MIM Kovar合金之燒結性質與玻璃封裝處理

Abstract

目前Kovar合金製程大部分為熔煉與鑄造，但需要形狀較複雜的Kovar工件時，常需額外進行二次機械加工，使整體成本相對提高。利用MIM(Metal Injection Molding)製程能一次成形出較複雜之工件，成本低，適合大量生產，機械性質亦優於傳統粉末冶金產品。Kovar合金一般與硬玻璃進行接合，屬於匹配型接合(Matched Seal)，即金屬與玻璃彼此之熱膨脹係數相當接近，是利用金屬表面經預氧化處理形成之氧化層與玻璃反應形成化學鍵結，因此具有良好的熱穩定性。製作MIM Kovar合金時，常使用預合金粉與混合元素粉兩種，其中混合元素粉之成本較低且燒結密度較高，但燒結後有可能發生成分不均勻的現象，導致α相生成，影響熱膨脹係數，故需提高燒結溫度與延長燒結時間，本研究中發現若在氫氣中以1380℃燒結4小時，不論預合金粉或混合元素粉均能均質化，成為單一γ相組織且具有高燒結密度，分別為96%與99.6%。 Kovar合金在與玻璃接合前需先進行預氧化處理，若在空氣中進行預氧化可發現所形成之氧化層主要為Fe2O3並有少量之Fe3O4，造成預氧化層之附著性及連續性皆不佳；若在溼氮氣中進行預氧化，由於氧分壓遠較空氣中低，故在800℃持溫45分鐘所形成之氧化層幾乎都為連續性佳且附著良好之Fe3O4並具有4.5 μm的適當厚度，利於後續接合處理。除了預氧化層之組成形態外，接合時所選用之氣氛也將大大影響接合完成後Kovar合金與玻璃之接合面狀態與強度，本研究發現使用緻密且連續性良好之氧化層，如在溼氮氣中以800℃持溫45分鐘所形成之氧化層，並在真空下以1100℃持溫20分鐘與玻璃接合能得到最佳之界面狀態、強度且玻璃能緻密化。預合金粉系統與混合元素粉系統接合後測試其抗剪強度分別為165 MPa及177 MPa；而這些接合工件經過500次溫度循環後仍維持160 MPa之抗剪強度且能承受至少50次高低溫熱衝擊而不發生界面破壞。

Kovar alloy products are usually made by melting and casting. These parts may need secondary machining and thus leading to high cost. One of the advantages of using MIM(Metal Injection Molding) process is that it can produce components with complex shapes and better mechanical properties than those manufactured by traditional PM process. MIM Kovar parts are made in this study from prealloyed powders and admixed powders. Admixed powders cost less and still can obtain high sintered density. However, composition inhomogeneity may happen after sintering and induce an increase in CTE(Coefficient of Thermal Expansion). Higher sintering temperature and longer holding time are the solutions to this problem. Both prealloyed powders and admixed powders can reach a high density of 96% and 99.6%, respectively, with uniform alloy distribution after sintering at 1380℃ for 4 hours in hydrogen. Kovar alloys need to be preoxidized before sealing with glass. After being preoxidized in air, it is obvious that the oxide layer on the surface of Kovar alloys is composed mainly of loosely-bonded Fe2O3 and some Fe3O4 due to the high partial pressure of O2. However, the dense Fe3O4 layer with a thickness of 4.5 μm and good adherence to the metal substrate could be attained when the preoxidation process is carried out at 800℃ for 45 minutes in wet nitrogen. The interfacial condition and strength after sealing depend on not only the characteristics of the oxide layer but also the sealing atmosphere. In this study, it has been shown that Kovar parts covered with dense Fe3O4 layer and with a thickness of 4.5 μm can produce the best interfacial condition and highest shear strength after sealing with glass at 1100℃ for 20 minutes in vacuum. These samples also show good reliability under thermal shock and temperature cycling tests.