Wear of ceramic-based dental materials

Effect of rapid cooling on residual stress and surface fracture toughness of dental zirconia

Short-time sintering of dental zirconia not only improves manufacturing efficiency of zirconia prosthetics, but also enables an attractive situation in which prosthetic treatment can be completed within a single visit. Although many studies have clarified the effects of heating rate and dwell time on the properties of dental zirconia during short-time sintering, there are only a few studies on rapid cooling. In this study, we investigated the effect of cooling rate on dental zirconia. It was found that the cooling rate had no effect on the three-point flexural strength, but a fast cooling rate improved fracture toughness at the material surface. Raman piezo-spectroscopy showed that a compressive stress layer formed in the neighborhood of the zirconia surface and that its thickness increased with increasing cooling rate. From the above results, it was concluded that the compressive stress layer formed on the surface by rapid cooling improved the apparent fracture toughness at the material surface.

Surface roughness, wear, and abrasiveness of printed and milled occlusal veneers after thermomechanical aging

STATEMENT OF PROBLEM

Occlusal veneers are a conservative method of reestablishing vertical dimensions of occlusion (VDO) for worn teeth. A restorative material used for occlusal veneers should have a smooth surface, adequate wear resistance, and low abrasiveness to the antagonists, as total wear of occlusal veneers and their antagonists determines the stability of the reestablished VDO. Studies on roughness, wear, and abrasiveness of occlusal veneers are scarce.

PURPOSE

The purpose of this in vitro study was to assess the surface roughness, wear, and abrasiveness of printed and milled occlusal veneers after thermomechanical aging against natural cusps and restorative materials.

MATERIAL AND METHODS

Forty-eight extracted mandibular first molars were prepared for occlusal veneers and scanned with an intraoral scanner. The scans were exported to a computer-aided design program to design the occlusal veneers in 3 groups (n=16) according to the restorative material: group IP: milled lithium disilicate; group EN: milled hybrid ceramic (Vita Enamic), group VA: printed hybrid ceramic (Varseosmile Crown plus). The occlusal veneers in each group were bonded to their corresponding abutments and subjected to 250 000 mechanical cycles and a simultaneous 1000 thermal cycles in a mastication simulator. During thermomechanical aging, half of the specimens of each study group (n=8) were opposed by natural cusps (antagonist C) and the other half by antagonist cusps fabricated from the same restorative material as the occlusal veneers (antagonist R). The wear of the occlusal veneers and their antagonists was assessed with a 3-dimensional processing software program. The surface roughness of the veneers was assessed with a contact profilometer. The results were statistically analyzed with a 2-way ANOVA followed by the post hoc Tukey HSD test (α=.05).

RESULTS

The 2-way ANOVA indicated a significant effect for the material and the antagonist on surface roughness, wear, and abrasiveness (P<.05). When opposed by antagonist C, VA showed significantly higher surface roughness than IP and EN (P<.001). VA had significantly higher surface wear followed by EN and IP (P<.001). IP caused significantly higher wear to antagonist C than EN and VA (P<.001). In addition, IP had significantly higher total wear (combined wear of veneers and their antagonists) followed by VA and EN (P<.001). When opposed by antagonist R, no significant difference was found among the 3 materials for surface roughness (P=.08), material wear (P=.12), opposing wear (P=.11), or total wear (P=.11).

CONCLUSIONS

Both material and antagonist had a significant effect on surface roughness and occlusal stability when occlusal veneers were fabricated to restore VDO. VA had significantly more surface roughness and wear than EN and IP when opposed by natural cusps. IP abraded natural cusps significantly more than EN and VA. The 3 tested materials showed similar roughness, wear, and abrasiveness when opposed by the same material.

Evaluation of Hardness and Wear of Conventional and Transparent Zirconia Ceramics, Feldspathic Ceramic, Glaze, and Enamel

The aim of the study was to compare the hardness, coefficient of friction, and wear experienced by four different ceramic samples: 3Y-TZP zirconium oxide ceramics-Zi-Ceramill Zi (Amman Girrbach), 5Y-PSZ transparent zirconium oxide ceramics-Zol-Ceramill Zolid (Amman Girrbach), Sak-feldspathic ceramics-Sakura Interaction (Elephant), and Glaze (Amman Girrbach). The Vickers hardness of the samples was measured. Friction tests ball-on-disc were performed between the discs of four ceramics and a zirconia ceramic ball, then a premolar tooth as a counter-sample. The mass loss and the friction coefficients of the ceramic samples were determined. The tooth counter-samples were 3D scanned, and enamel attrition depths and mass were measured. The following hardness values (HV1) were obtained: 1454 ± 46 HV1 for Zi, 1439 ± 62 HV1 for Zol, 491 ± 16 HV1 for Sak, 593 ± 16 HV1 for Glaze, and 372 ± 41 HV1 for enamel. The mass losses of the teeth in contact with ceramics were 0.1 mg for Zi, 0.1 mg for Zol, 5.5 mg for Sak, and 4 mg for Glaze. Conventional and transparent zirconium oxide ceramics are four times harder than enamel and three times harder than veneering ceramics. Zirconia ceramics exhibit lower wear and a more homogenous, smoother surface than the other ceramics. Tooth tissues are subject to greater attrition in contact with veneering ceramics than with polished zirconium oxide ceramics.

Recent Advances in Biomimetics for the Development of Bio-Inspired Prosthetic Limbs

Recent advancements in biomimetics have spurred significant innovations in prosthetic limb development by leveraging the intricate designs and mechanisms found in nature. Biomimetics, also known as "nature-inspired engineering", involves studying and emulating biological systems to address complex human challenges. This comprehensive review provides insights into the latest trends in biomimetic prosthetics, focusing on leveraging knowledge from natural biomechanics, sensory feedback mechanisms, and control systems to closely mimic biological appendages. Highlighted breakthroughs include the integration of cutting-edge materials and manufacturing techniques such as 3D printing, facilitating seamless anatomical integration of prosthetic limbs. Additionally, the incorporation of neural interfaces and sensory feedback systems enhances control and movement, while technologies like 3D scanning enable personalized customization, optimizing comfort and functionality for individual users. Ongoing research efforts in biomimetics hold promise for further advancements, offering enhanced mobility and integration for individuals with limb loss or impairment. This review illuminates the dynamic landscape of biomimetic prosthetic technology, emphasizing its transformative potential in rehabilitation and assistive technologies. It envisions a future where prosthetic solutions seamlessly integrate with the human body, augmenting both mobility and quality of life.

Effect of Post-Processing Heat Treatment on Micro-Contact Damage of Zirconia-Reinforced Lithium Silicate Dental Materials

Zirconia-reinforced lithium silicate (ZLS) is utilized as a material for prosthetic tooth crowns, offering enhanced strength compared to other dental glass-ceramics. In this study, we investigate a commercial ZLS material, provided in a fully crystallized form. We examine the effects of an optional post-processing heat treatment on micro-contact damage using controlled indentation tests simulating the primary modes of contact during chewing: axial and sliding. Our findings indicate that the heat treatment does not affect mechanical properties such as the elastic modulus, hardness and indentation fracture toughness. However, it does enhance the resistance to contact damage by fracture and chipping in both axial and sliding modes, as well as the resistance to crack initiation measured from sliding tests. This improvement is attributed to the refinement of the flaw population achieved through the heat treatment. The results are analysed using principles of contact and fracture mechanics theory, discussing their significance in prosthetic dentistry.

Wear of lithium disilicate opposed by various ceramic materials

OBJECTIVES

To compare volumetric wear of lithium disilicate against different ceramic (3 mol% yttria-stabilized (3Y) zirconia, 5 mol% yttria-stabilized (5Y) zirconia, lithium disilicate, porcelain and enamel antagonists).

MATERIALS AND METHODS

Forty lithium disilicate (e.max CAD) specimens (n = 8/antagonist) were wet sanded to 1200grit SiC and mounted into a UAB wear device. Antagonist spheres (diameter = 4.75 mm) were made from polished 3Y zirconia, 5Y zirconia, lithium disilicate, porcelain and human enamel. A two-body wear test was performed with 20 N load and 1.5 mm slide for 400,000 cycles at 1 Hz. 33% glycerin was used as a lubricant. Wear facets were measured with optical profilometry. Wear scar areas of antagonists were measured with digital microscopy. Scanning electron microscopy was performed on wear facets and scars. Vicker's microhardness was measured of all antagonist materials. All data were compared with 1-way ANOVA and Tukey post-hoc analysis.

RESULTS

Significant differences in lithium disilicate volumetric wear (mm3 ) occurred with various antagonist materials: 0.38 ± 0.01a (3Y zirconia), 0.33 ± 0.01b, (5Y zirconia), 0.16 ± 0.01c (lithium disilicate), 0.11 ± 0.03d, (enamel), and 0.07 ± 0.01e (porcelain). The lithium disilicate antagonist demonstrated a larger wear scar than other materials. Zirconia was the hardest material and enamel the least hard.

CONCLUSIONS

Zirconia causes significant wear on lithium disilicate and lithium disilicate causes significant wear against itself.

CLINICAL SIGNIFICANCE

When selecting a material to oppose an existing lithium disilicate crown, a porcelain or lithium disilicate surface would cause significantly less wear to the existing crown. If an existing zirconia crown exists opposed to a prepared tooth, lithium disilicate may not be an ideal material selection to restore the tooth.

Effect of different ceramic systems on antagonist dental structure by microtomographic analysis

OBJECTIVES

This study aims to identify the two-dimensional and three-dimensional analyses and evaluate the loss of tooth structure in the tooth's different constituent elements and the ceramic antagonist's surface.

METHODS

In this study, three groups (n = 10) represented by different ceramic systems (lithium disilicate [GDis], lithium silicate reinforced with zirconia [GSil], and monolithic zirconia [GZir]) were evaluated. Each group obtained ten ceramic blocks and submitted them to the sintering/crystallization process. To carry out the wear test, healthy mandibular premolars were used as the specimens and the ceramic blocks as antagonists. The premolars were submitted to two-dimensional and three-dimensional analyses using a computerized microtomography (µTC) before and after the wear test. The wear test was performed with 30 N and 300,000 cycles load, with occlusion, laterality, and disocclusion movements.

RESULTS

The statistical analysis comparing the loss of two-dimensional tooth structure showed a statistically significant difference among all groups (p < 0.05). Statistical analysis comparing the percentage of loss of three-dimensional tooth structure showed a statistically significant difference between groups GDis and GSil and between groups GDis and GZir. However, when comparing GSil with GZir, no statistically significant difference was found. The qualitative analysis of the teeth showed that GDis showed considerable enamel loss and dentin exposure, GSil showed enamel wear with flattening the cusp without dentin exposure, and GZir showed minimal enamel wear without dentin exposure. In the qualitative analysis of ceramic antagonists, more significant wear of the ceramic material for GDis was observed, followed by the GSil and GZir groups, respectively.

SIGNIFICANCE

The use of the lithium disilicate should be cautious, restricting it to areas with lower masticatory forces. Areas of higher masticatory forces showed a large amount of antagonist wear, with dentin involvement. This can lead to dentinal hypersensitivity, risk of compromising patients' occlusion, by harming group disocclusion guides, causing pain and temporomandibular disorders.

An In Vitro Study regarding the Wear of Composite Materials Following the Use of Dental Bleaching Protocols

Composite materials used in dental restorations are considered resistant, long-lasting and aesthetic. As the wear of restorations is an important element in long-term use, the aim of this study was to evaluate the surface condition of nanohybrid and microfilled composite resins, after being subjected to the erosive action of dental bleaching protocols. This paper reflects a comparative study between one nanofilled composite and three microfilled composites used in restorations. For each composite, three sets of samples (under the form of composite discs) were created: a control group, an "office bleach" group with discs bleached with 40% hydrogen peroxide gel, and a "home bleach" group with discs bleached with 16% carbamide peroxide gel. Wear was numerically determined as the trace and the coefficients of friction obtained using a tribometer, the ball-on-disk test method, and two balls: alumina and sapphire. For all composite groups, there were statistically significant differences between the wear corresponding to the control and bleaching groups, for both testing balls. Regarding the composite type, the largest traces were recorded for GC Gradia direct anterior, for all groups, using the alumina ball. In contrast, for the sapphire ball, 3M ESPE Filtek Z550 was characterized by the largest traces. With respect to the friction coefficients, the "office bleach" group recorded the largest values, no matter the composite or the ball type used. The 3M ESPE Valux Plus composite recorded the largest friction coefficients for the alumina ball, and 3M ESPE Filtek Z550 for the sapphire ball. Overall, the "office bleach" group was characterized by higher composite wear, compared to the "home bleach" protocol or control group. Nanofilled composite resins showed superior wear resistance to microfilled resins after undergoing a bleaching protocol.

Comparative Evaluation of the Tribological Properties of Polymer Materials with Similar Shore Hardness Working in Metal-Polymer Friction Systems

This article presents comparative tests of contact strength and tribological wear resistance of polymer sliding materials of the polyamide group. The aim of this work was to study Shore hardness, indentation hardness, modulus, creep, relaxation, Martens hardness and sliding wear resistance of two commercial materials. One of these materials was produced with the recycling process in mind. Abrasion tests were performed against a stainless-steel ball (100CRr6) on a normal load of 5 N for 23,830 friction cycles. The samples were tested under dry friction conditions and taking into account the hydrothermal factor, the presence of which was assumed in the anticipated operating conditions. It was distilled water at a temperature of 50 °C. The volumetric wear of the samples under various environmental conditions was assessed and related to the mechanical properties, in particular, Shore hardness. This mechanical size, which characterizes the surface, was considered the most frequently used by engineers selecting polymeric materials for tribological applications in industry. The Shore hardness of both materials was similar, which may indicate similar tribological performance properties. However, research and analysis indicate the need to use measures that directly correspond to tribological wear. The friction and wear of both materials varied. The coefficient of friction in hydrothermal conditions was lower and the wear was higher than in the dry friction test. It seems that it was not hardness that determined the suitability in the anticipated operating conditions, but the ability to form a sliding layer on the friction surface. The properties of the material that has been envisaged as a replacement may be appropriate for the intended uses.

Impact of the occlusal contact pattern and occlusal adjustment on the wear and stability of crowns

OBJECTIVES

To investigate the impact of the occlusal contact situation and occlusal adjustment on wear, roughness, and fracture force of molar crowns.

MATERIALS AND METHODS

CAD/CAM crowns (lower right first molar, n = 64; 4 groups à 8, 3Y-TZP zirconia and resin composite) and corresponding antagonists (upper right first molar; 3Y-TZP zirconia) were manufactured. Crowns were constructed according to two principles of occlusion (group "T": Peter K. Thomas' "point-centric" cusp-to-fossa tripodization concept, with 15 contact points; group "RA" Sigurd P. Ramfjord and Major M. Ash, "freedom in centric" concept with four contacts). On one half of the crowns, occlusal adjustment was performed (groups "T adjusted" and "RA adjusted"). All crowns underwent combined thermal cycling (TC) and mechanical loading (ML) (ML: 1.2 × 10⁶ cycles, 50 N, 2 Hz, mouth opening 1 mm; TC: 2 × 3000 cycles, 5/55°C). Wear area and depth of each contact point on the occlusal surfaces of crowns and antagonists were determined using a digital microscope. Surface roughness (R_a, R_z) was measured in and besides (reference) the worn area (3D laser-scanning microscope). Fracture force of the crowns was determined (statistics: Levene-test, one-way-ANOVA; Bonferroni-post-hoc-test; between-subjects effects, Pearson correlation, α=0.05).

RESULTS

The resin composite crowns yielded significantly higher mean values for wear area and depth (p < 0.001) and lower fracture forces (p < 0.001). Resin composite surfaces showed increased roughness after TCML while zirconia exhibited smoothened surfaces. The occlusal design significantly impacted wear depth (p = 0.012) and fracture force (p < 0.001). Resin composite crowns with fewer contact points (group RA) showed more wear and lower fracture force. Adjusted resin composite crowns showed increased wear areas and depths (p = 0.009-0.013). For zirconia crowns, the adjustment impacted wear area (p = 0.013), wear depth (p = 0.008), and fracture force (p = 0.006), with adjusted zirconia crowns exhibiting more wear and lower maximum forces until fracture. Zirconia wear depth was also impacted by the occlusal design (p = 0.012). Antagonistic wear was influenced by the restorative material, the occlusal contact pattern, and the adjustment.

CONCLUSIONS

The investigated materials show strongly varying performances with zirconia being significantly influenced by the adjustment, while for resin composites, contact design and adjustment had a major impact.

CLINICAL RELEVANCE

The results show the necessity of adapting occlusal design and adjustment in order to improve roughness, wear, and stability of zirconia and resin composite crowns.

Comparative study on the impact-sliding wear behaviour of CAD/CAM resin-ceramic materials and tooth enamel

OBJECTIVES

To compare the impact-sliding wear of different CAD/CAM resin-ceramic materials and tooth enamel, and explore the corresponding wear damage mechanism.

METHODS

Human tooth enamel (EN), Vita ENAMIC (Vita, VE), Lava Ultimate (3 M, LU), and GC CERASMART (GC, CS) were used in this study. The hardness, elastic modulus, and roughness values of the samples were measured. Further, impact-sliding wear tests were performed in a ball-on-flat configuration with spherical zirconia antagonists and the coefficients of friction (CoF) were recorded simultaneously. Additionally, a white light interferometer was used to determine the volume losses and scanning electron microscopy was used to observe the wear morphology of the wear scars and the damage feature in the vertical sections to clarify the damage mechanism during the impact-sliding wear test.

RESULTS

EN exhibited the highest elastic modulus and CoF, followed by VE, LU, and CS. The hardness and roughness of EN and VE were similar and were higher than those of LU and CS. Throughout the wear tests, VE exhibited the highest volume loss, whereas CS exhibited the lowest. The wear damage characteristics of VE were similar to those of EN, displaying brittle fractures of inorganic substances and plastic deformation of organic substances in the impact part, exhibiting plough marks in the sliding parts. In the case of LU and CS, the entire wear areas displayed plastic deformation of the resin matrix, exfoliation of the filler particles, and plough marks.

SIGNIFICANCE

Enamel and polymer-infiltrated ceramic network materials exhibit similar wear damage modes. Additionally, the high-density nanocomposite resin material is the most resistant to impact-sliding wear from a tribological perspective.

Exploring Ductility in Dental Ceramics

Two damage regimes-"brittle" and "ductile"-have been identified in the literature on ceramic grinding, machining, grit blasting, and wear. In the brittle regime, the damage mechanism is essentially crack formation, while in the ductile region, it is quasiplasticity. Onset of the brittle mode poses the greater threat to strength, so it becomes important to understand the mechanics of ductile-brittle thresholds in these materials. Controlled microcontact tests with a sharp indenter are employed to establish such thresholds for a suite of contemporary computer-aided design/computer-aided manufacturing dental ceramics. Plots of flexural strength S versus indentation load P show a steep decline beyond the threshold, consistent with well-established contact mechanics relations. Threshold dimensions occur on a scale of order 1 µm and contact load of order 1 N, values pertinent to practical grit finishing protocols. The ductile side of ceramic shaping is accessed by reducing grit sizes, applied loads, and depths of cut below critical levels. It is advocated that critical conditions for ductile shaping may be most readily quantified on analogous S(P) plots, but with appropriate machining variable (grit size, depths of cut, infeed rate) replacing load P. Working in the ductile region offers the promise of compelling time and cost economies in prosthesis fabrication and preparation.

Advances in Ceramics for Dental Applications

The purpose of this study is to present current dental ceramic materials and processing methods. The clinical indication was emphasized on basis of the material's microstructure and composition. Studies of ceramic characterization were also discussed, as they impact the clinical indication and serve as a parameter for the development of new materials. The novel strategies were mostly found aiming to mimic the natural dental structures, provide mechanical reliability, and develop predictable restorations in terms of adaptation and design.

Effects of Streptococcus mutans and their metabolites on the wear behavior of dental restorative materials

The wear behavior of dental restorative materials is highly related to the biolubricating medium in the oral environment. Bacteria, along with their metabolic products, are essential substances in the oral cavity and have not been studied as a potential factor affecting lubrication performance during mastication. In this study, the effects of the Streptococcus mutans bacterial cells and their metabolites were investigated on the wear behavior of resin composites, polymer-infiltrated ceramic networks and zirconium-lithium silicate glass-ceramics. A reciprocating friction test and quantitative analysis of the wear morphology were utilized to determine the coefficient of friction (COF) and wear resistance of the test materials. The results showed that the bacterial metabolite medium significantly reduces the COF and wear rate of the three restorative materials and provide better protection against superficial abrasion. When tested under lactic acid medium, a key acid production in bacterial metabolites, similar wear reduction results were observed in the three materials, which confirmed that lactic acid should be accountable for the excellent lubricating property of bacterial metabolites. Furthermore, the resin composite with lower wettability exhibited a more significant wear reduction than the other two materials when lubricating with a bacterial metabolite medium. These findings provide novel insights into the biological basis of lubrication mechanisms in the oral cavity under high-loading and low-velocity conditions.

Wear of various restorative materials against 5Y-ZP zirconia

STATEMENT OF PROBLEM

The ceramic 5-mol% yttria-stabilized zirconia (5Y-ZP) has been developed for dental use in the esthetic zone with greater translucency than 3-mol% yttria-stabilized zirconia (3Y-ZP). However, studies on the wear behavior of 5Y-ZP zirconia against clinically relevant antagonist materials are lacking.

PURPOSE

The purpose of this in vitro study was to investigate the wear behavior of 5Y-ZP zirconia against the antagonists 5Y-ZP zirconia, lithium disilicate, palladium alloy, and human enamel.

MATERIALS AND METHODS

Flat specimens (n=8) were fabricated from 5Y-ZP zirconia, lithium disilicate, palladium alloy, and human central incisor enamel. A custom wear-simulating device with a sliding pin-on-plate configuration was used for a total of 120 000 wear cycles at 1.6-Hz frequency under a 49-N vertical load while submerged in distilled water at room temperature. The wear volume and maximum wear depth of flat specimens were evaluated with a 3D profilometer. Scanning electron microscopy was used to analyze the characteristics of the worn surfaces.

RESULTS

After wear simulation, the 5Y-ZP zirconia and palladium-silver alloy specimens exhibited the least amount of material loss, both in terms of maximum wear depth and wear volume (0.079 ±0.042 μm, 0.001 ±0.001 mm³ and 0.637 ±0.307 μm, 0.001 ±0.000 mm³, respectively). This was followed by human enamel (6.034 ±1.086 μm, 0.009 ±0.001 mm³) and by lithium disilicate, which showed excessive material loss (38.342 ±2.569 μm, 0.213 ±0.024 mm³). Scanning electron microscopy revealed variations in wear mechanisms among the materials.

CONCLUSIONS

The 5Y-ZP zirconia and palladium-silver alloy exhibited the lowest wear, followed by human enamel and lithium disilicate. Slight grain dislodgement was displayed on worn 5Y-ZP surfaces, while more apparent grain dislodgement and wear grooves were found on lithium disilicate. Plastic deformation of worn palladium-silver alloy accumulated at the end of wear track. Cracks were detected in the human enamel specimens.

Indirect repair of saliva-contaminated materials using veneering ceramics

PURPOSE

To analyze the in vitro efficacy of a surface conditioning liquid facilitating ceramic repairs of saliva-contaminated metal-ceramic and all-ceramic restorations.

MATERIALS AND METHODS

Specimens constructed from non-precious alloy (NPA), precious alloy (PA), lithium-disilicate (LD), zirconia (ZI), veneering ceramics for zirconia (VZI), veneering ceramics for lithium-disilicate (VLD), and veneering ceramics for metal alloys (VM) were manufactured (total: n = 168; each material n = 24). Veneering ceramic cylinders (thickness: 2mm) were hand-layered on top of the specimens. Shear bond strength (SBS) tests were performed, measuring the maximum bond strength (MBS) of the cylinders on the specimens. Following this, the specimens were artificially aged and stored in artificial saliva for 30 days at 37°C. After physical cleaning using aluminum oxide air abrasion, a new surface conditioning liquid was applied (test, n = 84) or not (control, n = 84). New ceramic cylinders were hand-layered followed by a second SBS test. Descriptive statistics, linear regression analyses, and a one-sample t-test (α = 0.05) were used to ascertain the differences within (pre- vs. post-repair) and between the groups.

RESULTS

All specimens in the test group could be repaired, whereas 18 repairs in the control group failed. After the repairs, an MBS decrease was observed for the NPA specimens of the control group (-15.5MPa, p = 0.004) but not among any of the test groups. Comparing the change in MBS between the test and control groups, the reduction was significantly higher in the repaired NPA specimens of the control group (mean difference 11.8 MPa, p = 0.017).

CONCLUSIONS

Using the analyzed surface conditioning liquid, metal-ceramic and all-ceramic materials can be repaired, while some repairs failed without the liquid. The initial bond strengths between core and veneering materials could be restored in all specimens when the new surface conditioning liquid was applied. This article is protected by copyright. All rights reserved.

Wear behavior and abrasiveness of monolithic CAD/CAM ceramics after simulated mastication

Objectives

To evaluate the wear resistance and abrasiveness of monolithic CAD/CAM ceramics.

Materials and methods

Rectangular-shaped specimens (12 mm × 6.5 mm × 1.5 mm) were sectioned from the following CAD/CAM blocks (n = 10); partially crystallized lithium disilicate (PLD), experimental fully crystallized lithium disilicate (FLD), zirconia-reinforced lithium silicate (ZLS), super-translucent monolithic zirconia (SMZ), and ultra-translucent monolithic zirconia (UMZ). Silicon carbide papers were used to mechanically flatten and polish the surfaces. PLD specimens were subjected to a combined crystallization/glazing firing cycle. Ceramic specimens were mounted to the wear device and tested for 200,000 cycles against human premolars at 20 N force and 2 mm sliding distance. Artificial saliva was used as a lubricant. The teeth were scanned using micro-CT before and after the wear test and the generated models were overlapped to determine the volumetric tooth loss. Before and after the test, specimens’ weights and surface roughness (R_a) values were measured, and the differences were calculated. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were utilized for microstructural and chemical analysis. Statistical analysis was performed using one-way ANOVA or an equivalent test for non-parametric results. Significance level was set at P ≤ 0.05.

Results

The type of ceramic material affected the ceramic and antagonist wear rates (P < 0.001). PLD and ZLS had the highest ceramic and antagonist wear, whereas UMZ and SMZ demonstrated the lowest wear values. The FLD group showed comparable antagonist wear and significantly less ceramic wear than PLD and ZLS.

Conclusions

Monolithic zirconia demonstrated the best wear resistance and least abrasiveness to the antagonist. The experimental lithium disilicate was more wear-resistant than other glass–ceramic groups.

Clinical relevance

Monolithic zirconia is wear-resistant and gentle on the antagonist. In contrast, glass–ceramics are more abrasive to enamel.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00784-022-04611-w.

Threshold damage mechanisms in brittle solids and their impact on advanced technologies

Threshold damage mechanisms in brittle covalent-ionic solids are outlined. Fracture and deformation modes are analyzed in terms of classical contact mechanics. Distinctions are made between brittle, ductile and quasiplastic mechanisms in both axial and translational contact. Special attention is devoted to the relatively unexplored subthreshold region where macrofracture is largely suppressed, a region of increasing relevance in the relentless move toward ever smaller devices and precision shaping technologies in the manufacturing sector. Cross-section micrographic images illustrate the fundamental nature of shear events within the hardness deformation zone responsible for crack initiation and propagation. Basic analytical relations for the strengths of surfaces with contact-induced damage in the postthreshold and subthreshold regions are presented, with emphasis on concept rather than fine detail. Strength data for a prototypical brittle material after sharp-indenter damage are presented to highlight the vital role of microstructure in determining transitions between brittle and quasiplastic responses. Pristine defect-free solids are shown to be highly vulnerable to contact damage, even in the subthreshold region. Heterogeneous solids with granular microstructures have lower initial strengths, but are more flaw tolerant. Brittle solids are also highly susceptible to degradation by surface removal processes in wear and machining settings, to a large extent depending again on microstructure. Implications of these findings concerning advanced technological applications of covalent-ionic solids are discussed.

Application Research of Tooth Arrangement Based on Rotation Matrix Calculation and Resistance Detection in Oral

The goal of this research was to provide a new approach for analyzing orthodontic teeth arrangement inside oral depending on the rotation matrix computation and resistance detection. The present method includes the following operations within a certain therapy period: first three-dimensional positions of the tooth were evaluated with a pierced laser beam and a three-dimensional system of surface-scanning. Second, the three-dimensional shape data was automatically registered at maxillary 1^st molars, and methods of coordinate had been normalized. Third, a translation vector and rotation matrix had been evaluated from automatic registration of two position data of a particular tooth. Fourth, the limited spiral axes of teeth had been measured as the zero rotational dislocation locus; and impressions for a model of the dental cast had been taken at five different points: shortly before and after device was fitted, and ten days, one month, and two months after the treatment started. The results showed that existing analysis approach could more quickly classify a specific tooth's movement by spinning all over and translating along a finite helical axis. It can provide statistical visual three-dimensional data on complex tooth arrangement throughout orthodontic therapy.

Wear Behavior of Different Generations of Zirconia: Present Literature

Objective. The wear behavior of the novel zirconia generation is less well understood and may be affected by compositional modifications compared to the conventional zirconia. Materials and Methods. Combinations of keywords such as “zirconia,” “high translucent,” and “wear” were searched in PubMed and Google Scholar databases up to May 2021. The total of 23 relevant articles was selected according to inclusion criteria. Results. Reports show comparable wear resistance of translucent zirconia to the conventional zirconia despite an increased cubic phase content and lower mean flexural strength. A meticulously polished surface creates the lowest surface roughness, producing favorable zirconia wear resistance and antagonist wear compared to a glazed surface. In comparison to other ceramic materials, zirconia produces the least wear on an enamel antagonist and almost undetectable wear when opposed by zirconia. Wear when paired against resin materials yields a favorable outcome, whereas wear behavior against a metal antagonist varies with the surface hardness of the metal. Conclusions. All zirconia generations are considered wear-friendly to all types of antagonists. Nonetheless, comparative studies on antagonist wear opposing zirconia of different compositions are still limited and further investigation is required.

Wear evaluation of CAD-CAM dental ceramic materials by chewing simulation

PURPOSE

To evaluate the wear of computer-aided design/computer-aided manufacturing (CAD-CAM) dental ceramic materials opposed by enamel as a function of increased chewing forces.

MATERIALS AND METHODS

The enamel cusps of healthy human third molar teeth (n = 40) opposed by materials from CAD-CAM dental ceramic groups (n = 10), including Vita Enamic® (ENA), a polymer-infiltrated ceramic network (PICN); GC Cerasmart® (CERA), a resin nanoceramic; Celtra® Duo (DUO), a zirconia-reinforced lithium silicate (ZLS) ceramic; and IPS e.max ZirCAD (ZIR), a polycrystalline zirconia, were exposed to chewing simulation (1,200,000 cycles; 120 N load; 1 Hz frequency; 0.7 mm lateral and 2 mm vertical motion). The wear of both enamel cusps and materials was quantified using a 3D laser scanner, and the wear mechanisms were evaluated by scanning electron microscopy (SEM). The results were analysed using Welch ANOVA and Kruskal Wallis test (α = .05).

RESULTS

ZIR showed lower volume loss (0.02 ± 0.01 mm³) than ENA, CERA and DUO (P = .001, P = .018 and P = .005, respectively). The wear of cusp/DUO [0.59 mm³ (0.50-1.63 mm³)] was higher than cusp/CERA[0.17 mm³ (0.04-0.41 mm³)] (P = .007). ZIR showed completely different wear mechanism in SEM.

CONCLUSION

Composite structured materials such as PICN and ZLS ceramic exhibit more abrasive effect on opposing enamel due to their loss against wear, compared to uniform structured zirconia. The resin nanoceramic causes the lowest enamel wear thanks to its flexible nano-ceramic microstructure. While zirconia appears to be an enamel-friendly material in wear volume loss, it can cause microstructural defects of enamel.

Investigation of indentation size effect and R-curve behaviour of Li2O-SiO2 and Li2O-2SiO2 glass ceramics

Indentation size effect (ISE) and R-curve behaviour of Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics are investigated using micro-indentation and indentation-strength (IS) techniques, respectively. Vickers micro-indentations were applied on both materials at the load of 0.10-19.6 N to determine the load influence on the measured hardness. For the IS-measured fracture toughness, the load ranged from 1.96 to 19.6 N. The hardness decreased with increasing load by 20% and 18% on Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics, respectively, indicating the ISE behaviour on both materials. The fracture toughness increased with the load by 27% and 59% on Li₂O-SiO₂ and Li₂O-2SiO₂ glass ceramics, respectively, signifying the R-curve behaviour. The ISE behaviour of both materials was analysed using the Meyer's, Hays-Kendall (HK), proportional specimen resistance (PSR), Nix-Gao (NG), modified PSR (MPSR) and elastic plastic deformation (EPD) models while the R-curve behaviour was analysed by the fractional power law. The Meyer's index of both materials was less than 2, strongly confirming the ISE existence. The HK, PSR and NG models were only suitable to determine intrinsic Vickers hardness for Li₂O-2SiO₂ glass ceramic while the MPSR and EPD models were successful for both materials. The fractional power law gave higher R-curve steepness for Li₂O-2SiO₂ than Li₂O-SiO₂ glass ceramics. Also, material and brittleness indices predicted, respectively, higher quasi-plasticity and better machinability for Li₂O-2SiO₂ than Li₂O-SiO₂ glass ceramics indicating superior performance in the former to the latter. Finally, this study presents a new significant insight into the micro-mechanisms of fracture tolerance behaviour of these glass ceramics which is critical to their functional performance as structural ceramics.

Effect of Artificial Aging on Mechanical and Tribological Properties of CAD/CAM Composite Materials Used in Dentistry

With easy-to-process 3D printing materials and fast production, the quality of dental services can be improved. In the conventional procedure, the dentist makes temporary crowns directly in the patient’s mouth, e.g., from the most commonly used bis-acrylic composites. Temporary crowns made directly in the office without the use of CAD/CAM are often of inferior quality, which directly results in impaired hygiene, poorer masticatory mechanics, greater deposition of plaque, calculus and sediment, and may adversely affect periodontal and gum health. The mechanical strength, resistance to aging and abrasion of 3D printing materials are higher than those of the soft materials used in conventional methods. This translates into durability. The patient leaves the surgery with a restoration of higher utility quality compared to the conventional method. The objective of the paper was to determine the influence of aging in artificial saliva of AM (additive manufacturing) orthodontic composites on their functional properties. For the purpose of the study, fillings well-known worldwide were selected. These were traditional UV-curable resins (M I, M II, M III, M V) and a hybrid material based on a UV-curable resin (M VI). Samples were stored in artificial saliva at 37 ± 1 °C in a thermal chamber for 6 months. Indentation hardness, frictional tests and sliding wear measurements were conducted. A comparison between various materials was made. Descriptive statistics, degradation coefficients, H²E, Archard wear and specific wear rate were calculated. The Weibull statistical test for indentation hardness was performed and Hertzian contact stresses for the frictional association were calculated for unaged (M I, M II, M III, M V, M VI) and aged (M I AS, M II AS, M III AS, M V AS, M VI AS) samples. M I exhibited the lowest average hardness among the unaged materials, while M III AS had the lowest average hardness among the aged materials. Comparably low hardness was demonstrated by the M I AS material. The coefficient of friction values for the aged samples were found to be higher. The lowest wear value was demonstrated by the M I material. The wear resistance of most of the tested materials deteriorated after aging. The M VI AS material had the highest increase in wear. According to the results provided, not only the chemical composition and structure, but also aging have a great impact on the indentation hardness and wear resistance of the tested orthodontic materials.

A Review of Human Exposure to Microplastics and Insights Into Microplastics as Obesogens

The ubiquitous exposure of humans to microplastics (MPs) through inhalation of particles in air and ingestion in dust, water, and diet is well established. Humans are estimated to ingest tens of thousands to millions of MP particles annually, or on the order of several milligrams daily. Available information suggests that inhalation of indoor air and ingestion of drinking water bottled in plastic are the major sources of MP exposure. Little is known on the occurrence of MPs in human diet. Evidence is accumulating that feeding bottles and medical devices can contribute to MP exposure in newborns and infants. Biomonitoring studies of human stool, fetus, and placenta provide direct evidence of MP exposure in infants and children. MPs <20 µm were reported to cross biological membranes. Although plastics were once perceived as inert materials, MP exposure in laboratory animals is linked to various forms of inflammation, immunological response, endocrine disruption, alteration of lipid and energy metabolism, and other disorders. Whereas exposure to MPs itself is a concern, MPs can also be sources of exposure to plastic additives and other toxicants. Exposure of human cell lines to MP additives such as phthalates, bisphenols, and organotins causes adverse effects through the activation of nuclear receptors, peroxisome proliferator-activated receptors (PPARs) α, β, and γ, and retinoid X receptor (RXR), leading to oxidative stress, cytotoxicity, immunotoxicity, thyroid hormone disruption, and altered adipogenesis and energy production. The size, shape, chemical composition, surface charge, and hydrophobicity of MPs influence their toxicity. Maternal transfer of MPs to the developing fetus has been demonstrated in exposed laboratory animals and through the analysis of human placenta. In laboratory animal studies, maternal exposure to MPs altered energy and lipid metabolism in offspring and subsequent generations. Moreover, concomitant with the global increase in plastics production, the prevalence of overweight and obesity in human populations has increased over the past five decades, and there is evidence to support the hypothesis that MPs and their additives are potential obesogens. Even though MP exposures are ubiquitous and toxic effects from such exposures are a concern, systematic studies on this topic remain urgently needed.

Human exposure to microplastics: A study in Iran

Exposure of microplastics (MPs) to a cohort of adults of various demographics from different regions of Iran has been quantitatively assessed. Specifically, MPs were retrieved from filtered washes of the hand and face skin, head hair and saliva of individuals (n = 2000) after an exposure period of 24 h and were counted and, in a selected number of cases, characterised for shape-form and size microscopically. A total of over 16,000 MPs were recorded in the study, with head hair returning the most samples (> 7000, or, on average, >3.5 MPs per individual per day), saliva returning the least samples (about 650, or on average 0.33 MPs per individual), and MPs about twice as high in males than females. The number of MPs was similar amongst residents of different urbanised regions, albeit with evidence of greater quantities captured in more humid settings, and was considerably lower in residents of a remote and sparsely populated area. Polyethylene-polyethylene terephthalate and polypropylene fibres of < 100 μm in length, likely derived from clothing and soft furnishings in the indoor setting and a wider range of sources in the exterior environment, were the most abundant type of MP in all body receptors. Daily sampling of receptors from six participants over a seven-day period revealed that, despite these broad trends, both inter- and intra-individual exposure was highly heterogeneous. Although the present study has demonstrated the ubiquity of MP exposure, the resulting impacts on human health are unknown.

Micromechanics of Machining and Wear in Hard and Brittle Materials

Hard and brittle solids with covalent/ionic bonding are used in a wide range of modern-day manufacturing technologies. Optimization of a shaping process can shorten manufacturing time and cost of component production, and at the same time extend component longevity. The same process may contribute to wear and fatigue degradation in service. Educated development of advanced finishing protocols for this class of solids requires a comprehensive understanding of damage mechanisms at small-scale contacts from a materials science perspective. In this article the fundamentals of brittle-ductile transitions in indentation stress fields are surveyed, with distinctions between axial and sliding loading and blunt and sharp contacts. Attendant deformation and removal mechanisms in microcontact processes are analyzed and discussed in the context of brittle and ductile machining and severe and mild wear. The central role of material microstructure in material removal modes is demonstrated.

Wear behavior and microstructural characterization of translucent multilayer zirconia

OBJECTIVE

To characterize the composition, microstructure and wear properties of a multilayer translucent zirconia relative to the conventional 3Y-TZP.

METHODS

Two types of ceramics were evaluated: a multilayer zirconia (MULTI, IPS e.max ZirCAD Multi, Ivoclar Vivadent) and a control 3Y-TZP (IPS e.max ZirCAD LT, Ivoclar Vivadent). Pre-sintered CAD-CAM blocks were cut, ground, sintered and polished to 1 μm finish. The phase fraction and grain size were measured using XRD and FE-SEM. For wear testing (n = 12), square-shaped specimens (16 × 16 × 1 mm) were adhesively bonded to a dentin analog. Sliding wear tests were performed using a spherical zirconia antagonist (r = 3.15 mm), with 30 N load at 1.5 Hz for 500,000 cycles in water. Optical and scanning electron microscopes and 3D laser scanner were used for quantitative wear analyses. Data were analyzed using Student's t-test (α = 0.05).

RESULTS

For MULTI, the enamel layer had the highest cubic content and the largest grain size, followed by the two transition layers, and the dentin layer. 3Y-TZP showed the smallest grain size and cubic content. A significant amount of wear was observed in both materials up to 50,000 cycles until it reached a plateau. MULTI showed higher volume loss and greater wear depth than 3Y-TZP (p < 0.01). The higher volume loss was associated with extensive lateral fracture, leading to material spalling from the surface of cubic-containing zirconias.

SIGNIFICANCE

The wear pattern in multi-layered zirconia was more severe than 3Y-TZP. Additionally, the different layers of the multi-layered zirconia had similar wear behavior.

Effect of ionizing radiation and chewing simulation on human enamel and zirconia

PURPOSE

To evaluate the effect of ionizing irradiation on human enamel and zirconia after chewing simulation.

METHODS

Twenty enamel and twenty translucent Yttria-stabilized zirconia (Y-PSZ) specimens were divided in 4 groups: Co (control) - no irradiation on enamel cusps/opposing zirconia slabs; E70 - irradiated (70 Gray) enamel cusps/opposing irradiated enamel slabs; Z70 - irradiated zirconia cusps/opposing irradiated zirconia slabs; EZ70 - irradiated enamel cusps/opposing irradiated zirconia slabs. Cusps were abraded against slabs in a chewing simulator (CS - one million cycles, 80 N, artificial saliva, 37˚C). Wear and roughness of zirconia and enamel were analyzed using a stylus profilometer. The abraded enamel was analyzed by Electron probe micro-analyzer (EPMA) and zirconia was characterized by nanoindentation and X-ray diffraction. One-way analysis of variance (ANOVA) and Tukey test were used for analysis of wear, Repeated Measures and Bonferroni test for roughness, and hardness and modulus values were compared using Wilcoxan Mann Whitney rank sum test (overall 5% significance).

RESULTS

Significantly higher volume loss was presented by cusps in the E70 group (p<0.001). Wear was similar between Co and EZ70 groups. There was no significant effect of irradiation on roughness of enamel or zirconia slabs (p=0.072). Irradiated Y-PSZ slabs had significantly higher hardness and modulus than non-irradiated ones and a 7% increase in m phase content was detected after irradiation.

CONCLUSIONS

The opposing surface characteristics played a more significant role on enamel wear than did ionizing radiation. However, radiation affects Y-PSZ crystalline composition, hardness and modulus of elasticity.

A State-of-the-Art Review on the Wear of the Occlusal Surfaces of Natural Teeth and Prosthetic Crowns

This review focuses on the wear mechanisms of natural and restorative dental materials, presenting a comprehensive description and analysis of the works published in the last two decades on the wear at the interface of occlusal surfaces. Different groups of tribological pairs were considered: tooth-tooth, tooth-restorative material (tooth-ceramic, tooth-resin-based-materials, and tooth-metal), and restorative-restorative materials. The lack of standardization of the wear tests impairs the direct comparison of the obtained results. However, it was possible to infer about the main wear mechanisms observed on the different classes of dental materials. Concerning ceramics, their toughness and surface finishing determines the wear of antagonist tooth. Abrasion revealed to be the main wear mechanisms at occlusal interface. In the case of resin-based composites, the cohesion of the organic matrix and the nature, shape, and amount of filler particles greatly influences the dental wear. The protruding and detachment of the filler particles are the main causes of abrasion of antagonist enamel. Metallic materials induce lower wear on antagonist enamel than the other classes of materials, because of their low hardness and high ductility. Most of the studies revealed plastic deformation and adhesive wear as the main wear mechanisms. Overall, more research in this area is needed for a better understanding of the mechanisms involved at the occlusal surfaces wear. This would be essential for the development of more suitable restoration materials.

Inverse correlations between wear and mechanical properties in biphasic dental materials with ceramic constituents

The objective of this study is to elucidate the interdependence of competing mechanical degradation processes in biphasic dental materials with ceramic constituents in the region of high-pressure occlusal loading. It is hypothesized that wear resistance in this region correlates inversely with basic material parameters (modulus, hardness, toughness, strength) evaluated from 'standardized' test specimens. Ball-on-flat wear tests in simulation of oral function are used to quantify susceptibility to protracted sliding contact damage. Wear rates for this class of dental material tend to increase with quasistatic parameter values, so the latter do not provide a reliable guide to longevity. The generation of severe-wear facets involves cumulative quasiplastic deformation and microcrack coalescence at the grain level. It is implied that interplay between wear and fracture mechanisms should be an important consideration in future microstructural design of dental ceramics, especially in the quest to balance durability against esthetics.

Hardness and Wear Resistance of Dental Biomedical Nanomaterials in a Humid Environment with Non-Stationary Temperatures

This study discusses a quantitative fatigue evaluation of polymer-ceramic composites for dental restorations, i.e., commercial material (Filtek Z550) and experimental materials Ex-nano (G), Ex-flow (G). Their evaluation is based on the following descriptors: microhardness, scratch resistance, and sliding wear. In order to reflect factors of environmental degradation conditions, thermal fatigue was simulated with a special computer-controlled device performing algorithms of thermocycling. Specimens intended for the surface strength and wear tests underwent 10⁴ hydrothermal fatigue cycles. Thermocycling was preceded by aging, which meant immersing the specimens in artificial saliva at 37 °C for 30 days. Microhardness tests were performed with the Vickers hardness test method. The scratch test was done with a Rockwell diamond cone indenter. Sliding ball-on-disc friction tests were performed against an alumina ball in the presence of artificial saliva. A direct positive correlation was found between thermocycling fatigue and microhardness. The dominant mechanism of the wear of the experimental composites after thermocycling is the removal of fragments of the materials in the form of flakes from the friction surface (spalling). Hydrothermal fatigue is synergistic with mechanical fatigue.

Optimization of mechanical and tribological properties of a dental SiO2-Al2O3-K2O-CaO-P2O5 glass-ceramic

Dental glass-ceramics with main crystal of fluorapatite are usually employed as veneering porcelain. However, failure of the porcelain happens clinically in the form of fracture or excessive wear. The aims of the study were to evaluate the influence of microstructures of a SiO₂-Al₂O₃-K₂O-CaO-P₂O₅ fluorapatite glass-ceramic on its mechanical properties and tribological behaviors, and to improve the comprehensive performance by adjusting content and sintering process. The glass-ceramics were fabricated by sintering method with different CaO contents and heat treatment regimes. Phase compositions and crystal length of specimens were characterized by X-ray diffractometer and scanning electron microscope. Combined with mechanical properties, friction and wear behaviors in both dry and artificial saliva lubrication conditions were investigated. The results show that different content of CaO and heat treatment temperature could change crystallinity of main fluorapatite crystal. Larger crystallinity improves the mechanical properties, significantly influencing friction and wear behaviors. The specimens with 6.0 wt % CaO and sintered at 1100 °C have the best comprehensive performance, which show excellent mechanical properties and wear resistance.

High-translucent yttria-stabilized zirconia ceramics are wear-resistant and antagonist-friendly

OBJECTIVES

To evaluate two-body wear of three zirconia ceramics stabilized with 3, 4 and 5mol% yttria and to compare their wear behavior with that of a lithium-disilicate glass-ceramic.

METHODS

Sixteen rectangular-shaped specimens made from three grades of zirconia ceramics and a lithium-disilicate glass-ceramic were polished and dynamically loaded in a chewing simulator (2kg vertical load, 2.1Hz) under water at 90°C for 1.2×10⁶ cycles (about 7 days) in the ball-on-plate mode against steatite antagonists. Surface roughness was measured before and after wear testing. Wear tracks were scanned with a non-contact 3D profilometer and super-impositions were used to determine wear loss of the antagonists. Wear surfaces were imaged by SEM. XRD and micro-Raman spectroscopy were used to characterize phase transformation and stress status in the worn and unworn areas of the zirconia ceramics.

RESULTS

Independent of fracture toughness, strength and aging-susceptibility, the three zirconia ceramics showed a similar and limited amount of wear (∼10μm in depth) and were more wear-resistant than the lithium-disilicate glass-ceramic (∼880μm in depth). Abrasive wear without obvious cracks was observed for all investigated zirconias, whereas the glass-ceramic with a lower fatigue threshold and high susceptibility to surface dissolution exhibited significant abrasion, fatigue and corrosion wear. All three zirconia ceramics yielded a lower antagonist wear than the glass-ceramic and no significant differences were found between the zirconia ceramics.

SIGNIFICANCE

In the context of this study, high-translucent zirconia ceramics stabilized with a higher yttria content, recently introduced in the dental field, were as wear-resistant and antagonist-friendly as conventional high-strength zirconia and suitable for monolithic restorations.

Improving the wear performance of feldspathic veneering porcelain by ion-exchange strengthening

OBJECTIVES

The present study examined the effects of Na⁺→K⁺ ion-exchange on the wear performance of feldspathic veneering porcelain.

METHODS

Bar and disk specimens were prepared using IPS classic as the feldspathic veneering porcelain. After ion-exchange by immersion of the specimens in melted KNO₃ at two temperatures for different time-periods, the bars were tested for flexural strength and Vickers surface hardness. The disks were paired with zirconia antagonists and tested with a pin-on-disk tribometer with 10 N for 70☓10⁴ wear cycles in artificial saliva. Wear analysis of the porcelain and zirconia was performed using 3D profilometer and analysed with one-way analysis of variance and Tukey's post-hoc pairwise comparison procedures. Worn surfaces were examined with scanning electron microscopy.

RESULTS

The feldspathic veneering porcelain exhibited strong time-dependent wear behaviour, with typical running-in and steady wear stages. Ion-exchange treatments at 380 °C and 440 °C both enhanced the mechanical properties, decreased the wear rates of running-in wear and steady wear. The wear performance of porcelain treated by ion-exchange at lower temperature (380 °C) was improved significantly, especially reducing the wear rate of the running-in stage.

CONCLUSION

A thicker ion-exchange layer with less stress relaxation may be obtained by ion-exchange at lower exchange temperature for a long processing time. Such a protocol improves the wear performance of the porcelain effectively.

CLINICAL SIGNIFICANCE

Restorations with veneering porcelain may fail prematurely due to excessive wear. It important to improve the wear performance of the porcelain. Ion-exchange has the potential to strengthen dental veneering porcelain. Understanding the effect of ion-exchange on the wear performance of porcelain provides insight improving the wear performance of these restorations.

The Effect of Thermocycling on Surface Layer Properties of Light Cured Polymer Matrix Ceramic Composites (PMCCs) Used in Sliding Friction Pair

This paper discusses the problem of thermocycling effect of light-curing polymer–ceramic composites. Cyclic thermal shocks were simulated in laboratory conditions. As a rule, these loads were supposed to reproduce the actual conditions of biomaterials exploitation. Periodically variable stresses occurring in dental restorations are associated with the wear of cold and hot foods and beverages. They lead to changes in the properties of composites, including the properties of the surface layer. The aim of the work was to assess the impact of cyclic hydrothermal interactions on the properties of the surface layer of composites relevant to the operational quality. Two commercial materials manufactured by the world’s leading producer (3M ESPE)—Filtek Z550, Filtek Flow and two experimental, micro-hybrid and flow type composites marked Ex-mhyb(P) and Ex-flow(P), respectively. All tests were carried out before and after hydro-thermal cycles (flowing water thermocycling). Micro-hardness test using the Vickers method, indentation hardness, and resistance to tribological wear in a ball–disc system in sliding friction conditions were performed. In addition, observations of the surface layer of composites on the SEM (scanning electron microscope) were carried out. It was noticed that semi-liquid composites, containing a smaller amount of filler, retain higher stability of mechanical and tribological properties of the surface layer under cyclic hydro-thermal loads. Coefficient of friction of samples after hydro-thermal cycles increased for micro-hybrid materials and Filtek Flow (FFlow) composite. In the case of Ex-flow(P) material, the coefficient of friction decreased. The microhardness of composites also changed, the variability of this size depended on the type of material. Composites with a higher content of filler particles were characterized by greater variability of microhardness under the influence of thermocycles. The resistance to tribological wear also changed in a similar way. Composites containing higher volume fraction of inorganic filler showed higher tribological wear after thermocycling. The wear resistance of flow composites changed to a lesser extent, after thermocycling increased. The paper also showed that, in real kinematic nodes, the surface layer of light-curing ceramic–polymer composites is exposed to significant non-tribological (erosive, thermal, and chemical) defects that synergize with tribological ones. In slip pairs loaded dynamically, under mixed friction conditions, tribological wear of PMCCs (polymer matrix ceramic composites) is manifested by spalling (spalling of the material flakes, in particular the polymer phase) and pitting (crushing wear caused by wear products, in particular large filler particles or clusters, previously adhesively extracted).