INTRODUCTION maintain accurate dimensions over longer periods of

INTRODUCTION

The automotive industry began with the revolutionary
invention of a feasible internal combustion engine. Since then, the industry has developed
greatly through advances in mechanical technologies and new and improved
materials. This is very much evident today with things like autonomous vehicles,
electric cars and 3D printing. We usually consider metals to be the
most important class of engineering materials when it comes to the automotive
industry. However, the
contribution of ceramic materials to automobile technologies has allowed for
the industry to develop substantially. Many ceramic components, such as knock sensors,
silicon nitride turbochargers and ceramic glow plugs have been successfully
applied to automobiles. This report will focus on the contribution of ceramics
to automotive technologies and the potential contributions in the future. It
will also discuss the different manufacturing processes for the particular
ceramic materials and what improvements ceramic materials offer from the
materials previously used for the same component.  

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BRIEF OVERVIEW OF CERAMICS AND
CERAMIC PROCESSING TECHNIQUES

 

Ceramics are important in the world of engineering due to their
mechanical and physical properties, which are quite different from those of
metals. A ceramic is defined as an inorganic compound; this means it contains
metallic and non-metallic elements. Furthermore, ceramics can be divided into
three categories: Traditional ceramics, new ceramics and glasses. Traditional ceramics, some of which have been used for thousands of
years, include: clay, alumina and silicon carbide. These types of ceramics are
made from minerals occurring in nature. New ceramics include some of the
materials listed previously, such as alumina, but with properties that are
enhanced through modern processing methods. For example, new ceramics can
include: metal carbides such as tungsten carbide and titanium carbide (these
are used for things like cutting tools and industrial machinery), and
nitrides—such as titanium nitride and boron nitride (used as cutting tools and
grinding abrasives). Traditional and new ceramics have a crystalline structure.

Glass is very different to traditional and new ceramics and it can be
distinguished by its non-crystalline structure.

 

The properties that make ceramics useful in engineering
are their high hardness and good electrical and thermal insulating
characteristics. In addition, the high yield stress of ceramics allows for the
production of precisely machined parts that can maintain accurate dimensions
over longer periods of time. They also have great chemical stability and high
melting temperatures. Some ceramics are translucent. However, the brittle
behaviour of ceramics has restricted their applications to structural
components. This can cause problems in the processing and performance of
ceramic products.

 

For processing purposes, ceramics can be divided into two
basic categories: molten ceramics and particulate ceramics. Different methods
of manufacturing are required for the two types. Molten ceramics require glass-forming
processes. These processes primarily involve solidification. Glass forming
processes can include pressing, blowing, drawing and fibre forming. Particulate
ceramics include traditional and new ceramics. Particulate forming processes
can include powder pressing, hydro-plastic forming, slip casting and tape
casting.

 

The processing of ceramics generally takes place in
four steps:

1.    
Powder processing (raw materials)

2.    
Forming

3.    
Sintering (firing)

4.    
Finishing (painting, electroplating), densification and
sizing, heating treatment (hardening and strengthening)

Powder processing is essentially the preparation of
the starting powders. This involves crushing and grinding to powders to the
desired size range. The condition of the starting raw powders and the manner in
which they are treated prior to sintering affects the final outcome of these
techniques. The characteristics of powders include: particle size, shape, size
distribution, degree of segregation and agglomeration. Finer and homogenous
particles are preferred. Once the powders a prepared they can go through
various forming methods. These can involve pressing, slip casting, tape cast
and injection moulding etc.

 

Now that we have discussed the basic ideology around
ceramics and processing ceramics lets discuss how ceramics have been
incorporated into the automotive industry, through to case study parts, and how
this has improved the industry as a whole.

 

CASE STUDY PART 1: CERAMIC TURBOCHARGER (1985)

 

The first example that I will discuss is the ceramic turbocharger. Since
the inventions of the internal combustion engine mechanical and automotive
engineers have been searching for ways 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 more to build and maintain. A more appropriate way
to add power to the engine was to force more air into the combustion chamber. Adding
a supercharger or a turbocharger can do this. These devices produce
high-pressure air in the engine cylinders and thus provide more fuel meaning a
bigger explosion and greater horsepower. The main difference between
turbochargers and superchargers is their source of energy. Superchargers can be
powered mechanically by means of a belt or chain that is connected to the
engine’s crankshaft. Turbochargers use a turbine rotor that is powered by the
gases from the engine exhaust manifolds. The turbocharger was developed in 1905
and began to be widely used in the mid 1960s. To make the turbocharger, two
impellers were needed; one was a turbine wheel and the other was a compression
impeller. These were fitted onto a shaft. As explained above, the turbocharger
was able to generate a large power output from a compact engine. However, it
was found that the turbocharger experienced something called turbo lag. This
was essentially, a slight delay between the intention to accelerate and the
actual acceleration of the vehicle. As a result, ceramics were used to reduce
the weight of the rotor and thus reduce the delay or turbo lag. In 1985, the
world’s first ceramic turbocharger was made and it incorporated silicon
nitride. Figure 1 shows a silicon nitride turbocharger. Silicon nitride was a
better material to use compared to the tradition nickel based super alloys. This
was due to silicon nitride ceramic having a much lower density compared to the
nickel based super alloys. The density of the silicon nitride ceramic was
approximately 3.2Mg/m3 and the density of the metallic alloy was 8.2Mg/m3.

As you can see from figure 2, the revolution speeds for a ceramic rotor and a
metal rotor are compared. The time taken to reach 10,000rpm is approximately
36% shorter for the ceramic rotor. As well as the silicon nitride turbocharger
being lightweight, it also had a high thermal resistance, which was important
in order to resist degradation in the high temperature exhaust gas.

The ceramic turbine wheel was manufactured by injection
moulding. The ceramic injection moulding process involves the following steps:
feedstock preparation, injection moulding, de-binding process and sintering.

This is shown in figure 3. In terms of the ceramic turbine wheel, a silicon
nitride powder is used. This powder is mixed with sintering additives such as
rare earth oxide. A moulding agent is then added to the mixture to make the
wheel. The wheel will then go through sintering and machining. Once this is
done the ceramic turbine wheel will be bonded to the metallic shaft. After
this, an aluminium impeller is fitted on the other side of the shaft using
screws.

 

 

 

 

 

 

 

 

 

CASE STUDY PART 2: CERAMIC GLOW PLUGS (1985)

 

The second
example I will discuss is the ceramic glow plug. These are primarily used in
diesel engines to aid starting the engine in low temperatures. Diesel engines
are compression-ignition engines and do not have spark plugs to start
combustion. As a result, ignition is performed by allowing for clean air to be
taken into the combustion chamber. The clean air is then compressed, making it
heat up to a temperature of around 700-900oC. Fuel is then injected
into the combustion chamber with the air that has been compressed. Due to the
air heating up during compression, the high temperature triggers auto-ignition
and the diesel engine starts. However, starting a diesel engine is difficult
when the temperature is low and the engine is cold, as the compressed intake
air will not have a high enough temperature. This problem can be solved with
the glow plug. The glow plug creates the ideal ignition condition for the
injected fuel by generating thermal energy through an electric current. Metal
glow plugs were used throughout the years and comprised of a metallic heating
coil inside a metallic tube. The heater element was filled with magnesia powder
and wrapped around the coil. Metal glow plugs were able to achieve a temperature
of 800oC in five seconds when a voltage of 11V was applied. Ceramic
glow plugs were first introduced in the mid 1980s. They were made by placing
the metallic heating coil inside an insulating sintered ceramic (silicon
nitride). The first generation ceramic glow plug was able to achieve a
temperature increase faster than that of metal glow plugs. The temperature
reached 800oC in three seconds after a voltage of 11V was applied
(two seconds faster than the metal glow plugs). The heat resistance of the
first generation ceramic glow plug was 1200oC. To stop unnecessary
temperature rise in the metal fitting a controlling resistance was fitted onto
the ceramic heating elements. Around 1990, the second generation of the ceramic
glow plug was made. These used a highly heat-resistant silicon nitride rather
than normal silicon nitride. They included ceramic resistors for high
temperature heating. The heat resistance of the second-generation ceramic glow
plug was substantially improved to 1350oC. This meant that the
controlling resistance, which was needed for the first generation ceramic glow
plug, could be removed. At this time it was
found that ceramic glow plugs were far more superior to metal glow plugs in
terms of heating temperature. This was due to ceramic materials having great
heat resistance and durability. In 2005 the third generation
ceramic glow plug was created and it achieved a rapid temperature increase of
1000oC in 2s with a voltage of 11V applied. This was done by
lowering the resistance value and maintaining the heat resistance from the
second-generation ceramic glow plug. As you can see, ceramic glow plugs were
able to provide higher temperatures to start diesel engines. A further
advantage of ceramic glow plugs is that the heating portion can be made much
smaller. This is beneficial as automotive industries are trying to develop
smaller engines and space can be severely restricted around the combustion
chamber.

The insulating ceramic material used in the production
of glow plugs was made by weighing silicon nitride and sintering additives, and
pulverizing and mixing them for some period of time in a trommel.