Wears Of Alumina Ceramic In Simulated Body Fluids Biology Essay

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Resistance to wear and bio-compatibility make the ceramic material is ideal for medical applications, such as planting. Over 30 years, pure artificial hip alumina and dominated the history of artificial hip ceramics and the interest of prosthetic hip alumina continues to grow, due to life relatively short polymer synthetic mineral, resulting mainly from osteolysis and reduce useless due to wear polymer debris.

Chapter 1: Introduction

A hip replacement operation is one of the most successful operations in Orthopaedic Surgery. Hundreds of thousands of these operations are now carried out every year worldwide with excellent results. Its modern form was invented here in the U.K. in the 1960s by Sir John Charnley

Hip replacement surgery becomes necessary when the hip joint has been badly damaged from any cause and the resulting pain cannot be satisfactorily controlled by nonsurgical means. The usual problems that can end up in the need for a hip replacement include any one of the many types of arthritis, malformation of the hip since birth or abnormal development and damage from injury.

The artificial hip operation is one of the options that have in trying to control the pain of a damaged hip. It is not a life saving operation and hence every patient should carefully consider the alternatives to the surgery in discussion with their GP. Various means can be utilized to try and control the hip pain of bad arthritis. The GP or Rheumatologist may suggest:

Losing some weight if the case is overweight. This is because losing weight will reduce the stresses on your diseased hip and may reduce the pain.

It is often helpful to use a walking stick in the hand opposite to the side of the painful hip as that can reduce the pain by lowering the forces on the bad hip.

Taking pain killers and anti-inflammatory medication

Hip Joint:

The hip joint consists of a ball and socket joint. The top of the thighbone (femur) is a largest bone of human body, called femur joins with the horizontal pelvic coxal bone and lower end of that is fixed at the knee. The ball (femoral head) at the top of the thighbone fits into a portion of the pelvic bone forms the cup (acetabulum) or socket as shown in Figure 1.Between joining surfaces of the acetabulum and femoral lies a smooth glassy substance called cartilage. It provides frictionless cushion for constrained motion to femoral head within acetabular socket as shown in Figure 2.

Any failure eminating from acetabulum to femoral bone produces most common hip joint diseases in human body. Humans suffer from the various hip joint problems namely osteolysis, osteoarthritis, avascular necrosis, rheumatoid arthritis, fracture neck of femur, other inflammatory arthritis, developmental dysplasia, Paget's disease, arthrodesis (fusion) takedown, tumour, road accidents, soldier's injuries etc. Meaning of the medical terminology of some of these diseases is discussed below.

Figure : Front View of Pelvic Bone [1] 

Figure : Front View of Pelvic Bone [2] 

Osteolysis: It is local loss of bone tissue and appears because of wear. Destruction of bone takes place especially by bone resorption through removal or loss of calcium. Osteolysis may be evident in neoplastic, infectious, metabolic, traumatic, vascular, congenital and articular disorders.

Osteoarthritis (OA): It is degenerative arthritis disease because, a "wearing out" involving the breakdown of cartilage in the joints and is one of the oldest and most common types of arthritis. It is characterized by breakdown of the joint's cartilage. Cartilage is part of joint and cushions ends of the mating bones. The bones get deformed, and even small movements will cause friction between the ball and the socket of the hip, causing severe pain.

Avascular Necrosis: This is caused by lack of blood supply into bone. This condition may ultimately lead to bone death. Pain usually develops gradually and may be mild initially. If avascular necrosis progresses, bone and the surrounding joint surface may collapse causing increase in pain.

Rheumatoid Arthritis (RA): This involves inflammation in the lining of the joints and/or other internal organs. RA produces chemical changes in the synovium that cause it to become thickened and inflamed. In turn, the synovial fluid destroys cartilage. Rheumatoid arthritis typically affects many different joints and it is a chronic inflammatory joint disorder.

Developmental Dysplasia: Developmental Dysplasia of the hip is a condition in which the femoral head has an abnormal relationship to the acetabulum. It includes frank dislocation (luxation), partial dislocation (subluxation), or instability of the hip, wherein the femoral head comes in and out of the socket. Radiographic abnormalities reflect inadequate formation of the acetabulum. Since many of these findings may not be present at birth, the term developmental more accurately reflects the biologic features than the term congenital.

Paget's disease: It is a metabolic bone disorder of unknown origin. This normally affects older people. Bone is a living tissue and is constantly being renewed. Paget's disease of bone causes increased and irregular formation of bone. The bone cells, which are responsible for dissolving body's old bones and replacing them with new ones, become out of control.

Alumina Ceramics Hip Replacement:

The French surgeon Pierre Boutin attempted to use alumina ceramic in 1970. In 1977, Laurent Sedel implanted alumina ceramic heads with a Ceraver Osteal one-piece alumina ceramic ace tabular cup. (Figure 3-A) In his series of 86 implants, the survivorship rate at eight years was 97.8%. The failures were attributed to technique rather than the prosthesis design or material. In the early 1980s, the Mittelmeier AUTOPHOR alumina ceramic device consisting of a femoral head and ace tabular shell was introduced in the United States. (Figure 3-B) From 1982 to 1985, Mahoney implanted a series of 42 of these devices. The failure rate of the device at an average of 4.25 years after surgery was 35%. The failures resulted from poor cup performance and poor fit between the femoral stem and the femoral canal 2.

(B)

Figure : (A) one-piece alumina ceramic ace tabular cup, (B) alumina ceramic device consisting of a femoral head and ace tabular shell [3] 

The surgeons concluded "that the ceramic articulation performed well and did not contribute to the unsatisfactory results." Another series of 69 hips were implanted with the AUTOPHOR by O'Leary. The average time to revision was 26.2 months and the overall revision rate was 27%. Again the surgeons attributed the failure of the prosthesis "to technical and prosthetic design considerations" and not to the ceramic bearing. Based on the limited success of ceramic-on-ceramic articulation, this concept was abandoned in the US for about 10 years. During that time, fixation techniques for cemented and press-fit implants were greatly improved and it became increasingly obvious that long term implant survival depended on the elimination of polyethylene wear debris. Concurrently, aluminum oxide ceramics were being improved.

As attention returned to ceramic-on-ceramic articulation, implant designs were optimized by taking advantage of improvements in materials and manufacturing processes, and experience gained from the earlier implants. The design enhancements included highly polished auricular surfaces, optimized clearance between the head and liner to provide a fluid boundary, improved sphericity, tightened tolerances for tapers and elimination of skirts on ceramic heads.

Alumina Ceramic Experience

Aluminum oxide ceramic has been in use in orthopedic implants since 1970 due to its excellent biocompatibility, corrosion resistance, high hardness, wear resistance, scratch resistance, low coefficient of friction, phase stability and sufficient mechanical strength to resist fatigue. The following table shows wear rates for various couplings [3].

Table : Types of coupling and approximate wear rates

TYPE OF COUPLING

APPROXIMATE WEAR RATES

Cobalt Chrome on Polyethylene

0.1 - 0.4 mm/year

Alumina Ceramic on Polyethylene

0.05 - 0.1 mm/year

Alumina Ceramic on Alumina Ceramic

0.002 mm/year

The annual linear wear rate of alumina-on-alumina is 50 to 200 times lower than that of cobalt chrome-on polyethylene. However, a concern with alumina ceramic implants is their fracture potential. Much effort has been spent to track and analyze all ceramic fractures. With over 30 years of use, the alumina ceramic implants can be divided into 1st, 2nd and 3rd generation. CeramTec, the leading manufacturer of alumina ceramic implants, has reported the following fracture rates based on more than 1.5 million femoral heads.

Table : Multi-generation wear rate results

1st Generation

2st Generation

3st Generation

0.026%

0.014%

0.004%

26:100,000

14:100,000

4:100,000

The 1st generation alumina ceramics were low-density materials with a coarse microstructure. Today's 3rd generation alumina ceramic material has a high density with a small grain size, high chemical purity and a stable crystalline structure. Additionally, the material is hot isostatic pressed which decreases grain size and every component is proof tested. In fact the current specifications (ISO 6474 and ASTM F 603) are so stringent that the earlier materials would not have met them .

Third Generation alumina Bearings

Over the past 30 years, the alumina ceramic material has been gradually improved. The density has been increased from 3.94 to 3.97 g/cm3 and the grain size has been decreased from 7μm to 1.8 μm. (Figure 2) These improvements have increased the fracture toughness of the material.

Furthermore, enhancements to the locking taper have been made including narrowing the tolerances of the tapers, limiting taper angles and selecting proper surface topography to provide an even distribution of load during impaction. Additionally, the manufacturing process has undergone various improvements. Use of clean room technology has led to higher chemical purity, which is an important improvement as impurities can lead to the formation of stress risers and subsequent fracture.

The material is subjected to hot isostatic pressing. Finally, laser etching has replaced mechanical engraving. Since 1995, all alumina ceramic products manufactured by CeramTec have been subjected to proof testing. During proof testing, every component is subjected to stress distributions similar to those experienced under physiological conditions. As the components are exposed to short-term stress, proof testing is a quick, reliable and objective means of identifying flawed parts. For the components, hydraulic pressure is applied to the taper. Proof testing is superior to x-ray testing in that the latter identifies by imperfections but requires an operator to make a subjective determination of the risk associated with the imperfection.

Figure : The alumina ceramic material grain [4] 

Clinic Results

Bench testing of materials and implants plays an important role in providing a basic assurance of device safety and may be used to predict in vivo results. However, the best indicator of a device's in vivo performance is data collected in well-controlled clinical trials. In Europe alumina ceramics have been used since 1970 by surgeons such as Laurent Sedel. Dr. Sedel has reported minimum 5 year follow up of 97.4% survivorship, and in the US, Ben Bierbaum and Stephen Murphy published results of two studies showing that alumina-on-alumina ceramic implants perform at least as well as cobalt chrome-on-polyethylene in the mid-term. In one study, 349 cases were implanted with alumina ceramic articulation compared to 165 cases implanted with cobalt chrome on polyethylene. The Harris Hip Scores and patient satisfaction were equal in both groups at a mean follow-up of 35.2 months. Revisions were performed for dislocation, postoperative traumatic fracture, and deep joint infection.14 Murphy reported results of 405 alumina-on-alumina ceramic THAs that had been followed for a minimum of two years (mean: 30.5 months). With one report of deep infection requiring revision, he states that "the low incidence of infection is statistically significantly lower than that reported by the Swedish Hip Registry." Murphy reports a study where bacteria adhered 2.5 times more strongly to polyethylene than alumina ceramic and suggests that this may result in lower infection rates for alumina ceramic bearings.

There were no bearing fractures or incidences of osteolysis reported. Murphy concluded that aluminaon- alumina ceramic is a reliable bearing surface at mid-term follow-up and predicted that a substantial increase in the longevity of fixation long-term will result from the use of alumina ceramic bearings.

Hip Replacement Testing

Voluntary national and international standards have been relied on by the orthopedic community as guidelines for the mechanical properties of medical grade UHMWPE. The two most widely used standards for medical grade UHMWPE, ASTM F-648 and ISO 5834, include specifications for the properties of the unconsolidated resin powder, as well as the properties of the consolidated stock material. For many years now, both the ASTM and ISO standards have been developed by the same the industrial participants and researchers. Thus, the ASTM and ISO standards are considered to be harmonized and consequently reflect a unified and international perspective on the properties of medical grade UHMWPE.

Arthritis is a chronic degenerative disease which will affect most people at some point during their lifetime. It results from inflammation of the joints and can come in over 100 different forms. Osteoarthritis is the most common form of arthritis in the UK with an estimated 8.5 million people suffering from the condition1. It can affect all joints within the body; and often affects the fingers, toes, knees, hips, and shoulders. Joint replacement surgery is a well-established technique used to help alleviate this condition. The first total hip replacement surgery was performed over 40 years ago and now benefits millions of people every year. Technological advancements in materials, design and techniques have made it one of the most predictable and reliable medical procedures available. Not surprisingly, leading manufacturers of orthopaedic implants have well-equipped research and development laboratories to ensure the safety and function of these devices. This article looks at how a materials testing machine is used to make some of these critical measurements.

Materials testing are a well-established technique used to determine the physical and mechanical properties of raw materials and components from a human hair to steel, composite materials and plastics. Table 1 shows typical parameters that can be measured using the technique.

Currently, hip-joint endoprostheses consist, as a rule, of modularly constructed systems. A metal shaft with a pin, on which a spherical head is placed, is anchored in the femur. The spherical head articulates against a socket or a socket insert. A socket is implanted directly in the femur, whilst a socket insert is first inserted in a socket housing which is then anchored in the pelvic bone.

Total hip replacement is an operation designed to replace a hip joint that has been damaged by arthritis. The natural hip joint is a ball and socket joint. The ball is formed by the head of the femur (thigh bone) and fits snugly into the socket (acetabulum). In a total hip replacement operation, the surgeon replaces the worn head of the thigh bone with a metal or ceramic ball mounted on a stem, while the socket is resurfaced. The material used to resurface the socket can be polyethylene (plastic), ceramic, metal; and various different cup designs exist. The prosthesis may be cemented in place with a filler or grout which is called bone cement, or securely pressed into place without using cement.

Table : Physical and mechanical properties that can be measured using universal testing machine.

Tensile Strength

Bond Strength

Elastic Limit

Compression

Adhesion Strength

Elongation

Flexure/Bend Strength

Break Load

Rupture Strength

Coefficient of Friction

Creep and Stress Relaxation

Young's Modulus

Puncture Strength

Crush Resistance

Toughness

Delamination Strength

Deformation Strength

Peel Strength

Tear Resistance

Ductility

Tear Strength

In addition to components for endoprostheses of the hip joint made of metal and plastics material, there are also components that are made of high-purity, high-density ceramic material. These components, present significant advantages, in contrast to components made of different materials, such as complete biocompatibility and maximum wear resistance. Doubts exist, however, regarding the mechanical strength of such components, since ceramic materials are brittle, which means that instances of non-homogeneity of the material, for example micro-cracks, represent an increased risk of fracture in the event of loading. The reliability against defects resulting from components that are risk-attendant can be increased if these components are successfully detected and eliminated out by means of a suitable test after they have been produced. It is not, however, possible to detect components that are risk-attendant with any certainty by means of the usual non-destructive testing methods, for example X-ray testing, ultrasonic testing or dye-penetration methods.

For these reasons, method has been developed with which it is possible to test, in particular, components of hip-joint endoprostheses that are made of ceramic materials. For example, a method for testing ceramic sockets or socket inserts of hip-joint endoprostheses is known from DE 197 18 615 A1, in which a force is allowed to act on the inner surface of the latter in such a way that all the volume elements of the socket or socket insert respectively that are under a load, when physiologically loaded, become loaded and stresses are generated thereby that are higher, by a defined factor, than the stresses that are generated in the case of the physiological load.

In the case of the known overload tests, the so-called proof tests, carried out on ceramic socket inserts, the difficulty lies in generating the same stress ratios during the test that prevail in the case of a socket insert that is inserted in a socket housing, which in turn is implanted in a hip joint, and is loaded with a spherical head.

It is not possible to insert the socket inserts into the socket housings for testing purposes in order to be able to carry out a proof test as a quality control after production. After testing, it would be impossible to remove the socket inserts from the respective socket housing again without damage. Moreover, on account of the manufacturing tolerances of socket inserts and socket housings, reproducibility of the contact ratios between the components would not be guaranteed. This is necessary, however, since in a proof test it must be possible to repeat the most unfavorable case of stress distribution caused by the contact of the components.

Chapter 2: Material Properties and Specification

Alumina Ceramics:

Alumina (aluminum oxide) is the most important, widely used and cost effective oxide ceramic material. The technical alumina ceramics contain at least 80% of aluminum oxide (AL2O3). Small amounts of silica (SiO2), magnesia (MgO) and zirconia (ZrO2) may be added to alumina ceramics. Addition of zirconia to alumina ceramic results in considerable increase of the material fracture toughness.

Alumina possesses strong ionic bonding, which determines the material properties:

High mechanical strength (flexural strength) and hardness

High wear resistance

High resistance to chemical attacks of strong acids and alkali even at high temperatures

High stiffness

Excellent insulating properties

Low coefficient of thermal expansion

Good fracture toughness

Good thermal conductivity

Good biocompatibility

Aluminum ceramics parts are manufactured by the following technologies: uniaxial (die) pressing, isostatic pressing, injection molding, extrusion and slip casting. The parts may be machined in "green" condition before sintering (firing).

Aluminum ceramics are widely used in electronics and electrical engineering, metallurgical processes, chemical technologies, medical technologies, mechanical engineering, military equipment.

Aluminum ceramics are used for manufacturing insulators, capacitors, resistors, furnace tubes, sealing refractory parts, foundry shapes, and wear pads, thermocouple protection tubes, cutting tools and polishing/grinding powders, ballistic armor, laboratory equipment, bio-ceramic parts for orthopedic and dental surgery.

Ionic and Covalent bonding:

Ceramics (ceramic materials) are non-metallic inorganic compounds formed from metallic (Al, Mg, Na, Ti, W) or semi-metallic (Si, B) and non-metallic (O, N, C) elements. Atoms of the elements are held together in a ceramic structure by one of the following bonding mechanism: Ionic Bonding, Covalent Bonding, Mixed Bonding (Ionic-Covalent).

Most of ceramic materials have a mixed bonding structure with various ratios between Ionic and Covalent components. This ratio is dependent on the difference in the electro negativities of the elements and determines which of the bonding mechanisms is dominating ionic or covalent.

Ionic Bonding:

Ionic bonding occurs between two elements with a large difference in their electro negativities (metallic and non-metallic), which become ions (negative and positive) as a result of transfer of the valence electron from the element with low electro negativity to the element with high electro negativity.

The typical example of a material with Ionic Bonding is sodium chloride (NaCl).

Electropositive sodium atom donates its valence electron to the electronegative chlorine atom, completing its outer electron level (eight electrons):

As a result of the electron transfer the sodium atom becomes a positively charged ion (cation) and the chlorine atom becomes a negatively charged ion (anion). The two ions attract to each other by Coulomb force, forming a compound (sodium chloride) with ionic bonding. Ionic bonding is non-directional.

Covalent Bonding

Covalent bonding occurs between two elements with low difference in their electro negativities (usually non-metallic), outer electrons of which are shared between the four neighboring atoms.

ionic bondcovelent

Figure : Ionic and Covalent Bond [5] 

Properties of some Alumina Ceramics:

Table : Alumina ceramic (94% AL2O3)

Chemical composition: 94% AL2O3

Property

Value in metric unit

Value in US unit

Density

3.69 *10³

kg/m³

230.4

lb/ft³

Modulus of elasticity

300

GPa

43500

ksi

Flexural strength

330

MPa

47900

psi

Compressive strength

2000

MPa

290000

psi

Fracture toughness

3.5

MPa*m½

3.5

MPa*m½

Hardness

1175

HV

1175

HV

Thermal expansion (20 °C)

7.3*10-6

°Cˉ¹

4.1*10-6

in/(in* °F)

Thermal conductivity

18

W/(m*K)

125

BTU*in/(hr*ft²*°F)

Specific heat capacity

880

J/(kg*K)

0.21

BTU/(lb*°F)

Max. working temperature

1600

°C

2910

°F

Dielectric strength (DC)

8.7

KV/mm

220

V/mil

Dielectric constant (1MHz)

9.0

-

9.0

-

Table : Alumina ceramic (97.5% AL2O3)

Chemical composition: 97.5% AL2O3

Property

Value in metric unit

Value in US unit

Density

3.85 *10³

kg/m³

240.3

lb/ft³

Modulus of elasticity

331

GPa

48000

ksi

Flexural strength

386

MPa

56000

psi

Compressive strength

2070

MPa

300000

psi

Fracture toughness

4.3

MPa*m½

4.3

MPa*m½

Hardness

1340

HV

1340

HV

Thermal expansion (20 °C)

7.6*10-6

°Cˉ¹

4.2*10-6

in/(in* °F)

Thermal conductivity

35.4

W/(m*K)

245

BTU*in/(hr*ft²*°F)

Specific heat capacity

880

J/(kg*K)

0.21

BTU/(lb*°F)

Max. working temperature

1650

°C

3000

°F

Dielectric strength (DC)

9.3

KV/mm

235

V/mil

Dielectric constant (1MHz)

9.4

-

9.4

-

Table : Alumina ceramic (99.8% AL2O3)

Chemical composition: 99.8% AL2O3

Property

Value in metric unit

Value in US unit

Density

3.89 *10³

kg/m³

242.8

lb/ft³

Modulus of elasticity

375

GPa

54400

ksi

Flexural strength

410

MPa

59500

psi

Compressive strength

2340

MPa

340000

psi

Fracture toughness

4

MPa*m½

4

MPa*m½

Hardness

1440

HV

1440

HV

Thermal expansion (20 °C)

7.9*10-6

°Cˉ¹

4.4*10-6

in/(in* °F)

Thermal conductivity

35.4

W/(m*K)

245

BTU*in/(hr*ft²*°F)

Specific heat capacity

880

J/(kg*K)

0.21

BTU/(lb*°F)

Max. working temperature

1700

°C

3090

°F

Dielectric strength (DC)

9.8

KV/mm

250

V/mil

Dielectric constant (1MHz)

9.7

-

9.7

-

Strength, fatigue resistance and fracture toughness of polycrystalline alumina are a function of grain size and concentration of sintering aid. Al2O3 with an average grain size of < 4 um and a purity of >99.7% exhibit good flexural strength and excellent compressive strength. High content of sintering aids must be avoided because of it remain in the grain boundaries and reduce the fatigue resistance especially in the corrosive physiological environment. Al2O3 devices implanted with a tight mechanical fit and loaded primarily in compression are successful clinically.

Types of abrasion test

A large number of different abrasion apparatus have been used for testing HIP and an even larger number of permutations of the various factors would be possible. The first division of test types can be to distinguish between those using a loose abradant and those using a 'solid' abradant.

A loose abrasive powder can be used to impinge on the HIP rather in the manner of a shot blasting machine, or tumbled with the HIP test pieces in a rotating drum. These are logical ways to simulate the action of sand or similar abradants impinging on the HIP in service, as may be the case with conveyor belts or tank linings, but this type of test is not very common.

'Solid' abradants could consist of almost anything but the most common are abrasive wheels (vitreous or resilient), abrasive papers or cloth, and metal 'knives'. The possible geometries by which the test piece and a solid abradant can be rubbed together are legion and it is not sensible to make any general classification.

Figure : Total Wear-Out of a Teflon Socket by after 3 Years use [6] 

A loose abradant can also be used between the two sliding surfaces in what could be considered as a hybrid of loose and solid abradant tests. This situation occurs in practice through contamination and as a result of the generation of wear debris from a 'solid' abradant. A car tire is an example of the situation where there is a combination of abrasion against a solid rough abradant, the road, together with a free flowing abradant in the form of grit particles.

It follows, that there cannot be a universal standard abrasion test for HIP and the test method and conditions have to be chosen to suit the end application. In some applications, for example tires, the range of conditions encountered is so complex that they cannot be matched by a single laboratory test. However, for many products meaningful results can be obtained by careful modification of standard abrasion tests, but great care has to be taken if the test is intended to provide a significant degree of acceleration.

Wear of Ceramic Materials

General Features of Wear:

There are four fairly consistent differences between metals and ceramic materials in sliding contact:

1. The coefficient of friction of ceramic materials is usually significantly higher than that of metals. A parallel behavior is that ceramic materials are much more likely to produce severe vibrations during sliding than do metals.

2. In repeat-pass sliding with the pin-on-disk specimen shape, the wear loss from the pin is greatest for metal combinations (unless the disk is much softer than the pin), whereas the wear loss from the disk is greatest for ceramic combinations. There is often little wear in the early stages of sliding, followed in time by a rising wear rate.

3. The wear rate often increases sharply at some point during an increase in sliding speed, probably due to thermal stress cycling.

4. The wear rate often increases sharply at some point during an increase in contact pressure. An explanation is given below under Wear Models for Ceramic Materials.

Ceramic materials are different from metals and polymers in two very important respects that influence wear and surface damage:

1. The grains are brittle (but do behave in a somewhat ductile manner under compressive stress). Ceramic materials are mostly either ionic or covalent structures. Thus there is an overall brittle behavior of macroscopic-size specimens in a tensile test or impact test.

2. Grain boundaries range in properties from very ductile to very brittle. The reason is that many generic ceramic materials are made with several different, often ductile, sintering aids that become thin second phases or intergranular (grain boundary) materials. Si3N4 often has MgO or Y2O3 grain boundaries. SiC usually has none and in such materials the anisotropic behavior of grains places a high stress on grain boundaries during temperature changes and with externally applied stresses. ZrO2 is an example of ceramic material that changes lattice structure under stress, from the tetragonal phase to the ≈5% less dense monoclinic phase under tension, reverting partially to the tetragonal phase under compression.

These distinct properties produce two effects in tribological applications that are less obvious in other applications:

1. The small scale nonhomogeneous strain fields induced in materials in sliding or erosion preferentially fracture brittle grain boundaries.

2. The anisotropic morphology of ceramic materials promotes failure in repeat stress applications, also known as fatigue behavior. Since many tribological situations involve repeat-pass sliding, repeat impacts, etc., a fatigue mode of ceramic wear may be prominent. In the ceramic materials with ductile grain boundaries, the fatigue mechanisms are similar to the low-cycle fatigue mechanisms in metals. In the ceramic materials with brittle grain boundaries, failure also occurs in few cycles but cracks propagate quickly because of high residual and anisotropically induced stresses.

Wear Models for Ceramic Materials:

The most formal thinking on wear mechanisms of ceramic materials focuses on their brittle behavior. Wear is assumed in many papers to occur by the damage mechanisms formed by a sharp static indenter. Cracks occur at the corners of indentations made when a load is applied upon a Vickers or Knoop indenter, producing planar cracks perpendicular to the surface. Cracks also appear at some depth below the surface when the load is removed from the indenter.

Figure : Microstructure of fired high alumina ceramic parts [7] 

Chapter 3: Problem Analysis and Implementation

One of the most popular tests regarding to the hip replacement is the wear tests using Wheel Abrasion test.

The terms wear and abrasion are used so loosely that confusion sometimes results. Wear is a very general term covering the loss of material by virtually any means. The dictionary says that abrasion is the wearing away by means of friction, although in everyday life we think of it as the rasping action of a rough surface. As wear usually occurs by the rubbing together of two surfaces, abrasion is often used as a general term to mean wear. The mechanisms by which wear occurs when a HIP is in moving contact with any material are somewhat complex, principally involving cutting of the HIP and fatiguing of the HIP. Nevertheless, we call the tests to measure this wear abrasion tests.

It is possible to categories wear mechanisms of HIP in various ways and one convenient system is to differentiate between three main factors:

Abrasive wear, which is caused by hard asperities cutting the HIP.

Fatigue wear, which is caused by particles of HIP being detached as a result of dynamic stressing on a localized scale.

Adhesive wear, which is the transfer of HIP to another surface as a result of adhesive forces between the two surfaces.

Abrasive sliding wear of alumina ceramic in simulated body fluids

The main purpose is to obtain the effect of wear phenomena in hip replacement made from alumina ceramic material using Abrasion-Corrosion Test. This test is an example of a test method which produces imposed wear scar geometry.

A hard Alumina ceramic sphere is used to rotate parallel against a selected sample placed in a holder in the presence of slurry of fine abrasive that are small and hard particles, which is silicon Carbide (SiC) drip-fed for every 45 second. The geometry of the wear scar is understood to procedure the spherical geometry of the ball and corrosive removes the protective corrosion product, exposing clean surface for further corrosive attack.

Then the wear volume may be calculated by measuring the crater diameter using a microscope and also the radius of the ball.

The wear volume is calculated using an approximate formula and that is:

Where;

V= Volume of Wear.

b=Diameter of the Crater.

R= is the Radius of the ball.

The Radius of the ball used is 12.5 mm where the diameter of the Crater is 12.7mm.

The test mainly divided into for Trail, below are the specification of each trail

The concentrate of silicon Carbide (SiC) used is 400 g/liter.

The concentrate of silicon Carbide (SiC) used is 500 g/liter.

The concentrate of silicon Carbide (SiC) used is 500 g/liter, and using Grinding Wheel 1µm for surface improvement

The concentrate of silicon Carbide (SiC) used is 500 g/liter, and using Grinding Wheel 6µm for surface improvement

The results of the experiments will be present in details in next chapter, presenting the relation between distance and diameter of carter, wear volume and ball diameter.

09083021C11.jpg

Figure : Grinding Wheel disc [8] 

F:\On Prog ress\ahmad test\DSC00318.JPG

Figure : The sample that used in the wear test

DSC00313.JPG

Figure : Microscope "Diameter measure device"

DSC00314.JPG

Figure : Abrasion-Corrosion apparatus

Chapter 4: Problem Results and Discussions

Experiment No.1 the concentrate of silicon Carbide (SiC) used is 400 g/litre

Table : Experiment No. 1 Results

Sic 400 g\lit

Time

[sec]

Distance

[m]

Radius

[mm]

Diameter

[mm]

B

[mm]

Wear volume

[mm3]

Speed

[m\s]

45

2.32

12.5

0.2

0.1

3.927E-07

0.0516

90

4.63

12.5

0.375

0.1875

0.000004854

0.0515

135

6.95

12.5

0.75

0.375

0.00007766

0.0515

180

9.3

12.5

1.61

0.805

0.0016491

0.0517

225

11.6

12.5

1.84

0.92

0.0028133

0.0516

Figure : B value (mm) V.s Distance (m) Experiment No.1: Sic 400 g\lit

Figure : Wear volume (mm3) V.s Distance (m) Experiment No.1: Sic 400 g\lit

Figure : Diameter (mm) V.s Distance (m) Experiment No.1: Sic 400 g\lit

The table 7 and the group of figures (12-14) are present the result of the first experiment, it is clear that the relation between the distance and the three variables "volume of wear, Diameter of the crater, and the radius of the ball is approximately Proportional, where the environment of experiment is to use a Silicon Carbide (SIC) with concentration of 400 g/lit.

For Diameter of crater "B" with distance relationship figure, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 0.82 mm.

The final volume of wear after 225 second of operation is 0.0028133 mm3, but as shown in figure 13 that the volume of wear increased slightly until reach 6.95 m distance then the volume start to increasing rapidly.

For ball diameter it is clearly shown that the relation is approximately liner with distance, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 0.82 mm.

Experiment No.2 the concentrate of silicon Carbide (SiC) used is 500 g/litre

Table : Experiment No. 2 Results

Sic 500 g\lit

Time [sec]

Distance [m]

Radius [mm]

Diameter[m[m]

B [mm]

Wear volume [mm3]

Speed [m\s]

45

2.32

12.5

1.01

0.505

0.0002554

0.0516

90

4.63

12.5

1.52

0.76

0.00131

0.0515

135

6.95

12.5

2.66

1.33

0.012286

0.0515

180

9.3

12.5

3.22

1.61

0.026385

0.0517

225

11.6

12.5

3.68

1.84

0.045

0.0516

Figure : B value (mm) V.s Distance (m) Experiment No.2: Sic 500 g\lit

Figure : Wear volume (mm3) V.s Distance (m) Experiment No.2: Sic 500 g\lit

Figure : Diameter (mm) V.s Distance (m) Experiment No.2: Sic 500 g\lit

The table 8 and the group of figures (15-17) are present the result of the first experiment, it is clear that the relation between the distance and the three variables "volume of wear, Diameter of the crater, and the radius of the ball is approximately Proportional, where the environment of experiment is to use a Silicon Carbide (SIC) with concentration of 500 g/lit.

For Diameter of crater "B" with distance relationship figure, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 1.335 mm.

The final volume of wear after 225 second of operation is 0.045 mm3, but as shown in figure 16 that the volume of wear increased slightly until reach 4.63 m distance then the volume start to increasing rapidly.

For ball diameter it is clearly shown that the relation is approximately liner with distance, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 2.67 mm.

Experiment No.3 the concentrate of silicon Carbide (SiC) used is 500 g/litre after the roughness of the surface using Grinding Wheel 1µm

Table : Experiment No. 3 Results

Sic 500 g\lit After the roughness of the surface using Grinding Wheel 1µm

Time

[sec]

Distance

[m]

Radius

[mm]

Diameter

[mm]

B

[mm]

Wear volume

[mm3]

Speed

[m\s]

45

2.32

12.5

2.5

1.25

0.0095874

0.0516

90

4.63

12.5

2.9

1.45

0.0174

0.0515

135

6.95

12.5

3.45

1.725

0.0348

0.0515

180

9.3

12.5

3.95

1.975

0.05975

0.0517

225

11.6

12.5

4.45

2.225

0.096245

0.0516

Figure : B value (mm) V.s Distance (m) Experiment No. 3: Sic 500 g\lit after the roughness of the surface using Grinding Wheel 1µm

Figure : Wear volume (mm3) V.s Distance (m) Experiment No.3: Sic 500 g\lit after the roughness of the surface using Grinding Wheel 1µm

Figure : Diameter (mm) V.s Distance (m) Experiment No. 3: Sic 500 g\lit after the roughness of the surface using Grinding Wheel 1µm

The table 9 and the group of figures (18-20) are present the result of the first experiment, it is clear that the relation between the distance and the three variables "volume of wear, Diameter of the crater, and the radius of the ball is approximately Proportional, where the environment of experiment is to use a Silicon Carbide (SIC) with concentration of 500 g/lit after the roughness of the surface using Grinding Wheel 1µm

For Diameter of crater "B" with distance relationship figure, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 0.975 mm.

The final volume of wear after 225 second of operation is 0.096245 mm3, but as shown in figure 18 that the volume of wear increased slightly all over the experiment

For ball diameter it is clearly shown that the relation is approximately liner with distance, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 1.95 mm.

Experiment No.3 the concentrate of silicon Carbide (SiC) used is 500 g/litre after the roughness of the surface using Grinding Wheel 6µm

Table : Experiment No. 4 Results

Sic 500 g\lit After the roughness of the surface using Grinding Wheel 6µm

Time [sec]

Distance [m]

Radius [mm]

Diameter

[mm]

B [mm]

Wear volume [mm3]

Speed [m\s]

45

2.32

12.5

2.9

1.45

0.00135

0.0516

90

4.63

12.5

3.7

1.85

0.045

0.0515

135

6.95

12.5

4.35

2.175

0.08

0.0515

180

9.3

12.5

5.05

2.525

0.101

0.0517

225

11.6

12.5

5.85

2.925

0.29

0.0516

Figure : B value (mm) V.s Distance (m) Experiment No. 4: Sic 500 g\lit after the roughness of the surface using Grinding Wheel 6µm

Figure : Wear volume (mm3) V.s Distance (m) Experiment No.4: Sic 500 g\lit after the roughness of the surface using Grinding Wheel 6µm

Figure : Diameter (mm) V.s Distance (m) Experiment No. 4: Sic 500 g\lit after the roughness of the surface using Grinding Wheel 6µm

The table 10 and the group of figures (21-23) are present the result of the first experiment, it is clear that the relation between the distance and the three variables "volume of wear, Diameter of the crater, and the radius of the ball is approximately Proportional, where the environment of experiment is to use a Silicon Carbide (SIC) with concentration of 500 g/lit after the roughness of the surface using Grinding Wheel 6µm

For Diameter of crater "B" with distance relationship figure, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 1.475 mm.

The final volume of wear after 225 second of operation is 0.29 mm3, but as shown in figure 22 that the volume of wear increased slightly until reach 9.3 m distance then the volume start to increasing rapidly.

For ball diameter it is clearly shown that the relation is approximately liner with distance, the difference between first reading of crater diameter after 45 second and final diameter after 225 second operation time is 2.95 mm.

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