Glass-ceramics are one of those materials that seem obvious after you take a few materials engineering courses. You start with a glass, which is useful because it can be melted, shaped, and processed. Then instead of avoiding crystallization, you force the glass to crystallize in a controlled way.
That is the key distinction. In normal glass processing, crystallization is usually a defect. It means the glass devitrified. In glass-ceramics, crystallization is the product. So you want something like:
$$ \text{glass} \xrightarrow{\text{controlled heat treatment}} \text{glass-ceramic} $$
Typically, the more useful state is a glass-ceramic with one or more crystalline phases embedded in a residual glassy matrix. The parent glass composition, nucleation process, growth process, crystal size, and residual glass are all essential aspects [1].
The thing I like about glass-ceramics is that they are a very clean process-structure-property example where kinetics matter. You are not just choosing a chemistry. You are choosing a chemistry that can first form a vitreous state, and then later crystallize into the phases you actually want. The workflow is then something like:
This controlled process drives structure-property relationships that lead to glass-ceramics that have applications in cookware, cooktops, dental restorations, optical mirror substrates, sealants, and bioactive materials. They look unrelated, but the trick is the same, i.e., controlled crystallization gives access to property combinations that are hard to get from either ordinary glass or conventional sintered ceramics [2].
|
| Figure 1. Corning Pyroceram glass-ceramic. Wikimedia Commons, public domain. |
CorningWare (see fig. Figure 1) was a glass-ceramic that turned out to have outstanding thermal shock properties and hence was used for cookware. Whereas many silica-rich glasses (for example soda-lime window glass) tolerate rapid temperature changes poorly; borosilicate glass is a common exception. The low-expansion glass-ceramic handles thermal shock much better because the crystallized microstructure changes the effective thermal expansion response.
The classical glass-ceramic idea is tied to S. Donald Stookey's work on catalyzed crystallization of glass [3]. The important point is not simply that glasses can crystallize. I mean ever vitreous material tends towards the thermodynamically stable state which usually is the crystalline phase. As stated above the important point is that the crystallization process is very caefully controlled. The way to think of this is there are two events, 1.) crystallite nucleation, 2.) growth/distribution of the crystallites. if you assume two rates for nucleation and growth, what you try to control is something like,
$$ \begin{equation} I(T) \not\equiv U(T) \label{eq:nucleation_growth} \end{equation} $$
where $I(T)$ is the nucleation rate and $U(T)$ is the crystal growth rate (different physical quantities with different dimensions; (\not\equiv) means their (T)-dependences differ). The expression is illustrative, not empirical. In practice you need the right density of nuclei, and proper crystal growth. If there are too few nuclei, the crystals can grow too large. If the crystals grow too much, the material can lose the desired properties. Ideally one is looking to find the right crystal size and number density to get useful glass-ceramic properties.
The other thing to think about is the actual glass-ceramic microstructure. The useful properties of glass-ceramics come from details like:
- the crystalline phase,
- the crystal volume fraction,
- the crystal size,
- the crystal shape,
- the residual glass composition,
- the elastic and thermal mismatch between phases.
For low thermal expansion glass-ceramics, the idea is often that the glassy phase has positive thermal expansion and the crystalline phase has negative thermal expansion. If the balance is right, those contributions nearly cancel. So you have some linear mixing like:
$$ \begin{equation} \alpha_{\text{eff}} \approx V_g \alpha_g + V_c \alpha_c \nonumber \label{eq:cte_mixture} \end{equation} $$
where $V_g$ and $V_c$ are the glass and crystal volume fractions, and $\alpha_g$ and $\alpha_c$ are the thermal expansion coefficients of the glassy and crystalline phases. This is obviously too simple because real microstructures have elastic constraint and anisotropy.
One example is ZERODUR (see fig. @fig:zerodur). It is a lithium aluminosilicate glass-ceramic used in precision optical applications because it can have extremely low thermal expansion. The low expansion comes from balancing the positive expansion of the residual glass with the negative expansion of the crystalline phase [4].
|
| Figure 2. SCHOTT CERAN glass-ceramics. Wikimedia Commons, CC BY-SA 4.0 |
Cooktops (see fig. Figure 2) are the less exotic version of the same basic idea. You want a smooth surface, thermal shock resistance, chemical durability, and enough mechanical robustness for normal use. Dental materials are another good example because a dental restoration needs strength, chemical durability, wear resistance, processability, and translucency. A glass might look good, but not be strong enough and a polycrystalline ceramic is strong, but it may be too opaque or difficult to process. Whereas glass-ceramic gives the right balance, such as lithium disilicate dental glass-ceramics. They contain interlocked lithium disilicate crystals in a glassy matrix, which helps improve mechanical properties while still preserving required optical properties [5].
References
