Sintered silicon carbide for slide bearings and seal rings

Technology insight

Sintered silicon carbide for slide bearings and seal rings b y H K n o c h , M F u n d u s , E l e k t r o s c h m e l z w e r k K e m p t e n GmbH, Fed Rep Germany In t h e past, t w o p r o b l e m s have p r e v e n t e d t h e w i d e s p r e a d u s e o f s i n t e r e d silicon c a r b i d e (SIC) c o m p o n e n t s f o r w e a r . r e d u c i n g applications: firstly a c o m p l i c a t e d p r o d u c t i o n p r o c e s s m a r k e d b y relatively h i g h costs a n d i n a d e q u a t e availability, a n d s e c o n d l y t h e b r i t t l e n e s s o f c e r a m i c materials. P r o g r e s s has b e e n m a d e in b o t h areas d u r i n g the past years. The t e c h n o l o g y u s e d for p r o d u c i n g s i n t e r e d SiC has b e c o m e c o s t c o m p e t i t i v e , a n d m u c h has b e e n l e a r n e d a b o u t ceramico r i e n t e d materials design. In t h e f o l l o w i n g article c o n c e p t s o f p r o d u c t d e v e l o p m e n t f o r i m p r o v e d reliability o f silicon c a r b i d e slldln 8 bearIngs a n d m e c h a n i c a l seals will b e discussed. Materials optimizat i o n steps will b e c o n c e n t r a t e d o n a n d t h e i r r e s u l t s o n trlbological p e r f o r m a n c e as w e l l as o n c e r a m i c d e s i g n criteria for reliably w o r k i n 8 pump components.

Concepts of product development Product development of sintered siticon carbide for slide bearings and mechanical seal rings must concentrate on improving its weak points without degrading its strong poInts. When the tribological reliability, in situations where the lubricant film breaks down or is mi~ing, is to be improved, one must carefully consider the key properties of sintered SiC, which are the basis for its successful use in bearings and seals, and maintain them. These key properties are: • universal corrosion resistance: any liquid media (alkalis, acids, organic solvents) can be used as a lubricant for mechanical seal rings and hermetically sealed pump bearings • outstanding mechanical wear resistance: liquids containing abrasive particles do not restrict the use of sintered SiC • thermal stabili~ty: corrosion resistance and wear resistance are not influenced by changing temperatures. Cracks in sliding faces, induced by thermal shock In boundary lubrication and short dry run situations, are unknown. Tables I and 2 also show properties of

6

sintered SiC. The coefficients of friction of hard and wear.resistant ceramic materials and of surface coatings are always > 0.1 under dry run conditions. This value is too high to allow the development of reliable performing dry running bearings and seals. Especially under high loads when frictional heat develops. Although sintered SiC easily resists the heat, high temperatures may cause high thermal stresses in the overall structure, so that fracture and failure occur. Since material research indicates the impossibility of the development of hard ceramic materials with coefficients of friction in dry run as low as in lubricated conditions ( < 0.01), the concepts for product development must take the following factors into consideration: • stabilization of the hydrodynamic lubricant film • prevention of dry run • reduction of friction and wear if hydrodynamics break down The stabUiTation of the hydrodynamic lubricant film of a running bearing can be achieved in two possible ways. One is the evaluation of the optimum design, including the topography of

the slide face. The other is the optimization of the SiC bearing material. Without changing the universal corrosion resistance, the microstructure of the material can be improved with regard to tribological performance by tailoring the grain morphology, Incorporating pores which act as lubricant pockets, or incorporating graphite particles which act as solid lubricants. Material aspects The reliability of any mechanical seal or sliding bearing depends on the presence and stability of the load carrying hydrodynamic lubricant film, The development of siutered SiC with improved tribological properties, therefore, must concentrate on the improvement of the stability of the lubricant film of the sliding face. Close attention must be given to the effects of surface texture on the slide face with respect to wear behaviour. In the presence of a hydrodynamic lubricant film, a smooth, level slide face may be assumed to represent the most appropriate solution. The situation changes immediately, however, when the hydrodynamic lubricant film breaks down, resulting in unlubricated operation. When this happens, a surface exhibiting a certain texture that is capable of providing some form of residual or forced lubrication will provide the better sliding properties. Such structures could consist of machined lubricating/cooling grooves or shallow holes coming from the LappIng process of the slide face. Another possibility is the controlled introduction of isolated pores or graphite particles into the material microstructure. Consecutive short periods of dry run or extended periods of boundary lubrication conditions must result in wear of the sliding face due to solid state interaction. If the topography of the sliding face changes, its tribological properties may change as well The better tribological material therefore is one which does not change its surface properties significantiy if wear occurs. Very good results have been achieved in this regard with EKasic D sintered u-SiC material (developed by Elektroschmelzwerk Kempten GmbH) with a bimodal grain structure

Sealing Technology No. 17

Technology insight Table 1. Mechanical and physical property values of EKeslc D slntered silicon carbide Property

Units

Temperature (°C)

Density

gcm "3

20

Porosity

Vol.-%

Hardness (Knoop 100)

Value 3.1 3.5

20

2700

Compressive Strength

MPa

20

2200

Bending Strength (4-pt)

MPa

20 1000 1400

410 410 410

20

10

Weibull Modulus Fracture Toughness (sharp crack)

MPax/m

20

3.2

Young's Modulus

GPa

20

410

20

0.17

20

10-100

20

1000

110 45

20...500 500...1000 1000,,1500

4.0 5.8 6.0

Poisson Number Electrical Resistivity

~cm

Thermal Conductivity

Wm-IK 1

Thermal Expansion

10"OK-~

Table 2. Chemical composition and corrosion behavlour of EKaslc D sintered SIC. Analysis

% wt

Corrosionbehavlour

SiC

98.5 t.0 0.3 0 traces

aqueous acids and mixtures alkaline solutions and mixtures organic solvents

Cfrei AI Sifre i

O, N

containing about 30 voL% of larger hexagonal platelets ( ~ 100 tam length) and about 70 vol.% of smaller grains ( ~ 10 gm length). This material shows superior performance especially ff paired against softer carbon material. The relief structure develops as a result of the SiC crystals anisotropic tribological properties, which are most conspicuous in the presence of relatively large crystals. The depressions in such a textured surface can effectively serve as reservoirs for lubricant, thus improving the component's emergency running properties for situations in which the lubricating film separates and produces a dry running condition. Figure 1 shows a

Sealing Technology No. 17

no no no no

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schematic representation of the process in question. The good tribological properties of the bimodal grain structure compared with other SiC materials are also demonstrated in dry run situations. Figure 2 shows the development of frictional heat in a dry run test of seal faces. The bimodal EKasic D shows the least development of heat, thus indicating lowest coefficients of friction. In an extended DoD/DARPA US study on "Tribological Fundamentals of Solid Lubricated Ceramics" the bimodal grain size/shape distribution was judged best in terms of wear resistance, when compared with other fine grained, dense sintered u-SiC materials.

Under boundary lubrication conditions and breakdown of the lubricant film of dense sintered SiC sliding pairs there is another possible problem. The sliding action during solid state contact causes a polishing effect on the surface. Polished fiat faces of dense sintered SiC show strong adhesion upon solid state contact. The adhesive forces can be strong enough to break out particles of the sliding face. If this happens, these SiC particles may quickly destroy the functional quality of the sliding face. In order to prevent adhesion of fiat surfaces, pores or graphite particles can be incorporated into the bulk SiC material. If the material wears, there are always pores or graphite inclusions present at the surface, to counteract the adhesion and assist lubrication. Materials based on the microstructure of the bimodal EKasic D were designed to contain either only pores or both graphite particles and pores. The pores act as lubricant pockets and are present at the surface even after wear. The material containing pores was named TRIBO 2000 and the one containing pores and graphite particles was named TRIBO 2000-1. Figure 4 shows some comparative properties of these two materials and EKasic D. Pores and graphite inclusions act as internal flaws. So it is no surprise that the strength is reduced when compared with a flawless standard material. On the other hand, since pores and graphite inclusions are intended "flaws" of intended size, the variation in strength is reduced, so that the Weibull modulus is increased, which is important for design. Young's modulus is slightly reduced by the pores or soft graphite inclusions. Friction under boundary lubrication conditions decreases, as intended. The corrosion resistance remains excellent.

Applications results The SiC materials containing pores, and pores and graphite were tested in comparison to EKasic D, the base material. The Stribeck testing method was used to evaluate the coefficient of friction as a function of sliding velocity. In the axial bearing test method a stationary circular segment sits lubricated on a rotating ring of the

7

Technology insight

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8

same materiaL The torque between segment and ring is measured. The load bearing capacity of the hydrodynamic lubricant film is of special interest. The better tribological material will show stable hydrodynamics down to lower sliding velocities, before the lubricant film breaks down and the bearing comes into solid state contact. Figure 5 shows an example. In this case the graphite-containing material shows a higher hydrodynamic friction level and a deviation of hydrodynamics at a higher velocity as compared with the material containing pores and EKasic D. Summarizing all Stribeck test data - load, media, up to 10 000 repeats - the material containing pores shows the best performance in stabilizing the hydrodynamic film down to lowest velocities. This is important for sliding bearings and has been confirmed many times. The poorer performance of the material containing graphite and pores in this sliding bearing test is not clearly understood, but it may be correlated with the poor wetting behaviour of graphite in water under normal pressure. In mechanical seal applications, under differential pressures of less than 15 bars the same ratakirtg of materials is observed in terms of friction and wear. Under extreme situations, however, like very high differential pressure (continuous boundary lubrication) or pump cavitation conditions (break down of hydrodynamic film under frequent uncontrolled loads), the material containing graphite and pores shows the best performance. Figure 6 shows the wear of the optimized materials, compared with EKasic D, in a seal application using identical material ring pairs. The comparative application tests show that the optimized sintered SiC containing pores and containing pores and graphite exhibit improved tribological properties that result in improved reliability in sliding bearing and rotating seal applications.

Design aspects: basic aspects of proper ceramic design Design very much influences reliability and safety. Ceramic parts are usually rather small components

Sealing Technology No, 17

Technology

which are used for their attractive features in a far bigger equipment. So the c o r r e c t constructive integration of ceramic c o m p o n e n t s into metallic structures is an essential requirement for successful applications o f engineering ceramics. Differences in elastic constants, thermal conductivity and expansion must always be taken into consideration. Metal parts usually are dimensioned against plastic deformation. Brittle materials like SiC s h o w n o plastic deformation at all. Therefore the design must b e based on a totally different philosophy - - dimensioning against brittle fracture. Ceramics show a volume-dependent fracture strength and a greater scatter in fracture strength than metals. At the same time ceramics can withstand m u c h higher compressive stresses t h e n tensile stresses. Taking all this into account some basic rules for a p r o p e r ceramic design can be formulated: • Prefer pressurje loads • Avoid point loads • Minimize tensile stresses • Avoid stress intensities caused by notches and o t h e r stress concentrators • Round inner edges • Chamfer o u t e r edges • Prefer simple shaped parts with approximately constant wall thickness • Realise function sharing w h e n c o m p o n e n t s o f different materials are combined Most important is the avoidance of stress concentrations. As m e n t i o n e d above, ceramics are not able to r e d u c e stress concentrations by plastic deformation. So a ceramic comp o n e n t under an acceptable nominal load may fail at stress concentrators. For that reason a p r o p e r design of inner edges with radii or relief grooves must be realized. Figure 7 shows a typical part u n d e r standard load conditions. The maximum tensile stresses ~max using different radfi are compared with nominal stress ~n in a circular disk u n d e r simple bending. The stresses have b e e n calculated by finite element analysis. Figure 8 shows the d e p e n d e n c e o f the stress concentration factor K = c~mzt/cYno n the radius in the inner edge. Radii smaller than 1 m m result in K-factors greater than 2. Radii larger than 4 m m

Sealing Technology No. 17

insight

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9

Technology insight do not reduce the K-factor stgnlficandy. Of course the optimum radius size depends on the other dimensions of the part too. In the example a radius between 2 and 4 mm would be a good solution. Independent of the part size, radii smaller than 1 mm must be avoided if a ceramic part is exposed to tensile stresses.

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S h r i n k f i t t i n g o f SIC r i n g s i n t o m e t a l casings A common procedure to join a SiC riv& to a metal casing is shrink flttiv@. But if a closer look is taken at stresses "5 2 and deformations which are a consequence of different elastic constants, thermal conductivities and expan1 sions (Table 3), it must be realized that even a simple shrink fit has its problems. One problem is the thermal shock 0 during shrinking. Usually the metal casing is heated up to shrink tem0 5 10 15 20 25 perature. The cold SiC sleeve is joined with the hot casing. The temperature close to the outer diameter of the SiC sleeve rises very fast, which results in Figure 5. Coefficient o f friction as a f u n c t i o n o f s l i d i n g v e l o c i t y for identical sliding pairs o f sintered SIC materials a high temperature gradient in the sleeve. In the cold region near the inner diameter both the axial and the hoop stress reach a maximum tensile stress after rather short times before they move into the compressive range. The height of the maximum tensile stress depends on the shrink temperature and the geometry of the shrink fit partners. Finite element ¢D analysis showed that critical tensile r" stresses may develop. There are three c2. ways to prevent critical stress: • Minimized shrink temperature minimizes tensile stresses + • An increase of the SiC ring wall o9 fl) thickness reduces the tensile stresk.. f13 o ses caused by thermal shock t.. o • Isothermal shrink fitting prevents U. + (D thermal shocks. Isothermal shrink + fittin~ is achieved by heating the SiC sleeve and steel housing to the same temperature before joining co t~ and w w caused by shrink fitting. As a result of the thermal expansion mismatch both the interference of the SiC sleeve steel housin8 and the deformations change with changing temperatures. This is demonstrated and explained with shrunk seal rings. The results can Figure 6. Relative wear v o l u m e o n seal rings o f sintered SiC materials be generalized for journal bearings

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Sealing Technology Na 17

Technology insight Figure 7. FE-model a n d b o u n d a r y c o n d i t i o n s (top) Figure 8. Stress c o n c e n t r a t i o n in t h e I n n e r edge ( b o t t o m )

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Units Temperature (°C) GPa 20 10-eK-1 WmIK -1

Sealing Technology No. 17

20 0.17 20-500 20

SiC

Steel

410 0.3 4.0 110

200 11-18 12-5o

and especially thrust bearings which show a comparable behaviour. Figure 9 shows the geometry and FEmodel of the basic version analyzed. An isothermal shrink fitting from a shrink temperature of 160 °C to a room temperature was simulated. Figure 10 shows the deformed model at 20 °C. The stiffness change for region 1 to region 4 leads to a convergent gap geometry (gap opens at the outer diameter). After shrink fitting the seal rings are lapped to get a plane seal face and parallel gap. The simulation of the external load results in a seal face deformation smaller then 1 I~m. The simulation of a unique working temperature of 130 °C however results in a seal face deformation of about 9 ;ma and a divergent gap (Figure 11) due to the release of shrink stresses caused by the thermal expansion mismatch. The obvious result is that the deformation caused by the shrink fit has much more influence on the seal face deformation than the applied external loads. Therefore the main goal must be to reduce the seal face deformation after shrink fitting and adjust the gap geometry to the service conditions. A variation of the shrink fit parmers can give some hints on how to improve the deformation behaviour. There are three starting points: • Reduce projecting height of the seal face beyond the housing to a minimum and reduce the seal face width. Figure 12 points out the effects on seal face deformation. • Change the casing stiffness. Fig. ure 13 shows different casings with the same SiC ring. With the versions 21 to 23 small divergent or convergent gaps can be realized. • Disconnect SiC ring and steel casing partially by relief grooves. Figure 14 shows the influence of refief groove length on the seal face deformation and gap geometry. Alternatives to s h r i n k f i t t i n g Especially when joining a SiC sleeve to a metal shaft by shrink fitting critical tensile stresses may occur in the

11

Technology insight Figure 9. Basic g e o m e t r y ( v e r s i o n 1) a n d FE-model ',II:Hi",I

sleeve, But there are seven1 other methods to join ceramic sleeves onto metallic structures. Various types of tnterlayers and spring elements are used to minimize deformation and stresses. Important examples are: • Rubber O-rings • Rubber cups • Sittcone rubber mastic • Steel tolerance rings • Slotted metal sleeves Whether a shrink lit solution or an elastic element gives better results l~aally depends on the assembly situation and service conditions.

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Bimodal sintered SiC In mechanical seals or In slide bearings runs successfully and reliably as long as lubrication can be guaranteed. If the hydrodynamic lubricant film breaks down, dry friction followed by fric. tional heat and mechanical wear may cause problems. Therefore the concepts for product development have concentrated on the stabilization of the hydrodynamic lubricant film. An unproved sIntered SiC material with optimized tribological properties has been developed based on standard bimodal EKasic D u-SiC. The universal corrosion resistance and reliability of this base material is maintained. Additional defined pores with a pore size In the range of 40 ttm act as emcient lubricant-filled pockets In the sliding face to help stabilize the hydrodynamic lubricant film and assist lubrication on short breakdowns of the film. This material shows advantages In slide bearing applications and mechanical seal applications In which steady hydrodynamic lubrication cannot be guaranteed and reliability improvement is desired. This material is available under the name EKasic TRIBO 2000. Another material showIng promising properties is a sintered

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Sealing Technology Ncx 17

Technology insight Figure 12. Seal face d e f o r m a t i o n after s h r i n k fitting

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SiC, also based on EKasic D, which contains graphite particles in the size range of 60 tim in addition to the pores. This material demonstrates improved performance in hard/hard paired seal rings which run under high pressure differentials or pump cavitation conditions. It shows potential for further development which is currently underway. Permanently dry running SiC bearings are not possible since the coefficients of friction (m 0.4) are too high. Hard surface coatings, and especially CVD diamond-like carbon can reduce the coefficients of friction down to values of 0.1 to 0.2 for some period of time. Upon development of frictional heat, however, the coating deteriorates, and the reliability of the bearing again depends on the inherent properties of the bulk sintered u-SiC, with or without microstructure modifications. Materials development results in better and more reliable performance only when the proper ceramic design criteria are used for the construction of components. Both material and design are important factors influencing the performance of ceramic components in machines. Therefore successful product development must take material and design into consideration.

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1. W. Dawihl/G. Altmeyer: Grundlagen des Verschlei~.s hochharter Werkstoffe; Wear 32 (1975) 291-308 2. Kelley et al.: Wear resistant materials for coal conversion components; Prec. Am. Conf. Mat. Coal Cony. UtiL 1979 3. D.IC Shetty: Coal slurry erosion of reaction bonded SiC; Wear 79 (1982) 275-279 4. W. Sch6pplein: Hue gas desulphurisation plants-sealing effects; World Pumps 5 (1986) 124-128 5. J.H. Eisner: Gleitringdichtungen,

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Sealing Technology No. 17

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Figure 14. Seal face d e f o r m a t i o n after s h r i n k fitting

13

Patents Chemie-~nlagen + Verfahren (1982) 46, 51, 54 6. H. Knoch, J. Kracker, g Schelkem SiC-Werkstoffe ffir erosiv und korrosiv beansprunchte Pumpenbauteile, Chemie-Anlagen + Verfahren 2 (1983) 28-30 7. H. KnoclL J. Kracker, & Schelken: SiC-Werkstoffe fiir erosiv und korrosiv beanspruchte Pumpenbantelle, Chemie-Anglagen + Verfahren 3 (1985) 101-104 8. H. Knoch, J. Kracker: Sintered silicon carbide - a material for corrosion and wear resistant components in sliding applications, cfl/ber. DKG 64 (1987) 159-163 9. W & Giaeser: Wear and friction of nonmetallic materials evaluation of wear testing, ASTM STP 446 (1969) 42-54 10. D.W. Richerson: Contact stress and coefficient of friction effects on ceramic interfaces, Mat. Sci. Rex. 14 (1981) 661-676 11. IL Miyosl~ Anisotropic tribological properties of SiC, Proc. Int. Conf. Wear of Mat., ASME, San Francisco 1981 12. H. Knoch, L.Sigl, B. Long: Product Development with Pressureless Sintered Silicon Carbide, Oct.1, 1990, 37th Sagamore Army Materials Research Conf. AMTL Watertown Mass. 13. BR1TE-EURAM-ProJecU Prop. No. 2231, Proj.No. RJ-Ib 295, 1992 14. M.N. Gardos, Hughes Aircraft: Determination of the Tribological Fundamentals of Solid Lubricated Ceramics, EL Segundo, CA 90245, 1990, WRDC-TR-90-4096 15. L.S. Sigl, M. Fundus: Festigkeitsaspekte beim Konstruieren mit gesintertem Siliciumcarbid.KEM $5 (1991) 44-46

EKasic D, TRIBO 2000 and TRIBO 2000-1 are trademarks o f Elektroschmelzwerk Kempten GmbH. Contact: H. Knoch, ESK Engineered Ceramics Wacker Chemicals (USA) Inc, 535 Connecticut Ave, Norwalk, CT 06854, USA. Tel: + 1 203 866 9400; Fax: + 1 203 866 9427.

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Patents This is a list of recently published patents covering designs and inventions of relevance to Sealing Technolo~D, readers. Countries can be identified by the code in front of the number: AT Austria US United States BE Belgium AU Australia CA Canada BR Brazil CH Switzerland D Germany EP European Patent ES Spain Fit France H Finland HU Hungary GB Britain IR Fare IL Israel Jp Japan IT Italy LI Liechtenstein KP North Korea NL Netherlands LU Luxembourg RO Romania NO Norway SU Former Soviet Union SE Sweden WO World Patent Full copies of the original patent documents can be obtained from the following:

The British Library Patent Express, 25 Southampton Buildings London WC2A lAW, UK Fax: + 44 171 323 7230 (The British Library will also supply a list of UK libraries included in the Patent Information Network.)

US Patent Office Scientific Library 15th and East Streets, NW Washington, DC 20231, USA The Japanese Patent Office 43 Kaksumigaseld 3-chome Chiyoda.ku, Tokyo 100, Japan

Centrifugal seal assembly Patent number: WO 94/21923 Publication date: 29 September 1994 (application) Inventor: Kevin Burgess Applicant: Warman International Ltd This invention relates to centrifugal seal assemblies suitable for use in centrifugal pumps. A typical pump assembly and the associated seal assembly is shown in Figure 1. The pump generally indicated at (10) comprises a pump casing (12) with a pump chamber (14). The pump includes a pump impeller (18) which has an impeller passageway (20), the impeller is mounted for rotation on the pump shaft (16) and an outer section (36) which is disc-like in structure. The seal device (31) is disposed within a seal chamber (42) which is in communication with the pump chamber via the passageway (43). It includes a number of vanes (38) which extend from the inner section (34) of the main body of the sealing assembly and terminate at the peripheral edge of the outer sectiorL The vanes are spaced apart from one another in the circumferential direction and include a curved leading edge with respect to the direction of rotation of the device. The centrifugal seal assembly (30) is used in conjunction with a main seal apparatus (46) which may be in the form of packings, lip seals or other types of seals. Shaft seal assemblies of this general type for centrifugal pumps are known. The rotating seal device generates a dynamic pressure at its periphery. During rotation, liquid within the seal chamber is forced to rotate with the device. This pressure helps to counterbalance the pressure generated from the pump impeller. The reduced pressure at the pump shaft permits the main seal apparatus to function as a low pressure seal and thereby improve the seal life. The purpose of the main shaft seal is to prevent leakage when the pump has stopped.

Sealing Technology No. 17