Plazma ile püskürtmede püskürtme tabakasının özelliklerinin incelenmesi

Abstract

Bu tez çalışmasında, ısıl püskürtme ile kaplama yöntemine ilişkin prosesler, kaplama malzemeleri ve bu yöntemle elde edilmiş kaplamalardan beklenenler anlatılmıştır. Bu çerçevede, özellikle yaygın bir uygulama alanına sahip olduğu için plazma arkıyla ısıl püskürtme prosesinin açıklanmasına ağırlık verilmiştir. Isıl püskürtmede yaygın olarak kullanılan kaplama malzemeleri hakında bilgiler verilmiştir. Ancak konunun daha net bir şekilde anlaşılması için seramik esaslı ve alüminyum esaslı malzemelerinin ısıl püskürtmede kaplama malzemesi olarak kullanımları hakında genişçe bilgiler verilmiştir. Isıl püskürtmeyle metal yüzeylerin kaplanması, toz, tel yada çubuk formundaki kaplama malzemesinin bir ısı menbaında ergitilerek yada yan ergimiş duruma getirilerek basınçlı havayla veya bir gazla zerecikler halinde kaplanacak metal yüzeyine püskürtülmesi, püskürtülen bu zereciklerin yüzeye çarpıp yasılaşarak bir tabak oluşturması olarak kısaca tanımlanabilir. Kaplama tabakası büyük ölçüde mekanik olarak esas metal yüzeyine tutunmaktadır. Bu sebeble kaplanacak metalin yüzeyinin önceden temizlenip, pürüzlendirilmelidir. Pürüzlendirmeden dolayı oluşan çıkıntılar püskürtülen ergimiş kaplama malzemesi zereciklerine tutunma alnlan oluşturmaktadır. Pürüzlendirme işlemi ısıl püskürtmeyle kaplamada çok önemlidir. Pürüzlendirme küçük bilyalar halindeki sert parçacıkların yüzeye basınçlı havayla püskürtülmesiyle gerçekleştirilir. Temizleme işlemi ise kimyasal, mekanik ve parçanm ısıtılması suretiyle yapılır. Isıl (termal) püskürtme, ısı menbaının oluşturulması yöntemlerine göre; Plazma arkıyla, alevle, elektirik arkıyla, patlatma tabancasıyla ve yüksek hızlı oksi-yakıt karışımıyla (HVOF) püskürtme diye proses guruplarına ayrılır. Isıl püskürtmeyle kaplama eldesi, esas metali korozyondan korumak, metal yüzeyini aşınmadan korumak, aşınmış parçaların eski tölarans sınırlarına getirilmesi, elektirik ve ısı yalıtkanlığı sağlamak amacıyla, hasas elektronik komponentleri magnetik akılardan korumak gibi amçlarla uygulanır. Bu yöntemde kaplama kalınlığı tüm yüzeyde eşittir. Kaplama parametreleri hasas olarak kontrol edilebimekte, çok dar alanlarda kaplama yapılmasına imkan vermektedir. Metal ve metal dışı bir çok malzeme bu yöntemle kaplama malzemesi olarak kullanılabilmektedir. Ayrıca aynı anda birden fazla toz halindeki kaplama malzemesinin aynı anda yüzeye püskürtülmesi sayesinde çok değişik kaplamalar elde etmek mevcuttur. Bu tür elde edilen kaplamalara "yalancı alaşımla kaplama" denmektedir ki bu terim oldukça yenidir. Iısıl püskürtmeyle oluşturulmuş kaplamaların en zayıf noktası; kaplamanın gözenekli bir yapıya sahip oluşudur. Bu gözeneklerin büyük bir bölümü yüzeye açıktır. Dolayısıyla korozif sıvı ve gazların bu gözeneklerden sızıp esas metali korozyona uğratmasına ve sonuçta kaplamanını düşüp işlevsiz kalmasına neden olmaktadır. Ancak bu gözenekler kaplamayı sünekleştirdiğinden kimi durumlarda yararlıdır.

Wear is a big problem which has been encountered in all industry sectors. If precaution aren't taken soon and systematically, its results will be big. To give an idea about dimensions of the problem, we can look at its cost; in Deutshland, the cost of wear is 2 billion DM in the metalurgy industry, 1.5 billion DM in the mine industry, 500 million DM in the manufacturing industry. Although all this importance, especially in Turkey, it isn't given enough importance to the problem. Why is it neglected? We can show two reasons: The first one; according to today, wear was less important effect in the calculation of the machine's useful life in the past years. For example; when we compare vapour cylinder which was manufactured in 1760s with todays car cylinders, although both machines' power are the same, their designs are very difirent from each other. 1760s' cylinder was working in the low pressure and with a 6-7 mm manufacturing error. And a 5-6 mm wear dosen't effect the performance. But today, wear which can be allowed is in very limited tolarences. The second reason, the rate of wear generally is very low and until today, there weren't probabilities to measure the wear. Corrosion causes to damege in metals too. The most effective way of protecting from corrosion and wear is coating. As a coating material, coatings materials which are resistance of wear and corrosion are used. In this thesis, the coating of metals by thermal spraying was examined. Thermal spray processes primarily used in the aerospace industry for applying thermal barriers and wear-reasistant coatings, this process is proving to be a versatile, cost-effective solution to a variety of engineering and maintance tasks. Uses for thermal spray processes and materials are expanding to include hundreds of engineered coatings that meet the strict performance standards of automotive, biomedical, turbomachinery and general manifacturing industries. Maintance departments worldwide are saving millions of dollars annually by aplplying corrosion and wear resistant coatings, nonskid surfacing, and reclaiming worn machinery parts using the thermal spray process. Thermal spraying is a generic term used to define a group of procsses that deposit finely divided metallic or non-metallic materials onto a prepared substrate to form a coating. The coating materialhnay be in powder, rod or wire form. IX The thermal spray gun uses a plasma arc, combustible gases or an electiric ark to generate the heat necessary to melt the coating material. The material changes to a plastic or molten state when heated, and is accelerated by process gases. The acceleration of the molte material produces a confined stream of particles that the process gases transfer to the substrate. The particles strike the substrate, flatten upon impact, and form thin platelets that bond to the substrate and to each other. The particles built up and cool into a lamellar structure that forms a coating. The thermal spray process is synergistic. System components, parameters and the desired coating functions and properties must be determined to select the type of material and equipment needed for the process. The process may be tailored for particular applications after establishing this information. Varios coating coatings can be achived by using different combinations of equipment and consumables. Basic thermal systems typically consist of a spary gun, power supply or gas control console, and a wire or powder feeder. Thermal spray offers distinct advantages over other coating, cladding and hardfacing processes. The low application temperatures reduce distortion and the ability to apply dissimilar materials makes it possible to enhance substrate surface properties. Thermal spray metal and ceramic coatings are used to produce: Corrosion resistance: Wire flame spray coatings of zinc or alüminyum are commonly used to protect structures from oxidation or salt water corrosion. Electrical conductivity: Plasma arc spray and flame spray have been used to apply metals for electrical contact areas for applications. Electrical resistance: Plasma ark and flame spray are widely used to apply oxide coatings as insulating layers in aplications such as electrical resistance heaters, capacitors. Oxidation protection: Nickel and cobalt base and nickel chrome and cobalt alloys are frequently used to provide oxidation protection. Dimensional builtup for salvage of worn parts: The most common use of thermal spray is salvagiging worn parts. All thermal spray processes are used in salvage work. Wear: The wear properties of low alloy steel and nonferrous metal surfaces can be improved by using the appropriate combinations of thermal spray processes and coating materials. Hard metal coatings such as the chrome-nickel-boron alloys and carbide bearing coatings are two mainstays in thermal spray coatings. Thermal barries: Zirconia, magnesia and alumina are used singly or together to provide thermal protection or to improve the efficiency of thermal systems. Rocket engines and adiabatic engines use thermal barier coatings to improve efficiency and reduce metal temperatures or cooling requirements. Free-standing shapes: Parts can be fabricated from hard to machine materials by building up a coating on a removable form. In this way, parts can be made to near final shape and dimention. Rocket nozzles and ion engine components are two examples of applications for this technique. Processes Plasma arc spray The devolepment of turbine and rocket engines during the postwar are of World War II presented new challenges for the thermal spray industry. Many materials nedded for coating high-strength, high tempareture parts contain oxides and carbides, which thermal spraying equipment that could produce temperatures higer than the capabilites of either the arc or flame spray processes. The plasma arc spray process uses a hot ionized gas (plasma) as the heat source to melt powdered materials. Plasma systems provide controllable temperatures higer than the melting range of most substances. In the plasma process, a gas or gas mixture passes through an arc created between a coaxially aligned tungusten cathode and an örfice in a copper anode. The gas partially ionizes during the heating process and produces a plasma. Injected into the plasma, the powder melts and the high velocity plasma stream proples it onto the substrate. The type of nozzle, arc current, gas mixture ratio and gas flow rate control the heat content, tempareture and velocity of the plasma stream. The primary plasma forming gas is either nitrogen or argon. A secondary gas, either hydrogen or helium, may be added to increase the heat content and velocity of the plasma. Plasma arc spray systems contain a central control unit regulates DC electiric power to the arc, cooling water unit, plasma gases unit and powder supply unit. Electric arc spray The high efficency of the arcfspray process makes two-wire electric arc spraying the fastest, least complicated and most inexpensive way to produce metal coatings. A wire feeder pushes two wires through the arc spray gun. The heat zone created by an arc melts the wires and coprassed air blows the molten particles onto the substrate. Arc spray equipment applies two basic types of coatings: Wear resistant coatings that provide barriers resistant to abrasion, corrosion, erosion, fretting, friction or galling; and resurfacing coatings used for rebuilding worn areas, salvaging improperly machined parts or improving the charasteristic of finished parts. Properly selected materials used in resurfacing coatings can result in parts that will outwear the orginals by factors of two or more. XI Flame Spray } The flame spray process uses the heat genereted by the controlled ignition of a combustible fuel gas to melt the wire. The het zone created by the gas ignition melts the wire and the copressed air blows the molten particles onto the substrate. The molten metal impinges the substrate and cools to the ambient substrate temperature. Applications for flame spray equipment are similar to those of electric arc spray equipment. Altough flame spray is not as efficient as electiric arc spray, it does offer the advantages of being more portable than most arc spray equipment. It can be used in the application of ceramics and fusible materials and is less capital intensive than the arc spray process. High- Velocity Oxygen Fuel Spray (HVOF) High-velocity oxygen fuel (HVOF) systems are relatively new innovations from the thermal spray industry. HVOF systems combine supersonic spray velocities with improved powder particle heating and melting characterictics. These systems produce high quality coatings with excellent density, hardness and bond strength. Various fuel gases inculidina propylene, hydrogen, acetylene and propane can be used. Materials with high melting points, like ceramics or refractory metals such as molybdenum, can be sprayed with some HVOF systems. Industries are finding many new uses for these systems, with the most notable being the application of tungusten carbide-cobalt coatings. The HVOF process is versatile and offers premium coatings for wear resistance, corrosion protection, thermal and electirical insulation. Spray parameters can be regulated over a wide range to provide the best temperature and velocity for specific coating applications. High depositation rates, shorten the spray time on large parts. HVOF systems are now being evaluated and modified for specific applications within the industry. Thermal spray technology is seeing increased intrest for a widw range of applications. New applications in surface technology and the productions of advanced materials are fueling much of this intrest. In fact, until recently, sources of autorathive information of the technology did not exist and education programs were not available. Simalar to welding technology, thermal spray uses much of the same equipment, yet there are important differences in feedstock and in the matellurgy of the final coated system. In technical universities, there are many lessen aboat welding on the other hand thermal spray industry has not reached this level of maturity. The market for thermal spray tecnology will expand the need for education and training as more engineers, deiseners and industries learn of its utility and potential for high quality. Xll The industry also needs to create and implement a set of minimum standarts that must be adopted by aplicators. These will assure users and future users of thermal sprayed products of high quality and raise thermal spray to the next level of application. Education, training, standards and certifiations will be the backbone of a strong and widely accepted technology and thus are an important need, especially as new processes, aplications, automation, or materials are introduced. Thermal spray equipment is well within the economic reach of most machine shops and manufacturing facilities and the number of uses for this technology is limited only by the imigination.