All ceramic restorations, referred to as metal-free restorations, are the most widely used prostheses in contemporary restorative dentistry [1], [2]. Among those ceramics is a family of yttria-stabilized zirconia polycrystals (YSZ). Over the years, a heavy focus has been placed on improving YSZ translucency and strength for a wider range of clinical applications [3]. Five generations have since been developed [4]: the original strong but opaque 3Y-TZP (1st-generation, 3 mol.% yttria-stabilized tetragonal zirconia polycrystals containing 0.25 wt% Al2O3 sintering aids and over 90% tetragonal zirconia phase); a partially translucent but still moderately strong 3YSZ (2nd-generation, 3 mol.% yttria partially stabilized zirconia with <0.05 wt% Al2O3 and under 90% tetragonal zirconia content); the more translucent but weaker 4Y-, 5Y-, and 6YSZ (3rd-generation); polychromatic multilayered structures (4th-generation, contiguous layers with various shades and/or compositions); and finally, graded compositions and shades (5th-generation, contiguous layers with composition and/or shade gradients at their interfaces). The cubic phase content of these YSZ variants varies from 5% to 80% [5], [6]; grain size from 0.2 µm to 5 µm [6]; flexural strength from 400 to 1200 MPa [5], [6]; appearance from predominantly opaque to substantially translucent. In general, flexural strength decreases and translucency increases with increasing Y content. Therefore, the wide range of clinical applications of YSZ hinges on a compromising approach between strength and translucency [7].

Zirconia restorations are produced using CAD/CAM technologies in either a dental office or laboratory [8]. Three fabrication routes are available: in-office chairside, in-lab fabrication, or centralized milling facilities [9], [10]; but all adopt a strategy of soft-machining partially sintered (pre-sintered) blanks. The downside of soft milling is the need for a prolonged post-mill sintering process (typically lasting 4 – 12 h), which inevitably becomes a major bottleneck in the workflow. Thus, soft-milled zirconia restorations cannot be considered as a chairside single-visit treatment option [11], [12]. To overcome this limitation, some attempts have been made to mill zirconia restoration out of fully sintered blocks—the so-called, “hard machining”. However, this approach has many disadvantages, such as: a lengthy milling time that typically takes around 45 – 55 mins to mill a single-unit crown and the fast wear of milling tools [13] owing to the high hardness and toughness of the fully sintered zirconia [14]. Furthermore, hard machining may produce defects, flaws, and cracks in final restoration [15], [16]. Additionally, surface heat associated with hard machining may induce an undesirable tetragonal-to-monoclinic phase transformation, potentially compromising the clinical longevity of YSZ restorations [17].

The recent introduction of speed and high-speed sintering protocols has paved the way to a new era of chairside zirconia fabrication. Speed sintering (SS) takes 30 – 120 min, which allows the procedure to be accomplished chairside in a single visit. SS also utilizes a conventional furnace. However, the SS furnace can generate heat at a faster heating rate (∼ 40 – 70 °C/min) by featuring a smaller heating chamber [13]. More recently, the development of high-speed sintering (HS) protocols has further reduced the sintering time from a couple of hours to just tens of minutes. This was achieved with the introduction of an induction-type furnace (CEREC SpeedFire, Dentsply Sirona). Induction heating technology is based on generating heat by supplying alternating current to a metal coil, which in turn generates an alternating magnetic field [18]. This electromagnetic field is extremely effective in heating electrically conductive materials such as metals and semiconductors. Unfortunately, since much of the information concerning power electronics, control algorithms, and magnetic component and heating chamber designs of dental speed fire furnaces remain proprietary, the effectiveness of induction sintering zirconia—an electrical insulator until ∼800 – 900 °C—is not fully understood [19].

Since the introduction of speed fire technologies, there has been a large volume of literature concerning speed sintering of dental zirconias [20], [21], [22], [23], [24], [25], [26]. The vast majority of these studies focused on the mechanical properties of speed-sintered zirconia. Only a handful of studies reported on the impact of speed sintering on densification behavior and optical properties. For reference, the key findings of previous speed sintering studies are summarized in Table 1. In addition, many previous studies used commercial zirconia materials with very limited information on starting powder compositions and employed different speed sintering protocols and furnaces. Thus, it is challenging to draw definitive conclusions, and more so, to elucidate the underlying mechanisms from analyzing literature.

Accordingly, the present study aimed to elucidate the impact of the currently recommended SS and HS protocols on the densification, microstructure, phase content, translucency, and flexural strength of 3YSZ, 4YSZ, and 5YSZ fabricated from the most studied and widely used raw zirconia powders (Tosoh Corp., Tokyo, Japan), using well-established sintering protocols and furnaces. The null hypothesis was that the currently recommended SS and HS protocols have no effect on the densification, optical, and mechanical properties of dental YSZs relative to conventional sintering (CS).