respectively (Figure 7c). These results demonstrate that both CFs and Ti-C significantly bolster the bending resistance of the PTFE matrix, thereby enabling it to more effectively withstand the effects of CAC loads.

Figure 7D presents the compressive strength values of the four types of PTFE and its composites. The results indicated that the compressive strength of the PTFE-2 composite reinforced with CFs exhibited the best performance, showing an improvement of approximately 78.27% compared to the PTFE-1 matrix (Figure 7d). This enhancement underscores the excellent load-bearing capacity of CFs. In comparison, the compressive strengths of PTFE-3 and PTFE-4 composites increased by approximately 21.48% and 50.59%, respectively, relative to the PTFE-1 matrix.

Figure 8 presents a comparative analysis of the mechanical properties of the four types of PTFE and its composites before and after a 24 h immersion in seawater.

Table 7. The tensile and flexural properties of PTFE and its composites after a 24 h immersion test in seawater.

Samples	Before Seawater Immersion	After Seawater Immersion	Decrease (%)
Shore hardness
PTFE-1	48.19 ± 0.53	47.44 ± 0.63	1.58
PTFE-2	57.53 ± 0.61	56.22 ± 0.57	2.33
PTFE-3	73.83 ± 0.69	70.29 ± 0.63	5.04
PTFE-4	66.12 ± 0.74	64.77 ± 0.69	2.08
Tensile strength (MPa)
PTFE-1	33.62 ± 0.72	33.21 ± 0.74	1.23
PTFE-2	49.27 ± 0.58	48.33 ± 0.65	1.94
PTFE-3	42.79 ± 0.49	38.84 ± 0.75	10.17
PTFE-4	47.42 ± 0.41	46.77 ± 0.56	1.39
Flexural strength (MPa)
PTFE-1	18.89 ± 0.52	18.63 ± 0.39	1.39
PTFE-2	34.99 ± 0.67	33.76 ± 0.46	3.64
PTFE-3	25.74 ± 0.43	23.41 ± 0.61	9.95
PTFE-4	29.71 ± 0.81	29.15 ± 0.68	1.92
Compressive strength (MPa)
PTFE-1	20.34 ± 0.71	19.98 ± 0.63	1.81
PTFE-2	36.26 ± 1.02	35.06 ± 0.81	3.42
PTFE-3	24.71 ± 0.86	22.19 ± 0.93	11.36
PTFE-4	30.63 ± 0.79	29.82 ± 1.07	2.72

shows the specific change values. The data elucidate that these materials’ tensile and flexural strengths experience a decline due to the synergistic effects of CAC loading and seawater immersion. Notably, the PTFE-3 composite, characterized by its higher water absorption, exhibited the most pronounced reduction in mechanical properties. Specifically, hardness decreased by approximately 5.04% (Figure 8a), the tensile and flexural strength decreased by approximately10.17% (Figure 8b) and 9.95% (Figure 8c), and the compressive strength was reduced by over 11% (Figure 8d). This decrement was attributed to the susceptibility of the binding gap between the Ti-C particles and the PTFE matrix in the PTFE-3 composite to seawater penetration, with the continuous extrusion and impact of the CAC loading exacerbating this gap and significantly diminishing the mechanical properties.

Interestingly, the PTFE-4 composite, which was synergistically enhanced with Ti-C/CFs, demonstrated only marginal decreases in hardness, tensile, flexural, and compressive strengths of approximately 2.08, 1.39%, 1.92%, and 2.72%, respectively. These reductions were slightly higher than those observed in the PTFE-1 matrix material but lower than those in the PTFE-2 and PTFE-3 composites. These findings suggest that combining Ti-C and CFs can effectively enhance the bonding and adhesion to the PTFE matrix, reducing the binding gap and mitigating seawater immersion into the material matrix. Additionally, the high hardness of Ti-C and the high strength of CFs significantly bolster the resistance of the PTFE matrix to the CAC loading, effectively preserving its mechanical properties.

3.3. Analysis of the Friction and Wear Test Results

Under the application of CAC loading, the frictional behavior of PTFE and its composites was examined under dry and seawater lubrication conditions, with the results depicted in Figure 9a,b (sliding velocity of 0.3 m/s). The data revealed that incorporating seawater substantially ameliorated the antifriction characteristics of PTFE and its composites. Specifically, the frictional forces for all four PTFE samples were reduced in seawater compared to dry friction, exhibiting a more stable pattern of frictional fluctuation. Furthermore, it was observed that the fiber-reinforced PTFE-2, PTFE-3, and PTFE-4 composites demonstrated elevated friction forces relative to the PTFE-1 matrix under both dry and seawater lubrication. This suggests that the interfacial bonding between the fibers and the PTFE matrix, which increases the surface roughness, adversely affects the composites’ antifriction performance. Notably, the PTFE-3 composite exhibited the most pronounced friction force, with maximum fluctuations reaching approximately 25 N under dry conditions and 17 N under seawater lubrication—values exceeding those of the other materials and accompanied by significant fluctuations. These findings further substantiate the notion that the sustained CAC loading induces the precipitation of Ti-C particles from the PTFE matrix, which act as abrasive particles in the friction process and thereby augment the overall friction [36,37]. It is noteworthy that, aside from the exceptionally smooth surface of the PTFE-1 matrix material, the PTFE-4 composite material, which has been synergistically enhanced with Ti-C/CFs, displayed the most stable friction performance under both lubrication conditions. This indicates that adding Ti-C/CFs can significantly contribute to reducing friction in PTFE materials, showcasing superior tribological properties across different lubrication environments.

Figure 10a,b present a comparative analysis of the average friction and wear behaviors of PTFE and its composites under dry friction and seawater lubrication conditions with CAC loading. Notably, incorporating seawater substantially reduced the average friction force for all four PTFE composites compared to dry friction scenarios. Intriguingly, the PTFE-3 composite exhibited a relatively modest reduction in average friction under seawater lubrication of approximately 30.2% as opposed to the other materials. This suggests a pronounced boundary lubrication effect between the PTFE-3 composite and 316L stainless steel during CAC loading sliding [38], where the thin lubrication film formed by the sliding friction surface is insufficient to prevent direct contact between the friction surfaces, thereby increasing the effective contact area and promoting boundary lubrication (Figure 10a).

In our experimental investigation, seawater’s incorporation as a lubricant markedly intensified the wear loss of the PTFE-3 composite, as illustrated in Figure 10b. The wear loss under seawater lubrication escalated by approximately 59% relative to dry friction scenarios. These findings suggest that the PTFE-3 composite, which was solely reinforced with Ti-C particles under CAC loading, does not demonstrate optimal wear resistance in a marine environment. The synergistic effects of CAC loading and seawater-induced corrosion may expedite the detachment of Ti-C particles from the PTFE matrix. In subsequent analyses, this hypothesis will be scrutinized to unravel the intrinsic mechanisms responsible for the exacerbated wear behavior.

3.4. Three-Dimensional Wear Topography Analysis

The examination of the 3D wear morphology and profile of PTFE and its composites under dry friction conditions revealed significant wear characteristics for each material variant (Figure 11A–D). PTFE-1 exhibited a predominantly abrasive wear pattern, characterized by numerous scratches and rills on its wear surface. This suggested that the material’s degree of wear under dry friction was notably high, with visible signs of surface wear (Figure 11A). The PTFE-2 composite stood out due to extensive CF exposure marks on the wear surfaces, alongside observable CF adhesion. This suggested a strong interfacial bonding between the carbon fibers and the PTFE matrix, which, upon exposure, helped to mitigate the impact of cyclic loading on the wear surface (Figure 11B). In contrast, PTFE-3, which incorporated Ti-C particles, displayed an augmented number of scratches and furrows, indicating that adding Ti-C particles did not effectively enhanced the wear resistance under dry conditions, with the particles becoming dislodged during friction and contributing to the abrasive wear process (Figure 11C). Most intriguingly, the PTFE-4 composite demonstrated a markedly different wear pattern, with only unidirectional scratches and small, uniformly distributed pits on the wear surface (Figure 11D). The relative flatness and smoothness of the wear surface hinted at a superior wear resistance. The inclusion of CFs was credited with significantly bolstering the material’s mechanical properties, and the interfacial bonding between Ti-C and PTFE matrix was enhanced. Furthermore, the presence of Ti-C particles is posited to enhance the hardness and wear resistance of the PTFE matrix, thereby providing more robust protection against cyclic loading on the friction surface. The synergistic effects of CFs and Ti-C particles are believed to reduce the interfacial binding gap and strengthen the adhesion between Ti-C and the PTFE matrix, ensuring effective stress transfer and improved tribological properties of the composites.

Under seawater lubrication, the surface scratches and grooves of PTFE-1 and the PTFE-3 composite were significantly reduced, resulting in a relatively flat wear surface (Figure 11a,c). For the PTFE-2 composite, large amounts of carbon fibers adhered evenly to the material’s surface and were embedded in scratches and gullies. This phenomenon can be attributed to the scouring and washing action of the seawater medium, which disperses the carbon fibers gathered under dry friction conditions. In the sliding friction process, the carbon fibers are evenly distributed throughout the sliding surface, which effectively reduces the direct contact between the friction surface and improves the lubrication effect. Thus, this results in a reduction in surface wear of the material (Figure 11b). In addition, the introduction of seawater also smoothed the wear surface of the PTFE-4 composite compared to dry friction and significantly reduced the surface scratches and craters (Figure 11d). It was found that the self-lubricating properties of the PTFE-4 composite containing Ti-C/CFs were excellent. Compared with the PTFE-1, PTFE-2, and PTFE-3 composites, the wear surface of the PTFE-4 composite was smoother and more uniform under the seawater medium’s combined lubrication effect and the composites’ self-lubricating properties. This enhancement was attributed to the superior lubricating effects of seawater, as it has been previously established that the Mg⁺ and Ca⁺ ions present in seawater contribute positively to the lubrication process [14]. When PTFE and its composites were utilized in seawater in conjunction with the corrosion-resistant material 316L stainless steel, the lubricating effect of seawater resulted in a decreased degree of wear on the wear surfaces of PTFE and its composites. This reduction in wear indicates a synergistic interaction between the seawater and the Ti-C/CF/PTFE composite, leading to an optimized wear resistance profile.

The impact of material surface hardness and roughness on abrasive wear is significant and cannot be overlooked [39,40]. To elucidate the distinct wear characteristics of the four PTFE composites, surface roughness measurements were conducted at four distinct locations on the wear surfaces using laser confocal microscopy (Figure 12A–D). Figure 12a–d illustrate the surface roughness profiles under dry friction conditions with CAC loading. It was observed that PTFE-1 and PTFE-3 (Figure 12a,c) exhibited higher overall surface roughness compared to PTFE-2 and PTFE-4 (Figure 12b,d). Notably, PTFE-3 displayed a gradient decrease in surface roughness from the top to the bottom of the wear area. This trend is attributed to the impact of the CAC loading on the Ti-C particles, which initially concentrate stress at the top of the wear area, resulting in pronounced ploughing effects. As sliding progresses, this effect diminishes, leading to a progressive decrease in surface roughness. Although PTFE-1 exhibited relatively uniform surface wear, its elevated overall surface roughness suggested significant wear under dry friction conditions. In contrast, PTFE-2 and PTFE-4 displayed lower surface roughness values, especially when it comes to PTFE-4, with PTFE-4 maintaining relatively smooth wear surfaces in most areas, despite some localized areas of pronounced wear (Figure 12d). This is due to the synergy between the excellent mechanical properties of CFs and the good hardness of Ti-C.

Under seawater lubrication conditions, pronounced reductions in the surface roughness values of the four PTFE composites were observed, again underscoring the beneficial impact of seawater’s superior lubricating action on diminishing friction and wear (Figure 13a–d). Interestingly, PTFE-1 and PTFE-3 exhibited surface roughness patterns akin to those under dry friction conditions, which corroborated our prior hypotheses (Figure 13a,c). In contrast, the wear surfaces of PTFE-2 and PTFE-4 were found to be relatively smooth and even, with consistently low surface roughness values. Similarly, particularly for PTFE-4, the smooth, flat wear surface and low surface roughness confirmed the synergistic effects of Ti-C and CFs (Figure 13b,d). This effect not only enhances the interfacial bonding between the matrix and fibers but also effectively leverages their exceptional mechanical properties and hardness, thereby providing the material’s robust protection against surface wear.

3.5. Wear Morphology Analysis

The scanning electron microscopy (SEM) analysis of the wear morphology of PTFE and its composites under dry friction and seawater under cyclic loading revealed distinct wear characteristics for each material (Figure 14A–D,a–d). The wear surface of PTFE-1 exhibited uniformly distributed scratches and fine grooves along the sliding direction, indicative of abrasive wear dominance in its wear mechanism (Figure 14A). The PTFE-2 composite presented exposed carbon fibers on the PTFE matrix surface, reflecting poor dispersion due to carbon fiber aggregation. The wear surface of PTFE-2 also showed pronounced plastic deformation traces, with the wear mechanism primarily involving plastic deformation and minor abrasive wear (Figure 14B). In contrast, the wear surface of the PTFE-3 composite was marked by extensive material flaking and scratch marks, suggesting that Ti-C particles detached from the PTFE surface during friction. Upon impact with cyclic loads, these particles fractured and formed abrasive grains that participated in the friction process, leading to severe three-body abrasive wear and exacerbating the material’s surface wear, where the wear mechanism was primarily attributed to the shedding of fiber particles and the subsequent three-body abrasive wear (Figure 14C). Notably, the PTFE-4 composite stood out with a relatively flat wear surface devoid of large-scale plastic deformation or material spalling. The results showed that the interfacial bonding between Ti-C and PTFE matrix could be improved by adding CFs in a certain proportion. This strong adhesion ensured that the Ti-C particles hardly detangled from the PTFE matrix, even under the continuous action of CAC loads, and the excellent mechanical properties of CFs and the high hardness of Ti-C effectively withstood the cyclic stress applied to each sliding friction surface (Figure 14D).

Under seawater lubrication, the wear degree of the PTFE-1 material surface was significantly mitigated compared to dry friction conditions. The wear surface exhibited marked reductions in scratch depth and numbers, presenting a smoother morphology characterized by faint scratches and abrasions. This observation suggests that the smooth surface of PTFE-1 facilitates the formation and retention of a lubricating film by the aqueous medium on the material surface, thereby achieving hydrodynamic lubrication (Figure 14a). Conversely, the wear degree of the PTFE-2 composite material was reduced under seawater lubrication. The CFs, which aggregated under dry friction, were evenly distributed across the material surface after being washed by seawater and effectively bore the cyclic stress from external sources (Figure 14b). Moreover, CFs can utilize seawater to form a lubricating film on the material surface, reducing direct contact between friction pairs and decreasing the degree of surface wear. In contrast, the wear surface of the PTFE-3 composite materials underwent severe wear under seawater lubrication. The surface revealed a substantial area covered by detached and fragmented Ti-C particles, which carried scratch marks on the material surface. This indicates that the surface roughness of combining Ti-C particles with the PTFE matrix is relatively high. Seawater fails to form an effective fluid lubrication film on the sliding surface of the friction pairs, and the aqueous medium also exploits the binding gap between Ti-C particles and the PTFE matrix to accelerate penetration into the PTFE matrix, intensifying the degree of shedding and consequently aggravating the material surface wear (Figure 14c). Interestingly, the worn surface of PTFE-4 composite showed minimal scratches and furrows, with only slight plastic flow marks observed, further demonstrating that incorporating CF particles could substantially enhance the robustness and uniformity of Ti-C bonding to the PTFE matrix. This also indicated that the Ti-C/CF-reinforced PTFE-4 composite was smoother than the PTFE-1, PTFE-2, and PTFE-3 composites and could more effectively form and maintain the hydrodynamic lubrication film formed by the aqueous medium on the sliding surface of the friction pairs. The wear characteristics were primarily slight plastic flow and micro-wear (Figure 14d).

3.6. EDS Analysis

An EDS analysis was conducted on the worn surfaces of the PTFE-2, PTFE-3, and PTFE-4 composites (refer to Figure 15a–c). The results revealed that the fluorine (F) content within the wear track of the PTFE-3 specimen was significantly lower than that of PTFE-2 and PTFE-4, suggesting the substantial degradation of the PTFE matrix during its interaction with 316L stainless steel under sliding friction. This degradation led to a pronounced loss of matrix elements. Concurrently, marked reductions in the carbon (C) and titanium (Ti) contents on the worn surface of the PTFE-3 specimen were observed, indicative of the detachment of Ti-C particles during sliding friction. These particles, once dislodged, either participated in the wear process as wear particles, were removed by the erosive action of seawater, or became embedded within the wear surface of the counter-facing 316L stainless steel, thereby contributing to the diminished C and Ti contents (Figure 15b). Interestingly, the carbon content on the worn surface of PTFE-4 was higher than that of PTFE-2 (Figure 15a), implying that incorporating Ti-C particles enhanced CF adhesion within the PTFE matrix (Figure 15c). This improved adhesion allowed CFs to act more effectively as a solid lubricant, reducing fiber transfer, material flow, plastic deformation, and abrasive wear [41]. The enhanced interfacial bonding between the Ti-C particles and the CF/PTFE is suggested to be responsible for the observed increase in wear resistance and the formation of a more stable transfer film, which is crucial for the reduced wear and improved tribological performance of the composites under seawater lubrication.

4. Discussion

4.1. Effects of the Binding Properties of the Fibers and Matrix on Seawater Permeability

Seawater immersion can exacerbate the bonding gap in fiber-reinforced polymer composites. When polymer composite materials are exposed to seawater, seawater penetrates the matrix through the binding gap between the fibers and the matrix, a phenomenon known as the capillary effect [42]. Previous studies have highlighted that fiber-reinforced polymer composites’ water absorption efficiency and diffusion coefficient are significantly higher than those of the polymer matrix alone. The fibers absorb substantial seawater within the PTFE composite and gradually diffuse it into the matrix. Ti-C typically exhibits lower water absorption as a ceramic material than CFs due to its higher chemical stability and lower porosity [43]. Interestingly, the water absorption of PTFE-2 is lower than that of PTFE-3, indicating that the interface bonding effect largely influences the water absorption of polymer composites. Polymer composites with more significant binding gaps and uneven binding undoubtedly accelerate seawater immersion, which is also one of the reasons for the higher water absorption of PTFE-2 composites. Ti-C can greatly improve the hardness of CFs; CFs can also make up for the binding defects of Ti-C and the PTFE matrix. Therefore, the synergistic effects of Ti-C and CFs make PTFE-4 composites have better mechanical and tribological properties.

4.2. Synergistic Enhancement Effects and Lubrication Effects of Ti-C/CFs and Seawater on PTFE

During the sliding interaction between the PTFE-4 composite and 316L stainless steel, CFs are precipitated initially due to CAC loading excitation and sliding wear. The exceptional self-lubricity and high strength of CFs enabled the exposed CFs to bear the primary stress imposed by CAC loading on the sliding friction surface, effectively mitigating the impact of CAC loading on the PTFE matrix and safeguarding the Ti-C from extensive detachment. As Chen [14] report, the exposed CFs are constantly thinning as they are impacted by loading, bearing the main loading between the sliding surfaces (Figure 16a–d). The protected Ti-C particles significantly enhance the wear resistance of the PTFE matrix during sliding friction owing to their higher hardness. In addition, under dry friction conditions, due to the uneven distribution of stress imposed by CAC loading on the friction pairs, part of the CFs is first exposed to a large stress concentration, and the accumulated CFs cannot effectively protect the entire PTFE matrix (Figure 16e). When immersed in seawater, the high scouring effect effectively disperses the accumulated CFs, and the relative sliding of friction pairs also makes CFs become evenly distributed on the sliding surface of the material, bearing the main tangential and normal forces. At this time, the synergy between the excellent mixing lubrication effect of seawater, the excellent self-lubrication performance of CFs, and the high hardness and wear resistance of Ti-C greatly reduces the occurrence of friction and wear (Figure 16f).

4.3. Stability of Enhanced Filler–Substrate Long-Term Adhesion in Marine Environments and the Effects of Radiation and Surface Treatment on Its Performance

The long-term stability of filler–matrix adhesion is a critical factor in the performance of composites, particularly in marine environments. In these environments, the long-term stability of filler–matrix adhesion is influenced by various factors, including the chemical composition of seawater, temperature variations, mechanical stress, and the interfacial properties between the filler and the matrix. Preliminary studies [44] have indicated that chloride ions and other chemical constituents present in seawater can penetrate the pores of composites, resulting in interfacial corrosion and a subsequent degradation of adhesion properties. Temperature fluctuations within the marine environment have been demonstrated to exert an influence on the adhesion characteristics between the filler and the matrix. The presence of elevated temperatures has been demonstrated to expedite the rate of chemical reactions, thereby inducing interfacial aging and the degradation of the material’s properties [45]. The impact of CAC loading has been shown to accelerate the interfacial fatigue of the material, resulting in the deterioration of adhesion properties. Furthermore, the impacts of irradiation and surface treatment on the properties of PTFE and its composites are more extensive [46,47]. Consequently, conducting long-term field tests is imperative to accurately assess the long-term stability of adhesion between Ti-C and CFs and the matrix. The surface treatment of materials to improve their properties and enhance their service life in seawater is also a priority for future research.

5. Conclusions

Through an in-depth investigation into the water absorption characteristics, mechanical properties, and tribological behaviors of four types of PTFE materials under dry friction and seawater lubrication conditions with CAC loading, several vital conclusions have been drawn.

The incorporation of a specific ratio of CFs into Ti-C-reinforced PTFE composites substantially enhances the interfacial bonding with the Ti-C and PTFE matrix. Relative to PTFE-1, the PTFE-4 composite, which incorporate both Ti-C and CFs, demonstrates superior mechanical and tribological properties, outperforming both the PTFE-2 and PTFE-3 composites.

When Ti-C is combined with PTFE matrix, immersion in seawater will expand these gaps, resulting in an increase in the water absorption of PTFE-2 composites and a significant decrease in the mechanical properties. In addition, the influence of CAC loading accelerates the shedding of Ti-C particles from the surface of PTFE, resulting in severe three-body wear. Conversely, while CF-reinforced PTFE composites exhibit relatively stable mechanical and tribological properties, their performance is deemed inadequate for sustained applications in marine environments. The impeding factor in question has been attributed to an inadequate enhancement of the hardness of the PTFE matrix by the CFs.

The synergistic enhancement effects of Ti-C and CFs effectively compensate for the shortcomings of the PTFE-2 and PTFE-3 composites. On one hand, the strong bonding between CFs and the PTFE matrix enables it to resist CAC loads and seawater immersion to a large extent, thereby enhancing the adhesion of Ti-C particles within the PTFE matrix. On the other hand, the high hardness and wear resistance of Ti-C effectively augment the ability of CFs to withstand CAC loads. Additionally, the washing and mixing lubrication effects of seawater contribute to improved tribological properties. The scouring action of seawater effectively disperses accumulated CFs, ensuring a uniform distribution across the sliding wear surface. A primary reason for the excellent mechanical and tribological properties of PTFE-4 composites is the synergy between the excellent mixing lubrication of seawater, the excellent self-lubricating properties of CFs, and the high hardness and wear resistance of Ti-C.