The Progress in Tribocorrosion Research (2010–21): Focused on the Orthopedics and Dental Implants

Abstract

Tribocorrosion is an integration of two areas—tribology and corrosion. It can be defined as the material degradation caused by the combined effect of corrosion and tribological process at the material interfaces. Significant development has occurred in the field of tribocorrosion over the past years. This development is due to its applications in various fields, such as aerospace, marine, biomedical, and space. Focusing on biomedical applications, tribocorrosion finds its applications in the implants used in cardiovascular, spine, orthopedics, trauma, and dental areas. It was reported that around 7.2 million Americans are living with joint implants. Implant surgery is a traumatic and expensive procedure. Tribocorrosion can affect the lifespan of the implants, thus leading to implant failure and a potential cause of revision surgery. Hence, it is essential to understand how tribocorrosion works, its interaction with the implants, and what procedures can be implemented to protect materials from tribocorrosion. This paper discusses how tribocorrosion research has evolved over the past 11 years (2010–2021). This is a comprehensive overview of tribocorrosion research in biomedical applications.

1. Introduction

Components in devices will degrade after utilization due to mechanical or chemical factors depending on the operating environment. In some cases, they need to face dual-hazard environment simultaneously, such as the metallic parts in the nuclear power plant [1, 2], where an additional concern arises. The mechanical degradation (wear) and the chemical damage (corrosion) share a synergistic effect, which induces more material losses from each other, called “tribocorrosion.”

Tribocorrosion is an irreversible transformation of metal subsurface layers due to mechanical and chemical interactions caused by friction contact in a corrosive environment [3, 4], in which wear or corrosion is not entirely responsible for material loss. The degradation of metals due to tribocorrosion can be classified into three parts: mechanical removal of passive film and substrate (mechanical damage), corrosion due to the absence of passive film (chemical damage), and an extra material loss due to the synergistic effect [5]. There are many applications, like marine engineering, aerospace engineering, and biomedical engineering, where the primary mechanism causing material deterioration is tribocorrosion. Hence, it is critical to understand the synergism of wear and corrosion to extend the lifetime of devices.

In 2009, Mathew et al. [6] reviewed the progress of tribocorrosion at the time and indicated several future aspects in the field. Since then, there has been more research in the tribocorrosion area. Figure 1 exhibits the number of publications found in three citation databases, Pubmed, Scopus, and Web of Science. The search keyword for Fig. 1a is “tribocorrosion” and the one for Fig. 1b is “tribocorrosion and implant,” Although the numbers between different databases vary, it is clear to observe the trend of increasing published articles in the field. Based on the databases and publishing year, the percentage of the implant of tribocorrosion research is between 20 and 80%. As expected in the previous review, biomedical implants have become the major subject in the tribocorrosion field.

Fig. 1.

The literature statistics in last 11 years from Pubmed, Scopus, and Web of Science. a The search with keyword “tribocorrosion”. b The search with keyword “Tribocorrosion and implants”

Therefore, this manuscript begins with the general development of tribocorrosion in other topics and mainly focuses on the progress of tribocorrosion in biomedical implants. The application of tribocorrosion in dental and hip implants has been considered, and more emphasis was given to the fretting corrosion at taper junctions of the hip implants. In implants, another primary concern is the toxicity of tribocorrosion products. In addition, this review covers the different surface coatings, how to apply the coatings to biomedical implants against tribocorrosion damage, and the current difficulties of coatings in implants. Introducing different coating technologies and surface coatings provides a solid foundation for understanding how these coatings can be employed as a solution to tribocorrosion damage. The review ended with current knowledge gaps and challenges in tribocorrosion research and provided the future direction of this research.

2. General Development in Tribocorrosion

2.1. Influential Factors of the Tribocorrosion Mechanism

Tribocorrosion can be affected by various mechanical and chemical behaviors, causing wear, erosion, corrosion, fretting, fatigue, abrasion, etc. With this, there are four main factors considered to influence tribocorrosion behaviors: (1) mechanical and operational factors, which involve mechanical properties such as forces and their velocities, how the tribocorrosion is aligned, and its shape and size, (2) electrochemical factors, which include properties, such as resistance, applied potential, and surface film growth, (3) physical factors, which include the material properties of the system, such as the system’s surface roughness or texture, its hardness, and wear debris, and (4) properties of the environmental solution that include the solution’s temperature, pH, viscosity, and conductivity [6, 7]. In knowing all dynamic material properties, further insight into the wear resistance of materials allows a more detailed study of the relationship between corrosion and material loss [8].

Designing and utilizing new materials for future equipment, especially in medical and clinical settings, to mitigate the operating costs and extend the life span of devices have been well demanded for a long time [9]. The recent innovations expanded the research field in the understanding of surface degradation processes as well as the impact of a corrosive environment. The development of in situ electrochemical techniques to examine surface film has been used in tribocorrosion experiments to study the corresponding corrosion reactions during tribological testing [8]. Interactions of corrosion and tribology can be illustrated in Fig. 2, in which biological systems also play a significant role.

Fig. 2.

This schematic illustrates the synergism of different corrosion and wear mechanisms, including abrasion, fretting, fatigue, and erosion

The essence of tribocorrosion is the accelerating degrading rate of components under wear damage in a corrosive environment. While the employed parts are immaturely impaired, they can cause more damage to the entire working system and potential safety risks for implant patients [10–12]. Sun and Rana [4] extensively examined the tribocorrosion effect on AISI 304 stainless steel in 3.5% salt water and verified the dominant corrosion wear mechanism with the pitting corrosion in the wear scar. In addition, Davoren et al. [13] tested the tribocorrosion resistance of the friction-stir welding Ti6Al4V alloy in a 3.5% NaCl solution. Due to friction-stir welding, several regions were generated in the adjoined alloy after the process, and the welded region was shown to obtain the highest tribocorrosive resistance. As a result, fully understanding the factors in tribocorrosion can avoid possible impairment and mitigate the threats from unexpected hazards.

2.2. Other Industrial Application

One of the most researched aspects of tribocorrosion is industrial application. Due to the corrosive seawater and the wavy flow with abrasive particles, it is straightforward to consider the occurrence of tribocorrosion in the industrial environment. Zhang et al. [14] studied the type S31254 steel in seawater for its tribocorrosive resistance, in which they identified the higher synergistic material loss at the pitting potential of the material. Wood et al. [15] published an extensive review of tribocorrosion in industrial applications in 2017. They collected several prior studies following the model proposed by Wood and Hutton [16], generating the mapping between two ratios, synergistic wear rate and corrosion wear rate, erosive wear rate, and corrosion rate. In addition, the factors of material’s tribocorrosion behavior can be more complicated than only considering wear and corrosion. Some shore-operated equipment sustained tribocorrosion with constant high or cyclic fatigue loading, which might shorten the machine’s longevity. Von der Ohe et al. [17, 18] designed combined equipment for tribocorrosion under 4-point bending and examined the tribocorrosion performance of austenitic and super duplex stainless steel under loading. It is concluded that the applied loading, static and cyclic, and electrochemical conditions affected the material loss due to the microstructure alteration.

3. Tribocorrosion in Biomedical Implants

Compared to the accelerated degradation of mechanical parts, tribocorrosion in different biomedical implants can cause more prominent issues, as shown in Fig. 3. The higher amounts of material loss from tribocorrosion can result in the instability of implants, systemic toxicity, and suffering in patients. In the past 11 years, tribocorrosion has been well accepted as a crucial factor in biomedical implants. This review focuses on developing tribocorrosion research in dental and hip applications this decade.

Fig. 3.

Tribocorrosion-affected areas in biomedical implants in dentistry and orthopedics

3.1. Dental Application

Tribocorrosion is one of the significant factors that cause dental implant failures. The factors such as mastication, saliva, changes in pH, and temperature trigger the tribocorrosion processes at the dental implant interfaces. More details are provided below.

Commercial pure titanium (CP-Ti) and titanium alloy are widely used in dental implants due to their high corrosion resistance, mechanical properties, and biocompatibility. The passive oxide layer naturally generated on CP-Ti and titanium alloy surfaces protects the metal substrate from corrosion damage. Nonetheless, volatile oral environments and different surface treatments on titanium can affect the corrosion-resistant nature of titanium. Barao et al. [19] compared the corrosion performances of CP-Ti and Ti6Al4V alloy in the artificial saliva with varied pH values and indicated the acceleration of corrosion reactions on both groups in the acidic condition. The increased corrosion damage is exhibited by fluoride ions on CP-Ti and titanium alloy, causing pitting corrosion and microcracks (Souza et al. [20]). CP-Ti with three surface morphologies was tested in four types of mouthwash. The mouthwashes consisting of 1.5% hydrogen peroxide and 0.2% sodium fluoride reduced the resistance of titanium, and the specimens after the sandblasting treatment presented the highest probability of corroding (Beline et al. [21]).

The impact of corrosion on dental implants is the instability resulting from the material loss and the release of metal ions coupled with corrosion reactions that might arouse some concerns. It was suggested that titanium ions could induce necrosis as the concentration is over 11 ppm and alter the gene expressions of gingival epithelial cells [22]. The release of titanium ions in the animal model enhanced the expression of toll-like receptor4 (TLR-4), chemokine (C–C motif) ligand 2 (CCL2), and the ratio of RANKL to OPG expressions from the retrieved tissues, which might result in the bone resorption (Wachi et al. [23]).

While external friction is considered another factor, combining mechanical and electrochemical interferences causes more damage, including material loss and ion release from tribocorrosion [24]. There are three interfaces in dental implants where the fretting corrosion might occur. As shown in Fig. 3, from the cyclic loading of the biting and chewing following interfaces of the dental implant might be affected: (i) the conjunctions between the crown and abutment, (ii) the abutment and implants, and (iii) the implant and bone can be subjected to micromotion. This caused the breakage of passive films and accelerated corrosion [25, 26]. This kind of micromotion-induced tribocorrosive damage is recognized as fretting corrosion. Tribocorrosion research in dental implants has been summarized in Table 1. The possibility of fretting corrosion on the dental implants indicated the behavior of fretting corrosion on ratios between on-time and off-time operation (Kumar et al. [27]). The concentrations of fluoride ions in artificial saliva affected the fretting corrosion behavior, as reported by Sivakumar et al. [28]. The increased damage was observed from the tribocorrosive effect on CP-titanium during the inflammation by adding lipopolysaccharide (LPS) in the artificial saliva [29]. A review of the tribocorrosive effect on the interface between the abutment and the dental implant points out that the possible cause might not merely result from the mechanics between materials published (Apaza-Bedoya et al. [30]). The structural designs of dental implants could be another solution to mitigating the risk of tribocorrosion. The fretting environment between the dental implants and the abutments was simulated by Sikora et al. [31]. They considered two kinds of interfaces, ceramic/metal and metal/metal, and two different dental implants made from Ti6Al4V alloy and Ti–Zr alloys (Roxolid, Straumann). The results demonstrated that the combination of ceramic and Roxolid exhibited the lowest potential drop and material loss, indicating the potential option for future design. Several combinations of materials in fretting corrosion testing with human saliva concluded that Yttria-stabilized zirconia and pure titanium showed the best performance by Corne et al. [32]

Table 1.

Tribocorrosion research in dental implants in the past 11 years

Author	Experimental model	Findings/comment
Sikora et al. [31]	Ball-on-disc (Implant–abutment interface) Ti group: Ti/Ti, Ti/Rox Zr group: Zr/Ti, Zr/Rox	Highest corrosion Ti/Ti, least corrosion Zr/Rox Wear Volume loss: Max = Ti/Ti, Min = Zr/Ti Wear and corrosion: Ti Group > Zr Group
Alrabeah et al. [91]	cpTi External hex, UCLA abutment-Platform matched, platform switched	Platform-switched groups showed less metal ion release compared with their platform-matched counterparts and enhanced tribocorrosion resistance
Sridhar et al. [92]	Retrieval analysis-failed implants. Bacteria immersion test	Discolorations, pitting, scratches, cracks, and mechanical fracture were present
Ogawa et al. [93]	cpTi disks, HCl + H2O2 H2SO4 + H2O2, Sb Al2O3	Acidic medium-Impaired corrosion behavior. Sandblasting – increased mechanical & structural properties but decreased corrosion resistance
Beline et al. [21]	cpTi disks, CHX, 0.05% cetylpyridinium chloride, 0.2% NaF, and 1.5% H2O2	Clinically, 0.2% NaF and 1.5% H2O2 not recommended for implant patients. 0.12% CHX & 0.05% cetylpyridinium chloride can be safely used
Souza et al. [20]	Ti6Al4V–Grade 5 and cpTi. pH 5.5 no F, pH 5.5–20, 30,227 ppm F, & 6.5 at 12,300 ppm F, artificial saliva	Low F (20, 30 ppm) – high corrosion resistance. 227 and 12,300 ppm – decrease corrosion resistance. 12,300 pp F – cpTi: pitting corrosion. 12,300 ppm F- Ti6Al4V: microcracks
Wachi et al. [23]	Ti standard solution of (NH4)2TiF6, NaF at 1000ppmF and pH 4.2, P. gingivalis-LPS	Cytotoxic (increased: CCL2, RANKL to OPG ratio, TLR-4 expression)
Wheelis et al. [24]	cpTi and the alloy Ti6Al4V disks – immersion and rubbing	Acidic environments coupled with rubbing are able to introduce noticeable morphological changes and corrosion
Mathew et al. [29]	cpTi and TiAlV alloy disks, artificial saliva, LPS	Clinically, patients with oral infections (presence of lipopolysaccharide) are more likely to have their dental implants corroded
Makihira et al. [22]	Ti-ICP standard solution, cell culture medium, Pg-LPS	> 13-ppm Epithelial cell necrosis Large Ti particles (956 nm) increased cytotoxicity and changes in gene expression

3.2. Hip Implant Application

Wear and corrosion are both crucial factors for the failure of hip implants. With regard to clinical importance, there are essentially four main categories of wear and corrosion: (1) polyethylene wear, (2) ceramic-on-ceramic (CoC) bearing wear, (3) metal-on-metal (MoM) tribocorrosion, and (4) fretting corrosion at the taper junction [33]. Especially, MoM tribocorrosion and fretting corrosion at the taper junction have caused several patient issues. Although CoCrMo alloy, the primary material for MoM hip implant, performs a superior tribological response, the synergism of wear and corrosion strikes it harshly, generating large amounts of particles and releasing metal ions. Mathew et al. [34] examined the low-carbon CoCrMo alloy of tribocorrosion properties by comparing two experimental set-ups. In the study, the experiments in protein-containing solution showed the generation of a protective layer, which lessened the damage. Mathew et al. [35] further identified that the protective feature of protein-containing solution mitigated the induced current during tribocorrosion reaction. The mitigation might result from the natural formation of tribolayer in patients’ bodies reported in the prior studies [36–39]. Wimmer et al. [40] demonstrated the beneficial effects of tribolayer on both low-carbon and high-carbon CoCrMo alloy against corrosion and tribocorrosion. Even though the tribolayer can reduce part of tribocorrosion damage, the net material loss of MoM is still too significant to be seen as safe for patients. As a result, the Food and Drug Administration (FDA) announced warnings and recalled several commercial products of MoM hip implants [41].

3.2.1. Fretting Corrosion at the Modular Junction of Hip Implants

While MoM hip implants are employed less in the market after the announcement of the FDA, tribocorrosion reaction still risks the hip implants. Clinically, the modular junction between the neck and head of the hip implant may undergo micromotions, which are on the order of 5–15 μm [42–44]. The micromotion in the corrosive biological system can trigger fretting corrosion [42, 43, 45–47], leading to implant loosening and the generation of degradation products. The products of fretting corrosion can lead to adverse local tissue reactions (ATLRs) and necrosis of surrounding tissue [48, 49]. Since the metal femoral ball head is still widely used in the metal-on-polymer (MoP) design, further research is essential to assess the danger of fretting corrosion products to surrounding tissues, reduce fretting corrosion damage, and improve current hip implants.

Prior studies have shown mixed damage modes of corrosion, wear, and fretting corrosion on the retrieved hip implants [50–52]. The signs of etching, pitting corrosion, and intergranular corrosion demonstrated chemically driven material loss. Evidence of material transfer and fretting wear on the same samples can be categorized as highly mechanically driven [50, 52]. The reported results suggested that fretting corrosion initiates a chain reaction of wear and corrosion [51, 52]. With mechanical deformation, fluid infiltration occurs and changes the microenvironment [45]. Fluid in the modular junction might result in several chemically dominated corrosion modes.

3.2.2. Factors Affecting the Fretting Corrosion at the Modular Junction

One of the crucial factors in fretting corrosion on the trunnion of hip implants is their designs. Implants have become increasingly modular over time, allowing customization to meet the patient’s needs [51]. The trunnion taper is chosen and designed to give the patient the best range of motion compared to a normal healthy hip. A study done by Rowan et al. [53] took two different taper designs (V40 and 12/14) and performed cyclic loading at varying loads using ceramic and CoCrMo heads. The variation in corrosion currents between the designs of tapers and the combination of ceramic and metal heads was measured. The load at which corrosion current differed significantly differed between taper designs; however, all loads showed cyclic motion indicating the passivation and repassivation of the trunnion. An attempt was made to identify the difference in damage among four different taper designs, 11/13, 12/14, 16/18, and V40, from the retrieval study (Siljander et al. [54]). In addition to taper geometry, the femoral head size was also considered. Due to taper design and head size, the load of the patient will be dispersed in different manners to the stem from the head and ball joint leading to different damage modes. They concluded that smaller taper geometries produced the lowest taper damage and vice versa. Hence, these differences in taper geometries may increase the micromotion, leading to more wear and fluid penetration into the modular junction and increasing fretting corrosion-related damage.

3.2.3. Recent Progress in the Fretting Corrosion at the Implant Interfaces

Even though there are many findings from the retrieval studies to the fretting corrosion on the hip implants, the in vitro studies are equally crucial to understanding the mechanisms in the complex on-site environment (Table 2). Gilbert et al. [55] demonstrated the fretting corrosion between the stainless steel hip stem and the CoCrMo ball head. They showed that the results of fretting corrosion were intervened with the applied loads and designs of implants. Royhman et al. [56] designed the custom-built apparatus to simulate the taper junction and tested the experiments in two different pH conditions. Titanium on titanium couple exhibited the least mechanical dissipated energy but the highest electrochemical responses among all tested groups. Compared to extended in vivo implant performance, there are always many limitations with the short testing duration. Royhman et al. [57] continued the research into the full-scale study, including two electrochemical conditions, two pH values, and five different displacements. The incremented displacements altered the fretting mechanism from the partial to the gross slip, generating more corrosion damage in both pH conditions. Fischer et al. [58] examined both CoCrMo and CoCrFe alloy against Ti6Al4V alloy in fretting corrosion to verify the role of molybdenum in the tribofilm formation. Even though the correlation of molybdenum to generate the tribofilm was still not clarified in the study, the inclusion of molybdenum showed less material loss than the compared group, which was attributed to the different contributions of tribocorrosion in the process. Panagiotidou et al. [59] considered the assembled force a significant factor in reducing the impact of fretting corrosion. With the custom-made experimental apparatus, they successfully proved their hypothesis that the higher assembly force generated low tribocorrosion currents. Royhman et al. [60] performed the fretting corrosion testing on a couple of Ti6Al4V rod/CoCrMo pins in the bovine calf serum (BCS) with different pH conditions. On the specimens from the group in pH 4.5, there was much tribolayer deposited on the surface, leading to less potential drop and material loss. An incremental cyclic fretting corrosion (ICFC) test examined the fretting corrosion on the modular head-neck test coupons (Ouellette et al. [61]). The head offsets and seating loads have been shown as two influential parameters to the induced fretting current and the extent of micromotion.

Table 2.

Development of fretting corrosion in biomedical implants in the past 11 years

Author	Experimental model	Findings/comment
Kumar et al. (2010) [94]	-CP-Ti (grade-2) -Ball-on-flat contact configuration of 8-mm alumina ball moving against stationary CP-Ti -Electrolyte Solution is Ringer’s Solution	-Corrosion resistance and biocompatibility of CP-Ti are nullified under fretting conditions -Passive oxide layer damage led to repassivation that was not instantaneous -Accumulation of debris was present -Test parameters implied a gross slip condition
Pellier et al. (2011) [95]	-A specific device conceived and developed in collaboration between EMSM-SE and Böse was used for fretting corrosion tests -Load cell is a piezoelectric transducer, and the displacement measurer is a capacitive sensor	-Under OCP conditions, increased chloride concentration leads to higher corrosion and a more difficult passive layer reconstruction during the friction phase -Albumin promotes mechanical degradation compared with corrosive wear under cathodic polarization
Kim et al. (2012) [96]	-Conducted in Ringer’s solution -Stainless steel (316L) and poly (methyl methacrylate) -Various potentials are applied in fretting corrosion tests and dissipated energy is determined with several cycles	-The damage rate constant and the damage exponent could determine the accumulation of dissipated energy change -The magnitude of applied potential shows a relation with wear volume -The relation between the applied potential and wear volume can be expressed as an exponential function
Geringer et al. (2013) [97]	-Cylinder–plane contact -Threshold displacement was kept between partial and gross slip -A test system with a pin-on-ball configuration constructed to mimic contact conditions in femoral joint MoM -Co–Cr–Mo cylindrical disc of 12 mm diameter -Ceramic ball of 28 mm diameter of 26-N contact load	-Albumin inhibits the corrosive wear of Type 316L stainless steel -Experiments show that albumin protects 316L stainless steel against fretting corrosion degradation -Between fretting and wear corrosion, the contribution of the synergistic term to the total wear volume is higher for fretting corrosion than for wear corrosion
Kim et al. (2013) [98]	-One parallelepiped specimen (9 mm × 9 mm × 20 mm) and one cylindrical pad (lengths 15 mm with a radius of 10 mm) were used for each fretting corrosion test -Femoral stem was made of stainless steel (316L) PMMA mechanical properties were as follows: Elastic modulus of 2.5 GPa Poisson’s ratio of 0.39 Yield and Tensile strengths of 65 MPa and 75 MPa, respectively -Normal force 127.5 N applied -Frequency of 1 Hz was applied	-In the case of fretting corrosion, the friction coefficient growth rate can be expressed as a power law function -Friction coefficient of -04 V/SCE is higher than that of open-circuit potential -The addition of albumin increases friction coefficient, but it decreases wear rate
Kurtz et al. (2013) [99]	-Priori power analysis was conducted -Total of 100 retrieval cases was judged to be adequate -Scoring system for fretting and corrosion was characterized using a previously published 4-point scoring technique (1 indicating minimal fretting or corrosion and 4 indicating severe damage)	-For the stems in the ceramic–metal when compared with the metal–metal taper cohort: Fretting and corrosion scores were lower -Decreased stem flexural rigidity -Lower corrosion scores were observed between the ceramic–metal and metal–metal taper cohorts
Molloy et al. (2014) [68]	-Review of fifteen patients who had received an ABG II dual-modular hip system -Time frame: From May 2007 to August 2008 -Anteroposterior radiographs of the pelvis were scored with regard to medical calcar erosion -MRI was performed -Retrieval analysis on implants removed at revision	-Mean duration of follow-up for patients was 42.3 months -Cobalt-ion levels were elevated in all patients -Chromium levels were within normal range -Medial femoral calcar erosion in seven of fifteen patients -ABG II dual-modular hip system is associated with a high rate of early failure secondary to fretting and corrosion at femoral neck stem taper -Component has been recalled and is no longer in use
De Martino et al. (2015) [100]	-Retrospective case series of 60 consecutively retrieved implants from 55 patients (all who had received a Rejuvenate modular hip system as part of primary total hip arthroplasty from Stryker Orthopedics (New Jersey) -Rejuvenate stem made of titanium alloy (Ti-12Mo-6Zr-2Fe or TMZF) -Modular neck made of Co–Cr–Mo -Bearing consisted of Co–Cr–Mo (V40 LFIT CoCr, Stryker) and ceramic (V40 BIOLOX delta, Stryker) -Implants were examined visually under a stereomicroscope (magnification 6 × to 10 ×, Wild Type 376,788 Microscope, Heerbrugg, Switzerland) -Zones assess each modular neck and stem according to Goldberg’s criteria on a scale from 1 to 4 (1 =none, 4 = severe) -Scanning Electron Microscopy -Statistical Analyses: Multiple linear regression using the GEE method	-Higher corrosion scores on the medial and distal lateral sides of the taper junction were consistent with cyclic cantilever bending of the neck and head portion, which is a major cause of fretting and subsequent corrosion -Positive correlation between LOI and fretting and corrosion scores suggests damage modes will increase with time
Royhman et al. (2015) [56]	-CoCrMo and Ti6A14V alloy were used -Flat on flat (pins on rod from both sides) -Machine compliance evaluation -Tested couples: Ti6A14V–Ti6A14V–Ti6A14V (Ti–Ti–Ti), Ti6A14V–Ti6A14V–CoCrMo (Ti–Ti–Co), and CoCrMo–Ti6A14V–CoCrMo (Co–Ti–Co) -Electrolyte is diluted BCS protein concentration of 30 g/L -Reference Electrode: SCE’ -2 pH levels are (pH 3.0 and 7.6)	-No significant difference in electrochemical or mechanical behavior in response to pH change -Ti6A14V–Ti6A14V–Ti6A14V displayed the earliest passivation and superior electrochemical behavior under fretting conditions -Findings suggest transitions in the degradation mechanisms at the modular junction as a function of material couples/contact
Dos Santos et al. (2016) [101]	2 models of modular hip prostheses were analyzed: -Model SS/SS cemented: prosthesis of long-term use with the femoral head and stem made of austenitic stainless steel (ASTM F138–13a)-Bone cement -Model SS/TI Cementless: prosthesis of long-term use with the femoral head made of ASTM F138 stainless steel and stem made of Ti-6Al-4 V alloy (ASTM F136–13) Electrolyte Solution: 0.90 NaCl in distilled water Vacuum filtration of 0.015 μm	-Localized corrosion in head–taper interface in the prostheses -Stainless steel prostheses more susceptible to corrosive attack -Fretting corrosion did not create cracks Particles: agglomerated/irregular in SS/Ti and smooth/irregular in SS/SS -Particles with the presence of elements from the implants -Model SS/Ti was more resistant to fretting corrosion
Royhman et al. (2016) [57]	-CoCrMO hip implant head on Ti6A14V hip implant stem -Representative contact geometry (flat-on-flat contact) -Reference Electrode is SCE (saturated calomel electrode) -Counter electrode is a graphite rod	-Drastic deviation (of potential drops at onset fretting, during fretting, and the onset of fretting to termination) from linearity at 100-μm displacement amplitude -Found a large number of Ti deposits on the CoCrMo pin surfaces -Increased levels of resistance of the system after induction of fretting motion
Esguerra-Arce et al. (2016) [102]	- Ti1–xAlxN films were deposited on AISI 304 stainless steel and Si (100) substrates by reactive magnetron co-sputtering at 250 °C	-Fretting corrosion against bone can damage the metal and coatings -The wear volume loss is related to the corrosion resistance and H/E ratio -Ti–Al–N improved the wear resistance of the stainless steel in simulated body fluid
Hui et al. (2017) [71]	-Custom fretting corrosion cell culture test instrument was built to allow for mechanical, electrochemical, and cell culture measurements in a pin-on-disk model -Piezoelectric actuator in conjunction with an output amplifier	-Fretting corrosion debris and cathodic potential excursions induced increased cell killing -Cathodic potentials below a threshold of –400 mV appear to be a strong effector of cytotoxic response -The specific electrochemical conditions associated with fretting corrosion may play an important role in how local cells and tissue respond to mechanically assisted corrosion processes in vivo
Kyomoto et al. (2017) [103]	-Co–Cr–Mo alloy specimens were machined from a Co-28Cr-6Mo alloy bar stock -Galvanic corrosion test with a mixture of 27% volume FBS with pH adjusted to 7.4 were evaluated according to ASTM G&1–81 -12 hip simulators where femoral heads were placed on a Ti-6A104V alloy trunnion and a 2.0-kN load according to ISO 7206–10 -Head-stem neck junction	-The galvanic current density for the Co–Cr–Mo alloy group during the initial 60 s was significantly higher than those in our cupboards -The galvanic current density for the Co–Cr–Mo alloy group during the initial 60 s was significantly higher than that for the ZTA ceramics -In contrast, the galvanic current densities did not differ significantly even after 7 days
Royhman et al. (2018) [60]	-Ti6A14V/CoCrMo couples -Custom-made setup was used to evaluate the fretting corrosion behavior of hip implant modular junctions -New-born calf serum solution (30 g/L protein content) -Sinusoidal fretting motion with displacement amplitude of + 50 μm applied to Ti alloy rod -Simulated periprosthetic pH variations were (pH levels 3.0, 4.5, 6.0, and 7.6)	-The pH level influenced potential drop under fretting conditions -There is a direct correlation between the potential drop (cathodic excursion in potential) and the material loss into the surrounding electrolyte -Fretting corrosion at all pH levels created damaged surfaces with specific wear and corrosion features
Semtse et al. (2019) [104]	-Sample: Ti-6Al-4 V alloy with spark plasma sintered and Ti-6Al-4 V composites with 5–10-wt% ZrO2 -Fretting corrosion between the femoral stem and neck adapter was measured -Simulated body fluid used for the experiments was the fetal bovine serum with protein contents of 30 g/l -Test duration for fretting was 57,600 cycles	Adding zirconia to Ti-6Al-4 V produced a modified microstructure containing globular zirconia-rich agglomerates -Additions of 5-wt% ZrO2 in Ti-6Al-4 V is ineffective in improving the tribocorrosion properties of Ti-6Al-4 V due to the high amount of surface degradation and the increased wear volume exhibited by this material -Ti-6Al-4 V composites showered a lower tendency to metal ion release than pure Ti-6Al-4 V
Smith et al. (2020) [105]	-Custom in vitro pin-on-disk fretting test setup used to test pin-disk couples -Implant alloys: Ti-6Al-4 V/Ti-6Al-4 V (ASTM-F1472), CoCrMo/CoCrMo (ASTM-F1537), and Ti-6Al-4 V/CoCrMo -3 pin contact geometries and compliances: (low (2 mm wide × 1 mm tall), medium (1.5 mm wide × 2 mm tall), and high (1 mm wide × 3 mm tall) -Variable-load test chosen to allow for the full range of fretting regimes (slip, stick–slip, and stick)	-Geometric changes to the metal–metal junction can reduce fretting corrosion damage -Modified surface contact geometries can create a more compliant interface -Compliant interfaces prevent oxide abrasion of metal surfaces, reducing damage -Compliant geometries elastically bend during fretting, creating a ‘stick’ regime -Fretting corrosion maps characterize contact mechanics over a range of conditions
Tsai et al. (2021) [106]	-CoCrMo alloy is cut into 1 cm lengths from a 16-mm-diameter rod -Polishing was done with 6- and l-μm diamond suspensions and 0.05-μm silica suspension -Temperature during both coating depositions was controlled at 380–420 °C -SEM and EDS -Biaxial and shear model	-ZrN and TiSiN coatings on CoCrMo substrates to improve fretting corrosion resistance -Both coatings show the lower Co ion release -No evidence of avoiding the delamination

3.3. Tribocorrosion-Induced Cell Toxicity

The research of MoM hip implants is becoming increasingly popular throughout the century as the release of degradation products (ions + products) during tribocorrosion can affect biological responses in patients. MoM differs from Metal on Polymer (MoP) in their wear particle proportions, in that MoM particles are nanometric in size, while MoP particles are sub-micrometric [62]. This accounts for the differences in their wear rates and wear resistance in the hip joint. A preliminary study on the implant interface to the bone was reported by Runa et al. [63], simulating the corrosion and tribocorrosion conditions. The results indicated the presence of osteoblast cells enhanced the resistance against tribocorrosion damage, and the mechanical loading negatively influenced the surrounding osteoblastic cells. Hanawa et al. [64] conducted a study for the oxide films on a CoCrMo alloy, consisting of mainly chromium oxide and molybdenum oxide, which was in the inner oxide layer. The results suggested that cobalt ions were released into the environment. Given the release of cobalt, however, many have expressed concern since too much metal ion release may cause aggressive effects on a patient’s health, possibly causing metal accumulation in the liver, allergic reactions, metal ion toxicity, and even cancer [65, 66]. The discovered potential risks increased the importance of studying tribocorrosion and the associated degradation products.

There have been increasing adverse local tissue reactions (ALTRs) associated with metal ion release in patients with MoP hip implants [67]. Studies have indicated that the metal release may have come from fretting corrosion at the head-neck junction of the joint. Perivascular T-cell infiltration/vasculitis indicated the changes in the morphology of the endothelial cells to rounded or prismatic. Giant multinucleated cells were associated with large corrosion-product (Cr-rich, no Co) particles. Necrotic areas usually surround t-cell infiltration zones with low vascularity and evidence of cell degeneration [68, 69]. The metal debris was also shown to induce a different kind of macrophages, believed to increase the corrosion damage on the surface [70]. Systemic damage from the metal particles and released ions are also clinically reported. Bijukumar et al. [10] thoroughly reviewed the degraded products generated from the total hip replacement in local and systemic toxicity. In a further study, they demonstrated the difference in degraded particles between as-synthesized and processed conditions, indicating the importance of maintaining the consistency of in vitro study in an in vivo environment. In addition, Bijukumar et al. [71] verified that the as-synthesized degraded particles from the CoCrMo alloy disrupted the DNA replication in neuronal cells compared to the processed particles. The results showed how the hip implants’ tribocorrosion products resulted in local and systemic neuronal damage.

There might be more potential risks from the products of tribocorrosion, which have not been discovered yet. Clinical findings usually result from the addition of several mechanisms, which becomes too complicated to analyze and figure out the right pathological pathways. In vitro testing provides the researcher with a controllable environment, but the critical issue is the resemblance between in vitro and in vivo examinations. It was suggested that include a cell study in the tribocorrosion experiment, which posited cells near metal or cells on metal (Radice et al. [72]). The standardization of methodology could provide comparable data in the lab to those obtained in the clinic, such as the analysis of retrieved implants. The combination of two major fields might be the future research in tribocorrosion for the next decade.

4. Protection Against Tribocorrosion

The damage from tribocorrosion can cause or accelerate the dysfunction of working components in every field. The impairment might not draw much attention if wrecking parts can be readily repaired. However, in biomedical applications, the malfunctions of implants represent both disabilities to patients and the release of more wear products inside patients’ bodies, endangering them with local and systemic toxicity. To prevent the occurrence of these drawbacks, the enhancement of tribocorrosion resistance through surface modification is a straightforward solution, including the deposition of protective coatings and surface treatments (Fig. 4).

Fig. 4.

Surface coatings/modifications to minimize tribocorrosion in implants

Tribocorrosion is a synergism of tribology and corrosion, for which preventive improvement should consider both criteria. The typical approach is the deposition of hard ceramic coatings, which perform high wear and corrosion resistance. The potential risk of abrupt fracture leading to the failure of arthroplasty is a significant concern due to the brittle nature of ceramic materials. If ceramic coatings fail in operation, the shards of the coatings might hurt the surrounding tissues, and it will be difficult for surgeons to clean the area during revision surgeries, prolonging the suffering of patients. With the development of coating technology, formulating a solid and tough ceramic coating on the implant’s surface is highly possible. Sathish et al. [73] evaluated the corrosion and wear properties of the laser-nitrided CP-Ti and Ti-13Nb-13Zr alloy. After the surface treatment, the anodic current and wear rate became lower, demonstrating the protection of titanium nitride on the surface. Zhu et al. [74] developed a TiC layer with the ion-enhanced triode plasma CVD to improve the surface material property by incrementing hardness. Luo et al. [75] applied thermal oxidation on Ti6Al4V alloy and examined the tribological performance in bovine serum. The increased hardness and reduced mass loss showed the improvement of the treatment. Berni et al. [76] deposited the yttria-stabilized zirconia (YSZ) coating and showed the adhesion and tribological performance of the coating. Even though the better performance of YSZ film was presented, further development of the deposition technique was recommended. However, since the electrochemical responses were not simultaneously recorded, the reports might not cover all the degrading mechanisms occurring in the processes. To fully evaluate all aspects of damage, the entire tribocorrosion examination must be required. Mathew et al. [77] prepared the titanium oxycarbide coatings with the reactive DC magnetron sputtering and examined the behavior of tribocorrosion in the artificial saliva. The wear corrosion loss volume was positively correlated to the applied loading, and the tribocorrosion mechanism map was generated. Alemón et al. [78] proposed the TiAlVCN/CNx coating to enhance the tribocorrosion resistance of CoCrMo alloy. The coating exhibited lower corrosion current, lower potential drop, and friction coefficient in the tribocorrosion experiments in bovine serum albumin. Cheng et al. [79] synthesized the titanium oxycarbide through the solid carburization on Ti6Al4V alloy and examined the tribocorrosion and biocompatibility. With the nanocrystalline graphite in the structure, the treated specimens presented a low friction coefficient and better tribocorrosion resistance without sacrificing biocompatibility. Cheng et al. [80] tested the thermally sprayed titanium alloy with three hard ceramic coatings and concluded that the bilayer coating provided the best performance in the lowest friction coefficient and evolutions of open-circuit potential and potentiostatic-induced currents.

Carbonaceous coatings are another exciting subject and are widely investigated in tribology-related fields. Generally, carbon is a chemically inert substance suitable for corrosion-resistant material. Moreover, based on the ratio between sp² and sp³ bonding, the controllability of material properties in carbonaceous coatings is probable [81]. Since high hardness is required for wear resistance, the diamond-like carbon (DLC) is considered to lessen the tribocorrosion damage with the low wear rate and friction coefficient. Liu et al. [82] deposited the diamond-like carbon (DLC) film on CoCrMo alloy with the titanium interlayer for better adhesion. The higher values during the entire OCP evolution exhibited the protection of DLC coating on CoCrMo alloy against tribocorrosion damage. Cui et al. [83] prepared the multilayer DLC through plasma-enhanced chemical vapor deposition (PECVD) and compared it to the steel substrate and single-layer DLC. The lowest corrosion current, potential drop, and friction coefficient were found in the group of multi-layered DLC. With the advancement of technology, a nanocrystalline diamond can be uniformly formulated on the selected substrate. Patel et al. [84] demonstrated the increment of corrosion resistance from the ultra nanocrystalline diamond (UNCD) on both CP-Ti and Ti6Al4V alloys. Gopal et al. [85] presented the improvement against tribocorrosion from the nanocrystalline diamond (NCD) coating and proposed the degrading mechanism of NCD coating during tribocorrosion experiments. Besides crystalline carbon, amorphous carbon can protect the substrate from tribocorrosion attack. Cheng et al. [86] developed the carbide-derived carbon (CDC) on the titanium alloy and tested it in a tribocorrosion environment with the bovine calf serum for short- and long-term experiments. The results exhibited the protection from the CDC and the sustainability of the CDC, for which the CDC remained in the wear scar after the long-term testing.

Though many studies have been dedicated to research coating technology on biomedical implant applications, the unexpected early failure and questionable in vitro testing hinder the application. The most common coatings failure is delamination, for which the coating disintegrates from the substrate. Due to the coating and substrate are not the same material, the connection between them is weak. While there is a high enough impact on the coating, the crack might generate at the interface and propagate, resulting in delamination. Some studies proposed that the interlayer between the major coating and substrate could enhance the adhesion, but the concern of delamination barely diminishes.

Moreover, even if the best combination of coating, interlayer, and substrate is discovered, the lack of promising in vitro testing to simulate the in vivo condition fully becomes a huge technical drawback to applying the studied surface modification. From the adhesion and tribological testing to the cell-containing environment, in vitro testing hardly duplicates the working environment for evaluating proposed coatings. Hence, the developed coatings can exhibit excellent performance in the lab but fail quickly in clinical conditions. Harman et al. [87] reported a case study of a titanium nitride-coated total hip replacement against a UHMWPE liner that failed after a one-year in vivo operation. The retrieved hip implants exhibited several delaminated asperities on the coating and some evidence of adhesive wear. Raimondi and Pietrabissa [88] also investigated four titanium nitride-coated prosthetic femoral heads against UHMWPE liners retrieved after revision surgeries. The damaged areas of coatings showed a saw-toothed shape with higher surface roughness, leading to more material loss from the polymer liners. Taeger et al. [89] conducted a clinical study between the diamond-like carbon (DLC) coating on CoCrMo and alumina femoral heads. The follow-up investigation revealed significantly lower survival rates on DLC-coated implants than alumina ones. The mechanical failures of the DLC coating, such as delamination, were the main reason for low survivability. The coating technology is still an excellent option to increase the resistance of components against tribocorrosion with the solution to resolve the delamination issue and shorten the gap between the results from in vitro studies to in vivo evaluation.

5. Knowledge Gaps/Challenges in Tribocorrosion Research

5.1. In Situ Cytotoxicity Examination

In Sect. 4, the cytotoxicity of products generated from implants through the tribocorrosive effect has been discussed, but there is a significant difference from the in vitro study to the metal ion/particle levels found in patients. It is common to employ commercial wear particles from companies to conduct the in vitro study of generated products to specific cells. The size distributions and shapes of particles might not be similar to those produced in patients’ bodies. Moreover, particles are not the only product of the tribocorrosion reactions but also the ions. The increased corrosion reactions should release more ions in the bloodstream and change the ions’ local and/or entire concentration. The adverse effects from the ions are challenging to simulate from the commercial wear particles and might only be mimicked by adding ions close to the approximate concentration through the clinical data. As for in vitro testing, in situ cytotoxicity examination can provide information to identify the mechanisms for the degraded products to the surrounding cells and the possibly systemic damage [90]. In order to have this kind of in situ testing, the coupled hip simulator and culturing cell chamber are necessary, which becomes a circulated dynamic cell culturing system, as shown in Fig. 5. The cell culturing chamber can be made of a microfluidic system simulating vessel material exchange. If such a system is built up, the adverse reactions from tribocorrosion products to different cells and tissues can be clarified, assisting the development of more efficient clinical treatment.

Fig. 5.

Tribocorrosion system evolution for better in vitro biological evaluation

5.2. Evolutions in the Experimental Setup

In this decade, the apparatus used in examining tribocorrosion has also developed with ongoing research (Fig. 5). The instruments of standard tribocorrosion studies consist of a tribometer with a corrosion cell. In literature, it is common to observe one pin-on-disc tribometer coupled with a three-electrode corrosion cell monitoring the alteration of electrochemical responses. Nonetheless, the uniform experimental set-up cannot satisfy the varied demands of different applications. In the tribology community, it is well known that the simulated tribometer or preliminary testing must be operated in a setting as close to the actual working condition as possible; otherwise, the results obtained in a lab will not be applicable. Thus, more and more customized tribocorrosion systems are developed for better simulations.

Designing a simulating system is a complicated task, including the mechanical analysis of the working condition, formulating the working environment, and, most importantly, the comparable results after the experiments. The testing in the lab hardly has the chance to operate the exact system used in the industry. Nevertheless, according to the analysis of mechanics, researchers can simplify the instrument and customize the analogical simulator to perform similar results. Besides, the working environment, such as dry/wet conditions or acid/neutral/base solutions, can majorly affect the design of simulators. Based on the reports from the targeted area, the employing conditions are determined so is the apparatus. Unfortunately, although the mechanical analysis and the environmental factors are kept the same, it is still possible that the obtained results are not similar to the simulated ones. For instance, as mentioned in the prior section, the fretting corrosion damage found on the retrieved taper junction of hip implants is still not replicated in the in vitro studies by date. There are several possibilities, such as the difficulty of mimicking micromotion, the inconsistent contact stress, or the existence of cellular activities, which might cause different results. Hence, developing simulators accessing tribocorrosion is a multidisciplinary mission in which researchers with distinctive backgrounds must contribute their expertise. In the next decade, it is expected to observe the development of successfully simulated equipment to help us understand more about tribocorrosion. Knowledge can facilitate studies for more efficient and safer protection against tribocorrosion.

5.3. Computational/Theoretical Models in Tribocorrosion

Although tribocorrosion can increase material loss in many different applications, the perfect prediction of material loss in a tribocorrosive environment is still difficult. Several models have been proposed and developed in the past two decades and are generalized in Table 3. In each computational model, restricted by the assumption and the scope of the study, the proposed equations present good agreements to experimental observations but still with some deviations. Some studies focus more on the mechanical aspect of the induced corrosion loss. Other groups emphasize the electrochemical responses after the mechanical removal of the protective film. Even though the model estimating synergistic material loss with the inclusion of fracture mechanics is proposed, factors are still not included. To completely simulate the tribocorrosion scenario, more fields of study should be considered.

Table 3.

Proposed tribocorrosion Models in the past 11 years

Author	Proposed models for tribocorrosion	Advantage	Disadvantage
Stack et al. [107]	Based on the specified assumptions, the mass loss of erosion and corrosion can be formulated	The equations can generate different maps between parameters, which can apply in a typical experimental setup	The formulated material losses of erosion and corrosion are deducted from a series of assumptions. If the assumptions are not fulfilled, the model might not be accurate
Goldberg and Gilbert [108]	The peak generated current due to the removal of a passive layer can be calculated by adding currents from the metal dissolution and film formation	According to the electrochemical aspect, the theoretical corrosion loss from the destruction of a passive film can be estimated	The proposed equation cannot predict the peak currents beyond a range of applied potentials, exhibiting the necessity of further research
Mischler et al. [109]	The generated current from the reciprocating sliding is computed with the proposed model	The analytical expression includes the effect of sliding frequency to calculate the induced corrosion loss	There is no component in the equation addressing the influence of overpotential on the incremented currents
Jiang and Stack [110]	Following Mischler’s work, a more detailed expression of corrosion loss from tribocorrosion is formulated. Besides, the wear components in the process of tribocorrosion are listed, and the total synergistic material loss from tribocorrosion is proposed	With the inclusion of fracture mechanics, more detailed corrosion-induced wear mechanisms are included and discussed, demonstrating the synergism between tribology and corrosion	Since the proposed model follows previous Mischler’s study, the effect of overpotential in electrochemistry is still underestimated. Besides, the study did not include experimental data to verify the model
Von der Ohe et al. [111]	Based on Jiang’s model and Archard’s wear equation, the authors proposed a multi-degradation mechanism for the tribocorrosion phenomenon on the hydraulic piston	Incorporating the hardness criteria clearly indicates wear mechanisms for the examined systems	The hardness criterion is obtained through empirical observation, which might not be applicable in different scenarios
Vieira et al. [112]	The authors proposed a galvanic model between the wear track and the intact surface	The model can simulate the change of potentials during the rubbing	The model is not considered material properties and tribological perspectives in the proposed model
Cao et al. [113]	The tribocorrosion model is proposed through mechanical analysis and prior studies’ electrochemical equations	The model exhibited comparable results to the experimental data and was proposed to apply to the prediction of material loss on total hip replacement	The influence of electrochemical potential is not included in the model, and the prediction is generally higher than the material loss from the experiments
Gilbert and Zhu [114]	Following the previous model, the authors included the high-field oxide growth model and presented an integral heredity model describing the tribocorrosion phenomenon	The model showed a close fit in the induced current and a similar potential drop in open-circuit potential	The mechanical component was considered in the simple condition, which might need further modification to apply to complicated applications

6. Conclusion

In the current review, we summarize the general development in tribocorrosion and focus more on the progress of tribocorrosion in biomedical devices. Many unknown correlations exist between tribocorrosion debris and the cytotoxicity of local tissues and remote organs. Since it has been shown that the toxicity of metal debris might be altered due to different processes, formulating a better simulated environment of tribocorrosion to the in vivo testing to obtain more comparable data becomes a significant challenge. Besides, the development of apparatus to mimic the medical devices in the body is another critical issue that needs to be addressed in future research Fig. 6 displays, the possible new directions in tribocorrosion research in implants. Researchers have made many major accomplishments in this field in the past decade. Based on the solid foundation, it is expected to discover all aspects of tribocorrosion, from fundamental modeling to the potential risks of products, in the next 10 years.

Fig. 6.

Future directions of tribocorrosion research

Data Availability

The datasets generated during and analyzed during the current study are available from the corresponding author on reasonable request.