The microstructure of a lithium aluminosilicate (LAS) glass ceramic has been modified by ultra-short pulsed laser radiation during the crystallization process. Laser pulses with 10 ps pulse duration, 1064 nm wavelength and a repetition rate of 50 kHz have been focused inside LAS glass ceramic using a microscope objective with a NA of 0.4. Before laser treatment the LAS glass ceramic was already transformed to a primary crystallization stage by using a heat treatment at 540°C and 660°C. Caused by nonlinear absorption processes energy is transferred from the photons to the lattice and leads to local melting. The fast cooling of the melted volume due to heat conduction enables the formation of an amorphous phase. After a second heat treatment at 830°C the laser irradiated area shows a different microstructure compared to the untreated area. Influences of the modified microstructure on mechanical and optical properties have been studied. Potential applications of this process are proposed.
Glass-ceramic materials are increasingly becoming the material of choice in the field of dental prosthetics, as they can feature both high strength and very good aesthetics. It is believed that their color, microstructure and mechanical properties can be tuned such as to achieve an optimal lifelike performance. In order to reach that ultimate perfection a controlled arrangement of amorphous and crystalline phases in the material is required. A phase transformation from amorphous to crystalline is achieved by a heat treatment at defined temperature levels. The traditional approach is to perform the heat treatment in a furnace. This, however, only allows a homogeneous degree of crystallization over the whole volume of the parent glass material. Here a novel approach using a local heat treatment by laser irradiation is presented. To investigate the potential of this approach the crystallization process of SiO<sub>2</sub>-Li<sub>2</sub>O-Al<sub>2</sub>O<sub>3</sub>-based glass has been studied with laser systems (pulsed and continuous wave) operating at different wavelengths. Our results show the feasibility of gradual and partial crystallization of the base material using continuous laser irradiation. A dental prosthesis machined from an amorphous glassy state can be effectively treated with laser irradiation and crystallized within a confined region of a few millimeters starting from the body surface. Very good aesthetics have been achieved. Preliminary investigation with pulsed nanosecond lasers of a few hundreds nanoseconds pulse width has enabled more refinement of crystallization and possibility to place start of phase change within the material bulk.