Tribological behavior of TiO2 PEEK composite and stainless steel for pediatric crowns

Citation: Arieira, A.; Madeira, S.; Rodrigues, F.; Silva, F. Tribological Behavior of TiO2PEEK Composite and Stainless Steel for Pediatric Crowns. Materials 2023,16, 2420. https://doi.org/10.3390/ ma16062420 Academic Editors: Nicola Scotti and Gaetano Paolone Received: 15 February 2023 Revised: 13 March 2023 Accepted: 16 March 2023 Published: 17 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Tribological Behavior of TiO2PEEK Composite and Stainless Steel for Pediatric Crowns Ana Arieira 1,2,* , Sara Madeira 1,2, Flávio Rodrigues 1,2 and Filipe Silva 1,2,* 1Center for MicroElectroMechanical Systems—CMEMS, Campus de Azurém, University of Minho, 4800-058 Guimarães, Portugal 2LABBELS—Associate Laboratory, 4800-058 Guimarães, Portugal *Correspondence: [email protected] (A.A.); [email protected] (F.S.); Tel.: +351-926-286-521 (A.A.) Abstract: Dental decay still presents a major health problem among children. Its treatment usually requires the use of stainless steel crowns. This study compares the wear behavior of 316 L stainless steel and polyetheretherketone (PEEK) composite under identical test conditions. The wear tests were conducted in a reciprocating ball-on-plate tribometer (Plint TE67/R) using alumina balls as a counterface and artificial saliva as a lubricant at 37 ◦ C to faithfully mimic oral conditions. The coefficient of friction (COF) and specific wear rate (k) values were determined and SEM/EDS examinations were performed to identify the predominant wear mechanisms. Results showed that PEEK exhibited a significantly lower coefficient of friction (COF = 0.094 ± 0.004) and thus lower wear volume ( ∆ V= 0.0078 ± 0.0125 mm 3 ) and higher wear resistance, with an average value of specific wear rate of k= 9.07 × 10 −6 mm 3 N −1 m −1 when compared to stainless steel (COF = 0.32 ± 0.03, ∆ V= 0.0125 ± 0.0029 mm 3 ,k= 1.45 × 10 −5 mm 3 N −1 m −1 ). PEEK was revealed to be a potential material for use in pediatric crowns due to its high wear resistance while overcoming the disadvantages associated with steel at both an aesthetic and biological level. Keywords: coefficient of friction; wear rate; stainless steel; polyetheretherketone (PEEK) 1. Introduction Despite improvements in oral health in high-income countries, dental decay still presents one of the most widespread and significant public health problems among children [ 1 ]. According to the 2022 WHO Global Oral Health Status Report, over 514 million children have primary teeth caries [ 2 ]. A survey conducted by Public Health England in 2019 on the oral health of 5-year-old children showed that one in four children of that age have experienced dental caries [3]. Dental caries are caused by the interaction of bacteria from the plaque that forms on the surface of a tooth and sugary foods on tooth enamel [ 2 , 4 ]. Untreated dental caries can lead to odontogenic infections; these are a common dental emergency in children’s hospitals, highlighting the consequences of primary tooth decay [ 1 , 5 ]. These conditions can have an important impact on children’s health, nutrition, growth, and general health and quality of life [ 4 , 5 ]. Untreated primary tooth decay can lead to pain, tissue inflammation, infection, dental abscess, malocclusion, and chewing disorders [1,5,6]. In order to maintain the primary teeth in the dental arch preceding the eruption of the permanent teeth, a variety of restorative solutions and materials have been used in pediatric dentistry to provide full coverage restorations [ 7 – 9 ]. One of the most used methods and restorative materials in the treatment of tooth decay in children is stainless steel crowns (SSCs), renowned for their durability, long-term retention, and high clinical success rates in restoring larger carious lesions on primary molars [ 6 – 8 ]. SSCs are prefabricated, adapted to individual teeth, and cemented with a biocompatible luting agent. Despite the considerable amount of literature that supports the success of SSCs, they have some drawbacks. One major drawback Materials 2023,16, 2420. https://doi.org/10.3390/ma16062420 https://www.mdpi.com/journal/materials Materials 2023,16, 2420 2 of 14 is their poor aesthetic appearance [ 8 ]. Moreover, several studies revealed severe adverse effects of the ionic composition of SSCs in children (especially regarding the significant percentage of nickel), including the development of local inflammation or an allergic reaction to the systematic distribution of the ions as well as cytotoxic or genotoxic effects [ 10 – 12 ]. Several alternatives have emerged to address the use of metals and the unappealing appearance of dental restorations; these include pre-veneered stainless-steel crowns (PVSSCs), crowns composed of polymers (mainly PMMA), pre-veneered aluminum crowns, and prefabricated zirconia crowns, all of which aim to imitate the natural color of teeth. Nevertheless, the use of PVSSCs carries the potential risk of nickel allergy and sensitivity, while polymer and pre-veneered aluminum crowns have poor mechanical and wear resistance. Zirconia crowns are costly and necessitate extensive tooth preparation for placement [6,8,13,14]. More recently, and beyond just academic investigations polyetheretherketone (PEEK) has been used and already commercialized for several dental devices, including dental implants, abutments, healing caps, orthodontic braces, and denture prosthetic frameworks [ 15 – 17 ]. Further, PEEK composites [ 17 – 19 ], in particular TiO 2 reinforced PEEK, have been proposed to overcome some wear limitations and achieve color improvement. A study conducted on the effect of nano-sized TiO 2 addition on the tribological behavior of PEEK composite [ 19 ] using a pin-on-disc test revealed the superiority of rigid TiO 2 nanoparticle-loaded PEEK in terms of specific wear rate compared to neat PEEK. Additionally, titanium dioxide structures are known for their superior mechanical properties and strong antibacterial action [18]. The purpose of this article is to carry out a comparative tribological study between PEEK composite reinforced with TiO 2 nanoparticles and 316 L stainless steel to assess the potential of PEEK composite as an alternative material to stainless steel. This alternative is primarily for use in pediatric crowns due to its color and easier adaptation to the tooth due to its elasticity as well as its biological advantages, including non-cytotoxicity. According to studies in the literature about the mechanical superiority of TiO 2 reinforced PEEK, it is expected that this material presents a good response to wear. 2. Materials and Methods 2.1. Materials This study comprises the comparison of two materials of distinct nature: a conventional material used in pediatric dental restorations, 316 L stainless steel (SS), and PEEK composite reinforced with 20 wt%-TiO 2 nanoparticles acquired from the Dental Direkt Company. The chemical composition (wt.%) of the stainless steel is Fe-62.7%, Cr-16.2%, Ni-9.6%, C-2.4%, Si-0.7%, Mo-2.1% and Mn-1.6%. The PEEK composite has the following elements in its chemical composition (wt.%): C-65.6% and Ti-14%. 2.2. Sample Preparation Samples of both materials (PEEK and SS) were cut into circular pieces (∅8 mm ×0.2 mm thickness) from commercial blocks. A thickness of 0.2 mm was used to simulate the thickness of the commercialized SS pediatric crowns to ensure that there is no bias caused by the difference in thickness regarding the results obtained from the different tests conducted on the different samples. To confirm that the results were not impacted by the finishing process, all specimens underwent an identical process, similar to other studies [ 13 , 20 ]. Silicon carbide (SiC) papers with grits ranging from 800 to 4000 were used to polish the samples under running water. The final surface roughness of each material is presented in Table 1. The specimens were subjected to a 5-min ultrasonic bath in isopropyl alcohol. Materials 2023,16, 2420 3 of 14 Table 1. Mean surface roughness of stainless steel and PEEK samples. Material Mean Surface Roughness (µm) 316 L Stainless Steel 0.677 ±0.077 PEEK composite 0.229 ±0.097 A study conducted by Sampaio et al. on the influence of PEEK thickness on contact stress showed that the COF and the wear rate increased with decreasing PEEK thickness, resulting from the increased contact stress with the material [ 21 ]. Considering the importance of these factors on the long-term success of PEEK crowns, all samples, with 0.2 mm thickness, were cemented onto 2 mm thick zirconia samples in other to represent the conditions of the use of dental crowns, where the zirconia represents human teeth. The samples were cemented using the Bifix Hybrid Abutment, a universal luting composite, from VOCO. The zirconia discs were previously sandblasted with 150 µ m alumina sand, at 2 bar, to create roughness on the surface, thus allowing improved cementation. During cementation, a weight of 2 kg was placed on top of each sample to ensure that there was the necessary load for the correct bonding of the material samples to the zirconia. Figure 1 represents the bonding of the SS and PEEK samples to the zirconia substrate. Materials 2023, 16, x FOR PEER REVIEW 3 of 15 Table 1. Mean surface roughness of stainless steel and PEEK samples. Material Mean Surface Roughness (µm) 316 L Stainless Steel 0.677 ± 0.077 PEEK composite 0.229 ± 0.097 A study conducted by Sampaio et al. on the influence of PEEK thickness on contact stress showed that the COF and the wear rate increased with decreasing PEEK thickness, resulting from the increased contact stress with the material [21]. Considering the importance of these factors on the long-term success of PEEK crowns, all samples, with 0.2 mm thickness, were cemented onto 2 mm thick zirconia samples in other to represent the conditions of the use of dental crowns, where the zirconia represents human teeth. The samples were cemented using the Bifix Hybrid Abutment, a universal luting composite, from VOCO. The zirconia discs were previously sandblasted with 150 µm alumina sand, at 2 bar, to create roughness on the surface, thus allowing improved cementation. During cementation, a weight of 2 kg was placed on top of each sample to ensure that there was the necessary load for the correct bonding of the material samples to the zirconia. Figure 1 represents the bonding of the SS and PEEK samples to the zirconia substrate. Figure 1. Representation of the stainless steel and PEEK samples glued to the zirconia discs and image of samples and tooth crowns of the respective materials (the image is not to scale). Prior to the sliding wear testing, the hardness of each material was assessed using a nanoindenter (NanoTest—Micro Materials) equipped with a Berkovich diamond indenter type. Nine indentations at the maximum load of 50 mN for stainless steel and 100 mN for PEEK, with a dwell time of 10 s, were created in each specimen. The load-unloading cycles were carried out at a loading rate of 0.1 mN/s. 2.3. Colour Measurement The shade and whitening values of the 20 wt%-TiO2 PEEK Composite and the conventional PEEK were measured for comparison purposes (Figure 1). The measurements were performed using a digital device, the Vita Easyshade, widely used in the area of dentistry. The final values were obtained using the Vita Classical shade guide and the Vita Bleached guide from the device. 2.4. Wear Testing Test conditions were selected according to a previous study conduicted by Amanda et al. [20]. Reciprocating sliding tests were performed in a ball-on-plate configuration using a Plint TE67/R tribometer. The tests involved loading samples of SS and PEEK against 10 mm diameter alumina balls, as shown in Figure 2. Similar to other studies [22–24], alumina was selected as the counterpart material due to its increased and improved Figure 1. Representation of the stainless steel and PEEK samples glued to the zirconia discs and image of samples and tooth crowns of the respective materials (the image is not to scale). Prior to the sliding wear testing, the hardness of each material was assessed using a nanoindenter (NanoTest—Micro Materials) equipped with a Berkovich diamond indenter type. Nine indentations at the maximum load of 50 mN for stainless steel and 100 mN for PEEK, with a dwell time of 10 s, were created in each specimen. The load-unloading cycles were carried out at a loading rate of 0.1 mN/s. 2.3. Colour Measurement The shade and whitening values of the 20 wt%-TiO 2 PEEK Composite and the conventional PEEK were measured for comparison purposes (Figure 1). The measurements were performed using a digital device, the Vita Easyshade, widely used in the area of dentistry. The final values were obtained using the Vita Classical shade guide and the Vita Bleached guide from the device. 2.4. Wear Testing Test conditions were selected according to a previous study conduicted by Amanda et al. [ 20 ]. Reciprocating sliding tests were performed in a ball-on-plate configuration using a Plint TE67/R tribometer. The tests involved loading samples of SS and PEEK against 10 mm diameter alumina balls, as shown in Figure 2. Similar to other studies [ 22 – 24 ], alumina was selected as the counterpart material due to its increased and improved Materials 2023,16, 2420 4 of 14 mechanical and chemical inertness qualities. The samples cemented to zirconia were carefully mounted on a 3D-printed PLA holder attached to the specimen support. Materials 2023, 16, x FOR PEER REVIEW 4 of 15 mechanical and chemical inertness qualities. The samples cemented to zirconia were carefully mounted on a 3D-printed PLA holder attached to the specimen support. Figure 2. Schematic representation of the wear test. All wear tests were conducted in lubricated conditions at 37 ± 3 °C [25] using Fusayama–Meyer’s artificial saliva solution to reliably replicate human oral conditions, as performed in previous works [26]. The composition of the artificial saliva is indicated in Table 2. The pH was corrected to the range of normal human saliva pH values (6.2–7.6) [27]. Table 2. Fusayama–Meyer’s artificial saliva solution composition. NaCl (g) KCl (g) CaCl 2 ·2H 2 O (g) NaH 2 PO 4 (g) Na 2 S·9H 2 O (g) Urea (g) Distilled Water (mL) 0.4 0.4 0.906 0.69 0.005 1 1000 To mimic clinical loads within the acceptable range, a normal load of 30 N was chosen [28] along with a reciprocating sliding frequency of 1 Hz and a 4 mm stroke length. Each test lasted 1 h and involved a total sliding distance of 28.8 m. To determine the average value, each test condition was performed three times. To quantify the running-in period and evaluate the friction coefficient in the steady-state friction regime, the COF was continually measured during sliding. A 3D optical profilometer (Filmetrics Inc., San Diego, CA, USA) was used to assess the profile of the wear tracks. The wear width and wear depth, measured using the profilometer, were used to calculate the wear volume of the samples. The wear track model used to calculate the wear volume is represented in Figure 3. Figure 2. Schematic representation of the wear test. All wear tests were conducted in lubricated conditions at 37 ± 3 ◦ C [ 25 ] using Fusayama–Meyer’s artificial saliva solution to reliably replicate human oral conditions, as performed in previous works [ 26 ]. The composition of the artificial saliva is indicated in Table 2. The pH was corrected to the range of normal human saliva pH values (6.2–7.6) [ 27 ]. Table 2. Fusayama–Meyer’s artificial saliva solution composition. NaCl (g) KCl (g) CaCl2·2H2O (g) NaH2PO4 (g) Na2S·9H2O (g) Urea (g) Distilled Water (mL) 0.4 0.4 0.906 0.69 0.005 1 1000 To mimic clinical loads within the acceptable range, a normal load of 30 N was chosen [ 28 ] along with a reciprocating sliding frequency of 1 Hz and a 4 mm stroke length. Each test lasted 1 h and involved a total sliding distance of 28.8 m. To determine the average value, each test condition was performed three times. To quantify the running-in period and evaluate the friction coefficient in the steady-state friction regime, the COF was continually measured during sliding. A 3D optical profilometer (Filmetrics Inc., San Diego, CA, USA) was used to assess the profile of the wear tracks. The wear width and wear depth, measured using the profilometer, were used to calculate the wear volume of the samples. The wear track model used to calculate the wear volume is represented in Figure 3. Materials 2023,16, 2420 5 of 14 Materials 2023, 16, x FOR PEER REVIEW 5 of 15 Figure 3. Wear track volume calculation model [29]. Empirical mathematical equations were used to perform the calculations, based on the assumption that wear tracks result from a perfect ball geometry. The calculation for the mid-zone area of the track was conducted according to the schematic representation illustrated in Figure 4. The total wear track volume was determined using the following equation: 𝛥𝑉𝐿  󰇩1 2𝑅 2sin󰇡𝑎 𝑅󰇢 𝑏 ℎ′ 2󰇪 𝜋  𝑏 64𝑅 (1) where the ΔV is the total volume loss of the wear track in mm 3 , R is the radius of the alumina ball in mm, a is half of the width of the wear track 󰇛𝑎𝑏 2 ⁄󰇜 in mm, L is the stroke length in mm, and h’ is the height of the triangle in mm (see Figure 4B). This method has been reported in previous similar studies [29,30]. Figure 4. Calculation of the area of the mid-zone of the wear track: (A) area of the section, (B) area of the triangle, and (C) real area of the wear track [29]. The following equation was used to calculate the specific wear rate (k): 𝑘 ∆𝑉 𝑊𝐿 (2) where ΔV is the worn volume in mm 3 , L is the total sliding distance in m, and W is the normal applied load in N. After coating the worn samples with gold via sputter-coating, SEM/EDS characterization was conducted to identify the primary friction and wear mechanisms. 3. Results and Discussion Regarding the shade and whiteness measurements of both PEEK samples, the results revealed that the 20 wt%—TiO2 PEEK composite presented a shade in the lighter tooth shade zone of the Vita Classical shade guide and an adequate whitening level; the conventional PEEK sample presented a significantly darker shade and a much higher value on the whitening scale. This is in line with the fact that it is a darker material. The Vita Bleached guide is composed of a scale from 1 to 29, where 1 represents the whitest shade Figure 3. Wear track volume calculation model [29]. Empirical mathematical equations were used to perform the calculations, based on the assumption that wear tracks result from a perfect ball geometry. The calculation for the midzone area of the track was conducted according to the schematic representation illustrated in Figure 4. The total wear track volume was determined using the following equation: ∆V=L×1 2×R2×2sin−1a R−b×h0 2+π×b4 64R(1) where the ∆ Vis the total volume loss of the wear track in mm 3 ,Ris the radius of the alumina ball in mm, ais half of the width of the wear track (a=b/2) in mm, Lis the stroke length in mm, and h 0 is the height of the triangle in mm (see Figure 4B). This method has been reported in previous similar studies [29,30]. Materials 2023, 16, x FOR PEER REVIEW 5 of 15 Figure 3. Wear track volume calculation model [29]. Empirical mathematical equations were used to perform the calculations, based on the assumption that wear tracks result from a perfect ball geometry. The calculation for the mid-zone area of the track was conducted according to the schematic representation illustrated in Figure 4. The total wear track volume was determined using the following equation: 𝛥𝑉𝐿  󰇩1 2𝑅 2sin󰇡𝑎 𝑅󰇢 𝑏 ℎ′ 2󰇪 𝜋  𝑏 64𝑅 (1) where the ΔV is the total volume loss of the wear track in mm 3 , R is the radius of the alumina ball in mm, a is half of the width of the wear track 󰇛𝑎𝑏 2 ⁄󰇜 in mm, L is the stroke length in mm, and h’ is the height of the triangle in mm (see Figure 4B). This method has been reported in previous similar studies [29,30]. Figure 4. Calculation of the area of the mid-zone of the wear track: (A) area of the section, (B) area of the triangle, and (C) real area of the wear track [29]. The following equation was used to calculate the specific wear rate (k): 𝑘 ∆𝑉 𝑊𝐿 (2) where ΔV is the worn volume in mm 3 , L is the total sliding distance in m, and W is the normal applied load in N. After coating the worn samples with gold via sputter-coating, SEM/EDS characterization was conducted to identify the primary friction and wear mechanisms. 3. Results and Discussion Regarding the shade and whiteness measurements of both PEEK samples, the results revealed that the 20 wt%—TiO2 PEEK composite presented a shade in the lighter tooth shade zone of the Vita Classical shade guide and an adequate whitening level; the conventional PEEK sample presented a significantly darker shade and a much higher value on the whitening scale. This is in line with the fact that it is a darker material. The Vita Bleached guide is composed of a scale from 1 to 29, where 1 represents the whitest shade Figure 4. Calculation of the area of the mid-zone of the wear track: ( A ) area of the section, ( B ) area of the triangle, and (C) real area of the wear track [29]. The following equation was used to calculate the specific wear rate (k): k=∆V W×L(2) where ∆ Vis the worn volume in mm 3 ,Lis the total sliding distance in m, and Wis the normal applied load in N. After coating the worn samples with gold via sputter-coating, SEM/EDS characterization was conducted to identify the primary friction and wear mechanisms. 3. Results and Discussion Regarding the shade and whiteness measurements of both PEEK samples, the results revealed that the 20 wt%—TiO 2 PEEK composite presented a shade in the lighter tooth shade zone of the Vita Classical shade guide and an adequate whitening level; the conventional PEEK sample presented a significantly darker shade and a much higher value on the whitening scale. This is in line with the fact that it is a darker material. The Vita Bleached guide is composed of a scale from 1 to 29, where 1 represents the whitest shade and 29 the darkest. This guide is used to plan and monitor the tooth whitening processes. The results are presented in Table 3. Materials 2023,16, 2420 6 of 14 Table 3. Shade guide and bleach guide of TiO 2 PEEK composite and conventional PEEK according to Vita. Material 20 wt%—TiO2PEEK Composite Materials 2023, 16, x FOR PEER REVIEW 6 of 15 and 29 the darkest. This guide is used to plan and monitor the tooth whitening processes. The results are presented in Table 3. Table 3. Shade guide and bleach guide of TiO2 PEEK composite and conventional PEEK according to Vita. Material 20 wt%—TiO2 PEEK Composite Conventional PEEK Vita Classical shade guide A1 C4 Vita Bleached guide 4 20 Figure 5 presents the typical evolution of the COF for both materials tested (SS and PEEK), obtained from the wear test. Figure 5. Evolution of the friction coefficient (COF) during sliding against the alumina ball in the presence of artificial saliva for the two tested (a sliding distance of 28.8 m was covered, equivalent to 1 h of sliding). The evolution of the COF of the thin disc samples (stainless steel and PEEK) was characterized by two distinct behaviors. In the case of the SS, the friction coefficient showed a lower value (about 0.075) up to 3 m, when it then increased to a value of 0.35 between the distance of 3 and 5 m. After this sliding distance, the stainless steel reached a steady-state regime with the COF value stabilizing around 0.32. The lower value of the COF until 3 m might be explained by the formation of an oxide film on the stainless steel. However, that oxide film was then torn, which allowed the alumina ball to completely touch the substrate, translating into higher values of COF (0.32 ± 0.03). This behavior of SS in wear tests has already been verified in previous studies [24]. Conversely, in the case of the PEEK composite, a well-defined stationary phase was achieved almost instantly at the beginning of the test; the COF value remained practically unchanged during both the running-in phase and in the steady-state regime. The friction 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 5 10 15 20 25 30 COF Sliding distance (m) Stainless Steel PEEK Conventional PEEK Materials 2023, 16, x FOR PEER REVIEW 6 of 15 and 29 the darkest. This guide is used to plan and monitor the tooth whitening processes. The results are presented in Table 3. Table 3. Shade guide and bleach guide of TiO2 PEEK composite and conventional PEEK according to Vita. Material 20 wt%—TiO2 PEEK Composite Conventional PEEK Vita Classical shade guide A1 C4 Vita Bleached guide 4 20 Figure 5 presents the typical evolution of the COF for both materials tested (SS and PEEK), obtained from the wear test. Figure 5. Evolution of the friction coefficient (COF) during sliding against the alumina ball in the presence of artificial saliva for the two tested (a sliding distance of 28.8 m was covered, equivalent to 1 h of sliding). The evolution of the COF of the thin disc samples (stainless steel and PEEK) was characterized by two distinct behaviors. In the case of the SS, the friction coefficient showed a lower value (about 0.075) up to 3 m, when it then increased to a value of 0.35 between the distance of 3 and 5 m. After this sliding distance, the stainless steel reached a steady-state regime with the COF value stabilizing around 0.32. The lower value of the COF until 3 m might be explained by the formation of an oxide film on the stainless steel. However, that oxide film was then torn, which allowed the alumina ball to completely touch the substrate, translating into higher values of COF (0.32 ± 0.03). This behavior of SS in wear tests has already been verified in previous studies [24]. Conversely, in the case of the PEEK composite, a well-defined stationary phase was achieved almost instantly at the beginning of the test; the COF value remained practically unchanged during both the running-in phase and in the steady-state regime. The friction 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 5 10 15 20 25 30 COF Sliding distance (m) Stainless Steel PEEK Vita Classical shade guide A1 C4 Vita Bleached guide 4 20 Figure 5presents the typical evolution of the COF for both materials tested (SS and PEEK), obtained from the wear test. Materials 2023, 16, x FOR PEER REVIEW 6 of 15 and 29 the darkest. This guide is used to plan and monitor the tooth whitening processes. The results are presented in Table 3. Table 3. Shade guide and bleach guide of TiO2 PEEK composite and conventional PEEK according to Vita. Material 20 wt%—TiO2 PEEK Composite Conventional PEEK Vita Classical shade guide A1 C4 Vita Bleached guide 4 20 Figure 5 presents the typical evolution of the COF for both materials tested (SS and PEEK), obtained from the wear test. Figure 5. Evolution of the friction coefficient (COF) during sliding against the alumina ball in the presence of artificial saliva for the two tested (a sliding distance of 28.8 m was covered, equivalent to 1 h of sliding). The evolution of the COF of the thin disc samples (stainless steel and PEEK) was characterized by two distinct behaviors. In the case of the SS, the friction coefficient showed a lower value (about 0.075) up to 3 m, when it then increased to a value of 0.35 between the distance of 3 and 5 m. After this sliding distance, the stainless steel reached a steady-state regime with the COF value stabilizing around 0.32. The lower value of the COF until 3 m might be explained by the formation of an oxide film on the stainless steel. However, that oxide film was then torn, which allowed the alumina ball to completely touch the substrate, translating into higher values of COF (0.32 ± 0.03). This behavior of SS in wear tests has already been verified in previous studies [24]. Conversely, in the case of the PEEK composite, a well-defined stationary phase was achieved almost instantly at the beginning of the test; the COF value remained practically unchanged during both the running-in phase and in the steady-state regime. The friction 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0 5 10 15 20 25 30 COF Sliding distance (m) Stainless Steel PEEK Figure 5. Evolution of the friction coefficient (COF) during sliding against the alumina ball in the presence of artificial saliva for the two tested (a sliding distance of 28.8 m was covered, equivalent to 1 h of sliding). The evolution of the COF of the thin disc samples (stainless steel and PEEK) was characterized by two distinct behaviors. In the case of the SS, the friction coefficient showed a lower value (about 0.075) up to 3 m, when it then increased to a value of 0.35 between the distance of 3 and 5 m. After this sliding distance, the stainless steel reached a steady-state regime with the COF value stabilizing around 0.32. The lower value of the COF until 3 m might be explained by the formation of an oxide film on the stainless steel. However, that oxide film was then torn, which allowed the alumina ball to completely touch the substrate, translating into higher values of COF (0.32 ± 0.03). This behavior of SS in wear tests has already been verified in previous studies [24]. Conversely, in the case of the PEEK composite, a well-defined stationary phase was achieved almost instantly at the beginning of the test; the COF value remained practically unchanged during both the running-in phase and in the steady-state regime. The friction coefficient of both materials was well stabilized at the end of the 1 h of sliding of the wear test. Figure 6illustrates the measured steady-state friction coefficient values for the reciprocating sliding of SS and PEEK against alumina while using synthetic saliva. As previously presented, the COF value for SS was around 0.32 ± 0.03, while the mean values for PEEK Materials 2023,16, 2420 7 of 14 are much lower, at around 0.094 ± 0.004. The COF of SS is ≈ 3.4 times higher than PEEK. The values are in line with those in the literature [29]. Materials 2023, 16, x FOR PEER REVIEW 7 of 15 coefficient of both materials was well stabilized at the end of the 1 h of sliding of the wear test. Figure 6 illustrates the measured steady-state friction coefficient values for the reciprocating sliding of SS and PEEK against alumina while using synthetic saliva. As previously presented, the COF value for SS was around 0.32 ± 0.03, while the mean values for PEEK are much lower, at around 0.094 ± 0.004. The COF of SS is ≈3.4 times higher than PEEK. The values are in line with those in the literature [29]. Figure 6. Steady-state coefficient of friction for stainless steel and PEEK against alumina in the presence of artificial saliva after 1 h of sliding. The hardness values obtained from both materials are recorded in Table 4, with stainless steel having the highest value. The load and lubrication conditions for the investigated materials with varying hardness were the same across all experiments. A linear correlation between steady-state COF and hardness values could not be established, as the material with the highest hardness did not exhibit the highest COF. PEEK with the lowest hardness presented COF values ≈3.4 times lower than SS. This behavior has already been observed in other studies [21]. Table 4. Mean hardness of stainless steel and PEEK samples. Material Mean Hardness (GPa) 316 L Stainless steel 2.56 ± 0.26 PEEK composite 1.26 ± 0.32 Figure 7 displays the 3D profiles of the wear track for both tested materials, as obtained through profilometry analysis. It should be noted that the figure only shows a segment of the wear track rather than the entire track, as, in the case of the stainless steel, the wear products obtained during the test and the crystallization of the saliva prevented the clear measurement of the track in its entirety. In the case of PEEK, even after gold plating, it was also not possible to read the complete track. Figure 6. Steady-state coefficient of friction for stainless steel and PEEK against alumina in the presence of artificial saliva after 1 h of sliding. The hardness values obtained from both materials are recorded in Table 4, with stainless steel having the highest value. The load and lubrication conditions for the investigated materials with varying hardness were the same across all experiments. A linear correlation between steady-state COF and hardness values could not be established, as the material with the highest hardness did not exhibit the highest COF. PEEK with the lowest hardness presented COF values ≈ 3.4 times lower than SS. This behavior has already been observed in other studies [21]. Table 4. Mean hardness of stainless steel and PEEK samples. Material Mean Hardness (GPa) 316 L Stainless steel 2.56 ±0.26 PEEK composite 1.26 ±0.32 Figure 7displays the 3D profiles of the wear track for both tested materials, as obtained through profilometry analysis. It should be noted that the figure only shows a segment of the wear track rather than the entire track, as, in the case of the stainless steel, the wear products obtained during the test and the crystallization of the saliva prevented the clear measurement of the track in its entirety. In the case of PEEK, even after gold plating, it was also not possible to read the complete track. It should also be noted that these images may not reflect reality accurately since the depth of the wear track appears to be much greater due to the distinct numerical scaling in the different axes automatically implemented in the software. However, it is possible to observe, through the graphs of the track profiles of both materials (Figure 7B,D), that there is a considerable difference in the depth of the wear track between steel and PEEK; the depth of the stainless steel track is about 2.1 times greater than that of PEEK. While the mean depth of the wear track of the SS sample is 5.9 ± 0.3 µ m, the mean depth of the wear track of the PEEK sample is considerably lower, at around 2.8 ± 1.1 µ m. This difference is also reflected in the wear volume value of each material and is in accordance with specific wear rate values, as seen in Figure 8. The mean width of the wear track is similar for both materials; the SS track width (610 ± 79.4 µ m) was only slightly greater than PEEK (583.3 ± 57.7 µ m). It is worth Materials 2023,16, 2420 8 of 14 mentioning that due to the X-axis compression that emphasizes possible small unevenness caused by the accumulation of wear products and saliva crystallization, the wear track may appear uneven. However, this does not represent the truth. Materials 2023, 16, x FOR PEER REVIEW 8 of 15 Figure 7. Optical profilometry scans of wear tracks of: (A), stainless steel, and (C), PEEK; and profiles of the wear track of: (B), stainless steel, and (D), PEEK. It should also be noted that these images may not reflect reality accurately since the depth of the wear track appears to be much greater due to the distinct numerical scaling in the different axes automatically implemented in the software. However, it is possible to observe, through the graphs of the track profiles of both materials (Figure 7B,D), that there is a considerable difference in the depth of the wear track between steel and PEEK; the depth of the stainless steel track is about 2.1 times greater than that of PEEK. While the mean depth of the wear track of the SS sample is 5.9 ± 0.3 µm, the mean depth of the wear track of the PEEK sample is considerably lower, at around 2.8 ± 1.1 µm. This difference is also reflected in the wear volume value of each material and is in accordance with specific wear rate values, as seen in Figure 8. The mean width of the wear track is similar for both materials; the SS track width (610 ± 79.4 µm) was only slightly greater than PEEK (583.3 ± 57.7 µm). It is worth mentioning that due to the X-axis compression that emphasizes possible small unevenness caused by the accumulation of wear products and saliva crystallization, the wear track may appear uneven. However, this does not represent the truth. Figure 7. Optical profilometry scans of wear tracks of: ( A ), stainless steel, and ( C ), PEEK; and profiles of the wear track of: (B), stainless steel, and (D), PEEK. Materials 2023, 16, x FOR PEER REVIEW 9 of 15 Figure 8. Wear volume and specific wear rate values for PEEK and stainless steel. Wear volume and specific wear rate are presented in Figure 8. PEEK showed a significantly lower wear volume (0.0078 ± 0.0125 mm 3 ) and specific wear rate (k = 9.07 × 10 −6 mm 3 N −1 m −1 ), being ≈1.6 times lower when compared to SS. PEEK presents substantially higher wear resistance than SS. A similar study conducted by Jacobs et al. [31] revealed values in the same order of magnitude for the specific wear rate of PEEK compounds containing carbon fibers, glass fibers, PTFE, and graphite against alumina (k = 7 × 10 −6 mm 3 N −1 m −1 ). No specific wear rate values were found in the literature for validation purposes for TiO2 PEEK composite under the same test conditions as in the present study. Additionally, Gao et al. [32] reported a specific wear rate value of k = 21.32 × 10 −5 mm 3 N −1 m −1 for SS. SEM micrographs depicting the morphological characteristics of the worn surfaces of the tested samples of stainless steel and PEEK are presented in Figures 9 and 10, respectively. Additional SEM images of a portion of the SS and PEEK samples without catching the wear track were also obtained for comparison reasons (Figures 9A and 10A). Figure 8. Wear volume and specific wear rate values for PEEK and stainless steel. Materials 2023,16, 2420 9 of 14 Wear volume and specific wear rate are presented in Figure 8. PEEK showed a significantly lower wear volume (0.0078 ± 0.0125 mm 3 ) and specific wear rate (k= 9.07 × 10 −6 mm 3 N −1 m −1 ), being ≈ 1.6 times lower when compared to SS. PEEK presents substantially higher wear resistance than SS. A similar study conducted by Jacobs et al. [ 31 ] revealed values in the same order of magnitude for the specific wear rate of PEEK compounds containing carbon fibers, glass fibers, PTFE, and graphite against alumina (k= 7 × 10 −6 mm 3 N −1 m −1 ). No specific wear rate values were found in the literature for validation purposes for TiO 2 PEEK composite under the same test conditions as in the present study. Additionally, Gao et al. [32] reported a specific wear rate value of k= 21.32 ×10−5mm3N−1m−1for SS. SEM micrographs depicting the morphological characteristics of the worn surfaces of the tested samples of stainless steel and PEEK are presented in Figures 9and 10, respectively. Additional SEM images of a portion of the SS and PEEK samples without catching the wear track were also obtained for comparison reasons (Figures 9A and 10A). Materials 2023, 16, x FOR PEER REVIEW 10 of 15 Figure 9. SEM micrographs of the: (A)—stainless steel sample surface without the wear track; and worn surface after 1 h sliding at a (B) 250× and (C) 5000× magnitude. Figure 9. SEM micrographs of the: ( A )—stainless steel sample surface without the wear track; and worn surface after 1 h sliding at a (B) 250×and (C) 5000×magnitude.