Wear behaviour of titanium based composites reinforced with niobium pentoxide in saline and acidic environments

Abstract

The present study reports on the wet wear behaviour of spark plasma sintered commercial pure titanium (Cp Ti) and Ti-based composites containing 5, 10, and 15 wt% Nb₂O₅ in acidic and saline environments. The wear properties in wet (3.5 wt% NaCl and 0.3 M H₂SO₄ solutions) environments were assessed using a tribometer. The wear volumes and wear rates, irrespective of the environment, decreased with an increase in the Nb2O5 wt.%, which was linked to the harder Nb₂O₅ particles. Furthermore, the wear rate was relatively higher in the acidic environment than in the saline environment. This was connected to the higher chemical attack likely in the acidic environment due to the aggressive nature of the SO₄⁻ ions compared to the less aggressive 3.5 wt% NaCl solution. Abrasive wear prevalence, combined with chemical attack-induced particle scratch-off and subdued adhesive wearing, were mechanisms acknowledged to be operational for wet environments.

1. Introduction

Titanium and its alloys are broadly employed to develop some of the most sensitive and specialized components used in civilian and military aircraft, biomedical devices, components, and equipment parts used in processing industries [1,2]. Their use in these diverse areas of applications has been linked to their attractive spectrum of properties, which include: high specific strength, low density, corrosion resistance, heat resistance, and outstanding biocompatibility [[3], [4], [5]].

The unique and outstanding properties of Ti and Ti-based material systems have buoyed interest in their utilization in broader applications. For instance, the application of Ti-based equipment and components in chemical processing has expanded from the initial soda and caustic soda industries to chlorate, ammonium chloride, sodium chloride, inorganic salts, pesticides, fertilizers, and fine chemicals, where they serve as components for water filters, mixing vessel and oil-fired boiler [6]. Also, in the marine sector, research has shown that Ti heat exchangers and ancillary systems such as flanges, valves, and pumps account for over 50% of Ti usage [7]. However, for most of these applications, the Ti-based systems' utilization is challenging due to their high coefficient of friction, low wear resistance, and high production costs [8]. Consequently, there has been a lot of interest in addressing these shortcomings through research focused on understanding the wear behaviour of Ti-based systems and evolution of strategies to enhance their wear resistance for improved service performance. A relatively well-explored strategy is developing Ti-based composites containing hard materials as reinforcements.

Chen et al. [9] investigated the friction and wear performance of Ti–6Al–4V reinforced with Al, TiN, Ni60 A, and Si powders in the ratio of 4:3:1:2 to meet the performance requirements expected of turbine blades, piston rings, and valves. The composites were coated with TiN, TiB, Ti5Si₃, and Al₃Ti, and the tests on friction and wear were conducted at three specific loads of 3, 6, and 9 N as well as three different temperatures of 25, 350, and 700 °C. In comparison to the Ti–6Al–4V alloy, the composite coatings' abrasion loss, the volume of wear, wear depth and wear ratio were all lower. Additionally, the furrow surface was shallower, with less wear overall. Fellah et al. [10] used a ball-on-disk-type oscillating tribometer to study the tribological behaviour of hot-isostatic-pressed titanium alloy under a wet environment using a physiological solution. The samples showed good tribological performance for all milling times, with the particle and crystallite size decreasing with increasing milling time. Falodun et al. [11] studied the microhardness, microstructure, densification, and wear performance of spark plasma sintered Ti–6 A l–4 V containing TiN as reinforcements. The results indicated that as the TiN content increased, the composite microhardness increased while their sintered density decreased. When TiN is utilized as reinforcement, the composite's worn surface exhibits better abrasive wear resistance, characterized by non-continuous grooves. From the above-reviewed articles, which are representative of the current state of the art as regards studies on the wear behaviour of Ti-based composites, hardly any could be found which studied wet wear of Ti-based composites in saline and acidic environments. These environments typify the service environments of marine and petrochemical equipment and components, which are considerably Ti-based.

Our previous studies have reported investigations on the densification, mechanical and wear characteristics of sintered CpTi based composites reinforced with Nb₂O₅ in dry environments [12,13]. The investigation revealed that as the Nb₂O₅ content increased, the hardness, elastic strain to failure, elastic recovery index, and wear characteristics increased in dry environments. The consideration of Nb₂O₅ as reinforcement in CpTi in our work relied on its characteristics and potential applications. The properties of Nb₂O₅ include outstanding thermodynamic, thermal, and chemical stability with excellent fracture toughness and mechanical properties. Additionally, it has been reported that Nb₂O is biocompatible and, when added as reinforcement, it preserves the base metal's inherent biocompatibility characteristics. Nb₂O₅ has not yet been fully explored in terms of its potential benefits and applications.

This present study, however, investigates the wet wear behaviour of CpTi-based composites reinforced with Nb₂O₅, which recent studies have shown may have potential technological applications in several sectors [14]. This study provides insight into structural characteristics and wear behaviour of CpTi composite in acidic and saline environments. It established a relationship pathway between wet environments and wear mechanisms.

2. Materials and methods

2.1. Materials and composite fabrication

The powders utilized for this research are micro CpTi (99.8% purity; APS 25 μm) sourced from TLS Technik GmbH & Co Niedernberg, Bayern, Germany, and Nb₂O₅ (99.9% purity; APS 50 μm), sourced from Sigma-Aldrich Missouri, United States of America. The morphology of the starting Nb₂O₅ in CpTi powders has been reported in our previous studies [12,13].

The designed composites have 5, 10, and 15 wt% of the Nb₂O₅ reinforcement dispersed into the CpTi powder. The required quantity of Nb₂O₅ powder was measured, dispersed, and mixed with the CpTi using the Retch 100 p.m. planetary ball milling machine for 4 h at 100 rpm without balls. Microstructural analysis from our previous study [12] revealed that the Nb₂O₅ is homogenously distributed in the CpTi Matrix. To avoid powder reactions and cold welding during mixing, a 10-min stop period was introduced. The admixed powder was positioned in a 20 mm graphite sheet lined die to remove the sintered composites easily. The punch and die assembly was placed into the sintering machine (model HHPD 25, FCT GmbH Germany). The admixed powder was densified using a direct electric current and 50 MPa pressure. Sintering was achieved in a vacuum at 1100 °C for 10 min at a heating rate of 100 °C/min. The sintered composite samples were withdrawn after sintering and sandblasted to remove sticky graphite sheets and other impurities from the surface of the composites.

2.2. Microstructural evaluation of the composite

To properly grasp the microstructural characteristics of the sintered CpTi-Nb₂O₅ composites, a JEOL 7001 F scanning electron microscope (SEM) was used for the investigation. Also, to assess the distribution and concentration of elements present in the CpTi and composites, the SEM was linked with an energy dispersive spectrometer (EDS). An X-ray diffractometer was used to analyze and assess the phases present in the CpTi-Nb₂O₅ composite. A PANalytical EMPYREAN diffractometer using Co Kα radiation was used for this purpose. Samples were scanned within two 2θ range of 20$°$ to 90$°$.

2.3. Wet abrasive wear tests in acidic and saline environments

Pin-on-disk tribometer TRB³ from Anton Paar GmbH was utilized to characterize the abrasive wear performance of the SPSed CpTi-Nb₂O₅ composites in acidic and saline wet environments in agreement with ASTM G99-17 [15]. The wet environments, 3.5 wt% NaCl and 0.3 M H₂SO₄ solutions were selected as the saline and acidic environments, respectively. In operation, the sample holder pins down the composite in the environment that houses the acidic and saline medium, while a 5 N load was applied to wear the composite samples. Zirconia ball was selected as the counter face material due to its fracture durability, high crushing strength, and enhanced wear resistance. The Zirconia ball has a smooth surface with good sphericity, while the composite is mounted in a resin with 20 mm diameter and 10 mm thickness. Due to the low speed, the wet wear test was conducted under the boundary lubrication regime. In this regime, the damage is prevented by protective additives that promote sliding rather than welding surface asperities. The wear experiment was carried out at room temperature for 30 min test time, 600 r/min, and 1800 mm sliding distance. The weight of the composites was recorded before immersion and after wet abrasion tests; the weight difference was used to estimate the mass loss. A repeat test was conducted on the composites to ensure repeatability. The wear volume and rates after wet abrasion tests were evaluated, and the results were compared with those obtained for the dry abrasion test reported by Alaneme et al. [12]. SEM was utilized to understand the wear mechanisms of the CpTi-Nb₂O₅ composites through analysis of the degraded or worn-out surface of the composites.

3. Results and discussion

3.1. Microstructure

The microstructures and EDS composition profiles of the CpTi-Nb₂O₅ composites developed are presented in Fig. 1, Fig. 2, Fig. 3, Fig. 4. It is observed from Fig. 1 that CpTi has a single phase (α-Ti) which consists essentially of elemental Ti.

Fig. 1.

(a) SEM micrograph and (b) EDS of the CpTi without Nb₂O₅ reinforcement.

Fig. 2.

(a) low magnification SEM (b) high magnification SEM (c) EDS of the CpTi reinforced with 5 wt% Nb₂O₅.

Fig. 3.

(a) low magnification SEM (b) high magnification SEM (c) EDS of the CpTi reinforced with 10 wt% Nb₂O₅.

Fig. 4.

(a) low magnification SEM (b) high magnification SEM (c) EDS of the CpTi reinforced with 15 wt% Nb₂O₅.

However, for Fig. 2, Fig. 3, Fig. 4, dual-phase structures are observed in the Ti– Nb₂O₅ based composites. The phases consist of α-Ti matrix surrounded with α + β lamella structure. It is noted from Table 1 that the amount of the β-Ti phase increased with an increase in the weight percentage of the Nb₂O₅. This suggests that the addition of the Nb₂O₅ particles facilitates the transition of the α-Ti to β-Ti phase and results in grain size reduction during the sintering process. Studies have reported that the presence of Nb2O5 enhances the nucleation and propagation of β -Ti, which occurs at high sintering temperature and is conserved at ambient temperature due to the fast cooling associated with SPS [13]. The ease of phase transition from the α-Ti to β-Ti could be attributed to the presence of niobium (Nb), a β stabilizing element [16].

Table 1.

Average Grain Size and phase proportion of the composites.

Composite	Average Grain Size	α- phase proportion
0 wt %	–	1.00
5 wt %	24.32 ± 0.22 μm	0.50
10 wt %	22.28 ± 0.20 μm	0.51
15 wt %	16.31 ± 0.30 μm	0.41

3.2. X-ray diffraction

The XRD results (Fig. 5) confirm that the CpTi composition consists of predominantly α-Ti, while the CpTi-Nb₂O₅ composite show peaks of α-Ti, β-Ti, and Nb₂O₅ phases. The XRD result confirms the presence of β-Ti, alongside the α-Ti phase, with the addition of Nb₂O₅. The β-Ti phase was detected at 2θ of approximately 38° in all the composites. Further addition of Nb₂O₅ contributed to the formation of more β-Ti phase, which is detected at 2θ angles of approximately 72° for the 10 and 15 wt% Nb₂O₅ reinforcements. The XRD results confirm that the Nb₂O₅ reinforcement must have facilitated the formation of the β-Ti phase. The presence of Nb in titanium alloys positively influences β phase stability. Studies have shown that Nb can increase the tendency for α-Ti to β-Ti transformations of Ti-based alloys [17]. The observed transformation of α-Ti to β-Ti phase may have been influenced by the thermo-mechanical characteristics (intense creep phenomena, mass diffusion, and plastic deformation) associated with the SPS process [18]. The patterns also revealed peaks corresponding to the Nb₂O₅ phase with no evolution of other intermediate phases in the CpTi matrix. The Nb₂O₅ peak detected at 2θ of approximately 40° was more conspicuous for the 15 wt % CpTi-Nb₂O₅ composition and appeared to diminish in intensity with a decrease in the Nb₂O₅ weight percent.

Fig. 5.

X-ray diffraction pattern of the Cp-Ti and Ti– Nb₂O₅ composites.

3.3. Coefficient of friction

The coefficient of friction (CoF) with respect to time under load of 5 N for the CpTi and CpTi-Nb₂O₅ composites in acidic and saline environments are presented in Fig. 6, Fig. 7. The frictional behaviour of the composites was comparable regardless of the quantity of reinforcement, with only modest variations in the COF values. All the samples' friction coefficients increased significantly during the first 60 s before attaining a steady state. The sharp increase in COF during the early stage of abrasive wear in acidic and saline environments could be ascribed to the degree of surface roughness and deformation of the asperities on the composite surfaces. The attainment of a steady state COF indicates a balance between deformation, wear rate, and work hardening [19]. The composite with 15 wt% Nb₂O₅ took a slightly longer time to reach a steady state compared to other grades of composites. This could be attributed to increased surface temperature during wear due to increased Nb₂O₅ content. It is observed that the steady state friction coefficient ranged from 0.45 to 0.6 and 0.47–0.49 for the acidic and saline environments, respectively. However, the friction coefficient in the wet environments was noted to increase with an increase in Nb₂O₅ reinforcement.

Fig. 6.

Frictional coefficient variation of the CpTi and CpTi-Nb₂O₅ based composites subjected to wet wear in 0.3 M H₂SO₄ environment.

Fig. 7.

Frictional coefficient variation of the CpTi and CpTi-Nb₂O₅ composites subjected to wet wear in 3.5 wt% NaCl environment.

The mean CoF of the composites in acidic and saline environments were compared with the mean CoF values of the dry wear obtained in Alaneme et al. [12]. The result is presented in Fig. 8. It revealed that the CoF values in the wet environments (acidic and saline) were generally lower compared with the dry environment. The reduction in friction coefficient in the wet environments could be attributed to the removal of part of the substantial quantity of heat generated on the composite surface by the acidic and saline solutions. The result also shows that for the CpTi-Nb₂O₅ composites, CoF was generally lower in the saline environment compared with the dry and acidic environments.

Fig. 8.

Mean coefficient of friction of CpTi and CpTi-Nb₂O₅ composites in dry, acidic, and saline environments.

3.4. Wear volume and wear rate

The wear behaviour is better appreciated by analyzing the wear volume and wear rate results presented in Fig. 9, Fig. 10. Fig. 9 shows that wear volume (material loss) reduces regardless of the environment when Nb₂O₅ wt. %. Increases. This can be attributed to the harder Nb₂O₅ particles, which will offer more resistance to the abrasive action of the counter surface during the wear test. The CpTi has lower indentation resistance as supported by the hardness results presented in Alaneme et al. [12], hence is expected to exhibit the highest wear loss. The same arguments suffice to explain the wear rate trend (Fig. 10), which is consistent with that of the wear volume (Fig. 9). Compared with the dry environment, the wear volume and wear rates are lower for the wet environments, suggesting that the mechanisms at play are slightly different. This will become evident from analyzing the wear surface morphologies for the acidic and saline environments (Fig. 11, Fig. 12). The wet media could serve as a lubricating layer between the CpTi and Ti–Nb₂O₅ surfaces and that of the ball counterface, thereby reducing the available amount of abraded material from the test samples [9]. This can also result in less plastic shearing during contact with the counterface – hence less surface adhesion, as in the case of both wet environments, compared with the dry environment where more surface roughness is noted. Some of the plastically sheared materials could be washed off by the wet environment during testing, resulting in fewer materials forming a more preponderant tribolayer.

Fig. 9.

Variation in wear volume of the CpTi and CpTi-Nb₂O₅ composites in dry, acidic, and saline environments.

Fig. 10.

Variation in wear rates of the CpTi and CpTi-Nb₂O₅ based composites in dry, acidic and saline environments.

Fig. 11.

SEM of wear tracks of CpTi reinforced with (a) 0 wt% (b) 5 wt% (c) 10 wt% (d) 15 wt% Nb₂O₅ additions in 0.3 M H₂SO₄ environment.

Fig. 12.

SEM of wear tracks of CpTi reinforced with (a) 0 wt% (b) 5 wt% (c) 10 wt% (d) 15 wt% Nb₂O₅ in the saline environment.

Furthermore, it is noted that the wear volume and wear rates are relatively higher in the acidic environment than in the saline environment. This can be linked to the higher chemical attack likely in the acidic environment due to the aggressive nature of the SO₄⁻ ions compared to the less aggressive 3.5 wt% NaCl solution. As already noted, the wear mechanisms in both wet environments are a mixture of chemical, abrasive wear, and lesser signs of adhesive wear.

3.5. Worn wear surface

A closer analysis of the wear track morphologies of the composites subjected to wear in wet environments compared to those subjected to dry conditions [12] suggests that other factors are at play that contributed to the higher wear resistance exhibited by the CpTi-Nb₂O₅ composites in wet environments. Fig. 11, Fig. 12 show the wear morphologies of the CpTi in acidic and saline environments, characterized by even and continuous grooves along sliding direction with wearing occurring essentially by abrasive wear mechanism. For both wet environments, adhesive wear effects are less significant due to the reduced contact friction from the lubricating impact created by the fluids, which reduces the plastic shearing of the sample surface by the ball counterface. The effect of a chemical attack by the solutions contributes to the visible signs of particles scratched off the sample surfaces due to the preferential chemical attack at the matrix/particle interfaces. Also, low-depth wear grooves formed by the abrasive action of the counterface on the sample surfaces characterize the worn surfaces.

Wearing the composite under dry conditions produces a considerable quantity of heat on the composite surface, causing a lot of damage due to peeling under heating-mechanical processes [20,21]. In contrast, under acidic and saline environments, the solution shares a substantial amount of the heat generated during sliding at the interface. Protective film formation in wet environments has also been reported to act as a lubricant that reduces peeling [22]. Hence, the surface damage due to wear of the composites in wet environments is lighter when compared to wear under dry conditions.

For the saline environment, a closer observation of the wear track at 200 μm (Fig. 13) indicates the presence of Nb₂O₅ that was scratched off the surface of the CpTi-based composite containing 15 wt% Nb₂O₅ and the corresponding EDS shows the existence of Ti, Nb, Al, and O₂. The other labeled part in the box indicates that Nb₂O₅ is not present in that part of the composite, and the corresponding EDS only suggests the presence of Ti, O₂, C, Na, and Cl.

Fig. 13.

EDS of wear tracks of CpTi reinforced 15 wt% Nb₂O₅ in the saline environment at 200 μm.

4. Conclusion

The structural characteristics and wear behaviour in dry, acidic, and saline environments of CpTi and Ti–Nb₂O₅ composites containing 5, 10, and 15 wt% of the Nb₂O₅ were examined. The following conclusions are made from the findings.

• •The CpTi consisted essentially of the α-Ti phase, but with the addition of the Nb₂O₅ particles, the Ti–Nb₂O₅ composite contained α-Ti, β-Ti, and Nb₂O₅ phases.
• •The CoF, wear volume and wear rates were generally lower in wet environments than in dry environments. However, wear volumes and wear rates, irrespective of the environment, decreased with an increase in the Nb2O5 wt.%, which was linked to the more rigid Nb₂O₅ particles, which offered more resistance to the abrasive action of the counterface.
• •The wear volume and rate were relatively higher in the acidic (H₂SO₄ solution) environment than the saline (3.5 wt% NaCl solution) environment, which was linked to the higher chemical attack likely in the acidic environment due to the aggressive nature of the SO₄⁻ ions than in the less aggressive 3.5 wt% NaCl solution.
• •The wear mechanism in both wet environments was predominantly abrasive wear combined with chemical attack-induced particle scratch-off and subdued adhesive wearing.

Author contribution statement

Kenneth Kanayo Alaneme: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Ifeoluwa Joy Ajani: Performed the experiments; Analyzed and interpreted the data.

Samuel Ranti Oke: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Funding statement

Prof. Kenneth Alaneme was supported by National Research Foundation, South Africa [138062].

Data availability statement

Data will be made available on request.

Declaration of interest's statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.