氧化鋯/Crofer合金異質接合之材料製程與氣密特性之研究

Abstract

本研究提出一種利用鍍膜技術改善金屬Crofer22與陶瓷3YSZ接合的方法。透過真空濺鍍技術，在Crofer 22和3YSZ表面鍍上厚度約1.5~6μm鈦銅的複合薄膜。而後以銀作為母材的RAB (Reactive Air Brazing)製程，探討以此材料設計提升陶瓷與金屬接合面的顯微組織與反應機制，比較不同前處理厚度對顯微組織的影響，並以氣密試驗評估其成效。 結果顯示鍍膜製程可顯著提升接合效果，鍍膜在硬銲製程過後，在填料與金屬以及填料和陶瓷間，皆形成良好的反應界面層。就金屬和填料之間的接合處來看，金屬側含鐵和鉻氧化物的生成，可促進鈦和銅的擴散反應。銅元素在3YSZ界面的擴散變化行為較鈦元素明顯許多，在8μm的範圍內，元素百分比最高55 %，最低僅約5 %，因此硬銲層和3YSZ之間的接合，銅元素占主要地位。氣密試驗結果顯示成效良好，在2psig的條件下無壓力下降情形，最高試驗時間超過300小時，即使在600°C高溫條件下，仍能保持壓力條件。鍍膜厚度方面，本研究發現在3μm鍍膜的條件下，界面表現最好，在缺陷與製程良率控制上可達到最佳之效果。 為改善材料成本也助於電廠副產物材料的循環永續使用，佐以飛灰作為硬銲填料的添加劑，硬銲結果顯示，飛灰添加與否以及添加量，均會直接影響金屬Crofer22/銀銲料/陶瓷3YSZ界面層的顯微組織，5wt%飛灰的添加，對於Crofer與銀銲料之間的界面無明顯的不良影響，在硬銲區或界面並未發現單獨飛灰的球狀型態，考量飛灰為煤炭高溫燃燒後的產物，在硬銲後並未發現獨立的球形飛灰，雖有少量缺陷但界面接合良好無大尺寸裂紋。 當飛灰添加量達10wt%時，雖可順利接合，但在金屬側有可見之缺陷，也並未達成良好氣密之效果，在短時間即發生壓降，其主要原因可能為飛灰富含的鋁、矽等元素，與銅和銀之間反應較佳，和鉻元素的相容性較差，由於Crofer在硬銲過程表面的鉻氧化物的生成，而飛灰富含的鋁、矽等元素不與鉻元素相容，當飛灰的添加比例提高時，其元素間的不相容性因此而擴大，材料間差異性造成少部分的缺陷，此外，形狀不規則的飛灰顆粒較脆，因此在硬銲過程造成局部的空隙。 在氣密效果上，添加5%飛灰樣品的硬銲製程仍表現出優異的氣密性能。即使將試驗溫度提升到700 ℃，仍具備一定的氣密效果，平均洩漏率為1.5610-4 mbar·L/s。當飛灰的添加量提升到10%，氣密試驗顯示較明顯可見的氣體滲漏現象，約10小時即降至1psig，顯示10%的飛灰添加對於氣密效果的影響較大且負面。 在量產製程上，將10%鈦銅粉添加5%飛灰硬銲樣品進行氣密試驗，常溫與高溫600 ℃壓降結果結果顯示試驗期間均無壓降情形，亦即在量產製程上，添加飛灰仍具備實際應用於高溫氣密之可能性。

This study proposes a method to improve the joining between metal Crofer 22 and ceramic 3YSZ using a coating process. A composite thin film of titanium-copper with a thickness of approximately 1.5 to 6 µm was deposited on the surfaces of Crofer 22 and 3YSZ via vacuum sputtering. Subsequently, the microstructure and reaction mechanism of the ceramic-metal interface were investigated using a silver-based Reactive Air Brazing (RAB) process. The influence of different pre-treatment thicknesses on the microstructure was compared, and the effectiveness of the process was evaluated through gas‐tightness tests. The results demonstrate that the coating process significantly enhances the joining performance. After the brazing process, a favorable reaction interface layer was formed between the filler and the metal and between the filler and the ceramic. In the metal-filler interface, forming iron and chromium oxides on the metal side facilitated the diffusion reaction of titanium and copper. The diffusion behavior of copper was more pronounced than that of titanium at the 3YSZ interface. Within an 8μm thickness, the percentage of copper varied significantly, from a maximum of 55 % to a minimum of approximately 5 %, indicating that copper plays a significant role in joining the brazing layer and 3YSZ. The gas-tightness tests showed excellent results, with no pressure drop observed under 2 psig conditions for over 300 hours, and the pressure was maintained even at 600°C. Regarding the coating thickness, the study found that the 3 µm coating provided the best interface performance, achieving optimal defect control and process yield. Fly ash was added to the brazing filler to reduce material costs and promote the sustainable use of power plant by-products. The brazing results showed that the addition and amount of fly ash directly affected the microstructure of the Crofer 22/silver filler/3YSZ interface. Adding 5 wt% fly ash did not significantly impair the Crofer-silver filler interface, and no individual spherical fly ash particles were observed in the brazing zone or interface. Given that fly ash is a product of high-temperature coal combustion, the absence of independent spherical fly ash particles after brazing, despite some minor defects, indicated good interface joining without large cracks. When the fly ash addition reached 10 wt%, successful joining was achieved. However, visible defects were observed on the metal side, and satisfactory gas-tightness was not attained, with a rapid pressure drop. This was likely due to the aluminum and silicon elements in fly ash reacting more favorably with copper and silver but having poor compatibility with chromium. As chromium oxide formed on the Crofer surface during brazing, the incompatibility between the elements increased with higher fly ash addition, resulting in defects due to material differences. The irregular and brittle fly ash particles also caused localized voids during brazing. The sample with 5 % fly ash addition exhibited excellent performance regarding the gas-tightness test. Even when the test temperature was raised to 700 °C, it maintained a certain degree of gas-tightness, with an average leak rate of 1.56×10⁻⁴ mbar·L/s. However, the sample with 10 % fly ash addition showed significant gas leakage, with the pressure dropping to 1 psig within approximately 10 hours, indicating a negative impact on gas-tightness. The brazing sample with 10 % titanium-copper powder and 5 % fly ash was subjected to gas-tightness tests at room temperature and 600 °C for mass production. The results showed no pressure drop during the tests, suggesting that fly ash addition remains a viable option for high-temperature sealing applications in mass production.