Effect of boron carbide reinforcement on properties of stainless-steel metal matrix composite for nuclear applications

1 Introduction

Mechanisms such as fueling machine, control rod drive, and shut down system in Pressurized Heavy Water Reactors at nuclear power facility utilize moving components like piston–cylinder arrangements, bush and roller bearings, ball screws, and gears. These mechanisms are categorized into two types: low pressure–high speed (e.g., bearing, piston, and cylinder) and high pressure–low speed (e.g., ball screws and gears) [1–3]. Since these mechanisms operate underwater, these components demand high corrosion resistance, along with nuclear radiation tolerance. Stainless steel (SS304) is one of the promising materials used for these applications because of its superior corrosion resistance, high strength, toughness, and good weldability [4,5]. However, this material suffers from insufficient wear resistance and radiation tolerance. The shortcomings of this metal alloy can be resolved by the incorporation of hard ceramic reinforcements. Ceramic reinforcements into the metal matrix improve the properties such as density, hardness, and wear resistance of the matrix material without impacting corrosion resistance [6–9]. Among these ceramics, boron carbide (B₄C) is the third hardest material, with good chemical stability, high strength, and low density (2.52 g/cc). In addition, because of its ability to absorb neutrons, B₄C-reinforced composites can be effectively implemented for applications in nuclear industries [10,11]. Aluminum–B₄C composites have gained immense interest in the last decade for the production of wear-resistant and neutron absorbent materials, but these materials sometimes suffer from seizure and galling at high-pressure working conditions [2,12,13]. Recently, Sun et al. [14] developed B₄C-reinforced SS304 composite with neutron shielding properties wherein the B₄C content was optimized through the genetic algorithm program. With the incorporation of spherical carbide particles, the density was reduced by 33% and a significant increase in radiation tolerance was achieved. As mentioned earlier, the mechanism used for nuclear applications works under dynamic contact stress conditions, because of which, the SS–B₄C composites still demand to test tribological parameters such as wear and friction coefficient.

The properties of the composite depend upon the manufacturing method, reinforcing material, its fraction and distribution in the matrix, and interaction between the components [4,10,15,16]. Conventional methods like stir casting, powder metallurgy (PM), hot pressing, and extrusion along with some advanced methods like spark plasma sintering and physical vapor deposition are available for the fabrication of ceramic-reinforced metal matrix composite (MMC) [17–21]. With better material utilization, simplicity in the incorporation of hard materials into the matrix and proper distribution of particulates into the matrix, PM is a cost-effective technique to produce ceramic-reinforced metal composites [5,20–23]. On the other hand, porosity is induced in the composite structure during the sintering process which impacts the hardness of the material adversely [20,21]. Binder is a temporary means to mold the powder in the required shape by wetting the surface of particles and developing holding force between the particles. The binder material is usually incorporated during the blending process to wet the surface of powders for bonding and this binder is later evaporated during the sintering process. Stearic acid is an effective binder for iron matrix composite [24]; its insolubility in hot and cold water makes it suitable for underwater application.

Wear is a progressive loss of material when two materials are in contact with each other and there is a relative motion between them. The wear behavior of the material in nuclear applications decides the maintenance cycle and overall life of the material in extreme conditions. Wear depends upon the type of material, its microstructure, roughness, porosity, and hardness [2]. The hardness of any material affects the wear properties. The inclusion of a soft matrix with hard reinforcement increases the hardness along with the improvement of other properties such as density. An increase in the hardness of SS304–B₄C composite up to 7.5%, with reinforcement of 3% was reported by Balakrishnan and Rajesh [5]. Although the percentage of B₄C particles covered in ASTM A887 is only up to 0.25%, composites with ∼16 and ∼25% of B₄C reinforcement have been developed through PM and infiltration casting [14,23]. Comprehensive studies on fabrication and characterization of SS–B₄C MMC have been carried out [5,6,14,15,20–23] but none so far on the tribological behavior of SS–B₄C composite. Hence, the current study is focused on the development and tribological testing of B₄C–SS304 composite material for high-speed low contact pressure nuclear applications. Composites with 0, 5, 10, and 15% B₄C volume reinforcement by PM method are produced. The levels are selected to establish a general trend of the impact of B₄C particles on the tribology behavior of composites. To avoid the significant impact of porosity on the hardness of composite the reinforcement is limited to 15%. The tribology study of the developed composite is done by the pin-on-disk method under the dry sliding conditions to evaluate the coefficient of friction and wear properties. The effect of this B₄C reinforcement on density and water absorption is also evaluated. The next section is focused on details of materials and methods used for composite fabrication, methods for different material property testing followed by results and discussion.

2 Materials and methods

SS304 (or 18-8 steel) and B₄C in spherical powder form with an average particle size of 36 µm were used for the fabrication of cylindrical samples for the study. The typical contents of SS304 are presented in Table 1 [5,23]. PM involves the blending of powders, compaction into a particular shape under pressure with or without heat, followed by a sintering process.

B₄C powder with 0, 5, 10, and 15% of total volume was mixed with SS304 powder in planetary ball mill at 120 rpm with a 10:1 ball to powder ratio for 300 min. Ethanol of 2% was added as a processing agent. The mixed powders were dried and a 4% volume of stearic acid was added into the mixture as a binder. Then the mixture was uniaxially cold-pressed in a die at a pressure of 400 MPa on a compression machine to obtain final cylindrical specimens with a size of 8 mm diameter and 30 mm length (Figure 1).

Figure 1

Time vs temperature vs pressure program followed for PM fabrication process which involves powder blending, cold compaction followed by vacuum sintering.

Sintering of cold-pressed specimen was conducted in vacuum furnace as follows: first the specimen was heated at the rate of 30^oC/min up to 350^oC and then kept for 5 min. Then, heated to the final temperature of 800^oC and kept at this temperature for 10 min. Finally, the sample was cooled down in the furnace down to room temperature.

2.5 Tribology test (wear and COF measurement)

The operating sliding speed <1 m/s, and contact pressure >1,000 MPa is considered for tribology testing of low-speed–high-pressure mechanisms in the nuclear industry [1,3]. As the study is focused on high-speed low contact pressure applications, sliding speed of 1 m/s and 5 m/s, and contact load of 10 N is considered for testing. Pin-on-disc setup was utilized to conduct the friction and wear tests at RIT, India (DUCOM, Bangalore, India, model: TR20-LE). Tests were conducted according to the ASTM G-99 standard. Before the test, the disc was cleaned thoroughly with solvent and clamped on a holder.

The cylindrical specimen (with 8 mm diameter and 30 mm length) was mounted in the holder above and in contact with the rotating disc as shown in Figure 2. The specimen was set above the wear disc using a height adjusting block. The test conditions are listed in Table 2, the sliding distance, track diameter, contact load, and ambient temperature were kept constant. Wear and frictional force were confirmed to zero before starting each test. During the test, the frictional force and wear were continuously recorded using a load cell and linear variable differential transformer sensor. The plots of wear, coefficient of friction (COF), and frictional force with time were displayed online during the test. Wear of the material in microns was noted down after each test. Three tests were conducted for each sample and an average was presented.

Figure 2

Pin-on-disc experimental arrangement for measurement of coefficient of friction and wear properties of specimen material. Here the disc is rotated while the specimen is stationary in the holder, the relative sliding motion between the disc and pin results in wear.

Table 2

Experimental conditions

Parameter	Operating conditions
Contact load	10 N
Track diameter	100 mm
Sliding velocity	1, 5 m/s
Sliding distance	200 m
Lubrication	Dry
Disc material	EN 31
Total disc diameter	200 mm
Roughness of disc material	1.6 µm Ra
Hardness of disc material	54 HRC
Environmental temperature	26oC
Relative humidity	60%

3 Results and discussion

Figure 3 shows the peaks in the XRD pattern of B₄C-reinforced composite. The peaks at 2θ values of 23.75, 35.54, and 37.60 correspond to B₄C on other hand peaks at 43.51, 75.40 correspond to Fe. At low reinforcement (5%) the peaks are hard to differentiate from the impurities; however, at high reinforcement (15%) these peaks are clearly seen. The increase in the intensity of peaks confirms the increase in the percentage of reinforcement.

Figure 3

XRD patterns of sintered cold-pressed composite with B₄C reinforcement: (a) 5%, (b) 10%, and (c) 15%.

The effect of percentage reinforcement on the density of the material is shown in Figure 4. The density of composite material depends upon the molecular interaction, porosity induced, and molecular configuration [3,7]. The composites with 0, 5, 10, and 15% B₄C reinforcement showed density of 5.71, 5.21, 4.20, and 2.96 g/cc, respectively. The incorporation of lighter reinforcement into the same volume of matrix results in a reduction of density with an increase in percentage reinforcement. Porosity is one of the undesirable properties associated with the PM technique which reduces the density of composite material. The porous structure is produced during the sintering process when the binder material is evaporated gradually, creating voids in the structure. However, this percentage of binder remained the same for all the compositions; hence, the porosity of material has negligible contribution to the density reduction of composite material as compared to the former reason.

Figure 4

Effect of reinforcement on density and hardness of SS304–B₄C composite.

As shown in Figure 4, the hardness of composite increases with the percentage of hard B₄C ceramic reinforcement. An increase in the hardness of composite is observed from 104 to 153 HV with 15% reinforcement of B₄C particles. The obvious increase in the hardness can be due to high strength B₄C reinforcement promoting good adhesion between steel matrix and B₄C, and uniform dispersion of reinforcement into the matrix [25]. However, during the cold compaction process, B₄C particles are displaced toward the sidewall, top, and bottom of the cylindrical sample under high compressive pressure. This higher presence of hard particles at the surface results in high surface hardness of composite materials [26]. Although the density and hardness of the given composite material are controlled by the light and hard B₄C particles, they are not directly related to each other.

Water absorption is one of the important properties when the material is working underwater environment. The large presence of water or moisture in the material affects the useful properties such as strength and wear resistance during the working period of the material. It can be seen from the graph shown in Figure 5 that as the percentage of reinforcement increases, there is an increment in the absorption of water by the composite material. The percentage of water absorbed by the material with 0, 5, 10, and 15% of reinforcement is 0.27, 0.34, 0.53, and 0.71, respectively. Due to induced porosity, the voids in the material are filled by water resulting in more water absorption. This property can affect the wear behavior of the composite when the material is under sliding motion. The type of materials used in composites, additives, and binders, and temperature and length of exposure are the factors affecting the water absorption property of the composite material.

Figure 5

Effect of reinforcement on the water absorption capacity of SS304–B₄C composite material.

To investigate the effect of B₄C particles on the wear of the composite, parameters such as sliding velocity and sliding distance are kept constant. From Figure 6 wear amount and coefficient of friction gradually reduce with an increase in B₄C percentage. At 15% B₄C, composite exhibits lowest wear (1.91 mm³@1 m/s and 2.51 mm³@5 m/s) and COF (0.021@1 m/s and 0.042@5 m/s), respectively. Whereas the pure SS304 sample exhibits the largest value of wear (5.67 mm³@1 m/s and 3.62 mm³@5 m/s) and COF (0.110@1 m/s and @5 m/s), respectively. The decrease in the wear and COF is contributed by the hardness of the composite, according to the well-known Archard equation [25]. As the contacting surfaces are iron alloys, and tests are performed under the presence of air, the oxygen from air forms an oxide layer on the surface of iron alloy. A mechanically mixed layer (MML) is developed that has a low coefficient of friction and this layer reduces the direct contact between pin and disc material and avoids the occurrence of adhesive mechanism [2,3,27]. Milling of powders during fabrication leads to stability of this tribological layer and improvement in the wear resistance property of the material [7,25]. The worn surfaces of highly reinforced composite pins after the test are covered with grooves which denote abrasive wear. Whereas the pure SS304 pins show plastic deformation and dominance of adhesive wear for softer composite structures. Thus, the hard particles in the structure transform the wear mechanism from adhesive to abrasive type. The B₄C particles actively participate in the load-bearing mechanism during contact and have a low tendency to adhere to the counter surface, reducing the possibility of stick–slip phenomena. From the discussion, the composite material with 15% of B₄C reinforcement exhibits better properties compared to other compositions. The comparison of wear volume for different composites found in literature is shown in Table 3.

Figure 6

The effect of B₄C vol% reinforcement on wear and COF of composite material.

The pure ceramic and ceramic composite coating approach have proved itself for improving wear resistant and friction coefficient properties of metals. The friction coefficient of the B₄C/a-C coatings under sliding condition is reported as 0.2 for less than 1,000 cycles [28]. Whereas the SS304–B₄C composite shows maximum friction coefficient of 0.11 for less than 1,000 cycles. Although the ceramic coating wear resistant approach appears promising in the B₄C-based coating, boric acid is formed at the sliding interface in the presence of water vapors. Since many of the applications in nuclear industry work underwater environment, coating-based approach could cause severe problems. On the other hand, the uniform distribution of B₄C particles in metal matrix does not promote this behavior and at the sliding interface is dominated by MML rather.

4 Conclusion

The work presents the effect of the percentage of B₄C reinforcement on the properties of SS304 mainly density, water absorption capacity, hardness, COF, and wear of composite material. The density of 15% B₄C-reinforced composite was found lowest (2.96 g/cc) compared to the density of SS304 metal matrix (5.71 g/cc). There was a ∼48% reduction in the density due to increasing lighter reinforcement into the same volume of the metal matrix. The water absorption capacity of the composite was increased with percentage reinforcement, and it was found 62% higher than pure SS304 due to the accumulation of moisture or water content at the available spaces within the material, generated because of porosity. A maximum microhardness of 153 HV was noted for 15% reinforced composite. Wear and COF of the composite are inversely proportional to the percentage reinforcement because of increased hardness. The proper distribution of B₄C particles through PM and the presence of an oxide layer at the contact during operation stabilizes the wear process due to which wear and COF at 15% of reinforcement is the lowest than the pure material at different sliding velocities.