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Direct ink writing (DIW) of high-temperature thin-film sensors holds significant potential for monitoring extreme environments. However, existing high-temperature inks face a trade-off between cost and performance. This study proposes a SiCN/RuO₂/TiB₂ composite ceramic ink. The added TiB₂, after annealing in a high-temperature atmospheric environment, forms B₂O₃ glass, which synergizes with the SiO₂ glass phase formed from the SiCN precursor to effectively encapsulate RuO₂ particles. This enhances the film’s density and adhesion to the substrate, preventing RuO₂ volatilization at high temperatures. Additionally, the high conductivity of TiB₂ improves the film’s overall conductivity. Test results indicate that the SiCN/RuO₂/TiB₂ film exhibits high linearity from room temperature to 900 °C, high stability (resistance drift rate of 0.1%/h at 800 °C), and high conductivity (4410 S/m). As a proof of concept, temperature sensors and a heat flux sensor were successfully fabricated on a metallic hemispherical surface. Performance tests in extreme environments using high-power lasers and flame guns verified that the conformal thin-film sensor can accurately measure spherical temperature and heat flux, with a heat flux sensor response time of 53 ms. In conclusion, the SiCN/RuO₂/TiB₂ composite ceramic ink developed in this study offers a high-performance and cost-effective solution for high-temperature conformal thin-film sensors in extreme environments.