INTRODUCTIONThe automotive industrybegan with the revolutionary invention of a feasible internal combustionengine. Since then, the industry has developed greatly throughadvances in mechanical technologies and new and improved materials. This isvery much evident today with things like autonomous vehicles, electric cars and3D printing. We usuallyconsider metals to be the most important class of engineering materials when itcomes to the automotive industry.
However, thecontribution of ceramic materials to automobile technologies has allowed forthe industry to develop substantially. Many ceramic components, such as knocksensors, silicon nitride turbochargers and ceramic glow plugs have beensuccessfully applied to automobiles. This report will focus on the contributionof ceramics to automotive technologies and the potential contributions in thefuture. It will also discuss the different manufacturing processes for theparticular ceramic materials and what improvements ceramic materials offer fromthe materials previously used for the same component. BRIEF OVERVIEW OF CERAMICS AND CERAMIC PROCESSING TECHNIQUES Ceramics are important in the world of engineering dueto their mechanical and physical properties, which are quite different fromthose of metals. A ceramic is defined as an inorganic compound; this means itcontains metallic and non-metallic elements.
Furthermore, ceramics can bedivided into three categories: Traditional ceramics, new ceramics and glasses. Traditional ceramics, some of whichhave been used for thousands of years, include: clay, alumina and silicon carbide.These types of ceramics are made from minerals occurring in nature.
Newceramics include some of the materials listed previously, such as alumina, butwith properties that are enhanced through modern processing methods. Forexample, new ceramics can include: metal carbides such as tungsten carbide andtitanium carbide (these are used for things like cutting tools and industrialmachinery), and nitrides—such as titanium nitride and boron nitride (used ascutting tools and grinding abrasives). Traditional and new ceramics have acrystalline structure. Glass is very different to traditional and new ceramicsand it can be distinguished by its non-crystalline structure. The properties that make ceramicsuseful in engineering are their high hardness and good electrical and thermalinsulating characteristics. In addition, the high yield stress of ceramicsallows for the production of precisely machined parts that can maintain accuratedimensions over longer periods of time. They also have great chemical stabilityand high melting temperatures. Some ceramics are translucent.
However, thebrittle behaviour of ceramics has restricted their applications to structuralcomponents. This can cause problems in the processing and performance ofceramic products. For processing purposes, ceramics can be divided into twobasic categories: molten ceramics and particulate ceramics. Different methodsof manufacturing are required for both types and this is shown in figure 1.Molten ceramics include glasses and require glass-forming processes. Glassforming processes can include pressing, blowing, drawing and fibre forming.
Pressing is a method of manufacture used to form shapes like plates and dishes.Figure 2 shows the basic steps in this method of manufacture. Blowing is usedto produce objects like jars, bottles and light bulbs. This is shown in figure3. Drawing is used to form lengthier objects like tubes and rods. Theseprocesses primarily involve solidification. Particulateceramics include traditional and new ceramics.
The processing of particulate ceramicsgenerally takes place in four steps: 1. Powder processing (raw materials)2. Forming 3. Sintering (firing)4. Finishing (painting, electroplating), densificationand sizing, heating treatment (hardening and strengthening)Powderprocessing is essentially the preparation of the starting powders. This involvescrushing and grinding the powders to the desired size range and adding anyadditives to enhance the properties of the ceramic used. Once the powders aprepared they can go through various forming methods.
Particulate formingprocesses can include powder pressing, hydro-plastic forming, slip casting andtape casting. Now that wehave discussed the basic ideology around ceramics and processing ceramics letsdiscuss how ceramics have been incorporated into the automotive industry,through two case study parts, and how this has improved the industry as awhole. CASE STUDY PART 1: CERAMICTURBOCHARGER (1985) The first example that I willdiscuss is the ceramic turbocharger. Since the inventions of the internalcombustion engine mechanical and automotive engineers have been searching forways to boost its power. One way to add power was to build a bigger engine.However, this wasn’t the best method as bigger engines will weigh and cost moreto build and maintain.
A more appropriate way to add power to the engine was toforce more air into the combustion chamber. Adding a supercharger or a turbocharger can dothis. These devices produce high-pressure air in the engine cylinders and thusprovide more fuel meaning a bigger explosion and greater horsepower. The maindifference between turbochargers and superchargers is their source of energy. Superchargerscan be powered mechanically by means of a belt or chain that is connected tothe engine’s crankshaft.
Turbochargers use a turbine rotor that is powered bythe gases from the engine exhaust manifolds. The turbocharger was developed in 1905and began to be widely used in the mid 1960s. To make the turbocharger, twoimpellers were needed; one was a turbine wheel, which was rotated by theexhaust gas. The second impeller was a compressor impeller. The compressorimpeller increased the pressure of the air. These were fitted onto a centrehousing/hub rotating assembly.
As explained above, the turbocharger was able togenerate a large power output from a compact engine. However, it was found thatthe turbocharger experienced something called turbo lag. This was essentially,a slight delay between the intention to accelerate and the actual accelerationof the vehicle.
As a result, ceramics were used for the turbine rotor and thisreduced the overall weight of the component as well as the rotational inertiamoment. Therefore, the turbo lag was reduced. In 1985, the world’s firstceramic turbocharger was made and it incorporated silicon nitride.
Figure 4shows a silicon nitride turbocharger and figure 5 shows the operation mechanism of the turbocharger. Silicon nitride was abetter material to use compared to the tradition nickel based super alloys. Thiswas due to silicon nitride ceramic having a much lower density compared to thenickel based super alloys. The density of the silicon nitride ceramic wasapproximately 3.2Mg/m3 and the density of the metallic alloy was8.
2Mg/m3. As you can see from figure 6, the revolution speeds for aceramic rotor and a metal rotor are compared. The time taken to reach 10,000rpmis approximately 36% shorter for the ceramic rotor. As well as the siliconnitride turbocharger being lightweight and having a low density, it also had a high thermal resistance, which wasimportant in order to resist degradation in the high temperature exhaust gas. The main steps, in the manufacturing of the siliconnitride turbo rotors are shown in figure 7. The rotor was made by bonding aceramic wheel with a metalling shaft (this was done by brazing and interferencefitting). The ceramic wheel was manufactured by ceramic injection moulding. Theceramic injection moulding process involves the following steps: powderpreparation, injection moulding, de-binding process and sintering.
The firststep in the injection moulding process for the ceramic turbine wheel is to mix asilicon nitride powder with sintering additives such as rare earth oxide. Amoulding agent/binder is then added to the mixture to make the wheel. Themoulding agent/binder is a key aspect to the moulding process as it has a lowermelting point than the ceramic powder. Therefore, it allows for the twomaterials to be separated at a later stage.
This mixture is then injected intoa mould cavity with the use of an extruder. The mould is then heated until thetemperature is high enough for the binder material to melt (but not theceramic). The wheel will then go through sintering and machining. Once this isdone the ceramic turbine wheel will be bonded to the metallic shaft. Afterthis, an aluminium impeller is fitted on the other side of the shaft usingscrews. In conclusion,turbochargers are an effective means of lowering an engine’s specific fuelconsumption and increasing its power output. Due to environmental concerns andthe focus on gas conservation, the future of turbochargers is in doubt.
As aresult, the ceramic turbocharger is not currently used in new automobiles andthe production is limited. Automobile manufacturers are beginning to researchfuel saving applications of the turbocharger. CASE STUDY PART 2: CERAMIC GLOWPLUGS (1985) The second example I will discuss isthe ceramic glow plug. These are primarily used in diesel engines to aidstarting the engine in low temperatures. Diesel engines arecompression-ignition engines and do not have spark plugs to start combustion. Asa result, ignition is performed by allowing for clean air to be taken into thecombustion chamber. The clean air is then compressed, making it heat up to atemperature of around 700-900oC. Fuel is then injected into thecombustion chamber with the air that has been compressed.
Due to the airheating up during compression, the high temperature triggers auto-ignition andthe diesel engine starts. However, starting a diesel engine is difficult whenthe temperature is low and the engine is cold, as the compressed intake airwill not have a high enough temperature. This problem can be solved with theglow plug.
The glow plug creates the ideal ignition condition for the injectedfuel by generating thermal energy through an electric current. Metal glow plugswere used throughout the years and comprised of a metallic heating coil insidea metallic tube. The heater element was filled with magnesia powder and wrappedaround the coil. Metal glow plugs were able to achieve a temperature of 800oCin five seconds when a voltage of 11V was applied. Ceramic glow plugs werefirst introduced in the mid 1980s. They were made by placing the metallicheating coil inside an insulating sintered ceramic (silicon nitride).
The firstgeneration ceramic glow plug was able to achieve a temperature increase faster thanthat of metal glow plugs. The temperature reached 800oC in threeseconds after a voltage of 11V was applied (two seconds faster than the metalglow plugs). The heat resistance of the first generation ceramic glow plug was1200oC. To stop unnecessary temperature rise in the metal fitting, acontrolling resistance was fitted onto the ceramic heating elements.
Figure 8shows a comparison of the structures of a standard metal glow plug and a firstgeneration ceramic glow plugs. Around 1990, the second generation of theceramic glow plug was made. These used a highly heat-resistant silicon nitriderather than normal silicon nitride. They included ceramic resistors for hightemperature heating. The heat resistance of the second-generation ceramic glowplug was substantially improved to 1350oC.
This meant that thecontrolling resistance, which was needed for the first generation ceramic glowplug, could be removed. At this time it was found that ceramic glow plugs werefar more superior to metal glow plugs in terms of heating temperature. This wasdue to ceramic materials having great heat resistance and durability. In 2005the third generation ceramic glow plug was created and it achieved a rapidtemperature increase of 1000oC in 2s with a voltage of 11V applied.This was done by lowering the resistance value and maintaining the heatresistance from the second-generation ceramic glow plug. Figure 9 shows thefirst, second and third generation ceramic glow plugs. As you can see, ceramicglow plugs were able to provide higher temperatures to start diesel engines. Thisis further shown in figure 10.
A further advantage of ceramic glow plugs isthat the heating portion can be made much smaller. This is beneficial asautomotive industries are trying to develop smaller engines and space can beseverely restricted around the combustion chamber. The main parts needed for the ceramic glow plug include: an insulator,thread, centre electrode, heating coil, ceramic casing and connection terminal,metal shell, taper seat and finally a contacting ring.
The insulating ceramicmaterial used in the production of glow plugs was made by weighing siliconnitride and sintering additives, and grinding and mixing them in a kind ofrotary sieve (trommel). An organic binder is also added and this is then spraydried. The powder is press formed by dies into shape. The first generationceramic glow plug was made by using the metallic heating coil and theinsulating ceramic material and pressing the two together in a mould.
As aresult, the coil can become enclosed in the ceramic. It can then be de-waxedand sintered by hot pressing. In terms of the second and third generationceramic glow plugs the materials for the internal resistors were weighed,grinded, mixed and spray dries in the same way the insulating ceramic materialfor the first generation ceramic glow plugs were processed. The demand for ceramic glow plugs is estimated toexpand in response to the increasing numbers of diesel engines of lowercompression ratios. The diesel engine is now continuously being improved interms of fuel consumption, power outputs and ultimately refinement. Companies likeVolkswagen and NGK are researching more ways to improve the cold startperformance of diesel engines with glow plugs.
They are trying to find out howthey can make the glow plug have a long service life and an extendedpost-heating time. In order to do this a breakthrough is needed so that theadvantages of ceramics may be utilized to a greater extent.