In the 1950s, Dr. Donald Stookey succeeded in the production of the first glass-ceramic of the lithium disilicate type worldwide (Stookey, 1959; Höland and Beall, 2012). His discovery and following research on this new class of materials paved the way for a number of innovative technical solutions affecting daily life, high-tech industry, and fundamental research in various fields. Stove tops and kitchenware belong to the most famous applications of glass-ceramics just as telescope mirrors, the nosecones of spaceships, and dental prostheses (Höland and Beall, 2012). In light of this wide field of applications for Donald Stookey’s invention, the impact of his work on not only the technological progress of humankind becomes obvious but also has his research motivated and driven the mind and work of a huge number of scientists in various fields of research ever since. In this spirit, the authors of the present publication want to recognize the outstanding activities of Dr. Donald Stookey.

To identify and use the nucleating effect of Ag clusters in the development of glass-ceramics in the SiO₂–Li₂O system, as well as transferring the general principles of nucleation and crystallization to other glass systems, was a crucial part of his great achievement (Stookey, 1959). Besides enhancing the mechanical strength and machinability of glasses, the precipitation of certain crystal phases helped to control a number of different material properties including optical and electrical properties of glass matrix materials (Beall, 1971, 2014; Mauro et al., 2013).

Initiated after the development of the first lithium disilicate glass-ceramic by Donald Stookey, this system was the subject matter in a multitude of fundamental research topics. A molar ratio SiO₂/Li₂O of 2, similar to the stoichiometric composition of lithium disilicate, was the basis for most of the compositions investigated (Höland and Beall, 2012). Moreover, glass-ceramics of the lithium disilicate type from multicomponent base glass systems were developed as well. There, the molar ratio of SiO₂/Li₂O considerably deviated from the stoichiometric value of 2. Technical glass-ceramics as well as consumer applications were developed this way (Beall, 1992, 2014; Echeverría, 1992). Likewise, Höland et al. (1994, 2005, 2007, 2008, 2009) succeeded in the development of lithium disilicate glass-ceramics for dental applications using such multicomponent base glass systems.

Based on this research and development of lithium disilicate glass-ceramics for dental applications and driven by the quest for innovative combinations of material properties, the authors of the present publication pursued the objective of the parallel controlled nucleation and crystallization of lithium disilicate and diopside CaMgSi₂O₆ from a multicomponent base glass. The fundamental extension of the chemical composition of the lithium disilicate base glass system by adding MgO and CaO was mandatory just as the enhancement of nucleation and crystallization mechanisms. From a glass-ceramic engineering point of view, the authors assumed that controlling two separate nucleation mechanisms is mandatory to precipitate both a layer silicate and a chain silicate from a base glass. Only the development of a powder compact processing facilitated the crystallization of diopside according to the nucleation and crystallization mechanisms reported by Kokubo et al. (1989), Müller et al. (2000), and Reinsch et al. (2008) for different glass systems. The influence of the parallel precipitation of lithium disilicate and diopside on nucleation and crystallization principles as well as on properties crucial for dental applications, e.g., biaxial strength and translucency, were investigated.

The extension of the chemical composition of the base glass by certain amounts of MgO and CaO is the prerequisite for the precipitation of CaMgSi₂O₆ as a minor crystal phase in Li₂Si₂O₅ glass-ceramics. Increasing the content of MgO and CaO from approximately 1.4 to 2.9 mol% yielded the precipitation of diopside in the present study. However, no internal nucleation and crystallization of diopside was found in the present base glasses, but surface crystallization of diopside could be verified in the preliminary study of the crystallization behavior of the bulk base glass D (Figure 4). Hence, an activation of the glass surface was mandatory to nucleate the crystallization of diopside. In the present study, the activation of the glass surface was achieved by means of ball milling. It is important to note that the magnitude of activation of a glass surface is strongly dependent on the method and processing parameters used. There exist a lot of different milling techniques as well as there are various alternative ways to initiate surface nucleation and crystallization for different material systems, e.g., by means of tribochemistry (Henicke, 1984). Hence, the concentration of MgO and CaO that is mandatory for the crystallization of diopside can deviate dependent on the method used for the activation of the glass surface.

The analysis of the base glasses by means of DSC (Figure 1; Table 2) reveals two rather sharp exothermal peaks for all the base glasses. According to the data obtained by HT-XRD (Table 3), these peaks can be correlated to the crystallization of Li₂SiO₃ and Li₂Si₂O₅. The rather sharp characteristic of these peaks suggests that an internal nucleation and crystallization mechanism is dominant. The effect of P₂O₅ as internal nucleating agent for lithium silicates at a glassy amorphous interphase was previously reported in literature (Höland and Beall, 2012). HT-XRD studies in the present glass-ceramics reveal that crystalline Li₃PO₄ can only be detected after heat treatments above 700°C, whereas the crystallization of Li₂SiO₃ already starts at ≥520°C (Table 3). Hence, a mechanism reported by Höland and Beall (2012) describing the nucleation of lithium silicate at the interphase of a glassy amorphous or disordered nanocrystalline early phase of Li₃PO₄ seems most likely.

A rather wide exothermal signal in the DSC curves of glass B–D between 780 and 860°C are an indication for a surface nucleation and crystallization mechanism. The data obtained by HT-XRD show that diopside forms in exactly this temperature range. Hence, the authors conclude that diopside crystallizes on the activated surface of the glass grains. Furthermore, surface nucleation and crystallization display well-known mechanisms found in the investigations on diopside glass-ceramics (Kokubo et al., 1989; Zanotto, 1991; Müller et al., 2000).

In summary, the precipitation of two different silica-based crystal phases from the present glass system requires the control of two essentially different nucleation mechanisms. On the one hand, the layer silicate Li₂Si₂O₅ is nucleated by an internal nucleation mechanism, and on the other hand, surface activation nucleates the crystallization of the chain silicate CaMgSi₂O₆.

To produce bulk glass-ceramics from internal and surface crystallizing base glasses, a powder compact process was developed. Crystallization of the glass powder and sintering of the powder compacts to achieve dense bulk glass-ceramics has to be controlled in parallel. Analyzing the sintering behavior of the glass powders by means of HMA (Figure 2), together with the results of the HT-XRD studies (Table 3), was the basis to develop an adequate firing cycle. According to the HMA curves, densification starts at approximately 500°C for all base glasses. Since the T_g of the base glass powders is between 450 and 460°C, the starting densification is clearly a result of the softening and beginning viscous flow of the glass. Crystallization of Li₂SiO₃ and Li₂Si₂O₅ between 540 and 700°C clearly increase the viscosity of the glasses and hence stop the densification as can be seen in the first plateau of the HMA curve. Base glasses B–D show a second plateau ending at approximately 855°C, which can be explained by the crystallization of diopside hindering the sintering process. Dissolution of crystal phases and melting of the glass-ceramics begins at temperatures >860°C.

Based on these results, the crystallization and sintering cycle of the powder compacts was developed. It starts with a fast heating to 480°C, where the temperature is kept for 20 min in order to internally nucleate lithium silicate at a glassy amorphous P₂O₅-rich interface (Höland and Beall, 2012). After internal nucleation, the powder compacts were heated to 860°C with a reduced heating rate of 10 K/min. Crystallization and sintering takes place in this temperature range. To obtain a dense glass-ceramic with minimal crystal phase dissolution and viscous deformation of the specimen, the sintering cycle stops after a dwell time of 5 min at 860°C. The micrographs of the glass-ceramics in Figure 6 reveal pores in the glass-ceramics B and C. Glass-ceramics A and D show a more homogeneous and dense microstructure than the other glass-ceramics. Based on the quantitative crystal phase analysis of the glass-ceramics (Table 4) and assuming that the entire ratio of amorphous P₂O₅–Li₂O phase precipitates to Li₃PO₄, one can roughly estimate the composition of the residual glass phases. The estimated composition of residual glass phase is shown in Table 6. According to this estimation, the residual glass phase of the glass-ceramic D comprises a considerably higher concentration of network modifiers such as Li₂O, MgO, and CaO. This would result in a reduced viscosity of the residual glass phases compared to glass-ceramics A–C and therefore displays an interesting point of discussion regarding the sintering properties of the glass-ceramics. Comparing the sintering properties of glass-ceramic A to B and C, the restraining influence of crystallization processes on the sintering process becomes obvious, since glass-ceramic A shows no diopside formation in contrast to B and C. A more dedicated investigation is necessary to resolve the different results of the sintering process. Furthermore, enhancing the powder processing, e.g., by granulation of the glass powder prior to uniaxial pressing, could improve the densification and sintering of the powder compacts.

The influence of the chemical composition of the base glasses on the quantitative crystal phase composition of the final glass-ceramics is shown in Table 4. Obviously, a concentration of ≤1.4 or 1.6 mol% of MgO and CaO in the base glasses is insufficient for the precipitation of CaMgSi₂O₆ in the present base glass system. However, as mentioned in the first paragraph of the discussion, this finding is only true for the activation of the glass powders surface by means of ball milling down to a mean grain size of about 20 μm. Increasing the concentration of MgO and CaO from 2.9 or 3.1 to 3.6 or 3.7 mol%, respectively, resulted in a considerable increase of the diopside content by 3.4 wt.%, while further increasing to 3.8 or 4 mol%, respectively, did not significantly change the concentration of diopside in the glass ceramic. The reduced content of SiO₂ together with the consumption of SiO₂ during the crystallization of Li₂Si₂O₅ could explain the stagnant precipitation of CaMgSi₂O₆, although higher concentrations of CaO and MgO are provided in the base glass.

Based on the preliminary study of crystallization (Figure 4) and comparing the micrographs B–D with the micrograph of the diopside-free glass-ceramic A (Figure 6), the crystals forming the microstructure of the glass-ceramics can be identified. Large (5–10 μm) bar-like diopside crystals are embedded in a matrix of rather small (<1 μm) plate-like Li₂Si₂O₅ crystals (Figure 6).

High concentrations of crystalline phases (>55 wt.%) (Table 4) as well as the arrangement of Li₂Si₂O₅ and CaMgSi₂O₆ crystals in an interlocking microstructure (Figure 6) contribute to the biaxial strength of the glass-ceramics (Dittmer et al., 2014). However, pores in the microstructure, as they were found particularly in glass-ceramics B and C (Figure 6), can decrease the biaxial strength of the materials. In a previous study on tailoring, the thermal expansion of Li₂Si₂O₅ glass-ceramics by precipitating a high CTE minor phase significant effects on the biaxial strength of the glass-ceramics due to the considerable mismatch in the CTEs of the involved crystal phases were reported (Rampf et al., 2015, 2016). However, in the glass-ceramic system investigated in the present study, such effects are unlikely since there is no such huge difference in the thermal expansion of the involved crystal phases. The CTE_200–800°C of diopside crystals was investigated to be approximately 7.4 × 10⁻⁶ K⁻¹ (Rigby and Green, 1942) and therefore very close to polycrystalline lithium disilicate (CTE_25–600°C = 11.4 × 10⁻⁶ K⁻¹) compared to the investigations by Rampf et al. (2015, 2016).

The precipitation of diopside as a minor phase in Li₂Si₂O₅ glass-ceramics reduces the translucency of the glass-ceramic materials due to the introduction of a further interface where scattering of light occurs (Nassau, 2001). The influence of a minor crystal phase Ca₅(PO₄)₃F on the optical properties of Li₂Si₂O₅ glass-ceramics was previously investigated by Rampf et al. (2015, 2016). They reported the optical properties, in particular the translucency, to be dominated by the Li₂Si₂O₅ crystals, which appeared to be significantly larger than the submicron sized apatite crystals. However, the apatite crystals having dimensions in the range of visible light did affect the opalescence of the materials. In the present system, the diopside crystals are significantly larger than the approximately 0.5–1 μm-sized Li₂Si₂O₅ crystals. Hence, the precipitation of diopside displays an effective tool to influence the translucency of Li₂Si₂O₅ glass-ceramics without affecting its opalescence properties.

Machining of the crystallized glass-ceramics using conventional CAD/CAM technology was feasible without problems. The minor diopside phase enhanced the machinability of the Li₂Si₂O₅ glass-ceramics probably by interfering with the high-strength interlocking Li₂Si₂O₅ microstructure.

Glass-ceramics comprising Li₂Si₂O₅ as main crystal phase and CaMgSi₂O₆ as minor crystal phase can be produced from base glasses in the SiO₂–Li₂O–P₂O₅–Al₂O₃–K₂O–MgO–CaO system. The parallel precipitation of a layer silicate (Li₂Si₂O₅) on the one side and crystals of the chain silicate type (CaMgSi₂O₆) on the other side require the control of two separate nucleation and crystallization mechanisms. Li₂Si₂O₅ can be nucleated internally at an amorphous glassy P₂O₅–Li₂O-rich interphase. The activation of the glass surface, e.g., by means of ball milling, is mandatory to efficiently nucleate the crystallization of CaMgSi₂O₆.

The chemical composition, in particular a sufficient concentration of MgO and CaO, of the base glass is essential for the precipitation of the CaMgSi₂O₆ minor phase. However, the mandatory concentration of MgO and CaO can deviate with the extent or method applied for surface activation.

A reproducible powder compact process, including uniaxial pressing and sintering, was developed to produce dense Li₂Si₂O₅–CaMgSi₂O₆ glass ceramic materials.

Li₂Si₂O₅–CaMgSi₂O₆ glass-ceramics could be machined by means of conventional CAD/CAM technology and reach a biaxial strength of 391 ± 98 MPa based on the interlocking microstructure and possible additional compressive stress formation.

The large (5–10 μm) bar-like CaMgSi₂O₆ crystals embedded in a matrix of interlocking (≤1 μm) Li₂Si₂O₅ crystals reduce the translucency of the material and hence display a further potential tool to create new combinations of material properties within Li₂Si₂O₅-based glass-ceramics.

MR: planning, organization, and execution of experiments; interpretation and discussion of results; and compilation of manuscript and artwork. MD: execution of Rietveld refinement; interpretation and discussion of results on XRD analysis; and assistance in compilation of the manuscript. CR: consulting in planning of experiments; interpretation and discussion of HMA and DSC; and assistance in compilation of the manuscript. WH: consulting regarding the state of the art literature on the topic; interpretation and discussion of results; and assistance in the compilation and proofreading of the manuscript.

The authors are employees of the company Ivoclar Vivadent AG. A patent was filed on the present subject. The reviewer LM and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.