Farklı Seramik Takviyeli Bakır Matrisli Kompozit Kaplamalar İle Elektrik Kontak Malzemelerinin Yüzey Özelliklerinin Geliştirilmesi

Abstract

Elektrik kontak malzemeleri elektrik devrelerini herhangi bir hasara yol açmadan birbirine bağlayan veya birbirinden ayıran metal esaslı elektro-mekanik parçalar olarak tanımlanabilirler. İdeal elektrik kontak malzemelerinin yüksek ısı ve elektrik iletkenliğine sahip olmaları beklenir. Bunun nedeni üzerinden geçen akıma karşı mümkün olduğunca az direnç göstererek ısı oluşumunu minimum seviyede tutmak ve direnç sebebiyle oluşan ısının hızla dağıtılmasını sağlamaktır. Bakır gümüşten daha düşük elektrik iletkenliğine sahip olmasına karşın ekonomik nedenlerden ötürü en yaygın olarak kullanılan elektrik kontak malzemesidir. Fakat bakır ve bakır esaslı malzemeler sahip oldukları düşük sertlik değerleri ve düşük aşınma dirençleri sebebiyle elektrik kontak malzemesi olarak kullanıldıklarında yüzeylerinde aşınma meydana gelir ve servis ömürleri kısalır. Bu nedenle elektrik kontak malzemesi olarak kullanılan bakırın mekanik özelliklerinin geliştirilmesi gerekmektedir. Bu da kütlesel sertlik arttırma ve yüzey geliştirme işlemleri ile yapılmaktadır. Bu çalışmada yüzey geliştirme tekniği olarak kullanılan soğuk gaz dinamik püskürtme tekniği ile bakır altlık üzerine yine bakır tozlara farklı seramik takviyeleri ilavesiyle oluşturulan toz karışımlarının kaplanabilirliği incelenmiştir. Elektrik kontak malzemelerinin sahip olması gereken en önemli özelliklerden biri olan elektrik iletkenlik değerini mümkün olduğunca koruyarak bakırın aşınma direnci, sertlik gibi mekanik özelliklerini geliştirmek hedeflenmiştir. Böylece elektrik kontak malzemesi olarak kullanılan bakırın servis ömrü uzatılacak ve bakım maliyetleri azaltılacaktır. Deneysel çalışmalarda öncelikle en uygun parametrelerin bulunması için optimizasyon çalışmaları yapılmıştır. Bu optimizasyon çalışmaları çerçevesinde hacimce %12.5 B4C takviyeli bakır tozları saf bakır altlık üzerine püskürtülmüş ve oluşan kaplamaların mikroyapı, sertlik gibi özellikleri değerlendirildikten sonra kaplama kalitesinin yeterli seviyede olmaması nedeniyle bakır altlıkların kumlanmasına karar verilmiştir. Kumlanmış bakır altlıklar üzerine üç farklı seramik (B4C, TiC, TiB2) takviyeli üç farklı toz karışımları püskürtülerek kaplamalar elde edilmiştir. Kaplamaların kesitlerinden alınan taramalı elektron mikroskobu ve optik mikroskop görüntüleriyle arayüzey kalitesi, porozite, kaplama kalınlığı ve seramik dağılımı gibi yapısal özellikler incelenmiştir ve bütün kaplamaların başarılı bir şekilde yeterli kaplama kalınlıklarında üretildiği gözlenmiştir. Üretilen kaplamalara 300°C ve 600°C sıcaklıklarında argon atmosferi altında ısıl işlemler uygulanmıştır. Böylece ısıl işlemin kaplamanın mekanik özelliklerine etkilerinin incelenmesi hedeflenmiştir. Isıl işlemsiz ve ısıl işlemli durumlardaki kaplamaların elektrik iletkenlik değerleri ölçülmüştür. Soğuk gaz dinamik püskürtme tekniği ile üretilen kaplamalarda elektrik iletkenlik değerinde düşüş gözlenmiştir ancak ısıl işlemle beraber elektrik iletkenlik değerleri daha makul seviyelere çıkmıştır. Kaplamaların yapısındaki fazların tespiti için XRD analizleri yapılmıştır. Bu analizler sonucunda kaplama yapısında farklı bir faza rastlanmamıştır. Kaplamaların mekanik özelliklerinin incelenmesi için ısıl işlemsiz durumdaki ve ısıl işlemli durumdaki kaplamalara aşınma deneyleri uygulanmış ve kaplamaların sertlik ölçümleri alınmıştır. Aşınma deneyleri sonucunda takviyesiz bakır kaplamanın ve TiB2 takviyeli kompozit kaplamanın her koşulda bakır altlığa gore daha yüksek aşınma direncine sahip olduğu görülmüştür. Sertlik ölçümleri de soğuk gaz dinamik püskürtme yöntemiyle üretilmiş kaplamaların ısıl işlemsiz durumda saf bakıra gore yaklaşık 1.5 kat daha yüksek sertlik değerlerine sahip olduklarını göstermiştir. Isıl işlemle beraber sertlik değerlerinde düşüş gözlenmiştir. Sonuç olarak, aşınma deneylerinin sonuçları, elektrik iletkenlik ve sertlik değerleri ışığında elektrik kontak malzemesi olarak kullanılan bakır malzemesinin yüzey özelliklerini geliştirmek amacıyla 300 °C ısıl işlem uygulanmış saf bakır tozları ile yapılmış kaplamalar tercih edilebilir. Eğer seramik takviyesi kullanılacaksa bu takviye TiB2 malzemesi olarak seçilebilir.

Electrical contact materials are defined as materials which provides connection or disconnection of electrical circuits without any damage. Best electrical contacts should have high thermal and electrical conductivity. Because they should have minimum resistance to current and distrubute the heat which arises with resistance rapidly. Copper is a widely used material in electrical contact applications. Not only it provides the requirements for being used as electrical contacts such as electrical and thermal conductivity but it also has some additional properties like easy workability and low cost. However, due to low mechanical properties when compared with silver, mechanical properties of copper need to be improved to extend service time and decrease maintenance costs. Cold gas dynamic spraying, simply cold spraying, is a relatively new coating process. This technique was developed by Anatolii Papyrin and his colleagues in the 1980’s. They coated several number of metals, composites and polymers on to different substrates. In this process, particles reach supersonic velocities (500-1200 m/s) and they are exposed to severe plastic deformation upon impact on the surface and deposited on the surface. Cold gas dynamic spraying technique is similar to thermal spraying technique. But the particles do not melt to deposit on the surface at cold spraying technique. Because of its low temperature characteristic, oxide-free coatings can be obtained with this process especially for easily oxidize materials such as copper and titanium. In this study, it is investigated that of the coatability of three different ceramic particles reinforced copper matrix composite coatings to the copper substrate which is used for electrical contacts. The target is the improvement mechanical properties of copper without any decrease of electrical conductivity of copper. So the service time of copper will be increased and the mainenance costs will be reduced. For experimental study, firstly feedstockpowder mixtures were prepared for the cold dynamic gas spraying process. Three different ceramic powders (B4C, TiB2 and TiC) were used for reinforcing the copper powders. These ceramic powders were mixed with a volume ratio of % 12.5 and %87.5 copper. Then, to determine the optimum parameters for cold dynamic gas spraying process, optimization studies were made. After coating process, B4C reinforced copper matrix composite coatings were investigated by hardness measurements, coating thicknesses and microstructural examination. According to the results of these studies, the quality of coatings were not enough. Therefore it was decided that copper substrates would be sandblasted to increase the roughness of substrates, so it can be easier for powders to bond the substrates. After the copper substrates were sandblasted, their roughness values increased almost 100 times. So powders bonded surface of substrates more easier. In optimization process, it was seen that lower traverse speed increase the coating thickness but also increase the oxidation of surface. The coating thickness is also increases with higher number of passes. After optimization process, coatings were prepared with determinated parameters by cold gas dynamic spray equipment. Then coatings were prepared with metallographic procedure to examine the microstructural properties and hardness measurements. The microstructures of samples which were prepared with metallographic procedure were investigated with optical and scanning electron microscopes. Scanning electron microscope images have revealed that coatings adhere to the substrate without any discontinuity at the interface and ceramic particles distribute uniformly within the Cu matrix. Coating thicknesses were measured with the software of the optical microscope. Among the coatings, TiB2 yielded the highest thickness where unreinforced copper coating had the second highest thickness. On the other hand, the thickness values of TiC and B4C reinforced composite coatings were measured significantly lower values, respectively. To observe the difference between the ceramic particle content in feedstock powder mixtures and in coatings, image analysis software was used. The results have shown that percentage of ceramic particle contents retained in the coatings’ structure were less than those of the feedstock powder mixtures. This suggests that during spraying ceramic particles do not deform plastically but are surrounded by the plastically deformed Cu particles. The highest ceramic particle content was obtained with TiB2 containing feedstock mixtures. The electrical conductivity values of the cold dynamic gas sprayed coatings are lower than that of the Cu substrate. This is probably due to cold worked microstructure and ceramic particle content of the coatings. Electrical conductivity values increased with heat treatment. This could be explained by the removal of the internal stresses in the coating. According to the XRD patterns, cold sprayed coatings and Cu powder have the same phases. No phase transformation was observed from the XRD patterns of all coatings. The ceramic particles other than TiB2 in the coatings could not be detected by XRD analysis due to their very low contents which are proven by the image analysis results. Only a small diffraction peak of TiB2 phase was observed. For crystallographic characterization of the coatings, the peak widths were observed. Littleextension of the peaks were observed after this examination. This extension is the result of internal stresses caused from the plastic deformation of copper particles as a result of the high velocity impact onto the substrates. After heat treatment the width of peaks were decreased because of the removal of the stresses in the coatings. The results of hardness measurements show that cold sprayed coatings have higher hardness values than that of copper substrate. At the same time, presence of ceramic particles in the coating structure increases hardness. Higher hardness of the cold sprayed coatings when compared with that of Cu substrate can be explained with strain hardening of the spray particles which undergo severe plastic deformation upon impact. On the other hand, for the composite coatings, uniformly distributed ceramic particles also contribute to the hardness as a result of dispersion hardening effect. Heat treatment reduced the hardness of coatings. This is the result of removing internal stresses in the coatings. To compare the wear resistances of coatings, wear track areas were calculated and arranged relatively to the wear track area of copper substrate. Unreinforced Cu coating exhibited the lowest wear rate in all conditions where the coating containing TiC showed the highest wear rate in all conditions. The detachments of the ceramic particles were seen in the scanning electron microscope images of the wear track of coatings. But these particles were less in the wear track of TiB2 reinforced composite coatings because of the bigger particle size than other ceramic particles Detached particles caused the third body abrasion and increase the wear rate during the test. This condition arises from the easier derachment of ceramic particle with the respect to the copper ones, because the adhesion of metal to metal is stronger than metal ceramic adhesion. Corrosion behaviours of coatings was observed by the potentiodynamic polarization curves obtained the electrochemical test in the 3.5 % NaCl solution. Electrolitic corrosion tests were done between the potential of OCP-0.8 and OCP+1.2 with the scan rate of 1 mV/s. All coatings show similar results and their performance is generally lower than that of the copper substrate. Corrosion started earlier at unreinforced copper coatings than composite coatings. But as soon as corrosion damage started, it proceeds faster at composite coatings because of the spaces around the ceramic particles. Heat treatment did not change the corrosion behaviours of coatings significantly. As a result of these processes, unreinforced copper coating which was heated at 300 °C can be produced for improvement of the surface properties of copper with respect to the results of wear tests, hardness and electrical conductivity measurements. Also if any ceramic reinforcement is necessary, TiB2 is the best option for reinforce material for copper matrix composite coatings.