Defects and Mechanical Properties of Silicon Nitride Ball Bearings for Electric Vehicle Reducers

Abstract

In this study, two types of Si₃N₄ ball bearings for integrated-type EV reducers developed via different manufacturing processes were analyzed to study the microstructure of the balls and the defects that may occur during the manufacturing process. Three types of defects were confirmed that can reduce the fatigue life of Si₃N₄ ball bearings in operating environments. The microstructure was analyzed to identify the main additive components of each bearing, and pore defects with a size of less than 1 μm, and the types (particle defects or surface defects) and sizes of defects, were analyzed using LSCM, OM, and SEM. Hardness and fracture toughness, which are representative mechanical properties of ceramic materials, were evaluated. The results, evaluated using a Vickers indentation crack-based method, were statistically analyzed to confirm differences in hardness and fracture toughness between the two samples.

1. Introduction

Silicon nitride (Si₃N₄) is a ceramic material with characteristics such as low density, high strength, heat resistance, and corrosion resistance [1,2,3,4]. It has recently been widely used in various industries such as aerospace, railway, and national defense, as well as for machine tools [5,6]. Si₃N₄ balls are used in bearings and high-speed operating environments; they offer advantages over existing metal balls.

Bearings used in electric vehicles (EVs), in particular, possess structural characteristics that can facilitate better electrical conduction compared to traditional engine vehicles. The current flowing from the motor to the input shaft of the reducer causes the micro-welding of metal balls to the metal raceway, resulting in current erosion that damages the balls and raceway. Ceramic hybrid bearings composed of the metal raceways and the ceramic balls prevent current erosion by impeding the current flow due to the higher electrical resistance of ceramic materials compared to air. The EV reducer used in this study contains a “hybrid bearing” comprising Si₃N₄ ceramic balls and a metal raceway, known as a silicon nitride hybrid bearing (SNHB). Si₃N₄ ceramics theoretically possess very high strength.

However, due to the brittleness of ceramic materials, surface defects are generated during processing (grinding and lapping) and inner defects are generated during sintering. Therefore, their actual strength is lower than the theoretically predicted value. Small defects, presumed to be sintering voids with a diameter of less than 2 μm, can be a potential cause of failure and are considered inherent defects in the material rather than manufacturing defects [7]. Surface defects in Si₃N₄ balls and their propagation in rolling contact fatigue (RCF) are concerning, as they cause spalling, delamination, and rolling contact wear [8]. The spalling of Si₃N₄ balls may involve the propagation of the original ring crack and secondary surface cracks being induced as the crack grows. Fatigue life cannot be simply determined in terms of the propagation life of the surface crack; subsequent surface damage also plays an important role in predicting the life of Si₃N₄ balls [8]. Potential defects, including inner and surface defects, can cause fatal damage to the bearing operating environment. Therefore, research has been conducted on non-destructive inspection methods, vibration analysis methods, and defect improvement methods to ensure the quality of Si₃N₄ balls and bearing life [9].

In this study, two types of Si₃N₄ ball bearings for integrated-type EV reducers developed via different manufacturing processes were analyzed to study the microstructure of the balls and the defects that may occur during the manufacturing process. The two types of Si₃N₄ ball bearings differ in their main additive components and whether they underwent the hot isostatic process (HIP) after sintering. In the case of specimen A, the density was increased through the HIP after sintering, while specimen B did not include HIP in its manufacturing process. The Si₃N₄ balls prepared using the HIP mainly consisted of $\beta$-grain, which can increase the toughness [10]. Additionally, the representative mechanical properties of ceramic materials, such as hardness and fracture toughness, were evaluated, and their correlation with microstructure was analyzed. Figure 1 shows the structures of the gear system, input shaft, and ball bearings located on both sides of the input shaft, which are the subjects of this study.

Figure 2a shows the actual image of the SNHB and Si₃N₄ ball. Figure 2b shows the low-magnification scanning electron microscopy (SEM) image of the undamaged state of the Si₃N₄ ball, and Figure 2c shows the low-magnification SEM image of the Si₃N₄ ball, where spalling has occurred during driving. Figure 2d shows the enlarged SEM images of the spalling area in Figure 2c. Figure 2d shows the highly magnified SEM images of the fracture-initiated region A and the secondary propagated spalling region B.

2. Experimental Methods

Si₃N₄ balls were polished with emery paper (#100–#1000 Grit) and diamond suspension (9, 3, and 1 μm) to examine their microstructure. Their microstructure and additive components were analyzed using field emission scanning electron microscopy (FE–SEM, Quanta FEG 250, FEI Company, Hillsboro, OR, USA). To measure the surface defects, the balls were cleaned via sonication for 30 min in acetone, and a cotton swab dipped in acetone was rubbed on their surface. The entire surface of the balls was rubbed with ethanol using a latex glove and was then dried for 10 min. Surface defects were analyzed using an optical microscope (OM, Imager.M2m, ZEISS, Munich, Germany) and a laser scanning confocal microscope (LSCM, VK-X100, KEYENCE, Osaka, Japen). The chemical composition of the cluster-shaped particles on the surface were analyzed using scanning electron microscopy–energy dispersive spectrometry (SEM-EDS, VEGA3 XMU, TESCAN, Brno, Czech Republic). After the Si₃N₄ balls were polished with emery paper and diamond suspension, their hardness and fracture toughness were tested using a Vickers hardness diamond indenter (Q10A, ATM Qness GmbH, Mammelzen, Germany) under an indentation load of 10 kgf. In the global market, Si₃N₄ ball bearings are classified into grades based on fracture toughness characteristics [11]. The evaluation of fracture toughness of small and tough specimens such as Si₃N₄ balls bearings is difficult to characterize. In such cases, the indentation fracture resistance (IFR) method might be considered as the most suitable method because it requires a small specimen size and simple experimental procedures, and it has been adopted as the evaluation method for fracture toughness in the American standard specification for Si₃N₄ balls bearings [12,13]. The fracture toughness was measured using the IFR method proposed by Niihara, and the crack length of the indentation surface was observed using the OM and SEM. The hardness and fracture toughness were calculated using Equations (1) and (2), respectively, and are shown in Figure 3.

$$HV=\mathrm{1,854,400} P/{(2a)}^2$$

(1)

$$IFR=10.4 (E^{0.4})(P^{0.6})(\frac{a^{0.8}}{c^{1.5}})$$

(2)

$E$ is Young’s modulus, with a value of 270 GPa (minimum value of typical mechanical properties, ASTM F2094/F2094M–18a); $P$ is the applied load of 10 kgf; $a$ is the half diagonal value in microns (μm); and $c$ is the half tip-to-tip crack length in microns (μm).

3. Results

3.1. Microstructure

The microstructure is one of the major factors influencing the mechanical properties of Si₃N₄ [14,15]. Figure 4 shows the SEM–EDS analysis results of the inner defects (Figure 4a,d) and additive components (Figure 4b,c,e,f). The inner defect (micro-pores) sizes of specimens A and B are less than 1 μm, but the porosity of specimen A is relatively lower. The phase of the additive can be distinguished using the back-scattered electron (BSE) mode. EDS was used for the elemental analysis of additives, which revealed Ti and W to be the mean elements in the additives of specimens A and B, respectively.

3.2. Defects

The defect types of Si₃N₄ balls are broadly divided into two types (particle defect and surface defect). Figure 5 and Figure 6 show the analysis results of particle defects. Cluster-shaped particles protruding from the surface were identified using the OM, and their length and height were measured using the OM and an LSCM. Finally, the components of particles and clean surfaces were analyzed using SEM–EDS. The length and height of the particles in the two specimens were approximately 14–15 μm and 2.2–2.5 μm, respectively. Elemental analysis revealed that the surface contained Si, Al, and N as the main components, similar to Si₃N₄ balls, and the intensities of C and O were relatively higher than that of the clean surface. Based on the shape, size, and elemental components, the protruding particles are presumed to be Si₃N₄ grains that protruded from the surface before processing and were attached to the surface after being damaged during processes such as grinding and lapping. These protruding particles can become a source of stress concentration when in contact with a counterpart such as a raceway, which can cause cracks on the surface and can ultimately cause surface damage.

Figure 7 shows the analysis results of surface defects (scratches, cracks, and pores). The shapes and lengths of the surface defects were measured using the OM, and their depths were measured using an LSCM. As shown in Figure 7a,d, the length was measured to be over 100 μm. An LSCM confirmed that the defect was not deep, implying that it was a scratch. Figure 7b,c,e,f show that the length was relatively small at 9–32 μm; however, an LSCM confirmed the depth to be approximately 3.9–4.4 μm. Thus, this defect was deemed as a crack or pore defect. Damage defects may have different causes depending on their length, depth, and shape, but they can occur during any manufacturing process of Si₃N₄ balls. Scratch damage may occur if the processing pressure is abnormal or if the quality of the abrasive particles is low during processes such as grinding or lapping. In the case of a crack or pore defect, determining their cause is difficult. However, it could be an inner defect that occurred during sintering, a defect exposed due to surface polishing, or a defect that occurred due to abnormal processing. Damage defects affect fracture toughness, flexural resistance, and impact resistance [16]. As Si₃N₄ balls are spherical, manual observation of these defects is difficult. Thus, identifying and classifying these surface defects are complicated, making it difficult to effectively detect each defect type [17].

3.3. Hardness and Fracture Toughness

The fatigue life of the ball bearing depends on the basic dynamic load rating, and the rating factor and coefficient of the basic dynamic load rating is influenced by the material and manufacturing quality. The hardness and fracture toughness are commonly highlighted strength characteristics of engineering ceramic materials. A Vickers indentation crack-based method has been proposed to evaluate the fracture toughness of brittle materials [18]. Under the same indentation loading conditions, hardness and fracture toughness were determined based on the numerical results in terms of the morphology of the indented zone (including cracks and plastic zone) [19]. Figure 8 shows the OM and SEM results of a plastic zone and crack formed due to an indentation.

Table 1. Hardness and fracture toughness values.

Mechanical Properties	A	B
Hardness [HV]	1561	1482
	1567	1461
	1561	1482
	1573	1482
	1537	1477
	Avg. 1560	Avg. 1476
Fracture toughness [$\mathrm{MPa}·m^{\frac{1}{2}}$]	10.7	8.8
	10.8	8.5
	10.7	8.8
	10.8	8.8
	10.1	8.7
	Avg. 10.6	Avg. 8.7

shows the hardness and fracture toughness of each of the five specimens.

The hardness of specimen A was approximately 5.3% higher than that of specimen B, and the fracture toughness of specimen A is about 18.7% higher than that of specimen B, indicating that specimen A has a higher dynamic load rating than specimen B. Specimen A, with a lower porosity, has a relatively larger hardness, which proves the correlation between relative density and hardness. In the condition of Si₃N₄ balls of the same chemical composition, the grain size (diameter of $\beta$-grain) is proportional to fracture toughness, and inversely proportional to strength [20,21]. Based on these several investigations, it can be assumed that the grain size (diameter of $\beta$-grain) of specimen A is larger than that of specimen B, and that their different sintering and heat treatment conditions affect their grain sizes [22]. Specimen A surpassed the minimum hardness and fracture toughness of Si₃N₄ balls specified for ASTM material class I. Figure 9 shows the confidence interval graphs and the p-values through t-tests for each result group. For the results in which the intervals do not overlap and the p-values are less than the significant level of 0.05, it proves that the difference in means is statistically significant, and that the mechanical properties are different.

4. Conclusions

The microstructure (pores and additives), defects, and mechanical properties of Si₃N₄ ball bearings for EV reducers were analyzed. The defects of the two specimens with different manufacturing processes were largely classified into pore defects (inner), particle defects, and surface defects, all of which are potential defects that can occur during the manufacturing process. These defects can cause damage due to crack growth driven by rolling contact fatigue and can reduce the fatigue life of bearings in the operating environment. Further studies are needed to determine how the types and sizes of the defects identified in this study affect the lifespan of Si₃N₄ ball bearings. The evaluation of mechanical properties revealed that specimen A showed a relatively higher hardness and fracture toughness than specimen B. This result proves that the mechanical properties of Si₃N₄ ball bearings with a high relative density and a dominant $\beta$-grain are high, and there may be differences in their fatigue life in actual driving environments. The application of the SNHB is the best approach for preventing current erosion in EV reducers. For the successful operation and long-term performance of SNHBs in driving environments, the exploration of various manufacturing techniques to minimize defects, and materials research to improve mechanical properties are essential.