Application of thermal spraying in the automobile industry
Barbezat Gérard
Abstract
New advanced thermal spray technology, provides wear resistant coatings on the cylinder surface of aluminum or magnesium engines. The special surface topography obtained after the finishing allows us to decrease significantly the coefficient of friction and to decrease the fuel consumption by an amount of 2 to 4%. Engine tests on diesel and gasoline engines have confirmed the value of this technology for energy saving. This coating technology was introduced 4 years ago in Europe by the manufacturers of high power diesel and gasoline engines. The combination of different MMC coating materials allows the development of new specific solutions for each type of engine. Coatings with improved corrosion resistance and abrasion resistance were also developed and are available now. A brief overview of other applications of thermal spraying in the automotive industry will also be given.
Keywords: Automotive engines; Friction reductions; Cylinder bores; Thermal spraying
1. Introduction
Advanced thermal spray technology is widely used in the automotive industry. Transmission and engine parts are coated in large volume using different processes. Some applications have already reached maturity. Other applications are new and represent an important market for the future. Coated transmission parts such as synchronizing rings and shift forks are widely coated using wire flame spraying with molybdenum to provide a constant coefficient of friction and prevent scuffing. Piston rings are coated using the plasma spray process in general and also with the HVOF spray process in the case of high performance piston rings for diesel engines. Metal matrix carbides containing coatings are used for piston rings. Over the last 10 years there has been intensive development in methods to coat the cylinder bore of aluminium cast engine blocks for the automotive industry [1–6].This paper gives an overview of the materials used for engine blocks, the thermal spray processes for cylinder bore coatings and also the coating materials for the internal plasma spray process.
2. Material for monolithic engine blocks
For more than 100 years cast iron with lamellar graphite has been the material used for engine blocks. This material re- presents a compromise between tribological properties, low costs of production with sand casting technology, good machinability, but limited mechanical properties, especially a relatively low toughness. New developments in foundry technology improving the casting tolerances bring renewed interest in this material because it allows some reduction of the weight. Cast iron with vermicular graphite was developed some 20 years ago and is already used in those high loaded diesel engines, when higher mechanical strength and better toughness are required. However cast iron with vermicular graphite has
lower machinability and requires a particular foundry design to ensure the precipitation of the graphite in the required shape. The solidification rate determines graphite precipitation (Table 1).In the case of monolithic cast aluminium alloy a high silicon content is required to secure the tribological properties. The necessary Si content is in the range of 17 to 20% by weight. Special machining of the liner surface is required to avoid contact between the aluminium matrix and the piston ring material. The size of the primary silicon crystals plays an important role in the tribological properties.
Low pressure die casting is required to ensure the integrity of the cast material in this case. The high content silicon has a negative effect on the machinability and the toughness of these hypereutectic AlSi cast alloys.
3. Requirement for improvement and hybrid solutions
To reduce the weight of engine blocks, hypoeutectic aluminium silicon cast alloys were developed and are widely used today. Today more than 60% of the engines for passenger cars are produced in cast material from this category.
However the hypoeutectic aluminium alloys have poor tribological properties and require liners of cast iron with lamellar graphite or other surface treatments. During the last 20 years first galvanic coatings and later thermal spray coatings
were the objects of intensive development. Nickel coating with addition of silicon carbide has found widespread application in small engines and special applications. However this process requires complex chemistry and is relatively expensive. Several thermal spray processes were developed for the cylinder bore application during the last 15 years [1–6]. But today only the plasma spray process has reached the status of high volume manufacturing. Regardingmaterials, plasma spraying offers a large range of possibilities and optimization potential. Some characteristics that limit the use of hybrid solutions for cylinder liners are given in Table 2.
4. Thermal spray processes for coating on cylinder bores
Thermal spray technologies are important in a variety of different industries for the deposition of anti wear coatings based on metallic, carbide containing, ceramic or composite materials. Plasma spraying has already found a wide range of applications in the automotive industry, especially for piston rings,alternator covers, ceramic overlay for oxygen sensor protection. The high thermal energy density available within the plasma used for melting the powder coupled to the ability to manufacture powder and design plasma guns for specific applications
with short spray distances has rapidly promoted the use of plasma spraying.
Different thermal spray processes are being used to provide coatings for cylinder bores. Plasma and HVOF processes are characterized differently with regard to two principal aspects.
These are the thermal and kinetic engines of the sprayed particles. A mparison of atmospheric plasma spraying APS and high velocity oxy-fuel (HVOF) spraying shows that the plasma processes offer higher plasma temperature at relative low velocities whereas the HVOF process is the opposite. The temperature of the HVOF spray stream does show some dependence on the choice of fuel gas. However the HVOF spray distance exceeds the plasma spray distance. When HVOF is used at shorter spray distances it can lead to overheating of the substrate. In the specific case of the coating of cylinder bores the transfer of heat into the substrate is major concern since engine blocks are manufactured from AlSi cast alloy and overheating
during coating can result in an unacceptable distortion or change in microstructure. Since most cylinder bores have an inside diameter of 60 to 100 mm, a short spray distance is required. In order to keep the heat transfer during spraying as low as possible it is necessary to select suitable plasma guns,
spray parameters, scanning speeds during spraying and appropriate cooling methods. The electric wire arc spraying process is also an alternative for the coatings of deposition in cylinder bores. It has some restrictions regarding the choice of
coating materials and the reliability of the melting process.
Table 3 compares the three main coating processes for cylinder bores in AlSi cast alloys. The high degree of freedom regarding the choice of coating materials, the reliability of the melting process and the low heat transfer to the engine blocks are the key advantages of the plasma processes.
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