Behavior of CuO as solid lubricant inside ZTA matrices

Advanced ceramics have become one of the most promising materials in modern materials research, with potential to outperform traditional substitutes. These ceramics have excellent properties such as amazing hardness, outstanding wear resistance, high durability, and remarkable reliability. Interestingly, their capacity to adapt to hostile settings confirms their status as state-of-the-art materials for a wide range of applications. Researchers’ main focus these days is on trying to comprehend advanced ceramics and realize their full potential. This emphasis stems from the need to understand the complex phenomena related to these materials so that they can be used in a variety of sectors and enterprises. The development of improved ceramics that can self-lubricate or self-heal is a significant breakthrough in materials science. These materials create new avenues for their integration in many applications by reducing the difficulties caused by the high Coefficient of Friction (COF) and brittleness. The current efforts in this field of study attempt to optimize and customize these materials for particular applications in addition to deciphering the fundamental laws driving these features. To sum up, the development of sophisticated ceramics and the investigation of variations in self-lubrication or self-repair are prime examples of the dynamic characteristics of materials research. The potential for revolutionary breakthroughs in dependability, durability, and adaptability to adverse circumstances continues to drive innovation in this discipline as researchers delve further into the nuances of these materials. The intrinsic limitations of ceramics, which are typified by a high Coefficient of Friction (COF) and brittleness, present formidable obstacles to their extensive application. To overcome these constraints, modern research has shifted its paradigm, concentrating on the creation and investigation of sophisticated ceramics with the potential to be self-lubricating or self-healing. Since these cutting-edge materials are especially designed to overcome and transcend the historical problems connected with traditional ceramics, this represents a significant milestone in the area. This forward-thinking strategy represents a significant advancement, laying out a fresh path for the development of ceramic materials and holding the potential to resolve and surpass enduring problems. Several decades ago, the groundbreaking innovation of self-lubricating ceramics marked a significant milestone, yet the full extent of their potential application remains largely untapped. The genesis of self-healing materials can be traced back to 2014 when geologist Marie Jackson,¹ drawing inspiration from the practices of the ancient Roman Empire, pioneered the concept of material healing. Subsequent to this pioneering work, a plethora of research efforts have been dedicated to unraveling increasingly viable mechanisms for the advancement of such materials.

In the initial phases of exploration, coating techniques were employed in the development of self-lubricating materials. However, a critical observation showed that these coating materials exhibited low shear strength and were prone to being easily removed from the surface, reverting the materials to their initial problematic state. In response to these challenges, researchers sought innovative solutions. One such strategy involved the incorporation of a soft solid lubricant into a hard matrix, introducing a second phase that could effectively address the inherent limitations. It became imperative for this secondary solid lubricant to possess not only low shear strength but also high compressive strength and adhesive properties to enhance overall performance. This evolutionary approach aimed to overcome the initial drawbacks and propel the development of self-lubricating ceramics into a realm of enhanced durability and effectiveness. Alexeyev and Jahanmir^2,3 embarked on the task of addressing the initial challenges associated with modeling self-lubricating phenomena. Their research shed light on the intricate mechanisms involved for the improvement of tribological phenomena. The investigation further revealed that under the influence of applied load, the solid lubricant undergoes a squeezing process and accumulates at the interacting interface. Subsequently, through the sliding or smearing action, the initially soft particles undergo a transformative process, evolving into a thin lubricating surface that effectively provides lubrication. This resultant thin surface layer, aptly termed the “patchy layer,” plays a pivotal role in enhancing the tribological properties of the system. Furthermore, the patchy layer acts as a crucial mediator between surfaces, facilitating smoother interactions and contributing significantly to the overall improvement in tribological performance.

Hence, this investigation provides a new approach for the self-lubrication phenomenon in composites during sliding action. Controlling factors such as design and optimization of the process parameters could also reduce the tribological properties to minimum values.^4–8 In contemporary materials science, the concept of self-lubrication has transcended its traditional boundaries and found application in Zirconia Toughened Alumina (ZTA) ceramics, renowned for their exceptional combination of high hardness and toughness.^9–11 This remarkable advancement involves the incorporation of solid lubricants such as CuO, ZnO, MoS₂, MnO₂, and B₂O₃ into the matrix of ZTA, leading to highly favorable outcomes in terms of tribological properties.¹² The exploration of the tribological characteristics of alumina ceramics reinforced with diverse metal oxides was initially undertaken with great diligence by Kerkwijk et al..^13–15 Their findings unveiled intriguing results, particularly highlighting the superior tribological properties achieved with CuO as a solid lubricant. Subsequent to this groundbreaking research, Valefiet et al.^16–18 expanded the scope by investigating the self-lubricating phenomenon at varying elevated temperatures in alumina and zirconia reinforced with CuO as a solid lubricant. The outcomes demonstrated a pronounced beneficial effect of temperature within the range of 300–800 °C on the tribological properties of CuO/ZTA composites. Furthering the exploration of self-lubricating materials, Zhu et al.¹⁹ delved into the realm of tribological properties by introducing ZnO/CuO doping into the NiAl matrix. Their research discerned that the composition exhibiting the most favorable tribological properties at 800 °C was the CuO-doped NiAl matrix. This superiority was attributed to the formation of a glazed film at the interface, consisting of CuO and MoO₃. Such findings underscore the intricate interplay of temperature, composition, and interface characteristics in shaping the tribological behavior of advanced materials, paving the way for enhanced applications in diverse industrial settings.

In the comprehensive study conducted by Bi et al.,²⁰ a diverse array of self-lubricating materials was meticulously compiled to showcase their effectiveness in ceramic matrices. The findings of this research proposed that achieving a coefficient of friction (COF) below 0.2 and a specific wear rate below 10⁻⁶ mm³/Nm is feasible, even when exposed to temperatures as high as 1000 °C, through the use of self-lubricating composites. In a parallel study, Zhang et al.²¹ examined two distinct types of self-lubricating composites, revealing that a laminated graded structure, incorporating CuO as a solid lubricant, provides superior lubrication. Dey et al.²² contributed to the discourse by developing a variety of composites involving ZTA doped with metal oxides such as CuO, ZnO, TiO₂, SnO₂, and CeO₂ using the co-precipitation method. The investigation aimed to analyze the impact of these additives on tribological properties. Notably, the synergistic effect of CuO and ZnO within the ZTA matrix resulted in a significant reduction in the coefficient of friction (0.35–0.38), attributed to the formation of softer phases such as CuAl₂O₄ and ZnAl₂O₄ at the interface. Furthermore, a minimum specific wear rate (9 × 10⁻⁷ mm³/Nm) was observed when CeO₂ and TiO₂ were present within ZTA. Teow and Noum²³ delved into the densification of CuO/ZTA composites with varying CuO contents at different sintering temperatures. Their investigation concluded that composites with 0.6 Vol. % CuO, sintered at 1500 °C, exhibited the highest densification of ∼99%, coupled with superior mechanical properties. A neutral effect on fracture toughness due to varying CuO contents was also noted. Ramesh et al.²⁴ focused on the preparation of CuO/YSZ composites with varying CuO percentages and analyzed their physical properties. Microwave technique-assisted densification resulted in composites with maximum fracture toughness (7.8 MPam1/2), maximum hardness (14.4 GPa), and a relative density of 99.8%, achieved for 0.2 wt. % CuO-Y-TZP sintered in the range of 1250–1300 °C. Akhtar²⁵ compiled a comprehensive overview of self-lubricating materials integrated with cutting inserts or modern ceramic tools. In addition, Singh²⁶ elucidated the intricate mechanisms occurring during the machining of AISI 4340 steel using CuO/ZTA self-lubricating cutting inserts, attributing the improvement in performance to the formation of a patchy layer at the interface. The collective findings from the aforementioned literature provide compelling evidence supporting the application of CuO in ZTA for the development of self-lubricating materials. Moreover, the literature sheds light on the additive nature of CuO, characterized by low shear strength and high adhesive properties, making it a promising candidate as a solid lubricant. To delve deeper into the physics behind the self-lubricating phenomenon, a comparative study between ZTA and CuO/ZTA was undertaken, exploring their physical and tribological properties. This investigation includes a comparison of tribological properties through FESEM images and XRD plots, ultimately culminating in conclusive remarks for the futuristic applications of these materials.

The co-precipitation process was strategically employed to fabricate composite materials involving Zirconia Toughened Alumina (ZTA) and CuO/ZTA. This method utilized chemicals in nitrate forms, with the selection of composite percentages mentioned in a prior research study conducted by the same researcher.²⁷ The findings from the earlier study indicated that a composition of 1.5 wt. % CuO/ZTA yielded optimal results in terms of both mechanical and tribological properties. The synthetic procedure commenced with the meticulous dilution of specific quantities of chemicals in distilled water. This aqueous solution was then gradually combined with 0.1M ammonia solution in a drop-wise manner. The mixing process persisted until the solution’s pH reached 9, signifying the complete formation of precipitates. Throughout this process, a constant temperature ranging between 60 and 70 °C was maintained. Following the precipitation stage, the solution was allowed to settle for a duration of 10–12 h. After settlement, the water was decanted, and the resultant cake was transferred to a filtration unit. Here, the formed cake underwent washing with warm water to eliminate any remaining nitrate ions. Upon successful removal of nitrate ions, the cake was collected and subjected to 24 h of oven-drying. The dried lumps were then ground using a mortar and pestle and subsequently placed in a high-temperature (HT) furnace for calcination at 800 °C. After calcination, the powders underwent milling in a pot milling machine with alumina balls under a wet condition. Subsequent to milling, the powders were once again calcined at the aforementioned temperature. The milled and dried powders were further mixed with a 5% polyvinyl alcohol (PVA) solution to facilitate proper granulation. After granulation, the powders were arranged in a graphite die-punched setup and subjected to Hot Isostatic Press (HIP) for densification. Densification was carried out at a temperature of 1500 °C and a pressure of 30 MPa, maintained for 5 min. The sintered samples were gradually cooled inside the furnace and carefully removed from the die. After removal, the samples underwent a multi-step polishing process, involving different grades of silicon powders (400, 600, and 800). Subsequently, the semi-finished samples were further polished on a Bainpol polisher using diamond paste, with polishing continuing until the surface roughness of the samples did not exceed 0.5 µm. Finally, the polished samples were subjected to a heat treatment at 800 °C to eliminate any stress and moisture accrued during the polishing process. These heat-treated samples were then utilized for mechanical and tribological tests, with ten repetitive tests conducted and the average results reported in the article, excluding the tribological tests.

The determination of bulk densities for the meticulously polished samples was accomplished by employing the fundamental principles of Archimedes. Subsequent to the milling process, an exhaustive examination of the particle size of the developed composites was conducted utilizing a state-of-the-art particle size analyzer. Comprehensive insights into the microstructure of all sintered samples, coupled with a detailed examination of crystal morphology, were gleaned from high-resolution images and XRD plots spanning the angular range of 2θ = 20°–70°. Following a meticulous morphological study, investigation into the mechanical properties was performed, utilizing the advanced Vickers micro-hardness testing machine. In addition to this, the tribological properties of the developed composites underwent a thorough evaluation employing a Universal Mechanical Tester (UTM-2, Bruker). The parameters selected for conducting comparative tribological tests were based on the prior research endeavors of the authors.²⁸ These parameters included a sliding velocity of 6 m/min, a normal load of 10 N, and the utilization of an alumina ball with a diameter of 6 mm as the counter surface. This stringent testing methodology ensured a robust and comparative assessment of the tribological behavior of the developed composites.

After sintering, the first analysis was carried out for evaluation of the bulk density using the Archimedes principle. The evaluated values of physical properties are summarized in Table I. A slight decrease in the bulk density was observed for the CuO/ZTA composite compared to the parent matrix. Aligned results were also observed in the case of particle size and crystallite size, through the XRD plot evaluation.

From Table I, it can be seen that both composites have a resembling microstructure in terms of grains, but the grain sizes of CuO/ZTA are slightly bigger than those of the ZTA composite. It is well established that the development of grains in ceramics is dependent on the sintering process, pore migration, grain boundary pinning, and pore elimination. The pore mobility during the densification process acts as a dynamic feature within the ceramic microstructure, influencing grain growth kinetics and contributing to the formation of abnormal grains. However, in the case of the HIP technique, as the grains are fused in the presence of pressure, the effectiveness of pores is absent, leading to finer grains after densification. Furthermore, the fusion in CuO/ZTA is more due to liquid phase sintering at 1500 °C. It was well noted by Ran et al.²⁹ that the CuO particles were transformed to a liquid phase during sintering. The transformation results in redistribution of CuO particles inside the cluster, but due to very high viscosity, the changes in distribution are not significant. The transformation from solid to liquid phases and re-solidification are responsible for high grain growth after solidification. Hence, it can be concluded that the presence of CuO enhances the grain size to some extant during densification. A neutral effect of CuO on the stabilization of tetragonal zirconia was observed from the XRD plots. The analysis shows the same peak intensity of monoclinic (m)-zirconia and tetragonal (t)-zirconia phases for both composites. The monoclinic (m)-zirconia and tetragonal (t)-zirconia phases are evaluated through the XRD plot shown in Fig. 1. Hence, the toughening phenomenon that aligned with the transformation from m-zirconia to t-zirconia is completely absent.

An interesting observation was found for mechanical properties, i.e., the value of fracture toughness improves at the cost of hardness. A similar report was earlier cited by Singh et al.,³⁰ which suggested that the enhancement in the size of grains along with the creation of an Cu-ion rich zone develops an impurity phase at the grain boundary, which is responsible for the compromise in the value of hardness. The improvement in fracture toughness is attributed neither to the crack bridging and crack deflection phenomenon nor to the toughening phenomenon. The crack bridging phenomenon was earlier described by Srivastava et al.³¹ Researchers illustrated that the local stress developed at the crack generation point by emitting acoustic waves. These waves act as a barrier for further propagation of cracks, responsible for improvement in fracture toughness.

A comparative study between ZTA and CuO/ZTA in terms of the COF is shown in Fig. 2. The analysis clearly showed a remarkable improvement of around 39.34%, i.e., from 0.455 (ZTA) to 0.276 (CuO/ZTA), in the COF with reinforcement of CuO as a solid lubricant inside the ZTA matrix.

The reason behind the improvement in the COF is dedicated to the formation of the smooth thin layer, known as the patchy layer, at the interface. The pictorial representation of the same is shown in Figs. 3(a)–3(c). Previous researchers revealed that the formation of a thin lubricating layer at the interface takes place during the sliding period responsible for minimization of friction. It is noticed that the formation of the thin layer allied with squeezing and smearing action during sliding. During sliding, the load is applied on the surface, resulting in the squeezing of a softer second phase. Due to this, the softer particles come out from the matrix and accumulate at the interface. After accumulation, the softer particles were smeared again by sliding action. Hence, these soft particles transform into thin films (patchy layer). In Fig. 3(b), the accumulation of soft particles or CuO particles is observed; later, these particles transformed into a thin film, as shown in Fig. 3(c).

A clear difference in the wear track is observed between the two developed composites. The formation of a patchy layer is responsible for the significant reduction in the COF. Hence, the formation of the patchy layer at the wear surface is analyzed through Raman spectroscopy. The Raman spectra for the wear track of CuO/ZTA are shown in Fig. 4. From Fig. 4, a clear scattering measurement at 617 cm⁻¹ is noticed, representing the presence of CuO particles at the surface.^32,33 The earlier studies on the average friction coefficient revealed that in the case of ZTA, the glazed surface is predominant, whereas the formation of the patchy layer is predominant for the CuO/ZTA composite, which is demonstrated though Raman spectroscopy.

Earlier literature analysis³⁴ of wear track also revealed that a large number of grains pulled out with laminar removal of grains and abrasion accompanied by micro-cracks are possible phenomena that cause wearing of the surface in ZTA. However, a large number of micro-cracks accompanied by grain pullout with laminar removal of grains are observed in the case of CuO/ZTA. The analysis clearly reveals the formation of a patchy layer with adhesive properties at the interface surface.

Furthermore, the investigation finally presents a comparative study in terms of scar diameter for both the composites, as shown in Fig. 5. It is found that the scar diameter formed at the counter surface in the case of ZTA is more than that in the case of CuO/ZTA. It may be due to the formation of the patchy layer, which restricts the erosion or abrasion of particles from the counter surface to some extent. Hence, finally it can be concluded that the presence of CuO inside the harder ZTA matrix provides effective lubrication during sliding.³⁵ 

This research endeavors to detail the synthesis of homogeneous ZTA (zirconia toughened alumina) and CuO/ZTA composites employing the co-precipitation route. The fabrication of these samples adheres meticulously to the established standards. While the incorporation of CuO exhibits a neutral effect on both density and crystallite size, a marginal increase in grain size is discerned. Interestingly, the toughening phenomenon is conspicuously absent in the composites, yet a prevalence of crack bridging and crack deflection mechanisms is evident, contributing significantly to the enhancement of fracture toughness. The presence of Cu ions at grain boundaries is identified as the key factor influencing the hardness of the CuO/ZTA composite. Noteworthy is the remarkable 39.34% improvement observed in the coefficient of friction (COF). This enhancement is ascribed to the formation of a distinct patchy layer at the interface. The development of this patchy layer is concomitant with the squeezing and smearing action exerted on softer particles. Consequently, the scar diameter formed on the encounter surface is notably smaller for CuO/ZTA than pristine ZTA, a phenomenon attributed to the formation of the aforementioned patchy layer at the interface. In summary, the synthesis of homogeneous ZTA and CuO/ZTA composites through the co-precipitation route, characterized by meticulous adherence to established standards, reveals intriguing effects on various material properties. The absence of the toughening phenomenon is compensated by the prevalence of crack bridging and deflection mechanisms, contributing to improved fracture toughness. Furthermore, the incorporation of CuO leads to significant enhancements in hardness and the coefficient of friction, attributed to the formation of a unique patchy layer at the interface and its associated interactions with softer particles.

The authors heartily thank the head and staff of the CSIR-MPML group for their support, where this work was carried out.

There is no funding provided by any institutions/organizations/funding agencies for this research work.

The authors have no conflicts to disclose.

Bipin Kumar Singh: Conceptualization (equal); Writing – original draft (equal). Amit Kumar: Methodology (equal); Software (equal). Robert Cep: Formal analysis (equal); Validation (equal). Ajay Kumar: Conceptualization (equal); Methodology (equal); Supervision (equal); Writing – review & editing (equal). Ashwini Kumar: Investigation (equal); Methodology (equal); Visualization (equal). Namrata Dogra: Methodology (equal); Visualization (equal). K. Logesh: Methodology (equal); Software (equal).

The authors declare that the data supporting the findings of this study are available within the paper.