Apparent fracture toughness of dental porcelain with a metal substructure

ADM guidance-Ceramics: Fracture toughness testing and method selection

OBJECTIVES

The objective is within the scope of the Academy of Dental Materials Guidance Project, which is to provide dental materials researchers with a critical analysis of fracture toughness (FT) tests such that the assessment of the FT of dental ceramics is conducted in a reliable, repeatable and reproducible way.

METHODS

Fracture mechanics theory and FT methodologies were critically reviewed to introduce basic fracture principles and determine the main advantages and disadvantages of existing FT methods from the standpoint of the dental researcher.

RESULTS

The recommended methods for FT determination of dental ceramics were the Single Edge "V" Notch Beam (SEVNB), Single Edge Precracked Beam (SEPB), Chevron Notch Beam (CNB), and Surface Crack in Flexure (SCF). SEVNB's main advantage is the ease of producing the notch via a cutting disk, SEPB allows for production of an atomically sharp crack generated by a specific precracking device, CNB is technically difficult, but based on solid fracture mechanics solutions, and SCF involves fracture from a clinically sized precrack. The IF test should be avoided due to heavy criticism that has arisen in the engineering field regarding the empirical nature of the calculations used for FT determination.

SIGNIFICANCE

Dental researchers interested in FT measurement of dental ceramics should start with a broad review of fracture mechanics theory to understand the underlying principles involved in fast fracture of ceramics. The choice of FT methodology should be based on the pros and cons of each test, as described in this literature review.

[Relative fracture toughness of differents dental ceramics]

Although ceramics present high compressive strength, they are brittle materials due to their low tensile strength so they have lower capacity to absorb shocks. This study evaluated the fracture toughness of different ceramic systems, which refers to the ability of a friable material to absorb defformation energy. Three ceramic systems were investigated. Ten cylindrical samples (5,0mm x 3,0mm), were obtained from each ceramic material as follows: G1- 10 samples of Vitadur Alpha (Vita-Zahnfabrik); G2- 10 samples of IPS Empress2 (Ivoclar-Vivadent); G3- 10 samples of In-Ceram Alumina (Vita-Zahnfabrik). Fracture toughness values were collected upon indentation tests that were performed under a heavy load. A microhardness tester (Digital Microhardness Tester FM) utilized a 500gf load cell during 10seconds to perform four impressions on each sample. Statistically significant results were observed (ANOVA and Kruskal-Wallis tests). In-Ceram Alumina presented the highest median toughness values (2,96N/m3/2), followed by Vitadur Alpha (2,08N/m3/2) and IPS Empress2 (1,05N/m3/2). It may be concluded that different ceramic systems present distinct fracture toughness values, thus In-Ceram is capable of absorbing superior stress when compared to Vitadur Alpha and IPS Empress2.

Comparison of three fracture toughness testing techniques using a dental glass and a dental ceramic

OBJECTIVES

Various methods aimed at determining the fracture toughness of ceramics in mode I (KIc) have been described in the literature. The accuracy, scatter and the interexaminer reproducibility of KIc depend strongly on the procedural approach, the test parameters used and the conditioning of the specimens. The purpose of the present study was to compare fracture toughness values obtained using two indentation methods as well as a newly established fracture mechanics test.

METHODS

The following methods for KIc determination were applied: (1) indentation fracture (IF), (2) indentation strength (IS) and (3) the single-edge-V-notched-beam test (SEVNB). The materials tested were a low-fusing dental glass (Duceram LFC) and a feldspar-based porcelain (IPS classic). Data were compared by ANOVA and Tukey's multiple comparison test (p < or = 0.05).

RESULTS

For both materials, KIc coefficients of variation ranged between 10 and 14% for IF and 7 and 10% for IS. The IS technique demonstrated a load dependency for the IPS porcelain which was not observed when using the IF method. The SEVNB test provided consistent results with coefficients of variation between 1 and 3%. SEVNB toughness values for the IPS porcelain were in agreement with the IS technique. However, halfpenny shaped cracks were observed at the tip of the notch of all LFC specimens thus leading to underestimated KIc values.

SIGNIFICANCE

The overall aim of this type of study is to select testing procedures that are as expedient and reliable as possible. This study has shown that all three methods agreed within 10%. However none of the procedures proved absolutely straightforward. Decision on which method to use should be based on a sound understanding of the conceptual limitations and technical difficulties inherent to each technique.

Relative fracture toughness and hardness of new dental ceramics

Dental ceramics can fail through growth of microscopic surface flaws that form during processing or from surface impact during service. New restorative dental ceramic materials have been developed to improve resistance to crack propagation. Eleven of these improved materials with the common feature of a considerable amount of crystalline phase in the glassy matrix were evaluated. The ceramic materials studied included fluormica-, leucite-, alumina-, and zirconia-reinforced glasses. The relative hardness and fracture toughness were determined by indentation technique. Alumina-reinforced materials resulted in the highest fracture toughness values, whereas the fluormica- and leucite-reinfoced materials showed more moderate but statistically significant greater values compared with those of control materials. The hardness values of ceramic materials with improved fracture toughness were both substantially higher or lower than those of the control groups and suggested a lack of direct correlation between these two properties. Selection of appropriate restorative materials depends on clinical application and requires consideration of several physical properties including fracture toughness.

A review of the strength properties of dental ceramics

New ceramic materials for restorative dentistry have been developed and introduced in recent years. This article reviews advantages and disadvantages of dental ceramics, concentrating on strength properties. Included are factors affecting the strength of dental ceramic materials and the most common mechanisms for increasing the strength of dental ceramics. The properties of presently available materials such as dispersion-strengthened ceramics, cast ceramics, and foil-reinforced materials are discussed. Current research efforts to improve the fracture resistance of ceramic restorative materials are reviewed. A description of methods to evaluate the strength of ceramics is included, as a caution concerning the interpretation of strength data reported in the literature.

Apparent fracture toughness of all-ceramic crown systems

This study measured apparent fracture toughness (Kc) and elastic modulus-to-hardness ratio (E/H) of dental restorations made with five all-ceramic crown systems. An indentation technique was used whereby Kc was calculated from the length of radial cracks formed on the release of a Vickers Diamond. Twenty-four crowns were made by using five different crown systems: Cerestore, DICOR, Hi-Ceram, Renaissance, and Vitadur-N. After the crowns were fired and glazed, they were embedded, polished, gold-coated, and indented under oil. In addition Knoop indentations were made for E/H calculation. Cerestore had a mean Kc of 1.79 MPa.m1/2 and E/H of 13.9, DICOR 2.10 and 26.8, Hi-Ceram 1.96 and 17.8, Renaissance 2.02 and 24.2, and Vitadur-N 1.96 and 22.3. Statistically significant differences (p less than 0.05) were found between DICOR and Cerestore materials for Kc and between DICOR, Renaissance, and Cerestore materials for E/H. For comparison, previous work found Vita VMK metal ceramic crowns to have a mean Kc of 1.91 MPa.m1/2 and E/H of 20.2.