This guidance document describes the specific issues involved in dental multilayer ceramic systems. The material interactions with regard to specific thermal and mechanical properties are reviewed and the characteristics of dental tooth-shaped processing parameters (sintering, geometry, thickness ratio, etc.) are discussed. Several techniques for the measurement of bond quality and residual stresses are presented with a detailed discussion of advantages and disadvantages. In essence no single technique is able to describe adequately the all-ceramic interface. Invasive or semi-invasive methods have been shown to distort the information regarding the residual stress state while non-invasive methods are limited due to resolution, field of focus or working depth. This guidance document has endeavored to provide a scientific basis for future research aimed at characterizing the ceramic interface of dental restorations. Along with the methodological discussion it is seeking to provide an introduction and guidance to relatively inexperienced researchers.
The definition of an “interface” in dentistry is not exclusively related to adhesive joints but also includes sintered, soldered and welded joints. Sintered interfaces are commonly found in prosthetic Dentistry, especially in fixed partial dentures (FPDs), where metal or ceramic core frameworks are coated with a veneering ceramic in order to enhance the overall esthetic quality of the final prosthesis. Such interfaces are very complex, and the literature repeatedly addresses issues related to the performance of metal-ceramic and ceramic-ceramic interfaces. Yet, the published research, giving rise to the scientific progress in this field, are mostly based on metal-ceramic systems. In contrast, this guidance document aims to review the principal knowledge and understanding of the specific topic of all-ceramic interfaces.
Know how on the process of veneering has been around long before its application in dentistry. Vitreous glazes and enameling technologies were invented in ancient Japan and applied to clay or metals either for aesthetic reasons or simply for sealing and making them waterproof and chemically stable. Nowadays, this type of coating layer has a major application on white wares; therefore there are many publications dealing with this subject. Hence, much knowledge has accumulated from this early research on glazes and enamels. Classic problems such as coating delamination has been identified and related to a mismatch in thermal gradients between framework and veneer ceramics . In contrast to industrial coatings, veneering layers in Dentistry are quite unique, since they show large variations in thickness and shape (curvatures) according to the design of each individual FPD ( Fig. 1 ).
Ceramic veneers serve many clinical purposes and improving the esthetic appearance of frameworks is one of them. In fact, adding a layer of a glassy feldspathic ceramic to the surface of a zirconia framework results in a very natural looking restoration. In addition, veneering materials are usually fine-grained, reinforced glass-ceramics, with the hardness and abrasiveness that more closely resembles that of natural, human enamel .
Characterization of multilayer Interfaces
Where tooth has been restored with a ceramic crown, this prosthesis can contain several interfaces with the core-veneer interface being a focal point of the fracture susceptibility of the restoration. The characterization and quantification of the interfacial adhesion and the assessment of internal stresses are of central interest in assessing the potential clinical success or failure of veneered restorations. Several techniques and protocols have been developed in order to measure the “strength” of the bond between the different layers of a prosthesis. Such methodologies are mostly indirect tests that promote crack initiation at the interface between two materials, either via mechanical or dynamic thermal loading. The tests most commonly reported in the dental literature are based on the so-called “crunch-the-crown” principles . This type of test, however, due to the complex geometry of veneered restorations, is not very discriminatory and sensitive to many variables, providing only little information regarding the underlying principles and mechanisms that lead to fracture of the specimen . Additionally, the load-to-failure data far exceed the maximum masticatory forces found in vivo, having limited clinical relevance. Yet, the “crunch-the-crown” test may be used to rank materials and compare different crown designs.
If a study’s purpose is to understand the detailed behavior of the interface then a standardized interface characterization needs to be carried out. This is usually achieved with uni- and biaxial strength testing of veneered flat specimens (bend bars or discs) . Such experimental designs allow for variations in layer thickness and orientation, crack initiation and crack path development.
Interfacial strength testing
Different types of clinical failures are reported for layered ceramic prostheses. While FPDs fail predominantly from the gingival side of the connector , single crowns most frequently show their weakest link either at the gingival margins or at the occlusal surface due to wear degradation. The underlying failure modes are thereby based on stress concentration associated with high bending moments , occlusal impact loading, resulting in either Hertzian cone cracks or chipping fractures originating from the contact loading site . A deeper insight in the fracture mechanisms is provided in a separate guidance document on clinical fractography. In vitro simulation of relevant clinical failure modes require a setup that more closely resembles the clinical situation. In the case of veneered or layered prostheses, several methodologies are available to assess the mechanical performance of different materials combinations. Although being far from a clinical relevance, uniaxial or biaxial bending tests , delamination tests via flexural bending techniques , and even structural testing of crowns and bridges are examples of standardized methodologies providing answers to specific and clinically relevant questions about the interface. Several test variables such as thickness ratio , loading speed and direction or mismatch between elastic and thermal properties are usually added to this type of experimental setup. In addition to the external loading scenario, the quality of a multilayered specimen and the corresponding interface is strongly dependent on the internal residual stress state . Therefore, when selecting an appropriate experimental study setup, one needs to consider all previously mentioned aspects in order to successfully simulate the clinical performance of multilayered all-ceramic restorations and draw relevant, clinically significant conclusions.
Interfaces play an important role in determining the mechanical performance and long-term durability of multilayer ceramic components, such as all-ceramic prostheses constituted of a veneered framework. The strength and fracture toughness of the interface is thereby of central importance and the failure mode may involve cracks running either along the interface (adhesive delamination) or being deflected within the veneering layer in the vicinity of the interface (cohesive failure of the veneering ceramic). The respective failure mode depends on geometrical factors and the loading scenario, but is mainly determined by the interfacial fracture toughness and the elastic moduli of the framework and veneering ceramics . Theoretically, a critical surface or subsurface crack could propagate straight through the interface once the interfacial toughness (stored fracture energy) exceeds the critical level; in fact, the bilayer behaves like a quasi-homogeneous material . Depending on other factors, such as the residual tensile stress distribution in the veneering layer, cracks may deflect before reaching the interface causing delamination, with the crack sometimes running parallel to the interface. . Such a fracture process is determined by the elastic mismatch that exists between the two layers when loaded . Any elastic mismatch between both layers, which can be high for veneered zirconia bilayers, results in a significant concentration of tensile stresses at the interface. In addition, high stress levels will build-up close to the interface upon external loading . When evaluating the mechanical behavior of ceramic bilayers in vitro, it is important to simulate the same crack propagation patterns found in clinical fractures, therefore this guidance document discusses the most relevant variables involved in this type of experimental work.
Uniaxial and biaxial testing of multilayers
The most common mechanical approach to assess the structural integrity of multilayer interfaces is derived from the standardized bending test. A separate ADM guidance document presents the principles of strength testing in detail. Standardized beam geometries may vary in order to produce layers at different thickness ratios. The experimental boundaries remain similar to those used for three- or four-point bending tests. However, the mathematical solution for a multilayer specimen changes due to the variation in elastic properties of the different layers. The equation for calculation of the maximum tensile stress σ of monolithic bend bars in the three-point bend test is based on the elastic beam theory in which both the moment of bending (M) and the moment of inertia (I) are related to the position of the neutral axis (Y) in the specimen:
σ = M Y I