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Introduction

Ceramic materials are increasingly used as dental restorations due to their excellent mechanical properties such as wear resistance and stiffness. They are also chemical inert, which allows them to have a good interaction with the physiological environment. . The most commonly used dental ceramics are zirconia, feldspathic porcelain, glass-ceramics and glass-infiltrated alumina which are esthetically similar to the teeth they are replacing .

Zirconia is the only dental ceramic that meets the value of flexural strength requirements for fixed partial dentures (FPDs) recommended by the International Organization for Standardization (ISO, 1999) . Zirconia has the potential to withstand higher functional stresses imposed within the oral cavity . Among different zirconia alloys, tetragonal zirconia doped with 3% mol of yttria (3Y-TZP) is recommended as a framework material for crowns and FPDs due to their high strength, fracture toughness and easy machinability in a pre-sintered state . Zirconia is also an alternative to the currently used ceramic-metal restorations . However, due to its limited translucency, it must be coated with porcelain to meet esthetic demands.

Feldspathic dental porcelains are used as monolithic restorations (inlays, onlays and veneers) and especially for veneering of crowns and FPDs, because of their appearance (color and translucency ). Zirconia crowns coated with feldspathic porcelains have an increased demand for their use as dental prosthesis because of their appearance is very similar to natural teeth . However some studies of zirconia-based restorations reported failure due to chipping and delamination of the veneering porcelain , with up to 30% of failures occurring after 2 to 5 years of use . Chipping is a typical failure of contact loadings, normally produced when a crack generated or propagated by contact loads deflects due to the presence of a free surface nearby . These failures are attributed to different factors, such as low fracture toughness, inappropriate framework support, low cohesiveness and shear forces between the zirconia framework and veneering porcelain .

However, among the different reasons for failure of these coatings, contact loads are the responsible for generating and propagating a crack that will later result in chipping. Contact loads are generated between opposing teeth during oral functions such as chewing, grinding (bruxing) and crunching . These contact loads result in cracks in the porcelain, which can act as initial flaws from which fracture of the dental prosthesis may be started.

During the mastication process, the magnitude of the forces range from around 3–364 N, with a duration of 0.25–0.70 s over cuspal radii of 2–4 mm. It is estimated that the contact loading period per each day is around 30 min . Some authors reported that loads as high as 600 N in normal chewing and close to 900 N in bruxism . Similarly, it has been reported that during normal chewing, the load applied at initial contact in the teeth is usually in the range 10–20 N, and increases to higher values depending mainly on the type of the food, age and sex of the person .

Due to the range of factors affecting the expected lifetime of the prosthesis, it is important to characterize the damage and cracks produced under various contact loading conditions to understand the causes of failure, and in turn design better components.

One of the main techniques for studying contact mechanics is spherical indentation (Hertzian Indentation), where a sphere is pressed against a flat surface, simulating the load conditions inside of mouth as masticatory force and cuspal curvature of the teeth . When a material is under contact load, it can deform plastically or fracture before deformation, and the crack produced propagates inside the material at an angle with the substrate, adopting the shape of a truncated cone. This crack can fracture the entire prosthesis, but in most of cases acts as an initial defect which can cause the complete failure of the prosthesis at a later date.

The comprehensive understanding of the shape and length of the crack, as well as the damage produced by indentation, is crucial for developing new materials with long term structural integrity and reliability for dental prostheses. Previous studies have reported on the damage generated by indentation of transparent model materials, such as, polycarbonate, epoxy resin and soda-lime glass . However, porcelain coatings are not transparent, contain a small degree of porosity and are composed of leucite crystalline grains in a glassy matrix, which makes this material different from the model materials previously used in the literature.

Other studies have observed the crack beneath the indentation using a bonded interface technique or polishing in the cross-section . In the case of the bonded interface samples, there is an inherent limitation caused by the fact that the stress state is not the same as in the bulk specimen. In addition, especially for layered materials, the reduced size of the samples makes it difficult to match the layers correctly during the bonding. In the transverse polishing method, the main drawback is the damage caused to the material during the polishing routine .

Tomographic reconstructions, on the other hand, provide full 3D information of the damage produced, and, if performed with care, the microstructural information is not altered. There are several techniques that are capable of providing data for tomographic reconstruction, with different resolutions and maximum volumes; these include atomic probe tomography, transmission electron microscopy, focused ion beam (FIB) tomography, X-Ray tomography and sequential polishing .

The objective of this work is to characterize the damage produced by contact loading in commercial feldspathic porcelain VITA-VM9 (VITA Zahnfabrik) coating on zirconia stabilized with 3 mol% yttria (3Y-TZP) by tomographic techniques. Spherical indentations will be examined by sequential mechanical polishing to observe long cracks in the millimeter range, and cube corner nanoindentations will be examined by FIB tomography to observe short cracks and the interaction of the crack with the small leucite particles in the micrometer range.