ABSTRACT
Zirconium dioxide, or zirconia, ZrO2, is the word in present-day dentistry. We may say that zirconia is a material of choice in contemporary restorative dentistry for several reasons. Moreover, restorative dentistry is about adhesion promotion and about durable bonding of restorations. Zirconia has found wide applications in dental restorations, such as bridges, crowns, dental implant abutments, and full dental implant systems. This is due to its special and unique properties: high fracture toughness, light color, radiopacity, and, most importantly, biocompatibility. Nevertheless, the restoration interfaces, porcelain-to-zirconia and zirconia-to-resin cement systems, have challenged the dental community and are discussed in this chapter, which also presents bonding mechanisms and the mechanical tests to evaluate these adhesive interfaces. With the advance in zirconia research and the development of its medical uses, the prospects of zirconia as a dental biomaterial are auspicious.
15.1.1 Zirconium and Its AlloysZirconium (Zr) is an element, a transition metal, with an atomic number of 40 (atomic mass 91.22) and with a strong tendency for the maximum valence, +IV (Fig. 15.1). Zirconium is a very strong, ductile, malleable, and lustrous silver-gray metal (Fig. 15.2). Interestingly, its chemical and physical properties are similar to those of titanium (Ti). Ti and Zr are the only two metals commonly used in implant dentistry that do not inhibit the growth of osteoblasts, the bone-forming cells that are essential for osseointegration. Zirconium found in nature is composed of five isotopes. 90Zr, 91Zr, 92Zr, and 94Zr are stable. 94Zr can undergo so-called double beta decay with a half-life of over 1.10 × 1017 years. 96Zr has a half-life of 2.4 × 1019 years, which makes it the longest-lived radioisotope of Zr. Of these natural isotopes, 90Zr is the most common, making up 51.45% of all zirconium. The isotope 96Zr is the least common, comprising only 2.80% of zirconium [1]. Zirconium is extremely resistant to corrosion and heat, it is lighter than steel, and its hardness is similar to copper (Cu). Surprisingly, when Zr is finely divided, the metal can spontaneously ignite in air, especially at elevated temperatures. Zirconium and its salts generally have low systemic toxicity: the estimated dietary intake is about 50 µg. Most passes through the gut without being adsorbed, and that which is adsorbed tends to accumulate in the skeleton. It has been claimed that the strength of pure titanium is insufficient for use as an artificial hip joint as compared to Co-Cr alloys. Ti-Zr binary alloys have been investigated to evaluate their possible use as biomedical materials with promising results [3]. The Ti-40Zr alloy was shown to have better mechanical properties than Ti, excellent elastic recovery capability, and improved grindability at low grinding speed. It was concluded that the Ti-40Zr alloy has a great potential for use as a dental machining alloy [4]. A recently launched Ti-Zr alloy, commercially called Roxolid™, has been claimed to combine high biomechanical strength with excellent osseointegration, and it might be suitable for dental subgingival implants with narrow diameters. As they claim, “. . . the combination
