Using visible-near infrared light to measure temperature during selective laser flash sintering of yttria stabilized zirconia conducted with an alternating electric field

Selective laser flash sintering (SLFS) is emerging as a promising route for binder-free additive manufacturing of ceramics. Monitoring local temperatures during SLFS allows correlations between processing variables and the temperatures required for SLFS. This study demonstrates a visible-near infrared (vis-NIR) imaging approach for in situ temperature measurement during SLFS of porous 8 mol% yttria-stabilized zirconia (8YSZ) powder compacts conducted with an alternating current (AC) electric field. A high-speed, grayscale camera was first calibrated, establishing a power-law relationship between pixel grayscale intensity and temperature. This calibrated optical thermometry was then applied to measure surface temperatures under the laser spot during scanning. Experiments were conducted to systematically vary process parameters and determine how they affect local temperatures. These experiments showed that quasi-steady-state temperatures increased approximately linearly with laser energy density and that these higher surface temperatures resulted in increasingly severe thermal shock cracks. Temperatures initially increased quickly with AC field strengths, but then showed limited additional increase at higher field strengths. Varying AC frequency from 10 Hz to 15,000 Hz revealed two distinct regimes: temperatures that were insensitive to frequency from 10 to 200 Hz, followed by a decreasing trend in quasi-steady-state temperature from 200 Hz to 15,000 Hz, with temperatures approaching the no-field baseline of 1040 K at the highest tested frequencies. Overall, cracking was observed even at the lowest quasi-steady-state temperature measured in this study (approximately 950 K), indicating that lower temperatures and the ability to measure them are required to eliminate cracking during SLFS.