Basic Bio-Tribological Performance of Insulating Si₃N₄-Based Ceramic as Human Body Replacement Joints

Abstract

The paper presents an in-depth study of the bio-tribological performance on silicon nitride matrix ceramic composites containing hexagonal boron nitride (hBN) with different content. Ultra-high molecular weight polyethylene (UHMWPE) under simulated body fluid lubrication, and the simulated body fluid-lubricated sliding tests were performed on a universal friction and wear tester. The results showed that the incorporation of hBN into silicon nitride matrix reduced the friction coefficients from 0.27 for Si₃N₄/UHMWPE pair to 0.16 for Si₃N₄-20%hBN/UHMWPE with full immersion in simulated body fluid lubrication. Scanning electron microscopy (SEM), laser scanning microscope, X-ray photoelectron spectroscopy (XPS) were utilized to characterize the wear surface. The analysis results indicated that, with simulated body fluid lubrication, the interfacial between hBN and Si₃N₄ facilitated the wear pits to form on the wear surface, and the residual wear particles deposited in the pits. Then, tribochemical products were formed on the wear surface, which protected and smoothed the wear surface of the sliding pair in the simulated body fluid.

1. Introduction

It is well known that, with the development of economy, technology and medicine, the mean lifespan of human constantly improve. Along with the extension of lifespan, the problem of osteoarthritis in aged becomes increasingly outstanding. Meanwhile, transportation accidents, industrial injuries and sport injuries may also bring about bone trauma [1]. Total hip arthroplasty is an effective means to relive pain and improve the life quality of the patients with end-stage arthritis of the hip joint [2]. According to statistics, every year, no fewer than 300,000 hip arthroplasties have been performed in the United States, and 800,000 hip arthroplasties have been performed around the world.

Metal, polymer and ceramic could be all utilized as the hip replacement bearing materials. Meanwhile, titanium alloys (e.g., Ti6Al4V) and cobalt-chromium alloy have been used clinically due to good biocompatibility, avirulence, and their high strength and toughness. However, the free metal ions (e.g., cobalt ions) would diffuse in the tissues around the hip replacement, resulting in anaphylaxis [3]. Also, the metal replacement may induce some problems with stress shielding and medical image distortion. At the same time, it is well documented that polyethylene wear debris contributes to osteolysis, resulting in loosening of the prosthesis and ultimate failure [4,5]. Si [6] indicated that ultra-high molecular weight polyethylene UHMWPE was suited to acetabular components of hip joint and cushions, while UHMWPE could not be applied for total joint. Furthermore, bioceramic materials have been investigated for joint replacement because of their reduced wear rates and an increased corrosion resistance, such as calcium phosphate materials [7,8], alumina ceramic [9] and zirconia [10]. However, brittleness fracture and poor machinability have been reported as major concerns for these ceramic materials.

Silicon nitride (Si₃N₄) has been traditionally applied in engines, cutting tools and ball bearings due to its good mechanical performance [11]. During recent decades, Si₃N₄ has been introduced into the biomedical field, e.g., used for spinal implants. In 2011, the first Si₃N₄ femoral head was implanted in USA [12]. Why was Si₃N₄ ceramic chosen as an implant material? It was motivated by its good biocompatibility, low wear and relatively high strength [13].

It is well known that, hexagonal boron nitride (hBN) is a kind of in-situ lubricating material with layered structure. The tribological properties and machinability of Si₃N₄ ceramics can be further improved by adding hBN to the ceramic matrix [14,15,16]. Previously, scholars paid more attention to mechanical properties and tribological performance of Si₃N₄-hBN composites under engineering environment. Skopp [17] claimed that, for self-mated Si₃N₄-hBN composites in laboratory air, the tribochemical products of BN with water molecule were of benefit to the tribological performance of Si₃N₄-hBN composites in the range 22–150 °C. Meanwhile, Carrapichano [18] also reported hBN had a slight influence on proving anti-friction effect at room temperature. We [19,20] also found that, for Si₃N₄-hBN sliding pairs in laboratory, the incorporation of hBN into Si₃N₄ ceramic resulted in a significant decrease of friction and wear, just because of the formation of tribochemical film. Generally, current research works mostly focus on the engineering tribological performance of Si₃N₄-based composites. In recent years, our team also focused on the bio-tribological properties, and found that addition of hBN significantly reduced the friction coefficient of Si₃N₄/TC4 bearing couple under simulated body fluid due to the formation of tribofilm [21]. It was also demonstrated that sliding velocity and contact load have obvious influence on the tribological behaviors of Si₃N₄-20%hBN/HXLPE bearing couple under simulated body fluid [22]. However, the influence of hBN content on the bio-tribological performance of Si₃N₄/HXLPE bearing couple under simulated body fluid has not been systematically studied. In particular, the anti-friction mechanism of hBN in Si₃N₄/HXLPE bearing couple still needs to be further deeply investigated under simulated body fluid.

At present paper, the tribological properties of Si₃N₄-hBN composite ceramics with different hBN contents sliding on UHMWPE were studied in simuated body fluid. Through measuring friction coefficient and wear rate, the biological friction properties of Si₃N₄-HBN /UHMWPE composites were analyzed and discussed. Meanwhile, the morphology of worn surface was observed, and the chemical composition was further analyzed. Moreover, the anti-friction of hBN was studied in this paper.

2. Experimental

2.1. Test Rig and Materials

The friction and wear of Si₃N₄-hBN/UHMWPE pair were carried out on a pin-on-disc tribological test rig. In this test rig, an upper pin slides against a stationary disc in simulated body fluid, and the wearing surface is fully immersed in the liquid. The schematic diagram of the equipment is shown in Figure 1. The fixed disc bears a normal load, and a strain gauge is used to continuously measure friction.

Five types of Si₃N₄ composite ceramics were hot-pressing sintered with varied hBN content of 0, 5, 10, 20 and 30 vol.% (corresponding to SN0, SN5, SN10, SN20 and SN30) under a pressure of 30MPa, at a temperature of 1800 °C for 30 min of dewell time. And Then, a ceramic disc with a size of Φ45 mm × 6 mm was obtained. The blanks were sanded and cut into the required samples. The detailed preparation process has been given in our previous study [23]. Figure 2 illustrates the microstructures of the Si₃N₄ composite ceramics. It can be seen that the pure Si₃N₄ ceramic was mainly composed of elongated β-Si₃N₄ grains (as shown in Figure 2a), and the composite ceramic was mainly composed of elongated β-Si₃N₄ and lamellar hBN (as shown in Figure 2b–e). Meanwhile, when boron nitride (BN) was added to the Si3N4 matrix, the densification of Si₃N₄ ceramics decreased due to the interface inertia and the difference of thermal expansion coefficient. At the same time, because the boron nitride density is relatively low, the silicon nitride ceramic density decreased significantly.

In this study, the size of the pins (made of ceramic composite material) was 5 mm × 5 mm × 10 mm, with a rounded squared end to form a flat contact surface. In such a setting, the nominal contact area is approximately 22 mm². The matching discs were made from UHMWPE material, which was purchased from Chunli Zhengda Medical Instruments Co. Ltd, Beijing, China. The size of disc was Φ44 mm × 6 mm, and the surface roughness was around Ra 0.1 μm by grinding. Meanwhile, the surface roughness of ceramic pin was around Ra 0.5 μm by grinding utilized a diamond grinding tool. To adjust for the water-absorbing effects of the polymer, the disc sample was immersed in deionized water for about a month to achieve water saturation prior to the friction and wear test, according to Ref. [24]. SBF (simulated body fluid) liquid was prepared, according to Ref. [25].

Table 1. Ion concentration of simulated body fluid (SBF) and plasma.

Constituents & pH	Na+ (mmol/L)	K+ (mmol/L)	Mg2+ (mmol/L)	Ca2+ (mmol/L)	Protein (g/L)	pH
Blood plasma [26]	142.0	5.0	1.5	2.5	64–83	7.2–7.4
SBF	142.0	5.0	1.5	2.5	0	7.4

gives the corresponding ion concentrations and pH value, similar to those in human blood plasma.

2.2. Procedure

Before and after friction and wear tests, the samples needed to be cleaned, dried and weighed, according to Ref. [27]. Specifically, the samples were ultrasonic washed for minutes, and dried in the air. Then, the sample was weighed three times by electronic balance, and the average value was taken. Subsequently, all the tests were carried out in SBF at a speed of 500 r/min (0.73 m/s), normal load of 10 N, temperature of 36.5 °C, and a maximum sliding distance of 1000 m. Meanwhile, the nominal contact pressure from the normal load was 0.45 MPa. The initial running-in period was not considered when calculating the friction coefficient and wear rate. The friction coefficient f was directly given by the tester. The wear rate k was calculated by the following equation.

$$k=\raisebox{1ex}{$\Delta m$}\!\left/ \!\raisebox{-1ex}{$(\rho wx)$}\right.$$

where, $\Delta m$ represents the mass wear volume assessed by weight loss using a microbalance (accuracy = 0.1 mg), $w$ is the normal load, $x$ is the friction distance, and $\rho$ is the density.

Three tribological tests were carried out for one test condition. Before and after each experiment, the silicon nitride ceramic composite pin and polymer disc were weighed three times, and the average value was taken. The morphology of the worn surface was observed by SEM, and the chemical characterization was analyzed by EDS (Energy Dispersive Spectrometer) and XPS (X-ray Photoelectron Spectroscopy).

3. Results and Discussion

Figure 3 illustrated the friction coefficients and wear rates of Si₃N₄-hBN/UHMWPE pair under SBF lubrication. Figure 3a indicates the friction coefficients of the specimens at a sliding distance of 1000 m. The incorporation of hBN reduced the friction coefficient from 0.27 for SN0/UHMWPE pair to 0.16 for SN20/UHMWPE pair. Figure 3b presents the wear rates of UHMWPE discs. In this study, the wear rates of Si₃N₄-hBN pin were too small to detect them. Obviously, the wear of the friction pairs was mainly originated from the wear loss of UHMWPE disc. From Figure 3, it can be easily found that, the best results were exhibited for the SN20/UHMWPE pair in SBF.

Figure 4 gives the SEM images of morphologies of the ceramic composite pins against UHMWPE in SBF lubricant. From the figure, the worn surfaces present great alteration with hBN content increasing. SEM observation of the pure Si₃N₄ ceramic pin manifests a slightly rough surface. With the increase of hBN content, the surface gradually became rougher and rougher. Meanwhile, the worn surface of Si₃N₄-20%hBN pin manifests a relatively smoother surface, which is composed of a smoother surface (area “A” in Figure 4d) and rough surface. The composite pin surface becomes rough once again with excessive hBN content of 30%.

Figure 5 shows the high magnification image of the pin surfaces of pure Si₃N₄ ceramic and Si₃N₄-20%hBN composite. Figure 5a denotes that the worn surface morphology of pure Si₃N₄ pin is covered by adherent wear debris layers, and some loose particles and fracture can be also observed. Figure 5b gives the high magnification of area “A” in the pin surface of Si₃N₄-20%hBN composite. From this figure, some smooth area can be further observed. EDS analysis results of the smooth area (area “1”) on the worn surface of Si₃N₄-20%hBN pin are shown in Figure 6. From the figure, oxygen atom, silicon atom and calcium atom can be found in the worn surface.

The Si₃N₄-20%hBN pin surface after sliding against UHMWPE disc was analyzed by using XPS. Figure 7 shows that the B_1s and Si_2p peaks on the worn surface can be decomposed into two peaks, respectively. According to the relevant standard, from Figure 7a, one is 193.6 eV, which is close to 193.1 corresponding to B₂O₃; the other is 192.2 eV, which corresponds to H₃BO₃. Meanwhile, the Si_2p peak can also be decomposed into two peaks (Figure 7b), which correspond to SiO₂ and Si₃N₄ respectively. Thus, it may be concluded that some tribo-chemical products (especially in the smooth area “A” in Figure 4d) formed during the friction process. This phenomenon was also found in our previous studies [19,20].

In our previous studies [28], it was verified that Si₃N₄ and BN could react with water molecules during the friction process. The incorporation of hBN into Si₃N₄ matrix was beneficial to provide some storage space for the tribochemical products and induce the formation of a film. In this study, the obvious tribofilm was not found on the worn surfaces of pin samples. However, some tribochemical products were detected on the worn surface of Si₃N₄-20%hBN pin. In particular, the Si₃N₄-20%hBN pin against UHMWPE disc shows a polyphyletic morphology composed of rough surface and smooth surface. Meanwhile, the Si₃N₄-20%hBN/UHMWPE pair shows better tribological characteristics.

Figure 8 shows the SEM morphologies of worn surfaces of UHMWPE discs against different ceramic pins. From the figures, it can be clearly found that some fine particles adhere to the worn surface of the UHMWPE disc against the pure Si₃N₄ pin, and the furrow morphology can be observed. When the UHMWPE disc slid against Si₃N₄-5%hBN and Si₃N₄-10%hBN, the disc presented a significant deformation and fracture feature, and surfaces became rougher. When the UHMWPE disc slid against Si₃N₄-20%hBN pin, the disc surface was relatively smooth, and obvious furrow morphology could be also observed. However, when the UHMWPE disc slid against the Si₃N₄-30%hBN pin, the disc surface again became rougher. The corresponding 3-D images of the UHMWPE discs against Si₃N₄, Si₃N₄-20%hBN and Si₃N₄-30%hBN are shown in Figure 9.

According to previous studies, we know that the incorporation of hBN would be detrimental to the mechanical properties of silicon nitride ceramic, which might be attributed to the interfacial failure between hBN and Si₃N₄ [23]. From Figure 2, it can be clearly seen that the pure Si₃N₄ ceramic presents a denser structure. In this study, under SBF lubrication, fracture occurred on the wear surfaces of the Si₃N₄/UHMWPE pair at the beginning stage of the friction process. A part of wear particle would be taken away from the wear interface, while a part of the residual particles would accumulate on the wear surface. Thus, some loose wear debris could be observed on the worn surface of the UHMWPE disc against pure Si₃N₄ (Figure 8a). With the increase of hBN content, more fracture and spalling occurred on the wearing surface of the Si₃N₄-hBN/UHMWPE pair. Even though some wear particles were taken away by the SBF fluid, more and more particles adhered to the wear surface (Figure 8b,c). When hBN content reached 20%, a balance between the residual particles and spalling pits were reached. Namely, the residual particles mainly deposited in the spalling pits, and reacted with SBF lubricants. The tribochemical products provided a protected and lubricated effect for the sliding pair, and lower friction and wear were obtained. When hBN content further increased, the surface of ceramic pin was too rough, and serious mechanical wear was the main wear mechanism.

In recent years, a lot of scholars have been working on the research of bio-tribology of bone implants. Samanta et al. [29] found the wear rate of multilayer Ti/TiN coating against UHMWPE was 8 × 10⁻⁵ mm³/Nm for bio-tribological application. Liu et al. [30] also indicated that Zr/a-C GMFs exhibited the lowest friction coefficient at 0.114, and a wear rate of 1.47–1.56 × 10⁻⁵ mm³/Nm. Meanwhile, Sahasrabudhe et al. [31] found that the multi-material Ti6Al4V-calcium phosphate-nitride coatings show a lower friction coefficient of 1.25 and a wear rate of 2.71 × 10⁻⁴ mm³/Nm. Obviously, compared with these finding, the tribological properties of silicon nitride matrix composite ceramics need to be further improved. Moreover, this paper only focused on the tribological performance in the SBF environment, and failed to pay sufficient attention to the potential effect of lubricant composition, protein adsorption, contact conformity and transient operating conditions (which are all important factors in biologic tribology [32,33,34]). Thus, in the later stage, the relevant research work will be carried out.

4. Conclusions

In this study, the bio-tribological performances of Si₃N₄-hBN/UHMWPE bearing pair under SBF lubrications were investigated. The following research findings were obtained.

• The optimal friction pair is Si₃N₄-20%hBN/UHMWPE. In such a setting, excellent bio-tribological performance was obtained. Too much or too little hBN content will lead to the wear of the UHMWPE disc.
• Tribochemical behaviors contributed greatly to good bio-tribological properties. In SBF lubricate, due to interfacial failure between hBN and Si₃N₄, some tribochemical products were formed in the wear pits. The products could play a continuous and stable lubrication role.

The bio-tribological information from the study can promote the utilization of Si₃N₄-20%hBN/UHMWPE pair in the design of artificial joint prosthesis.