Aşınmaya Karşı Plazma Püskürtme Ve HVOF (Yüksek Hızlı Oksi-yakıt) Püskürtme Yöntemleriyle Yüzey Hazırlama

Abstract

Bu çalışmada, uygulamada birçok türü kullanılan ısıl püskürtme yöntemlerinden; alevle püskürtme, elektrik ark ile püskürtme, patlamalı püskürtme, plazma püskürtme ve HVOF püskürtme yöntemleri detaylı olarak incelenmiştir. Daha sonra, teknolojik açıdan son gelişmiş yöntemler olan Plazma ve HVOF püskürtmenin üstünlükleri ve kendileri arasında karşılaştırmaları irdelenmiştir. Bu son teknoloji sistemlerinin klasik püskürtme yöntemlerine göre ne kadar üstün olduğu yapılan çalışmalar sonucunda elde edilmiştir: 1. Yüksek kaplama kalitesi; - Yüksek mukavemet, - Yüksek makro ve mikro sertlik, - Kaplamada düşük oksit içeriği. 2. Yüksek kaplama; - Yüksek püskürtme oranı, - Yüksek kaplama verimi, - HVOF yöntemiyle plazma püskürtmede karşılaşılan çok miktarda parametrelerde azalma. Yüksek ekonomik özellikler; Kaplama oranının daha yüksek olması, Mekanik özellikleri yüksek, temiz olarak kaplanmış malzemelerde son yüzey işlemleri için ekstra işlerin ortadan kalkması (zaman, işçilik, makinelerin aşınma olayının azalması vs. gibi).

Plasma and HVOF spraying are high-tech procss used in a wide range of industries to provide functionally effective surfaces. Applying this versatile, flexible, high quality and economic technology, it is possible to adapt the surface properties of machine components optimally to the stresses and enviroments found in operation. The increase of outputs efficiencies of machines brouht about by higher pressure, temperatures and mass flow often results in the max. permissible stresses of present day materials being approached. Modern engineeing practice not only imposes stringent requirements as far as individual properties concerned, but also makes demands on the versality of the materials. The development of new technologies and prosesses requires suitable materials for their realization even thoroughbred, sophisticated materials are often not up to the stringent in some cases opposing requirements. Since many forms of attack,such as: - corrosion - friction - wear - heat - impact - radiation occur on the surface of a component, or are transferred via the surface into the component, surface protection has a considerable significance as regard modern materials' technology. The purpose of the surface technology is to produce functionally effective surfaces. This implies matching the surface properties of a component to its particular operating stresses. The aim is to achieve improvement of operating live, reliability.economic performans, safety and output from products and processes. Spraying has a particulaly high potential in the solution of complete material problems. It is possible using this method to coat components with almost any type of material. Coatings can be applied technically onto all suitable base materials, irrespective or whether the component is small or large. A high degree of automation can be easily accomplished and the spray process integrated into fabrication sequences both for mass and individual part production. Xll Plasma spraying process: Functional principle: plasma spraying uses and electric arc maintained in a nozzle as source of heat. This arc heats a stream of inert gas, genrally argon, nitrogen or helium, to very high temperatures. Gas temperatures of more then 20.000°C occur. This cause dissociaton of the gasmolekules into atoms and ionization of the proportion of the latter. A large increase of gas volume occurs, due to the high temperature, so that the gas flows from the nozzle at high speed. In modern plasma spraying installations, the jet has a flow speed which is a multiple of the speed of sound. The coating forming material is injected in powder form with the aid of a carrier gas into this high energy plasma jet. The particles ae melted an projected onto the pretreated base material. Almost all materials which can be melted without degradation and which are available in suitable grain configuration can be converted into high quality coatings using plasma spraying. Plasma spraying has earned great technical and economic importance in the production of functionally effective surfaces. Advantage of process; - no distorsion, so that finish machined parts can be coated, - no change of microstructure in the basement material, - no oxidation of the parts during coating, - coating of parts made from low melting point alloys, such as Al, tin and zinc alloys, as well as from sellected plastics is possible. The process is divided into the some following classifications which are seeing the Fig. 11. Spray And Base Materials Spray Materials: All substances which comply with the following requirements are suitable as spray materials for the production of plasma spray coatings: -the spray materials must melt without changing in any undesired way, e.g. no degradation, sublimation or oxidation -the spray material must be producible as powder in a form suitable for spraying, e.g. grain shape, grain size and size distribution. At present, several hundred different materials are plasma sprayed, they can be classified as follow:. pure metals ( e.g. Mo, Ni, Ta, Al, Zn). alloys (NiCr, NiCrAlY, FeCr-BSiC, steels, bronzes). pseudo-alloys (e.g.Cu-W, bronze-steel, Al-Mo). ceramics (A1203, Cr203, Ti02, Zr02, WC). cermets(seramik+metal) (e.g. Cr3C2/NiCr, WC/Co, Zr02/NiAl). Plastics (polyester, polietilen) Xlll Base Materials: all metals and alloys are suitable as base materials for plasma coatings. The most important are: - all types of steel - grey and SG cast irons - nickel and cobalts alloys - light metals, such as aluminium, magnesium and titanium alloys - non-ferrous, heavy metals, such as copper and copper alloys Furthermore, ceramics and certain plastics can be also coated. Their suitability as carriers for plasma coatings is to be tested for individual specific applications. Applications: Up to now, plasma spraying coatings have been successfully used for the following applications:. wear protection against - abrasion - scuffing - erosion - fretting corrosion. corrosion protection against - oxidation - hot gas corrosion - atmospheric corrosion - immersion corrosion. thermal insulation. reconturing, thereby repairing parts by adding material onto worn locations, or reconstituting a part which has been rejected due to faulty material or incorrect machining. surfaces with special properties - electrically insulating or conducting - abradable coatings - increase of surface area and structuring of surfaces - diffusion barriers - active and inert surface coatings - surfaces with catalytic properties - ion conducting coatings - decorative coatings - coatings which are highly absorptive or reflective to light. special application, such as: XIV - production of moulds on dissolvable cores - bedding in and fastening of probes - production of sensors - refining and spherodizing of powders The most suitable spray powders to be used in the production of functionally effective surfaces by plasma spraying are chosen to match the particular application, i.e. function of the surface, base material, stressing under operating conditions, geometry and size of the part. HVOF; All HVOF systems are essentially similar in their broad concept, but differ consderably in their design. HVOF guns consists essentially of an internal combustion chamber in to which gaseous (propylene, propane, hydrogen, MAPP ) or liquid fuel and oxygen are injected at high 0.5 - 3.5 MPa pressures and high flow rates, up to 0.016m /sn. This products are then ignited and continuously combusted, the resulting flame being allowed to expand supersonically and exit to atmosphere through along nozzle similar to that of a rocket motor, as indicated by the presence of charecteristic "shock diamonds" in the flame. Powder is injected into, or downstream of, the combustion chamber and particles are heated and accelerated by he flame. The high back pressures generated by most HVOF system necessitates the use of pressurized powder feeders in order to convey the powder into the hot gas jets. The HVOF process has shown the following performance and economic advantages for spraying compared to plasma spray: 1. Superior coating quality. Higher lap shear and tensile bond strengh. Higher macro and micro hardness. Lower coating oxides 2. Superior Deposition. Higher spray rate. Higher deposit effeciency. Reduction in the number of critical parameters 3. Superior Economics. Two times improvement in deposit rate. Elimination of extra cleaning steps due to clean overspray.