Both natural and synthetic prosthetic teeth undergo mechanical degradation, impacting their durability. Experimental studies typically simulate dental contacts using simple configurations involving normal and lateral forces. While often necessary due to the constraints of apparatus set-ups and mathematical models, these assumptions oversimplify the complex conditions during mastication and ignore poorly understood but potentially important rotational forces, which occur when teeth are compressed into the alveolar bone. We investigate the influence of rotational forces on contact damage/wear in synthetic dental materials using advanced equipment with decoupled biaxial actuators. Cyclic contact loads combining compression (50 N) and rotation (30°) are applied to zirconia (Z), composite (CP), feldspathic (F) and lithium silicate based (ZLS) glass-ceramics. After 105 cycles, Z exhibits the greatest wear resistance (wear volume 4.16 × 10-4 mm3), followed by F (5.83 × 10-3 mm3), CP (9.17 × 10-3 mm3) and ZLS (1.64 × 10-2 mm3), with p-values 0.004 (Z-F), 0.631 (F-CP), 0.012 (F-ZLS) and 0.009 (CP-ZLS). Abrasion is the primary wear mode, with specific mechanisms such as plastic deformation and microfracture varying with material microstructure. Contact mechanics analysis indicates that rotational forces induce lower wear than non-rotational sliding. Potential implications in dentistry, biology and anthropology are discussed, including the design of culturally and behaviourally informed dental prosthetics.

Polymer-ceramic composites are widely employed in dental prostheses. However, due to low hardness values, their lifetime is often limited by wear. Here, the wear of commercially available dental CAD/CAM composites is explored through in-vitro tests involving extended sliding contact with a zirconia antagonist. The predominant wear mode is abrasion, with specific mechanisms including plastic deformation at the asperity level and microcracking. The extent of damage exhibits significant variability as a function of materials’ microstructure, resulting in wear volume differences of up to two orders of magnitude. The findings are analyzed within the framework of tribology and contact fracture mechanics. Strategies for enhancing durability are discussed, emphasizing potential microstructural engineering approaches—increasing hardness, improving particle-matrix adhesive strength, and optimizing particle aspect ratio.

Chipping caused by micrometric particles poses a threat to the structural integrity of modern dental prosthetic materials. It can degrade their fracture strength and cause wear of both artificial crowns and antagonist teeth. Here, surface chipping of the main types of commercial ceramic-based dental materials at the microcontact/particle level is investigated by means of indentation tests. Conical tips of different sizes (radii 20 and 200 mu m) under axial and sliding loading are employed to simulate individual microcontacts. Both decreasing particle size and adding a lateral contact force decrease the chipping load below typical bite forces. Specific damage mechanisms are identified as predominantly brittle fracture in ceramics with small, equiaxed crystals, with significant quasi-plastic damage in ceramics containing large, elongated crystals and composites. Critical loads for the occurrence of chipping are quantified (lowest values in equiaxed glass-ceramics; greatest in zirconia) and analyzed within the framework of fracture mechanics. The brittleness index (BI) is proposed as a simple indicator of the resistance to chipping of dental materials-the lower the BI, the greater the resistance. Special attention is paid to the effect of the materials' microstructure, which can result in transformation toughening (as in zirconia) or quasi-plastic behavior (as in lithium disilicate), both highly beneficial to increasing the chipping resistance. Finally, practical implications for the selection of current dental materials as well as for the development of novel materials with improved durability are discussed.

Human tooth enamel must withstand the cyclic contact forces, wear, and corrosion processes involved with typical oral functions. Furthermore, unlike other human tissues, dental enamel does not have a significant capacity for healing or self-repair and thus the longevity of natural teeth in the oral environment depends to a large degree on the fatigue and wear properties of enamel. The purpose of this review is to provide an overview of our understanding of the fatigue and wear mechanisms of human enamel and how they relate to in vivo observations of tooth damage in the complex oral environment. A key finding of this review is that fatigue and wear processes are closely related. For example, the presence of abrasive wear particles significantly lowers the forces needed to initiate contact fatigue cracking while subsurface fatigue crack propagation drives key delamination wear mechanisms during attrition or attrition-corrosion of enamel. Furthermore, this review seeks to bring a materials science and mechanical engineering perspective to fatigue and wear phenomena. In this regard, we see developing a mechanistic description of fatigue and wear, and understanding the interconnectivity of the processes, as essential for successfully modelling enamel fatigue and wear damage and developing strategies and treatments to improve the longevity of our natural teeth. Furthermore, we anticipate that this review will stimulate ideas for extending the lifetime of the natural tooth structure and will help highlight where our understanding is too limited and where additional research into fatigue and wear of human tooth enamel is warranted.

Titanium alloys are widely used in parts of dental implants, such as screws and abutments. In practice, unwanted relative sliding between contacting implant parts can cause excessive wear, which may lead to early failure. The effect of a submicron diamond-like carbon (DLC) coating on the friction and wear of Ti6Al4V alloys under sliding contact in artificial saliva was investigated. Critically, the DLC film suppressed adhesion between contacting surfaces, significantly lowered the coefficient of friction and contact stress, and ultimately the wear rate, relative to uncoated Ti6Al4V, while maintaining good film-substrate bonding and undergoing a limited extent of fracture. Results are explained within the framework of contact mechanics. Implications for the development of durable dental implants are discussed.

Aesthetic glass-ceramics are widely employed dental materials, both in bulk form and as veneers. As they are highly brittle, the durability of these materials is limited by fracture and wear processes originated from contacts with opposing dentition and/or third-body particles during mastication and bruxism. This work investigates the resistance to fracture of commercial dental glass-ceramics under sliding contact, simulated by means of scratch tests. It finds that materials with relatively larger crystals (feldspathic and leucite) require a lower stress to fracture upon sliding than lithium silicates containing smaller (but more elongated) crystals, due to their larger defects and lower toughness. Results are analyzed as a function of material microstructure within the framework of Weibull theory and fracture mechanics. Implications for materials selection and development in prosthetic dentistry are briefly discussed.

Zirconia-reinforced lithium silicates (ZLS) are a novel generation of aesthetic, lithium-based ceramics intended for use in dental restorations at high loads due to their enhanced fracture strength compared to the previous lithium disilicate (LS2) materials. However, under severe dental conditions, material durability in service may be limited by wear from repetitive, frictional contacts. Here, a comparison is made between commercial ZLS and LS2 dental ceramics of their severe wear behavior under sliding contact against hard zirconia antagonists by means of in-vitro pin-on-disk tests. Specific wear rates greater than 10(-6) mm(3)/N.m are obtained. The mechanisms responsible for material removal are a combination of deformation and fracture. Results are discussed in terms of the materials' microstructure and mechanical properties. In combination with analytical modelling, ZLS dental ceramics are estimated to be up to 5 times less durable than LS2 under conditions modelling heavy chewing/ bruxism. Finally, implications for the microstructural design of novel lithium silicate/disilicate materials with improved durability are considered.

Three-dimensional multi-scale finite element models were designed to examine the effects of geometrical structure variations on the damage onset in cortical bone at multiple structural scales. A cohesive zone finite element approach, together with anisotropic damage initiation criteria, is used to predict the onset of damage. The finite element models are developed to account for the onset of microdamage from the microscopic length scales consisting of collagen fibres, to the macroscopic level consisting of osteons and the Haversian canals. Numerical results indicated that the yield strain at the initiation of microcracks is independent of variations in the local mineral volume fraction at each structural scale. Further, the yield strain and strength properties of cortical bone are dependent on its structural anisotropy and hierarchical structure. A positive correlation is observed between bone strength and mineral content at each length scale.

Despite numerous published studies on the wear of dental composites, few have considered the influence of temperature on the two-body wear process. Additionally, no previous work has considered the influence of temperature on dominant wear mechanisms during the consumption of hot substances, hence the focus of this study. Reciprocating wear tests were carried out at varying artificial saliva lubricant temperatures (37 and 57°C) and material loss was quantified using profilometry. The wear tracks were analysed using FIB/SEM/TEM. Results reveal that the wear rate of a dental composite can significantly increase with temperature, with fatigue/delamination and ploughing acting as dominant mechanisms.

The fracture properties of 0.94(Na0.5Bi0.5)TiO3-0.06BaTiO(3) (NBT-6BT) relaxor ferroelectrics were investigated using the Vickers indentation method and computation of crack tip opening displacement. It was found that an unpoled sample had a fracture toughness of around 1.35 MPa m(1/2). In contrast, an electrically poled sample exhibited anisotropy with a lower fracture toughness perpendicular to the poling direction and a higher value in the parallel direction, as compared to the unpoled sample. Upon cyclic electrical loading (with applied electric field amplitudes between 0.73E(C) and 1.4E(C)), the indented surface crack was found to propagate. In general, the crack grew rapidly during the initial cycles followed by crack arrest, and the principal driving force for crack growth was proposed to be residual stress around the indentation, as evidenced by the limited field dependence of crack growth. There was also a contribution from the electromechanical strain, which played a role at high cycles (>100 cycles) and high fields (>1.3 E-C). Evidence of a saturation threshold of crack propagation is an advantage for the electromechanical reliability of relaxor ferroelectrics in devices.

Due to the complexity of the oral environment, understanding the relationship between the enamel microstructure and corrosion behavior of mammalian teeth is important for the development and bionic design of novel dental materials with corrosion resistance. In this paper, enamel microstructure and its influence on corrosion and tribocorrosion behavior of bovine and human molars are studied using focused ion beam & scanning electronic microscopy (FIB-SEM), transmission electron microscope (TEM), nanoindentation, and wear testing. It was found that the regions with transversal HAP nanocrystallites have better corrosion resistance than those with longitudinal crystallites. Apart from the chemical composition, the microstructure of enamel also involves in its surface corrosion. HAP nanocrystallite orientation changes the acid permeability into the enamel surface. Transversal nanocrystallites result in zigzagged acid permeation channels to increase the difficulty of acid penetration and then improve the corrosion resistance of enamel surface. The higher the fraction of transversal nanocrystallites on a surface, the better is the corrosion resistance. These findings extend our understanding of the corrosion/wear mechanisms of dental enamel with different microstructures.

The electric field response of the lead-free solid solution (1−x)Bi0.53Na0.47TiO3–xBaTiO3 (BNT–BT) in the higher BT composition range with x = 0.12 was investigated using in situ synchrotron X-ray powder diffraction. An introduced Bi-excess non-stoichiometry caused an extended morphotropic phase boundary, leading to an unexpected fully reversible relaxor to ferroelectric (R–FE) phase transformation behavior. By varying the field frequency in a broad range from 10−4 up to 102 Hz, BNT–12BT showed a frequency-dependent gradual suppression of the field induced ferroelectric phase transformation in favor of the relaxor state. A frequency triggered self-heating within the sample was found and the temperature increase exponentially correlated with the field frequency. The effects of a lowered phase transformation temperature TR–FE, caused by the non-stoichiometric composition, were observed in the experimental setup of the freestanding sample. This frequency-dependent investigation of an R–FE phase transformation is unlike previous macroscopic studies, in which heat dissipating metal contacts are used.

The occupationally disordered structures and associated local polar fluctuations in lead-free relaxors determine their electrical properties that are also sensitive to external stimuli. These stimuli can lead to phase transitions, and the associated enhancement in the electro-mechanical responses necessitate a better understanding of these transitions. Here we report a non-canonical spontaneous phase transition from a relaxor to a ferroelectric phase in (Na1/2Bi1/2)TiO3-BaTiO3 with temperature. With the help of experiments (dielectric permittivity, diffraction, differential scanning calorimetry, polarization and Raman spectroscopy), a complete picture of the temperature evolution of relaxor behavior leading to this spontaneous phase transition has been reported. Furthermore, it has been shown that internal chemical pressure from oxygen vacancies can be utilized to tailor these phase transitions. Finally, an electric field-temperature phase diagram has been proposed with an emphasis on the influence of the defect chemistry. This work provides new insights into the origin of these spontaneous phase transitions.

The reduction of degradation rate at high cycles indicates the subsequent contribution from micro-scale surface damage, leading to irreversible fatigue.

In this work, the lead-free composition (1-x)Bi0.5Na0.5TiO3–xBaTiO3 (BNT–BT) with x = 0.12 was investigated using in situ Synchrotron x-ray powder diffraction. With the applied electric field, the pseudo-cubic relaxor phase reversibly transforms to a ferroelectric state. The reversibility is still preserved after 104 bipolar electric field cycles. A Rietveld refinement with a structure, strain, and texture analysis using a model based on the atomic scale was applied for four frequencies from 10−4 to 101 Hz. The analysis allowed us to separately determine the two coexisting phases, their electric field dependent evolution, and the underlying strain mechanisms. For all the applied frequencies, we showed that domain switching is the only strain mechanism appearing in the tetragonal phase and the lattice strain is the only mechanism in the rhombohedral phase. The coercive field of the tetragonal phase (4 kV/mm) is found to be higher than that of the rhombohedral phase (3 kV/mm). This divergence has not been observed in previously investigated lead-containing materials and cannot be detected solely using macroscopic strain and polarization experiments. Moreover, the domain strain abruptly starts to occur only after a threshold field value and exhibits high hysteresis. The lattice strain, on the other hand, starts nearly from the beginning and increases more linearly during the bipolar field cycle. It could, therefore, be demonstrated that complex structural mechanisms underlie the apparent clear and continuous macroscopic strain curve. These findings are crucial for all actuator materials undergoing a relaxor to ferroelectric phase transformation and provide approaches and strategies to optimize lead-free materials for tailored applications.

This paper investigates the tribological and mechanical properties of silk-based nanocomposite coatings which are finding applications in optics, biomedicine and dentistry, thanks to the exceptional mechanical/optical properties and associated biocompatibility of silk. Three different nanocomposite formulations were synthesized, and thin films were prepared by spin coating at different thicknesses and with different post-deposition annealing processes. Ellipsometry, FTIR spectroscopy, AFM, nanoindentation, scratch testing, continuous/reciprocating wear testing, confocal microscopy and SEM were used to characterize the coatings. The results reveal that their hardness and elastic modulus are in the range 0.56-1.30 GPa and 23.6-55.4 GPa, respectively, which are much higher than those reported for other silk films in literature. Incorporation of titanate nanosheets also improved coatings' scratch resistance.

We demonstrate a unique capability to control the formation and properties of skin layer structures in relaxor ferroelectrics by adjusting defect concentration. It is shown that the skin layer is polar and both electrically and optically active.

Multi-scale finite element analysis is performed to ascertain the effect of geometrical changes at multiple structural scales on the mechanical properties of cortical bone. Finite element models are developed, with reference to experimental data from existing literature, to account for bone's viscoelastic behaviour and anisotropic structure from the most fundamental level of bone consisting of mineralised collagen fibrils, up to the macroscopic level consisting of osteons and the Haversian canals. A statistical approach is incorporated to perform sensitivity analyses on the effects of different geometrical parameters on the effective material properties of cortical bone at each length scale. Numerical results indicate that there is an exponential correlation between the mineral volume fraction and the effective stiffness constants at each length scale. This contributes to the exponential behaviour of the instantaneous moduli describing cortical bone's two-phase stress relaxation process: a fast and slow response relaxation behaviour. Results indicate that the fast response relaxation time is independent of bone's structural anisotropy, whilst being dependent on variations in the global mineral volume fraction between length scales. However, the slow response relaxation time is independent of the changes in mineral volume fraction. It is also observed that the slow response relaxation time varies with bone's anisotropic structure, and therefore, contributes to the anisotropic properties of bone.

Similar to several Pb-based relaxor ferroelectrics, 0.94Na0.5Bi0.5TiO3–0.06BaTiO3 is reported to have a micrometer-sized surface layer. We hypothesize that since this layer has structural properties distinct from the bulk, it would undergo a different property degradation than the bulk during cyclic electrical loading or fatigue. First, we show the existence of a surface layer by comparing X-ray diffraction patterns of the ceramic surface and powders. Then, we show that fatigue damage is mainly localized in the surface layers, and thus, property degradation due to fatigue can be recovered on removing the affected surface layer. We also show that ion migration may be occurring in the surface layer during fatigue experiments using secondary ion mass spectroscopy, where the ion sources may be the sample itself, the electrode layer or the insulating oil in which the experiment is performed. Finally, we show that permanent fatigue damage such as microcracks is dependent on the choice of electrodes. While permanent damage was observed for Pt electrodes, it was not present for oxide electrodes, suggesting that oxygen permeation and accumulation at the electrode/surface interface may play a role in the formation of observed microcracks. In summary, we have shown that fatigue is influenced by the surface layer, and surface layer damage can be controlled using the selection of electrodes.

This paper investigates the wear behaviour of remineralised human dental enamel. Polished enamel flat surface samples were first demineralised in an acid solution and then remineralised in whole human saliva. They were opposed by enamel cusps in wear tests. Wear depth was measured by a profilometer and the wear surface and subsurface were examined by electron microscopy. The results show that the pores in the top surface layer collapse during wear and form a particle layer which is subsequently removed by ploughing/delamination. For enamel underneath, the dominating wear mechanism is delamination. Remineralised enamel is extremely vulnerable since the measured wear rate is 4 - 15 times that for sound enamel.