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Ceramic Materials In Cutting Tools

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Keywords: ceramic cutting tools

Traditional cutting tools are mostly made of steels and irons, however, ceramic cutting tools are developed quickly these years. Ceramic cutting tools have a lot of advantages over other cutting tools in certain areas.

Ceramics possess a lot of required properties for cutting tools such as high strength and good thermal conductivity. The development of ceramic cutting tools has once been hindered by the high cost and difficulty in manufacturing ceramics. Thanks to the improvement of materials engineering and the advent of composites, both the cost and the difficulty of manufacture have been resolved.

The market share of ceramic tools has increased in recent years, which indicates that the ceramic cutting tools are well along in development.

With a comparison and evaluation of many processing routes, die pressing and sintering are chosen as the most convenient and appropriate way for the manufacture of ceramic cutting tools.

There are a lot of potential ceramic materials can be used in cutting tools and most of them are alumina or silica based. Composites have also attracted much attention with a relatively low cost and high performance.

Cutting tools

Cutting tools are designed to separate materials. They can be divided into different categories, each of which has its own characteristics and requirements for the materials used. The mostly used materials for cutting tools are steel and iron. However, with the development of materials science and engineering, ceramic cutting tools are playing a more and more important role.

Ceramics are usually very hard, heat resistant and have little reactivity with steels, which are properties vital for cutting tools. Hence they can be used at high cutting speed without deformation and dissolution.[1] However, ceramics have their own drawbacks that they lack toughness and are sensitive to mechanical or thermal shock, which has limited the use of ceramic materials in cutting area.

Humans have used ceramic cutting tools for more than 100 years. [2] The cutting objects vary from very soft substances (butter) to extreme hard things (steels). In the early days, the major composition of these ceramic tools was alumina. However, the toughness of alumina material then was low and softened by glassy phases, which were the reasons why their applications were limited. [1] With the effort of scientists, materials science and technology has dramatically developed so that the properties of ceramics have been greatly improved. They become harder by improving the purity, less brittle with some additives and tougher with reinforcements. [1] The first ceramic material used in cutting tools was Alumina. [1] A variety of other ceramics have also been produced to be specifically used in cutting tools' manufacture. There are three categories available, namely pure oxide ceramics, mixed oxide plus carbide or nitride and silicon nitride based material. [2] Whisker reinforced ceramic materials, with high toughness and hardness at high temperature, has pushed forwarded the development of ceramic cutting tools.

Figure 2.1 Cermet indexable milling tools [1]

There are a lot of aspects that should be considered when the materials are chosen. The following three aspects were taken into consideration in the design of the tool material. [2] Firstly, the surface layer should possess the highest heat conductivity that is favourable for dissipation of cutting heat. Secondly, the thermal expansion coefficient of the surface layer should be the lowest of all the layers of the materials so as to form residual compressive stresses in the surface layers in the fabricating processing, which may partially counteract the stresses resulting from external loading. Finally, the value of maximum Von Mises stress should be the lowest in order to guarantee the structural integrity of the compact. [2]

The properties of the ceramic are not the only factor involved in the material choice, the economic factor has also played an important role in the consideration of the material. [1] One of the main focuses in ceramic industry is how to cut the prime cost and the future developments most important direction must be cost reduction. New kind of raw materials and methods are needed to attain this goal.

In the future, with the development of material science and technology, more methods will be used in improving ceramics. The perfection of coating technology and the progresses in composites will promote the usage of ceramics in cutting industry. CMCs (ceramic matrix composites) are considered to be one of the most popular materials used for cutting tools in the future.

Current market situation

There are three main cutting tools markets. Solid tools and indexable inserts; milling, turning and drilling tools are investigated to show the trend of current market of cutting tools. Grades include carbide, ceramic, cermet, CBN/PCBN, diamond and steel. Products are analyzed by country and end-use markets.[3]

The consumer markets are specifically broken down by each of the following geographic regions: [3]

• China • United States • Germany

• Japan • Korea • Italy • France

• Taiwan • United Kingdom

• Brazil • Other EU • Other NAFTA

• Other Europe • Other Asia/Pacific

• Other Latin America • Rest of World

The geographic regions from top to bottom represent the biggest markets for cutting tools. The United States Cutting Tool Institute which was formed when the Metal Cutting Industry and Cutting Tool Manufactures Association merged is now said to 'represent 80% of the domestic cutting tool market'.[4]

Great strides continue to be made in the cutting tool market that result in reduced bench time and additional handwork, as well as heavier depths of cutting which leads to increased productivity and higher accuracy. Over the past ten years the cutting tools market has increased dramatically. The tools have taken on very specific roles depending on the application. Now there are specific carbide grades, coatings and geometries to match the customer's needs. The market is predicted to expand and grown and achieve new heights. [4]

The majority of end users in industry of cutting tools are listed below:

• Aerospace

• Automotive

• Chemical Processing

• Communications

• Construction

• Defense/Military

• Electronic

• General Machining

• Medical/Research

• Oil & Gas

• Paper & Pulp Industry

• Power Generation

Global market for cutting has been captured by the metal cutting tools for a long time. However, some special properties make the advanced ceramics tend to be more inexpensive, longer life, larger range of application, which become a competitive advantage for manufacturers.[4] Therefore ceramic cutting tools are expected to recapture the cutting market.

The competition among manufacturers is expected to remain intense, which forces the manufactuers to work harder to get a smaller market share. The three largest competitors focus on carbide-tipped tools and carbide inserts worldwide.

Current materials used

There are four main ceramic compositions used in the cutting tool industry have been outlined below, the most important aspects of each material have been highlighted. [5]

Sialons

A sialon is a class of ceramic based on silicon nitride. The ceramic is composed of four different ceramic powders; silica (SiO2), alumina (Al2O3) and silicon nitride (Si3N4). The fourth component is either yittria (Y2O3) or magnesium oxide (MgO). The yittria is used to aid in the sintering process to increase efficiency of the process. [5]

Sialons have very low coefficient of thermal expansion meaning that they are good in situations of thermal shock. Sialons are retain their hardness at temperatures of 800ÌŠ C to 1000ÌŠ C which enables them to be very effective when machining heat resisting alloys. They are also effective when machining hardened die steels and cast irons at high speed. [6]

Aluminium oxide-based ceramics

These ceramics are composed as their name suggests but are commonly found with small additions of zirconia (2-5%).the additional zirconia is added to increase fracture toughness.

Al2O3 based ceramics show an increase in mechanical properties above 800ÌŠC. Below this temperature the hard metals posses a greater strength than the ceramics.

The hardness of the ceramic can be increased further by 30-40% with the additions of titanium carbide or titanium nitride. These harder ceramics are generally used for finishing and harder metals.

Another common addition to the (Al2O3 + ZrO2) base are silicon carbide (SiC) whiskers at around 25%. The whiskers act as reinforcements to the structure and increase the toughness. This enables the ceramic to be used for cutting nickel based super alloys at high speed. [5]

Silicon Nitride

The ceramic has a density of around 3.2g/cm3 and the grain size is approximately 2-3 micrometres. Silicon nitride boasts good wear resistance and cutting edge strength as well as high resistance to thermal shock.

Si3N4 is generally used in the field of providing a roughing grade for turning and milling cast iron.

The bad solution wear properties of silicon nitride restrict the ceramic from being used as a cutting tool for machining steels.

The high resistance to thermal shock means the ceramic can be used with and without the use of coolants. Money can be saved on cooling equipment in such a controlled manner. [5]

Cubic Boron Nitride

Cubic boron nitride is a synthetic compound, in its natural state it has a soft hexagonal crystal structure. Ton achieve a cubic structure it is heated to 1400ÌŠC in 60kbar of pressure. This has a great effect on the hardness of structure, (Around 4000VDH) this is similar to diamonds hardness. [6]

Carbon boron nitride is a polycrystalline and is used in applications where hard metals become limited by the cutting speeds employed. Examples are;

high speed steels, tool steels, case hardened steels, chilled cast iron and satellite.

Carbon boron nitrides hardness is second only to diamonds, for this reason carbon boron nitride offers great wear resistance and the ceramic will not need to be replaced on cutting tools as much as others. Harder metals are also possible to machine with more efficiency, however carbon boron nitride offers no advantage when machining softer materials. [5]

Composites

The properties of ceramics are dramatically improved by using some sizes, such as TiC, Co and WC, as dispersed phase, named composite ceramics. The type composite ceramics is the Si3N4 composite ceramics, which hardness, bending strength and toughness is higher than the simple Si3N4-based ceramics. It also has a better thermal impact resistance.

Because of its special properties, it is widely used in machining cold hardened cast iron roller, and it can finish the work of coarse and precise machining of roughcast, for overcoming the low machining hardening of steel parts used in the mining facilities. Its disadvantage is poor abrasion machining. [6]

Coating material

Coating technology plays a very important role during the cutting tool manufacture. It is used to produce the perfect cutting tools which have both hard surface as well as high toughness. With the development in the last 10 years, this technology has made materials with high-speed cutting, hard cutting and dry machining possible.

Cutter coating can be divided into four kinds. They are chemical vapor deposition (CVD), Low-temperature chemical vapor deposition (PCVD), Middle temperature chemical vapor deposition and PVD.

However, cutting tools are expected to be coated differently to meet the demands for comprehensive mechanical properties. Therefore more attention should be paid to make coating thinner and coating temperature lower. [6]

The future materials

The future of ceramic cutting tools is dependent on the development of super hard coatings for the cutting tools. A new coating for a ceramic cutting tool is a titanium aluminum nitride coating. The coating helps to control the temperature of the cutting tool when it is used at high speeds. This allows for quicker cutting and therefore a more efficient process. [6]

The coating also adds to the lifespan of the tool, in some cases it can make the tool last several times longer. It is important to note that the addition of the coating also adds 10% to the cost of the tool although this is immediately recovered by lasting much longer. [7]

Possible processing route

Most of the ceramics are processed by shaping and sintering. The shaped ceramics are called green body. The shaping process of ceramics is crucial as it can influence the properties of the sintered products such as porosity. Nowadays, the main methods of forming ceramic green bodies include die pressing, slip casting, pressure casting, and injection molding.

Shaping

All the methods above start with a suspension where the ceramic particles are mixed with a liquid or a polymer melt, proper dispersant, and possibly further additives such as binders, plasticizers, and antifoaming agents so that a well-dispered, nonagglomerated ceramic slurry can be made.

Die pressing can be divided to many different pressing route such as dry pressing, cold isostatic pressing and hot isostatic pressing. Green bodies are manufactured by pouring powders into the die and these granules are formed under certain pressure. [8]The method of dry pressing is almost as the same as the cold isostatic pressing. The two methods are widely used in the industrial production of ceramic materials, accounting for their productivity ,and the development of cold isostatic pressing makes green bodies of higher density possible.

Slip casting and pressure casting are included in the drain casting techniques, which invovle a solid-liquid separation process to form a dense green body. [8] The driven force of flow liquid in the two castings is an external pressure gradient. [8] Slip casting is a low-pressure filtration of which the driving force provided for the green bodies' forming is the capillary suction.[9] slip casting is slow compared with die pressing but complex figure can be obtain. Pressure casting is usually used to fabricate the traditional clay-based ceramic materials, such as pottery and sanitary porcelain. [9] Pressure filtration is an improved method of slip casting, the casting rate is raised and the green bodies have a higher density. [9]

Both die pressing and casting are plagued by some genetic problems. The liquid flow of the suspension is required and the stress gradient may also lead to nonuniform densities of the green body which lead to the pockety mass.[9]

Injection molding is an excellent method to form smaller objects. The mixture of powders and binder are intruded to the mould and form green bodies. [9] However, the removal of the binder which can be burned at relative low temperature has been a problem. Crack and imperfections will be introduced during this procedure. [8]

There is also a technique called solid freeform fabrication(SFF) by which we can form a ceramic green body without using a mould. All the process is controlled by computer using 3D CAD designed before the process.[10] This method have been used to form complex figures, however, the surface finish is poor. [9] This method has not been widely used for its immature technology.

Sintering

After shaping, the density of a green body is usually about 50% of its theoretic density. Full densification can be achieved by sintering at temperature up to 1800°C.[11]

During this process, individual powder particles can get enough energy to bond together realize the transformation from the porosity present to the compaction stages.

Composites processing

Composites are consisted of two parts: the matrix phase and the reinforcing phase. Generally, the combination of the two parts is achieved through a melding event. After the melding event, the part shape is set.

According to the melding event, the methods of composites preparation can be classified into four main methods¼švacuum bag moulding, pressure bag moulding, autoclave moulding and resin transfer moulding (RTM).[12] Besides, there are some other types of moulding that include press moulding, pultrusion moulding, filament winding and so on. [12]

Composites can also be devided into metal matrix composites(MMC) and ceramic matrix composites(CMC) based on the matrix materials. According to the temperatrue of metallic matrix during processing, the processes of MMC can be classified into five categories: [14]

1).liquid-phase processes,

2).solid-liquid processes,

3).two-phase (solid-liquid) processes,

4.) deposition techniques

5) In situ processes.

The selection of materials and processing route

Then dry processing has much influence on the green microstructure and production rate. We must treat this opreation to avoid the problem of cracking and warping. The desired result in ceramic part production is fast drying, but fast drying causes cracking. It has been studied that decreased drying rates results in the increasing green densities. The size of power also affects ceramics sintering properties. These days, nonasized powder attracts much attention, because of its special properties (mechanical, optical, magnetic and electrical). The compacts of nanosized powder can be sintered at significantly reduced temperatures, thereby lowing the firing cost.[10] And it is possible to get high density ceramic body in the formation of nanosized particles.[11]

From the ceramics already discussed the best ceramic to use for this product would be the Aluminium oxide-based ceramic. It is already widely used in the industry so there is no need for testing to make sure it works. When cutting the cutting tools increase in heat rapidly, sometimes this has an effect however with Al2O3 their mechanical properties get better the hotter they get. The ceramics hardness will also be increased further with addition of silicon carbide whiskers. These whiskers add reinforcement to the structure and increase it toughness. Therefore the cutting tool can be used at higher speeds. Not only will our ceramic have added strength from the whiskers but with have a tough outer shell from a coating of TiAlN. This coating has a 'high bond strength, high hardness, abrasion resistance and high cutting speed.[8]

From all of the possible processing routes in the industry, it has been decided that Dry Pressing would be the route which is most beneficial to the company.

Dry Pressings most appealing feature is the productivity factor. Its fast productivity means that the product quantities are increased therefore meaning that costs will be reduced. Ben Franklin one said 'Time is money', he was right. Reducing production times reduces the amount of time staff needs to work, it reduces the amount of time that machines have to be on, and therefore it reduces the costs of production.

Dry Pressing is conducted with granulated or spray-died powder containing 0 to 4% moisture' which is where Dry Pressing gets its name from.[1] There are some disadvantages of using this process but the many disadvantages overrule this fact. One of these disadvantages is that high pressures are used in the process, but then again these high pressures are not as high as the ones used in pressure casting. Dry pressing can only gain good results if all the steps prior of the forming or pressing is kept under careful control. The product once leaved the die cavity is in a plastic condition and if not handled carefully, may deform.

With dry pressing there is no limit to the compositions which can be used because the plasticity of the body is not relied on to aid the forming. Under normal conditions and supervision it is possible to produce ceramics to very good tolerances. In special cases with very close controls tolerances can be kept which are considerably better than usual.

Slip casting is in need of too much careful control to be able to mass produce at a beneficial production rate, especially with the long time it takes to cast articles on the mould. Pressure casting is pretty much the same as slip casting but with added pressure to speed up the casting time. This added pressure is not justified because it is too expensive and does not take enough time off the process. Injection Molding has a few defects, which can occur to the product. There can be an incomplete fill within the mould, knit lines, and microcracking, which will all, have a visual defect on the finished body.

Concluding, the dry press process has the least amount of possible defects to occur in comparison to other process routes. It is the quickest route with great productivity and can offer very high tolerances. These are the main reason for our choice of dry pressing process route.

7. Reasoned argument for RAC

The ceramic that has been selected to be taken forward is Al2O3 + (2%Zr) + (20%SiC whiskers), there is no issue in using this composition legally. However the methods of using the cutting tool must be reviewed as such patent 4,879,277 which claims . "In a method of cutting metal wherein a cutting tool is brought into contact with a metal workpiece and the cutting tool and metal workpiece move relative to each other whereby metal is removed by the cutting tool from the metal workpiece, the improvement comprising using a sintered composite cutting tool having a matrix consisting essentially of alumina and 2-40 volume percent silicon carbide whiskers distributed therethrough." [14] A work around the patent would have to be implemented if the material is to be used in such a way. A variation in whisker content or physical machining could help to deviate from the patents claims.

RAC could create a competitive edge for themselves by proceeding with this option as it will help them to differentiate themselves to most of their competitors. The material selected is a very cutting edge material with extremely high levels of performance. Many of the competitor businesses would not have adopted this material and may continue to use a lesser and cheaper material such as unreinforced aluminium oxide based ceramics. RAC will be able to offer a faster and more efficient cutting solution to their clients. The superior technology is likely to attract new customers to the business leading to a larger market share. The harder cut edge and faster cutting speed will lead to a greater cutting quality also, the tools will be able to be smaller in size and so more precise machining is possible. The business will be able to supply their service to a wider range of clients requiring different finishing qualities.

From the market analysis carried out previously it is clear that the cutting tool industry is a blooming one. It is encouraging that the market has grown so dramatically over the last ten years and that there is an increased demand for new cutting tool technologies. The need for faster and more accurate methods will remain the same as the products on the market continue to develop in complexity.

To conclude the ceramic and manufacture process put forward can be projected to be a positive step for RAC after considering the factors involved. The only problem to resolve before proceeding with the decision is the patent on the ceramic and its method. Once this is resolved the decision to proceed is an easy one and should lead to business success.


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