FABRICATION OF CERAMIC COMPONENTS USING MOLD SHAPE DEPOSITION MANUFACTURING

Transcription

1 FABRICATION OF CERAMIC COMPONENTS USING MOLD SHAPE DEPOSITION MANUFACTURING A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Alexander G. Cooper August 1999

2 Copyright by Alexander Cooper 1999 All Rights Reserved ii

3 I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the Doctor of Philosophy. Fritz Prinz (Principal Advisor) I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the Doctor of Philosophy. Mark Cutkosky I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and quality, as a dissertation for the Doctor of Philosophy. Kosuke Ishii Approved for the University Committee on Graduate Studies: iii

4 Abstract Mold Shape Deposition Manufacturing (Mold SDM) is a new process for the fabrication of geometrically complex structural ceramic components. Mold SDM builds fugitive molds which are then used to produce ceramic parts via gelcasting. This thesis describes the development of the Mold SDM process, including process steps, materials selection and planning strategies. A prototype Mold SDM machine, which was built to automate the mold building phase of the process, is shown. Preliminary process characterization results are presented, as well as a range of current and potential applications for ceramic, metal and polymer parts made using Mold SDM. The high strength, stiffness and low density of ceramic materials make them attractive for structural applications, and their excellent high temperature properties make them particularly attractive in high temperature applications. The two primary obstacles to the wider use of ceramics are their low toughness and the limitations of current manufacturing processes. As designers gain more experience with ceramics and the materials themselves become more reliable, the low toughness issue will become less of a limitation. Current production processes are limited in terms of achievable shape complexity and also suffer from long lead times. Rapid prototyping processes have been adapted to the fabrication of ceramic parts but they generally produce parts with poor surface quality. There is therefore a need for methods that can quickly produce high quality, geometrically complex ceramic parts. Mold SDM was developed to address this need. Mold SDM is based on Shape Deposition Manufacturing (SDM) and uses SDM techniques to build fugitive wax molds. SDM is an additive-subtractive layered manufacturing process. The subtraction step, accomplished by milling, improves surface finish and increases material flexibility relative to rapid prototyping processes. In order to increase the capabilities of the Mold SDM process and to improve the quality of parts produced, a number of new build techniques were developed. These new techniques can also be used with the basic SDM process. iv

5 The materials selection work involved identification and testing of a variety of materials in order to optimize the performance of the Mold SDM process. Key materials properties were identified and used to guide the selection process. Candidate materials were evaluated, primarily in terms of machinability, shrinkage, heat resistance and chemical compatibility. A number of preferred materials combinations were developed based on these results and these were used to produce a range of ceramic, metal and polymer parts. Based on experience accumulated during process development and part building, a number of new process planning strategies were developed. The manufacturability analysis technique presented can be used to determine if a given part is manufacturable using Mold SDM. Guidelines are given for optimum build orientation selection based on an analysis of part features. New decomposition techniques are proposed to take advantage of process capabilities such as multiple build directions. The construction and operation of the experimental Mold SDM machine is described. The machine was built by adding material deposition equipment to a milling machine to produce a machine capable of automating the mold building phase of the Mold SDM process. Process characterization in terms of feature size capabilities, achievable accuracy and surface roughness, build rates and final part properties is presented. Common part defects and their causes are explained and methods of avoiding them are described. The process characteristics are compared to those of competing ceramic manufacturing processes and rapid prototyping techniques. The primary application for Mold SDM has been the fabrication of complex structural ceramic components for use in gas turbine engines. During the course of this research a number of other applications and potential applications have been identified for ceramic, metal and polymer parts made via Mold SDM. Characteristics of ideal applications are identified and used to evaluate these applications. Some proof of concept parts are shown. v

6 Acknowledgments I would like to thank my advisor, Professor Fritz Prinz for providing the environment which made all of this work possible. I would also like to thank him for his energetic support and for always encouraging me to aim higher. I would like to thank my thesis reading committee members, Professors Kosuke Ishii and Mark Cutkosky for their time and effort in providing insightful feedback and advice. I would like to thank Professor Tom Kenny for serving on my defense committee and Professor James Jucker for Chairing it. I would like to thank my colleagues in the Rapid Prototyping Laboratory and also the Center for Design Research for their contributions to this research and their friendship. I would particularly like to thank Dr. John Kietzman, Alex Nickel, Rudi Leitgeb, Dr. John Fessler, Tassos Golnas, Dr. Ju-Hsien Kao, Sangkyun Kang, Dr. Jim Stampfl, Byong-Ho Park, Miguel Pinilla, Dr. Mike Binnard, Eric Chamness, Dr. Robert Merz, Dr. Krishnan Ramaswami and Tom Hasler. Sylvia Walters and Lynn Hoschek provided essential administrative support and encouragement throughout this time. Thanks also go to my colleagues at Carnegie Mellon University, Dr. Lee Weiss, Larry Schultz and Dr. Gennady Neplotnik, for their advice and assistance. Dr. John Lombardi and David Dent, from Advanced Ceramics Research Inc., and Professor Paul Calvert, from the University of Arizona, provided much assistance, particularly on materials related issues. I would like to thank the Defense Advanced Research Projects Agency, in particular Dr. Bill Coblenz, for supporting this research under contracts N I-0625 and N I Part of my studies was also funded by a Department of Energy Integrated Manufacturing Pre-Doctoral Fellowship. Finally, I would like to thank all of my friends and family for their support and encouragement over the years. vi

7 Table of Contents Table of Contents...vii List of Tables...xii List of Figures...xiv 1 Introduction Overview Process Development Materials Selection and Optimization Process Planning Thesis Outline Individual / Group Research Statement Background Ceramics Structural Ceramics Issues Ceramic Manufacturing Processes Ceramic Forming Processes Powder Pressing Slip Casting Tape Casting Extrusion Injection Molding Green Machining Gelcasting Rapid Prototyping Of Ceramic Parts Overview Stereolithography (SLA) Fused Deposition Modeling (FDM) Selective Laser Sintering (SLS) Three Dimensional Printing (3DP) vii

8 2.4.6 Laminated Object Manufacturing (LOM) Direct Photo Shaping (DPS) Computer Aided Manufacturing of Laminated Engineering Materials Sanders Prototype Evaluation of Existing Processes Objectives for SDM and Mold SDM of Ceramics Process Development Shape Deposition Manufacturing (SDM) Process Description Comparison to Other Rapid Prototyping Processes SDM Application Examples Fabrication of Ceramic Green Parts Using SDM Mold Shape Deposition Manufacturing (Mold SDM) Process Description Mold SDM Compared to SDM and Other RP Processes New Build Techniques Freeform Layer Geometries Overcut-Fill-Trim-Backfill One Time Surface Machining Cut Through Machining Incremental Casting Multiple Build Directions Fabrication Of Pre-Assembled Mechanisms Process Example Materials Selection Materials Requirements Mold Building Materials Part Materials Materials For SDM Of Ceramic Green Parts Materials For Mold SDM Initial Materials Combination Reducing Cracking And Delamination Lower Temperature Wax Deposition Improving Gelcasting Slurry Cure Conditions viii

9 4.3.5 Switch To A Lower Temperature Wax Solvent Removal Of Wax Investigation of Alternative Waxes Wax Optimization Other Materials Considered Materials Testing Machinability Testing Shrinkage Testing Heat Resistance Current Mold SDM Materials Mold Materials Support Materials Part Materials Future Mold SDM Materials Mold Materials Support Materials Part Materials Process Planning Background Planning for Rapid Prototyping Processes Planning for the SDM and Mold SDM Processes Planner Development History The Current Planner Process Planning Enhancements Manufacturability Analysis Manufacturability Rules for SDM and Mold SDM Manufacturability Analysis Example Orientation Selection Orientation Selection For Rapid Prototyping Processes Orientation Selection For Mold SDM Decomposition Strategies Improved General Decomposition Decomposition For The New Build Techniques Decomposition Strategies For Multiple Build Directions Pitch Shaft Planning Example ix

10 5.6.1 Horizontal Build Orientation Vertical Build Orientation Multiple Build Orientations Process Automation Reasons for Automation Automation of the Mold SDM Process The Experimental Mold SDM Machine Hardware Description Issues Process Characterization Objectives Characterization Tolerances and Surface Quality Feature Sizes Build Rate Part Properties Defects and Methods for Eliminating Them Defects Resulting From Mold Fabrication Defects Resulting From Casting Defects Resulting From Curing Defects Resulting From Demolding Defects Resulting From Drying, Burnout and Sintering Comparison With Other Processes Materials Speed Accuracy Sintering Applications Application Objectives Mold SDM Process Characteristics General Application Characteristics Applications For Ceramics Heat Engines x

11 8.4.2 Structural Applications Ceramic Mechanisms Applications For Metals Applications For Polymers Prototyping and Short Run Production Applications Conclusions Summary Contributions Future Work References Glossary xi

12 List of Tables Background Table 2.1: Typical properties of structural ceramics Table 2.2: Typical properties of structural alloys Table 2.3: Material properties of silicon nitride and 4340 steel Table 2.4: Calculated failure loads Table 2.5: Properties reported for ceramics made by SLA Table 2.6: Properties reported for ceramics made by FDC Table 2.7: Properties reported for ceramics made by 3DP Table 2.8: Commercial licensees of 3DP Table 2.9: Properties reported for ceramics made by LOM Materials Selection Table 4.1: Waxes tested for use in Mold SDM Table 4.2: Measured shrinkage values for various waxes Table 4.3: ACR gelcasting slurry properties Table 4.4: Processing conditions for the silicon nitride Table 4.5: Polymer part materials Process Planning Table 5.1: First orientation edge list Table 5.2: Second orientation edge list Table 5.3: Third orientation edge list Process Automation Table 6.1: Milling machine specifications Table 6.2: Material deposition and curing equipment specifications Process Characterization Table 7.1: Polymer part material shrinkage values Table 7.2: Part size measurements Table 7.3: Inlet nozzle size measurements Table 7.4: Summary of surface roughness measurements Table 7.5: Factors driving process accuracy xii

13 Table 7.6: Estimated part and feature size capabilities Table 7.7: Summary of main build time drivers Table 7.8: Properties of polymeric part materials used in Mold SDM Table 7.9: Surface quality achieved using RP processes xiii

14 List of Figures Background Figure 2.1: Ashby charts illustrating material property ranges Figure 2.2: Beam with a surface flaw loaded in 3-point bending Figure 2.3: Ceramic fabrication process sequence Process Development Figure 3.1: Example SDM build sequence Figure 3.2: Example Mold SDM build sequence Figure 3.3: Inclined sharp edge Figure 3.4: Decomposition using a horizontal splitting plane Figure 3.5: Decomposition using non-horizontal splitting plane Figure 3.6: Thin walled cylinder build sequences Figure 3.7: Conventional milling sequence Figure 3.8: Overcut-fill-trim-backfill sequence Figure 3.9: One time surface machining Figure 3.10: One time surface machining example Figure 3.11: Cut through machining Figure 3.12: Incremental casting Figure 3.13: Multiple build directions example part Figure 3.14: Multiple build direction build sequence example Figure 3.15: Pre-assembled mechanisms made using Mold SDM Figure 3.16: Multiple cavity mold for a turbine mechanism part Figure 3.17: Turbine mechanism fabrication sequence Figure 3.18: Multimaterial turbine Figure 3.19: Sintered alumina turbine mechanisms Figure 3.20: Sintered alumina turbine before fixture removal Figure 3.21: CAD model of the shrouded fan Figure 3.22: Shrouded fan build steps Figure 3.23: Finished polyurethane shrouded fan Materials Selection Figure 4.1: Green silicon nitride slidey made by SDM Figure 4.2: Blistered alumina part xiv

15 Figure 4.3: Machinability spiral test pattern Figure 4.4: Machinability failure modes Figure 4.5: Machinability without air Figure 4.6: Machinability using room temperature air Figure 4.7: Machinability using cold air Figure 4.8: Wax shrinkage modes Figure 4.9: Shrinkage as a function of the fraction of machinable wax Figure 4.10: Heat resistance test geometry Figure 4.11: Alumina impellers and turbines Figure 4.12: Miniature alumina turbines Figure 4.13: Alumina pitch shaft Figure 4.14: Alumina vane doublet Figure 4.15: Silicon nitride impellers and pitch shaft Figure 4.16: Silicon nitride inlet nozzle Figure 4.17: Silicon nitride impellers Figure 4.18: Polymer impellers Figure 4.19: Polyurethane turbines and sectioned mold Figure 4.20: Polyurethane shrouded fan Figure 4.21: Multicolor polyurethane Inchworm Figure 4.22: Silicone artery model Figure 4.23: Multimaterial polymer mechanisms Figure 4.24: Gelcast steel parts Process Planning Figure 5.1: RP process planning Figure 5.2: Mold SDM planning Figure 5.3: Process planning flow chart Figure 5.4: Edge concavity classification Figure 5.5: General V-shaped edges Figure 5.6: Edges with a vertical surface Figure 5.7: Edges with both surfaces leaning to the same side Figure 5.8: Edges with horizontal or below horizontal surfaces Figure 5.9: Manufacturability of sharply curved surfaces Figure 5.10: Example part geometry Figure 5.11: First build orientation Figure 5.12: Second build orientation xv

16 Figure 5.13: Third build orientation Figure 5.14: Decomposition using planar layers Figure 5.15: Decomposition using non-planar layers Figure 5.16: Compact consolidation Figure 5.17: Compact consolidation using the CAG Figure 5.18: Pitch shaft geometry Figure 5.19: Horizontal build orientation Figure 5.20: Horizontal orientation decomposition Figure 5.21: Cylindrical section build sequence Figure 5.22: Pitch shaft built in a horizontal orientation Figure 5.23: Vertical build orientation Figure 5.24: Vertical orientation decomposition Figure 5.25: Pitch shaft built in a vertical orientation Figure 5.26: Pitch shaft divided into 2.5D sections Figure 5.27: Interaction between sections 1 and Figure 5.28: Multiple build orientation build sequence Figure 5.29: Pitch shaft built using multiple build directions Process Automation Figure 6.1: Mold SDM machine Figure 6.2: Mold SDM machine schematic Figure 6.3: Machine control schematic Figure 6.4: Teflon casting frame Figure 6.5: Casting using containment walls Process Characterization Figure 7.1: Engine vanes Figure 7.2: Silicon artery models Figure 7.3: Miniature alumina turbines Figure 7.4: Pitch shaft build time breakdown Figure 7.5: Surface ripples Figure 7.6: Slumped tops of uprights Figure 7.7: Impeller mold sections showing thermal distortion Figure 7.8: Mold with cracks due to residual stress buildup Figure 7.9: Christmas Tree surface profile Figure 7.10: Christmas Tree effect experiment xvi

17 Figure 7.11: Voids resulting from incomplete mold filling Figure 7.12: Mold features that are difficult to fill Figure 7.13: Flaking and cracking of green part during curing Applications Figure 8.1: Vane doublet from Rolls-Royce Pegasus engine Figure 8.2: M-DOT miniature gas turbine engine Figure 8.3: Inlet nozzle from the M-DOT engine Figure 8.4: Pitch shaft from a missile guidance system xvii

18 1 Introduction 1.1 Overview The goal of this research was to develop a new manufacturing process for the fabrication of geometrically complex, high quality structural ceramic components. The primary application is for components for use in the hot sections of gas turbine engines. In order to accomplish this a number of areas had to be addressed including process development, materials selection and process planning strategies Process Development Existing ceramic manufacturing processes have a number of limitations. They are not well suited to the fabrication of complex parts, for example engine vanes with internal cooling channels. Most processes are based on some type of tooling, which is expensive to fabricate and usually has a long lead time. This makes design and testing iteration for ceramic parts difficult and expensive. Research into the use of rapid prototyping processes to produce structural ceramic components has not been particularly successful because of the limitations in surface quality. Parts made from defect sensitive materials, such as ceramics, need to have smooth surfaces in order to achieve the best mechanical properties. 1

19 One of the primary goals of this research was therefore to develop a new manufacturing method that would combine the shape complexity advantages of the rapid prototyping methods together with the quality of the traditional manufacturing processes, without suffering from any of the disadvantages of either. The result was Mold Shape Deposition Manufacturing (Mold SDM) Materials Selection and Optimization Having identified a promising process, it was necessary to identify a set of materials that could be used to implement the process. Because the Mold SDM process is based on Shape Deposition Manufacturing (SDM), the starting materials were chosen from the range of materials already used to build polymer parts using SDM. As the process developed, experience indicated that these materials could be further optimized, or replaced by new materials, to improve the quality of parts produced. As a result of this work, essential material characteristics were identified and used to identify and evaluate a range of new materials. These were then tested and several best combinations were selected for use with Mold SDM Process Planning Initially, Mold SDM planning was essentially identical to SDM planning. However as parts were built it became apparent that there were a number of limitations inherent in the planning process. Parts were not built exactly as specified, but were instead built as close approximations. Sharp corners, for example, were often built with small fillets. The context of features was also not considered in the planning process and this made it difficult to optimize the build strategy to take advantage of part features to build better quality parts. Experience gained from building parts was used to formulate a number of new planning and build techniques to improve part quality. A number of new part decomposition schemes were also developed to take advantage of process capabilities. 2

20 1.2 Thesis Outline Chapter 2 presents background information on ceramic materials and applications and then explains the range of current manufacturing processes and what their limitations are. The chapter concludes with an outline of the goals for this research, based on the limitations of current manufacturing technologies. Chapter 3 explains the Mold SDM process and how it was developed as an extension of the original SDM process. The chapter also presents a number of new build techniques which were developed to improve part quality, build rate and the range of manufacturable features. Chapter 4 describes the materials selection process. The materials requirements and issues for the Mold SDM process are presented in terms of the process steps. The history of the material selection work is described together with a number of material testing procedures which were developed to evaluate the suitability of the materials. The current best material selection is presented and a number of example parts are shown. Chapter 5 introduces a manufacturability analysis as well as a number of new process planning strategies. These are all based on the concept of exact manufacturability, where a part is built exactly as specified, with no geometrical approximations due to systemic process limitations. The new planning strategies are designed to improve part quality as well as to reduce the build time and simplify fabrication. Process automation issues are addressed in Chapter 6. These typically involve new process strategies that make the process easier to automate. This chapter also describes the Mold SDM machine which has been built to automate the mold building phase of the process. Chapter 7 describes initial characterization of the Mold SDM process. Estimates of process capabilities are given based on measurements made on parts built using Mold SDM. A number of common part defects are illustrated and methods for avoid them are presented. The Mold SDM process capabilities are also briefly compared to those of other processes. Chapter 8 examines applications for Mold SDM. The process is evaluated from an appli- 3

21 cations point of view to identify the general characteristics of applications that are ideal for Mold SDM. The chapter then presents specific current applications and explores some of the potential future applications. Chapter 9 contains brief conclusions and pointers to possible future research directions. 1.3 Individual / Group Research Statement The Stanford University Rapid Prototyping Laboratory is a group environment where collaboration and interaction are strongly encouraged. Consequently much of the research is the result of work performed by multiple individuals. All of the work presented in this thesis was performed in this environment. The basic SDM process was developed at Carnegie Mellon University and further refined at Stanford University. The Mold SDM process was my idea and I carried out all of the early development work. The new build techniques are entirely my own work and were developed based on experience gained during part building. Initial Mold SDM process development used materials that had for the most part been previously identified by researchers at Carnegie Mellon University and John Kietzman. As the process developed further, I investigated a variety of new materials which had not been used previously. All of the materials testing procedures presented are my own work. The manufacturability analysis and new planning strategies are almost entirely my own work. Discussions with Dr. Ju-Hsien Kao and Miguel Pinilla, as well as their development of the Compact Adjacency Graph, did influence this work. The construction of the Mold SDM machine was a group effort. I was principally responsible for the development of the initial concept and for equipment selection. I also participated in various phases of the machine construction. 4

22 2 Background This chapter presents background information on ceramic materials and the issues associated with the fabrication of ceramic components and their use. The chapter begins with a brief overview of ceramic materials in general, and then discusses ceramic materials used in structural applications. Traditional ceramic part fabrication processes are explained and compared with some of the newer Rapid Prototyping processes which have been developed recently for the fabrication of ceramic parts. The chapter ends with a definition of the goals for this research based on the limitations and issues of the existing ceramic fabrication processes. 2.1 Ceramics In the most general terms ceramics are defined as inorganic, non-metallic materials. Naturally occurring materials, such as rocks and minerals, are usually excluded. Inorganic glasses can be considered to be a subset of ceramics, but they differ in that they do not display long range order in their structure. Ceramics are typically oxides, carbides, nitrides or borides. In most applications they are used in polycrystalline form rather than as single 5

23 crystals. Ceramic materials have a tremendous range of properties which are useful for mechanical, electrical, optical and thermal applications. Currently the most widespread uses for ceramic materials are in electrical applications where ceramics are used because of their dielectric, piezoelectric, magnetic and superconductive properties Structural Ceramics Structural ceramics are ceramics that are used in applications that involve mechanical loading. Compared to structural metals, ceramics are attractive because of their low densities and high strengths, high stiffnesses and excellent creep resistances. As the use temperature increases, ceramics become more attractive because they maintain their mechanical properties to higher temperatures than metals do. For high temperature applications ceramic materials also have the advantage that they do not contain strategic materials such as refractory metals. Ceramics usually have excellent chemical and oxidation resistance, as well as good abrasion resistance because of their hardness. The major limitations of ceramics for structural applications are their low toughnesses and high costs. The two Ashby charts shown in Figure 2.1 show where ceramics lie with respect to other engineering materials in terms of their properites [Ashby 92]. The first chart, Figure 2.1a, shows fracture toughness versus strength. Ceramcs are the strongest class of materials. This is even more evident if specific strength is considered, because of the relatively low densities of ceramics compared to metallic alloys. However the fracture toughnesses of ceramics are inferior to all but the most brittle alloys. The second chart, Figure 2.1b, shows strength versus operating temperature. This figure clearly illustrates the benefits of ceramics as they are not only the strongest materials, but they also have the highest operating temperatures. Silicon Nitride Silicon nitride is one of the primary structural ceramics because of its high strength, particularly at high temperatures, high thermal shock resistance and good oxidation resistance at high temperatures. Silicon nitride is the ceramic material of choice for use in high tem- 6

24 Fracture Toughness (MPa m 1/2 ) Polymer Foams Polymers ,000 Strength (MPa) a Alloys Composites 10,000 10,000 Strength (MPa) 1, Ceramics Composites Polymers Polymer Foams Temperature ( C) b Alloys Elastomers Ceramics Figure 2.1: Ashby charts illustrating material property ranges perature structural applications such as heat engine components. Application examples include gas turbine engine vanes and blades, turbocharger rotors, internal combustion engine valves and high temperature seals. Silicon nitride components can be used at temperatures up to about 1350 C. Its high strength, hardness and thermal shock resistance also make silicon nitride attractive as a cutting tool material. Silicon nitride is also used in general structural applications which do not experience high temperatures. Examples include ball bearings, thread guides and parts for paper making machines. Alumina Alumina is most often used in electrical and electronic applications because it is an excellent insulator and is relatively inexpensive. However it is also used in structural applications because of its good mechanical properties and relatively low cost. It is not as strong as silicon nitride and has much less thermal shock resistance. Alumina is very chemically resistant, and since it is an oxide ceramic it is oxidation resistant, which makes it ideal for use in harsh chemical environments. Alumina is also used in biomedical applications, such as hip prostheses, because of its 7

25 excellent biocompatibility, corrosion resistance, wear resistance and strength. Alumina has been used to make cutting tools since the 1920s but because of its low thermal shock resistance it is usually toughened using silicon carbide whiskers or zirconia. Silicon Carbide Silicon carbide is used in many of the same structural applications as silicon nitride. It has good mechanical properties, maintaining its strength to temperatures up to 1400 C, high thermal shock resistance and good oxidation resistance at high temperatures. Because of its excellent wear resistance silicon carbide is used in applications such as sand blasting nozzles, seal rings and bearings. Its high hardness makes it attractive for use in armor applications, and together with its excellent thermal shock resistance, as a cutting tool material. Silicon carbide is also frequently used as an abrasive in the form of grinding wheels and powders. Zirconia Partially stabilized zirconia, which is toughened by a phase transformation mechanism [Evans 90], is the toughest of the structural ceramics with a fracture toughness approximately twice that of silicon nitride. Above about 800 C, a phase transformation negates the toughening effect, so zirconia is usually used in lower temperature applications. Zirconia is used in the same biomedical applications as alumina. Advantages of zirconia over alumina are its higher toughness and its strength which can be approximately double that of alumina. Zirconia is also used in refractory applications, as grinding media, extrusion dies and as wear resistant components. Cubic zirconia is an excellent ionic conductor and is used for oxygen sensor applications. Property Comparisons Table 2.1 lists some of the properties of these ceramic materials and Table 2.2 lists the same properties for a selection of typical structural metals [ASM 97, ASM 91, ASM 90a, 8

26 ASM 90b, ASM 82]. Properties shown are measured at room temperature unless shown otherwise. The maximum operating temperatures are approximate values since these depend greatly on the loading and service life required. The maximum use temperature for the 7075-T6 aluminum is shown as 100 C because the strength drops sharply at temperatures above 100 C. Property Density (g/cm 3 ) Strength (MPa) Elastic Modulus (GPa) Fracture Toughness (MPa /m) Thermal Expansion (10-6/K) Thermal Conductivity (W/mK) Maximum Use Temperature ( C) Alumina Silicon Nitride Silicon Carbide Zirconia (PSZ) Table 2.1: Typical properties of structural ceramics Property Density (g/cm 3 ) Yield Strength (MPa) Elastic Modulus (GPa) Fracture Toughness (MPa /m) Thermal Expansion (10-6 /K) Thermal Conductivity (W/mK) Maximum Use Temperature ( C) Aluminum (7075-T6) Titanium (Ti6 Al4 V) Steel (AISI 4340) Inconel (IN-738) C C Table 2.2: Typical properties of structural alloys 9

27 2.1.2 Issues Despite their attractive properties ceramics are currently limited in their applications by a number of factors: Manufacturability: because of the limitations of current manufacturing processes it is difficult to make complex shapes in ceramic materials [Panzarino 96]. In most cases, due to the variability in sintering shrinkage (typically ± 2%) and the need for defect free surfaces, ceramic parts need to be machined and/or ground after sintering. Because of the hardness and defect sensitivity of ceramics this is a difficult and expensive process. Low Toughness: because of their low toughness ceramics are very defect sensitive. This is explained in more detail below. Design Rules: in a ceramic part failure occurs when the critical stress of one of the defects in the part is exceeded. All ceramic parts contain defects and since there is no practical way to determine the flaw distribution in a particular part, failure is probabilistic. Design practice must therefore be based on the probability of the existence of a given flaw distribution in the parts. This is a very different design approach to the traditional approach used for metal parts and because of this designers need to adapt their design strategies to ceramics. Cost: ceramic parts are expensive because of the high purity starting materials and the carefully controlled processing steps required. Surface Quality Requirements Under mechanical loading ceramic materials usually fail by fracture, although at high temperatures effects such as creep may also contribute to failure. Brittle failure results from the lack of ductility in ceramic materials at low temperatures. Brittle failure is governed by the principles of fracture mechanics. In any component there will be a population of defects spread throughout the component. These defects will be of various configurations, such as cracks and voids, and will be present in a range of sizes. For any given loading condition, each defect will have a critical stress above which it will grow rapidly and cause failure of the part. Part failure therefore occurs when the critical 10

28 stress of one of the defects is exceeded. There are two general types of defect: surface and volume defects. Surface defects are located on the surfaces of the part and are typically small cracks, scratches and surface roughness. Volume defects occur inside the part and are typically cracks, voids and foreign particle inclusions. The critical stress for a defect depends on its size and configuration. Larger and sharper defects generally have lower critical stresses. Sharp cracks, for example, are worse than spherical voids. The following simple calculation illustrates the sensitivity of ceramic materials to defects. The failure loads for a beam with a 100 µm crack on the underside will be calculated for a 3-point bending case as shown in Figure 2.2. For comparison purposes the failure loads F a = 100 µm h,b = 10 mm L = 20 mm L = 20 mm Figure 2.2: Beam with a surface flaw loaded in 3-point bending will be calculated for a beam made of silicon nitride and for one made of 4340 high strength steel. The properties of these materials are shown in Table 2.3. Material Failure Strength (MPa) Fracture Toughness (MPa m 1/2 ) 4340 steel 860 (yield) 100 Silicon nitride Table 2.3: Material properties of silicon nitride and 4340 steel 11

29 The 4340 steel was selected because it is a structural steel with a strength in the range of typical strengths for silicon nitride. The strength of the silicon nitride was set equal to that of the 4340 steel for the purposes of this comparison. Note that the failure strength used for the 4340 steel is its yield strength. The failure loads can be calculated as follows. Ignoring the defect, the beam failure load can be calculated from simple beam theory: The maximum stress found on the top and bottom surfaces of the beam is: σ = 3FL bh 2 The failure load is therefore given by: F = bh σ 3L For failure by fracture the following equation relates the critical stress to the crack size: K IC = σk πa Here K is the stress intensity factor, which for this configuration is very nearly 1. K IC is the critical stress intensity required for failure. This equation can be rearranged to give the failure load due to fracture by using the maximum stress equation from above: F = K IC bh L πa Table 2.4 lists the calculated failure loads. The 4340 steel beam will fail by yielding at a load of 14.3 kn. Failure by fracture occurs at a much higher load and will therefore not be seen in practice. The silicon nitride beam does not yield but fails at 4.7 kn due to fracture. 12

30 Compared to the steel beam, which has the same strength, this is only about a third the failure load and it results from a defect only 100 µm in size. The steel beam will also fail gracefully by yielding rather than by suddenly fracturing. Material Yield (kn) Fracture (kn) 4340 steel Silicon nitride Table 2.4: Calculated failure loads This simple calculation illustrates why it is so important to produce defect free ceramic parts. Because of the lack of ductility, ceramic parts can t relieve the high stresses around their defects and thus the defect population controls the mechanical strength of ceramic parts. This is the reason why surface quality is so critical for structural ceramic parts. 2.2 Ceramic Manufacturing Processes Most ceramic parts are currently made in a two step process as shown in Figure 2.3. First, ceramic powders are formed into green parts. These are porous bodies consisting of tightly packed ceramic powder particles. Then to achieve full density the green parts are sintered at high temperature. During the sintering process the powder particles fuse together and produce a strong, dense ceramic part. Ceramic Powder Form Green Part Sinter Finished Part Figure 2.3: Ceramic fabrication process sequence For advanced ceramics applications the starting ceramic powders are usually high purity powders with particle sizes of 1 micron or less. The small particle size is needed because the mechanical properties of ceramics increase with decreasing grain size. Finer powders are also easier to sinter because they sinter faster and at lower temperatures. The forming step compacts the powder into a porous shape, or green part. A binder, typi- 13

31 cally a polymer, must normally be added to the powder to give the green part some mechanical strength so that it can withstand handling. A wide range of forming processes are in use, some of the more common are described briefly in the following sections. The goal of all these processes is to achieve maximum packing density and uniformity because these factors directly impact the sintering process. Higher green density makes sintering easier and reduces the sintering shrinkage. Non-uniform green density results in non-uniform sintering rates and shrinkage which can lead to warping and cracking during sintering. Green parts generally contain fluids and organic components in addition to the ceramic powder. These are leftover from the forming process and must be removed before the ceramic powder can be densified by sintering. The organic components and ceramic powder typically form a porous network structure with the fluid filling the pores. The fluid is removed first by drying. This must be performed in a carefully controlled fashion to avoid damage to the part caused by capillary forces. Once the fluid has been removed the organic components are burnt out to leave a porous ceramic structure. At this point the ceramic particles have begun to fuse but the links are very weak. Because the drying and burnout steps tend to be time consuming it is desirable to minimize the amount of liquid and organic components in the green parts. Sintering is a high temperature process where the individual particles fuse together to form a dense part. In order to achieve high sintered densities, green parts typically need to have densities of 50% or greater. Higher density parts usually sinter more easily and shrink less during sintering. Packing uniformity is important because uneven shrinkage during sintering usually causes warping. Areas of low packing density may fail to sinter to full density leaving voids in the sintered part. Because the green parts are usually only 50-60% dense there is significant shrinkage during sintering. To compensate for this the green parts are made correspondingly larger. In most cases ceramic parts are machined or ground after sintering to achieve required geometrical tolerances. The amount of shrinkage and process variability makes it very difficult to achieve tight tolerances without this. 14