Pack Aluminizing of Zirconium 702 Alloy to Improve Surface Hardness and Wear Resistance

Öz

Zirconium 702 (Zr 702) alloy exhibits excellent corrosion resistance but suffers from low hardness and poor wear resistance, limiting its use in wear-intensive applications. In this study, a pack aluminizing treatment was applied to Zr 702 to form a hard aluminide surface layer aimed at improving its tribological performance. Characterization by X-ray diffraction (XRD) and cross-sectional scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) confirmed the formation of a continuous diffusion coating approximately 40 µm thick, composed predominantly of an Al-rich intermetallic phase (mainly Al3Zr). This aluminized layer was dense, well-adhered, and significantly harder than the substrate, with a Vickers microhardness of 703.1 ± 76.7 HV0.025 compared to 182.9 ± 20.7 HV0.025 for untreated Zr 702. In dry sliding reciprocating wear tests (2–4 N loads against an alumina counterface), the aluminized sample exhibited a much lower and more stable coefficient of friction (COF), reaching a steady value of about 0.35 under 4 N load, whereas the untreated Zr 702 stabilized around 0.6 under the same conditions. In comparison with the untreated Zr 702, the aluminized surface exhibited about a 70% reduction in wear volume under a 4 N load as a result of the improved hardness and modification of its surface with an intermetallic layer. Wear track analysis further revealed that the dominant wear mechanism shifted from severe adhesive and abrasive wear in the untreated Zr 702 to a mild abrasive/oxidative wear mode in the aluminized sample, as the hard intermetallic layer protected the underlying Zr from significant damage. Overall, the pack aluminizing process remarkably enhanced the surface hardness, frictional behavior, and wear resistance of Zr 702, demonstrating an effective surface engineering approach to improve the tribological performance of zirconium alloys.

Anahtar Kelimeler

• Zirconium
• Thermo-chemical diffusion treatment
• Aluminising
• Microstructure
• Hardness
• Wear resistance

702 Zirkonyum Alaşımının Yüzey Sertliğini ve Aşınma Direncini Artırmaya Yönelik Kutu Alüminizasyon Uygulaması

Öz

Zirkonyum 702 (Zr 702) alaşımı mükemmel korozyon direnci sergilemekle birlikte düşük sertlik ve zayıf aşınma direnci nedeniyle yoğun aşınma koşullarındaki uygulamalarda sınırlı kullanılmaktadır. Bu çalışmada, Zr 702’ye kutu aluminizasyonu işlemi uygulanarak tribolojik performansını artırmayı amaçlayan sert bir aluminit yüzey tabakası oluşturulmuştur. X-ışını kırınımı (XRD) ile taramalı elektron mikroskoskopisi/enerji dağılımlı spektroskopisi (SEM/EDS) karakterizasyonu, ağırlıklı olarak Al’ce zengin bir intermetalik fazdan (başlıca Al3Zr) oluşan, yaklaşık 40 µm kalınlığında bir difüzyon esaslı kaplamanın oluştuğunu doğrulamıştır. Bu tabaka yoğun, altlığa iyi yapışmış ve iç yapıdan belirgin şekilde daha serttir. Kaplamanın Vickers mikrosertliği 703,1 ± 76,7 HV0.025 olup, işlem görmemiş Zr 702’nin sertliği ise 182,9 ± 20,7 HV0.025 mertebesindedir. Kuru kaymalı geri-ileri aşınma testlerinde (2–4 N yük, alümina karşı yüzey) aluminizasyon uygulanmış numune çok daha düşük ve kararlı bir sürtünme katsayısına (COF) ulaşmış ve COF değeri 4 N yük altında yaklaşık 0,35 düzeyinde sabitlenirken işlem görmemiş Zr 702 aynı koşullarda yaklaşık 0,6’da stabilize olmuştur. Artan sertlik ve yüzeyin sert intermetalik bir tabaka ile modifiye edilmesi sayesinde, aluminizasyon uygulanmış yüzey 4 N yükte aşınma hacmini işlem görmemiş Zr 702’ye göre yaklaşık %70 oranında azaltmıştır. Aşınma izi analizleri, işlen görmemiş Zr 702’de gözlenen şiddetli adezif ve abrazif aşınma mekanizmalarının aluminizasyon uygulanmış numunede sert intermetalik tabakanın altlık Zr 702’yi koruması sayesinde hafif aşındırıcı/oksidatif aşınmaya dönüştüğünü ortaya koymuştur. Sonuç olarak, kutu aluminizasyonu işlemi Zr 702’nin yüzey sertliğini, sürtünme davranışını ve aşınma direncini dikkate değer ölçüde iyileştirerek zirkonyum alaşımlarının tribolojik performansını artırmak için etkili bir yüzey mühendisliği yaklaşımı sunmaktadır.

Anahtar Kelimeler

• Zirkonyum
• Termo-kimyasal difüzyon işlemi
• Alüminizasyon
• Mikroyapı
• Sertlik
• Aşınma direnci

Kaynakça

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