The manufacture of ceramic bearings
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Published: Mon, 5 Dec 2016
1. During this investigation it has been discovered that the most appropriate material to use in the manufacture of ceramic bearings is silicon nitride (Si3N4) because of the superior properties that it exhibits in the necessary service conditions, ie, hardness, toughness, the ability to be fabricated to a high smoothness, to operate at a low coefficient of friction and have high positional accuracy. This material is also able to withstand the different conditions that a bearing may have to withstand such as salt water and common acids.
The most appropriate way to manufacture silicon nitride ceramic bearings is using a process called hot isostatic pressing which uses multi-axial pressing to ensure a uniform component. The start-up costs and running costs of this process will be high due to the moulds that are needed and the high temperatures and high pressures that it runs at.
The size of the current market is vast which means that it may be difficult for RAC to integrate efficiently into the market and gain a large market share.
2. As the name suggests, bearings ‘bear’ loads, they are able to reduce friction by the use of balls or rollers and smooth surface which enables them to rotate smoothly with minimum friction. They are usually able to withstand two kinds of load, thrust and radial load1.
To be slightly more specific, rolling bearings consist of two rings, an inner and outer ring, with a set of rolling elements running in their tracks. The standard shapes of the rolling elements give rise to the different types of bearings; these are the ball, cylindrical roller, needle roller, tapered roller, symmetrical and unsymmetrical barrel roller. The rolling elements are placed in a cage which sits between the two rings and ensure uniform spacing and prevents mutual contact2.
The many desired properties ceramic bearings provide mean that they can be used for a wide range of applications.
Water, salt water and most common acids are the sorts of environmental conditions ceramic bearings can withstand. The high corrosion resistance of ceramic bearings means that they are ideal for use in semiconductor equipment where the environment is aggressive3. Other applications for its high corrosion resistance properties are in plating equipment, optical film equipment, and composite fibre equipment4. In wave generating machines, such as wind turbines, the corrosive effects of long term exposure to sea water to the bearings are overcome by their corrosive resistance.
One of the main advantages ceramic bearings have over bearings of other materials is their high heat resistance. At temperatures other materials lose their mechanical properties, the ceramic bearings excel. Applications where these properties are ideal are in; medical equipment, chemical fibre instruments and in several aircraft components. In contrast, ceramic bearings can also provide a rolling element in a cryogenic environment, such as in liquid oxygen3.
The non-magnetic properties of ceramic bearings allow them to be used in strong magnetic field exposure applications. These include superconductor and semiconductor equipment, aircraft components, electron beam processes and other sensitive instruments.
Ceramic bearings are light and can therefore help in reducing the overall weight of a product. Applications where its light weight is very much effective are in aerospace appliances and components, and in a motorcycle crankshaft. Weight reduction can reduce effects of vibration and stress in out of balance applications, and can also increase speed and efficiency.
The hardness and rigidity of ceramic bearings are important to insure the bearings can bear a load. The hardness and rigidity of these bearings are greater than that of other bearing materials used. This means ceramic bearings have a greater wear resistance, hence a longer service life. This is essential for all the different types of bearing applications; including machine tool spindles, turbo chargers, gas turbines, vacuum equipment and motors, to name a few.
Further development on future bearings are based on a few things. Some of these are; greater energy savings, ‘cleaner’ credentials and comfort6. Better ways of reducing friction using bearings are continuously being researched into, to help with obtaining greater energy savings.
3. The size of the current market is extremely vast. Silicon nitride bearings are manufactured and sold in many parts of the world, since they are used in so many different applications as shown above in section two.
Many ceramics are made from steel but, ceramic bearings are advantageous over steel bearings in many ways7, this makes a desirable market, a growing market with opportunities for other companies to take advantage of. But recent figures have shown that there is decline in the sales of bearings in the United States motor industry of about 7.4%, from October 2009 to November 20098. Whereas in Canada there is an increase in sales of mounted and unmounted bearings, 19.6% and 5.9% respectively, during the same time frame8.
The above facts and figures are not specifically based on ceramic bearings, so they are not completely representative. Due to the fact that ceramic bearings are a technological advancement on an original product that is already out on the market, there is due to be a growth in the market of the sales and applications of ceramic bearings. For instance in the sports industry, Erik Hightower, a Paralympic wheelchair athlete has made changes to his wheelchair, including the bearings. Switching to ceramic bearings was one of the factors that allowed him to increase his speed and consequently achieve a personal best9. This shows that there is potential for ceramic bearings in not only the machinery industry but in many others such as sports.
Major international competitors for this product include VBX Bearings, Boca Bearing, The Barden, Shuster, SFK, Velo Carbon, IBSCO and AC Technologies10.
4. The materials chosen for bearing manufacture must possess certain properties to enable the objects to be used in service conditions such as high temperatures, high revolutions (in excess of 6,100 thousand revolutions per minute) and corrosive environments. These properties include; hardness, toughness, the ability to be fabricated to a high smoothness, to operate at a low coefficient of friction and have high positional accuracy.
High performance steels have been widely used in the manufacture of bearings for many years but the first ceramics were used for low-load applications such as precision measuring instruments.
Ceramics possess the necessary properties to provide a good bearing for example; they are wear resistant, hard and can be machined to a high surface smoothness. In addition to this, when compared to a metal they have much higher strength in compression and undergo less plastic and elastic deformation.
Prior to the 1970s ceramics such as alumina and silicon carbide were experimented with to try and achieve high-load and high-speed bearing but they were all unsuccessful as they failed because of brittle fracture. Hot-pressed silicon nitride was the first successful attempt at such a bearing. Tests showed that this ceramic had “an L10 life of 12,000,000 cycles- eight times the life of steel”.
Unlike the pervious ceramics that were used, silicon nitride bearings did not fail catastrophically, instead they failed in a similarly to metals by “the development of surface spallation”. Surface spallation is the spontaneous separation of a surface or surface coating, it often occurs because of internal stresses. The major difference that is present in silicon nitride compared to alumina or silicon carbide is the fracture toughness, which is around 50% higher for hot-pressed silicon nitride.
In addition to the basic properties that silicon nitride possesses (which make it a good material for bearing manufacture) it is also possible to obtain a surface finish to smoothness and roundness of approximately one-half millionth of an inch; this is because of its high hardness and microstructure. Not only does this reduce friction, it also decreases friction at high speeds.
Silicon nitride has a specific gravity of 3.2 which makes it 60% lighter than steel and therefore silicon nitride bearings use 15-20% less energy than metal equivalents and can be ran at speeds of up to 80% higher.
5. Hot pressing was a former manufacturing method for silicon nitride. But this method brought in several problems for the product, for example there was no uniform structure, many surface defects, it required expensive finishing and even the final part did not satisfy the requirement of rolling-element bearings.
Later, silicon nitride was used with hot isostatic pressing (HIP) as its manufacturing process, which could provide many benefits, including stabilizing the silicon nitride, reducing residual stresses, increasing density and eliminating voids and occlusions, and so the properties and strength of the structure of silicon nitride were elevated. As mentioned above, ceramic bearings have superiority in hardness, corrosion resistance, abrasion resistance and fatigue resistance compared with steel bearings.
All raw silicon nitride contains microscopic voids and “bubbles” of gas which means the powder is regarded as porous. When a silicon nitride part is manufactured using HIP, there is a simultaneous application of high pressure of inert gases- argon, nitrogen, helium at elevated temperatures which are used in order to eliminate all internal voids and porosity instead of distorting external geometry in all grain surface through a combination of plastic deformation, creep, and diffusion bonding . The density can reach greater than 99% of the theoretical density, forming perfect diffusion bonds between them, as a result of improve the mechanical properties such as density, ductility and fatigue life.
The operating temperatures of hot isostatic pressing range from 500 to 2200°C (900-3990 °F) and the standard pressures 1035 to 3100 bar (15,000 to 45,000 psi) are acceptable to product a high quality product such as a bearing.
The difference between the original hot pressing process and hot isostatic pressing is that HIP involves pressure being applied uniformly in all three directions rather than unidirectional, so this process is called “hot isostatic pressing”. Compared with hot pressing, HIP is good at making complicated shapes form ceramic powders and minimizing surface defects and internal voids. Any porosity that remains will cause high rejection rates because of unacceptable property levels and surface finishing problems after machining.
Moreover due to the fact that the process enhances the “strong bonds between the powder boundaries of similar or dissimilar materials”, the characteristics of the parts can be suitable for many extreme environmental circumstances and newer engineering applications. Further, the hot isostatic pressing process also reduces the cost of bearings, because the parts can be cast to near net shape and therefore eliminate costly machining and additional machining stresses. In addition, using silicon nitride during the process reduces the final production cost by more than 30%, and also the final cast is expected to be thermally stable.
Since modern HIP chambers are designed to be very large (a typical HPI
machine.), multiple kinds of small components can be accommodated in the HIP machine, which reduces the cost of the process.
By employing the hot isostatic pressing method, greater flexibility, with regards to sample shape of silicon nitride bearings, can be achieved and because of plastic deformation, creep, and diffusion bonding at the interface, there is no need to be rigid to the surface preparation.
Just as shown in Fig. 1. An internal gas is used to prevent chemical reactions and the chamber is heated, causing the pressure inside the vessel to increase. The crucial aspect of hot isostatic pressing is that the interface must be separated from the gaseous pressure in case the pressing gases interact with surface or interconnected porosity within the component, so a capsule is usually used to prevent this happening.
6. The best material to use to produce ceramic bearings is Si3N4, whose properties are compared with steels in Table 1 below.
As a bearing, the properties like hardness, toughness, crack resistance, strength, thermal shock resistance and rolling contact fatigue life must reach a certain level, or the bearing will not function correctly whilst it is in service.
As with all other ceramics, silicon nitride molecules are joined by covalent bonds which give strong conjunction. At the first processing step, Si3N4 can be synthesized by silicon and nitrogen’s direct reaction between temperatures 1300 and 1400°C.
3 Si(s) + 2 N2(g) → Si3N4(s)
Then the powder is formed into the desired shape, which is called a green body. Following this process, the green body is sintered at a high temperature to obtain a sufficiently dense material. Finally, the sintered compact is finished into the final shape.
But the low density and high porosity of products whilst using this method is a great problem. So it is necessary to sinter the component to densification. There are two phases-transforming alpha and beta, when the densification happens. As mentioned above, the best method for densification is hot isostatic pressing, which provides high temperature and high pressure that ensures the density of silicon nitride components reach more than 99% of the theoretical density by HP.
The advantage of hot isostatic pressing is that it could be highly automated and commercialised it also forms a near-net shape and good density. Although the high costs of this process may be a limitation as the manufacture of a single mould is very expensive.
7. It is important to take market share into account whilst considering entering a market as it plays a very important role in profits. Although on a basic level the start up costs and running costs of this venture would be very large as hot isostatic pressing is an expensive process. It uses elevated temperatures and multi-axial high pressures are needed due to the precision of the product. An additional cost of hot isostatic pressing is the moulds needed to create a part, although these parts can be used multiple times they are extremely expensive to manufacture in the first place- hence creating a large start up cost. Consequently if the plans for the mould are incorrect (as a bearing is a very sensitive product- especially a rolling bearing) there would be even more cost accompanying the final product.
As mentioned previously, ceramic bearings are now being used in areas other than machinery, for example wheel chairs. The entry into an up and coming market segment may be the best way to gain market share as other industries may have existing relationships with suppliers. Alternative strategies may be to offer a more high performance product at a lower price to developed segments.
Due to the world’s depleting iron stores it is important to look to alternative production methods of steel components. Ceramic bearings are a good way to do this as they posses equal, if not better, qualities in service than their steel rivals. Therefore a move into this market may be seen as a corporate and social responsibility, which would increase popularity with customers.
It would be important for the company to invest heavily in research and development to ensure that they could gain and retain market share whilst also taking a marketing business approach. Research and development into future, more efficient materials would be an additional way to gain market share.
In our opinion RAC should progress into this market whist taking into account what has been mentioned in this section.
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14. Avraham Harnoy, Bearing Design in Machinery, chapter 13. 2003
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(Visit at 24/01/2010)
(Visit at 24/01/2010)
(Visit at 24/01/2010)
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